OneBeacon Insurance 10-K 28 Feb 14 2013 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2013	Jun. 30, 2013	Feb. 25, 2014	Feb. 25, 2014
			Class A common	Class B common
Entity Registrant Name	'OneBeacon Insurance Group, Ltd.	'	'	'
Entity Central Index Key	'0001369817	'	'	'
Document Type	'10-K	'	'	'
Document Period End Date	31-Dec-13	'	'	'
Amendment Flag	'false	'	'	'
Current Fiscal Year End Date	'--12-31	'	'	'
Entity Well-known Seasoned Issuer	'No	'	'	'
Entity Voluntary Filers	'No	'	'	'
Entity Current Reporting Status	'Yes	'	'	'
Entity Filer Category	'Accelerated Filer	'	'	'
Entity Public Float	'	$327,705,756	'	'
Entity Common Stock, Shares Outstanding	'	'	23,583,865	71,754,738
Document Fiscal Year Focus	'2013	'	'	'
Document Fiscal Period Focus	'FY	'	'	'

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fixed maturity investments, at fair value	'	'
Fixed maturity investments, at fair value	$1,700.90	$1,593.30
Short-term investments, at amortized cost (which approximates fair value)	157	232.8
Common equity securities, at fair value	336.9	259
Convertible fixed maturity investments, at fair value	30.5	62.6
Other investments	139.6	143.8
Total investment securities	2,364.90	2,291.50
Cash	168.1	43.9
Reinsurance recoverables	89.9	110.6
Premiums receivable	228.2	225.6
Deferred acquisition costs	103.7	123.9
Net deferred tax asset	90.6	137.8
Investment income accrued	10.1	12.1
Accounts receivable on unsettled investment sales	3.3	2.1
Other assets	272.7	227.2
Assets held for sale	1,880.10	2,226.80
Total assets	5,211.60	5,401.50
Liabilities	'	'
Unpaid loss and loss adjustment expense reserves	1,054.30	1,000
Unearned premiums	544.9	573.8
Debt	274.7	274.7
Accounts payable on unsettled investment purchases	11.6	6.2
Other liabilities	338.6	302.7
Liabilities held for sale	1,880.10	2,226.80
Total liabilities	4,104.20	4,384.20
OneBeacon's common shareholders' equity	'	'
Common shares and paid-in surplus (par value $0.01; authorized, 200,000,000 shares; issued and outstanding, 95,404,138 and 95,386,179 shares)	1,022.50	1,019.10
Retained earnings	75	9.2
Accumulated other comprehensive loss, after tax	6.8	-13.8
Total OneBeacon's common shareholders' equity	1,104.30	1,014.50
Total noncontrolling interests, after tax	3.1	2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,107.40	1,017.30
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$5,211.60	$5,401.50

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Statement of Financial Position [Abstract]	'	'
Common shares, par value (in dollars per share)	$0.01	$0.01
Common shares, authorized shares (in shares)	200,000,000	200,000,000
Common shares, issued shares (in shares)	95,404,138	95,386,179
Common shares, outstanding shares (in shares)	95,404,138	95,386,179

CONSOLIDATED_STATEMENTS_OF_OPE

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues	'	'	'
Earned premiums	$1,120.40	$1,132	$1,012.20
Net investment income	41.1	53.6	71.4
Net realized and change in unrealized investment gains	49.4	55.7	10.6
Net other revenues (expenses)	31.2	-0.5	-12.4
Total revenues	1,242.10	1,240.80	1,081.80
Expenses	'	'	'
Loss and loss adjustment expenses	622.1	650	548.3
Policy acquisition expenses	208.9	249.4	221.2
Other underwriting expenses	204.8	205.2	162.3
General and administrative expenses	12	13.4	9.8
Interest expense	13	16.9	20.5
Total expenses	1,060.80	1,134.90	962.1
Pre-tax income from continuing operations	181.3	105.9	119.7
Income tax expense	-34.3	-8.4	-14.8
Net income from continuing operations	147	97.5	104.9
Loss from discontinued operations, net of tax	-46.6	-24.3	-29.6
Gain (loss) from sale of discontinued operations, net of tax	46.6	-91	-19.2
Net income (loss), including noncontrolling interests	147	-17.8	56.1
Less: Net income attributable to noncontrolling interests	-1	-1.4	-1
Net income (loss) attributable to OneBeacon's common shareholders	146	-19.2	55.1
Net change in benefit plan assets and obligations, net of tax	20.6	-2.9	-11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	$166.60	($22.10)	$43.90
Earnings (loss) per share attributable to OneBeacon's common shareholdersbbasic and diluted	'	'	'
Net income (loss) from continuing operations per share	$1.52	$1	$1.08
Loss from discontinued operations, net of tax, per share	($0.49)	($0.25)	($0.30)
Loss from sale of discontinued operations, net of tax, per share (in dollars per share)	$0.49	($0.96)	($0.20)
Net income (loss) attributable to OneBeacon's common shareholders per share	$1.52	($0.21)	$0.58
Dividends declared and paid per OneBeacon's common share (in dollars per share)	$0.84	$0.84	$1.84

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMMON SHAREHOLDERS' EQUITY (USD $)	Total	Common shares and paid-in surplus	Retained earnings	Accum. other comprehensive income (loss)	Total OneBeacon common shareholders' equity	Noncontrolling interests, after tax
Balances at Dec. 31, 2010	$1,248,900,000	$1,000,500,000	$228,200,000	$300,000	$1,229,000,000	$19,900,000
Increase (Decrease) in Stockholders' Equity	'	'	'	'	'	'
Net income	56,100,000	0	55,100,000	0	55,100,000	1,000,000
Other comprehensive income item, after tax:	'	'	'	'	'	'
Net change in benefit plan assets and obligations, net of tax	-11,200,000	0	0	-11,200,000	-11,200,000	0
Total comprehensive income	44,900,000	0	55,100,000	-11,200,000	43,900,000	1,000,000
Amortization of restricted share and option awards	1,400,000	1,400,000	0	0	1,400,000	0
Issuance of common shares	600,000	300,000	0	0	300,000	300,000
Repurchases and retirements of common shares	-1,300,000	0	0	0	0	-1,300,000
Dividends	-175,700,000	0	-174,800,000	0	-174,800,000	-900,000
Contributions	-100,000	0	0	0	0	-100,000
Distributions	-5,000,000	0	0	0	0	-5,000,000
Balances at Dec. 31, 2011	1,113,900,000	1,002,200,000	108,500,000	-10,900,000	1,099,800,000	14,100,000
Increase (Decrease) in Stockholders' Equity	'	'	'	'	'	'
Net income	-17,800,000	0	-19,200,000	0	-19,200,000	1,400,000
Other comprehensive income item, after tax:	'	'	'	'	'	'
Net change in benefit plan assets and obligations, net of tax	-2,900,000	0	0	-2,900,000	-2,900,000	0
Total comprehensive income	-20,700,000	0	-19,200,000	-2,900,000	-22,100,000	1,400,000
Amortization of restricted share and option awards	2,600,000	2,600,000	0	0	2,600,000	0
Issuance of common shares	500,000	300,000	0	0	300,000	200,000
Dividends	-80,700,000	0	-80,100,000	0	-80,100,000	-600,000
Contributions	-200,000	0	0	0	0	-200,000
Gain on sale of OB Holdings Lux	14,000,000	14,000,000	0	0	14,000,000	0
Sale of investment with noncontrolling interest	-12,500,000	0	0	0	0	-12,500,000
Balances at Dec. 31, 2012	1,017,300,000	1,019,100,000	9,200,000	-13,800,000	1,014,500,000	2,800,000
Increase (Decrease) in Stockholders' Equity	'	'	'	'	'	'
Net income	147,000,000	0	146,000,000	0	146,000,000	1,000,000
Other comprehensive income item, after tax:	'	'	'	'	'	'
Net change in benefit plan assets and obligations, net of tax	20,600,000	0	0	20,600,000	20,600,000	0
Total comprehensive income	167,600,000	0	146,000,000	20,600,000	166,600,000	1,000,000
Amortization of restricted share and option awards	3,100,000	3,100,000	0	0	3,100,000	0
Issuance of common shares	400,000	300,000	0	0	300,000	100,000
Dividends	-81,000,000	0	-80,200,000	0	-80,200,000	-800,000
Distributions	0	0	0	0	0	0
Balances at Dec. 31, 2013	$1,107,400,000	$1,022,500,000	$75,000,000	$6,800,000	$1,104,300,000	$3,100,000

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash flows from operations:	'	'	'
Net income (loss), including noncontrolling interests	$147	($17.80)	$56.10
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:	'	'	'
Net loss from discontinued operations	46.6	24.3	29.6
Net (gain) loss from sale of discontinued operations	-46.6	91	19.2
Net realized and change in unrealized investment gains	-49.4	-55.7	-10.6
Net realized gain on sale of business	-23	-4.4	0
Net other realized losses	0	6.3	11.7
Deferred income tax expense (benefit)	12.8	16.4	31.6
Other operating items:	'	'	'
Net change in loss and LAE reserves	54.3	87.5	69.2
Net change in unearned premiums	-28.9	48.4	50.2
Net change in premiums receivable	-2.6	-14.6	-49.4
Net change in reinsurance recoverable on paid and unpaid losses	20.7	-3.1	-46.3
Net change in other assets and liabilities	74.5	-35.6	-79.2
Net cash provided from operationsbcontinuing operations	205.4	142.7	82.1
Net cash used for operationsbdiscontinued operations	-76.7	-196.4	-200.6
Net cash provided from (used for) operations	128.7	-53.7	-118.5
Cash flows from investing activities:	'	'	'
Net maturities, purchases and sales of short-term investments	72.2	14.6	-19.9
Maturities of fixed maturity investments	245.2	191.5	478.3
Sales of fixed maturity investments	1,400	1,717.20	1,414.80
Sales of common equity securities	196.7	112.5	101.9
Sales of convertible fixed maturity investments	45.7	29.3	37.1
Return of capital and distributions of other investments	21.8	63.9	38.6
Purchases of fixed maturity investments	-1,697	-1,862.50	-1,472.30
Purchases of common equity securities	-206.3	-91.5	-88.5
Purchases of convertible fixed maturity investments	-14.4	-9.4	-30.4
Contributions for other investments	-7.6	-44.9	-10.8
Proceeds from sale of business	31.3	15	0
Net change in unsettled investment purchases and sales	4.1	-18.2	13.5
Net acquisitions of property and equipment	-10.3	-1.8	-3.7
Net cash provided from investing activitiesbcontinuing operations	81.4	115.7	458.6
Net cash provided from investing activitiesbdiscontinued operations	0	0	0
Net cash provided from investing activities	81.4	115.7	458.6
Cash flows from financing activities:	'	'	'
Issuance of debt, net of debt issuance costs	0	271.9	0
Repurchases of debt	0	-275.9	-161.6
Cash dividends paid to common shareholders	-80.2	-80.1	-174.8
Proceeds on sale of OB Holdings Lux	0	14	0
Proceeds from (payments on) capital lease obligation	-5.7	-4.9	23.1
Net cash used for financing activitiesbcontinuing operations	-85.9	-75	-313.3
Net cash used for financing activitiesbdiscontinued operations	0	0	0
Net cash used for financing activities	-85.9	-75	-313.3
Net increase (decrease) in cash during period	124.2	-13	26.8
Cash reclassified to assets held for sale	0	5.5	-5.5
Cash transferred with sale of business	0	-3.5	0
Net increase (decrease) after reclassification of cash to assets held for sale	124.2	-11	21.3
Cash balance at beginning of period	43.9	54.9	33.6
Cash balance at end of period	$168.10	$43.90	$54.90

Nature_of_Operations_and_Summa

Nature of Operations and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2013
Accounting Policies [Abstract]	'
Nature of Operations and Summary of Significant Accounting Policies	'
	Nature of Operations and Summary of Significant Accounting Policies
	Basis of presentation
	The accompanying consolidated financial statements include the accounts of OneBeacon Insurance Group, Ltd. (the "Company" or the "Registrant") and its subsidiaries (collectively, "OneBeacon") and have been prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP"). The Company is an exempted Bermuda limited liability company. The OneBeacon U.S.-based operating companies are property and casualty insurance writers, most of which historically have operated in a multi-company pool or have participated in 100% quota share reinsurance agreements. OneBeacon formed an indirect wholly-owned subsidiary, Split Rock Insurance, Ltd. ("Split Rock"), a Bermuda-based reinsurance company, which entered into an agreement in 2013 to reinsure certain risks of an affiliated entity. OneBeacon offers a wide range of specialty insurance products and services primarily through independent agencies, regional and national brokers, wholesalers and managing general agencies.
	OneBeacon was acquired by White Mountains Insurance Group, Ltd. ("White Mountains") from Aviva plc ("Aviva") in 2001 (the "OneBeacon Acquisition"). White Mountains is a holding company whose businesses provide property and casualty insurance, reinsurance and certain other products. As of December 31, 2013, White Mountains owned 75.2% of the Company's common shares. Within this report, the term "OneBeacon" is used to refer to one or more entities within the consolidated organization, as the context requires. The Company's headquarters are located at 14 Wesley Street, 5th Floor, Hamilton HM 11, Bermuda. The Company's U.S. corporate headquarters are located at 601 Carlson Parkway, Minnetonka, Minnesota 55305 and its registered office is located at Clarendon House, 2 Church Street, Hamilton HM 11, Bermuda.
	In October 2012, one of OneBeacon's indirect wholly-owned subsidiaries, OneBeacon Insurance Group LLC ("OneBeacon LLC"), entered into a definitive agreement (as amended, the "Stock Purchase Agreement") with Trebuchet US Holdings, Inc. ("Trebuchet"), a wholly-owned subsidiary of Armour Group Holdings Limited (together with Trebuchet, “Armour”), to sell its run-off business. See Note 2—"Acquisitions and Dispositions" and Note 19—"Discontinued Operations." OneBeacon's run-off business includes the results of OneBeacon's remaining non-specialty commercial lines business and certain other run-off business, including the vast majority of asbestos and environmental reserves, as well as certain purchase accounting adjustments related to the run-off business and OneBeacon Acquisition (the “Runoff Business,” the sale of which is referred to as the “Runoff Transaction”). The assets and liabilities associated with the Runoff Business as of December 31, 2013 and 2012, have been presented in the balance sheet as held for sale assuming the investing and financing steps required to effect the sale were completed as of the current balance sheet date. The Runoff Business has been presented as discontinued operations in the consolidated statements of operations and cash flows, with the prior periods reclassified to conform to the current period's presentation. The Runoff Business disposal group excludes investing and financing activities from amounts classified as discontinued operations. OneBeacon's investing and financing operations are conducted on an overall consolidated level and, accordingly, there are no separately identifiable investing or financing cash flows associated with the Runoff Business. Pursuant to the terms of the Stock Purchase Agreement, the legal entities included in the sale and expected to be transferred to Armour will hold an agreed upon level of invested assets and capital at closing.
	In anticipation of the Runoff Transaction, and as means to separate the Runoff Business from the ongoing specialty business, OneBeacon sought and received various regulatory approvals to terminate, enter into or amend various intercompany reinsurance agreements which took effect on October 1, 2012.
	OneBeacon's reportable segments are Specialty Products, Specialty Industries and Investing, Financing and Corporate. The Specialty Products segment is comprised of eight underwriting operating segments, including a new Crop underwriting operating segment, as well as the Collector Cars and Boats underwriting operating segment that was exited in 2013, representing an aggregation based on those that offer distinct products and tailored coverages and services to a broad customer base across the United States. During 2013, the Company received approval to provide Multiple Peril Crop Insurance ("MPCI") through the federal crop insurance program administered by the U.S. Department of Agriculture's Risk Management Agency ("RMA"). The Company has entered into an exclusive agreement with a managing general agency, Climate Crop Insurance Agency LLC ("The Climate Corporation"), to provide coverages through the federal program and other supplemental coverages, including crop-hail. The Company began writing crop business in the fourth quarter of 2013.
	The Specialty Industries segment is comprised of six underwriting operating segments, including the Energy underwriting operating segment that was exited in 2013, representing an aggregation based on those that focus on solving the unique needs of a particular customer or industry group.
	The Investing, Financing and Corporate segment includes the investing and financing activities for OneBeacon on a consolidated basis, and certain other activities conducted through the Company and its intermediate subsidiaries.
	All significant intercompany transactions have been eliminated in consolidation. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
	Significant Accounting Policies
	Investment Securities
	OneBeacon adopted the fair value election, which allows companies to report financial assets and liabilities at fair value, on an individual basis. The election must be made at the inception of a transaction and may not be reversed. OneBeacon believes that reporting its investment results in this manner is consistent with one of its operating principles, namely to manage investments for total return.
	OneBeacon classifies its portfolio of fixed maturity investments and common equity securities, including convertible fixed maturity investments, held for general investment purposes as trading securities. Trading securities are reported at fair value as of the balance sheet date as determined by quoted market prices when available. Realized and changes in unrealized investment gains on trading securities are reported, on a pre-tax basis, in revenues as net realized and change in unrealized investment gains.
	Short-term investments consist of money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized cost, which approximated fair value as of December 31, 2013 and 2012.
	Other investments primarily include hedge funds and private equity funds. OneBeacon measures its investments in hedge funds and private equity funds at fair value with changes therein reported in total revenues as net realized and change in unrealized investment gains. Other investments also includes an investment in a community reinvestment vehicle which is accounted for at fair value and a tax advantaged federal affordable housing development fund which is accounted for under the equity method.
	Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants (an "exit price"). Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources ("observable inputs") and a reporting entity's internal assumptions based upon the best information available when external market data is limited or unavailable ("unobservable inputs"). Quoted prices in active markets for identical assets or liabilities have the highest priority ("Level 1"), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities ("Level 2") and unobservable inputs, including the reporting entity's estimates of the assumptions that market participants would use, having the lowest priority ("Level 3").
	As of both December 31, 2013 and 2012, approximately 92% of the investment portfolio recorded at fair value was priced based upon quoted market prices or other observable inputs. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, common equities and short-term investments, which include U.S. Treasury Bills. Investments valued using Level 2 inputs comprise fixed maturity investments including corporate debt, state and other governmental debt, convertible fixed maturity investments and mortgage and asset-backed securities. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include OneBeacon's investments in hedge funds and private equity funds, as well as certain investments in debt and equity securities, including asset-backed securities, where quoted market prices are unavailable or are not considered reasonable. OneBeacon determines when transfers between levels have occurred as of the beginning of the period.
	OneBeacon uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, OneBeacon uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services OneBeacon uses have indicated that they will only provide prices where observable inputs are available. In circumstances where quoted market prices are unavailable or are not considered reasonable, OneBeacon estimates the fair value using industry standard pricing models and observable inputs such as benchmark interest rates, matrix pricing, market comparables, broker quotes, issuer spreads, bids, offers, credit rating prepayment speeds and other relevant inputs.
	OneBeacon's process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, evaluation of model pricing methodologies, review of the pricing services' quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from different independent pricing vendors on at least an annual basis, monthly analytical reviews of certain prices and review of assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. OneBeacon also performs back-testing of selected purchases and sales activity to determine whether there are any significant differences between the market price used to value the security prior to purchase or sale and the actual purchase or sale price on at least an annual basis. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on the procedures are considered outliers. Also considered outliers are prices that haven’t changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of OneBeacon's review process does not appear to support the market price provided by the pricing services, OneBeacon challenges the price. During the past year, approximately 30 securities fell outside OneBeacon's expected results, thereby triggering the challenge with the pricing service. If OneBeacon cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question.
	OneBeacon's investments in debt securities, including asset-backed securities, are generally valued using matrix and other pricing models. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage-backed and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life.
	Other investments, which are primarily comprised of hedge funds and private equity funds for which the fair value option has been elected, are carried at fair value based upon OneBeacon's proportionate interest in the underlying fund's net asset value, which is deemed to approximate fair value. The fair value of OneBeacon's investments in hedge funds and private equity funds has been estimated using net asset value because it reflects the fair value of the funds' underlying investments. OneBeacon employs a number of procedures to assess the reasonableness of the fair value measurements, including obtaining and reviewing each fund's audited financial statements and discussing each fund's pricing with the fund's manager.
	In circumstances where the underlying investments are publicly traded, such as the investments made by hedge funds, the fair value of the underlying investments is determined using current market prices. In circumstances where the underlying investments are not publicly traded, such as the investments made by private equity funds, the private equity fund managers have considered the need for a liquidity discount on each of the underlying investments when determining the fund's net asset value. In circumstances where OneBeacon's portion of a fund's net asset value is deemed to differ from fair value due to illiquidity or other factors associated with OneBeacon's investment in the fund, including counterparty credit risk, the net asset value is adjusted accordingly. See Note 5—"Investment Securities."
	Cash
	Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statements of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company's consolidated subsidiaries.
	Insurance Operations
	Premiums written are recognized as revenues and are earned ratably over the term of the related policy. Unearned premiums represent the portion of premiums written that are applicable to future insurance coverage provided by policies. MPCI net written premiums are recognized at the sales closing date, with necessary adjustments made as the Company receives acreage reports from the policyholders. Premiums written on this business are recognized as revenues and are earned ratably over the period of risk commencing with the sales closing date and ending with the crop harvest date.
	Deferred acquisition costs represent commissions, premium taxes, brokerage expenses and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as policy acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. This limitation is referred to as a premium deficiency. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses ("LAE"), unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. There were no premium deficiencies recognized for any years presented.
	Loss and LAE are charged against income as incurred. Unpaid insurance loss and LAE are based on estimates (generally determined by claims adjusters, legal counsel and actuarial staff) of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid reinsurance loss and LAE reserves are based primarily on reports received from ceding companies and actuarial projections. Unpaid loss and LAE reserves represent management's best estimate of ultimate loss and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated and any adjustments resulting therefrom are reflected in the current period. The process of estimating unpaid loss and LAE reserves involves a considerable degree of judgment by management and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements.
	OneBeacon discounts certain of its long-term workers compensation loss and LAE reserves when such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual claim basis. OneBeacon discounts these reserves using an average discount rate which is determined based on various assumptions including consideration of when the claims will be settled (3.5% at both December 31, 2013 and 2012). As of December 31, 2013 and 2012, the discount on OneBeacon's workers' compensation loss and LAE reserves amounted to $3.0 million (excluding $61.7 million which relates to reserves that have been reclassified to liabilities held for sale on the December 31, 2013 balance sheet) and $4.6 million (excluding $77.9 million which relates to reserves that have been reclassified to liabilities held for sale on the December 31, 2012 balance sheet), respectively.
	OneBeacon's insurance subsidiaries enter into ceded reinsurance contracts from time to time to protect their businesses from losses due to concentration of risk, to manage their operating leverage ratios and to limit losses arising from catastrophic events. The majority of such reinsurance contracts are executed through excess of loss treaties and catastrophe contracts under which the reinsurer indemnifies for a specified part or all of certain types of losses over stipulated amounts arising from any one occurrence or event. OneBeacon has also entered into quota share treaties with reinsurers under which all risks meeting prescribed criteria are covered on a pro rata basis. The amount of each risk ceded by OneBeacon is subject to maximum limits which vary by line of business and type of coverage.
	Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. The collectibility of reinsurance recoverables is subject to the solvency of the reinsurers. OneBeacon is selective in regard to its reinsurers, principally placing reinsurance with those reinsurers with strong financial condition, reputation, industry ratings and underwriting ability. Management monitors the financial condition and ratings of its reinsurers on an ongoing basis.
	Reinsurance premiums, commissions, expense reimbursements and reserves related to reinsured business are accounted for on a basis consistent with those used in accounting for the original policies issued and the terms of the reinsurance contracts. Premiums ceded to other companies are reported as a reduction of premiums written. Expense allowances received in connection with reinsurance ceded have been accounted for as a reduction of the related policy acquisition costs and are deferred and amortized accordingly.
	Accounting for Mandatory Shared Market Mechanisms
	As a condition of its licenses to do business in certain states, OneBeacon's insurance operations are required to participate in various mandatory shared market mechanisms commonly referred to as "residual" or "involuntary" markets. These markets generally consist of risks considered to be undesirable from a standard or routine underwriting perspective. Each state dictates the levels of insurance coverage that are mandatorily assigned to participating insurers within these markets. The total amount of such business an insurer must accept in a particular state is generally based on that insurer's market share of voluntary business written within that state. In certain cases, OneBeacon is obligated to write business from shared market mechanisms at a future date based on its historical market share of all voluntary policies written within that state. Involuntary business generated from mandatory shared market mechanisms may be treated as assumed reinsurance depending on the structure of the mechanism.
	OneBeacon's market assignments are typically required to be written in the current period, although, in certain cases OneBeacon is required to accept policy assignments at a future date. Anticipated losses associated with future market assignments are recognized when the amount of such anticipated losses is determined to be probable and can be reasonably estimated.
	Accounting for Insurance Related Assessments
	Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. OneBeacon's insurance subsidiaries record guaranty fund assessments when such assessments are billed by the respective guaranty funds. In addition, OneBeacon will monitor other insurance company insolvencies and will accrue a liability when such losses are determined to be probable and the assessment amounts can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years.
	Capital Lease
	In December 2011, the Company entered into lease financing arrangements with US Bancorp Equipment Finance, Inc. ("US Bancorp") and Fifth Third Equipment Finance Company ("Fifth Third") whereby it sold furniture and equipment and capitalized software, respectively, at a cost equal to net book value. The Company then leased the fixed assets back from US Bancorp for a lease term of five years and leased the capitalized software back from Fifth Third for a lease term of four years. See Note 16—"Commitments and Contingencies."
	Deferred Software Costs
	OneBeacon capitalizes costs related to computer software developed for internal use during the application development stage of software development projects. These costs generally consist of certain external, payroll and payroll related costs. OneBeacon begins amortization of these costs once the project or the respective phase of the project is completed and ready for its intended use. Amortization is on a straight line basis and over the useful life which generally ranges from three to five years. At December 31, 2013 and 2012, OneBeacon had unamortized deferred software costs of $11.8 million and $7.3 million, respectively.
	Federal and Foreign Income Taxes
	The majority of the Company's subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States.
	Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized.
	Foreign Currency Exchange
	The U.S. dollar is the functional currency for all of OneBeacon's businesses. OneBeacon is subject to foreign currency fluctuations associated with foreign investment securities. Assets and liabilities recorded in foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the average exchange rates for the period. Net foreign exchange gains and losses arising from the translation are reported as a component of net income in the period in which they arise.
	Variable Interest Entities
	OneBeacon consolidates a reciprocal insurance exchange ("reciprocal") as a variable interest entity ("VIE"), which is an entity in which the equity investors do not have the characteristics of a controlling interest or do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties. The primary beneficiary of a VIE is required to consolidate the VIE in its financial statements. The primary beneficiary is an entity that has the controlling financial interest in the VIE when it has both of the following: (1) the power to direct the activities of a variable interest entity that most significantly impact the entity’s economic performance and (2) the obligation to absorb losses of the entity that could potentially be significant to the VIE or the right to receive benefits from the entity that could potentially be significant to the VIE. A reporting entity must assess whether it has an implicit financial responsibility to ensure that a VIE operates as designed when determining if it has the power to direct the activities of the VIE that most significantly affect the entity’s economic performance. Ongoing reassessments of whether a reporting entity is the primary beneficiary of a VIE are required. See Note 13—"Variable Interest Entities ("VIE")."
	Defined Benefit Plans
	OneBeacon sponsors defined benefit plans and recognizes the funded status of the difference between plan assets at fair value and the projected benefit obligation (for defined benefit pension plans) or the accumulated benefit obligation (for other postretirement benefit plans) in its statement of financial position. The Company recognizes amounts previously deferred and amortized in other comprehensive income in the period in which they occur and measure plan assets and obligations as of the fiscal year end. See Note 8—"Retirement Plans."
	Recently Adopted Changes in Accounting Principles
	Policy Acquisition Costs
	On January 1, 2012, OneBeacon adopted Accounting Standards Update (“ASU”) 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts (Accounting Standards Codification ("ASC") 944). ASU 2010-26 changes the types of policy acquisition costs that are eligible for deferral. Specifically, ASU 2010-26 limits deferrable costs to those that are incremental direct costs of contract acquisition and certain costs related to acquisition activities performed by the insurer, such as underwriting, policy issuance and processing, medical and inspection costs and sales force contract selling. ASU 2010-26 defines incremental direct costs as those costs that result directly from and were essential to the contract acquisition and would not have been incurred absent the acquisition. Accordingly, under ASU 2010-26, deferrable acquisition costs are limited to costs related to successful contract acquisitions. Acquisition costs that are not eligible for deferral are to be charged to expense in the period incurred.
	OneBeacon adopted ASU 2010-26 prospectively. As a result of adopting ASU 2010-26, $5.6 million of unamortized deferred acquisition costs as of January 1, 2012, primarily relating to a portion of profit sharing commission that had been deferred under prior guidance, were determined to no longer be deferrable and were recognized in expense over the original amortization period. During the year ended December 31, 2012, all of the $5.6 million of unamortized acquisitions costs as of January 1, 2012 were recognized in expense. If OneBeacon had followed ASU 2010-26 in 2011, $5.6 million of acquisition costs that were deferred would have been recognized in expense during the year ended December 31, 2011.
	Fair Value Measurements and Disclosures
	On January 1, 2012, OneBeacon adopted ASU 2011-04, Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in US GAAP and IFRS (ASC 820). ASU 2011-04 clarifies existing guidance with respect to the concepts of highest and best use and valuation premise and measuring instruments classified within a reporting entity's shareholders' equity. ASU 2011-04 also clarifies disclosure requirements, requiring disclosure of quantitative information about unobservable inputs used in Level 3 fair value measurements. ASU 2011-04 also amends existing guidance. In circumstances where a reporting entity manages a portfolio of financial assets and liabilities based on the net market and counterparty credit risk exposures, ASU 2011-04 permits determination of the fair value of those instruments to be based on the net risk exposure. In addition, ASU 2011-04 permits the application of premiums or discounts to be applied in a fair value measurement to the extent that market participants would consider them in valuing the financial instruments. ASU 2011-04 also expands the required disclosures for Level 3 measurements, requiring that reporting entities provide a narrative description of the sensitivity of Level 3 fair value measurements to changes in unobservable inputs and the interrelationships between those inputs, if any. As a result of adopting ASU 2011-04, OneBeacon expanded its fair value disclosures. See Note 5—"Investment Securities."
	Comprehensive Income
	On January 1, 2012, OneBeacon adopted ASU 2011-05, Comprehensive Income (ASC 220). ASU 2011-05 requires all components of comprehensive income to be reported in a continuous financial statement or in consecutive statements displaying the components of net income and the components of other comprehensive income. Since OneBeacon already presented comprehensive income in a continuous financial statement, adoption of ASU 2011-05 had no effect on OneBeacon's financial statement presentation.
	Recently Issued Accounting Pronouncements
	Qualified Affordable Housing Projects
	On January 15, 2014 the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects ("QAHP") (ASC 323). The new guidance eases the requirements for an investor to elect to account for its investment in a QAHP using the effective yield method. Prior to the issuance of the new guidance, investors had to have a letter of credit guaranteeing the availability of the tax credit allocable to the investor, had to demonstrate that the projected yield based solely on the cash flows from the guaranteed tax credits was positive and had to be a limited partner in the QAHP for both legal and tax purposes. Under the new guidance, the letter of credit requirement has been eliminated and instead, the investor must simply be able to demonstrate that the tax credit allocable to the investor will be available. Investments in QAHP not meeting the criteria in the new guidance would be accounted for under the equity method or the cost method. The election to use the effective yield method is considered an accounting policy decision that should be applied consistently to all QAHP investments. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2014. The guidance should be applied retrospectively, with early adoption permitted. The Company holds an investment in a QAHP which is accounted for under the equity method and does not expect adoption to significantly affect its financial position.
	Unrecognized Tax Benefits
On July 18, 2013, the FASB issued ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for an NOL carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryfoward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of the ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance becomes effective for fiscal periods beginning on or after December 15, 2013 and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective adoption is allowed. The Company does not expect adoption to significantly affect its financial position.

Acqusitions_and_Dispositions

Acqusitions and Dispositions	12 Months Ended
	Dec. 31, 2013
Business Combinations [Abstract]	'
Acquisitions and Dispositions	'
	Acquisitions and Dispositions
	Essentia Insurance Company
	Effective January 1, 2013, OneBeacon completed the sale of Essentia Insurance Company ("Essentia"), an indirect wholly-owned subsidiary which wrote the collector cars and boats business, to Markel Corporation. Concurrently therewith, OneBeacon and Hagerty Insurance Agency ("Hagerty") terminated their underwriting arrangement with respect to the collector cars and boats business. OneBeacon recognized a pre-tax gain on sale of $23.0 million ($15.0 million after tax) in the first quarter of 2013. For the years ended December 31, 2012 and 2011, the business associated with this agreement generated net written premiums of $179.7 million and $166.6 million, respectively, or 15.2% and 15.7%, respectively, of consolidated written premiums. Earned premiums for 2013, 2012 and 2011 were $89.1 million, $172.4 million and $159.3 million, respectively.
	Runoff Business
	As described in Note 1, in October 2012, OneBeacon entered into the Stock Purchase Agreement with respect to the sale of its Runoff Business to Armour. Pursuant to the terms of the Stock Purchase Agreement, at closing, OneBeacon will transfer to Armour all of the issued and outstanding shares of common stock of certain legal entities that will contain the assets, liabilities (including gross and ceded loss reserves) and capital supporting the business as well as certain elements of the Runoff Business infrastructure, including staff and office space. Additionally, as part of the Runoff Transaction, OneBeacon may provide, under certain scenarios, financing in the form of surplus notes. The Runoff Transaction is expected to close in mid-2014.
	The Runoff Transaction is subject to various closing conditions, primarily the receipt of regulatory approvals. The regulatory review process has included a third party actuarial review of the Runoff Business loss and LAE reserves, completed in September of 2013, as well as independent stochastic modeling of the future cash flows of the Runoff Business. At closing, Armour and/or OneBeacon Insurance Company ("OBIC") and certain legal entities within the ongoing OneBeacon structure will enter into various ancillary agreements, including the amendment of existing reinsurance agreements and administrative services agreements, to support the separation of the Runoff Business and subsequent transfer to Armour. Also as part of the Runoff Transaction, at closing, OneBeacon and Armour will enter into a Transition Services Agreement (“TSA”), pursuant to which OneBeacon will provide certain transition services to Armour during the term of the TSA, which has an initial term of one year. OneBeacon has concluded that continuing involvement after the closing of the transaction is insignificant relative to the business being sold.
	As described in Note 1, the Runoff Business is presented as held for sale as of December 31, 2013 and 2012, and as discontinued operations for the years ended December 31, 2013, 2012 and 2011. See Note 19—"Discontinued Operations" for further information regarding balances classified as held for sale and activity reported as discontinued operations.
	AutoOne
	On February 22, 2012, OneBeacon completed the sale of its AutoOne Insurance business ("AutoOne") to Interboro Holdings, Inc. ("Interboro") (the "AutoOne Transaction"). AutoOne had offered products and services to assigned risk markets primarily in New York and New Jersey. AutoOne has been presented as discontinued operations in the statements of operations and cash flows with the prior periods reclassified to conform to the current presentation. The AutoOne disposal group excludes investing and financing activities from amounts classified as discontinued operations. OneBeacon's investing and financing operations are conducted on an overall consolidated level and, accordingly, there are no separately identifiable investing or financing cash flows associated with AutoOne. Pursuant to the terms of the AutoOne Transaction, at closing, the legal entities included in the sale held an agreed upon level of invested assets and capital.
	OneBeacon Holdings (Luxembourg) S.à r.l.
	On January 24, 2012, OneBeacon sold all of the issued and outstanding shares of common stock of OneBeacon Holdings (Luxembourg) S.à r.l. (“OB Lux”) to White Sands Holdings (Luxembourg) S.à r.l (“White Sands”), a subsidiary of White Mountains, for $24.7 million. As a result of the sale, OneBeacon recorded a gain of $14.0 million as additional paid in capital. Net of transaction costs expensed through the statement of operations, the gain was $13.6 million.
	Except as described above, during the years ended December 31, 2013, 2012 and 2011, there were no significant acquisitions or dispositions.

Unpaid_Loss_and_Loss_Adjustmen

Unpaid Loss and Loss Adjustment Expense ("LAE") Reserves	12 Months Ended
	Dec. 31, 2013
Insurance Loss Reserves [Abstract]	'
Unpaid Loss and Loss Adjustment Expense (LAE) Reserves	'
	Unpaid Loss and Loss Adjustment Expense ("LAE") Reserves
	OneBeacon's insurance subsidiaries establish loss and LAE reserves that are estimates of amounts needed to pay claims and related expenses in the future for insured events that have already occurred. The process of estimating reserves involves a considerable degree of judgment by management and, as of any given date, is inherently uncertain.
	Reserves are typically comprised of (1) case reserves for claims reported and (2) reserves for losses that have occurred but for which claims have not yet been reported, referred to as incurred but not reported ("IBNR") reserves, which include a provision for expected future development on case reserves. Case reserves are estimated based on the experience and knowledge of claims staff regarding the nature and potential cost of each claim and are adjusted as additional information becomes known or payments are made. IBNR reserves are derived by subtracting paid loss and LAE and case reserves from estimates of ultimate loss and LAE. Actuaries estimate ultimate loss and LAE using various generally accepted actuarial methods applied to known losses and other relevant information. Like case reserves, IBNR reserves are adjusted as additional information becomes known or payments are made.
	Ultimate loss and LAE are generally determined by extrapolation of claim emergence and settlement patterns observed in the past that can reasonably be expected to persist into the future. In forecasting ultimate loss and LAE with respect to any line of business, past experience with respect to that line of business is the primary resource, but cannot be relied upon in isolation. OneBeacon's own experience, particularly claims development experience, such as trends in case reserves, payments on and closings of claims, as well as changes in business mix and coverage limits, is the most important information for estimating its reserves. External data, available from organizations such as statistical bureaus, consulting firms and reinsurance companies, is sometimes used to supplement or corroborate OneBeacon's own experience, and can be especially useful for estimating costs of new business. For some lines of business, such as "long-tail" coverages discussed below, claims data reported in the most recent accident year is often too limited to provide a meaningful basis for analysis due to the typical delay in reporting of claims. For this type of business, OneBeacon uses a selected loss ratio method for the initial accident year or years. This is a standard and accepted actuarial reserve estimation method in these circumstances in which the loss ratio is selected based upon information used in pricing policies for that line of business, as well as any publicly available industry data, such as industry pricing, experience and trends, for that line of business.
	Uncertainties in estimating ultimate loss and LAE are magnified by the time lag between when a claim actually occurs and when it is reported and settled. This time lag is sometimes referred to as the "claim-tail." The claim-tail for most property coverages is typically short (usually a few days up to a few months). The claim-tail for liability/casualty coverages, such as automobile liability, general liability, products liability, multiple peril coverage, and workers compensation, can be especially long as claims are often reported and ultimately paid or settled years, even decades, after the related loss events occur. During the long claims reporting and settlement period, additional facts regarding coverages written in prior accident years, as well as about actual claims and trends may become known and, as a result, OneBeacon may adjust its reserves. If management determines that an adjustment is appropriate, the adjustment is booked in the accounting period in which such determination is made. Accordingly, should reserves need to be increased or decreased in the future from amounts currently established, future results of operations would be negatively or positively impacted, respectively.
	In determining ultimate loss and LAE, the cost to indemnify claimants, provide needed legal defense and other services for insureds and administer the investigation and adjustment of claims are considered. These claim costs are influenced by many factors that change over time, such as expanded coverage definitions as a result of new court decisions, inflation in costs to repair or replace damaged property, inflation in the cost of medical services and legislated changes in statutory benefits, as well as by the particular, unique facts that pertain to each claim. As a result, the rate at which claims arose in the past and the costs to settle them may not always be representative of what will occur in the future. The factors influencing changes in claim costs are often difficult to isolate or quantify and developments in paid and incurred losses from historical trends are frequently subject to multiple and conflicting interpretations. Changes in coverage terms or claims handling practices may also cause future experience and/or development patterns to vary from the past. A key objective of actuaries in developing estimates of ultimate loss and LAE, and resulting IBNR reserves, is to identify aberrations and systemic changes occurring within historical experience and accurately adjust for them so that the future can be projected reliably. Because of the factors previously discussed, this process requires the use of informed judgment and is inherently uncertain.
	OneBeacon's actuaries use several generally accepted actuarial methods to evaluate its loss reserves, each of which has its own strengths and weaknesses. OneBeacon places more or less reliance on a particular method based on the facts and circumstances at the time the reserve estimates are made. These methods generally fall into one of the following categories or are hybrids of one or more of the following categories:
	•	Historical paid loss development methods:  These methods use historical loss payments over discrete periods of time to estimate future losses. Historical paid loss development methods assume that the ratio of losses paid in one period to losses paid in an earlier period will remain constant. These methods necessarily assume that factors that have affected paid losses in the past, such as inflation or the effects of litigation, will remain constant in the future. Because historical paid loss development methods do not use case reserves to estimate ultimate losses, they can be more reliable than the other methods discussed below that look to case reserves (such as actuarial methods that use incurred losses) in situations where there are significant changes in how case reserves are established by a company's claims adjusters. However, historical paid loss development methods are more leveraged, meaning that small changes in payments have a larger impact on estimates of ultimate losses, than actuarial methods that use incurred losses because cumulative loss payments take much longer to equal the expected ultimate losses than cumulative incurred amounts. In addition, and for similar reasons, historical paid loss development methods are often slow to react to situations when new or different factors arise than those that have affected paid losses in the past.
	•	Historical incurred loss development methods:  These methods, like historical paid loss development methods, assume that the ratio of losses in one period to losses in an earlier period will remain constant in the future. However, instead of using paid losses, these methods use incurred losses (i.e., the sum of cumulative historical loss payments plus outstanding case reserves) over discrete periods of time to estimate future losses. Historical incurred loss development methods can be preferable to historical paid loss development methods because they explicitly take into account open cases and the claims adjusters' evaluations of the cost to settle all known claims. However, historical incurred loss development methods necessarily assume that case reserving practices are consistently applied over time. Therefore, when there have been significant changes in how case reserves are established, using incurred loss data to project ultimate losses can be less reliable than other methods.
	•	Expected loss ratio methods:  These methods are based on the assumption that ultimate losses vary proportionately with premiums. Expected loss ratios are typically developed based upon the information used in pricing, and are multiplied by the total amount of premiums written to calculate ultimate losses. Expected loss ratio methods are useful for estimating ultimate losses in the early years of long-tailed lines of business, when little or no paid or incurred loss information is available.
	•	Adjusted historical paid and incurred loss development methods:  These methods take traditional historical paid and incurred loss development methods and adjust them for the estimated impact of changes from the past in factors such as inflation, the speed of claim payments or the adequacy of case reserves. Adjusted historical paid and incurred loss development methods are often more reliable methods of predicting ultimate losses in periods of significant change, provided the actuaries can develop methods to reasonably quantify the impact of changes.
	OneBeacon performs an actuarial review of its recorded reserves each quarter. OneBeacon's actuaries compare the previous quarter's estimates of paid loss and LAE, case reserves and IBNR to amounts indicated by actual experience. Differences between previous estimates and actual experience are evaluated to determine whether a given actuarial method for estimating loss and LAE should be relied upon to a greater or lesser extent than it had been in the past. While some variance is expected each quarter due to the inherent uncertainty in loss and LAE, persistent or large variances would indicate that prior assumptions and/or reliance on certain reserving methods may need to be revised going forward.
	Loss and LAE reserve summary - Ongoing Business
	The following table summarizes the loss and LAE reserve activities of OneBeacon's insurance subsidiaries for the years ended December 31, 2013, 2012, and 2011:
			Year ended December 31,
			2013				2012				2011
			($ in millions)
	Gross beginning balance		$	1,000.00			$	3,358.60			$	3,295.50
	Less beginning reinsurance recoverable on unpaid losses		(107.3		)		(2,167.5		)		(1,893.2		)
	Net beginning loss and LAE reserves		892.7				1,191.10				1,402.30
	Loss and LAE incurred from continuing operations relating to:
	Current year losses		622.1				657.4				578.1
	Prior year losses		—				(7.4		)		(29.8		)
	Total incurred loss and LAE from continuing operations		622.1				650				548.3
	Loss and LAE paid from continuing operations relating to:
	Current year losses		(188.6		)		(224.6		)		(216.9		)
	Prior year losses		(352.1		)		(340.5		)		(306.3		)
	Total loss and LAE payments from continuing operations		(540.7		)		(565.1		)		(523.2		)
	Net loss and LAE reserves		974.1				1,276.00				1,427.40
	Total incurred loss and LAE from discontinued operations		78.9				48.4				89.5
	Total loss and LAE payments from discontinued operations		(102.3		)		(220.8		)		(261.1		)
	Net loss and LAE reserves		950.7				1,103.60				1,255.80
	Net change in loss and LAE reserves reported in liabilities held for sale		23.4			(1)	(147.1		)	(2)	(64.7		)	(3)
	Net loss and LAE reserves sold		—				(63.8		)	(4)	—
	Net ending loss and LAE reserves		974.1				892.7				1,191.10
	Plus ending reinsurance recoverable on unpaid losses		80.2				107.3				2,167.50
	Gross ending loss and LAE reserves		$	1,054.30			$	1,000.00			$	3,358.60
	_______________________________________________________________________________
	(1)	Consists of the change in net loss and LAE reserves to $188.4 million from $211.8 million, which amounts were classified as held for sale as December 31, 2013 and 2012, respectively, in connection with the Runoff Transaction.
	(2)	Consists of the change in net loss and LAE reserves to $211.8 million from $64.7 million, which amounts were classified as held for sale as of December 31, 2012 and 2011, respectively, in connection with the Runoff Transaction and AutoOne Transaction, respectively.
	(3)	Consists of the net loss and LAE reserves of $64.7 million classified as held for sale as of December 31, 2011 in connection with the AutoOne Transaction.
	(4)	Relates to the AutoOne Transaction, which closed in February 2012.
	Loss and LAE development - Ongoing Business
	Loss and LAE development—2013
	During the year ended December 31, 2013, OneBeacon experienced no net loss and LAE reserve development on prior accident year reserves. The Company experienced unfavorable development primarily related to the property line in Specialty Property, Entertainment (primarily, general liability), and the A&H line within the Accident underwriting operating segment. This amount was offset by favorable development in other liability (claims made) within the managed care E&O business included in the Professional Insurance underwriting operating segment, and in the ocean marine line of business, included in the International Marine Underwriters underwriting operating segment.
	Loss and LAE development—2012
	During the year ended December 31, 2012, OneBeacon experienced $7.4 million of favorable loss and LAE reserve development on prior accident year reserves. During 2010, management began separately reviewing loss reserves for some business which had been previously managed as a part of OneBeacon's former commercial lines underwriting unit. As of December 31, 2011, the reserves for these businesses had been selected based on expected emergence which was based in large part on the historic loss development of the former commercial lines underwriting unit. The favorable reserve development was primarily due to emergence which continued to be lower than expected for these businesses, particularly in the workers' compensation, multiple peril liability and general liability lines. This favorable development was partially offset by adverse development on excess property claims.
	Loss and LAE development—2011
	During the year ended December 31, 2011, OneBeacon experienced $29.8 million of favorable loss and LAE reserve development on prior accident year loss reserves. The favorable loss reserve development was primarily due to lower than expected severity on non-catastrophe losses related to professional liability lines, multiple peril liability lines and other general liability lines.
	With respect to the favorable loss reserve development, at December 31, 2010, management had revised its expectations downward for future loss emergence in the professional liability business, which had initially been based on market analysis when this business was initiated in 2002 and 2003. However, during the year ended December 31, 2011, losses continued to be lower than these revised expectations. As a result, management lowered its selected reserves on the earliest years which affected more recent years as total loss expectations for those years are based in part on prior years' results. The impact of this revised estimate was a decrease to professional liability reserves of $11.5 million.
	During 2010, management began separately reviewing loss reserves for some business which had been previously managed as a part of OneBeacon's former commercial lines underwriting unit. As of December 31, 2010, the reserves for these businesses had been selected based on expected emergence which was based on the historic loss development of the former commercial lines underwriting unit. However, during 2011 the actual emerged experience for these businesses was significantly lower than the expected emergence. As a result of this favorable emergence, management lowered the loss reserves for these businesses by $14.0 million, which also affected the Specialty Industries and Specialty Products segments.
	In addition to the development described for the lines of business above, management also recorded a $4.3 million net decrease in reserves in other lines of business as a result of its review of loss reserves at December 31, 2011. The change in reserves for each other line of business was not individually significant.
	Discontinued Operations
	See Note 19—"Discontinued Operations" for a discussion of impacts to reserves for unpaid losses and LAE related to discontinued operations.
	Asbestos and Environmental
	Substantially all of OneBeacon's unpaid loss and LAE reserves for Asbestos and Environmental ("A&E") relates to operations that have been discontinued. See Note 19—"Discontinued Operations" for more detail on this exposure. The remaining unpaid loss and LAE reserves for Asbestos and Environmental related to continuing operations is less than $1.0 million on both a gross and net basis as of December 31, 2013.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2013
Reinsurance Disclosures [Abstract]	'
Reinsurance	'
	Reinsurance
	In the normal course of business, OneBeacon's insurance subsidiaries seek to limit losses that may arise from catastrophes or other events by reinsuring with third-party reinsurers. OneBeacon remains liable for risks reinsured even if the reinsurer does not honor its obligations under reinsurance contracts. See Note 19—"Discontinued Operations" for amounts related to the Runoff Business.
	The effects of reinsurance on OneBeacon's insurance subsidiaries' written and earned premiums and on loss and LAE, excluding the Runoff Business, were as follows:
		Year ended December 31,
		2013					2012			2011
		($ in millions)
	Written premiums:
	Direct	$	1,103.10				$	1,204.00		$	1,079.20
	Assumed	59.8					55.2			49.1
	Ceded	(74.3		)			(80.0		)	(65.6		)
	Net written premiums	$	1,088.60				$	1,179.20		$	1,062.70
	Earned premiums:
	Direct	$	1,043.30				$	1,158.30		$	1,035.90
	Assumed	148.5			(1)		52.8			42.3
	Ceded	(71.4		)			(79.1		)	(66.0		)
	Net earned premiums	$	1,120.40				$	1,132.00		$	1,012.20
	Loss and LAE:
	Direct	$	584.9				$	687.5		$	551.8
	Assumed	76.3			(1)		29.6			9.2
	Ceded	(39.1		)			(67.1		)	(12.7		)
	Net loss and LAE	$	622.1				$	650		$	548.3
	_______________________________________________________________________________
	(1) Includes assumed earned premiums and loss and LAE of $89.1 million and $41.9 million, respectively, related to the collector cars and boats business which although exited on January 1, 2013, was assumed on a runoff basis from Essentia.
	In the normal course of business, OneBeacon's insurance subsidiaries seek to limit losses that may arise from catastrophes or other events by reinsuring with third-party reinsurers. OneBeacon remains liable for risks reinsured even if the reinsurer does not honor its obligations under reinsurance contracts. See Note 19—"Discontinued Operations" for amounts related to the Runoff Business.
	The timing and size of catastrophe losses are unpredictable and the level of losses experienced in any year could be material to OneBeacon's operating results and financial condition. Examples of catastrophes include losses caused by earthquakes, wildfires, hurricanes and other types of storms and terrorist acts. The extent of losses caused by catastrophes is a function of the amount and type of insured exposure in the area affected by the event as well as the severity of the event. OneBeacon continually assesses and implements programs to manage its exposure to catastrophe losses through individual risk selection and by limiting its concentration of insurance written in catastrophe-prone areas such as coastal regions. In addition, OneBeacon imposes wind deductibles on existing coastal windstorm exposures. OneBeacon uses probable maximum loss ("PML") forecasting to quantify its exposure to catastrophic losses. PML is a statistical modeling technique that measures a company's catastrophic exposure as the maximum probable loss in a given time period.
	OneBeacon seeks to further reduce its potential loss from catastrophe exposures through the purchase of catastrophe reinsurance. Effective May 1, 2013, OneBeacon renewed its property catastrophe reinsurance program through April 30, 2014. The program provides coverage for OneBeacon's property business as well as certain acts of terrorism. Under the program, the first $20.0 million of losses resulting from any single catastrophe are retained and $117.0 million of the next $130.0 million of losses resulting from the catastrophe are reinsured in three layers. OneBeacon retains 50% of losses from $20.0 million to $30.0 million, 10% of losses from $30.0 million to $70.0 million, and 5% of losses from $70.0 million to $150.0 million. That part of a catastrophe loss in excess of $150.0 million would be retained in full. In the event of a catastrophe, OneBeacon's property catastrophe reinsurance program is reinstated for the remainder of the original contract term by paying a reinstatement premium that is based on the percentage of coverage reinstated and the original property catastrophe coverage premium.
	OneBeacon's property catastrophe reinsurance program does not cover property losses resulting from any nuclear events or biological, chemical or radiological terrorist attacks. Also excluded are losses resulting from acts of terrorism committed by an individual or individuals acting on behalf of any foreign person or foreign interest as defined under the Terrorism Risk Insurance Program Reauthorization Act (the "Terrorism Act," or "TRIPRA"), but only with respect to acts of terrorism committed by an individual or individuals acting on behalf of any foreign person or foreign interest.
	Since the terrorist attacks of September 11, 2001, OneBeacon has sought to mitigate the risk associated with any future terrorist attacks by limiting the aggregate insured value of policies in geographic areas with exposure to losses from terrorist attacks. This is accomplished by either limiting the total insured values exposed, or, where applicable, through the use of terrorism exclusions.
	In December 2007, the United States government extended the Terrorism Act until December 31, 2014. The Terrorism Act established a federal "backstop" for commercial property and casualty losses, including workers compensation, resulting from acts of terrorism by or on behalf of any foreign person or foreign interest. As extended, the law now also covers domestic acts of terrorism. The law limits the industry's aggregate liability by requiring the federal government to share 85% of certified losses once a company meets a specific retention or deductible as determined by its prior year's direct written premiums and limits the aggregate liability to be paid by the government and industry without further action by Congress at $100 billion. In exchange for this "backstop," primary insurers are required to make coverage available to commercial insureds for losses from acts of terrorism as specified in the Terrorism Act. The following types of coverage are excluded from the program: commercial automobile, burglary and theft, surety, farmowners multi-peril and all professional liability coverage except directors and officers coverage.
	OneBeacon estimates its individual retention level for commercial policies subject to the Terrorism Act to be approximately $100 million in 2014. The federal government will pay 85% of covered terrorism losses that exceed OneBeacon's or the industry's retention levels in 2014, up to a total of $100 billion.
	In addition to the corporate catastrophe reinsurance protection, the Company also purchases dedicated reinsurance protection for certain specific lines of business. OneBeacon also purchases property-per-risk reinsurance coverage to reduce large loss volatility. The property-per-risk reinsurance program reinsures losses in excess of $10.0 million up to $100.0 million. Individual risk facultative reinsurance is purchased above $100.0 million. OneBeacon retains 5% of losses in excess of $20.0 million up to $40.0 million and 10% of losses in excess of $40.0 million. The property-per-risk treaty provides one limit of reinsurance protection for losses in excess of $10.0 million up to $100.0 million on an individual risk basis for acts of foreign terrorism. However, any nuclear events, or biological, chemical or radiological terrorist attacks are not covered.
	OneBeacon also maintains a casualty reinsurance program that provides protection for individual policies involving general liability, automobile liability, professional liability or umbrella liability. OneBeacon's healthcare professional liability treaty covers losses in excess of $5.0 million up to $20.0 million in two layers. The first layer, $5.0 million in excess of $5.0 million has a 10% co-participation. All other casualty business is covered in a separate treaty covering losses in excess of $5.0 million up to $21.0 million. The first layer of the casualty treaty ($6.0 million excess of $5.0 million) has a 10% co-participation and a $3.0 million annual aggregate deductible. The second layer of the casualty treaty ($10.0 million excess of $11.0 million) has a 5% co-participation. OneBeacon also purchases a treaty to protect against large workers compensation losses that provides coverage for 100% of the loss in excess of $1.0 million up to $10.0 million per occurrence. Additionally, for casualty and/or workers compensation catastrophe losses, OneBeacon maintains a dedicated clash treaty, which provides coverage in the event that one loss event results in two or more claims, that covers up to $60.0 million in excess of a $10.0 million retention.
	OneBeacon purchases a per occurrence treaty for marine business – both inland and ocean – that protects against large occurrences, whether a single large claim or a catastrophe. The marine treaty attaches at $2.0 million per occurrence. The first layer of the marine treaty is $5.0 million in excess of $2.0 million, with annual aggregate deductibles of $1.5 million for individual ocean marine large claims, $1.5 million for individual inland marine large claims and $5.0 million for catastrophe losses. OneBeacon retains 60% of the loss from $2.0 million up to $7.0 million. Catastrophe coverage is provided up to $60.0 million. Retained catastrophe losses are subject to the corporate catastrophe treaty. Individual risk losses from inland marine exceeding $20.0 million are subject to the corporate property per risk treaty. Reinstatement premiums are paid in full or in part depending on the layer and the occurrence if the coverage is attached.
	OneBeacon also purchases reinsurance for its Surety underwriting operating segment. This treaty covers 100% of losses in excess of $5.0 million up to $30.0 million per bond and up to $60.0 million in aggregate.
	At December 31, 2013, OneBeacon had reinsurance recoverable on paid losses of $9.7 million and reinsurance recoverables on unpaid losses of $80.2 million. Reinsurance contracts do not relieve OneBeacon of its obligations. Therefore, collectibility of balances due from reinsurers is critical to OneBeacon's financial strength. The following table summarizes Standard & Poor's Financial Services, LLC ("Standard & Poor's") ratings for OneBeacon's reinsurers for its continuing insurance operations, excluding industry pools and associations, based upon reinsurance recoverable amounts on paid and unpaid losses and LAE:
		Balance at				% of total
		31-Dec-13
	Standard & Poor's Rating:	($ in millions)
	AA	$	26.4			30	%
	A	56.1				62	%
	BBB+, Not Rated and Other	7.4				8	%
	Total reinsurance recoverables	$	89.9			100	%
	_______________________________________________________________________________
	(1) Standard & Poor's ratings as detailed above are "AA" (Very strong), "A" (Strong) and "BBB+" (Adequate).

