The Hanover Insurance Group, Inc. (THG) 10-K 24 Feb 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 20, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Entity Registrant Name	HANOVER INSURANCE GROUP, INC.
Entity Central Index Key	944695
Current Fiscal Year End Date	-19
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$2,746,167,107
Entity Common Stock, Shares Outstanding		44,302,074

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues
Premiums	$4,710.30	$4,450.50	$4,239.10
Net investment income	270.3	269	276.6
Net realized investment gains (losses):
Net realized gains from sales and other	55.6	39.5	31.4
Net other-than-temporary impairment losses on investments recognized in earnings	-5.5	-6	-7.8
Total net realized investment gains	50.1	33.5	23.6
Fees and other income	36.9	40.7	51.4
Total revenues	5,067.60	4,793.70	4,590.70
Losses and expenses
Losses and loss adjustment expenses	2,927.50	2,761.10	2,974.40
Amortization of deferred acquisition costs	1,040	971	938.1
Interest expense	65.2	65.3	61.9
Other operating expenses	656.9	667.2	587.6
Total losses and expenses	4,689.60	4,464.60	4,562
Income before income taxes	378	329.1	28.7
Income tax expense:
Current	27.1	8.6	18.4
Deferred	68.6	74.8	-35.8
Total income tax expense (benefit)	95.7	83.4	-17.4
Income from continuing operations	282.3	245.7	46.1
Net (loss) gain from discontinued operations (net of income tax (benefit) expense of $(0.1), $4.1 and $(0.5) in 2014, 2013 and 2012)	-0.3	5.3	9.8
Net income	$282	$251	$55.90
Basic:
Income from continuing operations	$6.41	$5.58	$1.03
Net (loss) gain from discontinued operations		$0.12	$0.22
Net income per share	$6.41	$5.70	$1.25
Weighted average shares outstanding	44	44.1	44.7
Diluted:
Income from continuing operations	$6.29	$5.47	$1.02
Net (loss) gain from discontinued operations	($0.01)	$0.12	$0.21
Net income per share	$6.28	$5.59	$1.23
Weighted average shares outstanding	44.9	44.9	45.3

Consolidated_Statements_of_Inc1

Consolidated Statements of Income (Parenthetical) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Net loss from discontinued operations, income tax benefit (expense)	($0.10)	$4.10	($0.50)

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income	$282	$251	$55.90
Available-for-sale securities and derivative instruments:
Net appreciation (depreciation) during the period	40	-167.3	109.4
Change in other-than-temporary impairment losses recognized in other comprehensive income	1.6	0.6	7.9
Total available-for-sale securities and derivative instruments	41.6	-166.7	117.3
Pension and postretirement benefits:
Net actuarial (loss) gain arising in the period	-22.6	10.8	-15.9
Amortization recognized as net benefit and postretirement cost	14.4	9.7	6.1
Total pension and postretirement benefits	-8.2	20.5	-9.8
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	-4.6	-2	7.9
Total other comprehensive income (loss), net of tax	28.8	-148.2	115.4
Comprehensive income (loss)	$310.80	$102.80	$171.30

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets
Fixed maturities, at fair value (amortized cost of $7,145.7 and $6,815.2)	$7,378.10	$6,970.60
Equity securities, at fair value (cost of $506.6 and $366.5)	580.8	430.2
Other investments	291.4	192.5
Total investments	8,250.30	7,593.30
Cash and cash equivalents	373.3	486.2
Accrued investment income	66.9	68
Premiums and accounts receivable, net	1,360.90	1,324.60
Reinsurance recoverable on paid and unpaid losses and unearned premiums	2,268.20	2,335
Deferred acquisition costs	525.7	506
Deferred income taxes	131.2	239.7
Goodwill	184.6	184.9
Other assets	486.6	526.1
Assets of discontinued operations	112	114.9
Total assets	13,759.70	13,378.70
Liabilities
Loss and loss adjustment expense reserves	6,391.70	6,231.50
Unearned premiums	2,583.90	2,515.80
Expenses and taxes payable	695.4	637.2
Reinsurance premiums payable	226.8	374.7
Debt	903.5	903.9
Liabilities of discontinued operations	114.4	121.1
Total liabilities	10,915.70	10,784.20
Commitments and contingencies
Shareholders' Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,830.70	1,830.10
Accumulated other comprehensive income	206.4	177.6
Retained earnings	1,558.70	1,349.10
Treasury stock at cost (16.6 and 16.8 million shares)	-752.4	-762.9
Total shareholders' equity	2,844	2,594.50
Total liabilities and shareholders' equity	$13,759.70	$13,378.70

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Statement Of Financial Position [Abstract]
Fixed maturities, amortized cost	$7,145.70	$6,815.20
Equity securities, cost	$506.60	$366.50
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, issued	0	0
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	16,600,000	16,800,000

Consolidated_Statements_of_Sha

Consolidated Statements of Shareholders' Equity (USD $)	Common Stock	Additional Paid-in Capital	Accumulated Net Unrealized Investment Gain (Loss)	Accumulated Defined Benefit Plans Adjustment	Cumulative Foreign Currency Translation Adjustment:	Retained Earnings	Treasury Stock	Total
In Millions
Balance at beginning of year at Dec. 31, 2011	$0.60	$1,784.80	$308.70	($86.80)	($11.50)	$1,211.30	($723.10)
Net income						55.9		55.9
Shares purchased at cost							-20
Employee and director stock-based awards and other		2.3
Net appreciation (depreciation) on available-for-sale securities and derivative instruments			117.3					117.3
Net actuarial gain (loss) arising in the period, Net of Tax				-15.9				-15.9
Net amount recognized as net periodic benefit cost				6.1				-6.1
Amount recognized as cumulative foreign currency translation during the period					7.9			7.9
Dividends to shareholders						-55.1
Total accumulated other comprehensive income					325.8			115.4
Net shares reissued at cost under employee stock-based compensation plans							13.4
Stock-based compensation						-0.5
Balance at end of year at Dec. 31, 2012	0.6	1,787.10	426	-96.6	-3.6	1,211.60	-729.7	2,595.40
Net income						251		251
Shares purchased at cost							-78.2
Employee and director stock-based awards and other		43
Net appreciation (depreciation) on available-for-sale securities and derivative instruments			-166.7					-166.7
Net actuarial gain (loss) arising in the period, Net of Tax				10.8				10.8
Net amount recognized as net periodic benefit cost				9.7				-9.7
Amount recognized as cumulative foreign currency translation during the period					-2			-2
Dividends to shareholders						-60
Total accumulated other comprehensive income					177.6			-148.2
Net shares reissued at cost under employee stock-based compensation plans							45
Stock-based compensation						-53.5
Balance at end of year at Dec. 31, 2013	0.6	1,830.10	259.3	-76.1	-5.6	1,349.10	-762.9	2,594.50
Net income						282		282
Shares purchased at cost							-20.4
Employee and director stock-based awards and other		0.6
Net appreciation (depreciation) on available-for-sale securities and derivative instruments			41.6					41.6
Net actuarial gain (loss) arising in the period, Net of Tax				-22.6				-22.6
Net amount recognized as net periodic benefit cost				14.4				-14.4
Amount recognized as cumulative foreign currency translation during the period					-4.6			-4.6
Dividends to shareholders						-67
Total accumulated other comprehensive income					206.4			28.8
Net shares reissued at cost under employee stock-based compensation plans							30.9
Stock-based compensation						-5.4
Balance at end of year at Dec. 31, 2014		$1,830.70	$300.90	($84.30)	($10.20)	$1,558.70	($752.40)	$2,844

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash Flows From Operating Activities
Net income	$282	$251	$55.90
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on repurchase of debt	0.1	19.9	5.1
Net realized investment gains	-50.2	-33.5	-22.8
Gain on sale of Citizens Management, Inc			-10.8
Net amortization and depreciation	33.5	35	35.6
Stock-based compensation expense	15.1	12.4	12.8
Amortization of defined benefit plan costs	22.1	14.9	9.3
Deferred income taxes expense	68.6	74.9	-35.5
Change in deferred acquisition costs	-19.6	-16.3	-30.8
Change in premiums receivable, net of reinsurance premiums payable	-184.1	-107.4	-53.3
Change in loss, loss adjustment expense and unearned premium reserves	256.3	65.9	598.3
Change in reinsurance recoverable	115.9	148.8	-230.7
Change in expenses and taxes payable	12.5	-62.9	87.6
Other, net	12.5	-18.8	-12.5
Net cash provided by operating activities	564.7	383.9	408.2
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,323.10	1,394.30	1,684.80
Proceeds from disposals of equity securities and other investments	175.2	217.8	218.2
Purchase of fixed maturities	-1,710.10	-1,698.80	-2,170.40
Purchase of equity securities and other investments	-379.7	-249.1	-274.9
Capital expenditures	-11.2	-22.9	-21.3
Net payments related to derivative agreements	2		0.8
Net cash (used in) investing activities	-600.7	-358.7	-562.8
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	12.6	25	2.6
Proceeds from debt borrowings, net		168.6	7.4
Change in cash collateral related to securities lending program	7.4	-15.9	5.2
Dividends paid to shareholders	-67	-60	-55.1
Repurchases of debt	-0.7	-139.9	-73.1
Repurchases of common stock	-20.4	-78.2	-20
Other financing activities	-3.6	-5.1	-0.4
Net cash used in financing activities	-71.7	-105.5	-133.4
Effect of exchange rate changes on cash	-5.2	1.6	32.1
Net change in cash and cash equivalents	-112.9	-78.7	-255.9
Net change in cash related to discontinued operations		0.1	0.3
Cash and cash equivalents, beginning of period	486.2	564.8	820.4
Cash and cash equivalents, end of period	373.3	486.2	564.8
Supplemental Cash Flow Information
Interest payments	64.1	66.1	62.7
Income tax net payments (refunds)	$8.40	$9.50	($5.50)

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Summary of Significant Accounting Policies [Abstract]
Summary of Significant Accounting Policies	1. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
	A. Basis of Presentation and Principles of Consolidation
	The consolidated financial statements of The Hanover Insurance Group, Inc. (“THG” or the “Company”), include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America (“Citizens”), THG’s principal U.S. domiciled property and casualty companies; Chaucer Holdings Limited (“Chaucer”), a specialist insurance underwriting group which operates through the Society and Corporation of Lloyd’s (“Lloyd’s”); and certain other insurance and non-insurance subsidiaries. These legal entities conduct their operations through several business segments discussed in Note 14 – “Segment Information”. The consolidated financial statements also include the Company’s discontinued operations, consisting primarily of the Company’s former life insurance businesses, and its accident and health business. All intercompany accounts and transactions have been eliminated. During 2013, the Company increased additional paid-in capital and decreased retained earnings by $34 million to correct the classification of amounts relating to stock-based compensation in prior periods. This reclassification within shareholders’ equity had no impact on net income, assets, liabilities or total shareholders’ equity of the Company.
	The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America (“U.S. GAAP”) requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amount of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of the Company’s management these financial statements reflect all adjustments, consisting of normal recurring items necessary for a fair presentation of the financial position and results of operations.
	B. Valuation of Investments
	Fixed maturities and equity securities are classified as available-for-sale. Available-for-sale securities are carried at fair value, with the unrealized gains and losses, net of taxes, reported in accumulated other comprehensive income, a separate component of shareholders’ equity. The amortized cost of fixed maturities is adjusted for amortization of premiums and accretion of discounts to maturity. Such amortization is included in net investment income.
	Participations in commercial mortgage loan originations (“mortgage participations”) and other mortgage loans, included in other investments in the Consolidated Balance Sheets, are stated at unpaid principal balances, net of reserves. Interest income is accrued on the unpaid principal balances at the loans’ contractual interest rates. Reserves on mortgage participations and other mortgage loans are established and are collectively evaluated based on losses expected by the Company for loans that may not be collectible in full. In establishing reserves, the Company considers, among other things, the estimated fair value of the underlying collateral.
	Fixed maturities, mortgage participations and other loans that are delinquent are placed on non-accrual status, and thereafter interest income is recognized only when cash payments are received.
	Realized investment gains and losses are reported as a component of revenues based upon specific identification of the investment assets sold. When an other-than-temporary decline in value of a specific investment is deemed to have occurred, and a charge to earnings is required, the Company recognizes a realized investment loss.
	The Company reviews investments in an unrealized loss position to identify other-than-temporary declines in value. When it is determined that a decline in value of an equity security is other-than-temporary, the Company reduces the cost basis of the security to fair value with a corresponding charge to earnings. When an other-than-temporary decline in value of a debt security is deemed to have occurred, the Company must assess whether it intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis. If the debt security meets either of these two criteria, an other-than-temporary impairment (“OTTI”) is recognized in earnings equal to the entire difference between the security’s amortized cost basis and its fair value at the impairment measurement date. If the Company does not intend to sell the debt security and it is not more likely than not the Company will be required to sell the security before recovery of its amortized cost basis, the credit loss portion of an OTTI is recorded through earnings while the portion attributable to all other factors is recorded separately as a component of other comprehensive income. The amount of the OTTI that relates to credit is estimated by comparing the amortized cost of the fixed maturity security with the net present value of the security’s projected future cash flows, discounted at the effective interest rate implicit in the investment prior to impairment. The non-credit portion of the impairment is equal to the difference between the fair value and the net present value of the security’s cash flows at the impairment measurement date. Once an OTTI has been recognized, the new amortized cost basis of the security is equal to the previous amortized cost less the amount of OTTI recognized in earnings. For equity method investments, an impairment is recognized when evidence demonstrates that an other-than-temporary loss in value has occurred, including the absence of the ability to recover the carrying amount of the investment or the inability of the investee to sustain a level of earnings that would justify the carrying amount of the investment.
	C. Financial Instruments
	In the normal course of business, the Company may enter into transactions involving various types of financial instruments, including debt, investments such as fixed maturities, equity securities and mortgage loans, investment and loan commitments, swap contracts, option contracts, forward contracts and futures contracts. These instruments involve credit risk and could also be subject to risk of loss due to interest rate and foreign currency fluctuation. The Company evaluates and monitors each financial instrument individually and, when appropriate, obtains collateral or other security to minimize losses.
	D. Other Investments
	Other investments consist primarily of overseas deposits, mortgage participations and limited partnerships. Overseas deposits are investments maintained in overseas funds and managed exclusively by Lloyd’s. These funds are required in order to protect policyholders in overseas markets and enable the Company to operate in those markets. Overseas deposits are carried at fair value. Realized and unrealized gains and losses on overseas deposits, including the impact of foreign currency movements, are reflected in the income statement in the period the gain or loss was generated.
	Mortgage participations represent interests in investment grade commercial mortgage loans originated and serviced by a third party of which the Company shares, on a pro-rata basis, in all related cash flows of the underlying mortgage loans. Due to certain reacquisition rights retained by the third party in the loan participation, these investments are accounted for as secured borrowings under Accounting Standards Codification (“ASC”) 860, Transfers and Servicing (“ASC 860”).
	E. Cash and Cash Equivalents
	Cash and cash equivalents includes cash on hand, amounts due from banks and highly liquid debt instruments purchased with an original maturity of three months or less.
	F. Deferred Acquisition Costs
	Acquisition costs consist of commissions, underwriting costs and other costs, which vary with, and are primarily related to, the successful production of premiums. Acquisition costs are deferred and amortized over the terms of the insurance policies.
	Deferred acquisition costs (“DAC”) for each line of business are reviewed to determine if the costs are recoverable from future income, including investment income. If such costs are determined to be unrecoverable, they are expensed at the time of determination. Although recoverability of DAC is not assured, the Company believes it is more likely than not that all of these costs will be recovered. The amount of DAC considered recoverable, however, could be reduced in the near term if the estimates of total revenues discussed above are reduced or permanently impaired as a result of a disposition of a line of business. The amount of amortization of DAC could be revised in the near term if any of the estimates discussed above are revised.
	G. Reinsurance Recoverables
	The Company shares certain insurance risks it has underwritten, through the use of reinsurance contracts, with various insurance entities. Reinsurance accounting is followed for ceded transactions when the risk transfer provisions of ASC 944, Financial Services – Insurance (“ASC 944”), have been met. As a result, when the Company experiences loss or claims events that are subject to a reinsurance contract, reinsurance recoverables are recorded. The amount of the reinsurance recoverable can vary based on the terms of the reinsurance contract, the size of the individual loss or claim, or the aggregate amount of all losses or claims in a particular line or book of business or an aggregate amount associated with a particular accident year. The valuation of losses or claims recoverable depends on whether the underlying loss or claim is a reported loss or claim, or an incurred but not reported loss. For reported losses and claims, the Company values reinsurance recoverables at the time the underlying loss or claim is recognized, in accordance with contract terms. For incurred but not reported losses, the Company estimates the amount of reinsurance recoverables based on the terms of the reinsurance contracts and historical reinsurance recovery information and applies that information to the gross loss reserve. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured business and the balance is disclosed separately in the financial statements. However, the ultimate amount of the reinsurance recoverable is not known until all losses and claims are settled. Allowances are established for amounts deemed uncollectible and reinsurance recoverables are recorded net of these allowances. The Company evaluates the financial condition of its reinsurers and monitors concentration risk to minimize its exposure to significant credit losses from individual reinsurers.
	H. Property, Equipment and Capitalized Software
	Property, equipment, leasehold improvements and capitalized software are recorded at cost, less accumulated depreciation and amortization. Depreciation is generally provided using the straight-line method over the estimated useful lives of the related assets, which generally range from 3 to 30 years. The estimated useful life for capitalized software is generally 5 to 7 years. Amortization of leasehold improvements is provided using the straight-line method over the lesser of the term of the leases or the estimated useful life of the improvements.
	The Company tests for the recoverability of long-lived assets whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. The Company recognizes impairment losses only to the extent that the carrying amounts of long-lived assets exceed the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the assets. When an impairment loss occurs, the Company reduces the carrying value of the asset to fair value and no longer depreciates the asset. Fair values are estimated using discounted cash flow analysis.
	In 2013, the Company consolidated its operations in Howell, Michigan from two buildings into one building. This resulted in a plan to relocate the employees and pursue the sale of one of the buildings. During the fourth quarter of 2014 the building was sold. In 2013, the Company recognized a loss of $4.7 million in conjunction with the plan to dispose of the building. This was included in other operating expenses in the Consolidated Statements of Income during that year.
	I. GOODWILL AND INTANGIBLE ASSETS
	In accordance with the provisions of ASC 350, Intangibles- Goodwill and Other, the Company carries its goodwill at cost, net of amortization prior to January 1, 2002 and net of impairments. Increases to goodwill are generated through acquisition and represent the excess of the cost of an acquisition over the fair value of net assets acquired, including any intangibles acquired. Since January 1, 2002, goodwill is no longer amortized but rather, is reviewed for impairment. Additionally, acquisitions can also produce intangible assets, which have either a definite or indefinite life. Intangible assets with definite lives are amortized over that life, whereas those intangible assets determined to have an indefinite life are reviewed at least annually for impairment.
	The Company tests for the recoverability of goodwill and intangible assets with indefinite lives annually or whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. The Company recognizes impairment losses only to the extent that the carrying amounts of reporting units with goodwill exceed the fair value. The amount of the impairment loss that is recognized is determined based upon the excess of the carrying value of goodwill compared to the implied fair value of the goodwill, as determined with respect to all assets and liabilities of the reporting unit. The Company has performed its annual review of goodwill and intangible assets with indefinite lives for impairment in the fourth quarters of 2014 and 2013 with no impairments recognized. At December 31, 2014 and 2013, Chaucer held intangible assets with indefinite lives of $76.5 million and $81.3 million, respectively, which represents approximately 80% of the Company’s balance. In addition, at December 31, 2014 and 2013, goodwill held by Chaucer was $5.8 million and $6.3 million, respectively. The remaining balance relates to the U.S. Companies. Changes in the value of goodwill and intangible assets with indefinite lives from December 31, 2013 represent foreign exchange differences arising during 2014.
	J. LIABILITIES FOR LOSSES, LAE, AND UNEARNED PREMIUMS
	Liabilities for outstanding claims, losses and loss adjustment expenses (“LAE”) are estimates of payments to be made for reported losses and LAE and estimates of losses and LAE incurred but not reported. These liabilities are determined using case basis evaluations and statistical analyses of historical loss patterns and represent estimates of the ultimate cost of all losses incurred but not paid. These estimates are continually reviewed and adjusted as necessary; adjustments are reflected in current operations. Estimated amounts of salvage and subrogation on unpaid losses are deducted from the liability for unpaid claims.
	Premiums for direct and assumed business are reported as earned on a pro-rata basis over the contract period. The unexpired portion of these premiums is recorded as unearned premiums.
	All losses, LAE and unearned premium liabilities are based on the various estimates discussed above. Although the adequacy of these amounts cannot be assured, the Company believes that it is more likely than not that these liabilities and accruals will be sufficient to meet future obligations of policies in force. The amount of liabilities and accruals, however, could be revised in the near-term if the estimates discussed above are revised.
	K. Debt
	The Company’s debt at December 31, 2014 includes senior debentures, subordinated debentures, and collateralized borrowings with the Federal Home Loan Bank of Boston (“FHLBB”). The senior debentures are carried at principal amount borrowed, net of any applicable unamortized discounts. The subordinated debentures and borrowings under the FHLBB program are carried at principal amount borrowed (See Note 6 – “Debt and Credit Arrangements”).
	L. Premium, Premium Receivable, Fee Revenue and Related Expenses
	Insurance premiums written are generally recorded at the policy inception and are primarily earned on a pro rata basis over the terms of the policies for all products. Premiums written include estimates, primarily in the Chaucer segment, that are derived from multiple sources, which include the historical experience of the underlying business, similar businesses and available industry information. These estimates are regularly reviewed and updated, and any resulting adjustments are included in the current year’s results. Unearned premium reserves represent the portion of premiums written that relates to the unexpired terms of the underlying in-force insurance policies and reinsurance contracts. Premium receivables reflect the unpaid balance of premium written as of the balance sheet date. Premium receivables are generally short-term in nature and are reported net of an allowance for estimated uncollectible premium accounts. The Company reviews its receivables for collectability at the balance sheet date. The allowance for uncollectible accounts was not material as of December 31, 2014 and 2013. Ceded premiums are charged to income over the applicable term of the various reinsurance contracts with third party reinsurers. Reinsurance reinstatement premiums, when required, are recognized in the same period as the loss event that gave rise to the reinstatement premiums. Losses and related expenses are matched with premiums, resulting in their recognition over the lives of the contracts. This matching is accomplished through estimated and unpaid losses and amortization of deferred acquisition costs.
	M. Income Taxes
	The Company is subject to the tax laws and regulations of the U.S. and foreign countries in which it operates. The Company files a consolidated U.S. federal income tax return that includes the holding company and its U.S. subsidiaries. Generally, taxes are accrued at the U.S. statutory tax rate of 35% for income from the U.S. operations. The Company’s primary non-U.S. jurisdiction is the United Kingdom (“U.K.”). In July 2012, the U.K. statutory rate decreased from 26% to 24% effective April 1, 2012 and from 24% to 23% effective April 1, 2013. Further decreases were enacted in July 2013 to reduce the statutory rate from 23% to 21% effective April 1, 2014 and from 21% to 20% effective April 1, 2015. The Company accrues taxes on certain non-U.S. income that is subject to U.S. tax at the U.S. tax rate. Foreign tax credits, where available, are utilized to offset U.S. tax as permitted. Certain of our non-U.S. income is not subject to U.S. tax until repatriated. Foreign taxes on this non-U.S. income are accrued at the local foreign rate and do not have an accrual for U.S. deferred taxes since these earnings are intended to be indefinitely reinvested overseas.
	The Company’s accounting for income taxes represents its best estimate of various events and transactions.
	Deferred income taxes are generally recognized when assets and liabilities have different values for financial statement and tax reporting purposes, and for other temporary taxable and deductible differences as defined by ASC 740, Income Taxes (“ASC 740”). These temporary differences are measured at the balance sheet date using enacted tax rates expected to apply to taxable income in the years the temporary differences are expected to reverse. These differences result primarily from insurance reserves, deferred acquisition costs, tax credit carryforwards and deferred Lloyd’s underwriting income.
	The realization of deferred tax assets depends upon the existence of sufficient taxable income within the carryback or carryforward periods under the tax law in the applicable tax jurisdiction. Consideration is given to all available positive and negative evidence, including reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial operations. Valuation allowances are established if, based on available information, it is determined that it is more likely than not that all or some portion of the deferred tax assets will not be realized. Changes in valuation allowances are generally reflected in income tax expense or as an adjustment to other comprehensive income (loss) depending on the nature of the item for which the valuation allowance is being recorded.
	N. Stock-Based Compensation
	The Company recognizes the fair value of compensation costs for all share-based payments, including employee stock options, in the financial statements. Unvested awards are generally expensed on a straight line basis, by tranche, over the vesting period of the award. The Company’s stock-based compensation plans are discussed further in Note 11 – “Stock-Based Compensation Plans”.
	O. Earnings Per Share
	Earnings per share (“EPS”) for the years ended December 31, 2014, 2013 and 2012 is based on a weighted average of the number of shares outstanding during each year. Basic and diluted EPS is computed by dividing income available to common stockholders by the weighted average number of shares outstanding for the period. The weighted average shares outstanding used to calculate basic EPS differ from the weighted average shares outstanding used in the calculation of diluted EPS due to the effect of dilutive employee stock options, nonvested stock grants and other contingently issuable shares. If the effect of such items is antidilutive, the weighted average shares outstanding used to calculate diluted EPS are equal to those used to calculate basic EPS.
	Options to purchase shares of common stock whose exercise prices are greater than the average market price of the common shares are not included in the computation of diluted earnings per share because the effect would be antidilutive.
	P. Foreign Currency
	The Company’s reporting currency is the U.S. dollar. The functional currencies of the Company’s foreign operations are the U.K. pound sterling (“GBP”), U.S. dollar, and Canadian dollar. Assets and liabilities of foreign operations are translated into the U.S. dollar using the exchange rates in effect at the balance sheet date. Revenues and expenses of foreign operations are translated using the average exchange rate for the period. Gains or losses from translating the financial statements of foreign operations are recorded in the cumulative translation adjustment, as a separate component of accumulated other comprehensive income. Gains and losses arising from transactions denominated in a foreign currency, other than the Company’s functional currencies, are included in net income (loss), except for the Company’s foreign currency denominated available-for-sale investments. The Company’s foreign currency denominated available-for-sale investments’ change in exchange rates between the local currency and the functional currency at each balance sheet date represents an unrealized appreciation or depreciation in value of these securities, and is included as a component of accumulated other comprehensive income.
	The Company manages its exposure to foreign currency risk primarily by matching assets and liabilities denominated in the same currency. To the extent that assets and liabilities in foreign currencies are not matched, the Company is exposed to foreign currency risk. For functional currencies, the related exchange rate fluctuations are reflected in other comprehensive income (loss). The Company translated Chaucer’s balance sheet at December 31, 2014 and 2013 from GBP to U.S. dollars using a conversion rate of 1.56 and 1.66, respectively. The Company recognized $4.2 million,  $4.2 million and $6.3 million in foreign currency transaction gains in the Consolidated Statements of Income during the years ended December 31, 2014, 2013 and 2012, respectively.
	Q. New Accounting Pronouncements
	Recently Implemented Standards
	In July 2013, the Financial Accounting Standards Board (“FASB”) issued ASC Update No. 2013-11 (Topic 740) Presentation of an Unrecognized Tax Benefit When a Net Operating Loss Carryforward, a Similar Tax Loss, or a Tax Credit Carryforward Exists (a consensus of the FASB Emerging Issues Task Force).  This ASC update clarifies the applicable guidance for the presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. An unrecognized tax benefit, or a portion of an unrecognized tax benefit, should be presented in the financial statements as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward as long as it is available, at the reporting date under the tax law of the applicable jurisdiction, to settle any additional income taxes that would result from the disallowance of a tax position (with certain exceptions). The assessment of whether a deferred tax asset is available is based on the unrecognized tax benefit and deferred tax asset that exist at the reporting date and should be made presuming disallowance of the tax position at the reporting date. This guidance was applicable for reporting periods beginning after December 15, 2013, with early adoption permitted, and was to be applied prospectively to all unrecognized tax benefits that existed at the effective date. Retrospective application to all prior periods upon the date of adoption was permitted. The Company implemented this guidance effective January 1, 2014. The effect of implementing this guidance was not material to the Company’s financial position or results of operations.
	In March 2013, the FASB issued ASC Update No. 2013-05 (Topic 830) Foreign Currency Matters-Parent’s Accounting for the Cumulative Translation Adjustment upon Derecognition of Certain Subsidiaries or Groups of Assets within a Foreign Entity or of an Investment in a Foreign Entity (a consensus of the FASB Emerging Issues Task Force).  This ASC update clarifies the applicable guidance for the release of the cumulative translation adjustment into net income when a parent either sells all or a portion of its investment in a foreign entity. This guidance is also required to be applied when an entity no longer holds a controlling financial interest in a subsidiary or group of assets that is a nonprofit activity or a business within a foreign entity (with certain exceptions). Additionally, this update clarifies that the sale of an investment in a foreign entity includes events that result in an acquirer obtaining control of an acquiree in which it held an equity interest immediately before the acquisition date in a business combination achieved in stages. This guidance was applicable for reporting periods beginning after December 15, 2013, with early adoption permitted, and was to be applied prospectively to derecognition events occurring after the effective date. The Company implemented this guidance effective January 1, 2014. The effect of implementing this guidance was not material to the Company’s financial position or results of operations.
	In February 2013, the FASB issued ASC Update No. 2013-02 (Topic 220) Comprehensive Income Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income (“ASC Update No. 2013-02”). This guidance requires an entity to provide information about the amounts reclassified out of accumulated other comprehensive income (“AOCI”) either on the face of the Statement of Income or in the Notes to the Consolidated Financial Statements. Significant amounts reclassified out of AOCI should be provided by the respective line items of net income but only if the amount reclassified is required under U.S. GAAP to be reclassified in its entirety to net income in the same reporting period. For amounts not required to be reclassified in their entirety to net income, a cross-reference to other disclosures provided for in accordance with U.S. GAAP is required. This guidance was applicable for reporting periods beginning after December 15, 2012. The Company implemented the guidance effective January 1, 2013. The effect of implementing the guidance relates to financial statement presentation and disclosures. (See disclosures in Note 10 – Other Comprehensive Income.)
	Recently Issued Standards
	In April 2014, the FASB issued ASC Update No. 2014-08 (Topic 205 and Topic 360) Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity. This ASC update modifies the definition of discontinued operations by limiting discontinued operations reporting to disposals of components of an entity that represent strategic shifts that have (or will have) a major effect on an entity’s operations and financial results. Also, this update requires additional financial statement disclosures about discontinued operations, as well as disposals of an individually significant component of an entity that do not qualify for discontinued operations presentation. This ASC update is effective for all disposals (or classifications as held for sale) of components of an entity that occur within annual and interim periods beginning on or after December 15, 2014 and for all businesses that, on acquisition, are classified as held for sale that also occur within interim and annual periods beginning on or after December 15, 2014. Early adoption is permitted for disposals (or classifications as held for sale) that have not been reported in financial statements previously issued or available for issuance. The Company does not expect the adoption of ASC Update 2014-08 to have a material impact on its financial position or results of operations.
	In May 2014, the FASB issued ASC Update No. 2014-09 (Topic 606) Revenue from Contracts with Customers. This ASC was issued to clarify the principles for recognizing revenue. Insurance Contracts and financial instrument transactions are not within the scope of this updated guidance, and; therefore, only an insignificant amount of the Company’s revenue is subject to this updated guidance. The updated guidance is effective for periods beginning after December 15, 2016 and is not expected to have a material effect on the Company’s financial position or results of operations.
	In August 2014, the FASB issued ASC update No. 2014-15 (Subtopic 205-40) Presentation of Financial Statements – Going Concern. This ASC update provides guidance on determining when and how to disclose going concern uncertainties in the financial statements, and requires management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date the financial statements are issued. The updated guidance is effective for annual periods ending after December 15, 2016 and interim periods thereafter. Early adoption is permitted. The Company does not expect the adoption of ASC update 2014-15 to have a material impact on its financial position or results of operations.
	R. Reclassifications
	Certain prior year amounts have been reclassified to conform to the current year presentation.

Discontinued_Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2014
Discontinued Operations [Abstract]
Discontinued Operations	2. DISCONTINUED OPERATIONS
	Discontinued operations primarily consist of the Company’s former life insurance businesses, which were sold prior to 2009, and its discontinued accident and health business.
	Our former life insurance businesses include indemnity obligations for which we have established reserves.
	During 1999, the Company exited its accident and health insurance business, consisting of its Employee Benefit Services business, its Affinity Group Underwriters business and its accident and health assumed reinsurance pool business. Prior to 1999, these businesses comprised substantially all of the former Corporate Risk Management Services segment. Accordingly, the operating results of the discontinued segment have been reported in accordance with Accounting Principles Board Opinion No. 30, Reporting the Results of Operations—Reporting the Effects of Disposal of a Segment of a Business, and Extraordinary, Unusual and Infrequently Occurring Events and Transactions (“APB Opinion No. 30”). On January 2, 2009, Hanover Insurance directly assumed a portion of the accident and health business; and therefore continues to apply APB Opinion No. 30 to this business. In addition, the remainder of the Discontinued First Allmerica Financial Life Insurance Company (“FAFLIC”) accident and health business was reinsured by Hanover Insurance in connection with the sale of FAFLIC to Commonwealth Annuity, and has been reported in accordance with ASC 205, Presentation of Financial Statements.
	At December 31, 2014 and 2013, the portion of the discontinued accident and health business that was directly assumed had assets of $65.2 million and $66.6 million, respectively, consisting primarily of invested assets, and liabilities of $49.7 million and $51.3 million, respectively, consisting primarily of policy liabilities. At December 31, 2014 and 2013, the assets and liabilities of this business, as well as those of the reinsured portion of the accident and health business are classified as assets and liabilities of discontinued operations in the Consolidated Balance Sheets.
	Discontinued operations for the years ended December 31, 2014 and 2013 resulted in losses of $0.3 million and gains of $5.3 million, respectively, net of tax. The 2013 benefit associated with the Company’s former life insurance businesses was primarily due to an insurance settlement related to a class action lawsuit.
	On April 30, 2012, the Company completed the sale of its third party administration subsidiary, Citizens Management, Inc. (“CMI”). The Company recognized net gains of $10.8 million after taxes related to this transaction during the year ended December 31, 2012. Included in this amount was a contingent gain with a fair value of $1.7 million that was entirely contributed to the Company’s charitable foundation.

