OneBeacon Insurance 10-Q 28 Oct 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 24, 2013	Oct. 24, 2013
		Class A common	Class B common
Entity Registrant Name	OneBeacon Insurance Group, Ltd.
Entity Central Index Key	1369817
Document Type	10-Q
Document Period End Date	30-Sep-13
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		23,649,400	71,754,738
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q3

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investment Securities:
Fixed maturity investments, at fair value	$1,626.10	$1,593.30
Short-term investments, at amortized cost (which approximates fair value)	162	232.8
Common equity securities, at fair value	327.5	259
Convertible fixed maturity investments, at fair value	31.9	62.6
Other investments	146.2	143.8
Total investments	2,293.70	2,291.50
Cash	173.8	43.9
Reinsurance recoverables	86.1	110.6
Premiums receivable	255.9	225.6
Deferred acquisition costs	107.7	123.9
Net deferred tax asset	127.2	137.8
Investment income accrued	10.6	12.1
Accounts receivable on unsettled investment sales	9.4	2.1
Other assets	203.5	227.2
Assets held for sale	1,957.20	2,226.80
Total assets	5,225.10	5,401.50
Liabilities
Unpaid loss and loss adjustment expense reserves	1,052.90	1,000
Unearned premiums	552.7	573.8
Debt	274.7	274.7
Accounts payable on unsettled investment purchases	5.9	6.2
Other liabilities	316.7	302.7
Liabilities held for sale	1,957.20	2,226.80
Total liabilities	4,160.10	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)	1,021.70	1,019.10
Retained earnings	53.7	9.2
Accumulated other comprehensive loss, after tax	-13.3	-13.8
Total OneBeacon's common shareholders' equity	1,062.10	1,014.50
Total noncontrolling interests, after tax	2.9	2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,065	1,017.30
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$5,225.10	$5,401.50

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 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 $)	3 Months Ended		9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues
Earned premiums	$278.90	$293.90	$846.20	$846
Net investment income	10.1	12.8	30.9	41.5
Net realized and change in unrealized investment gains	17	40	19.9	57.9
Net other revenues (expenses)	5.5	-0.4	30.1	-0.1
Total revenues	311.5	346.3	927.1	945.3
Expenses
Loss and loss adjustment expenses	167.8	164.7	473.7	452.5
Policy acquisition expenses	53.6	66.6	160.9	185.6
Other underwriting expenses	46.8	47.4	150.7	146.2
General and administrative expenses	4.1	4.4	11	9.7
Interest expense	3.3	4	9.8	12.1
Total expenses	275.6	287.1	806.1	806.1
Pre-tax income from continuing operations	35.9	59.2	121	139.2
Income tax benefit (expense)	2.9	-14.2	-15.9	-28.1
Net income from continuing operations	38.8	45	105.1	111.1
Income (loss) from discontinued operations, net of tax	0.3	-15.8	0.3	-24.8
Loss from sale of discontinued operations, net of tax	0	-91	0	-91
Net income (loss), including noncontrolling interests	39.1	-61.8	105.4	-4.7
Less: Net income attributable to noncontrolling interests	0	-0.4	-0.8	-1.2
Net income (loss) attributable to OneBeacon's common shareholders	39.1	-62.2	104.6	-5.9
Net change in benefit plan assets and obligations, net of tax	0.2	-2	0.5	-1.7
Comprehensive income (loss) attributable to OneBeacon's common shareholders	$39.30	($64.20)	$105.10	($7.60)
Earnings (loss) per share attributable to OneBeacon's common shareholdersbbasic and diluted
Net income from continuing operations, per share	$0.41	$0.47	$1.09	$1.15
Loss from discontinued operations, net of tax, per share	$0	($0.17)	$0	($0.27)
Loss from sale of discontinued operations, net of tax, per share	$0	($0.95)	$0	($0.95)
Net income (loss) attributable to OneBeacon's common shareholders per share	$0.41	($0.65)	$1.09	($0.07)
Dividends declared and paid per OneBeacon's common share	$0.21	$0.21	$0.63	$0.63

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
In Millions, unless otherwise specified
Balances at Dec. 31, 2011	$1,113.90	$1,002.20	$108.50	($10.90)	$1,099.80	$14.10
Increase (Decrease) in Stockholders' Equity
Net income	-4.7	0	-5.9	0	-5.9	1.2
Other comprehensive income item, after tax:
Net change in benefit plan assets and obligations, net of tax	-1.7	0	0	-1.7	-1.7	0
Total comprehensive income	-6.4	0	-5.9	-1.7	-7.6	1.2
Amortization of restricted share and option awards	1.9	1.9	0	0	1.9	0
Issuance of common shares	0.5	0.3	0	0	0.3	0.2
Dividends	-60.6	0	-60	0	-60	-0.6
Contributions	0.2	0	0	0	0	0.2
Gain on sale of OB Holdings Lux	14	14	0	0	14	0
Balances at Sep. 30, 2012
Balances at Dec. 31, 2012	1,017.30	1,019.10	9.2	-13.8	1,014.50	2.8
Increase (Decrease) in Stockholders' Equity
Net income	105.4	0	104.6	0	104.6	0.8
Other comprehensive income item, after tax:
Net change in benefit plan assets and obligations, net of tax	0.5	0	0	0.5	0.5	0
Total comprehensive income	105.9	0	104.6	0.5	105.1	0.8
Amortization of restricted share and option awards	2.3	2.3	0	0	2.3	0
Issuance of common shares	0.4	0.3	0	0	0.3	0.1
Dividends	-60.9	0	-60.1	0	-60.1	-0.8
Balances at Sep. 30, 2013	$1,065	$1,021.70	$53.70	($13.30)	$1,062.10	$2.90

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operations:
Net income (loss) including noncontrolling interests	$105.40	($4.70)
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Net (income) loss from discontinued operations	-0.3	24.8
Loss from sale of discontinued operations, net of tax	0	91
Net realized and change in unrealized investment gains	-19.9	-57.9
Net realized gain on sale of business	-23	0
Deferred income tax expense	12.3	12.3
Other operating items:
Net change in loss and LAE reserves	52.9	-0.9
Net change in unearned premiums	-21.1	85.2
Net change in premiums receivable	-30.3	-45.7
Net change in reinsurance recoverable on paid and unpaid losses	24.5	64.4
Net change in other assets and liabilities	77.8	-32.3
Net cash provided from operationsbcontinuing operations	178.3	136.2
Net cash used for operationsbdiscontinued operations	-98	-155.6
Net cash provided from (used for) operations	80.3	-19.4
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	67.3	-22.7
Maturities of fixed maturity investments	196.6	147.5
Sales of fixed maturity investments	1,208.60	1,305.90
Sales of common equity securities	142	37.2
Sales of convertible fixed maturity investments	39.5	16
Distributions and redemptions of other investments	13.1	6.9
Purchases of fixed maturity investments	-1,381.80	-1,317.20
Purchases of common equity securities	-168.5	-49.3
Purchases of convertible fixed maturity investments	-12	-6.8
Contributions for other investments	-7.1	-3.1
Proceeds from sale of business	31.3	0
Net change in unsettled investment purchases and sales	-7.7	-69.1
Net acquisitions of property and equipment	-7.6	-1.2
Net cash provided from investing activitiesbcontinuing operations	113.7	44.1
Net cash provided from investing activitiesbdiscontinued operations	0	0
Net cash provided from investing activities	113.7	44.1
Cash flows from financing activities:
Cash dividends paid to common shareholders	-60.1	-60
Proceeds on sale of OB Holdings Lux	0	14
Payments on capital lease obligation	-4	-3.6
Net cash used for financing activitiesbcontinuing operations	-64.1	-49.6
Net cash used for financing activitiesbdiscontinued operations	0	0
Net cash used for financing activities	-64.1	-49.6
Net increase (decrease) in cash during period	129.9	-24.9
Cash reclassified to assets held for sale	0	5.5
Cash transferred with sale of business	0	-3.5
Net increase (decrease) after reclassification of cash to assets held for sale	129.9	-22.9
Cash balance at beginning of period	43.9	54.9
Cash balance at end of period	$173.80

Nature_of_Operations_and_Summa

Nature of Operations and Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 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
	These interim 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., a Bermuda-based reinsurance company, which entered into an agreement during the three months ended September 30, 2013 to reinsure certain risks of an affiliated entity. OneBeacon offers a wide range of specialty insurance products and services 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 September 30, 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.
	On October 17, 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 15—"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 September 30, 2013 and December 31, 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.
	The Company revised its reporting segment structure in the fourth quarter of 2012 into Specialty Products, Specialty Industries and Investing, Financing and Corporate. Certain amounts in the prior period financial statements have been reclassified to conform to the current presentation. The Specialty Products segment is comprised of eight underwriting operating segments, including the Collector Cars and Boats underwriting operating segment that was exited in the first quarter of 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 the second quarter of 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, a firm that specializes in the crop business, to provide coverages through the federal program and other supplemental coverages, including crop-hail. The operations associated with this new underwriting operating segment have been included in the Specialty Products reportable segment.
	The Specialty Industries segment is comprised of six underwriting operating segments, including one 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 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. These interim financial statements include all adjustments, consisting of a normal recurring nature, considered necessary by management to fairly state the financial position, results of operations and cash flows of OneBeacon. These interim financial statements may not be indicative of financial results for the full year and should be read in conjunction with the Company’s 2012 Annual Report on Form 10-K. 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. Refer to the Company’s 2012 Annual Report on Form 10-K for a complete discussion regarding OneBeacon’s significant accounting policies. As described above, certain amounts in the prior period financial statements have been reclassified to conform to the current presentation.
	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 three and nine months ended September 30, 2012, $0.9 million and $5.3 million, respectively, of the $5.6 million of unamortized acquisitions costs as of January 1, 2012 were recognized in expense, with the remainder of the $5.6 million balance expensed during the year ended December 31, 2012 such that no incremental impact was recognized in the three and nine months ended September 30, 2013.
	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
	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 balance sheet.

Acqusitions_and_Dispositions

Acqusitions and Dispositions	9 Months Ended
	Sep. 30, 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 car and boat business, to Markel Corporation. Concurrently therewith, OneBeacon and Hagerty Insurance Agency ("Hagerty") terminated their underwriting arrangement with respect to the collector car and boat 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 three and nine months ended September 30, 2012, the business associated with this agreement generated net written premiums of $52.5 million and $145.8 million, respectively, or 15.7% for both periods, of consolidated written premiums.
	Runoff Business
	As described in Note 1, on October 17, 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 the second quarter of 2014.
	The Runoff Transaction is subject to various closing conditions, primarily the receipt of regulatory approvals, which includes a third party actuarial review which has now been completed. 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 September 30, 2013 and December 31, 2012 and as discontinued operations for the three and nine months ended September 30, 2013 and 2012. See Note 15—"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.

Reserves_for_Unpaid_Losses_and

Reserves for Unpaid Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2013
Insurance Loss Reserves [Abstract]
Loss and LAE reserve summary - Ongoing Business	Unpaid Loss and Loss Adjustment Expense (LAE) Reserves
	Loss and LAE reserve summary - Ongoing Business
	The following table summarizes the loss and LAE reserve activities of OneBeacon's insurance subsidiaries for the three and nine months ended September 30, 2013 and 2012:
			Three months ended								Nine months ended
			September 30,								September 30,
			2013				2012				2013				2012
			($ in millions)								($ in millions)
	Gross beginning balance		$	1,027.80			$	3,175.30			$	1,000.00			$	3,358.60
	Less beginning reinsurance recoverable on unpaid losses		(81.8		)		(2,040.7		)		(107.3		)		(2,167.5		)
	Net beginning loss and LAE reserves		946				1,134.60				892.7				1,191.10
	Loss and LAE incurred relating to:
	Current year losses		164				167				473.8				460.1
	Prior year losses		3.8				(2.3		)		(0.1		)		(7.6		)
	Total incurred loss and LAE from continuing operations		167.8				164.7				473.7				452.5
	Loss and LAE paid relating to:
	Current year losses		(54.1		)		(59.6		)		(115.1		)		(123.0		)
	Prior year losses		(87.1		)		(73.4		)		(278.7		)		(266.2		)
	Total loss and LAE payments from continuing operations		(141.2		)		(133.0		)		(393.8		)		(389.2		)
	Net loss and LAE reserves		972.6				1,166.30				972.6				1,254.40
	Total incurred loss and LAE from discontinued operations		—				27.7				7.5				48.4
	Total loss and LAE payments from discontinued operations		(17.2		)		(67.4		)		(88.6		)		(177.1		)
	Net loss and LAE reserves		955.4				1,126.60				891.5				1,125.70
	Net change in loss and LAE reserves reported in liabilities held for sale		17.3			(1)	(256.3		)	(2)	81.2			(3)	(191.6		)	(4)
	Net loss and LAE reserves sold		—				—				—				(63.8		)	(5)
	Net ending loss and LAE reserves		972.7				870.3				972.7				870.3
	Plus ending reinsurance recoverable on unpaid losses		80.2				41.3				80.2				41.3
	Gross ending loss and LAE reserves		$	1,052.90			$	911.6			$	1,052.90			$	911.6
	_______________________________________________________________________________
	(1)	Consists of the change in net loss and LAE reserves from $147.9 million to $130.6 million, which amounts were classified as held for sale as of June 30, 2013 and September 30, 2013, respectively, in connection with the Runoff Transaction.
	(2)	Consists of the net loss and LAE reserves recorded in the three months ended September 30, 2012 in connection with the Runoff Transaction.
	(3)	Consists of the change in net loss and LAE reserves from $211.8 million to $130.6 million, which amounts were classified as held for sale as of December 31, 2012 and September 30, 2013, respectively, in connection with the Runoff Transaction.
	(4)	Consists of the net loss and LAE reserves of $256.3 million recorded in the nine months ended September 30, 2012 in connection with the Runoff Transaction, offset by net loss and LAE reserves of $64.7 million reclassified from held for sale upon the closing of the AutoOne Transaction in February 2012.
	(5)	Relates to the AutoOne Transaction, which closed in February 2012.
	During the three and nine months ended September 30, 2013, OneBeacon recorded $3.8 million and $(0.1) million, respectively, of unfavorable (favorable) loss and LAE reserve development on prior accident year reserves. Unfavorable development during the three months ended September 30, 2013 was primarily related to the Specialty Property and Entertainment underwriting operating segments, which was partially offset by favorable development in its ocean marine business, included in the International Marine Underwriters ("IMU") underwriting operating segment, and in the healthcare business included in the Professional Insurance underwriting operating segment. Unfavorable development for the nine months ended September 30, 2013, primarily related to the Specialty Property underwriting operating segment, was more than offset by favorable development in other underwriting operating segments, primarily IMU and Professional Insurance. During the three and nine months ended September 30, 2012, OneBeacon recorded $2.3 million and $7.6 million, respectively, of favorable loss and LAE reserve development on prior accident year loss reserves, primarily resulting from lower than expected severity on non-catastrophe losses related to professional liability lines, multiple peril liability lines and other general liability lines.
	See Note 15—"Discontinued Operations" for a discussion of impacts to reserves for unpaid losses and LAE related to discontinued operations.

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 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 15—"Discontinued Operations" for amounts related to the Runoff Business.
	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 the next $130.0 million of losses resulting from the catastrophe are reinsured in three layers, although OneBeacon retains a co-participation of 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. Any loss above $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.
	In addition to the corporate catastrophe reinsurance protection, OneBeacon may also purchase dedicated reinsurance protection for specific businesses. OneBeacon purchased a per occurrence treaty for its IMU underwriting operating segment that protects against large occurrences, whether a single large claim or a catastrophe. The IMU treaty attaches at $2.0 million per occurrence. Coverage is provided up to $60.0 million. The first layer of the marine treaty is $5.0 million in excess of $2.0 million, with an annual aggregate deductible of $1.5 million for large losses and $5.0 million for catastrophes losses. Losses not covered by the marine treaty are subject to the corporate property and per risk treaties after the Company's retention. Reinstatement premiums are paid in full or in part depending on the layer and the occurrence if the coverage is attached. Losses retained under marine reinsurance treaties are subject to the corporate catastrophe treaty. Additionally, in 2013 OneBeacon purchased reinsurance for its Surety underwriting operating segment. This treaty covers losses in excess of $5.0 million up to $30.0 million and had co-participation of 17% through September 30, 2013, which was revised through a new treaty effective October 1, 2013 to have no co-participation.
	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 may be purchased above $100.0 million where OneBeacon deems it appropriate. OneBeacon retains a co-participation of 5% for losses in excess of $20.0 million up to $40.0 million and a co-participation of 10% for losses in excess of $40.0 million. The property-per-risk treaty also 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 casualty treaty has a 10% co-participation in the first layer ($6.0 million in excess of $5.0 million) and a 5% co-participation in the second layer of $10.0 million in excess of $11.0 million. OneBeacon purchases a treaty to protect against large workers compensation losses that covers 100% of the loss in excess of $1.0 million up to $10.0 million per person. In addition, for casualty losses involving more than one insured, OneBeacon maintains a dedicated treaty that covers up to $60.0 million in excess of a $10.0 million retention.
	At September 30, 2013, OneBeacon had reinsurance recoverable on paid losses of $5.9 million and reinsurance recoverables on unpaid losses of $80.2 million. Reinsurance contracts do not relieve OneBeacon of its obligations. Therefore, collectibility of balances due from reinsurers is critical to OneBeacon's financial strength. The following table provides a listing of OneBeacon's top reinsurers for its continuing insurance operations, excluding industry pools and associations, based upon reinsurance recoverable amounts on paid and unpaid losses, the percentage of the total balance, and the reinsurer's A.M. Best Company, Inc. ("A.M. Best") ratings:
	($ in millions)		Balance at		% of total		A.M. Best
			30-Sep-13				Rating(1)
	Hannover Ruckversich		$	10.3	12	%	A+
	Munich Reinsurance America		6.6		8	%	A+
	Platinum Underwriters		5.6		7	%	A
	Hartford Steam Boiler		4.8		6	%	A++
	Swiss Reinsurance America Corp		4.6		5	%	A+
	_______________________________________________________________________________
	(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) and "A" (Excellent, which is the third highest of sixteen financial strength ratings).
	Greater than 90% of reinsurance recoverables on paid and unpaid losses are from reinsurers rated A or better.