Investment_Securities

Investment Securities	12 Months Ended
	Dec. 31, 2013
Schedule of Investments [Abstract]	'
Investment Securities	'
	Investment Securities
	OneBeacon's net investment income is comprised primarily of interest income associated with OneBeacon's fixed maturity investments, dividend income from its equity investments, and interest income from its short-term investments. Other investments primarily include hedge funds and private equity funds. Net investment income for the years ended December 31, 2013, 2012, and 2011 consisted of the following:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Fixed maturity investments		$	40.6		$	49.1		$	69.7
	Short-term investments		—			0.1			0.1
	Common equity securities		6.2			6.8			5.3
	Convertible fixed maturity investments		1.2			4.6			3.5
	Other investments		(0.1		)	(0.4		)	(0.3		)
	Gross investment income		47.9			60.2			78.3
	Less investment expenses		(6.8		)	(6.6		)	(6.9		)
	Net investment income		$	41.1		$	53.6		$	71.4
	The composition of net realized investment gains consisted of the following:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Fixed maturity investments		$	2.1		$	27.1		$	32.1
	Short-term investments		—			—			—
	Common equity securities		36.1			2.1			16.7
	Convertible fixed maturity investments		0.3			0.4			(0.4		)
	Other investments		5.7			21.2			8.4
	Net realized investment gains, pre-tax		44.2			50.8			56.8
	Income taxes		(20.5		)	(17.2		)	(19.9		)
	Net realized investment gains, after tax		$	23.7		$	33.6		$	36.9
	OneBeacon recognized gross realized investment gains of $70.1 million, $68.4 million and $77.9 million and gross realized investment losses of $25.9 million, $17.6 million and $21.1 million on sales and other-than-temporary impairment charges on investment securities during the years ended December 31, 2013, 2012 and 2011, respectively.
	As of December 31, 2013, and 2012, OneBeacon reported $11.6 million, and $6.2 million, respectively, in accounts payable on unsettled investment purchases and $3.3 million, and $2.1 million, respectively, in accounts receivable on unsettled investment sales.
	The net changes in fair value for the years ended December 31, 2013, 2012, and 2011 are as follows:
			Year ended December 31, 2013
			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected
			gains(1)			translation			in revenues(1)
						gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	(34.4	)	$	—		$	(34.4	)
	Short-term investments		0.1			—			0.1
	Common equity securities		32			0.1			32.1
	Convertible fixed maturity investments		1.1			—			1.1
	Other investments		6.3			—			6.3
	Net change, pre-tax		$	5.1		$	0.1		$	5.2
			Year ended December 31, 2012
			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected
			gains(1)			translation			in revenues(1)
						gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	7.2		$	0.1		$	7.3
	Short-term investments		—			—			—
	Common equity securities		11.5			—			11.5
	Convertible fixed maturity investments		(0.4		)	—			(0.4		)
	Other investments(2)		(13.5		)	—			(13.5		)
	Net change, pre-tax		$	4.8		$	0.1		$	4.9
			Year ended December 31, 2011
			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected
			gains(1)			translation			in revenues(1)
						gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	(18.7	)	$	(0.1	)	$	(18.8	)
	Short-term investments		—			0.1			0.1
	Common equity securities		(22.1		)	(0.1		)	(22.2		)
	Convertible fixed maturity investments		(8.4		)	—			(8.4		)
	Other investments		3.1			—			3.1
Net change, pre-tax		$	(46.1	)	$	(0.1	)	$	(46.2	)
_______________________________________________________________________________
(1)	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.8) million, $(0.2) million, and $(5.2) million for the years ended December 31, 2013, 2012, and 2011, respectively.
(2)	The year ended December 31, 2012, includes a change in net unrealized gains of $(11.6) million related to the sale of a limited partnership that had been consolidated into OneBeacon's results. See Note 15—"Related Party Disclosures."
The components of OneBeacon's ending net unrealized investment gains and losses, excluding the impact of net unrealized foreign currency translation gains and losses, on its investment portfolio as of December 31, 2013 and 2012 were as follows:
		December 31,
		2013			2012
		($ in millions)
Investment securities:
Gross unrealized investment gains		$	133.7		$	127.4
Gross unrealized investment losses		(9.3		)	(8.7		)
Total net unrealized investment gains, pre-tax		124.4			118.7
Income taxes		(33.8		)	(39.4		)
Total net unrealized investment gains, after tax		$	90.6		$	79.3
The cost or amortized cost, gross unrealized pre-tax investment gains and losses, net foreign currency losses and carrying values of OneBeacon's fixed maturity investments as of December 31, 2013 and 2012 were as follows:
		December 31, 2013(1)
		Cost or			Gross			Gross			Net foreign			Carrying
		amortized			unrealized			unrealized			currency			value
		cost			gains			losses			losses
		($ in millions)
U.S. Government and agency obligations		$	131		$	0.2		$	(0.1	)	$	—		$	131.1
Debt securities issued by industrial corporations		741.7			14.1			(1.3		)	—			754.5
Municipal obligations		16.9			—			(0.4		)	—			16.5
Asset-backed securities		951.5			1.2			(3.2		)	—			949.5
Foreign government obligations		2			0.3			—			—			2.3
Preferred stocks		78.3			5			—			—			83.3
Total fixed maturity investments		$	1,921.40		$	20.8		$	(5.0	)	$	—		$	1,937.20
		December 31, 2012(1)
		Cost or			Gross			Gross			Net foreign			Carrying
		amortized			unrealized			unrealized			currency			value
		cost			gains			losses			losses
		($ in millions)
U.S. Government and agency obligations		$	197.1		$	0.5		$	—		$	—		$	197.6
Debt securities issued by industrial corporations		678.7			32.8			—			—			711.5
Municipal obligations		3.3			—			(0.1		)	—			3.2
Asset-backed securities		918.5			9.7			(0.3		)	—			927.9
Foreign government obligations		6			0.5			—			—			6.5
Preferred stocks		78.3			6.4			—			—			84.7
Total fixed maturity investments		$	1,881.90		$	49.9		$	(0.4	)	$	—		$	1,931.40
_______________________________________________________________________________
(1)	Includes carrying value of $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheets as part of the Runoff Transaction as of December 31, 2013 and 2012, respectively.
The cost or amortized cost and carrying value of OneBeacon's fixed maturity investments and convertible fixed maturity investments at December 31, 2013 is presented below by contractual maturity. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties.
		December 31, 2013(1)
		Cost or			Carrying
		amortized			value
		cost
		($ in millions)
Due in one year or less		$	184.9		$	188
Due after one year through five years		664.7			674.6
Due after five years through ten years		59.1			61.3
Due after ten years		9.2			11
Asset-backed securities		951.5			949.5
Preferred stocks		78.3			83.3
Total		$	1,947.70		$	1,967.70
_______________________________________________________________________________
-1	Includes carrying value of $236.3 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheet as part of the Runoff Transaction as of December 31, 2013
The cost or amortized cost, gross unrealized pre-tax investment gains and losses, net foreign currency gains and carrying values of OneBeacon's common equity securities, convertible fixed maturity investments and other investments as of December 31, 2013 and 2012 were as follows:
		31-Dec-13
		Cost or			Gross			Gross			Net foreign			Carrying
		amortized			unrealized			unrealized			currency			value
		cost			gains			losses			gains
		($ in millions)
Common equity securities		$	267.2		$	70.4		$	(0.9	)	$	0.2		$	336.9
Convertible fixed maturity investments		26.3			4.5			(0.3		)	—			30.5
Other investments		104.7			38			(3.1		)	—			139.6
Total common equity securities, convertible fixed maturity investments and other investments		$	398.2		$	112.9		$	(4.3	)	$	0.2		$	507
		31-Dec-12
		Cost or			Gross			Gross			Net foreign			Carrying
		amortized			unrealized			unrealized			currency			value
		cost			gains			losses			gains
		($ in millions)
Common equity securities		$	221.6		$	39.8		$	(2.4	)	$	—		$	259
Convertible fixed maturity investments		59.5			3.2			(0.1		)	—			62.6
Other investments		115.1			34.5			(5.8		)	—			143.8
Total common equity securities, convertible fixed maturity investments and other investments		$	396.2		$	77.5		$	(8.3	)	$	—		$	465.4
Sales and maturities of investments, excluding short-term investments and other investments, totaled $1,887.7 million, $2,050.5 million and $2,032.1 million for the years ended December 31, 2013, 2012 and 2011, respectively. There were no non-cash exchanges or involuntary sales of investment securities during the years ended December 31, 2013, 2012 and 2011.
OneBeacon's consolidated insurance operations are required to maintain deposits with certain insurance regulatory agencies in order to maintain their insurance licenses. The fair value of such deposits totaled $152.2 million and $174.3 million, respectively, as of December 31, 2013 and 2012.
As of December 31, 2013 and 2012, investments of $38.4 million and $50.7 million, respectively, were held in trusts required to be maintained in relation to various reinsurance agreements and will be transferred to Armour upon the closing of the Runoff Transaction. Trust balances as of December 31, 2013 and 2012 include $9.9 million and $15.3 million, respectively, related to investments held in a trust established in conjunction with the sale of the personal lines business in 2010. As of December 31, 2013 and 2012, the trust balance also included $22.6 million and $25.9 million related to investments held in a trust established in conjunction with White Mountains' sale of Esurance Holdings, Inc. ("Esurance Holdings") and its subsidiaries (collectively, "Esurance Insurance"). See Note 15.
As of December 31, 2013 and 2012, the Company held unrestricted collateral from its customers, which is included in cash and invested assets, relating to its Surety business of $63.3 million and $5.9 million, respectively. The obligation to return these funds is included in other liabilities in the consolidated balance sheets.
Fair value measurements
As of both December 31, 2013 and 2012, approximately 92% of the investment portfolio recorded at fair value was priced based upon observable inputs.
The fair values of OneBeacon's investments in hedge funds and private equity funds have been classified as Level 3 under the fair value hierarchy since the fund managers do not provide sufficient information to independently evaluate the pricing inputs and methods for each underlying investment, and therefore the inputs are considered to be unobservable. At December 31, 2013 and 2012, OneBeacon did not record a liquidity adjustment to the net asset value related to its investments in hedge funds or private equity funds.
As of both December 31, 2013 and 2012, other investments reported at fair value represented approximately 5% of the investment portfolio recorded at fair value. Other investments accounted for at fair value as of December 31, 2013 and 2012 were comprised of $49.4 million and $47.3 million, respectively, in hedge funds, $56.1 million and $61.3 million, respectively, in private equity funds and $14.4 million and $14.1 million, respectively, in an investment in a community reinvestment vehicle. As of December 31, 2013 and 2012, OneBeacon held investments in 8 hedge funds for both periods. OneBeacon held investments in 19 and 17 private equity funds at December 31, 2013 and 2012, respectively. The largest investment in a single fund was $14.9 million and $12.9 million at December 31, 2013 and 2012, respectively. As of December 31, 2013 and 2012, other investments also included $19.7 million and $21.1 million, respectively, of an investment in a tax advantaged federal affordable housing development fund which is accounted for using the equity method.
The following tables summarize the Company's fair value measurements for investments at December 31, 2013 and 2012 by level:
		Fair value at			Level 1			Level 2			Level 3
		December 31, 2013(2)
		($ in millions)
Fixed maturity investments:
U.S. Government and agency obligations		$	131.1		$	131.1		$	—		$	—
Debt securities issued by corporations:
Consumer		239.6			—			239.6			—
Industrial		106			—			106			—
Financial		145.5			—			145.5			—
Communications		91.5			—			91.5			—
Energy		40.5			—			40.5			—
Basic materials		64.4			—			64.4			—
Utilities		38			—			38			—
Technology		29			—			29			—
Debt securities issued by corporations		754.5			—			754.5			—
Municipal obligations		16.5			—			16.5			—
Asset-backed securities		949.5			—			938.6			10.9
Foreign government obligations		2.3			1.6			0.7			—
Preferred stocks		83.3			—			12.3			71
Fixed maturity investments		1,937.20			132.7			1,722.60			81.9
Short-term investments		157			155.9			1.1			—
Common equity securities:
Financials		83.3			83.2			—			0.1
Basic Materials		20.3			20.3			—			—
Consumer		122.6			122.6			—			—
Energy		34.7			34.7			—			—
Utilities		8.3			8.3			—			—
Other		67.7			67.7			—			—
Common equity securities		336.9			336.8			—			0.1
Convertible fixed maturity investments		30.5			—			30.5			—
Other investments(1)		119.9			—			—			119.9
Total(1)		$	2,581.50		$	625.4		$	1,754.20		$	201.9
_______________________________________________________________________________
-1	Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of December 31, 2013.
-2	Fair value includes $236.3 million of fixed maturity investments reclassified to assets held for sale in the December 31, 2013 consolidated balance sheets as part of the Runoff Transaction.
		Fair value at			Level 1			Level 2			Level 3
		December 31, 2012(2)
		($ in millions)
Fixed maturity investments:
U.S. Government and agency obligations		$	197.6		$	197.6		$	—		$	—
Debt securities issued by corporations:
Consumer		249.9			—			249.9			—
Industrial		103.5			—			103.5			—
Financial		92.3			—			92.3			—
Communications		70			—			70			—
Energy		57.1			—			57.1			—
Basic materials		77.8			—			77.8			—
Utilities		46.5			—			46.5			—
Technology		14.4			—			14.4			—
Debt securities issued by corporations		711.5			—			711.5			—
Municipal obligations		3.2			—			3.2			—
Asset-backed securities		927.9			—			922.6			5.3
Foreign government obligations		6.5			5.8			0.7			—
Preferred stocks		84.7			—			13.9			70.8
Fixed maturity investments		1,931.40			203.4			1,651.90			76.1
Short-term investments		232.8			232.8			—			—
Common equity securities:
Financials		48.4			48.3			—			0.1
Basic Materials		42.9			42.9			—			—
Consumer		80.5			80.5			—			—
Energy		38.1			38.1			—			—
Utilities		17			17			—			—
Other		32.1			32.1			—			—
Common equity securities		259			258.9			—			0.1
Convertible fixed maturity investments		62.6			—			62.6			—
Other investments(1)		122.7			—			—			122.7
Total(1)		$	2,608.50		$	695.1		$	1,714.50		$	198.9
_______________________________________________________________________________
-1	Excludes the carrying value of $21.1 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of December 31, 2012.
-2	Fair value includes $338.1 million of fixed maturity investments reclassified to assets held for sale in the December 31, 2012 consolidated balance sheets as part of the Runoff Transaction.
At December 31, 2013 and 2012, OneBeacon held one private preferred stock that represented approximately 85% and 84%, respectively, of its preferred stock portfolio. OneBeacon used quoted market prices for similar securities that were adjusted to reflect management's best estimate of fair value; this security is classified as a Level 3 measurement.
The following table summarizes the ratings of OneBeacon's corporate debt securities as of December 31, 2013 and 2012:
		December 31,
		2013			2012
		($ in millions)
AA		$	51.2		$	37
A		357.9			301.6
BBB		345.4			362.5
BB		—			7
Other		—			3.4
Debt securities issued by corporations		$	754.5		$	711.5
Rollforwards of Fair Value Measurements by Level
The changes in Level 1 fair value measurements for the year ended December 31, 2013 are as follows:
		Fixed			Common			Convertible			Other			Total(1)
		maturity			equity			fixed			investments
		investments			securities			maturity
								investments
		($ in millions)
Balance at January 1, 2013		$	203.4		$	258.9		$	—		$	—		$	462.3
Amortization/accretion		(0.1		)	—			—			—			(0.1		)
Net realized and unrealized gains (losses)		(2.4		)	68.2			—			—			65.8
Purchases		179			206.4			—			—			385.4
Sales		(247.2		)	(196.7		)	—			—			(443.9		)
Transfers in		—			—			—			—			—
Transfers out		—			—			—			—			—
Balance at December 31, 2013		$	132.7		$	336.8		$	—		$	—		$	469.5
_______________________________________________________________________________
(1)	Excludes short-term investments which are deemed to have a Level 1 designation.
The changes in Level 2 fair value measurements for the year ended December 31, 2013 are as follows:
		Fixed			Common			Convertible			Other			Total(1)
		maturity			equity			fixed			investments
		investments			securities			maturity
								investments
		($ in millions)
Balance at January 1, 2013		$	1,651.90		$	—		$	62.6		$	—		$	1,714.50
Amortization/accretion		(11.3		)	—			(0.1		)	—			(11.4		)
Net realized and unrealized gains (losses)		(28.3		)	—			1.4			—			(26.9		)
Purchases		2,174.10			—			14.4			—			2,188.50
Sales		(2,077.4		)	—			(47.8		)	—			(2,125.2		)
Transfers in		74.9			—			—			—			74.9
Transfer out		(61.3		)	—			—			—			(61.3		)
Balance at December 31, 2013		$	1,722.60		$	—		$	30.5		$	—		$	1,753.10
_______________________________________________________________________________
-1	Excludes short-term investments which are deemed to have a Level 2 designation.
The changes in Level 3 fair value measurements for the year ended December 31, 2013 are as follows:
		Fixed			Common			Convertible			Other			Total(1)
		maturity			equity			fixed			investments(1)
		investments			securities			maturity
								investments
		($ in millions)
Balance at January 1, 2013		$	76.1		$	0.1		$	—		$	122.7		$	198.9
Amortization/accretion		—			—			—			—			—
Net realized and unrealized gains (losses)		(1.5		)	—			—			11.9			10.4
Purchases		21.4			—			—			7.1			28.5
Sales		(0.5		)	—			—			(21.8		)	(22.3		)
Transfers in		61.3			—			—			—			61.3
Transfers out		(74.9		)	—			—			—			(74.9		)
Balance at December 31, 2013		$	81.9		$	0.1		$	—		$	119.9		$	201.9
_______________________________________________________________________________
(1)	Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method.
“Transfers in” to Level 3 fixed maturity investments of $61.3 million for the year ended December 31, 2013 was comprised of one agency commercial mortgage-backed security for which the estimated fair value was determined using industry standard pricing models that take into account unobservable inputs such as prepayment rate, default rate and swap spread.
“Transfers out” of Level 3 fixed maturity investments of $74.9 million for the year ended December 31, 2013 were comprised of three mortgage-backed securities which had been previously classified as a Level 3 measurement and were recategorized as a Level 2 measurement when quoted market prices for similar securities that were considered reliable and could be validated against an alternative source became available.
Significant Unobservable Inputs
As previously described, in certain circumstances, OneBeacon estimates the fair value of investments using industry standard pricing models and both observable and unobservable inputs.
The following summarizes significant unobservable inputs used in estimating the fair value of fixed maturity securities classified within Level 3 other than hedge funds and private equities at December 31, 2013. The fair value of investments in hedge funds and private equity funds, which are classified with Level 3, are estimated using the net asset value of the funds.
Description		Fair Value			Rating(1)		Valuation Technique	Unobservable Inputs		Range(2)
Preferred stock		$	71		N/R		Discounted cash flow	Discount yield		7.40%
Asset-backed securities		$	10.9		AA+		Broker pricing	Broker quote		N/A
_________________________________________________________________________
(1)	Credit ratings are assigned based on the following hierarchy: 1) Standard & Poor's and 2) Moody's Investors Service, Inc. ("Moody's")
(2) As of December 31, 2013, each asset type consists of one security.
The assumed prepayment rate is a significant unobservable input used to estimate the fair value of investments in agency commercial mortgage-backed securities ( agency “CMBS”). Generally for bonds priced at a premium, increases in prepayment speeds will result in a lower fair value, while decreases in prepayment speed may result in a higher fair value.
The following table summarizes the change in net unrealized gains or losses for assets designated as Level 3 for the year ended December 31, 2013, 2012, and 2011:
		Year ended December 31,
		2013			2012			2011
		($ in millions)
Fixed maturity investments		$	(1.5	)	$	6.7		$	(7.5	)
Short-term investments		—			—			—
Common equity securities		—			—			—
Convertible fixed maturity investments		—			—			—
Other investments		6.3			(13.5		)	3.1
Total		$	4.8		$	(6.8	)	$	(4.4	)
Asset-backed Securities
OneBeacon purchases commercial and residential mortgage-backed securities to maximize its risk adjusted returns in the context of a diversified portfolio. OneBeacon's non-agency commercial mortgage-backed securities (non-agency "CMBS") are generally short tenor and structurally senior, with approximately 30 points of subordination on average for fixed rate and floating rate CMBS as of December 31, 2013. In general, subordination represents the percentage of principal loss on the underlying collateral that would have to occur before the security incurs a loss. These collateral losses, instead, are first absorbed by other securities lower in the capital structure. OneBeacon believes this structural protection mitigates the risk of loss tied to refinancing challenges facing the commercial real estate market. As of December 31, 2013, on average less than 1% of the underlying loans were reported as non-performing for both agency and non-agency CMBS held by OneBeacon. OneBeacon is not an originator of residential mortgage loans. OneBeacon did not hold any RMBS categorized as sub-prime as of December 31, 2013. OneBeacon's investments in hedge funds and private equity funds contain negligible amounts of sub-prime mortgage-backed securities as of December 31, 2013. OneBeacon considers sub-prime mortgage-backed securities to be those that have underlying loan pools that exhibit weak credit characteristics or are issued from dedicated sub-prime shelves or dedicated second-lien shelf registrations (i.e., OneBeacon considers investments backed primarily by second-liens to be sub-prime risks regardless of credit scores or other metrics).
There are also mortgage backed securities that OneBeacon categorizes as "non-prime" (also called "Alt A" or "A-") that are backed by collateral that has overall credit quality between prime and sub-prime, as determined based on OneBeacon's review of the characteristics of their underlying mortgage loan pools, such as credit scores and financial ratios. As of December 31, 2013, OneBeacon held one mortgage-backed security with a market value of $7.3 million that was classified as non-prime. OneBeacon's non-agency residential mortgage-backed portfolio is generally of moderate average life, fixed rate and structurally senior. OneBeacon does not own any collateralized debt obligations, including residential mortgage-backed collateralized debt obligations.
The following table summarizes the carrying value of OneBeacon's asset-backed securities as of December 31, 2013 and 2012:
		December 31,
		2013									2012
		Fair Value			Level 2			Level 3			Fair Value			Level 2			Level 3
		($ in millions)
Mortgage-backed securities:
Agency:
GNMA		$	321.8		$	321.8		$	—		$	551.2		$	551.2		$	—
FNMA		36.5			36.5			—			13.9			13.9			—
FHLMC		24.3			24.3			—			10.5			10.5			—
Total agency(1)		382.6			382.6			—			575.6			575.6			—
Non-agency:
Residential		51.5			51.5			—			38.1			38.1			—
Commercial		155			155			—			175.4			175.4			—
Total Non-agency		206.5			206.5			—			213.5			213.5			—
Total mortgage-backed securities		589.1			589.1			—			789.1			789.1			—
Other asset-backed securities:
Credit card receivables		124.2			113.3			10.9			49			43.7			5.3
Vehicle receivables		231.8			231.8			—			81.5			81.5			—
Other		4.4			4.4			—			8.3			8.3			—
Total other asset-backed securities		360.4			349.5			10.9			138.8			133.5			5.3
Total asset-backed securities		$	949.5		$	938.6		$	10.9		$	927.9		$	922.6		$	5.3
_______________________________________________________________________________
(1)	Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Non-agency Mortgage-backed Securities
The security issuance years of OneBeacon's investments in non-agency RMBS and non-agency CMBS securities as of December 31, 2013 are as follows:
		Fair Value			Security Issuance Year
					2004			2005			2006			2007			2008		2010		2011		2012		2013
		($ in millions)
Total non-agency RMBS		$	51.5		$	6.8		$	10.1		$	5.4		$	—		$	—	$	11.1	$	—	$	—	$	18.1
Total non-agency CMBS		155			—			—			—			1.4			16		5.8		15.7		81.8		34.3
Total non-agency		$	206.5		$	6.8		$	10.1		$	5.4		$	1.4		$	16	$	16.9	$	15.7	$	81.8	$	52.4
Non-agency Residential Mortgage-backed Securities
The classification of the underlying collateral quality and the tranche levels of OneBeacon's non-agency RMBS securities are as follows as of December 31, 2013:
		Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
		($ in millions)
Prime		$	44.2		$	12.7		$	31.5		$	—
Non-prime		7.3			—			7.3			—
Total non-agency RMBS		$	51.5		$	12.7		$	38.8		$	—
_______________________________________________________________________________
(1)	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch Ratings ("Fitch") and were senior to other AAA or Aaa bonds.
(2)	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were senior to non-AAA or non-Aaa bonds.
(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were junior to other bonds.
Non-agency Commercial Mortgage-backed Securities
The amount of fixed and floating rate securities and their tranche levels are as follows as of December 31, 2013:
		Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
		($ in millions)
Fixed rate CMBS		$	113.9		$	84.3		$	24.9		$	4.7
Floating rate CMBS		41.1			1.4			16			23.7
Total non-agency CMBS		$	155		$	85.7		$	40.9		$	28.4
_______________________________________________________________________________
(1)	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's or AAA by Fitch and were senior to other AAA or Aaa bonds.
(2)	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were senior to non-AAA or non-Aaa bonds.
(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were junior to other bonds.
Other Investments
OneBeacon holds investments in hedge funds and private equity funds which are included in other investments. The fair value of these investments has been estimated using the net asset value of the funds. The following table summarizes investments in hedge funds and private equity funds at December 31, 2013 and 2012:
		December 31,
		2013						2012
		Fair			Unfunded			Fair			Unfunded
		Value			Commitments			Value			Commitments
		($ in millions)
Hedge funds
Long/short credit and distressed		$	8.1		$	—		$	8.4		$	—
Long bank loan		0.1			—			0.1			—
Long/short equity		39.1			—			36.8			—
Long/short equity activist		2.1			—			2			—
Total hedge funds		49.4			—			47.3			—
Private equity funds
Insurance		2.3			0.1			3.1			0.1
Distressed residential real estate		0.2			—			7.9			—
Energy infrastructure and services		26			5.9			20.7			7.5
Healthcare		2.8			1.4			2.1			2.7
Multi-sector		14.5			2.5			14.9			3.7
Private equity secondaries		6.4			2.1			7			2
Real estate		3.9			0.1			5.6			0.1
Total private equity funds		56.1			12.1			61.3			16.1
Total hedge funds and private equity funds(1)		$	105.5		$	12.1		$	108.6		$	16.1
_______________________________________________________________________________
(1)	Excluded from the above table are other investments totaling $14.4 million and $14.1 million of an investment in a community reinvestment vehicle as of December 31, 2013 and 2012, respectively, and $19.7 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of December 31, 2013 and 2012, respectively.
Redemptions of investments in certain funds are subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. The following summarizes the December 31, 2013 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:
		Hedge Funds—Active Funds
		30 - 59 days			60 - 89 days			90 - 119 days			120+ days			Total
		notice			notice			notice			notice
		($ in millions)
Redemption frequency
Monthly		$	—		$	—		$	—		$	5.8		$	5.8
Quarterly		26.9			8.1			6.4			—			41.4
Annual		—			—			2.1			0.1			2.2
Total hedge funds		$	26.9		$	8.1		$	8.5		$	5.9		$	49.4
Certain hedge fund investments are no longer active and are in the process of disposing of their underlying investments. Distributions from such funds are remitted to investors as the fund's underlying investments are liquidated. At December 31, 2013, $1.3 million of hedge funds were in liquidation. The actual amount of the final distribution is subject to market fluctuations. The date at which such distributions will be received is not determinable at December 31, 2013.
OneBeacon has also submitted redemption requests for certain of its investments in active hedge funds. At December 31, 2013, redemptions of $2.1 million were outstanding. The date at which all these redemptions will be received is not determinable at December 31, 2013. Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations.
Investments in private equity funds are generally subject to "lock-up" periods during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund's underlying investment. In addition, certain private equity funds provide an option to extend the lock-up period at either the sole discretion of the fund manager or upon agreement between the fund and the investors. At December 31, 2013, investments in private equity funds were subject to lock-up periods as follows:
		1 - 3 years			3 - 5 years			5 - 10 years			>10 years			Total
		($ in millions)
Private Equity Funds—expected lock-up period remaining		$	2.5		$	20.2		$	33.4		$	—		$	56.1

Debt

Debt	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Debt	'
	Debt
	OneBeacon's debt outstanding as of December 31, 2013 and 2012 consisted of the following:
		December 31,
		2013			2012
		($ in millions)
	Senior unsecured notes, at face value	$	275		$	275
	Unamortized original issue discount	(0.3		)	(0.3		)
	Senior unsecured notes, carrying value	$	274.7		$	274.7
	Contractual repayments of $275.0 million for OneBeacon's outstanding debt are due more than five years after December 31, 2013.
	2012 Senior Notes
	In November 2012, OneBeacon U.S. Holdings, Inc. ("OBH") issued $275.0 million face value of senior unsecured notes ("2012 Senior Notes") through a public offering, at an issue price of 99.9% and received $272.9 million of proceeds. The 2012 Senior Notes bear an annual interest rate of 4.6% payable semi-annually in arrears on May 9 and November 9, until maturity on November 9, 2022, and are fully and unconditionally guaranteed as to the payment of principal and interest by the Company. OBH incurred $2.8 million in expenses related to the issuance of the 2012 Senior Notes (including the $1.8 million underwriting discount), which were deferred and are being recognized into interest expense over the life of the 2012 Senior Notes. Taking into effect the amortization of the original issue discount and all underwriting and issuance expenses, the 2012 Senior Notes have an effective yield to maturity of approximately 4.7% per annum. The proceeds from the 2012 Senior Notes were utilized to repurchase and retire, for $275.9 million, the remaining $269.8 million balance outstanding on the Company's previously held senior notes issued in 2003.
	2003 Senior Notes
	In May 2003, OBH issued $700.0 million face value of senior unsecured notes ("2003 Senior Notes") through a public offering. In December 2012, OBH repurchased and retired the remaining $269.8 million of 2003 Senior Notes for $275.9 million, which resulted in a $6.3 million loss, including transaction fees and the write-off of $0.2 million in remaining unamortized deferred costs and original issue discount at the time of repurchase, in the year ended December 31, 2012.
	On March 24, 2011, OBH commenced a cash tender offer that resulted in the retirement of $150.0 million aggregate principal amount of the 2003 Senior Notes for $161.6 million, which resulted in a $12.0 million pre-tax loss, including transaction fees.
	Debt Covenants
	At December 31, 2013, OneBeacon was in compliance with all of the covenants under the 2012 Senior Notes.
	Interest
	Total interest expense incurred by OneBeacon for its indebtedness was $13.0 million, $16.9 million and $20.5 million, respectively, during the years ended December 31, 2013, 2012 and 2011. Total cash interest paid by OneBeacon for its indebtedness was $12.7 million, $16.4 million and $20.4 million, respectively, during the years ended December 31, 2013, 2012 and 2011.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	OneBeacon and its Bermuda-domiciled subsidiaries are not subject to Bermuda income tax under current Bermuda law. In the event that there is a change in the current law such that taxes are imposed, OneBeacon and its Bermuda-domiciled subsidiaries would be exempt from such tax until March 31, 2035, pursuant to the Bermuda Exempted Undertakings Tax Protection Act of 1966. OneBeacon also has subsidiaries that operate in Gibraltar, Luxembourg and the United States. U.S. operations are financed with a combination of debt and equity and the financing income currently accounts for the majority of non-U.S. earnings.
	OneBeacon's U.S. subsidiaries join in the filing of a federal consolidated tax return. The consolidated U.S. parent is OneBeacon U.S. Financial Services, Inc. ("OBFS"). For all years, the companies included within the U.S. consolidated tax return are parties to a tax sharing agreement which provides that each company pays the amount of income taxes or estimated tax or receives refunds that it would have to make or be entitled to if it filed its own separate tax return. As a result, certain companies have made payments, and received refunds from the consolidated U.S. parent that are different than amounts payable to the Internal Revenue Service ("IRS"). The companies that are domiciled outside of the U.S. file separate returns for the appropriate jurisdictions.
	The total income tax expense from continuing operations for the years ended December 31, 2013, 2012 and 2011 consisted of the following:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Current tax (expense) benefit:
	Federal		$	(20.3	)	$	10.1		$	18.5
	State		(0.8		)	(1.5		)	(1.6		)
	Non-U.S.		(0.4		)	(0.6		)	(0.1		)
	Total current tax (expense) benefit		(21.5		)	8			16.8
	Deferred tax expense:
	Federal		(12.8		)	(16.4		)	(31.6		)
	State		—			—			—
	Non-U.S.		—			—			—
	Total deferred tax expense		(12.8		)	(16.4		)	(31.6		)
	Total income tax expense		$	(34.3	)	$	(8.4	)	$	(14.8	)
	A reconciliation of taxes calculated using the 35% U.S. statutory rate (the tax rate at which the majority of OneBeacon's worldwide operations are taxed) to the income tax expense on pre-tax income from continuing operations follows:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Tax expense at the U.S. statutory rate		$	(63.5	)	$	(37.1	)	$	(41.9	)
	Differences in taxes resulting from:
	Non-U.S. earnings, net of foreign taxes		20.9			26			25.5
	Tax reserve adjustments		(1.0		)	(1.2		)	(1.9		)
	Tax exempt interest and dividends		2.4			2.3			2.1
	Change in valuation allowance		5.9			(0.7		)	(0.7		)
	Other, net		1			2.3			2.1
	Total income tax expense on pre-tax income from continuing operations		$	(34.3	)	$	(8.4	)	$	(14.8	)
	The non-U.S. component of pre-tax income from continuing operations which, as described above, primarily relates to interest income, was $60.3 million, $75.6 million and $73.3 million, respectively, for the years ended December 31, 2013, 2012 and 2011. The change in valuation allowance was driven by a $6.6 million decrease due to the restructuring of a surplus note with Houston General Insurance Exchange ("HGIE"), a reciprocal which is included in the Company's consolidated results as a VIE.
	The income tax expense related to pre-tax income from continuing operations for the years ended December 31, 2013, 2012 and 2011 represented net effective tax rates of 18.9%, 7.9% and 12.4%, respectively. For the years ended December 31, 2013, 2012 and 2011, the effective tax rate on non-U.S. income was 0.5%, 0.6% and 0.3%, respectively, and the effective tax rate on U.S. income was 28.1%, 26.1% and 31.6%, respectively.
	Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes. An outline of the significant components of OneBeacon's deferred tax assets and liabilities follows:
			December 31,
			2013			2012
			($ in millions)
	Deferred income tax assets related to:
	Discounting of loss and LAE reserves		$	35.3		$	41.6
	Unearned premiums		31.6			39.8
	Sale of Runoff Business		24.2			49.2
	Compensation and bonus accruals		20.3			15.6
	U.S. net operating loss carryforwards		13			42.6
	Tax credit carryforwards		11.1			6.2
	Investment basis differences		8.9			4.6
	Deferred compensation plans		8			8.4
	Fixed assets		4.7			3
	Accrued rent		3.6			3.7
	Allowance for doubtful accounts		1.1			1.2
	Other accrued compensation		0.5			0.7
	Non-U.S. net operating loss carryforwards		0.3			0.2
	Involuntary pool and guaranty fund accruals		0.1			0.1
	Pension and benefit accruals		—			8
	Other items		1.5			9.2
	Total gross deferred income tax assets		164.2			234.1
	Less valuation allowance		(1.9		)	(7.8		)
	Total net deferred income tax assets		162.3			226.3
	Deferred income tax liabilities related to:
	Net unrealized investment gains		33.8			39.4
	Deferred acquisition costs		25.2			43.4
	Prepaid pension		6.3			—
	Other items		6.4			5.7
	Total deferred income tax liabilities		71.7			88.5
	Net deferred tax asset		$	90.6		$	137.8
OneBeacon's deferred tax assets are net of federal and non-U.S. valuation allowances and, to the extent they relate to non-U.S. jurisdictions, they are shown at year-end exchange rates. Of the $1.9 million valuation allowance at December 31, 2013, $0.3 million relates to deferred tax assets on net operating losses (“NOLs”) in Luxembourg subsidiaries that are not expected to have significant income in the future, and $1.6 million relates to deferred tax assets of HGIE, which files its own tax return.
On January 24, 2012, OneBeacon sold the issued and outstanding shares of common stock of OneBeacon Holdings (Luxembourg) S.à r.l. ("OB Lux") to White Sands Holdings (Luxembourg) S.à r.l ("White Sands"), a subsidiary of White Mountains, for $24.7 million. As a result of this sale, the Luxembourg NOL carryovers from OB Lux will no longer be available for use by OneBeacon. See Note 15—"Related Party Disclosures."
OneBeacon believes that, based upon its prior earnings history, expected future earnings, reversing temporary differences and capacity for carry-back of losses, it is more likely than not that the net deferred tax asset balances (net of valuation allowance) carried at December 31, 2013 and 2012 will be realized. It is possible that projected earnings may not be sufficient to utilize the entire deferred tax asset, which could result in material changes to OneBeacon's valuation allowance on deferred tax assets and tax expense.
Net operating loss carryforwards as of December 31, 2013 and the expiration dates are as follows:
		December 31, 2013
		United States			Luxembourg			Total
		($ in millions)
Net operating loss carryforwards by year:
2014		$	—		$	—		$	—
From years 2015 to 2022		1.3			—			1.3
From years 2023 to 2033		35.8			—			35.8
No expiration date		—			1			1
Total net operating loss carryforwards		$	37.1		$	1		$	38.1
		December 31, 2013
		United States			Luxembourg			Total
		($ in millions)
Gross deferred tax asset		$	13		$	0.3		$	13.3
Valuation allowance		(1.3		)	(0.3		)	(1.6		)
Net deferred tax asset		$	11.7		$	—		$	11.7
OneBeacon does not anticipate future taxable income in Luxembourg. Therefore, OneBeacon does not believe that it is more likely than not that these losses will be realized and has recorded a full valuation allowance against the tax benefits associated with these NOLs. Effective January 1, 2013, the Luxembourg statutory tax rate increased from 28.80% to 29.22%.
At December 31, 2013, there were U.S. net operating loss carryforwards of approximately $37.1 million, which begin to expire in 2021. Included in these tax losses are losses of $0.5 million subject to an annual limitation on utilization under Internal Revenue Code Section 382. In addition, the losses include NOLs of $3.8 million related to HGIE, a reciprocal, which files its own tax return. At December 31, 2013, OBFS also had alternative minimum tax credit carryovers of $10.6 million which do not expire.
Recognition of the benefit of a given tax position is based upon whether or not a company determines that it is more likely than not that a tax position will be sustained upon examination based upon the technical merits of the position. In evaluating the more-likely-than-not recognition threshold, OneBeacon must presume that the tax position will be subject to examination by a taxing authority with full knowledge of all relevant information. If the recognition threshold is met, then the tax position is measured at the largest amount of benefit that is more than 50% likely of being realized upon ultimate settlement.
A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:
		Permanent			Temporary			Interest and			Total
		differences(1)			differences(2)			penalties(3)
		($ in millions)
January 1, 2011		$	7.6		$	58.5		$	4.7		$	70.8
Changes in prior year tax positions		—			—			1.9			1.9
Tax positions taken during the current year		—			(21.0		)	—			(21.0		)
Lapse in statute of limitations		—			—			—			—
Settlements with tax authorities		—			—			—			—
December 31, 2011		$	7.6		$	37.5		$	6.6		$	51.7
Changes in prior year tax positions		—			—			1.2			1.2
Tax positions taken during the current year		—			(13.9		)	—			(13.9		)
Lapse in statute of limitations		—			—			—			—
Settlements with tax authorities		—			—			—			—
December 31, 2012		$	7.6		$	23.6		$	7.8		$	39
Changes in prior year tax positions		—			—			1			1
Tax positions taken during the current year		—			(7.0		)	—			(7.0		)
Lapse in statute of limitations		—			—			—			—
Settlements with tax authorities		—			—			(0.4		)	(0.4		)
December 31, 2013		$	7.6		$	16.6		$	8.4		$	32.6
_______________________________________________________________________________
-1	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.
-2	Represents the amount of unrecognized tax benefits that, if recognized, would create a temporary difference between the reported amount of an item in OneBeacon's Consolidated Balance Sheet and its tax basis.
-3	Net of tax benefit.
If recognized, $16.0 million (tax plus interest and penalties) would be recorded as a reduction to income tax expense. Also included in the balance at December 31, 2013 are $16.6 million of tax positions for which ultimate deductibility is highly certain but the timing of deductibility is uncertain. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of the shorter deductibility period would not affect the effective tax rate but would accelerate the payment of cash to the taxing authority to an earlier period. OneBeacon believes that it is reasonably possible that the gross amount of unrecognized tax benefits may decrease by $5.2 million within the next twelve months due to the resolution of its 2005 and 2006 IRS exam.
OneBeacon classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. During the years ended December 31, 2013, 2012 and 2011, OneBeacon recognized $1.0 million, $1.2 million and $1.9 million, respectively, in interest expense (benefit), net of federal benefit. The balance of accrued interest at December 31, 2013 and 2012 is $8.4 million and $7.8 million, respectively, net of any tax benefit.
With few exceptions, OneBeacon is no longer subject to U.S. federal, state or non-U.S. income tax examinations by tax authorities for years before 2005. In October 2008, the IRS commenced examination of OneBeacon's U.S. income tax returns for 2005 and 2006. Subsequent to the year end 2013, on February 14, 2014, OneBeacon received Form 870-AD (Offer to Waive Restrictions on Assessment and Collection Tax Deficiency and to Accept Over assessment) from the IRS Appeals Office relating to the examination of tax years 2005 and 2006. All disputed items have now been agreed to and resolved with the Joint Committee. The total assessment, including interest is $3.3 million. However, $2.7 million of the adjustments relate to items for which the expense deduction has been disallowed in a year being examined, but ultimate deductibility is highly certain to occur in a later period. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of the deduction in the exam period will not affect the effective tax rate. As the receipt of the Form 870-AD described above represents formal settlement, OneBeacon expects to record a tax benefit of approximately $5.0 million in the first quarter of 2014 relating to the settlement of the IRS examination for tax years 2005 and 2006.
On July 28, 2011, the IRS commenced an examination of OneBeacon's U.S. income tax returns for 2007, 2008 and 2009. On July 17, 2013, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2007, 2008 and 2009. The estimated total assessment, including interest, utilization of alternative minimum and foreign tax credit carryovers and capital loss carrybacks, is $68.3 million. However, $60.2 million of the proposed adjustments relate to items for which the expense deduction has been disallowed in a year being examined, but ultimate deductibility is highly certain to occur in a later period. Because of the impact of deferred tax accounting, other than interest and penalties, the disallowance of the deduction in the exam period would not affect the effective tax rate but would accelerate the payment of cash to the taxing authority. OneBeacon disagrees with the adjustments proposed by the IRS and is defending its position. Although the timing of the resolution of these issues is uncertain, it is reasonably possible that the resolution could occur within the next twelve months. An estimate of the range of potential outcomes cannot be made at this time. When ultimately settled, OneBeacon does not expect the resolution of this examination to result in a material change to its financial position.
On September 2, 2013, the IRS commenced an examination of OneBeacon's U.S. income tax returns for 2010, 2011 and 2012. OneBeacon does not expect the resolution of this examination to result in a material change to its financial position.
Net cash (refunds) payments for federal, state and non-U.S. income taxes, including tax sharing payments to related companies, totaled $5.6 million, $0.8 million and $(1.6) million for the years ended December 31, 2013, 2012 and 2011, respectively.

Retirement_Plans

Retirement Plans	12 Months Ended
	Dec. 31, 2013
Compensation and Retirement Disclosure [Abstract]	'
Retirement Plans	'
	Retirement Plans
	OneBeacon sponsors qualified and non-qualified, non-contributory, defined benefit pension plans covering substantially all employees who were employed as of December 31, 2001 and former employees who had met the eligibility requirements, as well as retirees. Current plans include the OneBeacon qualified pension plan (the "Qualified Plan") and the OneBeacon non-qualified pension plan (the "Non-qualified Plan") (collectively the "Plans"). OneBeacon's Plans were frozen and curtailed in 2002 and, as a result, the projected benefit obligation is equal to the accumulated benefit obligation.
	The benefits for the Plans are based primarily on years of service and employees’ compensation through December 31, 2002. OneBeacon’s funding policy is consistent with the funding requirements of U.S. federal laws and regulations.
	The following tables set forth the obligations and funded status, assumptions, plan assets and cash flows associated with the Plans at December 31, 2013 and 2012:
			December 31,
			2013				2012
			($ in millions)
	Change in projected benefit obligation:
	Projected benefit obligation at beginning of year		$	119.5			$	113
	Service cost		0.8				0.7
	Interest cost		4.2				4.7
	Settlement gain		—				(0.7		)
	Special termination benefits expense		0.3				0.6
	Assumption changes		(13.0		)		11.2
	Actuarial gain		(0.5		)		(0.2		)
	Benefits and expenses paid with plan assets		(6.2		)		(7.5		)
	Benefits paid directly by OneBeacon		(2.2		)		(2.3		)
	Projected benefit obligation at end of year		$	102.9			$	119.5
	Change in plan assets:
	Fair value of plan assets at beginning of year		$	124.7			$	120.8
	Actual return on plan assets		24.3				11.4
	Employer contributions		—				—
	Benefits and expenses paid		(6.2		)		(7.5		)
	Fair value of plan assets at end of year		$	142.8			$	124.7
	Funded status at end of year		$	39.9			$	5.2
	The funded status of the consolidated pension plans at December 31, 2013 was $39.9 million, which represents an over-funding of $64.1 million related to the Qualified Plan and an under-funding of $24.2 million related to the Non-qualified Plan. The Non-qualified Plan, which is unfunded, does not hold any assets. OneBeacon has set aside $16.2 million in an irrevocable rabbi trust for the benefit of Non-qualified Plan participants. Assets held in the rabbi trust are not reflected in the funded status of the consolidated pension plans as presented.
	Amounts recognized in the financial statements as of December 31, 2013 and 2012 consist of:
			December 31,
			2013				2012
			($ in millions)
	Net balance sheet asset recorded in other assets		$	64.1			$	32.8
	Net balance sheet liability recorded in other liabilities		(24.2		)		(27.6		)
	Net amount recognized in the financial statements		$	39.9			$	5.2
	Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets, was as follows:
			December 31,
			2013				2012
			($ in millions)
	Projected benefit obligation		$	24.2			$	27.6
	Accumulated benefit obligation		$	24.2			$	27.6
	Fair value of plan assets		$	—			$	—
	Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets, was as follows:
			December 31,
			2013				2012
			($ in millions)
	Projected benefit obligation		$	78.7			$	91.9
	Accumulated benefit obligation		$	78.7			$	91.9
	Fair value of plan net assets		$	142.8			$	124.7
	The amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis for the years ended December 31, 2013 and 2012 were as follows:
			December 31,
			2013				2012
			($ in millions)
	Accumulated other comprehensive loss at beginning of year		$	(21.2	)		$	(16.7	)
	Increase (decrease) in accumulated other comprehensive income (loss):
	Amortization of net actuarial losses recognized during the year		0.9				0.8
	Net actuarial gains (losses) occurring during the year(1)		30.8				(5.9		)
	Other adjustments		—				0.6
	Accumulated other comprehensive income (loss) at end of year		$	10.5			$	(21.2	)
	_______________________________________________________________________________
	-1	Net actuarial gains (losses) resulted from investment returns and demographic experience different than assumed, as well as changes in assumptions in estimating the projected benefit obligation in the years ended December 31, 2013 and 2012.
	The amount in accumulated other comprehensive income, on a pre-tax basis, that has not yet been recognized as a component of net periodic benefit cost for the year ended December 31, 2013 is attributable to net gains. During the year ended December 31, 2014, OneBeacon expects $0.3 million will be amortized from accumulated other comprehensive income into net periodic benefit cost.
	The components of net periodic benefit cost for the years ended December 31, 2013, 2012 and 2011 were as follows:
			December 31,
			2013				2012			2011
		($ in millions)
Service cost		$	0.8			$	0.7		$	0.8
Interest cost		4.2				4.7			5.2
Expected return on plan assets		(7.1		)		(6.9		)	(7.6		)
Amortization of prior service benefit		—				—			—
Amortization of net actuarial losses recognized during the year		0.9				0.8			0.5
Net periodic pension cost (income) before settlements, curtailments and special termination benefits		(1.2		)		(0.7		)	(1.1		)
Settlement loss		—				0.6			0.5
Special termination benefits expense(1)		0.3				0.6			0.8
Total net periodic benefit cost (income)		$	(0.9	)		$	0.5		$	0.2
_______________________________________________________________________________
-1	Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.
Assumptions
The weighted average assumptions used to determine benefit obligations at December 31, 2013 and 2012 were as follows:
		December 31,
		2013			2012
Discount rate		4.66	%		3.64	%
The weighted average assumptions used to determine net periodic benefit cost for the years ended December 31, 2013 and 2012 were as follows:
		December 31,
		2013			2012
Discount rate		3.64	%		4.38	%
Expected long-term rate of return on plan assets		5.75	%		5.75	%
OneBeacon's discount rate assumptions used to account for the Plans reflect the rates at which the benefit obligations could be effectively settled. In addition to consideration of published yields for high quality long-term corporate bonds, U.S. Treasuries and insurance company annuity contract pricings, consideration was given to cash flow matching analyses.
OneBeacon performed an analysis of expected long-term rates of return based on the allocation of its Qualified Plan assets at December 31, 2012 and 2011 to develop expected rates of return for 2013 and 2012, respectively, for each significant asset class or economic indicator. A range of returns was developed based both on forecasts and on broad-market historical benchmarks for expected return, correlation, and volatility for each asset class.
Plan Assets
The majority of the Qualified Plan's assets are managed by Prospector Partners, LLC ("Prospector"), a related party (see Note 15). The investment policy places an emphasis on preserving invested assets through a diversified portfolio of high-quality income producing investments and equity investments.
The investment management process integrates the risks and returns available in the investment arena with the risks and returns available to the Qualified Plan in establishing the proper allocation of invested assets. The asset classes may include fixed maturity, equity, convertible fixed maturity investments, and cash and short-term investments. The factors examined in establishing the appropriate investment mix include the outlook for risk and return in the various investment markets and sectors, and the long-term need for capital growth.
The Qualified Plan's investments are stated at fair value. Many factors are considered in arriving at fair market value. In general, fixed maturity investments such as corporate bonds and government securities are valued based on yields currently available on comparable securities of issuers with similar credit ratings. Shares of common and preferred stock are valued at quoted market prices when available. Convertible fixed maturity investments are valued based on quoted market prices, analysis of listed markets and use of sensitivity analyses. Registered investment companies are valued at the net asset value as reported by the fund at year-end.
The fair value of the Qualified Plan's assets and their related inputs at December 31, 2013 and 2012 by asset category were as follows:
		Fair value at				Level 1			Level 2			Level 3
		December 31, 2013
Fixed maturity investments		$	—			$	—		$	—		$	—
Short-term investments		8.9				8.8			0.1			—
Common equity securities:
Financials		15.8				15.8			—			—
Basic Materials		9.1				9.1			—			—
Consumer		31.6				31.6			—			—
Energy		15.6				15.6			—			—
Utilities		4.2				4.2			—			—
Other		27				27			—			—
Common equity securities		103.3				103.3			—			—
Convertible fixed maturity investments:
Financials		2				—			2			—
Basic Materials		7.4				—			7.4			—
Consumer		10.4				—			10.4			—
Energy		0.6				—			0.6			—
Utilities		—				—			—			—
Other		9.5				—			9.5			—
Convertible fixed maturity investments		29.9				—			29.9			—
Total		$	142.1			$	112.1		$	30		$	—
		Fair value at				Level 1			Level 2			Level 3
		December 31, 2012
Fixed maturity investments:
Utilities		$	1.9			$	—		$	1.9		$	—
Fixed maturity investments		1.9				—			1.9			—
Short-term investments		8.1				8.1			—			—
Common equity securities:
Financials		10.7				10.7			—			—
Basic Materials		10.7				10.7			—			—
Consumer		26.6				26.6			—			—
Energy		12				12			—			—
Utilities		1.9				1.9			—			—
Other		17.6				17.6			—			—
Common equity securities		79.5				79.5			—			—
Convertible fixed maturity investments:
Financials		0.9				—			0.9			—
Basic Materials		10.6				—			10.6			—
Consumer		16.7				—			16.7			—
Energy		0.1				—			0.1			—
Utilities		—				—			—			—
Other		6.2				—			6.2			—
Convertible fixed maturity investments		34.5				—			34.5			—
Total		$	124			$	87.6		$	36.4		$	—
There were no transfers between Levels 1, 2 or 3 during the years ended December 31, 2013 and 2012.
The Qualified Plan's asset allocations at December 31, 2013 and 2012 by asset category were as follows:
		Plan Assets at
		December 31,
		2013			2012
Fixed maturity investments		—	%		1.5	%
Common equity securities		72.7			64.2
Convertible fixed maturity investments		21.1			27.8
Cash and short-term investments		6.2			6.5
Total		100	%		100	%
As described above, the Qualified Plan's investment securities are exposed to various risks such as interest rate, market, and credit risks. Market prices of common equity securities, in general, are subject to fluctuations which would cause the amount to be realized upon sale or exercise of the instruments to differ significantly from the current reported value. The fluctuations may result from perceived changes in the underlying economic characteristics of the investee, the relative price of alternative investments, general market conditions and supply and demand imbalances for a particular security. Increases and decreases in prevailing interest rates generally translate into decreases and increases in fair values of fixed maturity and convertible fixed maturity investments, respectively. Additionally, fair values of interest rate sensitive instruments may be affected by the creditworthiness of the issuer, prepayment options, relative values of alternative investments, the liquidity of the instrument and other general market conditions.
Cash Flows
OneBeacon does not expect to make a contribution to its Qualified Plan in 2014. OneBeacon anticipates contributing $2.2 million to the Non-qualified Plan in 2014, for which OneBeacon has assets held in a rabbi trust.
The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid:
	Expected Benefit
	Payments
	($ in millions)
2014	$	5.1
2015	5.4
2016	5.6
2017	5.8
2018	6.1
2019 - 2023	33.9
Other Benefit Plans
OneBeacon sponsors an employee savings plan (defined contribution plan) covering the majority of employees. The contributory plan provides qualifying employees with matching contributions of 50% of the first 6% of salary (subject to federal limits on allowable contributions in a given year). Total expense for the plan was $2.3 million, $2.8 million and $3.0 million in the years ended December 31, 2013, 2012 and 2011, respectively. The employee savings plan includes an employee stock ownership component. See Note 9.
OneBeacon had a post-employment benefit liability related to disability and health benefits available to former employees that are no longer employed by the Company, of $5.8 million and $7.1 million at December 31, 2013 and 2012, respectively.

Employee_ShareBased_Incentive_

Employee Share-Based Incentive Compensation Plans	12 Months Ended
	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Employee Share-Based Incentive Compensation Plans	'
	Employee Share-Based Incentive Compensation Plans
	The OneBeacon Long-Term Incentive Plan (the "Incentive Plan") provides for granting various types of share-based incentive awards including performance shares, options, share appreciation rights and restricted shares to certain key employees of OneBeacon. The Incentive Plan was adopted by the Board of Directors (the "Board") in October 2006. In 2007, the Board and shareholders approved the 2007 OneBeacon Long-Term Incentive Plan (the "2007 Incentive Plan"). The 2007 Incentive Plan provides for all of the awards referenced above as well as restricted stock units. Awards are granted under the 2007 Incentive Plan, which also reflects amendments from 2011.
	OneBeacon's share-based compensation plans include performance shares, restricted shares, and previously included stock options and restricted stock units. OneBeacon's share-based compensation plans are designed to maximize shareholder value over long periods of time by aligning the financial interests of its management with those of its owners. Performance shares are payable only upon achievement of pre-defined business goals and are valued based on the market value of OneBeacon's common shares at the time awards are earned. See "Performance Shares" below. Performance shares are typically paid in cash, though, in some instances, they may be paid in common shares or may be deferred in accordance with the terms of OneBeacon's deferred compensation plan. OneBeacon expenses the full cost of all its share-based compensation over the requisite service period. For the years ended December 31, 2013, 2012 and 2011, the Company recognized expense of $11.9 million, $6.3 million and $9.2 million, respectively, related to its share-based compensation plans.
	Performance Shares
	Performance shares are conditional grants of a specified maximum number of common shares or an equivalent amount of cash. In general, grants are earned, subject to the attainment of pre-specified performance goals, at the end of a three-year period or as otherwise determined by the Compensation Committee of the Board (the "Compensation Committee") and are valued based on the market price of an underlying OneBeacon common share at the time awards are paid ("OB Performance Shares"). Results that significantly exceed pre-specified targets can result in a performance share payout of up to 200% of granted shares whereas results significantly below target could result in no payout. Compensation expense, based on the estimated performance share payout, is recognized ratably over the performance period.
	For awards granted during the year ended December 31, 2011, the targeted performance goal for full payment of the outstanding performance shares is the attainment of a growth in book value per share ("GBVPS") of 11% for the 2011-2013 performance cycle. At a GBVPS of 4% or less, no performance shares would be earned and at a GBVPS of 18% or more, 200% of performance shares would be earned.
	For awards granted during the year ended December 31, 2012, the targeted performance goal for full payment of the outstanding performance shares is the attainment of a GBVPS of 10% for the 2012-2014 performance cycle. At a GBVPS of 3% or less, no performance shares would be earned and at a GBVPS of 17% or more, 200% of performance shares would be earned.
	For awards granted during the year ended December 31, 2013, the targeted performance goal for full payment of the outstanding performance shares is the attainment of a GBVPS of 13% for the performance cycle 2013-2015. At a GBVPS of 6% or less, no performance shares would be earned and at a GBVPS of 20% or more, 200% of performance shares would be earned.
	The following summarizes performance share activity for OB Performance Shares for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013					2012					2011
			Target OB		Accrued			Target OB		Accrued			Target OB		Accrued
			Performance		expense			Performance		expense			Performance		expense
			Shares					Shares					Shares
			outstanding					outstanding					outstanding
			($ in millions)
	Beginning of period		563,190		$	1.2		642,667		$	9.7		1,464,295		$	18.5
	Payments and deferrals(1)		(238,658	)	—			(258,901	)	(7.7		)	(936,150	)	(10.5		)
	New awards		179,000		—			181,290		—			194,900		—
	Forfeitures and net change in assumed forfeitures		(10,111	)	(0.1		)	(1,866	)	—			(80,378	)	(0.5		)
	Expense recognized		—		2.9			—		(0.8		)	—		2.2
	End of period		493,421		$	4		563,190		$	1.2		642,667		$	9.7
	_______________________________________________________________________________
	(1)	No payments were made in 2013 for the 2010-2012 performance cycle as the performance factor was zero. Performance share payments in 2012 for the 2009-2011 performance cycle were based upon a performance factor of 138.6%. Performance share payments in 2011 for the 2008-2010 performance cycle were based upon a performance factor of 68.5%.
	The following summarizes performance shares outstanding and accrued performance share expense at December 31, 2013 for each performance cycle:
			Target OB		Accrued
			Performance		expense
			Shares
			outstanding
			($ in millions)
	Performance cycle:
	2011 - 2013		142,138		$	1
	2012 - 2014		181,290		2.1
	2013 - 2015		179,000		1
	Subtotal		502,428		4.1
	Assumed forfeitures		(9,007	)	(0.1		)
	Total at December 31, 2013		493,421		$	4
	If 100% of the outstanding performance shares had been vested on December 31, 2013, the total additional compensation cost to be recognized would have been $2.9 million, based on current accrual factors (common share price, accumulated dividends and payout assumptions) at December 31, 2013.
	All performance shares earned and paid were settled in cash or by deferral into OneBeacon's deferred compensation plan.
	Restricted Shares
	On March 1, 2012, OneBeacon issued 300,000 shares of restricted stock to certain employees that vest in equal installments on February 28, 2014 and 2015. On May 25, 2011, OneBeacon issued 630,000 shares of restricted stock to its CEO that vest in equal installments on February 22, 2014, 2015, 2016 and 2017. Concurrently with the 2011 grant of restricted stock, 35,000 performance shares issued to the CEO for the 2011-2013 performance share cycle were forfeited. Annual performance share awards to the CEO through 2016 are being reduced by 35,000 shares. The restricted shares contain dividend participation features and therefore are considered participating securities.
	The following summarizes restricted shares activity for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013					2012					2011
			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
			($ in millions)
	Beginning of period		927,000		$	9.6		630,000		$	7.7		—		$	—
	New awards		—		—			300,000		4.5			630,000		8.6
	Forfeitures		(3,000	)	—			(2,333	)	—			—		—
	Vested		(9,000	)	—			(667	)	—			—		—
	Expense recognized		—		(3.1		)	—		(2.6		)	—		(0.9		)
	End of period		915,000		$	6.5		927,000		$	9.6		630,000		$	7.7
	Restricted shares that vested during the year ended December 31, 2013 had a grant date fair value of $0.1 million. No significant amount of restricted shares vested during the years ended December 31, 2012 and 2011. As of December 31, 2013, unrecognized compensation expense of $6.5 million related to restricted stock awards is expected to be recognized over a weighted-average period of 2.7 years.
	Stock Options
	In November 2006, in connection with the initial public offering, OneBeacon issued 1,420,000 options to acquire common shares of the Company at an above-market fixed exercise price to certain key employees as a one-time incentive. No options remained outstanding as of December 31, 2013 and 2012 as the remaining 740,870 options outstanding as of December 31, 2011 expired unexercised during 2012.
	No expense was recognized for the years ended December 31, 2013 and 2012 as the options were fully vested prior to 2012. The company recognized compensation expense of $0.5 million in connection with these options during the year ended December 31, 2011.
	Restricted Stock Units
	The options granted in connection with OneBeacon's initial public offering did not include a mechanism in the options to reflect the contribution to total return from the regular quarterly dividend. As a result, during the first quarter of 2008, OneBeacon granted 116,270 Restricted Stock Units ("RSUs") to actively employed option holders. The expense associated with the RSUs was recognized ratably over the vesting period. No expense was recognized for the year ended December 31, 2013 and 2012 as the RSUs were fully vested in 2011. For the year ended December 31, 2011 OneBeacon recognized expense of $0.1 million. All vested RSUs were distributed in May 2012.
	Other Share-Based Compensation
	OneBeacon sponsors a defined contribution plan, the OneBeacon 401(k) Savings and Employee Stock Ownership Plan ("KSOP"). Under the KSOP, participants have the ability to invest their balances in several different investment options, including the common shares of White Mountains and the common shares of the Company.
	The employee stock ownership component of the KSOP provides all participants with an annual base contribution in common shares of the Company equal to 3% of their salary, up to the applicable Social Security wage base (or $113,700 with respect to 2013). Additionally, those participants not otherwise eligible to receive certain other Company benefits can earn a variable contribution up to an additional 6% of their salary, capped at the annual covered compensation limits ($250,000 for 2013), contingent upon OneBeacon's performance. The variable contribution amounts for eligible participants constituted approximately 4%, 3% and 2%, respectively, of salary for the years ended December 31, 2013, 2012 and 2011. OneBeacon has recorded $6.0 million, $4.5 million and $6.0 million, respectively, in compensation expense to pay benefits and allocate common shares to participants' accounts for the years ended December 31, 2013, 2012 and 2011.
	As of December 31, 2013 and 2012, the KSOP owned less than 3% of either of the total White Mountains common shares outstanding or the total Company common shares outstanding. All common shares held by the KSOP are considered outstanding for earnings (loss) per share computations.
	As of December 31, 2013, the Company has no outstanding share appreciation rights.

Common_Shareholders_Equity

Common Shareholders' Equity	12 Months Ended
	Dec. 31, 2013
Stockholders' Equity Note [Abstract]	'
Common Shareholders' Equity	'
	Common Shareholders' Equity
	Common Shares Repurchased and Retired
	On August 22, 2007, the Company's Board authorized the repurchase of up to $200.0 million of its Class A common shares from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. This authorization does not have a stated expiration date. During the years ended December 31, 2013, 2012 and 2011, no shares were repurchased under the share repurchase authorization. The amount of authorization remaining is $87.7 million as of December 31, 2013.
	During the year ended December 31, 2013, the Company repurchased 3,300 common shares to satisfy employee income tax withholding, pursuant to employee benefit plans. No shares were repurchased during the years ended December 31, 2012 and 2011. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above.
	Dividends on Common Shares
	During the years ended December 31, 2013, 2012 and 2011, the Company declared and paid cash dividends totaling $80.2 million, $80.1 million and $174.8 million, respectively, or $0.84 per common share of regular quarterly cash dividends for each year, plus a $1.00 per common share special dividend for the year ended December 31, 2011.
	Accumulated Other Comprehensive Income (Loss)
	For the years ended December 31, 2013, 2012 and 2011, OneBeacon recorded changes to accumulated other comprehensive income (loss) for net increases in net benefit plan assets and obligations of $20.6 million, $(2.9) million and $(11.2) million, which were net of tax (expense) benefit of $(11.1) million, $1.6 million and $6.0 million, respectively. See Note 8—"Retirement Plans."
	Share Repurchase Authorization
	On August 22, 2007, the Company's Board authorized the repurchase of up to $200.0 million of its Class A common shares from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. This authorization does not have a stated expiration date. During the years ended December 31, 2013, 2012 and 2011, no shares were repurchased. The amount of authorization remaining is $87.7 million.

Statutory_Capital_and_Surplus

Statutory Capital and Surplus	12 Months Ended
	Dec. 31, 2013
Insurance [Abstract]	'
Statutory Capital and Surplus	'
	Statutory Capital and Surplus
	OneBeacon's U.S. insurance operations are subject to regulation and supervision in each of the states where they are domiciled and licensed to conduct business. Generally, state regulatory authorities have broad supervisory and administrative powers over such matters as licenses, standards of solvency, premium rates, policy forms, investments, security deposits, methods of accounting, form and content of financial statements, reserves for unpaid loss and LAE, reinsurance, minimum capital and surplus requirements, dividends and other distributions to shareholders, periodic examinations and annual and other report filings. In general, such regulation is for the protection of policyholders rather than shareholders. Currently, all state insurance departments have established the insurer solvency laws and regulatory infrastructure to maintain accredited status with the National Association of Insurance Commissioners ("NAIC"). A key solvency-driven NAIC accreditation requirement is a state's adoption of risk-based capital ("RBC") requirements. At December 31, 2013, OneBeacon's active U.S. insurance operating subsidiaries met their respective RBC requirements.
	The Insurance Act 1978 of Bermuda and related regulations, as amended (the “Insurance Act”), regulates the insurance businesses of Split Rock. Under the Insurance Act, Split Rock is required to maintain available statutory capital and surplus at a level equal to or in excess of its enhanced capital requirement which is established by reference to either a Bermuda Solvency Capital Requirement ("BSCR") model or an approved internal capital model. At December 31, 2013, Split Rock met Bermuda’s statutory capital and surplus requirements.
	OneBeacon’s combined statutory surplus (including U.S. statutory surplus and Bermuda statutory capital and surplus for Split Rock) was $1.0 billion and $0.9 billion as of December 31, 2013 and 2012, respectively. OneBeacon's U.S. combined statutory surplus was $0.9 billion as of both December 31, 2013 and 2012. OneBeacon's combined U.S. statutory net income for the years ended December 31, 2013, 2012 and 2011 was $100.1 million, $82.0 million and $119.6 million, respectively. The minimum policyholders' surplus necessary to satisfy OneBeacon's regulatory requirements was $164.7 million at December 31, 2013, which equals the authorized control level of the NAIC risk-based capital of OneBeacon’s primary top tier regulated operating subsidiary.
	OneBeacon’s statutory capital and surplus for Split Rock was $96.4 million as of December 31, 2013. Split Rock’s statutory net loss for the year ended December 31, 2013 was $38.7 million. As Split Rock was formed during 2013, there are no equivalent amounts for 2012 or 2011.
	The principal differences between OneBeacon's combined U.S. statutory amounts and Split Rock, and the amounts reported in accordance with GAAP include deferred acquisition costs, deferred taxes and market value adjustments for debt securities. OneBeacon's insurance subsidiaries' statutory policyholders' surplus at December 31, 2013 was in excess of the minimum requirements of relevant state and Bermuda insurance regulations.
	Dividend Capacity
	Under the insurance laws of the jurisdictions under which OneBeacon's insurance operating subsidiaries are domiciled, an insurer is restricted with respect to the timing or the amount of dividends it may pay without prior approval by regulatory authorities. Accordingly, there can be no assurance regarding the amount of such dividends that may be paid by such subsidiaries in the future.
	Generally, OneBeacon's top tier regulated U.S. insurance operating subsidiaries have the ability to pay dividends during any 12-month period without the prior approval of regulatory authorities in an amount set by formula based on the greater of prior year statutory net income or 10% of prior year end statutory surplus, subject to the availability of unassigned funds. OneBeacon Insurance Company ("OBIC"), OneBeacon's primary top tier regulated U.S. insurance operating subsidiary, has the ability to pay $86.6 million of dividends during 2014 without prior approval of regulatory authorities, subject to the availability of unassigned funds. The amount of dividends available to be paid by OBIC in any given year is also subject to cash flow and earnings generated by OBIC's business, which now just comprises the Runoff Business, as well as to dividends received from its subsidiaries, including Atlantic Specialty Insurance Company ("ASIC"), the lead U.S. insurance operating subsidiary for the Ongoing Business. At December 31, 2013, OBIC had $0.6 billion of unassigned funds and $0.9 billion of statutory surplus.
	As disclosed in Note 2—"Acquisitions and Dispositions," during the fourth quarter of 2012, OneBeacon executed various intercompany reinsurance agreements which, along with other internal capital transactions among the Company's regulated U.S. insurance operating subsidiaries, resulted in ASIC becoming the lead insurance company for the ongoing specialty business and OBIC becoming the lead insurance company for the Runoff Business. Notwithstanding these restructuring transactions, OneBeacon continues to manage its statutory capital on a combined basis. Although OBIC remains the primary top tier regulated U.S. insurance operating subsidiary and maintains sufficient statutory capital to support the Runoff Business, the majority of the group's statutory capital is now included in ASIC, which is currently a subsidiary of OBIC, to support the ongoing specialty business. Prior to the closing of the Runoff Transaction, and subject to regulatory approval, OBIC will distribute its investment in ASIC to its immediate parent, OneBeacon LLC.
	ASIC has the ability to pay dividends during any 12-month period without the prior approval of regulatory authorities in an amount set by formula based on the lesser of net investment income, as defined by statute, or 10% of statutory surplus, in both cases as most recently reported to regulatory authorities, subject to the availability of earned surplus, and subject to dividends paid in prior periods. ASIC has the ability to pay $23.9 million of dividends during 2014 without prior approval of regulatory authorities, subject to the availability of earned surplus. Given the changes in structure noted above, and in order for ASIC to pay dividends consistent with being OneBeacon’s lead insurance company for its Ongoing Business, ASIC may require prior approval by regulatory authorities in order to make additional distributions until it builds up a historical net investment income stream and earned surplus balance under its new structure. At December 31, 2013, ASIC had $94.6 million of earned surplus and $0.7 billion of statutory surplus.
	Split Rock has the ability to declare or pay dividends during any 12-month period without the prior approval of Bermuda regulatory authorities on condition that any such declaration or payment of such dividend does not cause a breach of any of its regulatory solvency and liquidity requirements. If Split Rock fails to meet its regulatory solvency or liquidity requirements on the last day of any financial year, it is prohibited from declaring or paying any dividends during the next financial year without the approval of the Bermuda Monetary Authority.
	In addition, under the Companies Act 1981 of Bermuda (the Companies Act), Split Rock is prohibited from declaring or paying a dividend, or making a distribution out of contributed surplus, if there are reasonable grounds for believing that in each case:
	• Split Rock is, or would after the payment be, unable to pay its liabilities as they become due; or
	• the realizable value of Split Rock's assets would thereby be less than its liabilities.
	During 2014, Split Rock has the ability to make capital distributions without the prior approval of regulatory authorities, subject to meeting all appropriate liquidity and solvency requirements, of up to $20.3 million, which is equal to 15% of its December 31, 2013 statutory capital, excluding earned surplus. During the year ended December 31, 2013, OneBeacon Ltd., through an intermediary holding company, contributed $135.1 million to Split Rock. Split Rock did not pay any dividends in 2013.
	During the year ended December 31, 2013, ASIC distributed $190.0 million to OBIC by way of an extraordinary return of capital, which was approved by the New York Department of Financial Services. OBIC, in turn, distributed the $190.0 million to its immediate parent. Subsequently, the Company's intermediary holding companies contributed $35.0 million to OBIC in late 2013.
	During the year ended December 31, 2012, the Company's top tier regulated U.S. insurance operating subsidiaries distributed $173.1 million to their immediate parent, representing $114.7 million of dividends, which included the distribution of a regulated insurance subsidiary with a value of $34.0 million, and a return of capital of $58.4 million.
	During the years ended December 31, 2013 and 2012, the Company's unregulated insurance operating subsidiaries paid $17.3 million and $4.9 million, respectively, of dividends to their immediate parent. At December 31, 2013, OneBeacon's unregulated insurance operating subsidiaries had $57.9 million of net unrestricted cash, short-term investments and fixed maturity investments.
	As described in Note 10, during the years ended December 31, 2013 and 2012, OneBeacon Ltd. declared and paid dividends totaling $80.2 million and $80.1 million, respectively, of regular quarterly dividends to its common shareholders. During the year ended December 31, 2011, the Company declared and paid dividends totaling $174.8 million, including $79.7 million of regular quarterly dividends and $95.1 million of a special dividend.
	At December 31, 2013, OneBeacon Ltd. and its intermediate holding companies held $217.5 million of net unrestricted cash, short-term investments and fixed maturity investments and $89.8 million of common equity securities and convertible fixed maturity investments outside of its regulated and unregulated insurance operating subsidiaries.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2013
Segment Reporting [Abstract]	'
Segment Information	'
	Segment Information
	The Company has fourteen underwriting operating segments, including two that were exited in 2013, which are managed by the chief operating decision maker and are aggregated into two underwriting reportable segments. The two underwriting reportable segments were determined based on the nature of products or services, the production process, method of distribution and the nature of the regulatory environment. The principal difference between the reportable segments is the type or class of customer.
	The Specialty Products segment is comprised of eight operating segments, including a new Crop underwriting operating segment, as well as the Collector Cars and Boats underwriting operating segment that was exited in the first quarter of 2013 (see Note 2—"Acquisitions and Dispositions"), representing an aggregation based on those that offer distinct products and tailored coverages and services to a broad customer base across the United States. In addition to Crop and Collector Cars and Boats, the Specialty Products segment includes the Professional Insurance, Specialty Property, Environmental, Tuition Reimbursement, Programs, and Surety underwriting operating segments. During 2013, the Company received approval to provide multiple peril crop insurance through the federal crop insurance program administered by the U.S. Department of Agriculture's Risk Management Agency. The Company has entered into an exclusive agreement with a managing general agency, The Climate Corporation, provide coverages through the federal program and other supplemental coverages, including crop-hail. The Company began writing crop business in the fourth quarter of 2013.
	The Specialty Industries segment is comprised of six underwriting operating segments, including the Energy underwriting operating segment that was exited in the first quarter of 2013, representing an aggregation based on those that focus on solving the unique needs of a particular customer or industry group. The Specialty Industries segment includes the International Marine Underwriters, Technology, Accident, Government Risks, and Entertainment underwriting operating segments.
	The Investing, Financing and Corporate segment includes the investing and financing activities for OneBeacon on a consolidated basis, and certain other activities conducted through the Company and its intermediate subsidiaries.
	Invested assets are not allocated to the Specialty Products and Specialty Industries segments since OneBeacon does not manage them by segment. Invested assets, net investment income and net realized and change in unrealized investment gains related to OneBeacon's Specialty Products and Specialty Industries segments are included in the Investing, Financing and Corporate segment since these assets are available for payment of losses and expenses for all segments. Debt and the related interest expense on debt also are not allocated to or managed by segment and are also included in the Investing, Financing and Corporate segment.
	Substantially all of the Company's revenue is generated from customers located in the United States.
	Financial information for OneBeacon's reportable segments is as follows:
			Insurance Operations						Investing,
									Financing
									and
			Specialty			Specialty			Corporate			Consolidated
			Products			Industries
			($ in millions)
	Year ended December 31, 2013
	Earned premiums		$	553.5		$	566.9		$	—		$	1,120.40
	Loss and loss adjustment expense		(312.3		)	(309.8		)	—			(622.1		)
	Policy acquisition expenses		(106.3		)	(102.6		)	—			(208.9		)
	Other underwriting expenses		(97.4		)	(107.4		)	—			(204.8		)
	Total underwriting income		37.5			47.1			—			84.6
	Net investment income		—			—			41.1			41.1
	Net realized and change in unrealized investment gains		—			—			49.4			49.4
	Net other revenues		0.3			1.1			29.8			31.2
	General and administrative expenses		—			(2.4		)	(9.6		)	(12.0		)
	Interest expense		—			—			(13.0		)	(13.0		)
	Pre-tax income from continuing operations		$	37.8		$	45.8		$	97.7		$	181.3
	Year ended December 31, 2012
	Earned premiums		$	604		$	528		$	—		$	1,132.00
	Loss and loss adjustment expense		(345.6		)	(304.4		)	—			(650.0		)
	Policy acquisition expenses		(150.3		)	(99.1		)	—			(249.4		)
	Other underwriting expenses		(96.2		)	(109.0		)	—			(205.2		)
	Total underwriting income		11.9			15.5			—			27.4
	Net investment income		—			—			53.6			53.6
	Net realized and change in unrealized investment gains		—			—			55.7			55.7
	Net other revenues (expenses)		0.4			(0.8		)	(0.1		)	(0.5		)
	General and administrative expenses		—			(1.9		)	(11.5		)	(13.4		)
	Interest expense		—			—			(16.9		)	(16.9		)
	Pre-tax income from continuing operations		$	12.3		$	12.8		$	80.8		$	105.9
	Year ended December 31, 2011
	Earned premiums		$	549.8		$	462.4		$	—		$	1,012.20
	Loss and loss adjustment expense		(281.7		)	(266.6		)	—			(548.3		)
	Policy acquisition expenses		(129.1		)	(92.1		)	—			(221.2		)
	Other underwriting expenses		(77.1		)	(85.2		)	—			(162.3		)
	Total underwriting income		61.9			18.5			—			80.4
	Net investment income		—			—			71.4			71.4
	Net realized and change in unrealized investment gains		—			—			10.6			10.6
	Net other revenues (expenses)		—			0.6			(13.0		)	(12.4		)
	General and administrative expenses		—			(1.7		)	(8.1		)	(9.8		)
	Interest expense		—			—			(20.5		)	(20.5		)
	Pre-tax income from continuing operations		$	61.9		$	17.4		$	40.4		$	119.7
			Insurance Operations						Investing,
									Financing
									and
			Specialty			Specialty			Corporate(1)			Consolidated
			Products			Industries
			($ in millions)
	31-Dec-13
	Total investment securities		$	—		$	—		$	2,364.90		$	2,364.90
	Reinsurance recoverables		35.1			54.8			—			89.9
	Deferred acquisition costs		53.6			50.1			—			103.7
	Unpaid loss and loss adjustment expense reserves		607.8			446.5			—			1,054.30
	Unearned premiums		275.4			269.5			—			544.9
	Debt		—			—			274.7			274.7
31-Dec-12
Total investment securities		$	—		$	—		$	2,291.50		$	2,291.50
Reinsurance recoverables		52.3			58.3			—			110.6
Deferred acquisition costs		75.1			48.8			—			123.9
Unpaid loss and loss adjustment expense reserves		561.9			438.1			—			1,000.00
Unearned premiums		318.7			255.1			—			573.8
Debt		—			—			274.7			274.7
_______________________________________________________________________________
(1)As described in Note 2, balances related to the the Runoff Business are presented as held for sale. Total investment securities excludes $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale as of December 31, 2013 and December 31, 2012, respectively.
The following tables provide net written premiums, earned premiums and underwriting ratios for OneBeacon's insurance operations by reportable segment and in total for the years ended December 31, 2013, 2012 and 2011:
		Insurance Operations
($ in millions)		Specialty			Specialty			Consolidated
		Products			Industries
Year ended December 31, 2013
Net written premiums		$	509.6		$	579		$	1,088.60
Earned premiums		553.5			566.9			1,120.40
Underwriting ratios:(1)
Loss and LAE		56.4		%	54.7		%	55.5		%
Expense		36.8			37			36.9
Total combined ratio		93.2		%	91.7		%	92.4		%
Year ended December 31, 2012
Net written premiums		$	630.9		$	548.3		$	1,179.20
Earned premiums		604			528			1,132.00
Underwriting ratios:(1)
Loss and LAE		57.2		%	57.7		%	57.4		%
Expense		40.7			39.4			40.1
Total combined ratio		97.9		%	97.1		%	97.5		%
Year ended December 31, 2011
Net written premiums		$	571.2		$	491.5		$	1,062.70
Earned premiums		549.8			462.4			1,012.20
Underwriting ratios:(1)
Loss and LAE		51.2		%	57.7		%	54.2		%
Expense		37.5			38.3			37.9
Total combined ratio		88.7		%	96		%	92.1		%
_______________________________________________________________________________
(1)	Underwriting ratios are used to measure the components of underwriting profitability and include: The loss and LAE ratio, calculated by dividing loss and LAE by earned premiums; the expense ratio, calculated by dividing policy acquisition and other underwriting expenses by earned premiums; and the combined ratio, the sum of the loss and LAE ratio and the expense ratio.