Investments

Investments	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Investments	3. INVESTMENTS
	A. FIXED MATURITIES AND EQUITY SECURITIES
	The amortized cost and fair value of available-for-sale fixed maturities and the cost and fair value of equity securities were as follows:
	31-Dec-14
	(in millions)		Amortized		Gross		Gross				OTTI
			Cost or		Unrealized		Unrealized				Unrealized
			Cost		Gains		Losses		Fair Value		Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	516.3	$	7.6	$	3.5	$	520.4	$	-
	Foreign government		349.4		5.2		0.6		354.0		-
	Municipal		1,079.6		62.4		4.0		1,138.0		-
	Corporate		3,746.3		166.3		31.8		3,880.8		7.4
	Residential mortgage-backed		770.4		21.7		3.0		789.1		0.4
	Commercial mortgage-backed		516.7		12.4		1.3		527.8		-
	Asset-backed		167.0		1.2		0.2		168.0		-
	Total fixed maturities	$	7,145.7	$	276.8	$	44.4	$	7,378.1	$	7.8
	Equity securities	$	506.6	$	76.8	$	2.6	$	580.8	$	-
	31-Dec-13
	(in millions)		Amortized		Gross		Gross				OTTI
			Cost or		Unrealized		Unrealized				Unrealized
			Cost		Gains		Losses		Fair Value		Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	417.5	$	3.3	$	14.2	$	406.6	$	-
	Foreign government		304.5		2.1		1.6		305.0		-
	Municipal		1,108.0		37.4		19.1		1,126.3		-
	Corporate		3,690.2		171.5		37.5		3,824.2		8.6
	Residential mortgage-backed		722.8		20.1		14.1		728.8		1.6
	Commercial mortgage-backed		405.9		10.5		4.8		411.6		-
	Asset-backed		166.3		2.0		0.2		168.1		-
	Total fixed maturities	$	6,815.2	$	246.9	$	91.5	$	6,970.6	$	10.2
	Equity securities	$	366.5	$	66.9	$	3.2	$	430.2	$	-
	OTTI unrealized losses in the tables above represent OTTI recognized in accumulated other comprehensive income. This amount excludes net unrealized gains on impaired securities relating to changes in the value of such securities subsequent to the impairment measurement date of $12.3 million and $16.4 million as of December 31, 2014 and 2013, respectively.
	The Company participates in a security lending program for the purpose of enhancing income. Securities on loan to various counterparties had a fair value of $20.9 million and $13.6 million at December 31, 2014 and 2013, respectively, and were fully collateralized by cash. The fair value of the loaned securities is monitored on a daily basis, and the collateral is maintained at a level of at least 102% of the fair value of the loaned securities. Securities lending collateral is recorded by the Company in cash and cash equivalents, with an offsetting liability included in expenses and taxes payable.
	At December 31, 2014 and 2013, fixed maturities with fair values of $211.2 million and $142.4 million, respectively, and amortized cost of $196.4 million and $135.2 million, respectively, were on deposit with various state and governmental authorities.
	In accordance with Lloyd’s operating guidelines, the Company deposits funds at Lloyd’s to support underwriting operations. These funds are available only to fund claim obligations. At December 31, 2014 and 2013, fixed maturities with a fair value of approximately $450 million and $334 million, respectively, and cash of $2 million and $1 million, respectively, were on deposit with Lloyd’s.
	The Company enters into various agreements that may require its fixed maturities to be held as collateral by others. At December 31, 2014 and 2013, fixed maturities with a fair value of $208.2 million and $160.9 million, respectively, were held as collateral for collateralized borrowings and other arrangements. Of these amounts, $195.7 million and $148.1 million related to the FHLBB collateralized borrowing program at December 31, 2014 and 2013, respectively. See Note 6—“Debt and Credit Arrangements” for additional information related to the Company’s FHLBB program.
	The amortized cost and fair value by maturity periods for fixed maturities are shown below. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers.
	31-Dec		2014
	(in millions)		Amortized
			Cost		Fair Value
	Due in one year or less	$	498.1	$	502.5
	Due after one year through five years		2,300.0		2,388.0
	Due after five years through ten years		2,267.3		2,337.5
	Due after ten years		626.2		665.2
			5,691.6		5,893.2
	Mortgage-backed and asset-backed securities		1,454.1		1,484.9
	Total fixed maturities	$	7,145.7	$	7,378.1
	B. DERIVATIVE INSTRUMENTS
	The Company maintains an overall risk management strategy that incorporates the use of derivative instruments, as necessary, to manage significant unplanned fluctuations in earnings that may be caused by foreign currency exchange and interest rate volatility.
	The Company did not use derivative instruments in 2014 or 2013. In 2012, the Company realized a loss of $5.1 million on futures contracts relating to the realization, for tax purposes only, of unrealized gains in its investment portfolio. Additionally, the Company utilized a foreign currency forward contract to mitigate changes in fair value caused by foreign currency fluctuation in converting GBP denominated securities into their U.S. dollar denominated equivalent. The contract was terminated in March 2012 and the Company recognized a gain of $0.7 million.
	C. UNREALIZED GAINS AND LOSSES
	Unrealized gains and losses on available-for-sale and other securities are summarized in the following table.
	YEARS ENDED DECEMBER 31
	(in millions)				Equity
			Fixed		Securities and
	2014		Maturities		Other		Total
	Net appreciation, beginning of year	$	212.1	$	47.2	$	259.3
	Net appreciation on available-for-sale securities		76.3		10.2		86.5
	Change in OTTI losses recognized in other comprehensive income		2.4		-		2.4
	Provision for deferred income taxes		-40.8		-6.5		-47.3
			37.9		3.7		41.6
	Net appreciation, end of year	$	250.0	$	50.9	$	300.9
	2013
	Net appreciation, beginning of year	$	410.1	$	15.9	$	426.0
Net (depreciation) appreciation on available-for-sale securities		-273.6		48.1		-225.5
Change in OTTI losses recognized in other comprehensive income		0.8		-		0.8
Benefit (provision) for deferred income taxes		74.8		-16.8		58.0
		-198		31.3		-166.7
Net appreciation, end of year	$	212.1	$	47.2	$	259.3
2012
Net appreciation, beginning of year	$	300.2	$	8.5	$	308.7
Net appreciation on available-for-sale securities and derivative instruments		140.7		10.6		151.3
Change in OTTI losses recognized in other comprehensive income		12.2		-		12.2
Provision for deferred income taxes		-43		-3.2		-46.2
		109.9		7.4		117.3
Net appreciation, end of year	$	410.1	$	15.9	$	426.0
Equity securities and other balances at December 31, 2014, 2013 and 2012 include after-tax net appreciation on other invested assets of $2.6 million, $2.9 million and $2.1 million, respectively.
D. SECURITIES IN AN UNREALIZED LOSS POSITION
The following tables provide information about the Company’s fixed maturities and equity securities that were in an unrealized loss position at December 31, 2014 and 2013 including the length of time the securities have been in an unrealized loss position:
31-Dec-14		12 months or less				Greater than 12 months				Total
(in millions)		Gross				Gross				Gross
		Unrealized		Fair		Unrealized		Fair		Unrealized		Fair
		Losses		Value		Losses		Value		Losses		Value
Fixed maturities:
Investment grade:
U.S. Treasury and government
agencies	$	-	$	52.2	$	3.5	$	137.9	$	3.5	$	190.1
Foreign governments		0.4		20.8		0.2		24.2		0.6		45.0
Municipal		0.3		57.1		3.7		140.2		4.0		197.3
Corporate		7.8		393.3		9.3		217.4		17.1		610.7
Residential mortgage-backed		0.2		36.4		2.8		98.0		3.0		134.4
Commercial mortgage-backed		0.4		90.4		0.9		60.8		1.3		151.2
Asset-backed		0.1		46.6		0.1		13.2		0.2		59.8
Total investment grade		9.2		696.8		20.5		691.7		29.7		1,388.5
Below investment grade:
Corporate		12.2		114.9		2.5		28.3		14.7		143.2
Total fixed maturities		21.4		811.7		23.0		720.0		44.4		1,531.7
Equity securities		2.2		130.2		0.4		3.9		2.6		134.1
Total	$	23.6	$	941.9	$	23.4	$	723.9	$	47.0	$	1,665.8
31-Dec-13		12 months or less				Greater than 12 months				Total
(in millions)		Gross				Gross				Gross
		Unrealized		Fair		Unrealized		Fair		Unrealized		Fair
		Losses		Value		Losses		Value		Losses		Value
Fixed maturities:
Investment grade:
U.S. Treasury and government
agencies	$	12.3	$	247.9	$	1.9	$	18.8	$	14.2	$	266.7
Foreign governments		1.5		129.0		0.1		17.3		1.6		146.3
Municipal		14.8		345.3		4.3		39.9		19.1		385.2
Corporate		21.4		872.7		11.6		87.7		33.0		960.4
Residential mortgage-backed		10.3		321.1		3.4		29.5		13.7		350.6
Commercial mortgage-backed		4.2		155.4		0.6		10.2		4.8		165.6
Asset-backed		0.2		31.0		-		0.3		0.2		31.3
Total investment grade		64.7		2,102.4		21.9		203.7		86.6		2,306.1
Below investment grade:
Corporate		2.9		71.9		1.6		21.4		4.5		93.3
Residential mortgage-backed		0.1		2.0		0.3		1.5		0.4		3.5
Total below investment grade		3.0		73.9		1.9		22.9		4.9		96.8
Total fixed maturities		67.7		2,176.3		23.8		226.6		91.5		2,402.9
Equity securities		2.8		45.2		0.4		0.7		3.2		45.9
Total	$	70.5	$	2,221.5	$	24.2	$	227.3	$	94.7	$	2,448.8
The Company views gross unrealized losses on fixed maturities and equity securities as being temporary since it is its assessment that these securities will recover in the near term, allowing the Company to realize the anticipated long-term economic value. The Company employs a systematic methodology to evaluate declines in fair value below amortized cost for fixed maturity securities or cost for equity securities. In determining OTTI of fixed maturity and equity securities, the Company evaluates several factors and circumstances, including the issuer’s overall financial condition; the issuer’s credit and financial strength ratings; the issuer’s financial performance, including earnings trends, dividend payments and asset quality; any specific events which may influence the operations of the issuer; the general outlook for market conditions in the industry or geographic region in which the issuer operates; and the length of time and the degree to which the fair value of an issuer’s securities remains below the Company’s cost. With respect to fixed maturity investments, the Company considers any factors that might raise doubt about the issuer’s ability to make contractual payments as they come due and whether the Company expects to recover the entire amortized cost basis of the security. With respect to equity securities, the Company considers its ability and intent to hold the investment for a period of time to allow for a recovery in value.
E. OTHER INVESTMENTS
The Company’s mortgage participations and other mortgage loans were $94.9 million and $2.5 million at December 31, 2014 and 2013, respectively.
In 2014, the Company purchased $93.4 million of participating interests in investment grade commercial mortgage loans originated and serviced by a third party.  The Company shares, on a pro-rata basis, in all related cash flows of the underlying mortgages. Due to certain reacquisition rights retained by the third party in the loan participation arrangement, the Company accounts for its participating interests in mortgage loans as secured borrowings under ASC 860.
Mortgage participations and other mortgage loans included the following property type and geographic locations. These investments are reflected in other investments in the Consolidated Balance Sheets.
31-Dec-14	Carrying Value
(in millions)
Property Type:
Office	$	40.0
Apartments		33.4
Retail and other		21.7
Valuation allowance		-0.2
Total	$	94.9
31-Dec-14	Carrying Value
(in millions)
Geographic Region:
South Atlantic	$	30.0
West South Central		25.0
Pacific		20.0
East North Central		11.4
Mid-Atlantic		8.7
Valuation allowance		-0.2
Total	$	94.9
At December 31, 2014, scheduled maturities of mortgage participations and other mortgage loans were as follows: due in one year - $0.3 million; two years - $1.4 million; five to ten years - $73.4 million; and thereafter - $19.8 million. Actual maturities could differ from contractual maturities because borrowers may have the right to prepay obligations with or without the prepayment penalties and loans may be refinanced.
During 2014, the Company did not refinance any loans based on terms that differed from current market rates.
The Company held overseas deposits of $132.6 million and $149.6  million at December 31, 2014 and 2013, respectively, which are investments held in overseas funds and managed exclusively by Lloyd’s. These investments are also reflected in other investments in the Consolidated Balance Sheets.
F. OTHER
The Company had no concentration of investments in a single investee that exceeded 10% of shareholders’ equity except for fixed maturities invested in Federal Home Loan Mortgage Corp. (“FHLMC”) and Federal National Mortgage Association (“FNMA”). At December 31, 2014, the Company held FHLMC and FNMA with fair values of $394.4 million and $315.8 million, respectively. At December 31, 2013, the Company held FHLMC and FNMA with fair values of $341.8 million and $253.2 million, respectively.
In 2012, the Company sold an office building and adjacent garage that it had developed for $75.9 million, and recognized an estimated gain of $3.3 million. In 2013, upon receipt of final construction costs, the gain was adjusted by $(0.9) million, to $2.4 million. The project was financed, in part, with $46.3 million of FHLBB advances through the Company’s membership in the FHLBB, which was repaid in January 2013.
At December 31, 2014, there were contractual investment commitments of up to $83.5 million.

Investment_Income_and_Gains_an

Investment Income and Gains and Losses	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Investment Income and Gains and Losses	4. INVESTMENT INCOME AND GAINS AND LOSSES
	A. NET INVESTMENT INCOME
	The components of net investment income were as follows:
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Fixed maturities	$	255.8	$	254.8	$	264.2
	Equity securities		16.5		15.6		15.3
	Other investments		9.0		8.8		7.0
	Gross investment income		281.3		279.2		286.5
	Less investment expenses		-11		-10.2		-9.9
	Net investment income	$	270.3	$	269.0	$	276.6
	The carrying values of fixed maturity securities on non-accrual status at December 31, 2014 and 2013 were not material. The effects of non-accruals for the years ended December 31, 2014, 2013 and 2012, compared with amounts of net investment income that would have been recognized in accordance with the original terms of the fixed maturities, were reductions of $2.1 million,  $2.3 million and $2.5 million, respectively.
	B. NET REALIZED INVESTMENT GAINS AND LOSSES
	Net realized gains (losses) on investments were as follows:
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Equity securities	$	45.8	$	26.7	$	15.5
	Fixed maturities		4.5		8.6		9.7
	Real estate		-		-0.9		3.3
	Derivative instruments		-		-		-4.4
	Other investments		-0.2		-0.9		-0.5
	Net realized investment gains	$	50.1	$	33.5	$	23.6
	Included in the net realized investment gains (losses) were OTTI of investment securities recognized in earnings totaling $5.5 million, $6.0 million and $7.8 million in 2014, 2013 and 2012, respectively.
	Other-than-temporary-impairments
	For 2014, total OTTI was $5.4 million. Of this amount, $5.5 million was recognized in earnings including $0.1 million which was transferred from unrealized losses in accumulated other comprehensive income.  The $5.5 million of OTTI recognized in earnings was related to fixed maturity securities that the Company intended to sell.
	For 2013, total OTTI was $7.0 million. Of this amount, $6.0 million was recognized in earnings and $1.0 million was recorded as unrealized losses in accumulated other comprehensive income.
	For 2012, total OTTI was $7.3 million. Of this amount, $7.8 million was recognized in earnings, including $0.5 million that was transferred from unrealized losses in accumulated other comprehensive income.
	There were no credit impairments in 2014. The methodology and significant inputs used to measure the amount of credit losses on fixed maturities in 2013 and 2012 were as follows:
	Corporate bonds - the Company utilized a financial model that derives expected cash flows based on probability-of-default factors by credit rating and asset duration and loss-given-default factors based on security type. These factors are based on historical data provided by an independent third-party rating agency.
	Asset-backed securities, including commercial and residential mortgage-backed securities - the Company utilized cash flow estimates based on bond specific facts and circumstances that include collateral characteristics, expectations of delinquency and default rates, loss severity, prepayment speeds and structural support, including subordination and guarantees.
	Municipals - the Company utilized cash flow estimates based on bond specific facts and circumstances that may include the political subdivision’s taxing authority, the issuer’s ability to adjust user fees or other sources of revenue to satisfy its debt obligations and the ability to access insurance or guarantees.
	The following table provides rollforwards of the cumulative amounts related to the Company’s credit loss portion of the OTTI losses on fixed maturity securities for which the non-credit portion of the loss is included in other comprehensive income.
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Credit losses as of the beginning of the year	$	7.8	$	8.6	$	14.5
	Credit losses for which an OTTI was not previously recognized		-		1.1		0.6
	Additional credit losses on securities for which an OTTI was previously
	recognized		-		0.3		0.6
	Reductions for securities sold, matured or called		-3.2		-2.2		-6.9
	Reductions for securities reclassified as intend to sell		-0.4		-		-0.2
	Credit losses as of the end of the year	$	4.2	$	7.8	$	8.6
	The proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses on those sales, were as follows:
	YEARS ENDED DECEMBER 31
	(in millions)
			Proceeds		Gross		Gross
	2014		from Sales		Gains		Losses
	Fixed maturities	$	349.5	$	5.8	$	2.6
	Equity securities	$	156.1	$	46.2	$	0.8
	2013
	Fixed maturities	$	450.4	$	6.0	$	4.0
	Equity securities	$	193.5	$	31.2	$	0.4
	2012
	Fixed maturities	$	616.3	$	12.1	$	2.0
	Equity securities	$	141.3	$	17.5	$	0.7

Fair_Value

Fair Value	12 Months Ended
	Dec. 31, 2014
Fair Value [Abstract]
Fair Value	5. FAIR VALUE
	Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability, i.e., exit price, in an orderly transaction between market participants. The Company emphasizes the use of observable market data whenever available in determining fair value. Fair values presented for certain financial instruments are estimates which, in many cases, may differ significantly from the amounts that could be realized upon immediate liquidation. A hierarchy of the three broad levels of fair value are as follows, with the highest priority given to Level 1 as these are the most observable, and the lowest priority given to Level 3:
	Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities.
	Level 2 – Quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data, including model-derived valuations.
	Level 3 – Unobservable inputs that are supported by little or no market activity.
	When more than one level of input is used to determine fair value, the financial instrument is classified as Level 2 or 3 according to the lowest level input that has a significant impact on the fair value measurement.
	The following methods and assumptions were used to estimate the fair value of each class of financial instruments and have not changed since last year.
	CASH AND CASH EQUIVALENTS
	The carrying amount approximates fair value. Cash equivalents primarily consist of money market instruments, which are generally valued using unadjusted quoted prices in active markets that are accessible for identical assets and are classified as Level 1.
	FIXED MATURITIES
	Level 1 securities generally include U.S. Treasury issues and other securities that are highly liquid and for which quoted market prices are available. Level 2 securities are valued using pricing for similar securities and pricing models that incorporate observable inputs including, but not limited to yield curves and issuer spreads. Level 3 securities include issues for which little observable data can be obtained, primarily due to the illiquid nature of the securities, and for which significant inputs used to determine fair value are based on the Company’s own assumptions. Non-binding broker quotes are also included in Level 3.
	The Company utilizes a third party pricing service for the valuation of the majority of its fixed maturity securities and receives one quote per security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. Since fixed maturities other than U.S. Treasury securities generally do not trade on a daily basis, the pricing service prepares estimates of fair value for those securities using pricing applications based on a market approach. Inputs into the fair value pricing common to all asset classes include: benchmark U.S. Treasury security yield curves; reported trades of identical or similar fixed maturity securities; broker/dealer quotes of identical or similar fixed maturity securities and structural characteristics such as maturity date, coupon, mandatory principal payment dates, frequency of interest and principal payments, and optional redemption features. Inputs into the fair value applications that are unique by asset class include, but are not limited to:
		·		U.S. government agencies – determination of direct versus indirect government support and whether any contingencies exist with respect to the timely payment of principal and interest.
		·		Foreign government – estimates of appropriate market spread versus underlying related sovereign treasury curve(s) dependent on liquidity and direct or contingent support.
		·		Municipals – overall credit quality, including assessments of the level and variability of: sources of payment such as income, sales or property taxes, levies or user fees; credit support such as insurance; state or local economic and political base; natural resource availability; and susceptibility to natural or man-made catastrophic events such as hurricanes, earthquakes or acts of terrorism.
		·		Corporate fixed maturities – overall credit quality, including assessments of the level and variability of: economic sensitivity; liquidity; corporate financial policies; management quality; regulatory environment; competitive position; ownership; restrictive covenants; and security or collateral.
		·		Residential mortgage-backed securities – estimates of prepayment speeds based upon: historical prepayment rate trends; underlying collateral interest rates; geographic concentration; vintage year; borrower credit quality characteristics; interest rate and yield curve forecasts; government or monetary authority support programs; tax policies; delinquency/default trends; and, in the case of non-agency collateralized mortgage obligations, severity of loss upon default and length of time to recover proceeds following default.
		·		Commercial mortgage-backed securities – overall credit quality, including assessments of the value and supply/demand characteristics of: collateral type such as office, retail, residential, lodging, or other; geographic concentration by region, state, metropolitan statistical area and locale; vintage year; historical collateral performance including defeasance, delinquency, default and special servicer trends; and capital structure support features.
		·		Asset-backed securities – overall credit quality, including assessments of the underlying collateral type such as credit card receivables, auto loan receivables and equipment lease receivables; geographic diversification; vintage year; historical collateral performance including delinquency, default and casualty trends; economic conditions influencing use rates and resale values; and contract structural support features.
	Generally, all prices provided by the pricing service, except actively traded securities with quoted market prices, are reported as Level 2.
	The Company holds privately placed fixed maturity securities and certain other fixed maturity securities that do not have an active market and for which the pricing service cannot provide fair values. The Company determines fair values for these securities using either matrix pricing utilizing the market approach or broker quotes. The Company will use observable market data as inputs into the fair value applications, as discussed in the determination of Level 2 fair values, to the extent it is available, but is also required to use a certain amount of unobservable judgment due to the illiquid nature of the securities involved. Unobservable judgment reflected in the Company’s matrix model accounts for estimates of additional spread required by market participants for factors such as issue size, structural complexity, high bond coupon, long maturity term or other unique features. These matrix-priced securities are reported as Level 2 or Level 3, depending on the significance of the impact of unobservable judgment on the security’s value. Additionally, the Company may obtain non-binding broker quotes which are reported as Level 3.
	EQUITY SECURITIES
	Level 1 consists of publicly traded securities, including exchange traded funds, valued at quoted market prices. Level 2 includes securities that are valued using pricing for similar securities and pricing models that incorporate observable inputs. Level 3 consists of common or preferred stock of private companies for which observable inputs are not available. Non-binding broker quotes are also included in Level 3.
	The Company utilizes a third party pricing service for the valuation of the majority of its equity securities and receives one quote for each equity security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. The Company holds certain equity securities that have been issued by privately-held entities that do not have an active market and for which the pricing service cannot provide fair values. Generally, the Company estimates fair value for these securities based on the issuer’s book value and market multiples. These securities are reported as Level 2 or Level 3 depending on the significance of the impact of unobservable judgment on the security’s value.
	OTHER INVESTMENTS
	Other investments consist primarily of overseas trust funds, for which fair values are provided by the investment manager based on quoted prices for similar instruments in active markets and are reported as Level 2. Also included in other investments are mortgage participations and other mortgage loans as well as cost basis limited partnerships. Fair values of mortgage participations and other mortgage loans are estimated by discounting the future contractual cash flows using the current rates at which similar loans would be made to borrowers with similar credit ratings and are reported as Level 3. The fair values of cost basis limited partnerships are based on the net asset value provided by the general partner and recent financial information and are reported as Level 3.
	DEBT
	The fair value of debt is estimated based on quoted market prices. If a quoted market price is not available, fair values are estimated using discounted cash flows that are based on current interest rates and yield curves for debt issuances with maturities and credit risks consistent with the debt being valued. Debt is reported as Level 2.
	The estimated fair value of the financial instruments were as follows:
	31-Dec			2014						2013
	(in millions)			Carrying			Fair			Carrying		Fair
				Value			Value			Value		Value
	Financial Assets
	Cash and cash equivalents		$	373.3		$	373.3		$	486.2	$	486.2
	Fixed maturities			7,378.1			7,378.1			6,970.6		6,970.6
	Equity securities			580.8			580.8			430.2		430.2
	Other investments			267.4			272.2			173.1		173.7
	Total financial assets		$	8,599.6		$	8,604.4		$	8,060.1	$	8,060.7
	Financial Liabilities
	Debt		$	903.5		$	1,021.7		$	903.9	$	961.7
	The Company has processes designed to ensure that the values received from its third party pricing service are accurately recorded, that the data inputs and valuation techniques utilized are appropriate and consistently applied and that the assumptions are reasonable and consistent with the objective of determining fair value. The Company performs a review of the fair value hierarchy classifications and of prices received from its pricing service on a quarterly basis. The Company reviews the pricing services’ policies describing its methodology, processes, practices and inputs, including various financial models used to value securities. Also, the Company reviews the portfolio pricing, including securities with changes in prices that exceed a defined threshold are verified to independent sources, if available. If upon review, the Company is not satisfied with the validity of a given price, a pricing challenge would be submitted to the pricing service along with supporting documentation for its review. The Company does not adjust quotes or prices obtained from the pricing service unless the pricing service agrees with the Company’s challenge. During 2014 and 2013, the Company did not adjust any prices received from brokers or its pricing service.
	Changes in the observability of valuation inputs may result in a reclassification of certain financial assets or liabilities within the fair value hierarchy. Reclassifications between levels of the fair value hierarchy are reported as of the beginning of the period in which the reclassification occurs. As previously discussed, the Company utilizes a third party pricing service for the valuation of the majority of its fixed maturities and equity securities. The pricing service has indicated that it will only produce an estimate of fair value if there is objectively verifiable information to produce a valuation. If the pricing service discontinues pricing an investment, the Company will use observable market data to the extent it is available, but may also be required to make assumptions for market based inputs that are unavailable due to market conditions.
	The following tables provide, for each hierarchy level, the Company’s assets that were measured at fair value on a recurring basis.
			31-Dec-14
	(in millions)			Total			Level 1			Level 2		Level 3
	Fixed maturities:
	U.S. Treasury and government agencies		$	520.4		$	233.5		$	286.9	$	-
	Foreign government			354.0			43.3			310.7		-
	Municipal			1,138.0			-			1,112.3		25.7
	Corporate			3,880.8			-			3,871.2		9.6
	Residential mortgage-backed, U.S. agency backed			689.2			-			689.2		-
	Residential mortgage-backed, non-agency			99.9			-			99.9		-
	Commercial mortgage-backed			527.8			-			506.4		21.4
	Asset backed			168.0			-			168.0		-
	Total fixed maturities			7,378.1			276.8			7,044.6		56.7
	Equity securities			571.5			570.3			-		1.2
	Other investments			136.4			-			132.6		3.8
	Total investment assets at fair value		$	8,086.0		$	847.1		$	7,177.2	$	61.7
			31-Dec-13
	(in millions)			Total			Level 1			Level 2		Level 3
	Fixed maturities:
	U.S. Treasury and government agencies		$	406.6		$	167.2		$	239.4	$	-
	Foreign government			305.0			45.6			259.4		-
	Municipal			1,126.3			-			1,100.7		25.6
Corporate			3,824.2			-			3,811.2		13.0
Residential mortgage-backed, U.S. agency backed			573.2			-			573.2		-
Residential mortgage-backed, non-agency			155.6			-			155.1		0.5
Commercial mortgage-backed			411.6			-			388.7		22.9
Asset backed			168.1			-			168.1		-
Total fixed maturities			6,970.6			212.8			6,695.8		62.0
Equity securities			420.9			382.3			-		38.6
Other investments			153.2			-			149.6		3.6
Total investment assets at fair value		$	7,544.7		$	595.1		$	6,845.4	$	104.2
The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value.
		31-Dec-14
(in millions)			Total			Level 1			Level 2		Level 3
Assets:
Cash and cash equivalents		$	373.3		$	373.3		$	-	$	-
Equity securities			9.3			-			9.3		-
Other investments			135.8			-			-		135.8
Liabilities:
Debt		$	1,021.7		$	-		$	1,021.7	$	-
		31-Dec-13
(in millions)			Total			Level 1			Level 2		Level 3
Assets:
Cash and cash equivalents		$	486.2		$	486.2		$	-	$	-
Equity securities			9.3			-			9.3		-
Other investments			20.5			-			2.7		17.8
Liabilities:
Debt		$	961.7		$	-		$	961.7	$	-
The tables below provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).
YEAR ENDED DECEMBER 31, 2014		Fixed Maturities
									Residential
									mortgage-		Commercial							Equity
									backed,		mortgage-			Asset-				and		Total
(in millions)			Municipal			Corporate			non-agency		backed			backed		Total		Other		Assets
Balance at beginning of year		$	25.6		$	13.0		$	0.5	$	22.9		$	-	$	62.0	$	42.2	$	104.2
Transfers into Level 3			2.2			2.2			-		-			-		4.4		-		4.4
Transfers out of Level 3			-2.6			-5.3			-		-			-		-7.9		-		-7.9
Total gains (losses):																-				-
Included in earnings			-			0.1			-		-			-		0.1		-		0.1
Included in other
comprehensive income - net appreciation (depreciation) on available-for-sale securities			0.8			-0.2			-		0.4			-		1.0		0.2		1.2
Purchases and sales:
Purchases			2.5			-			-		-			-		2.5		-		2.5
Sales			-2.8			-0.2			-0.5		-1.9			-		-5.4		-37.4		-42.8
Balance at end of year		$	25.7		$	9.6		$	-	$	21.4		$	-	$	56.7	$	5.0	$	61.7
YEAR ENDED DECEMBER 31, 2013		Fixed Maturities
									Residential
									mortgage-		Commercial							Equity
									backed,		mortgage-			Asset-				and		Total
(in millions)			Municipal			Corporate			non-agency		backed			backed		Total		Other		Assets
Balance at beginning of year		$	19.4		$	26.4		$	0.7	$	26.7		$	1.5	$	74.7	$	28.0	$	102.7
Transfers into Level 3			9.7			0.2			-		-			-		9.9		-		9.9
Transfers out of Level 3			-			-2.2			-		-			-1.5		-3.7		-0.9		-4.6
Total gains (losses):
Included in earnings			-			1.5			-		-			-		1.5		-		1.5
Included in other
comprehensive income - net appreciation (depreciation) on available-for-sale securities			-0.8			-1.6			-		-1.3			-		-3.7		15.1		11.4
Sales			-2.7			-11.3			-0.2		-2.5			-		-16.7		-		-16.7
Balance at end of year		$	25.6		$	13.0		$	0.5	$	22.9		$	-	$	62.0	$	42.2	$	104.2
During the years ended December 31, 2014 and 2013, the Company transferred fixed maturities between Level 2 and Level 3 primarily as a result of assessing the significance of unobservable inputs on the fair value measurement. There were no transfers between Level 1 and Level 2 during 2014 or 2013.
Net realized investment gains and losses due to changes in fair value that are recorded in net income for Level 3 assets were $0.1 million and $1.5 million for the years ended December 31, 2014 and 2013, respectively.
There were no Level 3 liabilities held by the Company for years ended December 31, 2014 and 2013.
The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets. Where discounted cash flows were used in the valuation of fixed maturities, the internally-developed discount rate was adjusted by the significant unobservable inputs shown in the table. Valuations for securities based on broker quotes for which there was a lack of transparency as to inputs used to develop the valuations have been excluded.
							31-Dec-14				31-Dec-13
		Valuation		Significant			Fair		Range		Fair		Range
(in millions)		Technique		Unobservable Inputs			Value		(Wtd Average)		Value		(Wtd Average)
Fixed maturities:
Municipal		Discounted		Discount for:		$	25.7			$	25.6
		cash flow		Small issue size			0.6-4.5% (2.0%)				1.0-4.0%  (2.3%)
				Above-market coupon			0.3-1.0% (0.4%)				0.3-1.0%  (0.5%)
Corporate		Discounted		Discount for:			9.4				12.8
		cash flow		Small issue size			0.5-1.0% (0.7%)				0.3-1.0%  (0.5%)
				Above-market coupon			0.3-0.8% (0.6%)				0.3-0.8%  (0.6%)
Residential mortgage-backed,
non-agency		Discounted		Discount for:			-				0.5
		cash flow		Small issue size					N/A				0.5% (0.5%)
Commercial mortgage-backed		Discounted		Discount for:			21.4				22.9
		cash flow		Above-market coupon			0.5-0.8% (0.5%)				0.5-0.8%  (0.6%)
				Credit stress					0.5% (0.5%)		0.5% (0.5%)
				Small issue size					0.5% (0.5%)		0.5% (0.5%)
				Lease structure					0.3% (0.3%)		0.3% (0.3%)
Equity securities		Market		Net tangible asset			1.1				38.5
		comparables		market multiples					1.0X (1.0X)				1.3X (1.3X)
Other		Discounted		Discount rate			3.8	18.0% (18.0%)			3.6	18.0% (18.0%)
		cash flow
Significant increases (decreases) in any of the above inputs in isolation would result in a significantly lower (higher) fair value measurement. There were no interrelationships between these inputs which might magnify or mitigate the effect of changes in unobservable inputs on the fair value measurement.