Investment_Securities

Investment Securities	9 Months Ended
	Sep. 30, 2013
Schedule of Investments [Abstract]
Investment Securities	Investment Securities
	OneBeacon's invested assets are comprised of securities and other investments held for general investment purposes. Refer to the Company's 2012 Annual Report on Form 10-K for a complete discussion.
	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 change in unrealized investment gains and losses on trading securities are reported in total revenues as net realized and unrealized investment gains (losses) in revenues on a pre-tax basis.
	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 approximates fair value.
	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 unrealized investment gains (losses). Other investments also include 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.
	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 three and nine months ended September 30, 2013, and 2012 consisted of the following:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Fixed maturity investments		$	10.4		$	12		$	30.7		$	38.6
	Short-term investments		—			—			—			0.1
	Common equity securities		1.5			1.5			4.4			4.8
	Convertible fixed maturity investments		0.2			1.1			0.9			3.4
	Other investments		(0.4		)	(0.2		)	—			(0.4		)
	Gross investment income		11.7			14.4			36			46.5
	Less investment expenses		(1.6		)	(1.6		)	(5.1		)	(5.0		)
	Net investment income		$	10.1		$	12.8		$	30.9		$	41.5
	The composition of net realized investment gains (losses) consisted of the following:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Fixed maturity investments		$	(5.0	)	$	10.6		$	2.8		$	26.7
	Short-term investments		—			—			0.1			—
	Common equity securities		2.7			2.9			20.6			(0.2		)
	Convertible fixed maturity investments		—			0.6			(0.4		)	2
	Other investments		(1.9		)	(0.4		)	2.8			1.6
	Net realized investment gains (losses), pre-tax		$	(4.2	)	$	13.7		$	25.9		$	30.1
	As of September 30, 2013 and December 31, 2012, OneBeacon reported $5.9 million and $6.2 million, respectively, in accounts payable on unsettled investment purchases and $9.4 million and $2.1 million, respectively, in accounts receivable on unsettled investment sales.
	The net changes in fair value for the three and nine months ended September 30, 2013, and 2012 are as follows:
			Three months ended September 30, 2013									Nine months ended September 30, 2013
			Changes in net			Changes in net			Total net changes			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected			unrealized investment			foreign currency			in fair value reflected
			gains (losses)(1)			translation			in revenues(1)			gains (losses)(1)			translation			in revenues(1)
						gains (losses)(1)									gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	0.5		$	0.1		$	0.6		$	(33.7	)	$	—		$	(33.7	)
	Short-term investments		—			—			—			—			—			—
	Common equity securities		15.8			0.1			15.9			21.3			0.1			21.4
	Convertible fixed maturity investments		1			—			1			(0.6		)	—			(0.6		)
	Other investments		3.7			—			3.7			6.9			—			6.9
	Net change, pre-tax		$	21		$	0.2		$	21.2		$	(6.1	)	$	0.1		$	(6.0	)
			Three months ended September 30, 2012									Nine months ended September 30, 2012
			Changes in net			Changes in net			Total net changes			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected			unrealized investment			foreign currency			in fair value reflected
			gains (losses)(2)			translation			in revenues(2)			gains (losses)(2)			translation			in revenues(2)
						gains (losses)(2)									gains (losses)(2)
			($ in millions)
	Fixed maturity investments		$	5.9		$	0.2		$	6.1		$	8.6		$	0.2		$	8.8
	Short-term investments		—			—			—			—			—			—
	Common equity securities		18.7			—			18.7			17			—			17
	Convertible fixed maturity investments		(0.6		)	—			(0.6		)	(2.0		)	—			(2.0		)
	Other investments		2.1			—			2.1			4			—			4
	Net change, pre-tax		$	26.1		$	0.2		$	26.3		$	27.6		$	0.2		$	27.8
	_______________________________________________________________________________
	(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 $(0.7) million and $(0.8) million for the three and nine months ended September 30, 2013.
	(2)	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.2) million for the nine months ended September 30, 2012. There were no such changes in net deferred gains and losses for the three months ended September 30, 2012.
	The components of OneBeacon's ending net unrealized investment gains and losses, excluding the impact of net unrealized foreign currency translation gains and losses, on its trading investment portfolio as of September 30, 2013 and December 31, 2012 were as follows:
			September 30,			December 31,
			2013			2012
			($ in millions)
	Investment securities:
	Gross unrealized investment gains		$	124.6		$	127.4
Gross unrealized investment losses		(11.4		)	(8.7		)
Total net unrealized investment gains, pre-tax		113.2			118.7
Income taxes		(36.7		)	(39.4		)
Total net unrealized investment gains, after tax		$	76.5		$	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 September 30, 2013 and December 31, 2012 were as follows:
		September 30, 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		$	137.4		$	0.2		$	—		$	—		$	137.6
Debt securities issued by industrial corporations		717.3			16.3			(1.4		)	—			732.2
Municipal obligations		3.3			—			(0.3		)	—			3
Asset-backed securities		911.1			1.3			(4.5		)	—			907.9
Foreign government obligations		2			0.4			—			—			2.4
Preferred stocks		78.3			4.4			—			—			82.7
Total fixed maturity investments		$	1,849.40		$	22.6		$	(6.2	)	$	—		$	1,865.80
		December 31, 2012(1)
		Cost or			Gross			Gross			Net foreign			Carrying
		amortized			unrealized			unrealized			currency			value
		cost			gains			losses			gains
		($ 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 $239.7 million and $338.1 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheets as part of the Runoff Transaction as of September 30, 2013 and December 31, 2012, respectively.
The cost or amortized cost, gross unrealized pre-tax investment gains and losses, net foreign currency gains and carrying values of OneBeacon's common equity securities, convertible fixed maturity investments and other investments as of September 30, 2013 and December 31, 2012 were as follows:
		30-Sep-13
		Cost or			Gross			Gross			Net foreign			Carrying
		amortized			unrealized			unrealized			currency			value
		cost			gains			losses			gains
		($ in millions)
Common equity securities		$	268.6		$	59.3		$	(0.5	)	$	0.1		$	327.5
Convertible fixed maturity investments		29.4			2.6			(0.1		)	—			31.9
Other investments		110.7			40.1			(4.6		)	—			146.2
Total common equity securities, convertible fixed maturity investments and other investments		$	408.7		$	102		$	(5.2	)	$	0.1		$	505.6
		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
As of September 30, 2013 and December 31, 2012, approximately 90% and 92% of the investment portfolio recorded at fair value was priced based upon observable inputs, respectively.
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 September 30, 2013 and December 31, 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 September 30, 2013 and December 31, 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 September 30, 2013 and December 31, 2012 were comprised of $48.2 million and $47.3 million, respectively, in hedge funds, $63.7 million and $61.3 million, respectively, in private equity funds and $14.1 million for both periods in an investment in a community reinvestment vehicle. As of September 30, 2013 and December 31, 2012, OneBeacon held investments in 8 hedge funds for both periods and 19 and 17 private equity funds, respectively. The largest investment in a single fund was $14.3 million and $12.9 million at September 30, 2013 and December 31, 2012, respectively. As of September 30, 2013 and December 31, 2012, other investments also included $20.2 million and $21.1 million, respectively, of an investment in a tax advantaged federal affordable housing development fund which is accounted for using the equity method.
The following tables summarize the Company's fair value measurements for investments at September 30, 2013 and December 31, 2012 by level:
		Fair value at			Level 1			Level 2			Level 3
		September 30, 2013(2)
		($ in millions)
Fixed maturity investments:
U.S. Government and agency obligations		$	137.6		$	137.6		$	—		$	—
Debt securities issued by corporations:
Consumer		214.5			—			214.5			—
Industrial		95.6			—			95.6			—
Financial		138.6			—			138.6			—
Communications		92.6			—			92.6			—
Energy		40.9			—			40.9			—
Basic materials		80			—			80			—
Utilities		41.1			—			41.1			—
Technology		28.9			—			28.9			—
Debt securities issued by corporations		732.2			—			732.2			—
Municipal obligations		3			—			3			—
Asset-backed securities		907.9			—			843.3			64.6
Foreign government obligations		2.4			1.7			0.7			—
Preferred stocks		82.7			—			12.1			70.6
Fixed maturity investments		1,865.80			139.3			1,591.30			135.2
Short-term investments		162			160.9			1.1			—
Common equity securities:
Financials		77			76.9			—			0.1
Basic Materials		24.5			24.5			—			—
Consumer		114.3			114.3			—			—
Energy		37.3			37.3			—			—
Utilities		8.4			8.4			—			—
Other		66			66			—			—
Common equity securities		327.5			327.4			—			0.1
Convertible fixed maturity investments		31.9			—			31.9			—
Other investments(1)		126			—			—			126
Total(1)		$	2,513.20		$	627.6		$	1,624.30		$	261.3
_______________________________________________________________________________
-1	Excludes the carrying value of $20.2 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of September 30, 2013.
-2	Fair value includes $239.7 million of fixed maturity investments reclassified to assets held for sale in the September 30, 2013 consolidated balance sheets as part of the Runoff Transaction.
		Fair value at			Level 1			Level 2			Level 3
		December 31, 2012(2)
		($ 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 $338.1 million of fixed maturity investments reclassified to assets held for sale in the December 31, 2012 consolidated balance sheets as part of the Runoff Transaction.
At September 30, 2013 and December 31, 2012, OneBeacon held one private preferred stock that represented approximately 85% and 84% , respectively, of its preferred stock portfolio. OneBeacon used quoted market prices for similar securities that were adjusted to reflect management's best estimate of fair value; this security is classified as a Level 3 measurement.
The following table summarizes the ratings of OneBeacon's corporate debt securities as of September 30, 2013 and 2012:
		September 30,			December 31,
		2013			2012
		($ in millions)
AA		$	46.5		$	37
A		353.4			301.6
BBB		324.2			362.5
BB		6.9			7
Other		1.2			3.4
Debt securities issued by corporations		$	732.2		$	711.5
Rollforwards of Fair Value Measurements by Level
The changes in Level 1 fair value measurements for the three and nine months ended September 30, 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		(0.2		)	—			—			—			(0.2		)
Net realized and unrealized gains (losses)		(0.1		)	24.6			—			—			24.5
Purchases		59.3			102.4			—			—			161.7
Sales		(98.8		)	(89.7		)	—			—			(188.5		)
Transfers in		—			—			—			—			—
Transfers out		—			—			—			—			—
Balance at March 31, 2013		$	163.6		$	296.2		$	—		$	—		$	459.8
Amortization/accretion		—			—			—			—			—
Net realized and unrealized losses		(2.2		)	(1.2		)	—			—			(3.4		)
Purchases		71.6			23.4			—			—			95
Sales		(11.1		)	(19.9		)	—			—			(31.0		)
Transfers in		—			—			—			—			—
Transfers out		—			—			—			—			—
Balance at June 30, 2013		$	221.9		$	298.5		$	—		$	—		$	520.4
Amortization/accretion		—			—			—			—			—
Net realized and unrealized gains (losses)		(0.1		)	18.6			—			—			18.5
Purchases		30.1			42.7			—			—			72.8
Sales		(112.6		)	(32.4		)	—			—			(145.0		)
Transfers in		—			—			—			—			—
Transfers out		—			—			—			—			—
Balance at September 30, 2013		$	139.3		$	327.4		$	—		$	—		$	466.7
_______________________________________________________________________________
(1)	Excludes short-term investments which are deemed to have a Level 1 designation.
The changes in Level 2 fair value measurements for the three and nine months ended September 30, 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		(3.1		)	—			—			—			(3.1		)
Net realized and unrealized gains (losses)		(0.5		)	—			0.5			—			—
Purchases		531.4			—			5			—			536.4
Sales		(560.3		)	—			(29.6		)	—			(589.9		)
Transfers in		—			—			—			—			—
Transfer out		—			—			—			—			—
Balance at March 31, 2013		$	1,619.40		$	—		$	38.5		$	—		$	1,657.90
Amortization/accretion		(2.7		)	—			(0.1		)	—			(2.8		)
Net realized and unrealized losses		(23.5		)	—			(2.6		)	—			(26.1		)
Purchases		319			—			3.7			—			322.7
Sales		(316.6		)	—			(9.7		)	—			(326.3		)
Transfers in		1.8			—			—			—			1.8
Transfer out		—			—			—			—			—
Balance at June 30, 2013		$	1,597.40		$	—		$	29.8		$	—		$	1,627.20
Amortization/accretion		(2.8		)	—			—			—			(2.8		)
Net realized and unrealized gains (losses)		(2.8		)	—			1.1			—			(1.7		)
Purchases		272.8			—			3.3			—			276.1
Sales		(225.8		)	—			(2.3		)	—			(228.1		)
Transfers in		13.8			—			—			—			13.8
Transfer out		(61.3		)	—			—			—			(61.3		)
Balance at September 30, 2013		$	1,591.30		$	—		$	31.9		$	—		$	1,623.20
_______________________________________________________________________________
-1	Excludes short-term investments which are deemed to have a Level 2 designation.
The changes in Level 3 fair value measurements for the three and nine months ended September 30, 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		0.1			—			—			3.7			3.8
Purchases		1.8			—			—			1.8			3.6
Sales		—			—			—			(4.5		)	(4.5		)
Transfers in		—			—			—			—			—
Transfers out		—			—			—			—			—
Balance at March 31, 2013		$	78		$	0.1		$	—		$	123.7		$	201.8
Amortization/accretion		—			—			—			—			—
Net realized and unrealized gains (losses)		(0.2		)	—			—			4.2			4
Purchases		14			—			—			3.4			17.4
Sales		—			—			—			(6.7		)	(6.7		)
Transfers in		—			—			—			—			—
Transfers out		(1.8		)	—			—			—			(1.8		)
Balance at June 30, 2013		$	90		$	0.1		$	—		$	124.6		$	214.7
Amortization/accretion		—			—			—			—			—
Net realized and unrealized gains (losses)		(1.8		)	—			—			1.9			0.1
Purchases		—			—			—			1.3			1.3
Sales		(0.5		)	—			—			(1.8		)	(2.3		)
Transfers in		61.3			—			—			—			61.3
Transfers out		(13.8		)	—			—			—			(13.8		)
Balance at September 30, 2013		$	135.2		$	0.1		$	—		$	126		$	261.3
_______________________________________________________________________________
(1)	Excludes the carrying value of $20.2 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 nine months ended September 30, 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 $15.6 million for the nine months ended September 30, 2013 were comprised of two 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 investment securities classified within Level 3 at September 30, 2013.
Description		Fair Value			Rating		Valuation Technique	Unobservable Inputs		Range(1)
Preferred stock		$	70.6		N/R		Discounted cash flow	Discount yield		7.80%
Agency commercial mortgage-backed securities		$	59.3		AA+		Discounted cash flow	Prepayment/default rate		5 CPJ(2)
								Swap spread		0.70%
Asset-backed securities		$	5.3		AA+		Broker pricing	Broker quote		N/A
_________________________________________________________________________
(1)	As of September 30, 2013, each asset type consists of one security.
(2)	CPJ refers to the market convention assumptions for prepayment and default vector.
The assumed prepayment rate is a significant unobservable input used to estimate the fair value of investments in agency commercial mortgage-backed securities (“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 three and nine months ended September 30, 2013 and 2012:
		Three months ended						Nine months ended
		September 30,						September 30,
		2013			2012			2013			2012
		($ in millions)
Fixed maturity investments		$	(1.8	)	$	0.4		$	(1.8	)	$	6.6
Short-term investments		—			—			—			—
Common equity securities		—			(0.1		)	—			(0.1		)
Convertible fixed maturity investments		—			—			—			—
Other investments		3.7			2.1			6.9			4
Total		$	1.9		$	2.4		$	5.1		$	10.5
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 ("CMBS") are generally short tenor and structurally senior, with approximately 30 points of subordination on average for fixed rate CMBS and approximately 30 points of subordination on average for floating rate CMBS as of September 30, 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 September 30, 2013, on average less than 1% of the underlying loans were reported as non-performing for all 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 September 30, 2013. OneBeacon's investments in hedge funds and private equity funds contain negligible amounts of sub-prime mortgage-backed securities as of September 30, 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 September 30, 2013, OneBeacon held one mortgage-backed security with a market value of $8.7 million that was classified as non-prime. OneBeacon's non-agency residential mortgage-backed portfolio is generally of moderate average life, fixed rate and structurally senior. OneBeacon does not own any collateralized debt obligations, including residential mortgage-backed collateralized debt obligations.
The following table summarizes the carrying value of OneBeacon's asset-backed securities as of September 30, 2013 and December 31, 2012:
		September 30, 2013									December 31, 2012
		Fair Value			Level 2			Level 3			Fair Value			Level 2			Level 3
		($ in millions)
Mortgage-backed securities:
Agency:
GNMA		$	342		$	282.7		$	59.3		$	551.2		$	551.2		$	—
FNMA		2.7			2.7			—			13.9			13.9			—
FHLMC		8.5			8.5			—			10.5			10.5			—
Total agency(1)		353.2			293.9			59.3			575.6			575.6			—
Non-agency:
Residential		52.2			52.2			—			38.1			38.1			—
Commercial		160.6			160.6			—			175.4			175.4			—
Total Non-agency		212.8			212.8			—			213.5			213.5			—
Total mortgage-backed securities		566			506.7			59.3			789.1			789.1			—
Other asset-backed securities:
Credit card receivables		122.4			117.1			5.3			49			43.7			5.3
Vehicle receivables		219.5			219.5			—			81.5			81.5			—
Other		—			—			—			8.3			8.3			—
Total other asset-backed securities		341.9			336.6			5.3			138.8			133.5			5.3
Total asset-backed securities		$	907.9		$	843.3		$	64.6		$	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 September 30, 2013 are as follows:
		Fair Value			Security Issuance Year
					2004			2005			2006			2007			2008			2010		2011		2012		2013
		($ in millions)
Total non-agency RMBS		$	52.2		$	7.2		$	7.8		$	5.3		$	—		$	—		$	12.9	$	—	$	—	$	19
Total non-agency CMBS		160.6			—			—			—			2.9			17.5			6		16.3		83.7		34.2
Total non-agency		$	212.8		$	7.2		$	7.8		$	5.3		$	2.9		$	17.5		$	18.9	$	16.3	$	83.7	$	53.2
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 September 30, 2013:
		Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
		($ in millions)
Prime		$	43.5		$	13.2		$	30.3		$	—
Non-prime		8.7			—			8.7			—
Total		$	52.2		$	13.2		$	39		$	—
_______________________________________________________________________________
(1)	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's Financial Services LLC ("Standard & Poor's") or Aaa by Moody's Investors Service, Inc. ("Moody's") 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 or Aaa by Moody's and were senior to non-AAA or non-Aaa bonds.
(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's or Aaa by Moody's 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 September 30, 2013:
		Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
		($ in millions)
Fixed rate CMBS		$	116.6		$	86.5		$	25.4		$	4.7
Floating rate CMBS		44			2.9			17.5			23.6
Total		$	160.6		$	89.4		$	42.9		$	28.3
_______________________________________________________________________________
(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 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 or Aaa by Moody's and were senior to non-AAA or non-Aaa bonds.
(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's or Aaa by Moody's 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 September 30, 2013 and December 31, 2012:
		30-Sep-13						December 31, 2012
		Fair			Unfunded			Fair			Unfunded
		Value			Commitments			Value			Commitments
		($ in millions)
Hedge funds
Long/short credit and distressed		$	8		$	—		$	8.4		$	—
Long bank loan		0.1			—			0.1			—
Long/short equity		38			—			36.8			—
Long/short equity activist		2.1			—			2			—
Total hedge funds		48.2			—			47.3			—
Private equity funds
Insurance		2.4			0.1			3.1			0.1
Distressed residential real estate		4.7			—			7.9			—
Energy infrastructure and services		26			6.1			20.7			7.5
Healthcare		3.5			1.4			2.1			2.7
Multi-sector		16.2			2.6			14.9			3.7
Private equity secondaries		6.4			2			7			2
Real estate		4.5			0.1			5.6			0.1
Total private equity funds		63.7			12.3			61.3			16.1
Total hedge funds and private equity funds(1)		$	111.9		$	12.3		$	108.6		$	16.1
_______________________________________________________________________________
(1)	Other investments also includes $14.1 million of an investment in a community reinvestment vehicle as of September 30, 2013 and December 31, 2012, and $20.2 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of September 30, 2013 and December 31, 2012, respectively.
Redemptions of investments in certain funds are subject to restrictions including lock-up periods where no redemptions or withdrawals are allowed, restrictions on redemption frequency and advance notice periods for redemptions. Amounts requested for redemptions remain subject to market fluctuations until the redemption effective date, which generally falls at the end of the defined redemption period. The following summarizes the September 30, 2013 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:
		Hedge Funds—Active Funds
		30 - 59 days			60 - 89 days			90 - 119 days			120+ days			Total
		notice			notice			notice			notice
		($ in millions)
Redemption frequency
Monthly		$	—		$	—		$	—		$	5.6		$	5.6
Quarterly		26.3			8			6.1			—			40.4
Annual		—			—			2.1			0.1			2.2
Total		$	26.3		$	8		$	8.2		$	5.7		$	48.2
Certain of the 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 September 30, 2013, $1.2 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 September 30, 2013.
OneBeacon has also submitted redemption requests for certain of its investments in active hedge funds. At September 30, 2013, redemptions of $2.1 million were outstanding. The date at which all these redemptions will be received is not determinable at September 30, 2013. Redemptions are recorded as receivables when the investment is no longer subject to market fluctuations.
Investments in private equity funds are generally subject to "lock-up" periods during which investors may not request a redemption. Distributions prior to the expected termination date of the fund may be limited to dividends or proceeds arising from the liquidation of the fund's underlying investment. In addition, certain private equity funds provide an option to extend the lock-up period at either the sole discretion of the fund manager or upon agreement between the fund and the investors. At September 30, 2013, investments in private equity funds were subject to lock-up periods as follows:
		1 - 3 years			3 - 5 years			5 - 10 years			>10 years			Total
		($ in millions)
Private Equity Funds—expected lock-up period remaining		$	7.1		$	22.1		$	34.5		$	—		$	63.7

Debt

Debt	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]
Debt	Debt
	OneBeacon's debt outstanding as of September 30, 2013 and December 31, 2012 consisted of the following:
		September 30,			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
	2012 Senior Notes
	In November 2012, OneBeacon U.S. Holdings, Inc. ("OBH") issued $275.0 million face value of senior unsecured debt ("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").

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]
Segment Information	Segment Information
	As a result of recent transactions (including the Runoff Transaction and AutoOne sale), the Company revised its reporting segment structure in the fourth quarter of 2012 into Specialty Products, Specialty Industries and Investing, Financing and Corporate.
	The Company has fourteen underwriting operating segments, including two that have been recently exited, 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 Insurance underwriting operating segment that was approved during the second quarter of 2013, 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 Insurance 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 the second quarter of 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, a firm that specializes in the crop business, to provide coverages through the federal program and other supplemental coverages, including crop-hail.
	The Specialty Industries segment is comprised of six underwriting operating segments, including one 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, Entertainment, and Energy (which has been exited) 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.
	Previously, the Company reported its insurance operations through a Specialty Insurance Operations segment and an Other Insurance Operations segment. The former Specialty Insurance Operations segment was comprised of twelve underwriting operating segments that were aggregated into a single reportable segment, with supplemental disclosures of three major underwriting units for financial reporting: MGA Business, Specialty Industries and Specialty Products. The former Other Insurance Operations segment consisted of substantially all operations classified as discontinued operations as of September 30, 2013 and December 31, 2012, including AutoOne, other run-off business, and certain purchase accounting adjustments relating to the run-off business resulting from the OneBeacon Acquisition. Prior periods have been reclassified to conform to the current presentation.
	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 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.
	Financial information for OneBeacon's reportable segments is as follows:
		Insurance Operations						Investing,
								Financing
								and
		Specialty			Specialty			Corporate			Consolidated
		Products			Industries
		($ in millions)
	Three months ended September 30, 2013
	Earned premiums	$	132.2		$	146.7		$	—		$	278.9
	Loss and loss adjustment expense	(84.2		)	(83.6		)	—			(167.8		)
	Policy acquisition expenses	(27.4		)	(26.2		)	—			(53.6		)
	Other underwriting expenses	(21.2		)	(25.6		)	—			(46.8		)
	Total underwriting income (loss)	(0.6		)	11.3			—			10.7
	Net investment income	—			—			10.1			10.1
	Net realized and change in unrealized investment gains	—			—			17			17
	Net other revenues	—			0.6			4.9			5.5
	General and administrative expenses	—			(1.1		)	(3.0		)	(4.1		)
	Interest expense	—			—			(3.3		)	(3.3		)
	Pre-tax income (loss) from continuing operations	$	(0.6	)	$	10.8		$	25.7		$	35.9
	Three months ended September 30, 2012
	Earned premiums	$	157.4		$	136.5		$	—		$	293.9
	Loss and loss adjustment expense	(90.4		)	(74.3		)	—			(164.7		)
	Policy acquisition expenses	(40.3		)	(26.3		)	—			(66.6		)
	Other underwriting expenses	(21.9		)	(25.5		)	—			(47.4		)
	Total underwriting income	4.8			10.4			—			15.2
	Net investment income	—			—			12.8			12.8
	Net realized and change in unrealized investment gains	—			—			40			40
	Net other revenues (expenses)	0.1			(1.1		)	0.6			(0.4		)
	General and administrative expenses	—			(0.6		)	(3.8		)	(4.4		)
	Interest expense	—			—			(4.0		)	(4.0		)
	Pre-tax income from continuing operations	$	4.9		$	8.7		$	45.6		$	59.2
		Insurance Operations						Investing,
								Financing
								and
		Specialty			Specialty			Corporate			Consolidated
		Products			Industries
		($ in millions)
	Nine months ended September 30, 2013
	Earned premiums	$	426.9		$	419.3		$	—		$	846.2
	Loss and loss adjustment expense	(240.6		)	(233.1		)	—			(473.7		)
	Policy acquisition expenses	(84.3		)	(76.6		)	—			(160.9		)
	Other underwriting expenses	(72.5		)	(78.2		)	—			(150.7		)
	Total underwriting income	29.5			31.4			—			60.9
	Net investment income	—			—			30.9			30.9
	Net realized and change in unrealized investment gains	—			—			19.9			19.9
	Net other revenues	0.3			0.9			28.9			30.1
	General and administrative expenses	(0.1		)	(2.3		)	(8.6		)	(11.0		)
	Interest expense	—			—			(9.8		)	(9.8		)
	Pre-tax income from continuing operations	$	29.7		$	30		$	61.3		$	121
	Nine months ended September 30, 2012
	Earned premiums	$	449.2		$	396.8		$	—		$	846
	Loss and loss adjustment expense	(245.7		)	(206.8		)	—			(452.5		)
	Policy acquisition expenses	(111.6		)	(74.0		)	—			(185.6		)
	Other underwriting expenses	(67.7		)	(78.5		)	—			(146.2		)
	Total underwriting income	24.2			37.5			—			61.7
	Net investment income	—			—			41.5			41.5
Net realized and change in unrealized investment gains	—			—			57.9			57.9
Net other revenues (expenses)	0.4			(1.0		)	0.5			(0.1		)
General and administrative expenses	—			(1.4		)	(8.3		)	(9.7		)
Interest expense	—			—			(12.1		)	(12.1		)
Pre-tax income from continuing operations	$	24.6		$	35.1		$	79.5		$	139.2
	Insurance Operations						Investing,
							Financing
							and
	Specialty			Specialty			Corporate(1)			Consolidated
	Products			Industries
	($ in millions)
30-Sep-13
Total investments	$	—		$	—		$	2,293.70		$	2,293.70
Reinsurance recoverables	32.4			53.7			—			86.1
Deferred acquisition costs	56.5			51.2			—			107.7
Unpaid loss and loss adjustment expense reserves	608.9			444			—			1,052.90
Unearned premiums	282.6			270.1			—			552.7
Debt	—			—			274.7			274.7
31-Dec-12
Total investments	$	—		$	—		$	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 investments excludes $239.7 million and $338.1 million of fixed maturity investments reclassified to assets held for sale as of September 30, 2013 and December 31, 2012.
The following tables provide net written premiums, earned premiums and underwriting ratios for OneBeacon's insurance operations by reportable segment and in total, for the three and nine months ended September 30, 2013, and 2012:
	Insurance Operations
($ in millions)	Specialty			Specialty			Consolidated
	Products			Industries
Three months ended September 30, 2013
Net written premiums	$	172		$	142.1		$	314.1
Earned premiums	132.2			146.7			278.9
Underwriting ratios:
Loss and LAE	63.7		%	56.9		%	60.1		%
Expense	36.7			35.4			36
Total combined ratio	100.4		%	92.3		%	96.1		%
Three months ended September 30, 2012
Net written premiums	$	203.9		$	131.3		$	335.2
Earned premiums	157.4			136.5			293.9
Underwriting ratios:
Loss and LAE	57.4		%	54.4		%	56		%
Expense	39.5			37.9			38.8
Total combined ratio	96.9		%	92.3		%	94.8		%
	Insurance Operations
($ in millions)	Specialty			Specialty			Consolidated
	Products			Industries
Nine months ended September 30, 2013
Net written premiums	$	393.3		$	432.8		$	826.1
Earned premiums	426.9			419.3			846.2
Underwriting ratios:
Loss and LAE	56.4		%	55.6		%	56		%
Expense	36.7			36.9			36.8
Total combined ratio	93.1		%	92.5		%	92.8		%
Nine months ended September 30, 2012
Net written premiums	$	510.3		$	420.1		$	930.4
Earned premiums	449.2			396.8			846
Underwriting ratios:
Loss and LAE	54.7		%	52.1		%	53.5		%
Expense	39.9			38.4			39.2
Total combined ratio	94.6		%	90.5		%	92.7		%

Retirement_Plans

Retirement Plans	9 Months Ended
	Sep. 30, 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 components of net periodic benefit cost for the three and nine months ended September 30, 2013 and 2012 were as follows:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Service cost		$	0.2		$	0.2		$	0.6		$	0.6
	Interest cost		1.1			1.2			3.2			3.6
	Expected return on plan assets		(1.8		)	(1.7		)	(5.4		)	(5.1		)
	Amortization of unrecognized loss		0.2			0.2			0.7			0.6
	Net periodic pension income before settlement and special termination benefits		(0.3		)	(0.1		)	(0.9		)	(0.3		)
	Special termination benefits expense(1)		—			0.1			0.3			0.2
	Settlement expense		—			0.5			—			0.5
	Total net periodic benefit expense (income)		$	(0.3	)	$	0.5		$	(0.6	)	$	0.4
	_______________________________________________________________________________
	(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.
	OneBeacon does not expect to make a contribution to its fully-funded Qualified Plan in 2013. OneBeacon anticipates contributing $2.2 million to the Non-qualified Plan in 2013, for which OneBeacon has assets held in a rabbi trust. As of September 30, 2013, $1.7 million in contributions have been made to the Non-qualified Plan.
	During the third quarter of 2012, the Qualified Plan experienced a partial plan settlement which required re-measurement of the remaining accumulated plan benefits. OneBeacon’s discount rate assumption used to account for the Qualified Plan reflects the rate at which the benefit obligation could effectively be settled. Based on consideration of published yields for high quality long-term corporate bonds, U.S. Treasuries, insurance company annuity contract pricings and cash flow matching analyses, the discount rate utilized for the revaluation of the Qualified Plan obligation was 3.75% as of September 30, 2012, as compared to 4.50% at December 31, 2011. As a result of the partial settlement and re-measurement, the Company recognized a loss of $0.5 million through pre-tax income and a pre-tax loss of $3.7 million through other comprehensive income in the three and nine months ended September 30, 2012. At the time of settlement, the remaining Qualified Plan liabilities, which were attributable to Qualified Plan participants who were actively employed by OneBeacon, terminated vested participants and retired participants, were approximately $92.4 million.

Employee_ShareBased_Incentive_

Employee Share-Based Incentive Compensation Plans	9 Months Ended
	Sep. 30, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Employee Share-Based Incentive Compensation Plans	Employee Share-Based Incentive Compensation Plans
	OneBeacon's share-based compensation plans include performance shares and restricted shares. 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.
	Performance Shares
	The following summarizes performance share activity for OB Performance Shares for the three and nine months ended September 30, 2013 and 2012:
			Three months ended September 30,
			2013					2012
			Target OB		Accrued			Target OB		Accrued
			Performance		expense			Performance		expense
			Shares					Shares
			outstanding					outstanding
			($ in millions)
	Beginning of period		489,867		$	2.4		560,577		$	3.1
	Payments and deferrals(1)		—		—			—		—
	New awards		—		—			—		—
	Forfeitures and net change in assumed forfeitures		—		—			(3,354	)	—
	Expense (benefit) recognized		—		0.1			—		(2.2		)
	End of period		489,867		$	2.5		557,223		$	0.9
			Nine months ended September 30,
			2013					2012
			Target OB		Accrued			Target OB		Accrued
			Performance		expense			Performance		expense
			Shares					Shares
			outstanding					outstanding
			($ in millions)
	Beginning of period		563,190		$	1.2		642,667		$	9.7
	Payments and deferrals(1)		(238,658	)	—			(258,901	)	(7.8		)
	New awards		179,000		—			181,290		—
	Forfeitures and net change in assumed forfeitures		(13,665	)	(0.1		)	(7,833	)	—
	Expense recognized		—		1.4			—		(1.0		)
	End of period		489,867		$	2.5		557,223		$	0.9
	_______________________________________________________________________________
	(1)	No payments were made in 2013 for the 2010-2012 performance cycle as the performance factor was zero percent. Performance share payments in 2012 for the 2009-2011 performance cycle were based upon a performance factor of 138.6%.
	The following summarizes performance shares outstanding and accrued performance share expense at September 30, 2013 for each performance cycle:
			Target OB		Accrued
			Performance		expense
			Shares
			outstanding
			($ in millions)
	Performance cycle:
	2011 - 2013		142,138		$	0.5
	2012 - 2014		181,290		1.4
	2013 - 2015		179,000		0.7
	Subtotal		502,428		2.6
	Assumed forfeitures		(12,561	)	(0.1		)
	Total at September 30, 2013		489,867		$	2.5
	If 100% of the outstanding performance shares had been vested on September 30, 2013, the total additional compensation cost to be recognized would have been $3.1 million, based on current accrual factors (common share price, accumulated dividends and payout assumptions) at September 30, 2013.
	All performance shares earned and paid for the 2009-2011 performance cycles were settled in cash or by deferral into OneBeacon's deferred compensation plan.
	Restricted Shares
	On March 1, 2012, OneBeacon issued 300,000 shares of restricted stock to certain employees that vest in equal installments on February 28, 2014 and 2015. On May 25, 2011, OneBeacon issued 630,000 shares of restricted stock to its CEO that vest in equal installments on February 22, 2014, 2015, 2016 and 2017. Concurrently with the 2011 grant of restricted stock, 35,000 performance shares issued to the CEO for the 2011-2013 performance share cycle were forfeited. Performance share awards to the CEO for each of the next five years are being reduced by 35,000 shares. The restricted shares contain dividend participation features, and therefore, are considered participating securities.
	The following summarizes restricted shares activity for the three and nine months ended September 30, 2013 and 2012:
			Three months ended September 30,
			2013					2012
			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
			($ in millions)
	Beginning of period		918,000		$	8		930,000		$	11.2
	New awards		—		—			—		—
	Forfeitures		(3,000	)	—			—		—
	Vested		—		—			—		—
	Expense recognized		—		(0.7		)	—		(0.8		)
	End of period		915,000		$	7.3		930,000		$	10.4
			Nine months ended September 30,
			2013					2012
			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		4.6
Forfeitures		(3,000	)	—			—		—
Vested		(9,000	)	—			—		—
Expense recognized		—		(2.3		)	—		(1.9		)
End of period		915,000		$	7.3		930,000		$	10.4

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 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. income.
	OneBeacon's income tax expense related to pre-tax income from continuing operations for the three months ended September 30, 2013 and 2012 represented net effective tax rates of (8.1)% and 24.0%, respectively. The net effective tax rates for the nine months ended September 30, 2013 and 2012 were 13.1% and 20.2%, respectively. The effective tax rates for the three and nine months ended September 30, 2013 were lower than the U.S. statutory rate of 35% due to a reduction in the valuation allowance as a result of the restructuring of the surplus note issued by Houston General Insurance Exchange and income generated in jurisdictions other than the United States, principally representing interest income taxed in a jurisdiction with a lower effective tax rate. The effective tax rates for the three and nine months ended September 30, 2012 were lower than the U.S. statutory rate of 35% due to income generated in jurisdictions other than the United States, principally representing interest income taxed in a jurisdiction with a lower effective tax rate. For the three months ended September 30, 2013 and 2012, the effective tax rate on non-U.S. income was 0.5% and 0.7%, respectively, and the effective tax rate on U.S. income was (19.7)% and 37.4%, respectively. For the nine months ended September 30, 2013 and 2012, the effective tax rate on non-U.S. income was 0.3% and 0.7%, respectively, and the effective tax rate on U.S. income was 24.3% and 34.0%, respectively.
	In arriving at the effective tax rate for the three months and nine months ended September 30, 2013 and 2012, OneBeacon forecasted all income and expense items including the change in unrealized investment gains and realized investment gains for the years ending December 31, 2013 and 2012, and included these gains in the effective tax rate calculation pursuant to ASC 740-270.
	OneBeacon classifies all interest and penalties on unrecognized tax benefits as part of income tax expense. 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. On January 5, 2011, OneBeacon received a revised Form 4549-A (Income Tax Discrepancy Adjustments) from the IRS relating to the examination of tax years 2005 and 2006. The estimated total assessment, including interest and utilization of alternative minimum and foreign tax credit carryovers, is $18.7 million. OneBeacon disagrees with the adjustments proposed by the IRS and intends to defend its position. The timing of the resolution of these issues is uncertain, however, 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 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 $64.5 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.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 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, other than financial guarantees and investment contracts.
	At September 30, 2013 and December 31, 2012, the fair value of OneBeacon's 2012 Senior Notes (its fixed-rate, long-term indebtedness) was $272.2 million and $282.4 million, respectively, which compared to a carrying value of $274.7 million as of both September 30, 2013 and December 31, 2012. The fair value measurement of the Senior Notes is classified as Level 2 in the valuation hierarchy.