Variable_Interest_Entity_Notes

Variable Interest Entity (Notes)	12 Months Ended
	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Variable Interest Entities (VIE)	'
	Variable Interest Entities ("VIE")
	Reciprocals
	Reciprocals are policyholder-owned insurance carriers organized as unincorporated associations. Each policyholder insured by the reciprocal shares risk with the other policyholders. Policyholders share profits and losses in the same proportion as the amount of insurance purchased but are not subject to assessment for net losses of the reciprocal.
	OneBeacon has one reciprocal that was capitalized by loaning funds to them in exchange for surplus notes. In 2004, OneBeacon formed Houston General Management Company ("HGMC") to provide management services for a fee to the reciprocal, Houston General Insurance Exchange ("HGIE"). During 2004, OneBeacon contributed $2.0 million of capital to HGIE and in 2005, contributed one of its subsidiaries, Houston General Insurance Company ("HGIC") with assets of $149.4 million and liabilities of $127.6 million, to HGIE (together "Houston General Insurance"). Subsequent to the contribution of HGIC, HGIE issued a surplus note of $23.7 million to HGMC. During 2012, HGIE sold HGIC to OBIC. During 2013, HGIE repaid $19.0 million of the interest owed on the surplus note, as well as $1.0 million of the outstanding principal balance. Subsequent to this payment, the surplus note was amended such that the remaining balance under existing surplus notes was reduced to $4.0 million. The principal and interest on the remaining surplus note is repayable to HGMC only with regulatory approval. The obligation to repay principal on the note is subordinated to all other liabilities including obligations to policyholders and claimants for benefits under insurance policies. OneBeacon has no ownership interest in the reciprocals.
	OneBeacon has determined that HGIE qualifies as a VIE. Furthermore, OneBeacon has determined that it is the primary beneficiary as it has both the power to direct the activities of the VIE that most significantly impact the entity's economic performance and the obligation to absorb losses or receive benefits of the entity that could potentially be significant to the VIE as a result of the management services provided to the reciprocal and the funds loaned to it. Accordingly, OneBeacon consolidates HGIE.
	At December 31, 2013 and 2012, consolidated amounts related to HGIE included total assets of $2.6 million and $22.6 million, respectively, and total liabilities of $4.2 million and $43.8 million, respectively. At December 31, 2013, the net amount of capital at risk is equal to the surplus note of $4.0 million less the accumulated losses of $1.6 million which includes accrued interest on the surplus note of $0.1 million which eliminates in consolidation.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2013
Fair Value Disclosures [Abstract]	'
Fair Value of Financial Instruments	'
	Fair Value of Financial Instruments
	OneBeacon carries its financial instruments on its balance sheet at fair value with the exception of its equity method investments and fixed-rate, long-term indebtedness. For certain financial instruments where quoted market prices are not available, other independent valuation techniques and assumptions are used. Because considerable judgment is used, these estimates are not necessarily indicative of amounts that could be realized in a current market exchange. Certain financial instruments are excluded from disclosure, including insurance contracts.
	At December 31, 2013 and 2012, the fair value of OneBeacon's 2012 Senior Notes (its fixed-rate, long-term indebtedness) was $269.8 million and $282.4 million, respectively, which compared to a carrying value of $274.7 million as of both December 31, 2013 and 2012. The fair value measurement of the Senior Notes is classified as Level 2 in the valuation hierarchy.

Related_Party_Disclosures

Related Party Disclosures	12 Months Ended
	Dec. 31, 2013
Related Party Transactions [Abstract]	'
Related Party Disclosures	'
	Related Party Disclosures
	White Mountains
	During the fourth quarter of 2006, White Mountains sold 27.6 million or 27.6% of the Company's common shares in an initial public offering. Prior to the initial public offering, OneBeacon was a wholly-owned subsidiary of White Mountains. As of December 31, 2013, White Mountains owned 75.2% of the Company's outstanding common shares.
	Separation Agreement
	In connection with the initial public offering, the Company entered into a separation agreement dated November 14, 2006 with White Mountains (the "Separation Agreement") to address a number of operational, administrative and financial matters relating to the fact that OneBeacon would no longer be a wholly-owned subsidiary of White Mountains. These matters included, among others, the administration of payroll, employee benefits programs, deferred compensation and 401(k) plans, OneBeacon's travel and logistics office, certain information technology assets and functions and certain agreements with respect to finance and tax arrangements. Pursuant to the Separation Agreement, White Mountains has agreed to indemnify the Company and its subsidiaries, as well as their current and former officers, directors and employees to the extent permitted by law, for any and all claims or actions resulting in losses, expenses or damages relating to or arising out of the business, operation or ownership of any subsidiary company or business owned by the Company or its subsidiaries that, subsequent to OneBeacon's ownership, was a subsidiary or business of White Mountains (but no longer a subsidiary or business of OneBeacon).
	For the years ended December 31, 2013, 2012 and 2011, OneBeacon recorded expenses of $0.4 million, $0.3 million and $0.4 million, respectively, and recorded revenues of $2.0 million, $2.1 million and $2.4 million, respectively, for services under the Separation Agreement that OneBeacon received from or provided to White Mountains and its subsidiaries.
	Registration Rights Agreement
	In connection with the initial public offering, the Company entered into a registration rights agreement dated November 14, 2006 with White Mountains that provides that White Mountains can demand that the Company register the distribution of its common shares owned by White Mountains ("demand" registration rights). In addition, White Mountains has "piggyback" registration rights, which means that White Mountains may include its shares in any future registrations of the Company's common equity securities, whether or not that registration relates to a primary offering by the Company or a secondary offering by or on behalf of any of the Company's shareholders. These registration rights are transferable by White Mountains. The Company will pay all costs and expenses in connection with each such registration, except underwriting discounts and commissions applicable to the common shares sold by White Mountains. The registration rights agreement contains customary terms and provisions with respect to, among other things, registration procedures and rights to indemnification in connection with the registration of the common shares on behalf of White Mountains. The Company will register sales of its common shares owned by employees and directors of White Mountains pursuant to employee share or option plans, but only to the extent such registration is required for the shares to be freely tradable.
	Investment Management Agreement with WM Advisors
	Prior to the initial public offering, White Mountains Advisors LLC ("WM Advisors") managed the majority of OneBeacon's investments, including the investments of the employee benefit plan portfolios. Prospector served as a discretionary advisor with respect to certain assets, specifically publicly-traded common equity and convertible securities, through a sub-advisory agreement with WM Advisors.
	Pursuant to a Master Investment Management Agreement dated as of October 1, 2010, as amended (the "Master Investment Management Agreement"), WM Advisors supervises and directs the fixed income and other investments portions of OneBeacon's investment portfolio in accordance with the investment objectives, policies and restrictions described in OneBeacon's investment guidelines (the "Investment Guidelines"). The Master Investment Management Agreement replaced agreements dated as of November 2006 and 2007 entered into subsequent to the initial public offering.
	In November 2009, WM Advisors terminated the investment management agreements with respect to the management of the employee benefit plan portfolios, including two of the three proprietary funds offered in the KSOP which Prospector managed through a sub-advisory agreement which was also terminated. New investment management agreements were entered into with Prospector to manage the portfolios of the Qualified Plan and the KSOP. See "Related Party Disclosures—Prospector."
	Under the Master Investment Management Agreement, WM Advisors has full discretion and authority to make all investment decisions in respect of the fixed income and other investments portions of OneBeacon's investment portfolio, and to do anything which WM Advisors deems is required, appropriate or advisable in connection with the foregoing, subject to and in accordance with Investment Guidelines. The assets of OneBeacon's portfolio are held in one or more separately identifiable accounts in the custody of a bank or similar entity designated by OneBeacon and acceptable to WM Advisors. The agreement is terminable by WM Advisors or OneBeacon upon 60 days prior written notice.
	OneBeacon is responsible for custodial arrangements and the payment of all custodial charges and fees. OneBeacon has agreed to pay annual investment management fees generally based on the quarter-end market values held under custody as set forth in the table below:
	Assets Under Management	Annual Fee
	Investment Grade Fixed Income:
	—Up to $1 billion	10.0 basis points
	—Next $1 billion	8.5 basis points
	—Next $3 billion	7.5 basis points
	—Greater than $5 billion	2.5 basis points
	High Yield Fixed Income	25.0 basis points
	Equities	100.0 basis points
	Hedge Funds	100.0 basis points
	Private Equities & Deferreds
	First 2 years of fund's life (committed)	100.0 basis points
	Thereafter (fair value)	100.0 basis points
	Affordable housing credit funds
	First year of fund's life (committed)	100.0 basis points
	Thereafter (fair value)	10.0 basis points
	OneBeacon incurred $3.4 million, $3.5 million and $3.9 million in total fees for investment management services provided by WM Advisors under these investment management agreements during the years ended December 31, 2013, 2012 and 2011, respectively. WM Advisors is also paid a quarterly fee for treasury management services computed at the annual rate of 1.75 basis points (0.0175%) of the aggregate value of net assets on an annual basis. OneBeacon incurred $0.4 million, $0.4 million and $0.5 million in treasury management fees during the years ended December 31, 2013, 2012 and 2011, respectively.
	Star & Shield Insurance Exchange
	ASIC and Split Rock each entered into a quota share agreement effective as of November 1, 2013, to reinsure certain risks of Star & Shield Insurance Exchange (“Star & Shield”), a reciprocal insurance exchange which provides private passenger auto insurance. ASIC and Split Rock recorded written premiums totaling a combined $5.6 million in 2013. After ASIC and Split Rock entered into the quota share agreements with Star & Shield, in December 2013, a subsidiary of White Mountains entered into an agreement to acquire Star & Shield Risk Management LLC, the attorney-in-fact for Star & Shield, and the acquisition transaction closed in January 2014.
	OBH Guarantee
	Prior to the repayment of the remaining 2003 Senior Notes during the fourth quarter of 2012, White Mountains provided an irrevocable and unconditional guarantee as to the payment of principal and interest (the "Guarantee") on the 2003 Senior Notes. See Note 6.
	In consideration of this Guarantee, OneBeacon paid a specified fee to White Mountains in the amount of 25 basis points per annum on the outstanding principal amount of the 2003 Senior Notes. Such payments were made on a semi-annual basis in arrears. The Company incurred $0.8 million and $0.9 million in fees for the provision of the Guarantee during the years ended December 31, 2012 and 2011, respectively.
	The indenture documents governing the 2003 Senior Notes provided that, if OneBeacon or White Mountains as guarantor of the 2003 Senior Notes defaulted under a credit agreement, mortgage or similar debt agreement with a principal amount greater than $25 million, and such default results in the acceleration of such debt, there would have been a default under the 2003 Senior Notes (commonly referred to as a "cross default"). Such a default would have resulted in a trigger of the cross default provisions in the indenture documents governing the 2003 Senior Notes resulting in a required repayment of the 2003 Senior Notes.
	The Company's 2012 Senior Notes are not guaranteed by White Mountains. See Note 6.
	Sale of OneBeacon Holdings (Luxembourg) S.à r.l. to White Mountains
	On January 24, 2012, OneBeacon Holdings Gibraltar Limited, an indirect wholly-owned subsidiary of the Company ("OneBeacon Gibraltar"), sold all of the issued and outstanding shares of OB Lux to White Sands, an indirect wholly-owned subsidiary of White Mountains, for $24.7 million, or $14.0 million over GAAP equity of $10.7 million.
	Esurance
	On October 7, 2011, White Mountains completed the sale of Esurance Insurance ("Esurance") and Answer Financial Inc. and its subsidiaries to The Allstate Corporation (the "Esurance Transaction"). Accordingly, effective as of the closing date of the Esurance Transaction, White Mountains ceased to own the Esurance business and it is no longer considered a related party to OneBeacon.
	During 2011, OBIC subleased to Esurance approximately 4,200 square feet of an approximately 14,000 square foot office facility in Brooklyn, NY, and Esurance paid OBIC $0.2 million for the office space.
	During the third quarter of 2006, OneBeacon sold one of its inactive licensed subsidiaries, Homeland Central Insurance Company, to Esurance Holdings. In connection with the sale, Esurance Holdings agreed to pay $2.9 million of the total purchase price in periodic annual installments based upon a percentage of an annually recognized tax benefit with respect to premium tax in the state of New Jersey. In the event of a sale of Esurance Holdings to a third party, the remainder of the tax benefit would become due and payable to OneBeacon. For the year ended December 31, 2011, OneBeacon received $1.2 million related to these filings. In connection with the closing of the Esurance Transaction in 2011, OneBeacon received $0.6 million, representing payment in full of the remainder of the tax benefit.
	Federal Insurance Indemnity
	In December 2003, White Mountains and OBH (then named Fund American Companies, Inc.), entered into a General Agreement of Indemnity with Federal Insurance Company ("Federal"), under which Federal agreed to execute judicial and similar bonds on behalf of White Mountains and OBH and their respective subsidiaries. Under the General Agreement of Indemnity, White Mountains and OBH agreed to jointly and severally indemnify Federal for any losses under the bonds. Included within the Separation Agreement is a provision under which each of White Mountains and the Company will indemnify the other for losses arising out of or in connection with bonds to the extent for the benefit of White Mountains or the Company, respectively. In 2006, the Company entered into a new agreement with Federal providing a similar bonding capacity to which White Mountains is not a party. At December 31, 2013, 2012 and 2011, total exposure under the General Agreement of Indemnity for outstanding bonds was $0.4 million, $0.9 million and $1.1 million, respectively, substantially all of which related to bonds issued on behalf of OBH or its subsidiaries.
	Prospector
	Investment Management Agreement with Prospector
	Prior to the initial public offering, Prospector managed most of the publicly-traded common equity and convertible securities in OneBeacon's portfolio, as well as certain assets of the employee benefit plan portfolios, through a sub-advisory agreement with WM Advisors.
	In connection with the initial public offering, OneBeacon entered into an investment management agreement with Prospector in November 2006, as amended in November 2007, pursuant to which Prospector supervises and directs the publicly-traded common equity and convertible securities portion of OneBeacon's investment portfolio in accordance with the investment objectives, policies and restrictions described in OneBeacon's investment guidelines. The investment management agreement was renewed in March 2011 upon the same terms and conditions as the November 2006 agreement including an initial fixed term of three years which may be extended for an additional two year term.
	Prospector served as a discretionary advisor to WM Advisors under sub-advisory agreements with respect to specified assets in OneBeacon's employee benefit plan portfolios until the fourth quarter of 2009. As described above under "Related Party Disclosures—WM Advisors," in November 2009, WM Advisors terminated the investment management agreements pursuant to which WM Advisors provided services with respect to the employee benefit plan portfolios. Effective as of November 1, 2009, the Qualified Plan entered into a new investment management agreement with Prospector with respect to the management of the plan assets. Effective as of December 1, 2009, OneBeacon entered into a new investment management agreement with Prospector with respect to the management of two proprietary funds under the KSOP.
	In December 2010, OneBeacon and Prospector entered into an investment management agreement pursuant to which Prospector supervises and directs the assets held in trust to fund the Company's obligations under the Non-qualified Plan. The terms of the agreement are substantially similar to the terms of the March 2011 agreement described below.
	Under the agreements, Prospector has discretion and authority with respect to the portfolios it manages for OneBeacon that are substantially similar to WM Advisors' discretion and authority under its agreements. The assets of OneBeacon's portfolios are held in one or more separately identifiable accounts in the custody of a bank or similar entity designated by OneBeacon and acceptable to Prospector. OneBeacon is responsible for custodial arrangements and the payment of all custodial charges and fees.
	Under the agreements, OneBeacon pays annual management fees to Prospector based on aggregate net assets under management according to the following schedule: 1.00% on the first $200 million; 0.50% on the next $200 million; and 0.25% on amounts over $400 million. The agreements are terminable by OneBeacon only (i) for cause (including material non-performance by Prospector), (ii) if either John D. Gillespie or Richard P. Howard are no longer affiliated with Prospector, (iii) if there is a change in control of Prospector (for this purpose, a change in control represents 50% or greater change in voting interest of Prospector), or (iv) if White Mountains' voting interest in the Company falls below 50%. OneBeacon reviews periodically the performance of and the fees paid to Prospector under the agreements.
	For the years ended December 31, 2013, 2012 and 2011, OneBeacon incurred $2.0 million, $2.1 million and $1.9 million, respectively, in fees for investment management services provided by Prospector. For the years ended December 31, 2013, 2012 and 2011, the employee benefit plans incurred $1.2 million, $1.3 million and $1.4 million, respectively, in fees for investment management services provided by Prospector.
	Richard P. Howard, a portfolio manager of Prospector, is a director of the Company.
	Prospector Managed Limited Partnerships
	At December 31, 2013 and 2012, OneBeacon had $14.9 million and $12.9 million, respectively, invested in limited partnerships managed by Prospector. Under the limited partnership agreements, Prospector serves as general partner and general manager of the funds and is paid a management fee by OneBeacon. In addition, OneBeacon allocates a portion of its earnings from OneBeacon's limited partnership interests to Prospector as an incentive fee. For the years ended December 31, 2013, 2012 and 2011, OneBeacon incurred $0.2 million, $0.4 million and $0.4 million, respectively, in management fees. For the years ended December 31, 2013 and 2012, OneBeacon incurred $0.5 million and $0.3 million, respectively, in incentive fees. No incentive fees were incurred for the year ended December 31, 2011.
	In December 2012, OneBeacon sold its investment in the Prospector Turtle Fund, L.P. ("Turtle Fund"). The Turtle Fund, one of the limited partnerships managed by Prospector, was sold to an affiliate of White Mountains for $31.6 million, representing the net asset value of OneBeacon's interest in the Turtle Fund on the date of sale. Prior to the sale, the Turtle Fund was consolidated by OneBeacon.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Commitments and Contingencies	'
	Commitments and Contingencies
	Leases
	OneBeacon leases certain office space under noncancellable operating leases expiring at various dates through 2021. Net rental expense for all of OneBeacon's locations was approximately $11.5 million, $9.7 million and $13.1 million, respectively, for the years ended December 31, 2013, 2012 and 2011. OneBeacon's future annual minimum rental payments required under noncancellable leases primarily for office space are $6.6 million, $5.8 million, $5.5 million, $4.6 million and $7.6 million for 2014, 2015, 2016, 2017 and 2018 and thereafter, respectively. As of December 31, 2012 and 2011 OneBeacon had $0.1 million and $1.4 million, respectively, net of anticipated sub-lease income for leased space which OneBeacon has ceased using and had none accrued as of December 31, 2013.
	As described in Note 1, in December 2011, the Company sold the majority of its fixed assets and capitalized software. The Company entered into lease financing arrangements with US Bancorp and Fifth Third whereby it sold furniture and equipment and capitalized software, respectively, at a cost equal to net book value. The Company then leased the fixed assets back from US Bancorp for a lease term of five years and leased the capitalized software back from Fifth Third for a lease term of four years. The Company received cash proceeds of $23.1 million as a result of entering into the sale-leaseback transactions. At the end of the lease terms, the Company will have the obligation to purchase the leased assets for a nominal fee, after which all rights, title and interest would transfer back to it. As of December 31, 2013 and 2012, the Company had a capital lease obligation of $12.5 million and $18.2 million, respectively, included within other liabilities and a capital lease asset of $10.9 million and $16.1 million, respectively, included within other assets. The underlying assets will continue to be depreciated over their respective useful lives. OneBeacon's future annual minimum rental payments are $5.3 million for each of the years ended December 31, 2014 and 2015 and $1.9 million for the year ended December 31, 2016.
	OneBeacon also has various other lease obligations which are immaterial in the aggregate.
	Other Investments
	OneBeacon has future binding commitments to fund certain limited partnership investments which do not have fixed funding dates and totaled $12.1 million as of December 31, 2013.
	Assigned Risks
	As a condition of OneBeacon's license to do business in certain states, OneBeacon's insurance operations are required to participate in mandatory shared market mechanisms. Each state dictates the types of insurance and the level of coverage that must be provided. The total amount of such business an insurer is required to accept is based on its market share of voluntary business in the state. In certain cases, OneBeacon is obligated to write business from mandatory shared market mechanisms at some time in the future based on the market share of voluntary policies it is currently writing. Underwriting results related to assigned risk plans are typically adverse and are not subject to the predictability associated with OneBeacon's voluntarily written business.
	Guaranty Funds
	Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. OneBeacon's insurance subsidiaries record guaranty fund assessments when such assessments are billed by the respective guaranty funds. In addition, OneBeacon will monitor other insurance company insolvencies and will accrue a liability when such losses are determined to be probable and the assessment amounts can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years. At December 31, 2013, the reserve for such assessments at OneBeacon's insurance subsidiaries totaled $13.0 million.
	Legal Contingencies
	OneBeacon, and the insurance and reinsurance industry in general, is routinely subject to claims-related litigation and arbitration in the normal course of business, as well as litigation and arbitration that do not arise from, or directly relate to, claims activity. OneBeacon's estimates of the costs of settling matters routinely encountered in claims activity are reflected in the reserves for unpaid loss and LAE. See Note 3—"Unpaid Loss and Loss Adjustment Expense (LAE) Reserves."
	OneBeacon evaluates its exposure to non-claims related litigation and arbitration and establishes accruals for litigation and arbitration if it is probable that a loss has been incurred and it can be reasonably estimated. Disclosure of litigation and arbitration is made if it is probable that a loss has been incurred or if there is a reasonable possibility that a loss may have been incurred. Although the ultimate outcome of claims and non-claims related litigation and arbitration, and the amount or range of potential loss at any particular time, is often inherently uncertain, management does not believe that the ultimate outcome of such claims and non-claims related litigation and arbitration will have a material adverse effect on OneBeacon's financial condition, full year results of operations, or cash flows.
	The following summarizes significant ongoing non-claims related litigation or arbitration as of December 31, 2013:
	Deutsche Bank Litigation
	In June 2011, Deutsche Bank Trust Company Americas, Law Debenture Company of New York and Wilmington Trust Company (collectively referred to as “Plaintiffs”), in their capacity as trustees for certain senior notes issued by the Tribune Company (“Tribune”), filed lawsuits in various jurisdictions (the “Noteholder Actions”) against numerous defendants including OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune seeking recovery of the proceeds from the sale of common stock of Tribune in connection with Tribune's leveraged buyout in 2007 (the “LBO”). Tribune filed for bankruptcy in 2008 in the Delaware bankruptcy court (the “Bankruptcy Court”). The Bankruptcy Court granted Plaintiffs permission to commence these LBO-related actions, and in 2011, the Judicial Panel on Multidistrict Litigation granted a motion to consolidate the actions for pretrial matters and transferred all such proceedings to the United States District Court for the Southern District of New York. Plaintiffs seek recovery of the proceeds received by the former Tribune shareholders on a theory of constructive fraudulent transfer asserting that Tribune purchased or repurchased its common shares without receiving fair consideration at a time when it was, or as a result of the purchases of shares, was rendered, insolvent. OneBeacon has entered into a joint defense agreement with other affiliates of White Mountains that are defendants in the action. OneBeacon and OneBeacon-sponsored benefit plans received approximately $32 million for Tribune common stock tendered in connection with the LBO.
	The Court granted an omnibus motion to dismiss the Noteholder Actions in September 2013 and Plaintiffs have filed a notice of appeal.
	In addition, OneBeacon, OneBeacon-sponsored benefit plans and other affiliates of White Mountains in their capacity as former shareholders of Tribune, along with thousands of former Tribune shareholders, have been named as defendants in an adversary proceeding brought by the Official Committee of Unsecured Creditors of the Tribune Company (the “Committee”), on behalf of the Tribune Company, which seeks to avoid the repurchase of shares by Tribune in the LBO on a theory of intentional fraudulent transfer (the “Committee Action”). Tribune emerged from bankruptcy in 2012, and a litigation trustee replaced the Committee as plaintiff in the Committee Action. This matter was consolidated for pretrial matters with the Noteholder Actions in the United States District Court for the Southern District of New York and was stayed pending the motion to dismiss in the Noterholder Action. The Committee Action will proceed upon the lifting of the stay and a scheduling order from the court.

Earnings_per_Share

Earnings per Share	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Earnings per Share	'
	Earnings per Share
	Basic and diluted earnings per share amounts are based on the weighted average number of common shares outstanding, including unvested restricted shares that are considered participating securities. Diluted earnings per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding.
	The following table outlines the Company's computation of earnings per share for net income from continuing operations attributable to OneBeacon's common shareholders for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013			2012			2011
	Earnings attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders		$	146		$	96.1		$	103.9
	Allocation of income for participating unvested restricted common shares		(1.4		)	(0.9		)	(0.4		)
	Dividends paid on participating restricted common shares		(0.8		)	(0.8		)	(1.0		)
	Total allocation to restricted common shares		(2.2		)	(1.7		)	(1.4		)
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	143.8		$	94.4		$	102.5
	Undistributed (over-distributed) net earnings (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	143.8		$	94.4		$	102.5
	Dividends paid, net of restricted common share amounts		(79.4		)	(79.3		)	(173.8		)
	Total undistributed (over-distributed) net earnings, net of restricted common share amounts		$	64.4		$	15.1		$	(71.3	)
	Earnings per share denominator—basic and diluted (in millions):
	Total weighted average common shares outstanding		95.4			95.4			94.8
	Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)	(0.4		)
	Basic and diluted earnings per share denominator(2)		94.5			94.5			94.4
	Earnings per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
	Net income from continuing operations attributable to OneBeacon's common shareholders		$	1.52		$	1		$	1.08
	Dividends declared and paid		(0.84		)	(0.84		)	(1.84		)
	Undistributed (over-distributed) earnings		$	0.68		$	0.16		$	(0.76	)
	_______________________________________________________________________________
	-1	Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event (see Note 9).
	(2)	Common shares issuable upon exercise of stock options (Note 9) were not included in the years ended December 31, 2012 and 2011 as their inclusion would be anti-dilutive. There were no options outstanding during the year ended December 31, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.
	Basic and diluted income (loss) per share amounts for discontinued operations are included in Note 19—"Discontinued Operations."

Share_Repurchase_Authorization

Share Repurchase Authorization	12 Months Ended
	Dec. 31, 2013
Equity [Abstract]	'
Share Repurchase Authorization	'
	Common Shareholders' Equity
	Common Shares Repurchased and Retired
	On August 22, 2007, the Company's Board authorized the repurchase of up to $200.0 million of its Class A common shares from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. This authorization does not have a stated expiration date. During the years ended December 31, 2013, 2012 and 2011, no shares were repurchased under the share repurchase authorization. The amount of authorization remaining is $87.7 million as of December 31, 2013.
	During the year ended December 31, 2013, the Company repurchased 3,300 common shares to satisfy employee income tax withholding, pursuant to employee benefit plans. No shares were repurchased during the years ended December 31, 2012 and 2011. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above.
	Dividends on Common Shares
	During the years ended December 31, 2013, 2012 and 2011, the Company declared and paid cash dividends totaling $80.2 million, $80.1 million and $174.8 million, respectively, or $0.84 per common share of regular quarterly cash dividends for each year, plus a $1.00 per common share special dividend for the year ended December 31, 2011.
	Accumulated Other Comprehensive Income (Loss)
	For the years ended December 31, 2013, 2012 and 2011, OneBeacon recorded changes to accumulated other comprehensive income (loss) for net increases in net benefit plan assets and obligations of $20.6 million, $(2.9) million and $(11.2) million, which were net of tax (expense) benefit of $(11.1) million, $1.6 million and $6.0 million, respectively. See Note 8—"Retirement Plans."
	Share Repurchase Authorization
	On August 22, 2007, the Company's Board authorized the repurchase of up to $200.0 million of its Class A common shares from time to time, subject to market conditions. Shares may be repurchased on the open market or through privately negotiated transactions. This authorization does not have a stated expiration date. During the years ended December 31, 2013, 2012 and 2011, no shares were repurchased. The amount of authorization remaining is $87.7 million.

Discontinued_Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2013
Discontinued Operations and Disposal Groups [Abstract]	'
Discontinued Operations	'
	Discontinued Operations
	Runoff Business
	As described in Note 1 and Note 2, in October 2012, OneBeacon entered into the Stock Purchase Agreement with respect to the sale of its Runoff Business to Armour. Pursuant to the terms of the Stock Purchase Agreement, at closing, OneBeacon will transfer to Armour all of the issued and outstanding shares of common stock of certain legal entities that will contain the assets, liabilities (including gross and ceded loss reserves) and capital supporting the business as well as certain elements of the Runoff Business infrastructure, including staff and office space. Additionally, as part of the Runoff Transaction, OneBeacon may provide, under certain scenarios, financing in the form of surplus notes.
	In anticipation of the Runoff Transaction, OneBeacon received regulatory approval as required from various state departments of insurance effective October 1, 2012 to terminate the then-existing pooling agreement and intercompany 100% quota share reinsurance agreements and to enter into new 100% quota share reinsurance agreements. The result is that the Runoff Business is assumed and retained by OBIC, one of the legal entities that will be transferred to Armour at closing, and that the ongoing specialty business is assumed and retained by Atlantic Specialty Insurance Company ("ASIC"), one of the entities that OneBeacon will continue to own post-closing.
	The Pennsylvania Insurance Department (“PID”) is required to conduct an examination of the Runoff Business as part of its regulatory review of the Runoff Transaction. Pursuant to this examination, the PID required a third party actuarial review to provide an independent actuarial assessment of the loss reserves associated with the Runoff Business, which is a normal requirement associated with such examinations. The independent actuarial review was completed in September 2013, at which time the PID posted the summary review to its web site. The independent actuarial review produced a range of total statutory net loss and LAE reserves of $215 million to $668 million as of March 31, 2013. This compared to the Company’s recorded statutory net loss and LAE reserves of $166 million as of March 31, 2013. Since March 31, 2013, the Company increased the Runoff Business loss and LAE reserves by $78.9 million.
	During the fourth quarter of 2013, and as part of the Company's annual certification process, the Company completed a comprehensive actuarial analysis of the non-A&E loss and LAE reserves associated with the Runoff Business. In addition to the Company's internal actuaries taking into account the differing assumptions, methods, and analyses produced by the independent actuarial review and other factors, management considered other sources of information, including runoff claims staffing models and related costs. For A&E reserve estimates associated with the Runoff Business, the Company primarily relies on the internal study of its legacy A&E exposures completed in 2011 and on its subsequent monitoring of quarterly A&E activity, including the comparison of that activity against what was assumed in that most recent study.
	As a result of the comprehensive actuarial analysis conducted by the Company's actuaries during the fourth quarter of 2013, the Company recorded $71.5 million of unfavorable prior year non-A&E loss and LAE development related to the Runoff Business. The increase in loss reserves was concentrated in the workers compensation, personal auto liability, and excess liability lines of business. In addition, the Company increased its estimate of adjusting and other expenses, a component of LAE reserves. The Company has not revised its estimate of net ultimate A&E payments.
	Workers compensation unpaid loss reserves increased by $36.6 million due to changes in how the Company evaluates various estimated settlement rates, mortality, and medical inflation assumptions. These three key assumptions, which were previously evaluated implicitly as part of overall case incurred activity, were separately analyzed and then explicitly reviewed under varying assumptions and an array of resulting reserve estimates, to generate an actuarial indication which management selected for its best estimate. For personal auto liability, a $16.9 million loss provision was recorded based on a ground-up analysis of unlimited medical automobile no-fault claims from the 1970s and 1980s, which produced a range of estimates at varying medical inflation rates. The remaining $5.4 million loss reserve increase was driven by adverse prior year loss development recorded on a few large excess liability claims. Finally, the Company recorded a provision to increase its LAE reserves by $12.6 million for adjusting and other expenses due to a change in assumptions of staff efficiency associated with handling and settling runoff claims.
	For the full year 2013, the Company recorded $78.9 million loss and LAE provision for the Runoff Business, which includes a $7.4 million increase in loss and LAE reserves recorded in the second quarter of 2013. The $78.9 million loss and LAE adverse development recorded in 2013 was partially offset by other revenue of $7.8 million associated with a settlement award in the second quarter of 2013 in the Safeco v. American International Group, Inc. (AIG) class action related to AIG's alleged underreporting of workers' compensation premiums to the National Workers' Compensation Reinsurance Pool.
	As of December 31, 2013, the recorded net unpaid loss and LAE reserves associated with the Runoff Business totaled $188.4 million. Management believes that the recorded net loss and LAE reserves reflect a reasonable provision for expected future loss and LAE payments and represent management’s best estimate within a range of reasonable estimates.
	The $71.5 million ($46.5 million after tax) increase in Runoff Business loss and LAE reserves was recorded in the fourth quarter of 2013 as a component of discontinued operations and offset by an equal after tax amount which decreased the estimated ultimate loss on sale of the Runoff Business. The terms of the Stock Purchase Agreement prescribe that the buyer has assumed the risk that loss and LAE reserves develop unfavorably from September 30, 2012 onward, resulting in the offset.
	During the fourth quarter of 2013, the Company also increased the estimated pre-tax transaction costs associated with the Runoff Transaction which was partially offset by the accretion of interest on the original purchase price and, coupled with the $46.5 million after tax provision for loss and LAE, resulted in a $46.6 million after-tax reduction in the ultimate loss on sale from discontinued operations in the fourth quarter and full year ended December 31, 2013. This reduction in the ultimate loss on sale was essentially offset by a $46.6 million after tax loss included in loss from discontinued operations for the fourth quarter, driven by the adverse reserve development. The Company's current estimated ultimate loss on sale of the Runoff Business is $69.0 million pre-tax, or $44.9 million after tax.
	Although the Stock Purchase Agreement stipulates the amount of reserves and surplus to be transferred to Armour at closing, the PID may require additional reserves and/or surplus as a closing condition. In that event, and to respond to such a closing condition, the Stock Purchase Agreement provides that the Company would invest in surplus notes issued by the transferring companies, subject to certain limits on the amount of surplus notes issued. The Company believes that the transferred reserves and surplus plus the funding requirements/limitations agreed to in the Stock Purchase Agreement cover the full range of claim projections produced in the independent actuarial review. Currently, we expect to provide financing by way of surplus notes in an amount that falls under the provisions of the Stock Purchase Agreement.
	In October 2013, OneBeacon and Armour amended the Stock Purchase Agreement to extend the date by which either party may terminate the Stock Purchase Agreement to July 31, 2014. If the required regulatory approval to close the Runoff Transaction has not been obtained on or prior to July 31, 2014, either OneBeacon or Armour may unilaterally extend the termination date of the Stock Purchase Agreement by no more than 90 days. The Company expects the Runoff Transaction to close in mid-2014.
	AutoOne
	On February 22, 2012, OneBeacon completed the sale of the AutoOne business to Interboro. The AutoOne Transaction included the sale of two insurance entities, AutoOne Insurance Company ("AOIC") and AutoOne Select Insurance Company ("AOSIC"), through which substantially all of the AutoOne business was written on a direct basis. The AutoOne Transaction also included the execution of a reinsurance agreement with certain subsidiaries of the Company pursuant to which OneBeacon cedes, on a 100% quota share basis, AutoOne business not directly written by AOIC and AOSIC.
	Summary of Reclassified Balances and Related Items
	As of December 31, 2013 and 2012, the Runoff Transaction met the criteria for held for sale accounting. As a result, the assets and liabilities associated with the businesses being sold, after effecting the various steps contemplated by the Stock Purchase Agreement, are presented separately as single line items in the asset and liability sections of the consolidated balance sheets as of December 31, 2013 and 2012. The following summarizes the major categories of assets and liabilities associated with the business classified as held for sale:
			December 31,
			2013				2012
			($ in millions)
	Assets:
	Investments		$	236.3			$	338.1
	Premiums receivable		9.1				11
	Reinsurance recoverable on unpaid losses(1)		1,604.70				1,840.80
	Reinsurance recoverable on paid losses		10.7				15.6
	Net deferred tax asset		3.3				5.1
	Other assets		16				16.2
	Total assets held for sale		$	1,880.10			$	2,226.80
	Liabilities:
	Unpaid loss and loss adjustment expense reserves(1)		$	1,793.10			$	2,052.60
	Unearned premiums		0.2				0.5
	Ceded reinsurance payable		12.3				21.9
	Other liabilities(2)		74.5				151.8
	Total liabilities held for sale		$	1,880.10			$	2,226.80
	_______________________________________________________________________________
	(1)	The December 31, 2013 and 2012 balances include the remaining purchase accounting fair value adjustments of $136.9 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of December 31, 2013 and 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,741.6 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,930.0 million and $2,202.7 million for each period.
	(2)	Other liabilities as of December 31, 2013 and 2012 include the accrual related to the pre-tax loss on sale of the Runoff Business of $69.0 million and $140.7 million, respectively.
	As described in Note 1 and Note 2, the results of operations for the Runoff Business and AutoOne have been classified as discontinued operations and are presented as such, net of related income taxes, in the statements of operations and comprehensive income (loss) and cash flows for all periods. Investing and financing activities for OneBeacon are managed on a consolidated basis reported within the Investing, Financing and Corporate segment. Therefore, no investment or financing activity is included in discontinued operations.
	The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations:
			Year ended December 31,
			2013				2012				2011
			($ in millions)
	Net written premiums		$	0.4			$	3.5			$	58
	Revenues
	Earned premiums		$	0.8			$	10.6			$	70.5
	Other revenue		6.3				—				1.7
	Total revenues		7.1				10.6				72.2
	Expenses
	Loss and loss adjustment expenses		78.9				48.4				89.5
	Policy acquisition expenses (benefit)		—				(2.1		)		5.3
	Other underwriting expenses		(0.2		)		1.7				22.8
	Total expenses		78.7				48				117.6
	Pre-tax loss		(71.6		)		(37.4		)		(45.4		)
	Income tax benefit		25				13.1				15.8
	Loss from discontinued operations, net of tax		(46.6		)		(24.3		)		(29.6		)
	Gain (loss) from sale of discontinued operations, net of tax		46.6				(91.0		)		(19.2		)
	Net loss from discontinued operations, net of tax		$	—			$	(115.3	)		$	(48.8	)
	Loss per Share Related to Discontinued Operations
	Basic loss per share amounts are based on the weighted average number of common shares outstanding including unvested restricted shares that are considered participating securities. Diluted loss per share amounts are based on the weighted average number of common shares including unvested restricted shares and the net effect of potentially dilutive common shares outstanding.
	The following table outlines the computation of loss per share for discontinued operations attributable to OneBeacon's common shareholders for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013				2012				2011
Loss attributable to OneBeacon's common shareholders—basic and diluted (in millions):
Net loss from discontinued operations attributable to OneBeacon's common shareholders		$	—			$	(115.3	)		$	(48.8	)
Allocation of loss for participating unvested restricted common shares		—				1.1				0.2
Net loss from discontinued operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	—			$	(114.2	)		$	(48.6	)
Loss per share denominator—basic and diluted (in millions):
Total weighted average common shares outstanding		95.4				95.4				94.8
Weighted average unvested restricted common shares(1)		(0.9		)		(0.9		)		(0.4		)
Basic and diluted loss per share denominator(2)		94.5				94.5				94.4
Loss per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
Net loss from discontinued operations attributable to OneBeacon's common shareholders per share		$	—			$	(1.21	)		$	(0.50	)
_______________________________________________________________________________
(1)	Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event.
(2)	Common shares issuable upon exercise of stock options (Note 9) were not included in the years ended December 31, 2012 and 2011 as their inclusion would be anti-dilutive. There were no options outstanding during the year ended December 31, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.
Additional Disclosures
Due to the relative significance of the transactions described above, OneBeacon has expanded the disclosures herein to provide additional insight into the balances and related activity reclassified to held for sale and discontinued operations.
Results of Discontinued Operations
For the full year 2013, the Company recorded $78.9 million loss and LAE provision for the Runoff Business, which includes a $7.4 million increase in loss and LAE reserves recorded in the second quarter of 2013. The $78.9 million loss and LAE adverse development recorded in 2013 was partially offset by other revenue of $7.8 million associated with a settlement award in the second quarter of 2013 in the Safeco v. American International Group, Inc. (AIG) class action related to AIG's alleged underreporting of workers' compensation premiums to the National Workers' Compensation Reinsurance Pool.
As of December 31, 2013, the recorded net unpaid loss and LAE reserves associated with the Runoff Business totaled $188.4 million. Management believes that the recorded net loss and LAE reserves reflect a reasonable provision for expected future loss and LAE payments and represent management’s best estimate within a range of reasonable estimates.
The $71.5 million ($46.5 million after tax) increase in Runoff Business loss and LAE reserves was recorded in the fourth quarter of 2013 as a component of discontinued operations and offset by an equal after tax amount which decreased the estimated ultimate loss on sale of the Runoff Business. The terms of the Stock Purchase Agreement prescribe that the buyer has assumed the risk that loss and LAE reserves develop unfavorably from September 30, 2012 onward, resulting in the offset.
During the fourth quarter of 2013, the Company also increased the estimated pre-tax transaction costs associated with the Runoff Transaction which was partially offset by the accretion of interest on the original purchase price and, coupled with the $46.5 million after tax provision for loss and LAE, resulted in a $46.6 million after-tax reduction in the ultimate loss on sale from discontinued operations in the fourth quarter and full year ended December 31, 2013. This reduction in the ultimate loss on sale was essentially offset by a $46.6 million after tax loss included in loss from discontinued operations for the fourth quarter, driven by the adverse reserve development. The Company's current estimated ultimate loss on sale of the Runoff Business is $69.0 million pre-tax, or $44.9 million after tax.
Loss from discontinued operations, net of tax, was $24.3 million for 2012, driven by adverse development related to losses incurred on a legacy assumed reinsurance treaty, case incurred development on a small number of claims related to multiple peril liability lines and general liability lines and also the impact of an adverse ruling in Mississippi regarding a disputed assessment from an involuntary pool for hurricane Katrina claims. In addition, $9.0 million of after tax ($15.2 million of pre-tax) incurred loss and LAE, reported as adverse loss development, was related to a change in the workers' compensation tabular discount rate. For the year ended December 31, 2012, management evaluated the interest rate used in calculating the workers' compensation discount, consistent with prior years, and as a result lowered the rate from 4.5% to 3.5% during 2012, resulting in an increase of $15.2 million to the reserves. The discount rate remained unchanged at 3.5% for 2013.
For the year ended December 31, 2012, OneBeacon recognized a loss on sale of discontinued operations of $91.0 million in connection with the Runoff Transaction and AutoOne Transaction. During the year ended December 31, 2012, the Company recorded a $91.5 million after tax ($140.7 million pre-tax) estimated loss on sale of the Runoff Business. Also during the year ended December 31, 2012, OneBeacon and Interboro reached conclusion on post-closing adjustments to the closing balance sheet resulting in OneBeacon recording a net gain of $0.5 million after tax, reflecting a true up of the estimated loss on sale of AutoOne.
During the year ended December 31, 2011, OneBeacon recorded a net charge of $29.6 million pre-tax, $19.2 million after tax, driven by adverse loss reserve development resulting from a detailed review of run-off expenses, which factored in the revised definition of run-off claims to include the non-specialty commercial lines business, and reflecting the estimated loss on sale of the AutoOne business, which included the $25.0 million of net assets held for sale as well as estimates for transaction costs. This after tax net charge is included in loss from sale of discontinued operations, net of tax, in the statement of operations and comprehensive income for the year ended December 31, 2011.
Fair Value Adjustment
In connection with purchase accounting for the OneBeacon Acquisition, the Company was required to adjust to fair value the loss and LAE reserves and the related reinsurance recoverables. Loss and LAE reserves and the related reinsurance recoverable presented in the summary of reclassified balances within assets and liabilities held for sale as of December 31, 2013 and 2012, are net of $136.9 million and $150.1 million, respectively, related to the outstanding pre-tax unaccreted adjustment.
Asbestos and Environmental (A&E) Loss and LAE Reserve Activity
OneBeacon's reserves include provisions made for claims that assert damages from A&E related exposures. These reserves have been reclassified to liabilities held for sale as of December 31, 2013 and 2012, as they are part of the Runoff Business.
The following tables summarize reported A&E loss and LAE reserve activities (gross and net of reinsurance) for OneBeacon for the years ended December 31, 2013, 2012 and 2011, respectively.
	Year ended December 31,
	2013												2012									2011
	Gross				Pre-				Net				Gross			Pre-			Net			Gross			Pre-			Net
					NICO											NICO									NICO
					Net(1)											Net(1)									Net(1)
	($ in millions)
Asbestos:
Beginning balance	$	929.4			$	602.5			$	2.4			$	1,074.30		$	681.2		$	2.2		$	904		$	647.3		$	6.4
Incurred loss and LAE (2)	—				—				—				(0.3		)	(0.5		)	(0.5		)	256.8			32.2			(4.0		)
Paid loss and LAE (2)	(84.3		)		(79.3		)		(0.5		)		(144.6		)	(78.2		)	0.7			(86.5		)	1.7			(0.2		)
Ending balance (3)	$	845.1			$	523.2			$	1.9			$	929.4		$	602.5		$	2.4		$	1,074.30		$	681.2		$	2.2
Environmental:
Beginning balance	$	233			$	125.4			$	6.4			$	279.8		$	151.6		$	9		$	119		$	93.8		$	9.2
Incurred loss and LAE (2)	—				—				—				(0.9		)	(0.5		)	(0.5		)	231.8			62.2			10
Paid loss and LAE (2)	(64.3		)		(33.0		)		(2.6		)		(45.9		)	(25.7		)	(2.1		)	(71.0		)	(4.4		)	(10.2		)
Ending balance (3)	$	168.7			$	92.4			$	3.8			$	233		$	125.4		$	6.4		$	279.8		$	151.6		$	9
Total asbestos and environmental:
Beginning balance	$	1,162.40			$	727.9			$	8.8			$	1,354.10		$	832.8		$	11.2		$	1,023.00		$	741.1		$	15.6
Incurred loss and LAE (2)	—				—				—				(1.2		)	(1.0		)	(1.0		)	488.6			94.4			6
Paid loss and LAE (2)	(148.6		)		(112.3		)		(3.1		)		(190.5		)	(103.9		)	(1.4		)	(157.5		)	(2.7		)	(10.4		)
Ending balance (3)	$	1,013.80			$	615.6			$	5.7			$	1,162.40		$	727.9		$	8.8		$	1,354.10		$	832.8		$	11.2
__________________________________
-1	Represents A&E reserve activity, net of third-party reinsurance, but prior to the NICO Cover (defined below).
-2	Substantially all of the paid and incurred loss and LAE for all periods presented relate to the Runoff Business.
-3	The ending balances presented include the unpaid A&E loss and LAE reserves, substantially all of which are related to the Runoff Business. The net unpaid loss and LAE reserves reclassified as liabilities held for sale on the December 31, 2013 and 2012 balance sheets for Asbestos were $1.9 million and $2.4 million, respectively; Environmental were $3.8 million and $6.4 million, respectively; and total A&E were $5.7 million and $8.8 million, respectively.
Reinsurance
As described in Note 4—"Reinsurance," in the normal course of business, OneBeacon's insurance subsidiaries seek to limit losses that may arise from catastrophes or other events by reinsuring with third-party reinsurers. OneBeacon remains liable for risks reinsured even if the reinsurer does not honor its obligations under reinsurance contracts.
In connection with the OneBeacon Acquisition, Aviva caused OneBeacon to purchase two reinsurance contracts from subsidiaries of Berkshire Hathaway Inc.: a reinsurance contract with National Indemnity Company (“NICO”) for up to $2.5 billion in old A&E claims and certain other exposures (the “NICO Cover”) and an adverse loss reserve development cover from General Reinsurance Corporation (“GRC”) for up to $570.0 million, comprised of $400.0 million of adverse loss reserve development occurring in years 2000 and prior (the “GRC Cover”) in addition to $170.0 million of reserves ceded as of the date of the OneBeacon Acquisition. The NICO Cover and GRC Cover, which were contingent on and occurred contemporaneously with the OneBeacon Acquisition, were put in place in lieu of a seller guarantee of loss and LAE reserves and are therefore accounted for under GAAP as a seller guarantee.
NICO Cover
Under the terms of the NICO Cover, NICO receives the economic benefit of reinsurance recoverables from certain of OneBeacon’s third party reinsurers (“Third Party Reinsurers”) in existence at the time the NICO Cover was executed (“Third Party Recoverables”). As a result, the underlying Third Party Recoverables serve to protect the $2.5 billion limit of NICO coverage for the benefit of OneBeacon. Any amounts uncollectible from third-party reinsurers due to dispute or the reinsurers' financial inability to pay are covered by NICO under its agreement with OneBeacon. Third-Party Recoverables are typically for the amount of loss in excess of a stated level each year. Of claim payments in the past 14 years, approximately 45.8% of A&E losses have been recovered under the historical third-party reinsurance.
During 2011, OneBeacon completed a study of its legacy A&E exposures. Reasonable estimates of potential adverse scenarios continue to be within the $2.5 billion reinsurance cover issued by NICO. Based on the results of the study, OneBeacon increased the central estimate of incurred losses ceded to NICO from $2.2 billion to $2.3 billion, an increase of $121.9 million, net of underlying reinsurance. Due to the NICO Cover, there was no impact to income or equity from the change in estimate. The Company reviews A&E activity each quarter and compares that activity to what was assumed in the most recently completed A&E study. Through December 31, 2013, that activity has been in line with expectations, so the Company has not revised its estimate of ultimate payments.
OneBeacon estimates that on an incurred basis it has used approximately $2.3 billion of the coverage provided by NICO at December 31, 2013. Since entering into the NICO Cover, approximately 10% of the $2.3 billion of utilized coverage relates to uncollectible Third-Party Recoverables and settlements on Third-Party Recoverables through December 31, 2013. Net losses paid totaled approximately $1.6 billion as of December 31, 2013. To the extent that actual experience differs from OneBeacon’s estimate of ultimate A&E losses and Third-Party Recoverables, future losses could exceed the $198.3 million of protection remaining under the NICO Cover at December 31, 2013.
OneBeacon's reserves for A&E losses at December 31, 2013 represent management's best estimate of its ultimate liability based on information currently available. However, significant uncertainties, including but not limited to case law developments, medical and clean up cost increases and industry settlement practices, limit OneBeacon's ability to accurately estimate ultimate liability and OneBeacon may be subject to A&E losses beyond currently estimated amounts. In addition, OneBeacon remains liable for risks reinsured in the event that a reinsurer does not honor its obligations under reinsurance contracts. OneBeacon cannot reasonably estimate at the present time loss reserve additions arising from any such future adverse loss reserve developments and cannot be sure that allocated loss reserves, plus the remaining capacity under the NICO Cover and other reinsurance contracts, will be sufficient to cover additional liability arising from any such adverse loss reserve developments.
GRC Cover
Pursuant to the GRC Cover, OneBeacon is not entitled to recover losses to the full contract limit if such losses are reimbursed by GRC more quickly than anticipated at the time the contract was signed. OneBeacon intends to seek reimbursement from GRC only for claims which result in payment patterns similar to those supporting its recoverables recorded pursuant to the GRC Cover. The economic cost of not submitting certain other eligible claims to GRC is primarily the investment spread between the rate credited by GRC and the rate achieved by OneBeacon on its own investments. This cost, if any, is expected to be nominal. As of December 31, 2013, OneBeacon has ceded estimated incurred losses of $562.0 million to GRC under the GRC Cover. As of December 31, 2013, OneBeacon has $369.7 million of reinsurance recoverable on unpaid losses outstanding under the GRC Cover.
The effects of reinsurance on OneBeacon's insurance subsidiaries' written and earned premiums and on incurred loss and LAE related to discontinued operations were as follows:
		Year ended December 31,
		2013				2012				2011(1)
		($ in millions)
Written premiums:
Direct		$	0.2			$	5.8			$	56.2
Assumed		0.2				(0.1		)		(0.9		)
Ceded		—				(2.2		)		2.7
Net written premiums		$	0.4			$	3.5			$	58
Earned premiums:
Direct		$	0.4			$	12.1			$	169.8
Assumed		0.4				0.9				0.8
Ceded		—				(2.4		)		(100.1		)
Net earned premiums		$	0.8			$	10.6			$	70.5
Loss and LAE:
Direct		$	82			$	52.8			$	656.3
Assumed		5.2				14.5				26.8
Ceded		(8.3		)		(18.9		)		(593.6		)
Net loss and LAE		$	78.9			$	48.4			$	89.5
__________________________________
(1)	During the year ended December 31, 2011, OneBeacon ceded written premiums of $0.6 million, earned premiums of $96.6 million, and loss and LAE of $62.3 million, pursuant to the sale of the renewal rights to the Company's non-specialty commercial lines business in 2009.
At December 31, 2013, OneBeacon had $10.7 million of reinsurance recoverable on paid losses and $1,741.6 million (gross of $136.9 million in purchase accounting adjustments, as described above) that will become recoverable if claims are paid in accordance with current reserve estimates, related to the Runoff Business that have been reclassified to assets held for sale. Reinsurance contracts do not relieve OneBeacon of its obligations. Therefore, collectibility of balances due from reinsurers is critical to OneBeacon's financial strength. The following table provides a listing of the top reinsurers related to the Runoff Business reported in assets held for sale, excluding industry pools and associations, based on reinsurance recoverable amounts on paid and unpaid losses, the percentage of the total reported as held for sale (gross of the $136.9 million in purchase accounting adjustment), and the reinsurers' A.M. Best ratings.
($ in millions)		Balance at				% of total			A.M. Best
		31-Dec-13							Rating(1)
National Indemnity Company ("NICO") and General Reinsurance Corporation(2)		$	1,243.70			71	%		A++
Hanover Insurance Company		43.9				3	%		A
Tokio Marine and Nichido Fire(3)		26.2				1	%		A++
Munich Reinsurance America		16.3				1	%		A+
Tower Insurance Company		10.2				1	%		B(4)
_______________________________________________________________________________
(1)	A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of sixteen financial strength ratings), “A+” (Superior, which is the second highest of sixteen financial strength ratings), “A” (Excellent, which is the third highest of sixteen financial strength ratings) and “B” (Fair, which is the seventh highest of sixteen financial strength ratings).
(2)	Includes $198.3 million of Third Party Recoverables, which NICO would pay under the terms of the NICO Cover if they are unable to collect from third party reinsurers.
(3)	Excludes $21.8 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.
(4)	Under review with developing implications.