Debt_and_Credit_Arrangements

Debt and Credit Arrangements	12 Months Ended
	Dec. 31, 2014
Debt and Credit Arrangements [Abstract]
Debt and Credit Arrangements
	6. DEBT AND CREDIT ARRANGEMENTS
	Debt consists of the following:
	31-Dec		2014		2013
	(in millions)
	Senior debentures maturing June 15, 2021	$	300.0	$	300.0
	Senior debentures maturing March 1, 2020		164.6		165.1
	Senior debentures maturing October 15, 2025		81.1		81.2
	Subordinated debentures maturing March 30, 2053		175.0		175.0
	Subordinated debentures maturing February 3, 2027		59.7		59.7
	FHLBB borrowings (secured)		125.0		125.0
	Total principal debt	$	905.4	$	906.0
	Unamortized debt discount		-1.9		-2.1
	Total	$	903.5	$	903.9
	On March 20, 2013, the Company issued $175.0 million aggregate principal amount of 6.35% subordinated unsecured debentures due March 30, 2053. These debentures pay interest quarterly. The Company may redeem these debentures in whole at any time, or in part from time to time, on or after March 30, 2018, at a redemption price equal to their principal amount plus accrued and unpaid interest.
	On June 17, 2011, the Company issued $300.0 million aggregate principal amount of 6.375% senior unsecured debentures due June 15, 2021. Additionally, on February 23, 2010, the Company issued $200.0 million aggregate principal amount of 7.50% senior unsecured debentures. At December 31, 2014 and 2013, the remaining 7.50% senior debentures have a par value of $164.6 million and $165.1 million, respectively, and mature on March 1, 2020. The Company also issued 7.625% senior unsecured debentures with a par value of $200.0 million on October 16, 1995. As of December 31, 2014 and 2013, the remaining 7.625% senior debentures have a par value of $81.1 million and $81.2 million, respectively, and mature on October 15, 2025. All of the Company’s senior debentures are subject to certain restrictive covenants, including limitations on the issuance or disposition of stock of restricted subsidiaries and limitations on liens. These debentures pay interest semi-annually. In addition, the Company’s subordinated debentures maturing February 3, 2027 have a par value of $59.7 million as of December 31, 2014 and 2013 and pay cumulative dividends semi-annually at 8.207%.
	In 2013, the Company repurchased senior debentures maturing October 15, 2025, with a net carrying value of $39.2 million at a cost of $50.5 million, resulting in a loss of $11.3 million. Also in 2013, the Company repurchased senior debentures maturing March 1, 2020, with a net carrying value of $34.6 million at a cost of $43.2 million, resulting in a loss of $8.6 million.
	In 2009, Hanover Insurance received a $125.0 million FHLBB advance through its membership in the FHLBB. This collateralized advance bears interest at a fixed rate of 5.50% per annum over a twenty-year term. In July 2010, Hanover Insurance committed to an additional $46.3 million of FHLBB advances. These advances were drawn in several increments from July 2010 to January 2012 and carried fixed interest rates with a weighted average of 3.88%. In January 2013, Hanover Insurance repaid the $46.3 million of FHLBB advances plus prepayment fees of $7.8 million for a total payment of $54.1 million. These advances would have matured on July 30, 2020.
	As collateral to FHLBB, Hanover Insurance pledged government agency securities with a fair value of $195.7 million and $148.1 million, for the aggregate borrowings of $125.0 million as of December 31, 2014 and December 31, 2013, respectively. The fair value of the collateral pledged must be maintained at certain specified levels of the borrowed amount, which can vary depending on the type of assets pledged. If the fair value of this collateral declines below these specified levels, Hanover Insurance would be required to pledge additional collateral or repay outstanding borrowings. Hanover Insurance is permitted to voluntarily repay the outstanding borrowings at any time, subject to a repayment fee. As a requirement of membership in the FHLBB, Hanover Insurance maintains a certain level of investment in FHLBB stock. Total holdings of FHLBB stock were $9.3 million at December 31, 2014 and 2013.
	At December 31, 2014, the Company had a $200.0 million credit agreement which expires in November 2018. The Company had no borrowings under this agreement. Additionally, the Company had a Standby Letter of Credit Facility not to exceed £130.0 million (or $202.8 million based on the foreign currency exchange rate of 1.56 at December 31, 2014). This Letter of Credit Facility provides regulatory capital supporting Chaucer’s underwriting activities for the 2014 and 2015 years of account and each prior open year of account. Simultaneous with this agreement, THG entered into a Guaranty Agreement with Lloyds Bank plc, as Facility Agent and Security Agent, pursuant to which, THG unconditionally guarantees the obligations of Chaucer under the Letter of Credit Facility.
	Interest expense was $65.2 million,  $65.3 million, and $61.9 million in 2014, 2013 and 2012, respectively. At December 31, 2014, the Company was in compliance with the covenants associated with all of its debt indentures and credit arrangements.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Taxes [Abstract]
Income Taxes	7. INCOME TAXES
	Provisions for income taxes have been calculated in accordance with the provisions of ASC 740. A summary of the components of income before income taxes and income tax expense (benefit) in the Consolidated Statements of Income are shown below:
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Income (loss) before income taxes:
	U.S.	$	185.6		$	183.5		$	-99.1
	Non-U.S.		192.4			145.6			127.8
		$	378.0		$	329.1		$	28.7
	Income tax expense (benefit) includes the following components:
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Current:
	U.S.	$	4.5		$	8.0		$	2.3
	Non-U.S.		22.6			0.6			16.1
	Total current		27.1			8.6			18.4
	Deferred:
	U.S.		51.1			52.9			-42.7
	Non-U.S.		17.5			21.9			6.9
	Total deferred		68.6			74.8			-35.8
	Total income tax expense (benefit)	$	95.7		$	83.4		$	-17.4
	The income tax expense attributable to the consolidated results of continuing operations is different from the amount determined by multiplying income from continuing operations before income taxes by the U.S. statutory federal income tax rate of 35%. The sources of the difference and the tax effects of each were as follows:
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Expected income tax expense	$	132.3		$	115.2		$	10.0
	Tax difference related to investment disposals and maturities		-16.2			-17.5			-14.3
	Effect of foreign operations		-8.1			-10.8			-3.5
	Foreign exchange losses		-6.9			-			-
	Dividend received deduction		-2.6			-2.5			-2.8
	Tax-exempt interest		-1.7			-1.7			-1.5
	Nondeductible expenses		1.8			1.1			2.8
	Change in foreign tax rate		-			-0.7			-0.4
	Change in valuation allowance		-2.9			-			-7.7
	Other, net		-			0.3			-
	Income tax expense (benefit)	$	95.7		$	83.4		$	-17.4
	Effective tax rate		25.3	%		25.3	%		-60.6	%
	The following are the components of the Company’s deferred tax assets and liabilities (excluding those associated with its discontinued operations).
	31-Dec		2014			2013
	(in millions)
	Deferred tax assets:
	Loss, LAE and unearned premium reserves, net	$	175.9		$	185.0
	Tax credit carryforwards		125.8			123.1
	Employee benefit plans		45.7			38.2
	Operating loss carryforwards		29.1			64.5
	Other		10.6			74.9
			387.1			485.7
	Less: Valuation allowance		-			2.9
			387.1			482.8
	Deferred tax liabilities:
	Deferred acquisition costs		129.6			123.5
	Deferred Lloyd's underwriting income		54.0			13.8
	Investments, net		44.4			8.4
	Software capitalization		27.3			30.1
	Other		0.6			67.3
			255.9			243.1
	Net deferred tax asset	$	131.2		$	239.7
	Deferred tax assets are reduced by a valuation allowance if it is more likely than not that all or some portion of the deferred tax assets will not be realized.
	Included in the Company’s deferred tax asset as of December 31, 2014 is an asset of $29.1 million related to operating loss carryforwards. The pre-tax operating loss of $81.9 million was generated in the U.K. and has no expiration date. It is the Company’s opinion that the U.K. operations will generate sufficient future taxable income to utilize these loss carryforwards. The Company’s estimate of the amount and likely realization of loss carryforwards may change over time.
	The Company’s deferred tax asset as of December 31, 2014 also included assets of $110.0 million related to alternative minimum tax credit carryforwards and $15.8 million related to foreign tax credit carryforwards. The alternative minimum tax credit carryforwards have no expiration date and the foreign tax credit carryforwards will expire beginning in 2022. The Company may utilize the credits to offset regular federal income taxes due from future income, and although the Company believes that these assets are fully recoverable, there can be no certainty that future events will not affect their recoverability. The Company believes, based on objective evidence, it is more likely than not that the remaining deferred tax assets will be realized.
	During 2014, the Company released the valuation allowance related to its deferred tax assets of $2.9 million which was recorded in 2013. The write off of this valuation allowance resulted from appreciation in the Company’s investment portfolio. The Company recorded the decrease to its valuation allowance of $2.9 million as an adjustment to income tax expense.
	In prior years, the Company completed several transactions which resulted in the realization, for tax purposes only, of unrealized gains in its investment portfolio. The Company realized $69.6 million in 2012 as a result of such transactions. These transactions enabled the Company to realize capital loss carryforwards to offset these gains, and resulted in the release of the valuation allowance the Company held against the deferred tax asset related to these capital loss carryforwards. In 2012, the Company released $24.4 million of its valuation allowance which was recorded as a benefit in accumulated other comprehensive income. During 2014, 2013 and 2012, the Company recognized in income from continuing operations, related to non-operating income, $16.2 million, $17.5 million and $14.3 million, respectively. The remaining amount of  $91.5 million in accumulated other comprehensive income will be released into income from continuing operations in future years, as the investment securities subject to these transactions are sold or mature. During 2012, the Company released $35.9 million of its valuation allowance related to its deferred tax asset held at the beginning of the year. The release of this valuation allowance resulted from the aforementioned transaction which utilized capital loss carryforwards, unrealized appreciation in its investment portfolio, and net realized capital gains in its Consolidated Statements of Income. Accordingly, the Company recorded decreases in its valuation allowance of $25.3 million as an adjustment to accumulated other comprehensive income,  $7.7 million as an adjustment to income tax expense from continuing operations, and $2.9 million in discontinued operations.
	Although most of the Company’s non – U.S. income is subject to U.S. federal income tax, certain of its non-U.S. income is not subject to U.S. federal income tax until repatriated. Foreign taxes on this non – U.S. income are accrued at the local foreign tax rate, as opposed to the higher U.S. statutory tax rate, since these earnings currently are expected to be indefinitely reinvested overseas. This assumption could change, as a result of a sale of the subsidiaries, the receipt of dividends from the subsidiaries, a change in management’s intentions, or as a result of various other events. The Company has not made a provision for U.S. taxes on $23.3 million,  $20.3 million and $10.0 million of non-U.S. income for 2014, 2013 and 2012, respectively. However, in the future, if such earnings were distributed to the Company, taxes of $21.1 million would be payable on such undistributed earnings and would be reflected in the tax provision for the year in which these earnings are no longer intended to be indefinitely reinvested overseas, assuming all foreign tax credits are realized.
	The table below provides a reconciliation of the beginning and ending liability for uncertain tax positions as follows:
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
Liability at beginning of year, net	$	1.4		$	0.8		$	0.8
Additions for tax positions of prior years		-0.1			5.4			-
Settlements		-4.8			-			-
Deferred deductions		4.8			-4.8			-
Liability at end of year, net	$	1.3		$	1.4		$	0.8
In 2014, the IRS audits of the years 2007 through 2010 were settled with no material impact to the Company’s financial position or results of operations.
There are no tax positions at December 31, 2014 and 2012 for which the ultimate deductibility is highly certain, but for which there is uncertainty about the timing of such deductibility. At December 31, 2013, the ultimate deductibility of a  $4.8 million liability was highly certain, but there was uncertainty about the timing of such deductibility. Because of the impact of deferred tax accounting, other than interest and penalties, a change in the timing of deductions would not impact the annual effective tax rate.
The Company recognizes interest and penalties related to unrecognized tax benefits in federal income tax expense. The Company had accrued interest of  $0.5 million and $1.2 million as of December 31, 2014 and 2013, respectively. For the years ended December 31, 2014, 2013 and 2012 the Company recognized interest expense of $0.5 million, $0.8 million and $0.1 million, respectively. The Company has not recognized any penalties associated with unrecognized tax benefits.
The Company or its subsidiaries files income tax returns in the U.S. federal jurisdiction and various state jurisdictions, as well as foreign jurisdictions. The Company and its subsidiaries are subject to U.S. federal income tax examinations by tax authorities for years after 2010, U.S. state income tax examinations for years after 2006 and foreign examinations for years after 2011.

Pension_Plans

Pension Plans	12 Months Ended
	Dec. 31, 2014
Pension Plans [Abstract]
Pension Plans	8. PENSION PLANS
	DEFINED BENEFIT PLANS
	The Company recognizes the funded status of its defined benefit plans in its Consolidated Balance Sheets. The funded status is measured as the difference between the fair value of plan assets and the projected benefit obligation of the Company’s defined benefit plans. The Company is required to aggregate separately all overfunded plans from all underfunded plans.
	U.S. Defined Benefit Plans
	Prior to 2005, THG provided retirement benefits to substantially all of its employees under defined benefit pension plans. These plans were based on a defined benefit cash balance formula, whereby the Company annually provided an allocation to each covered employee based on a percentage of that employee’s eligible salary, similar to a defined contribution plan arrangement. In addition to the cash balance allocation, certain transition group employees who had met specified age and service requirements as of December 31, 1994 were eligible for a grandfathered benefit based primarily on the employees’ years of service and compensation during their highest five consecutive plan years of employment. The Company’s policy for the plans is to fund at least the minimum amount required by the Employee Retirement Income Security Act of 1974 (“ERISA”).
	As of January 1, 2005, the defined benefit pension plans were frozen and since that date, no further cash balance allocations have been credited to participants. Participants’ accounts are credited with interest daily, based upon the General Agreement of Trades and Tariffs. In addition, the grandfathered benefits for the transition group were also frozen at January 1, 2005 levels with an annual transition pension adjustment calculated at an interest rate equal to 5% per year up to 35 years of completed service, and 3% thereafter. As of December 31, 2014, based on current estimates of plan liabilities and other assumptions, the projected benefit obligation of the qualified defined benefit pension plans exceeds plan assets by approximately $25 million.
	Chaucer Pension Plan
	Prior to 2002, the Chaucer segment provided defined benefit pension retirement benefits to certain of its employees. As of December 31, 2001, the defined benefit pension plan was closed to new members. The defined benefit obligation for this plan is based on the employees’ years of service and final pensionable salary. Contributions are made at least annually to this plan by both the Company and by employees. As of December 31, 2014, based on current estimates of plan liabilities and other assumptions, the projected benefit obligation of the qualified defined benefit pension plan exceeds plan assets by approximately $6 million.
	Assumptions
	In order to measure the expense associated with these plans, management must make various estimates and assumptions, including discount rates used to value liabilities, assumed rates of return on plan assets, employee turnover rates and anticipated mortality rates, for example. The estimates used by management are based on the Company’s historical experience, as well as current facts and circumstances. In addition, the Company uses outside actuaries to assist in measuring the expense and liability associated with these plans.
	The Company measures the funded status of its plans as of the date of its year-end statement of financial position. The Company utilizes a measurement date of December 31st to determine its benefit obligations, consistent with the date of its Consolidated Balance Sheets.
	Weighted average assumptions used to determine pension benefit obligations are as follows:
	31-Dec			2014			2013		2012
	U.S.
	Discount rate - qualified plan			4.38%			5.00%		4.25%
	Discount rate - non-qualified plan			4.25%			5.00%		4.13%
	Cash balance interest crediting rate			3.50%			3.50%		3.50%
	Chaucer
	Discount rate			3.75%			4.50%		4.80%
	Rate of increase in future compensation			3.00%			3.15%		4.20%
	The Company utilizes a measurement date of January 1st to determine its periodic pension costs. Weighted average assumptions used to determine net periodic pension costs for the defined benefit plans are as follows:
	YEARS ENDED DECEMBER 31		2014		2013		2012
	U.S.:
	Qualified plan
	Discount rate		5.00%		4.25%		5.13%
	Expected return on plan assets		5.50%		5.25%		6.00%
	Cash balance interest crediting rate		3.50%		3.50%		4.00%
	Non-qualified plan
	Discount rate		5.00%		4.13%		5.00%
	Chaucer:
	Discount rate		4.50%		4.80%		4.90%
	Rate of increase in future compensation		3.15%		4.20%		4.30%
	Expected return on plan assets		6.55%		6.70%		7.50%
	The expected rates of return were determined by using historical mean returns for each asset class, adjusted for certain factors believed to have an impact on future returns. These returns are generally weighted to the plan’s actual asset allocation. For the U.S. defined benefit plans, expectations with respect to the interest rate market resulted in a slight increase in the expected return on plan assets for 2014 to 5.50%. Regarding the Chaucer defined benefit plan, because the Company began shifting the allocation of assets between equities and fixed maturities, the historical mean returns were adjusted downward slightly to reflect the plan’s asset mix. Due to the shift in asset composition, the adjusted mean returns were weighted to the plan’s expected target asset allocation at December 31, 2014, resulting in an expected rate of return on plan assets for 2014 of 6.55%. The Company reviews and updates, at least annually, its expected return on plan assets based on changes in the actual assets held by the plans.
	Plan Assets
	U.S. Qualified Defined Benefit Plans
	The Company utilizes a target allocation strategy, which focuses on creating a mix of assets that will generate modest growth from equity securities while minimizing volatility in the Company’s earnings from changes in the markets and economic environment. Various factors are taken into consideration in determining the appropriate asset mix, such as census data, actuarial valuation information and capital market assumptions. During 2014 and 2013, the plan assets were invested 85% and 84% in fixed income securities and 15% and 16% in equity securities, respectively. The Company reviews and updates, at least annually, the target allocation and makes changes periodically.
	The following table provides 2014 target allocations and actual invested asset allocations for 2014 and 2013.
	31-Dec		2014 TARGET LEVELS		2014		2013
	Fixed income securities:
	Fixed maturities		83%		84%		83%
	Money market funds		2%		1%		1%
	Total fixed income securities		85%		85%		84%
	Equity securities:
	Domestic		12%		12%		12%
	International		3%		3%		4%
	Total equity securities		15%		15%		16%
	Total plan assets		100%		100%		100%
	The following tables present for each hierarchy level the U.S. qualified defined benefit plan’s investment assets that are measured at fair value at December 31, 2014 and 2013. (Refer to Note 5 – “Fair Value” for a description of the different levels in the Fair Value Hierarchy).
	31-Dec		2014									2013
	(in millions)			Total			Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
	Fixed income securities:
	Fixed maturities		$	432.5		$	1.3	$	400.5	$	30.7	$	449.6	$	2.1	$	447.5	$	-
	Money market funds			3.9			3.9		-		-		5.3		5.3		-		-
	Total fixed income securities			436.4			5.2		400.5		30.7		454.9		7.4		447.5		-
	Equity securities:
	Domestic			60.6			-		60.6		-		67.8		0.2		67.6		-
	International			16.4			-		16.4		-		19.3		0.1		19.2		-
	Total equity securities			77.0			-		77.0		-		87.1		0.3		86.8		-
	Total investments at fair value		$	513.4		$	5.2	$	477.5	$	30.7	$	542.0	$	7.7	$	534.3	$	-
	Fixed Income Securities
	Securities classified as Level 1 at December 31, 2014 and 2013 include actively traded mutual funds and publicly traded securities, which are valued at quoted market prices. Securities classified as Level 2 at December 31, 2014 and 2013 include a separate investment account, which is invested entirely in the Vanguard Total Bond Market Index Fund, a mutual fund that in turn invests in investment grade fixed maturities, as well as commingled pools and investment grade fixed income securities that are held in a custom fund. The fair value of each of the Level 2 investments is determined daily as the Net Asset Value (“NAV”) based on the value of the underlying investments, which is determined independently by the investment manager. The daily NAV, which is not published as a quoted market price for these investments, is used as the basis for transactions. Redemption of these funds is not subject to restriction. Securities classified as Level 3 at December 31, 2014 include assets held in a fixed account of an insurance company. The fair value of the investment is estimated using a comparable public market financial institution derived fair value curve that uses non-observable inputs for market liquidity and unique credit characteristics of its underlying securities.
	Equity Securities
	Securities classified as Level 1 at December 31, 2014 and 2013 include actively traded mutual funds that are publicly traded and which primarily invest in equity securities that are valued at quoted market prices. Securities classified as Level 2 include investments in commingled pools that primarily invest in publicly traded common stocks and international equity securities. The fair value of each of the Level 2 investments is determined daily as the NAV based on the value of the underlying investments, which is determined independently by the investment manager. The daily NAV, which is not published as a quoted market price for these investments, is used as the basis for transactions. Redemption of these funds is not subject to restriction.
The table below provides a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).
YEAR ENDED DECEMBER 31			2014
(in millions)
Balance at beginning of period		$	-
Purchases			30.0
Actual return on plan assets related to assets still held			0.7
Balance at end of year		$	30.7
Chaucer Pension Plan
The investment strategy of the Chaucer defined benefit pension plan is to invest primarily in growth assets in the form of equity funds which are expected to provide a positive return that exceeds inflation over the longer term in order to protect the existing and future liabilities of the pension plan. In order to reduce volatility and diversify the portfolio, the target allocation includes an exposure to corporate bond and commercial property funds. In 2013, the Company modified its investment allocation strategy and began shifting assets from equity securities into fixed maturities. The Company shifted assets in 2014 to essentially achieve its current ultimate target level of 60% equities, 30% fixed maturities and 10% real estate funds. This allocation plan is reviewed annually and target allocation changes are made as appropriate.
The following table provides 2014 target allocations and actual invested asset allocations for 2014 and 2013.
31-Dec		2014 TARGET LEVELS		2014		2013
Fixed income securities:
Fixed maturities		30%		32%		19%
Equity securities:
Domestic (United Kingdom)		15%		14%		26%
International		45%		43%		48%
Total equity securities		60%		57%		74%
Real estate funds		10%		11%		7%
Total plan assets		100%		100%		100%
Included in total plan assets of $131.8 million at December 31, 2014 were $130.3 million of invested assets carried at fair value and $1.5 million of cash and equivalents. Total plan assets at December 31, 2013 of $120.2 million included $119.9 million of invested assets carried at fair value and $0.3 million of cash and equivalents.
The following table presents for each hierarchy level the Chaucer defined benefit plan’s investment assets that are measured at fair value at December 31, 2014 and 2013.
31-Dec		2014									2013
(in millions)			Total			Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
Fixed income securities:
Fixed maturities		$	40.9		$	-	$	40.9	$	-	$	22.8	$	-	$	22.8	$	-
Equity securities:
Domestic (United Kingdom)			18.5			-		18.5		-		30.5		-		30.5		-
International			56.9			-		56.9		-		57.8		-		57.8		-
Total equity securities			75.4			-		75.4		-		88.3		-		88.3		-
Real estate funds			14.0			-		-		14.0		8.8		-		-		8.8
Total investments at fair value		$	130.3		$	-	$	116.3	$	14.0	$	119.9	$	-	$	111.1	$	8.8
Fixed Income and Equity Securities
Securities classified as Level 2 at December 31, 2014 and 2013 include pooled funds which are valued at the close of business using a third party pricing service. These values are adjusted by the fund manager to reflect outstanding dividends, taxes and investment fees and other expenses to calculate the NAV.
Real Estate Funds
Real estate fund investments classified as Level 3 at December 31, 2014 and 2013 are valued based upon the values of the net assets of the fund. Although the NAV is calculated daily, transactions also consider cash inflows and outflows of the fund. The price where units are transacted includes the NAV, which is adjusted for investment charges and other estimated acquisition costs such as legal fees, taxes, planning and architect fees, survey and agent fees, among others.
The table below provides a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).
YEARS ENDED DECEMBER 31			2014			2013
(in millions)
Balance at beginning of period		$	8.8		$	8.2
Purchases			4.2			-
Actual return on plan assets related to assets still held			1.9			0.5
Foreign currency translation			-0.9			0.1
Balance at end of year		$	14.0		$	8.8
Obligations and Funded Status
The Company recognizes the current net underfunded status of its plans in its Consolidated Balance Sheets. Changes in the funded status of the plans are reflected as components of accumulated other comprehensive loss or income. The components of accumulated other comprehensive loss or income are reflected as either a net actuarial gain or loss or a net prior service cost.
The following table reflects the benefit obligations, fair value of plan assets and funded status of the plans at December 31, 2014 and 2013.
31-Dec		U.S. Qualified Pension Plans					U.S. Non-Qualified     Pension Plans				Chaucer Pension Plan
(in millions)			2014			2013		2014		2013		2014		2013
Accumulated benefit obligation		$	538.5		$	548.8	$	40.6	$	37.4	$	134.1	$	126.4
Change in benefit obligation:
Projected benefit obligation, beginning of
period			548.8			607.0		37.4		41.2		128.5		116.6
Employee contributions			-			-		-		-		0.4		0.4
Service cost - benefits earned during the
period			-			-		-		-		1.5		1.5
Interest cost			26.0			24.8		1.8		1.6		5.9		5.4
Actuarial losses (gains)			53.0			-36.5		4.6		-2.3		12.4		3.6
Benefits paid (1)			-89.3			-46.5		-3.2		-3.1		-2.6		-1.5
Foreign currency translation			-			-		-		-		-8.6		2.5
Projected benefit obligation, end of year			538.5			548.8		40.6		37.4		137.5		128.5
Change in plan assets:
Fair value of plan assets, beginning of period			542.0			591.6		-		-		120.2		90.9
Actual return on plan assets			59.4			-3.1		-		-		14.8		17.1
Contributions			1.3			-		3.2		3.1		7.7		10.6
Benefits paid (1)			-89.3			-46.5		-3.2		-3.1		-2.6		-1.5
Foreign currency translation			-			-		-		-		-8.3		3.1
Fair value of plan assets, end of year			513.4			542.0		-		-		131.8		120.2
Funded status of the plans		$	-25.1		$	-6.8	$	-40.6	$	-37.4	$	-5.7	$	-8.3
	-1		Includes $59.0 million paid to participants in 2014 in settlement of their pension obligations, essentially as a result of a voluntary lump-sum pension payout program and the termination of a small qualified defined benefit plan.
Contributions include $0.4 million of Chaucer employee contributions during 2014 and 2013. All other contributions for all plans were made by the Company.
Components of Net Periodic Pension Cost
The components of total net periodic pension cost are as follows:
YEARS ENDED DECEMBER 31			2014			2013		2012
(in millions)
Service cost - benefits earned during the period		$	1.5		$	1.5	$	1.6
Interest cost			33.7			31.9		35.2
Expected return on plan assets			-36.6			-35.9		-38.7
Recognized net actuarial loss			11.7			14.7		12.8
Amortization of prior service cost			0.1			-		0.1
Settlement loss			12.1			0.4		-
Net periodic pension cost		$	22.5		$	12.6	$	11.0
During 2014, the Company announced a program to offer voluntary lump-sum pension payout options to eligible former employees that, if accepted, would settle the Company’s obligation to them. The program provided participants with a one-time choice of electing to receive a lump-sum settlement of their remaining pension benefit. As part of this voluntary lump-sum program, the Company settled $55.1 million of its pension obligations with an equal amount paid from plan assets. As a result, the Company recorded settlement losses of $10.8 million, reflecting the accelerated recognition of unamortized losses in the plan proportionate to the obligation that was settled. These settlement charges were recorded in other operating expenses with a corresponding balance sheet reduction in accumulated other comprehensive income. Additionally, the Company terminated another of its small qualified plans, accelerating the recognition of unamortized losses of $1.3 million.
The following table reflects the total amounts recognized in accumulated other comprehensive income relating to both the U.S. defined benefit pension plans and the Chaucer pension plan as of December 31, 2014 and 2013.
31-Dec			2014			2013
(in millions)
Net actuarial loss		$	126.9		$	118.7
Net prior service cost			-			0.1
		$	126.9		$	118.8
The unrecognized net actuarial gains (losses) which exceed 10% of the greater of the projected benefit obligation or the fair value of plan assets are amortized as a component of net periodic pension cost over the next five years. The following table reflects the total estimated amount of actuarial losses that will be amortized from accumulated other comprehensive income into net periodic pension cost in 2014:
ESTIMATED AMORITZATION IN 2015			Expense
(in millions)
Net actuarial loss		$	12.0
Contributions
In accordance with ERISA guidelines, the Company is not required to fund its U.S. qualified benefit plan in 2015. The Company expects to contribute $3.5 million to its U.S. non-qualified pension plan to fund 2015 benefit payments and $1.3 million to the Chaucer pension plan. At this time, no additional discretionary contributions are expected to be made to the plans during 2015 and the Company does not expect that any funds will be returned from the plans to the Company during 2015.
Benefit Payments
The Company estimates that benefit payments over the next 10 years will be as follows:
YEARS ENDED DECEMBER 31			2015			2016		2017		2018		2019		2020-2024
(in millions)
U.S. qualified pension plan		$	38.7		$	38.4	$	39.4	$	38.6	$	38.3	$	186.5
U.S. non-qualified pension plans		$	3.5		$	3.2	$	3.1	$	3.1	$	3.0	$	14.2
Chaucer pension plan		$	2.7		$	1.6	$	1.7	$	1.7	$	1.8	$	9.7
The benefit payments are based on the same assumptions used to measure the Company’s benefit obligations at the end of 2014. Benefit payments related to the U.S. qualified plan and the Chaucer plan will be made from plan assets, whereas those payments related to the non-qualified plans will be provided for by the Company. Expected benefits related to the Chaucer pension plan were converted to U.S. dollars at an exchange rate of 1.56.
DEFINED CONTRIBUTION PLAN
In addition to the defined benefit plans, THG provides a defined contribution 401(k) plan for its U.S. employees, whereby the Company matches employee elective 401(k) contributions, up to a maximum of 6% of eligible compensation in 2014, 2013, and 2012. The Company’s expense for this matching provision was $19.9 million, $18.3 million and $17.9 million for 2014, 2013 and 2012, respectively. In addition to this matching provision, the Company can elect to make an annual contribution to employees’ accounts. Additional contributions amounted to $2.1 million and $2.8 million for the 2014 and 2013 plan years, respectively. There were no additional contributions in 2012.
Chaucer also provides a defined contribution plan for its employees which provides for employer provided contributions. The Company’s expense for 2014, 2013 and 2012 was $6.1 million,  $5.0 million and $4.8 million, respectively.

Other_Postretirement_Benefit_P

Other Postretirement Benefit Plans	12 Months Ended
	Dec. 31, 2014
Other Postretirement Benefit Plans [Abstract]
Other Postretirement Benefit Plans
	9. OTHER POSTRETIREMENT BENEFIT PLANS
	In addition to the Company’s pension plans, the Company also has postretirement medical benefits that it provides to former agents and retirees and their dependents, and a limited number of full-time employees. The medical plans provide benefits including hospital and major medical, with certain limits, and have varying co-payments and deductibles, depending on the plan. Generally, employees who were actively employed on December 31, 1995 became eligible with at least 15 years of service after the age of 40. Effective January 1, 1996, the Company revised these benefits so as to establish limits on future benefit payments to beneficiaries of retired participants and to restrict eligibility to then current employees. In 2009, the Company changed the postretirement medical benefits, only as they relate to current employees who still qualified for participation in the plan under the above formula. For these participants, the plan now provides for only post age 65 benefits. The population of agents receiving postretirement benefits was frozen as of December 31, 2002, when the Company ceased its distribution of proprietary life and annuity products. This plan is unfunded.
	Prior to June 2013, the Company also had postretirement death benefits providing for payment at death up to the retirees’ final annual salary. In May 2013, the Company settled and defeased the life insurance portion of its postretirement benefits by decreasing the level of death benefits and concurrently fully funding the remaining benefits through the purchase of life insurance policies for the plan beneficiaries from an unaffiliated life insurer, resulting in a net settlement gain. This plan was also unfunded.
	The Company has recognized the funded status of its postretirement benefit plans in its Consolidated Balance Sheets. Since the plans are unfunded, the amount recognized in the Consolidated Balance Sheets is equal to the accumulated benefit obligation of the plans. The components of accumulated other comprehensive income or loss are reflected as either a net actuarial gain or loss or a net prior service cost.
	Obligation and Funded Status
	The following table reflects the funded status of these plans:
	31-Dec		2014			2013
	(in millions)
	Change in benefit obligation:
	Accumulated postretirement benefit obligation, beginning of year	$	17.1		$	45.2
	Service cost		0.1			0.1
	Interest cost		0.8			1.2
	Net actuarial loss (gain)		1.5			-1
	Benefits paid		-2.5			-2.8
	Plan amendment		-			-8.4
	Settlement of death benefits		-			-17.2
	Accumulated postretirement benefit obligation, end of year		17.0			17.1
	Fair value of plan assets, end of year		-			-
	Funded status of plans	$	-17		$	-17.1
	Benefit Payments
	The Company estimates that benefit payments over the next 10 years will be as follows:
	YEARS ENDING DECEMBER 31
	(in millions)
	2015	$	2.1
	2016		1.8
	2017		1.7
	2018		1.5
	2019		1.3
	2020-2024		5.4
	The benefit payments are based on the same assumptions used to measure the Company’s benefit obligation at the end of 2014 and reflect benefits attributable to estimated future service.
	Components of Net Periodic Postretirement Expense (Benefit)
	The components of net periodic postretirement expense (benefit) were as follows:
	YEARS ENDED DECEMBER 31		2014			2013		2012
	(in millions)
	Service cost	$	0.1		$	0.1	$	0.1
	Interest cost		0.8			1.2		2.1
	Recognized net actuarial loss		0.1			0.2		0.2
	Amortization of prior service cost		-1.9			-3.7		-3.8
	Net settlement gain		-			-1.6		-
	Net periodic postretirement benefit	$	-0.9		$	-3.8	$	-1.4
	The following table reflects the balances in accumulated other comprehensive income relating to the Company’s postretirement benefit plans:
	31-Dec		2014			2013
	(in millions)
	Net actuarial loss	$	3.4		$	1.9
	Net prior service cost		-1.7			-3.5
		$	1.7		$	-1.6
	The following table reflects the estimated amortization to be recognized in net periodic benefit cost in 2015:
	Estimated Amortization in 2015		Expense (Benefit)
	(in millions)
	Net actuarial loss	$	0.2
	Net prior service cost		-0.6
		$	-0.4
	Assumptions
	Employers are required to measure the funded status of their plans as of the date of their year-end statement of financial position. As such, the Company has utilized a measurement date of December 31, 2014 and 2013, to determine its postretirement benefit obligations, consistent with the date of its Consolidated Balance Sheets. Weighted average discount rate assumptions used to determine postretirement benefit obligations and periodic postretirement costs are as follows:
	YEARS ENDED DECEMBER 31	2014		2013
	Postretirement benefit obligations discount rate	4.38%		5.00%
	Postretirement benefit cost discount rate	5.00%		4.25%
	Assumed health care cost trend rates are as follows:
	31-Dec	2014		2013
	Health care cost trend rate assumed for next year	7.00%		7.00%
	Rate to which the cost trend is assumed to decline (ultimate trend rate)	5.00%		5.00%
	Year the rate reaches the ultimate trend rate	2020		2020
	A one-percentage point change in assumed health care cost trend rates in each year would have an immaterial effect on net periodic benefit cost during 2014 and accumulated postretirement benefit obligation at December 31, 2014.

Other_Comprehensive_Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Other Comprehensive Income	10. OTHER COMPREHENSIVE INCOME
	The following table provides changes in other comprehensive income.
	YEARS ENDED DECEMBER 31	2014						2013						2012
	(in millions)		Pre-Tax		Tax Benefit (Expense)		Net of Tax		Pre-Tax		Tax Benefit (Expense)		Net of Tax		Pre-Tax		Tax Benefit (Expense)		Net of Tax
	Unrealized gains (losses) on
	available-for-sale securities and derivative instruments:
	Unrealized gains (losses) arising
	during period (net of pre-tax, ceded unrealized gains (losses) of $0.8 million and ($1.2) million  for the years ended December 31, 2014 and 2013)	$	138.7	$	-45.7	$	93.0	$	-189.4	$	63.1	$	-126.3	$	186.4	$	-40.2	$	146.2
	Amount of realized gains
	from sales and other		-55.3		0.3		-55		-41.3		-3		-44.3		-31.8		-3.3		-35.1
	Portion of other-than-
	temporary impairment losses recognized in earnings		5.5		-1.9		3.6		6.0		-2.1		3.9		8.9		-2.7		6.2
	Net unrealized gains (losses)		88.9		-47.3		41.6		-224.7		58.0		-166.7		163.5		-46.2		117.3
	Pension and postretirement
	benefits:
	Net actuarial (loss) gain arising
	in the period		-33.6		11.0		-22.6		15.0		-4.2		10.8		-24.9		9.0		-15.9
	Loss on settlement of
	pension obligation		12.1		-4.2		7.9		-		-		-		-		-		-
	Amortization of net actuarial
	loss and prior service cost recognized as net periodic benefit cost		10.0		-3.5		6.5		14.9		-5.2		9.7		9.3		-3.2		6.1
	Cumulative foreign currency
	translation adjustment:
	Foreign currency translation
	recognized during the period		-7.1		2.5		-4.6		-3.1		1.1		-2		12.2		-4.3		7.9
	Other comprehensive income (loss)	$	70.3	$	-41.5	$	28.8	$	-197.9	$	49.7	$	-148.2	$	160.1	$	-44.7	$	115.4
	Reclassifications out of accumulated other comprehensive income were as follows:
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Details about Accumulated Other	Amount Reclassified from Accumulated						Affected Line Item in the Statement
	Comprehensive Income Components	Other Comprehensive Income						Where Net Income is Presented
	Unrealized gains (losses) on available-for-
	sale securities and derivative instruments	$	55.2	$	41.3	$	31.8	Net realized gains from sales and other
								Net other-than-temporary impairment losses
			-5.5		-6		-7.8	on investments recognized in earnings
			49.7		35.3		24.0	Total before tax
			1.6		5.1		6.0	Tax benefit
			51.3		40.4		30.0
			0.1		-		-0.9	Discontinued operations, net of tax
			51.4		40.4		29.1	Net of tax
	Amortization of defined benefit pension							Loss adjustment expenses and other
	and postretirement plans		-22.1		-14.9		-9.3	operating expenses
			7.7		5.2		3.2	Tax benefit
			-14.4		-9.7		-6.1	Net of tax
	Total reclassifications for the period	$	37.0	$	30.7	$	23.0	Net of tax
	The amount reclassified from accumulated other comprehensive income for the pension and postretirement benefits was allocated approximately 40% to loss adjustment expenses and 60% to other operating expenses for each of the years ended December 31, 2014, 2013 and 2012. In 2014, $12.1 million of this amount represented the accelerated recognition of unamortized losses due to the settlement of pension plan obligations.