Legal_Contingencies

Legal Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]
Legal Contingencies	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 September 30, 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, OBIC-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. 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 OBIC-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, OBIC-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. The Committee Action is proceeding, pending lifting of the stay and entry of a further scheduling order.

Earnings_per_Share

Earnings per Share	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2013 and 2012:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
	Earnings attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders		$	38.8		$	44.6		$	104.3		$	109.9
	Allocation of income for participating unvested restricted common shares		(0.4		)	(0.4		)	(1.0		)	(1.0		)
	Dividends paid on participating restricted common shares		(0.2		)	(0.2		)	(0.6		)	(0.6		)
	Total allocation to restricted common shares		(0.6		)	(0.6		)	(1.6		)	(1.6		)
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	38.2		$	44		$	102.7		$	108.3
	Undistributed net earnings (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	38.2		$	44		$	102.7		$	108.3
	Dividends paid, net of restricted common share amounts		(19.8		)	(19.8		)	(59.5		)	(59.5		)
	Total undistributed net earnings, net of restricted common share amounts		$	18.4		$	24.2		$	43.2		$	48.8
	Earnings per share denominator—basic and diluted (in millions):
	Total weighted average common shares outstanding		95.4			95.4			95.4			95.3
	Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)	(0.9		)	(0.8		)
	Basic and diluted earnings per share denominator(2)		94.5			94.5			94.5			94.5
	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		$	0.41		$	0.47		$	1.09		$	1.15
	Dividends declared and paid		(0.21		)	(0.21		)	(0.63		)	(0.63		)
	Undistributed earnings		$	0.2		$	0.26		$	0.46		$	0.52
	_______________________________________________________________________________
	(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 the options were not included in the three and nine months ended September 30, 2012 as the exercise price was greater than the average market price of common shares. There were no options outstanding during the three and nine months ended September 30, 2013.
	Basic and diluted income (loss) per share amounts for discontinued operations are included in Note 15—"Discontinued Operations."

Common_Shareholders_Equity

Common Shareholders' Equity	9 Months Ended
	Sep. 30, 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 nine months ended September 30, 2013 and 2012, no shares were repurchased under the share repurchase authorization. The amount of authorization remaining is $87.7 million as of September 30, 2013.
	During the nine months ended September 30, 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 nine months ended September 30, 2012. Shares repurchased pursuant to employee benefit plans do not fall under the board authorizations referred to above.
	Dividends on Common Shares
	During the nine months ended September 30, 2013 and 2012, the Company declared and paid cash dividends totaling $60.1 million and $60.0 million, respectively, or $0.63 per common share for each nine month period.
	Accumulated Other Comprehensive Income (Loss)
	For the nine months ended September 30, 2013 and 2012, OneBeacon recorded pre-tax changes to accumulated other comprehensive income (loss) for net increases in net benefit plan assets and obligations of $0.7 million. As a result of the partial settlement and re-measurement discussed in Note 8—"Retirement Plans," the Company recognized a pre-tax loss of $3.7 million through other comprehensive income in the nine months ended September 30, 2012.

Discontinued_Operations

Discontinued Operations	9 Months Ended
	Sep. 30, 2013
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations
	Runoff Business
	As described in Note 1 and Note 2, on October 17, 2012, OneBeacon entered into the Stock Purchase Agreement to sell the Runoff Business to Armour. Pursuant to the terms of the Stock Purchase Agreement, the Runoff Transaction includes the sale 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. The Runoff Transaction is expected to close in the second quarter of 2014, subject to regulatory approvals, which includes a third party actuarial review which has been completed.
	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.
	Regulatory Approval Process and Independent Actuarial Analysis
	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 on September 9, 2013; the summary report was filed with the PID on September 17, 2013; and the PID posted the summary report to its public web site on September 19, 2013, which the Company referenced in a Form 8-K filed on the same date. 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 compares to the Company’s recorded statutory net loss and LAE reserves of $166 million as of March 31, 2013.
	Two items cause the majority of the difference between the Company’s recorded net loss and LAE reserves and the low end of the range of estimates produced by the independent actuarial review. First, the independent actuaries did not assume that historically favorable loss experience would repeat in the future, causing the low end estimate to be approximately $46 million higher. Second, the provision for estimated costs to administer the runoff claims, as developed by the independent actuarial review, focused mainly on Armour’s estimated cost structure versus the Company’s cost structure which would have generated a $17 million lower cost estimate.
	The high end of the range produced by the independent actuarial review results from the accumulation of conservative selections in many other key assumptions, such as medical inflation costs, mortality experience, lump sum settlement rates, tail factors, and other judgmental items, which are more fully described in our Critical Accounting Estimates on pages 65-76 of the Company’s 2012 Annual Report on Form 10-K.
	As of September 30, 2013, the net loss and LAE reserves associated with the Runoff Business totaled $131 million, a reduction due to normal claim runoff. Management believes that the recorded net loss and LAE reserves continue to reflect a reasonable provision for expected future loss and LAE payments and represent management’s best estimate within a range of reasonable estimates.
	During the fourth quarter of 2013, the Company’s actuaries will complete additional analyses of the loss and LAE reserves associated with the Runoff Business. In addition to the internal actuaries taking into account the differing assumptions, methods, and analyses produced by the independent actuarial review and other factors, management will consider other sources of information, including runoff claims staffing models and related costs as well as perspectives provided by the PID during the ongoing regulatory approval process. Management will also consider that, for the two most recent quarters since the date of the independent actuarial review, actual loss emergence on the runoff reserves has continued to be consistent with or lower than the expected losses used to estimate its recorded net loss and LAE reserves. This recent claims experience was not considered in the results of the independent actuarial review described above.
	While management believes that the recorded loss and LAE reserves make a reasonable provision for future loss and LAE payments related to the Runoff Business, once all factors are considered, including the information contained in the independent actuarial review and recent claims experience, it is possible that the fourth quarter or subsequent internal actuarial analyses may cause the actuarial indications to increase, which could result in an increase in management’s best estimate of loss and LAE reserves associated with the Runoff Business. In the event that the Company records an increase in the loss and LAE reserves associated with the Runoff Business in the fourth quarter or at any time prior to the closing of the Runoff Transaction, the estimated loss on sale of the Runoff business would be reduced by an equal and offsetting amount in the same period. The offset to the estimated loss on sale would reflect the terms of the Stock Purchase Agreement, under which Armour has assumed the risk that loss and LAE reserves develop unfavorably from September 30, 2012 onward.
	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. The Company now expects the closing date of the Runoff Transaction to occur in the second quarter of 2014. Accordingly, OneBeacon and Armour have amended the Stock Purchase Agreement to extend the date on which the Stock Purchase Agreement may be terminated by the parties to July 31, 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 September 30, 2013 and December 31, 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 September 30, 2013 and December 31, 2012, respectively. The following summarizes the major categories of assets and liabilities associated with the business classified as held for sale:
			September 30,			December 31,
			2013			2012
			($ in millions)
	Investments		$	239.7		$	338.1
	Premiums receivable		9.8			11
	Reinsurance recoverable on unpaid losses(1)		1,666.50			1,840.80
	Reinsurance recoverable on paid losses		21.8			15.6
	Net deferred tax asset		3.2			5.1
	Other assets		16.2			16.2
	Total assets held for sale		$	1,957.20		$	2,226.80
	Unpaid loss and loss adjustment expense reserves(1)		$	1,797.10		$	2,052.60
	Unearned premiums		0.2			0.5
	Ceded reinsurance payable		12.8			21.9
	Other liabilities(2)		147.1			151.8
	Total liabilities held for sale		$	1,957.20		$	2,226.80
	_______________________________________________________________________________
	(1)	The September 30, 2013 and December 31, 2012 balances include the remaining purchase accounting fair value adjustments of $140.2 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of September 30, 2013 and December 31, 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,806.7 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,937.3 million and $2,202.7 million for each period.
	(2)	Other liabilities as of September 30, 2013 and December 31, 2012 include the accrual related to the pre-tax loss on sale of the Runoff Business of $140.7 million.
	As described in Note 1, the results of operations for the Runoff Business have been classified as discontinued operations and are presented as such, net of related income taxes, in the statements of operations and comprehensive income and cash flows for all periods. Investing and financing activities for OneBeacon are managed on a consolidated basis reported within the Investing, Financing and Corporate segment. Therefore, no investment or financing activity is included in discontinued operations.
	The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations:
			Three months ended							Nine months ended
			September 30,							September 30,
			2013			2012				2013			2012
			($ in millions)
	Net written premiums		$	1		$	(1.2	)		$	0.4		$	0.6
	Revenues
	Earned premiums		$	1.1		$	(0.4	)		$	0.7		$	10
	Other revenue		—			—				7.8			—
	Total revenues		1.1			(0.4		)		8.5			10
	Expenses
	Loss and loss adjustment expenses		0.1			27.7				7.6			48.4
	Policy acquisition expenses		0.1			(0.8		)		0.1			(1.3		)
	Other underwriting expenses (income)		0.4			(1.1		)		0.3			1.1
	Total expenses		0.6			25.8				8			48.2
	Pre-tax income (loss)		0.5			(26.2		)		0.5			(38.2		)
	Income tax benefit (expense)		(0.2		)	10.4				(0.2		)	13.4
	Income (loss) from discontinued operations, net of tax		0.3			(15.8		)		0.3			(24.8		)
	Loss from sale of discontinued operations, net of tax		—			(91.0		)		—			(91.0		)
	Net income (loss) from discontinued operations, net of tax		$	0.3		$	(106.8	)		$	0.3		$	(115.8	)
	Income (Loss) per Share Related to Discontinued Operations
	Basic income (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 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 computation of income (loss) per share for discontinued operations attributable to OneBeacon's common shareholders for the three and nine months ended September 30, 2013 and 2012:
			Three months ended							Nine months ended
			September 30,							September 30,
			2013			2012				2013			2012
	Income (loss) attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders		$	0.3		$	(106.8	)		$	0.3		$	(115.8	)
	Allocation of loss for participating unvested restricted common shares		—			1				—			1
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	0.3		$	(105.8	)		$	0.3		$	(114.8	)
Income (loss) per share denominator—basic and diluted (in millions):
Total weighted average common shares outstanding		95.4			95.4				95.4			95.3
Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)		(0.9		)	(0.9		)
Basic and diluted income (loss) per share denominator(2)		94.5			94.5				94.5			94.4
Income (loss) per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders per share		$	—		$	(1.12	)		$	—		$	(1.22	)
_______________________________________________________________________________
(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 the options were not included in the three and nine months ended September 30, 2012 as their inclusion would be anti-dilutive. There were no options outstanding during the three and nine months ended September 30, 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
Income from discontinued operations, net of tax, for both the three and nine months ended September 30, 2013 was $0.3 million. The results of discontinued operations for the nine months ended September 30, 2013 are net of 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. In light of the ongoing regulatory review of the Runoff Business and the Runoff Transaction, which includes a third party actuarial review, the Company decided to increase loss and LAE reserves in the second quarter of 2013 by approximately the same amount of the benefit resulting from the settlement.
Loss from discontinued operations, net of tax, for the three and nine months ended September 30, 2012 was $15.8 million and $24.8 million, respectively, primarily related to the Runoff Business. The three and nine months ended September 30, 2012 included $9.0 million of after tax incurred loss and LAE relating to an adjustment to the workers compensation discount rate applied to the loss reserves being transferred in connection with the Runoff Transaction. Additionally, OneBeacon recorded $6.5 million of after tax underwriting losses in connection with the Runoff Business, primarily related to adverse prior year loss reserve development related to a legacy assumed reinsurance treaty. The nine months ended September 30, 2012 also included case incurred development on a small number of claims related to multiple peril liability lines and general liability lines and the impact of an adverse court ruling in Mississippi regarding a disputed assessment from an involuntary pool for hurricane Katrina claims.
For the three and nine months ended September 30, 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 three months ended September 30, 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 three months ended September 30, 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.
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 September 30, 2013 and December 31, 2012, are net of $140.2 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 September 30, 2013 and December 31, 2012, as they are part of the Runoff Business.
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 asbestos and environmental (“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.
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. OneBeacon estimates that on an incurred basis it has used approximately $2.3 billion of the coverage provided by NICO at September 30, 2013. Net losses paid totaled approximately $1.6 billion as of September 30, 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 September 30, 2013.
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. OneBeacon has ceded estimated incurred losses of $562.0 million to GRC under the GRC Cover. As of September 30, 2013, OneBeacon has $384.3 million of reinsurance recoverable on unpaid losses outstanding under the GRC Cover.
At September 30, 2013, OneBeacon had $21.8 million of reinsurance recoverable on paid losses and $1,806.7 million (gross of $140.2 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 $140.2 million in purchase accounting adjustment), and the reinsurers' A.M. Best ratings.
($ in millions)		Balance at			% of total			A.M. Best
		30-Sep-13						Rating(1)
National Indemnity Company ("NICO") and General Reinsurance Corporation(2)		$	1,292.20		71	%		A++
Hanover Insurance Company		54.4			3	%		A
Tokio Marine and Nichido Fire(3)		25.8			1	%		A++
Munich Reinsurance America		19.9			1	%		A+
Tower Insurance Company		13			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++” (Good, which is the fifth 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 $28.0 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.
(4)	Under review with negative implications.

Consolidating_Financial_Inform

Consolidating Financial Information	9 Months Ended
	Sep. 30, 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 September 30, 2013 and December 31, 2012 and statements of operations and comprehensive income and cash flows for the three and nine months ended September 30, 2013 and 2012. 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 September 30, 2013				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,586.20		$	64.6		$	(24.7	)	$	1,626.10
	Short-term investments, at amortized cost (which approximates fair value)	4.2			153.2			4.6			—			162
	Common equity securities, at fair value	—			327.5			—			—			327.5
	Convertible fixed maturity investments, at fair value	—			31.9			—			—			31.9
	Other investments	—			146.2			—			—			146.2
	Total investment securities	4.2			2,245.00			69.2			(24.7		)	2,293.70
	Cash	—			173.4			0.4			—			173.8
	Reinsurance recoverables	—			86.1			—			—			86.1
	Premiums receivable	—			255.9			—			—			255.9
	Deferred acquisition costs	—			107.7			—			—			107.7
	Net deferred tax asset	—			129.5			(2.0		)	(0.3		)	127.2
	Investment income accrued	—			11			0.1			(0.5		)	10.6
	Accounts receivable on unsettled investment sales	—			9.4			—			—			9.4
	Investments in subsidiaries	1,059.00			—			1,131.70			(2,190.7		)	—
	Other assets	(1.2		)	199.6			5.1			—			203.5
	Assets held for sale	—			1,957.20			—			—			1,957.20
	Total assets	$	1,062.00		$	5,174.80		$	1,204.50		$	(2,216.2	)	$	5,225.10
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,052.90		$	—		$	—		$	1,052.90
	Unearned premiums	—			552.7			—			—			552.7
	Debt	—			—			299.7			(25.0		)	274.7
	Accounts payable on unsettled investment purchases	—			5.9			—			—			5.9
	Other liabilities	(0.1		)	309.9			7.4			(0.5		)	316.7
	Liabilities held for sale	—			1,957.20			—			—			1,957.20
	Total liabilities	(0.1		)	3,878.60			307.1			(25.5		)	4,160.10
	OneBeacon's common shareholders' equity and noncontrolling interests
	Total OneBeacon's common shareholders' equity	1,062.10			1,293.30			897.4			(2,190.7		)	1,062.10
	Total noncontrolling interests	—			2.9			—			—			2.9
	Total OneBeacon's common shareholders' equity and noncontrolling interests	1,062.10			1,296.20			897.4			(2,190.7		)	1,065.00
	Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,062.00		$	5,174.80		$	1,204.50		$	(2,216.2	)	$	5,225.10
	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
Three months ended September 30, 2013	(in millions)
Revenues
Earned premiums	$	—		$	278.9		$	—		$	—		$	278.9
Net investment income (expense)	—			10.2			0.2			(0.3		)	10.1
Net realized and change in unrealized investment gains	—			17.2			(0.4		)	0.2			17
Net other revenues	—			5.5			—			—			5.5
Total revenues	—			311.8			(0.2		)	(0.1		)	311.5
Expenses
Loss and loss adjustment expenses	—			167.8			—			—			167.8
Policy acquisition expenses	—			53.6			—			—			53.6
Other underwriting expenses	—			46.8			—			—			46.8
General and administrative expenses	1.7			2.2			0.2			—			4.1
Interest expense	—			—			3.5			(0.2		)	3.3
Total expenses	1.7			270.4			3.7			(0.2		)	275.6
Pre-tax (loss) income from continuing operations	(1.7		)	41.4			(3.9		)	0.1			35.9
Income tax benefit (expense)	—			0.5			2.5			(0.1		)	2.9
Net (loss) income from continuing operations	(1.7		)	41.9			(1.4		)	—			38.8
Loss from discontinued operations, net of tax	—			0.3			—			—			0.3
(Loss) income before equity in earnings of unconsolidated affiliates	(1.7		)	42.2			(1.4		)	—			39.1
Equity in earnings of subsidiaries, net of tax	44.5			—			34			(78.5		)	—
Net income (loss) including noncontrolling interests	42.8			42.2			32.6			(78.5		)	39.1
Less: Net income attributable to noncontrolling interests	—			—			—			—			—
Net income (loss) attributable to OneBeacon's common shareholders	42.8			42.2			32.6			(78.5		)	39.1
Net change in benefit plan assets and obligations, net of tax	0.2			—			0.2			(0.2		)	0.2
Comprehensive income (loss) attributable to OneBeacon's common shareholders	$	43		$	42.2		$	32.8		$	(78.7	)	$	39.3
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Three months ended September 30, 2012	(in millions)
Revenues
Earned premiums	$	—		$	293.9		$	—		$	—		$	293.9
Net investment income	—			13.5			0.1			(0.8		)	12.8
Net realized and change in unrealized investment gains	—			38.7			1			0.3			40
Net other revenues (expenses)	—			(0.2		)	(0.2		)	—			(0.4		)
Total revenues	—			345.9			0.9			(0.5		)	346.3
Expenses
Loss and loss adjustment expenses	—			164.7			—			—			164.7
Policy acquisition expenses	—			66.6			—			—			66.6
Other underwriting expenses	—			47.4			—			—			47.4
General and administrative expenses	1.5			2.7			0.2			—			4.4
Interest expense	—			(0.1		)	4.8			(0.7		)	4
Total expenses	1.5			281.3			5			(0.7		)	287.1
Pre-tax (loss) income from continuing operations	(1.5		)	64.6			(4.1		)	0.2			59.2
Income tax (expense) benefit	—			(16.1		)	2			(0.1		)	(14.2		)
Net (loss) income from continuing operations	(1.5		)	48.5			(2.1		)	0.1			45
Loss from discontinued operations, net of tax	—			(15.8		)	—			—			(15.8		)
Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
Net loss before equity in earnings of unconsolidated affiliates	(1.5		)	(58.3		)	(2.1		)	0.1			(61.8		)
Equity in loss of subsidiaries, net of tax	(60.7		)	—			(69.9		)	130.6			—
Net loss including noncontrolling interests	(62.2		)	(58.3		)	(72.0		)	130.7			(61.8		)
Less: Net income attributable to noncontrolling interests	—			(0.4		)	—			—			(0.4		)
Net loss attributable to OneBeacon's common shareholders	(62.2		)	(58.7		)	(72.0		)	130.7			(62.2		)
Net change in benefit plan assets and obligations, net of tax	(2.0		)	—			(2.0		)	2			(2.0		)
Comprehensive loss attributable to OneBeacon's common shareholders	$	(64.2	)	$	(58.7	)	$	(74.0	)	$	132.7		$	(64.2	)
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Nine months ended September 30, 2013	(in millions)
Revenues
Earned premiums	$	—		$	846.2		$	—		$	—		$	846.2
Net investment income (expense)	—			32.1			(0.3		)	(0.9		)	30.9
Net realized and change in unrealized investment gains	—			19.7			(0.8		)	1			19.9
Net other revenues	—			30.1						—			30.1
Total revenues	—			928.1			(1.1		)	0.1			927.1
Expenses
Loss and loss adjustment expenses	—			473.7			—			—			473.7
Policy acquisition expenses	—			160.9			—			—			160.9
Other underwriting expenses	—			150.7			—			—			150.7
General and administrative expenses	3.6			7.5			(0.1		)	—			11
Interest expense	—			—			10.6			(0.8		)	9.8
Total expenses	3.6			792.8			10.5			(0.8		)	806.1
Pre-tax (loss) income from continuing operations	(3.6		)	135.3			(11.6		)	0.9			121
Income tax benefit (expense)	0.1			(23.3		)	7.7			(0.4		)	(15.9		)
Net (loss) income from continuing operations	(3.5		)	112			(3.9		)	0.5			105.1
Income from discontinued operations, net of tax	—			0.3			—			—			0.3
(Loss) income before equity in earnings of unconsolidated affiliates	(3.5		)	112.3			(3.9		)	0.5			105.4
Equity in earnings of subsidiaries, net of tax	111.8			—			91.6			(203.4		)	—
Net income including noncontrolling interests	108.3			112.3			87.7			(202.9		)	105.4
Less: Net income attributable to noncontrolling interests	—			(0.8		)	—			—			(0.8		)
Net income attributable to OneBeacon's common shareholders	108.3			111.5			87.7			(202.9		)	104.6
Net change in benefit plan assets and obligations, net of tax	0.5			—			0.5			(0.5		)	0.5
Comprehensive income attributable to OneBeacon's common shareholders	$	108.8		$	111.5		$	88.2		$	(203.4	)	$	105.1
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Nine months ended September 30, 2012	(in millions)
Revenues
Earned premiums	$	—		$	846		$	—		$	—		$	846
Net investment income	—			43.8			0.1			(2.4		)	41.5
Net realized and change in unrealized investment gains	—			56.5			1			0.4			57.9
Net other revenues (expenses)	—			0.5			(0.6		)	—			(0.1		)
Total revenues	—			946.8			0.5			(2.0		)	945.3
Expenses
Loss and loss adjustment expenses	—			452.5			—			—			452.5
Policy acquisition expenses	—			185.6			—			—			185.6
Other underwriting expenses	—			146.2			—			—			146.2
General and administrative expenses	4.5			4.7			0.5			—			9.7
Interest expense	—			(0.1		)	14.4			(2.2		)	12.1
Total expenses	4.5			788.9			14.9			(2.2		)	806.1
Pre-tax (loss) income from continuing operations	(4.5		)	157.9			(14.4		)	0.2			139.2
Income tax (expense) benefit	—			(35.1		)	7.1			(0.1		)	(28.1		)
Net (loss) income from continuing operations	(4.5		)	122.8			(7.3		)	0.1			111.1
Loss from discontinued operations, net of tax	—			(24.8		)	—			—			(24.8		)
Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
(Loss) income before equity in earnings of unconsolidated affiliates	(4.5		)	7			(7.3		)	0.1			(4.7		)
Equity in loss of subsidiaries, net of tax	(1.4		)	—			(20.5		)	21.9			—
Net (loss) income including noncontrolling interests	(5.9		)	7			(27.8		)	22			(4.7		)
Less: Net income attributable to noncontrolling interests	—			(1.2		)	—			—			(1.2		)
Net (loss) income attributable to OneBeacon's common shareholders	(5.9		)	5.8			(27.8		)	22			(5.9		)
Net change in benefit plan assets and obligations, net of tax	(1.7		)	—			(1.7		)	1.7			(1.7		)
Comprehensive (loss) income attributable to OneBeacon's common shareholders	$	(7.6	)	$	5.8		$	(29.5	)	$	23.7		$	(7.6	)
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Nine months ended September 30, 2013	($ in millions)
Cash flows from operations:
Net income including noncontrolling interests	$	108.3		$	112.3		$	87.7		$	(202.9	)	$	105.4
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	(111.8		)	—			(91.6		)	203.4			—
Net income from discontinued operations	—			(0.3		)	—			—			(0.3		)
Net loss from sale of discontinued operations	—			—			—			—			—
Net realized and change in unrealized investment gains	—			(19.7		)	0.8			(1.0		)	(19.9		)
Net other realized losses	—			—			—			—			—
Net realized gain on sale of business	—			(23.0		)	—			—			(23.0		)
Deferred income tax expense	—			12.5			(0.5		)	0.3			12.3
Dividends received from subsidiaries	62			—			23			(85.0		)	—
Other operating items:
Net change in loss and LAE reserves	—			52.9			—			—			52.9
Net change in unearned premiums	—			(21.1		)	—			—			(21.1		)
Net change in premiums receivable	—			(30.3		)	—			—			(30.3		)
Net change in reinsurance recoverable on paid and unpaid losses	—			24.5			—			—			24.5
Net change in other assets and liabilities	3.1			70.4			4.1			0.2			77.8
Net cash provided from operations—continuing operations	61.6			178.2			23.5			(85.0		)	178.3
Net cash used for operations—discontinued operations	—			(98.0		)	—			—			(98.0		)
Net cash provided from operations	61.6			80.2			23.5			(85.0		)	80.3
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(1.5		)	13.2			55.6			—			67.3
Maturities of fixed maturity investments	—			185.3			11.3			—			196.6
Sales of fixed maturity investments	—			1,114.60			129.7			(35.7		)	1,208.60
Sales of common equity securities	—			142			—			—			142
Sales of convertible fixed maturity investments	—			39.5			—			—			39.5
Distributions and redemptions of other investments	—			13.1			—			—			13.1
Purchases of fixed maturity investments	—			(1,227.8		)	(189.7		)	35.7			(1,381.8		)
Purchases of common equity securities	—			(168.5		)	—			—			(168.5		)
Purchases of convertible fixed maturity investments	—			(12.0		)	—			—			(12.0		)
Contributions for other investments	—			(7.1		)	—			—			(7.1		)
Proceeds from sale of business	—			31.3			—			—			31.3
Net change in unsettled investment purchases and sales	—			(7.7		)	—			—			(7.7		)
Net acquisitions of property and equipment	—			(7.6		)	—			—			(7.6		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			30			—			(30.0		)	—
Net cash (used for) provided from investing activities—continuing operations	(1.5		)	138.3			6.9			(30.0		)	113.7
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(1.5		)	138.3			6.9			(30.0		)	113.7
Cash flows from financing activities:
Cash dividends paid to common shareholders	(60.1		)	—			—			—			(60.1		)
Cash dividends paid to parent	—			(85.0		)	—			85			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—						(30.0		)	30			—
Payments on capital lease obligation	—			(4.0		)	—			—			(4.0		)
Net cash used for financing activities—continuing operations	(60.1		)	(89.0		)	(30.0		)	115			(64.1		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(60.1		)	(89.0		)	(30.0		)	115			(64.1		)
Net increase in cash during period	—			129.5			0.4			—			129.9
Cash reclassified to assets held for sale	—			—			—			—			—
Cash transferred with sale of business	—			—			—			—			—
Net increase after reclassification of cash to assets held for sale	—			129.5			0.4			—			129.9
Cash balance at beginning of period	—			43.9			—			—			43.9
Cash balance at end of period	$	—		$	173.4		$	0.4		$	—		$	173.8
Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Nine months ended September 30, 2012	($ in millions)
Cash flows from operations:
Net (loss) income including noncontrolling interests	$	(5.9	)	$	7		$	(27.8	)	$	22		$	(4.7	)
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed loss from subsidiaries	1.4			—			20.5			(21.9		)	—
Net loss from discontinued operations	—			24.8			—			—			24.8
Net loss from sale of discontinued operations	—			91			—			—			91
Net realized and change in unrealized investment gains	—			(56.5		)	(1.0		)	(0.4		)	(57.9		)
Net other realized losses	—			—			—			—			—
Net realized gain on sale of business	—			—			—			—			—
Deferred income tax expense	—			9.1			3.1			0.1			12.3
Dividends received from subsidiaries	62			15.5			101.1			(178.6		)	—
Other operating items:
Net change in loss and LAE reserves	—			(0.9		)	—			—			(0.9		)
Net change in unearned premiums	—			85.2			—			—			85.2
Net change in premiums receivable	—			(45.7		)	—			—			(45.7		)
Net change in reinsurance recoverable on paid and unpaid losses	—			64.4			—			—			64.4
Net change in other assets and liabilities	2.9			(36.3		)	0.9			0.2			(32.3		)
Net cash provided from operations—continuing operations	60.4			157.6			96.8			(178.6		)	136.2
Net cash used for operations—discontinued operations	—			(155.6		)	—			—			(155.6		)
Net cash provided from operations	60.4			2			96.8			(178.6		)	(19.4		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(0.4		)	12.9			(35.2		)	—			(22.7		)
Maturities of fixed maturity investments	—			147.5			—			—			147.5
Sales of fixed maturity investments	—			1,306.30			—			(0.4		)	1,305.90
Sales of common equity securities	—			62.4			0.2			(25.4		)	37.2
Sales of convertible fixed maturity investments	—			23.5			0.1			(7.6		)	16
Distributions and redemptions of other investments	—			6.9			—			—			6.9
Purchases of fixed maturity investments	—			(1,317.2		)	(0.4		)	0.4			(1,317.2		)
Purchases of common equity securities	—			(48.5		)	(26.2		)	25.4			(49.3		)
Purchases of convertible fixed maturity investments	—			(6.7		)	(7.7		)	7.6			(6.8		)
Contributions for other investments	—			(3.1		)	—			—			(3.1		)
Proceeds from sale of business	—			—			—			—			—
Net change in unsettled investment purchases and sales	—			(69.1		)	—			—			(69.1		)
Net acquisitions of property and equipment	—			(1.2		)	—			—			(1.2		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			14.5			—			(14.5		)	—
Net cash (used for) provided from investing activities—continuing operations	(0.4		)	128.2			(69.2		)	(14.5		)	44.1
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(0.4		)	128.2			(69.2		)	(14.5		)	44.1
Cash flows from financing activities:
Cash dividends paid to common shareholders	(60.0		)	—			—			—			(60.0		)
Cash dividends paid to parent	—			(163.1		)	(15.5		)	178.6			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(14.5		)	14.5			—
Payments on capital lease obligation	—			(3.6		)	—			—			(3.6		)
Proceeds on sale of OneBeacon Holdings (Luxembourg) S.à r.l.	—			14			—			—			14
Net cash used for financing activities—continuing operations	(60.0		)	(152.7		)	(30.0		)	193.1			(49.6		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(60.0		)	(152.7		)	(30.0		)	193.1			(49.6		)
Net (decrease) in cash during period	—			(22.5		)	(2.4		)	—			(24.9		)
Cash reclassified to 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	—			(20.5		)	(2.4		)	—			(22.9		)
Cash balance at beginning of period	—			52.5			2.4			—			54.9
Cash balance at end of period	$	—		$	32		$	—		$	—		$	32

Nature_of_Operations_and_Summa1

Nature of Operations and Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
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 three and nine months ended September 30, 2012, $0.9 million and $5.3 million, respectively, of the $5.6 million of unamortized acquisitions costs as of January 1, 2012 were recognized in expense, with the remainder of the $5.6 million balance expensed during the year ended December 31, 2012 such that no incremental impact was recognized in the three and nine months ended September 30, 2013.
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.