Consolidating_Financial_Inform

Consolidating Financial Information	12 Months Ended
	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Consolidating Financial Information	'
	Consolidating Financial Information
	The Company has fully and unconditionally guaranteed the 2012 Senior Notes issued by its 100% owned subsidiary, OBH. The following tables present OneBeacon's consolidating balance sheets as of December 31, 2013 and 2012, and statements of operations and comprehensive income and cash flows for the years ended December 31, 2013, 2012 and 2011. These financial statements reflect the Company's ("guarantor") financial position, results of operations and cash flows on a stand-alone basis, that of OBH ("the issuer") and of the Company's other entities ("non-guarantor subsidiaries") as well as the necessary consolidating adjustments to eliminate intercompany balances and transactions.
	Consolidating Balance Sheet	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	as of December 31, 2013				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,725.40		$	—		$	(24.5	)	$	1,700.90
	Short-term investments, at amortized cost (which approximates fair value)	13			140			4			—			157
	Common equity securities, at fair value	—			336.9			—			—			336.9
	Convertible fixed maturity investments, at fair value	—			30.5			—			—			30.5
	Other investments	—			139.6			—			—			139.6
	Total investment securities	13			2,372.40			4			(24.5		)	2,364.90
	Cash	—			166.6			1.5			—			168.1
	Reinsurance recoverables	—			89.9			—			—			89.9
	Premiums receivable	—			228.2			—			—			228.2
	Deferred acquisition costs	—			103.7			—			—			103.7
	Net deferred tax asset	—			93.1			(2.4		)	(0.1		)	90.6
	Investment income accrued	—			10.3			—			(0.2		)	10.1
	Accounts receivable on unsettled investment sales	—			3.3			—			—			3.3
	Investments in subsidiaries	1,092.00			—			1,034.90			(2,126.9		)	—
	Other assets	(0.8		)	272.4			1.1			—			272.7
	Assets held for sale	—			1,880.10			—			—			1,880.10
	Total assets	$	1,104.20		$	5,220.00		$	1,039.10		$	(2,151.7	)	$	5,211.60
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,054.30		$	—		$	—		$	1,054.30
	Unearned premiums	—			544.9			—			—			544.9
	Debt	—			—			299.7			(25.0		)	274.7
	Accounts payable on unsettled investment purchases	—			11.6			—			—			11.6
	Other liabilities	(0.1		)	335			3.9			(0.2		)	338.6
	Liabilities held for sale	—			1,880.10			—			—			1,880.10
	Total liabilities	(0.1		)	3,825.90			303.6			(25.2		)	4,104.20
	OneBeacon's common shareholders' equity and noncontrolling interests
	Total OneBeacon's common shareholders' equity	1,104.30			1,391.00			735.5			(2,126.5		)	1,104.30
	Total noncontrolling interests	—			3.1			—			—			3.1
	Total OneBeacon's common shareholders' equity and noncontrolling interests	1,104.30			1,394.10			735.5			(2,126.5		)	1,107.40
	Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,104.20		$	5,220.00		$	1,039.10		$	(2,151.7	)	$	5,211.60
	Consolidating Balance Sheet	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	as of December 31, 2012				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,602.40		$	16.6		$	(25.7	)	$	1,593.30
	Short-term investments, at amortized cost (which approximates fair value)	2.7			169.9			60.2			—			232.8
	Common equity securities, at fair value	—			259			—			—			259
	Convertible fixed maturity investments, at fair value	—			62.6			—			—			62.6
	Other investments	—			143.8			—			—			143.8
	Total investment securities	2.7			2,237.70			76.8			(25.7		)	2,291.50
	Cash	—			43.9			—			—			43.9
	Reinsurance recoverables	—			110.6			—			—			110.6
	Premiums receivable	—			225.6			—			—			225.6
	Deferred acquisition costs	—			123.9			—			—			123.9
	Net deferred tax asset	—			140.2			(2.5		)	0.1			137.8
	Investment income accrued	—			12.3			—			(0.2		)	12.1
	Accounts receivable on unsettled investment sales	—			1.4			0.7			—			2.1
	Investments in subsidiaries	1,012.40			—			1,062.60			(2,075.0		)	—
	Other assets	0.3			219.8			7.1			—			227.2
	Assets held for sale	—			2,226.80			—			—			2,226.80
	Total assets	$	1,015.40		$	5,342.20		$	1,144.70		$	(2,100.8	)	$	5,401.50
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,000.00		$	—		$	—		$	1,000.00
	Unearned premiums	—			573.8			—			—			573.8
	Debt	—			—			299.6			(24.9		)	274.7
	Accounts payable on unsettled investment purchases	—			6.2			—			—			6.2
	Other liabilities	0.9			295.9			6			(0.1		)	302.7
Liabilities held for sale	—			2,226.80			—			—			2,226.80
Total liabilities	0.9			4,102.70			305.6			(25.0		)	4,384.20
OneBeacon's common shareholders' equity and noncontrolling interests
Total OneBeacon's common shareholders' equity	1,014.50			1,236.70			839.1			(2,075.8		)	1,014.50
Total noncontrolling interests	—			2.8			—			—			2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,014.50			1,239.50			839.1			(2,075.8		)	1,017.30
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,015.40		$	5,342.20		$	1,144.70		$	(2,100.8	)	$	5,401.50
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2013	(in millions)
Revenues
Earned premiums	$	—		$	1,120.40		$	—		$	—		$	1,120.40
Net investment income (expense)	—			42.6			(0.3		)	(1.2		)	41.1
Net realized and change in unrealized investment gains	—			48.9			(0.7		)	1.2			49.4
Net other revenues	—			31.2			—			—			31.2
Total revenues	—			1,243.10			(1.0		)	—			1,242.10
Expenses
Loss and loss adjustment expenses	—			622.1			—			—			622.1
Policy acquisition expenses	—			208.9			—			—			208.9
Other underwriting expenses	—			204.8			—			—			204.8
General and administrative expenses	5			7.1			(0.1		)	—			12
Interest expense	—			—			14.2			(1.2		)	13
Total expenses	5			1,042.90			14.1			(1.2		)	1,060.80
Pre-tax (loss) income from continuing operations	(5.0		)	200.2			(15.1		)	1.2			181.3
Income tax benefit (expense)	0.1			(40.4		)	6.2			(0.2		)	(34.3		)
Net (loss) income from continuing operations	(4.9		)	159.8			(8.9		)	1			147
Loss from discontinued operations, net of tax	—			(46.6		)	—			—			(46.6		)
Gain from sale of discontinued operations, net of tax	—			46.6			—			—			46.6
(Loss) income before equity in earnings of unconsolidated affiliates	(4.9		)	159.8			(8.9		)	1			147
Equity in earnings of subsidiaries, net of tax	150.9			—			134.7			(285.6		)	—
Net income including noncontrolling interests	146			159.8			125.8			(284.6		)	147
Less: Net income attributable to noncontrolling interests	—			(1.0		)	—			—			(1.0		)
Net income attributable to OneBeacon's common shareholders	146			158.8			125.8			(284.6		)	146
Net change in benefit plan assets and obligations, net of tax	20.6			—			20.6			(20.6		)	20.6
Comprehensive income attributable to OneBeacon's common shareholders	$	166.6		$	158.8		$	146.4		$	(305.2	)	$	166.6
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2012	(in millions)
Revenues
Earned premiums	$	—		$	1,132.00		$	—		$	—		$	1,132.00
Net investment income	—			56.2			0.4			(3.0		)	53.6
Net realized and change in unrealized investment gains	—			56.5			(1.4		)	0.6			55.7
Net other revenues (expenses)	—			6.6			(7.1		)	—			(0.5		)
Total revenues	—			1,251.30			(8.1		)	(2.4		)	1,240.80
Expenses
Loss and loss adjustment expenses	—			650			—			—			650
Policy acquisition expenses	—			249.4			—			—			249.4
Other underwriting expenses	—			205.2			—			—			205.2
General and administrative expenses	5.4			7.4			0.6			—			13.4
Interest expense	—			—			19.7			(2.8		)	16.9
Total expenses	5.4			1,112.00			20.3			(2.8		)	1,134.90
Pre-tax (loss) income from continuing operations	(5.4		)	139.3			(28.4		)	0.4			105.9
Income tax benefit (expense)	0.1			(18.6		)	10.1			—			(8.4		)
Net (loss) income from continuing operations	(5.3		)	120.7			(18.3		)	0.4			97.5
Loss from discontinued operations, net of tax	—			(24.3		)	—			—			(24.3		)
Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
Net (loss) income before equity in earnings of unconsolidated affiliates	(5.3		)	5.4			(18.3		)	0.4			(17.8		)
Equity in loss of subsidiaries, net of tax	(13.9		)	—			(28.9		)	42.8			—
Net (loss) income including noncontrolling interests	(19.2		)	5.4			(47.2		)	43.2			(17.8		)
Less: Net income attributable to noncontrolling interests	—			(1.4		)	—			—			(1.4		)
Net (loss) income attributable to OneBeacon's common shareholders	(19.2		)	4			(47.2		)	43.2			(19.2		)
Net change in benefit plan assets and obligations, net of tax	(2.9		)	—			(2.9		)	2.9			(2.9		)
Comprehensive (loss) income attributable to OneBeacon's common shareholders	$	(22.1	)	$	4		$	(50.1	)	$	46.1		$	(22.1	)
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2011	(in millions)
Revenues
Earned premiums	$	—		$	1,012.20		$	—		$	—		$	1,012.20
Net investment income	—			72.7			0.5			(1.8		)	71.4
Net realized and change in unrealized investment gains	—			10			(0.1		)	0.7			10.6
Net other revenues (expense)	—			0.7			(13.1		)	—			(12.4		)
Total revenues	—			1,095.60			(12.7		)	(1.1		)	1,081.80
Expenses
Loss and loss adjustment expenses	—			548.3			—			—			548.3
Policy acquisition expenses	—			221.2			—			—			221.2
Other underwriting expenses	—			162.3			—			—			162.3
General and administrative expenses	5.1			4.1			0.6			—			9.8
Interest expense	—			—			22.1			(1.6		)	20.5
Total expenses	5.1			935.9			22.7			(1.6		)	962.1
Pre-tax (loss) income from continuing operations	(5.1		)	159.7			(35.4		)	0.5			119.7
Income tax (expense) benefit	(0.2		)	(26.9		)	12.4			(0.1		)	(14.8		)
Net (loss) income from continuing operations	(5.3		)	132.8			(23.0		)	0.4			104.9
Loss from discontinued operations, net of tax	—			(29.6		)	—			—			(29.6		)
Loss from sale of discontinued operations, net of tax	—			(19.2		)	—			—			(19.2		)
(Loss) income before equity in earnings of unconsolidated affiliates	(5.3		)	84			(23.0		)	0.4			56.1
Equity in earnings of subsidiaries, net of tax	60.4			—			50.9			(111.3		)	—
Net income including noncontrolling interests	55.1			84			27.9			(110.9		)	56.1
Less: Net income attributable to noncontrolling interests	—			(1.0		)	—			—			(1.0		)
Net income attributable to OneBeacon's common shareholders	55.1			83			27.9			(110.9		)	55.1
Net change in benefit plan assets and obligations, net of tax	(11.2		)	—			(11.2		)	11.2			(11.2		)
Comprehensive income attributable to OneBeacon's common shareholders	$	43.9		$	83		$	16.7		$	(99.7	)	$	43.9
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2013	($ in millions)
Cash flows from operations:
Net income including noncontrolling interests	$	146		$	159.8		$	125.8		$	(284.6	)	$	147
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed earnings from subsidiaries	(150.9		)	—			(134.7		)	285.6			—
Net loss from discontinued operations	—			46.6			—			—			46.6
Net gain from sale of discontinued operations	—			(46.6		)	—			—			(46.6		)
Net realized and change in unrealized investment gains	—			(48.9		)	0.7			(1.2		)	(49.4		)
Net realized gain on sale of business	—			(23.0		)	—			—			(23.0		)
Deferred income tax expense	—			12.7			(0.1		)	0.2			12.8
Dividends received from subsidiaries	92			—			218			(310.0		)	—
Other operating items:
Net change in loss and LAE reserves	—			54.3			—			—			54.3
Net change in unearned premiums	—			(28.9		)	—			—			(28.9		)
Net change in premiums receivable	—			(2.6		)	—			—			(2.6		)
Net change in reinsurance recoverable on paid and unpaid losses	—			20.7			—			—			20.7
Net change in other assets and liabilities	3.4			100.5			(29.4		)	—			74.5
Net cash provided from operations—continuing operations	90.5			244.6			180.3			(310.0		)	205.4
Net cash used for operations—discontinued operations	—			(76.7		)	—			—			(76.7		)
Net cash provided from operations	90.5			167.9			180.3			(310.0		)	128.7
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(10.3		)	26.3			56.2			—			72.2
Maturities of fixed maturity investments	—			223.7			21.5			—			245.2
Sales of fixed maturity investments	—			1,496.70			386.7			(483.4		)	1,400.00
Sales of common equity securities	—			196.7			—			—			196.7
Sales of convertible fixed maturity investments	—			45.7			—			—			45.7
Return of capital and distributions of other investments	—			21.8			—			—			21.8
Purchases of fixed maturity investments	—			(1,787.2		)	(393.2		)	483.4			(1,697.0		)
Purchases of common equity securities	—			(206.3		)	—			—			(206.3		)
Purchases of convertible fixed maturity investments	—			(14.4		)	—			—			(14.4		)
Contributions for other investments	—			(7.6		)	—			—			(7.6		)
Proceeds from sale of business	—			31.3			—			—			31.3
Net change in unsettled investment purchases and sales	—			4.1			—			—			4.1
Net acquisitions of property and equipment	—			(10.3		)	—			—			(10.3		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			250			—			(250.0		)	—
Net cash (used for) provided from investing activities—continuing operations	(10.3		)	270.5			71.2			(250.0		)	81.4
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(10.3		)	270.5			71.2			(250.0		)	81.4
Cash flows from financing activities:
Cash dividends paid to common shareholders	(80.2		)	—			—			—			(80.2		)
Cash dividends paid to parent	—			(310.0		)	—			310			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(250.0		)	250			—
Payments on capital lease obligation	—			(5.7		)	—			—			(5.7		)
Net cash used for financing activities—continuing operations	(80.2		)	(315.7		)	(250.0		)	560			(85.9		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(80.2		)	(315.7		)	(250.0		)	560			(85.9		)
Net increase in cash during period	—			122.7			1.5			—			124.2
Cash reclassified to assets held for sale	—			—			—			—			—
Cash transferred with sale of business	—			—			—			—			—
Net increase after reclassification of cash to assets held for sale	—			122.7			1.5			—			124.2
Cash balance at beginning of period	—			43.9			—			—			43.9
Cash balance at end of period	$	—		$	166.6		$	1.5		$	—		$	168.1
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2012	($ in millions)
Cash flows from operations:
Net (loss) income including noncontrolling interests	$	(19.2	)	$	5.4		$	(47.2	)	$	43.2		$	(17.8	)
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed loss from subsidiaries	13.9			—			28.9			(42.8		)	—
Net loss from discontinued operations	—			24.3			—			—			24.3
Net loss from sale of discontinued operations	—			91			—			—			91
Net realized and change in unrealized investment gains	—			(56.5		)	1.4			(0.6		)	(55.7		)
Net other realized losses	—			—			6.3			—			6.3
Net realized gain on sale of business	—			(4.4		)	—			—			(4.4		)
Deferred income tax expense	—			15.2			0.8			0.4			16.4
Dividends received from subsidiaries	83			15.5			160.1			(258.6		)	—
Other operating items:
Net change in loss and LAE reserves	—			87.5			—			—			87.5
Net change in unearned premiums	—			48.4			—			—			48.4
Net change in premiums receivable	—			(14.6		)	—			—			(14.6		)
Net change in reinsurance recoverable on paid and unpaid losses	—			(3.1		)	—			—			(3.1		)
Net change in other assets and liabilities	2.3			(38.8		)	1.1			(0.2		)	(35.6		)
Net cash provided from operations—continuing operations	80			169.9			151.4			(258.6		)	142.7
Net cash used for operations—discontinued operations	—			(196.4		)	—			—			(196.4		)
Net cash provided from (used for) operations	80			(26.5		)	151.4			(258.6		)	(53.7		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	0.1			74.7			(60.2		)	—			14.6
Maturities of fixed maturity investments	—			191.4			0.1			—			191.5
Sales of fixed maturity investments	—			1,783.90			24.6			(91.3		)	1,717.20
Sales of common equity securities	—			114.8			23.1			(25.4		)	112.5
Sales of convertible fixed maturity investments	—			33.1			7.6			(11.4		)	29.3
Return of capital and distributions of other investments	—			63.9			—			—			63.9
Purchases of fixed maturity investments	—			(1,887.6		)	(42.0		)	67.1			(1,862.5		)
Purchases of common equity securities	—			(90.1		)	(26.8		)	25.4			(91.5		)
Purchases of convertible fixed maturity investments	—			(13.0		)	(7.8		)	11.4			(9.4		)
Contributions for other investments	—			(44.9		)	—			—			(44.9		)
Proceeds from sale of business	—			15			—			—			15
Net change in unsettled investment purchases and sales	—			(18.2		)	—			—			(18.2		)
Net acquisitions of property and equipment	—			(1.8		)	—			—			(1.8		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			28.7			—			(28.7		)	—
Net cash provided from (used for) from investing activities—continuing operations	0.1			249.9			(81.4		)	(52.9		)	115.7
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash provided from (used for) investing activities	0.1			249.9			(81.4		)	(52.9		)	115.7
Cash flows from financing activities:
Issuance of debt	—			—			296.9			(25.0		)	271.9
Repurchases of debt	—			—			(325.1		)	49.2			(275.9		)
Cash dividends paid to common shareholders	(80.1		)	—			—			—			(80.1		)
Cash dividends paid to parent	—			(243.1		)	(15.5		)	258.6			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(28.7		)	28.7			—
Payments on capital lease obligation	—			(4.9		)	—			—			(4.9		)
Proceeds on sale of OneBeacon Holdings (Luxembourg) S.à r.l.	—			14			—			—			14
Net cash used for financing activities—continuing operations	(80.1		)	(234.0		)	(72.4		)	311.5			(75.0		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(80.1		)	(234.0		)	(72.4		)	311.5			(75.0		)
Net (decrease) in cash during period	—			(10.6		)	(2.4		)	—			(13.0		)
Cash reclassified from assets held for sale	—			5.5			—			—			5.5
Cash transferred with sale of business	—			(3.5		)	—			—			(3.5		)
Net decrease after reclassification of cash to assets held for sale	—			(8.6		)	(2.4		)	—			(11.0		)
Cash balance at beginning of period	—			52.5			2.4			—			54.9
Cash balance at end of period	$	—		$	43.9		$	—		$	—		$	43.9
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2011	($ in millions)
Cash flows from operations:
Net income (loss) including noncontrolling interests	$	55.1		$	84		$	27.9		$	(110.9	)	$	56.1
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed earnings from subsidiaries	(60.4		)	—			(50.9		)	111.3			—
Net loss from discontinued operations	—			29.6			—			—			29.6
Net loss from sale of discontinued operations	—			19.2			—			—			19.2
Net realized and change in unrealized investment gains	—			(10.0		)	0.1			(0.7		)	(10.6		)
Net other realized losses	—			(0.3		)	12			—			11.7
Deferred income tax expense	—			31.3			0.1			0.2			31.6
Dividends received from subsidiaries	181			34.4			155			(370.4		)	—
Other operating items:
Net change in loss and LAE reserves	—			69.2			—			—			69.2
Net change in unearned premiums	—			50.2			—			—			50.2
Net change in premiums receivable	—			(49.4		)	—			—			(49.4		)
Net change in reinsurance recoverable on paid and unpaid losses	—			(46.3		)	—			—			(46.3		)
Net change in other assets and liabilities	0.1			(92.1		)	12.7			0.1			(79.2		)
Net cash provided from operations—continuing operations	175.8			119.8			156.9			(370.4		)	82.1
Net cash used for operations—discontinued operations	—			(200.6		)	—			—			(200.6		)
Net cash provided from (used for) operations	175.8			(80.8		)	156.9			(370.4		)	(118.5		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(1.0		)	(55.7		)	36.8			—			(19.9		)
Maturities of fixed maturity investments	—			475.3			3			—			478.3
Sales of fixed maturity investments	—			1,406.70			139.8			(131.7		)	1,414.80
Sales of common equity securities	—			101.9			—			—			101.9
Sales of convertible fixed maturity investments	—			37.1			—			—			37.1
Return of capital and distributions of other investments	—			38.6			—			—			38.6
Purchases of fixed maturity investments	—			(1,476.3		)	(127.7		)	131.7			(1,472.3		)
Purchases of common equity securities	—			(88.5		)	—			—			(88.5		)
Purchases of convertible fixed maturity investments	—			(30.4		)	—			—			(30.4		)
Contributions for other investments	—			(10.8		)	—			—			(10.8		)
Net change in unsettled investment purchases and sales	—			13.5			—			—			13.5
Net acquisitions of property and equipment	—			(3.7		)	—			—			(3.7		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			10.6			—			(10.6		)	—
Net cash (used for) provided from investing activities—continuing operations	(1.0		)	418.3			51.9			(10.6		)	458.6
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(1.0		)	418.3			51.9			(10.6		)	458.6
Cash flows from financing activities:
Repurchases of debt	—			—			(161.6		)	—			(161.6		)
Cash dividends paid to common shareholders	(174.8		)	—			—			—			(174.8		)
Cash dividends paid to parent	—			(336.0		)	(34.4		)	370.4			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(10.6		)	10.6			—
Payments on capital lease obligation	—			23.1			—			—			23.1
Net cash used for financing activities—continuing operations	(174.8		)	(312.9		)	(206.6		)	381			(313.3		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(174.8		)	(312.9		)	(206.6		)	381			(313.3		)
Net increase in cash during period	—			24.6			2.2			—			26.8
Cash reclassified to assets held for sale	—			(5.5		)	—			—			(5.5		)
Net increase after reclassification of cash to assets held for sale	—			19.1			2.2			—			21.3
Cash balance at beginning of period	—			33.4			0.2			—			33.6
Cash balance at end of period	$	—		$	52.5		$	2.4		$	—		$	54.9
SCHEDULE II
ONEBEACON INSURANCE GROUP, LTD.
(Registrant Only)
CONDENSED BALANCE SHEETS
	December 31,
	2013			2012
	($ in millions)
Assets:
Investments	$	13		$	2.7
Investments in subsidiaries	1,092.00			1,012.40
Other assets	(0.8		)	0.3
Total assets	$	1,104.20		$	1,015.40
Liabilities	$	(0.1	)	$	0.9
OneBeacon's common shareholders' equity	1,104.30			1,014.50
Total liabilities and OneBeacon's common shareholders' equity	$	1,104.20		$	1,015.40
CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS)
	Year ended December 31,
	2013			2012			2011
	($ in millions)
Revenues	$	—		$	—		$	—
Expenses	5			5.4			5.1
Pre-tax loss	(5.0		)	(5.4		)	(5.1		)
Income tax benefit (expense)	0.1			0.1			(0.2		)
Net loss	(4.9		)	(5.3		)	(5.3		)
Equity in earnings (loss) from subsidiaries, net of tax	150.9			(13.9		)	60.4
Net income (loss) attributable to OneBeacon's common shareholders	146			(19.2		)	55.1
Other comprehensive income (loss) items, after tax	20.6			(2.9		)	(11.2		)
Comprehensive income (loss) attributable to OneBeacon's common shareholders	$	166.6		$	(22.1	)	$	43.9
SCHEDULE II (continued)
ONEBEACON INSURANCE GROUP, LTD.
(Registrant Only)
CONDENSED STATEMENTS OF CASH FLOWS
	Year ended December 31,
	2013			2012			2011
	($ in millions)
Net income (loss)	$	146		$	(19.2	)	$	55.1
Charges (credits) to reconcile net income to net cash from operations:
Undistributed (earnings) loss from subsidiaries	(150.9		)	13.9			(60.4		)
Dividends received from subsidiaries	92			83			181
Net change in other assets and liabilities	3.4			2.3			0.1
Net cash provided from operations	90.5			80			175.8
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(10.3		)	0.1			(1.0		)
Net cash (used for) provided from investing activities	(10.3		)	0.1			(1.0		)
Cash flows from financing activities:
Cash dividends paid to common shareholders	(80.2		)	(80.1		)	(174.8		)
Net cash used for financing activities	(80.2		)	(80.1		)	(174.8		)
Net change in cash during the year	—			—			—
Cash balance at beginning of year	—			—			—
Cash balance at end of year	$	—		$	—		$	—
ONEBEACON INSURANCE GROUP, LTD.
(Registrant Only)
NOTES TO CONDENSED FINANCIAL INFORMATION
1. General
The financial statements of the Registrant should be read in conjunction with the consolidated financial statements and notes thereto included in Item 8.
2. Long-term Obligations and Guarantees
The Company has fully and unconditionally guaranteed the 2012 Senior Notes maturing on November 9, 2022 presented in Note 6—"Debt" that were issued by its 100% owned subsidiary, OBH.
3. Supplemental Cash Flow Information
Dividends received from subsidiaries were $92.0 million, $83.0 million and $181.0 million for the years ended December 31, 2013, 2012, and 2011, respectively.

Subsequent_Event

Subsequent Event	12 Months Ended
	Dec. 31, 2013
Subsequent Events [Abstract]	'
Subsequent Events	'
	Subsequent Events
	On February 26, 2014, the Board declared a dividend of $0.21 per common share, payable on March 28, 2014 to shareholders of record on March 14, 2014

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Quarterly Financial Data (Unaudited)	'
	Selected Quarterly Financial Data (Unaudited)
	Selected quarterly financial data for 2013 and 2012 is shown in the following table. The quarterly financial data includes, in the opinion of management, all recurring adjustments necessary for a fair presentation of the results of operations for the interim periods. Financial information for the prior periods has been reclassified to conform to this presentation.
		2013 Three months ended											2012 Three months ended
	Millions, except per share amounts	Mar. 31			June 30			Sept. 30		Dec. 31			Mar. 31			June 30			Sept. 30			Dec. 31
	Revenues	$	348.3		$	267.3		$	311.5	$	315		$	316.4		$	282.6		$	346.3		$	295.5
	Expenses	260.2			270.3			275.6		254.7			248.8			270.2			287.1			328.8
	Pre-tax income (loss) from continuing operations	88.1			(3.0		)	35.9		60.3			67.6			12.4			59.2			(33.3		)
	Tax (expense) benefit	(19.8		)	1			2.9		(18.4		)	(13.5		)	(0.4		)	(14.2		)	19.7
	Net income (loss) from continuing operations	68.3			(2.0		)	38.8		41.9			54.1			12			45			(13.6		)
	Income (loss) from discontinued operations, net of tax	0.5			(0.5		)	0.3		(46.9		)	(9.5		)	0.5			(15.8		)	0.5
	Gain (loss) from sale of discontinued operations, net of tax	—			—			—		46.6			—			—			(91.0		)	—
	Net income (loss) including noncontrolling interests	68.8			(2.5		)	39.1		41.6			44.6			12.5			(61.8		)	(13.1		)
	Less: Net income attributable to noncontrolling interests	(0.4		)	(0.4		)	—		(0.2		)	(0.6		)	(0.2		)	(0.4		)	(0.2		)
	Net income (loss) attributable to OneBeacon's common shareholders	$	68.4		$	(2.9	)	$	39.1	$	41.4		$	44		$	12.3		$	(62.2	)	$	(13.3	)
	Earnings (loss) per share attributable to OneBeacon's common shareholders—basic and diluted:(1)
	Net income (loss) from continuing operations per share	$	0.71		$	(0.03	)	$	0.41	$	0.43		$	0.56		$	0.12		$	0.47		$	(0.15	)
	Loss from discontinued operations, net of tax, per share	—			—			—		(0.49		)	(0.10		)	—			(0.17		)	0.01
	Gain (loss) from sale of discontinued operations, net of tax, per share	—			—			—		0.49			—			—			(0.95		)	—
	Net income (loss) attributable to OneBeacon's common shareholders per share	$	0.71		$	(0.03	)	$	0.41	$	0.43		$	0.46		$	0.12		$	(0.65	)	$	(0.14	)
	(1) Due to the averaging of shares, quarterly earnings per share may not add to the total for the full year.

SCHEDULE_I_SUMMARY_OF_INVESTME

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2013
Summary of Investments, Other than Investments in Related Parties [Abstract]	'
SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES	'
	SCHEDULE I
	ONEBEACON INSURANCE GROUP, LTD.
	SUMMARY OF INVESTMENTS—OTHER THAN
	INVESTMENTS IN RELATED PARTIES
	At December 31, 2013
			Amortized		Fair		Carrying
			Cost		Value		Value
			($ in millions)
	Investments:
	Fixed maturities:
	Bonds:
	U.S. Government and government agencies and authorities		$	131	$	131.1	$	131.1
	Corporate bonds and asset-backed securities		1,693.20		1,704.00		1,704.00
	States, municipalities and political subdivisions		16.9		16.5		16.5
	Foreign governments		2		2.3		2.3
	Redeemable preferred stocks		78.3		83.3		83.3
	Total fixed maturities(1)		1,921.40		1,937.20		1,937.20
	Short-term investments		157		157		157
	Common equity securities:
	Banks, trust and insurance companies		66.6		83.3		83.3
	Public utilities		8.3		8.3		8.3
	Industrial, miscellaneous and other		192.3		245.3		245.3
	Total common equity securities		267.2		336.9		336.9
	Convertible fixed maturity investments		26.3		30.5		30.5
	Other investments		104.7		139.6		139.6
	Total investments		$	2,476.60	$	2,601.20	$	2,601.20
	_______________________________________________________________________________
	-1	Includes $236.3 million of fixed maturity investments reclassified to assets held for sale in the December 31, 2013 consolidated balance sheet as part of the Runoff Transaction.

SCHEDULE_II_CONDENSED_FINANCIA

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT [Schedule]	12 Months Ended
	Dec. 31, 2013
Condensed Financial Information of Parent Company Only Disclosure [Abstract]	'
SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT	'
	Consolidating Financial Information
	The Company has fully and unconditionally guaranteed the 2012 Senior Notes issued by its 100% owned subsidiary, OBH. The following tables present OneBeacon's consolidating balance sheets as of December 31, 2013 and 2012, and statements of operations and comprehensive income and cash flows for the years ended December 31, 2013, 2012 and 2011. These financial statements reflect the Company's ("guarantor") financial position, results of operations and cash flows on a stand-alone basis, that of OBH ("the issuer") and of the Company's other entities ("non-guarantor subsidiaries") as well as the necessary consolidating adjustments to eliminate intercompany balances and transactions.
	Consolidating Balance Sheet	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	as of December 31, 2013				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,725.40		$	—		$	(24.5	)	$	1,700.90
	Short-term investments, at amortized cost (which approximates fair value)	13			140			4			—			157
	Common equity securities, at fair value	—			336.9			—			—			336.9
	Convertible fixed maturity investments, at fair value	—			30.5			—			—			30.5
	Other investments	—			139.6			—			—			139.6
	Total investment securities	13			2,372.40			4			(24.5		)	2,364.90
	Cash	—			166.6			1.5			—			168.1
	Reinsurance recoverables	—			89.9			—			—			89.9
	Premiums receivable	—			228.2			—			—			228.2
	Deferred acquisition costs	—			103.7			—			—			103.7
	Net deferred tax asset	—			93.1			(2.4		)	(0.1		)	90.6
	Investment income accrued	—			10.3			—			(0.2		)	10.1
	Accounts receivable on unsettled investment sales	—			3.3			—			—			3.3
	Investments in subsidiaries	1,092.00			—			1,034.90			(2,126.9		)	—
	Other assets	(0.8		)	272.4			1.1			—			272.7
	Assets held for sale	—			1,880.10			—			—			1,880.10
	Total assets	$	1,104.20		$	5,220.00		$	1,039.10		$	(2,151.7	)	$	5,211.60
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,054.30		$	—		$	—		$	1,054.30
	Unearned premiums	—			544.9			—			—			544.9
	Debt	—			—			299.7			(25.0		)	274.7
	Accounts payable on unsettled investment purchases	—			11.6			—			—			11.6
	Other liabilities	(0.1		)	335			3.9			(0.2		)	338.6
	Liabilities held for sale	—			1,880.10			—			—			1,880.10
	Total liabilities	(0.1		)	3,825.90			303.6			(25.2		)	4,104.20
	OneBeacon's common shareholders' equity and noncontrolling interests
	Total OneBeacon's common shareholders' equity	1,104.30			1,391.00			735.5			(2,126.5		)	1,104.30
	Total noncontrolling interests	—			3.1			—			—			3.1
	Total OneBeacon's common shareholders' equity and noncontrolling interests	1,104.30			1,394.10			735.5			(2,126.5		)	1,107.40
	Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,104.20		$	5,220.00		$	1,039.10		$	(2,151.7	)	$	5,211.60
	Consolidating Balance Sheet	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	as of December 31, 2012				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,602.40		$	16.6		$	(25.7	)	$	1,593.30
	Short-term investments, at amortized cost (which approximates fair value)	2.7			169.9			60.2			—			232.8
	Common equity securities, at fair value	—			259			—			—			259
	Convertible fixed maturity investments, at fair value	—			62.6			—			—			62.6
	Other investments	—			143.8			—			—			143.8
	Total investment securities	2.7			2,237.70			76.8			(25.7		)	2,291.50
	Cash	—			43.9			—			—			43.9
	Reinsurance recoverables	—			110.6			—			—			110.6
	Premiums receivable	—			225.6			—			—			225.6
	Deferred acquisition costs	—			123.9			—			—			123.9
	Net deferred tax asset	—			140.2			(2.5		)	0.1			137.8
	Investment income accrued	—			12.3			—			(0.2		)	12.1
	Accounts receivable on unsettled investment sales	—			1.4			0.7			—			2.1
	Investments in subsidiaries	1,012.40			—			1,062.60			(2,075.0		)	—
	Other assets	0.3			219.8			7.1			—			227.2
	Assets held for sale	—			2,226.80			—			—			2,226.80
	Total assets	$	1,015.40		$	5,342.20		$	1,144.70		$	(2,100.8	)	$	5,401.50
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,000.00		$	—		$	—		$	1,000.00
	Unearned premiums	—			573.8			—			—			573.8
	Debt	—			—			299.6			(24.9		)	274.7
	Accounts payable on unsettled investment purchases	—			6.2			—			—			6.2
	Other liabilities	0.9			295.9			6			(0.1		)	302.7
Liabilities held for sale	—			2,226.80			—			—			2,226.80
Total liabilities	0.9			4,102.70			305.6			(25.0		)	4,384.20
OneBeacon's common shareholders' equity and noncontrolling interests
Total OneBeacon's common shareholders' equity	1,014.50			1,236.70			839.1			(2,075.8		)	1,014.50
Total noncontrolling interests	—			2.8			—			—			2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,014.50			1,239.50			839.1			(2,075.8		)	1,017.30
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,015.40		$	5,342.20		$	1,144.70		$	(2,100.8	)	$	5,401.50
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2013	(in millions)
Revenues
Earned premiums	$	—		$	1,120.40		$	—		$	—		$	1,120.40
Net investment income (expense)	—			42.6			(0.3		)	(1.2		)	41.1
Net realized and change in unrealized investment gains	—			48.9			(0.7		)	1.2			49.4
Net other revenues	—			31.2			—			—			31.2
Total revenues	—			1,243.10			(1.0		)	—			1,242.10
Expenses
Loss and loss adjustment expenses	—			622.1			—			—			622.1
Policy acquisition expenses	—			208.9			—			—			208.9
Other underwriting expenses	—			204.8			—			—			204.8
General and administrative expenses	5			7.1			(0.1		)	—			12
Interest expense	—			—			14.2			(1.2		)	13
Total expenses	5			1,042.90			14.1			(1.2		)	1,060.80
Pre-tax (loss) income from continuing operations	(5.0		)	200.2			(15.1		)	1.2			181.3
Income tax benefit (expense)	0.1			(40.4		)	6.2			(0.2		)	(34.3		)
Net (loss) income from continuing operations	(4.9		)	159.8			(8.9		)	1			147
Loss from discontinued operations, net of tax	—			(46.6		)	—			—			(46.6		)
Gain from sale of discontinued operations, net of tax	—			46.6			—			—			46.6
(Loss) income before equity in earnings of unconsolidated affiliates	(4.9		)	159.8			(8.9		)	1			147
Equity in earnings of subsidiaries, net of tax	150.9			—			134.7			(285.6		)	—
Net income including noncontrolling interests	146			159.8			125.8			(284.6		)	147
Less: Net income attributable to noncontrolling interests	—			(1.0		)	—			—			(1.0		)
Net income attributable to OneBeacon's common shareholders	146			158.8			125.8			(284.6		)	146
Net change in benefit plan assets and obligations, net of tax	20.6			—			20.6			(20.6		)	20.6
Comprehensive income attributable to OneBeacon's common shareholders	$	166.6		$	158.8		$	146.4		$	(305.2	)	$	166.6
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2012	(in millions)
Revenues
Earned premiums	$	—		$	1,132.00		$	—		$	—		$	1,132.00
Net investment income	—			56.2			0.4			(3.0		)	53.6
Net realized and change in unrealized investment gains	—			56.5			(1.4		)	0.6			55.7
Net other revenues (expenses)	—			6.6			(7.1		)	—			(0.5		)
Total revenues	—			1,251.30			(8.1		)	(2.4		)	1,240.80
Expenses
Loss and loss adjustment expenses	—			650			—			—			650
Policy acquisition expenses	—			249.4			—			—			249.4
Other underwriting expenses	—			205.2			—			—			205.2
General and administrative expenses	5.4			7.4			0.6			—			13.4
Interest expense	—			—			19.7			(2.8		)	16.9
Total expenses	5.4			1,112.00			20.3			(2.8		)	1,134.90
Pre-tax (loss) income from continuing operations	(5.4		)	139.3			(28.4		)	0.4			105.9
Income tax benefit (expense)	0.1			(18.6		)	10.1			—			(8.4		)
Net (loss) income from continuing operations	(5.3		)	120.7			(18.3		)	0.4			97.5
Loss from discontinued operations, net of tax	—			(24.3		)	—			—			(24.3		)
Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
Net (loss) income before equity in earnings of unconsolidated affiliates	(5.3		)	5.4			(18.3		)	0.4			(17.8		)
Equity in loss of subsidiaries, net of tax	(13.9		)	—			(28.9		)	42.8			—
Net (loss) income including noncontrolling interests	(19.2		)	5.4			(47.2		)	43.2			(17.8		)
Less: Net income attributable to noncontrolling interests	—			(1.4		)	—			—			(1.4		)
Net (loss) income attributable to OneBeacon's common shareholders	(19.2		)	4			(47.2		)	43.2			(19.2		)
Net change in benefit plan assets and obligations, net of tax	(2.9		)	—			(2.9		)	2.9			(2.9		)
Comprehensive (loss) income attributable to OneBeacon's common shareholders	$	(22.1	)	$	4		$	(50.1	)	$	46.1		$	(22.1	)
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2011	(in millions)
Revenues
Earned premiums	$	—		$	1,012.20		$	—		$	—		$	1,012.20
Net investment income	—			72.7			0.5			(1.8		)	71.4
Net realized and change in unrealized investment gains	—			10			(0.1		)	0.7			10.6
Net other revenues (expense)	—			0.7			(13.1		)	—			(12.4		)
Total revenues	—			1,095.60			(12.7		)	(1.1		)	1,081.80
Expenses
Loss and loss adjustment expenses	—			548.3			—			—			548.3
Policy acquisition expenses	—			221.2			—			—			221.2
Other underwriting expenses	—			162.3			—			—			162.3
General and administrative expenses	5.1			4.1			0.6			—			9.8
Interest expense	—			—			22.1			(1.6		)	20.5
Total expenses	5.1			935.9			22.7			(1.6		)	962.1
Pre-tax (loss) income from continuing operations	(5.1		)	159.7			(35.4		)	0.5			119.7
Income tax (expense) benefit	(0.2		)	(26.9		)	12.4			(0.1		)	(14.8		)
Net (loss) income from continuing operations	(5.3		)	132.8			(23.0		)	0.4			104.9
Loss from discontinued operations, net of tax	—			(29.6		)	—			—			(29.6		)
Loss from sale of discontinued operations, net of tax	—			(19.2		)	—			—			(19.2		)
(Loss) income before equity in earnings of unconsolidated affiliates	(5.3		)	84			(23.0		)	0.4			56.1
Equity in earnings of subsidiaries, net of tax	60.4			—			50.9			(111.3		)	—
Net income including noncontrolling interests	55.1			84			27.9			(110.9		)	56.1
Less: Net income attributable to noncontrolling interests	—			(1.0		)	—			—			(1.0		)
Net income attributable to OneBeacon's common shareholders	55.1			83			27.9			(110.9		)	55.1
Net change in benefit plan assets and obligations, net of tax	(11.2		)	—			(11.2		)	11.2			(11.2		)
Comprehensive income attributable to OneBeacon's common shareholders	$	43.9		$	83		$	16.7		$	(99.7	)	$	43.9
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2013	($ in millions)
Cash flows from operations:
Net income including noncontrolling interests	$	146		$	159.8		$	125.8		$	(284.6	)	$	147
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed earnings from subsidiaries	(150.9		)	—			(134.7		)	285.6			—
Net loss from discontinued operations	—			46.6			—			—			46.6
Net gain from sale of discontinued operations	—			(46.6		)	—			—			(46.6		)
Net realized and change in unrealized investment gains	—			(48.9		)	0.7			(1.2		)	(49.4		)
Net realized gain on sale of business	—			(23.0		)	—			—			(23.0		)
Deferred income tax expense	—			12.7			(0.1		)	0.2			12.8
Dividends received from subsidiaries	92			—			218			(310.0		)	—
Other operating items:
Net change in loss and LAE reserves	—			54.3			—			—			54.3
Net change in unearned premiums	—			(28.9		)	—			—			(28.9		)
Net change in premiums receivable	—			(2.6		)	—			—			(2.6		)
Net change in reinsurance recoverable on paid and unpaid losses	—			20.7			—			—			20.7
Net change in other assets and liabilities	3.4			100.5			(29.4		)	—			74.5
Net cash provided from operations—continuing operations	90.5			244.6			180.3			(310.0		)	205.4
Net cash used for operations—discontinued operations	—			(76.7		)	—			—			(76.7		)
Net cash provided from operations	90.5			167.9			180.3			(310.0		)	128.7
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(10.3		)	26.3			56.2			—			72.2
Maturities of fixed maturity investments	—			223.7			21.5			—			245.2
Sales of fixed maturity investments	—			1,496.70			386.7			(483.4		)	1,400.00
Sales of common equity securities	—			196.7			—			—			196.7
Sales of convertible fixed maturity investments	—			45.7			—			—			45.7
Return of capital and distributions of other investments	—			21.8			—			—			21.8
Purchases of fixed maturity investments	—			(1,787.2		)	(393.2		)	483.4			(1,697.0		)
Purchases of common equity securities	—			(206.3		)	—			—			(206.3		)
Purchases of convertible fixed maturity investments	—			(14.4		)	—			—			(14.4		)
Contributions for other investments	—			(7.6		)	—			—			(7.6		)
Proceeds from sale of business	—			31.3			—			—			31.3
Net change in unsettled investment purchases and sales	—			4.1			—			—			4.1
Net acquisitions of property and equipment	—			(10.3		)	—			—			(10.3		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			250			—			(250.0		)	—
Net cash (used for) provided from investing activities—continuing operations	(10.3		)	270.5			71.2			(250.0		)	81.4
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(10.3		)	270.5			71.2			(250.0		)	81.4
Cash flows from financing activities:
Cash dividends paid to common shareholders	(80.2		)	—			—			—			(80.2		)
Cash dividends paid to parent	—			(310.0		)	—			310			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(250.0		)	250			—
Payments on capital lease obligation	—			(5.7		)	—			—			(5.7		)
Net cash used for financing activities—continuing operations	(80.2		)	(315.7		)	(250.0		)	560			(85.9		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(80.2		)	(315.7		)	(250.0		)	560			(85.9		)
Net increase in cash during period	—			122.7			1.5			—			124.2
Cash reclassified to assets held for sale	—			—			—			—			—
Cash transferred with sale of business	—			—			—			—			—
Net increase after reclassification of cash to assets held for sale	—			122.7			1.5			—			124.2
Cash balance at beginning of period	—			43.9			—			—			43.9
Cash balance at end of period	$	—		$	166.6		$	1.5		$	—		$	168.1
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2012	($ in millions)
Cash flows from operations:
Net (loss) income including noncontrolling interests	$	(19.2	)	$	5.4		$	(47.2	)	$	43.2		$	(17.8	)
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed loss from subsidiaries	13.9			—			28.9			(42.8		)	—
Net loss from discontinued operations	—			24.3			—			—			24.3
Net loss from sale of discontinued operations	—			91			—			—			91
Net realized and change in unrealized investment gains	—			(56.5		)	1.4			(0.6		)	(55.7		)
Net other realized losses	—			—			6.3			—			6.3
Net realized gain on sale of business	—			(4.4		)	—			—			(4.4		)
Deferred income tax expense	—			15.2			0.8			0.4			16.4
Dividends received from subsidiaries	83			15.5			160.1			(258.6		)	—
Other operating items:
Net change in loss and LAE reserves	—			87.5			—			—			87.5
Net change in unearned premiums	—			48.4			—			—			48.4
Net change in premiums receivable	—			(14.6		)	—			—			(14.6		)
Net change in reinsurance recoverable on paid and unpaid losses	—			(3.1		)	—			—			(3.1		)
Net change in other assets and liabilities	2.3			(38.8		)	1.1			(0.2		)	(35.6		)
Net cash provided from operations—continuing operations	80			169.9			151.4			(258.6		)	142.7
Net cash used for operations—discontinued operations	—			(196.4		)	—			—			(196.4		)
Net cash provided from (used for) operations	80			(26.5		)	151.4			(258.6		)	(53.7		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	0.1			74.7			(60.2		)	—			14.6
Maturities of fixed maturity investments	—			191.4			0.1			—			191.5
Sales of fixed maturity investments	—			1,783.90			24.6			(91.3		)	1,717.20
Sales of common equity securities	—			114.8			23.1			(25.4		)	112.5
Sales of convertible fixed maturity investments	—			33.1			7.6			(11.4		)	29.3
Return of capital and distributions of other investments	—			63.9			—			—			63.9
Purchases of fixed maturity investments	—			(1,887.6		)	(42.0		)	67.1			(1,862.5		)
Purchases of common equity securities	—			(90.1		)	(26.8		)	25.4			(91.5		)
Purchases of convertible fixed maturity investments	—			(13.0		)	(7.8		)	11.4			(9.4		)
Contributions for other investments	—			(44.9		)	—			—			(44.9		)
Proceeds from sale of business	—			15			—			—			15
Net change in unsettled investment purchases and sales	—			(18.2		)	—			—			(18.2		)
Net acquisitions of property and equipment	—			(1.8		)	—			—			(1.8		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			28.7			—			(28.7		)	—
Net cash provided from (used for) from investing activities—continuing operations	0.1			249.9			(81.4		)	(52.9		)	115.7
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash provided from (used for) investing activities	0.1			249.9			(81.4		)	(52.9		)	115.7
Cash flows from financing activities:
Issuance of debt	—			—			296.9			(25.0		)	271.9
Repurchases of debt	—			—			(325.1		)	49.2			(275.9		)
Cash dividends paid to common shareholders	(80.1		)	—			—			—			(80.1		)
Cash dividends paid to parent	—			(243.1		)	(15.5		)	258.6			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(28.7		)	28.7			—
Payments on capital lease obligation	—			(4.9		)	—			—			(4.9		)
Proceeds on sale of OneBeacon Holdings (Luxembourg) S.à r.l.	—			14			—			—			14
Net cash used for financing activities—continuing operations	(80.1		)	(234.0		)	(72.4		)	311.5			(75.0		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(80.1		)	(234.0		)	(72.4		)	311.5			(75.0		)
Net (decrease) in cash during period	—			(10.6		)	(2.4		)	—			(13.0		)
Cash reclassified from assets held for sale	—			5.5			—			—			5.5
Cash transferred with sale of business	—			(3.5		)	—			—			(3.5		)
Net decrease after reclassification of cash to assets held for sale	—			(8.6		)	(2.4		)	—			(11.0		)
Cash balance at beginning of period	—			52.5			2.4			—			54.9
Cash balance at end of period	$	—		$	43.9		$	—		$	—		$	43.9
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2011	($ in millions)
Cash flows from operations:
Net income (loss) including noncontrolling interests	$	55.1		$	84		$	27.9		$	(110.9	)	$	56.1
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed earnings from subsidiaries	(60.4		)	—			(50.9		)	111.3			—
Net loss from discontinued operations	—			29.6			—			—			29.6
Net loss from sale of discontinued operations	—			19.2			—			—			19.2
Net realized and change in unrealized investment gains	—			(10.0		)	0.1			(0.7		)	(10.6		)
Net other realized losses	—			(0.3		)	12			—			11.7
Deferred income tax expense	—			31.3			0.1			0.2			31.6
Dividends received from subsidiaries	181			34.4			155			(370.4		)	—
Other operating items:
Net change in loss and LAE reserves	—			69.2			—			—			69.2
Net change in unearned premiums	—			50.2			—			—			50.2
Net change in premiums receivable	—			(49.4		)	—			—			(49.4		)
Net change in reinsurance recoverable on paid and unpaid losses	—			(46.3		)	—			—			(46.3		)
Net change in other assets and liabilities	0.1			(92.1		)	12.7			0.1			(79.2		)
Net cash provided from operations—continuing operations	175.8			119.8			156.9			(370.4		)	82.1
Net cash used for operations—discontinued operations	—			(200.6		)	—			—			(200.6		)
Net cash provided from (used for) operations	175.8			(80.8		)	156.9			(370.4		)	(118.5		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(1.0		)	(55.7		)	36.8			—			(19.9		)
Maturities of fixed maturity investments	—			475.3			3			—			478.3
Sales of fixed maturity investments	—			1,406.70			139.8			(131.7		)	1,414.80
Sales of common equity securities	—			101.9			—			—			101.9
Sales of convertible fixed maturity investments	—			37.1			—			—			37.1
Return of capital and distributions of other investments	—			38.6			—			—			38.6
Purchases of fixed maturity investments	—			(1,476.3		)	(127.7		)	131.7			(1,472.3		)
Purchases of common equity securities	—			(88.5		)	—			—			(88.5		)
Purchases of convertible fixed maturity investments	—			(30.4		)	—			—			(30.4		)
Contributions for other investments	—			(10.8		)	—			—			(10.8		)
Net change in unsettled investment purchases and sales	—			13.5			—			—			13.5
Net acquisitions of property and equipment	—			(3.7		)	—			—			(3.7		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			10.6			—			(10.6		)	—
Net cash (used for) provided from investing activities—continuing operations	(1.0		)	418.3			51.9			(10.6		)	458.6
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(1.0		)	418.3			51.9			(10.6		)	458.6
Cash flows from financing activities:
Repurchases of debt	—			—			(161.6		)	—			(161.6		)
Cash dividends paid to common shareholders	(174.8		)	—			—			—			(174.8		)
Cash dividends paid to parent	—			(336.0		)	(34.4		)	370.4			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(10.6		)	10.6			—
Payments on capital lease obligation	—			23.1			—			—			23.1
Net cash used for financing activities—continuing operations	(174.8		)	(312.9		)	(206.6		)	381			(313.3		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(174.8		)	(312.9		)	(206.6		)	381			(313.3		)
Net increase in cash during period	—			24.6			2.2			—			26.8
Cash reclassified to assets held for sale	—			(5.5		)	—			—			(5.5		)
Net increase after reclassification of cash to assets held for sale	—			19.1			2.2			—			21.3
Cash balance at beginning of period	—			33.4			0.2			—			33.6
Cash balance at end of period	$	—		$	52.5		$	2.4		$	—		$	54.9
SCHEDULE II
ONEBEACON INSURANCE GROUP, LTD.
(Registrant Only)
CONDENSED BALANCE SHEETS
	December 31,
	2013			2012
	($ in millions)
Assets:
Investments	$	13		$	2.7
Investments in subsidiaries	1,092.00			1,012.40
Other assets	(0.8		)	0.3
Total assets	$	1,104.20		$	1,015.40
Liabilities	$	(0.1	)	$	0.9
OneBeacon's common shareholders' equity	1,104.30			1,014.50
Total liabilities and OneBeacon's common shareholders' equity	$	1,104.20		$	1,015.40
CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS)
	Year ended December 31,
	2013			2012			2011
	($ in millions)
Revenues	$	—		$	—		$	—
Expenses	5			5.4			5.1
Pre-tax loss	(5.0		)	(5.4		)	(5.1		)
Income tax benefit (expense)	0.1			0.1			(0.2		)
Net loss	(4.9		)	(5.3		)	(5.3		)
Equity in earnings (loss) from subsidiaries, net of tax	150.9			(13.9		)	60.4
Net income (loss) attributable to OneBeacon's common shareholders	146			(19.2		)	55.1
Other comprehensive income (loss) items, after tax	20.6			(2.9		)	(11.2		)
Comprehensive income (loss) attributable to OneBeacon's common shareholders	$	166.6		$	(22.1	)	$	43.9
SCHEDULE II (continued)
ONEBEACON INSURANCE GROUP, LTD.
(Registrant Only)
CONDENSED STATEMENTS OF CASH FLOWS
	Year ended December 31,
	2013			2012			2011
	($ in millions)
Net income (loss)	$	146		$	(19.2	)	$	55.1
Charges (credits) to reconcile net income to net cash from operations:
Undistributed (earnings) loss from subsidiaries	(150.9		)	13.9			(60.4		)
Dividends received from subsidiaries	92			83			181
Net change in other assets and liabilities	3.4			2.3			0.1
Net cash provided from operations	90.5			80			175.8
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(10.3		)	0.1			(1.0		)
Net cash (used for) provided from investing activities	(10.3		)	0.1			(1.0		)
Cash flows from financing activities:
Cash dividends paid to common shareholders	(80.2		)	(80.1		)	(174.8		)
Net cash used for financing activities	(80.2		)	(80.1		)	(174.8		)
Net change in cash during the year	—			—			—
Cash balance at beginning of year	—			—			—
Cash balance at end of year	$	—		$	—		$	—
ONEBEACON INSURANCE GROUP, LTD.
(Registrant Only)
NOTES TO CONDENSED FINANCIAL INFORMATION
1. General
The financial statements of the Registrant should be read in conjunction with the consolidated financial statements and notes thereto included in Item 8.
2. Long-term Obligations and Guarantees
The Company has fully and unconditionally guaranteed the 2012 Senior Notes maturing on November 9, 2022 presented in Note 6—"Debt" that were issued by its 100% owned subsidiary, OBH.
3. Supplemental Cash Flow Information
Dividends received from subsidiaries were $92.0 million, $83.0 million and $181.0 million for the years ended December 31, 2013, 2012, and 2011, respectively.

SCHEDULE_III_SUPPLEMENTARY_INS

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2013
Supplementary Insurance Information [Abstract]	'
SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	'
	SCHEDULE III
	ONEBEACON INSURANCE GROUP, LTD.
	SUPPLEMENTARY INSURANCE INFORMATION(1)
	Column A		Column B		Column C		Column D		Column E		Column F		Column G		Column H			Column I			Column J			Column K
	Segment		Deferred		Future policy		Unearned		Other		Premiums		Net		Benefits,			Amortization			Other			Premiums
			acquisition		benefits,		premiums		policy		earned		investment		claims, losses,			of policy			operating			written
			costs		losses,				claims and				income(3)		and			acquisition			expenses
					claims and				benefits						settlement			expenses
					loss expenses				payable						expenses
			($ in millions)
	Years ended:
	December 31, 2013:
	Specialty Industries		$	50.1	$	446.5	$	269.5	$	—	$	566.9	$	—	$	(309.8	)	$	(102.6	)	$	(107.4	)	$	579
	Specialty Products		53.6		607.8		275.4		—		553.5		—		(312.3		)	(106.3		)	(97.4		)	509.6
	Investing, Financing and Corporate(2)		—		—		—		—		—		41.1		—			—			—			—
	December 31, 2012:
	Specialty Industries		$	48.8	$	438.1	$	255.1	$	—	$	528	$	—	$	(304.4	)	$	(99.1	)	$	(109.0	)	$	548.3
	Specialty Products		75.1		561.9		318.7		—		604		—		(345.6		)	(150.3		)	(96.2		)	630.9
	Investing, Financing and Corporate(2)		—		—		—		—		—		53.6		—			—			—			—
	December 31, 2011:
	Specialty Industries		$	48.7	$	363.6	$	234.5	$	—	$	462.4	$	—	$	(266.6	)	$	(92.1	)	$	(85.2	)	$	491.5
	Specialty Products		74.6		505		291.1		—		549.8		—		(281.7		)	(129.1		)	(77.1		)	571.2
	Investing, Financing and Corporate(2)		0.2		2,490.00		2.4		—		—		71.4		—			—			—			—
	__________________________________________________________
	-1	Schedule excludes activity related to discontinued operations for all periods presented.
	-2	Schedule excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013 and 2012 and to the AutoOne Transaction as of December 31, 2011.
	-3	Invested assets are not allocated to Specialty Industries or Specialty Products since OneBeacon does not manage its assets by segment. Net investment income related to OneBeacon's Specialty Industries and Specialty Products segments are included in the Investing, Financing and Corporate segment since these assets are available for payment of losses and expenses for all segments.

SCHEDULE_IV_REINSURANCE

SCHEDULE IV REINSURANCE	12 Months Ended
	Dec. 31, 2013
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]	'
SCHEDULE IV REINSURANCE	'
	SCHEDULE IV
	ONEBEACON INSURANCE GROUP, LTD.
	REINSURANCE
	Column A	Column B		Column C			Column D		Column E		Column F
	Premiums earned	Gross		Ceded to			Assumed		Net		Percentage
		amount		other			from		amount		of amount
				companies			other				assumed to
							companies				net
		($ in millions)
	Years ended:
	31-Dec-13
	Continuing Operations	$	1,043.30	$	(71.4	)	$	148.5	$	1,120.40	13.3	%
	Discontinued Operations	0.4		—			0.4		0.8		50
	31-Dec-12
	Continuing Operations	$	1,158.30	$	(79.1	)	$	52.8	$	1,132.00	4.7	%
	Discontinued Operations	12.1		(2.4		)	0.9		10.6		8.5
	31-Dec-11
	Continuing Operations	$	1,035.90	$	(66.0	)	$	42.3	$	1,012.20	4.2	%
	Discontinued Operations	169.8		(100.1		)	0.8		70.5		1.1

SCHEDULE_V_VALUATION_AND_QUALI

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2013
Valuation and Qualifying Accounts [Abstract]	'
SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	'
	SCHEDULE V
	ONEBEACON INSURANCE GROUP, LTD.
	VALUATION AND QUALIFYING ACCOUNTS(1)
			Column A		Column B		Column C			Column D			Column E
			Additions (subtractions)
	Description		Balance at		Charged		Charged			Other			Balance at
			beginning		(credited)		(credited) to			additions			end of
			of period		to costs and		other			(deductions)			period
					expenses		accounts(2)			described(3)
			($ in millions)
	Years ended:
	December 31, 2013(4)
	Reinsurance recoverable on paid and unpaid losses:
	Allowance for reinsurance balances		$	25.6	$	—	$	—		(11.3		)	$	14.3
	Premiums receivable:
	Allowance for uncollectible accounts		4.4		0.3		—			(1.6		)	3.1
	31-Dec-12
	Reinsurance recoverable on paid and unpaid losses:
	Allowance for reinsurance balances		$	24.4	$	—	$	—		$	1.2		$	25.6
	Premiums receivable:
	Allowance for uncollectible accounts		3.4		1.1		—			(0.1		)	4.4
	31-Dec-11
	Reinsurance recoverable on paid and unpaid losses:
	Allowance for reinsurance balances		$	17.5	$	10.7	$	—		$	(3.8	)	$	24.4
	Premiums receivable:
	Allowance for uncollectible accounts		3.9		—		(0.5		)	—			3.4
	_______________________________________________________________________________
	-1	Schedule includes activity related to discontinued operations. Balances as of December 31, 2013 and 2012 include an Allowance for reinsurance balances of $11.3 million and $23.1 million, respectively, and an Allowance for uncollectible premiums receivable of $0.1 million and $0.9 million, respectively, related to the Runoff Transaction that were reclassified to held for sale in the consolidated balance sheet. Balance as of December 31, 2011 includes an Allowance for uncollectible premiums receivable of $0.4 million related to the AutoOne Transaction that was reclassified to held for sale in the consolidated balance sheet.
	-2	Amount credited to other accounts represents a reduction in the Allowance for uncollectible premiums receivable which was offset by a corresponding reduction in Gross premiums receivable. There was no impact to Premiums receivable as presented in the consolidated balance sheet.
	-3	Represents net collections (charge offs) of balances receivable.
	(4)	Excludes $0.1 million of allowance for uncollectible deductibles as of December 31, 2013, with no allowance for uncollectible deductibles as of December 31, 2012 and 2011.

SCHEDULE_VI_SUPPLEMENTAL_INFOR

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS	12 Months Ended
	Dec. 31, 2013
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]	'
SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS	'
	SCHEDULE VI
	ONEBEACON INSURANCE GROUP, LTD.
	SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS(1), (2)
	Column A		Column B		Column C		Column D			Column E		Column F		Column G		Column H					Column I			Column J		Column K
			Deferred		Reserves		Discount,			Unearned		Earned		Net		Claims and Claims					Amortization of			Paid		Premiums
			acquisition		for Unpaid		if any,			Premiums		Premiums		investment		Adjustment					deferred			Claims and		written
			costs		Claims and		deducted in							income(5)		Expenses Incurred					policy			Claims
					Claims		Column C									Related to					acquisition			Adjustment
					Adjustment																costs			Expenses
	Affiliation with registrant				Expenses											Current		Prior
																Year(1)		Year
			($ in millions)
	Specialty Industries:
	2013		$	50.1	$	446.5	$	2.8	(3)	$	269.5	$	566.9	$	—	$	315.7	$	(5.9	)	$	(102.6	)	$	268.5	$	579
	2012		48.8		438.1		4.1		(3)	255.1		528		—		309.9		(5.5		)	(99.1		)	253		548.3
	2011		48.7		363.6		4.2		(3)	234.5		462.4		—		279.8		(13.2		)	(92.1		)	241.6		491.5
	Specialty Products:
	2013		$	53.6	$	607.8	$	0.2	(3)	$	275.4	$	553.5	$	—	$	306.4	$	5.9		$	(106.3	)	$	272.2	$	509.6
	2012		75.1		561.9		0.5		(3)	318.7		604		—		347.5		(1.9		)	(150.3		)	312.1		630.9
	2011		74.6		505		0.8		(3)	291.1		549.8		—		298.3		(16.6		)	(129.1		)	281.6		571.2
	Investing, Financing and Corporate:(5)
	2013		$	—	$	—	$	—	(3)(4)	$	—	$	—	$	41.1	$	—	$	—		$	—		$	—	$	—
	2012		—		—		—		(3)(4)	—		—		53.6		—		—			—			—		—
	2011		0.2		2,490.00		266.6		(3)(4)	2.4		—		71.4		—		—			—			—		—
	__________________________________________________________
	-1	Schedule excludes activity related to discontinued operations for all periods presented.
	-2	Schedule excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013 and 2012, and related to the AutoOne Transaction as of December 31, 2011.
	-3	The amounts shown represent and/or include OneBeacon's discount on its long-term workers compensation loss and LAE reserves, as such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual basis. OneBeacon discounts these reserves using an average discount rate which is determined based on various assumptions including consideration of when the claims will be settled (3.5%, 3.5% and 4.5%, respectively, at December 31, 2013, 2012 and 2011).
	-4	The years ended December 31, 2013, 2012 and 2011 also include unamortized fair value adjustments of $136.9 million, $150.1 million, and $163.3 million, respectively, to reserves for unpaid claims and claims adjustment expenses made in purchase accounting as a result of the OneBeacon Acquisition.
	-5	Invested assets are not allocated to Specialty Industries or Specialty Products since OneBeacon does not manage its assets by segment. Net investment income related to OneBeacon's Specialty Industries and Specialty Products segments are included in the Investing, Financing and Corporate segment since these assets are available for payment of losses and expenses for all segments.

Nature_of_Operations_and_Summa1

Nature of Operations and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Investment Securities	'
	Investment Securities
	OneBeacon adopted the fair value election, which allows companies to report financial assets and liabilities at fair value, on an individual basis. The election must be made at the inception of a transaction and may not be reversed. OneBeacon believes that reporting its investment results in this manner is consistent with one of its operating principles, namely to manage investments for total return.
	OneBeacon classifies its portfolio of fixed maturity investments and common equity securities, including convertible fixed maturity investments, held for general investment purposes as trading securities. Trading securities are reported at fair value as of the balance sheet date as determined by quoted market prices when available. Realized and changes in unrealized investment gains on trading securities are reported, on a pre-tax basis, in revenues as net realized and change in unrealized investment gains.
	Short-term investments consist of money market funds, certificates of deposit and other securities which, at the time of purchase, mature or become available for use within one year. Short-term investments are carried at amortized cost, which approximated fair value as of December 31, 2013 and 2012.
	Other investments primarily include hedge funds and private equity funds. OneBeacon measures its investments in hedge funds and private equity funds at fair value with changes therein reported in total revenues as net realized and change in unrealized investment gains. Other investments also includes an investment in a community reinvestment vehicle which is accounted for at fair value and a tax advantaged federal affordable housing development fund which is accounted for under the equity method.
	Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants (an "exit price"). Fair value measurements are categorized into a hierarchy that distinguishes between inputs based on market data from independent sources ("observable inputs") and a reporting entity's internal assumptions based upon the best information available when external market data is limited or unavailable ("unobservable inputs"). Quoted prices in active markets for identical assets or liabilities have the highest priority ("Level 1"), followed by observable inputs other than quoted prices, including prices for similar but not identical assets or liabilities ("Level 2") and unobservable inputs, including the reporting entity's estimates of the assumptions that market participants would use, having the lowest priority ("Level 3").
	As of both December 31, 2013 and 2012, approximately 92% of the investment portfolio recorded at fair value was priced based upon quoted market prices or other observable inputs. Investments valued using Level 1 inputs include fixed maturity investments, primarily investments in U.S. Treasuries, common equities and short-term investments, which include U.S. Treasury Bills. Investments valued using Level 2 inputs comprise fixed maturity investments including corporate debt, state and other governmental debt, convertible fixed maturity investments and mortgage and asset-backed securities. Fair value estimates for investments that trade infrequently and have few or no observable market prices are classified as Level 3 measurements. Level 3 fair value estimates based upon unobservable inputs include OneBeacon's investments in hedge funds and private equity funds, as well as certain investments in debt and equity securities, including asset-backed securities, where quoted market prices are unavailable or are not considered reasonable. OneBeacon determines when transfers between levels have occurred as of the beginning of the period.
	OneBeacon uses brokers and outside pricing services to assist in determining fair values. For investments in active markets, OneBeacon uses the quoted market prices provided by outside pricing services to determine fair value. The outside pricing services OneBeacon uses have indicated that they will only provide prices where observable inputs are available. In circumstances where quoted market prices are unavailable or are not considered reasonable, OneBeacon estimates the fair value using industry standard pricing models and observable inputs such as benchmark interest rates, matrix pricing, market comparables, broker quotes, issuer spreads, bids, offers, credit rating prepayment speeds and other relevant inputs.
	OneBeacon's process to assess the reasonableness of the market prices obtained from the outside pricing sources covers substantially all of its fixed maturity investments and includes, but is not limited to, evaluation of model pricing methodologies, review of the pricing services' quality control processes and procedures on at least an annual basis, comparison of market prices to prices obtained from different independent pricing vendors on at least an annual basis, monthly analytical reviews of certain prices and review of assumptions utilized by the pricing service for selected measurements on an ad hoc basis throughout the year. OneBeacon also performs back-testing of selected purchases and sales activity to determine whether there are any significant differences between the market price used to value the security prior to purchase or sale and the actual purchase or sale price on at least an annual basis. Prices provided by the pricing services that vary by more than 5% and $1.0 million from the expected price based on the procedures are considered outliers. Also considered outliers are prices that haven’t changed from period to period and prices that have trended unusually compared to market conditions. In circumstances where the results of OneBeacon's review process does not appear to support the market price provided by the pricing services, OneBeacon challenges the price. During the past year, approximately 30 securities fell outside OneBeacon's expected results, thereby triggering the challenge with the pricing service. If OneBeacon cannot gain satisfactory evidence to support the challenged price, it relies upon its own pricing methodologies to estimate the fair value of the security in question.
	OneBeacon's investments in debt securities, including asset-backed securities, are generally valued using matrix and other pricing models. Key inputs include benchmark yields, benchmark securities, reported trades, issuer spreads, bids, offers, credit ratings and prepayment speeds. Income on mortgage-backed and asset-backed securities is recognized using an effective yield based on anticipated prepayments and the estimated economic life of the securities. When actual prepayments differ significantly from anticipated prepayments, the estimated economic life is recalculated and the remaining unamortized premium or discount is amortized prospectively over the remaining economic life.
	Other investments, which are primarily comprised of hedge funds and private equity funds for which the fair value option has been elected, are carried at fair value based upon OneBeacon's proportionate interest in the underlying fund's net asset value, which is deemed to approximate fair value. The fair value of OneBeacon's investments in hedge funds and private equity funds has been estimated using net asset value because it reflects the fair value of the funds' underlying investments. OneBeacon employs a number of procedures to assess the reasonableness of the fair value measurements, including obtaining and reviewing each fund's audited financial statements and discussing each fund's pricing with the fund's manager.
	In circumstances where the underlying investments are publicly traded, such as the investments made by hedge funds, the fair value of the underlying investments is determined using current market prices. In circumstances where the underlying investments are not publicly traded, such as the investments made by private equity funds, the private equity fund managers have considered the need for a liquidity discount on each of the underlying investments when determining the fund's net asset value. In circumstances where OneBeacon's portion of a fund's net asset value is deemed to differ from fair value due to illiquidity or other factors associated with OneBeacon's investment in the fund, including counterparty credit risk, the net asset value is adjusted accordingly. See Note 5—"Investment Securities."
Cash	'
	Cash
	Cash includes amounts on hand and demand deposits with banks and other financial institutions. Amounts presented in the statements of cash flows are shown net of balances acquired and sold in the purchase or sale of the Company's consolidated subsidiaries.
Insurance Operations	'
	Insurance Operations
	Premiums written are recognized as revenues and are earned ratably over the term of the related policy. Unearned premiums represent the portion of premiums written that are applicable to future insurance coverage provided by policies. MPCI net written premiums are recognized at the sales closing date, with necessary adjustments made as the Company receives acreage reports from the policyholders. Premiums written on this business are recognized as revenues and are earned ratably over the period of risk commencing with the sales closing date and ending with the crop harvest date.
	Deferred acquisition costs represent commissions, premium taxes, brokerage expenses and other costs which are directly attributable to and vary with the production of business. These costs are deferred and amortized to the extent they relate to successful contract acquisitions over the applicable premium recognition period as policy acquisition expenses. Deferred acquisition costs are limited to the amount expected to be recovered from future earned premiums and anticipated investment income. This limitation is referred to as a premium deficiency. A premium deficiency is recognized if the sum of expected loss and loss adjustment expenses ("LAE"), unamortized acquisition costs, and maintenance costs exceeds related unearned premiums and anticipated investment income. A premium deficiency is recognized by charging any unamortized acquisition costs to expense to the extent required in order to eliminate the deficiency. If the premium deficiency exceeds unamortized acquisition costs then a liability is accrued for the excess deficiency. There were no premium deficiencies recognized for any years presented.
	Loss and LAE are charged against income as incurred. Unpaid insurance loss and LAE are based on estimates (generally determined by claims adjusters, legal counsel and actuarial staff) of the ultimate costs of settling claims, including the effects of inflation and other societal and economic factors. Unpaid reinsurance loss and LAE reserves are based primarily on reports received from ceding companies and actuarial projections. Unpaid loss and LAE reserves represent management's best estimate of ultimate loss and LAE, net of estimated salvage and subrogation recoveries, if applicable. Such estimates are regularly reviewed and updated and any adjustments resulting therefrom are reflected in the current period. The process of estimating unpaid loss and LAE reserves involves a considerable degree of judgment by management and the ultimate amount of expense to be incurred could be considerably greater than or less than the amounts currently reflected in the financial statements.
	OneBeacon discounts certain of its long-term workers compensation loss and LAE reserves when such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual claim basis. OneBeacon discounts these reserves using an average discount rate which is determined based on various assumptions including consideration of when the claims will be settled (3.5% at both December 31, 2013 and 2012). As of December 31, 2013 and 2012, the discount on OneBeacon's workers' compensation loss and LAE reserves amounted to $3.0 million (excluding $61.7 million which relates to reserves that have been reclassified to liabilities held for sale on the December 31, 2013 balance sheet) and $4.6 million (excluding $77.9 million which relates to reserves that have been reclassified to liabilities held for sale on the December 31, 2012 balance sheet), respectively.
	OneBeacon's insurance subsidiaries enter into ceded reinsurance contracts from time to time to protect their businesses from losses due to concentration of risk, to manage their operating leverage ratios and to limit losses arising from catastrophic events. The majority of such reinsurance contracts are executed through excess of loss treaties and catastrophe contracts under which the reinsurer indemnifies for a specified part or all of certain types of losses over stipulated amounts arising from any one occurrence or event. OneBeacon has also entered into quota share treaties with reinsurers under which all risks meeting prescribed criteria are covered on a pro rata basis. The amount of each risk ceded by OneBeacon is subject to maximum limits which vary by line of business and type of coverage.
	Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policies. The collectibility of reinsurance recoverables is subject to the solvency of the reinsurers. OneBeacon is selective in regard to its reinsurers, principally placing reinsurance with those reinsurers with strong financial condition, reputation, industry ratings and underwriting ability. Management monitors the financial condition and ratings of its reinsurers on an ongoing basis.
	Reinsurance premiums, commissions, expense reimbursements and reserves related to reinsured business are accounted for on a basis consistent with those used in accounting for the original policies issued and the terms of the reinsurance contracts. Premiums ceded to other companies are reported as a reduction of premiums written. Expense allowances received in connection with reinsurance ceded have been accounted for as a reduction of the related policy acquisition costs and are deferred and amortized accordingly.
Accounting for Mandatory Shared Market Mechanisms	'
	Accounting for Mandatory Shared Market Mechanisms
	As a condition of its licenses to do business in certain states, OneBeacon's insurance operations are required to participate in various mandatory shared market mechanisms commonly referred to as "residual" or "involuntary" markets. These markets generally consist of risks considered to be undesirable from a standard or routine underwriting perspective. Each state dictates the levels of insurance coverage that are mandatorily assigned to participating insurers within these markets. The total amount of such business an insurer must accept in a particular state is generally based on that insurer's market share of voluntary business written within that state. In certain cases, OneBeacon is obligated to write business from shared market mechanisms at a future date based on its historical market share of all voluntary policies written within that state. Involuntary business generated from mandatory shared market mechanisms may be treated as assumed reinsurance depending on the structure of the mechanism.
	OneBeacon's market assignments are typically required to be written in the current period, although, in certain cases OneBeacon is required to accept policy assignments at a future date. Anticipated losses associated with future market assignments are recognized when the amount of such anticipated losses is determined to be probable and can be reasonably estimated.
Accounting for Insurance Related Assessments	'
	Accounting for Insurance Related Assessments
	Under existing guaranty fund laws in all states, insurers licensed to do business in those states can be assessed for certain obligations of insolvent insurance companies to policyholders and claimants. OneBeacon's insurance subsidiaries record guaranty fund assessments when such assessments are billed by the respective guaranty funds. In addition, OneBeacon will monitor other insurance company insolvencies and will accrue a liability when such losses are determined to be probable and the assessment amounts can be reasonably estimated. The actual amount of such assessments will depend upon the final outcome of rehabilitation proceedings and will be paid over several years.
Capital Lease	'
	Capital Lease
	In December 2011, the Company entered into lease financing arrangements with US Bancorp Equipment Finance, Inc. ("US Bancorp") and Fifth Third Equipment Finance Company ("Fifth Third") whereby it sold furniture and equipment and capitalized software, respectively, at a cost equal to net book value. The Company then leased the fixed assets back from US Bancorp for a lease term of five years and leased the capitalized software back from Fifth Third for a lease term of four years. See Note 16—"Commitments and Contingencies."
Deferred Software Costs	'
	Deferred Software Costs
	OneBeacon capitalizes costs related to computer software developed for internal use during the application development stage of software development projects. These costs generally consist of certain external, payroll and payroll related costs. OneBeacon begins amortization of these costs once the project or the respective phase of the project is completed and ready for its intended use. Amortization is on a straight line basis and over the useful life which generally ranges from three to five years. At December 31, 2013 and 2012, OneBeacon had unamortized deferred software costs of $11.8 million and $7.3 million, respectively.
Federal and Foreign Income Taxes	'
	Federal and Foreign Income Taxes
	The majority of the Company's subsidiaries file consolidated tax returns in the United States. Income earned or losses generated by companies outside the United States are generally subject to an overall effective tax rate lower than that imposed by the United States.
	Deferred tax assets and liabilities are recorded when a difference between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts for tax purposes exists, and for other temporary differences. The deferred tax asset or liability is recorded based on tax rates expected to be in effect when the difference reverses. The deferred tax asset is recognized when it is more likely than not that it will be realized.
Foreign Currency Exchange	'
	Foreign Currency Exchange
	The U.S. dollar is the functional currency for all of OneBeacon's businesses. OneBeacon is subject to foreign currency fluctuations associated with foreign investment securities. Assets and liabilities recorded in foreign currencies are translated into U.S. dollars at exchange rates in effect at the balance sheet date, and revenues and expenses are converted using the average exchange rates for the period. Net foreign exchange gains and losses arising from the translation are reported as a component of net income in the period in which they arise.
Variable Interest Entities	'
	Variable Interest Entities
	OneBeacon consolidates a reciprocal insurance exchange ("reciprocal") as a variable interest entity ("VIE"), which is an entity in which the equity investors do not have the characteristics of a controlling interest or do not have sufficient equity at risk for the entity to finance its activities without additional subordinated financial support from other parties. The primary beneficiary of a VIE is required to consolidate the VIE in its financial statements. The primary beneficiary is an entity that has the controlling financial interest in the VIE when it has both of the following: (1) the power to direct the activities of a variable interest entity that most significantly impact the entity’s economic performance and (2) the obligation to absorb losses of the entity that could potentially be significant to the VIE or the right to receive benefits from the entity that could potentially be significant to the VIE. A reporting entity must assess whether it has an implicit financial responsibility to ensure that a VIE operates as designed when determining if it has the power to direct the activities of the VIE that most significantly affect the entity’s economic performance. Ongoing reassessments of whether a reporting entity is the primary beneficiary of a VIE are required. See Note 13—"Variable Interest Entities ("VIE")."
Defined Benefit Plans	'
	Defined Benefit Plans
	OneBeacon sponsors defined benefit plans and recognizes the funded status of the difference between plan assets at fair value and the projected benefit obligation (for defined benefit pension plans) or the accumulated benefit obligation (for other postretirement benefit plans) in its statement of financial position. The Company recognizes amounts previously deferred and amortized in other comprehensive income in the period in which they occur and measure plan assets and obligations as of the fiscal year end. See Note 8—"Retirement Plans."
Policy Acquisition Costs	'
	Policy Acquisition Costs
	On January 1, 2012, OneBeacon adopted Accounting Standards Update (“ASU”) 2010-26, Accounting for Costs Associated with Acquiring or Renewing Insurance Contracts (Accounting Standards Codification ("ASC") 944). ASU 2010-26 changes the types of policy acquisition costs that are eligible for deferral. Specifically, ASU 2010-26 limits deferrable costs to those that are incremental direct costs of contract acquisition and certain costs related to acquisition activities performed by the insurer, such as underwriting, policy issuance and processing, medical and inspection costs and sales force contract selling. ASU 2010-26 defines incremental direct costs as those costs that result directly from and were essential to the contract acquisition and would not have been incurred absent the acquisition. Accordingly, under ASU 2010-26, deferrable acquisition costs are limited to costs related to successful contract acquisitions. Acquisition costs that are not eligible for deferral are to be charged to expense in the period incurred.
	OneBeacon adopted ASU 2010-26 prospectively. As a result of adopting ASU 2010-26, $5.6 million of unamortized deferred acquisition costs as of January 1, 2012, primarily relating to a portion of profit sharing commission that had been deferred under prior guidance, were determined to no longer be deferrable and were recognized in expense over the original amortization period. During the year ended December 31, 2012, all of the $5.6 million of unamortized acquisitions costs as of January 1, 2012 were recognized in expense.
Fair Value Measurements and Disclosure	'
	Fair Value Measurements and Disclosures
	On January 1, 2012, OneBeacon adopted ASU 2011-04, Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in US GAAP and IFRS (ASC 820). ASU 2011-04 clarifies existing guidance with respect to the concepts of highest and best use and valuation premise and measuring instruments classified within a reporting entity's shareholders' equity. ASU 2011-04 also clarifies disclosure requirements, requiring disclosure of quantitative information about unobservable inputs used in Level 3 fair value measurements. ASU 2011-04 also amends existing guidance. In circumstances where a reporting entity manages a portfolio of financial assets and liabilities based on the net market and counterparty credit risk exposures, ASU 2011-04 permits determination of the fair value of those instruments to be based on the net risk exposure. In addition, ASU 2011-04 permits the application of premiums or discounts to be applied in a fair value measurement to the extent that market participants would consider them in valuing the financial instruments. ASU 2011-04 also expands the required disclosures for Level 3 measurements, requiring that reporting entities provide a narrative description of the sensitivity of Level 3 fair value measurements to changes in unobservable inputs and the interrelationships between those inputs, if any. As a result of adopting ASU 2011-04, OneBeacon expanded its fair value disclosures. See Note 5—"Investment Securities.
Comprehensive Income	'
	Comprehensive Income
	On January 1, 2012, OneBeacon adopted ASU 2011-05, Comprehensive Income (ASC 220). ASU 2011-05 requires all components of comprehensive income to be reported in a continuous financial statement or in consecutive statements displaying the components of net income and the components of other comprehensive income. Since OneBeacon already presented comprehensive income in a continuous financial statement, adoption of ASU 2011-05 had no effect on OneBeacon's financial statement presentation.
Recently Issued Accounting Pronouncements	'
	Recently Issued Accounting Pronouncements
	Qualified Affordable Housing Projects
	On January 15, 2014 the FASB issued ASU 2014-01, Accounting for Investments in Qualified Affordable Housing Projects ("QAHP") (ASC 323). The new guidance eases the requirements for an investor to elect to account for its investment in a QAHP using the effective yield method. Prior to the issuance of the new guidance, investors had to have a letter of credit guaranteeing the availability of the tax credit allocable to the investor, had to demonstrate that the projected yield based solely on the cash flows from the guaranteed tax credits was positive and had to be a limited partner in the QAHP for both legal and tax purposes. Under the new guidance, the letter of credit requirement has been eliminated and instead, the investor must simply be able to demonstrate that the tax credit allocable to the investor will be available. Investments in QAHP not meeting the criteria in the new guidance would be accounted for under the equity method or the cost method. The election to use the effective yield method is considered an accounting policy decision that should be applied consistently to all QAHP investments. The new guidance is effective for annual and interim reporting periods beginning after December 15, 2014. The guidance should be applied retrospectively, with early adoption permitted. The Company holds an investment in a QAHP which is accounted for under the equity method and does not expect adoption to significantly affect its financial position.
	Unrecognized Tax Benefits
	On July 18, 2013, the FASB issued ASU 2013-11, Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (ASC 740). The new ASU requires balance sheet presentation of an unrecognized tax benefit as a reduction of a deferred tax asset for an NOL carryforward or tax credit carryforward rather than as a liability. The exception is in circumstances where a carryfoward is not available to settle the additional taxes that might arise upon disallowance of the tax position under the tax law of the applicable jurisdiction. Prior to the issuance of the ASU 2013-11, the guidance for unrecognized tax benefits under ASC 740 did not provide explicit guidance on whether an entity should present an unrecognized tax benefit as a liability or as a reduction of NOL carryforwards or other tax credits. In circumstances where an NOL carryforward is not available to offset settlement of any additional taxes arising from a disallowed tax position, the unrecognized tax benefit should be presented as a liability. The new guidance becomes effective for fiscal periods beginning on or after December 15, 2013 and should be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective adoption is allowed. The Company does not expect adoption to significantly affect its financial position.