Stockbased_Compensation

Stock-based Compensation	12 Months Ended
	Dec. 31, 2014
Stock-based Compensation [Abstract]
Stock-based Compensation	11. STOCK-BASED COMPENSATION PLANS
	On May 20, 2014 the shareholders approved The Hanover Insurance Group 2014 Long-Term Incentive Plan (the “2014 Stock Plan”). With respect to new share-based award issuances, the 2014 Stock Plan replaced The Hanover Insurance Group, Inc. 2006 Long-Term Incentive Plan (the “2006 Stock Plan”) and authorized the issuance of 6,100,000 shares in a new share pool plus any shares subject to outstanding awards under the 2006 Stock Plan that may become available for reissuance as a result of the cash settlement, forfeiture, expiration or cancellation of such awards. The 2014 Stock Plan provides for the granting of the same types of awards as the 2006 Stock Plan, which includes stock options and stock appreciation rights (“SARS”), restricted and unrestricted stock, stock units, performance and market-based stock awards, and cash awards. In accordance with the 2014 Stock Plan, the issuance of one share of common stock in the form of an option or SAR will reduce the share pool by one share, whereas the issuance of one share of common stock for the other types of stock awards provided by the plan will reduce the pool by 3.8 shares. As of December 31, 2014, there were 6,138,403 shares available for grants under the 2014 Stock Plan.
	Additionally, on May 20, 2014, the shareholders approved The Hanover Insurance Group 2014 Employee Stock Purchase Plan (the “ESPP Plan”) and the Chaucer Share Incentive Plan (the “SIP Plan”), authorizing the issuance of 2,500,000 and 750,000 shares, respectively, under such plans. As of December 31, 2014,  32,681 shares and 18,778 shares have been issued under the ESPP Plan and the SIP Plan, respectively.
	Compensation cost for the years ended December 31, 2014, 2013, and 2012 totaled $15.1 million, $12.4 million and $12.8 million, respectively. Related tax benefits were $5.3 million, $4.3 million and $4.5 million, respectively.
	STOCK OPTIONS
	Under the 2014 Stock Plan, options may be granted to eligible employees, directors or consultants at an exercise price equal to the market price of the Company’s common stock on the date of grant. Option shares may be exercised subject to the terms prescribed by the Compensation Committee of the Board of Directors (the “Committee”) at the time of grant. Options granted in 2014 and 2013 generally vest over 3 years with 33 1/3% vesting in each year, whereas options granted in 2012 generally vest over 4 years with a 50% vesting rate in the third year and a 50% vesting rate in the fourth year. Options must be exercised not later than ten years from the date of grant.
	Information on the Company’s stock options is summarized below.
	YEARS ENDED DECEMBER 31		2014					2013					2012
	(in whole shares and dollars)		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
	Outstanding, beginning of year		2,049,173		$	41.18		2,892,882		$	38.28		2,715,430		$	38.57
	Granted		687,700			58.06		537,800			42.53		529,500			36.84
	Exercised		-444,200			39.33		-1,192,134			34.70		-90,624			25.47
	Forfeited or cancelled		-56,053			45.99		-189,375			41.54		-101,574			41.45
	Expired		-			-		-			-		-159,850			44.05
	Outstanding, end of year		2,236,620		$	46.61		2,049,173		$	41.18		2,892,882		$	38.28
	Exercisable, end of year		726,321		$	43.55		783,873		$	41.16		1,630,382		$	37.18
	Cash received for options exercised for the years ended December 31, 2014, 2013 and 2012 was $10.9 million, $24.4 million and $2.3 million, respectively. The intrinsic value of options exercised for the years ended December 31, 2014, 2013 and 2012 was $10.6 million,  $19.9 million and $1.2 million, respectively.
	The excess tax expense realized from options exercised for the years ended December 31, 2014, 2013, and 2012 was $5.9 million, $9.0 million, and $0.4 million, respectively. The aggregate intrinsic value at December 31, 2014 for shares outstanding and shares exercisable was $55.2 million and $20.2 million, respectively. At December 31, 2014, the weighted average remaining contractual life for shares outstanding and shares exercisable was 6.9 years and 4.4 years, respectively. Additional information about employee options outstanding and exercisable at December 31, 2014 is included in the following table:
					Options Outstanding							Options Currently Exercisable
	Range of Exercise Prices				Number		Weighted Average Remaining Contractual Lives			Weighted Average Exercise Price		Number			Weighted Average Exercise Price
	$	34.19 to $36.50			114,500		3.84		$	34.37		114,500		$	34.37
	$	36.81			422,500		7.06			36.81		-			-
	$	37.77 to $42.15			126,300		5.15			42.14		126,300			42.14
	$	42.49			417,683		8.16			42.49		109,452			42.49
	$	45.21 to $46.28			120,000		1.97			45.83		120,000			45.83
	$	46.47 to $47.41			215,500		6.15			46.57		106,250			46.67
	$	48.46 to $54.41			151,487		2.23			48.48		149,819			48.47
	$	57.99 to $73.30			668,650		9.15			58.06		-			-
	The fair value of each option is estimated on the date of grant using the Black-Scholes option pricing model. For all options granted through December 31, 2014, the exercise price equaled the market price on the grant date. Compensation cost related to options is based upon the grant date fair value and expensed on a straight-line basis over the service period for each separately vesting portion of the option as if the option was, in substance, multiple awards.
	The weighted average grant date fair value of options granted during the years ended December 31, 2014, 2013 and 2012 was $8.96,  $7.69 and $8.87, respectively.
	The following significant assumptions were used to determine the fair value for options granted in the years indicated.
			2014			2013			2012
	Dividend yield		2.07% to 2.55	%		2.43% to 3.11	%		3.13% to 3.26	%
	Expected volatility		18.07% to 24.38	%		21.77% to 32.02	%		32.86% to 35.27	%
	Weighted average expected volatility		23.00	%		27.53	%		34.05	%
	Risk-free interest rate		0.53% - 1.92	%		0.29% - 2.02	%		0.61% - 1.06	%
	Expected term, in years		2.5 to 5.5			3.0 to 6.0			4.5 to 5.5
	The expected dividend yield is based on the Company’s dividend payout rate(s), in the year noted. Expected volatility is based generally on the Company’s historical daily stock price volatility. The risk-free rate for periods within the contractual life of the option is based on the U.S. Treasury yield curve in effect at the time of grant. The expected term of options granted represents the period of time that options are expected to be outstanding and is derived primarily using historical exercise, forfeit and cancellation behavior, along with certain other factors expected to differ from historical data.
	The fair value of shares that vested during the years ended December 31, 2014 and 2013 was $7.8 million and $2.3 million, respectively. As of December 31, 2014, the Company had unrecognized compensation expense of $4.8 million related to unvested stock options that is expected to be recognized over a weighted average period of 1.4 years.
	RESTRICTED STOCK UNITS
	Stock grants may be awarded to eligible employees at a price established by the Committee (which may be zero). Under the 2014 Stock Plan, the Company may award shares of restricted stock, restricted stock units, as well as shares of unrestricted stock. Restricted stock grants may vest based upon performance criteria, market criteria or continued employment and be in the form of shares or units. Vesting periods are established by the Committee.
	The Company granted market-based restricted share units in 2014, 2013 and 2012 and performance-based restricted share units in 2011 to certain employees. Share units granted in 2014 and 2013 generally vest after 3 years of continued employment and after the achievement of certain stock performance targets and share units granted in 2012 and 2011 vest after the achievement of certain performance targets at a rate of 50% after 3 years and the remaining 50% after 4 years of continued employment. The Company also granted restricted stock units to eligible employees in 2014 and 2013 that generally vest after 3 years of continued employment. For those restricted stock units granted in 2012, vesting is at a rate of 50% after 3 years and the remaining 50% after 4 years of continued employment. The following table summarizes information about employee restricted stock units:
	YEARS ENDED		2014					2013					2012
	31-Dec
			Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
	Time-based restricted stock
	units:
	Outstanding, beginning of
	year		525,980		$	41.20		750,837		$	40.15		768,529		$	40.17
	Granted		102,131			58.78		141,073			43.31		174,841			37.01
	Vested		-231,078			41.45		-298,388			39.51		-137,194			36.19
	Forfeited		-12,110			43.67		-67,542			41.33		-55,339			40.47
	Outstanding, end of year		384,923		$	45.63		525,980		$	41.20		750,837		$	40.15
	Performance and market-based
	restricted stock units:
	Outstanding, beginning of
	year		184,626		$	40.42		132,775		$	39.97		69,500		$	45.37
	Granted		60,338			55.73		82,795			41.85		96,150			36.91
	Vested		-22,826			44.78		-			-		-			-
	Forfeited		-3,800			37.90		-30,944			42.33		-32,875			43.28
	Outstanding, end of year		218,338		$	44.24		184,626		$	40.42		132,775		$	39.97
	In 2014, 2013 and 2012, the Company granted market-based awards totaling 56,625,  76,175,  and 92,400, respectively, to certain members of senior management, which are included in the table above as performance and market-based restricted stock activity. The vesting of these stock units is based on the relative total shareholder return (“TSR”) of the Company. This metric is generally based on relative TSR for a three year period as compared to a Property and Casualty Index of peer companies. The fair value of market based awards was estimated at the date of grant using a valuation model. These units have the potential to range from 0% to 150% of the shares disclosed. Performance-based restricted stock units are based upon the achievement of the performance metric at 100%. These units have the potential to range from 0% to 200% of the shares disclosed, which varies based on grant year and individual participation level. Increases above the 100% target level are reflected as granted in the period in which performance-based stock unit goals are achieved. Decreases below the 100% target level are reflected as forfeited.
	In 2014, performance and market-based stock units of 2,888 were included as granted due to completion levels in excess of 100% for units granted in 2012. The weighted average grant date fair value of these awards was $43.31. In 2013, performance-based stock units of  8,244 were included as forfeited due to completion levels less than 100% for units granted in 2011. The weighted average grant date fair value for these awards was $46.47. In 2012, performance-based stock units of 31,750 were included as forfeited due to completion levels of less than the threshold achievement level for shares granted in 2010. The weighted average grant date fair value for these awards was $42.15.
	The intrinsic value, which is equal to the fair value for restricted stock and for restricted stock units that vested during the years ended December 31, 2014, 2013 and 2012 was $4.2 million,  $1.8  million and $1.7 million, respectively. The intrinsic value for performance-based restricted stock units that vested in 2014 was $0.3 million. For performance-based restricted stock units that would have vested in 2013 and 2012, there was no intrinsic value since these awards were forfeited due to the aforementioned completion levels of less than threshold. Forfeitures in 2014 related to the above mentioned market-based restricted stock units.
At December 31, 2014, the aggregate intrinsic value of restricted stock and restricted stock units was $27.5 million and the weighted average remaining contractual life was 1.2  years. The aggregate intrinsic value of performance and market-based restricted stock units was $15.6 million and the weighted average remaining contractual life was 1.3 years. As of December 31, 2014, there was $11.9 million of total unrecognized compensation cost related to unvested restricted stock units and performance and market-based restricted stock units, assuming performance and market-based restricted stock units are achieved at  100% of the performance metric. The cost is expected to be recognized over a weighted average period of 1.7  years. Compensation cost associated with restricted stock, restricted stock units and performance and market-based restricted stock units is generally calculated based upon grant date fair value, which is determined using current market prices.

Earnings_Per_Share_and_Shareho

Earnings Per Share and Shareholders' Equity Transactions	12 Months Ended
	Dec. 31, 2014
Earnings Per Share and Shareholdersb Equity Transactions [Abstract]
Earnings Per Share and Shareholders' Equity Transactions	12. EARNINGS PER SHARE AND SHAREHOLDERS’ EQUITY TRANSACTIONS
	The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share:
	31-Dec		2014		2013		2012
	(in millions, except per share data)
	Basic shares used in the calculation of earnings per share		44.0		44.1		44.7
	Dilutive effect of securities:
	Employee stock options		0.5		0.3		0.2
	Non-vested stock grants		0.4		0.5		0.4
	Diluted shares used in the calculation of earnings per share		44.9		44.9		45.3
	Per share effect of dilutive securities on income from continuing operations	$	-0.12	$	-0.11	$	-0.01
	Per share effect of dilutive securities on net income	$	-0.13	$	-0.11	$	-0.02
	Diluted earnings per share during 2014, 2013 and 2012 excludes 0.7 million,  0.1 million and 1.8 million, respectively, of common shares issuable under the Company’s stock compensation plans, because their effect would be antidilutive.
	The Company’s Board of Directors has authorized aggregate repurchases of the Company’s common stock of up to $600 million. As of December 31, 2014, the Company has $116.5 million available for repurchases under these repurchase authorizations. Repurchases may be executed using open market purchases, privately negotiated transactions, accelerated repurchase programs or other transactions. The Company is not required to purchase any specific number of shares or to make purchases by any certain date under this program. During 2014, the Company purchased 0.3 million shares of the Company’s common stock at a cost of $20.4 million.

Dividend_Restrictions

Dividend Restrictions	12 Months Ended
	Dec. 31, 2014
Dividend Restrictions [Abstract]
Dividend Restrictions	13. DIVIDEND RESTRICTIONS
	U.S. INSURANCE SUBSIDIARIES
	The individual law of all states, including New Hampshire and Michigan, where Hanover Insurance and Citizens are domiciled, respectively, restrict the payment of dividends to stockholders by insurers. These laws affect the dividend paying ability of Hanover Insurance and Citizens.
	Pursuant to New Hampshire’s statute, the maximum dividends and other distributions that an insurer may pay in any twelve month period, without prior approval of the New Hampshire Insurance Commissioner, is limited to 10% of such insurer’s statutory policyholder surplus as of the preceding December 31. No dividends were declared by Hanover Insurance in 2014, 2013 or 2012. During 2015, the maximum dividend payable without prior approval is $205.2 million.
	Pursuant to Michigan’s statute, the maximum dividends and other distributions that an insurer may pay in any twelve month period, without prior approval of the Michigan Insurance Commissioner, is limited to the greater of 10% of policyholders’ surplus as of December 31 of the immediately preceding year or the statutory net income less net realized gains, for the immediately preceding calendar year. Citizens declared dividends to its parent, Hanover Insurance, totaling $66.0 million, $68.0 million and $70.0 million in 2014, 2013 and 2012, respectively. Citizens cannot pay a further dividend to its parent without prior approval until December 2015, at which time the maximum dividend payable without prior approval would be $63.4 million.
	The statutes in both New Hampshire and Michigan require that prior notice to the respective Insurance Commissioner of any proposed dividend be provided and such Commissioner may, in certain circumstances, prohibit the payment of the proposed dividend.
	CHAUCER
	Dividend payments from Chaucer to its parent are regulated by U.K. law. Dividends from Chaucer are dependent on dividends from its subsidiaries. Annual dividend payments from Chaucer are limited to retained earnings that are not restricted by capital and other requirements for business at Lloyd’s. Also, Chaucer must provide advance notice to the U.K.’s Prudential Regulation Authority (“PRA”), one of the successors to the Financial Services Authority, of certain proposed dividends or other payments from PRA regulated entities. Dividend payments to THG from Chaucer entities totaled $68.7 million and $13.9 million in 2014 and 2013, respectively.  No dividends were declared by Chaucer in 2012.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Information [Abstract]
Segment Information	14. SEGMENT INFORMATION
	The Company’s primary business operations include insurance products and services provided through four operating segments. The domestic operating segments are Commercial Lines, Personal Lines and Other, and the Company’s international operating segment is Chaucer. Commercial Lines includes commercial multiple peril, commercial automobile, workers’ compensation, and other commercial coverages, such as specialty program business, inland marine, management and professional liability and surety. Personal Lines includes personal automobile, homeowners and other personal coverages. Chaucer includes marine and aviation, energy, property, U.K. motor, and casualty and other coverages (which includes international liability, specialist coverages, and syndicate participations). Included in Other are Opus Investment Management, Inc., which markets investment management services to institutions, pension funds and other organizations; earnings on holding company assets; and, a discontinued voluntary pools business. The separate financial information is presented consistent with the way results are regularly evaluated by the chief operating decision maker in deciding how to allocate resources and in assessing performance.
	The Company reports interest expense related to debt separately from the earnings of its operating segments. This consists of interest on the Company’s senior debentures, subordinated debentures, collateralized borrowings with the Federal Home Loan Bank of Boston, and letter of credit facility. Management evaluates the results of the aforementioned segments based on operating income before taxes which also excludes interest expense on debt. Operating income before taxes excludes certain items which are included in net income, such as net realized investment gains and losses (including net gains and losses on certain derivative instruments). Such gains and losses are excluded since they are determined by interest rates, financial markets and the timing of sales. Also, operating income before taxes excludes net gains and losses on disposals of businesses and real estate assets, gains and losses related to the repayment of debt, discontinued operations, costs to acquire businesses, restructuring costs, the cumulative effect of accounting changes and certain other items. Although the items excluded from operating income before taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income before taxes enhances an investor’s understanding of the Company’s results of operations by highlighting net income attributable to the core operations of the business. However, operating income before taxes should not be construed as a substitute for income before income taxes and operating income should not be construed as a substitute for net income.
	Summarized below is financial information with respect to the Company’s business segments.
	YEARS ENDED DECEMBER 31		2014			2013		2012
	(in millions)
	Operating revenues:
	Commercial Lines	$	2,239.0		$	2,109.5	$	1,966.2
	Personal Lines		1,491.0			1,542.5		1,560.0
	Chaucer		1,279.7			1,098.2		1,030.0
	Other		7.8			10.0		10.9
	Total		5,017.5			4,760.2		4,567.1
	Net realized investment gains		50.1			33.5		23.6
	Total revenues	$	5,067.6		$	4,793.7	$	4,590.7
	Operating income (loss) before interest expense and income taxes:
	Commercial Lines:
	GAAP underwriting loss	$	-8.3		$	-11	$	-224.2
	Net investment income		149.4			143.5		142.4
	Other (expense) income		-1.2			-0.1		1.5
	Commercial Lines operating income (loss)		139.9			132.4		-80.3
	Personal Lines:
	GAAP underwriting income (loss)		22.1			37.6		-67.3
	Net investment income		71.9			75.8		86.5
	Other income		5.0			5.2		6.3
	Personal Lines operating income		99.0			118.6		25.5
	Chaucer:
	GAAP underwriting income		122.5			107.7		93.5
	Net investment income		44.2			42.7		40.2
	Other income		10.9			-		3.1
	Chaucer operating income		177.6			150.4		136.8
	Other:
	GAAP underwriting loss		-3.1			-3.4		-3.5
	Net investment income		4.8			7.0		7.2
	Other net expenses		-12			-11.6		-10.6
	Other operating loss		-10.3			-8		-6.9
	Operating income before interest expense and income taxes		406.2			393.4		75.1
	Interest on debt		-65.2			-65.3		-61.9
	Operating income before income taxes		341.0			328.1		13.2
	Non-operating income items:
	Net realized investment gains		50.1			33.5		23.6
	Loss from settlement of pension obligation		-12.1			-		-
	Net loss from repayment of debt		-0.1			-27.7		-5.1
	Loss from disposal of real estate		-			-4.7		-
	Other non-operating items		-0.9			-0.1		-3
	Income before income taxes	$	378.0		$	329.1	$	28.7
	The following table provides identifiable assets for the Company’s business segments and discontinued operations:
	31-Dec		2014			2013
	(in millions)	Identifiable Assets
	U.S. Companies	$	9,418.4		$	8,962.6
	Chaucer		4,229.3			4,301.2
	Discontinued operations		112.0			114.9
	Total	$	13,759.7		$	13,378.7
	The Company reviews the assets of its U.S. Companies collectively and does not allocate them among the Commercial Lines, Personal Lines and Other segments.
	GEOGRAPHIC CONCENTRATIONS
	The following table presents the Company’s gross premiums written (“GPW”) based on the location of the risk:
	YEARS ENDED DECEMBER 31	2014		2013		2012
		% of Total GPW
	United States	78%		78%		76%
	United Kingdom	6%		6%		6%
	Worldwide and other	16%		16%		18%
	Total	100%		100%		100%
	The worldwide and other category includes insured risks that move across multiple geographic areas, including the U.S. and U.K., due to their mobile nature or insured risks that are fixed in locations that span more than one geographic area, and risks located in a single country outside the U.S. and U.K. These contracts include, for example, marine and aviation, hull, satellite, offshore energy exploration and production risks that can move across multiple geographic areas and assumed risks where the cedant insures risks in two or more geographic zones. These risks may include U.S. and U.K. insured risks.
	Long-lived assets located outside the U.S. were not material for the years ended December 31, 2014 or 2013. The Company does not have revenue from transactions with a single agent or broker amounting to 10 percent or more of its consolidated revenue.

Lease_Commitments

Lease Commitments	12 Months Ended
	Dec. 31, 2014
Lease Commitments [Abstract]
Lease Commitments	15. LEASE COMMITMENTS
	Rental expenses for operating leases amounted to $21.4 million, $21.6 million and $22.4 million in 2014, 2013 and 2012, respectively. These expenses relate primarily to building leases of the Company. At December 31, 2014, future minimum rental payments under non-cancelable operating leases were $62.7 million, payable as follows: 2015 - $17.3 million; 2016 - $14.3 million; 2017 - $11.5 million; 2018 - $8.0 million and 2019 and thereafter - $11.6 million. It is expected that in the normal course of business, leases that expire may be renewed or replaced by leases on other property and equipment.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2014
Reinsurance [Abstract]
Reinsurance	16. REINSURANCE
	In the normal course of business, the Company seeks to reduce the losses that may arise from catastrophes or other events that cause unfavorable underwriting results by reinsuring certain levels of risk in various areas of exposure with other insurance enterprises or reinsurers. Reinsurance transactions are accounted for in accordance with the provisions of ASC 944.
	Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policy. Reinsurance contracts do not relieve the Company from its obligations to policyholders. Failure of reinsurers to honor their obligations could result in losses to the Company; consequently, allowances are established for amounts deemed uncollectible. The Company determines the appropriate amount of reinsurance based on evaluations of the risks accepted and analyses prepared by consultants and on market conditions (including the availability and pricing of reinsurance). The Company also believes that the terms of its reinsurance contracts are consistent with industry practice in that they contain standard terms with respect to lines of business covered, limit and retention, arbitration and occurrence. The Company believes that its reinsurers are financially sound. This belief is based upon an ongoing review of its reinsurers’ financial statements, reported financial strength ratings from rating agencies, reputations in the marketplace, and the analysis and guidance of THG’s reinsurance advisors.
	As a condition to conduct certain business in various states, the Company is required to participate in residual market mechanisms, facilities and pooling arrangements such as the Michigan Catastrophic Claims Association (“MCCA”). The Company is subject to concentration of risk with respect to reinsurance ceded to the MCCA. Funding for MCCA comes from assessments against automobile insurers based upon their share of insured automobiles in the state. Insurers are allowed to pass along this cost to Michigan automobile policyholders. The Company ceded to the MCCA premiums earned and losses and LAE incurred of $73.4 million and $99.2 million in 2014, $72.2 million and $76.6 million in 2013 and $74.0 million and $99.9 million in 2012, respectively. MCCA, which represented 39.7% of the total reinsurance receivable balance at December 31, 2014, is the Company’s only reinsurer representing at least 10% of its reinsurance assets. Reinsurance recoverables related to MCCA were $899.5 million and $867.0 million at December 31, 2014 and 2013, respectively. Because the MCCA is supported by assessments permitted by statute, and there have been no significant uncollectible balances from MCCA identified during the three years ending December 31, 2014, the Company believes that it has no significant exposure to uncollectible reinsurance balances from this entity.
	The following table provides the effects of reinsurance.
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Premiums written:
	Direct	$	4,777.3		$	4,599.2		$	4,515.1
	Assumed (1)		688.1			602.5			689.1
	Ceded (2)		-655.3			-649			-835.8
	Net premiums written	$	4,810.1		$	4,552.7		$	4,368.4
	Premiums earned:
	Direct	$	4,679.8		$	4,546.3		$	4,352.9
	Assumed (1)		696.4			617.7			687.0
	Ceded (2)		-665.9			-713.5			-800.8
	Net premiums earned	$	4,710.3		$	4,450.5		$	4,239.1
	Percentage of assumed to net premiums earned		14.78	%		13.88	%		16.21	%
	Losses and LAE:
	Direct	$	3,019.4		$	2,939.4		$	3,251.4
	Assumed (1)		266.1			172.3			311.4
	Ceded (2)		-358			-350.6			-588.4
	Net losses and LAE	$	2,927.5		$	2,761.1		$	2,974.4
	(1)Assumed reinsurance activity primarily relates to the Chaucer segment.
	(2)In 2013, we did not renew the capital provision reinsurance treaty with Flagstone Re. This reduced ceded premiums written, premiums earned and losses and LAE for the year ended December 31, 2013.

Liabilities_For_Outstanding_Cl

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2014
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Abstract]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses
	17. LIABILITIES FOR OUTSTANDING CLAIMS, LOSSES AND LOSS ADJUSTMENT EXPENSES
	The Company regularly updates its reserve estimates as new information becomes available and further events occur which may impact the resolution of unsettled claims. Reserve adjustments are reflected in results of operations as adjustments to losses and LAE. Often these adjustments are recognized in periods subsequent to the period in which the underlying policy was written and loss event occurred. These types of subsequent adjustments are described as “prior years’ loss reserves”. Such development can be either favorable or unfavorable to the Company’s financial results and may vary by line of business.
	The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses.
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Gross loss and LAE reserves, beginning of year	$	6,231.5	$	6,197.0	$	5,760.3
	Reinsurance recoverable on unpaid losses		2,030.4		2,074.3		1,931.8
	Net loss and LAE reserves, beginning of year		4,201.1		4,122.7		3,828.5
	Net incurred losses and LAE in respect of losses occurring in:
	Current year		3,026.6		2,837.4		2,990.2
	Prior years		-99.1		-76.3		-15.8
	Total incurred losses and LAE		2,927.5		2,761.1		2,974.4
	Net payments of losses and LAE in respect of losses occurring in:
	Current year		1,328.7		1,213.5		1,317.6
	Prior years		1,398.9		1,469.8		1,396.5
	Total payments		2,727.6		2,683.3		2,714.1
	Commutation of Chaucer Flagstone reinsurance agreement		85.7		-		-
	Effect of foreign exchange rate changes		-78		0.6		33.9
	Net reserve for losses and LAE, end of year		4,408.7		4,201.1		4,122.7
	Reinsurance recoverable on unpaid losses		1,983.0		2,030.4		2,074.3
	Gross reserve for losses and LAE, end of year	$	6,391.7	$	6,231.5	$	6,197.0
	As part of an ongoing process, reserves have been re-estimated for all prior accident years and were decreased by $99.1 million,  $76.3 million and $15.8 million in 2014, 2013 and 2012, respectively. For the years ended December 31, 2014, 2013 and 2012, these amounts include favorable loss and LAE reserve development of $104.6 million, $94.6 million and $72.6 million, respectively for Chaucer. Chaucer’s favorable development during 2014 was primarily the result of lower than expected losses in the marine and aviation lines, primarily in the 2011 through 2013 accident years, within casualty  and other lines, specialty liability lines, primarily in the 2010 and 2013 accident years, and the property line, primarily in the 2010 through 2013 accident years. Chaucer’s favorable development was also partially attributable to the favorable impact of foreign exchange rate movements on prior years’ loss reserves. For Commercial and Personal Lines, the unfavorable loss and LAE development during 2014 was primarily the result of higher than expected large losses within the commercial automobile coverages,  which includes the AIX Holdings, Inc. (“AIX”) program business, primarily related to liability coverage in the 2009 through 2012 accident years.  Partially offsetting the unfavorable development was lower than expected losses within the personal automobile line, primarily related to the 2013 accident year, lower than expected losses within the workers’ compensation line, primarily related to the 2007 through 2012 accident years, and lower than expected losses within the commercial multiple peril line, primarily related to the 2012 and 2013 accident years.
	The $76.3 million favorable loss and LAE reserve development during the year ended December 31, 2013 included favorable loss and LAE reserve development of $94.6 million for Chaucer. Chaucer’s favorable development during 2013 was primarily the result of lower than expected losses in the energy line, primarily in the 2009 through 2012 accident years, property line, primarily in the 2011 and 2012 accident years, within casualty and other lines, specialty liability lines, primarily in the 2008 accident year, and the marine and aviation line, primarily in the 2010 through 2012 accident years. For Commercial and Personal Lines, the unfavorable loss and LAE development during 2013 was primarily the result of higher than expected losses within the personal automobile line, due to severity in bodily injury coverage in the 2010 through 2012 accident years, higher than expected large losses in the commercial automobile line, primarily related to liability coverage in the 2009 through 2011 accident years, and higher than expected losses within other commercial lines, primarily in the 2010 through 2012 accident years. Partially offsetting the unfavorable development was lower than expected losses within the workers’ compensation line, primarily in the 2006 through 2011 accident years and lower involuntary pool losses, including a $3.2 million benefit from the settlement of a legal proceeding, and lower than expected losses within the commercial multi-peril line, primarily in the 2012 accident year.
	The $15.8 million favorable loss and LAE reserve development during the year ended December 31, 2012 included favorable loss and LAE reserve development of $72.6 million for Chaucer. Chaucer’s favorable development during 2012 was primarily the result of lower than expected losses in the energy line, primarily in the 2008 through 2011 accident years, marine and aviation lines, primarily in the 2007 through 2011 accident years, casualty lines, primarily in the 2010 and 2011 accident years, and the property lines, primarily in the 2009 through 2011 accident years. For Commercial and Personal Lines, the unfavorable loss and LAE development during 2012 was primarily the result of higher than expected losses within other commercial lines, primarily in the contract surety line due to the challenging macroeconomic environment for contractors, and to a lesser extent, the AIX program business, primarily related to unexpected severity in commercial automobile liability, commercial multiple peril and general liability in a limited number of programs, and from higher than expected large losses within the commercial automobile line, primarily related to liability coverage in the 2011 accident year. In addition, the Company experienced higher than expected losses within the personal automobile line, primarily related to bodily injury severity in the 2010 and 2011 accident years, and higher than expected homeowners property losses from non-catastrophe weather related activity in the 2011 accident year. Partially offsetting the unfavorable development was lower than expected losses within the commercial multiple peril line, related to the 2008 through 2011 accident years.
	Loss and LAE reserves related to asbestos and environmental damage liability, primarily in other commercial lines, were $60.6 million, $61.9 million and $60.5 million as of December 31, 2014, 2013 and 2012, respectively. Ending loss and LAE reserves for all direct business written by the Company related to asbestos and environmental damage liability, included in the reserve for losses and LAE, were $10.1 million, $11.5 million and $9.8 million, net of reinsurance of $21.4 million, $20.6 million and $20.4 million as of December 31, 2014, 2013 and 2012, respectively. As a result of the Company’s historical direct underwriting mix of Commercial Lines policies toward smaller and middle market risks, past asbestos and environmental damage liability loss experience has remained minimal in relation to the Company’s total loss and LAE incurred experience. In addition, the Company has established gross loss and LAE reserves for its run-off voluntary assumed reinsurance pool business with asbestos and environmental damage liability of $29.1 million, $29.8 million and $30.3 million at December 31, 2014, 2013 and 2012, respectively. These reserves relate to pools in which the Company has terminated its participation; however, the Company continues to be subject to claims related to years in which it was a participant. Because of the inherent uncertainty regarding the types of claims in these pools, the Company cannot provide assurance that its reserves will be sufficient.
	The Company estimates its ultimate liability for asbestos, environmental and toxic tort liability claims, whether resulting from direct business, or assumed reinsurance and pool business, based upon currently known facts, reasonable assumptions where the facts are not known, current law and methodologies currently available. Although these outstanding claims are not significant, their existence gives rise to uncertainty and are discussed because of the possibility that they may become significant. The Company believes that, notwithstanding the evolution of case law expanding liability in asbestos and environmental claims, recorded reserves related to these claims are adequate. The asbestos, environmental and toxic tort liability could be revised in the near term if the estimates used in determining the liability are revised, and any such revisions could have a material adverse effect on the Company’s results of operations for a particular quarterly or annual period or its financial position.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies [Abstract]
Commitments and Contingencies
	18. COMMITMENTS AND CONTINGENCIES
	Legal Proceedings
	Durand Litigation
	On March 12, 2007, a putative class action suit captioned Jennifer A. Durand v. The Hanover Insurance Group, Inc., and The Allmerica Financial Cash Balance Pension Plan was filed in the United States District Court for the Western District of Kentucky. The named plaintiff, a former employee who received a lump sum distribution from the Company’s Cash Balance Plan (the “Plan”) at or about the time of her termination, claims that she and others similarly situated did not receive the appropriate lump sum distribution because in computing the lump sum, the Company and the Plan understated the accrued benefit in the calculation. The plaintiff claims that the Plan underpaid her distributions and those of similarly situated participants by failing to pay an additional so-called “whipsaw” amount reflecting the present value of an estimate of future interest credits from the date of the lump sum distribution to each participant’s retirement age of 65.
	The plaintiff filed an Amended Complaint adding two new named plaintiffs and additional claims on December 11, 2009. In response, the Company filed a Motion to Dismiss on January 30, 2010. In addition to the pending claim challenging the calculation of lump sum distributions, the Amended Complaint included: (a) a claim that the Plan failed to calculate participants’ account balances and lump sum payments properly because interest credits were based solely upon the performance of each participant’s selection from among various hypothetical investment options (as the Plan provided) rather than crediting the greater of that performance or the 30 year Treasury rate; (b) a claim that the 2004 Plan amendment, which changed interest crediting for all participants from the performance of participant’s investment selections to the 30 year Treasury rate, reduced benefits in violation of the Employee Retirement Income Security Act of 1974 (“ERISA”) for participants who had account balances as of the amendment date by not continuing to provide them performance-based interest crediting on those balances; and (c) claims against the Company for breach of fiduciary duty and ERISA notice requirements arising from the various interest crediting and lump sum distribution matters of which plaintiffs complain. On March 31, 2011, the District Court granted the Company and the Plan’s Motion to Dismiss on statute of limitations grounds the additional claims set forth in (a) and (b) above, however, in response to a motion for reconsideration, the Court allowed the new breach of fiduciary duty claim to stand, but only as to plaintiffs’ “whipsaw” claim that remained in the case. On June 22, 2012, the Company and the Plan filed a Partial Motion for Summary Judgment to dismiss the “whipsaw” claim of one of the named plaintiffs who received his lump sum distribution after December 31, 2003. On October 2, 2013, the Court granted the Company and the Plan’s Partial Motion for Summary Judgment and dismissed with prejudice the “whipsaw” claim of the named plaintiff who received a lump sum distribution after December 31, 2003 and the similar claims of the putative class members he sought to represent. On December 17, 2013, the Court entered an order certifying a class to bring “whipsaw” and related breach of fiduciary duty claims consisting of all persons who received a lump sum distribution between March 1, 1997 and December 31, 2003, and a subclass to bring such claims consisting of all persons who received lump sum distributions between March 1, 1997 and March 12, 2002. On December 17, 2013, the Court also granted plaintiffs’ motion for entry of a final order allowing an immediate appeal by the two named plaintiffs added in the Amended Complaint of their dismissed claims that the 2004 Plan amendment reduced benefits in violation of ERISA, and for one of them, that his post-2003 lump sum distribution should have been greater. On January 14, 2014, the Company filed a Motion to Alter or Amend the Court’s December 17, 2013 Order requesting that the Court reverse its order making the dismissed claims final and appealable or, in the alternative, stay merits discovery on the claims remaining in the district court pending resolution of the dismissed plaintiffs’ appeal. The Court denied this motion on April 30, 2014. The appeal of the dismissal of the claims of the two named plaintiffs added in the Amended Complaint was filed on May 30, 2014 and is pending. The Company recently filed a Summary Judgment motion to dismiss another class of plaintiffs that were added with the Amended Complaint. Plaintiffs have objected to this motion pending the outcome of the foregoing appeal.
	At this time, the Company is unable to provide a reasonable estimate of the potential range of ultimate liability if the outcome of the suit is unfavorable. The extent to which any of the plaintiffs’ multiple theories of liability, some of which are overlapping and others of which are quite complex and novel, are accepted and upheld on appeal will significantly affect the Plan’s or the Company’s potential liability. The statute of limitations applicable to the class has not yet been finally determined and the extent of potential liability, if any, will depend on this final determination. In addition, assuming for these purposes that the plaintiffs prevail with respect to claims that benefits accrued or payable under the Plan were understated, then there are numerous possible theories and other variables upon which any revised calculation of benefits as requested under plaintiffs’ claims could be based. Any adverse judgment in this case against the Plan would be expected to create a liability for the Plan, with resulting effects on the Plan’s assets available to pay benefits. The Company’s future required funding of the Plan could also be impacted by such a liability.
	Other Matters
	The Company has been named a defendant in various other legal proceedings arising in the normal course of business. In addition, the Company is involved, from time to time, in examinations, investigations and proceedings by governmental and self-regulatory agencies. The potential outcome of any such action or regulatory proceedings in which the Company has been named a defendant or the subject of an inquiry or investigation, and its ultimate liability, if any, from such action or regulatory proceedings, is difficult to predict at this time. The ultimate resolutions of such proceedings are not expected to have a material effect on its financial position, although they could have a material effect on the results of operations for a particular quarter or annual period.
	Residual Markets
	The Company is required to participate in residual markets in various states, which generally pertain to high risk insureds, disrupted markets or lines of business or geographic areas where rates are regarded as excessive. The results of the residual markets are not subject to the predictability associated with the Company’s own managed business, and are significant to both the personal and commercial automobile lines of business, the workers’ compensation line of business, and the homeowners line of business.