Reserves_for_Unpaid_Losses_and1

Reserves for Unpaid Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2013 and 2012:
			Three months ended								Nine months ended
			September 30,								September 30,
			2013				2012				2013				2012
			($ in millions)								($ in millions)
	Gross beginning balance		$	1,027.80			$	3,175.30			$	1,000.00			$	3,358.60
	Less beginning reinsurance recoverable on unpaid losses		(81.8		)		(2,040.7		)		(107.3		)		(2,167.5		)
	Net beginning loss and LAE reserves		946				1,134.60				892.7				1,191.10
	Loss and LAE incurred relating to:
	Current year losses		164				167				473.8				460.1
	Prior year losses		3.8				(2.3		)		(0.1		)		(7.6		)
	Total incurred loss and LAE from continuing operations		167.8				164.7				473.7				452.5
	Loss and LAE paid relating to:
	Current year losses		(54.1		)		(59.6		)		(115.1		)		(123.0		)
	Prior year losses		(87.1		)		(73.4		)		(278.7		)		(266.2		)
	Total loss and LAE payments from continuing operations		(141.2		)		(133.0		)		(393.8		)		(389.2		)
	Net loss and LAE reserves		972.6				1,166.30				972.6				1,254.40
	Total incurred loss and LAE from discontinued operations		—				27.7				7.5				48.4
	Total loss and LAE payments from discontinued operations		(17.2		)		(67.4		)		(88.6		)		(177.1		)
	Net loss and LAE reserves		955.4				1,126.60				891.5				1,125.70
	Net change in loss and LAE reserves reported in liabilities held for sale		17.3			(1)	(256.3		)	(2)	81.2			(3)	(191.6		)	(4)
	Net loss and LAE reserves sold		—				—				—				(63.8		)	(5)
	Net ending loss and LAE reserves		972.7				870.3				972.7				870.3
	Plus ending reinsurance recoverable on unpaid losses		80.2				41.3				80.2				41.3
	Gross ending loss and LAE reserves		$	1,052.90			$	911.6			$	1,052.90			$	911.6
	_______________________________________________________________________________
	(1)	Consists of the change in net loss and LAE reserves from $147.9 million to $130.6 million, which amounts were classified as held for sale as of June 30, 2013 and September 30, 2013, respectively, in connection with the Runoff Transaction.
	(2)	Consists of the net loss and LAE reserves recorded in the three months ended September 30, 2012 in connection with the Runoff Transaction.
	(3)	Consists of the change in net loss and LAE reserves from $211.8 million to $130.6 million, which amounts were classified as held for sale as of December 31, 2012 and September 30, 2013, respectively, in connection with the Runoff Transaction.
	(4)	Consists of the net loss and LAE reserves of $256.3 million recorded in the nine months ended September 30, 2012 in connection with the Runoff Transaction, offset by net loss and LAE reserves of $64.7 million reclassified from held for sale upon the closing of the AutoOne Transaction in February 2012.
	(5)	Relates to the AutoOne Transaction, which closed in February 2012.

Reinsurance_Tables

Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2013
Reinsurance Disclosures [Abstract]
Listing of top reinsurers	The following table provides a listing of OneBeacon's top reinsurers for its continuing insurance operations, excluding industry pools and associations, based upon reinsurance recoverable amounts on paid and unpaid losses, the percentage of the total balance, and the reinsurer's A.M. Best Company, Inc. ("A.M. Best") ratings:
	($ in millions)		Balance at		% of total		A.M. Best
			30-Sep-13				Rating(1)
	Hannover Ruckversich		$	10.3	12	%	A+
	Munich Reinsurance America		6.6		8	%	A+
	Platinum Underwriters		5.6		7	%	A
	Hartford Steam Boiler		4.8		6	%	A++
	Swiss Reinsurance America Corp		4.6		5	%	A+
	_______________________________________________________________________________
	(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) and "A" (Excellent, which is the third highest of sixteen financial strength ratings).
	At September 30, 2013, OneBeacon had $21.8 million of reinsurance recoverable on paid losses and $1,806.7 million (gross of $140.2 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 $140.2 million in purchase accounting adjustment), and the reinsurers' A.M. Best ratings.
	($ in millions)		Balance at		% of total		A.M. Best
			30-Sep-13				Rating(1)
	National Indemnity Company ("NICO") and General Reinsurance Corporation(2)		$	1,292.20	71	%	A++
	Hanover Insurance Company		54.4		3	%	A
	Tokio Marine and Nichido Fire(3)		25.8		1	%	A++
	Munich Reinsurance America		19.9		1	%	A+
	Tower Insurance Company		13		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++” (Good, which is the fifth 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 $28.0 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire

Investment_Securities_Tables

Investment Securities (Tables)	9 Months Ended
	Sep. 30, 2013
Schedule of Investments [Abstract]
Net investment income, pre-tax	Net investment income for the three and nine months ended September 30, 2013, and 2012 consisted of the following:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Fixed maturity investments		$	10.4		$	12		$	30.7		$	38.6
	Short-term investments		—			—			—			0.1
	Common equity securities		1.5			1.5			4.4			4.8
	Convertible fixed maturity investments		0.2			1.1			0.9			3.4
	Other investments		(0.4		)	(0.2		)	—			(0.4		)
	Gross investment income		11.7			14.4			36			46.5
	Less investment expenses		(1.6		)	(1.6		)	(5.1		)	(5.0		)
	Net investment income		$	10.1		$	12.8		$	30.9		$	41.5
Net realized and unrealized investment gains (losses), pre-tax	The composition of net realized investment gains (losses) consisted of the following:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Fixed maturity investments		$	(5.0	)	$	10.6		$	2.8		$	26.7
	Short-term investments		—			—			0.1			—
	Common equity securities		2.7			2.9			20.6			(0.2		)
	Convertible fixed maturity investments		—			0.6			(0.4		)	2
	Other investments		(1.9		)	(0.4		)	2.8			1.6
	Net realized investment gains (losses), pre-tax		$	(4.2	)	$	13.7		$	25.9		$	30.1
Net unrealized investment gains (losses)	The net changes in fair value for the three and nine months ended September 30, 2013, and 2012 are as follows:
			Three months ended September 30, 2013									Nine months ended September 30, 2013
			Changes in net			Changes in net			Total net changes			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected			unrealized investment			foreign currency			in fair value reflected
			gains (losses)(1)			translation			in revenues(1)			gains (losses)(1)			translation			in revenues(1)
						gains (losses)(1)									gains (losses)(1)
			($ in millions)
	Fixed maturity investments		$	0.5		$	0.1		$	0.6		$	(33.7	)	$	—		$	(33.7	)
	Short-term investments		—			—			—			—			—			—
	Common equity securities		15.8			0.1			15.9			21.3			0.1			21.4
	Convertible fixed maturity investments		1			—			1			(0.6		)	—			(0.6		)
	Other investments		3.7			—			3.7			6.9			—			6.9
	Net change, pre-tax		$	21		$	0.2		$	21.2		$	(6.1	)	$	0.1		$	(6.0	)
			Three months ended September 30, 2012									Nine months ended September 30, 2012
			Changes in net			Changes in net			Total net changes			Changes in net			Changes in net			Total net changes
			unrealized investment			foreign currency			in fair value reflected			unrealized investment			foreign currency			in fair value reflected
			gains (losses)(2)			translation			in revenues(2)			gains (losses)(2)			translation			in revenues(2)
						gains (losses)(2)									gains (losses)(2)
			($ in millions)
	Fixed maturity investments		$	5.9		$	0.2		$	6.1		$	8.6		$	0.2		$	8.8
	Short-term investments		—			—			—			—			—			—
	Common equity securities		18.7			—			18.7			17			—			17
	Convertible fixed maturity investments		(0.6		)	—			(0.6		)	(2.0		)	—			(2.0		)
	Other investments		2.1			—			2.1			4			—			4
	Net change, pre-tax		$	26.1		$	0.2		$	26.3		$	27.6		$	0.2		$	27.8
	_______________________________________________________________________________
	(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 $(0.7) million and $(0.8) million for the three and nine months ended September 30, 2013.
	(2)	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.2) million for the nine months ended September 30, 2012.
Gross unrealized investment gains or losses	The components of OneBeacon's ending net unrealized investment gains and losses, excluding the impact of net unrealized foreign currency translation gains and losses, on its trading investment portfolio as of September 30, 2013 and December 31, 2012 were as follows:
			September 30,			December 31,
			2013			2012
			($ in millions)
	Investment securities:
	Gross unrealized investment gains		$	124.6		$	127.4
	Gross unrealized investment losses		(11.4		)	(8.7		)
	Total net unrealized investment gains, pre-tax		113.2			118.7
	Income taxes		(36.7		)	(39.4		)
	Total net unrealized investment gains, after tax		$	76.5		$	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 September 30, 2013 and December 31, 2012 were as follows:
			September 30, 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		$	137.4		$	0.2		$	—		$	—		$	137.6
	Debt securities issued by industrial corporations		717.3			16.3			(1.4		)	—			732.2
	Municipal obligations		3.3			—			(0.3		)	—			3
	Asset-backed securities		911.1			1.3			(4.5		)	—			907.9
	Foreign government obligations		2			0.4			—			—			2.4
	Preferred stocks		78.3			4.4			—			—			82.7
	Total fixed maturity investments		$	1,849.40		$	22.6		$	(6.2	)	$	—		$	1,865.80
			December 31, 2012(1)
			Cost or			Gross			Gross			Net foreign			Carrying
			amortized			unrealized			unrealized			currency			value
			cost			gains			losses			gains
			($ 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 $239.7 million and $338.1 million of fixed maturity investments reclassified to assets held for sale in the consolidated balance sheets as part of the Runoff Transaction as of September 30, 2013 and December 31, 2012, respectively.
Investment holdings, common equity securities, convertible fixed maturity investments and other investments	The cost or amortized cost, gross unrealized pre-tax investment gains and losses, net foreign currency gains and carrying values of OneBeacon's common equity securities, convertible fixed maturity investments and other investments as of September 30, 2013 and December 31, 2012 were as follows:
			30-Sep-13
			Cost or			Gross			Gross			Net foreign			Carrying
			amortized			unrealized			unrealized			currency			value
			cost			gains			losses			gains
			($ in millions)
	Common equity securities		$	268.6		$	59.3		$	(0.5	)	$	0.1		$	327.5
	Convertible fixed maturity investments		29.4			2.6			(0.1		)	—			31.9
	Other investments		110.7			40.1			(4.6		)	—			146.2
	Total common equity securities, convertible fixed maturity investments and other investments		$	408.7		$	102		$	(5.2	)	$	0.1		$	505.6
			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 September 30, 2013 and December 31, 2012 by level:
			Fair value at			Level 1			Level 2			Level 3
			September 30, 2013(2)
			($ in millions)
	Fixed maturity investments:
	U.S. Government and agency obligations		$	137.6		$	137.6		$	—		$	—
	Debt securities issued by corporations:
	Consumer		214.5			—			214.5			—
	Industrial		95.6			—			95.6			—
	Financial		138.6			—			138.6			—
	Communications		92.6			—			92.6			—
	Energy		40.9			—			40.9			—
	Basic materials		80			—			80			—
	Utilities		41.1			—			41.1			—
	Technology		28.9			—			28.9			—
	Debt securities issued by corporations		732.2			—			732.2			—
	Municipal obligations		3			—			3			—
	Asset-backed securities		907.9			—			843.3			64.6
	Foreign government obligations		2.4			1.7			0.7			—
	Preferred stocks		82.7			—			12.1			70.6
	Fixed maturity investments		1,865.80			139.3			1,591.30			135.2
	Short-term investments		162			160.9			1.1			—
	Common equity securities:
	Financials		77			76.9			—			0.1
	Basic Materials		24.5			24.5			—			—
	Consumer		114.3			114.3			—			—
	Energy		37.3			37.3			—			—
	Utilities		8.4			8.4			—			—
	Other		66			66			—			—
	Common equity securities		327.5			327.4			—			0.1
	Convertible fixed maturity investments		31.9			—			31.9			—
	Other investments(1)		126			—			—			126
	Total(1)		$	2,513.20		$	627.6		$	1,624.30		$	261.3
	_______________________________________________________________________________
	-1	Excludes the carrying value of $20.2 million associated with a tax advantaged federal affordable housing development fund accounted for using the equity method as of September 30, 2013.
	-2	Fair value includes $239.7 million of fixed maturity investments reclassified to assets held for sale in the September 30, 2013 consolidated balance sheets as part of the Runoff Transaction.
			Fair value at			Level 1			Level 2			Level 3
			December 31, 2012(2)
			($ 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
Debt securities, credit ratings	The following table summarizes the ratings of OneBeacon's corporate debt securities as of September 30, 2013 and 2012:
			September 30,			December 31,
			2013			2012
			($ in millions)
	AA		$	46.5		$	37
	A		353.4			301.6
	BBB		324.2			362.5
	BB		6.9			7
	Other		1.2			3.4
	Debt securities issued by corporations		$	732.2		$	711.5
Fair value, significant unobservable inputs	The following summarizes significant unobservable inputs used in estimating the fair value of investment securities classified within Level 3 at September 30, 2013.
	Description		Fair Value			Rating		Valuation Technique	Unobservable Inputs		Range(1)
	Preferred stock		$	70.6		N/R		Discounted cash flow	Discount yield		7.80%
	Agency commercial mortgage-backed securities		$	59.3		AA+		Discounted cash flow	Prepayment/default rate		5 CPJ(2)
									Swap spread		0.70%
	Asset-backed securities		$	5.3		AA+		Broker pricing	Broker quote		N/A
	_________________________________________________________________________
	(1)	As of September 30, 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 three and nine months ended September 30, 2013 and 2012:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Fixed maturity investments		$	(1.8	)	$	0.4		$	(1.8	)	$	6.6
	Short-term investments		—			—			—			—
	Common equity securities		—			(0.1		)	—			(0.1		)
	Convertible fixed maturity investments		—			—			—			—
	Other investments		3.7			2.1			6.9			4
	Total		$	1.9		$	2.4		$	5.1		$	10.5
Mortgage-backed and asset-backed securities	The following table summarizes the carrying value of OneBeacon's asset-backed securities as of September 30, 2013 and December 31, 2012:
			September 30, 2013									December 31, 2012
			Fair Value			Level 2			Level 3			Fair Value			Level 2			Level 3
			($ in millions)
	Mortgage-backed securities:
	Agency:
	GNMA		$	342		$	282.7		$	59.3		$	551.2		$	551.2		$	—
	FNMA		2.7			2.7			—			13.9			13.9			—
	FHLMC		8.5			8.5			—			10.5			10.5			—
	Total agency(1)		353.2			293.9			59.3			575.6			575.6			—
	Non-agency:
	Residential		52.2			52.2			—			38.1			38.1			—
	Commercial		160.6			160.6			—			175.4			175.4			—
	Total Non-agency		212.8			212.8			—			213.5			213.5			—
	Total mortgage-backed securities		566			506.7			59.3			789.1			789.1			—
	Other asset-backed securities:
	Credit card receivables		122.4			117.1			5.3			49			43.7			5.3
	Vehicle receivables		219.5			219.5			—			81.5			81.5			—
	Other		—			—			—			8.3			8.3			—
	Total other asset-backed securities		341.9			336.6			5.3			138.8			133.5			5.3
	Total asset-backed securities		$	907.9		$	843.3		$	64.6		$	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 September 30, 2013 are as follows:
			Fair Value			Security Issuance Year
						2004			2005			2006			2007			2008			2010		2011		2012		2013
			($ in millions)
	Total non-agency RMBS		$	52.2		$	7.2		$	7.8		$	5.3		$	—		$	—		$	12.9	$	—	$	—	$	19
	Total non-agency CMBS		160.6			—			—			—			2.9			17.5			6		16.3		83.7		34.2
	Total non-agency		$	212.8		$	7.2		$	7.8		$	5.3		$	2.9		$	17.5		$	18.9	$	16.3	$	83.7	$	53.2
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 September 30, 2013:
			Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
			($ in millions)
	Prime		$	43.5		$	13.2		$	30.3		$	—
	Non-prime		8.7			—			8.7			—
	Total		$	52.2		$	13.2		$	39		$	—
	_______________________________________________________________________________
	(1)	At issuance, Super Senior, or in the case of resecuritization, the underlying securities, were rated AAA by Standard & Poor's Financial Services LLC ("Standard & Poor's") or Aaa by Moody's Investors Service, Inc. ("Moody's") 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 or Aaa by Moody's and were senior to non-AAA or non-Aaa bonds.
	(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's or Aaa by Moody's 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 September 30, 2013:
			Fair Value			Super Senior(1)			Senior(2)			Subordinate(3)
			($ in millions)
	Fixed rate CMBS		$	116.6		$	86.5		$	25.4		$	4.7
	Floating rate CMBS		44			2.9			17.5			23.6
	Total		$	160.6		$	89.4		$	42.9		$	28.3
	_______________________________________________________________________________
	(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 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 or Aaa by Moody's and were senior to non-AAA or non-Aaa bonds.
	(3)	At issuance, Subordinate were not rated AAA by Standard & Poor's or Aaa by Moody's and were junior to other bonds.
Other investments	The following table summarizes investments in hedge funds and private equity funds at September 30, 2013 and December 31, 2012:
			30-Sep-13						December 31, 2012
			Fair			Unfunded			Fair			Unfunded
			Value			Commitments			Value			Commitments
			($ in millions)
	Hedge funds
	Long/short credit and distressed		$	8		$	—		$	8.4		$	—
	Long bank loan		0.1			—			0.1			—
	Long/short equity		38			—			36.8			—
	Long/short equity activist		2.1			—			2			—
	Total hedge funds		48.2			—			47.3			—
	Private equity funds
	Insurance		2.4			0.1			3.1			0.1
	Distressed residential real estate		4.7			—			7.9			—
	Energy infrastructure and services		26			6.1			20.7			7.5
	Healthcare		3.5			1.4			2.1			2.7
	Multi-sector		16.2			2.6			14.9			3.7
	Private equity secondaries		6.4			2			7			2
	Real estate		4.5			0.1			5.6			0.1
	Total private equity funds		63.7			12.3			61.3			16.1
	Total hedge funds and private equity funds(1)		$	111.9		$	12.3		$	108.6		$	16.1
	_______________________________________________________________________________
	(1)	Other investments also includes $14.1 million of an investment in a community reinvestment vehicle as of September 30, 2013 and December 31, 2012, and $20.2 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of September 30, 2013 and December 31, 2012, respectively.
Fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds	The following summarizes the September 30, 2013 fair value of hedge funds subject to restrictions on redemption frequency and advance notice period requirements for investments in active hedge funds:
			Hedge Funds—Active Funds
			30 - 59 days			60 - 89 days			90 - 119 days			120+ days			Total
			notice			notice			notice			notice
			($ in millions)
	Redemption frequency
	Monthly		$	—		$	—		$	—		$	5.6		$	5.6
	Quarterly		26.3			8			6.1			—			40.4
	Annual		—			—			2.1			0.1			2.2
	Total		$	26.3		$	8		$	8.2		$	5.7		$	48.2
Fair Value of private equity funds subject to lock-up periods	At September 30, 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		$	7.1		$	22.1		$	34.5		$	—		$	63.7
Fair Value Assets Measured on Recurring Basis Measurement Input Reconciliation [Table Text Block]	The changes in Level 1 fair value measurements for the three and nine months ended September 30, 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		(0.2		)	—			—			—			(0.2		)
	Net realized and unrealized gains (losses)		(0.1		)	24.6			—			—			24.5
	Purchases		59.3			102.4			—			—			161.7
	Sales		(98.8		)	(89.7		)	—			—			(188.5		)
	Transfers in		—			—			—			—			—
	Transfers out		—			—			—			—			—
	Balance at March 31, 2013		$	163.6		$	296.2		$	—		$	—		$	459.8
	Amortization/accretion		—			—			—			—			—
	Net realized and unrealized losses		(2.2		)	(1.2		)	—			—			(3.4		)
	Purchases		71.6			23.4			—			—			95
	Sales		(11.1		)	(19.9		)	—			—			(31.0		)
	Transfers in		—			—			—			—			—
	Transfers out		—			—			—			—			—
	Balance at June 30, 2013		$	221.9		$	298.5		$	—		$	—		$	520.4
	Amortization/accretion		—			—			—			—			—
	Net realized and unrealized gains (losses)		(0.1		)	18.6			—			—			18.5
	Purchases		30.1			42.7			—			—			72.8
	Sales		(112.6		)	(32.4		)	—			—			(145.0		)
	Transfers in		—			—			—			—			—
	Transfers out		—			—			—			—			—
	Balance at September 30, 2013		$	139.3		$	327.4		$	—		$	—		$	466.7
	_______________________________________________________________________________
	(1)	Excludes short-term investments which are deemed to have a Level 1 designation.
	The changes in Level 2 fair value measurements for the three and nine months ended September 30, 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		(3.1		)	—			—			—			(3.1		)
	Net realized and unrealized gains (losses)		(0.5		)	—			0.5			—			—
	Purchases		531.4			—			5			—			536.4
	Sales		(560.3		)	—			(29.6		)	—			(589.9		)
	Transfers in		—			—			—			—			—
	Transfer out		—			—			—			—			—
	Balance at March 31, 2013		$	1,619.40		$	—		$	38.5		$	—		$	1,657.90
	Amortization/accretion		(2.7		)	—			(0.1		)	—			(2.8		)
	Net realized and unrealized losses		(23.5		)	—			(2.6		)	—			(26.1		)
	Purchases		319			—			3.7			—			322.7
	Sales		(316.6		)	—			(9.7		)	—			(326.3		)
	Transfers in		1.8			—			—			—			1.8
	Transfer out		—			—			—			—			—
	Balance at June 30, 2013		$	1,597.40		$	—		$	29.8		$	—		$	1,627.20
	Amortization/accretion		(2.8		)	—			—			—			(2.8		)
	Net realized and unrealized gains (losses)		(2.8		)	—			1.1			—			(1.7		)
	Purchases		272.8			—			3.3			—			276.1
	Sales		(225.8		)	—			(2.3		)	—			(228.1		)
	Transfers in		13.8			—			—			—			13.8
	Transfer out		(61.3		)	—			—			—			(61.3		)
	Balance at September 30, 2013		$	1,591.30		$	—		$	31.9		$	—		$	1,623.20
	_______________________________________________________________________________
	-1	Excludes short-term investments which are deemed to have a Level 2 designation.
	The changes in Level 3 fair value measurements for the three and nine months ended September 30, 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		0.1			—			—			3.7			3.8
Purchases		1.8			—			—			1.8			3.6
Sales		—			—			—			(4.5		)	(4.5		)
Transfers in		—			—			—			—			—
Transfers out		—			—			—			—			—
Balance at March 31, 2013		$	78		$	0.1		$	—		$	123.7		$	201.8
Amortization/accretion		—			—			—			—			—
Net realized and unrealized gains (losses)		(0.2		)	—			—			4.2			4
Purchases		14			—			—			3.4			17.4
Sales		—			—			—			(6.7		)	(6.7		)
Transfers in		—			—			—			—			—
Transfers out		(1.8		)	—			—			—			(1.8		)
Balance at June 30, 2013		$	90		$	0.1		$	—		$	124.6		$	214.7
Amortization/accretion		—			—			—			—			—
Net realized and unrealized gains (losses)		(1.8		)	—			—			1.9			0.1
Purchases		—			—			—			1.3			1.3
Sales		(0.5		)	—			—			(1.8		)	(2.3		)
Transfers in		61.3			—			—			—			61.3
Transfers out		(13.8		)	—			—			—			(13.8		)
Balance at September 30, 2013		$	135.2		$	0.1		$	—		$	126		$	261.3
_______________________________________________________________________________
(1)	Excludes the carrying value of $20.2 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 nine months ended September 30, 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 $15.6 million for the nine months ended September 30, 2013 were comprised of two 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.