Unpaid_Loss_and_Loss_Adjustmen1

Unpaid Loss and Loss Adjustment Expense ("LAE") Reserves (Tables)	12 Months Ended
	Dec. 31, 2013
Insurance Loss Reserves [Abstract]	'
Loss and loss adjustment expense (LAE) reserve activities	'
	The following table summarizes the loss and LAE reserve activities of OneBeacon's insurance subsidiaries for the years ended December 31, 2013, 2012, and 2011:
			Year ended December 31,
			2013				2012				2011
			($ in millions)
	Gross beginning balance		$	1,000.00			$	3,358.60			$	3,295.50
	Less beginning reinsurance recoverable on unpaid losses		(107.3		)		(2,167.5		)		(1,893.2		)
	Net beginning loss and LAE reserves		892.7				1,191.10				1,402.30
	Loss and LAE incurred from continuing operations relating to:
	Current year losses		622.1				657.4				578.1
	Prior year losses		—				(7.4		)		(29.8		)
	Total incurred loss and LAE from continuing operations		622.1				650				548.3
	Loss and LAE paid from continuing operations relating to:
	Current year losses		(188.6		)		(224.6		)		(216.9		)
	Prior year losses		(352.1		)		(340.5		)		(306.3		)
	Total loss and LAE payments from continuing operations		(540.7		)		(565.1		)		(523.2		)
	Net loss and LAE reserves		974.1				1,276.00				1,427.40
	Total incurred loss and LAE from discontinued operations		78.9				48.4				89.5
	Total loss and LAE payments from discontinued operations		(102.3		)		(220.8		)		(261.1		)
	Net loss and LAE reserves		950.7				1,103.60				1,255.80
	Net change in loss and LAE reserves reported in liabilities held for sale		23.4			(1)	(147.1		)	(2)	(64.7		)	(3)
	Net loss and LAE reserves sold		—				(63.8		)	(4)	—
	Net ending loss and LAE reserves		974.1				892.7				1,191.10
	Plus ending reinsurance recoverable on unpaid losses		80.2				107.3				2,167.50
	Gross ending loss and LAE reserves		$	1,054.30			$	1,000.00			$	3,358.60
	_______________________________________________________________________________
	(1)	Consists of the change in net loss and LAE reserves to $188.4 million from $211.8 million, which amounts were classified as held for sale as December 31, 2013 and 2012, respectively, in connection with the Runoff Transaction.
	(2)	Consists of the change in net loss and LAE reserves to $211.8 million from $64.7 million, which amounts were classified as held for sale as of December 31, 2012 and 2011, respectively, in connection with the Runoff Transaction and AutoOne Transaction, respectively.
	(3)	Consists of the net loss and LAE reserves of $64.7 million classified as held for sale as of December 31, 2011 in connection with the AutoOne Transaction.
	(4)	Relates to the AutoOne Transaction, which closed in February 2012.

Reinsurance_Tables

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2013
Reinsurance Disclosures [Abstract]	'
Effects of Reinsurance	'
	The effects of reinsurance on OneBeacon's insurance subsidiaries' written and earned premiums and on loss and LAE, excluding the Runoff Business, were as follows:
			Year ended December 31,
			2013					2012			2011
			($ in millions)
	Written premiums:
	Direct		$	1,103.10				$	1,204.00		$	1,079.20
	Assumed		59.8					55.2			49.1
	Ceded		(74.3		)			(80.0		)	(65.6		)
	Net written premiums		$	1,088.60				$	1,179.20		$	1,062.70
	Earned premiums:
	Direct		$	1,043.30				$	1,158.30		$	1,035.90
	Assumed		148.5			(1)		52.8			42.3
	Ceded		(71.4		)			(79.1		)	(66.0		)
	Net earned premiums		$	1,120.40				$	1,132.00		$	1,012.20
	Loss and LAE:
	Direct		$	584.9				$	687.5		$	551.8
	Assumed		76.3			(1)		29.6			9.2
	Ceded		(39.1		)			(67.1		)	(12.7		)
	Net loss and LAE		$	622.1				$	650		$	548.3
	_______________________________________________________________________________
	(1) Includes assumed earned premiums and loss and LAE of $89.1 million and $41.9 million, respectively, related to the collector cars and boats business which although exited on January 1, 2013, was assumed on a runoff basis from Essentia.
Reinsurer by S&P Rating	'
	The following table summarizes Standard & Poor's Financial Services, LLC ("Standard & Poor's") ratings for OneBeacon's reinsurers for its continuing insurance operations, excluding industry pools and associations, based upon reinsurance recoverable amounts on paid and unpaid losses and LAE:
			Balance at				% of total
			31-Dec-13
	Standard & Poor's Rating:		($ in millions)
	AA		$	26.4			30	%
	A		56.1				62	%
	BBB+, Not Rated and Other		7.4				8	%
	Total reinsurance recoverables		$	89.9			100	%
	At December 31, 2013, OneBeacon had $10.7 million of reinsurance recoverable on paid losses and $1,741.6 million (gross of $136.9 million in purchase accounting adjustments, as described above) that will become recoverable if claims are paid in accordance with current reserve estimates, related to the Runoff Business that have been reclassified to assets held for sale. Reinsurance contracts do not relieve OneBeacon of its obligations. Therefore, collectibility of balances due from reinsurers is critical to OneBeacon's financial strength. The following table provides a listing of the top reinsurers related to the Runoff Business reported in assets held for sale, excluding industry pools and associations, based on reinsurance recoverable amounts on paid and unpaid losses, the percentage of the total reported as held for sale (gross of the $136.9 million in purchase accounting adjustment), and the reinsurers' A.M. Best ratings.
	($ in millions)		Balance at				% of total			A.M. Best
			31-Dec-13							Rating(1)
	National Indemnity Company ("NICO") and General Reinsurance Corporation(2)		$	1,243.70			71	%		A++
	Hanover Insurance Company		43.9				3	%		A
	Tokio Marine and Nichido Fire(3)		26.2				1	%		A++
	Munich Reinsurance America		16.3				1	%		A+
	Tower Insurance Company		10.2				1	%		B(4)
	_______________________________________________________________________________
	(1)	A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of sixteen financial strength ratings), “A+” (Superior, which is the second highest of sixteen financial strength ratings), “A” (Excellent, which is the third highest of sixteen financial strength ratings) and “B” (Fair, which is the seventh highest of sixteen financial strength ratings).
	(2)	Includes $198.3 million of Third Party Recoverables, which NICO would pay under the terms of the NICO Cover if they are unable to collect from third party reinsurers.
	(3)	Excludes $21.8 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.
	(4)	Under review with developing implications.

Investment_Securities_Tables

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2013
Schedule of Investments [Abstract]	'
Net investment income, pre-tax	'
	Net investment income for the years ended December 31, 2013, 2012, and 2011 consisted of the following:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Fixed maturity investments		$	40.6		$	49.1		$	69.7
	Short-term investments		—			0.1			0.1
	Common equity securities		6.2			6.8			5.3
	Convertible fixed maturity investments		1.2			4.6			3.5
	Other investments		(0.1		)	(0.4		)	(0.3		)
	Gross investment income		47.9			60.2			78.3
	Less investment expenses		(6.8		)	(6.6		)	(6.9		)
	Net investment income		$	41.1		$	53.6		$	71.4
Net realized investment gains (losses), pre-tax	'
	The composition of net realized investment gains consisted of the following:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Fixed maturity investments		$	2.1		$	27.1		$	32.1
	Short-term investments		—			—			—
	Common equity securities		36.1			2.1			16.7
	Convertible fixed maturity investments		0.3			0.4			(0.4		)
	Other investments		5.7			21.2			8.4
	Net realized investment gains, pre-tax		44.2			50.8			56.8
	Income taxes		(20.5		)	(17.2		)	(19.9		)
	Net realized investment gains, after tax		$	23.7		$	33.6		$	36.9
Net unrealized investment gains (losses)	'
	The net changes in fair value for the years ended December 31, 2013, 2012, and 2011 are as follows:
			Year ended December 31, 2013
			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected
			gains(1)			translation			in revenues(1)
						gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	(34.4	)	$	—		$	(34.4	)
	Short-term investments		0.1			—			0.1
	Common equity securities		32			0.1			32.1
	Convertible fixed maturity investments		1.1			—			1.1
	Other investments		6.3			—			6.3
	Net change, pre-tax		$	5.1		$	0.1		$	5.2
			Year ended December 31, 2012
			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected
			gains(1)			translation			in revenues(1)
						gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	7.2		$	0.1		$	7.3
	Short-term investments		—			—			—
	Common equity securities		11.5			—			11.5
	Convertible fixed maturity investments		(0.4		)	—			(0.4		)
	Other investments(2)		(13.5		)	—			(13.5		)
	Net change, pre-tax		$	4.8		$	0.1		$	4.9
			Year ended December 31, 2011
			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected
			gains(1)			translation			in revenues(1)
						gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	(18.7	)	$	(0.1	)	$	(18.8	)
	Short-term investments		—			0.1			0.1
	Common equity securities		(22.1		)	(0.1		)	(22.2		)
	Convertible fixed maturity investments		(8.4		)	—			(8.4		)
	Other investments		3.1			—			3.1
	Net change, pre-tax		$	(46.1	)	$	(0.1	)	$	(46.2	)
	_______________________________________________________________________________
	(1)	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.8) million, $(0.2) million, and $(5.2) million for the years ended December 31, 2013, 2012, and 2011, respectively.
	(2)	The year ended December 31, 2012, includes a change in net unrealized gains of $(11.6) million related to the sale of a limited partnership that had been consolidated into OneBeacon's results. See Note 15—"Related Party Disclosures.
Gross unrealized investment gains or losses	'
	The components of OneBeacon's ending net unrealized investment gains and losses, excluding the impact of net unrealized foreign currency translation gains and losses, on its investment portfolio as of December 31, 2013 and 2012 were as follows:
			December 31,
			2013			2012
			($ in millions)
	Investment securities:
	Gross unrealized investment gains		$	133.7		$	127.4
	Gross unrealized investment losses		(9.3		)	(8.7		)
	Total net unrealized investment gains, pre-tax		124.4			118.7
	Income taxes		(33.8		)	(39.4		)
	Total net unrealized investment gains, after tax		$	90.6		$	79.3
Investment holdings, fixed maturity investments	'
	The cost or amortized cost, gross unrealized pre-tax investment gains and losses, net foreign currency losses and carrying values of OneBeacon's fixed maturity investments as of December 31, 2013 and 2012 were as follows:
			December 31, 2013(1)
			Cost or			Gross			Gross			Net foreign			Carrying
			amortized			unrealized			unrealized			currency			value
			cost			gains			losses			losses
			($ in millions)
	U.S. Government and agency obligations		$	131		$	0.2		$	(0.1	)	$	—		$	131.1
	Debt securities issued by industrial corporations		741.7			14.1			(1.3		)	—			754.5
	Municipal obligations		16.9			—			(0.4		)	—			16.5
	Asset-backed securities		951.5			1.2			(3.2		)	—			949.5
	Foreign government obligations		2			0.3			—			—			2.3
	Preferred stocks		78.3			5			—			—			83.3
	Total fixed maturity investments		$	1,921.40		$	20.8		$	(5.0	)	$	—		$	1,937.20
			December 31, 2012(1)
			Cost or			Gross			Gross			Net foreign			Carrying
			amortized			unrealized			unrealized			currency			value
			cost			gains			losses			losses
			($ in millions)
	U.S. Government and agency obligations		$	197.1		$	0.5		$	—		$	—		$	197.6
	Debt securities issued by industrial corporations		678.7			32.8			—			—			711.5
	Municipal obligations		3.3			—			(0.1		)	—			3.2
	Asset-backed securities		918.5			9.7			(0.3		)	—			927.9
	Foreign government obligations		6			0.5			—			—			6.5
	Preferred stocks		78.3			6.4			—			—			84.7
	Total fixed maturity investments		$	1,881.90		$	49.9		$	(0.4	)	$	—		$	1,931.40
	_______________________________________________________________________________
	(1)	Includes carrying value of $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheets as part of the Runoff Transaction as of December 31, 2013 and 2012, respectively.
Investments Classified by Contractual Maturity Date	'
	The cost or amortized cost and carrying value of OneBeacon's fixed maturity investments and convertible fixed maturity investments at December 31, 2013 is presented below by contractual maturity. Actual maturities could differ from contractual maturities because borrowers may have the right to call or prepay certain obligations with or without call or prepayment penalties.
			December 31, 2013(1)
			Cost or			Carrying
			amortized			value
			cost
			($ in millions)
	Due in one year or less		$	184.9		$	188
	Due after one year through five years		664.7			674.6
	Due after five years through ten years		59.1			61.3
	Due after ten years		9.2			11
	Asset-backed securities		951.5			949.5
	Preferred stocks		78.3			83.3
	Total		$	1,947.70		$	1,967.70
	_______________________________________________________________________________
	-1	Includes carrying value of $236.3 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheet as part of the Runoff Transaction as of December 31, 2013
Investment holdings, common equity securities, convertible fixed maturity investments and other investments	'
	The cost or amortized cost, gross unrealized pre-tax investment gains and losses, net foreign currency gains and carrying values of OneBeacon's common equity securities, convertible fixed maturity investments and other investments as of December 31, 2013 and 2012 were as follows:
			31-Dec-13
			Cost or			Gross			Gross			Net foreign			Carrying
			amortized			unrealized			unrealized			currency			value
			cost			gains			losses			gains
			($ in millions)
	Common equity securities		$	267.2		$	70.4		$	(0.9	)	$	0.2		$	336.9
	Convertible fixed maturity investments		26.3			4.5			(0.3		)	—			30.5
	Other investments		104.7			38			(3.1		)	—			139.6
	Total common equity securities, convertible fixed maturity investments and other investments		$	398.2		$	112.9		$	(4.3	)	$	0.2		$	507
			31-Dec-12
			Cost or			Gross			Gross			Net foreign			Carrying
			amortized			unrealized			unrealized			currency			value
			cost			gains			losses			gains
			($ in millions)
	Common equity securities		$	221.6		$	39.8		$	(2.4	)	$	—		$	259
	Convertible fixed maturity investments		59.5			3.2			(0.1		)	—			62.6
	Other investments		115.1			34.5			(5.8		)	—			143.8
	Total common equity securities, convertible fixed maturity investments and other investments		$	396.2		$	77.5		$	(8.3	)	$	—		$	465.4
Fair value measurements by level, investment securities	'
	The following tables summarize the Company's fair value measurements for investments at December 31, 2013 and 2012 by level:
			Fair value at			Level 1			Level 2			Level 3
			December 31, 2013(2)
			($ in millions)
	Fixed maturity investments:
	U.S. Government and agency obligations		$	131.1		$	131.1		$	—		$	—
	Debt securities issued by corporations:
	Consumer		239.6			—			239.6			—
	Industrial		106			—			106			—
	Financial		145.5			—			145.5			—
	Communications		91.5			—			91.5			—
	Energy		40.5			—			40.5			—
	Basic materials		64.4			—			64.4			—
	Utilities		38			—			38			—
	Technology		29			—			29			—
	Debt securities issued by corporations		754.5			—			754.5			—
	Municipal obligations		16.5			—			16.5			—
	Asset-backed securities		949.5			—			938.6			10.9
	Foreign government obligations		2.3			1.6			0.7			—
	Preferred stocks		83.3			—			12.3			71
	Fixed maturity investments		1,937.20			132.7			1,722.60			81.9
	Short-term investments		157			155.9			1.1			—
	Common equity securities:
	Financials		83.3			83.2			—			0.1
	Basic Materials		20.3			20.3			—			—
	Consumer		122.6			122.6			—			—
	Energy		34.7			34.7			—			—
	Utilities		8.3			8.3			—			—
	Other		67.7			67.7			—			—
	Common equity securities		336.9			336.8			—			0.1
	Convertible fixed maturity investments		30.5			—			30.5			—
	Other investments(1)		119.9			—			—			119.9
	Total(1)		$	2,581.50		$	625.4		$	1,754.20		$	201.9
	_______________________________________________________________________________
	-1	Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of December 31, 2013.
	-2	Fair value includes $236.3 million of fixed maturity investments reclassified to assets held for sale in the December 31, 2013 consolidated balance sheets as part of the Runoff Transaction.
			Fair value at			Level 1			Level 2			Level 3
			December 31, 2012(2)
			($ in millions)
	Fixed maturity investments:
	U.S. Government and agency obligations		$	197.6		$	197.6		$	—		$	—
	Debt securities issued by corporations:
	Consumer		249.9			—			249.9			—
	Industrial		103.5			—			103.5			—
	Financial		92.3			—			92.3			—
	Communications		70			—			70			—
	Energy		57.1			—			57.1			—
	Basic materials		77.8			—			77.8			—
	Utilities		46.5			—			46.5			—
	Technology		14.4			—			14.4			—
	Debt securities issued by corporations		711.5			—			711.5			—
	Municipal obligations		3.2			—			3.2			—
	Asset-backed securities		927.9			—			922.6			5.3
	Foreign government obligations		6.5			5.8			0.7			—
	Preferred stocks		84.7			—			13.9			70.8
	Fixed maturity investments		1,931.40			203.4			1,651.90			76.1
	Short-term investments		232.8			232.8			—			—
	Common equity securities:
	Financials		48.4			48.3			—			0.1
	Basic Materials		42.9			42.9			—			—
	Consumer		80.5			80.5			—			—
	Energy		38.1			38.1			—			—
	Utilities		17			17			—			—
Other		32.1			32.1			—			—
Common equity securities		259			258.9			—			0.1
Convertible fixed maturity investments		62.6			—			62.6			—
Other investments(1)		122.7			—			—			122.7
Total(1)		$	2,608.50		$	695.1		$	1,714.50		$	198.9
_______________________________________________________________________________
-1	Excludes the carrying value of $21.1 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of December 31, 2012.
-2	Fair value includes $338.1 million of fixed maturity investments reclassified to assets held for sale in the December 31, 2012 consolidated balance sheets as part of the Runoff Transaction.
Debt securities, credit ratings	'
	The following table summarizes the ratings of OneBeacon's corporate debt securities as of December 31, 2013 and 2012:
			December 31,
			2013			2012
			($ in millions)
	AA		$	51.2		$	37
	A		357.9			301.6
	BBB		345.4			362.5
	BB		—			7
	Other		—			3.4
	Debt securities issued by corporations		$	754.5		$	711.5
Fair Value Assets Measured on Recurring Basis Measurement Input Reconciliation	'
	The changes in Level 1 fair value measurements for the year ended December 31, 2013 are as follows:
			Fixed			Common			Convertible			Other			Total(1)
			maturity			equity			fixed			investments
			investments			securities			maturity
									investments
			($ in millions)
	Balance at January 1, 2013		$	203.4		$	258.9		$	—		$	—		$	462.3
	Amortization/accretion		(0.1		)	—			—			—			(0.1		)
	Net realized and unrealized gains (losses)		(2.4		)	68.2			—			—			65.8
	Purchases		179			206.4			—			—			385.4
	Sales		(247.2		)	(196.7		)	—			—			(443.9		)
	Transfers in		—			—			—			—			—
	Transfers out		—			—			—			—			—
	Balance at December 31, 2013		$	132.7		$	336.8		$	—		$	—		$	469.5
	_______________________________________________________________________________
	(1)	Excludes short-term investments which are deemed to have a Level 1 designation.
	The changes in Level 2 fair value measurements for the year ended December 31, 2013 are as follows:
			Fixed			Common			Convertible			Other			Total(1)
			maturity			equity			fixed			investments
			investments			securities			maturity
									investments
			($ in millions)
	Balance at January 1, 2013		$	1,651.90		$	—		$	62.6		$	—		$	1,714.50
	Amortization/accretion		(11.3		)	—			(0.1		)	—			(11.4		)
	Net realized and unrealized gains (losses)		(28.3		)	—			1.4			—			(26.9		)
	Purchases		2,174.10			—			14.4			—			2,188.50
	Sales		(2,077.4		)	—			(47.8		)	—			(2,125.2		)
	Transfers in		74.9			—			—			—			74.9
	Transfer out		(61.3		)	—			—			—			(61.3		)
	Balance at December 31, 2013		$	1,722.60		$	—		$	30.5		$	—		$	1,753.10
	_______________________________________________________________________________
	-1	Excludes short-term investments which are deemed to have a Level 2 designation.
	The changes in Level 3 fair value measurements for the year ended December 31, 2013 are as follows:
			Fixed			Common			Convertible			Other			Total(1)
			maturity			equity			fixed			investments(1)
			investments			securities			maturity
									investments
			($ in millions)
	Balance at January 1, 2013		$	76.1		$	0.1		$	—		$	122.7		$	198.9
	Amortization/accretion		—			—			—			—			—
	Net realized and unrealized gains (losses)		(1.5		)	—			—			11.9			10.4
	Purchases		21.4			—			—			7.1			28.5
	Sales		(0.5		)	—			—			(21.8		)	(22.3		)
	Transfers in		61.3			—			—			—			61.3
	Transfers out		(74.9		)	—			—			—			(74.9		)
	Balance at December 31, 2013		$	81.9		$	0.1		$	—		$	119.9		$	201.9
	_______________________________________________________________________________
	(1)	Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method.
Fair value, significant unobservable inputs	'
	The following summarizes significant unobservable inputs used in estimating the fair value of fixed maturity securities classified within Level 3 other than hedge funds and private equities at December 31, 2013. The fair value of investments in hedge funds and private equity funds, which are classified with Level 3, are estimated using the net asset value of the funds.
	Description		Fair Value			Rating(1)		Valuation Technique	Unobservable Inputs		Range(2)
	Preferred stock		$	71		N/R		Discounted cash flow	Discount yield		7.40%
	Asset-backed securities		$	10.9		AA+		Broker pricing	Broker quote		N/A
	_________________________________________________________________________
	(1)	Credit ratings are assigned based on the following hierarchy: 1) Standard & Poor's and 2) Moody's Investors Service, Inc. ("Moody's")
	(2) As of December 31, 2013, each asset type consists of one security.
Net unrealized gains or losses for Level 3 investments	'
	The following table summarizes the change in net unrealized gains or losses for assets designated as Level 3 for the year ended December 31, 2013, 2012, and 2011:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Fixed maturity investments		$	(1.5	)	$	6.7		$	(7.5	)
	Short-term investments		—			—			—
	Common equity securities		—			—			—
	Convertible fixed maturity investments		—			—			—
	Other investments		6.3			(13.5		)	3.1
	Total		$	4.8		$	(6.8	)	$	(4.4	)
Mortgage-backed and asset-backed securities	'
	The following table summarizes the carrying value of OneBeacon's asset-backed securities as of December 31, 2013 and 2012:
			December 31,
			2013									2012
			Fair Value			Level 2			Level 3			Fair Value			Level 2			Level 3
			($ in millions)
	Mortgage-backed securities:
	Agency:
	GNMA		$	321.8		$	321.8		$	—		$	551.2		$	551.2		$	—
	FNMA		36.5			36.5			—			13.9			13.9			—
	FHLMC		24.3			24.3			—			10.5			10.5			—
	Total agency(1)		382.6			382.6			—			575.6			575.6			—
	Non-agency:
	Residential		51.5			51.5			—			38.1			38.1			—
	Commercial		155			155			—			175.4			175.4			—
	Total Non-agency		206.5			206.5			—			213.5			213.5			—
	Total mortgage-backed securities		589.1			589.1			—			789.1			789.1			—
	Other asset-backed securities:
	Credit card receivables		124.2			113.3			10.9			49			43.7			5.3
	Vehicle receivables		231.8			231.8			—			81.5			81.5			—
	Other		4.4			4.4			—			8.3			8.3			—
	Total other asset-backed securities		360.4			349.5			10.9			138.8			133.5			5.3
	Total asset-backed securities		$	949.5		$	938.6		$	10.9		$	927.9		$	922.6		$	5.3
	_______________________________________________________________________________
	(1)	Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e., GNMA) or are guaranteed by a government sponsored entity (i.e., FNMA, FHLMC).
Schedule of security issuance years of investments in non-agency RMBS and non-agency CMBS securities	'
	The security issuance years of OneBeacon's investments in non-agency RMBS and non-agency CMBS securities as of December 31, 2013 are as follows:
			Fair Value			Security Issuance Year
						2004			2005			2006			2007			2008		2010		2011		2012		2013
			($ in millions)
	Total non-agency RMBS		$	51.5		$	6.8		$	10.1		$	5.4		$	—		$	—	$	11.1	$	—	$	—	$	18.1
	Total non-agency CMBS		155			—			—			—			1.4			16		5.8		15.7		81.8		34.3
	Total non-agency		$	206.5		$	6.8		$	10.1		$	5.4		$	1.4		$	16	$	16.9	$	15.7	$	81.8	$	52.4
Non-agency RMBS, collateral quality and tranche levels	'
	The classification of the underlying collateral quality and the tranche levels of OneBeacon's non-agency RMBS securities are as follows as of December 31, 2013:
			Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
			($ in millions)
	Prime		$	44.2		$	12.7		$	31.5		$	—
	Non-prime		7.3			—			7.3			—
	Total non-agency RMBS		$	51.5		$	12.7		$	38.8		$	—
	_______________________________________________________________________________
	(1)	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch Ratings ("Fitch") and were senior to other AAA or Aaa bonds.
	(2)	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were senior to non-AAA or non-Aaa bonds.
	(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were junior to other bonds.
Non-agency CMBS, type of interest rate and tranche levels	'
	The amount of fixed and floating rate securities and their tranche levels are as follows as of December 31, 2013:
			Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
			($ in millions)
	Fixed rate CMBS		$	113.9		$	84.3		$	24.9		$	4.7
	Floating rate CMBS		41.1			1.4			16			23.7
	Total non-agency CMBS		$	155		$	85.7		$	40.9		$	28.4
	_______________________________________________________________________________
	(1)	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's or AAA by Fitch and were senior to other AAA or Aaa bonds.
	(2)	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were senior to non-AAA or non-Aaa bonds.
	(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's, Aaa by Moody's, or AAA by Fitch and were junior to other bonds.
Other investments	'
	The following table summarizes investments in hedge funds and private equity funds at December 31, 2013 and 2012:
			December 31,
			2013						2012
			Fair			Unfunded			Fair			Unfunded
			Value			Commitments			Value			Commitments
			($ in millions)
	Hedge funds
	Long/short credit and distressed		$	8.1		$	—		$	8.4		$	—
	Long bank loan		0.1			—			0.1			—
	Long/short equity		39.1			—			36.8			—
	Long/short equity activist		2.1			—			2			—
	Total hedge funds		49.4			—			47.3			—
	Private equity funds
	Insurance		2.3			0.1			3.1			0.1
	Distressed residential real estate		0.2			—			7.9			—
	Energy infrastructure and services		26			5.9			20.7			7.5
	Healthcare		2.8			1.4			2.1			2.7
	Multi-sector		14.5			2.5			14.9			3.7
	Private equity secondaries		6.4			2.1			7			2
	Real estate		3.9			0.1			5.6			0.1
	Total private equity funds		56.1			12.1			61.3			16.1
	Total hedge funds and private equity funds(1)		$	105.5		$	12.1		$	108.6		$	16.1
	_______________________________________________________________________________
	(1)	Excluded from the above table are other investments totaling $14.4 million and $14.1 million of an investment in a community reinvestment vehicle as of December 31, 2013 and 2012, respectively, and $19.7 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of December 31, 2013 and 2012, respectively.
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	'
	The following summarizes the December 31, 2013 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:
			Hedge Funds—Active Funds
			30 - 59 days			60 - 89 days			90 - 119 days			120+ days			Total
			notice			notice			notice			notice
			($ in millions)
	Redemption frequency
	Monthly		$	—		$	—		$	—		$	5.8		$	5.8
	Quarterly		26.9			8.1			6.4			—			41.4
	Annual		—			—			2.1			0.1			2.2
	Total hedge funds		$	26.9		$	8.1		$	8.5		$	5.9		$	49.4
Fair Value of private equity funds subject to lock-up periods	'
	At December 31, 2013, investments in private equity funds were subject to lock-up periods as follows:
			1 - 3 years			3 - 5 years			5 - 10 years			>10 years			Total
			($ in millions)
	Private Equity Funds—expected lock-up period remaining		$	2.5		$	20.2		$	33.4		$	—		$	56.1

Debt_Tables

Debt (Tables)	12 Months Ended
	Dec. 31, 2013
Debt Disclosure [Abstract]	'
Schedule of debt outstanding	'
	OneBeacon's debt outstanding as of December 31, 2013 and 2012 consisted of the following:
		December 31,
		2013			2012
		($ in millions)
	Senior unsecured notes, at face value	$	275		$	275
	Unamortized original issue discount	(0.3		)	(0.3		)
	Senior unsecured notes, carrying value	$	274.7		$	274.7

Income_Taxes_Income_Taxes_Tabl

Income Taxes Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]	'
Schedule of Components of Income Tax Expense (Benefit)	'
	The total income tax expense from continuing operations for the years ended December 31, 2013, 2012 and 2011 consisted of the following:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Current tax (expense) benefit:
	Federal		$	(20.3	)	$	10.1		$	18.5
	State		(0.8		)	(1.5		)	(1.6		)
	Non-U.S.		(0.4		)	(0.6		)	(0.1		)
	Total current tax (expense) benefit		(21.5		)	8			16.8
	Deferred tax expense:
	Federal		(12.8		)	(16.4		)	(31.6		)
	State		—			—			—
	Non-U.S.		—			—			—
	Total deferred tax expense		(12.8		)	(16.4		)	(31.6		)
	Total income tax expense		$	(34.3	)	$	(8.4	)	$	(14.8	)
Schedule of Effective Income Tax Rate Reconciliation	'
	A reconciliation of taxes calculated using the 35% U.S. statutory rate (the tax rate at which the majority of OneBeacon's worldwide operations are taxed) to the income tax expense on pre-tax income from continuing operations follows:
			Year ended December 31,
			2013			2012			2011
			($ in millions)
	Tax expense at the U.S. statutory rate		$	(63.5	)	$	(37.1	)	$	(41.9	)
	Differences in taxes resulting from:
	Non-U.S. earnings, net of foreign taxes		20.9			26			25.5
	Tax reserve adjustments		(1.0		)	(1.2		)	(1.9		)
	Tax exempt interest and dividends		2.4			2.3			2.1
	Change in valuation allowance		5.9			(0.7		)	(0.7		)
	Other, net		1			2.3			2.1
	Total income tax expense on pre-tax income from continuing operations		$	(34.3	)	$	(8.4	)	$	(14.8	)
Schedule of Deferred Tax Assets and Liabilities	'
	An outline of the significant components of OneBeacon's deferred tax assets and liabilities follows:
			December 31,
			2013			2012
			($ in millions)
	Deferred income tax assets related to:
	Discounting of loss and LAE reserves		$	35.3		$	41.6
	Unearned premiums		31.6			39.8
	Sale of Runoff Business		24.2			49.2
	Compensation and bonus accruals		20.3			15.6
	U.S. net operating loss carryforwards		13			42.6
	Tax credit carryforwards		11.1			6.2
	Investment basis differences		8.9			4.6
	Deferred compensation plans		8			8.4
	Fixed assets		4.7			3
	Accrued rent		3.6			3.7
	Allowance for doubtful accounts		1.1			1.2
	Other accrued compensation		0.5			0.7
	Non-U.S. net operating loss carryforwards		0.3			0.2
	Involuntary pool and guaranty fund accruals		0.1			0.1
	Pension and benefit accruals		—			8
	Other items		1.5			9.2
	Total gross deferred income tax assets		164.2			234.1
	Less valuation allowance		(1.9		)	(7.8		)
	Total net deferred income tax assets		162.3			226.3
	Deferred income tax liabilities related to:
	Net unrealized investment gains		33.8			39.4
	Deferred acquisition costs		25.2			43.4
	Prepaid pension		6.3			—
	Other items		6.4			5.7
	Total deferred income tax liabilities		71.7			88.5
	Net deferred tax asset		$	90.6		$	137.8
Summary of Operating Loss Carryforwards	'
	Net operating loss carryforwards as of December 31, 2013 and the expiration dates are as follows:
			December 31, 2013
			United States			Luxembourg			Total
			($ in millions)
	Net operating loss carryforwards by year:
	2014		$	—		$	—		$	—
	From years 2015 to 2022		1.3			—			1.3
	From years 2023 to 2033		35.8			—			35.8
	No expiration date		—			1			1
	Total net operating loss carryforwards		$	37.1		$	1		$	38.1
			December 31, 2013
			United States			Luxembourg			Total
			($ in millions)
	Gross deferred tax asset		$	13		$	0.3		$	13.3
	Valuation allowance		(1.3		)	(0.3		)	(1.6		)
	Net deferred tax asset		$	11.7		$	—		$	11.7
Schedule of Unrecognized Tax Benefits Roll Forward	'
	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:
			Permanent			Temporary			Interest and			Total
			differences(1)			differences(2)			penalties(3)
			($ in millions)
	January 1, 2011		$	7.6		$	58.5		$	4.7		$	70.8
	Changes in prior year tax positions		—			—			1.9			1.9
	Tax positions taken during the current year		—			(21.0		)	—			(21.0		)
	Lapse in statute of limitations		—			—			—			—
	Settlements with tax authorities		—			—			—			—
	December 31, 2011		$	7.6		$	37.5		$	6.6		$	51.7
	Changes in prior year tax positions		—			—			1.2			1.2
	Tax positions taken during the current year		—			(13.9		)	—			(13.9		)
	Lapse in statute of limitations		—			—			—			—
	Settlements with tax authorities		—			—			—			—
	December 31, 2012		$	7.6		$	23.6		$	7.8		$	39
	Changes in prior year tax positions		—			—			1			1
	Tax positions taken during the current year		—			(7.0		)	—			(7.0		)
	Lapse in statute of limitations		—			—			—			—
	Settlements with tax authorities		—			—			(0.4		)	(0.4		)
	December 31, 2013		$	7.6		$	16.6		$	8.4		$	32.6
	_______________________________________________________________________________
	-1	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.
	-2	Represents the amount of unrecognized tax benefits that, if recognized, would create a temporary difference between the reported amount of an item in OneBeacon's Consolidated Balance Sheet and its tax basis.
	-3	Net of tax benefit.

Retirement_Plans_Tables

Retirement Plans (Tables)	12 Months Ended
	Dec. 31, 2013
Compensation and Retirement Disclosure [Abstract]	'
Schedule of Net Funded Status	'
	The following tables set forth the obligations and funded status, assumptions, plan assets and cash flows associated with the Plans at December 31, 2013 and 2012:
			December 31,
			2013				2012
			($ in millions)
	Change in projected benefit obligation:
	Projected benefit obligation at beginning of year		$	119.5			$	113
	Service cost		0.8				0.7
	Interest cost		4.2				4.7
	Settlement gain		—				(0.7		)
	Special termination benefits expense		0.3				0.6
	Assumption changes		(13.0		)		11.2
	Actuarial gain		(0.5		)		(0.2		)
	Benefits and expenses paid with plan assets		(6.2		)		(7.5		)
	Benefits paid directly by OneBeacon		(2.2		)		(2.3		)
	Projected benefit obligation at end of year		$	102.9			$	119.5
	Change in plan assets:
	Fair value of plan assets at beginning of year		$	124.7			$	120.8
	Actual return on plan assets		24.3				11.4
	Employer contributions		—				—
	Benefits and expenses paid		(6.2		)		(7.5		)
	Fair value of plan assets at end of year		$	142.8			$	124.7
	Funded status at end of year		$	39.9			$	5.2
Schedule of Amounts Recognized in Balance Sheet	'
	Amounts recognized in the financial statements as of December 31, 2013 and 2012 consist of:
			December 31,
			2013				2012
			($ in millions)
	Net balance sheet asset recorded in other assets		$	64.1			$	32.8
	Net balance sheet liability recorded in other liabilities		(24.2		)		(27.6		)
	Net amount recognized in the financial statements		$	39.9			$	5.2
Schedule of components of net periodic benefit cost (income)	'
	The components of net periodic benefit cost for the years ended December 31, 2013, 2012 and 2011 were as follows:
			December 31,
			2013				2012			2011
			($ in millions)
	Service cost		$	0.8			$	0.7		$	0.8
	Interest cost		4.2				4.7			5.2
	Expected return on plan assets		(7.1		)		(6.9		)	(7.6		)
	Amortization of prior service benefit		—				—			—
	Amortization of net actuarial losses recognized during the year		0.9				0.8			0.5
	Net periodic pension cost (income) before settlements, curtailments and special termination benefits		(1.2		)		(0.7		)	(1.1		)
	Settlement loss		—				0.6			0.5
	Special termination benefits expense(1)		0.3				0.6			0.8
	Total net periodic benefit cost (income)		$	(0.9	)		$	0.5		$	0.2
	_______________________________________________________________________________
	-1	Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.
Schedule of Accumulated Benefit Obligations in Excess of Fair Value of Plan Assets	'
	Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets, was as follows:
			December 31,
			2013				2012
			($ in millions)
	Projected benefit obligation		$	24.2			$	27.6
	Accumulated benefit obligation		$	24.2			$	27.6
	Fair value of plan assets		$	—			$	—
	Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets, was as follows:
			December 31,
			2013				2012
			($ in millions)
	Projected benefit obligation		$	78.7			$	91.9
	Accumulated benefit obligation		$	78.7			$	91.9
	Fair value of plan net assets		$	142.8			$	124.7
Schedule of Amounts Recognized in Other Comprehensive Income (Loss)	'
	The amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis for the years ended December 31, 2013 and 2012 were as follows:
			December 31,
			2013				2012
			($ in millions)
	Accumulated other comprehensive loss at beginning of year		$	(21.2	)		$	(16.7	)
	Increase (decrease) in accumulated other comprehensive income (loss):
	Amortization of net actuarial losses recognized during the year		0.9				0.8
	Net actuarial gains (losses) occurring during the year(1)		30.8				(5.9		)
	Other adjustments		—				0.6
	Accumulated other comprehensive income (loss) at end of year		$	10.5			$	(21.2	)
	_______________________________________________________________________________
	-1	Net actuarial gains (losses) resulted from investment returns and demographic experience different than assumed, as well as changes in assumptions in estimating the projected benefit obligation in the years ended December 31, 2013 and 2012.
Schedule of Expected Benefit Payments	'
	The following benefit payments, which reflect expected future service, as appropriate, are expected to be paid:
		Expected Benefit
		Payments
		($ in millions)
	2014	$	5.1
	2015	5.4
	2016	5.6
	2017	5.8
	2018	6.1
	2019 - 2023	33.9
Schedule of Defined Benefit Plans Disclosures	'
	The weighted average assumptions used to determine benefit obligations at December 31, 2013 and 2012 were as follows:
			December 31,
			2013			2012
	Discount rate		4.66	%		3.64	%
	The weighted average assumptions used to determine net periodic benefit cost for the years ended December 31, 2013 and 2012 were as follows:
			December 31,
			2013			2012
	Discount rate		3.64	%		4.38	%
	Expected long-term rate of return on plan assets		5.75	%		5.75	%
Schedule of Allocation of Plan Assets	'
	The fair value of the Qualified Plan's assets and their related inputs at December 31, 2013 and 2012 by asset category were as follows:
			Fair value at				Level 1			Level 2			Level 3
			December 31, 2013
	Fixed maturity investments		$	—			$	—		$	—		$	—
	Short-term investments		8.9				8.8			0.1			—
	Common equity securities:
	Financials		15.8				15.8			—			—
	Basic Materials		9.1				9.1			—			—
	Consumer		31.6				31.6			—			—
	Energy		15.6				15.6			—			—
	Utilities		4.2				4.2			—			—
	Other		27				27			—			—
	Common equity securities		103.3				103.3			—			—
	Convertible fixed maturity investments:
	Financials		2				—			2			—
	Basic Materials		7.4				—			7.4			—
	Consumer		10.4				—			10.4			—
	Energy		0.6				—			0.6			—
	Utilities		—				—			—			—
	Other		9.5				—			9.5			—
	Convertible fixed maturity investments		29.9				—			29.9			—
	Total		$	142.1			$	112.1		$	30		$	—
			Fair value at				Level 1			Level 2			Level 3
			December 31, 2012
	Fixed maturity investments:
	Utilities		$	1.9			$	—		$	1.9		$	—
	Fixed maturity investments		1.9				—			1.9			—
	Short-term investments		8.1				8.1			—			—
	Common equity securities:
	Financials		10.7				10.7			—			—
	Basic Materials		10.7				10.7			—			—
	Consumer		26.6				26.6			—			—
	Energy		12				12			—			—
	Utilities		1.9				1.9			—			—
	Other		17.6				17.6			—			—
	Common equity securities		79.5				79.5			—			—
	Convertible fixed maturity investments:
	Financials		0.9				—			0.9			—
	Basic Materials		10.6				—			10.6			—
	Consumer		16.7				—			16.7			—
	Energy		0.1				—			0.1			—
	Utilities		—				—			—			—
	Other		6.2				—			6.2			—
	Convertible fixed maturity investments		34.5				—			34.5			—
	Total		$	124			$	87.6		$	36.4		$	—
	The Qualified Plan's asset allocations at December 31, 2013 and 2012 by asset category were as follows:
			Plan Assets at
			December 31,
			2013			2012
	Fixed maturity investments		—	%		1.5	%
	Common equity securities		72.7			64.2
	Convertible fixed maturity investments		21.1			27.8
	Cash and short-term investments		6.2			6.5
	Total		100	%		100	%

Employee_ShareBased_Incentive_1

Employee Share-Based Incentive Compensation Plans (Tables)	12 Months Ended
	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'
Summary of performance share activity	'
	The following summarizes performance share activity for OB Performance Shares for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013					2012					2011
			Target OB		Accrued			Target OB		Accrued			Target OB		Accrued
			Performance		expense			Performance		expense			Performance		expense
			Shares					Shares					Shares
			outstanding					outstanding					outstanding
			($ in millions)
	Beginning of period		563,190		$	1.2		642,667		$	9.7		1,464,295		$	18.5
	Payments and deferrals(1)		(238,658	)	—			(258,901	)	(7.7		)	(936,150	)	(10.5		)
	New awards		179,000		—			181,290		—			194,900		—
	Forfeitures and net change in assumed forfeitures		(10,111	)	(0.1		)	(1,866	)	—			(80,378	)	(0.5		)
	Expense recognized		—		2.9			—		(0.8		)	—		2.2
	End of period		493,421		$	4		563,190		$	1.2		642,667		$	9.7
	_______________________________________________________________________________
	(1)	No payments were made in 2013 for the 2010-2012 performance cycle as the performance factor was zero. Performance share payments in 2012 for the 2009-2011 performance cycle were based upon a performance factor of 138.6%.
Summary of performance shares outstanding and accrued expense for performance shares awarded under the OB Incentive Plan	'
	The following summarizes performance shares outstanding and accrued performance share expense at December 31, 2013 for each performance cycle:
			Target OB		Accrued
			Performance		expense
			Shares
			outstanding
			($ in millions)
	Performance cycle:
	2011 - 2013		142,138		$	1
	2012 - 2014		181,290		2.1
	2013 - 2015		179,000		1
	Subtotal		502,428		4.1
	Assumed forfeitures		(9,007	)	(0.1		)
	Total at December 31, 2013		493,421		$	4
Summary of restricted shares activity	'
	The following summarizes restricted shares activity for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013					2012					2011
			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
			($ in millions)
	Beginning of period		927,000		$	9.6		630,000		$	7.7		—		$	—
	New awards		—		—			300,000		4.5			630,000		8.6
	Forfeitures		(3,000	)	—			(2,333	)	—			—		—
	Vested		(9,000	)	—			(667	)	—			—		—
	Expense recognized		—		(3.1		)	—		(2.6		)	—		(0.9		)
	End of period		915,000		$	6.5		927,000		$	9.6		630,000		$	7.7

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2013
Segment Reporting [Abstract]	'
Financial information for OneBeacon's segments	'
	Financial information for OneBeacon's reportable segments is as follows:
			Insurance Operations						Investing,
									Financing
									and
			Specialty			Specialty			Corporate			Consolidated
			Products			Industries
			($ in millions)
	Year ended December 31, 2013
	Earned premiums		$	553.5		$	566.9		$	—		$	1,120.40
	Loss and loss adjustment expense		(312.3		)	(309.8		)	—			(622.1		)
	Policy acquisition expenses		(106.3		)	(102.6		)	—			(208.9		)
	Other underwriting expenses		(97.4		)	(107.4		)	—			(204.8		)
	Total underwriting income		37.5			47.1			—			84.6
	Net investment income		—			—			41.1			41.1
	Net realized and change in unrealized investment gains		—			—			49.4			49.4
	Net other revenues		0.3			1.1			29.8			31.2
	General and administrative expenses		—			(2.4		)	(9.6		)	(12.0		)
	Interest expense		—			—			(13.0		)	(13.0		)
	Pre-tax income from continuing operations		$	37.8		$	45.8		$	97.7		$	181.3
	Year ended December 31, 2012
	Earned premiums		$	604		$	528		$	—		$	1,132.00
	Loss and loss adjustment expense		(345.6		)	(304.4		)	—			(650.0		)
	Policy acquisition expenses		(150.3		)	(99.1		)	—			(249.4		)
	Other underwriting expenses		(96.2		)	(109.0		)	—			(205.2		)
	Total underwriting income		11.9			15.5			—			27.4
	Net investment income		—			—			53.6			53.6
	Net realized and change in unrealized investment gains		—			—			55.7			55.7
	Net other revenues (expenses)		0.4			(0.8		)	(0.1		)	(0.5		)
	General and administrative expenses		—			(1.9		)	(11.5		)	(13.4		)
	Interest expense		—			—			(16.9		)	(16.9		)
	Pre-tax income from continuing operations		$	12.3		$	12.8		$	80.8		$	105.9
	Year ended December 31, 2011
	Earned premiums		$	549.8		$	462.4		$	—		$	1,012.20
	Loss and loss adjustment expense		(281.7		)	(266.6		)	—			(548.3		)
	Policy acquisition expenses		(129.1		)	(92.1		)	—			(221.2		)
	Other underwriting expenses		(77.1		)	(85.2		)	—			(162.3		)
	Total underwriting income		61.9			18.5			—			80.4
	Net investment income		—			—			71.4			71.4
	Net realized and change in unrealized investment gains		—			—			10.6			10.6
	Net other revenues (expenses)		—			0.6			(13.0		)	(12.4		)
	General and administrative expenses		—			(1.7		)	(8.1		)	(9.8		)
	Interest expense		—			—			(20.5		)	(20.5		)
	Pre-tax income from continuing operations		$	61.9		$	17.4		$	40.4		$	119.7
Schedule of selected balance sheet of the segments	'
			Insurance Operations						Investing,
									Financing
									and
			Specialty			Specialty			Corporate(1)			Consolidated
			Products			Industries
			($ in millions)
	31-Dec-13
	Total investment securities		$	—		$	—		$	2,364.90		$	2,364.90
	Reinsurance recoverables		35.1			54.8			—			89.9
	Deferred acquisition costs		53.6			50.1			—			103.7
	Unpaid loss and loss adjustment expense reserves		607.8			446.5			—			1,054.30
	Unearned premiums		275.4			269.5			—			544.9
	Debt		—			—			274.7			274.7
	31-Dec-12
	Total investment securities		$	—		$	—		$	2,291.50		$	2,291.50
	Reinsurance recoverables		52.3			58.3			—			110.6
	Deferred acquisition costs		75.1			48.8			—			123.9
	Unpaid loss and loss adjustment expense reserves		561.9			438.1			—			1,000.00
	Unearned premiums		318.7			255.1			—			573.8
	Debt		—			—			274.7			274.7
	_______________________________________________________________________________
	(1)As described in Note 2, balances related to the the Runoff Business are presented as held for sale. Total investment securities excludes $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale as of December 31, 2013 and December 31, 2012, respectively.
Schedule of net written premiums, earned premiums and ratios for OneBeacon's insurance operations by segment and in total	'
	The following tables provide net written premiums, earned premiums and underwriting ratios for OneBeacon's insurance operations by reportable segment and in total for the years ended December 31, 2013, 2012 and 2011:
			Insurance Operations
	($ in millions)		Specialty			Specialty			Consolidated
			Products			Industries
	Year ended December 31, 2013
	Net written premiums		$	509.6		$	579		$	1,088.60
	Earned premiums		553.5			566.9			1,120.40
	Underwriting ratios:(1)
	Loss and LAE		56.4		%	54.7		%	55.5		%
	Expense		36.8			37			36.9
	Total combined ratio		93.2		%	91.7		%	92.4		%
	Year ended December 31, 2012
	Net written premiums		$	630.9		$	548.3		$	1,179.20
	Earned premiums		604			528			1,132.00
	Underwriting ratios:(1)
	Loss and LAE		57.2		%	57.7		%	57.4		%
	Expense		40.7			39.4			40.1
	Total combined ratio		97.9		%	97.1		%	97.5		%
	Year ended December 31, 2011
	Net written premiums		$	571.2		$	491.5		$	1,062.70
	Earned premiums		549.8			462.4			1,012.20
	Underwriting ratios:(1)
	Loss and LAE		51.2		%	57.7		%	54.2		%
	Expense		37.5			38.3			37.9
	Total combined ratio		88.7		%	96		%	92.1		%
	_______________________________________________________________________________
	(1)	Underwriting ratios are used to measure the components of underwriting profitability and include: The loss and LAE ratio, calculated by dividing loss and LAE by earned premiums; the expense ratio, calculated by dividing policy acquisition and other underwriting expenses by earned premiums; and the combined ratio, the sum of the loss and LAE ratio and the expense ratio.

Related_Party_Disclosures_Tabl

Related Party Disclosures (Tables)	12 Months Ended
	Dec. 31, 2013
Related Party Transactions [Abstract]	'
Related Party Management Fees	'
	OneBeacon is responsible for custodial arrangements and the payment of all custodial charges and fees. OneBeacon has agreed to pay annual investment management fees generally based on the quarter-end market values held under custody as set forth in the table below:
	Assets Under Management	Annual Fee
	Investment Grade Fixed Income:
	—Up to $1 billion	10.0 basis points
	—Next $1 billion	8.5 basis points
	—Next $3 billion	7.5 basis points
	—Greater than $5 billion	2.5 basis points
	High Yield Fixed Income	25.0 basis points
	Equities	100.0 basis points
	Hedge Funds	100.0 basis points
	Private Equities & Deferreds
	First 2 years of fund's life (committed)	100.0 basis points
	Thereafter (fair value)	100.0 basis points
	Affordable housing credit funds
	First year of fund's life (committed)	100.0 basis points
	Thereafter (fair value)	10.0 basis points

Earnings_per_Share_Tables

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2013
Earnings Per Share [Abstract]	'
Schedule of computation of earnings per share	'
	The following table outlines the Company's computation of earnings per share for net income from continuing operations attributable to OneBeacon's common shareholders for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013			2012			2011
	Earnings attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders		$	146		$	96.1		$	103.9
	Allocation of income for participating unvested restricted common shares		(1.4		)	(0.9		)	(0.4		)
	Dividends paid on participating restricted common shares		(0.8		)	(0.8		)	(1.0		)
	Total allocation to restricted common shares		(2.2		)	(1.7		)	(1.4		)
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	143.8		$	94.4		$	102.5
	Undistributed (over-distributed) net earnings (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	143.8		$	94.4		$	102.5
	Dividends paid, net of restricted common share amounts		(79.4		)	(79.3		)	(173.8		)
	Total undistributed (over-distributed) net earnings, net of restricted common share amounts		$	64.4		$	15.1		$	(71.3	)
	Earnings per share denominator—basic and diluted (in millions):
	Total weighted average common shares outstanding		95.4			95.4			94.8
	Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)	(0.4		)
	Basic and diluted earnings per share denominator(2)		94.5			94.5			94.4
	Earnings per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
	Net income from continuing operations attributable to OneBeacon's common shareholders		$	1.52		$	1		$	1.08
	Dividends declared and paid		(0.84		)	(0.84		)	(1.84		)
	Undistributed (over-distributed) earnings		$	0.68		$	0.16		$	(0.76	)
	_______________________________________________________________________________
	-1	Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event (see Note 9).
	(2)	Common shares issuable upon exercise of stock options (Note 9) were not included in the years ended December 31, 2012 and 2011 as their inclusion would be anti-dilutive. There were no options outstanding during the year ended December 31, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.

Discontinued_Operations_Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2013
Discontinued Operations and Disposal Groups [Abstract]	'
Schedule of discontinued operations balance sheet statement of operations and other disclosures	'
	The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations:
			Year ended December 31,
			2013			2012				2011
			($ in millions)
	Net written premiums		$	0.4		$	3.5			$	58
	Revenues
	Earned premiums		$	0.8		$	10.6			$	70.5
	Other revenue		6.3			—				1.7
	Total revenues		7.1			10.6				72.2
	Expenses
	Loss and loss adjustment expenses		78.9			48.4				89.5
	Policy acquisition expenses (benefit)		—			(2.1		)		5.3
	Other underwriting expenses		(0.2		)	1.7				22.8
	Total expenses		78.7			48				117.6
	Pre-tax loss		(71.6		)	(37.4		)		(45.4		)
	Income tax benefit		25			13.1				15.8
	Loss from discontinued operations, net of tax		(46.6		)	(24.3		)		(29.6		)
	Gain (loss) from sale of discontinued operations, net of tax		46.6			(91.0		)		(19.2		)
	Net loss from discontinued operations, net of tax		$	—		$	(115.3	)		$	(48.8	)
	As of December 31, 2013 and 2012, the Runoff Transaction met the criteria for held for sale accounting. As a result, the assets and liabilities associated with the businesses being sold, after effecting the various steps contemplated by the Stock Purchase Agreement, are presented separately as single line items in the asset and liability sections of the consolidated balance sheets as of December 31, 2013 and 2012. The following summarizes the major categories of assets and liabilities associated with the business classified as held for sale:
			December 31,
			2013			2012
			($ in millions)
	Assets:
	Investments		$	236.3		$	338.1
	Premiums receivable		9.1			11
	Reinsurance recoverable on unpaid losses(1)		1,604.70			1,840.80
	Reinsurance recoverable on paid losses		10.7			15.6
	Net deferred tax asset		3.3			5.1
	Other assets		16			16.2
	Total assets held for sale		$	1,880.10		$	2,226.80
	Liabilities:
	Unpaid loss and loss adjustment expense reserves(1)		$	1,793.10		$	2,052.60
	Unearned premiums		0.2			0.5
	Ceded reinsurance payable		12.3			21.9
	Other liabilities(2)		74.5			151.8
	Total liabilities held for sale		$	1,880.10		$	2,226.80
	_______________________________________________________________________________
	(1)	The December 31, 2013 and 2012 balances include the remaining purchase accounting fair value adjustments of $136.9 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of December 31, 2013 and 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,741.6 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,930.0 million and $2,202.7 million for each period.
	(2)	Other liabilities as of December 31, 2013 and 2012 include the accrual related to the pre-tax loss on sale of the Runoff Business of $69.0 million and $140.7 million, respectively.
	The following table outlines the computation of loss per share for discontinued operations attributable to OneBeacon's common shareholders for the years ended December 31, 2013, 2012 and 2011:
			Year ended December 31,
			2013			2012				2011
	Loss attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net loss from discontinued operations attributable to OneBeacon's common shareholders		$	—		$	(115.3	)		$	(48.8	)
	Allocation of loss for participating unvested restricted common shares		—			1.1				0.2
	Net loss from discontinued operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	—		$	(114.2	)		$	(48.6	)
	Loss per share denominator—basic and diluted (in millions):
	Total weighted average common shares outstanding		95.4			95.4				94.8
	Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)		(0.4		)
	Basic and diluted loss per share denominator(2)		94.5			94.5				94.4
	Loss per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
	Net loss from discontinued operations attributable to OneBeacon's common shareholders per share		$	—		$	(1.21	)		$	(0.50	)
	_______________________________________________________________________________
	(1)	Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event.
	(2)	Common shares issuable upon exercise of stock options (Note 9) were not included in the years ended December 31, 2012 and 2011 as their inclusion would be anti-dilutive. There were no options outstanding during the year ended December 31, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.
Reinsurer by S&P Rating	'
	The following table summarizes Standard & Poor's Financial Services, LLC ("Standard & Poor's") ratings for OneBeacon's reinsurers for its continuing insurance operations, excluding industry pools and associations, based upon reinsurance recoverable amounts on paid and unpaid losses and LAE:
			Balance at			% of total
			31-Dec-13
	Standard & Poor's Rating:		($ in millions)
	AA		$	26.4		30	%
	A		56.1			62	%
	BBB+, Not Rated and Other		7.4			8	%
	Total reinsurance recoverables		$	89.9		100	%
	At December 31, 2013, OneBeacon had $10.7 million of reinsurance recoverable on paid losses and $1,741.6 million (gross of $136.9 million in purchase accounting adjustments, as described above) that will become recoverable if claims are paid in accordance with current reserve estimates, related to the Runoff Business that have been reclassified to assets held for sale. Reinsurance contracts do not relieve OneBeacon of its obligations. Therefore, collectibility of balances due from reinsurers is critical to OneBeacon's financial strength. The following table provides a listing of the top reinsurers related to the Runoff Business reported in assets held for sale, excluding industry pools and associations, based on reinsurance recoverable amounts on paid and unpaid losses, the percentage of the total reported as held for sale (gross of the $136.9 million in purchase accounting adjustment), and the reinsurers' A.M. Best ratings.
	($ in millions)		Balance at			% of total			A.M. Best
			31-Dec-13						Rating(1)
	National Indemnity Company ("NICO") and General Reinsurance Corporation(2)		$	1,243.70		71	%		A++
	Hanover Insurance Company		43.9			3	%		A
	Tokio Marine and Nichido Fire(3)		26.2			1	%		A++
	Munich Reinsurance America		16.3			1	%		A+
	Tower Insurance Company		10.2			1	%		B(4)
	_______________________________________________________________________________
	(1)	A.M. Best ratings as detailed above are: “A++” (Superior, which is the highest of sixteen financial strength ratings), “A+” (Superior, which is the second highest of sixteen financial strength ratings), “A” (Excellent, which is the third highest of sixteen financial strength ratings) and “B” (Fair, which is the seventh highest of sixteen financial strength ratings).
	(2)	Includes $198.3 million of Third Party Recoverables, which NICO would pay under the terms of the NICO Cover if they are unable to collect from third party reinsurers.
	(3)	Excludes $21.8 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.
	(4)	Under review with developing implications.