Statutory_Financial_Informatio

Statutory Financial Information	12 Months Ended
	Dec. 31, 2014
Statutory Financial Information [Abstract]
Statutory Financial Information	19. STATUTORY FINANCIAL INFORMATION
	The Company’s U.S. insurance subsidiaries are required to file annual statements with state regulatory authorities prepared on an accounting basis prescribed or permitted by such authorities (statutory basis), as codified by the National Association of Insurance Commissioners. Permitted statutory accounting practices encompass all accounting practices that are not prescribed; such practices differ from state to state, may differ from company to company within a state, and may change in the future. The Company’s U.S. insurance subsidiaries did not have any permitted practices as of or for the years ended December 31, 2014, 2013 and 2012.
	Statutory capital and surplus differs from shareholders’ equity reported in accordance with generally accepted accounting principles primarily because under the statutory basis of accounting, deferred acquisition costs are expensed when incurred, the recognition of deferred tax assets is based on different recoverability assumptions and prior to 2013, postretirement benefit costs were based on different participant assumptions.
	The following table provides statutory net income for the year ended December 31 and statutory capital and surplus as of December 31 for the periods indicated:
	(in millions)		2014		2013		2012
	Statutory Net Income (Loss)
	U.S. Insurance Subsidiaries	$	204.3	$	193.2	$	-47.3
	Statutory Capital and Surplus
	U.S. Insurance Subsidiaries	$	2,057.1	$	1,834.3	$	1,523.4
	The minimum statutory capital and surplus necessary to satisfy the Company’s regulatory requirements was $374.6 million, $354.5 million and $329.0 million, which equals the Authorized Control Level at December 31, 2014, 2013 and 2012, respectively.

Quarterly_Results_of_Operation

Quarterly Results of Operations	12 Months Ended
	Dec. 31, 2014
Quarterly Results of Operations [Abstract]
Quarterly Results of Operations
	20. QUARTERLY RESULTS OF OPERATIONS (UNAUDITED)
	The quarterly results of operations for 2014 and 2013 are summarized below.
	THREE MONTHS ENDED
	(in millions, except per share data)
	2014		31-Mar		30-Jun		Sept. 30		Dec. 31
	Total revenues	$	1,243.9	$	1,273.0	$	1,265.6	$	1,285.1
	Income from continuing operations	$	54.7	$	82.5	$	55.0	$	90.1
	Net income	$	54.6	$	82.6	$	54.9	$	89.9
	Income from continuing operations per share:
	Basic	$	1.25	$	1.87	$	1.25	$	2.05
	Diluted	$	1.22	$	1.84	$	1.22	$	2.01
	Net income per share:
	Basic	$	1.24	$	1.88	$	1.25	$	2.04
	Diluted	$	1.22	$	1.84	$	1.22	$	2.00
	Dividends declared per share	$	0.37	$	0.37	$	0.37	$	0.41
	THREE MONTHS ENDED
	(in millions, except per share data)
	2013		31-Mar		30-Jun		Sept. 30		Dec. 31
	Total revenues	$	1,180.3	$	1,182.6	$	1,201.8	$	1,229.0
	Income from continuing operations	$	66.4	$	53.1	$	61.3	$	64.9
	Net income	$	66.2	$	53.4	$	61.3	$	70.1
	Income from continuing operations per share:
	Basic	$	1.49	$	1.21	$	1.40	$	1.48
	Diluted	$	1.47	$	1.19	$	1.37	$	1.45
	Net income per share:
	Basic	$	1.49	$	1.21	$	1.40	$	1.60
	Diluted	$	1.46	$	1.19	$	1.37	$	1.57
	Dividends declared per share	$	0.33	$	0.33	$	0.33	$	0.37
	Due to the use of weighted average shares outstanding when calculating earnings per common share, the sum of the quarterly per common share data may not equal the per common share data for the year.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	21. SUBSEQUENT EVENTS
	There were no subsequent events requiring adjustment to the financial statements and no additional disclosures required in the notes to the consolidated financial statements.

SCHEDULE_I_SUMMARY_OF_INVESTME

SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2014
SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES [Abstract]
SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES
	SCHEDULE I
	THE HANOVER INSURANCE GROUP, INC.
	SUMMARY OF INVESTMENTS – OTHER THAN INVESTMENTS IN RELATED PARTIES
	31-Dec-14
	(in millions)
	Type of investment			Cost (1)		Value		Amount at which shown in the balance sheet
	Fixed maturities:
	Bonds:
	United States Government and agencies and authorities		$	1,137.4	$	1,154.9	$	1,154.9
	States, municipalities and political subdivisions			1,078.7		1,137.1		1,137.1
	Foreign governments			339.2		343.8		343.8
	Public utilities			348.5		372.0		372.0
	All other corporate bonds			4,158.8		4,287.2		4,287.2
	Total fixed maturities			7,062.6		7,295.0		7,295.0
	Equity securities:
	Common stocks:
	Public utilities			92.7		116.4		116.4
	Banks, trusts and insurance companies			10.3		10.6		10.6
	Industrial, miscellaneous and all other			393.5		435.0		435.0
	Nonredeemable preferred stocks			10.1		18.8		18.8
	Total equity securities			506.6		580.8		580.8
	Mortgage loans on real estate (2)			94.9		XXXXX		94.9
	Real estate			10.0		XXXXX		10.0
	Other long-term investments (3)			182.9		XXXXX		186.5
	Short-term investments			83.1		XXXXX		83.1
	Total investments		$	7,940.1	$	XXXXX	$	8,250.3
		-1		For equity securities, represents original cost, and for fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.
		-2		Includes $93.4 million of participating interests in investment grade commercial mortgage loans originated and serviced by a third party. The Company shares, on a pro rata basis, in all related cash flows of the underlying mortgages. Due to certain reacquisition rights retained by the third party in the loan participation arrangement, the Company accounted for its participatory interest in mortgage loans as secured borrowings.
		-3		The cost of other long-term investments differs from the carrying value due to market value changes in the Company's equity ownership of limited partnership investments.

SCHEDULE_II_CONDENSED_FINANCIA

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY	12 Months Ended
	Dec. 31, 2014
SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY [Abstract]
SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY STATEMENTS OF INCOME	SCHEDULE II
	THE HANOVER INSURANCE GROUP, INC.
	CONDENSED FINANCIAL INFORMATION OF REGISTRANT
	PARENT COMPANY ONLY
	STATEMENTS OF INCOME
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Revenues
	Net investment income	$	3.9	$	5.5	$	6.6
	Net realized gains from sales and other		-		0.8		2.5
	Interest income from loan to subsidiary		22.5		22.5		22.5
	Other income		0.1		0.1		0.1
	Total revenues		26.5		28.9		31.7
	Expenses
	Interest expense		54.1		55.3		48.6
	Loss from retirement of debt		0.1		19.9		-
	Employee benefit related expenses		11.1		6.2		5.3
	Costs (benefit) related to acquired businesses		-		-6.4		2.6
	Other operating expenses		8.1		8.0		8.0
	Total expenses		73.4		83.0		64.5
	Net loss before income taxes and equity in income of unconsolidated subsidiaries		-46.9		-54.1		-32.8
	Income tax benefit		52.1		37.3		40.5
	Equity in income of unconsolidated subsidiaries		277.4		262.5		47.5
	Income from continuing operations		282.6		245.7		55.2
	(Loss) income from discontinued operations (net of income tax (benefit) expense
	of $(0.2),  $4.1 and $(0.1) in 2014, 2013 and 2012)		-0.6		5.3		0.7
	Net income	$	282.0	$	251.0	$	55.9
	The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto.
	SCHEDULE II (CONTINUED)
	THE HANOVER INSURANCE GROUP, INC.
	CONDENSED FINANCIAL INFORMATION OF REGISTRANT
	PARENT COMPANY ONLY
	BALANCE SHEETS
	31-Dec		2014		2013
	(in millions, except per share data)
	Assets
	Fixed maturities - at fair value (amortized cost of $102.1 and $98.5)	$	105.2	$	101.7
	Equity securities - at fair value (cost of $1.0)		1.1		1.1
	Cash and cash equivalents		14.5		20.7
	Investments in unconsolidated subsidiaries		3,129.8		2,876.0
	Net receivable from subsidiaries		20.4		23.0
	Deferred income taxes		62.8		49.1
	Current income tax receivable		-		8.3
	Loan receivable from subsidiary		300.0		300.0
	Other assets		14.0		17.6
	Total assets	$	3,647.8	$	3,397.5
	Liabilities
	Expenses and state taxes payable	$	15.0	$	16.0
	Current income tax payable		2.2		-
	Interest payable		8.1		8.1
	Debt		778.5		778.9
	Total liabilities		803.8		803.0
	Shareholders' Equity
	Preferred stock, par value $0.01 per share, 20.0 million shares authorized, none issued		-		-
	Common stock, par value $0.01 per share, 300.0 million shares authorized, 60.5 million shares issued		0.6		0.6
	Additional paid-in-capital		1,830.7		1,830.1
	Accumulated other comprehensive income		206.4		177.6
	Retained earnings		1,558.7		1,349.1
	Treasury stock at cost (16.6 and 16.8 million)		-752.4		-762.9
	Total shareholders' equity		2,844.0		2,594.5
	Total liabilities and shareholders' equity	$	3,647.8	$	3,397.5
	The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto.
	SCHEDULE II (CONTINUED)
	THE HANOVER INSURANCE GROUP, INC.
CONDENSED FINANCIAL INFORMATION OF REGISTRANT
PARENT COMPANY ONLY
STATEMENTS OF CASH FLOWS
YEARS ENDED DECEMBER 31		2014		2013		2012
(in millions)
Cash flows from operating activities
Net income	$	282.0	$	251.0	$	55.9
Adjustments to reconcile net income to net cash
provided by operating activities:
Loss (gain) from discontinued operations		0.6		2.5		-0.7
Loss from retirement of debt		0.1		19.9		0.1
Equity in net income of unconsolidated subsidiaries		-277.4		-262.5		-47.5
Net realized investment gains		-		-0.8		-2.5
Dividends received from unconsolidated subsidiaries		70.2		12.5		1.0
Deferred income tax (benefit) expense		-29.9		-32.3		3.9
Change in expenses and taxes payable		11.4		0.1		6.2
Change in net payable from subsidiaries		16.9		16.5		30.7
Other, net		4.3		3.4		2.8
Net cash provided by operating activities		78.2		10.3		49.9
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities		25.6		122.9		44.2
Purchase of fixed maturities		-28.6		-90.8		-2.2
Net cash used for business acquisitions		-2.3		-2.2		-3.4
Net cash (used in) provided by investing activities		-5.3		29.9		38.6
Cash flows from financing activities
Proceeds from debt borrowings		-		168.6		-
Repurchases of debt		-0.7		-93.6		-0.7
Dividends paid to shareholders		-67		-60		-55.1
Repurchase of common stock		-20.4		-78.2		-20
Proceeds from exercise of employee stock options		12.6		25.0		2.6
Other financing activities		-3.6		-5.1		-0.4
Net cash used in financing activities		-79.1		-43.3		-73.6
Net change in cash and cash equivalents		-6.2		-3.1		14.9
Cash and cash equivalents, beginning of year		20.7		23.8		8.9
Cash and cash equivalents, end of year	$	14.5	$	20.7	$	23.8
Included in other operating cash flows was the cash portion of dividends received from unconsolidated subsidiaries. Cash payments of $70.2  million, $12.5 million and $1.0 million in 2014, 2013 and 2012, and investment assets of $1.0 million and $17.9 million were transferred to the parent company in 2014 and 2012, respectively, to settle dividend and other intercompany balances. There were no assets transferred to the parent company from its subsidiaries in 2013 to settle balances.
The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto.

SCHEDULE_III_SUPPLEMENTARY_INS

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2014
SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION [Abstract]
SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	SCHEDULE III
	THE HANOVER INSURANCE GROUP, INC.
	SUPPLEMENTARY INSURANCE INFORMATION
	31-Dec-14
	(in millions)
	Segments		Deferred acquisition costs		Future policy benefits, losses, claims and loss expenses		Unearned premiums		Other policy claims and benefits payable		Premium revenue		Net investment income		Benefits, claims, losses and settlement expenses		Amortization of deferred acquisition costs		Other operating expenses		Premiums written
	Commercial,
	Personal	$	370.3	$	3,960.8	$	1,815.8	$	5.1	$	3,488.5	$	226.1	$	2,293.8	$	748.5	$	481.0	$	3,578.7
	and Other
	Chaucer		155.4		2,425.8		768.1		-		1,221.8		44.2		633.7		291.5		182.9		1,231.4
	Interest on Debt		-		-		-		-		-		-		-		-		65.2		-
	Eliminations		-		-		-		-		-		-		-		-		-7		-
	Total	$	525.7	$	6,386.6	$	2,583.9	$	5.1	$	4,710.3	$	270.3	$	2,927.5	$	1,040.0	$	722.1	$	4,810.1
	31-Dec-13
	(in millions)
	Segments		Deferred acquisition costs		Future policy benefits, losses, claims and loss expenses		Unearned premiums		Other policy claims and benefits payable		Premium revenue		Net investment income		Benefits, claims, losses and settlement expenses		Amortization of deferred acquisition costs		Other operating expenses		Premiums written
	Commercial,
	Personal	$	354.2	$	3,845.9	$	1,727.1	$	4.1	$	3,412.6	$	226.3	$	2,222.9	$	729.8	$	503.0	$	3,435.2
	and Other
	Chaucer		151.8		2,381.5		788.7		-		1,037.9		42.7		538.2		241.2		170.8		1,117.5
	Interest on Debt		-		-		-		-		-		-		-		-		65.3		-
	Eliminations		-		-		-		-		-		-		-		-		-6.6		-
	Total	$	506.0	$	6,227.4	$	2,515.8	$	4.1	$	4,450.5	$	269.0	$	2,761.1	$	971.0	$	732.5	$	4,552.7
	31-Dec-12
	(in millions)
	Segments		Deferred acquisition costs		Future policy benefits, losses, claims and loss expenses		Unearned premiums		Other policy claims and benefits payable		Premium revenue		Net investment income		Benefits, claims, losses and settlement expenses		Amortization of deferred acquisition costs		Other operating expenses		Premiums written
	Commercial,
	Personal	$	346.6	$	3,784.9	$	1,711.1	$	4.1	$	3,272.3	$	236.1	$	2,467.2	$	704.2	$	436.5	$	3,377.9
	and Other
	Chaucer		142.9		2,408.0		763.7		-		966.8		40.2		507.2		233.9		157.6		990.5
	Interest on Debt		-		-		-		-		-		-		-		-		61.9		-
	Eliminations		-		-		-		-		-		0.3		-		-		-6.5		-
	Total	$	489.5	$	6,192.9	$	2,474.8	$	4.1	$	4,239.1	$	276.6	$	2,974.4	$	938.1	$	649.5	$	4,368.4

SCHEDULE_IV_REINSURANCE

SCHEDULE IV REINSURANCE	12 Months Ended
	Dec. 31, 2014
SCHEDULE IV REINSURANCE [Abstract]
SCHEDULE IV REINSURANCE	SCHEDULE IV
	THE HANOVER INSURANCE GROUP, INC.
	REINSURANCE
	Incorporated herein by reference to Note 16 —“Reinsurance” in the Notes of the Consolidated Financial Statements included in Financial Statements and Supplemental Data of this Form 10-K.

SCHEDULE_V_VALUATION_AND_QUALI

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2014
SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS [Abstract]
SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS
	SCHEDULE V
	THE HANOVER INSURANCE GROUP, INC.
	VALUATION AND QUALIFYING ACCOUNTS
	31-Dec
					Additions
	(in millions)			Balance at beginning of period		Charged to costs and expenses		Charged to other accounts(1)		Deductions		Balance at end of period
	Description
	2014
	Allowance for doubtful accounts		$	3.1	$	9.4	$	-	$	-9.3	$	3.2
	Allowance for uncollectible reinsurance recoverables			20.7		0.9		-0.3		-5.4		15.9
			$	23.8	$	10.3	$	-0.3	$	-14.7	$	19.1
	2013
	Allowance for doubtful accounts		$	2.8	$	8.1	$	-	$	-7.8	$	3.1
	Allowance for uncollectible reinsurance recoverables			16.9		3.6		0.5		-0.3		20.7
			$	19.7	$	11.7	$	0.5	$	-8.1	$	23.8
	2012
	Allowance for doubtful accounts		$	2.3	$	8.1	$	-	$	-7.6	$	2.8
	Allowance for uncollectible reinsurance recoverables			16.4		0.4		0.9		-0.8		16.9
			$	18.7	$	8.5	$	0.9	$	-8.4	$	19.7
		-1		Amount charged to other accounts includes foreign exchange gains and losses.

SCHEDULE_VI_SUPPLEMENTAL_INFOR

SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	12 Months Ended
	Dec. 31, 2014
SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS [Abstract]
SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	SCHEDULE VI
	THE HANOVER INSURANCE GROUP, INC.
	SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS
	YEARS ENDED DECEMBER 31
	(in millions)
	Affiliation with Registrant			Deferred acquisition costs		Reserves for unpaid claims and claim adjustment expenses (1)		Discount, if any, deducted from previous column (2)		Unearned premiums (1)		Earned premiums		Net investment income
	Consolidated Property and Casualty
	Subsidiaries
	2014		$	525.7	$	6,391.7	$	-	$	2,583.9	$	4,710.3	$	270.3
	2013		$	506.0	$	6,231.5	$	-	$	2,515.8	$	4,450.5	$	269.0
	2012		$	489.5	$	6,197.0	$	-	$	2,474.8	$	4,239.1	$	276.6
					Claims and claim adjustment expenses incurred related to
						Current year		Prior years		Amortization of deferred acquisition costs		Paid claims and claim adjustment expenses		Premiums written
	2014				$	3,026.6	$	-99.1	$	1,040.0	$	2,727.6	$	4,810.1
	2013				$	2,837.4	$	-76.3	$	971.0	$	2,683.3	$	4,552.7
	2012				$	2,990.2	$	-15.8	$	938.1	$	2,714.1	$	4,368.4
		-1		Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,983.0 million, $2,030.4 million and $2,074.3 million of reinsurance recoverable on unpaid losses in 2014, 2013 and 2012, respectively. Unearned premiums are shown gross of prepaid premiums of $197.8 million, $219.0 million and $275.4 million in 2014, 2013 and 2012, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.
		-2		The Company does not use discounting techniques.

Investments_Tables

Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Schedule of Available-for-sale Securities Reconciliation
	31-Dec-14
	(in millions)		Amortized		Gross		Gross				OTTI
			Cost or		Unrealized		Unrealized				Unrealized
			Cost		Gains		Losses		Fair Value		Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	516.3	$	7.6	$	3.5	$	520.4	$	-
	Foreign government		349.4		5.2		0.6		354.0		-
	Municipal		1,079.6		62.4		4.0		1,138.0		-
	Corporate		3,746.3		166.3		31.8		3,880.8		7.4
	Residential mortgage-backed		770.4		21.7		3.0		789.1		0.4
	Commercial mortgage-backed		516.7		12.4		1.3		527.8		-
	Asset-backed		167.0		1.2		0.2		168.0		-
	Total fixed maturities	$	7,145.7	$	276.8	$	44.4	$	7,378.1	$	7.8
	Equity securities	$	506.6	$	76.8	$	2.6	$	580.8	$	-
	31-Dec-13
	(in millions)		Amortized		Gross		Gross				OTTI
			Cost or		Unrealized		Unrealized				Unrealized
			Cost		Gains		Losses		Fair Value		Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	417.5	$	3.3	$	14.2	$	406.6	$	-
	Foreign government		304.5		2.1		1.6		305.0		-
	Municipal		1,108.0		37.4		19.1		1,126.3		-
	Corporate		3,690.2		171.5		37.5		3,824.2		8.6
	Residential mortgage-backed		722.8		20.1		14.1		728.8		1.6
	Commercial mortgage-backed		405.9		10.5		4.8		411.6		-
	Asset-backed		166.3		2.0		0.2		168.1		-
	Total fixed maturities	$	6,815.2	$	246.9	$	91.5	$	6,970.6	$	10.2
	Equity securities	$	366.5	$	66.9	$	3.2	$	430.2	$	-
Investments Classified by Contractual Maturity Date
	31-Dec		2014
	(in millions)		Amortized
			Cost		Fair Value
	Due in one year or less	$	498.1	$	502.5
	Due after one year through five years		2,300.0		2,388.0
	Due after five years through ten years		2,267.3		2,337.5
	Due after ten years		626.2		665.2
			5,691.6		5,893.2
	Mortgage-backed and asset-backed securities		1,454.1		1,484.9
	Total fixed maturities	$	7,145.7	$	7,378.1
Unrealized Gains and Losses on Available-For-Sale and Other Securities
	YEARS ENDED DECEMBER 31
	(in millions)				Equity
			Fixed		Securities and
	2014		Maturities		Other		Total
	Net appreciation, beginning of year	$	212.1	$	47.2	$	259.3
	Net appreciation on available-for-sale securities		76.3		10.2		86.5
	Change in OTTI losses recognized in other comprehensive income		2.4		-		2.4
	Provision for deferred income taxes		-40.8		-6.5		-47.3
			37.9		3.7		41.6
	Net appreciation, end of year	$	250.0	$	50.9	$	300.9
	2013
	Net appreciation, beginning of year	$	410.1	$	15.9	$	426.0
	Net (depreciation) appreciation on available-for-sale securities		-273.6		48.1		-225.5
	Change in OTTI losses recognized in other comprehensive income		0.8		-		0.8
	Benefit (provision) for deferred income taxes		74.8		-16.8		58.0
			-198		31.3		-166.7
	Net appreciation, end of year	$	212.1	$	47.2	$	259.3
	2012
	Net appreciation, beginning of year	$	300.2	$	8.5	$	308.7
	Net appreciation on available-for-sale securities and derivative instruments		140.7		10.6		151.3
	Change in OTTI losses recognized in other comprehensive income		12.2		-		12.2
	Provision for deferred income taxes		-43		-3.2		-46.2
			109.9		7.4		117.3
	Net appreciation, end of year	$	410.1	$	15.9	$	426.0
Schedule of Unrealized Loss on Investments
	31-Dec-14		12 months or less				Greater than 12 months				Total
	(in millions)		Gross				Gross				Gross
			Unrealized		Fair		Unrealized		Fair		Unrealized		Fair
			Losses		Value		Losses		Value		Losses		Value
	Fixed maturities:
	Investment grade:
	U.S. Treasury and government
	agencies	$	-	$	52.2	$	3.5	$	137.9	$	3.5	$	190.1
	Foreign governments		0.4		20.8		0.2		24.2		0.6		45.0
	Municipal		0.3		57.1		3.7		140.2		4.0		197.3
	Corporate		7.8		393.3		9.3		217.4		17.1		610.7
	Residential mortgage-backed		0.2		36.4		2.8		98.0		3.0		134.4
	Commercial mortgage-backed		0.4		90.4		0.9		60.8		1.3		151.2
	Asset-backed		0.1		46.6		0.1		13.2		0.2		59.8
	Total investment grade		9.2		696.8		20.5		691.7		29.7		1,388.5
	Below investment grade:
	Corporate		12.2		114.9		2.5		28.3		14.7		143.2
	Total fixed maturities		21.4		811.7		23.0		720.0		44.4		1,531.7
	Equity securities		2.2		130.2		0.4		3.9		2.6		134.1
	Total	$	23.6	$	941.9	$	23.4	$	723.9	$	47.0	$	1,665.8
	31-Dec-13		12 months or less				Greater than 12 months				Total
	(in millions)		Gross				Gross				Gross
			Unrealized		Fair		Unrealized		Fair		Unrealized		Fair
			Losses		Value		Losses		Value		Losses		Value
	Fixed maturities:
	Investment grade:
	U.S. Treasury and government
	agencies	$	12.3	$	247.9	$	1.9	$	18.8	$	14.2	$	266.7
	Foreign governments		1.5		129.0		0.1		17.3		1.6		146.3
	Municipal		14.8		345.3		4.3		39.9		19.1		385.2
	Corporate		21.4		872.7		11.6		87.7		33.0		960.4
	Residential mortgage-backed		10.3		321.1		3.4		29.5		13.7		350.6
	Commercial mortgage-backed		4.2		155.4		0.6		10.2		4.8		165.6
	Asset-backed		0.2		31.0		-		0.3		0.2		31.3
	Total investment grade		64.7		2,102.4		21.9		203.7		86.6		2,306.1
	Below investment grade:
	Corporate		2.9		71.9		1.6		21.4		4.5		93.3
	Residential mortgage-backed		0.1		2.0		0.3		1.5		0.4		3.5
	Total below investment grade		3.0		73.9		1.9		22.9		4.9		96.8
	Total fixed maturities		67.7		2,176.3		23.8		226.6		91.5		2,402.9
	Equity securities		2.8		45.2		0.4		0.7		3.2		45.9
	Total	$	70.5	$	2,221.5	$	24.2	$	227.3	$	94.7	$	2,448.8

Investment_Income_and_Gains_an1

Investment Income and Gains and Losses (Tables)	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Components of Net Investment Income
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Fixed maturities	$	255.8	$	254.8	$	264.2
	Equity securities		16.5		15.6		15.3
	Other investments		9.0		8.8		7.0
	Gross investment income		281.3		279.2		286.5
	Less investment expenses		-11		-10.2		-9.9
	Net investment income	$	270.3	$	269.0	$	276.6
Schedule of Net Realized Gains (Losses) on Investments
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Equity securities	$	45.8	$	26.7	$	15.5
	Fixed maturities		4.5		8.6		9.7
	Real estate		-		-0.9		3.3
	Derivative instruments		-		-		-4.4
	Other investments		-0.2		-0.9		-0.5
	Net realized investment gains	$	50.1	$	33.5	$	23.6
Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Credit losses as of the beginning of the year	$	7.8	$	8.6	$	14.5
	Credit losses for which an OTTI was not previously recognized		-		1.1		0.6
	Additional credit losses on securities for which an OTTI was previously
	recognized		-		0.3		0.6
	Reductions for securities sold, matured or called		-3.2		-2.2		-6.9
	Reductions for securities reclassified as intend to sell		-0.4		-		-0.2
	Credit losses as of the end of the year	$	4.2	$	7.8	$	8.6
Schedule of Realized Gain (Loss)
	YEARS ENDED DECEMBER 31
	(in millions)
			Proceeds		Gross		Gross
	2014		from Sales		Gains		Losses
	Fixed maturities	$	349.5	$	5.8	$	2.6
	Equity securities	$	156.1	$	46.2	$	0.8
	2013
	Fixed maturities	$	450.4	$	6.0	$	4.0
	Equity securities	$	193.5	$	31.2	$	0.4
	2012
	Fixed maturities	$	616.3	$	12.1	$	2.0
	Equity securities	$	141.3	$	17.5	$	0.7

Fair_Value_Tables

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value [Abstract]
Fair Value of Financial Instruments
	31-Dec		2014						2013
	(in millions)		Carrying			Fair			Carrying		Fair
			Value			Value			Value		Value
	Financial Assets
	Cash and cash equivalents	$	373.3		$	373.3		$	486.2	$	486.2
	Fixed maturities		7,378.1			7,378.1			6,970.6		6,970.6
	Equity securities		580.8			580.8			430.2		430.2
	Other investments		267.4			272.2			173.1		173.7
	Total financial assets	$	8,599.6		$	8,604.4		$	8,060.1	$	8,060.7
	Financial Liabilities
	Debt	$	903.5		$	1,021.7		$	903.9	$	961.7
Fair Value, Assets Measured on Recurring Basis
		31-Dec-14
	(in millions)		Total			Level 1			Level 2		Level 3
	Fixed maturities:
	U.S. Treasury and government agencies	$	520.4		$	233.5		$	286.9	$	-
	Foreign government		354.0			43.3			310.7		-
	Municipal		1,138.0			-			1,112.3		25.7
	Corporate		3,880.8			-			3,871.2		9.6
	Residential mortgage-backed, U.S. agency backed		689.2			-			689.2		-
	Residential mortgage-backed, non-agency		99.9			-			99.9		-
	Commercial mortgage-backed		527.8			-			506.4		21.4
	Asset backed		168.0			-			168.0		-
	Total fixed maturities		7,378.1			276.8			7,044.6		56.7
	Equity securities		571.5			570.3			-		1.2
	Other investments		136.4			-			132.6		3.8
	Total investment assets at fair value	$	8,086.0		$	847.1		$	7,177.2	$	61.7
		31-Dec-13
	(in millions)		Total			Level 1			Level 2		Level 3
	Fixed maturities:
	U.S. Treasury and government agencies	$	406.6		$	167.2		$	239.4	$	-
	Foreign government		305.0			45.6			259.4		-
	Municipal		1,126.3			-			1,100.7		25.6
	Corporate		3,824.2			-			3,811.2		13.0
	Residential mortgage-backed, U.S. agency backed		573.2			-			573.2		-
	Residential mortgage-backed, non-agency		155.6			-			155.1		0.5
	Commercial mortgage-backed		411.6			-			388.7		22.9
	Asset backed		168.1			-			168.1		-
	Total fixed maturities		6,970.6			212.8			6,695.8		62.0
	Equity securities		420.9			382.3			-		38.6
	Other investments		153.2			-			149.6		3.6
	Total investment assets at fair value	$	7,544.7		$	595.1		$	6,845.4	$	104.2
Estimated Fair Values of Financial Instruments Not Carried at Fair Value
		31-Dec-14
	(in millions)		Total			Level 1			Level 2		Level 3
	Assets:
	Cash and cash equivalents	$	373.3		$	373.3		$	-	$	-
	Equity securities		9.3			-			9.3		-
	Other investments		135.8			-			-		135.8
	Liabilities:
	Debt	$	1,021.7		$	-		$	1,021.7	$	-
		31-Dec-13
	(in millions)		Total			Level 1			Level 2		Level 3
	Assets:
	Cash and cash equivalents	$	486.2		$	486.2		$	-	$	-
	Equity securities		9.3			-			9.3		-
	Other investments		20.5			-			2.7		17.8
	Liabilities:
	Debt	$	961.7		$	-		$	961.7	$	-
Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)
	YEAR ENDED DECEMBER 31, 2014	Fixed Maturities
									Residential
									mortgage-		Commercial							Equity
									backed,		mortgage-			Asset-				and		Total
	(in millions)		Municipal			Corporate			non-agency		backed			backed		Total		Other		Assets
	Balance at beginning of year	$	25.6		$	13.0		$	0.5	$	22.9		$	-	$	62.0	$	42.2	$	104.2
	Transfers into Level 3		2.2			2.2			-		-			-		4.4		-		4.4
	Transfers out of Level 3		-2.6			-5.3			-		-			-		-7.9		-		-7.9
	Total gains (losses):															-				-
	Included in earnings		-			0.1			-		-			-		0.1		-		0.1
	Included in other
	comprehensive income - net appreciation (depreciation) on available-for-sale securities		0.8			-0.2			-		0.4			-		1.0		0.2		1.2
	Purchases and sales:
	Purchases		2.5			-			-		-			-		2.5		-		2.5
	Sales		-2.8			-0.2			-0.5		-1.9			-		-5.4		-37.4		-42.8
	Balance at end of year	$	25.7		$	9.6		$	-	$	21.4		$	-	$	56.7	$	5.0	$	61.7
	YEAR ENDED DECEMBER 31, 2013	Fixed Maturities
									Residential
									mortgage-		Commercial							Equity
									backed,		mortgage-			Asset-				and		Total
	(in millions)		Municipal			Corporate			non-agency		backed			backed		Total		Other		Assets
	Balance at beginning of year	$	19.4		$	26.4		$	0.7	$	26.7		$	1.5	$	74.7	$	28.0	$	102.7
	Transfers into Level 3		9.7			0.2			-		-			-		9.9		-		9.9
	Transfers out of Level 3		-			-2.2			-		-			-1.5		-3.7		-0.9		-4.6
	Total gains (losses):
	Included in earnings		-			1.5			-		-			-		1.5		-		1.5
	Included in other
	comprehensive income - net appreciation (depreciation) on available-for-sale securities		-0.8			-1.6			-		-1.3			-		-3.7		15.1		11.4
	Sales		-2.7			-11.3			-0.2		-2.5			-		-16.7		-		-16.7
	Balance at end of year	$	25.6		$	13.0		$	0.5	$	22.9		$	-	$	62.0	$	42.2	$	104.2
Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3
							31-Dec-14				31-Dec-13
		Valuation		Significant			Fair		Range		Fair		Range
	(in millions)	Technique		Unobservable Inputs			Value		(Wtd Average)		Value		(Wtd Average)
	Fixed maturities:
	Municipal	Discounted		Discount for:		$	25.7			$	25.6
		cash flow		Small issue size			0.6-4.5% (2.0%)				1.0-4.0%  (2.3%)
				Above-market coupon			0.3-1.0% (0.4%)				0.3-1.0%  (0.5%)
	Corporate	Discounted		Discount for:			9.4				12.8
		cash flow		Small issue size			0.5-1.0% (0.7%)				0.3-1.0%  (0.5%)
				Above-market coupon			0.3-0.8% (0.6%)				0.3-0.8%  (0.6%)
	Residential mortgage-backed,
	non-agency	Discounted		Discount for:			-				0.5
		cash flow		Small issue size					N/A				0.5% (0.5%)
	Commercial mortgage-backed	Discounted		Discount for:			21.4				22.9
		cash flow		Above-market coupon			0.5-0.8% (0.5%)				0.5-0.8%  (0.6%)
				Credit stress					0.5% (0.5%)		0.5% (0.5%)
				Small issue size					0.5% (0.5%)		0.5% (0.5%)
				Lease structure					0.3% (0.3%)		0.3% (0.3%)
	Equity securities	Market		Net tangible asset			1.1				38.5
		comparables		market multiples					1.0X (1.0X)				1.3X (1.3X)
	Other	Discounted		Discount rate			3.8	18.0% (18.0%)			3.6	18.0% (18.0%)
		cash flow