Debt_Tables

Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]
Schedule of debt outstanding	OneBeacon's debt outstanding as of September 30, 2013 and December 31, 2012 consisted of the following:
		September 30,			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

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 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)
	Three months ended September 30, 2013
	Earned premiums	$	132.2		$	146.7		$	—		$	278.9
	Loss and loss adjustment expense	(84.2		)	(83.6		)	—			(167.8		)
	Policy acquisition expenses	(27.4		)	(26.2		)	—			(53.6		)
	Other underwriting expenses	(21.2		)	(25.6		)	—			(46.8		)
	Total underwriting income (loss)	(0.6		)	11.3			—			10.7
	Net investment income	—			—			10.1			10.1
	Net realized and change in unrealized investment gains	—			—			17			17
	Net other revenues	—			0.6			4.9			5.5
	General and administrative expenses	—			(1.1		)	(3.0		)	(4.1		)
	Interest expense	—			—			(3.3		)	(3.3		)
	Pre-tax income (loss) from continuing operations	$	(0.6	)	$	10.8		$	25.7		$	35.9
	Three months ended September 30, 2012
	Earned premiums	$	157.4		$	136.5		$	—		$	293.9
	Loss and loss adjustment expense	(90.4		)	(74.3		)	—			(164.7		)
	Policy acquisition expenses	(40.3		)	(26.3		)	—			(66.6		)
	Other underwriting expenses	(21.9		)	(25.5		)	—			(47.4		)
	Total underwriting income	4.8			10.4			—			15.2
	Net investment income	—			—			12.8			12.8
	Net realized and change in unrealized investment gains	—			—			40			40
	Net other revenues (expenses)	0.1			(1.1		)	0.6			(0.4		)
	General and administrative expenses	—			(0.6		)	(3.8		)	(4.4		)
	Interest expense	—			—			(4.0		)	(4.0		)
	Pre-tax income from continuing operations	$	4.9		$	8.7		$	45.6		$	59.2
		Insurance Operations						Investing,
								Financing
								and
		Specialty			Specialty			Corporate			Consolidated
		Products			Industries
		($ in millions)
	Nine months ended September 30, 2013
	Earned premiums	$	426.9		$	419.3		$	—		$	846.2
	Loss and loss adjustment expense	(240.6		)	(233.1		)	—			(473.7		)
	Policy acquisition expenses	(84.3		)	(76.6		)	—			(160.9		)
	Other underwriting expenses	(72.5		)	(78.2		)	—			(150.7		)
	Total underwriting income	29.5			31.4			—			60.9
	Net investment income	—			—			30.9			30.9
	Net realized and change in unrealized investment gains	—			—			19.9			19.9
	Net other revenues	0.3			0.9			28.9			30.1
	General and administrative expenses	(0.1		)	(2.3		)	(8.6		)	(11.0		)
	Interest expense	—			—			(9.8		)	(9.8		)
	Pre-tax income from continuing operations	$	29.7		$	30		$	61.3		$	121
	Nine months ended September 30, 2012
	Earned premiums	$	449.2		$	396.8		$	—		$	846
	Loss and loss adjustment expense	(245.7		)	(206.8		)	—			(452.5		)
	Policy acquisition expenses	(111.6		)	(74.0		)	—			(185.6		)
	Other underwriting expenses	(67.7		)	(78.5		)	—			(146.2		)
	Total underwriting income	24.2			37.5			—			61.7
	Net investment income	—			—			41.5			41.5
	Net realized and change in unrealized investment gains	—			—			57.9			57.9
	Net other revenues (expenses)	0.4			(1.0		)	0.5			(0.1		)
	General and administrative expenses	—			(1.4		)	(8.3		)	(9.7		)
	Interest expense	—			—			(12.1		)	(12.1		)
	Pre-tax income from continuing operations	$	24.6		$	35.1		$	79.5		$	139.2
Schedule of selected balance sheet of the segments
		Insurance Operations						Investing,
								Financing
								and
		Specialty			Specialty			Corporate(1)			Consolidated
		Products			Industries
		($ in millions)
	30-Sep-13
	Total investments	$	—		$	—		$	2,293.70		$	2,293.70
	Reinsurance recoverables	32.4			53.7			—			86.1
	Deferred acquisition costs	56.5			51.2			—			107.7
	Unpaid loss and loss adjustment expense reserves	608.9			444			—			1,052.90
	Unearned premiums	282.6			270.1			—			552.7
	Debt	—			—			274.7			274.7
	31-Dec-12
	Total investments	$	—		$	—		$	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 investments excludes $239.7 million and $338.1 million of fixed maturity investments reclassified to assets held for sale as of September 30, 2013 and December 31, 2012.
Schedule of net written premiums, earned premiums and ratios for OneBeacon's insurance operations by segment and in total, as well as for the three major underwriting units within Specialty Insurance Operations	The following tables provide net written premiums, earned premiums and underwriting ratios for OneBeacon's insurance operations by reportable segment and in total, for the three and nine months ended September 30, 2013, and 2012:
		Insurance Operations
	($ in millions)	Specialty			Specialty			Consolidated
		Products			Industries
	Three months ended September 30, 2013
	Net written premiums	$	172		$	142.1		$	314.1
	Earned premiums	132.2			146.7			278.9
	Underwriting ratios:
	Loss and LAE	63.7		%	56.9		%	60.1		%
	Expense	36.7			35.4			36
	Total combined ratio	100.4		%	92.3		%	96.1		%
	Three months ended September 30, 2012
	Net written premiums	$	203.9		$	131.3		$	335.2
	Earned premiums	157.4			136.5			293.9
	Underwriting ratios:
	Loss and LAE	57.4		%	54.4		%	56		%
	Expense	39.5			37.9			38.8
	Total combined ratio	96.9		%	92.3		%	94.8		%
		Insurance Operations
	($ in millions)	Specialty			Specialty			Consolidated
		Products			Industries
	Nine months ended September 30, 2013
	Net written premiums	$	393.3		$	432.8		$	826.1
	Earned premiums	426.9			419.3			846.2
	Underwriting ratios:
	Loss and LAE	56.4		%	55.6		%	56		%
	Expense	36.7			36.9			36.8
	Total combined ratio	93.1		%	92.5		%	92.8		%
	Nine months ended September 30, 2012
	Net written premiums	$	510.3		$	420.1		$	930.4
	Earned premiums	449.2			396.8			846
	Underwriting ratios:
	Loss and LAE	54.7		%	52.1		%	53.5		%
	Expense	39.9			38.4			39.2
	Total combined ratio	94.6		%	90.5		%	92.7		%

Retirement_Plans_Tables

Retirement Plans (Tables)	9 Months Ended
	Sep. 30, 2013
Compensation and Retirement Disclosure [Abstract]
Schedule of components of net periodic benefit cost (income)	The components of net periodic benefit cost for the three and nine months ended September 30, 2013 and 2012 were as follows:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
			($ in millions)
	Service cost		$	0.2		$	0.2		$	0.6		$	0.6
	Interest cost		1.1			1.2			3.2			3.6
	Expected return on plan assets		(1.8		)	(1.7		)	(5.4		)	(5.1		)
	Amortization of unrecognized loss		0.2			0.2			0.7			0.6
	Net periodic pension income before settlement and special termination benefits		(0.3		)	(0.1		)	(0.9		)	(0.3		)
	Special termination benefits expense(1)		—			0.1			0.3			0.2
	Settlement expense		—			0.5			—			0.5
	Total net periodic benefit expense (income)		$	(0.3	)	$	0.5		$	(0.6	)	$	0.4
	_______________________________________________________________________________
	(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.

Employee_ShareBased_Incentive_1

Employee Share-Based Incentive Compensation Plans (Tables)	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2013 and 2012:
			Three months ended September 30,
			2013					2012
			Target OB		Accrued			Target OB		Accrued
			Performance		expense			Performance		expense
			Shares					Shares
			outstanding					outstanding
			($ in millions)
	Beginning of period		489,867		$	2.4		560,577		$	3.1
	Payments and deferrals(1)		—		—			—		—
	New awards		—		—			—		—
	Forfeitures and net change in assumed forfeitures		—		—			(3,354	)	—
	Expense (benefit) recognized		—		0.1			—		(2.2		)
	End of period		489,867		$	2.5		557,223		$	0.9
			Nine months ended September 30,
			2013					2012
			Target OB		Accrued			Target OB		Accrued
			Performance		expense			Performance		expense
			Shares					Shares
			outstanding					outstanding
			($ in millions)
	Beginning of period		563,190		$	1.2		642,667		$	9.7
	Payments and deferrals(1)		(238,658	)	—			(258,901	)	(7.8		)
	New awards		179,000		—			181,290		—
	Forfeitures and net change in assumed forfeitures		(13,665	)	(0.1		)	(7,833	)	—
	Expense recognized		—		1.4			—		(1.0		)
	End of period		489,867		$	2.5		557,223		$	0.9
	_______________________________________________________________________________
	(1)	No payments were made in 2013 for the 2010-2012 performance cycle as the performance factor was zero percent. 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 September 30, 2013 for each performance cycle:
			Target OB		Accrued
			Performance		expense
			Shares
			outstanding
			($ in millions)
	Performance cycle:
	2011 - 2013		142,138		$	0.5
	2012 - 2014		181,290		1.4
	2013 - 2015		179,000		0.7
	Subtotal		502,428		2.6
	Assumed forfeitures		(12,561	)	(0.1		)
	Total at September 30, 2013		489,867		$	2.5
Summary of restricted shares activity	The following summarizes restricted shares activity for the three and nine months ended September 30, 2013 and 2012:
			Three months ended September 30,
			2013					2012
			Restricted Shares		Unamortized Issue Date Fair Value			Restricted Shares		Unamortized Issue Date Fair Value
			($ in millions)
	Beginning of period		918,000		$	8		930,000		$	11.2
	New awards		—		—			—		—
	Forfeitures		(3,000	)	—			—		—
	Vested		—		—			—		—
	Expense recognized		—		(0.7		)	—		(0.8		)
	End of period		915,000		$	7.3		930,000		$	10.4
			Nine months ended September 30,
			2013					2012
			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		4.6
	Forfeitures		(3,000	)	—			—		—
	Vested		(9,000	)	—			—		—
	Expense recognized		—		(2.3		)	—		(1.9		)
	End of period		915,000		$	7.3		930,000		$	10.4

Earnings_per_Share_Tables

Earnings per Share (Tables)	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2013 and 2012:
			Three months ended						Nine months ended
			September 30,						September 30,
			2013			2012			2013			2012
	Earnings attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders		$	38.8		$	44.6		$	104.3		$	109.9
	Allocation of income for participating unvested restricted common shares		(0.4		)	(0.4		)	(1.0		)	(1.0		)
	Dividends paid on participating restricted common shares		(0.2		)	(0.2		)	(0.6		)	(0.6		)
	Total allocation to restricted common shares		(0.6		)	(0.6		)	(1.6		)	(1.6		)
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	38.2		$	44		$	102.7		$	108.3
	Undistributed net earnings (in millions):
	Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	38.2		$	44		$	102.7		$	108.3
	Dividends paid, net of restricted common share amounts		(19.8		)	(19.8		)	(59.5		)	(59.5		)
	Total undistributed net earnings, net of restricted common share amounts		$	18.4		$	24.2		$	43.2		$	48.8
	Earnings per share denominator—basic and diluted (in millions):
	Total weighted average common shares outstanding		95.4			95.4			95.4			95.3
	Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)	(0.9		)	(0.8		)
	Basic and diluted earnings per share denominator(2)		94.5			94.5			94.5			94.5
	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		$	0.41		$	0.47		$	1.09		$	1.15
	Dividends declared and paid		(0.21		)	(0.21		)	(0.63		)	(0.63		)
	Undistributed earnings		$	0.2		$	0.26		$	0.46		$	0.52
	_______________________________________________________________________________
	(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 the options were not included in the three and nine months ended September 30, 2012 as the exercise price was greater than the average market price of common shares. There were no options outstanding during the three and nine months ended September 30, 2013.

Discontinued_Operations_Tables

Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2013
Discontinued Operations and Disposal Groups [Abstract]
Schedule of discontinued operations balance sheet statement of operations and other disclosures	As of September 30, 2013 and December 31, 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 September 30, 2013 and December 31, 2012, respectively. The following summarizes the major categories of assets and liabilities associated with the business classified as held for sale:
			September 30,			December 31,
			2013			2012
			($ in millions)
	Investments		$	239.7		$	338.1
	Premiums receivable		9.8			11
	Reinsurance recoverable on unpaid losses(1)		1,666.50			1,840.80
	Reinsurance recoverable on paid losses		21.8			15.6
	Net deferred tax asset		3.2			5.1
	Other assets		16.2			16.2
	Total assets held for sale		$	1,957.20		$	2,226.80
	Unpaid loss and loss adjustment expense reserves(1)		$	1,797.10		$	2,052.60
	Unearned premiums		0.2			0.5
	Ceded reinsurance payable		12.8			21.9
	Other liabilities(2)		147.1			151.8
	Total liabilities held for sale		$	1,957.20		$	2,226.80
	_______________________________________________________________________________
	(1)	The September 30, 2013 and December 31, 2012 balances include the remaining purchase accounting fair value adjustments of $140.2 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of September 30, 2013 and December 31, 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,806.7 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,937.3 million and $2,202.7 million for each period.
	(2)	Other liabilities as of September 30, 2013 and December 31, 2012 include the accrual related to the pre-tax loss on sale of the Runoff Business of $140.7 million.
	The following summarizes the results of operations, including related income taxes associated with the business classified as discontinued operations:
			Three months ended							Nine months ended
			September 30,							September 30,
			2013			2012				2013			2012
			($ in millions)
	Net written premiums		$	1		$	(1.2	)		$	0.4		$	0.6
	Revenues
	Earned premiums		$	1.1		$	(0.4	)		$	0.7		$	10
	Other revenue		—			—				7.8			—
	Total revenues		1.1			(0.4		)		8.5			10
	Expenses
	Loss and loss adjustment expenses		0.1			27.7				7.6			48.4
	Policy acquisition expenses		0.1			(0.8		)		0.1			(1.3		)
	Other underwriting expenses (income)		0.4			(1.1		)		0.3			1.1
	Total expenses		0.6			25.8				8			48.2
	Pre-tax income (loss)		0.5			(26.2		)		0.5			(38.2		)
	Income tax benefit (expense)		(0.2		)	10.4				(0.2		)	13.4
	Income (loss) from discontinued operations, net of tax		0.3			(15.8		)		0.3			(24.8		)
	Loss from sale of discontinued operations, net of tax		—			(91.0		)		—			(91.0		)
	Net income (loss) from discontinued operations, net of tax		$	0.3		$	(106.8	)		$	0.3		$	(115.8	)
	The following table outlines the computation of income (loss) per share for discontinued operations attributable to OneBeacon's common shareholders for the three and nine months ended September 30, 2013 and 2012:
			Three months ended							Nine months ended
			September 30,							September 30,
			2013			2012				2013			2012
	Income (loss) attributable to OneBeacon's common shareholders—basic and diluted (in millions):
	Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders		$	0.3		$	(106.8	)		$	0.3		$	(115.8	)
	Allocation of loss for participating unvested restricted common shares		—			1				—			1
	Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders, net of restricted common share amounts		$	0.3		$	(105.8	)		$	0.3		$	(114.8	)
	Income (loss) per share denominator—basic and diluted (in millions):
	Total weighted average common shares outstanding		95.4			95.4				95.4			95.3
	Weighted average unvested restricted common shares(1)		(0.9		)	(0.9		)		(0.9		)	(0.9		)
	Basic and diluted income (loss) per share denominator(2)		94.5			94.5				94.5			94.4
	Income (loss) per share attributable to OneBeacon's common shareholders—basic and diluted (in dollars):
	Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders per share		$	—		$	(1.12	)		$	—		$	(1.22	)
	_______________________________________________________________________________
	(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 the options were not included in the three and nine months ended September 30, 2012 as their inclusion would be anti-dilutive. There were no options outstanding during the three and nine months ended September 30, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.
Listing of top reinsurers	The following table provides a listing of OneBeacon's top reinsurers for its continuing insurance operations, excluding industry pools and associations, based upon reinsurance recoverable amounts on paid and unpaid losses, the percentage of the total balance, and the reinsurer's A.M. Best Company, Inc. ("A.M. Best") ratings:
	($ in millions)		Balance at			% of total			A.M. Best
			30-Sep-13						Rating(1)
	Hannover Ruckversich		$	10.3		12	%		A+
	Munich Reinsurance America		6.6			8	%		A+
	Platinum Underwriters		5.6			7	%		A
	Hartford Steam Boiler		4.8			6	%		A++
	Swiss Reinsurance America Corp		4.6			5	%		A+
	_______________________________________________________________________________
	(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) and "A" (Excellent, which is the third highest of sixteen financial strength ratings).
	At September 30, 2013, OneBeacon had $21.8 million of reinsurance recoverable on paid losses and $1,806.7 million (gross of $140.2 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 $140.2 million in purchase accounting adjustment), and the reinsurers' A.M. Best ratings.
	($ in millions)		Balance at			% of total			A.M. Best
			30-Sep-13						Rating(1)
	National Indemnity Company ("NICO") and General Reinsurance Corporation(2)		$	1,292.20		71	%		A++
	Hanover Insurance Company		54.4			3	%		A
	Tokio Marine and Nichido Fire(3)		25.8			1	%		A++
	Munich Reinsurance America		19.9			1	%		A+
	Tower Insurance Company		13			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++” (Good, which is the fifth 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 $28.0 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire

Consolidating_Financial_Inform1

Consolidating Financial Information (Tables)	9 Months Ended
	Sep. 30, 2013
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Consolidating Balance Sheet	The following tables present OneBeacon's consolidating balance sheets as of September 30, 2013 and December 31, 2012 and statements of operations and comprehensive income and cash flows for the three and nine months ended September 30, 2013 and 2012. 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 September 30, 2013				(in millions)
	Assets
	Investment Securities:
	Fixed maturity investments, at fair value	$	—		$	1,586.20		$	64.6		$	(24.7	)	$	1,626.10
	Short-term investments, at amortized cost (which approximates fair value)	4.2			153.2			4.6			—			162
	Common equity securities, at fair value	—			327.5			—			—			327.5
	Convertible fixed maturity investments, at fair value	—			31.9			—			—			31.9
	Other investments	—			146.2			—			—			146.2
	Total investment securities	4.2			2,245.00			69.2			(24.7		)	2,293.70
	Cash	—			173.4			0.4			—			173.8
	Reinsurance recoverables	—			86.1			—			—			86.1
	Premiums receivable	—			255.9			—			—			255.9
	Deferred acquisition costs	—			107.7			—			—			107.7
	Net deferred tax asset	—			129.5			(2.0		)	(0.3		)	127.2
	Investment income accrued	—			11			0.1			(0.5		)	10.6
	Accounts receivable on unsettled investment sales	—			9.4			—			—			9.4
	Investments in subsidiaries	1,059.00			—			1,131.70			(2,190.7		)	—
	Other assets	(1.2		)	199.6			5.1			—			203.5
	Assets held for sale	—			1,957.20			—			—			1,957.20
	Total assets	$	1,062.00		$	5,174.80		$	1,204.50		$	(2,216.2	)	$	5,225.10
	Liabilities
	Unpaid loss and loss adjustment expense reserves	$	—		$	1,052.90		$	—		$	—		$	1,052.90
	Unearned premiums	—			552.7			—			—			552.7
	Debt	—			—			299.7			(25.0		)	274.7
	Accounts payable on unsettled investment purchases	—			5.9			—			—			5.9
	Other liabilities	(0.1		)	309.9			7.4			(0.5		)	316.7
	Liabilities held for sale	—			1,957.20			—			—			1,957.20
	Total liabilities	(0.1		)	3,878.60			307.1			(25.5		)	4,160.10
	OneBeacon's common shareholders' equity and noncontrolling interests
	Total OneBeacon's common shareholders' equity	1,062.10			1,293.30			897.4			(2,190.7		)	1,062.10
	Total noncontrolling interests	—			2.9			—			—			2.9
	Total OneBeacon's common shareholders' equity and noncontrolling interests	1,062.10			1,296.20			897.4			(2,190.7		)	1,065.00
	Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	$	1,062.00		$	5,174.80		$	1,204.50		$	(2,216.2	)	$	5,225.10
	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
	Three months ended September 30, 2013	(in millions)
	Revenues
	Earned premiums	$	—		$	278.9		$	—		$	—		$	278.9
	Net investment income (expense)	—			10.2			0.2			(0.3		)	10.1
	Net realized and change in unrealized investment gains	—			17.2			(0.4		)	0.2			17
	Net other revenues	—			5.5			—			—			5.5
	Total revenues	—			311.8			(0.2		)	(0.1		)	311.5
	Expenses
	Loss and loss adjustment expenses	—			167.8			—			—			167.8
	Policy acquisition expenses	—			53.6			—			—			53.6
	Other underwriting expenses	—			46.8			—			—			46.8
	General and administrative expenses	1.7			2.2			0.2			—			4.1
	Interest expense	—			—			3.5			(0.2		)	3.3
	Total expenses	1.7			270.4			3.7			(0.2		)	275.6
	Pre-tax (loss) income from continuing operations	(1.7		)	41.4			(3.9		)	0.1			35.9
	Income tax benefit (expense)	—			0.5			2.5			(0.1		)	2.9
	Net (loss) income from continuing operations	(1.7		)	41.9			(1.4		)	—			38.8
	Loss from discontinued operations, net of tax	—			0.3			—			—			0.3
	(Loss) income before equity in earnings of unconsolidated affiliates	(1.7		)	42.2			(1.4		)	—			39.1
	Equity in earnings of subsidiaries, net of tax	44.5			—			34			(78.5		)	—
	Net income (loss) including noncontrolling interests	42.8			42.2			32.6			(78.5		)	39.1
	Less: Net income attributable to noncontrolling interests	—			—			—			—			—
	Net income (loss) attributable to OneBeacon's common shareholders	42.8			42.2			32.6			(78.5		)	39.1
	Net change in benefit plan assets and obligations, net of tax	0.2			—			0.2			(0.2		)	0.2
	Comprehensive income (loss) attributable to OneBeacon's common shareholders	$	43		$	42.2		$	32.8		$	(78.7	)	$	39.3
	Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Three months ended September 30, 2012	(in millions)
	Revenues
	Earned premiums	$	—		$	293.9		$	—		$	—		$	293.9
	Net investment income	—			13.5			0.1			(0.8		)	12.8
	Net realized and change in unrealized investment gains	—			38.7			1			0.3			40
	Net other revenues (expenses)	—			(0.2		)	(0.2		)	—			(0.4		)
	Total revenues	—			345.9			0.9			(0.5		)	346.3
	Expenses
	Loss and loss adjustment expenses	—			164.7			—			—			164.7
	Policy acquisition expenses	—			66.6			—			—			66.6
	Other underwriting expenses	—			47.4			—			—			47.4
	General and administrative expenses	1.5			2.7			0.2			—			4.4
	Interest expense	—			(0.1		)	4.8			(0.7		)	4
	Total expenses	1.5			281.3			5			(0.7		)	287.1
	Pre-tax (loss) income from continuing operations	(1.5		)	64.6			(4.1		)	0.2			59.2
	Income tax (expense) benefit	—			(16.1		)	2			(0.1		)	(14.2		)
	Net (loss) income from continuing operations	(1.5		)	48.5			(2.1		)	0.1			45
	Loss from discontinued operations, net of tax	—			(15.8		)	—			—			(15.8		)
	Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
	Net loss before equity in earnings of unconsolidated affiliates	(1.5		)	(58.3		)	(2.1		)	0.1			(61.8		)
	Equity in loss of subsidiaries, net of tax	(60.7		)	—			(69.9		)	130.6			—
	Net loss including noncontrolling interests	(62.2		)	(58.3		)	(72.0		)	130.7			(61.8		)
	Less: Net income attributable to noncontrolling interests	—			(0.4		)	—			—			(0.4		)
	Net loss attributable to OneBeacon's common shareholders	(62.2		)	(58.7		)	(72.0		)	130.7			(62.2		)
	Net change in benefit plan assets and obligations, net of tax	(2.0		)	—			(2.0		)	2			(2.0		)
	Comprehensive loss attributable to OneBeacon's common shareholders	$	(64.2	)	$	(58.7	)	$	(74.0	)	$	132.7		$	(64.2	)
	Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Nine months ended September 30, 2013	(in millions)
	Revenues
	Earned premiums	$	—		$	846.2		$	—		$	—		$	846.2
	Net investment income (expense)	—			32.1			(0.3		)	(0.9		)	30.9
	Net realized and change in unrealized investment gains	—			19.7			(0.8		)	1			19.9
	Net other revenues	—			30.1						—			30.1
	Total revenues	—			928.1			(1.1		)	0.1			927.1
	Expenses
	Loss and loss adjustment expenses	—			473.7			—			—			473.7
Policy acquisition expenses	—			160.9			—			—			160.9
Other underwriting expenses	—			150.7			—			—			150.7
General and administrative expenses	3.6			7.5			(0.1		)	—			11
Interest expense	—			—			10.6			(0.8		)	9.8
Total expenses	3.6			792.8			10.5			(0.8		)	806.1
Pre-tax (loss) income from continuing operations	(3.6		)	135.3			(11.6		)	0.9			121
Income tax benefit (expense)	0.1			(23.3		)	7.7			(0.4		)	(15.9		)
Net (loss) income from continuing operations	(3.5		)	112			(3.9		)	0.5			105.1
Income from discontinued operations, net of tax	—			0.3			—			—			0.3
(Loss) income before equity in earnings of unconsolidated affiliates	(3.5		)	112.3			(3.9		)	0.5			105.4
Equity in earnings of subsidiaries, net of tax	111.8			—			91.6			(203.4		)	—
Net income including noncontrolling interests	108.3			112.3			87.7			(202.9		)	105.4
Less: Net income attributable to noncontrolling interests	—			(0.8		)	—			—			(0.8		)
Net income attributable to OneBeacon's common shareholders	108.3			111.5			87.7			(202.9		)	104.6
Net change in benefit plan assets and obligations, net of tax	0.5			—			0.5			(0.5		)	0.5
Comprehensive income attributable to OneBeacon's common shareholders	$	108.8		$	111.5		$	88.2		$	(203.4	)	$	105.1
Consolidating Statement of Operations and Comprehensive Income	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
Nine months ended September 30, 2012	(in millions)
Revenues
Earned premiums	$	—		$	846		$	—		$	—		$	846
Net investment income	—			43.8			0.1			(2.4		)	41.5
Net realized and change in unrealized investment gains	—			56.5			1			0.4			57.9
Net other revenues (expenses)	—			0.5			(0.6		)	—			(0.1		)
Total revenues	—			946.8			0.5			(2.0		)	945.3
Expenses
Loss and loss adjustment expenses	—			452.5			—			—			452.5
Policy acquisition expenses	—			185.6			—			—			185.6
Other underwriting expenses	—			146.2			—			—			146.2
General and administrative expenses	4.5			4.7			0.5			—			9.7
Interest expense	—			(0.1		)	14.4			(2.2		)	12.1
Total expenses	4.5			788.9			14.9			(2.2		)	806.1
Pre-tax (loss) income from continuing operations	(4.5		)	157.9			(14.4		)	0.2			139.2
Income tax (expense) benefit	—			(35.1		)	7.1			(0.1		)	(28.1		)
Net (loss) income from continuing operations	(4.5		)	122.8			(7.3		)	0.1			111.1
Loss from discontinued operations, net of tax	—			(24.8		)	—			—			(24.8		)
Loss from sale of discontinued operations, net of tax	—			(91.0		)	—			—			(91.0		)
(Loss) income before equity in earnings of unconsolidated affiliates	(4.5		)	7			(7.3		)	0.1			(4.7		)
Equity in loss of subsidiaries, net of tax	(1.4		)	—			(20.5		)	21.9			—
Net (loss) income including noncontrolling interests	(5.9		)	7			(27.8		)	22			(4.7		)
Less: Net income attributable to noncontrolling interests	—			(1.2		)	—			—			(1.2		)
Net (loss) income attributable to OneBeacon's common shareholders	(5.9		)	5.8			(27.8		)	22			(5.9		)
Net change in benefit plan assets and obligations, net of tax	(1.7		)	—			(1.7		)	1.7			(1.7		)
Comprehensive (loss) income attributable to OneBeacon's common shareholders	$	(7.6	)	$	5.8		$	(29.5	)	$	23.7		$	(7.6	)
Consolidating Statement of Cash Flows
	Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Nine months ended September 30, 2013	($ in millions)
	Cash flows from operations:
	Net income including noncontrolling interests	$	108.3		$	112.3		$	87.7		$	(202.9	)	$	105.4
	Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
	Undistributed earnings from subsidiaries	(111.8		)	—			(91.6		)	203.4			—
	Net income from discontinued operations	—			(0.3		)	—			—			(0.3		)
	Net loss from sale of discontinued operations	—			—			—			—			—
	Net realized and change in unrealized investment gains	—			(19.7		)	0.8			(1.0		)	(19.9		)
	Net other realized losses	—			—			—			—			—
	Net realized gain on sale of business	—			(23.0		)	—			—			(23.0		)
	Deferred income tax expense	—			12.5			(0.5		)	0.3			12.3
	Dividends received from subsidiaries	62			—			23			(85.0		)	—
	Other operating items:
	Net change in loss and LAE reserves	—			52.9			—			—			52.9
	Net change in unearned premiums	—			(21.1		)	—			—			(21.1		)
	Net change in premiums receivable	—			(30.3		)	—			—			(30.3		)
	Net change in reinsurance recoverable on paid and unpaid losses	—			24.5			—			—			24.5
	Net change in other assets and liabilities	3.1			70.4			4.1			0.2			77.8
	Net cash provided from operations—continuing operations	61.6			178.2			23.5			(85.0		)	178.3
	Net cash used for operations—discontinued operations	—			(98.0		)	—			—			(98.0		)
	Net cash provided from operations	61.6			80.2			23.5			(85.0		)	80.3
	Cash flows from investing activities:
	Net maturities, purchases and sales of short-term investments	(1.5		)	13.2			55.6			—			67.3
	Maturities of fixed maturity investments	—			185.3			11.3			—			196.6
	Sales of fixed maturity investments	—			1,114.60			129.7			(35.7		)	1,208.60
	Sales of common equity securities	—			142			—			—			142
	Sales of convertible fixed maturity investments	—			39.5			—			—			39.5
	Distributions and redemptions of other investments	—			13.1			—			—			13.1
	Purchases of fixed maturity investments	—			(1,227.8		)	(189.7		)	35.7			(1,381.8		)
	Purchases of common equity securities	—			(168.5		)	—			—			(168.5		)
	Purchases of convertible fixed maturity investments	—			(12.0		)	—			—			(12.0		)
	Contributions for other investments	—			(7.1		)	—			—			(7.1		)
	Proceeds from sale of business	—			31.3			—			—			31.3
	Net change in unsettled investment purchases and sales	—			(7.7		)	—			—			(7.7		)
	Net acquisitions of property and equipment	—			(7.6		)	—			—			(7.6		)
	Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			30			—			(30.0		)	—
	Net cash (used for) provided from investing activities—continuing operations	(1.5		)	138.3			6.9			(30.0		)	113.7
	Net cash provided from investing activities—discontinued operations	—			—			—			—			—
	Net cash (used for) provided from investing activities	(1.5		)	138.3			6.9			(30.0		)	113.7
	Cash flows from financing activities:
	Cash dividends paid to common shareholders	(60.1		)	—			—			—			(60.1		)
	Cash dividends paid to parent	—			(85.0		)	—			85			—
	Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—						(30.0		)	30			—
	Payments on capital lease obligation	—			(4.0		)	—			—			(4.0		)
	Net cash used for financing activities—continuing operations	(60.1		)	(89.0		)	(30.0		)	115			(64.1		)
	Net cash used for financing activities—discontinued operations	—			—			—			—			—
	Net cash used for financing activities	(60.1		)	(89.0		)	(30.0		)	115			(64.1		)
	Net increase in cash during period	—			129.5			0.4			—			129.9
	Cash reclassified to assets held for sale	—			—			—			—			—
	Cash transferred with sale of business	—			—			—			—			—
	Net increase after reclassification of cash to assets held for sale	—			129.5			0.4			—			129.9
	Cash balance at beginning of period	—			43.9			—			—			43.9
	Cash balance at end of period	$	—		$	173.4		$	0.4		$	—		$	173.8
	Consolidating Statement of Cash Flows	The Company (guarantor)			Non-guarantor subsidiaries			OBH (issuer)			Consolidating adjustments			Consolidated
	Nine months ended September 30, 2012	($ in millions)
	Cash flows from operations:
	Net (loss) income including noncontrolling interests	$	(5.9	)	$	7		$	(27.8	)	$	22		$	(4.7	)
	Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
	Undistributed loss from subsidiaries	1.4			—			20.5			(21.9		)	—
	Net loss from discontinued operations	—			24.8			—			—			24.8
	Net loss from sale of discontinued operations	—			91			—			—			91
	Net realized and change in unrealized investment gains	—			(56.5		)	(1.0		)	(0.4		)	(57.9		)
Net other realized losses	—			—			—			—			—
Net realized gain on sale of business	—			—			—			—			—
Deferred income tax expense	—			9.1			3.1			0.1			12.3
Dividends received from subsidiaries	62			15.5			101.1			(178.6		)	—
Other operating items:
Net change in loss and LAE reserves	—			(0.9		)	—			—			(0.9		)
Net change in unearned premiums	—			85.2			—			—			85.2
Net change in premiums receivable	—			(45.7		)	—			—			(45.7		)
Net change in reinsurance recoverable on paid and unpaid losses	—			64.4			—			—			64.4
Net change in other assets and liabilities	2.9			(36.3		)	0.9			0.2			(32.3		)
Net cash provided from operations—continuing operations	60.4			157.6			96.8			(178.6		)	136.2
Net cash used for operations—discontinued operations	—			(155.6		)	—			—			(155.6		)
Net cash provided from operations	60.4			2			96.8			(178.6		)	(19.4		)
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	(0.4		)	12.9			(35.2		)	—			(22.7		)
Maturities of fixed maturity investments	—			147.5			—			—			147.5
Sales of fixed maturity investments	—			1,306.30			—			(0.4		)	1,305.90
Sales of common equity securities	—			62.4			0.2			(25.4		)	37.2
Sales of convertible fixed maturity investments	—			23.5			0.1			(7.6		)	16
Distributions and redemptions of other investments	—			6.9			—			—			6.9
Purchases of fixed maturity investments	—			(1,317.2		)	(0.4		)	0.4			(1,317.2		)
Purchases of common equity securities	—			(48.5		)	(26.2		)	25.4			(49.3		)
Purchases of convertible fixed maturity investments	—			(6.7		)	(7.7		)	7.6			(6.8		)
Contributions for other investments	—			(3.1		)	—			—			(3.1		)
Proceeds from sale of business	—			—			—			—			—
Net change in unsettled investment purchases and sales	—			(69.1		)	—			—			(69.1		)
Net acquisitions of property and equipment	—			(1.2		)	—			—			(1.2		)
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			14.5			—			(14.5		)	—
Net cash (used for) provided from investing activities—continuing operations	(0.4		)	128.2			(69.2		)	(14.5		)	44.1
Net cash provided from investing activities—discontinued operations	—			—			—			—			—
Net cash (used for) provided from investing activities	(0.4		)	128.2			(69.2		)	(14.5		)	44.1
Cash flows from financing activities:
Cash dividends paid to common shareholders	(60.0		)	—			—			—			(60.0		)
Cash dividends paid to parent	—			(163.1		)	(15.5		)	178.6			—
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	—			—			(14.5		)	14.5			—
Payments on capital lease obligation	—			(3.6		)	—			—			(3.6		)
Proceeds on sale of OneBeacon Holdings (Luxembourg) S.à r.l.	—			14			—			—			14
Net cash used for financing activities—continuing operations	(60.0		)	(152.7		)	(30.0		)	193.1			(49.6		)
Net cash used for financing activities—discontinued operations	—			—			—			—			—
Net cash used for financing activities	(60.0		)	(152.7		)	(30.0		)	193.1			(49.6		)
Net (decrease) in cash during period	—			(22.5		)	(2.4		)	—			(24.9		)
Cash reclassified to 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	—			(20.5		)	(2.4		)	—			(22.9		)
Cash balance at beginning of period	—			52.5			2.4			—			54.9
Cash balance at end of period	$	—		$	32		$	—		$	—		$	32