Consolidating_Financial_Inform1

Consolidating Financial Information (Tables)	12 Months Ended
	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]	'
Consolidating Balance Sheet	'
	The following tables present OneBeacon's consolidating balance sheets as of December 31, 2013 and 2012, and statements of operations and comprehensive income and cash flows for the years ended December 31, 2013, 2012 and 2011. These financial statements reflect the Company's ("guarantor") financial position, results of operations and cash flows on a stand-alone basis, that of OBH ("the issuer") and of the Company's other entities ("non-guarantor subsidiaries") as well as the necessary consolidating adjustments to eliminate intercompany balances and transactions.
	Consolidating Balance Sheet	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	as of December 31, 2013				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,725.40		$	—		$	(24.5	)	$	1,700.90
	Short-term investments, at amortized cost (which approximates fair value)	13			140			4			—			157
	Common equity securities, at fair value	—			336.9			—			—			336.9
	Convertible fixed maturity investments, at fair value	—			30.5			—			—			30.5
	Other investments	—			139.6			—			—			139.6
	Total investment securities	13			2,372.40			4			(24.5		)	2,364.90
	Cash	—			166.6			1.5			—			168.1
	Reinsurance recoverables	—			89.9			—			—			89.9
	Premiums receivable	—			228.2			—			—			228.2
	Deferred acquisition costs	—			103.7			—			—			103.7
	Net deferred tax asset	—			93.1			(2.4		)	(0.1		)	90.6
	Investment income accrued	—			10.3			—			(0.2		)	10.1
	Accounts receivable on unsettled investment sales	—			3.3			—			—			3.3
	Investments in subsidiaries	1,092.00			—			1,034.90			(2,126.9		)	—
	Other assets	(0.8		)	272.4			1.1			—			272.7
	Assets held for sale	—			1,880.10			—			—			1,880.10
	Total assets	$	1,104.20		$	5,220.00		$	1,039.10		$	(2,151.7	)	$	5,211.60
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,054.30		$	—		$	—		$	1,054.30
	Unearned premiums	—			544.9			—			—			544.9
	Debt	—			—			299.7			(25.0		)	274.7
	Accounts payable on unsettled investment purchases	—			11.6			—			—			11.6
	Other liabilities	(0.1		)	335			3.9			(0.2		)	338.6
	Liabilities held for sale	—			1,880.10			—			—			1,880.10
	Total liabilities	(0.1		)	3,825.90			303.6			(25.2		)	4,104.20
	OneBeacon's common shareholders' equity and noncontrolling interests
	Total OneBeacon's common shareholders' equity	1,104.30			1,391.00			735.5			(2,126.5		)	1,104.30
	Total noncontrolling interests	—			3.1			—			—			3.1
	Total OneBeacon's common shareholders' equity and noncontrolling interests	1,104.30			1,394.10			735.5			(2,126.5		)	1,107.40
	Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,104.20		$	5,220.00		$	1,039.10		$	(2,151.7	)	$	5,211.60
	Consolidating Balance Sheet	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	as of December 31, 2012				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,602.40		$	16.6		$	(25.7	)	$	1,593.30
	Short-term investments, at amortized cost (which approximates fair value)	2.7			169.9			60.2			—			232.8
	Common equity securities, at fair value	—			259			—			—			259
	Convertible fixed maturity investments, at fair value	—			62.6			—			—			62.6
	Other investments	—			143.8			—			—			143.8
	Total investment securities	2.7			2,237.70			76.8			(25.7		)	2,291.50
	Cash	—			43.9			—			—			43.9
	Reinsurance recoverables	—			110.6			—			—			110.6
	Premiums receivable	—			225.6			—			—			225.6
	Deferred acquisition costs	—			123.9			—			—			123.9
	Net deferred tax asset	—			140.2			(2.5		)	0.1			137.8
	Investment income accrued	—			12.3			—			(0.2		)	12.1
	Accounts receivable on unsettled investment sales	—			1.4			0.7			—			2.1
	Investments in subsidiaries	1,012.40			—			1,062.60			(2,075.0		)	—
	Other assets	0.3			219.8			7.1			—			227.2
	Assets held for sale	—			2,226.80			—			—			2,226.80
	Total assets	$	1,015.40		$	5,342.20		$	1,144.70		$	(2,100.8	)	$	5,401.50
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,000.00		$	—		$	—		$	1,000.00
	Unearned premiums	—			573.8			—			—			573.8
	Debt	—			—			299.6			(24.9		)	274.7
	Accounts payable on unsettled investment purchases	—			6.2			—			—			6.2
	Other liabilities	0.9			295.9			6			(0.1		)	302.7
	Liabilities held for sale	—			2,226.80			—			—			2,226.80
Total liabilities	0.9			4,102.70			305.6			(25.0		)	4,384.20
OneBeacon's common shareholders' equity and noncontrolling interests
Total OneBeacon's common shareholders' equity	1,014.50			1,236.70			839.1			(2,075.8		)	1,014.50
Total noncontrolling interests	—			2.8			—			—			2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,014.50			1,239.50			839.1			(2,075.8		)	1,017.30
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,015.40		$	5,342.20		$	1,144.70		$	(2,100.8	)	$	5,401.50
Consolidating Statement of Operations and Comprehensive (Loss) Income	'
	Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Year ended December 31, 2013	(in millions)
	Revenues
	Earned premiums	$	—		$	1,120.40		$	—		$	—		$	1,120.40
	Net investment income (expense)	—			42.6			(0.3		)	(1.2		)	41.1
	Net realized and change in unrealized investment gains	—			48.9			(0.7		)	1.2			49.4
	Net other revenues	—			31.2			—			—			31.2
	Total revenues	—			1,243.10			(1.0		)	—			1,242.10
	Expenses
	Loss and loss adjustment expenses	—			622.1			—			—			622.1
	Policy acquisition expenses	—			208.9			—			—			208.9
	Other underwriting expenses	—			204.8			—			—			204.8
	General and administrative expenses	5			7.1			(0.1		)	—			12
	Interest expense	—			—			14.2			(1.2		)	13
	Total expenses	5			1,042.90			14.1			(1.2		)	1,060.80
	Pre-tax (loss) income from continuing operations	(5.0		)	200.2			(15.1		)	1.2			181.3
	Income tax benefit (expense)	0.1			(40.4		)	6.2			(0.2		)	(34.3		)
	Net (loss) income from continuing operations	(4.9		)	159.8			(8.9		)	1			147
	Loss from discontinued operations, net of tax	—			(46.6		)	—			—			(46.6		)
	Gain from sale of discontinued operations, net of tax	—			46.6			—			—			46.6
	(Loss) income before equity in earnings of unconsolidated affiliates	(4.9		)	159.8			(8.9		)	1			147
	Equity in earnings of subsidiaries, net of tax	150.9			—			134.7			(285.6		)	—
	Net income including noncontrolling interests	146			159.8			125.8			(284.6		)	147
	Less: Net income attributable to noncontrolling interests	—			(1.0		)	—			—			(1.0		)
	Net income attributable to OneBeacon's common shareholders	146			158.8			125.8			(284.6		)	146
	Net change in benefit plan assets and obligations, net of tax	20.6			—			20.6			(20.6		)	20.6
	Comprehensive income attributable to OneBeacon's common shareholders	$	166.6		$	158.8		$	146.4		$	(305.2	)	$	166.6
	Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Year ended December 31, 2012	(in millions)
	Revenues
	Earned premiums	$	—		$	1,132.00		$	—		$	—		$	1,132.00
	Net investment income	—			56.2			0.4			(3.0		)	53.6
	Net realized and change in unrealized investment gains	—			56.5			(1.4		)	0.6			55.7
	Net other revenues (expenses)	—			6.6			(7.1		)	—			(0.5		)
	Total revenues	—			1,251.30			(8.1		)	(2.4		)	1,240.80
	Expenses
	Loss and loss adjustment expenses	—			650			—			—			650
	Policy acquisition expenses	—			249.4			—			—			249.4
	Other underwriting expenses	—			205.2			—			—			205.2
	General and administrative expenses	5.4			7.4			0.6			—			13.4
	Interest expense	—			—			19.7			(2.8		)	16.9
	Total expenses	5.4			1,112.00			20.3			(2.8		)	1,134.90
	Pre-tax (loss) income from continuing operations	(5.4		)	139.3			(28.4		)	0.4			105.9
	Income tax benefit (expense)	0.1			(18.6		)	10.1			—			(8.4		)
	Net (loss) income from continuing operations	(5.3		)	120.7			(18.3		)	0.4			97.5
	Loss from discontinued operations, net of tax	—			(24.3		)	—			—			(24.3		)
	Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
	Net (loss) income before equity in earnings of unconsolidated affiliates	(5.3		)	5.4			(18.3		)	0.4			(17.8		)
	Equity in loss of subsidiaries, net of tax	(13.9		)	—			(28.9		)	42.8			—
	Net (loss) income including noncontrolling interests	(19.2		)	5.4			(47.2		)	43.2			(17.8		)
	Less: Net income attributable to noncontrolling interests	—			(1.4		)	—			—			(1.4		)
	Net (loss) income attributable to OneBeacon's common shareholders	(19.2		)	4			(47.2		)	43.2			(19.2		)
	Net change in benefit plan assets and obligations, net of tax	(2.9		)	—			(2.9		)	2.9			(2.9		)
	Comprehensive (loss) income attributable to OneBeacon's common shareholders	$	(22.1	)	$	4		$	(50.1	)	$	46.1		$	(22.1	)
	Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Year ended December 31, 2011	(in millions)
	Revenues
	Earned premiums	$	—		$	1,012.20		$	—		$	—		$	1,012.20
	Net investment income	—			72.7			0.5			(1.8		)	71.4
	Net realized and change in unrealized investment gains	—			10			(0.1		)	0.7			10.6
	Net other revenues (expense)	—			0.7			(13.1		)	—			(12.4		)
	Total revenues	—			1,095.60			(12.7		)	(1.1		)	1,081.80
	Expenses
Loss and loss adjustment expenses	—			548.3			—			—			548.3
Policy acquisition expenses	—			221.2			—			—			221.2
Other underwriting expenses	—			162.3			—			—			162.3
General and administrative expenses	5.1			4.1			0.6			—			9.8
Interest expense	—			—			22.1			(1.6		)	20.5
Total expenses	5.1			935.9			22.7			(1.6		)	962.1
Pre-tax (loss) income from continuing operations	(5.1		)	159.7			(35.4		)	0.5			119.7
Income tax (expense) benefit	(0.2		)	(26.9		)	12.4			(0.1		)	(14.8		)
Net (loss) income from continuing operations	(5.3		)	132.8			(23.0		)	0.4			104.9
Loss from discontinued operations, net of tax	—			(29.6		)	—			—			(29.6		)
Loss from sale of discontinued operations, net of tax	—			(19.2		)	—			—			(19.2		)
(Loss) income before equity in earnings of unconsolidated affiliates	(5.3		)	84			(23.0		)	0.4			56.1
Equity in earnings of subsidiaries, net of tax	60.4			—			50.9			(111.3		)	—
Net income including noncontrolling interests	55.1			84			27.9			(110.9		)	56.1
Less: Net income attributable to noncontrolling interests	—			(1.0		)	—			—			(1.0		)
Net income attributable to OneBeacon's common shareholders	55.1			83			27.9			(110.9		)	55.1
Net change in benefit plan assets and obligations, net of tax	(11.2		)	—			(11.2		)	11.2			(11.2		)
Comprehensive income attributable to OneBeacon's common shareholders	$	43.9		$	83		$	16.7		$	(99.7	)	$	43.9
Consolidating Statement of Cash Flows	'
	Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Year ended December 31, 2013	($ in millions)
	Cash flows from operations:
	Net income including noncontrolling interests	$	146		$	159.8		$	125.8		$	(284.6	)	$	147
	Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
	Undistributed earnings from subsidiaries	(150.9		)	—			(134.7		)	285.6			—
	Net loss from discontinued operations	—			46.6			—			—			46.6
	Net gain from sale of discontinued operations	—			(46.6		)	—			—			(46.6		)
	Net realized and change in unrealized investment gains	—			(48.9		)	0.7			(1.2		)	(49.4		)
	Net realized gain on sale of business	—			(23.0		)	—			—			(23.0		)
	Deferred income tax expense	—			12.7			(0.1		)	0.2			12.8
	Dividends received from subsidiaries	92			—			218			(310.0		)	—
	Other operating items:
	Net change in loss and LAE reserves	—			54.3			—			—			54.3
	Net change in unearned premiums	—			(28.9		)	—			—			(28.9		)
	Net change in premiums receivable	—			(2.6		)	—			—			(2.6		)
	Net change in reinsurance recoverable on paid and unpaid losses	—			20.7			—			—			20.7
	Net change in other assets and liabilities	3.4			100.5			(29.4		)	—			74.5
	Net cash provided from operations—continuing operations	90.5			244.6			180.3			(310.0		)	205.4
	Net cash used for operations—discontinued operations	—			(76.7		)	—			—			(76.7		)
	Net cash provided from operations	90.5			167.9			180.3			(310.0		)	128.7
	Cash flows from investing activities:
	Net maturities, purchases and sales of short-term investments	(10.3		)	26.3			56.2			—			72.2
	Maturities of fixed maturity investments	—			223.7			21.5			—			245.2
	Sales of fixed maturity investments	—			1,496.70			386.7			(483.4		)	1,400.00
	Sales of common equity securities	—			196.7			—			—			196.7
	Sales of convertible fixed maturity investments	—			45.7			—			—			45.7
	Return of capital and distributions of other investments	—			21.8			—			—			21.8
	Purchases of fixed maturity investments	—			(1,787.2		)	(393.2		)	483.4			(1,697.0		)
	Purchases of common equity securities	—			(206.3		)	—			—			(206.3		)
	Purchases of convertible fixed maturity investments	—			(14.4		)	—			—			(14.4		)
	Contributions for other investments	—			(7.6		)	—			—			(7.6		)
	Proceeds from sale of business	—			31.3			—			—			31.3
	Net change in unsettled investment purchases and sales	—			4.1			—			—			4.1
	Net acquisitions of property and equipment	—			(10.3		)	—			—			(10.3		)
	Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			250			—			(250.0		)	—
	Net cash (used for) provided from investing activities—continuing operations	(10.3		)	270.5			71.2			(250.0		)	81.4
	Net cash provided from investing activities—discontinued operations	—			—			—			—			—
	Net cash (used for) provided from investing activities	(10.3		)	270.5			71.2			(250.0		)	81.4
	Cash flows from financing activities:
	Cash dividends paid to common shareholders	(80.2		)	—			—			—			(80.2		)
	Cash dividends paid to parent	—			(310.0		)	—			310			—
	Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(250.0		)	250			—
	Payments on capital lease obligation	—			(5.7		)	—			—			(5.7		)
	Net cash used for financing activities—continuing operations	(80.2		)	(315.7		)	(250.0		)	560			(85.9		)
	Net cash used for financing activities—discontinued operations	—			—			—			—			—
	Net cash used for financing activities	(80.2		)	(315.7		)	(250.0		)	560			(85.9		)
	Net increase in cash during period	—			122.7			1.5			—			124.2
	Cash reclassified to assets held for sale	—			—			—			—			—
	Cash transferred with sale of business	—			—			—			—			—
	Net increase after reclassification of cash to assets held for sale	—			122.7			1.5			—			124.2
	Cash balance at beginning of period	—			43.9			—			—			43.9
	Cash balance at end of period	$	—		$	166.6		$	1.5		$	—		$	168.1
	Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Year ended December 31, 2012	($ in millions)
	Cash flows from operations:
	Net (loss) income including noncontrolling interests	$	(19.2	)	$	5.4		$	(47.2	)	$	43.2		$	(17.8	)
	Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
	Undistributed loss from subsidiaries	13.9			—			28.9			(42.8		)	—
	Net loss from discontinued operations	—			24.3			—			—			24.3
	Net loss from sale of discontinued operations	—			91			—			—			91
	Net realized and change in unrealized investment gains	—			(56.5		)	1.4			(0.6		)	(55.7		)
	Net other realized losses	—			—			6.3			—			6.3
Net realized gain on sale of business	—			(4.4		)	—			—			(4.4		)
Deferred income tax expense	—			15.2			0.8			0.4			16.4
Dividends received from subsidiaries	83			15.5			160.1			(258.6		)	—
Other operating items:
Net change in loss and LAE reserves	—			87.5			—			—			87.5
Net change in unearned premiums	—			48.4			—			—			48.4
Net change in premiums receivable	—			(14.6		)	—			—			(14.6		)
Net change in reinsurance recoverable on paid and unpaid losses	—			(3.1		)	—			—			(3.1		)
Net change in other assets and liabilities	2.3			(38.8		)	1.1			(0.2		)	(35.6		)
Net cash provided from operations—continuing operations	80			169.9			151.4			(258.6		)	142.7
Net cash used for operations—discontinued operations	—			(196.4		)	—			—			(196.4		)
Net cash provided from (used for) operations	80			(26.5		)	151.4			(258.6		)	(53.7		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	0.1			74.7			(60.2		)	—			14.6
Maturities of fixed maturity investments	—			191.4			0.1			—			191.5
Sales of fixed maturity investments	—			1,783.90			24.6			(91.3		)	1,717.20
Sales of common equity securities	—			114.8			23.1			(25.4		)	112.5
Sales of convertible fixed maturity investments	—			33.1			7.6			(11.4		)	29.3
Return of capital and distributions of other investments	—			63.9			—			—			63.9
Purchases of fixed maturity investments	—			(1,887.6		)	(42.0		)	67.1			(1,862.5		)
Purchases of common equity securities	—			(90.1		)	(26.8		)	25.4			(91.5		)
Purchases of convertible fixed maturity investments	—			(13.0		)	(7.8		)	11.4			(9.4		)
Contributions for other investments	—			(44.9		)	—			—			(44.9		)
Proceeds from sale of business	—			15			—			—			15
Net change in unsettled investment purchases and sales	—			(18.2		)	—			—			(18.2		)
Net acquisitions of property and equipment	—			(1.8		)	—			—			(1.8		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			28.7			—			(28.7		)	—
Net cash provided from (used for) from investing activities—continuing operations	0.1			249.9			(81.4		)	(52.9		)	115.7
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash provided from (used for) investing activities	0.1			249.9			(81.4		)	(52.9		)	115.7
Cash flows from financing activities:
Issuance of debt	—			—			296.9			(25.0		)	271.9
Repurchases of debt	—			—			(325.1		)	49.2			(275.9		)
Cash dividends paid to common shareholders	(80.1		)	—			—			—			(80.1		)
Cash dividends paid to parent	—			(243.1		)	(15.5		)	258.6			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(28.7		)	28.7			—
Payments on capital lease obligation	—			(4.9		)	—			—			(4.9		)
Proceeds on sale of OneBeacon Holdings (Luxembourg) S.à r.l.	—			14			—			—			14
Net cash used for financing activities—continuing operations	(80.1		)	(234.0		)	(72.4		)	311.5			(75.0		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(80.1		)	(234.0		)	(72.4		)	311.5			(75.0		)
Net (decrease) in cash during period	—			(10.6		)	(2.4		)	—			(13.0		)
Cash reclassified from assets held for sale	—			5.5			—			—			5.5
Cash transferred with sale of business	—			(3.5		)	—			—			(3.5		)
Net decrease after reclassification of cash to assets held for sale	—			(8.6		)	(2.4		)	—			(11.0		)
Cash balance at beginning of period	—			52.5			2.4			—			54.9
Cash balance at end of period	$	—		$	43.9		$	—		$	—		$	43.9
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Year ended December 31, 2011	($ in millions)
Cash flows from operations:
Net income (loss) including noncontrolling interests	$	55.1		$	84		$	27.9		$	(110.9	)	$	56.1
Charges (credits) to reconcile net income to cash flows provided from (used for) operations:
Undistributed earnings from subsidiaries	(60.4		)	—			(50.9		)	111.3			—
Net loss from discontinued operations	—			29.6			—			—			29.6
Net loss from sale of discontinued operations	—			19.2			—			—			19.2
Net realized and change in unrealized investment gains	—			(10.0		)	0.1			(0.7		)	(10.6		)
Net other realized losses	—			(0.3		)	12			—			11.7
Deferred income tax expense	—			31.3			0.1			0.2			31.6
Dividends received from subsidiaries	181			34.4			155			(370.4		)	—
Other operating items:
Net change in loss and LAE reserves	—			69.2			—			—			69.2
Net change in unearned premiums	—			50.2			—			—			50.2
Net change in premiums receivable	—			(49.4		)	—			—			(49.4		)
Net change in reinsurance recoverable on paid and unpaid losses	—			(46.3		)	—			—			(46.3		)
Net change in other assets and liabilities	0.1			(92.1		)	12.7			0.1			(79.2		)
Net cash provided from operations—continuing operations	175.8			119.8			156.9			(370.4		)	82.1
Net cash used for operations—discontinued operations	—			(200.6		)	—			—			(200.6		)
Net cash provided from (used for) operations	175.8			(80.8		)	156.9			(370.4		)	(118.5		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(1.0		)	(55.7		)	36.8			—			(19.9		)
Maturities of fixed maturity investments	—			475.3			3			—			478.3
Sales of fixed maturity investments	—			1,406.70			139.8			(131.7		)	1,414.80
Sales of common equity securities	—			101.9			—			—			101.9
Sales of convertible fixed maturity investments	—			37.1			—			—			37.1
Return of capital and distributions of other investments	—			38.6			—			—			38.6
Purchases of fixed maturity investments	—			(1,476.3		)	(127.7		)	131.7			(1,472.3		)
Purchases of common equity securities	—			(88.5		)	—			—			(88.5		)
Purchases of convertible fixed maturity investments	—			(30.4		)	—			—			(30.4		)
Contributions for other investments	—			(10.8		)	—			—			(10.8		)
Net change in unsettled investment purchases and sales	—			13.5			—			—			13.5
Net acquisitions of property and equipment	—			(3.7		)	—			—			(3.7		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			10.6			—			(10.6		)	—
Net cash (used for) provided from investing activities—continuing operations	(1.0		)	418.3			51.9			(10.6		)	458.6
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(1.0		)	418.3			51.9			(10.6		)	458.6
Cash flows from financing activities:
Repurchases of debt	—			—			(161.6		)	—			(161.6		)
Cash dividends paid to common shareholders	(174.8		)	—			—			—			(174.8		)
Cash dividends paid to parent	—			(336.0		)	(34.4		)	370.4			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(10.6		)	10.6			—
Payments on capital lease obligation	—			23.1			—			—			23.1
Net cash used for financing activities—continuing operations	(174.8		)	(312.9		)	(206.6		)	381			(313.3		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(174.8		)	(312.9		)	(206.6		)	381			(313.3		)
Net increase in cash during period	—			24.6			2.2			—			26.8
Cash reclassified to assets held for sale	—			(5.5		)	—			—			(5.5		)
Net increase after reclassification of cash to assets held for sale	—			19.1			2.2			—			21.3
Cash balance at beginning of period	—			33.4			0.2			—			33.6
Cash balance at end of period	$	—		$	52.5		$	2.4		$	—		$	54.9

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]	'
Schedule of Quarterly Financial Information [Table Text Block]	'
	Selected quarterly financial data for 2013 and 2012 is shown in the following table. The quarterly financial data includes, in the opinion of management, all recurring adjustments necessary for a fair presentation of the results of operations for the interim periods. Financial information for the prior periods has been reclassified to conform to this presentation.
		2013 Three months ended											2012 Three months ended
	Millions, except per share amounts	Mar. 31			June 30			Sept. 30		Dec. 31			Mar. 31			June 30			Sept. 30			Dec. 31
	Revenues	$	348.3		$	267.3		$	311.5	$	315		$	316.4		$	282.6		$	346.3		$	295.5
	Expenses	260.2			270.3			275.6		254.7			248.8			270.2			287.1			328.8
	Pre-tax income (loss) from continuing operations	88.1			(3.0		)	35.9		60.3			67.6			12.4			59.2			(33.3		)
	Tax (expense) benefit	(19.8		)	1			2.9		(18.4		)	(13.5		)	(0.4		)	(14.2		)	19.7
	Net income (loss) from continuing operations	68.3			(2.0		)	38.8		41.9			54.1			12			45			(13.6		)
	Income (loss) from discontinued operations, net of tax	0.5			(0.5		)	0.3		(46.9		)	(9.5		)	0.5			(15.8		)	0.5
	Gain (loss) from sale of discontinued operations, net of tax	—			—			—		46.6			—			—			(91.0		)	—
	Net income (loss) including noncontrolling interests	68.8			(2.5		)	39.1		41.6			44.6			12.5			(61.8		)	(13.1		)
	Less: Net income attributable to noncontrolling interests	(0.4		)	(0.4		)	—		(0.2		)	(0.6		)	(0.2		)	(0.4		)	(0.2		)
	Net income (loss) attributable to OneBeacon's common shareholders	$	68.4		$	(2.9	)	$	39.1	$	41.4		$	44		$	12.3		$	(62.2	)	$	(13.3	)
	Earnings (loss) per share attributable to OneBeacon's common shareholders—basic and diluted:(1)
	Net income (loss) from continuing operations per share	$	0.71		$	(0.03	)	$	0.41	$	0.43		$	0.56		$	0.12		$	0.47		$	(0.15	)
	Loss from discontinued operations, net of tax, per share	—			—			—		(0.49		)	(0.10		)	—			(0.17		)	0.01
	Gain (loss) from sale of discontinued operations, net of tax, per share	—			—			—		0.49			—			—			(0.95		)	—
	Net income (loss) attributable to OneBeacon's common shareholders per share	$	0.71		$	(0.03	)	$	0.41	$	0.43		$	0.46		$	0.12		$	(0.65	)	$	(0.14	)

Nature_of_Operations_and_Summa2

Nature of Operations and Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended					12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2013
	segment		Accounting Standards Update 2010-26	Accounting Standards Update 2010-26	White Mountains Insurance Group Ltd	Specialty Products	Specialty Industries
						segment	segment
Basis of presentation	'	'	'	'	'	'	'
Participation percentage in quota share reinsurance agreements	100.00%	'	'	'	'	'	'
Ownership interest (as a percent)	'	'	'	'	75.20%	'	'
Number of underwriting segments	14	'	'	'	'	8	6
Unamortized discount	'	'	$5.60	$5.60	'	'	'
Percentage of fair value investments using observable inputs	92.00%	92.00%	'	'	'	'	'
Minimum Percentage of Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	5.00%	'	'	'	'	'	'
Minimum Variation from Expected Price Required to Treat Prices of Investments Provided by Pricing Services as Outliers	1	'	'	'	'	'	'
Supplemental Information for Property, Casualty Insurance Underwriters, Interest Rate at which Discount Computed	3.50%	3.50%	'	'	'	'	'
Workers Compensation Loss and Loss Adjustment Reserves, Discount Amount	3	4.6	'	'	'	'	'
Loss and Loss Adjustment Reserves, Liabilities of Assets Held for Sale	61.7	77.9	'	'	'	'	'
Capitalized Computer Software, Net	$11.80	$7.30	'	'	'	'	'

Acqusitions_and_Dispositions_D

Acqusitions and Dispositions (Details) (USD $)

0 Months Ended

3 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Jan. 24, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jan. 02, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jan. 24, 2012

Dec. 31, 2012

Dec. 31, 2012

Essentia Insurance Company

Essentia Insurance Company

Essentia Insurance Company

Essentia Insurance Company

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

Additional Paid-in Capital [Member]

OneBeacon Holdings (Luxembourg) S.a r.l.

OneBeacon Holdings (Luxembourg) S.a r.l.

White Mountains Insurance Group Ltd

OneBeacon Holdings (Luxembourg) S.a r.l.

Dispositions

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Gain on sale of OB Holdings Lux

'

'

'

'

'

'

'

'

'

'

$14

'

'

'

'

'

'

$13.60

$14

Gain on sale of discontinued operation, pre-tax

'

'

'

'

'

'

'

'

'

'

'

'

23

'

'

'

'

'

'

Gain (loss) from sale of discontinued operations, net of tax

'

46.6

0

0

0

0

-91

0

0

46.6

-91

-19.2

15

'

'

'

'

'

'

Earned premiums

'

'

'

'

'

'

'

'

'

1,120.40

1,132

1,012.20

'

'

179.7

166.6

'

'

'

Premiums written, percentage of total

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15.20%

15.70%

'

'

'

Net written premiums

'

'

'

'

'

'

'

'

'

1,088.60

1,179.20

1,062.70

'

89.1

172.4

159.3

'

'

'

Proceeds from sale of business

$24.70

'

'

'

'

'

'

'

'

$31.30

$15

$0

'

'

'

'

$24.70

'

'

Unpaid_Loss_and_Loss_Adjustmen2

Unpaid Loss and Loss Adjustment Expense ("LAE") Reserves (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]	'		'		'
Gross beginning balance	$1,000		$3,358.60		$3,295.50
Less beginning reinsurance recoverable on unpaid losses	-107.3		-2,167.50		-1,893.20
Net ending loss and LAE reserves	892.7		1,191.10		1,402.30
Loss and LAE incurred relating to Current year losses	622.1		657.4		578.1
Loss and LAE incurred relating to prior year losses	0		-7.4		-29.8
Total incurred loss and LAE from continuing operations	622.1		650		548.3
Loss and LAE paid relating to Current year losses	-188.6		-224.6		-216.9
Loss and LAE paid relating to Prior year losses	-352.1		-340.5		-306.3
Total loss and LAE payments from continuing operations	-540.7		-565.1		-523.2
Net loss and LAE reserves	974.1		1,276		1,427.40
Total incurred loss and LAE from discontinued operations	78.9		48.4		89.5
Total loss and LAE payments from discontinued operations	-102.3		-220.8		-261.1
Net loss and LAE reserves	950.7		1,103.60		1,255.80
Net change in loss and LAE reserves reported in liabilities held for sale	23.4	[1]	-147.1	[2]	-64.7	[3]
Net loss and LAE reserves sold	0		-63.8	[4]	0
Net ending loss and LAE reserves	974.1		892.7		1,191.10
Plus ending reinsurance recoverable on unpaid losses	-80.2		-107.3		-2,167.50
Gross ending balance	1,054.30		1,000		3,358.60
Runoff	'		'		'
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]	'		'		'
Net ending loss and LAE reserves	188.4		'		'
Loss and Loss Adjustment Expense Reserves Reclassified to from Held For Sale	-188.4	[1],[4]	-211.8	[1],[4]	-64.7	[1],[4]
Professional Malpractice Liability Insurance [Member]	'		'		'
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]	'		'		'
Net loss and LAE reserves	'		'		-11.5
Property and Casualty, Commercial Insurance Product Line [Member]	'		'		'
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]	'		'		'
Net loss and LAE reserves	'		'		-14
Other Lines of Business [Member]	'		'		'
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]	'		'		'
Loss and LAE incurred relating to prior year losses	'		'		$4.30
[1]	Consists of the change in net loss and LAE reserves to $188.4 million from $211.8 million, which amounts were classified as held for sale as DecemberB 31, 2013 and 2012, respectively, in connection with the Runoff Transaction
[2]	Consists of the change in net loss and LAE reserves to $211.8 million from $64.7 million, which amounts were classified as held for sale as of DecemberB 31, 2012 and 2011, respectively, in connection with the Runoff Transaction and AutoOne Transaction, respectively.
[3]	Consists of the net loss and LAE reserves of $64.7 million classified as held for sale as of DecemberB 31, 2011 in connection with the AutoOne Transaction.
[4]	Relates to the AutoOne Transaction, which closed in February 2012.

Unpaid_Loss_and_Loss_Adjustmen3

Unpaid Loss and Loss Adjustment Expense ("LAE") Reserves - Narrative (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'	'
Loss and LAE incurred relating to prior year losses	$0	($7.40)	($29.80)
Loss and LAE Loss Incurred	-974.1	-1,276	-1,427.40
Liability for asbestos and environmental claims, gross and net	'	1	'
Professional Malpractice Liability Insurance [Member]	'	'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'	'
Loss and LAE Loss Incurred	'	'	11.5
Property and Casualty, Commercial Insurance Product Line [Member]	'	'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'	'
Loss and LAE Loss Incurred	'	'	14
Other Lines of Business [Member]	'	'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'	'
Loss and LAE incurred relating to prior year losses	'	'	$4.30

Reinsurance_Effects_of_Reinsur

Reinsurance - Effects of Reinsurance (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'		'	'
Direct	$1,103.10		$1,204	$1,079.20
Assumed	59.8		55.2	49.1
Ceded	-74.3		-80	-65.6
Net written premiums	1,088.60		1,179.20	1,062.70
Direct	1,043.30		1,158.30	1,035.90
Assumed	148.5	[1]	52.8	42.3
Ceded	-71.4		-79.1	-66
Net earned premiums	1,120.40		1,132	1,012.20
Direct	584.9		687.5	551.8
Assumed	76.3	[1]	29.6	9.2
Ceded	-39.1		-67.1	-12.7
Net loss and LAE	622.1		650	548.3
Collector Cars and Boat Business	'		'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'		'	'
Assumed	89.1		'	'
Assumed	$41.90		'	'
[1]	Includes assumed earned premiums and loss and LAE of $89.1 million and $41.9 million, respectively, related to the collector cars and boats business which although exited on January 1, 2013, was assumed on a runoff basis from Essentia.

Reinsurance_Details

Reinsurance (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	85.00%	'	'	'
Reinsurance recoverable on paid losses	$9,700,000	'	'	'
Reinsurance recoverable on unpaid losses	80,200,000	107,300,000	2,167,500,000	1,893,200,000
Reinsurance recoverables	89,900,000	110,600,000	'	'
% of total	100.00%	'	'	'
Property Catastrophe Reinsurance	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	20,000,000	'	'	'
Property Catastrophe Reinsurance | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	117,000,000	'	'	'
Property Catastrophe Reinsurance | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	130,000,000	'	'	'
Property Per Risk Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	10,000,000	'	'	'
Property Per Risk Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	100,000,000	'	'	'
Property Per-Risk, Acts of Terrorism	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	100,000,000	'	'	'
Reinsurance reinsured risk, percentage	85.00%	'	'	'
Property Per-Risk, Acts of Terrorism | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	10,000,000	'	'	'
Property Per-Risk, Acts of Terrorism | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	100,000,000,000	'	'	'
Reinsurance program reinsured losses	100,000,000,000	'	'	'
Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount reinsured for more than one insured	10,000,000	'	'	'
Casualty Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount reinsured for more than one insured	60,000,000	'	'	'
Layer One | Property Catastrophe Reinsurance	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	50.00%	'	'	'
Layer One | Property Catastrophe Reinsurance | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	20,000,000	'	'	'
Layer One | Property Catastrophe Reinsurance | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	30,000,000	'	'	'
Layer One | Property Per Risk Reinsurance Program	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	5.00%	'	'	'
Layer One | Property Per Risk Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	20,000,000	'	'	'
Layer One | Property Per Risk Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	40,000,000	'	'	'
Layer Two | Property Catastrophe Reinsurance	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	10.00%	'	'	'
Layer Two | Property Catastrophe Reinsurance | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	30,000,000	'	'	'
Layer Two | Property Catastrophe Reinsurance | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	70,000,000	'	'	'
Layer Two | Property Per Risk Reinsurance Program	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	10.00%	'	'	'
Layer Two | Property Per Risk Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	40,000,000	'	'	'
Layer Three | Property Catastrophe Reinsurance	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	5.00%	'	'	'
Layer Three | Property Catastrophe Reinsurance | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	70,000,000	'	'	'
Layer Three | Property Catastrophe Reinsurance | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	150,000,000	'	'	'
Healthcare Professional Liability | Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	5,000,000	'	'	'
Healthcare Professional Liability | Casualty Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	20,000,000	'	'	'
Healthcare Professional Liability | Layer One | Casualty Reinsurance Program	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	5,000,000	'	'	'
Reinsurance reinsured risk, percentage	10.00%	'	'	'
Healthcare Professional Liability | Layer One | Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	5,000,000	'	'	'
Casualty Business | Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	5,000,000	'	'	'
Casualty Business | Casualty Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	21,000,000	'	'	'
Casualty Business | Layer One | Casualty Reinsurance Program	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	6,000,000	'	'	'
Reinsurance reinsured risk, percentage	10.00%	'	'	'
Casualty Business | Layer One | Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	5,000,000	'	'	'
Insurance deductible for individual large claims	3,000,000	'	'	'
Casualty Business | Layer Two | Casualty Reinsurance Program	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	5.00%	'	'	'
Casualty Business | Layer Two | Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	10,000,000	'	'	'
Casualty Business | Layer Two | Casualty Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	11,000,000	'	'	'
Workers Compensation Losses | Casualty Reinsurance Program	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance reinsured risk, percentage	100.00%	'	'	'
Workers Compensation Losses | Casualty Reinsurance Program | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	1,000,000	'	'	'
Workers Compensation Losses | Casualty Reinsurance Program | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	10,000,000	'	'	'
Inland and Ocean Marine | Marine Treaty Per Occurrence	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	2,000,000	'	'	'
Inland and Ocean Marine | Marine Treaty | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	2,000,000	'	'	'
Inland and Ocean Marine | Marine Treaty | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	60,000,000	'	'	'
Inland and Ocean Marine | Property Per Risk Treaty | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	20,000,000	'	'	'
Inland and Ocean Marine | Layer One | Marine Treaty	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Reinsurance amount retained	5,000,000	'	'	'
Amount retained, per risk	2,000,000	'	'	'
Reinsurance reinsured risk, percentage	60.00%	'	'	'
Inland and Ocean Marine | Layer One | Marine Treaty | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	7,000,000	'	'	'
Inland and Ocean Marine | Layer One | Ocean Marine	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Insurance deductible for individual large claims	1,500,000	'	'	'
Inland and Ocean Marine | Layer One | Inland Marine	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Insurance deductible for individual large claims	1,500,000	'	'	'
Inland and Ocean Marine | Layer One | Catastrophic Losses	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Insurance deductible for individual large claims	5,000,000	'	'	'
Surety Underwriting | Insurance Per Bond	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	5,000,000	'	'	'
Reinsurance reinsured risk, percentage	100.00%	'	'	'
Surety Underwriting | Insurance Per Bond | Minimum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	30,000,000	'	'	'
Surety Underwriting | Insurance Per Bond | Maximum	'	'	'	'
Reinsurance Retention Policy [Line Items]	'	'	'	'
Amount retained, per risk	$60,000,000	'	'	'

Reinsurance_Credit_Risk_Detail

Reinsurance - Credit Risk (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'
Reinsurance recoverables	$89.90	$110.60
% of total	100.00%	'
AA	'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'
Reinsurance recoverables	26.4	'
% of total	30.00%	'
A	'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'
Reinsurance recoverables	56.1	'
% of total	62.00%	'
BBB, Not Rated and Other	'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'	'
Reinsurance recoverables	$7.40	'
% of total	8.00%	'

Investment_Securities_Net_inve

Investment Securities (Net investment income) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Gross investment income	$47.90	$60.20	$78.30
Less investment expenses	-6.8	-6.6	-6.9
Net investment income	41.1	53.6	71.4
Fixed maturity investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Gross investment income	40.6	49.1	69.7
Short-term investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Gross investment income	0	0.1	0.1
Common equity securities	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Gross investment income	6.2	6.8	5.3
Convertible fixed maturity investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Gross investment income	1.2	4.6	3.5
Other investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Gross investment income	($0.10)	($0.40)	($0.30)

Investment_Securities_Net_real

Investment Securities (Net realized investment gains (losses)) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Net realized investment gains, pre-tax	$44.20	$50.80	$56.80
Trading Securities, Tax Realized Holding Gain (Loss) on Investments	-20.5	-17.2	-19.9
Trading Securities, Tax Realized Holding Gain (Loss) on Investments, After Tax	23.7	33.6	36.9
Trading Securities, Realized Gain	70.1	68.4	77.9
Trading Securities, Realized Loss	-25.9	-17.6	-21.1
Accounts payable on unsettled investment purchases	11.6	6.2	'
Accounts receivable on unsettled investment sales	3.3	2.1	'
Fixed maturity investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Net realized investment gains, pre-tax	2.1	27.1	32.1
Short-term investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Net realized investment gains, pre-tax	0	0	0
Common equity securities	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Net realized investment gains, pre-tax	36.1	2.1	16.7
Convertible fixed maturity investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Net realized investment gains, pre-tax	0.3	0.4	-0.4
Other investments	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'
Net realized investment gains, pre-tax	$5.70	$21.20	$8.40

Investment_Securities_Changes_

Investment Securities (Changes in fair value) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	$5.10	[1]	$4.80	[1]	($46.10)	[1]
Changes in net foreign currency translation gains (losses)	0.1	[1]	0.1	[1]	-0.1	[1]
Total net changes in fair value reflected in revenues	5.2	[1]	4.9	[1]	-46.2	[1]
Change in deferred gains (losses)	-0.8		-0.2		-5.2
Fixed maturity investments	'		'		'
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	-34.4	[1]	7.2	[1]	-18.7	[1]
Changes in net foreign currency translation gains (losses)	0	[1]	0.1	[1]	-0.1	[1]
Total net changes in fair value reflected in revenues	-34.4	[1]	7.3	[1]	-18.8	[1]
Short-term investments	'		'		'
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	0.1	[1]	0	[1]	0	[1]
Changes in net foreign currency translation gains (losses)	0	[1]	0	[1]	0.1	[1]
Total net changes in fair value reflected in revenues	0.1	[1]	0	[1]	0.1	[1]
Common equity securities	'		'		'
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	32	[1]	11.5	[1]	-22.1	[1]
Changes in net foreign currency translation gains (losses)	0.1	[1]	0	[1]	-0.1	[1]
Total net changes in fair value reflected in revenues	32.1	[1]	11.5	[1]	-22.2	[1]
Convertible fixed maturity investments	'		'		'
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	1.1	[1]	-0.4	[1]	-8.4	[1]
Changes in net foreign currency translation gains (losses)	0	[1]	0	[1]	0	[1]
Total net changes in fair value reflected in revenues	1.1	[1]	-0.4	[1]	-8.4	[1]
Other investments	'		'		'
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	6.3	[1]	-13.5	[1],[2]	3.1	[1]
Changes in net foreign currency translation gains (losses)	0	[1]	0	[1],[2]	0	[1]
Total net changes in fair value reflected in revenues	6.3	[1]	-13.5	[1],[2]	3.1	[1]
OneBeacon Holdings (Luxembourg) S.a r.l.	'		'		'
Total net changes in fair value reflected in revenues	'		'		'
Changes in net unrealized investment gains (losses)	'		($11.60)		'
[1]	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.8) million, $(0.2) million, and $(5.2) million for the years ended DecemberB 31, 2013, 2012, and 2011, respectively.
[2]	The year ended December 31, 2012, includes a change in net unrealized gains of $(11.6) million related to the sale of a limited partnership that had been consolidated into OneBeacon's results. See Note 15b"Related Party Disclosures."

Investment_Securities_Net_unre

Investment Securities (Net unrealized investment gains and losses) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investments [Abstract]	'	'
Gross unrealized investment gains	$133.70	$127.40
Gross unrealized investment losses	9.3	8.7
Total net unrealized investment gains, pre-tax	124.4	118.7
Income taxes	-33.8	-39.4
Total net unrealized investment gains, after tax	$90.60	$79.30

Investment_Securities_Fixed_ma

Investment Securities (Fixed maturity investment gains and losses) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Gross unrealized gains	$133.70		$127.40
Gross unrealized losses	-9.3		-8.7
Carrying value	1,700.90		1,593.30
Total investment securities	2,364.90		2,291.50
U.S. Government and agency obligations	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	131	[1]	197.1	[1]
Gross unrealized gains	0.2	[1]	0.5	[1]
Gross unrealized losses	-0.1	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	131.1	[1]	197.6	[1]
Debt securities issued by industrial corporations	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	741.7	[1]	678.7	[1]
Gross unrealized gains	14.1	[1]	32.8	[1]
Gross unrealized losses	-1.3	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	754.5	[1]	711.5	[1]
Municipal obligations	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	16.9		3.3	[1]
Gross unrealized gains	0	[1]	0	[1]
Gross unrealized losses	-0.4	[1]	-0.1	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	16.5	[1]	3.2	[1]
Asset-backed securities	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	951.5	[1]	918.5	[1]
Gross unrealized gains	1.2	[1]	9.7	[1]
Gross unrealized losses	-3.2	[1]	-0.3	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	949.5	[1]	927.9	[1]
Foreign government obligations	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	2	[1]	6	[1]
Gross unrealized gains	0.3	[1]	0.5	[1]
Gross unrealized losses	0	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	2.3	[1]	6.5	[1]
Preferred stocks	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	78.3	[1]	78.3	[1]
Gross unrealized gains	5	[1]	6.4	[1]
Gross unrealized losses	0	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	83.3	[1]	84.7	[1]
Total fixed maturity investments	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Cost or amortized cost	1,921.40	[1]	1,881.90	[1]
Gross unrealized gains	20.8	[1]	49.9	[1]
Gross unrealized losses	-5	[1]	-0.4	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	1,937.20	[1]	1,931.40	[1]
Segment, Discontinued Operations | Runoff	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Total investment securities	$236.30		$338.10
[1]	Includes carrying value of $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheets as part of the Runoff Transaction as of DecemberB 31, 2013 and 2012, respectively.

Investment_Securities_Cost_or_

Investment Securities (Cost or amortized cost fixed maturity investments) (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Trading Securities by Maturity Cost or Amortized Cost [Abstract]	'		'
Total	$1,947.70	[1]	'
Trading Securities by Maturity Fair Value [Abstract]	'		'
Total	1,967.70	[1]	'
Total investment securities	2,364.90		2,291.50
Fixed Maturities and Convertible Fixed Maturities Excluding Mortgage backed securities, Asset backed securities and Preferred Stock [Member]	'		'
Trading Securities by Maturity Cost or Amortized Cost [Abstract]	'		'
Due in one year or less	184.9	[1]	'
Due after one year through five years	664.7	[1]	'
Due after five years through ten years	59.1	[1]	'
Due after ten years	9.2	[1]	'
Trading Securities by Maturity Fair Value [Abstract]	'		'
Due in one year or less	188	[1]	'
Due after one year through five years	674.6	[1]	'
Due after five years through ten years	61.3	[1]	'
Due after ten years	11	[1]	'
Asset-backed securities	'		'
Trading Securities by Maturity Cost or Amortized Cost [Abstract]	'		'
Without Single Maturity Date	951.5	[1]	'
Trading Securities by Maturity Fair Value [Abstract]	'		'
Without Single Maturity Date	949.5	[1]	'
Preferred stocks	'		'
Trading Securities by Maturity Cost or Amortized Cost [Abstract]	'		'
Without Single Maturity Date	78.3	[1]	'
Trading Securities by Maturity Fair Value [Abstract]	'		'
Without Single Maturity Date	83.3	[1]	'
Runoff | Segment, Discontinued Operations	'		'
Trading Securities by Maturity Fair Value [Abstract]	'		'
Total investment securities	$236.30		$338.10
[1]	Includes carrying value of $236.3 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheet as part of the Runoff Transaction as of DecemberB 31, 2013

Investment_Securities_Converti

Investment Securities (Convertible fixed maturity investments and other investments) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Gross unrealized gains	$133.70		$127.40		'
Gross unrealized investment losses	9.3		8.7		'
Carrying value	1,700.90		1,593.30		'
Proceeds from Sale, Maturity and Collection of Long-term Investments	1,887.70		2,050.50		2,032.10
Assets Held by Insurance Regulators	152.2		174.3		'
Investments Held in Trusts	38.4		50.7		'
Trust Balances of Investments Held in Trusts	9.9		15.3		'
Unrestricted collateral from customers	63.3		5.9		'
Fixed maturity investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	1,921.40	[1]	1,881.90	[1]	'
Gross unrealized gains	20.8	[1]	49.9	[1]	'
Gross unrealized investment losses	5	[1]	0.4	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	1,937.20	[1]	1,931.40	[1]	'
Preferred stocks	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	78.3	[1]	78.3	[1]	'
Gross unrealized gains	5	[1]	6.4	[1]	'
Gross unrealized investment losses	0	[1]	0	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	83.3	[1]	84.7	[1]	'
Asset-backed securities	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	951.5	[1]	918.5	[1]	'
Gross unrealized gains	1.2	[1]	9.7	[1]	'
Gross unrealized investment losses	3.2	[1]	0.3	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	949.5	[1]	927.9	[1]	'
Debt securities issued by industrial corporations	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	741.7	[1]	678.7	[1]	'
Gross unrealized gains	14.1	[1]	32.8	[1]	'
Gross unrealized investment losses	1.3	[1]	0	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	754.5	[1]	711.5	[1]	'
U.S. Government and agency obligations	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	131	[1]	197.1	[1]	'
Gross unrealized gains	0.2	[1]	0.5	[1]	'
Gross unrealized investment losses	0.1	[1]	0	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	131.1	[1]	197.6	[1]	'
Common equity securities	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	267.2		221.6		'
Gross unrealized gains	70.4		39.8		'
Gross unrealized investment losses	0.9		2.4		'
Net foreign currency gains	0.2		0		'
Carrying value	336.9		259		'
Convertible fixed maturity investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	26.3		59.5		'
Gross unrealized gains	4.5		3.2		'
Gross unrealized investment losses	0.3		0.1		'
Net foreign currency gains	0		0		'
Carrying value	30.5		62.6		'
Other investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	104.7		115.1		'
Gross unrealized gains	38		34.5		'
Gross unrealized investment losses	3.1		5.8		'
Net foreign currency gains	0		0		'
Carrying value	139.6		143.8		'
Total common equity securities, convertible fixed maturity investments and other investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	398.2		396.2		'
Gross unrealized gains	112.9		77.5		'
Gross unrealized investment losses	4.3		8.3		'
Net foreign currency gains	0.2		0		'
Carrying value	507		465.4		'
Municipal obligations	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	16.9		3.3	[1]	'
Gross unrealized gains	0	[1]	0	[1]	'
Gross unrealized investment losses	0.4	[1]	0.1	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	16.5	[1]	3.2	[1]	'
Foreign government obligations	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Cost or amortized cost	2	[1]	6	[1]	'
Gross unrealized gains	0.3	[1]	0.5	[1]	'
Gross unrealized investment losses	0	[1]	0	[1]	'
Net foreign currency gains	0	[1]	0	[1]	'
Carrying value	2.3	[1]	6.5	[1]	'
Esurance [Member]	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Assets Held by Insurance Regulators	$22.60		$25.90		'
[1]	Includes carrying value of $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheets as part of the Runoff Transaction as of DecemberB 31, 2013 and 2012, respectively.

Investment_Securities_Fair_val

Investment Securities (Fair value measurements) (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Percentage of fair value investments using observable inputs	92.00%		92.00%
Tax development fund	$19.70		$21.10
Debt securities issued by corporations	139.6		143.8
Percentage of private preferred stock	85.00%		84.00%
Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	469.5	[1]	462.3	[1]
Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1,753.10	[2]	1,714.50	[2]
Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	201.9	[3]	198.9	[3]
Hedge funds	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Number of Investments	8		8
Debt securities issued by corporations	49.4		47.3
Private equity funds	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Number of Investments	19		17
Debt securities issued by corporations	56.1		61.3
Community Reinvestment Vehicle [Member]	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	14.4		14.1
Single Fund	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	14.9		12.9
Debt securities issued by industrial corporations	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	754.5		711.5
Asset-backed securities | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	949.5		927.9
Asset-backed securities | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	938.6		922.6
Asset-backed securities | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	10.9		5.3
Fixed maturity investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	132.7		203.4
Fixed maturity investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1,722.60		1,651.90
Fixed maturity investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	81.9		76.1
Convertible fixed maturity investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Convertible fixed maturity investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	30.5		62.6
Convertible fixed maturity investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Other investments	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Percentage of investments based on observable inputs	5.00%		5.00%
Other investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Other investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Other investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	119.9	[3]	122.7	[3]
Recurring basis | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	2,581.50	[4],[5]	2,608.50	[4],[5]
Recurring basis | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	625.4	[5]	695.1	[5]
Recurring basis | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1,754.20	[5]	1,714.50	[5]
Recurring basis | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	201.9	[5]	198.9	[5]
Recurring basis | U.S. Government and agency obligations | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	131.1	[4]	197.6	[4]
Recurring basis | U.S. Government and agency obligations | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	131.1		197.6
Recurring basis | U.S. Government and agency obligations | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | U.S. Government and agency obligations | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	754.5	[4]	711.5	[4]
Recurring basis | Debt securities issued by industrial corporations | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	754.5		711.5
Recurring basis | Debt securities issued by industrial corporations | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Consumer | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	239.6	[4]	249.9	[4]
Recurring basis | Debt securities issued by industrial corporations | Consumer | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Consumer | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	239.6		249.9
Recurring basis | Debt securities issued by industrial corporations | Consumer | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Industrial | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	106	[4]	103.5	[4]
Recurring basis | Debt securities issued by industrial corporations | Industrial | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Industrial | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	106		103.5
Recurring basis | Debt securities issued by industrial corporations | Industrial | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Financial | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	145.5	[4]	92.3	[4]
Recurring basis | Debt securities issued by industrial corporations | Financial | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Financial | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	145.5		92.3
Recurring basis | Debt securities issued by industrial corporations | Financial | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Communications | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	91.5	[4]	70	[4]
Recurring basis | Debt securities issued by industrial corporations | Communications | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Communications | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	91.5		70
Recurring basis | Debt securities issued by industrial corporations | Communications | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Energy | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	40.5	[4]	57.1	[4]
Recurring basis | Debt securities issued by industrial corporations | Energy | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Energy | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	40.5		57.1
Recurring basis | Debt securities issued by industrial corporations | Energy | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Basic materials | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	64.4	[4]	77.8	[4]
Recurring basis | Debt securities issued by industrial corporations | Basic materials | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Basic materials | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	64.4		77.8
Recurring basis | Debt securities issued by industrial corporations | Basic materials | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Utilities | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	38	[4]	46.5	[4]
Recurring basis | Debt securities issued by industrial corporations | Utilities | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Utilities | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	38		46.5
Recurring basis | Debt securities issued by industrial corporations | Utilities | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Technology | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	29	[4]	14.4	[4]
Recurring basis | Debt securities issued by industrial corporations | Technology | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Debt securities issued by industrial corporations | Technology | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	29		14.4
Recurring basis | Debt securities issued by industrial corporations | Technology | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Municipal obligations | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	16.5	[4]	3.2	[4]
Recurring basis | Municipal obligations | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Municipal obligations | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	16.5		3.2
Recurring basis | Municipal obligations | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Asset-backed securities | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	949.5	[4]	927.9	[4]
Recurring basis | Asset-backed securities | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Asset-backed securities | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	938.6		922.6
Recurring basis | Asset-backed securities | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	10.9		5.3
Recurring basis | Foreign government obligations | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	2.3	[4]	6.5	[4]
Recurring basis | Foreign government obligations | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1.6		5.8
Recurring basis | Foreign government obligations | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0.7		0.7
Recurring basis | Foreign government obligations | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Preferred stocks | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	83.3	[4]	84.7	[4]
Recurring basis | Preferred stocks | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Preferred stocks | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	12.3		13.9
Recurring basis | Preferred stocks | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	71		70.8
Recurring basis | Fixed maturity investments | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1,937.20	[4]	1,931.40	[4]
Recurring basis | Fixed maturity investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	132.7		203.4
Recurring basis | Fixed maturity investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1,722.60		1,651.90
Recurring basis | Fixed maturity investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	81.9		76.1
Recurring basis | Short-term investments | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	157	[4]	232.8	[4]
Recurring basis | Short-term investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	155.9		232.8
Recurring basis | Short-term investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	1.1		0
Recurring basis | Short-term investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	336.9	[4]	259	[4]
Recurring basis | Common equity securities | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	336.8		258.9
Recurring basis | Common equity securities | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0.1		0.1
Recurring basis | Common equity securities | Consumer | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	122.6	[4]	80.5	[4]
Recurring basis | Common equity securities | Consumer | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	122.6		80.5
Recurring basis | Common equity securities | Consumer | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Consumer | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Financial | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	83.3	[4]	48.4	[4]
Recurring basis | Common equity securities | Financial | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	83.2		48.3
Recurring basis | Common equity securities | Financial | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Financial | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0.1		0.1
Recurring basis | Common equity securities | Energy | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	34.7	[4]	38.1	[4]
Recurring basis | Common equity securities | Energy | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	34.7		38.1
Recurring basis | Common equity securities | Energy | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Energy | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Basic materials | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	20.3	[4]	42.9	[4]
Recurring basis | Common equity securities | Basic materials | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	20.3		42.9
Recurring basis | Common equity securities | Basic materials | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Basic materials | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Utilities | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	8.3	[4]	17	[4]
Recurring basis | Common equity securities | Utilities | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	8.3		17
Recurring basis | Common equity securities | Utilities | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Utilities | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Other | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	67.7	[4]	32.1	[4]
Recurring basis | Common equity securities | Other | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	67.7		32.1
Recurring basis | Common equity securities | Other | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Common equity securities | Other | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Convertible fixed maturity investments | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	30.5	[4]	62.6	[4]
Recurring basis | Convertible fixed maturity investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Convertible fixed maturity investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	30.5		62.6
Recurring basis | Convertible fixed maturity investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0		0
Recurring basis | Other investments | Fair Value	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	119.9	[4],[5]	122.7	[4],[5]
Recurring basis | Other investments | Level 1 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0	[5]	0	[5]
Recurring basis | Other investments | Level 2 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	0	[5]	0	[5]
Recurring basis | Other investments | Level 3 Inputs	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Fair value of investments	119.9	[5]	122.7	[5]
Segment, Discontinued Operations	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Securities reclassified to assets held for sale	$236.30		$338.10
[1]	Excludes short-term investments which are deemed to have a LevelB 1 designation.
[2]	Excludes short-term investments which are deemed to have a LevelB 2 designation.
[3]	Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method.
[4]	1)Excludes the carrying value of $21.1 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of DecemberB 31, 2012.
[5]	(2)Fair value includes $338.1 million of fixed maturity investments reclassified to assets held for sale in the DecemberB 31, 2012 consolidated balance sheets as part of the Runoff Transaction.

Investment_Securities_Corporat

Investment Securities (Corporate debt securites) (Details) (Debt securities issued by industrial corporations, USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'
Debt securities issued by corporations	$754.50	$711.50
AA	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'
Debt securities issued by corporations	51.2	37
A	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'
Debt securities issued by corporations	357.9	301.6
BBB	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'
Debt securities issued by corporations	345.4	362.5
BB	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'
Debt securities issued by corporations	0	7
Other	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'
Debt securities issued by corporations	$0	$3.40

Investment_Securities_Fair_val1

Investment Securities (Fair value rollforward) (Details) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2013
Level 1 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		$462.30	[1]
Amortization/accretion	'		-0.1	[1]
Total net realized and unrealized gains (losses)	'		65.8	[1]
Purchases	'		385.4	[1]
Sales	'		-443.9	[1]
Transfers in	'		0	[1]
Transfers out	'		0	[1]
Investments at fair value ending balance	469.5	[1]	469.5	[1]
Level 2 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		1,714.50	[2]
Amortization/accretion	'		-11.4	[2]
Total net realized and unrealized gains (losses)	'		-26.9	[2]
Purchases	'		2,188.50	[2]
Sales	'		-2,125.20	[2]
Transfers in	'		74.9	[2]
Transfers out	'		61.3	[2]
Investments at fair value ending balance	1,753.10	[2]	1,753.10	[2]
Level 3 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		198.9	[3]
Amortization/accretion	'		0	[3]
Total net realized and unrealized gains (losses)	'		10.4	[3]
Purchases	'		28.5	[3]
Sales	'		-22.3	[3]
Transfers in	'		61.3	[3]
Transfers out	'		74.9	[3]
Investments at fair value ending balance	201.9	[3]	201.9	[3]
Fixed maturity investments | Level 1 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		203.4
Amortization/accretion	'		-0.1
Total net realized and unrealized gains (losses)	'		-2.4
Purchases	'		179
Sales	'		-247.2
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	132.7		132.7
Fixed maturity investments | Level 2 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		1,651.90
Amortization/accretion	'		-11.3
Total net realized and unrealized gains (losses)	'		-28.3
Purchases	'		2,174.10
Sales	'		-2,077.40
Transfers in	'		74.9
Transfers out	'		61.3
Investments at fair value ending balance	1,722.60		1,722.60
Fixed maturity investments | Level 3 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		76.1
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		-1.5
Purchases	'		21.4
Sales	'		-0.5
Transfers in	61.3		61.3
Transfers out	'		74.9
Investments at fair value ending balance	81.9		81.9
Transfers out recategorized	-74.9		'
Common equity securities | Level 1 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		258.9
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		68.2
Purchases	'		206.4
Sales	'		-196.7
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	336.8		336.8
Common equity securities | Level 2 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		0
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		0
Purchases	'		0
Sales	'		0
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	0		0
Common equity securities | Level 3 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		0.1
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		0
Purchases	'		0
Sales	'		0
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	0.1		0.1
Convertible fixed maturity investments | Level 1 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		0
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		0
Purchases	'		0
Sales	'		0
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	0		0
Convertible fixed maturity investments | Level 2 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		62.6
Amortization/accretion	'		-0.1
Total net realized and unrealized gains (losses)	'		1.4
Purchases	'		14.4
Sales	'		-47.8
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	30.5		30.5
Convertible fixed maturity investments | Level 3 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		0
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		0
Purchases	'		0
Sales	'		0
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	0		0
Other investments | Level 1 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		0
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		0
Purchases	'		0
Sales	'		0
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	0		0
Other investments | Level 2 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		0
Amortization/accretion	'		0
Total net realized and unrealized gains (losses)	'		0
Purchases	'		0
Sales	'		0
Transfers in	'		0
Transfers out	'		0
Investments at fair value ending balance	0		0
Other investments | Level 3 Inputs	'		'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'		'
Investments at fair value beginning balance	'		122.7	[3]
Amortization/accretion	'		0	[3]
Total net realized and unrealized gains (losses)	'		11.9	[3]
Purchases	'		7.1	[3]
Sales	'		-21.8	[3]
Transfers in	'		0	[3]
Transfers out	'		0	[3]
Investments at fair value ending balance	$119.90	[3]	$119.90	[3]
[1]	Excludes short-term investments which are deemed to have a LevelB 1 designation.
[2]	Excludes short-term investments which are deemed to have a LevelB 2 designation.
[3]	Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method.

Investment_Securities_Securiti

Investment Securities (Securities classified as level 3) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
Significant unobservable inputs	'
Number of Securities in each Asset type	1
Standard Poors N R Rating [Member] | Total Non-agency | AA	'
Significant unobservable inputs	'
Fair Value	$10.90
Standard Poors N R Rating [Member] | Preferred stocks | Discounted cash flow	'
Significant unobservable inputs	'
Fair Value	$71
Discount Rate	7.40%	[1]
[1]	As of DecemberB 31, 2013, each asset type consists of one security.

Investment_Securities_Change_i

Investment Securities (Change in net unrealized gains or losses for Level 3 assets) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	$5.10	[1]	$4.80	[1]	($46.10)	[1]
Fixed maturity investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	-34.4	[1]	7.2	[1]	-18.7	[1]
Short-term investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	0.1	[1]	0	[1]	0	[1]
Convertible fixed maturity investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	1.1	[1]	-0.4	[1]	-8.4	[1]
Other investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	6.3	[1]	-13.5	[1],[2]	3.1	[1]
Level 3	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	4.8		-6.8		-4.4
Level 3 | Fixed maturity investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	-1.5		6.7		-7.5
Level 3 | Short-term investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	0		0		0
Level 3 | Common equity securities	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	0		0		0
Level 3 | Convertible fixed maturity investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	0		0		0
Level 3 | Other investments	'		'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'		'
Unrealized gains (losses) on unsold securities	$6.30		($13.50)		$3.10
[1]	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.8) million, $(0.2) million, and $(5.2) million for the years ended DecemberB 31, 2013, 2012, and 2011, respectively.
[2]	The year ended December 31, 2012, includes a change in net unrealized gains of $(11.6) million related to the sale of a limited partnership that had been consolidated into OneBeacon's results. See Note 15b"Related Party Disclosures."

Investment_Securities_Asset_ba

Investment Securities (Asset backed securities) (Details) (USD $)

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

In Millions, unless otherwise specified

Level 2

Level 2

Level 3

Level 3

Fixed rate CMBS

Fixed rate CMBS

Floating rate CMBS

GNMA

GNMA

GNMA

GNMA

GNMA

GNMA

FNMA

FNMA

FNMA

FNMA

FNMA

FNMA

FHLMC

FHLMC

FHLMC

FHLMC

FHLMC

FHLMC

Agency commercial mortgage-backed securities

Agency commercial mortgage-backed securities

Agency commercial mortgage-backed securities

Agency commercial mortgage-backed securities

Agency commercial mortgage-backed securities

Agency commercial mortgage-backed securities

Residential

Residential

Residential

Residential

Residential

Residential

Residential

Commercial

Commercial

Commercial

Commercial

Commercial

Commercial

Total Non-agency

Total Non-agency

Total Non-agency

Total Non-agency

Total Non-agency

Total Non-agency

Total mortgage-backed securities

Total mortgage-backed securities

Total mortgage-backed securities

Total mortgage-backed securities

Total mortgage-backed securities

Total mortgage-backed securities

Credit card receivables

Credit card receivables

Credit card receivables

Credit card receivables

Credit card receivables

Credit card receivables

Vehicle receivables

Vehicle receivables

Vehicle receivables

Vehicle receivables

Vehicle receivables

Vehicle receivables

Other

Other

Other

Other

Other

Other

Total other asset-backed securities

Total other asset-backed securities

Total other asset-backed securities

Total other asset-backed securities

Total other asset-backed securities

Total other asset-backed securities

Total asset-backed securities

Total asset-backed securities

Total asset-backed securities

Total asset-backed securities

Total asset-backed securities

Total asset-backed securities

Non-Prime Residential Mortgage Backed Security

Maximum

Fair Value

Fair Value

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Fair Value

Level 2

Level 2

Level 3

Level 3

Fair Value

Commercial

Schedule of Trading Securities and Other Trading Assets [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Average basis points of subordination

'

'

'

'

30

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

'

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'

'

'

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'

'

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'

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'

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'

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'

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'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of underlying loans reported as non-performing

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

'

'

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'

'

'

'

'

'

'

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'

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'

'

'

'

'

'

'

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'

'

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'

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'

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'

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'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.00%

Debt securities issued by corporations

$1,753.10

[1]

$1,714.50

[1]

$201.90

[2]

$198.90

[2]

'

$113.90

$41.10

$321.80

$551.20

$321.80

$551.20

$0

$0

$36.50

$13.90

$36.50

$13.90

$0

$0

$24.30

$10.50

$24.30

$10.50

$0

$0

$382.60

[3]

$575.60

[3]

$382.60

[3]

$575.60

[3]

$0

[3]

$0

[3]

$51.50

$51.50

$38.10

$51.50

$38.10

$0

$0

$155

$175.40

$155

$175.40

$0

$0

$206.50

$213.50

$206.50

$213.50

$0

$0

$589.10

$789.10

$589.10

$789.10

$0

$0

$124.20

$49

$113.30

$43.70

$10.90

$5.30

$231.80

$81.50

$231.80

$81.50

$0

$0

$4.40

$8.30

$4.40

$8.30

$0

$0

$360.40

$138.80

$349.50

$133.50

$10.90

$5.30

$949.50

$927.90

$938.60

$922.60

$10.90

$5.30

$7.30

'

[1]

Excludes short-term investments which are deemed to have a LevelB 2 designation.

[2]

Excludes the carrying value of $19.7 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method.

[3]

Represents publicly traded mortgage-backed securities which carry the full faith and credit guaranty of the U.S. government (i.e.,B GNMA) or are guaranteed by a government sponsored entity (i.e.,B FNMA, FHLMC).

Investment_Securities_Nonagenc

Investment Securities (Non-agency securities) (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	$51.50		'
Super Senior | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	12.7	[1]	'
Senior | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	38.8	[2]	'
Subordinate | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0	[3]	'
Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	51.5		38.1
Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	155		175.4
Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	206.5		213.5
Fair Value | Commercial Mortgage Backed Securities Fixed Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	113.9		'
Fair Value | Commercial Mortgage Backed Securities Floating Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	41.1		'
Fair Value | Super Senior | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	85.7	[4]	'
Fair Value | Super Senior | Commercial Mortgage Backed Securities Fixed Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	84.3	[4]	'
Fair Value | Super Senior | Commercial Mortgage Backed Securities Floating Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	1.4	[4]	'
Fair Value | Senior | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	40.9	[2]	'
Fair Value | Senior | Commercial Mortgage Backed Securities Fixed Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	24.9	[2]	'
Fair Value | Senior | Commercial Mortgage Backed Securities Floating Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	16	[2]	'
Fair Value | Subordinate | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	28.4	[3]	'
Fair Value | Subordinate | Commercial Mortgage Backed Securities Fixed Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	4.7	[2]	'
Fair Value | Subordinate | Commercial Mortgage Backed Securities Floating Rate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	23.7	[3]	'
Securities Issued in 2004 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	6.8		'
Securities Issued in 2004 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities Issued in 2004 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	6.8		'
Securities Issued in 2005 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	10.1		'
Securities Issued in 2005 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities Issued in 2005 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	10.1		'
Securities Issued in 2006 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	5.4		'
Securities Issued in 2006 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities Issued in 2006 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	5.4		'
Securities issued in 2007 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities issued in 2007 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	1.4		'
Securities issued in 2007 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	1.4		'
Securities Issued in 2008 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities Issued in 2008 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	16		'
Securities Issued in 2008 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	16		'
Securities issued in 2010 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	11.1		'
Securities issued in 2010 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	5.8		'
Securities issued in 2010 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	16.9		'
Securities issued in 2011 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities issued in 2011 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	15.7		'
Securities issued in 2011 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	15.7		'
Securities issued in 2012 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0		'
Securities issued in 2012 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	81.8		'
Securities issued in 2012 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	81.8		'
Securities Issued in 2013 | Fair Value | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	18.1		'
Securities Issued in 2013 | Fair Value | Non-agency CMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	34.3		'
Securities Issued in 2013 | Fair Value | Total	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	52.4		'
Prime | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	44.2		'
Prime | Super Senior | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	12.7	[1]	'
Prime | Senior | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	31.5	[2]	'
Prime | Subordinate | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0	[3]	'
Non-prime | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	7.3		'
Non-prime | Super Senior | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	0	[1]	'
Non-prime | Senior | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	7.3	[2]	'
Non-prime | Subordinate | Non-agency RMBS	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Debt securities issued by corporations	$0	[3]	'
[1]	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by StandardB & Poor's, Aaa by Moody's, or AAA by Fitch Ratings ("Fitch") and were senior to other AAA or Aaa bonds.
[2]	At issuance, Senior, or in the case of resecuritization, the underlying securities, were rated AAA by StandardB & Poor's, Aaa by Moody's, or AAA by Fitch and were senior to non-AAA or non-Aaa bonds.
[3]	At issuance, Subordinate were not rated AAA by StandardB & Poor's, Aaa by Moody's, or AAA by Fitch and were junior to other bonds.
[4]	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by StandardB & Poor's, Aaa by Moody's or AAA by Fitch and were senior to other AAA or Aaa bonds.