Debt_and_Credit_Arrangements_T

Debt and Credit Arrangements (Tables)	12 Months Ended
	Dec. 31, 2014
Debt and Credit Arrangements [Abstract]
Schedule of Long-Term Debt
	31-Dec		2014		2013
	(in millions)
	Senior debentures maturing June 15, 2021	$	300.0	$	300.0
	Senior debentures maturing March 1, 2020		164.6		165.1
	Senior debentures maturing October 15, 2025		81.1		81.2
	Subordinated debentures maturing March 30, 2053		175.0		175.0
	Subordinated debentures maturing February 3, 2027		59.7		59.7
	FHLBB borrowings (secured)		125.0		125.0
	Total principal debt	$	905.4	$	906.0
	Unamortized debt discount		-1.9		-2.1
	Total	$	903.5	$	903.9

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Taxes [Abstract]
Components of Income Before Income Taxes
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Income (loss) before income taxes:
	U.S.	$	185.6		$	183.5		$	-99.1
	Non-U.S.		192.4			145.6			127.8
		$	378.0		$	329.1		$	28.7
Components of Income Tax Expense (Benefit)
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Current:
	U.S.	$	4.5		$	8.0		$	2.3
	Non-U.S.		22.6			0.6			16.1
	Total current		27.1			8.6			18.4
	Deferred:
	U.S.		51.1			52.9			-42.7
	Non-U.S.		17.5			21.9			6.9
	Total deferred		68.6			74.8			-35.8
	Total income tax expense (benefit)	$	95.7		$	83.4		$	-17.4
Schedule of Effective Income Tax Rate Reconciliation
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Expected income tax expense	$	132.3		$	115.2		$	10.0
	Tax difference related to investment disposals and maturities		-16.2			-17.5			-14.3
	Effect of foreign operations		-8.1			-10.8			-3.5
	Foreign exchange losses		-6.9			-			-
	Dividend received deduction		-2.6			-2.5			-2.8
	Tax-exempt interest		-1.7			-1.7			-1.5
	Nondeductible expenses		1.8			1.1			2.8
	Change in foreign tax rate		-			-0.7			-0.4
	Change in valuation allowance		-2.9			-			-7.7
	Other, net		-			0.3			-
	Income tax expense (benefit)	$	95.7		$	83.4		$	-17.4
	Effective tax rate		25.3	%		25.3	%		-60.6	%
Schedule of Deferred Tax Assets and Liabilities
	31-Dec		2014			2013
	(in millions)
	Deferred tax assets:
	Loss, LAE and unearned premium reserves, net	$	175.9		$	185.0
	Tax credit carryforwards		125.8			123.1
	Employee benefit plans		45.7			38.2
	Operating loss carryforwards		29.1			64.5
	Other		10.6			74.9
			387.1			485.7
	Less: Valuation allowance		-			2.9
			387.1			482.8
	Deferred tax liabilities:
	Deferred acquisition costs		129.6			123.5
	Deferred Lloyd's underwriting income		54.0			13.8
	Investments, net		44.4			8.4
	Software capitalization		27.3			30.1
	Other		0.6			67.3
			255.9			243.1
	Net deferred tax asset	$	131.2		$	239.7
Summary of Income Tax Uncertainties
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Liability at beginning of year, net	$	1.4		$	0.8		$	0.8
	Additions for tax positions of prior years		-0.1			5.4			-
	Settlements		-4.8			-			-
	Deferred deductions		4.8			-4.8			-
	Liability at end of year, net	$	1.3		$	1.4		$	0.8

Pension_Plans_Tables

Pension Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Schedule of Weighted Average Assumptions	Weighted average assumptions used to determine pension benefit obligations are as follows:
	31-Dec		2014			2013		2012
	U.S.
	Discount rate - qualified plan		4.38%			5.00%		4.25%
	Discount rate - non-qualified plan		4.25%			5.00%		4.13%
	Cash balance interest crediting rate		3.50%			3.50%		3.50%
	Chaucer
	Discount rate		3.75%			4.50%		4.80%
	Rate of increase in future compensation		3.00%			3.15%		4.20%
	The Company utilizes a measurement date of January 1st to determine its periodic pension costs. Weighted average assumptions used to determine net periodic pension costs for the defined benefit plans are as follows:
	YEARS ENDED DECEMBER 31	2014		2013		2012
	U.S.:
	Qualified plan
	Discount rate	5.00%		4.25%		5.13%
	Expected return on plan assets	5.50%		5.25%		6.00%
	Cash balance interest crediting rate	3.50%		3.50%		4.00%
	Non-qualified plan
	Discount rate	5.00%		4.13%		5.00%
	Chaucer:
	Discount rate	4.50%		4.80%		4.90%
	Rate of increase in future compensation	3.15%		4.20%		4.30%
	Expected return on plan assets	6.55%		6.70%		7.50%
Summary of Plan Assets Investment Measured at Fair Value
	31-Dec	2014									2013
	(in millions)		Total			Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
	Fixed income securities:
	Fixed maturities	$	432.5		$	1.3	$	400.5	$	30.7	$	449.6	$	2.1	$	447.5	$	-
	Money market funds		3.9			3.9		-		-		5.3		5.3		-		-
	Total fixed income securities		436.4			5.2		400.5		30.7		454.9		7.4		447.5		-
	Equity securities:
	Domestic		60.6			-		60.6		-		67.8		0.2		67.6		-
	International		16.4			-		16.4		-		19.3		0.1		19.2		-
	Total equity securities		77.0			-		77.0		-		87.1		0.3		86.8		-
	Total investments at fair value	$	513.4		$	5.2	$	477.5	$	30.7	$	542.0	$	7.7	$	534.3	$	-
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans
	31-Dec	U.S. Qualified Pension Plans					U.S. Non-Qualified     Pension Plans				Chaucer Pension Plan
	(in millions)		2014			2013		2014		2013		2014		2013
	Accumulated benefit obligation	$	538.5		$	548.8	$	40.6	$	37.4	$	134.1	$	126.4
	Change in benefit obligation:
	Projected benefit obligation, beginning of
	period		548.8			607.0		37.4		41.2		128.5		116.6
	Employee contributions		-			-		-		-		0.4		0.4
	Service cost - benefits earned during the
	period		-			-		-		-		1.5		1.5
	Interest cost		26.0			24.8		1.8		1.6		5.9		5.4
	Actuarial losses (gains)		53.0			-36.5		4.6		-2.3		12.4		3.6
	Benefits paid (1)		-89.3			-46.5		-3.2		-3.1		-2.6		-1.5
	Foreign currency translation		-			-		-		-		-8.6		2.5
	Projected benefit obligation, end of year		538.5			548.8		40.6		37.4		137.5		128.5
	Change in plan assets:
	Fair value of plan assets, beginning of period		542.0			591.6		-		-		120.2		90.9
	Actual return on plan assets		59.4			-3.1		-		-		14.8		17.1
	Contributions		1.3			-		3.2		3.1		7.7		10.6
	Benefits paid (1)		-89.3			-46.5		-3.2		-3.1		-2.6		-1.5
	Foreign currency translation		-			-		-		-		-8.3		3.1
	Fair value of plan assets, end of year		513.4			542.0		-		-		131.8		120.2
	Funded status of the plans	$	-25.1		$	-6.8	$	-40.6	$	-37.4	$	-5.7	$	-8.3
Components of Net Periodic Pension Cost
	YEARS ENDED DECEMBER 31		2014			2013		2012
	(in millions)
	Service cost - benefits earned during the period	$	1.5		$	1.5	$	1.6
	Interest cost		33.7			31.9		35.2
	Expected return on plan assets		-36.6			-35.9		-38.7
	Recognized net actuarial loss		11.7			14.7		12.8
	Amortization of prior service cost		0.1			-		0.1
	Settlement loss		12.1			0.4		-
	Net periodic pension cost	$	22.5		$	12.6	$	11.0
Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans
	31-Dec		2014			2013
	(in millions)
	Net actuarial loss	$	126.9		$	118.7
	Net prior service cost		-			0.1
		$	126.9		$	118.8
Summary of Estimated Amount Amortized From Accumulated Other Comprehensive Income (Loss) Into Net Periodic Pension Cost
	ESTIMATED AMORITZATION IN 2015		Expense
	(in millions)
	Net actuarial loss	$	12.0
Schedule of Expected Benefit Payments
	YEARS ENDED DECEMBER 31		2015			2016		2017		2018		2019		2020-2024
	(in millions)
	U.S. qualified pension plan	$	38.7		$	38.4	$	39.4	$	38.6	$	38.3	$	186.5
	U.S. non-qualified pension plans	$	3.5		$	3.2	$	3.1	$	3.1	$	3.0	$	14.2
	Chaucer pension plan	$	2.7		$	1.6	$	1.7	$	1.7	$	1.8	$	9.7
U.S. Qualified Defined Benefit Plans [Member]
Summary of Target Allocations and Invested Asset Allocations
	31-Dec	2014 TARGET LEVELS		2014		2013
	Fixed income securities:
	Fixed maturities	83%		84%		83%
	Money market funds	2%		1%		1%
	Total fixed income securities	85%		85%		84%
	Equity securities:
	Domestic	12%		12%		12%
	International	3%		3%		4%
	Total equity securities	15%		15%		16%
	Total plan assets	100%		100%		100%
Summary of Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs
	YEAR ENDED DECEMBER 31		2014
	(in millions)
	Balance at beginning of period	$	-
	Purchases		30.0
	Actual return on plan assets related to assets still held		0.7
	Balance at end of year	$	30.7
Chaucer Pension Plan [Member]
Summary of Target Allocations and Invested Asset Allocations
	31-Dec	2014 TARGET LEVELS		2014		2013
	Fixed income securities:
	Fixed maturities	30%		32%		19%
	Equity securities:
	Domestic (United Kingdom)	15%		14%		26%
	International	45%		43%		48%
	Total equity securities	60%		57%		74%
	Real estate funds	10%		11%		7%
	Total plan assets	100%		100%		100%
Summary of Plan Assets Investment Measured at Fair Value
	31-Dec	2014									2013
	(in millions)		Total			Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3
	Fixed income securities:
	Fixed maturities	$	40.9		$	-	$	40.9	$	-	$	22.8	$	-	$	22.8	$	-
	Equity securities:
	Domestic (United Kingdom)		18.5			-		18.5		-		30.5		-		30.5		-
	International		56.9			-		56.9		-		57.8		-		57.8		-
	Total equity securities		75.4			-		75.4		-		88.3		-		88.3		-
	Real estate funds		14.0			-		-		14.0		8.8		-		-		8.8
	Total investments at fair value	$	130.3		$	-	$	116.3	$	14.0	$	119.9	$	-	$	111.1	$	8.8
Summary of Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs
	YEARS ENDED DECEMBER 31		2014			2013
	(in millions)
	Balance at beginning of period	$	8.8		$	8.2
	Purchases		4.2			-
	Actual return on plan assets related to assets still held		1.9			0.5
	Foreign currency translation		-0.9			0.1
	Balance at end of year	$	14.0		$	8.8

Other_Postretirement_Benefit_P1

Other Postretirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans
	31-Dec	U.S. Qualified Pension Plans					U.S. Non-Qualified     Pension Plans				Chaucer Pension Plan
	(in millions)		2014			2013		2014		2013		2014		2013
	Accumulated benefit obligation	$	538.5		$	548.8	$	40.6	$	37.4	$	134.1	$	126.4
	Change in benefit obligation:
	Projected benefit obligation, beginning of
	period		548.8			607.0		37.4		41.2		128.5		116.6
	Employee contributions		-			-		-		-		0.4		0.4
	Service cost - benefits earned during the
	period		-			-		-		-		1.5		1.5
	Interest cost		26.0			24.8		1.8		1.6		5.9		5.4
	Actuarial losses (gains)		53.0			-36.5		4.6		-2.3		12.4		3.6
	Benefits paid (1)		-89.3			-46.5		-3.2		-3.1		-2.6		-1.5
	Foreign currency translation		-			-		-		-		-8.6		2.5
	Projected benefit obligation, end of year		538.5			548.8		40.6		37.4		137.5		128.5
	Change in plan assets:
	Fair value of plan assets, beginning of period		542.0			591.6		-		-		120.2		90.9
	Actual return on plan assets		59.4			-3.1		-		-		14.8		17.1
	Contributions		1.3			-		3.2		3.1		7.7		10.6
	Benefits paid (1)		-89.3			-46.5		-3.2		-3.1		-2.6		-1.5
	Foreign currency translation		-			-		-		-		-8.3		3.1
	Fair value of plan assets, end of year		513.4			542.0		-		-		131.8		120.2
	Funded status of the plans	$	-25.1		$	-6.8	$	-40.6	$	-37.4	$	-5.7	$	-8.3
Schedule of Expected Benefit Payments
	YEARS ENDED DECEMBER 31		2015			2016		2017		2018		2019		2020-2024
	(in millions)
	U.S. qualified pension plan	$	38.7		$	38.4	$	39.4	$	38.6	$	38.3	$	186.5
	U.S. non-qualified pension plans	$	3.5		$	3.2	$	3.1	$	3.1	$	3.0	$	14.2
	Chaucer pension plan	$	2.7		$	1.6	$	1.7	$	1.7	$	1.8	$	9.7
Components of Net Periodic Pension Cost
	YEARS ENDED DECEMBER 31		2014			2013		2012
	(in millions)
	Service cost - benefits earned during the period	$	1.5		$	1.5	$	1.6
	Interest cost		33.7			31.9		35.2
	Expected return on plan assets		-36.6			-35.9		-38.7
	Recognized net actuarial loss		11.7			14.7		12.8
	Amortization of prior service cost		0.1			-		0.1
	Settlement loss		12.1			0.4		-
	Net periodic pension cost	$	22.5		$	12.6	$	11.0
Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans
	31-Dec		2014			2013
	(in millions)
	Net actuarial loss	$	126.9		$	118.7
	Net prior service cost		-			0.1
		$	126.9		$	118.8
Summary of Estimated Amount Amortized From Accumulated Other Comprehensive Income (Loss) Into Net Periodic Pension Cost
	ESTIMATED AMORITZATION IN 2015		Expense
	(in millions)
	Net actuarial loss	$	12.0
Postretirement Plans
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans
	31-Dec		2014			2013
	(in millions)
	Change in benefit obligation:
	Accumulated postretirement benefit obligation, beginning of year	$	17.1		$	45.2
	Service cost		0.1			0.1
	Interest cost		0.8			1.2
	Net actuarial loss (gain)		1.5			-1
	Benefits paid		-2.5			-2.8
	Plan amendment		-			-8.4
	Settlement of death benefits		-			-17.2
	Accumulated postretirement benefit obligation, end of year		17.0			17.1
	Fair value of plan assets, end of year		-			-
	Funded status of plans	$	-17		$	-17.1
Schedule of Expected Benefit Payments
	YEARS ENDING DECEMBER 31
	(in millions)
	2015	$	2.1
	2016		1.8
	2017		1.7
	2018		1.5
	2019		1.3
	2020-2024		5.4
Components of Net Periodic Pension Cost
	YEARS ENDED DECEMBER 31		2014			2013		2012
	(in millions)
	Service cost	$	0.1		$	0.1	$	0.1
	Interest cost		0.8			1.2		2.1
	Recognized net actuarial loss		0.1			0.2		0.2
	Amortization of prior service cost		-1.9			-3.7		-3.8
	Net settlement gain		-			-1.6		-
	Net periodic postretirement benefit	$	-0.9		$	-3.8	$	-1.4
Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans
	31-Dec		2014			2013
	(in millions)
	Net actuarial loss	$	3.4		$	1.9
	Net prior service cost		-1.7			-3.5
		$	1.7		$	-1.6
Summary of Estimated Amount Amortized From Accumulated Other Comprehensive Income (Loss) Into Net Periodic Pension Cost
	Estimated Amortization in 2015		Expense (Benefit)
	(in millions)
	Net actuarial loss	$	0.2
	Net prior service cost		-0.6
		$	-0.4
Summary of Weighted-Average Assumptions Used to Determine Pension Benefit Obligations
	YEARS ENDED DECEMBER 31	2014		2013
	Postretirement benefit obligations discount rate	4.38%		5.00%
	Postretirement benefit cost discount rate	5.00%		4.25%
Assumed Health Care Cost Trend Rates
	31-Dec	2014		2013
	Health care cost trend rate assumed for next year	7.00%		7.00%
	Rate to which the cost trend is assumed to decline (ultimate trend rate)	5.00%		5.00%
	Year the rate reaches the ultimate trend rate	2020		2020

Other_Comprehensive_Income_Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Changes in Other Comprehensive Income
	YEARS ENDED DECEMBER 31	2014						2013						2012
	(in millions)		Pre-Tax		Tax Benefit (Expense)		Net of Tax		Pre-Tax		Tax Benefit (Expense)		Net of Tax		Pre-Tax		Tax Benefit (Expense)		Net of Tax
	Unrealized gains (losses) on
	available-for-sale securities and derivative instruments:
	Unrealized gains (losses) arising
	during period (net of pre-tax, ceded unrealized gains (losses) of $0.8 million and ($1.2) million  for the years ended December 31, 2014 and 2013)	$	138.7	$	-45.7	$	93.0	$	-189.4	$	63.1	$	-126.3	$	186.4	$	-40.2	$	146.2
	Amount of realized gains
	from sales and other		-55.3		0.3		-55		-41.3		-3		-44.3		-31.8		-3.3		-35.1
	Portion of other-than-
	temporary impairment losses recognized in earnings		5.5		-1.9		3.6		6.0		-2.1		3.9		8.9		-2.7		6.2
	Net unrealized gains (losses)		88.9		-47.3		41.6		-224.7		58.0		-166.7		163.5		-46.2		117.3
	Pension and postretirement
	benefits:
	Net actuarial (loss) gain arising
	in the period		-33.6		11.0		-22.6		15.0		-4.2		10.8		-24.9		9.0		-15.9
	Loss on settlement of
	pension obligation		12.1		-4.2		7.9		-		-		-		-		-		-
	Amortization of net actuarial
	loss and prior service cost recognized as net periodic benefit cost		10.0		-3.5		6.5		14.9		-5.2		9.7		9.3		-3.2		6.1
	Cumulative foreign currency
	translation adjustment:
	Foreign currency translation
	recognized during the period		-7.1		2.5		-4.6		-3.1		1.1		-2		12.2		-4.3		7.9
	Other comprehensive income (loss)	$	70.3	$	-41.5	$	28.8	$	-197.9	$	49.7	$	-148.2	$	160.1	$	-44.7	$	115.4
Reclassifications Out of Accumulated Other Comprehensive Income
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Details about Accumulated Other	Amount Reclassified from Accumulated						Affected Line Item in the Statement
	Comprehensive Income Components	Other Comprehensive Income						Where Net Income is Presented
	Unrealized gains (losses) on available-for-
	sale securities and derivative instruments	$	55.2	$	41.3	$	31.8	Net realized gains from sales and other
								Net other-than-temporary impairment losses
			-5.5		-6		-7.8	on investments recognized in earnings
			49.7		35.3		24.0	Total before tax
			1.6		5.1		6.0	Tax benefit
			51.3		40.4		30.0
			0.1		-		-0.9	Discontinued operations, net of tax
			51.4		40.4		29.1	Net of tax
	Amortization of defined benefit pension							Loss adjustment expenses and other
	and postretirement plans		-22.1		-14.9		-9.3	operating expenses
			7.7		5.2		3.2	Tax benefit
			-14.4		-9.7		-6.1	Net of tax
	Total reclassifications for the period	$	37.0	$	30.7	$	23.0	Net of tax

Stockbased_Compensation_Tables

Stock-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Stock-based Compensation [Abstract]
Summary of Stock Option Plan Activity
	YEARS ENDED DECEMBER 31		2014					2013					2012
	(in whole shares and dollars)		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price		Shares			Weighted Average Exercise Price
	Outstanding, beginning of year		2,049,173		$	41.18		2,892,882		$	38.28		2,715,430		$	38.57
	Granted		687,700			58.06		537,800			42.53		529,500			36.84
	Exercised		-444,200			39.33		-1,192,134			34.70		-90,624			25.47
	Forfeited or cancelled		-56,053			45.99		-189,375			41.54		-101,574			41.45
	Expired		-			-		-			-		-159,850			44.05
	Outstanding, end of year		2,236,620		$	46.61		2,049,173		$	41.18		2,892,882		$	38.28
	Exercisable, end of year		726,321		$	43.55		783,873		$	41.16		1,630,382		$	37.18
Schedule of Stock Options by Exercise Price Range
					Options Outstanding							Options Currently Exercisable
	Range of Exercise Prices				Number		Weighted Average Remaining Contractual Lives			Weighted Average Exercise Price		Number			Weighted Average Exercise Price
	$	34.19 to $36.50			114,500		3.84		$	34.37		114,500		$	34.37
	$	36.81			422,500		7.06			36.81		-			-
	$	37.77 to $42.15			126,300		5.15			42.14		126,300			42.14
	$	42.49			417,683		8.16			42.49		109,452			42.49
	$	45.21 to $46.28			120,000		1.97			45.83		120,000			45.83
	$	46.47 to $47.41			215,500		6.15			46.57		106,250			46.67
	$	48.46 to $54.41			151,487		2.23			48.48		149,819			48.47
	$	57.99 to $73.30			668,650		9.15			58.06		-			-
Schedule of Stock Option Valuation Assumptions
			2014			2013			2012
	Dividend yield		2.07% to 2.55	%		2.43% to 3.11	%		3.13% to 3.26	%
	Expected volatility		18.07% to 24.38	%		21.77% to 32.02	%		32.86% to 35.27	%
	Weighted average expected volatility		23.00	%		27.53	%		34.05	%
	Risk-free interest rate		0.53% - 1.92	%		0.29% - 2.02	%		0.61% - 1.06	%
	Expected term, in years		2.5 to 5.5			3.0 to 6.0			4.5 to 5.5
Summary of Restricted Stock Activity
	YEARS ENDED		2014					2013					2012
	31-Dec
			Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
	Time-based restricted stock
	units:
	Outstanding, beginning of
	year		525,980		$	41.20		750,837		$	40.15		768,529		$	40.17
	Granted		102,131			58.78		141,073			43.31		174,841			37.01
	Vested		-231,078			41.45		-298,388			39.51		-137,194			36.19
	Forfeited		-12,110			43.67		-67,542			41.33		-55,339			40.47
	Outstanding, end of year		384,923		$	45.63		525,980		$	41.20		750,837		$	40.15
	Performance and market-based
	restricted stock units:
	Outstanding, beginning of
	year		184,626		$	40.42		132,775		$	39.97		69,500		$	45.37
	Granted		60,338			55.73		82,795			41.85		96,150			36.91
	Vested		-22,826			44.78		-			-		-			-
	Forfeited		-3,800			37.90		-30,944			42.33		-32,875			43.28
	Outstanding, end of year		218,338		$	44.24		184,626		$	40.42		132,775		$	39.97

Earnings_Per_Share_and_Shareho1

Earnings Per Share and Shareholders' Equity Transactions (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share and Shareholdersb Equity Transactions [Abstract]
Information Regarding Basic and Diluted Earnings Per Share
	31-Dec		2014		2013		2012
	(in millions, except per share data)
	Basic shares used in the calculation of earnings per share		44.0		44.1		44.7
	Dilutive effect of securities:
	Employee stock options		0.5		0.3		0.2
	Non-vested stock grants		0.4		0.5		0.4
	Diluted shares used in the calculation of earnings per share		44.9		44.9		45.3
	Per share effect of dilutive securities on income from continuing operations	$	-0.12	$	-0.11	$	-0.01
	Per share effect of dilutive securities on net income	$	-0.13	$	-0.11	$	-0.02

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Information [Abstract]
Financial Information with Respect to Business Segments
	YEARS ENDED DECEMBER 31		2014			2013		2012
	(in millions)
	Operating revenues:
	Commercial Lines	$	2,239.0		$	2,109.5	$	1,966.2
	Personal Lines		1,491.0			1,542.5		1,560.0
	Chaucer		1,279.7			1,098.2		1,030.0
	Other		7.8			10.0		10.9
	Total		5,017.5			4,760.2		4,567.1
	Net realized investment gains		50.1			33.5		23.6
	Total revenues	$	5,067.6		$	4,793.7	$	4,590.7
	Operating income (loss) before interest expense and income taxes:
	Commercial Lines:
	GAAP underwriting loss	$	-8.3		$	-11	$	-224.2
	Net investment income		149.4			143.5		142.4
	Other (expense) income		-1.2			-0.1		1.5
	Commercial Lines operating income (loss)		139.9			132.4		-80.3
	Personal Lines:
	GAAP underwriting income (loss)		22.1			37.6		-67.3
	Net investment income		71.9			75.8		86.5
	Other income		5.0			5.2		6.3
	Personal Lines operating income		99.0			118.6		25.5
	Chaucer:
	GAAP underwriting income		122.5			107.7		93.5
	Net investment income		44.2			42.7		40.2
	Other income		10.9			-		3.1
	Chaucer operating income		177.6			150.4		136.8
	Other:
	GAAP underwriting loss		-3.1			-3.4		-3.5
	Net investment income		4.8			7.0		7.2
	Other net expenses		-12			-11.6		-10.6
	Other operating loss		-10.3			-8		-6.9
	Operating income before interest expense and income taxes		406.2			393.4		75.1
	Interest on debt		-65.2			-65.3		-61.9
	Operating income before income taxes		341.0			328.1		13.2
	Non-operating income items:
	Net realized investment gains		50.1			33.5		23.6
	Loss from settlement of pension obligation		-12.1			-		-
	Net loss from repayment of debt		-0.1			-27.7		-5.1
	Loss from disposal of real estate		-			-4.7		-
	Other non-operating items		-0.9			-0.1		-3
	Income before income taxes	$	378.0		$	329.1	$	28.7
Identifiable Assets by Business Segment
	31-Dec		2014			2013
	(in millions)	Identifiable Assets
	U.S. Companies	$	9,418.4		$	8,962.6
	Chaucer		4,229.3			4,301.2
	Discontinued operations		112.0			114.9
	Total	$	13,759.7		$	13,378.7
Gross Written Premium by Geographical Area
	YEARS ENDED DECEMBER 31	2014		2013		2012
		% of Total GPW
	United States	78%		78%		76%
	United Kingdom	6%		6%		6%
	Worldwide and other	16%		16%		18%
	Total	100%		100%		100%

Reinsurance_Tables

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2014
Reinsurance [Abstract]
Schedule of Effects of Reinsurance
	YEARS ENDED DECEMBER 31		2014			2013			2012
	(in millions)
	Premiums written:
	Direct	$	4,777.3		$	4,599.2		$	4,515.1
	Assumed (1)		688.1			602.5			689.1
	Ceded (2)		-655.3			-649			-835.8
	Net premiums written	$	4,810.1		$	4,552.7		$	4,368.4
	Premiums earned:
	Direct	$	4,679.8		$	4,546.3		$	4,352.9
	Assumed (1)		696.4			617.7			687.0
	Ceded (2)		-665.9			-713.5			-800.8
	Net premiums earned	$	4,710.3		$	4,450.5		$	4,239.1
	Percentage of assumed to net premiums earned		14.78	%		13.88	%		16.21	%
	Losses and LAE:
	Direct	$	3,019.4		$	2,939.4		$	3,251.4
	Assumed (1)		266.1			172.3			311.4
	Ceded (2)		-358			-350.6			-588.4
	Net losses and LAE	$	2,927.5		$	2,761.1		$	2,974.4
	(1)Assumed reinsurance activity primarily relates to the Chaucer segment.
	(2)In 2013, we did not renew the capital provision reinsurance treaty with Flagstone Re. This reduced ceded premiums written, premiums earned and losses and LAE for the year ended December 31, 2013.

Liabilities_For_Outstanding_Cl1

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Abstract]
Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses
	YEARS ENDED DECEMBER 31		2014		2013		2012
	(in millions)
	Gross loss and LAE reserves, beginning of year	$	6,231.5	$	6,197.0	$	5,760.3
	Reinsurance recoverable on unpaid losses		2,030.4		2,074.3		1,931.8
	Net loss and LAE reserves, beginning of year		4,201.1		4,122.7		3,828.5
	Net incurred losses and LAE in respect of losses occurring in:
	Current year		3,026.6		2,837.4		2,990.2
	Prior years		-99.1		-76.3		-15.8
	Total incurred losses and LAE		2,927.5		2,761.1		2,974.4
	Net payments of losses and LAE in respect of losses occurring in:
	Current year		1,328.7		1,213.5		1,317.6
	Prior years		1,398.9		1,469.8		1,396.5
	Total payments		2,727.6		2,683.3		2,714.1
	Commutation of Chaucer Flagstone reinsurance agreement		85.7		-		-
	Effect of foreign exchange rate changes		-78		0.6		33.9
	Net reserve for losses and LAE, end of year		4,408.7		4,201.1		4,122.7
	Reinsurance recoverable on unpaid losses		1,983.0		2,030.4		2,074.3
	Gross reserve for losses and LAE, end of year	$	6,391.7	$	6,231.5	$	6,197.0

Statutory_Financial_Informatio1

Statutory Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
Statutory Financial Information [Abstract]
Statutory Accounting Practices Disclosure
	(in millions)		2014		2013		2012
	Statutory Net Income (Loss)
	U.S. Insurance Subsidiaries	$	204.3	$	193.2	$	-47.3
	Statutory Capital and Surplus
	U.S. Insurance Subsidiaries	$	2,057.1	$	1,834.3	$	1,523.4

Recovered_Sheet1

Summary Of Significant Accounting Policies (Details) (USD $)	12 Months Ended			0 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 01, 2014	Mar. 31, 2014	Apr. 01, 2013	Mar. 31, 2013	Apr. 01, 2012	Mar. 31, 2012	Apr. 01, 2015	Mar. 31, 2015
		item
Summary Of Significant Accounting Policies [Line Items]
Prior period reclassification adjustment		$34
Number of buildings before consolidation		2
Number of buildings after consolidation		1
Loss on sale of asset		4.7
Goodwill	184.6	184.9
U.S. federal income tax rate	35.00%
Current exchange rate between GBP and US dollars	1.56	1.66
Net foreign currency transaction gains	4.2	4.2	6.3
Chaucer [Member]
Summary Of Significant Accounting Policies [Line Items]
Goodwill	5.8	6.3
Indefinite-Lived Intangible Assets (Excluding Goodwill)	$76.50	$81.30
U.K. [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-U.S. jurisdiction income tax rate				21.00%	23.00%	23.00%	24.00%	24.00%	26.00%
U.K. [Member] | Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-U.S. jurisdiction income tax rate										20.00%	21.00%
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives	3 years
Minimum [Member] | Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives	5 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives	30 years
Maximum [Member] | Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives	7 years

Discontinued_Operations_Detail

Discontinued Operations (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Significant Acquisitions and Disposals [Line Items]
Assets of discontinued operations	$112	$114.90
Liabilities of discontinued operations	114.4	121.1
Net (loss) gain from discontinued operations (net of income tax (benefit) expense of $(0.1), $4.1 and $(0.5) in 2014, 2013 and 2012)	-0.3	5.3	9.8
Contribution to charitable foundation that offset gain from disposal of discontinued operation			1.7
Discontinued Accident And Health Insurance Business [Member]
Significant Acquisitions and Disposals [Line Items]
Assets of discontinued operations	65.2	66.6
Liabilities of discontinued operations	49.7	51.3
Citizens Managment Inc. [Member]
Significant Acquisitions and Disposals [Line Items]
Gain on discontinued operations, after taxes			$10.80

Investments_Narrative_Details

Investments (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities	$12.30	$16.40
Cash Collateral Maintained Under Securities Lending Program As Percentage Of Fair Value Of Securities Loaned	102.00%
Restricted assets, fixed maturities	450	334
Restricted assets, cash and cash equivalents	2	1
Net appreciation on other invested assets	2.6	2.9	2.1
Concentration of Investment in a Single Investee, Maximum	10.00%
Sale of office building and adjacent garage			75.9
Gain recognized on sale of office building and adjacent garage		2.4	3.3
Adjustments made to gain on sale of office building and adjacent garage		-0.9
Contractual investment commitments	83.5
Other Investments	291.4	192.5
Due in one year or less, Amortized Cost	498.1
Due after one year through five years, Amortized Cost	2,300
Due after five years through ten years, Amortized Cost	2,267.30
Due after ten years, Amortized Cost	626.2
Overseas
Schedule of Investments [Line Items]
Other Investments	132.6	149.6
Federal Home Loan Mortgage Corp.
Schedule of Investments [Line Items]
Marketable Securities, Fixed Maturities	394.4	341.8
Federal National Mortgage Association [Member]
Schedule of Investments [Line Items]
Marketable Securities, Fixed Maturities	315.8	253.2
Capital Loss Carryforward
Schedule of Investments [Line Items]
Loss on derivative			5.1
Derivative instruments, gain recognized			0.7
Security Lending Program
Schedule of Investments [Line Items]
Securities on loan, fair value	20.9	13.6
Unites States Government and agencies and authorities
Schedule of Investments [Line Items]
Securities on loan, fair value	211.2	142.4
Fixed maturities deposit, amortized cost	196.4	135.2
Collateralized Borrowings And Other Arrangements
Schedule of Investments [Line Items]
Securities on loan, fair value	208.2	160.9
Federal Home Loan Bank of Boston
Schedule of Investments [Line Items]
Securities on loan, fair value	195.7	148.1
Advance received			46.3
Mortgage loans on real estate
Schedule of Investments [Line Items]
Other Investments	94.9	2.5
Due in one year or less, Amortized Cost	0.3
Due after one year through five years, Amortized Cost	1.4
Due after five years through ten years, Amortized Cost	73.4
Due after ten years, Amortized Cost	19.8
Loan Participations and Assignments [Member]
Schedule of Investments [Line Items]
Other Investments	$93.40