Nature_of_Operations_and_Summa2

Nature of Operations and Summary of Significant Accounting Policies (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2011
Basis of presentation
Participation percentage in quota share reinsurance agreements			100.00%
Number of underwriting segments			14
Number of segements sold			1
Accounting Standards Update 2010-26
Basis of presentation
Unamortized discount					$5.60
Amortization expense	$0	$0.90	$0	$5.30
White Mountains Insurance Group Ltd
Basis of presentation
Ownership interest (as a percent)	75.20%		75.20%
Specialty Industries
Basis of presentation
Number of underwriting segments			6
Number of segements sold			1

Acqusitions_and_Dispositions_D

Acqusitions and Dispositions (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended	3 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jan. 02, 2013	Sep. 30, 2012	Sep. 30, 2012	Jan. 24, 2012	Sep. 30, 2012
					Essentia Insurance Company	Essentia Insurance Company	Essentia Insurance Company	OneBeacon Holdings (Luxembourg) S.a r.l.	OneBeacon Holdings (Luxembourg) S.a r.l.
Dispositions
Gain on sale of discontinued operation, pre-tax					$23
Loss from sale of discontinued operations, net of tax	0	-91	0	-91	15
Net written premiums						52.5	145.8
Premiums written, percentage of total						15.70%	15.70%
Proceeds from sale of subsidiary								24.7
Gain on sale of OneBeacon Holdings (Luxembourg) S.a r.l.									14
Gain on sale of subsidiary, net of transaction costs								$13.60

Reserves_for_Unpaid_Losses_and2

Reserves for Unpaid Losses and Loss Adjustment Expenses (Details) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Feb. 22, 2012		Sep. 30, 2013		Jun. 30, 2013		Dec. 31, 2012		Sep. 30, 2012
														AutoOne business		Runoff		Runoff		Runoff		Runoff
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross beginning balance			$911.60		$1,000		$3,358.60		$1,027.80			$3,175.30
Less beginning reinsurance recoverable on unpaid losses			-41.3		-107.3		-2,167.50		-81.8			-2,040.70
Loss and LAE incurred relating to Current year losses	164		167		473.8		460.1
Loss and LAE incurred relating to prior year losses	3.8		-2.3		-0.1		-7.6
Total incurred loss and LAE from continuing operations	167.8		164.7		473.7		452.5
Loss and LAE paid relating to Current year losses	-54.1		-59.6		-115.1		-123
Loss and LAE paid relating to Prior year losses	-87.1		-73.4		-278.7		-266.2
Total loss and LAE payments from continuing operations	-141.2		-133		-393.8		-389.2
Net loss and LAE reserves	972.6		1,166.30		972.6		1,254.40
Total incurred loss and LAE from discontinued operations	0		27.7		7.5		48.4
Total loss and LAE payments from discontinued operations	-17.2		-67.4		-88.6		-177.1
Net loss and LAE reserves	955.4		1,126.60		891.5		1,125.70
Net change in loss and LAE reserves reported in liabilities held for sale	17.3	[1],[2]	-256.3	[1],[2]	81.2	[1],[2]	-191.6	[1],[2]
Net loss and LAE reserves sold	0		0		0		-63.8
Net ending loss and LAE reserves	972.7				972.7				946	892.7	870.3	1,134.60	1,191.10
Plus ending reinsurance recoverable on unpaid losses	80.2				80.2				81.8			2,040.70
Gross ending balance	1,052.90				1,052.90				1,027.80			3,175.30
Loss and Loss Adjustment Expense Reserves Reclassified to from Held For Sale														($64.70)	[1],[2]	($130.60)	[1],[2]	($147.90)	[1],[2]	$211.80	[1],[2]	$256.30	[1],[2]
[1]	Consists of the net loss and LAE reserves of $256.3 million recorded in the nine months ended September 30, 2012 in connection with the Runoff Transaction, offset by net loss and LAE reserves of $64.7 million reclassified from held for sale upon the closing of the AutoOne Transaction in February 2012.
[2]	Consists of the change in net loss and LAE reserves from $147.9 million to $130.6 million, which amounts were classified as held for sale as of June 30, 2013 and SeptemberB 30, 2013, respectively, in connection with the Runoff Transaction.

Reinsurance_Details

Reinsurance (Details) (USD $)

Sep. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Jun. 30, 2012

Mar. 31, 2012

Dec. 31, 2011

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

In Millions, unless otherwise specified

AM Best, A Plus Plus Rating

AM Best, A Plus Plus Rating

AM Best, A Plus Plus Rating

AM Best, A Plus Plus Rating

AM Best, A Rating

Property Catastrophe Reinsurance

Property Per Risk Reinsurance Program

Property Per-Risk, Acts of Terrorism

Property Per-Risk, Acts of Terrorism

Casualty Reinsurance Program

Casualty Reinsurance Program

Layer One

Layer One

Layer One

Layer One

Layer One

Layer One

Layer Two

Layer Two

Layer Two

Layer Two

Layer Two

Layer Three

Layer Three

Layer Three

IMU Treaty

IMU Treaty

IMU Treaty

Surety Product Line

Surety Product Line

Surety Product Line

Surety Product Line

Healthcare Professional Liability

Healthcare Professional Liability

Healthcare Professional Liability

Healthcare Professional Liability

Casualty Business

Casualty Business

Casualty Business

Casualty Business

Casualty Business

Casualty Business

Casualty Business

Casualty Business

Workers Compensation Losses

Workers Compensation Losses

Workers Compensation Losses

Segment, Discontinued Operations

Segment, Discontinued Operations

Segment, Discontinued Operations

Hannover Ruckversich

Munich Reinsurance America

Swiss Reinsurance America Corp

Hartford Steam Boiler

Platinum Underwriters

Minimum

Minimum

Maximum

Minimum

Maximum

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Per Risk Reinsurance Program

Property Per Risk Reinsurance Program

Property Per Risk Reinsurance Program

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Per Risk Reinsurance Program

Property Per Risk Reinsurance Program

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Layer One

Layer One

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Casualty Reinsurance Program

Casualty Reinsurance Program

Layer One

Layer One

Casualty Reinsurance Program

Casualty Reinsurance Program

Layer One

Layer One

Layer One

Layer Two

Layer Two

Layer Two

Casualty Reinsurance Program

Casualty Reinsurance Program

Casualty Reinsurance Program

Munich Reinsurance America

Minimum

Maximum

Minimum

Maximum

Minimum

Maximum

Minimum

Minimum

Maximum

Property Catastrophe Reinsurance

Property Catastrophe Reinsurance

Minimum

Maximum

Minimum

Maximum

Casualty Reinsurance Program

Casualty Reinsurance Program

Minimum

Maximum

Casualty Reinsurance Program

Casualty Reinsurance Program

Casualty Reinsurance Program

Casualty Reinsurance Program

Casualty Reinsurance Program

Casualty Reinsurance Program

Minimum

Maximum

Minimum

Maximum

Minimum

Minimum

Maximum

Minimum

Maximum

Reinsurance Retention Policy [Line Items]

Reinsurance amount retained

$20

$70

$150

Property catastrophe reinsurance program second layer

130

10

10

100

20

30

20

40

30

70

40

60

2

5

5

30

5

20

5

5

5

21

5

6

10

11

1

10

Reinsurance reinsured risk, percentage

50.00%

5.00%

10.00%

10.00%

5.00%

0.00%

17.00%

10.00%

10.00%

5.00%

100.00%

Property catastrophe treaty aggregate deductible for large losses

1.5

Property Catastrophe Treaty Aggregate Deductible for Catastrophic Losses

5

Reinsurance amount reinsured for more than one insured

10

60

Reinsurance recoverable on paid losses

5.9

Reinsurance recoverable on unpaid losses

80.2

81.8

107.3

41.3

2,040.70

2,167.50

1,806.70

1,990.90

Reinsurance recoverables

$86.10

$110.60

$10.30

$6.60

$4.60

$4.80

$5.60

$19.90

Percentage of total paid and unpaid reinsurance recoverables

12.00%

8.00%

5.00%

6.00%

7.00%

1.00%

Number of ratings in A.M. Best ratings

16

Percent of reinsurance receoverables rated A or better

90.00%

Investment_Securities_Net_inve

Investment Securities (Net investment income) (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross investment income	$11.70	$14.40	$36	$46.50
Less investment expenses	-1.6	-1.6	-5.1	-5
Net investment income	10.1	12.8	30.9	41.5
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross investment income	10.4	12	30.7	38.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross investment income	0	0	0	0.1
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross investment income	1.5	1.5	4.4	4.8
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross investment income	0.2	1.1	0.9	3.4
Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross investment income	($0.40)	($0.20)	$0	($0.40)

Investment_Securities_Net_real

Investment Securities (Net realized investment gains (losses)) (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	($4.20)	$13.70	$25.90	$30.10
Accounts payable on unsettled investment purchases	5.9		5.9		6.2
Accounts receivable on unsettled investment sales	9.4		9.4		2.1
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	-5	10.6	2.8	26.7
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	0	0	0.1	0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	2.7	2.9	20.6	-0.2
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	0	0.6	-0.4	2
Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	($1.90)	($0.40)	$2.80	$1.60

Investment_Securities_Changes_

Investment Securities (Changes in fair value) (Details) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	$21	[1]	$26.10	[2]	($6.10)	[1]	$27.60	[2]
Changes in net foreign currency translation gains (losses)	0.2	[1]	0.2	[2]	0.1	[1]	0.2	[2]
Total net changes in fair value reflected in revenues	21.2	[1]	26.3	[2]	-6	[1]	27.8	[2]
Change in deferred gains (losses)	-0.7		0		-0.8		-0.2
Fixed maturity investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	0.5	[1]	5.9	[2]	-33.7	[1]	8.6	[2]
Changes in net foreign currency translation gains (losses)	0.1	[1]	0.2	[2]	0	[1]	0.2	[2]
Total net changes in fair value reflected in revenues	0.6	[1]	6.1	[2]	-33.7	[1]	8.8	[2]
Short-term investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	0	[1]	0	[2]	0	[1]	0	[2]
Changes in net foreign currency translation gains (losses)	0	[1]	0	[2]	0	[1]	0	[2]
Total net changes in fair value reflected in revenues	0	[1]	0	[2]	0	[1]	0	[2]
Common equity securities
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	15.8	[1]	18.7	[2]	21.3	[1]	17	[2]
Changes in net foreign currency translation gains (losses)	0.1	[1]	0	[2]	0.1	[1]	0	[2]
Total net changes in fair value reflected in revenues	15.9	[1]	18.7	[2]	21.4	[1]	17	[2]
Convertible fixed maturity investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	1	[1]	-0.6	[2]	-0.6	[1]	-2	[2]
Changes in net foreign currency translation gains (losses)	0	[1]	0	[2]	0	[1]	0	[2]
Total net changes in fair value reflected in revenues	1	[1]	-0.6	[2]	-0.6	[1]	-2	[2]
Other investments
Total net changes in fair value reflected in revenues
Changes in net unrealized investment gains (losses)	3.7	[1]	2.1	[2]	6.9	[1]	4	[2]
Changes in net foreign currency translation gains (losses)	0	[1]	0	[2]	0	[1]	0	[2]
Total net changes in fair value reflected in revenues	$3.70	[1]	$2.10	[2]	$6.90	[1]	$4	[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 $(0.7) million and $(0.8) million for the three and nine months ended SeptemberB 30, 2013.
[2]	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.2) million for the nine months ended September 30, 2012. There were no such changes in net deferred gains and losses for the three months ended September 30, 2012.

Investment_Securities_Net_unre

Investment Securities (Net unrealized investment gains and losses) (Details) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Investments [Abstract]
Gross unrealized investment gains	$124.60	$127.40
Gross unrealized investment losses	-11.4	-8.7
Gross unrealized investment losses	113.2	118.7
Income taxes	-36.7	-39.4
Total net unrealized investment gains, after tax	$76.50	$79.30

Investment_Securities_Fixed_ma

Investment Securities (Fixed maturity investment gains and losses) (Details) (USD $)	9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross unrealized gains	$124.60		$127.40
Gross unrealized investment losses	-11.4		-8.7
Carrying value	1,626.10		1,593.30
Total investments	2,293.70		2,291.50
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	137.4	[1]	197.1	[1]
Gross unrealized gains	0.2	[1]	0.5	[1]
Gross unrealized investment losses	0	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	137.6	[1]	197.6	[1]
Debt securities issued by industrial corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	717.3	[1]	678.7	[1]
Gross unrealized gains	16.3	[1]	32.8	[1]
Gross unrealized investment losses	-1.4	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	732.2	[1]	711.5	[1]
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	3.3	[1]	3.3	[1]
Gross unrealized gains	0	[1]	0	[1]
Gross unrealized investment losses	-0.3	[1]	-0.1	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	3	[1]	3.2	[1]
Asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	911.1	[1]	918.5	[1]
Gross unrealized gains	1.3	[1]	9.7	[1]
Gross unrealized investment losses	-4.5	[1]	-0.3	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	907.9	[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	0.4	[1]	0.5	[1]
Gross unrealized investment losses	0	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	2.4	[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	4.4	[1]	6.4	[1]
Gross unrealized investment losses	0	[1]	0	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	82.7	[1]	84.7	[1]
Total fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,849.40	[1]	1,881.90	[1]
Gross unrealized gains	22.6	[1]	49.9	[1]
Gross unrealized investment losses	-6.2	[1]	-0.4	[1]
Net foreign currency losses	0	[1]	0	[1]
Carrying value	1,865.80	[1]	1,931.40	[1]
Segment, Discontinued Operations | Runoff
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total investments	$239.70		$338.10
[1]	Includes carrying value of $239.7 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 September 30, 2013 and December 31, 2012, respectively.

Investment_Securities_Converti

Investment Securities (Convertible fixed maturity investments and other investments) (Details) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Gross unrealized gains	$124.60	$127.40
Trading Securities, Unrealized Holding Loss	11.4	8.7
Carrying value	1,626.10	1,593.30
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	268.6	221.6
Gross unrealized gains	59.3	39.8
Trading Securities, Unrealized Holding Loss	0.5	2.4
Net foreign currency gains	0.1	0
Carrying value	327.5	259
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	29.4	59.5
Gross unrealized gains	2.6	3.2
Trading Securities, Unrealized Holding Loss	0.1	0.1
Net foreign currency gains	0	0
Carrying value	31.9	62.6
Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	110.7	115.1
Gross unrealized gains	40.1	34.5
Trading Securities, Unrealized Holding Loss	4.6	5.8
Net foreign currency gains	0	0
Carrying value	146.2	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	408.7	396.2
Gross unrealized gains	102	77.5
Trading Securities, Unrealized Holding Loss	5.2	8.3
Net foreign currency gains	0.1	0
Carrying value	$505.60	$465.40

Investment_Securities_Fair_val

Investment Securities (Fair value measurements) (Details) (USD $)	Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of investments based on observable inputs	90.00%						92.00%
Community reinvestment vehicle	$14.10						$14.10
Tax development fund	20.2						21.1
Debt securities issued by corporations	146.2						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	466.7	[1]	520.4	[1]	459.8	[1]	462.3	[1]
Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	1,623.20	[2]	1,627.20	[2]	1,657.90	[2]	1,714.50	[2]
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	261.3	[3]	214.7	[3]	201.8	[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	48.2						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	63.7						61.3
Single Fund
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	14.3						12.9
Debt securities issued by industrial corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	732.2						711.5
Asset-backed securities | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	907.9						927.9
Asset-backed securities | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	843.3						922.6
Asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	64.6						5.3
Fixed maturity investments | Level 1 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	139.3		221.9		163.6		203.4
Fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	1,591.30		1,597.40		1,619.40		1,651.90
Fixed maturity investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	135.2		90		78		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		0		0
Convertible fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	31.9		29.8		38.5		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		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		0		0
Other investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	0		0		0		0
Other investments | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	126	[3]	124.6	[3]	123.7	[3]	122.7	[3]
Recurring basis | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	2,513.20	[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	627.6	[5]					695.1	[5]
Recurring basis | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	1,624.30	[5]					1,714.50	[5]
Recurring basis | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	261.3	[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	137.6	[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	137.6						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	732.2	[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	732.2						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	214.5	[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	214.5						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	95.6	[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	95.6						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	138.6	[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	138.6						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	92.6	[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	92.6						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	40.9	[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	40.9						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	80	[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	80						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	41.1	[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	41.1						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	28.9	[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	28.9						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	3	[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	3						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	907.9	[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	843.3						922.6
Recurring basis | Asset-backed securities | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	64.6						5.3
Recurring basis | Foreign government obligations | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	2.4	[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	1.7						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	82.7	[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	12.1						13.9
Recurring basis | Preferred stocks | Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	70.6						70.8
Recurring basis | Fixed maturity investments | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	1,865.80	[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	139.3						203.4
Recurring basis | Fixed maturity investments | Level 2 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	1,591.30						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	135.2						76.1
Recurring basis | Short-term investments | Fair Value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value of investments	162	[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	160.9						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	327.5	[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	327.4						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	114.3	[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	114.3						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	77	[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	76.9						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	37.3	[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	37.3						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	24.5	[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	24.5						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	8.4	[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	8.4						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	66	[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	66						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	31.9	[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	31.9						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	126	[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	126	[5]					122.7	[5]
Runoff | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities reclassified to assets held for sale	$239.70						$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 $20.2 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 $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$732.20	$711.50
AA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	46.5	37
A
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	353.4	301.6
BBB
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	324.2	362.5
BB
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	6.9	7
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$1.20	$3.40

Investment_Securities_Fair_val1

Investment Securities (Fair value rollforward) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013
Level 1 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value beginning balance	$520.40	[1]	$459.80	[1]	$462.30	[1]
Amortization/accretion	0	[1]	0	[1]	-0.2	[1]
Total net realized and unrealized gains (losses)	18.5	[1]	-3.4	[1]	24.5	[1]
Purchases	72.8	[1]	95	[1]	161.7	[1]
Sales	-145	[1]	-31	[1]	-188.5	[1]
Transfers in	0	[1]	0	[1]	0	[1]
Transfers out	0	[1]	0	[1]	0	[1]
Investments at fair value ending balance	466.7	[1]	520.4	[1]	459.8	[1]
Level 2 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value beginning balance	1,627.20	[2]	1,657.90	[2]	1,714.50	[2]
Amortization/accretion	-2.8	[2]	-2.8		-3.1	[2]
Total net realized and unrealized gains (losses)	-1.7	[2]	-26.1		0	[2]
Purchases	276.1	[2]	322.7		536.4	[2]
Sales	-228.1	[2]	-326.3		-589.9	[2]
Transfers in	13.8	[2]	1.8		0	[2]
Transfers out	61.3	[2]	0		0	[2]
Investments at fair value ending balance	1,623.20	[2]	1,627.20	[2]	1,657.90	[2]
Level 3 Inputs
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Investments at fair value beginning balance	214.7	[3]	201.8	[3]	198.9	[3]
Amortization/accretion	0	[3]	0		0	[3]
Total net realized and unrealized gains (losses)	0.1	[3]	4	[3]	3.8	[3]
Purchases	1.3	[3]	17.4		3.6	[3]
Sales	-2.3	[3]	-6.7		-4.5	[3]
Transfers in	61.3	[3]	0		0	[3]
Transfers out	13.8	[3]	1.8		0	[3]
Investments at fair value ending balance	261.3	[3]	214.7	[3]	201.8	[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	221.9		163.6		203.4
Amortization/accretion	0		0		-0.2
Total net realized and unrealized gains (losses)	-0.1		-2.2		-0.1
Purchases	30.1		71.6		59.3
Sales	-112.6		-11.1		-98.8
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	139.3		221.9		163.6
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	1,597.40		1,619.40		1,651.90
Amortization/accretion	-2.8		-2.7		-3.1
Total net realized and unrealized gains (losses)	-2.8		-23.5		-0.5
Purchases	272.8		319		531.4
Sales	-225.8		-316.6		-560.3
Transfers in	13.8		1.8		0
Transfers out	61.3		0		0
Investments at fair value ending balance	1,591.30		1,597.40		1,619.40
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	90		78		76.1
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	-1.8		-0.2		0.1
Purchases	0		14		1.8
Sales	-0.5		0		0
Transfers in	61.3		0		0
Transfers out	13.8		1.8		0
Investments at fair value ending balance	135.2		90		78
Transfers out recategorized	-15.6
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	298.5		296.2		258.9
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	18.6		-1.2		24.6
Purchases	42.7		23.4		102.4
Sales	-32.4		-19.9		-89.7
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	327.4		298.5		296.2
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		0		0
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	0		0		0
Purchases	0		0		0
Sales	0		0		0
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	0		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		0.1		0.1
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	0		0		0
Purchases	0		0		0
Sales	0		0		0
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	0.1		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		0		0
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	0		0		0
Purchases	0		0		0
Sales	0		0		0
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	0		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	29.8		38.5		62.6
Amortization/accretion	0		-0.1		0
Total net realized and unrealized gains (losses)	1.1		-2.6		0.5
Purchases	3.3		3.7		5
Sales	-2.3		-9.7		-29.6
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	31.9		29.8		38.5
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		0		0
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	0		0		0
Purchases	0		0		0
Sales	0		0		0
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	0		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		0		0
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	0		0		0
Purchases	0		0		0
Sales	0		0		0
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	0		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		0		0
Amortization/accretion	0		0		0
Total net realized and unrealized gains (losses)	0		0		0
Purchases	0		0		0
Sales	0		0		0
Transfers in	0		0		0
Transfers out	0		0		0
Investments at fair value ending balance	0		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	124.6	[3]	123.7	[3]	122.7	[3]
Amortization/accretion	0	[3]	0		0	[3]
Total net realized and unrealized gains (losses)	1.9	[3]	4.2		3.7	[3]
Purchases	1.3	[3]	3.4		1.8	[3]
Sales	-1.8	[3]	-6.7		-4.5	[3]
Transfers in	0	[3]	0		0	[3]
Transfers out	0	[3]	0		0	[3]
Investments at fair value ending balance	$126	[3]	$124.60	[3]	$123.70	[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 $20.2 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 $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Significant unobservable inputs
Number of Securities in each Asset type	1
Standard & Poor's, AA+ Rating [Member] | Asset-backed securities
Significant unobservable inputs
Fair Value	$59.30
Default rate	0.70%
Standard Poors N R Rating [Member] | Total Non-agency | Standard & Poor's, AA+ Rating [Member]
Significant unobservable inputs
Fair Value	5.3
Standard Poors N R Rating [Member] | Preferred stocks | Discounted cash flow
Significant unobservable inputs
Fair Value	$70.60
Discount Rate	7.80%	[1]
[1]	As of September 30, 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 $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	$21	[1]	$26.10	[2]	($6.10)	[1]	$27.60	[2]
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0.5	[1]	5.9	[2]	-33.7	[1]	8.6	[2]
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0	[1]	0	[2]	0	[1]	0	[2]
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	1	[1]	-0.6	[2]	-0.6	[1]	-2	[2]
Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	3.7	[1]	2.1	[2]	6.9	[1]	4	[2]
Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	1.9		2.4		5.1		10.5
Level 3 | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	-1.8		0.4		-1.8		6.6
Level 3 | Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	0		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		-0.1		0		-0.1
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		0
Level 3 | Other investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on unsold securities	$3.70		$2.10		$6.90		$4
[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 $(0.7) million and $(0.8) million for the three and nine months ended SeptemberB 30, 2013.
[2]	Includes pre-tax changes in net deferred gains and losses on sales of investments between OneBeacon and entities under White Mountains' common control of $(0.2) million for the nine months ended September 30, 2012. There were no such changes in net deferred gains and losses for the three months ended September 30, 2012.