Investment_Securities_Other_in

Investment Securities (Other investments) (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Community reinvestment vehicle	$14.40		'
Investment in tax advantaged federal affordable housing development fund	19.7		21.1
Long/short credit and distressed	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	8.1		8.4
Long bank loan	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0.1		0.1
Long/short equity	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	39.1		36.8
Long/short equity activist	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	2.1		2
Hedge funds	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	49.4		47.3
Distributions from inactive hedge funds	1.3		'
Redemption requests	2.1		'
Hedge funds | 30 - 59 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	26.9		'
Hedge funds | 60 - 89 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	8.1		'
Hedge funds | 90 - 119 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	8.5		'
Hedge funds | 120+ days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	5.9		'
Hedge funds | Monthly	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	5.8		'
Hedge funds | Monthly | 30 - 59 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds | Monthly | 60 - 89 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds | Monthly | 90 - 119 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds | Monthly | 120+ days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	5.8		'
Hedge funds | Quarterly	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	41.4		'
Hedge funds | Quarterly | 30 - 59 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	26.9		'
Hedge funds | Quarterly | 60 - 89 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	8.1		'
Hedge funds | Quarterly | 90 - 119 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	6.4		'
Hedge funds | Quarterly | 120+ days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds | Annual	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	2.2		'
Hedge funds | Annual | 30 - 59 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds | Annual | 60 - 89 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds | Annual | 90 - 119 days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	2.1		'
Hedge funds | Annual | 120+ days notice	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0.1		'
Insurance	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	2.3		3.1
Net asset value per share of unfunded commitments	0.1		0.1
Distressed residential real estate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0.2		7.9
Net asset value per share of unfunded commitments	0		0
Energy infrastructure and services	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	26		20.7
Net asset value per share of unfunded commitments	5.9		7.5
Healthcare	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	2.8		2.1
Net asset value per share of unfunded commitments	1.4		2.7
Multi-sector	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	14.5		14.9
Net asset value per share of unfunded commitments	2.5		3.7
Private equity secondaries	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	6.4		7
Net asset value per share of unfunded commitments	2.1		2
Real estate	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	3.9		5.6
Net asset value per share of unfunded commitments	0.1		0.1
Private equity funds	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	56.1		61.3
Net asset value per share of unfunded commitments	12.1		16.1
Private equity funds | 1 - 3 years	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	2.5		'
Private equity funds | 3 - 5 years	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	20.2		'
Private equity funds | 5 - 10 years	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	33.4		'
Private equity funds | >10 years	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	0		'
Hedge funds and private equity funds	'		'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'		'
Other investments at fair value	105.5	[1]	108.6	[1]
Net asset value per share of unfunded commitments	$12.10	[1]	$16.10	[1]
[1]	Excluded from the above table are other investments totaling $14.4 million and $14.1 million of an investment in a community reinvestment vehicle as of DecemberB 31, 2013 and 2012, respectively, and $19.7 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of DecemberB 31, 2013 and 2012, respectively.

Debt_Details

Debt (Details) (USD $)

12 Months Ended

1 Months Ended

1 Months Ended

0 Months Ended

1 Months Ended

3 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Nov. 30, 2012

Dec. 31, 2013

Dec. 31, 2012

Nov. 30, 2011

31-May-03

Mar. 24, 2011

Apr. 20, 2011

Dec. 31, 2010

OBH (issuer)

OBH (issuer)

OBH Senior Notes

OBH Senior Notes

2012 OBH Senior Notes

2012 OBH Senior Notes

2003 OBH Senior Notes

2003 OBH Senior Notes

2003 OBH Senior Notes

Senior Notes [Member]

Senior Notes [Member]

Senior Notes [Member]

OBH (issuer)

OBH (issuer)

OBH (issuer)

OBH (issuer)

OBH (issuer)

OBH (issuer)

OBH (issuer)

2003 OBH Senior Notes

2003 OBH Senior Notes

2003 OBH Senior Notes

OBH (issuer)

OBH (issuer)

OBH (issuer)

Debt

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Senior unsecured notes, at face value

'

'

'

'

'

$275

$275

'

'

'

'

$700

'

'

'

Unamortized original issue discount

'

'

'

-0.3

-0.3

'

'

'

'

'

'

'

'

'

'

Senior unsecured notes, carrying value

'

'

'

274.7

274.7

'

'

'

'

'

'

'

'

'

'

Repayments after year five

275

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Percentage of par value at which debt was issued

'

'

'

'

'

'

'

99.90%

'

'

'

'

'

'

'

Issuance of debt, net of debt issuance costs

0

271.9

0

'

'

'

'

272.9

'

'

'

'

'

'

'

Interest rate (as a percent)

'

'

'

'

'

'

'

'

4.60%

'

'

'

'

'

'

Debt issue costs

'

'

'

'

'

'

'

2.8

'

'

'

'

'

'

'

Underwriting discount

'

'

'

'

'

'

'

1.8

'

'

'

'

'

'

'

Effective yield (as a percent)

'

'

'

'

'

'

'

'

4.70%

'

'

'

'

'

'

Debt

274.7

274.7

'

'

'

'

'

'

'

'

269.8

'

'

'

'

Aggregate principal amount of senior notes purchased and retired

'

'

'

'

'

'

'

'

'

275.9

'

'

'

'

'

Loss on debt extinguishment

'

'

'

'

'

'

'

'

'

'

'

'

'

-12

6.3

Write off of debt issuance

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.2

Interest expense

13

16.9

20.5

'

'

'

'

'

'

'

'

'

'

'

'

Interest Paid

12.7

16.4

20.4

'

'

'

'

'

'

'

'

'

'

'

'

Extinguishment of debt, tender offer

'

'

'

'

'

'

'

'

'

'

'

'

150

'

'

Repurchases of debt

$0

$275.90

$161.60

'

'

'

'

'

'

'

'

'

'

$161.60

'

Income_Taxes_Details

Income Taxes (Details) (USD $)

0 Months Ended

3 Months Ended

12 Months Ended

12 Months Ended

2 Months Ended

12 Months Ended

2 Months Ended

In Millions, unless otherwise specified

Jan. 24, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Feb. 28, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Feb. 28, 2013

Temporary Differences [Member]

Foreign Tax Authority [Member]

Foreign Tax Authority [Member]

Foreign Tax Authority [Member]

Domestic Tax Authority

Domestic Tax Authority

Domestic Tax Authority

Houston General Insurance Exchange [Member]

LUXEMBOURG

LUXEMBOURG

LUXEMBOURG

HGIE [Member]

HGIE [Member]

HGIE [Member]

Tax Year 2005 to 2006 [Member]

Tax Assessment Year 2007 to 2009 [Member]

Subsequent Event [Member]

Domestic Tax Authority

LUXEMBOURG

Internal Revenue Service (IRS) [Member]

Internal Revenue Service (IRS) [Member]

Tax Year 2005 to 2006 [Member]

Internal Revenue Service (IRS) [Member]

Income Tax Contingency [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Income (Loss) from Continuing Operations before Income Taxes, Foreign

'

'

'

'

'

'

'

'

'

$60.30

$75.60

$73.30

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Change in valuation allowance

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

6.6

'

'

'

'

'

'

'

'

'

Less valuation allowance

'

1.9

'

'

'

7.8

'

'

'

1.9

7.8

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Gross deferred tax asset

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.3

13.3

13

0.3

'

'

'

Deferred Tax Assets, Gross

'

164.2

'

'

'

234.1

'

'

'

164.2

234.1

'

'

'

'

'

'

'

'

'

'

'

'

'

1.6

'

'

'

'

'

Proceeds from sale of business

24.7

'

'

'

'

'

'

'

'

31.3

15

0

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.50%

0.60%

0.30%

'

'

'

'

29.22%

28.80%

'

'

'

'

'

'

'

Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent

'

'

'

'

'

'

'

'

'

35.00%

35.00%

35.00%

'

'

'

'

'

28.10%

26.10%

31.60%

'

'

'

'

'

'

'

'

'

'

Unrecognized Tax Benefits that Would Impact Effective Tax Rate

'

16

'

'

'

'

'

'

'

16

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Income Taxes Paid, Net

'

'

'

'

'

'

'

'

'

5.6

0.8

-1.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Unrecognized Tax Benefits

'

32.6

'

'

'

39

'

'

'

32.6

39

51.7

70.8

16.6

'

'

'

8.4

7.8

'

'

'

'

'

'

'

'

'

'

'

Income Tax Examination, Increase (Decrease) in Liability from Prior Year

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

3.3

68.3

'

Income Tax Examination, Increase (Decrease) in Liability from Prior Year, Related to Items with Unallowable Deductions

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2.7

60.2

'

Income tax benefit

'

18.4

-2.9

-1

19.8

-19.7

14.2

0.4

13.5

34.3

8.4

14.8

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5

Unrecognized Tax Benefits, Potential Decrease in Next Twelve Months

'

'

'

'

'

'

'

'

'

5.2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Unrecognized Tax Benefits, Interest on Income Taxes Expense

'

'

'

'

'

'

'

'

'

1

1.2

1.9

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total net operating loss carryforwards

'

38.1

'

'

'

'

'

'

'

38.1

'

'

'

'

'

'

'

37.1

'

'

'

'

'

1

'

'

'

'

'

'

Losses Included in Loss Carryforwards Subject to Annual Limitation on Untilzation

'

-0.5

'

'

'

'

'

'

'

-0.5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Operating Income (Loss)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-3.8

'

'

'

'

'

'

'

'

'

Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax

'

$10.60

'

'

'

'

'

'

'

$10.60

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Effective Income Tax Rate Reconciliation, Percent

'

'

'

'

'

'

'

'

'

-18.90%

-7.90%

-12.40%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Income_Taxes_Income_Tax_Expens

Income Taxes - Income Tax Expense (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Deferred tax expense:	'	'	'	'	'	'	'	'	'	'	'
Total deferred tax expense	'	'	'	'	'	'	'	'	$12.80	$16.40	$31.60
Total income tax expense	18.4	-2.9	-1	19.8	-19.7	14.2	0.4	13.5	34.3	8.4	14.8
Continuing Operations [Member]	'	'	'	'	'	'	'	'	'	'	'
Current tax (expense) benefit:	'	'	'	'	'	'	'	'	'	'	'
Federal	'	'	'	'	'	'	'	'	20.3	-10.1	-18.5
State	'	'	'	'	'	'	'	'	0.8	1.5	1.6
Non-U.S.	'	'	'	'	'	'	'	'	0.4	0.6	0.1
Total current tax (expense) benefit	'	'	'	'	'	'	'	'	21.5	-8	-16.8
Deferred tax expense:	'	'	'	'	'	'	'	'	'	'	'
Federal	'	'	'	'	'	'	'	'	12.8	16.4	31.6
State	'	'	'	'	'	'	'	'	0	0	0
Non-U.S.	'	'	'	'	'	'	'	'	0	0	0
Total deferred tax expense	'	'	'	'	'	'	'	'	12.8	16.4	31.6
Total income tax expense	'	'	'	'	'	'	'	'	$34.30	$8.40	$14.80

Income_Taxes_Reconciliation_De

Income Taxes - Reconciliation (Details) (Continuing Operations [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Continuing Operations [Member]	'	'	'
Income Tax Contingency [Line Items]	'	'	'
Tax expense at the U.S. statutory rate	$63.50	$37.10	$41.90
Non-U.S. earnings, net of foreign taxes	-20.9	-26	-25.5
Tax reserve adjustments	1	1.2	1.9
Tax exempt interest and dividends	2.4	2.3	2.1
Change in valuation allowance	-5.9	0.7	0.7
Other, net	-1	-2.3	-2.1
Total income tax expense on pre-tax income from continuing operations	$34.30	$8.40	$14.80

Income_Taxes_Deferred_Income_T

Income Taxes - Deferred Income Taxes (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Deferred Tax Assets, Gross [Abstract]	'	'
Discounting of loss and LAE reserves	$35.30	$41.60
Unearned premiums	31.6	39.8
Sale of Runoff Business	24.2	49.2
Compensation and bonus accruals	20.3	15.6
U.S. net operating loss carryforwards	13	42.6
Tax credit carryforwards	11.1	6.2
Investment basis differences	8.9	4.6
Deferred compensation plans	8	8.4
Fixed assets	4.7	3
Accrued rent	3.6	3.7
Allowance for doubtful accounts	1.1	1.2
Other accrued compensation	0.5	0.7
Non-U.S. net operating loss carryforwards	0.3	0.2
Involuntary pool and guaranty fund accruals	0.1	0.1
Pension and benefit accruals	0	8
Other items	1.5	9.2
Total gross deferred income tax assets	164.2	234.1
Less valuation allowance	1.9	7.8
Total net deferred income tax assets	162.3	226.3
Deferred Tax Liabilities, Gross [Abstract]	'	'
Net unrealized investment gains	33.8	39.4
Deferred acquisition costs	25.2	43.4
Prepaid pension	6.3	0
Other items	6.4	5.7
Total deferred income tax liabilities	71.7	88.5
Net deferred tax asset	$90.60	$137.80

Income_Taxes_Carry_Forwards_De

Income Taxes - Carry Forwards (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	$38.10	'
Total net deferred income tax assets	162.3	226.3
Domestic Tax Authority	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	37.1	'
LUXEMBOURG	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	1	'
Gross deferred tax asset	0.3	'
Tax Year 2014	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	0	'
Tax Year 2014 | Domestic Tax Authority	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	0	'
Tax Year 2014 | LUXEMBOURG	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	0	'
Tax Years 2015 to 2022	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	1.3	'
Tax Years 2015 to 2022 | Domestic Tax Authority	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	1.3	'
Tax Years 2015 to 2022 | LUXEMBOURG	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	0	'
Tax Years 2023 to 2033	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	35.8	'
Tax Years 2023 to 2033 | Domestic Tax Authority	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	35.8	'
Tax Years 2023 to 2033 | LUXEMBOURG	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	0	'
No Expiration Date	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	1	'
No Expiration Date | Domestic Tax Authority	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	0	'
No Expiration Date | LUXEMBOURG	'	'
Income Tax Contingency [Line Items]	'	'
Total net operating loss carryforwards	1	'
HGIE [Member]	'	'
Income Tax Contingency [Line Items]	'	'
Gross deferred tax asset	13.3	'
Valuation allowance	-1.6	'
Total net deferred income tax assets	11.7	'
HGIE [Member] | Domestic Tax Authority	'	'
Income Tax Contingency [Line Items]	'	'
Gross deferred tax asset	13	'
Valuation allowance	-1.3	'
Total net deferred income tax assets	11.7	'
HGIE [Member] | LUXEMBOURG	'	'
Income Tax Contingency [Line Items]	'	'
Gross deferred tax asset	0.3	'
Valuation allowance	-0.3	'
Total net deferred income tax assets	$0	'

Income_Taxes_Unrecognized_Tax_

Income Taxes - Unrecognized Tax Benefits (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]	'		'		'
Unrecognized Tax Benefits	$39		$51.70		$70.80
Changes in prior year tax positions	1		1.2		1.9
Tax positions taken during the current year	-7		-13.9		-21
Lapse in statute of limitations	0		0		0
Settlements with tax authorities	0.4		0		0
Unrecognized Tax Benefits	32.6		39		51.7
Permanent Differences [Member]	'		'		'
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]	'		'		'
Unrecognized Tax Benefits	7.6	[1]	7.6	[1]	7.6	[1]
Changes in prior year tax positions	0	[1]	0	[1]	0	[1]
Tax positions taken during the current year	0	[1]	0	[1]	0	[1]
Lapse in statute of limitations	0	[1]	0	[1]	0	[1]
Settlements with tax authorities	0	[1]	0	[1]	0	[1]
Unrecognized Tax Benefits	7.6	[1]	7.6	[1]	7.6	[1]
Temporary Differences [Member]	'		'		'
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]	'		'		'
Unrecognized Tax Benefits	23.6	[2]	37.5	[2]	58.5	[2]
Changes in prior year tax positions	0	[2]	0	[2]	0	[2]
Tax positions taken during the current year	-7	[2]	-13.9	[2]	-21	[2]
Lapse in statute of limitations	0	[2]	0	[2]	0	[2]
Settlements with tax authorities	0	[2]	0	[2]	0	[2]
Unrecognized Tax Benefits	16.6	[2]	23.6	[2]	37.5	[2]
Interest and Penalties [Member]	'		'		'
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]	'		'		'
Unrecognized Tax Benefits	7.8	[3]	6.6	[3]	4.7	[3]
Changes in prior year tax positions	1	[3]	1.2	[3]	1.9	[3]
Tax positions taken during the current year	0	[3]	0	[3]	0	[3]
Lapse in statute of limitations	0	[3]	0	[3]	0	[3]
Settlements with tax authorities	0.4	[3]	0	[3]	0	[3]
Unrecognized Tax Benefits	$8.40	[3]	$7.80	[3]	$6.60	[3]
[1]	Represents the amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate.
[2]	(2)Represents the amount of unrecognized tax benefits that, if recognized, would create a temporary difference between the reported amount of an item in OneBeacon's Consolidated Balance Sheet and its tax basis.
[3]	Net of tax benefit.

Retirement_Plans_Obligations_a

Retirement Plans - Obligations and Funded Status (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Change in projected benefit obligation:	'		'		'
Projected benefit obligation at beginning of year	$119.50		$113		'
Service cost	0.8		0.7		0.8
Interest cost	4.2		4.7		5.2
Settlement gain	0		-0.7		'
Special termination benefits expense	0.3	[1]	0.6	[1]	0.8	[1]
Assumption changes	-13		11.2		'
Actuarial gain	0.5		0.2		'
Benefits and expenses paid with plan assets	-6.2		-7.5		'
Benefits paid directly by OneBeacon	-2.2		-2.3		'
Projected benefit obligation at end of year	102.9		119.5		113
Change in plan assets:	'		'		'
Fair value of plan assets at beginning of year	124.7		120.8		'
Actual return on plan assets	24.3		11.4		'
Employer contributions	0		0		'
Funded status at end of year	39.9		5.2		'
Fair value of plan assets at end of year	142.8		124.7		120.8
Qualified Plan [Member]	'		'		'
Change in plan assets:	'		'		'
Funded status at end of year	64.1		32.8		'
Fair value of plan assets at end of year	142.1		124		'
Non Qualified Plan [Member]	'		'		'
Change in plan assets:	'		'		'
Funded status at end of year	-24.2		-27.6		'
Defined Benefit Plan, Amount Set Aside Under Irrevocable Trust for the Benefit of Plan Participant	$16.20		'		'
[1]	Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.

Retirement_Plans_Funded_Status

Retirement Plans - Funded Status (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Defined Benefit Plan Disclosure [Line Items]	'	'
Funded status at end of year	$39.90	$5.20
Qualified Plan [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Funded status at end of year	64.1	32.8
Non Qualified Plan [Member]	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Funded status at end of year	($24.20)	($27.60)

Retirement_Plans_Nonqualified_

Retirement Plans - Non-qualified (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Compensation and Retirement Disclosure [Abstract]	'	'
Projected benefit obligation	$24.20	$27.60
Accumulated benefit obligation	24.2	27.6
Fair value of plan assets	$0	$0

Retirement_Plans_Qualified_Pla

Retirement Plans - Qualified Plan (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Compensation and Retirement Disclosure [Abstract]	'	'
Projected benefit obligation	$78.70	$91.90
Accumulated benefit obligation	78.7	91.9
Fair value of plan net assets	$142.80	$124.70

Retirement_Plans_Accumulated_O

Retirement Plans - Accumulated Other Comprehensive Income (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan, Accumulated Other Comprehensive Income [Roll Forward]	'		'
Accumulated other comprehensive loss at beginning of year	($21.20)		($16.70)
Amortization of net actuarial losses recognized during the year	0.9		0.8
Net actuarial gains (losses) occurring during the year(1)	30.8	[1]	-5.9	[1]
Other adjustments	0		0.6
Accumulated other comprehensive income (loss) at end of year	$10.50		($21.20)
[1]	Net actuarial gains (losses) resulted from investment returns and demographic experience different than assumed, as well as changes in assumptions in estimating the projected benefit obligation in the years ended December 31, 2013 and 2012.

Retirement_Plans_Net_Benefit_C

Retirement Plans - Net Benefit Cost (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Compensation and Retirement Disclosure [Abstract]	'		'		'
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost	$0.30		'		'
Defined Benefit Plan, Net Periodic Benefit Cost [Abstract]	'		'		'
Service cost	0.8		0.7		0.8
Interest cost	4.2		4.7		5.2
Expected return on plan assets	7.1		6.9		7.6
Amortization of prior service benefit	0		0		0
Amortization of net actuarial losses recognized during the year	-0.9		-0.8		-0.5
Net periodic pension cost (income) before settlements, curtailments and special termination benefits	-1.2		-0.7		-1.1
Settlement loss	0		0.6		0.5
Special termination benefits expense	0.3	[1]	0.6	[1]	0.8	[1]
Total net periodic benefit cost (income)	($0.90)		$0.50		$0.20
[1]	Special termination benefits represent additional payments made from the Qualified Plan to certain vested participants when their employment was terminated due to a reduction in force.

Retirement_Plans_Assumptions_D

Retirement Plans - Assumptions (Details)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Compensation and Retirement Disclosure [Abstract]	'	'
Discount rate	4.66%	3.64%
Discount rate	3.64%	4.38%
Expected long-term rate of return on plan assets	5.75%	5.75%

Retirement_Plans_Fair_Value_De

Retirement Plans - Fair Value (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	$142.80	$124.70	$120.80
Qualified Plan [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	142.1	124	'
Qualified Plan [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	112.1	87.6	'
Qualified Plan [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	30	36.4	'
Qualified Plan [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Fixed Maturity Utility Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	'	1.9	'
Qualified Plan [Member] | Fixed Maturity Utility Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	'	0	'
Qualified Plan [Member] | Fixed Maturity Utility Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	'	1.9	'
Qualified Plan [Member] | Fixed Maturity Utility Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	'	0	'
Qualified Plan [Member] | Fixed maturity investments	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	1.9	'
Qualified Plan [Member] | Fixed maturity investments | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Fixed maturity investments | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	1.9	'
Qualified Plan [Member] | Fixed maturity investments | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Short-term investments	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	8.9	8.1	'
Qualified Plan [Member] | Short-term investments | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	8.8	8.1	'
Qualified Plan [Member] | Short-term investments | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0.1	0	'
Qualified Plan [Member] | Short-term investments | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Financial Investment [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	15.8	10.7	'
Qualified Plan [Member] | Equity Financial Investment [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	15.8	10.7	'
Qualified Plan [Member] | Equity Financial Investment [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Financial Investment [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Basic Material Investment [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	9.1	10.7	'
Qualified Plan [Member] | Equity Basic Material Investment [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	9.1	10.7	'
Qualified Plan [Member] | Equity Basic Material Investment [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Basic Material Investment [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Consumer Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	31.6	26.6	'
Qualified Plan [Member] | Equity Consumer Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	31.6	26.6	'
Qualified Plan [Member] | Equity Consumer Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Consumer Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Energy Consumer Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	15.6	12	'
Qualified Plan [Member] | Energy Consumer Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	15.6	12	'
Qualified Plan [Member] | Energy Consumer Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Energy Consumer Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Utilities Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	4.2	1.9	'
Qualified Plan [Member] | Equity Utilities Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	4.2	1.9	'
Qualified Plan [Member] | Equity Utilities Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Utilities Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Other Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	27	17.6	'
Qualified Plan [Member] | Equity Other Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	27	17.6	'
Qualified Plan [Member] | Equity Other Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Equity Other Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Common equity securities	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	103.3	79.5	'
Qualified Plan [Member] | Common equity securities | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	103.3	79.5	'
Qualified Plan [Member] | Common equity securities | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Common equity securities | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Financial Debt Securities [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	2	0.9	'
Qualified Plan [Member] | Convertible Financial Debt Securities [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Financial Debt Securities [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	2	0.9	'
Qualified Plan [Member] | Convertible Financial Debt Securities [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Basic Materials Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	7.4	10.6	'
Qualified Plan [Member] | Convertible Basic Materials Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Basic Materials Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	7.4	10.6	'
Qualified Plan [Member] | Convertible Basic Materials Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Consumer Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	10.4	16.7	'
Qualified Plan [Member] | Convertible Consumer Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Consumer Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	10.4	16.7	'
Qualified Plan [Member] | Convertible Consumer Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Energy Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0.6	0.1	'
Qualified Plan [Member] | Convertible Energy Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Energy Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0.6	0.1	'
Qualified Plan [Member] | Convertible Energy Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Utilities Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Utilities Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Utilities Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Utilities Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Other Investments [Member]	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	9.5	6.2	'
Qualified Plan [Member] | Convertible Other Investments [Member] | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible Other Investments [Member] | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	9.5	6.2	'
Qualified Plan [Member] | Convertible Other Investments [Member] | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible fixed maturity investments	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	29.9	34.5	'
Qualified Plan [Member] | Convertible fixed maturity investments | Level 1 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	0	0	'
Qualified Plan [Member] | Convertible fixed maturity investments | Level 2 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	29.9	34.5	'
Qualified Plan [Member] | Convertible fixed maturity investments | Level 3 Inputs	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'
Defined Benefit Plan, Fair Value of Plan Assets	$0	$0	'

Retirement_Plans_Plan_Assets_D

Retirement Plans - Plan Assets (Details) (Qualified Plan [Member])	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]	'	'
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%	100.00%
Debt Securities	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Defined Benefit Plan, Actual Plan Asset Allocations	0.00%	1.50%
Common equity securities	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Defined Benefit Plan, Actual Plan Asset Allocations	72.70%	64.20%
Convertible fixed maturity investments	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Defined Benefit Plan, Actual Plan Asset Allocations	21.10%	27.80%
Cash and short-term investments	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'
Defined Benefit Plan, Actual Plan Asset Allocations	6.20%	6.50%

Retirement_Plans_Cash_Flow_Det

Retirement Plans - Cash Flow (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
Postemployment Benefits [Abstract]	'
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year	$2.20
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	5.1
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	5.4
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	5.6
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	5.8
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	6.1
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	$33.90

Retirement_Plans_Other_Benefit

Retirement Plans - Other Benefit Plans (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Compensation and Retirement Disclosure [Abstract]	'	'	'
Percentage of matching contribution made by company	50.00%	'	'
Employer matching contribution, percent of employees' gross pay	6.00%	'	'
Cost recognized	$2.30	$2.80	$3
Noncurrent benefit liability	$5.80	$7.10	'

Employee_ShareBased_Incentive_2

Employee Share-Based Incentive Compensation Plans Employee Share-Based Incentive Compensation Plans (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]	'	'	'
Expense recognized	$11.90	$6.30	$9.20

Employee_ShareBased_Incentive_3

Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Expense amortized	'		'		'
Expense recognized	$11.90		$6.30		$9.20
Additional disclosures	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%		'		'
OneBeacon Long Term Incentive Plan [Member] | Performance shares	'		'		'
Additional disclosures	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage	200.00%		200.00%		200.00%
Share-based compensation Arrangement by Share-based Payment Award, Targeted Performance Goal for Personnel Expressed as Percentage of Growth in Intrinsic Business Value Per Share	13.00%		10.00%		11.00%
Share-based Compensation Arragement by Share-based Payment Award, Performance Goal Percentage for Minimum Payout	6.00%		3.00%		4.00%
Share-based Compensation Arrangement by Share-based Payment Award Performance Goal Percentage for Maximum Payout	20.00%		17.00%		18.00%
Long Term Incentive Plan | Performance shares	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Nonvested Number before Forfeitures	493,421		'		'
Activity in plan	'		'		'
Beginning of period (in shares)	563,190		642,667		1,464,295
Payments and deferrals (in shares)	-238,658	[1]	-258,901	[1]	-936,150	[1]
New awards (in shares)	179,000		181,290		194,900
Forfeitures and net change in assumed forfeitures (in shares)	-10,111		-1,866		-80,378
End of period (in shares)	493,421		563,190		642,667
Expense amortized	'		'		'
Accrued expense at beginning of period	1.2		9.7		18.5
Payments and deferrals	0	[1]	-7.7	[1]	-10.5	[1]
Forfeitures and net change in assumed forfeitures	-0.1		0		0.5
Expense recognized	2.9		-0.8		2.2
Accrued expense at end of period	4		1.2		9.7
Additional disclosures	'		'		'
Assumed forfeitures (in shares)	-9,007		'		'
Assumed forfeitures	-0.1		'		'
Additional compensation cost that would be recognized if all outstanding performance shares vested	2.9		'		'
Long Term Incentive Plan | Performance shares | Performance cycle 2010 - 2012	'		'		'
Additional disclosures	'		'		'
Performance share payments factor	0.00%		'		'
Long Term Incentive Plan | Performance shares | Performance cycle 2009 - 2011	'		'		'
Additional disclosures	'		'		'
Performance share payments factor	138.60%		'		'
Long Term Incentive Plan | Performance shares | Performance Cycle 2008 to [Member]	'		'		'
Additional disclosures	'		'		'
Performance share payments factor	68.50%		'		'
Long Term Incentive Plan | Performance shares | Performance cycle 2011 - 2013	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Nonvested Number before Forfeitures	142,138		'		'
Expense amortized	'		'		'
Accrued expense at end of period	1		'		'
Long Term Incentive Plan | Performance shares | Performance cycle 2012 - 2014	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Nonvested Number before Forfeitures	181,290		'		'
Expense amortized	'		'		'
Accrued expense at end of period	2.1		'		'
Long Term Incentive Plan | Performance shares | Performance cycle 2013 - 2015	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Nonvested Number before Forfeitures	179,000		'		'
Expense amortized	'		'		'
Accrued expense at end of period	1		'		'
Long Term Incentive Plan | Performance shares | Performance cycle 2010 - 2014	'		'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'		'
Share Based Compensation Arrangement by Share Based Payment Award Equity Instruments Other than Options Nonvested Number before Forfeitures	502,428		'		'
Expense amortized	'		'		'
Accrued expense at end of period	$4.10		'		'
[1]	No payments were made in 2013 for the 2010-2012 performance cycle as the performance factor was zero. Performance share payments in 2012 for the 2009-2011 performance cycle were based upon a performance factor of 138.6%. Performance share payments in 2011 for the 2008-2010 performance cycle were based upon a performance factor of 68.5%.

Employee_ShareBased_Incentive_4

Employee Share-Based Incentive Compensation Plans (Restricted) (Details) (USD $)	12 Months Ended						0 Months Ended	12 Months Ended	0 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	25-May-11	Dec. 31, 2012	25-May-11
				Long Term Incentive Plan	Long Term Incentive Plan	Long Term Incentive Plan	Long Term Incentive Plan	Long Term Incentive Plan	Long Term Incentive Plan
				Restricted Stock Units (RSUs) [Member]	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares
									Performance cycle 2011 - 2013
Activity in plan	'	'	'	'	'	'	'	'	'
Beginning of period (in shares)	'	'	'	0	927,000	630,000	'	'	'
New awards (in shares)	'	'	'	630,000	0	'	630,000	300,000	'
Forfeitures (in shares)	'	'	'	0	-3,000	-2,333	'	'	-35,000
Vested (in shares)	'	'	'	0	9,000	667	'	'	'
End of period (in shares)	'	'	'	630,000	915,000	927,000	'	'	'
Unamortized grant date fair value	'	'	'	'	'	'	'	'	'
Unamortized grant date fair value, beginning of period	'	'	'	$0	$9.60	$7.70	'	'	'
Issued	'	'	'	8.6	0	4.5	'	'	'
Expense recognized	-11.9	-6.3	-9.2	-0.9	-3.1	-2.6	'	'	'
Unamortized grant date fair value, end of period	'	'	'	7.7	6.5	9.6	'	'	'
Grand date fair value	'	'	'	'	($0.10)	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Weighted Average Remaining Contractual Terms	'	'	'	'	'2 years 8 months 12 days	'	'	'	'

Employee_ShareBased_Incentive_5

Employee Share-Based Incentive Compensation Plans - Stock Options (Details) (USD $)	1 Months Ended	12 Months Ended
In Millions, except Share data, unless otherwise specified	Nov. 30, 2006	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Partners' Capital Account, Units, Sold in Public Offering	1,420,000	'	'	'
Expense recognized	'	$11.90	$6.30	$9.20
Nonqualified Stock Options [Member]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	'	'	'	740,870
Expense recognized	'	'	'	$0.50

Employee_ShareBased_Incentive_6

Employee Share-Based Incentive Compensation Plans - Restricted Stock Units (Details) (USD $)	12 Months Ended				0 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011	Feb. 26, 2008
				Restricted Stock Units (RSUs) [Member]	OneBeacon Long Term Incentive Plan [Member]
					Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'	'
New awards (in shares)	'	'	'	'	116,270
Expense recognized	$11.90	$6.30	$9.20	$0.10	'

Employee_ShareBased_Incentive_7

Employee Share-Based Incentive Compensation Plans - Other Share-Based Compensation (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Expense recognized	$11,900,000	$6,300,000	$9,200,000
Percentage Owned of Common Stock	'	3.00%	'
KSOP [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Employee Stock Ownership Plan (ESOP), Annual Contribution Provided to Participants Expressed as Percentage of Salary	3.00%	'	'
Employee Stock Ownership Plan (ESOP), Social Security Wage Base Amount Maximum	113,700	'	'
Employee Stock Ownership Plan (ESOP), Variable Contribution Provided to Participants Not Eligible to Certain Benefits Expressed as Percentage of Salary	6.00%	'	'
Annual Compensation Cap	250,000	'	'
Expense recognized	$6,000,000	$4,500,000	$6,000,000
Performance 1 [Member] | KSOP [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Employee Stock Ownership Plan (ESOP), Variable Contribution Provided to Participants Not Eligible to Certain Benefits Expressed as Percentage of Salary	4.00%	'	'
Performance 2 [Member] | KSOP [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Employee Stock Ownership Plan (ESOP), Variable Contribution Provided to Participants Not Eligible to Certain Benefits Expressed as Percentage of Salary	3.00%	'	'
Performance 3 [Member] | KSOP [Member]	'	'	'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'
Employee Stock Ownership Plan (ESOP), Variable Contribution Provided to Participants Not Eligible to Certain Benefits Expressed as Percentage of Salary	2.00%	'	'

Common_Shareholders_Equity_Det

Common Shareholders' Equity (Details) (USD $)

12 Months Ended

0 Months Ended

1 Months Ended

12 Months Ended

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Aug. 22, 2007

Dec. 31, 2010

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Class A common

Class A common

Common Stock

Common Stock

One Beacon

One Beacon

One Beacon

Accumulated Other Comprehensive Income (Loss) [Member]

Accumulated Other Comprehensive Income (Loss) [Member]

Accumulated Other Comprehensive Income (Loss) [Member]

Equity, Class of Treasury Stock [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock repurchase, authorized amount

'

'

'

$200,000,000

$87,700,000

'

'

'

'

'

'

'

'

Number of shares authorized to be repurchased

87,700,000

'

'

'

'

'

'

'

'

'

'

'

'

Repurchases and retirements of common shares

'

'

1,300,000

'

'

3,300

0

'

'

0

'

'

0

Dividends

81,000,000

80,700,000

175,700,000

'

'

'

'

80,200,000

80,100,000

174,800,000

0

0

0

Dividends, per share, cash paid

$0.84

$0.84

$1.84

'

'

'

'

$0.84

$0.84

'

'

'

'

Common Stock, Dividends, Per Share, Cash Paid, Special

'

'

$1

'

'

'

'

'

'

'

'

'

'

Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax

-20,600,000

2,900,000

11,200,000

'

'

'

'

-20,600,000

2,900,000

11,200,000

-20,600,000

2,900,000

11,200,000

Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax, Portion Attributable to Parent

'

'

'

'

'

'

'

'

'

'

($11,100,000)

$1,600,000

$6,000,000

Statutory_Capital_and_Surplus_

Statutory Capital and Surplus (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Statutory Accounting Practices [Line Items]	'		'	'
Statutory Accounting Practices, Statutory Capital and Surplus Required	$164,700,000		'	'
Dividend Payment without Prior Approval of Regulatory Authorities Percentage of Prior Year Statutory Surplus	10.00%		'	'
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval	86,600,000		'	'
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	900,000,000		'	'
Statutory Accounting Practices, Statutory Net Income Amount	100,100,000		82,000,000	119,600,000
Statutory Accounting Practices, Statutory Capital, Percent	15.00%		'	'
Proceeds from Contributions from Affiliates	135,100,000		'	'
Cash dividends paid to parent	0		0	0
Payments of Dividends	80,200,000		80,100,000	'
Net Unrestricted Cash, Short Term and Fixed Maturity Investments	217,500,000		'	'
Total	1,967,700,000	[1]	'	'
Payments of Ordinary Dividends, Common Stock	80,200,000		80,100,000	174,800,000
Payments of Dividends Common Stock Regular	'		'	79,700,000
Payments of Dividends Common Stock Special	'		'	95,100,000
Dividends received from subsidiaries	0		0	0
Partners' Capital Account, Return of Capital	'		58,400,000	'
Regulated Operation [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Retained Earnings, Unappropriated	600,000,000		'	'
Proceeds from Contributions from Affiliates	35,000,000		'	'
Cash dividends paid to parent	190,000,000		173,100,000	'
Dividends received from subsidiaries	'		34,000,000	'
Unregulated Operation [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Payments of Dividends	17,300,000		4,900,000	'
Net Unrestricted Cash, Short Term and Fixed Maturity Investments	57,900,000		'	'
U.S and Bermuda [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	1,000,000,000		900,000,000	'
UNITED STATES	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	900,000,000		'	'
BERMUDA	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Statutory Accounting Practices, Statutory Capital and Surplus Required	-38,700,000		'	'
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	96,400,000		'	'
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulated Operation [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Dividend Payment without Prior Approval of Regulatory Authorities Percentage of Prior Year Statutory Surplus	10.00%		'	'
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval	23,900,000		'	'
Statutory Accounting Practices Earned, Surplus	94,600,000		'	'
Statutory Accounting Practices, Statutory Capital and Surplus, Balance	-700,000,000		'	'
Subsidiaries [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Cash dividends paid to parent	'		114,700,000	'
Retained earnings | Atlantic Speciality Insurance Company (ASIC) [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval	20,300,000		'	'
Equity Securities and Convertible Fixed Maturity Investments [Member]	'		'	'
Statutory Accounting Practices [Line Items]	'		'	'
Total	$89,800,000		'	'
[1]	Includes carrying value of $236.3 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheet as part of the Runoff Transaction as of DecemberB 31, 2013

Segment_Information_Details

Segment Information (Details) (USD $)

3 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

segment

Segment Information

'

'

'

'

'

'

'

'

'

'

'

'

Earned premiums

'

'

'

'

'

'

'

'

$1,120.40

$1,132

$1,012.20

'

Loss and loss adjustment expense

'

'

'

'

'

'

'

'

-622.1

-650

-548.3

'

Policy acquisition expenses

'

'

'

'

'

'

'

'

-208.9

-249.4

-221.2

'

Other underwriting expenses

'

'

'

'

'

'

'

'

-204.8

-205.2

-162.3

'

Total underwriting income (loss)

'

'

'

'

'

'

'

'

84.6

27.4

80.4

'

Net investment income

'

'

'

'

'

'

'

'

41.1

53.6

71.4

'

Net realized and change in unrealized investment gains

'

'

'

'

'

'

'

'

49.4

55.7

10.6

'

Net other revenues

'

'

'

'

'

'

'

'

31.2

-0.5

-12.4

'

General and administrative expenses

'

'

'

'

'

'

'

'

-12

-13.4

-9.8

'

Interest expense

'

'

'

'

'

'

'

'

-13

-16.9

-20.5

'

Pre-tax income from continuing operations

60.3

35.9

-3

88.1

-33.3

59.2

12.4

67.6

181.3

105.9

119.7

'

Segment Information, Additional Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

Total investment securities

2,364.90

'

'

'

2,291.50

'

'

'

2,364.90

2,291.50

'

'

Reinsurance recoverables

89.9

'

'

'

110.6

'

'

'

89.9

110.6

'

'

Deferred acquisition costs

103.7

'

'

'

123.9

'

'

'

103.7

123.9

'

'

Unpaid loss and loss adjustment expense reserves

1,054.30

'

'

'

1,000

'

'

'

1,054.30

1,000

3,358.60

3,295.50

Unearned premiums

544.9

'

'

'

573.8

'

'

'

544.9

573.8

'

'

Debt

274.7

'

'

'

274.7

'

'

'

274.7

274.7

'

'

Net written premiums

'

'

'

'

'

'

'

'

1,088.60

1,179.20

1,062.70

'

Loss and LAE Ratio

'

'

'

'

'

'

'

'

55.50%

[1]

57.40%

54.20%

'

Expense Ratio

'

'

'

'

'

'

'

'

36.90%

[1]

40.10%

37.90%

'

Combined Ratio

'

'

'

'

'

'

'

'

92.40%

[1]

97.50%

92.10%

'

Number of underwriting segments

'

'

'

'

'

'

'

'

14

'

'

'

Underwriting Segment [Member]

'

'

'

'

'

'

'

'

'

'

'

'

Segment Information, Additional Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

Number of underwriting segments

'

'

'

'

'

'

'

'

2

'

'

'

Specialty Products

'

'

'

'

'

'

'

'

'

'

'

'

Segment Information

'

'

'

'

'

'

'

'

'

'

'

'

Earned premiums

'

'

'

'

'

'

'

'

553.5

604

549.8

'

Loss and loss adjustment expense

'

'

'

'

'

'

'

'

-312.3

-345.6

-281.7

'

Policy acquisition expenses

'

'

'

'

'

'

'

'

-106.3

-150.3

-129.1

'

Other underwriting expenses

'

'

'

'

'

'

'

'

-97.4

-96.2

-77.1

'

Total underwriting income (loss)

'

'

'

'

'

'

'

'

37.5

11.9

61.9

'

Net investment income

'

'

'

'

'

'

'

'

0

0

0

'

Net realized and change in unrealized investment gains

'

'

'

'

'

'

'

'

0

0

0

'

Net other revenues

'

'

'

'

'

'

'

'

0.3

0.4

0

'

General and administrative expenses

'

'

'

'

'

'

'

'

0

0

0

'

Interest expense

'

'

'

'

'

'

'

'

0

0

0

'

Pre-tax income from continuing operations

'

'

'

'

'

'

'

'

37.8

12.3

61.9

'

Segment Information, Additional Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

Total investment securities

0

'

'

'

0

'

'

'

0

0

'

'

Reinsurance recoverables

35.1

'

'

'

52.3

'

'

'

35.1

52.3

'

'

Deferred acquisition costs

53.6

'

'

'

'

'

'

'

53.6

'

'

'

Deferred acquisition costs

'

'

'

'

75.1

'

'

'

'

75.1

'

'

Unpaid loss and loss adjustment expense reserves

607.8

'

'

'

561.9

'

'

'

607.8

561.9

'

'

Unearned premiums

275.4

[2]

'

'

'

'

'

'

'

275.4

[2]

'

'

'

Unearned premiums

'

'

'

'

318.7

'

'

'

'

318.7

'

'

Debt

0

'

'

'

0

'

'

'

0

0

'

'

Net written premiums

'

'

'

'

'

'

'

'

509.6

630.9

571.2

'

Loss and LAE Ratio

'

'

'

'

'

'

'

'

56.40%

[1]

57.20%

51.20%

'

Expense Ratio

'

'

'

'

'

'

'

'

36.80%

[1]

40.70%

37.50%

'

Combined Ratio

'

'

'

'

'

'

'

'

93.20%

[1]

97.90%

88.70%

'

Number of underwriting segments

'

'

'

'

'

'

'

'

8

'

'

'

Specialty Industries

'

'

'

'

'

'

'

'

'

'

'

'

Segment Information

'

'

'

'

'

'

'

'

'

'

'

'

Earned premiums

'

'

'

'

'

'

'

'

566.9

528

462.4

'

Loss and loss adjustment expense

'

'

'

'

'

'

'

'

-309.8

-304.4

-266.6

'

Policy acquisition expenses

'

'

'

'

'

'

'

'

-102.6

-99.1

-92.1

'

Other underwriting expenses

'

'

'

'

'

'

'

'

-107.4

-109

-85.2

'

Total underwriting income (loss)

'

'

'

'

'

'

'

'

47.1

15.5

18.5

'

Net investment income

'

'

'

'

'

'

'

'

0

0

0

'

Net realized and change in unrealized investment gains

'

'

'

'

'

'

'

'

0

0

0

'

Net other revenues

'

'

'

'

'

'

'

'

1.1

-0.8

0.6

'

General and administrative expenses

'

'

'

'

'

'

'

'

-2.4

-1.9

-1.7

'

Interest expense

'

'

'

'

'

'

'

'

0

0

0

'

Pre-tax income from continuing operations

'

'

'

'

'

'

'

'

45.8

12.8

17.4

'

Segment Information, Additional Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

Total investment securities

0

'

'

'

0

'

'

'

0

0

'

'

Reinsurance recoverables

54.8

'

'

'

58.3

'

'

'

54.8

58.3

'

'

Deferred acquisition costs

50.1

'

'

'

'

'

'

'

50.1

'

'

'

Deferred acquisition costs

'

'

'

'

48.8

'

'

'

'

48.8

'

'

Unpaid loss and loss adjustment expense reserves

446.5

'

'

'

438.1

'

'

'

446.5

438.1

'

'

Unearned premiums

269.5

'

'

'

'

'

'

'

269.5

'

'

'

Unearned premiums

'

'

'

'

255.1

'

'

'

'

255.1

'

'

Debt

0

'

'

'

0

'

'

'

0

0

'

'

Net written premiums

'

'

'

'

'

'

'

'

579

548.3

491.5

'

Loss and LAE Ratio

'

'

'

'

'

'

'

'

54.70%

[1]

57.70%

57.70%

'

Expense Ratio

'

'

'

'

'

'

'

'

37.00%

[1]

39.40%

38.30%

'

Combined Ratio

'

'

'

'

'

'

'

'

91.70%

[1]

97.10%

96.00%

'

Number of underwriting segments

'

'

'

'

'

'

'

'

6

'

'

'

Investing, Financing and Corporate

'

'

'

'

'

'

'

'

'

'

'

'

Segment Information

'

'

'

'

'

'

'

'

'

'

'

'

Earned premiums

'

'

'

'

'

'

'

'

0

0

0

'

Loss and loss adjustment expense

'

'

'

'

'

'

'

'

0

0

0

'

Policy acquisition expenses

'

'

'

'

'

'

'

'

0

0

0

'

Other underwriting expenses

'

'

'

'

'

'

'

'

0

0

0

'

Total underwriting income (loss)

'

'

'

'

'

'

'

'

0

0

0

'

Net investment income

'

'

'

'

'

'

'

'

41.1

53.6

71.4

'

Net realized and change in unrealized investment gains

'

'

'

'

'

'

'

'

49.4

55.7

10.6

'

Net other revenues

'

'

'

'

'

'

'

'

29.8

-0.1

-13

'

General and administrative expenses

'

'

'

'

'

'

'

'

-9.6

-11.5

-8.1

'

Interest expense

'

'

'

'

'

'

'

'

-13

-16.9

-20.5

'

Pre-tax income from continuing operations

'

'

'

'

'

'

'

'

97.7

80.8

40.4

'

Segment Information, Additional Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

Total investment securities

2,364.90

[3]

'

'

'

2,291.50

[3]

'

'

'

2,364.90

[3]

2,291.50

[3]

'

'

Reinsurance recoverables

0

[3]

'

'

'

0

[3]

'

'

'

0

[3]

0

[3]

'

'

Deferred acquisition costs

0

[3]

'

'

'

0

[3]

'

'

'

0

[3]

0

[3]

'

'

Unpaid loss and loss adjustment expense reserves

0

[3]

'

'

'

0

[3]

'

'

'

0

[3]

0

[3]

'

'

Unearned premiums

0

[3]

'

'

'

0

[3]

'

'

'

0

[3]

0

[3]

'

'

Debt

274.7

[3]

'

'

'

274.7

[3]

'

'

'

274.7

[3]

274.7

[3]

'

'

Segment, Discontinued Operations

'

'

'

'

'

'

'

'

'

'

'

'

Segment Information

'

'

'

'

'

'

'

'

'

'

'

'

Earned premiums

'

'

'

'

'

'

'

'

0.8

10.6

70.5

'

Loss and loss adjustment expense

'

'

'

'

'

'

'

'

-78.9

-48.4

-89.5

'

Policy acquisition expenses

'

'

'

'

'

'

'

'

0.2

-1.7

-22.8

'

Net other revenues

'

'

7.8

'

'

'

'

'

6.3

0

1.7

'

Securities reclassified to assets held for sale

236.3

'

'

'

338.1

'

'

'

236.3

338.1

'

'

Segment Information, Additional Disclosure

'

'

'

'

'

'

'

'

'

'

'

'

Unpaid loss and loss adjustment expense reserves

1,930

'

'

'

2,202.70

'

'

'

1,930

2,202.70

'

'

Net written premiums

'

'

'

'

'

'

'

'

$0.40

$3.50

$58

'

[1]

Underwriting ratios are used to measure the components of underwriting profitability and include: The loss and LAE ratio, calculated by dividing loss and LAE by earned premiums; the expense ratio, calculated by dividing policy acquisition and other underwriting expenses by earned premiums; and the combined ratio, the sum of the loss and LAE ratio and the expense ratio.

[2]

Schedule excludes activity related to discontinued operations for all periods presented.

[3]

As described in Note 2, balances related to the the Runoff Business are presented as held for sale. Total investment securities excludes $236.3 million and $338.1 million of fixed maturity investments reclassified to assets held for sale as of DecemberB 31, 2013 and DecemberB 31, 2012, respectively.

Variable_Interest_Entity_Detai

Variable Interest Entity (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2004	Dec. 31, 2012	Dec. 31, 2005
Variable Interest Entity [Line Items]	'	'	'	'
Retained earnings	$75	'	$9.20	'
Reciprocals [Member]	'	'	'	'
Variable Interest Entity [Line Items]	'	'	'	'
Number of Reciprocals Capitalized	1	'	'	'
Houston General Insurance [Member]	'	'	'	'
Variable Interest Entity [Line Items]	'	'	'	'
Partners' Capital Account, Contributions	'	2	'	'
Variable Interest Entity, Consolidated, Carrying Amount, Assets	2.6	'	22.6	149.4
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	4.2	'	43.8	127.6
Surplus Notes	4	'	'	23.7
Interest Paid	19	'	'	'
Principle Payment on Surplus Note	1	'	'	'
Retained earnings	1.6	'	'	'
Accrued Interest on Surplus Notes	$0.10	'	'	'

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value of Financial Instruments	'	'
Total net operating loss carryforwards	$38.10	'
OBH (issuer)	'	'
Fair Value of Financial Instruments	'	'
Senior unsecured notes, carrying value	274.7	274.7
Fair Value | OBH (issuer)	'	'
Fair Value of Financial Instruments	'	'
Senior unsecured notes, carrying value	269.8	282.4
Carrying Value | OBH (issuer)	'	'
Fair Value of Financial Instruments	'	'
Senior unsecured notes, carrying value	274.7	274.7
LUXEMBOURG	'	'
Fair Value of Financial Instruments	'	'
Total net operating loss carryforwards	1	'
Domestic Tax Authority	'	'
Fair Value of Financial Instruments	'	'
Total net operating loss carryforwards	$37.10	'

Related_Party_Disclosures_Deta

Related Party Disclosures (Details) (USD $)

0 Months Ended

3 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

0 Months Ended

3 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

Share data in Millions, unless otherwise specified

Jan. 24, 2012

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Jan. 24, 2012

Dec. 31, 2006

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

White Mountains Advisors L L C [Member]

White Mountains Advisors L L C [Member]

White Mountains Advisors L L C [Member]

Star and Shield Insurance Exchange [Member]

Limited Liability Company [Member]

Limited Liability Company [Member]

Limited Liability Company [Member]

White Mountains Insurance Group Ltd

Propsector Turtle Fund, L.P. [Member]

Homeland Central Insurance Company [Member]

OneBeacon Holdings (Luxembourg) S.a r.l.

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

LUXEMBOURG

Esurance [Member]

Federal Insurance Company [Member]

Federal Insurance Company [Member]

Federal Insurance Company [Member]

Prospective Partners LLC [Member]

Prospective Partners LLC [Member]

Prospective Partners LLC [Member]

Esurance [Member]

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

White Mountains Insurance Group Ltd

Limited Liability Company [Member]

Limited Liability Company [Member]

Limited Liability Company [Member]

sqft

Related Party Transaction [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Guarantor Obligations, Maximum Exposure, Undiscounted

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$25,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock Issued During Period, Shares, New Issues

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

27.6

'

'

'

'

'

'

'

'

'

'

'

Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

27.60%

'

'

'

'

'

'

'

'

'

'

'

Ownership interest (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

75.20%

'

'

'

'

'

'

'

'

Expenses Recognized Under Seperation Agreement

'

'

'

'

'

'

'

'

'

400,000

300,000

400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Revenue Recognized Under Seperation Agreement

'

'

'

'

'

'

'

'

'

2,000,000

2,100,000

2,400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Asset Management Fees

'

'

'

'

'

'

'

'

'

'

'

'

3,400,000

3,500,000

3,900,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Asset Managment Fee, Treasury Percentage

'

'

'

'

'

'

'

'

'

'

'

'

0.02%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Asset Managment Fee, Treasury

'

'

'

'

'

'

'

'

'

'

'

'

400,000

400,000

500,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total noncontrolling interests, after tax

'

3,100,000

'

'

'

2,800,000

'

'

'

3,100,000

2,800,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10,700,000

'

'

'

'

'

'

'

Proceeds from sale of business

24,700,000

'

'

'

'

'

'

'

'

31,300,000

15,000,000

0

'

'

'

'

'

'

'

'

31,600,000

'

24,700,000

'

'

'

'

'

'

'

'

'

'

'

'

Area of Real Estate Property Leased

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

4,200

'

'

'

'

'

'

Area of Real Estate Property

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

14,000

'

'

'

'

'

'

Operating Leases, Rent Expense, Net

'

'

'

'

'

'

'

'

'

11,500,000

9,700,000

13,100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

200,000

'

'

'

'

'

'

Business Acquisition, Cost of Acquired Entity, Purchase Price (Deprecated 2013-01-31)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,900,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Income tax benefit

'

18,400,000

-2,900,000

-1,000,000

19,800,000

-19,700,000

14,200,000

400,000

13,500,000

34,300,000

8,400,000

14,800,000

'

'

'

'

'

'

'

'

'

1,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Proceeds Received From Related Party For Tax Benefits

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

Indemnity Bonds, Outstanding Amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

400,000

900,000

1,100,000

'

'

'

Investment Advisory Fees on First Threshold of Assets Under Management in Basis Points

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.00%

'

'

First Specified Threshold Limit of Assets Under Management

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

200,000,000

'

'

Investment Advisory Fees on Second Threshold of Assets Under Management in Basis Points

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.50%

'

'

Second Specified Threshold Limit of Assets Under Management

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

200,000,000

'

'

Investment Advisory Fees on Third Threshold of Assets Under Management in Basis Points

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0.25%

'

'

Specified Threshold Limit of Assets under Management

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

400,000,000

'

Noninterest Expense Investment Advisory Fees

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2,000,000

2,100,000

1,900,000

Share-based Compensation

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1,200,000

1,300,000

1,400,000

Limited Partnership Investments Under Management, Carrying Amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

14,900,000

12,900,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Related Party Transaction Management Fees from Transactions with Related Party

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

200,000

400,000

400,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Related Party Transaction Incentive Fees from Transactions with Related Party

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

500,000

300,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Net written premiums

'

'

'

'

'

'

'

'

'

1,088,600,000

1,179,200,000

1,062,700,000

'

'

'

5,600,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Investment Advisory, Management and Administrative Fees

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$800,000

$900,000

'

'

'

'

'

'

'

'

'

Related_Party_Disclosures_Asse

Related Party Disclosures - Asset Management (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
Up to $1 Billion	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from transactions with related party, amount of asset portfolio	$1,000
Related party transaction, expenses from Transactions with related party, percent	0.10%
Next $1 billion	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from transactions with related party, amount of asset portfolio	1,000
Related party transaction, expenses from Transactions with related party, percent	0.09%
Next $3 Billion	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from transactions with related party, amount of asset portfolio	3,000
Related party transaction, expenses from Transactions with related party, percent	0.08%
Greater than $5 billion	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from transactions with related party, amount of asset portfolio	$5,000
Related party transaction, expenses from Transactions with related party, percent	0.03%
High Yield	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	0.25%
Equities	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	1.00%
Hedge funds	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	1.00%
First Two Years of Funds Life	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	1.00%
Fair Value After Two Years	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	1.00%
First Year of Funds Life	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	1.00%
Fair Value After One Year	'
Related Party Transaction [Line Items]	'
Related party transaction, expenses from Transactions with related party, percent	1.00%

Commitments_and_Contingencies_

Commitments and Contingencies (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Loss Contingencies [Line Items]	'		'		'
Operating Leases, Rent Expense, Net	$11.50		$9.70		$13.10
Operating Leases, Future Minimum Payments Due, Next Twelve Months	6.6		'		'
Operating Leases, Future Minimum Payments, Due in Two Years	5.8		'		'
Operating Leases, Future Minimum Payments, Due in Three Years	5.5		'		'
Operating Leases, Future Minimum Payments, Due in Four Years	4.6		'		'
Operating Leases, Future Minimum Payments, Due Thereafter	7.6		'		'
Operating Leases, Income Statement, Sublease Revenue	'		0.1		1.4
Capital Lease Obligations	12.5		18.2		'
Capital Leased Assets, Gross	10.9		16.1		'
Capital Leases, Future Minimum Payments Due, Next Twelve Months	5.3		'		'
Capital Leases, Future Minimum Payments Due in Four Years	1.9		'		'
Pending Guranty Fund Assessments	13		'		'
Pending Litigation | Tribune Company Litigation	'		'		'
Loss Contingencies [Line Items]	'		'		'
Proceeds common stock, litigation settlement	32		'		'
OB Services [Member]	'		'		'
Loss Contingencies [Line Items]	'		'		'
Sale Leaseback Transaction, Net Proceeds	'		'		23.1
Hedge funds and private equity funds	'		'		'
Loss Contingencies [Line Items]	'		'		'
Net asset value per share of unfunded commitments	$12.10	[1]	$16.10	[1]	'
[1]	Excluded from the above table are other investments totaling $14.4 million and $14.1 million of an investment in a community reinvestment vehicle as of DecemberB 31, 2013 and 2012, respectively, and $19.7 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of DecemberB 31, 2013 and 2012, respectively.

Earnings_per_Share_Details

Earnings per Share (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Earnings attributable to OneBeacon's common shareholdersbbasic and diluted (in millions):	'		'		'
Net income from continuing operations attributable to OneBeacon's common shareholders	$146		$96.10		$103.90
Allocation of loss (income) for participating unvested restricted common shares	-1.4		-0.9		-0.4
Dividends paid on participating restricted common shares	-0.8		-0.8		-1
Total allocation to restricted common shares	-2.2		-1.7		-1.4
Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts	143.8		94.4		102.5
Undistributed Earnings, Basic [Abstract]	'		'		'
Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts	143.8		94.4		102.5
Dividends paid, net of restricted common share amounts	-79.4		-79.3		-173.8
Total undistributed (over-distributed) net earnings, net of restricted common share amounts	$64.40		$15.10		($71.30)
Weighted Average Number of Shares Outstanding, Basic [Abstract]	'		'		'
Total weighted average common shares outstanding	95.4		95.4		94.8
Weighted average unvested restricted common shares	-0.9	[1]	-0.9	[1]	-0.4	[1]
Basic and diluted earnings (loss) per share denominator	94.5	[2]	94.5	[2]	94.4	[2]
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]	'		'		'
Net income from continuing operations attributable to OneBeacon's common shareholders	$1.52		$1		$1.08
Dividends, per share, cash paid	($0.84)		($0.84)		($1.84)
Undistributed (over-distributed) earnings	$0.68		$0.16		($0.76)
[1]	Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event (see NoteB 9).
[2]	Common shares issuable upon exercise of stock options (Note 9) were not included in the years ended December 31, 2012 and 2011 as their inclusion would be anti-dilutive. There were no options outstanding during the year ended DecemberB 31, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.

Share_Repurchase_Authorization1

Share Repurchase Authorization (Details) (Class A common, USD $)	0 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Aug. 22, 2007	Dec. 31, 2010
Class A common	'	'
Class of Stock [Line Items]	'	'
Stock repurchase, authorized amount	$200	$87.70

Discontinued_Operations_Additi

Discontinued Operations (Additional Disclosures) (Details) (USD $)

3 Months Ended

12 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

In Millions, unless otherwise specified

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Dec. 31, 2013

Dec. 31, 2011

Dec. 31, 2012

Jun. 30, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Dec. 31, 2013

31-May-01

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Mar. 31, 2013

Dec. 31, 2011

Dec. 31, 2001

Mar. 31, 2013

Mar. 31, 2013

NICO Cover

NICO Cover

NICO Cover

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Runoff

Runoff

Runoff

Runoff

Runoff

Runoff

Runoff

Runoff

Auto One

General Reinsurance Corporation Cover

Minimum

Maximum

GRC Cover

GRC Cover

NICO Cover

NICO Cover

National Indemnity Company (NICO) and General Reinsurance Corporation

Hanover Insurance Company

Tokio Marine and Nichido Fire

Munich Reinsurance America

Tower Insurance Company

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Runoff

Runoff

GRC Cover

Segment, Discontinued Operations

Segment, Discontinued Operations

Dispositions

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Reinsurance agreement percentage

100.00%

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Unamortized amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$136.90

$150.10

$163.30

'

'

$570

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Reinsurance recoverable on unpaid losses

80.2

'

'

'

107.3

'

'

'

80.2

107.3

2,167.50

1,893.20

'

'

'

'

1,741.60

1,990.90

'

'

'

'

2,500

'

'

'

'

'

'

'

'

1,840.80

[1]

1,604.70

[1]

1,604.70

[1]

1,840.80

[1]

'

'

'

'

'

'

Unpaid loss and loss adjustment expense reserves

1,054.30

'

'

'

1,000

'

'

'

1,054.30

1,000

3,358.60

3,295.50

'

'

'

'

1,930

2,202.70

'

'

'

'

'

'

'

'

'

'

'

'

'

2,052.60

[1]

1,793.10

[1]

1,793.10

[1]

2,052.60

[1]

'

166

'

'

215

668

Increase (Decrease) in Environmental and Liabilities Reserves

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

78.9

'

'

'

'

'

'

'

'

Pre-tax loss

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

-71.6

-37.4

-45.4

'

'

'

'

'

'

'

'

'

'

'

'

'

'

69

140.7

'

'

'

'

'

'

Net Loss from discontinued operations, net of tax

'

'

'

'

'

'

'

'

-46.6

-24.3

-29.6

'

'

'

'

'

0

-115.3

-48.8

'

'

'

'

'

'

'

'

'

'

'

-46.6

'

'

'

'

'

'

'

'

'

'

Net other revenues (expenses)

'

'

'

'

'

'

'

'

31.2

-0.5

-12.4

'

'

'

'

7.8

6.3

0

1.7

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Liability for Unpaid Claims and Claims Adjustment Expense, Net

974.1

'

'

'

892.7

'

'

'

974.1

892.7

1,191.10

1,402.30

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

188.4

'

'

'

'

'

'

'

'

'

'

Income (loss) from discontinued operations, net of tax

-46.9

0.3

-0.5

0.5

0.5

-15.8

0.5

-9.5

'

'

'

'

'

'

'

'

-46.6

-24.3

-29.6

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

71.5

'

'

'

'

'

Workers Compensation Loss and Loss Adjustment Reserves, Discount Amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

36.6

'

'

'

'

'

'

'

'

'

'

Provision Personal Auto Liability

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

16.9

'

'

'

'

'

'

'

'

'

'

Loss Reserves Increase From Large Excess Liability Claims

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5.4

'

'

'

'

'

'

'

'

'

'

Accrual for Environmental Loss Contingencies, Provision for New Losses

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

12.6

'

'

'

'

'

'

'

'

'

'

Loss and loss adjustment expenses

'

'

'

'

'

'

'

'

622.1

650

548.3

'

'

'

'

'

78.9

48.4

89.5

'

'

'

'

'

'

'

'

'

'

9

'

'

'

'

'

'

'

'

'

'

'

Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense, Amount

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

7.4

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Workers Compensation Loss and Loss Adjustment Reserves, Discount Amount

3

'

'

'

4.6

'

'

'

3

4.6

'

'

'

'

'

'

'

15.2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Workers' Compensation Discount, Percent

'

'

'

'

'

'

'

'

3.50%

3.50%

'

'

'

'

'

'

'

3.50%

4.50%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Gain (loss) from sale of discontinued operations, net of tax

46.6

0

0

0

0

-91

0

0

46.6

-91

-19.2

'

'

'

'

'

46.6

-91

-19.2

'

'

'

'

'

'

'

'

'

'

91.5

'

0.5

'

'

'

46.5

'

'

'

'

'

Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Estimated

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

69

'

'

'

'

'

Assets Held-for-sale, Current

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

25

'

'

'

Adverse loss reserve development on losses occurring in years 2000 and prior

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

400

'

'

Loss reserves ceded at acquisition

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

170

'

'

LAE claims, net

'

'

'

'

'

'

'

'

'

'

'

'

'

2,200

2,300

'

5.7

8.8

11.2

15.6

'

'

'

2,300

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Utilized Coverage Relating to Uncollectible Third Party Recoverables Percentage

'

'

'

'

'

'

'

'

'

'

'

'

10.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Liability for Asbestos and Environmental Claims, Net Incurred Loss Ceded, Increase

'

'

'

'

'

'

'

'

'

'

'

'

'

121.9

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Reinsurance recoverables

89.9

'

'

'

110.6

'

'

'

89.9

110.6

'

'

'

'

'

'

'

'

'

'

369.7

'

1,600

'

1,243.70

[2]

43.9

26.2

[3]

16.3

10.2

'

'

'

'

'

'

'

'

'

'

'

'

LAE claims, estimated future losses

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

198.3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Estimated incurred losses ceded

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

562

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Reinsurance recoverable on paid losses

9.7

'

'

'

'

'

'

'

9.7

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15.6

10.7

10.7

15.6

'

'

'

'

'

'

% of total

100.00%

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

71.00%

[2]

3.00%

1.00%

[3]

1.00%

1.00%

[4]

'

'

'

'

'

'

'

'

'

'

'

'

Number of ratings in A.M. Best ratings

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

16

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Reinsurance recoverables

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

198.3

[3]

'

21.8

[3]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Period during which Asbestos and Environmental Losses was Recovered under Third Party Reinsurance

'

'

'

'

'

'

'

'

'

'

'

'

'14 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Asbestos and Environmental Losses Recovered under Third-Party Reinsurance Percentage

'

'

'

'

'

'

'

'

'

'

'

'

45.80%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax, Estimated

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$44.90

'

'

'

'

'

'

'

[1]

The DecemberB 31, 2013 and 2012 balances include the remaining purchase accounting fair value adjustments of $136.9 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of DecemberB 31, 2013 and 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,741.6 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,930.0 million and $2,202.7 million for each period.

[2]

Includes $198.3 million of Third Party Recoverables, which NICO would pay under the terms of the NICO Cover if they are unable to collect from third party reinsurers.

[3]

Excludes $21.8 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.

[4]

Under review with developing implications.

Discontinued_Operations_Summar

Discontinued Operations (Summary of Reclassified Balances and Related Items) (Details) (USD $)

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2013

Dec. 31, 2012

Mar. 31, 2013

In Millions, unless otherwise specified

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Runoff

Runoff

Runoff

Assets

'

'

'

'

'

'

'

'

'

'

Investments

$2,364.90

$2,291.50

'

'

'

'

'

$236.30

$338.10

'

Premiums receivable

228.2

225.6

'

'

'

'

'

9.1

11

'

Reinsurance recoverable on unpaid losses

80.2

107.3

2,167.50

1,893.20

1,741.60

1,990.90

'

1,604.70

[1]

1,840.80

[1]

'

Reinsurance recoverable on paid losses

9.7

'

'

'

'

'

'

10.7

15.6

'

Net deferred tax asset

162.3

226.3

'

'

'

'

'

3.3

5.1

'

Other assets

272.7

227.2

'

'

'

'

'

16

16.2

'

Total assets

5,211.60

5,401.50

'

'

'

'

'

1,880.10

2,226.80

'

Liabilities

'

'

'

'

'

'

'

'

'

'

Unpaid loss and loss adjustment expense reserves

1,054.30

1,000

3,358.60

3,295.50

1,930

2,202.70

'

1,793.10

[1]

2,052.60

[1]

166

Unearned premiums

544.9

573.8

'

'

'

'

'

0.2

0.5

'

Ceded reinsurance payable

'

'

'

'

'

'

'

12.3

21.9

'

Other liabilities

338.6

302.7

'

'

'

'

'

74.5

[2]

151.8

[2]

'

Total liabilities

4,104.20

4,384.20

'

'

'

'

'

1,880.10

2,226.80

'

Pre-tax loss

'

'

'

'

($71.60)

($37.40)

($45.40)

$69

$140.70

'

[1]

The DecemberB 31, 2013 and 2012 balances include the remaining purchase accounting fair value adjustments of $136.9 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of DecemberB 31, 2013 and 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,741.6 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,930.0 million and $2,202.7 million for each period.