Investments_Schedule_of_Availa

Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	$7,145.70	$6,815.20
Gross Unrealized Gains	276.8	246.9
Gross Unrealized Loss	44.4	91.5
Fair Value	7,378.10	6,970.60
OTTI Unrealized Losses	7.8	10.2
Unites States Government and agencies and authorities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	516.3	417.5
Gross Unrealized Gains	7.6	3.3
Gross Unrealized Loss	3.5	14.2
Fair Value	520.4	406.6
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	349.4	304.5
Gross Unrealized Gains	5.2	2.1
Gross Unrealized Loss	0.6	1.6
Fair Value	354	305
Municipal
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	1,079.60	1,108
Gross Unrealized Gains	62.4	37.4
Gross Unrealized Loss	4	19.1
Fair Value	1,138	1,126.30
Corporate
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	3,746.30	3,690.20
Gross Unrealized Gains	166.3	171.5
Gross Unrealized Loss	31.8	37.5
Fair Value	3,880.80	3,824.20
OTTI Unrealized Losses	7.4	8.6
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	770.4	722.8
Gross Unrealized Gains	21.7	20.1
Gross Unrealized Loss	3	14.1
Fair Value	789.1	728.8
OTTI Unrealized Losses	0.4	1.6
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	516.7	405.9
Gross Unrealized Gains	12.4	10.5
Gross Unrealized Loss	1.3	4.8
Fair Value	527.8	411.6
Asset-backed Securities, Securitized Loans and Receivables [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	167	166.3
Gross Unrealized Gains	1.2	2
Gross Unrealized Loss	0.2	0.2
Fair Value	168	168.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	506.6	366.5
Gross Unrealized Gains	76.8	66.9
Gross Unrealized Loss	2.6	3.2
Fair Value	$580.80	$430.20

Investments_Investments_Classi

Investments (Investments Classified by Contractual Maturity Date) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Investments [Abstract]
Due in one year or less, Amortized Cost	$498.10
Due after one year through five years, Amortized Cost	2,300
Due after five years through ten years, Amortized Cost	2,267.30
Due after ten years, Amortized Cost	626.2
Gross fixed maturities, Amortized Cost	5,691.60
Mortgage-backed and asset-backed securities, Amortized Cost	1,454.10
Total fixed maturities, Amortized Cost	7,145.70	6,815.20
Due in one year or less, Fair Value	502.5
Due after one year through five years, Fair Value	2,388
Due after five years through ten years, Fair Value	2,337.50
Due after ten years, Fair Value	665.2
Gross fixed maturities, Fair Value	5,893.20
Mortgage-backed and asset-backed securities, Fair Value	1,484.90
Fixed maturities, Fair Value	$7,378.10	$6,970.60

Investments_Unrealized_Gains_a

Investments (Unrealized Gains and Losses on Available-For-Sale and Other Securities) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Ending balance	$2.60	$2.90	$2.10
Accumulated Net Unrealized Investment Gain (Loss)
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	259.3	426	308.7
Net appreciation (depreciation) on available-for-sale securities and derivative instruments	86.5	-225.5	151.3
Change in OTTI losses recognized in other comprehensive income	2.4	0.8	12.2
Benefit (provision) for deferred income taxes	-47.3	58	-46.2
Total adjustment	41.6	-166.7	117.3
Ending balance	300.9	259.3	426
Accumulated Net Unrealized Investment Gain (Loss) | Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	212.1	410.1	300.2
Net appreciation (depreciation) on available-for-sale securities and derivative instruments	76.3	-273.6	140.7
Change in OTTI losses recognized in other comprehensive income	2.4	0.8	12.2
Benefit (provision) for deferred income taxes	-40.8	74.8	-43
Total adjustment	37.9	-198	109.9
Ending balance	250	212.1	410.1
Accumulated Net Unrealized Investment Gain (Loss) | Equity Securities And Other
Schedule of Available-for-sale Securities [Line Items]
Beginning balance	47.2	15.9	8.5
Net appreciation (depreciation) on available-for-sale securities and derivative instruments	10.2	48.1	10.6
Benefit (provision) for deferred income taxes	-6.5	-16.8	-3.2
Total adjustment	3.7	31.3	7.4
Ending balance	$50.90	$47.20	$15.90

Investments_Schedule_of_Unreal

Investments (Schedule of Unrealized Loss on Investments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$23.60	$70.50
12 months or less, Fair Value	941.9	2,221.50
Greater than 12 months, Gross Unrealized Losses	23.4	24.2
Greater than 12 months, Fair Value	723.9	227.3
Total, Gross Unrealized Losses	47	94.7
Total, Fair Value	1,665.80	2,448.80
Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	9.2	64.7
12 months or less, Fair Value	696.8	2,102.40
Greater than 12 months, Gross Unrealized Losses	20.5	21.9
Greater than 12 months, Fair Value	691.7	203.7
Total, Gross Unrealized Losses	29.7	86.6
Total, Fair Value	1,388.50	2,306.10
Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		3
12 months or less, Fair Value		73.9
Greater than 12 months, Gross Unrealized Losses		1.9
Greater than 12 months, Fair Value		22.9
Total, Gross Unrealized Losses		4.9
Total, Fair Value		96.8
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	21.4	67.7
12 months or less, Fair Value	811.7	2,176.30
Greater than 12 months, Gross Unrealized Losses	23	23.8
Greater than 12 months, Fair Value	720	226.6
Total, Gross Unrealized Losses	44.4	91.5
Total, Fair Value	1,531.70	2,402.90
Unites States Government and agencies and authorities | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		12.3
12 months or less, Fair Value	52.2	247.9
Greater than 12 months, Gross Unrealized Losses	3.5	1.9
Greater than 12 months, Fair Value	137.9	18.8
Total, Gross Unrealized Losses	3.5	14.2
Total, Fair Value	190.1	266.7
Foreign government | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	1.5
12 months or less, Fair Value	20.8	129
Greater than 12 months, Gross Unrealized Losses	0.2	0.1
Greater than 12 months, Fair Value	24.2	17.3
Total, Gross Unrealized Losses	0.6	1.6
Total, Fair Value	45	146.3
Municipal | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	14.8
12 months or less, Fair Value	57.1	345.3
Greater than 12 months, Gross Unrealized Losses	3.7	4.3
Greater than 12 months, Fair Value	140.2	39.9
Total, Gross Unrealized Losses	4	19.1
Total, Fair Value	197.3	385.2
Corporate | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	7.8	21.4
12 months or less, Fair Value	393.3	872.7
Greater than 12 months, Gross Unrealized Losses	9.3	11.6
Greater than 12 months, Fair Value	217.4	87.7
Total, Gross Unrealized Losses	17.1	33
Total, Fair Value	610.7	960.4
Corporate | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	12.2	2.9
12 months or less, Fair Value	114.9	71.9
Greater than 12 months, Gross Unrealized Losses	2.5	1.6
Greater than 12 months, Fair Value	28.3	21.4
Total, Gross Unrealized Losses	14.7	4.5
Total, Fair Value	143.2	93.3
Residential mortgage-backed | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	10.3
12 months or less, Fair Value	36.4	321.1
Greater than 12 months, Gross Unrealized Losses	2.8	3.4
Greater than 12 months, Fair Value	98	29.5
Total, Gross Unrealized Losses	3	13.7
Total, Fair Value	134.4	350.6
Residential mortgage-backed | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.1
12 months or less, Fair Value		2
Greater than 12 months, Gross Unrealized Losses		0.3
Greater than 12 months, Fair Value		1.5
Total, Gross Unrealized Losses		0.4
Total, Fair Value		3.5
Commercial mortgage-backed | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	4.2
12 months or less, Fair Value	90.4	155.4
Greater than 12 months, Gross Unrealized Losses	0.9	0.6
Greater than 12 months, Fair Value	60.8	10.2
Total, Gross Unrealized Losses	1.3	4.8
Total, Fair Value	151.2	165.6
Asset-backed Securities, Securitized Loans and Receivables [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	0.2
12 months or less, Fair Value	46.6	31
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	13.2	0.3
Total, Gross Unrealized Losses	0.2	0.2
Total, Fair Value	59.8	31.3
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.2	2.8
12 months or less, Fair Value	130.2	45.2
Greater than 12 months, Gross Unrealized Losses	0.4	0.4
Greater than 12 months, Fair Value	3.9	0.7
Total, Gross Unrealized Losses	2.6	3.2
Total, Fair Value	$134.10	$45.90

Investments_Schedule_of_Other_

Investments (Schedule of Other Investments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Investments [Line Items]
Other Investments	$291.40	$192.50
Loan Participations and Assignments [Member]
Schedule of Investments [Line Items]
Other Investments	93.4
Mortgage loans on real estate
Schedule of Investments [Line Items]
Other Investments	94.9	2.5
Mortgage loans on real estate | Office [Member]
Schedule of Investments [Line Items]
Other Investments	40
Mortgage loans on real estate | Apartments [Member]
Schedule of Investments [Line Items]
Other Investments	33.4
Mortgage loans on real estate | Retail and Other [Member]
Schedule of Investments [Line Items]
Other Investments	21.7
Mortgage loans on real estate | Valuation Allowance [Member]
Schedule of Investments [Line Items]
Other Investments	-0.2
Mortgage loans on real estate | South Atlantic [Member]
Schedule of Investments [Line Items]
Other Investments	30
Mortgage loans on real estate | West South Central [Member]
Schedule of Investments [Line Items]
Other Investments	25
Mortgage loans on real estate | Pacific [Member]
Schedule of Investments [Line Items]
Other Investments	20
Mortgage loans on real estate | East North Central [Member]
Schedule of Investments [Line Items]
Other Investments	11.4
Mortgage loans on real estate | Mid-Atlantic [Member]
Schedule of Investments [Line Items]
Other Investments	$8.70

Investment_Income_and_Gains_an2

Investment Income and Gains and Losses (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Non-accruals of fixed maturity securities	$2.10	$2.30	$2.50
Net other-than-temporary impairment losses on securities recognized in earnings	5.5	6	7.8
Other-than-temporary impairments	5.4	7	7.3
Net other-than-temporary impairment losses recorded in accumulated other comprehensive income		1	0.5
Portion Of Impairment Loss Recognized In Earnings That Were Transferred From AOCI	0.1
Fixed Maturities Held-for-Sale
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings	$5.50

Investment_Income_and_Gains_an3

Investment Income and Gains and Losses (Components of Net Investment Income) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Gross investment income	$281.30	$279.20	$286.50
Less investment expenses	-11	-10.2	-9.9
Net Investment Income, Total	270.3	269	276.6
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Gross investment income	255.8	254.8	264.2
Equity Securities
Gain (Loss) on Investments [Line Items]
Gross investment income	16.5	15.6	15.3
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$9	$8.80	$7

Investment_Income_and_Gains_an4

Investment Income and Gains and Losses (Schedule of Net Realized Gains Losses on Investments) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Net realized investment gains	$50.10	$33.50	$23.60
Equity Securities
Gain (Loss) on Investments [Line Items]
Net realized investment gains	45.8	26.7	15.5
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Net realized investment gains	4.5	8.6	9.7
Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Net realized investment gains		-0.9	3.3
Derivative Instruments
Gain (Loss) on Investments [Line Items]
Net realized investment gains			-4.4
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Net realized investment gains	($0.20)	($0.90)	($0.50)

Investment_Income_and_Gains_an5

Investment Income and Gains and Losses (Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments [Abstract]
Credit losses at beginning of period	$7.80	$8.60	$14.50
Credit losses for which an OTTI was not previously recognized		1.1	0.6
Additional credit losses on securities for which an OTTI was previously recognized		0.3	0.6
Reductions for securities sold, matured or called	-3.2	-2.2	-6.9
Reductions for securities reclassified as intended to sell	-0.4		-0.2
Credit losses at the end of the year	$4.20	$7.80	$8.60

Investment_Income_and_Gains_an6

Investment Income and Gains and Losses (Proceeds from Sale of Available for Sale Securities) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fixed Maturities
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$349.50	$450.40	$616.30
Gross Gains	5.8	6	12.1
Gross Losses	2.6	4	2
Equity Securities
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	156.1	193.5	141.3
Gross Gains	46.2	31.2	17.5
Gross Losses	$0.80	$0.40	$0.70

Fair_Value_Narrative_Details

Fair Value (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value [Abstract]
Included in earnings	$0.10	$1.50
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$0	$0

Fair_Value_Fair_Value_of_Finan

Fair Value (Fair Value of Financial Instruments) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$373.30	$486.20	$564.80	$820.40
Fixed maturities	7,378.10	6,970.60
Equity securities	580.8	430.2
Other investments	291.4	192.5
Debt	903.5	903.9
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	373.3	486.2
Fixed maturities	7,378.10	6,970.60
Equity securities	580.8	430.2
Other investments	267.4	173.1
Total financial assets	8,599.60	8,060.10
Debt	903.5	903.9
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	9.3	9.3
Estimate of Fair Value, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	373.3	486.2
Fixed maturities	7,378.10	6,970.60
Equity securities	580.8	430.2
Other investments	272.2	173.7
Total financial assets	8,604.40	8,060.70
Debt	$1,021.70	$961.70

Fair_Value_Assets_and_Liabilit

Fair Value (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$7,378.10	$6,970.60
Equity securities	580.8	430.2
Other investments	291.4	192.5
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	136.4	153.2
Total investment assets at fair value	8,086	7,544.70
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value	847.1	595.1
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	132.6	149.6
Total investment assets at fair value	7,177.20	6,845.40
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	3.8	3.6
Total investment assets at fair value	61.7	104.2
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,378.10	6,970.60
Fair Value, Measurements, Recurring | Fixed Maturities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	276.8	212.8
Fair Value, Measurements, Recurring | Fixed Maturities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,044.60	6,695.80
Fair Value, Measurements, Recurring | Fixed Maturities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	56.7	62
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	520.4	406.6
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	233.5	167.2
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	286.9	239.4
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	354	305
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	43.3	45.6
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	310.7	259.4
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,138	1,126.30
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,112.30	1,100.70
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	25.7	25.6
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,880.80	3,824.20
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,871.20	3,811.20
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	9.6	13
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	689.2	573.2
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	689.2	573.2
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	99.9	155.6
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	99.9	155.1
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities		0.5
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	527.8	411.6
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	506.4	388.7
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	21.4	22.9
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	168	168.1
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	168	168.1
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	571.5	420.9
Fair Value, Measurements, Recurring | Equity Securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	570.3	382.3
Fair Value, Measurements, Recurring | Equity Securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$1.20	$38.60

Fair_Value_Estimated_Fair_Valu

Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	$580.80	$430.20
Estimate of Fair Value, Fair Value Disclosure
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	580.8	430.2
Estimate of Fair Value, Fair Value Disclosure | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	373.3	486.2
Estimate of Fair Value, Fair Value Disclosure | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	9.3	9.3
Other investments		2.7
Debt	1,021.70	961.7
Estimate of Fair Value, Fair Value Disclosure | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other investments	135.8	17.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	373.3	486.2
Equity securities	9.3	9.3
Other investments	135.8	20.5
Debt	$1,021.70	$961.70

Fair_Value_Fair_Value_on_Recur

Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	$104.20	$102.70
Transfers into Level 3	4.4	9.9
Transfers out of Level 3	-7.9	-4.6
Included in earnings	0.1	1.5
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	1.2	11.4
Purchases	2.5
Sales	-42.8	-16.7
Balance at end of year	61.7	104.2
Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	62	74.7
Transfers into Level 3	4.4	9.9
Transfers out of Level 3	-7.9	-3.7
Included in earnings	0.1	1.5
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	1	-3.7
Purchases	2.5
Sales	-5.4	-16.7
Balance at end of year	56.7	62
Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	25.6	19.4
Transfers into Level 3	2.2	9.7
Transfers out of Level 3	-2.6
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.8	-0.8
Purchases	2.5
Sales	-2.8	-2.7
Balance at end of year	25.7	25.6
Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	13	26.4
Transfers into Level 3	2.2	0.2
Transfers out of Level 3	-5.3	-2.2
Included in earnings	0.1	1.5
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	-0.2	-1.6
Sales	-0.2	-11.3
Balance at end of year	9.6	13
Fixed Maturities | Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	0.5	0.7
Sales	-0.5	-0.2
Balance at end of year		0.5
Fixed Maturities | Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	22.9	26.7
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.4	-1.3
Sales	-1.9	-2.5
Balance at end of year	21.4	22.9
Fixed Maturities | Asset-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year		1.5
Transfers out of Level 3		-1.5
Equity Securities and Other Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	42.2	28
Transfers out of Level 3		-0.9
Included in other comprehensive income - net appreciation (depreciation) on available-for-sale securities	0.2	15.1
Sales	-37.4
Balance at end of year	$5	$42.20

Fair_Value_Schedule_of_Additio

Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value [Line Items]
Fair Value	61.7	104.2	$102.70
Corporate | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.50%
Corporate | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)	1.00%
Corporate | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.70%
Fixed Maturities | Municipal | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Fair Value	25.7	25.6
Fixed Maturities | Municipal | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.60%	1.00%
Fixed Maturities | Municipal | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)	4.50%	4.00%
Fixed Maturities | Municipal | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)	2.00%	2.30%
Fixed Maturities | Municipal | Discount For Above Market Coupon | Level 3 | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.30%
Fixed Maturities | Municipal | Discount For Above Market Coupon | Level 3 | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)	-1.00%
Fixed Maturities | Municipal | Discount For Above Market Coupon | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.50%
Fixed Maturities | Municipal | Discount For Above Market Coupon | Income Approach Valuation Technique | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.30%
Fixed Maturities | Municipal | Discount For Above Market Coupon | Income Approach Valuation Technique | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)	1.00%
Fixed Maturities | Municipal | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.40%
Fixed Maturities | Corporate | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Fair Value	9.4	12.8
Fixed Maturities | Corporate | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)		0.30%
Fixed Maturities | Corporate | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)		-1.00%
Fixed Maturities | Corporate | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)		0.50%
Fixed Maturities | Corporate | Discount For Above Market Coupon | Income Approach Valuation Technique | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)		0.30%
Fixed Maturities | Corporate | Discount For Above Market Coupon | Income Approach Valuation Technique | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)		-0.80%
Fixed Maturities | Corporate | Discount For Above Market Coupon | Income Approach Valuation Technique | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)		0.60%
Fixed Maturities | Corporate | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.30%
Fixed Maturities | Corporate | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.80%
Fixed Maturities | Corporate | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.60%
Fixed Maturities | Residential mortgage-backed | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Fair Value		0.5
Fixed Maturities | Residential mortgage-backed | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Range (Weighted Average)		0.50%
Fixed Maturities | Residential mortgage-backed | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)		0.50%
Fixed Maturities | Commercial mortgage-backed | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Fair Value	21.4	22.9
Fixed Maturities | Commercial mortgage-backed | Discount For Small Issue Size | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Range (Weighted Average)	0.50%	0.50%
Fixed Maturities | Commercial mortgage-backed | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Minimum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.50%	0.50%
Fixed Maturities | Commercial mortgage-backed | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Maximum [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.80%	-0.80%
Fixed Maturities | Commercial mortgage-backed | Discount For Above Market Coupon | Income Approach Valuation Technique | Level 3 | Weighted Average [Member]
Fair Value [Line Items]
Range (Weighted Average)	0.50%	0.60%
Fixed Maturities | Commercial mortgage-backed | Discount For Credit Stress | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Range (Weighted Average)	0.50%	0.50%
Fixed Maturities | Commercial mortgage-backed | Discount For Lease Structure | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Range (Weighted Average)	0.30%	0.30%
Equity Securities | Net Tangible Asset Market Multiples | Market comparables [Member] | Level 3
Fair Value [Line Items]
Fair Value	1.1	38.5
Fair value measurement market multiples	1	1.3
Other securities | Level 3
Fair Value [Line Items]
Range (Weighted Average)		3.60%
Other securities | Income Approach Valuation Technique | Level 3
Fair Value [Line Items]
Fair Value	3.8	3.6
Range (Weighted Average)	18.00%	18.00%

Debt_and_Credit_Arrangements_S

Debt and Credit Arrangements (Schedule of Long Term Debt) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Debt Instrument [Line Items]
Total principal debt	$905.40	$906
Unamortized debt discount	-1.9	-2.1
Long-term Debt, Total	903.5	903.9
Senior debentures maturing June 15, 2021 [Member]
Debt Instrument [Line Items]
Total principal debt	300	300
Senior debentures maturing March 1, 2020 [Member]
Debt Instrument [Line Items]
Total principal debt	164.6	165.1
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt	81.1	81.2
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Total principal debt	175	175
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt	59.7	59.7
Federal Home Loan Bank of Boston
Debt Instrument [Line Items]
Total principal debt	$125	$125

Debt_and_Credit_Arrangements_N

Debt and Credit Arrangements (Narrative) (Details)

12 Months Ended

1 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2014

Dec. 31, 2014

Jan. 31, 2013

Jul. 31, 2010

Dec. 31, 2009

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

USD ($)

USD ($)

USD ($)

Standby Letters of Credit [Member]

Standby Letters of Credit [Member]

Federal Home Loan Bank of Boston

Federal Home Loan Bank of Boston

Federal Home Loan Bank of Boston

Collateralized Borrowing Program [Member]

Collateralized Borrowing Program [Member]

Subordinated debentures maturing March 30, 2053 [Member]

Subordinated debentures maturing March 30, 2053 [Member]

Senior Unsecured Notes Issued On June 17, 2011 [Member]

Senior unsecured Notes Issued On February 23, 2010 [Member]

Senior Unsecured Notes Issued On October 16, 1995 [Member]

Senior Unsecured Notes Issued On October 16, 1995 [Member]

8.207% Subordinated Debentures [Member]

8.207% Subordinated Debentures [Member]

Senior debentures maturing October 15, 2025 [Member]

Senior debentures maturing October 15, 2025 [Member]

Senior debentures maturing March 1, 2020 [Member]

Senior debentures maturing March 1, 2020 [Member]

The Hanover Insurance Group [Member]

The Hanover Insurance Group [Member]

The Hanover Insurance Group [Member]

USD ($)

GBP (£)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

USD ($)

Debt Instrument [Line Items]

Debt issuance date

20-Mar-13

17-Jun-11

23-Feb-10

16-Oct-95

Aggregate principal amount

$175,000,000

$300,000,000

$200,000,000

$200,000,000

$59,700,000

$59,700,000

Debt instrument interest rate

5.50%

6.35%

6.38%

7.50%

7.63%

7.50%

Maturity date

30-Jul-20

30-Mar-53

15-Jun-21

1-Mar-20

15-Oct-25

3-Feb-27

15-Oct-25

1-Mar-20

Debt Instrument Redemption Date

30-Mar-18

Interest rate of Series B Subordinated Deferrable Interest Debentures

8.21%

Repurchase of senior debentures

39,200,000

34,600,000

Repurchase of senior debentures at cost

50,500,000

43,200,000

Gain (loss) on the repurchase of debt

-100,000

-27,700,000

-5,100,000

11,300,000

8,600,000

-100,000

-19,900,000

-100,000

Advance received

46,300,000

125,000,000

125,000,000

Length of debt instrument

20 years

Weighted average fixed interest rate

3.88%

Repayments of Federal Home Loan Bank Borrowings

46,300,000

Federal Home Loan Bank Advances General Debt Obligations Disclosures Penalties

7,800,000

Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties

54,100,000

Securities Held As Collateral At Fair Value

195,700,000

148,100,000

Federal Home Loan Bank Stock

9,300,000

Long-term Debt, Gross

905,400,000

906,000,000

175,000,000

175,000,000

81,100,000

81,200,000

81,200,000

81,100,000

165,100,000

164,600,000

Line of Credit Facility, Maximum Borrowing Capacity

200,000,000

202,800,000

130,000,000

Line Of Credit Facility Maturity Month And Year

2018-11

Foreign Currency Exchange Rate Translation 1

1.56

1.66

Interest Expense Debt

$65,200,000

$65,300,000

$61,900,000

$54,100,000

$55,300,000

$48,600,000

Income_Taxes_Narrative_Details

Income Taxes (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
U.S. statutory federal income tax rate	35.00%
Deferred tax assets, U.S. net operating loss carryforwards	$29.10
Pre-tax operating loss carryforward	81.9
Deferred tax asset, alternative minimum tax carryforward	110
Deferred tax assets, Foreign tax credit carryforwards	15.8
Foreign tax credit carryforwards expiration date	beginning in 2022
Valuation Allowance		2.9
Valuation allowance, deferred tax asset, reduction (increase) in amount	-2.9		-35.9
Realized Gains In Accumulated Other Comprehensive Income To Be Released Into Income From Continuing Operations In Future	91.5
Unrealized gains in our investment portfolio			69.6
Net realized investment gains	50.1	33.5	23.6
Foreign income permanently reinvested	23.3	20.3	10
Estimated taxes payable on undistributed earnings	21.1
Deferred deductions	4.8
Liability for uncertain tax positions		4.8
Accrued interest	0.5	1.2
Unrecognized Tax Benefits, Interest on Income Taxes Expense	0.5	0.8	0.1
Income Tax Expense (Benefit) [Member]
Income Taxes [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount	-2.9
Accumulated Other Comprehensive Incoome [Member]
Income Taxes [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			-24.4
Non-Operating Income [Member]
Income Taxes [Line Items]
Income recognized in continuing operations related to non-segment income	16.2	17.5	14.3
Valuation Allowance, Capital Loss Carryforwards, Unrealized Appreciation, And Net Realized Capital Gains [Member] | Income Tax Expense (Benefit) [Member]
Income Taxes [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			-7.7
Valuation Allowance, Capital Loss Carryforwards, Unrealized Appreciation, And Net Realized Capital Gains [Member] | Accumulated Other Comprehensive Incoome [Member]
Income Taxes [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			-25.3
Valuation Allowance, Capital Loss Carryforwards, Unrealized Appreciation, And Net Realized Capital Gains [Member] | Discontinued Operations [Member]
Income Taxes [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			($2.90)

Income_Taxes_Components_of_Inc

Income Taxes (Components of Income Before Income Taxes) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Abstract]
U.S.	$185.60	$183.50	($99.10)
Non-U.S.	192.4	145.6	127.8
Income before income taxes	$378	$329.10	$28.70

Income_Taxes_Components_of_Inc1

Income Taxes (Components of Income Tax Expense (Benefit)) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Abstract]
Current income taxes, U.S.	$4.50	$8	$2.30
Current income taxes, Non-U.S.	22.6	0.6	16.1
Current Income Tax Expense (Benefit), Total	27.1	8.6	18.4
Deferred income taxes, U.S.	51.1	52.9	-42.7
Deferred income taxes, Non-U.S.	17.5	21.9	6.9
Deferred Income Tax Expense (Benefit), Total	68.6	74.8	-35.8
Total income tax expense (benefit)	$95.70	$83.40	($17.40)

Income_Taxes_Schedule_of_Effec

Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Abstract]
Expected income tax expense	$132.30	$115.20	$10
Tax difference related to investment disposals and maturities	-16.2	-17.5	-14.3
Effect of foreign operations	-8.1	-10.8	-3.5
Foreign exchange losses	-6.9
Dividend received deduction	-2.6	-2.5	-2.8
Tax-exempt interest	-1.7	-1.7	-1.5
Nondeductible expenses	1.8	1.1	2.8
Change in foreign tax rate		-0.7	-0.4
Change in valuation allowance	-2.9		-7.7
Other, net		0.3
Income tax expense (benefit)	$95.70	$83.40	($17.40)
Effective tax rate	25.30%	25.30%	-60.60%

Income_Taxes_Schedule_of_Defer

Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Income Taxes [Abstract]
Loss, LAE and unearned premium reserves, net	$175.90	$185
Tax credit carryforwards	125.8	123.1
Operating loss carryforwards	29.1	64.5
Employee benefit plans	45.7	38.2
Other	10.6	74.9
Deferred tax assets, gross	387.1	485.7
Valuation Allowance		2.9
Deferred tax assets, total	387.1	482.8
Deferred acquisition costs	129.6	123.5
Software capitalization	27.3	30.1
Deferred Lloyd's underwriting income	54	13.8
Investments, net	44.4	8.4
Other	0.6	67.3
Deferred tax liabilities, total	255.9	243.1
Deferred Tax Assets, Net of Valuation Allowance, Total	$131.20	$239.70

Income_Taxes_Summary_of_Income

Income Taxes (Summary of Income Tax Uncertainties) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011
Income Taxes [Abstract]
Unrecognized Tax Benefits, Beginning Balance	$1.40	$0.80	$0.80
Additions for tax positions of prior years	-0.1	5.4
Settlements	-4.8
Deferred deductions	4.8	-4.8
Unrecognized Tax Benefits, Ending Balance	$1.30	$1.40	$0.80

Pension_Plan_Narrative_Details

Pension Plan (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended
	Jan. 01, 2005	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 2005
Retirement Plans [Line Items]
Interest rate used calculating annual transition pension adjustment for specified completed period of service	5.00%				5.00%
Defined benefit plan, maximum number of years of completed service used in calculating the annual transition pension adjustment	35 years
Interest rate used calculating annual transition pension adjustment, thereafter	3.00%				3.00%
Projected benefit obligation, defined benefit pension plans exceeds plan assets		$25,000,000
Fair value of total plan asset		131,800,000	120,200,000
Invested assets, fair value		130,300,000	119,900,000
Cash and cash equivalents, fair value		1,500,000	300,000
Employee contributions		400,000	400,000
Lump-sum settlements		55,100,000
Settlement loss		12,100,000	400,000
Accelerated recognition of gains (losses)		1,300,000
Estimated future contributions in next fiscal year		0
Maximum percentage contributed by employer		6.00%	6.00%	6.00%
Defined contribution plan expense		19,900,000	18,300,000	17,900,000
Additional cost contribution		2,800,000	2,100,000	0
Additional cost contribution		6,100,000	5,000,000	4,800,000
Defined Benefit Plan Foreign Currency Exchange Rate		1.56
Minimum [Member]
Retirement Plans [Line Items]
Percentage of unrecognized net actuarial gains (losses) as projected benefit obligation, amortized as component of net periodic pension cost		10.00%
Chaucer Pension Plan [Member]
Retirement Plans [Line Items]
Projected benefit obligation, defined benefit pension plans exceeds plan assets		6,000,000
Expected rate of return on plan assets		6.55%	6.70%	7.50%
Defined benefit plan, target allocation percentage		100.00%	100.00%
Fair value of total plan asset		131,800,000	120,200,000	90,900,000
Estimated future contributions in next fiscal year		1,300,000
Chaucer Pension Plan [Member] | Equity Securities
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		57.00%	74.00%
Fair value of total plan asset		75,400,000	88,300,000
Chaucer Pension Plan [Member] | Equity Securities | Ultimate Target Level
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		60.00%
Chaucer Pension Plan [Member] | Fixed Maturities
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		32.00%	19.00%
Fair value of total plan asset		40,900,000	22,800,000
Chaucer Pension Plan [Member] | Fixed Maturities | Ultimate Target Level
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		30.00%
Chaucer Pension Plan [Member] | Real Estate Funds
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		11.00%	7.00%
Fair value of total plan asset		14,000,000	8,800,000
Chaucer Pension Plan [Member] | Real Estate Funds | Ultimate Target Level
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		10.00%
U.S. Qualified Defined Benefit Plans [Member]
Retirement Plans [Line Items]
Expected rate of return on plan assets		5.50%
Defined benefit plan, target allocation percentage		100.00%	100.00%
Fair value of total plan asset		513,400,000	542,000,000	591,600,000
Settlement loss		10,800,000
Estimated future contributions in next fiscal year		3,500,000
U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		85.00%	84.00%
Fair value of total plan asset		436,400,000	454,900,000
U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		15.00%	16.00%
Fair value of total plan asset		77,000,000	87,100,000
U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Retirement Plans [Line Items]
Defined benefit plan, target allocation percentage		84.00%	83.00%
Fair value of total plan asset		$432,500,000	$449,600,000

Pension_Plans_Schedule_of_Weig

Pension Plans (Schedule of Weighted Average Assumptions Used to Determine Pension Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
U.S. Qualified Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Cash balance interest crediting rate	3.50%	3.50%	3.50%
Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.38%	5.00%	4.25%
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.25%	5.00%	4.13%
Rate of increase in future compensation	3.00%	3.15%	4.20%
Chaucer Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.75%	4.50%	4.80%

Pension_Plans_Schedule_of_Weig1

Pension Plans (Schedule of Weighted Average Assumptions Used to Determine Net Periodic Pension Costs) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
U.S. Qualified Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	5.50%
Cash balance interest crediting rate	3.50%	3.50%	3.50%
Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.00%	4.25%	5.13%
Expected return on plan assets	5.50%	5.25%	6.00%
Cash balance interest crediting rate	3.50%	3.50%	4.00%
Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.00%	4.13%	5.00%
Chaucer Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.50%	4.80%	4.90%
Expected return on plan assets	6.55%	6.70%	7.50%
Rate of increase in future compensation	3.15%	4.20%	4.30%

Pension_Plans_Summary_of_Targe

Pension Plans (Summary of Target Allocations and Invested Asset Allocations) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
U.S. Qualified Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	100.00%
Total Fixed Income and Equity Securities	100.00%	100.00%
U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	83.00%
Total Fixed Income and Equity Securities	84.00%	83.00%
U.S. Qualified Defined Benefit Plans [Member] | Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	2.00%
Total Fixed Income and Equity Securities	1.00%	1.00%
U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	85.00%
Total Fixed Income and Equity Securities	85.00%	84.00%
U.S. Qualified Defined Benefit Plans [Member] | Domestic
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	12.00%
Total Fixed Income and Equity Securities	12.00%	12.00%
U.S. Qualified Defined Benefit Plans [Member] | International
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	3.00%
Total Fixed Income and Equity Securities	3.00%	4.00%
U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	15.00%
Total Fixed Income and Equity Securities	15.00%	16.00%
Chaucer Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	100.00%
Total Fixed Income and Equity Securities	100.00%	100.00%
Chaucer Pension Plan [Member] | Fixed Maturities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	30.00%
Total Fixed Income and Equity Securities	32.00%	19.00%
Chaucer Pension Plan [Member] | Domestic
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	15.00%
Total Fixed Income and Equity Securities	14.00%	26.00%
Chaucer Pension Plan [Member] | International
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	45.00%
Total Fixed Income and Equity Securities	43.00%	48.00%
Chaucer Pension Plan [Member] | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	60.00%
Total Fixed Income and Equity Securities	57.00%	74.00%
Chaucer Pension Plan [Member] | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	10.00%
Total Fixed Income and Equity Securities	11.00%	7.00%

Pension_Plans_Summary_of_Plan_

Pension Plans (Summary of Plan Assets Investment Measured At Fair Value) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$130.30	$119.90
Invested assets, fair value	131.8	120.2
U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	513.4	542	591.6
U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	432.5	449.6
U.S. Qualified Defined Benefit Plans [Member] | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	3.9	5.3
U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	436.4	454.9
U.S. Qualified Defined Benefit Plans [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	60.6	67.8
U.S. Qualified Defined Benefit Plans [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	16.4	19.3
U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	77	87.1
Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	131.8	120.2	90.9
Chaucer Pension Plan [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	40.9	22.8
Chaucer Pension Plan [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.5	30.5
Chaucer Pension Plan [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	56.9	57.8
Chaucer Pension Plan [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	75.4	88.3
Chaucer Pension Plan [Member] | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	14	8.8
Level 1 | U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	5.2	7.7
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	1.3	2.1
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	3.9	5.3
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	5.2	7.4
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value		0.2
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value		0.1
Level 1 | U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value		0.3
Level 2 | U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	477.5	534.3
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	400.5	447.5
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	400.5	447.5
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	60.6	67.6
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	16.4	19.2
Level 2 | U.S. Qualified Defined Benefit Plans [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	77	86.8
Level 2 | Chaucer Pension Plan [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	40.9	22.8
Level 2 | Chaucer Pension Plan [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	18.5	30.5
Level 2 | Chaucer Pension Plan [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	56.9	57.8
Level 2 | Chaucer Pension Plan [Member] | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	75.4	88.3
Level 3 | U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	30.7
Level 3 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	30.7
Level 3 | U.S. Qualified Defined Benefit Plans [Member] | Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	30.7
Level 3 | Chaucer Pension Plan [Member] | Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	14	8.8
Estimate of Fair Value, Fair Value Disclosure | Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	130.3	119.9
Estimate of Fair Value, Fair Value Disclosure | Level 2 | Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	116.3	111.1
Estimate of Fair Value, Fair Value Disclosure | Level 3 | Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$14	$8.80