Investment_Securities_Asset_ba

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

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

In Millions, unless otherwise specified

Level 2

Level 2

Level 2

Level 2

Level 3

Level 3

Level 3

Level 3

Fixed rate CMBS

Fixed rate CMBS

Floating 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]

Average basis points of subordination

30

30

Percentage of underlying loans reported as non-performing

1.00%

Debt securities issued by corporations

$1,623.20

[1]

$1,627.20

[1]

$1,657.90

[1]

$1,714.50

[1]

$261.30

[2]

$214.70

[2]

$201.80

[2]

$198.90

[2]

$116.60

$44

$342

$551.20

$282.70

$551.20

$59.30

$0

$2.70

$13.90

$2.70

$13.90

$0

$0

$8.50

$10.50

$8.50

$10.50

$0

$0

$353.20

[3]

$575.60

[3]

$293.90

[3]

$575.60

[3]

$59.30

[3]

$0

[3]

$52.20

$52.20

$38.10

$52.20

$38.10

$0

$0

$160.60

$175.40

$160.60

$175.40

$0

$0

$212.80

$213.50

$212.80

$213.50

$0

$0

$566

$789.10

$506.70

$789.10

$59.30

$0

$122.40

$49

$117.10

$43.70

$5.30

$5.30

$219.50

$81.50

$219.50

$81.50

$0

$0

$0

$8.30

$0

$8.30

$0

$0

$341.90

$138.80

$336.60

$133.50

$5.30

$5.30

$907.90

$927.90

$843.30

$922.60

$64.60

$5.30

$8.70

[1]

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

[2]

Excludes the carrying value of $20.2 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 $)	Sep. 30, 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	$52.20
Super Senior | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	13.2	[1]
Senior | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	39	[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	52.2		38.1
Fair Value | Non-agency CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	160.6		175.4
Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	212.8		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	116.6
Fair Value | Commercial Mortgage Backed Securities Floating Rate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	44
Fair Value | Super Senior | Non-agency CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	89.4	[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	86.5	[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	2.9	[4]
Fair Value | Senior | Non-agency CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	42.9	[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	25.4	[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	17.5	[2]
Fair Value | Subordinate | Non-agency CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	28.3	[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	23.6	[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	7.2
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	7.2
Securities Issued in 2005 | Fair Value | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	7.8
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	7.8
Securities Issued in 2006 | Fair Value | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	5.3
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	5.3
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	2.9
Securities issued in 2007 | Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	2.9
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	17.5
Securities Issued in 2008 | Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	17.5
Securities issued in 2010 | Fair Value | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	12.9
Securities issued in 2010 | Fair Value | Non-agency CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	6
Securities issued in 2010 | Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	18.9
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	16.3
Securities issued in 2011 | Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	16.3
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	83.7
Securities issued in 2012 | Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	83.7
Securities Issued in 2013 | Fair Value | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	19
Securities Issued in 2013 | Fair Value | Non-agency CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	34.2
Securities Issued in 2013 | Fair Value | Total
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	53.2
Prime | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	43.5
Prime | Super Senior | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	13.2	[1]
Prime | Senior | Non-agency RMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	30.3	[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	8.7
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	8.7	[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 Financial ServicesB LLC ("StandardB & Poor's") or Aaa by Moody's Investors Service,B Inc. ("Moody's") 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 or Aaa by Moody's and were senior to non-AAA or non-Aaa bonds.
[3]	At issuance, Subordinate were not rated AAA by StandardB & Poor's or Aaa by Moody's 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 Ratings and were senior to other AAA or Aaa bonds.

Investment_Securities_Other_in

Investment Securities (Other investments) (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Trading Securities and Other Trading Assets [Line Items]
Community reinvestment vehicle	$14.10		$14.10
Investment in tax advantaged federal affordable housing development fund	20.2		21.1
Long/short credit and distressed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	8		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	38		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	48.2		47.3
Distributions from inactive hedge funds	1.2
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	26.3
Hedge funds | 60 - 89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	8
Hedge funds | 90 - 119 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	8.2
Hedge funds | 120+ days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	5.7
Hedge funds | Monthly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	5.6
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	5.6
Hedge funds | Quarterly
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	40.4
Hedge funds | Quarterly | 30 - 59 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	26.3
Hedge funds | Quarterly | 60 - 89 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	8
Hedge funds | Quarterly | 90 - 119 days notice
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	6.1
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	2.4		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	4.7		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	6.1		7.5
Healthcare
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	3.5		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	16.2		14.9
Net asset value per share of unfunded commitments	2.6		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		2
Real estate
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	4.5		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	63.7		61.3
Net asset value per share of unfunded commitments	12.3		16.1
Private equity funds | 1 - 3 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	7.1
Private equity funds | 3 - 5 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	22.1
Private equity funds | 5 - 10 years
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other investments at fair value	34.5
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	111.9	[1]	108.6	[1]
Net asset value per share of unfunded commitments	$12.30	[1]	$16.10	[1]
[1]	Other investments also includes $14.1 million of an investment in a community reinvestment vehicle as of SeptemberB 30, 2013 and DecemberB 31, 2012, and $20.2 million and $21.1 million, of an investment in a tax advantaged federal affordable housing development fund as of SeptemberB 30, 2013 and DecemberB 31, 2012, respectively.

Debt_Details

Debt (Details) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012	Nov. 30, 2012	Sep. 30, 2013	Dec. 31, 2012	Nov. 30, 2011
In Millions, unless otherwise specified			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
					OBH (issuer)	OBH (issuer)	OBH (issuer)	OBH (issuer)	OBH (issuer)	OBH (issuer)
Debt
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
Percentage of par value at which debt was issued							99.90%
Issuance of debt, net of debt issuance costs							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									$275.90

Segment_Information_Details

Segment Information (Details) (USD $)	3 Months Ended		9 Months Ended							9 Months Ended			3 Months Ended			9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012
			segment							Underwriting Segment [Member]	Specialty Insurance Operations	Specialty Insurance Operations	Specialty Products	Specialty Products	Specialty Products	Specialty Products	Specialty Products	Specialty Industries	Specialty Industries	Specialty Industries	Specialty Industries	Specialty Industries	Investing, Financing and Corporate	Investing, Financing and Corporate	Investing, Financing and Corporate	Investing, Financing and Corporate	Investing, Financing and Corporate	Runoff	Runoff
										segment	segment	segment		segment						segment								Fixed maturity investments	Fixed maturity investments
Segment Information
Securities reclassified to assets held for sale																												$239.70	$338.10
Earned premiums	278.9	293.9	846.2	846									132.2		157.4	426.9	449.2	146.7	136.5	419.3	396.8		0	0	0	0
Loss and loss adjustment expense	-167.8	-164.7	-473.7	-452.5									-84.2		-90.4	-240.6	-245.7	-83.6	-74.3	-233.1	-206.8		0	0	0	0
Policy acquisition expenses	-53.6	-66.6	-160.9	-185.6									-27.4		-40.3	-84.3	-111.6	-26.2	-26.3	-76.6	-74		0	0	0	0
Other underwriting expenses	-46.8	-47.4	-150.7	-146.2									-21.2		-21.9	-72.5	-67.7	-25.6	-25.5	-78.2	-78.5		0	0	0	0
Total underwriting income (loss)	10.7	15.2	60.9	61.7									-0.6		4.8	29.5	24.2	11.3	10.4	31.4	37.5		0	0	0	0
Net investment income	10.1	12.8	30.9	41.5									0		0	0	0	0	0	0	0		10.1	12.8	30.9	41.5
Net realized and change in unrealized investment gains	17	40	19.9	57.9									0		0	0	0	0	0	0	0		17	40	19.9	57.9
Net other revenues	5.5	-0.4	30.1	-0.1									0		0.1	0.3	0.4	0.6	-1.1	0.9	-1		4.9	0.6	28.9	0.5
General and administrative expenses	-4.1	-4.4	-11	-9.7									0		0	-0.1	0	-1.1	-0.6	-2.3	-1.4		-3	-3.8	-8.6	-8.3
Interest expense	-3.3	-4	-9.8	-12.1									0		0	0	0	0	0	0	0		-3.3	-4	-9.8	-12.1
Pre-tax income from continuing operations	35.9	59.2	121	139.2									-0.6		4.9	29.7	24.6	10.8	8.7	30	35.1		25.7	45.6	61.3	79.5
Segment Information, Additional Disclosure
Total investments	2,293.70		2,293.70			2,291.50							0	0		0		0		0		0	2,293.70		2,293.70		2,291.50
Reinsurance recoverables	86.1		86.1			110.6							32.4	52.3		32.4		53.7		53.7		58.3	0		0		0
Deferred acquisition costs	107.7		107.7			123.9							56.5	75.1		56.5		51.2		51.2		48.8	0		0		0
Unpaid loss and loss adjustment expense reserves	1,052.90		1,052.90		1,027.80	1,000	911.6	3,175.30	3,358.60				608.9	561.9		608.9		444		444		438.1	0		0		0
Unearned premiums	552.7		552.7			573.8							282.6	318.7		282.6		270.1		270.1		255.1	0		0		0
Debt	274.7		274.7			274.7							0	0		0		0		0		0	274.7		274.7		274.7
Net written premiums	$314.10	$335.20	$826.10	$930.40									$172		$203.90	$393.30	$510.30	$142.10	$131.30	$432.80	$420.10
Loss and LAE Ratio	60.10%	56.00%	56.00%	53.50%									63.70%		57.40%	56.40%	54.70%	56.90%	54.40%	55.60%	52.10%
Expense Ratio	36.00%	38.80%	36.80%	39.20%									36.70%		39.50%	36.70%	39.90%	35.40%	37.90%	36.90%	38.40%
Combined Ratio	96.10%	94.80%	92.80%	92.70%									100.40%		96.90%	93.10%	94.60%	92.30%	92.30%	92.50%	90.50%
Number of underwriting segments			14							2		12		8						6
Number of segements sold			1																	1
Number of major underwriting units											3

Retirement_Plans_Periodic_Bene

Retirement Plans (Periodic Benefit Cost) (Details) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2011
Compensation and Retirement Disclosure [Abstract]
Service cost	$0.20		$0.20		$0.60		$0.60
Interest cost	1.1		1.2		3.2		3.6
Expected return on plan assets	-1.8		-1.7		-5.4		-5.1
Amortization of unrecognized loss	0.2		0.2		0.7		0.6
Net periodic pension income before settlement and special termination benefits	-0.3		-0.1		-0.9		-0.3
Special termination benefits expense	0	[1]	0.1	[1]	0.3	[1]	0.2	[1]
Total net periodic benefit cost	-0.3		0.5		-0.6		0.4
Contributions to non-qualified plan in 2013					2.2
Pension contributions					1.7
Discount rate utilized for revaluation (as a percent)			3.75%				3.75%		4.50%
Settlement expense	0		0.5		0		0.5
Recognized loss due to settlement and re-measurement throgh other comrehensive income			3.7				3.7
Remaining Qualified Plan liabilites at time of settlement			$92.40				$92.40
[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

Employee_ShareBased_Incentive_2

Employee Share-Based Incentive Compensation Plans (Performance Shares) (Details) (Long Term Incentive Plan, Performance shares, USD $)	3 Months Ended				9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
Activity in plan
Beginning of period (in shares)			557,223						489,867	489,867			560,577
Payments and deferrals (in shares)	0	[1]	0	[1]	-238,658	[1]	-258,901	[1]
New awards (in shares)	0		0		179,000		181,290
Forfeitures and net change in assumed forfeitures (in shares)	0		-3,354		-13,665		-7,833
Target performance shares outstanding, end of period (in shares)	489,867				489,867						563,190	557,223		642,667
Expense amortized
Accrued expense at beginning of period			$0.90		$1.20		$9.70			$2.40			$3.10
Payments and deferrals	0	[1]	0	[1]	0	[1]	-7.8	[1]
Forfeitures and net change in assumed forfeitures	0		0		-0.1		0
Expense recognized	0.1		-2.2		1.4		-1
Accrued expense at end of period	2.5				2.5					2.4			3.1
Additional disclosures
Assumed forfeitures (in shares)	-12,561				-12,561
Assumed forfeitures	-0.1				-0.1
Additional compensation cost that would be recognized if all outstanding performance shares vested	3.1				3.1
Performance cycle 2010 - 2012
Additional disclosures
Performance share payments factor					0.00%
Performance cycle 2009 - 2011
Additional disclosures
Performance share payments factor					138.60%
Performance cycle 2011 - 2013
Activity in plan
Target performance shares outstanding, end of period (in shares)	142,138				142,138				142,138
Expense amortized
Accrued expense at beginning of period									0.5
Accrued expense at end of period	0.5				0.5				0.5
Performance cycle 2012 - 2014
Activity in plan
Target performance shares outstanding, end of period (in shares)	181,290				181,290				181,290
Expense amortized
Accrued expense at beginning of period									1.4
Accrued expense at end of period	1.4				1.4				1.4
Performance cycle 2013 - 2015
Activity in plan
Target performance shares outstanding, end of period (in shares)	179,000				179,000				179,000
Expense amortized
Accrued expense at beginning of period									0.7
Accrued expense at end of period	0.7				0.7				0.7
Performance cycle 2010 - 2014
Activity in plan
Target performance shares outstanding, end of period (in shares)	502,428				502,428				502,428
Expense amortized
Accrued expense at beginning of period									2.6
Accrued expense at end of period	$2.60				$2.60				$2.60
[1]	No payments were made in 2013 for the 2010-2012 performance cycle as the performance factor was zero percent. Performance share payments in 2012 for the 2009-2011 performance cycle were based upon a performance factor of 138.6%.

Employee_ShareBased_Incentive_3

Employee Share-Based Incentive Compensation Plans (Restricted) (Details) (Long Term Incentive Plan, USD $)	3 Months Ended		9 Months Ended				0 Months Ended	9 Months Ended	0 Months Ended	9 Months Ended
In Millions, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013	Mar. 31, 2012	25-May-11	Sep. 30, 2012	25-May-11	Sep. 30, 2013
	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Restricted Shares	Chief Executive Officer
									Performance cycle 2011 - 2013	Restricted Shares
Activity in plan
Beginning of period (in shares)		930,000	927,000	630,000	918,000	930,000
New awards (in shares)	0	0	0				630,000	300,000
Forfeitures (in shares)	-3,000	0	-3,000	0					-35,000
Years Performance Shares will be Forfeited Period										5 years
Vested (in shares)	0	0	-9,000	0
End of period (in shares)	915,000		915,000		918,000	930,000
Unamortized grant date fair value
Unamortized grant date fair value, beginning of period		$10.40	$9.60	$7.70	$8	$11.20
Issued	0	0	0	4.6
Expense recognized	-0.7	-0.8	-2.3	-1.9
Unamortized grant date fair value, end of period	$7.30		$7.30		$8	$11.20

Income_Taxes_Details

Income Taxes (Details) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Jan. 05, 2011

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Sep. 30, 2013

Sep. 30, 2012

Apr. 28, 2011

Domestic Tax Authority

Domestic Tax Authority

Domestic Tax Authority

Domestic Tax Authority

Foreign Tax Authority

Foreign Tax Authority

Foreign Tax Authority

Foreign Tax Authority

Tax Assessment Year 2007 to 2009

Internal Revenue Service (IRS)

Income Tax Contingency [Line Items]

Effective tax rate

-8.10%

-24.00%

-13.10%

-20.20%

-19.70%

37.40%

24.30%

34.00%

0.50%

0.70%

0.30%

0.70%

Federal statutory rate

35.00%

35.00%

Foreign tax credit carryforwards

$18.70

$64.50

Tax assessment related to disallowed deductions

$60.20

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Details) (OBH (issuer), USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value of Financial Instruments
Senior unsecured notes, carrying value	$274.70	$274.70
Fair Value
Fair Value of Financial Instruments
Senior unsecured notes, carrying value	272.2	282.4
Carrying Value
Fair Value of Financial Instruments
Senior unsecured notes, carrying value	$274.70	$274.70

Legal_Contingencies_Details

Legal Contingencies (Details) (Pending Litigation, Tribune Company Litigation, USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Pending Litigation | Tribune Company Litigation
Loss Contingencies [Line Items]
Proceeds common stock, litigation settlement	$32

Earnings_per_Share_Details

Earnings per Share (Details) (USD $)	3 Months Ended				9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Earnings attributable to OneBeacon's common shareholdersbbasic and diluted (in millions):
Net income from continuing operations attributable to OneBeacon's common shareholders	$38.80		$44.60		$104.30		$109.90
Allocation of loss (income) for participating unvested restricted common shares	-0.4		-0.4		-1		-1
Dividends paid on participating restricted common shares	-0.2		-0.2		-0.6		-0.6
Total allocation to restricted common shares	-0.6		-0.6		-1.6		-1.6
Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts	38.2		44		102.7		108.3
Undistributed Earnings, Basic [Abstract]
Net income from continuing operations attributable to OneBeacon's common shareholders, net of restricted common share amounts	38.2		44		102.7		108.3
Dividends paid, net of restricted common share amounts	-19.8		-19.8		-59.5		-59.5
Total undistributed net earnings, net of restricted common share amounts	$18.40		$24.20		$43.20		$48.80
Weighted Average Number of Shares Outstanding, Basic [Abstract]
Total weighted average common shares outstanding	95.4		95.4		95.4		95.3
Weighted average unvested restricted common shares	-0.9	[1]	-0.9	[1]	-0.9	[1]	-0.8	[1]
Basic and diluted earnings (loss) per share denominator	94.5	[2]	94.5	[2]	94.5	[2]	94.5	[2]
Earnings Per Share, Basic and Diluted, Other Disclosures [Abstract]
Net income from continuing operations attributable to OneBeacon's common shareholders	$0.41		$0.47		$1.09		$1.15
Dividends declared and paid	($0.21)		($0.21)		($0.63)		($0.63)
Undistributed earnings	$0.20		$0.26		$0.46		$0.52
[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 the options were not included in the three and nine months ended September 30, 2012 as the exercise price was greater than the average market price of common shares. There were no options outstanding during the three and nine months ended September 30, 2013.

Common_Shareholders_Equity_Det

Common Shareholders' Equity (Details) (USD $)	3 Months Ended		9 Months Ended			9 Months Ended
Share data in Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Aug. 22, 2007	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
					Class A common	Common Stock	Common Stock	Total OneBeacon common shareholders' equity	Total OneBeacon common shareholders' equity
Equity, Class of Treasury Stock [Line Items]
Number of shares authorized to be repurchased					200
Remaining authorized repurchase amount			$87,700,000
Repurchases and retirements of common shares						3,300	0
Dividends			60,900,000	60,600,000				60,100,000	60,000,000
Dividends, per share, cash paid	$0.21	$0.21	$0.63	$0.63				$0.63	$0.63
Pre-tax changes to accumulated other comprehensive income			700,000
Recognized loss due to settlement and re-measurement throgh other comrehensive income		$3,700,000		$3,700,000

Discontinued_Operations_Summar

Discontinued Operations (Summary of Reclassified Balances and Related Items) (Details) (USD $)	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 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		Segment, Discontinued Operations		Segment, Discontinued Operations
												Runoff		Runoff		Runoff
Assets
Investments	$2,293.70		$2,291.50									$239.70		$338.10
Premiums receivable	255.9		225.6									9.8		11
Reinsurance recoverable on unpaid losses	80.2	81.8	107.3	41.3	2,040.70	2,167.50	1,806.70		1,806.70		1,990.90	1,666.50	[1]	1,840.80	[1]
Reinsurance recoverable on paid losses	5.9											21.8		15.6
Net deferred tax asset												3.2		5.1
Other assets	203.5		227.2									16.2		16.2
Total assets	5,225.10		5,401.50									1,957.20		2,226.80
Liabilities
Unpaid loss and loss adjustment expense reserves	1,052.90	1,027.80	1,000	911.6	3,175.30	3,358.60	1,937.30		1,937.30		2,202.70	1,797.10	[1]	2,052.60	[1]	166
Unearned premiums	552.7		573.8									0.2		0.5
Ceded reinsurance payable												12.8		21.9
Other liabilities	316.7		302.7									147.1	[2]	151.8	[2]
Total liabilities	4,160.10		4,384.20									1,957.20		2,226.80
Pre-tax income (loss)							$0.50	($26.20)	$0.50	($38.20)		$140.70		$140.70
[1]	The SeptemberB 30, 2013 and DecemberB 31, 2012 balances include the remaining purchase accounting fair value adjustments of $140.2 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of SeptemberB 30, 2013 and DecemberB 31, 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,806.7 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,937.3 million and $2,202.7 million for each period.
[2]	Other liabilities as of SeptemberB 30, 2013 and DecemberB 31, 2012 include the accrual related to the pre-tax loss on sale of the Runoff Business of $140.7 million.

Discontinued_Operations_Summar1

Discontinued Operations (Summary of the Results of Operations) (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2013	Dec. 31, 2012
					Segment, Discontinued Operations	Segment, Discontinued Operations	Segment, Discontinued Operations	Segment, Discontinued Operations	Runoff	Runoff	Runoff	Runoff
										Segment, Discontinued Operations	Segment, Discontinued Operations	Segment, Discontinued Operations
Discontinued Operations
Net written premiums					$1	($1.20)	$0.40	$0.60
Revenues
Earned premiums	278.9	293.9	846.2	846	1.1	-0.4	0.7	10
Other revenue	5.5	-0.4	30.1	-0.1	0	0	7.8	0
Total revenues	311.5	346.3	927.1	945.3	1.1	-0.4	8.5	10
Expenses
Loss and loss adjustment expenses	167.8	164.7	473.7	452.5	0.1	27.7	7.6	48.4	9
Policy acquisition expenses					0.1	-0.8	0.1	-1.3
Other underwriting expenses (income)	53.6	66.6	160.9	185.6	0.4	-1.1	0.3	1.1
Total expenses	275.6	287.1	806.1	806.1	0.6	25.8	8	48.2
Pre-tax income (loss)					0.5	-26.2	0.5	-38.2			140.7	140.7
Income tax benefit (expense)	-2.9	14.2	15.9	28.1	-0.2	10.4	-0.2	13.4
Income (loss) from discontinued operations, net of tax					0.3	-15.8	0.3	-24.8
Loss from sale of discontinued operations, net of tax	0	-91	0	-91	0	-91	0	-91	91.5	0.5
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent					$0.30	($106.80)	$0.30	($115.80)

Discontinued_Operations_Loss_p

Discontinued Operations (Loss per Share Related to Discontinued Operations) (Details) (USD $)	3 Months Ended				9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Income (loss) attributable to OneBeacon's common shareholdersbbasic and diluted (in millions):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	$0.30		($15.80)		$0.30		($24.80)
Allocation of loss for participating unvested restricted common shares	-0.4		-0.4		-1		-1
Net income attributable to OneBeacon's common shareholders	39.1		-62.2		104.6		-5.9
Income (loss) per share denominatorbbasic and diluted (in millions):
Total weighted average common shares outstanding	95.4		95.4		95.4		95.3
Weighted average unvested restricted common shares	-0.9	[1]	-0.9	[1]	-0.9	[1]	-0.8	[1]
Basic and diluted earnings (loss) per share denominator	94.5	[2]	94.5	[2]	94.5	[2]	94.5	[2]
Income (loss) per share attributable to OneBeacon's common shareholdersbbasic and diluted (in dollars):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders per share	$0.41		($0.65)		$1.09		($0.07)
Segment, Discontinued Operations
Income (loss) attributable to OneBeacon's common shareholdersbbasic and diluted (in millions):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0.3		-106.8		0.3		-115.8
Allocation of loss for participating unvested restricted common shares	0		1		0		1
Net income attributable to OneBeacon's common shareholders	$0.30		($105.80)		$0.30		($114.80)
Income (loss) per share denominatorbbasic and diluted (in millions):
Total weighted average common shares outstanding	95.4		95.4		95.4		95.3
Weighted average unvested restricted common shares	-0.9	[1]	-0.9	[1]	-0.9	[1]	-0.9	[1]
Basic and diluted earnings (loss) per share denominator	94.5	[3]	94.5	[3]	94.5	[3]	94.4	[3]
Income (loss) per share attributable to OneBeacon's common shareholdersbbasic and diluted (in dollars):
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders per share	$0		($1.12)		$0		($1.22)
[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 the options were not included in the three and nine months ended September 30, 2012 as the exercise price was greater than the average market price of common shares. There were no options outstanding during the three and nine months ended September 30, 2013.
[3]	Common shares issuable upon exercise of the options were not included in the three and nine months ended September 30, 2012 as their inclusion would be anti-dilutive. There were no options outstanding during the three and nine months ended SeptemberB 30, 2013, as the remaining outstanding options expired unexercised during the year ended December 31, 2012.