[2]

Other liabilities as of DecemberB 31, 2013 and 2012 include the accrual related to the pre-tax loss on sale of the Runoff Business of $69.0 million and $140.7 million, respectively.

Discontinued_Operations_Summar1

Discontinued Operations (Summary of the Results of Operations) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Discontinued Operations	'	'	'	'	'	'	'	'	'	'	'
Net written premiums	'	'	'	'	'	'	'	'	$1,088.60	$1,179.20	$1,062.70
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	1,120.40	1,132	1,012.20
Other revenue	'	'	'	'	'	'	'	'	31.2	-0.5	-12.4
Revenues	315	311.5	267.3	348.3	295.5	346.3	282.6	316.4	1,242.10	1,240.80	1,081.80
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	622.1	650	548.3
Other underwriting expenses	'	'	'	'	'	'	'	'	208.9	249.4	221.2
Total expenses	254.7	275.6	270.3	260.2	328.8	287.1	270.2	248.8	1,060.80	1,134.90	962.1
Income tax benefit	18.4	-2.9	-1	19.8	-19.7	14.2	0.4	13.5	34.3	8.4	14.8
Income (loss) from discontinued operations, net of tax	-46.9	0.3	-0.5	0.5	0.5	-15.8	0.5	-9.5	'	'	'
Gain (loss) from sale of discontinued operations, net of tax	46.6	0	0	0	0	-91	0	0	46.6	-91	-19.2
Segment, Discontinued Operations	'	'	'	'	'	'	'	'	'	'	'
Discontinued Operations	'	'	'	'	'	'	'	'	'	'	'
Net written premiums	'	'	'	'	'	'	'	'	0.4	3.5	58
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	0.8	10.6	70.5
Other revenue	'	'	7.8	'	'	'	'	'	6.3	0	1.7
Revenues	'	'	'	'	'	'	'	'	7.1	10.6	72.2
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	78.9	48.4	89.5
Policy acquisition expenses (benefit)	'	'	'	'	'	'	'	'	0	-2.1	5.3
Other underwriting expenses	'	'	'	'	'	'	'	'	-0.2	1.7	22.8
Total expenses	'	'	'	'	'	'	'	'	78.7	48	117.6
Pre-tax loss	'	'	'	'	'	'	'	'	-71.6	-37.4	-45.4
Income tax benefit	'	'	'	'	'	'	'	'	25	13.1	15.8
Income (loss) from discontinued operations, net of tax	'	'	'	'	'	'	'	'	-46.6	-24.3	-29.6
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	'	'	'	'	46.6	-91	-19.2
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	'	'	'	'	'	'	'	'	0	-115.3	-48.8
Runoff	'	'	'	'	'	'	'	'	'	'	'
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	9	'	'	'	'	'	'
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	91.5	'	'	'	'	'	'
Runoff | Segment, Discontinued Operations	'	'	'	'	'	'	'	'	'	'	'
Expenses	'	'	'	'	'	'	'	'	'	'	'
Pre-tax loss	'	'	'	'	'	'	'	'	69	140.7	'
Income (loss) from discontinued operations, net of tax	'	'	'	'	'	'	'	'	'	'	71.5
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	$0.50	'	'	'	'	'	$46.50

Discontinued_Operations_Loss_p

Discontinued Operations (Loss per Share Related to Discontinued Operations) (Details) (USD $)

3 Months Ended

12 Months Ended

In Millions, except Per Share data, unless otherwise specified

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

Loss attributable to OneBeacon's common shareholdersbbasic and diluted (in millions):

'

'

'

'

'

'

'

'

'

'

'

Net loss from discontinued operations attributable to OneBeacon's common shareholders

'

'

'

'

'

'

'

'

($46.60)

($24.30)

($29.60)

Allocation of loss for participating unvested restricted common shares

'

'

'

'

'

'

'

'

-1.4

-0.9

-0.4

Net income (loss) attributable to OneBeacon's common shareholders

41.4

39.1

-2.9

68.4

-13.3

-62.2

12.3

44

146

-19.2

55.1

Loss per share denominatorbbasic and diluted (in millions):

'

'

'

'

'

'

'

'

'

'

'

Total weighted average common shares outstanding

'

'

'

'

'

'

'

'

95.4

95.4

94.8

Weighted average unvested restricted common shares

'

'

'

'

'

'

'

'

-0.9

[1]

-0.9

[1]

-0.4

[1]

Basic and diluted earnings (loss) per share denominator

'

'

'

'

'

'

'

'

94.5

[2]

94.5

[2]

94.4

[2]

Loss per share attributable to OneBeacon's common shareholdersbbasic and diluted (in dollars):

'

'

'

'

'

'

'

'

'

'

'

Net loss from discontinued operations attributable to OneBeacon's common shareholders per share

$0.43

[3]

$0.41

[3]

($0.03)

[3]

$0.71

[3]

($0.14)

[3]

($0.65)

[3]

$0.12

[3]

$0.46

[3]

$1.52

($0.21)

$0.58

Segment, Discontinued Operations

'

'

'

'

'

'

'

'

'

'

'

Loss attributable to OneBeacon's common shareholdersbbasic and diluted (in millions):

'

'

'

'

'

'

'

'

'

'

'

Net loss from discontinued operations attributable to OneBeacon's common shareholders

'

'

'

'

'

'

'

'

0

-115.3

-48.8

Allocation of loss for participating unvested restricted common shares

'

'

'

'

'

'

'

'

0

1.1

0.2

Net income (loss) attributable to OneBeacon's common shareholders

'

'

'

'

'

'

'

'

$0

($114.20)

($48.60)

Loss per share denominatorbbasic and diluted (in millions):

'

'

'

'

'

'

'

'

'

'

'

Total weighted average common shares outstanding

'

'

'

'

'

'

'

'

95.4

95.4

94.8

Weighted average unvested restricted common shares

'

'

'

'

'

'

'

'

-0.9

[4]

-0.9

[4]

-0.4

[4]

Basic and diluted earnings (loss) per share denominator

'

'

'

'

'

'

'

'

94.5

[2]

94.5

[2]

94.4

[2]

Loss per share attributable to OneBeacon's common shareholdersbbasic and diluted (in dollars):

'

'

'

'

'

'

'

'

'

'

'

Net loss from discontinued operations attributable to OneBeacon's common shareholders per share

'

'

'

'

'

'

'

'

$0

($1.21)

($0.50)

[1]

Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event (see NoteB 9).

[2]

Common shares issuable upon exercise of stock options (Note 9) were not included in the years ended December 31, 2012 and 2011 as their inclusion would be anti-dilutive. There were no options outstanding during the year ended DecemberB 31, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.

[3]

Due to the averaging of shares, quarterly earnings per share may not add to the total for the full year.

[4]

Restricted shares outstanding vest in equal installments upon a stated date or upon the occurrence of a specified event.

Discontinued_Operations_Loss_a

Discontinued Operations (Loss and LAE Exposure) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discontinued Operations	'	'	'	'
Liability for asbestos and environmental claims, gross and net	'	$1	'	'
Segment, Discontinued Operations	'	'	'	'
Discontinued Operations	'	'	'	'
Liability for asbestos and environmental claims, gross and net	1,013.80	1,162.40	1,354.10	1,023
Liability for Asbestos and Environmental Claims, Net, Before nico	615.6	727.9	832.8	741.1
LAE claims, net	5.7	8.8	11.2	15.6
Liability for Asbestos and Environmental Claims, Gross, Incurred Loss	0	-1.2	488.6	'
Liability for Asbestos and Environmental Claims, Net, Incurred Loss before nico	0	-1	94.4	'
Liability for Asbestos and Environmental Claims, Net, Incurred Loss	0	-1	6	'
Liability for Asbestos and Environmental Claims, Gross, Claims Paid	-148.6	-190.5	-157.5	'
Liability for Asbestos and Environmental Claims, Net, Claims Paid before nico	-112.3	-103.9	-2.7	'
Liability for Asbestos and Environmental Claims, Net, Claims Paid	-3.1	-1.4	-10.4	'
Segment, Discontinued Operations | Asbestos Issue [Member]	'	'	'	'
Discontinued Operations	'	'	'	'
Liability for asbestos and environmental claims, gross and net	845.1	929.4	1,074.30	904
Liability for Asbestos and Environmental Claims, Net, Before nico	523.2	602.5	681.2	647.3
LAE claims, net	1.9	2.4	2.2	6.4
Liability for Asbestos and Environmental Claims, Gross, Incurred Loss	0	-0.3	256.8	'
Liability for Asbestos and Environmental Claims, Net, Incurred Loss before nico	0	-0.5	32.2	'
Liability for Asbestos and Environmental Claims, Net, Incurred Loss	0	-0.5	-4	'
Liability for Asbestos and Environmental Claims, Gross, Claims Paid	-84.3	-144.6	-86.5	'
Liability for Asbestos and Environmental Claims, Net, Claims Paid before nico	-79.3	-78.2	1.7	'
Liability for Asbestos and Environmental Claims, Net, Claims Paid	-0.5	0.7	-0.2	'
Segment, Discontinued Operations | Environmental Issue [Member]	'	'	'	'
Discontinued Operations	'	'	'	'
Liability for asbestos and environmental claims, gross and net	168.7	233	279.8	119
Liability for Asbestos and Environmental Claims, Net, Before nico	92.4	125.4	151.6	93.8
LAE claims, net	3.8	6.4	9	9.2
Liability for Asbestos and Environmental Claims, Gross, Incurred Loss	0	-0.9	231.8	'
Liability for Asbestos and Environmental Claims, Net, Incurred Loss before nico	0	-0.5	62.2	'
Liability for Asbestos and Environmental Claims, Net, Incurred Loss	0	-0.5	10	'
Liability for Asbestos and Environmental Claims, Gross, Claims Paid	-64.3	-45.9	-71	'
Liability for Asbestos and Environmental Claims, Net, Claims Paid before nico	-33	-25.7	-4.4	'
Liability for Asbestos and Environmental Claims, Net, Claims Paid	($2.60)	($2.10)	($10.20)	'

Discontinued_Operations_Writte

Discontinued Operations (Written and Earned Premiums) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Discontinued Operations	'		'	'
Direct	$1,103.10		$1,204	$1,079.20
Assumed	59.8		55.2	49.1
Ceded	-74.3		-80	-65.6
Net written premiums	1,088.60		1,179.20	1,062.70
Direct	1,043.30		1,158.30	1,035.90
Assumed	148.5	[1]	52.8	42.3
Ceded	-71.4		-79.1	-66
Net earned premiums	1,120.40		1,132	1,012.20
Direct	584.9		687.5	551.8
Assumed	76.3	[1]	29.6	9.2
Ceded	-39.1		-67.1	-12.7
Net loss and LAE	622.1		650	548.3
Segment, Discontinued Operations	'		'	'
Discontinued Operations	'		'	'
Direct	0.2		5.8	56.2
Assumed	0.2		-0.1	-0.9
Ceded	0		-2.2	2.7
Net written premiums	0.4		3.5	58
Direct	0.4		12.1	169.8
Assumed	0.4		0.9	0.8
Ceded	0		-2.4	-100.1
Net earned premiums	0.8		10.6	70.5
Direct	82		52.8	656.3
Assumed	5.2		14.5	26.8
Ceded	-8.3		-18.9	-593.6
Net loss and LAE	78.9		48.4	89.5
Segment, Discontinued Operations | Non - Specialty Commercial Lines [Member]	'		'	'
Discontinued Operations	'		'	'
Ceded	'		'	0.6
Net earned premiums	'		'	96.6
Net loss and LAE	'		'	$62.30
[1]	Includes assumed earned premiums and loss and LAE of $89.1 million and $41.9 million, respectively, related to the collector cars and boats business which although exited on January 1, 2013, was assumed on a runoff basis from Essentia.

Consolidating_Financial_Inform2

Consolidating Financial Information (Consolidating Balance Sheet) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
In Millions, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]	'	'	'	'
Fixed maturity investments, at fair value	$1,700.90	$1,593.30	'	'
Short-term investments, at amortized cost (which approximates fair value)	157	232.8	'	'
Common equity securities, at fair value	336.9	259	'	'
Convertible fixed maturity investments, at fair value	30.5	62.6	'	'
Other investments	139.6	143.8	'	'
Total investment securities	2,364.90	2,291.50	'	'
Cash	168.1	43.9	54.9	33.6
Reinsurance recoverables	89.9	110.6	'	'
Premiums receivable	228.2	225.6	'	'
Deferred acquisition costs	103.7	123.9	'	'
Net deferred tax asset	90.6	137.8	'	'
Investment income accrued	10.1	12.1	'	'
Accounts receivable on unsettled investment sales	3.3	2.1	'	'
Investments in subsidiaries	0	0	'	'
Other assets	272.7	227.2	'	'
Assets held for sale	1,880.10	2,226.80	'	'
Total assets	5,211.60	5,401.50	'	'
Liabilities	'	'	'	'
Unpaid loss and loss adjustment expense reserves	1,054.30	1,000	3,358.60	3,295.50
Unearned premiums	544.9	573.8	'	'
Debt	274.7	274.7	'	'
Accounts payable on unsettled investment purchases	11.6	6.2	'	'
Other liabilities	338.6	302.7	'	'
Liabilities held for sale	1,880.10	2,226.80	'	'
Total liabilities	4,104.20	4,384.20	'	'
Equity [Abstract]	'	'	'	'
Total OneBeacon's common shareholders' equity	1,104.30	1,014.50	'	'
Total noncontrolling interests	3.1	2.8	'	'
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,107.40	1,017.30	1,113.90	1,248.90
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,211.60	5,401.50	'	'
One Beacon	'	'	'	'
Investments, Debt and Equity Securities [Abstract]	'	'	'	'
Fixed maturity investments, at fair value	0	0	'	'
Short-term investments, at amortized cost (which approximates fair value)	13	2.7	'	'
Common equity securities, at fair value	0	0	'	'
Convertible fixed maturity investments, at fair value	0	0	'	'
Other investments	0	0	'	'
Total investment securities	13	2.7	'	'
Cash	0	0	0	0
Reinsurance recoverables	0	0	'	'
Premiums receivable	0	0	'	'
Deferred acquisition costs	0	0	'	'
Net deferred tax asset	0	0	'	'
Investment income accrued	0	0	'	'
Accounts receivable on unsettled investment sales	0	0	'	'
Investments in subsidiaries	1,092	1,012.40	'	'
Other assets	-0.8	0.3	'	'
Assets held for sale	0	0	'	'
Total assets	1,104.20	1,015.40	'	'
Liabilities	'	'	'	'
Unpaid loss and loss adjustment expense reserves	0	0	'	'
Unearned premiums	0	0	'	'
Debt	0	0	'	'
Accounts payable on unsettled investment purchases	0	0	'	'
Other liabilities	-0.1	0.9	'	'
Liabilities held for sale	0	0	'	'
Total liabilities	-0.1	0.9	'	'
Equity [Abstract]	'	'	'	'
Total OneBeacon's common shareholders' equity	1,104.30	1,014.50	'	'
Total noncontrolling interests	0	0	'	'
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,104.30	1,014.50	'	'
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	1,104.20	1,015.40	'	'
Non-guarantor subsidiaries	'	'	'	'
Investments, Debt and Equity Securities [Abstract]	'	'	'	'
Fixed maturity investments, at fair value	1,725.40	1,602.40	'	'
Short-term investments, at amortized cost (which approximates fair value)	140	169.9	'	'
Common equity securities, at fair value	336.9	259	'	'
Convertible fixed maturity investments, at fair value	30.5	62.6	'	'
Other investments	139.6	143.8	'	'
Total investment securities	2,372.40	2,237.70	'	'
Cash	166.6	43.9	52.5	33.4
Reinsurance recoverables	89.9	110.6	'	'
Premiums receivable	228.2	225.6	'	'
Deferred acquisition costs	103.7	123.9	'	'
Net deferred tax asset	93.1	140.2	'	'
Investment income accrued	10.3	12.3	'	'
Accounts receivable on unsettled investment sales	3.3	1.4	'	'
Investments in subsidiaries	0	0	'	'
Other assets	272.4	219.8	'	'
Assets held for sale	1,880.10	2,226.80	'	'
Total assets	5,220	5,342.20	'	'
Liabilities	'	'	'	'
Unpaid loss and loss adjustment expense reserves	1,054.30	1,000	'	'
Unearned premiums	544.9	573.8	'	'
Debt	0	0	'	'
Accounts payable on unsettled investment purchases	11.6	6.2	'	'
Other liabilities	335	295.9	'	'
Liabilities held for sale	1,880.10	2,226.80	'	'
Total liabilities	3,825.90	4,102.70	'	'
Equity [Abstract]	'	'	'	'
Total OneBeacon's common shareholders' equity	1,391	1,236.70	'	'
Total noncontrolling interests	3.1	2.8	'	'
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,394.10	1,239.50	'	'
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,220	5,342.20	'	'
OBH (issuer)	'	'	'	'
Investments, Debt and Equity Securities [Abstract]	'	'	'	'
Fixed maturity investments, at fair value	0	16.6	'	'
Short-term investments, at amortized cost (which approximates fair value)	4	60.2	'	'
Common equity securities, at fair value	0	0	'	'
Convertible fixed maturity investments, at fair value	0	0	'	'
Other investments	0	0	'	'
Total investment securities	4	76.8	'	'
Cash	1.5	0	2.4	0.2
Reinsurance recoverables	0	0	'	'
Premiums receivable	0	0	'	'
Deferred acquisition costs	0	0	'	'
Net deferred tax asset	-2.4	-2.5	'	'
Investment income accrued	0	0	'	'
Accounts receivable on unsettled investment sales	0	0.7	'	'
Investments in subsidiaries	1,034.90	1,062.60	'	'
Other assets	1.1	7.1	'	'
Assets held for sale	0	0	'	'
Total assets	1,039.10	1,144.70	'	'
Liabilities	'	'	'	'
Unpaid loss and loss adjustment expense reserves	0	0	'	'
Unearned premiums	0	0	'	'
Debt	299.7	299.6	'	'
Accounts payable on unsettled investment purchases	0	0	'	'
Other liabilities	3.9	6	'	'
Liabilities held for sale	0	0	'	'
Total liabilities	303.6	305.6	'	'
Equity [Abstract]	'	'	'	'
Total OneBeacon's common shareholders' equity	735.5	839.1	'	'
Total noncontrolling interests	0	0	'	'
Total OneBeacon's common shareholders' equity and noncontrolling interests	735.5	839.1	'	'
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	1,039.10	1,144.70	'	'
Consolidating adjustments	'	'	'	'
Investments, Debt and Equity Securities [Abstract]	'	'	'	'
Fixed maturity investments, at fair value	-24.5	-25.7	'	'
Short-term investments, at amortized cost (which approximates fair value)	0	0	'	'
Common equity securities, at fair value	0	0	'	'
Convertible fixed maturity investments, at fair value	0	0	'	'
Other investments	0	0	'	'
Total investment securities	-24.5	-25.7	'	'
Cash	0	0	0	0
Reinsurance recoverables	0	0	'	'
Premiums receivable	0	0	'	'
Deferred acquisition costs	0	0	'	'
Net deferred tax asset	-0.1	0.1	'	'
Investment income accrued	-0.2	-0.2	'	'
Accounts receivable on unsettled investment sales	0	0	'	'
Investments in subsidiaries	-2,126.90	-2,075	'	'
Other assets	0	0	'	'
Assets held for sale	0	0	'	'
Total assets	-2,151.70	-2,100.80	'	'
Liabilities	'	'	'	'
Unpaid loss and loss adjustment expense reserves	0	0	'	'
Unearned premiums	0	0	'	'
Debt	-25	-24.9	'	'
Accounts payable on unsettled investment purchases	0	0	'	'
Other liabilities	-0.2	-0.1	'	'
Liabilities held for sale	0	0	'	'
Total liabilities	-25.2	-25	'	'
Equity [Abstract]	'	'	'	'
Total OneBeacon's common shareholders' equity	-2,126.50	-2,075.80	'	'
Total noncontrolling interests	0	0	'	'
Total OneBeacon's common shareholders' equity and noncontrolling interests	-2,126.50	-2,075.80	'	'
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	($2,151.70)	($2,100.80)	'	'

Consolidating_Financial_Inform3

Consolidating Financial Information (Consolidating Statement of Operations and Comprehensive (Loss)) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	$1,120.40	$1,132	$1,012.20
Net investment income (expense)	'	'	'	'	'	'	'	'	41.1	53.6	71.4
Net realized and change in unrealized investment gains	'	'	'	'	'	'	'	'	49.4	55.7	10.6
Net other revenues	'	'	'	'	'	'	'	'	31.2	-0.5	-12.4
Total revenues	315	311.5	267.3	348.3	295.5	346.3	282.6	316.4	1,242.10	1,240.80	1,081.80
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	622.1	650	548.3
Policy acquisition expenses	'	'	'	'	'	'	'	'	208.9	249.4	221.2
Other underwriting expenses	'	'	'	'	'	'	'	'	204.8	205.2	162.3
General and administrative expenses	'	'	'	'	'	'	'	'	12	13.4	9.8
Interest expense	'	'	'	'	'	'	'	'	13	16.9	20.5
Total expenses	254.7	275.6	270.3	260.2	328.8	287.1	270.2	248.8	1,060.80	1,134.90	962.1
Pre-tax (loss) income from continuing operations	60.3	35.9	-3	88.1	-33.3	59.2	12.4	67.6	181.3	105.9	119.7
Income tax expense	-18.4	2.9	1	-19.8	19.7	-14.2	-0.4	-13.5	-34.3	-8.4	-14.8
Net income from continuing operations	41.9	38.8	-2	68.3	-13.6	45	12	54.1	147	97.5	104.9
Net loss from discontinued operations attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	-46.6	-24.3	-29.6
Gain (loss) from sale of discontinued operations, net of tax	46.6	0	0	0	0	-91	0	0	46.6	-91	-19.2
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	'	'	'	'	'	'	'	'	147	-17.8	56.1
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	0	0	0
Net income (loss), including noncontrolling interests	41.6	39.1	-2.5	68.8	-13.1	-61.8	12.5	44.6	147	-17.8	56.1
Less: Net income attributable to noncontrolling interests	-0.2	0	-0.4	-0.4	-0.2	-0.4	-0.2	-0.6	-1	-1.4	-1
Net income (loss) attributable to OneBeacon's common shareholders	41.4	39.1	-2.9	68.4	-13.3	-62.2	12.3	44	146	-19.2	55.1
Net change in benefit plan assets and obligations, net of tax	'	'	'	'	'	'	'	'	20.6	-2.9	-11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	166.6	-22.1	43.9
One Beacon	'	'	'	'	'	'	'	'	'	'	'
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	0	0	0
Net investment income (expense)	'	'	'	'	'	'	'	'	0	0	0
Net realized and change in unrealized investment gains	'	'	'	'	'	'	'	'	0	0	0
Net other revenues	'	'	'	'	'	'	'	'	0	0	0
Total revenues	'	'	'	'	'	'	'	'	0	0	0
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	0	0	0
Policy acquisition expenses	'	'	'	'	'	'	'	'	0	0	0
Other underwriting expenses	'	'	'	'	'	'	'	'	0	0	0
General and administrative expenses	'	'	'	'	'	'	'	'	5	5.4	5.1
Interest expense	'	'	'	'	'	'	'	'	0	0	0
Total expenses	'	'	'	'	'	'	'	'	5	5.4	5.1
Pre-tax (loss) income from continuing operations	'	'	'	'	'	'	'	'	-5	-5.4	-5.1
Income tax expense	'	'	'	'	'	'	'	'	0.1	0.1	-0.2
Net income from continuing operations	'	'	'	'	'	'	'	'	-4.9	-5.3	-5.3
Net loss from discontinued operations attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	0	0	0
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	'	'	'	'	0	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	'	'	'	'	'	'	'	'	-4.9	-5.3	-5.3
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	150.9	-13.9	60.4
Net income (loss), including noncontrolling interests	'	'	'	'	'	'	'	'	146	-19.2	55.1
Less: Net income attributable to noncontrolling interests	'	'	'	'	'	'	'	'	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	146	-19.2	55.1
Net change in benefit plan assets and obligations, net of tax	'	'	'	'	'	'	'	'	20.6	-2.9	-11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	166.6	-22.1	43.9
Non-guarantor subsidiaries	'	'	'	'	'	'	'	'	'	'	'
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	1,120.40	1,132	1,012.20
Net investment income (expense)	'	'	'	'	'	'	'	'	42.6	56.2	72.7
Net realized and change in unrealized investment gains	'	'	'	'	'	'	'	'	48.9	56.5	10
Net other revenues	'	'	'	'	'	'	'	'	31.2	6.6	0.7
Total revenues	'	'	'	'	'	'	'	'	1,243.10	1,251.30	1,095.60
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	622.1	650	548.3
Policy acquisition expenses	'	'	'	'	'	'	'	'	208.9	249.4	221.2
Other underwriting expenses	'	'	'	'	'	'	'	'	204.8	205.2	162.3
General and administrative expenses	'	'	'	'	'	'	'	'	7.1	7.4	4.1
Interest expense	'	'	'	'	'	'	'	'	0	0	0
Total expenses	'	'	'	'	'	'	'	'	1,042.90	1,112	935.9
Pre-tax (loss) income from continuing operations	'	'	'	'	'	'	'	'	200.2	139.3	159.7
Income tax expense	'	'	'	'	'	'	'	'	-40.4	-18.6	-26.9
Net income from continuing operations	'	'	'	'	'	'	'	'	159.8	120.7	132.8
Net loss from discontinued operations attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	-46.6	-24.3	-29.6
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	'	'	'	'	46.6	-91	-19.2
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	'	'	'	'	'	'	'	'	159.8	5.4	84
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	0	0	0
Net income (loss), including noncontrolling interests	'	'	'	'	'	'	'	'	159.8	5.4	84
Less: Net income attributable to noncontrolling interests	'	'	'	'	'	'	'	'	-1	-1.4	-1
Net income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	158.8	4	83
Net change in benefit plan assets and obligations, net of tax	'	'	'	'	'	'	'	'	0	0	0
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	158.8	4	83
OBH (issuer)	'	'	'	'	'	'	'	'	'	'	'
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	0	0	0
Net investment income (expense)	'	'	'	'	'	'	'	'	-0.3	0.4	0.5
Net realized and change in unrealized investment gains	'	'	'	'	'	'	'	'	-0.7	-1.4	-0.1
Net other revenues	'	'	'	'	'	'	'	'	0	-7.1	-13.1
Total revenues	'	'	'	'	'	'	'	'	-1	-8.1	-12.7
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	0	0	0
Policy acquisition expenses	'	'	'	'	'	'	'	'	0	0	0
Other underwriting expenses	'	'	'	'	'	'	'	'	0	0	0
General and administrative expenses	'	'	'	'	'	'	'	'	-0.1	0.6	0.6
Interest expense	'	'	'	'	'	'	'	'	14.2	19.7	22.1
Total expenses	'	'	'	'	'	'	'	'	14.1	20.3	22.7
Pre-tax (loss) income from continuing operations	'	'	'	'	'	'	'	'	-15.1	-28.4	-35.4
Income tax expense	'	'	'	'	'	'	'	'	6.2	10.1	12.4
Net income from continuing operations	'	'	'	'	'	'	'	'	-8.9	-18.3	-23
Net loss from discontinued operations attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	0	0	0
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	'	'	'	'	0	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	'	'	'	'	'	'	'	'	-8.9	-18.3	-23
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	134.7	-28.9	50.9
Net income (loss), including noncontrolling interests	'	'	'	'	'	'	'	'	125.8	-47.2	27.9
Less: Net income attributable to noncontrolling interests	'	'	'	'	'	'	'	'	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	125.8	-47.2	27.9
Net change in benefit plan assets and obligations, net of tax	'	'	'	'	'	'	'	'	20.6	-2.9	-11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	146.4	-50.1	16.7
Consolidating adjustments	'	'	'	'	'	'	'	'	'	'	'
Revenues	'	'	'	'	'	'	'	'	'	'	'
Earned premiums	'	'	'	'	'	'	'	'	0	0	0
Net investment income (expense)	'	'	'	'	'	'	'	'	-1.2	-3	-1.8
Net realized and change in unrealized investment gains	'	'	'	'	'	'	'	'	1.2	0.6	0.7
Net other revenues	'	'	'	'	'	'	'	'	0	0	0
Total revenues	'	'	'	'	'	'	'	'	0	-2.4	-1.1
Expenses	'	'	'	'	'	'	'	'	'	'	'
Loss and loss adjustment expenses	'	'	'	'	'	'	'	'	0	0	0
Policy acquisition expenses	'	'	'	'	'	'	'	'	0	0	0
Other underwriting expenses	'	'	'	'	'	'	'	'	0	0	0
General and administrative expenses	'	'	'	'	'	'	'	'	0	0	0
Interest expense	'	'	'	'	'	'	'	'	-1.2	-2.8	-1.6
Total expenses	'	'	'	'	'	'	'	'	-1.2	-2.8	-1.6
Pre-tax (loss) income from continuing operations	'	'	'	'	'	'	'	'	1.2	0.4	0.5
Income tax expense	'	'	'	'	'	'	'	'	-0.2	0	-0.1
Net income from continuing operations	'	'	'	'	'	'	'	'	1	0.4	0.4
Net loss from discontinued operations attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	0	0	0
Gain (loss) from sale of discontinued operations, net of tax	'	'	'	'	'	'	'	'	0	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	'	'	'	'	'	'	'	'	1	0.4	0.4
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	-285.6	42.8	-111.3
Net income (loss), including noncontrolling interests	'	'	'	'	'	'	'	'	-284.6	43.2	-110.9
Less: Net income attributable to noncontrolling interests	'	'	'	'	'	'	'	'	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	-284.6	43.2	-110.9
Net change in benefit plan assets and obligations, net of tax	'	'	'	'	'	'	'	'	-20.6	2.9	11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	($305.20)	$46.10	($99.70)

Consolidating_Financial_Inform4

Consolidating Financial Information (Consolidating Statement of Cash Flows) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]	'	'	'
Net income (loss), including noncontrolling interests	$147	($17.80)	$56.10
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:	'	'	'
Undistributed earnings from subsidiaries	0	0	0
Net loss from discontinued operations	46.6	24.3	29.6
Net loss from sale of discontinued operations	-46.6	91	19.2
Net realized and change in unrealized investment gains	-49.4	-55.7	-10.6
Net other realized losses	0	6.3	11.7
Net realized gain on sale of business	-23	-4.4	0
Deferred income tax expense (benefit)	12.8	16.4	31.6
Dividends received from subsidiaries	0	0	0
Other operating items:	'	'	'
Net change in loss and LAE reserves	54.3	87.5	69.2
Net change in unearned premiums	-28.9	48.4	50.2
Net change in premiums receivable	-2.6	-14.6	-49.4
Net change in reinsurance recoverable on paid and unpaid losses	20.7	-3.1	-46.3
Net change in other assets and liabilities	74.5	-35.6	-79.2
Net cash provided from operationsbcontinuing operations	205.4	142.7	82.1
Net cash used for operationsbdiscontinued operations	-76.7	-196.4	-200.6
Net cash provided from (used for) operations	128.7	-53.7	-118.5
Cash flows from investing activities:	'	'	'
Net maturities, purchases and sales of short-term investments	72.2	14.6	-19.9
Maturities of fixed maturity investments	245.2	191.5	478.3
Sales of fixed maturity investments	1,400	1,717.20	1,414.80
Sales of common equity securities	196.7	112.5	101.9
Sales of convertible fixed maturity investments	45.7	29.3	37.1
Return of capital and distributions of other investments	21.8	63.9	38.6
Purchases of fixed maturity investments	-1,697	-1,862.50	-1,472.30
Purchases of common equity securities	-206.3	-91.5	-88.5
Purchases of convertible fixed maturity investments	-14.4	-9.4	-30.4
Contributions for other investments	-7.6	-44.9	-10.8
Proceeds from sale of business	31.3	15	0
Net change in unsettled investment purchases and sales	4.1	-18.2	13.5
Net acquisitions of property and equipment	-10.3	-1.8	-3.7
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0	0
Net cash provided from investing activitiesbcontinuing operations	81.4	115.7	458.6
Net cash provided from investing activitiesbdiscontinued operations	0	0	0
Net cash provided from investing activities	81.4	115.7	458.6
Cash flows from financing activities:	'	'	'
Issuance of debt, net of debt issuance costs	0	271.9	0
Repurchases of debt	0	-275.9	-161.6
Cash dividends paid to common shareholders	-80.2	-80.1	-174.8
Cash dividends paid to parent	0	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0	0
Payments on capital lease obligation	-5.7	-4.9	23.1
Proceeds on sale of OB Holdings Lux	0	14	0
Net cash used for financing activitiesbcontinuing operations	-85.9	-75	-313.3
Net cash used for financing activitiesbdiscontinued operations	0	0	0
Net cash used for financing activities	-85.9	-75	-313.3
Net increase in cash during period	124.2	-13	26.8
Cash reclassified to assets held for sale	0	5.5	-5.5
Cash transferred with sale of business	0	-3.5	'
Net increase (decrease) after reclassification of cash to assets held for sale	124.2	-11	21.3
Cash balance at beginning of period	43.9	54.9	33.6
Cash balance at end of period	168.1	43.9	54.9
One Beacon	'	'	'
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]	'	'	'
Net income (loss), including noncontrolling interests	146	-19.2	55.1
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:	'	'	'
Undistributed earnings from subsidiaries	-150.9	13.9	-60.4
Net loss from discontinued operations	0	0	0
Net loss from sale of discontinued operations	0	0	0
Net realized and change in unrealized investment gains	0	0	0
Net other realized losses	'	0	0
Net realized gain on sale of business	0	0	'
Deferred income tax expense (benefit)	0	0	0
Dividends received from subsidiaries	92	83	181
Other operating items:	'	'	'
Net change in loss and LAE reserves	0	0	0
Net change in unearned premiums	0	0	0
Net change in premiums receivable	0	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0	0
Net change in other assets and liabilities	3.4	2.3	0.1
Net cash provided from operationsbcontinuing operations	90.5	80	175.8
Net cash used for operationsbdiscontinued operations	0	0	0
Net cash provided from (used for) operations	90.5	80	175.8
Cash flows from investing activities:	'	'	'
Net maturities, purchases and sales of short-term investments	-10.3	0.1	-1
Maturities of fixed maturity investments	0	0	0
Sales of fixed maturity investments	0	0	0
Sales of common equity securities	0	0	0
Sales of convertible fixed maturity investments	0	0	0
Return of capital and distributions of other investments	0	0	0
Purchases of fixed maturity investments	0	0	0
Purchases of common equity securities	0	0	0
Purchases of convertible fixed maturity investments	0	0	0
Contributions for other investments	0	0	0
Proceeds from sale of business	0	0	'
Net change in unsettled investment purchases and sales	0	0	0
Net acquisitions of property and equipment	0	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0	0
Net cash provided from investing activitiesbcontinuing operations	-10.3	0.1	-1
Net cash provided from investing activitiesbdiscontinued operations	0	0	0
Net cash provided from investing activities	-10.3	0.1	-1
Cash flows from financing activities:	'	'	'
Issuance of debt, net of debt issuance costs	'	0	'
Repurchases of debt	'	0	0
Cash dividends paid to common shareholders	-80.2	-80.1	-174.8
Cash dividends paid to parent	0	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0	0
Payments on capital lease obligation	0	0	0
Proceeds on sale of OB Holdings Lux	'	0	'
Net cash used for financing activitiesbcontinuing operations	-80.2	-80.1	-174.8
Net cash used for financing activitiesbdiscontinued operations	0	0	0
Net cash used for financing activities	-80.2	-80.1	-174.8
Net increase in cash during period	0	0	0
Cash reclassified to assets held for sale	0	0	0
Cash transferred with sale of business	0	0	'
Net increase (decrease) after reclassification of cash to assets held for sale	0	0	0
Cash balance at beginning of period	0	0	0
Cash balance at end of period	0	0	0
Non-guarantor subsidiaries	'	'	'
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]	'	'	'
Net income (loss), including noncontrolling interests	159.8	5.4	84
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:	'	'	'
Undistributed earnings from subsidiaries	0	0	0
Net loss from discontinued operations	46.6	24.3	29.6
Net loss from sale of discontinued operations	-46.6	91	19.2
Net realized and change in unrealized investment gains	-48.9	-56.5	-10
Net other realized losses	'	0	-0.3
Net realized gain on sale of business	-23	-4.4	'
Deferred income tax expense (benefit)	12.7	15.2	31.3
Dividends received from subsidiaries	0	15.5	34.4
Other operating items:	'	'	'
Net change in loss and LAE reserves	54.3	87.5	69.2
Net change in unearned premiums	-28.9	48.4	50.2
Net change in premiums receivable	-2.6	-14.6	-49.4
Net change in reinsurance recoverable on paid and unpaid losses	20.7	-3.1	-46.3
Net change in other assets and liabilities	100.5	-38.8	-92.1
Net cash provided from operationsbcontinuing operations	244.6	169.9	119.8
Net cash used for operationsbdiscontinued operations	-76.7	-196.4	-200.6
Net cash provided from (used for) operations	167.9	-26.5	-80.8
Cash flows from investing activities:	'	'	'
Net maturities, purchases and sales of short-term investments	26.3	74.7	-55.7
Maturities of fixed maturity investments	223.7	191.4	475.3
Sales of fixed maturity investments	1,496.70	1,783.90	1,406.70
Sales of common equity securities	196.7	114.8	101.9
Sales of convertible fixed maturity investments	45.7	33.1	37.1
Return of capital and distributions of other investments	21.8	63.9	38.6
Purchases of fixed maturity investments	-1,787.20	-1,887.60	-1,476.30
Purchases of common equity securities	-206.3	-90.1	-88.5
Purchases of convertible fixed maturity investments	-14.4	-13	-30.4
Contributions for other investments	-7.6	-44.9	-10.8
Proceeds from sale of business	31.3	15	'
Net change in unsettled investment purchases and sales	4.1	-18.2	13.5
Net acquisitions of property and equipment	-10.3	-1.8	-3.7
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	250	28.7	10.6
Net cash provided from investing activitiesbcontinuing operations	270.5	249.9	418.3
Net cash provided from investing activitiesbdiscontinued operations	0	0	0
Net cash provided from investing activities	270.5	249.9	418.3
Cash flows from financing activities:	'	'	'
Issuance of debt, net of debt issuance costs	'	0	'
Repurchases of debt	'	0	0
Cash dividends paid to common shareholders	0	0	0
Cash dividends paid to parent	-310	-243.1	-336
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0	0
Payments on capital lease obligation	-5.7	-4.9	23.1
Proceeds on sale of OB Holdings Lux	'	14	'
Net cash used for financing activitiesbcontinuing operations	-315.7	-234	-312.9
Net cash used for financing activitiesbdiscontinued operations	0	0	0
Net cash used for financing activities	-315.7	-234	-312.9
Net increase in cash during period	122.7	-10.6	24.6
Cash reclassified to assets held for sale	0	5.5	-5.5
Cash transferred with sale of business	0	-3.5	'
Net increase (decrease) after reclassification of cash to assets held for sale	122.7	-8.6	19.1
Cash balance at beginning of period	43.9	52.5	33.4
Cash balance at end of period	166.6	43.9	52.5
OBH (issuer)	'	'	'
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]	'	'	'
Net income (loss), including noncontrolling interests	125.8	-47.2	27.9
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:	'	'	'
Undistributed earnings from subsidiaries	-134.7	28.9	-50.9
Net loss from discontinued operations	0	0	0
Net loss from sale of discontinued operations	0	0	0
Net realized and change in unrealized investment gains	0.7	1.4	0.1
Net other realized losses	'	6.3	12
Net realized gain on sale of business	0	0	'
Deferred income tax expense (benefit)	-0.1	0.8	0.1
Dividends received from subsidiaries	218	160.1	155
Other operating items:	'	'	'
Net change in loss and LAE reserves	0	0	0
Net change in unearned premiums	0	0	0
Net change in premiums receivable	0	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0	0
Net change in other assets and liabilities	-29.4	1.1	12.7
Net cash provided from operationsbcontinuing operations	180.3	151.4	156.9
Net cash used for operationsbdiscontinued operations	0	0	0
Net cash provided from (used for) operations	180.3	151.4	156.9
Cash flows from investing activities:	'	'	'
Net maturities, purchases and sales of short-term investments	56.2	-60.2	36.8
Maturities of fixed maturity investments	21.5	0.1	3
Sales of fixed maturity investments	386.7	24.6	139.8
Sales of common equity securities	0	23.1	0
Sales of convertible fixed maturity investments	0	7.6	0
Return of capital and distributions of other investments	0	0	0
Purchases of fixed maturity investments	-393.2	-42	-127.7
Purchases of common equity securities	0	-26.8	0
Purchases of convertible fixed maturity investments	0	-7.8	0
Contributions for other investments	0	0	0
Proceeds from sale of business	0	0	'
Net change in unsettled investment purchases and sales	0	0	0
Net acquisitions of property and equipment	0	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0	0
Net cash provided from investing activitiesbcontinuing operations	71.2	-81.4	51.9
Net cash provided from investing activitiesbdiscontinued operations	0	0	0
Net cash provided from investing activities	71.2	-81.4	51.9
Cash flows from financing activities:	'	'	'
Issuance of debt, net of debt issuance costs	'	296.9	'
Repurchases of debt	'	-325.1	-161.6
Cash dividends paid to common shareholders	0	0	0
Cash dividends paid to parent	0	-15.5	-34.4
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	-250	-28.7	-10.6
Payments on capital lease obligation	0	0	0
Proceeds on sale of OB Holdings Lux	'	0	'
Net cash used for financing activitiesbcontinuing operations	-250	-72.4	-206.6
Net cash used for financing activitiesbdiscontinued operations	0	0	0
Net cash used for financing activities	-250	-72.4	-206.6
Net increase in cash during period	1.5	-2.4	2.2
Cash reclassified to assets held for sale	0	0	0
Cash transferred with sale of business	0	0	'
Net increase (decrease) after reclassification of cash to assets held for sale	1.5	-2.4	2.2
Cash balance at beginning of period	0	2.4	0.2
Cash balance at end of period	1.5	0	2.4
Consolidating adjustments	'	'	'
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]	'	'	'
Net income (loss), including noncontrolling interests	-284.6	43.2	-110.9
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:	'	'	'
Undistributed earnings from subsidiaries	285.6	-42.8	111.3
Net loss from discontinued operations	0	0	0
Net loss from sale of discontinued operations	0	0	0
Net realized and change in unrealized investment gains	-1.2	-0.6	-0.7
Net other realized losses	'	0	0
Net realized gain on sale of business	0	0	'
Deferred income tax expense (benefit)	0.2	0.4	0.2
Dividends received from subsidiaries	-310	-258.6	-370.4
Other operating items:	'	'	'
Net change in loss and LAE reserves	0	0	0
Net change in unearned premiums	0	0	0
Net change in premiums receivable	0	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0	0
Net change in other assets and liabilities	0	-0.2	0.1
Net cash provided from operationsbcontinuing operations	-310	-258.6	-370.4
Net cash used for operationsbdiscontinued operations	0	0	0
Net cash provided from (used for) operations	-310	-258.6	-370.4
Cash flows from investing activities:	'	'	'
Net maturities, purchases and sales of short-term investments	0	0	0
Maturities of fixed maturity investments	0	0	0
Sales of fixed maturity investments	-483.4	-91.3	-131.7
Sales of common equity securities	0	-25.4	0
Sales of convertible fixed maturity investments	0	-11.4	0
Return of capital and distributions of other investments	0	0	0
Purchases of fixed maturity investments	483.4	67.1	131.7
Purchases of common equity securities	0	25.4	0
Purchases of convertible fixed maturity investments	0	11.4	0
Contributions for other investments	0	0	0
Proceeds from sale of business	0	0	'
Net change in unsettled investment purchases and sales	0	0	0
Net acquisitions of property and equipment	0	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	-250	-28.7	-10.6
Net cash provided from investing activitiesbcontinuing operations	-250	-52.9	-10.6
Net cash provided from investing activitiesbdiscontinued operations	0	0	0
Net cash provided from investing activities	-250	-52.9	-10.6
Cash flows from financing activities:	'	'	'
Issuance of debt, net of debt issuance costs	'	-25	'
Repurchases of debt	'	49.2	0
Cash dividends paid to common shareholders	0	0	0
Cash dividends paid to parent	310	258.6	370.4
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	250	28.7	10.6
Payments on capital lease obligation	0	0	0
Proceeds on sale of OB Holdings Lux	'	0	'
Net cash used for financing activitiesbcontinuing operations	560	311.5	381
Net cash used for financing activitiesbdiscontinued operations	0	0	0
Net cash used for financing activities	560	311.5	381
Net increase in cash during period	0	0	0
Cash reclassified to assets held for sale	0	0	0
Cash transferred with sale of business	0	0	'
Net increase (decrease) after reclassification of cash to assets held for sale	0	0	0
Cash balance at beginning of period	0	0	0
Cash balance at end of period	$0	$0	$0

Subsequent_Event_Details

Subsequent Event (Details) (Subsequent Event [Member], USD $)	0 Months Ended
	Feb. 26, 2014
Subsequent Event [Member]	'
Subsequent Event [Line Items]	'
Dividends declared (in dollars per share)	$0.21

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data (Unaudited) (Details) (USD $)	3 Months Ended																12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]	'		'		'		'		'		'		'		'		'	'	'
Revenues	$315		$311.50		$267.30		$348.30		$295.50		$346.30		$282.60		$316.40		$1,242.10	$1,240.80	$1,081.80
Expenses	254.7		275.6		270.3		260.2		328.8		287.1		270.2		248.8		1,060.80	1,134.90	962.1
Pre-tax income from continuing operations	60.3		35.9		-3		88.1		-33.3		59.2		12.4		67.6		181.3	105.9	119.7
Tax (expense) benefit	-18.4		2.9		1		-19.8		19.7		-14.2		-0.4		-13.5		-34.3	-8.4	-14.8
Net income from continuing operations	41.9		38.8		-2		68.3		-13.6		45		12		54.1		147	97.5	104.9
Income (loss) from discontinued operations, net of tax	-46.9		0.3		-0.5		0.5		0.5		-15.8		0.5		-9.5		'	'	'
Gain (loss) from sale of discontinued operations, net of tax	46.6		0		0		0		0		-91		0		0		46.6	-91	-19.2
Net income (loss), including noncontrolling interests	41.6		39.1		-2.5		68.8		-13.1		-61.8		12.5		44.6		147	-17.8	56.1
Less: Net income attributable to noncontrolling interests	-0.2		0		-0.4		-0.4		-0.2		-0.4		-0.2		-0.6		-1	-1.4	-1
Net income (loss) attributable to OneBeacon's common shareholders	$41.40		$39.10		($2.90)		$68.40		($13.30)		($62.20)		$12.30		$44		$146	($19.20)	$55.10
Net income (loss) from continuing operations per share	$0.43	[1]	$0.41	[1]	($0.03)	[1]	$0.71	[1]	($0.15)	[1]	$0.47	[1]	$0.12	[1]	$0.56	[1]	$1.52	$1	$1.08
Loss from discontinued operations, net of tax, per share	($0.49)	[1]	$0	[1]	$0	[1]	$0	[1]	$0.01	[1]	($0.17)	[1]	$0	[1]	($0.10)	[1]	($0.49)	($0.25)	($0.30)
Gain (loss) from sale of discontinued operations, net of tax, per share	$0.49	[1]	$0	[1]	$0	[1]	$0	[1]	$0	[1]	($0.95)	[1]	$0	[1]	$0	[1]	'	'	'
Net income (loss) attributable to OneBeacon's common shareholders per share	$0.43	[1]	$0.41	[1]	($0.03)	[1]	$0.71	[1]	($0.14)	[1]	($0.65)	[1]	$0.12	[1]	$0.46	[1]	$1.52	($0.21)	$0.58
[1]	Due to the averaging of shares, quarterly earnings per share may not add to the total for the full year.

SCHEDULE_II_CONDENSED_FINANCIA1

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT [Schedule] SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT (CONDENSED BALANCE SHEETS) (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Condensed Balance Sheet Statements, Captions [Line Items]	'	'
Short-term investments, at amortized cost (which approximates fair value)	$157	$232.80
Investments in subsidiaries	0	0
Other assets	272.7	227.2
Total assets	5,211.60	5,401.50
Liabilities	4,104.20	4,384.20
Total OneBeacon's common shareholders' equity	1,104.30	1,014.50
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,211.60	5,401.50
One Beacon	'	'
Condensed Balance Sheet Statements, Captions [Line Items]	'	'
Short-term investments, at amortized cost (which approximates fair value)	13	2.7
Investments in subsidiaries	1,092	1,012.40
Other assets	-0.8	0.3
Total assets	1,104.20	1,015.40
Liabilities	-0.1	0.9
Total OneBeacon's common shareholders' equity	1,104.30	1,014.50
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$1,104.20	$1,015.40

SCHEDULE_I_SUMMARY_OF_INVESTME1

SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) (USD $)	Dec. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Total investment securities	$2,364.90		$2,291.50
Summary of Investments, Other than Investments in Related Parties, Cost	2,476.60		'
Summary of Investments, Other than Investments in Related Parties, Fair Value	2,601.20		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	2,601.20		'
U.S. Government and agency obligations	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	131		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	131.1		'
Debt securities issued by industrial corporations	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	1,693.20		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	1,704		'
Municipal obligations	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	16.9		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	16.5		'
Foreign government obligations	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	2		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	2.3		'
Preferred stocks	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	78.3		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	83.3		'
Fixed maturity investments	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	1,921.40	[1]	'
Summary of Investments, Other than Investments in Related Parties, Fair Value	1,937.20	[1]	'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	1,937.20	[1]	'
Short-term investments	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	157		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	157		'
Banks, Trust and Insurance, Equities [Member]	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	66.6		'
Summary of Investments, Other than Investments in Related Parties, Fair Value	83.3		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	83.3		'
Public Utility, Equities [Member]	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	8.3		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	8.3		'
Industrial, Miscellaneous, and All Others [Member]	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	192.3		'
Summary of Investments, Other than Investments in Related Parties, Fair Value	245.3		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	245.3		'
Common equity securities	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	267.2		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	336.9		'
Convertible fixed maturity investments	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	26.3		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	30.5		'
Other investments	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Summary of Investments, Other than Investments in Related Parties, Cost	104.7		'
Summary of Investments, Other than Investments in Related Parties, Carrying Amount	139.6		'
Segment, Discontinued Operations | Runoff	'		'
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]	'		'
Total investment securities	$236.30		$338.10
[1]	Includes $236.3 million of fixed maturity investments reclassified to assets held for sale in the DecemberB 31, 2013 consolidated balance sheet as part of the Runoff Transaction.

SCHEDULE_II_CONDENSED_FINANCIA2

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT [Schedule] SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT (CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Income Statements, Captions [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Revenues	$315	$311.50	$267.30	$348.30	$295.50	$346.30	$282.60	$316.40	$1,242.10	$1,240.80	$1,081.80
Expenses	254.7	275.6	270.3	260.2	328.8	287.1	270.2	248.8	1,060.80	1,134.90	962.1
Pre-tax income from continuing operations	60.3	35.9	-3	88.1	-33.3	59.2	12.4	67.6	181.3	105.9	119.7
Tax (expense) benefit	-18.4	2.9	1	-19.8	19.7	-14.2	-0.4	-13.5	-34.3	-8.4	-14.8
Net income from continuing operations	41.9	38.8	-2	68.3	-13.6	45	12	54.1	147	97.5	104.9
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	41.4	39.1	-2.9	68.4	-13.3	-62.2	12.3	44	146	-19.2	55.1
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	'	'	'	'	'	'	'	'	-20.6	2.9	11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	166.6	-22.1	43.9
One Beacon	'	'	'	'	'	'	'	'	'	'	'
Condensed Income Statements, Captions [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Revenues	'	'	'	'	'	'	'	'	0	0	0
Expenses	'	'	'	'	'	'	'	'	5	5.4	5.1
Pre-tax income from continuing operations	'	'	'	'	'	'	'	'	-5	-5.4	-5.1
Tax (expense) benefit	'	'	'	'	'	'	'	'	0.1	0.1	-0.2
Net income from continuing operations	'	'	'	'	'	'	'	'	-4.9	-5.3	-5.3
Equity in earnings of subsidiaries, net of tax	'	'	'	'	'	'	'	'	150.9	-13.9	60.4
Net income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	146	-19.2	55.1
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	'	'	'	'	'	'	'	'	-20.6	2.9	11.2
Other comprehensive income (loss) items, after tax	'	'	'	'	'	'	'	'	20.6	-2.9	-11.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	'	'	'	'	'	'	'	'	$166.60	($22.10)	$43.90

SCHEDULE_II_CONDENSED_FINANCIA3

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT [Schedule] SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT (CONDENSED STATEMENTS OF CASH FLOWS) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Cash Flow Statements, Captions [Line Items]	'	'	'
Net income (loss), including noncontrolling interests	$147	($17.80)	$56.10
Equity in earnings of subsidiaries, net of tax	0	0	0
Dividends received from subsidiaries	0	0	0
Net change in other assets and liabilities	74.5	-35.6	-79.2
Net cash provided from (used for) operations	128.7	-53.7	-118.5
Net maturities, purchases and sales of short-term investments	72.2	14.6	-19.9
Net cash provided from investing activities	81.4	115.7	458.6
Cash dividends paid to common shareholders	-80.2	-80.1	-174.8
Net cash used for financing activities	-85.9	-75	-313.3
Net increase in cash during period	124.2	-13	26.8
Cash	168.1	43.9	54.9
One Beacon	'	'	'
Condensed Cash Flow Statements, Captions [Line Items]	'	'	'
Net income (loss), including noncontrolling interests	146	-19.2	55.1
Equity in earnings of subsidiaries, net of tax	150.9	-13.9	60.4
Dividends received from subsidiaries	92	83	181
Net change in other assets and liabilities	3.4	2.3	0.1
Net cash provided from (used for) operations	90.5	80	175.8
Net maturities, purchases and sales of short-term investments	-10.3	0.1	-1
Net cash provided from investing activities	-10.3	0.1	-1
Cash dividends paid to common shareholders	-80.2	-80.1	-174.8
Net cash used for financing activities	-80.2	-80.1	-174.8
Net increase in cash during period	0	0	0
Cash	$0	$0	$0

SCHEDULE_II_CONDENSED_FINANCIA4

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT [Schedule] - Additional Information (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Cash Flow Statements, Captions [Line Items]	'	'	'
Dividends received from subsidiaries	$0	$0	$0
Cash dividends paid to parent	0	0	0
One Beacon	'	'	'
Condensed Cash Flow Statements, Captions [Line Items]	'	'	'
Dividends received from subsidiaries	92	83	181
Cash dividends paid to parent	$0	$0	$0

SCHEDULE_III_SUPPLEMENTARY_INS1

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Supplementary Insurance Information, by Segment [Line Items]	'		'		'
Earned Premiums	$566.90		'		'
Specialty Industries	'		'		'
Supplementary Insurance Information, by Segment [Line Items]	'		'		'
Deferred acquisition costs	50.1	[1]	48.8	[1]	48.7	[1]
Future policy benefits, losses, claims and loss expenses	446.5	[1]	438.1	[1]	363.6	[1]
Unearned premiums	269.5	[1]	255.1	[1]	234.5	[1]
Other policy claims and benefits payable	0	[1]	0	[1]	0	[1]
Earned Premiums	566.9	[1]	528	[1]	462.4	[1]
Net investment income(3)	0	[1],[2]	0	[1],[2]	0	[1],[2]
Benefits, claims, losses, and settlement expenses	-309.8	[1]	-304.4	[1]	-266.6	[1]
Amortization of policy acquisition expenses	-102.6	[1]	-99.1	[1]	-92.1	[1]
Other operating expenses	-107.4	[1]	-109	[1]	-85.2	[1]
Net written premiums	579	[1]	548.3	[1]	491.5	[1]
Specialty Products	'		'		'
Supplementary Insurance Information, by Segment [Line Items]	'		'		'
Deferred acquisition costs	53.6	[1]	75.1	[1]	74.6	[1]
Future policy benefits, losses, claims and loss expenses	607.8	[1]	561.9	[1]	505	[1]
Unearned premiums	275.4	[1]	318.7	[1]	291.1	[1]
Other policy claims and benefits payable	0	[1]	0	[1]	0	[1]
Earned Premiums	553.5	[1]	604	[1]	549.8	[1]
Net investment income(3)	0	[1],[2]	0	[1],[2]	0	[1],[2]
Benefits, claims, losses, and settlement expenses	-312.3	[1]	-345.6	[1]	-281.7	[1]
Amortization of policy acquisition expenses	-106.3	[1]	-150.3	[1]	-129.1	[1]
Other operating expenses	-97.4	[1]	-96.2	[1]	-77.1	[1]
Net written premiums	509.6	[1]	630.9	[1]	571.2	[1]
Investing, Financing and Corporate	'		'		'
Supplementary Insurance Information, by Segment [Line Items]	'		'		'
Deferred acquisition costs	0	[1],[2],[3]	0	[1],[2],[3]	0.2	[1],[2],[3]
Future policy benefits, losses, claims and loss expenses	0	[1],[3]	0	[1],[3]	2,490	[1],[3]
Unearned premiums	0	[1],[2],[3]	0	[1],[2],[3]	2.4	[1],[2],[3]
Other policy claims and benefits payable	0	[1],[3]	0	[1],[3]	0	[1],[3]
Earned Premiums	0	[1],[2],[3]	0	[1],[2],[3]	0	[1],[2],[3]
Net investment income(3)	41.1	[2],[3]	53.6	[1],[2],[3]	71.4	[1],[2],[3]
Benefits, claims, losses, and settlement expenses	0	[1],[3]	0	[1],[3]	0	[1],[3]
Amortization of policy acquisition expenses	0	[1],[2],[3]	0	[1],[2],[3]	0	[1],[2],[3]
Other operating expenses	0	[1],[3]	0	[1],[3]	0	[1],[3]
Net written premiums	$0	[1],[3]	$0	[1],[3]	$0	[1],[3]
[1]	Schedule excludes activity related to discontinued operations for all periods presented.
[2]	Invested assets are not allocated to Specialty Industries or Specialty Products since OneBeacon does not manage its assets by segment. Net investment income related to OneBeacon's Specialty Industries and Specialty Products segments are included in the Investing, Financing and Corporate segment since these assets are available for payment of losses and expenses for all segments.
[3]	Schedule excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013 and 2012 and to the AutoOne Transaction as of December 31, 2011.

SCHEDULE_IV_REINSURANCE_Detail

SCHEDULE IV REINSURANCE (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'		'	'
Direct	$1,043.30		$1,158.30	$1,035.90
Ceded	-71.4		-79.1	-66
Assumed	148.5	[1]	52.8	42.3
Net earned premiums	1,120.40		1,132	1,012.20
Continuing Operations [Member]	'		'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'		'	'
Direct	1,043.30		1,158.30	1,035.90
Ceded	-71.4		-79.1	-66
Assumed	148.5		52.8	42.3
Net earned premiums	1,120.40		1,132	1,012.20
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net (Deprecated 2013-01-31)	13.30%		4.70%	4.20%
Segment, Discontinued Operations	'		'	'
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]	'		'	'
Direct	0.4		12.1	169.8
Ceded	0		-2.4	-100.1
Assumed	0.4		0.9	0.8
Net earned premiums	$0.80		$10.60	$70.50
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net (Deprecated 2013-01-31)	50.00%		8.50%	1.10%
[1]	Includes assumed earned premiums and loss and LAE of $89.1 million and $41.9 million, respectively, related to the collector cars and boats business which although exited on January 1, 2013, was assumed on a runoff basis from Essentia.

SCHEDULE_V_VALUATION_AND_QUALI1

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Valuation and Qualifying Accounts Disclosure [Line Items]	'		'		'		'
Allowance for Doubtful Accounts Receivable	$0.10		'		'		'
Allowance for Reinsurance Recoverable [Member]	'		'		'		'
Valuation and Qualifying Accounts Disclosure [Line Items]	'		'		'		'
Valuation Allowances and Reserves, Balance	14.3	[1],[2]	25.6	[1],[2]	24.4	[1]	17.5	[1]
Valuation Allowances and Reserves, Charged to Cost and Expense	0	[1],[2]	0	[1]	10.7	[1]	'
Valuation Allowances and Reserves, Charged to Other Accounts	0	[1],[2],[3]	0	[1],[3]	0	[1],[3]	'
Valuation Allowances and Reserves, Deductions	-11.3	[1],[2],[4]	1.2	[1],[4]	-3.8	[1],[4]	'
Allowance for Uncollectible Premiums Receivable [Member]	'		'		'		'
Valuation and Qualifying Accounts Disclosure [Line Items]	'		'		'		'
Valuation Allowances and Reserves, Balance	3.1	[1],[2]	4.4	[1],[2]	3.4	[1]	3.9	[1]
Valuation Allowances and Reserves, Charged to Cost and Expense	0.3	[1],[2]	1.1	[1]	0	[1]	'
Valuation Allowances and Reserves, Charged to Other Accounts	0	[1],[2],[3]	0	[1],[3]	-0.5	[1],[3]	'
Valuation Allowances and Reserves, Deductions	-1.6	[1],[2],[4]	-0.1	[1],[4]	0	[1],[4]	'
Valuation Allowance for Reinsurance Balances	11.3		23.1		'		'
Runoff | Allowance for Uncollectible Premiums Receivable [Member]	'		'		'		'
Valuation and Qualifying Accounts Disclosure [Line Items]	'		'		'		'
Valuation Allowances and Reserves, Balance	$0.10		$0.90		$0.40		'
[1]	Schedule includes activity related to discontinued operations. Balances as of December 31, 2013 and 2012 include an Allowance for reinsurance balances of $11.3 million and $23.1 million, respectively, and an Allowance for uncollectible premiums receivable of $0.1 million and $0.9 million, respectively, related to the Runoff Transaction that were reclassified to held for sale in the consolidated balance sheet. Balance as of December 31, 2011 includes an Allowance for uncollectible premiums receivable of $0.4 million related to the AutoOne Transaction that was reclassified to held for sale in the consolidated balance sheet.
[2]	Excludes $0.1 million of allowance for uncollectible deductibles as of December 31, 2013, with no allowance for uncollectible deductibles as of December 31, 2012 and 2011.
[3]	Amount credited to other accounts represents a reduction in the Allowance for uncollectible premiums receivable which was offset by a corresponding reduction in Gross premiums receivable. There was no impact to Premiums receivable as presented in the consolidated balance sheet.
[4]	Represents net collections (charge offs) of balances receivable.

SCHEDULE_VI_SUPPLEMENTAL_INFOR1

SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]	'		'		'
Earned Premiums	$566.90		'		'
Net investment income	41.1		53.6		71.4
Interest rate at which discount computed	3.50%		3.50%		4.50%
Specialty Industries	'		'		'
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]	'		'		'
Deferred acquisition costs	50.1	[1]	48.8	[1]	48.7	[1]
Reserves for Unpaid Claims and Claims Adjustment Expenses	446.5		438.1		363.6
Discount, if any, deducted in Column C	2.8	[2]	4.1	[2]	4.2	[2]
Unearned premiums	269.5	[1]	255.1	[1]	234.5	[1]
Earned Premiums	566.9	[1]	528	[1]	462.4	[1]
Net investment income	0	[3]	0	[3]	0	[3]
Claims and Claims Adjustment Expenses Incurred Related to	315.7	[1]	309.9	[1]	279.8	[1]
Claims and Claims Adjustment Expenses Incurred Related to Prior Year	-5.9		-5.5		-13.2
Amortization of policy acquisition expenses	-102.6	[1]	-99.1	[1]	-92.1	[1]
Paid Claims and Claims Adjustment Expenses	268.5		253		241.6
Premiums written	579		548.3		491.5
Other Insurance Operations [Member]	'		'		'
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]	'		'		'
Deferred acquisition costs	53.6		75.1		74.6
Reserves for Unpaid Claims and Claims Adjustment Expenses	607.8		561.9		505
Discount, if any, deducted in Column C	0.2	[2]	0.5	[2]	0.8	[2]
Unearned premiums	275.4		318.7		291.1
Earned Premiums	553.5		604		549.8
Net investment income	0	[3]	0	[3]	0	[3]
Claims and Claims Adjustment Expenses Incurred Related to	306.4	[1]	347.5	[1]	298.3	[1]
Claims and Claims Adjustment Expenses Incurred Related to Prior Year	5.9		-1.9		-16.6
Amortization of policy acquisition expenses	-106.3		-150.3		-129.1
Paid Claims and Claims Adjustment Expenses	272.2		312.1		281.6
Premiums written	509.6		630.9		571.2
Investing, Financing and Corporate	'		'		'
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]	'		'		'
Deferred acquisition costs	0	[1],[3],[4]	0	[1],[3],[4]	0.2	[1],[3],[4]
Reserves for Unpaid Claims and Claims Adjustment Expenses	0	[3]	0	[3]	2,490	[3]
Discount, if any, deducted in Column C	0	[2],[3],[5]	0	[2],[3],[5]	266.6	[2],[3],[5]
Unearned premiums	0	[1],[3],[4]	0	[1],[3],[4]	2.4	[1],[3],[4]
Earned Premiums	0	[1],[3],[4]	0	[1],[3],[4]	0	[1],[3],[4]
Net investment income	41.1	[3]	53.6	[3]	71.4	[3]
Claims and Claims Adjustment Expenses Incurred Related to	0	[1],[3]	0	[1],[3]	0	[1],[3]
Claims and Claims Adjustment Expenses Incurred Related to Prior Year	0	[3]	0	[3]	0	[3]
Amortization of policy acquisition expenses	0	[1],[3],[4]	0	[1],[3],[4]	0	[1],[3],[4]
Paid Claims and Claims Adjustment Expenses	0	[3]	0	[3]	0	[3]
Premiums written	0	[3]	0	[3]	0	[3]
Segment, Discontinued Operations	'		'		'
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]	'		'		'
Unamortized amount	$136.90		$150.10		$163.30
[1]	Schedule excludes activity related to discontinued operations for all periods presented.
[2]	The amounts shown represent and/or include OneBeacon's discount on its long-term workers compensation loss and LAE reserves, as such liabilities constitute unpaid but settled claims under which the payment pattern and ultimate costs are fixed and determinable on an individual basis. OneBeacon discounts these reserves using an average discount rate which is determined based on various assumptions including consideration of when the claims will be settled (3.5%, 3.5% and 4.5%, respectively, at DecemberB 31, 2013, 2012 and 2011).
[3]	Invested assets are not allocated to Specialty Industries or Specialty Products since OneBeacon does not manage its assets by segment. Net investment income related to OneBeacon's Specialty Industries and Specialty Products segments are included in the Investing, Financing and Corporate segment since these assets are available for payment of losses and expenses for all segments.
[4]	Schedule excludes balances reclassified to held for sale in the consolidated balance sheets related to the Runoff Transaction as of December 31, 2013 and 2012 and to the AutoOne Transaction as of December 31, 2011.
[5]	The years ended December 31, 2013, 2012 and 2011 also include unamortized fair value adjustments of $136.9 million, $150.1 million, and $163.3 million, respectively, to reserves for unpaid claims and claims adjustment expenses made in purchase accounting as a result of the OneBeacon Acquisition.