Discontinued_Operations_Additi

Discontinued Operations (Additional Disclosures) (Details) (USD $)	3 Months Ended		9 Months Ended							3 Months Ended		9 Months Ended													3 Months Ended								3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	31-May-01	Sep. 30, 2013	Sep. 30, 2013		Sep. 30, 2013	Sep. 30, 2013		Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012		Sep. 30, 2013		Jun. 30, 2013		Dec. 31, 2012		Sep. 30, 2012	Sep. 30, 2013		Dec. 31, 2012		Mar. 31, 2013	Sep. 30, 2013	Sep. 30, 2013	Mar. 31, 2013	Mar. 31, 2013	Sep. 30, 2013	Sep. 30, 2013
										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	General Reinsurance Corporation Cover	General Reinsurance Corporation Cover	Minimum	Maximum	Item One	Item Two
															GRC Cover	GRC 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	Runoff	Runoff	Runoff	Runoff
																																							GRC Cover		Segment, Discontinued Operations	Segment, Discontinued Operations	Segment, Discontinued Operations	Segment, Discontinued Operations
Dispositions
Reinsurance agreement percentage	100.00%		100.00%
Unamortized amount										$140,200,000		$140,200,000		$150,100,000		$570,000,000
Reinsurance recoverable on unpaid losses	80,200,000		80,200,000		81,800,000	107,300,000	41,300,000	2,040,700,000	2,167,500,000	1,806,700,000		1,806,700,000		1,990,900,000			2,500,000,000																	1,666,500,000	[1]	1,840,800,000	[1]
Unpaid loss and loss adjustment expense reserves	1,052,900,000		1,052,900,000		1,027,800,000	1,000,000,000	911,600,000	3,175,300,000	3,358,600,000	1,937,300,000		1,937,300,000		2,202,700,000																				1,797,100,000	[1]	2,052,600,000	[1]	166,000,000			215,000,000	668,000,000
Difference between net loss and LAE and actuarial estimates																																											46,000,000	17,000,000
Pre-tax income (loss)										500,000	-26,200,000	500,000	-38,200,000																					140,700,000		140,700,000
Income (loss) from discontinued operations, net of tax	300,000	-15,800,000	300,000	-24,800,000						300,000	-106,800,000	300,000	-115,800,000
Net other revenues (expenses)	5,500,000	-400,000	30,100,000	-100,000						0	0	7,800,000	0
Income (loss) from discontinued operations, net of tax										300,000	-15,800,000	300,000	-24,800,000
Loss and loss adjustment expenses	167,800,000	164,700,000	473,700,000	452,500,000						100,000	27,700,000	7,600,000	48,400,000												9,000,000
Loss and LAE incurred relating to prior year losses	3,800,000	-2,300,000	-100,000	-7,600,000							6,500,000
Loss from sale of discontinued operations, net of tax	0	-91,000,000	0	-91,000,000						0	-91,000,000	0	-91,000,000												91,500,000								500,000
Adverse loss reserve development on losses occurring in years 2000 and prior																																								400,000,000
Loss reserves ceded at acquisition																																							170,000,000
LAE claims, net																	2,300,000,000
Reinsurance recoverables	86,100,000		86,100,000			110,600,000									384,300,000		1,600,000,000	1,292,200,000	[2]	54,400,000	25,800,000	[3]	19,900,000	13,000,000
LAE claims, estimated future losses																	198,300,000
Estimated incurred losses ceded															562,000,000
Reinsurance recoverable on paid losses	5,900,000		5,900,000																															21,800,000		15,600,000
Percentage of total paid and unpaid reinsurance recoverables																		71.00%	[2]	3.00%	1.00%	[3]	1.00%	1.00%
Number of ratings in A.M. Best ratings										16		16
Reinsurance recoverables																		198,300,000	[3]		28,000,000	[3]
Loss and Loss Adjustment Expense Reserves Reclassified to from Held For Sale																									$256,300,000	[4],[5]	($130,600,000)	[4],[5]	($147,900,000)	[4],[5]	$211,800,000	[4],[5]
[1]	The SeptemberB 30, 2013 and DecemberB 31, 2012 balances include the remaining purchase accounting fair value adjustments of $140.2 million and $150.1 million, respectively, relating to the OneBeacon Acquisition. As of SeptemberB 30, 2013 and DecemberB 31, 2012, reinsurance recoverable on unpaid losses, gross of purchase accounting adjustments, were $1,806.7 million and $1,990.9 million, respectively, and unpaid loss and LAE reserves, gross of purchase accounting adjustments, were $1,937.3 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 $28.0 million of reinsurance recoverables from the various reinsurers that are guaranteed by Tokio Marine and Nichido Fire.
[4]	Consists of the net loss and LAE reserves of $256.3 million recorded in the nine months ended September 30, 2012 in connection with the Runoff Transaction, offset by net loss and LAE reserves of $64.7 million reclassified from held for sale upon the closing of the AutoOne Transaction in February 2012.
[5]	Consists of the change in net loss and LAE reserves from $147.9 million to $130.6 million, which amounts were classified as held for sale as of June 30, 2013 and SeptemberB 30, 2013, respectively, in connection with the Runoff Transaction.

Consolidating_Financial_Inform2

Consolidating Financial Information (Consolidating Balance Sheet) (Details) (USD $)	Sep. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	$1,626.10		$1,593.30
Short-term investments, at amortized cost (which approximates fair value)	162		232.8
Common equity securities, at fair value	327.5		259
Convertible fixed maturity investments, at fair value	31.9		62.6
Other investments	146.2		143.8
Total investments	2,293.70		2,291.50
Cash	173.8		43.9	32		54.9
Reinsurance recoverables	86.1		110.6
Premiums receivable	255.9		225.6
Deferred acquisition costs	107.7		123.9
Net deferred tax asset	127.2		137.8
Investment income accrued	10.6		12.1
Accounts receivable on unsettled investment sales	9.4		2.1
Investments in subsidiaries	0		0
Other assets	203.5		227.2
Assets held for sale	1,957.20		2,226.80
Total assets	5,225.10		5,401.50
Liabilities
Unpaid loss and loss adjustment expense reserves	1,052.90	1,027.80	1,000	911.6	3,175.30	3,358.60
Unearned premiums	552.7		573.8
Debt	274.7		274.7
Accounts payable on unsettled investment purchases	5.9		6.2
Other liabilities	316.7		302.7
Liabilities held for sale	1,957.20		2,226.80
Total liabilities	4,160.10		4,384.20
Equity [Abstract]
OneBeacon's common shareholders' equity	1,062.10		1,014.50
Total noncontrolling interests	2.9		2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,065		1,017.30	1,063.50		1,113.90
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,225.10		5,401.50
The Company (guarantor)
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	0		0
Short-term investments, at amortized cost (which approximates fair value)	4.2		2.7
Common equity securities, at fair value	0		0
Convertible fixed maturity investments, at fair value	0		0
Other investments	0		0
Total investments	4.2		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	1,059		1,012.40
Other assets	-1.2		0.3
Assets held for sale	0		0
Total assets	1,062		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]
OneBeacon's common shareholders' equity	1,062.10		1,014.50
Total noncontrolling interests	0		0
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,062.10		1,014.50
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	1,062		1,015.40
Non-guarantor subsidiaries
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	1,586.20		1,602.40
Short-term investments, at amortized cost (which approximates fair value)	153.2		169.9
Common equity securities, at fair value	327.5		259
Convertible fixed maturity investments, at fair value	31.9		62.6
Other investments	146.2		143.8
Total investments	2,245		2,237.70
Cash	173.4		43.9	32		52.5
Reinsurance recoverables	86.1		110.6
Premiums receivable	255.9		225.6
Deferred acquisition costs	107.7		123.9
Net deferred tax asset	129.5		140.2
Investment income accrued	11		12.3
Accounts receivable on unsettled investment sales	9.4		1.4
Investments in subsidiaries	0		0
Other assets	199.6		219.8
Assets held for sale	1,957.20		2,226.80
Total assets	5,174.80		5,342.20
Liabilities
Unpaid loss and loss adjustment expense reserves	1,052.90		1,000
Unearned premiums	552.7		573.8
Debt	0		0
Accounts payable on unsettled investment purchases	5.9		6.2
Other liabilities	309.9		295.9
Liabilities held for sale	1,957.20		2,226.80
Total liabilities	3,878.60		4,102.70
Equity [Abstract]
OneBeacon's common shareholders' equity	1,293.30		1,236.70
Total noncontrolling interests	2.9		2.8
Total OneBeacon's common shareholders' equity and noncontrolling interests	1,296.20		1,239.50
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	5,174.80		5,342.20
OBH (issuer)
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	64.6		16.6
Short-term investments, at amortized cost (which approximates fair value)	4.6		60.2
Common equity securities, at fair value	0		0
Convertible fixed maturity investments, at fair value	0		0
Other investments	0		0
Total investments	69.2		76.8
Cash	0.4		0	0		2.4
Reinsurance recoverables	0		0
Premiums receivable	0		0
Deferred acquisition costs	0		0
Net deferred tax asset	-2		-2.5
Investment income accrued	0.1		0
Accounts receivable on unsettled investment sales	0		0.7
Investments in subsidiaries	1,131.70		1,062.60
Other assets	5.1		7.1
Assets held for sale	0		0
Total assets	1,204.50		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	7.4		6
Liabilities held for sale	0		0
Total liabilities	307.1		305.6
Equity [Abstract]
OneBeacon's common shareholders' equity	897.4		839.1
Total noncontrolling interests	0		0
Total OneBeacon's common shareholders' equity and noncontrolling interests	897.4		839.1
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	1,204.50		1,144.70
Consolidating adjustments
Investments, Debt and Equity Securities [Abstract]
Fixed maturity investments, at fair value	-24.7		-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 investments	-24.7		-25.7
Cash	0		0	0		0
Reinsurance recoverables	0		0
Premiums receivable	0		0
Deferred acquisition costs	0		0
Net deferred tax asset	-0.3		0.1
Investment income accrued	-0.5		-0.2
Accounts receivable on unsettled investment sales	0		0
Investments in subsidiaries	-2,190.70		-2,075
Other assets	0		0
Assets held for sale	0		0
Total assets	-2,216.20		-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	-0.5		-0.1
Liabilities held for sale	0		0
Total liabilities	-25.5		-25
Equity [Abstract]
OneBeacon's common shareholders' equity	-2,190.70		-2,075.80
Total noncontrolling interests	0		0
Total OneBeacon's common shareholders' equity and noncontrolling interests	-2,190.70		-2,075.80
Total liabilities, OneBeacon's common shareholders' equity and noncontrolling interests	($2,216.20)		($2,100.80)

Consolidating_Financial_Inform3

Consolidating Financial Information (Consolidating Statement of Operations and Comprehensive (Loss)) (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues
Earned premiums	$278.90	$293.90	$846.20	$846
Net investment income (expense)	10.1	12.8	30.9	41.5
Net realized and change in unrealized investment gains	17	40	19.9	57.9
Net other revenues	5.5	-0.4	30.1	-0.1
Total revenues	311.5	346.3	927.1	945.3
Expenses
Loss and loss adjustment expenses	167.8	164.7	473.7	452.5
Policy acquisition expenses	53.6	66.6	160.9	185.6
Other underwriting expenses	46.8	47.4	150.7	146.2
General and administrative expenses	4.1	4.4	11	9.7
Interest expense	3.3	4	9.8	12.1
Total expenses	275.6	287.1	806.1	806.1
Pre-tax (loss) income from continuing operations	35.9	59.2	121	139.2
Income tax benefit (expense)	2.9	-14.2	-15.9	-28.1
Net income from continuing operations	38.8	45	105.1	111.1
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0.3	-15.8	0.3	-24.8
Loss from sale of discontinued operations, net of tax	0	-91	0	-91
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	39.1	-61.8	105.4	-4.7
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income (loss), including noncontrolling interests	39.1	-61.8	105.4	-4.7
Less: Net income attributable to noncontrolling interests	0	-0.4	-0.8	-1.2
Net income (loss) attributable to OneBeacon's common shareholders	39.1	-62.2	104.6	-5.9
Net change in benefit plan assets and obligations, net of tax	0.2	-2	0.5	-1.7
Comprehensive income (loss) attributable to OneBeacon's common shareholders	39.3	-64.2	105.1	-7.6
The Company (guarantor)
Revenues
Earned premiums	0	0	0	0
Net investment income (expense)	0	0	0	0
Net realized and change in unrealized investment gains	0	0	0	0
Net other revenues	0	0	0	0
Total revenues	0	0	0	0
Expenses
Loss and loss adjustment expenses	0	0	0	0
Policy acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
General and administrative expenses	1.7	1.5	3.6	4.5
Interest expense	0	0	0	0
Total expenses	1.7	1.5	3.6	4.5
Pre-tax (loss) income from continuing operations	-1.7	-1.5	-3.6	-4.5
Income tax benefit (expense)	0	0	0.1	0
Net income from continuing operations	-1.7	-1.5	-3.5	-4.5
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0	0	0	0
Loss from sale of discontinued operations, net of tax		0	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	-1.7	-1.5	-3.5	-4.5
Equity in earnings of subsidiaries, net of tax	44.5	-60.7	111.8	-1.4
Net income (loss), including noncontrolling interests	42.8	-62.2	108.3	-5.9
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	42.8	-62.2	108.3	-5.9
Net change in benefit plan assets and obligations, net of tax	0.2	-2	0.5	-1.7
Comprehensive income (loss) attributable to OneBeacon's common shareholders	43	-64.2	108.8	-7.6
Non-guarantor subsidiaries
Revenues
Earned premiums	278.9	293.9	846.2	846
Net investment income (expense)	10.2	13.5	32.1	43.8
Net realized and change in unrealized investment gains	17.2	38.7	19.7	56.5
Net other revenues	5.5	-0.2	30.1	0.5
Total revenues	311.8	345.9	928.1	946.8
Expenses
Loss and loss adjustment expenses	167.8	164.7	473.7	452.5
Policy acquisition expenses	53.6	66.6	160.9	185.6
Other underwriting expenses	46.8	47.4	150.7	146.2
General and administrative expenses	2.2	2.7	7.5	4.7
Interest expense	0	-0.1	0	-0.1
Total expenses	270.4	281.3	792.8	788.9
Pre-tax (loss) income from continuing operations	41.4	64.6	135.3	157.9
Income tax benefit (expense)	0.5	-16.1	-23.3	-35.1
Net income from continuing operations	41.9	48.5	112	122.8
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0.3	-15.8	0.3	-24.8
Loss from sale of discontinued operations, net of tax		-91	0	-91
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	42.2	-58.3	112.3	7
Equity in earnings of subsidiaries, net of tax	0	0	0	0
Net income (loss), including noncontrolling interests	42.2	-58.3	112.3	7
Less: Net income attributable to noncontrolling interests	0	-0.4	-0.8	-1.2
Net income (loss) attributable to OneBeacon's common shareholders	42.2	-58.7	111.5	5.8
Net change in benefit plan assets and obligations, net of tax	0	0	0	0
Comprehensive income (loss) attributable to OneBeacon's common shareholders	42.2	-58.7	111.5	5.8
OBH (issuer)
Revenues
Earned premiums	0	0	0	0
Net investment income (expense)	0.2	0.1	-0.3	0.1
Net realized and change in unrealized investment gains	-0.4	1	-0.8	1
Net other revenues	0	-0.2		-0.6
Total revenues	-0.2	0.9	-1.1	0.5
Expenses
Loss and loss adjustment expenses	0	0	0	0
Policy acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
General and administrative expenses	0.2	0.2	-0.1	0.5
Interest expense	3.5	4.8	10.6	14.4
Total expenses	3.7	5	10.5	14.9
Pre-tax (loss) income from continuing operations	-3.9	-4.1	-11.6	-14.4
Income tax benefit (expense)	2.5	2	7.7	7.1
Net income from continuing operations	-1.4	-2.1	-3.9	-7.3
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0	0	0	0
Loss from sale of discontinued operations, net of tax		0	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	-1.4	-2.1	-3.9	-7.3
Equity in earnings of subsidiaries, net of tax	34	-69.9	91.6	-20.5
Net income (loss), including noncontrolling interests	32.6	-72	87.7	-27.8
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	32.6	-72	87.7	-27.8
Net change in benefit plan assets and obligations, net of tax	0.2	-2	0.5	-1.7
Comprehensive income (loss) attributable to OneBeacon's common shareholders	32.8	-74	88.2	-29.5
Consolidating adjustments
Revenues
Earned premiums	0	0	0	0
Net investment income (expense)	-0.3	-0.8	-0.9	-2.4
Net realized and change in unrealized investment gains	0.2	0.3	1	0.4
Net other revenues	0	0	0	0
Total revenues	-0.1	-0.5	0.1	-2
Expenses
Loss and loss adjustment expenses	0	0	0	0
Policy acquisition expenses	0	0	0	0
Other underwriting expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Interest expense	-0.2	-0.7	-0.8	-2.2
Total expenses	-0.2	-0.7	-0.8	-2.2
Pre-tax (loss) income from continuing operations	0.1	0.2	0.9	0.2
Income tax benefit (expense)	-0.1	-0.1	-0.4	-0.1
Net income from continuing operations	0	0.1	0.5	0.1
Net income (loss) from discontinued operations attributable to OneBeacon's common shareholders	0	0	0	0
Loss from sale of discontinued operations, net of tax		0	0	0
(Loss) income before equity in (losses) earnings of unconsolidated affiliates	0	0.1	0.5	0.1
Equity in earnings of subsidiaries, net of tax	-78.5	130.6	-203.4	21.9
Net income (loss), including noncontrolling interests	-78.5	130.7	-202.9	22
Less: Net income attributable to noncontrolling interests	0	0	0	0
Net income (loss) attributable to OneBeacon's common shareholders	-78.5	130.7	-202.9	22
Net change in benefit plan assets and obligations, net of tax	-0.2	2	-0.5	1.7
Comprehensive income (loss) attributable to OneBeacon's common shareholders	($78.70)	$132.70	($203.40)	$23.70

Consolidating_Financial_Inform4

Consolidating Financial Information (Consolidating Statement of Cash Flows) (Details) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	$105.40	($4.70)
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	0	0
Net loss from discontinued operations	-0.3	24.8
Net loss from sale of discontinued operations	0	91
Net realized and change in unrealized investment gains	-19.9	-57.9
Net other realized losses	0	0
Net realized gain on sale of business	-23	0
Deferred income tax expense	12.3	12.3
Dividends received from subsidiaries	0	0
Other operating items:
Net change in loss and LAE reserves	52.9	-0.9
Net change in unearned premiums	-21.1	85.2
Net change in premiums receivable	-30.3	-45.7
Net change in reinsurance recoverable on paid and unpaid losses	24.5	64.4
Net change in other assets and liabilities	77.8	-32.3
Net cash provided from operationsbcontinuing operations	178.3	136.2
Net cash used for operationsbdiscontinued operations	-98	-155.6
Net cash provided from (used for) operations	80.3	-19.4
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	67.3	-22.7
Maturities of fixed maturity investments	196.6	147.5
Sales of fixed maturity investments	1,208.60	1,305.90
Sales of common equity securities	142	37.2
Sales of convertible fixed maturity investments	39.5	16
Distributions and redemptions of other investments	13.1	6.9
Purchases of fixed maturity investments	-1,381.80	-1,317.20
Purchases of common equity securities	-168.5	-49.3
Purchases of convertible fixed maturity investments	-12	-6.8
Contributions for other investments	-7.1	-3.1
Proceeds from sale of business	31.3	0
Net change in unsettled investment purchases and sales	-7.7	-69.1
Net acquisitions of property and equipment	-7.6	-1.2
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0
Net cash provided from investing activitiesbcontinuing operations	113.7	44.1
Net cash provided from investing activitiesbdiscontinued operations	0	0
Net cash provided from investing activities	113.7	44.1
Cash flows from financing activities:
Cash dividends paid to common shareholders	-60.1	-60
Cash dividends paid to parent	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0
Payments on capital lease obligation	-4	-3.6
Proceeds on sale of OB Holdings Lux	0	14
Net cash used for financing activitiesbcontinuing operations	-64.1	-49.6
Net cash used for financing activitiesbdiscontinued operations	0	0
Net cash used for financing activities	-64.1	-49.6
Net increase in cash during period	129.9	-24.9
Cash reclassified to assets held for sale	0	5.5
Cash transferred with sale of business	0	-3.5
Net increase after reclassification of cash to assets held for sale	129.9	-22.9
Cash balance at beginning of period	43.9	54.9
Cash balance at end of period	173.8
The Company (guarantor)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	108.3	-5.9
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	-111.8	1.4
Net loss from discontinued operations	0	0
Net loss from sale of discontinued operations	0	0
Net realized and change in unrealized investment gains	0	0
Net other realized losses	0	0
Net realized gain on sale of business	0	0
Deferred income tax expense	0	0
Dividends received from subsidiaries	62	62
Other operating items:
Net change in loss and LAE reserves	0	0
Net change in unearned premiums	0	0
Net change in premiums receivable	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0
Net change in other assets and liabilities	3.1	2.9
Net cash provided from operationsbcontinuing operations	61.6	60.4
Net cash used for operationsbdiscontinued operations	0	0
Net cash provided from (used for) operations	61.6	60.4
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	-1.5	-0.4
Maturities of fixed maturity investments	0	0
Sales of fixed maturity investments	0	0
Sales of common equity securities	0	0
Sales of convertible fixed maturity investments	0	0
Distributions and redemptions of other investments	0	0
Purchases of fixed maturity investments	0	0
Purchases of common equity securities	0	0
Purchases of convertible fixed maturity investments	0	0
Contributions for other investments	0	0
Proceeds from sale of business	0	0
Net change in unsettled investment purchases and sales	0	0
Net acquisitions of property and equipment	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0
Net cash provided from investing activitiesbcontinuing operations	-1.5	-0.4
Net cash provided from investing activitiesbdiscontinued operations	0	0
Net cash provided from investing activities	-1.5	-0.4
Cash flows from financing activities:
Cash dividends paid to common shareholders	-60.1	-60
Cash dividends paid to parent	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0
Payments on capital lease obligation	0	0
Proceeds on sale of OB Holdings Lux		0
Net cash used for financing activitiesbcontinuing operations	-60.1	-60
Net cash used for financing activitiesbdiscontinued operations	0	0
Net cash used for financing activities	-60.1	-60
Net increase in cash during period	0	0
Cash reclassified to assets held for sale	0	0
Cash transferred with sale of business	0	0
Net increase after reclassification of cash to assets held for sale	0	0
Cash balance at beginning of period	0	0
Cash balance at end of period	0
Non-guarantor subsidiaries
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	112.3	7
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	0	0
Net loss from discontinued operations	-0.3	24.8
Net loss from sale of discontinued operations	0	91
Net realized and change in unrealized investment gains	-19.7	-56.5
Net other realized losses	0	0
Net realized gain on sale of business	-23	0
Deferred income tax expense	12.5	9.1
Dividends received from subsidiaries	0	15.5
Other operating items:
Net change in loss and LAE reserves	52.9	-0.9
Net change in unearned premiums	-21.1	85.2
Net change in premiums receivable	-30.3	-45.7
Net change in reinsurance recoverable on paid and unpaid losses	24.5	64.4
Net change in other assets and liabilities	70.4	-36.3
Net cash provided from operationsbcontinuing operations	178.2	157.6
Net cash used for operationsbdiscontinued operations	-98	-155.6
Net cash provided from (used for) operations	80.2	2
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	13.2	12.9
Maturities of fixed maturity investments	185.3	147.5
Sales of fixed maturity investments	1,114.60	1,306.30
Sales of common equity securities	142	62.4
Sales of convertible fixed maturity investments	39.5	23.5
Distributions and redemptions of other investments	13.1	6.9
Purchases of fixed maturity investments	-1,227.80	-1,317.20
Purchases of common equity securities	-168.5	-48.5
Purchases of convertible fixed maturity investments	-12	-6.7
Contributions for other investments	-7.1	-3.1
Proceeds from sale of business	31.3	0
Net change in unsettled investment purchases and sales	-7.7	-69.1
Net acquisitions of property and equipment	-7.6	-1.2
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	30	14.5
Net cash provided from investing activitiesbcontinuing operations	138.3	128.2
Net cash provided from investing activitiesbdiscontinued operations	0	0
Net cash provided from investing activities	138.3	128.2
Cash flows from financing activities:
Cash dividends paid to common shareholders	0	0
Cash dividends paid to parent	-85	-163.1
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.		0
Payments on capital lease obligation	-4	-3.6
Proceeds on sale of OB Holdings Lux		14
Net cash used for financing activitiesbcontinuing operations	-89	-152.7
Net cash used for financing activitiesbdiscontinued operations	0	0
Net cash used for financing activities	-89	-152.7
Net increase in cash during period	129.5	-22.5
Cash reclassified to assets held for sale	0	5.5
Cash transferred with sale of business	0	-3.5
Net increase after reclassification of cash to assets held for sale	129.5	-20.5
Cash balance at beginning of period	43.9	52.5
Cash balance at end of period	173.4
OBH (issuer)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	87.7	-27.8
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	-91.6	20.5
Net loss from discontinued operations	0	0
Net loss from sale of discontinued operations	0	0
Net realized and change in unrealized investment gains	0.8	-1
Net other realized losses	0	0
Net realized gain on sale of business	0	0
Deferred income tax expense	-0.5	3.1
Dividends received from subsidiaries	23	101.1
Other operating items:
Net change in loss and LAE reserves	0	0
Net change in unearned premiums	0	0
Net change in premiums receivable	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0
Net change in other assets and liabilities	4.1	0.9
Net cash provided from operationsbcontinuing operations	23.5	96.8
Net cash used for operationsbdiscontinued operations	0	0
Net cash provided from (used for) operations	23.5	96.8
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	55.6	-35.2
Maturities of fixed maturity investments	11.3	0
Sales of fixed maturity investments	129.7	0
Sales of common equity securities	0	0.2
Sales of convertible fixed maturity investments	0	0.1
Distributions and redemptions of other investments	0	0
Purchases of fixed maturity investments	-189.7	-0.4
Purchases of common equity securities	0	-26.2
Purchases of convertible fixed maturity investments	0	-7.7
Contributions for other investments	0	0
Proceeds from sale of business	0	0
Net change in unsettled investment purchases and sales	0	0
Net acquisitions of property and equipment	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	0	0
Net cash provided from investing activitiesbcontinuing operations	6.9	-69.2
Net cash provided from investing activitiesbdiscontinued operations	0	0
Net cash provided from investing activities	6.9	-69.2
Cash flows from financing activities:
Cash dividends paid to common shareholders	0	0
Cash dividends paid to parent	0	-15.5
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	-30	-14.5
Payments on capital lease obligation	0	0
Proceeds on sale of OB Holdings Lux		0
Net cash used for financing activitiesbcontinuing operations	-30	-30
Net cash used for financing activitiesbdiscontinued operations	0	0
Net cash used for financing activities	-30	-30
Net increase in cash during period	0.4	-2.4
Cash reclassified to assets held for sale	0	0
Cash transferred with sale of business	0	0
Net increase after reclassification of cash to assets held for sale	0.4	-2.4
Cash balance at beginning of period	0	2.4
Cash balance at end of period	0.4
Consolidating adjustments
Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]
Net income (loss) including noncontrolling interests	-202.9	22
Charges (credits) to reconcile net income to cash flows (used for) provided from operations:
Undistributed earnings from subsidiaries	203.4	-21.9
Net loss from discontinued operations	0	0
Net loss from sale of discontinued operations	0	0
Net realized and change in unrealized investment gains	-1	-0.4
Net other realized losses	0	0
Net realized gain on sale of business	0	0
Deferred income tax expense	0.3	0.1
Dividends received from subsidiaries	-85	-178.6
Other operating items:
Net change in loss and LAE reserves	0	0
Net change in unearned premiums	0	0
Net change in premiums receivable	0	0
Net change in reinsurance recoverable on paid and unpaid losses	0	0
Net change in other assets and liabilities	0.2	0.2
Net cash provided from operationsbcontinuing operations	-85	-178.6
Net cash used for operationsbdiscontinued operations	0	0
Net cash provided from (used for) operations	-85	-178.6
Cash flows from investing activities:
Net maturities, purchases and sales of short-term investments	0	0
Maturities of fixed maturity investments	0	0
Sales of fixed maturity investments	-35.7	-0.4
Sales of common equity securities	0	-25.4
Sales of convertible fixed maturity investments	0	-7.6
Distributions and redemptions of other investments	0	0
Purchases of fixed maturity investments	35.7	0.4
Purchases of common equity securities	0	25.4
Purchases of convertible fixed maturity investments	0	7.6
Contributions for other investments	0	0
Proceeds from sale of business	0	0
Net change in unsettled investment purchases and sales	0	0
Net acquisitions of property and equipment	0	0
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	-30	-14.5
Net cash provided from investing activitiesbcontinuing operations	-30	-14.5
Net cash provided from investing activitiesbdiscontinued operations	0	0
Net cash provided from investing activities	-30	-14.5
Cash flows from financing activities:
Cash dividends paid to common shareholders	0	0
Cash dividends paid to parent	85	178.6
Return of capital to OneBeacon U.S. Enterprises Holdings, Inc.	30	14.5
Payments on capital lease obligation	0	0
Proceeds on sale of OB Holdings Lux		0
Net cash used for financing activitiesbcontinuing operations	115	193.1
Net cash used for financing activitiesbdiscontinued operations	0	0
Net cash used for financing activities	115	193.1
Net increase in cash during period	0	0
Cash reclassified to assets held for sale	0	0
Cash transferred with sale of business	0	0
Net increase after reclassification of cash to assets held for sale	0	0
Cash balance at beginning of period	0	0
Cash balance at end of period	$0