Pension_Plans_Summary_for_Asse

Pension Plans (Summary for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	$104.20	$102.70
Transfers into Level 3	4.4	9.9
Balance at end of year	61.7	104.2
Real Estate Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year	8.8	8.2
Transfers into Level 3	4.2
Actual return on plan assets related to assets still held	1.9	0.5
Foreign currency translation	-0.9	0.1
Balance at end of year	14	8.8
U.S. Qualified Defined Benefit Plans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year
Transfers into Level 3	30
Actual return on plan assets related to assets still held	0.7
Balance at end of year	30.7
Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Foreign currency translation	($8.30)	$3.10

Pension_Plans_Fair_Value_of_Pl

Pension Plans (Fair Value of Plan Assets and Funded Status of Plans) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$1.50	$1.50	$1.60
Interest cost	33.7	31.9	35.2
Actuarial (gains) losses	11.7	14.7	12.8
Fair value of plan assets, beginning of period	120.2
Settlements	55.1
Fair value of plan assets, end of year	131.8	120.2
Defined Benefit Plan, Payment to Terminate Plan	59
U.S. Qualified Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	538.5	548.8
Projected benefit obligation, beginning of period	548.8	607
Interest cost	26	24.8
Actuarial (gains) losses	-53	36.5
Benefits paid	-89.3	-46.5
Projected benefit obligation, end of year	538.5	548.8
Fair value of plan assets, beginning of period	542	591.6
Actual return on plan assets	59.4	-3.1
Contributions	1.3
Benefits paid	-89.3	-46.5
Fair value of plan assets, end of year	513.4	542
Funded status of the plans	-25.1	-6.8
U.S. Non Qualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	40.6	37.4
Projected benefit obligation, beginning of period	37.4	41.2
Interest cost	1.8	1.6
Actuarial (gains) losses	-4.6	2.3
Benefits paid	-3.2	-3.1
Projected benefit obligation, end of year	40.6	37.4
Contributions	3.2	3.1
Benefits paid	-3.2	-3.1
Funded status of the plans	-40.6	-37.4
Chaucer Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	134.1	126.4
Projected benefit obligation, beginning of period	128.5	116.6
Employee contributions	0.4	0.4
Service cost - benefits earned during the period	1.5	1.5
Interest cost	5.9	5.4
Actuarial (gains) losses	-12.4	-3.6
Benefits paid	-2.6	-1.5
Foreign currency translation	-8.6	2.5
Projected benefit obligation, end of year	137.5	128.5
Fair value of plan assets, beginning of period	120.2	90.9
Actual return on plan assets	14.8	17.1
Contributions	7.7	10.6
Benefits paid	-2.6	-1.5
Foreign currency translation	-8.3	3.1
Fair value of plan assets, end of year	131.8	120.2
Funded status of the plans	($5.70)	($8.30)

Pension_Plans_Components_of_Ne

Pension Plans (Components of Net Periodic Benefit Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans [Abstract]
Service cost - benefits earned during the period	$1.50	$1.50	$1.60
Interest cost	33.7	31.9	35.2
Expected return on plan assets	-36.6	-35.9	-38.7
Recognized net actuarial loss	11.7	14.7	12.8
Amortization of prior service cost	0.1		0.1
Settlement loss	12.1	0.4
Net periodic pension cost	$22.50	$12.60	$11

Pension_Plans_Summary_of_Amoun

Pension Plans (Summary of Amounts Recognized in Accumulated Other Comprehensive Income Loss) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Pension Plans [Abstract]
Net actuarial loss	$126.90	$118.70
Net prior service cost		0.1
Amounts recognized in accumulated other comprehensive income	$126.90	$118.80

Pension_Plans_Summary_of_Estim

Pension Plans (Summary of Estimated Amount Amortized from Accumulated Other Comprehensive Income Loss into Net Periodic Pension Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Pension Plans [Abstract]
Net actuarial loss	$12

Pension_Plans_Summary_of_Estim1

Pension Plans (Summary of Estimated Benefit Payments) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
U.S. Qualified Defined Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	$38.70
2016	38.4
2017	39.4
2018	38.6
2019	38.3
2020-2024	186.5
U.S. Non Qualified Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	3.5
2016	3.2
2017	3.1
2018	3.1
2019	3
2020-2024	14.2
Chaucer Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	2.7
2016	1.6
2017	1.7
2018	1.7
2019	1.8
2020-2024	$9.70

Other_Postretirement_Benefit_P2

Other Postretirement Benefit Plans (Narrative) (Details) (Postretirement Plans)	12 Months Ended
	Dec. 31, 2014
1995 Postretirement Health Coverage [Member]
Retirement Plans [Line Items]
Eligibility age	40 years
2009 Postretirement Health Coverage [Member]
Retirement Plans [Line Items]
Eligibility age	65 years
Minimum [Member] | 1995 Postretirement Health Coverage [Member]
Retirement Plans [Line Items]
Service period required to avail postretirement benefits (in years)	15 years

Other_Postretirement_Benefit_P3

Other Postretirement Benefit Plans (Fair Value of Plan Assets and Funded Status of Plans) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$1.50	$1.50	$1.60
Interest cost	33.7	31.9	35.2
Net actuarial loss (gain)	-11.7	-14.7	-12.8
Fair value of plan assets, end of year	131.8	120.2
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation, beginning of year	17.1	45.2
Service cost - benefits earned during the period	0.1	0.1	0.1
Interest cost	0.8	1.2	2.1
Net actuarial loss (gain)	1.5	-1
Benefits paid	-2.5	-2.8
Plan amendment		-8.4
Settlement loss		-17.2
Accumulated benefit obligation, end of year	17	17.1	45.2
Funded status of the plans	($17)	($17.10)

Other_Postretirement_Benefit_P4

Other Postretirement Benefit Plans (Schedule of Expected Benefit Payments) (Details) (Postretirement Plans, USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	$2.10
2016	1.8
2017	1.7
2018	1.5
2019	1.3
2020-2024	$5.40

Other_Postretirement_Benefit_P5

Other Postretirement Benefit Plans (Components of Net Periodic Pension Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost - benefits earned during the period	$1.50	$1.50	$1.60
Interest cost	33.7	31.9	35.2
Amortization of prior service cost	0.1		0.1
Net settlement gain	12.1	0.4
Net periodic pension cost	22.5	12.6	11
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost - benefits earned during the period	0.1	0.1	0.1
Interest cost	0.8	1.2	2.1
Recognized net actuarial loss	0.1	0.2	0.2
Amortization of prior service cost	-1.9	-3.7	-3.8
Net settlement gain		-1.6
Net periodic pension cost	($0.90)	($3.80)	($1.40)

Other_Postretirement_Benefit_P6

Other Postretirement Benefit Plans (Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	$126.90	$118.70
Net prior service cost		0.1
Amounts recognized in accumulated other comprehensive income	126.9	118.8
Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net actuarial loss	3.4	1.9
Net prior service cost	-1.7	-3.5
Amounts recognized in accumulated other comprehensive income	$1.70	($1.60)

Other_Postretirement_Benefit_P7

Other Postretirement Benefit Plans (Summary of Estimated Amount Amortized From Accumulated Other Comprehensive Income (Loss) Into Net Periodic Pension Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$12
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	0.2
Net prior service cost	-0.6
Estimated amount of actuarial losses that will be amortized	($0.40)

Other_Postretirement_Benefit_P8

Other Postretirement Benefit Plans (Summary of Weighted-Average Assumptions Used to Determine Pension Benefit Obligations) (Details) (Postretirement Plans)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefit obligations discount rate	4.38%	5.00%
Postretirement benefit cost discount rate	5.00%	4.25%

Other_Postretirement_Benefit_P9

Other Postretirement Benefit Plans (Assumed Health Care Cost Trend Rates) (Details) (Postretirement Plans)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next year	7.00%	7.00%
Rate to which the cost trend is assumed to decline (ultimate trend rate)	5.00%	5.00%
Year the rate reaches the ultimate trend rate	2020	2020

Other_Comprehensive_Income_Nar

Other Comprehensive Income (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Loss Adjustment Expense	40.00%	40.00%	40.00%
Percentage Of Other Operating Expenses	60.00%	60.00%	60.00%
Accelerated recognition of unamortized losses	$12.10

Other_Comprehensive_Income_Cha

Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Unrealized gains (losses) arising during period, Pre-Tax	$138.70	($189.40)	$186.40
Amount of realized gains from sales and other, Pre-tax	-55.3	-41.3	-31.8
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	5.5	6	8.9
Net unrealized gains (losses), Pre-tax	88.9	-224.7	163.5
Net actuarial gain (loss) arising during the period, Pre-Tax	-33.6	15	-24.9
Loss on settlement of pension obligation, Pre-Tax	12.1
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Pre-tax	10	14.9	9.3
Foreign currency translation recognized during the period, Pre-tax	-7.1	-3.1	12.2
Other comprehensive income (loss), Pre-tax	70.3	-197.9	160.1
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	-45.7	63.1	-40.2
Amount of realized gains from sales and other, Tax Benefit (Expense)	0.3	-3	-3.3
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	-1.9	-2.1	-2.7
Net unrealized gains (losses), Tax Benefit (Expense)	-47.3	58	-46.2
Net actuarial gain (loss) arising during the period, Tax Benefit (Expense)	11	-4.2	9
Loss on settlement of pension obligation, Tax Benefit (Expense)	-4.2
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Tax Benefit (Expense)	-3.5	-5.2	-3.2
Foreign currency translation recognized during the period, Tax Benefit (Expense)	2.5	1.1	-4.3
Other comprehensive income (loss), Tax Benefit (Expense)	-41.5	49.7	-44.7
Unrealized gains (losses) arising during period, Net of Tax	93	-126.3	146.2
Amount of realized gains from sales and other, Net of Tax	-55	-44.3	-35.1
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	3.6	3.9	6.2
Net unrealized gains (losses), Net of Tax	41.6	-166.7	117.3
Net actuarial gain (loss) arising in the period, Net of Tax	-22.6	10.8	-15.9
Loss on settlement of pension obligation, Net of Tax	7.9
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Net of Tax	14.4	9.7	6.1
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Net of Tax	6.5
Foreign currency translation recognized during the period, Net of Tax	-4.6	-2	7.9
Other comprehensive income (loss), Net of Tax	28.8	-148.2	115.4
Net of pre-tax, ceded unrealized gains (losses)	$0.80	($1.20)

Other_Comprehensive_Income_Rec

Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other									$55.60	$39.50	$31.40
Total losses and expenses									-4,689.60	-4,464.60	-4,562
Net other-than-temporary impairment losses on investments recognized in earnings									-5.5	-6	-7.8
Total before tax									378	329.1	28.7
Tax benefit (expense)									-95.7	-83.4	17.4
Income from continuing operations	90.1	55	82.5	54.7	64.9	61.3	53.1	66.4	282.3	245.7	46.1
Income Loss From Discontinued Operations Net Of Tax									-0.3	5.3	9.8
Net of tax	89.9	54.9	82.6	54.6	70.1	61.3	53.4	66.2	282	251	55.9
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									37	30.7	23
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains from sales and other									55.2	41.3	31.8
Net other-than-temporary impairment losses on investments recognized in earnings									-5.5	-6	-7.8
Total before tax									49.7	35.3	24
Tax benefit (expense)									1.6	5.1	6
Income from continuing operations									51.3	40.4	30
Income Loss From Discontinued Operations Net Of Tax									0.1		-0.9
Net of tax									51.4	40.4	29.1
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total losses and expenses									-22.1	-14.9	-9.3
Tax benefit (expense)									7.7	5.2	3.2
Net of tax									($14.40)	($9.70)	($6.10)

Stock_Based_Compensation_Narra

Stock Based Compensation (Narrative) (Details) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	20-May-14
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$15.10	$12.40	$12.80
Tax benefit from compensation expense	5.3	4.3	4.5
Vesting period	3 years	3 years	4 years
Vesting rate	33.33%	33.33%
Award expiration Period	10 years
Cash received for options exercised	10.9	24.4	2.3
Intrinsic value of options exercised	10.6	19.9	1.2
Aggregate intrinsic value, outstanding	55.2
Aggregate intrinsic value, exercisable	20.2
Weighted average remaining contractual life, outstanding (in years)	6 years 10 months 24 days
Weighted average remaining contractual life, exercisable (in years)	4 years 4 months 24 days
Weighted average grant date fair value of options granted	$8.96	$7.69	$8.87
Fair value of vested shares	7.8	2.3
Share-based Compensation Arrangement by Share-Based Payment Award, Options, Tax Expense Realized from Options Exercised	5.9	9	0.4
Share-based Compensation Arrangement by Share-based Payment Award, Reduction in Shares Available for Grant per Share Issued	3.8
2014 Stock Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under plan	6,100,000
Number of shares available for grant	6,138,403
ESPP Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under plan				2,500,000
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	32,681
SIP Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized under plan				750,000
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	18,778
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	4.8
Unrecognized compensation expense, weighted average period of recognition	1 year 4 months 24 days
Employee Stock Option | Vesting Rate After Three Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			50.00%
Employee Stock Option | Vesting Rate After Four Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate			50.00%
Performance-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance metric for performance based restricted stock units	100.00%
Performance metric for performance based restricted stock units minimum potential range	0.00%
Performance metric for performance based restricted stock units maximum potential range	200.00%
Increase as grant in period, percentage	100.00%
Decrease as forfeited, percentage	100.00%
Intrinsic value of awards vested	0.3
Performance-based restricted stock units | 2012 and 2011 Grants [Member] | Vesting Rate After Three Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Vesting rate	50.00%
Performance-based restricted stock units | 2012 and 2011 Grants [Member] | Vesting Rate After Four Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Vesting rate	50.00%
Performance-based restricted stock units | 2011 Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards forfeited		8,244
Weighted average grant date fair value of awards forfeited		$46.47
Performance-based restricted stock units | 2010 Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards forfeited			31,750
Weighted average grant date fair value of awards forfeited			$42.15
Performance And Market-Based Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	11.9
Unrecognized compensation expense, weighted average period of recognition	1 year 8 months 12 days
Performance metric for performance based restricted stock units	100.00%
Performance metric for performance based restricted stock units minimum potential range	0.00%
Performance metric for performance based restricted stock units maximum potential range	150.00%
Aggregate instrinsic value	15.6
Weighted average remaining contractual life	1 year 3 months 18 days
Performance And Market-Based Restricted Stock Units | 2013 Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance And Market-Based Restricted Stock Units | 2013 Grants [Member] | Vesting Rate After Three Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Performance And Market-Based Restricted Stock Units | 2012 and 2011 Grants [Member] | Vesting Rate After Three Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Vesting rate	50.00%
Performance And Market-Based Restricted Stock Units | 2012 and 2011 Grants [Member] | Vesting Rate After Four Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	4 years
Vesting rate	50.00%
Performance And Market-Based Restricted Stock Units | 2012 Grants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares	2,888
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value	$43.31
Market-based awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted, Shares	56,625	76,175	92,400
Restricted Stock and Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of awards vested	4.2	1.8	1.7
Aggregate instrinsic value	$27.50
Weighted average remaining contractual life	1 year 2 months 12 days

Stockbased_Compensation_Summar

Stock-based Compensation (Summary of Stock Option Plan Activity) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shares
Outstanding, beginning of period, Shares	2,049,173	2,892,882	2,715,430
Granted, Shares	687,700	537,800	529,500
Exercised, Shares	-444,200	-1,192,134	-90,624
Forfeited or cancelled, Shares	-56,053	-189,375	-101,574
Expired, Shares			-159,850
Outstanding, end of period, Shares	2,236,620	2,049,173	2,892,882
Weighted Average Exercise Price
Outstanding, beginning of period, Weighted Average Exercise Price	$41.18	$38.28	$38.57
Granted, Weighted Average Exercise Price	$58.06	$42.53	$36.84
Exercised, Weighted Average Exercise Price	$39.33	$34.70	$25.47
Forfeited or cancelled, Weighted Average Exercise Price	$45.99	$41.54	$41.45
Expired, Weighted Average Exercise Price			$44.05
Outstanding, end of period, Weighted Average Exercise Price	$46.61	$41.18	$38.28
Additional Disclosures
Exercisable, end of year (Shares)	726,321	783,873	1,630,382
Exercisable, end of year (Weighted Average Exercise Price)	$43.55	$41.16	$37.18

Stockbased_Compensation_Schedu

Stock-based Compensation (Schedule of Stock Options by Exercise Price Range) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Exercise Price Range One [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	114,500
Options Outstanding, Weighted Average Remaining Contractual Lives	3 years 10 months 2 days
Options Outstanding, Weighted Average Exercise Price	$34.37
Options Currently Exercisable, Number	114,500
Options Currently Exercisable, Weighted Average Exercise Price	$34.37
Exercise Price Range Two [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	422,500
Options Outstanding, Weighted Average Remaining Contractual Lives	7 years 22 days
Options Outstanding, Weighted Average Exercise Price	$36.81
Exercise Price Range Three [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	126,300
Options Outstanding, Weighted Average Remaining Contractual Lives	5 years 1 month 24 days
Options Outstanding, Weighted Average Exercise Price	$42.14
Options Currently Exercisable, Number	126,300
Options Currently Exercisable, Weighted Average Exercise Price	$42.14
Exercise Price Range Four [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	417,683
Options Outstanding, Weighted Average Remaining Contractual Lives	8 years 1 month 28 days
Options Outstanding, Weighted Average Exercise Price	$42.49
Options Currently Exercisable, Number	109,452
Options Currently Exercisable, Weighted Average Exercise Price	$42.49
Exercise Price Range Five [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	120,000
Options Outstanding, Weighted Average Remaining Contractual Lives	1 year 11 months 19 days
Options Outstanding, Weighted Average Exercise Price	$45.83
Options Currently Exercisable, Number	120,000
Options Currently Exercisable, Weighted Average Exercise Price	$45.83
Exercise Price Range Six [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	215,500
Options Outstanding, Weighted Average Remaining Contractual Lives	6 years 1 month 24 days
Options Outstanding, Weighted Average Exercise Price	$46.57
Options Currently Exercisable, Number	106,250
Options Currently Exercisable, Weighted Average Exercise Price	$46.67
Exercise Price Range Seven [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	151,487
Options Outstanding, Weighted Average Remaining Contractual Lives	2 years 2 months 23 days
Options Outstanding, Weighted Average Exercise Price	$48.48
Options Currently Exercisable, Number	149,819
Options Currently Exercisable, Weighted Average Exercise Price	$48.47
Exercise Price Range Eight [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, Number	668,650
Options Outstanding, Weighted Average Remaining Contractual Lives	9 years 1 month 24 days
Options Outstanding, Weighted Average Exercise Price	$58.06

Stockbased_Compensation_Schedu1

Stock-based Compensation (Schedule of Stock Option Valuation Assumptions) (Details) (Employee Stock Option)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	18.07%	21.77%	32.86%
Expected volatility, maximum	24.38%	32.02%	35.27%
Weighted average expected volatility	23.00%	27.53%	34.05%
Risk-free interest rate, minimum	0.53%	0.29%	0.61%
Risk-free interest rate, maximum	1.92%	2.02%	1.06%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield	2.07%	2.43%	3.13%
Expected term, in years	2 years 6 months	3 years	4 years 6 months
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield	2.55%	3.11%	3.26%
Expected term, in years	5 years 6 months	6 years	5 years 6 months

Stockbased_Compensation_Summar1

Stock-based Compensation (Summary of Restricted Stock Activity) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	525,980	750,837	768,529
Granted, Shares	102,131	141,073	174,841
Vested, Shares	-231,078	-298,388	-137,194
Forfeited, Shares	-12,110	-67,542	-55,339
Outstanding, end of period, Shares	384,923	525,980	750,837
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$41.20	$40.15	$40.17
Granted, Weighted Average Grant Date Fair Value	$58.78	$43.31	$37.01
Vested, Weighted Average Grant Date Fair Value	$41.45	$39.51	$36.19
Forfeited, Weighted Average Grant Date Fair Value	$43.67	$41.33	$40.47
Outstanding, end of period, Weighted Average Grant Date Fair Value	$45.63	$41.20	$40.15
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	184,626	132,775	69,500
Granted, Shares	60,338	82,795	96,150
Vested, Shares	-22,826
Forfeited, Shares	-3,800	-30,944	-32,875
Outstanding, end of period, Shares	218,338	184,626	132,775
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$40.42	$39.97	$45.37
Granted, Weighted Average Grant Date Fair Value	$55.73	$41.85	$36.91
Vested, Weighted Average Grant Date Fair Value	$44.78
Forfeited, Weighted Average Grant Date Fair Value	$37.90	$42.33	$43.28
Outstanding, end of period, Weighted Average Grant Date Fair Value	$44.24	$40.42	$39.97

Earnings_Per_Share_and_Shareho2

Earnings Per Share and Shareholders Equity Transactions (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share and Shareholdersb Equity Transactions [Abstract]
Antidilutive securities excluded from calculation of earnings per share	0.7	0.1	1.8
Repurchases common stock, authorized	$600
Repurchases common stock, available for repurchases	116.5
Repurchases common stock, shares	0.3
Repurchases common stock, value	$20.40

Earnings_Per_Share_and_Shareho3

Earnings Per Share and Shareholdersb Equity Transactions (Information Regarding Basic and Diluted Earnings Per Share) (Details) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic shares used in the calculation of earnings per share	44	44.1	44.7
Diluted shares used in the calculation of earnings per share	44.9	44.9	45.3
Per share effect of dilutive securities on income from continuing operations	($0.12)	($0.11)	($0.01)
Per share effect of dilutive securities on income from net income	($0.13)	($0.11)	($0.02)
Employee Stock Option
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.5	0.3	0.2
Non-Vested Stock Grants
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.4	0.5	0.4

Dividend_Restrictions_Details

Dividend Restrictions (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
NEW HAMPSHIRE
Statutory Accounting Practices [Line Items]
Percentage of policyholder's surplus distributes as dividends and other distributions	10.00%
Dividends declared	$0	$0	$0
Maximum dividends payable without prior approval of state regulators	205.2
MICHIGAN
Statutory Accounting Practices [Line Items]
Percentage of policyholder's surplus distributes as dividends and other distributions	10.00%
Dividends declared	66	68	70
Maximum dividends payable without prior approval of state regulators	63.4
U.K. [Member]
Statutory Accounting Practices [Line Items]
Dividends declared	$68.70	$13.90	$0

Segment_Information_Narrative_

Segment Information (Narrative) (Details)	12 Months Ended
	Dec. 31, 2014
	segment
Segment Information [Abstract]
Operating segments	4

Segment_Information_Financial_

Segment Information (Financial Information with Respect to Business Segments) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Operating revenues									$5,017.50	$4,760.20	$4,567.10
Net realized investment gains									50.1	33.5	23.6
Total revenues	1,285.10	1,265.60	1,273	1,243.90	1,229	1,201.80	1,182.60	1,180.30	5,067.60	4,793.70	4,590.70
Net investment income									270.3	269	276.6
Operating income (loss) before interest expense and income taxes									406.2	393.4	75.1
Interest on debt									-65.2	-65.3	-61.9
Operating income before income taxes									341	328.1	13.2
Loss from settlement of pension obligation									-12.1
Net loss from repayment of debt									-0.1	-27.7	-5.1
Net costs related to acquired businesses									-0.9	-0.1	-3
Loss on real estate										-4.7
Income before income taxes									378	329.1	28.7
Commercial Lines
Segment Reporting Information [Line Items]
Operating revenues									2,239	2,109.50	1,966.20
GAAP underwriting loss									-8.3	-11	-224.2
Net investment income									149.4	143.5	142.4
Other income (expense)									-1.2	-0.1	1.5
Operating income (loss) before interest expense and income taxes									139.9	132.4	-80.3
Personal Lines
Segment Reporting Information [Line Items]
Operating revenues									1,491	1,542.50	1,560
GAAP underwriting loss									22.1	37.6	-67.3
Net investment income									71.9	75.8	86.5
Other income (expense)									5	5.2	6.3
Operating income (loss) before interest expense and income taxes									99	118.6	25.5
Chaucer [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,279.70	1,098.20	1,030
GAAP underwriting loss									122.5	107.7	93.5
Net investment income									44.2	42.7	40.2
Other income (expense)									10.9		3.1
Operating income (loss) before interest expense and income taxes									177.6	150.4	136.8
Other
Segment Reporting Information [Line Items]
Operating revenues									7.8	10	10.9
GAAP underwriting loss									-3.1	-3.4	-3.5
Net investment income									4.8	7	7.2
Other net expenses									-12	-11.6	-10.6
Operating income (loss) before interest expense and income taxes									$10.30	$8	$6.90

Segment_Information_Identifiab

Segment Information (Identifiable Assets by Business Segment) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Segment Reporting Information [Line Items]
Discontinued operations	$112	$114.90
Identifiable assets	13,759.70	13,378.70
U.S. Companies
Segment Reporting Information [Line Items]
Identifiable assets	9,418.40	8,962.60
Chaucer
Segment Reporting Information [Line Items]
Identifiable assets	$4,229.30	$4,301.20

Segment_Information_Gross_Writ

Segment Information (Gross Written Premium by Geographical Area) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross written premium, percentage	100.00%	100.00%	100.00%
UNITED STATES
Segment Reporting Information [Line Items]
Gross written premium, percentage	78.00%	78.00%	76.00%
U.K. [Member]
Segment Reporting Information [Line Items]
Gross written premium, percentage	6.00%	6.00%	6.00%
Worldwide And Other
Segment Reporting Information [Line Items]
Gross written premium, percentage	16.00%	16.00%	18.00%

Lease_Commitments_Details

Lease Commitments (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Lease Commitments [Abstract]
Rental expenses for operating leases	$21.40	$21.60	$22.40
Future minimum rental payments	62.7
Future minimum rental payments, 2015	17.3
Future minimum rental payments, 2016	14.3
Future minimum rental payments, 2017	11.5
Future minimum rental payments, 2018	8
Future minimum rental payments, 2019 and thereafter	$11.60

Reinsurance_Narrative_Details

Reinsurance (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded	$665.90	[1]	$713.50	[1]	$800.80	[1]
Property and casualty losses and LAE, Ceded	358	[1]	350.6	[1]	588.4	[1]
Reinsurance recoverable on paid and unpaid losses and unearned premiums	2,268.20		2,335
MCCA [Member]
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded	73.4		72.2		74
Property and casualty losses and LAE, Ceded	99.2		76.6		99.9
Percentage of reinsurance receivable represented by segment	39.70%
Percentage of reinsurance assets represented by segment, minimum	10.00%
Reinsurance recoverable on paid and unpaid losses and unearned premiums	$899.50		$867
[1]	In 2013, we did not renew the capital provision reinsurance treaty with Flagstone Re. This reduced ceded premiums written, premiums earned and losses and LAE for the year ended DecemberB 31, 2013.

Reinsurance_Schedule_of_Effect

Reinsurance (Schedule of Effects of Reinsurance) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Reinsurance [Abstract]
Property and casualty premiums written, Direct	$4,777.30		$4,599.20		$4,515.10
Property and casualty premiums written, Assumed	688.1	[1]	602.5	[1]	689.1	[1]
Property and casualty premiums written, Ceded	-655.3	[2]	-649	[2]	-835.8	[2]
Property and casualty premiums written, Net	4,810.10		4,552.70		4,368.40
Property and casualty premiums earned, Direct	4,679.80		4,546.30		4,352.90
Property and casualty premiums earned, Assumed	696.4	[1]	617.7	[1]	687	[1]
Property and casualty premiums earned, Ceded	-665.9	[2]	-713.5	[2]	-800.8	[2]
Premiums Earned, Net, Total	4,710.30		4,450.50		4,239.10
Percentage of assumed to net premiums earned	14.78%		13.88%		16.21%
Property and casualty losses and LAE, Direct	3,019.40		2,939.40		3,251.40
Property and casualty losses and LAE, Assumed	266.1	[1]	172.3	[1]	311.4	[1]
Property and casualty losses and LAE, Ceded	-358	[2]	-350.6	[2]	-588.4	[2]
Policyholder Benefits and Claims Incurred, Net, Total	$2,927.50		$2,761.10		$2,974.40
[1]	Assumed reinsurance activity primarily relates to the Chaucer segment.
[2]	In 2013, we did not renew the capital provision reinsurance treaty with Flagstone Re. This reduced ceded premiums written, premiums earned and losses and LAE for the year ended DecemberB 31, 2013.

Liabilities_For_Outstanding_Cl2

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Decrease in reserves after re-estimation	$99.10	$76.30	$15.80
Benefit from settlement of legal proceeding			3.2
Reinsurance of loss and LAE reserves	21.4	20.6	20.4
Liability for Asbestos and Environmental Claims, Gross	60.6	61.9	60.5
Commercial Lines
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Loss and LAE reserves of asbestos and environmental damage liability	10.1	11.5	9.8
Run-off Voluntary Assumed Reinsurance Pool [Member]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Liability for Asbestos and Environmental Claims, Gross	29.1	29.8	30.3
Chaucer Business [Member]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Decrease in reserves after re-estimation		94.6	72.6
Favorable loss and LAE reserve development	$104.60	$94.60	$72.60

Liabilities_For_Outstanding_Cl3

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Gross, Beginning Balance	$6,231.50	$6,197	$5,760.30
Reinsurance recoverable on unpaid losses, beginning of year	2,030.40	2,074.30	1,931.80
Net loss and LAE reserves, beginning of year	4,201.10	4,122.70	3,828.50
Net incurred losses and LAE in respect of losses occurring in Current year	3,026.60	2,837.40	2,990.20
Net incurred losses and LAE in respect of losses occurring in prior year	-99.1	-76.3	-15.8
Total incurred losses and LAE	2,927.50	2,761.10	2,974.40
Net payments of losses and LAE in respect of losses occurring in Current year	1,328.70	1,213.50	1,317.60
Net payments of losses and LAE in respect of losses occurring in Prior years	1,398.90	1,469.80	1,396.50
Total payments	2,727.60	2,683.30	2,714.10
Chaucer Flagstone Reinsurance commutation impact	85.7
Effect of foreign exchange rate changes	-78	0.6	33.9
Net reserve for losses and LAE, end of year	4,408.70	4,201.10	4,122.70
Reinsurance recoverable on unpaid losses, end of year	1,983	2,030.40	2,074.30
Liability for Unpaid Claims and Claims Adjustment Expense, Gross, Ending Balance	6,391.70	6,231.50	6,197
Chaucer Business [Member]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Net incurred losses and LAE in respect of losses occurring in prior year		($94.60)	($72.60)

Commitments_and_Contingencies_

Commitments and Contingencies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 17, 2013
		plaintiff
Commitments and Contingencies [Abstract]
Participant's retirement age	65 years
Treasury rate, years	30 years
Named plaintiffs		2

Statutory_Financial_Informatio2

Statutory Financial Information (Narrative) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Statutory Financial Information [Abstract]
Minimum staturtory capital and surplus required	$374.60	$354.50	$329

Statutory_Financial_Informatio3

Statutory Financial Information (Statutory Accounting Practices Disclosure) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statutory Financial Information [Abstract]
Statutory Net Income (Loss)	$204.30	$193.20	($47.30)
Statutory Capital and Surplus	$2,057.10	$1,834.30	$1,523.40

Quarterly_Results_of_Operation1

Quarterly Results of Operations (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Results of Operations [Abstract]
Total revenues	$1,285.10	$1,265.60	$1,273	$1,243.90	$1,229	$1,201.80	$1,182.60	$1,180.30	$5,067.60	$4,793.70	$4,590.70
Income from continuing operations	90.1	55	82.5	54.7	64.9	61.3	53.1	66.4	282.3	245.7	46.1
Net income	$89.90	$54.90	$82.60	$54.60	$70.10	$61.30	$53.40	$66.20	$282	$251	$55.90
Income from continuing operations	$2.05	$1.25	$1.87	$1.25	$1.48	$1.40	$1.21	$1.49	$6.41	$5.58	$1.03
Income from continuing operations	$2.01	$1.22	$1.84	$1.22	$1.45	$1.37	$1.19	$1.47	$6.29	$5.47	$1.02
Net income per share, Basic	$2.04	$1.25	$1.88	$1.24	$1.60	$1.40	$1.21	$1.49	$6.41	$5.70	$1.25
Net income per share, Diluted	$2	$1.22	$1.84	$1.22	$1.57	$1.37	$1.19	$1.46	$6.28	$5.59	$1.23
Dividends declared per share	$0.41	$0.37	$0.37	$0.37	$0.37	$0.33	$0.33	$0.33

SCHEDULE_I_SUMMARY_OF_INVESTME1

SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$7,940.10	[1]
Amount at which shown in the balance sheet	8,250.30
Mortgage loans on real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	94.9	[1],[2]
Amount at which shown in the balance sheet	94.9	[2]
Real estate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10	[1]
Amount at which shown in the balance sheet	10
Other Long-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	182.9	[1],[3]
Amount at which shown in the balance sheet	186.5	[3]
Short-term Investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	83.1	[1]
Amount at which shown in the balance sheet	83.1
Loan Participations and Assignments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	93.4
Fixed Maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7,062.60	[1]
Value	7,295
Amount at which shown in the balance sheet	7,295
Fixed Maturities | Unites States Government and agencies and authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,137.40	[1]
Value	1,154.90
Amount at which shown in the balance sheet	1,154.90
Fixed Maturities | States, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,078.70	[1]
Value	1,137.10
Amount at which shown in the balance sheet	1,137.10
Fixed Maturities | Foreign governments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	339.2	[1]
Value	343.8
Amount at which shown in the balance sheet	343.8
Fixed Maturities | Public utility bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	348.5	[1]
Value	372
Amount at which shown in the balance sheet	372
Fixed Maturities | All other corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,158.80	[1]
Value	4,287.20
Amount at which shown in the balance sheet	4,287.20
Equity Securities Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	506.6	[1]
Value	580.8
Amount at which shown in the balance sheet	580.8
Equity Securities Investment Summary [Member] | Public utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	92.7	[1]
Value	116.4
Amount at which shown in the balance sheet	116.4
Equity Securities Investment Summary [Member] | Banks, trust and insurance companies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10.3	[1]
Value	10.6
Amount at which shown in the balance sheet	10.6
Equity Securities Investment Summary [Member] | Industrial, miscellaneous and all other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	393.5	[1]
Value	435
Amount at which shown in the balance sheet	435
Equity Securities Investment Summary [Member] | Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	10.1	[1]
Value	18.8
Amount at which shown in the balance sheet	$18.80
[1]	For equity securities, represents original cost, and for fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.
[2]	Includes $93.4 million of participating interests in investment grade commercial mortgage loans originated and serviced by a third party. The Company shares, on a pro rata basis, in all related cash flows of the underlying mortgages. Due to certain reacquisition rights retained by the third party in the loan participation arrangement, the Company accounted for its participatory interest in mortgage loans as secured borrowings.
[3]	The cost of other long-term investments differs from the carrying value due to market value changes in the Company's equity ownership of limited partnership investments.

SCHEDULE_II_CONDENSED_FINANCIA1

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF INCOME) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net investment income									$270.30	$269	$276.60
Net realized gains from sales and other									55.6	39.5	31.4
Total revenues	1,285.10	1,265.60	1,273	1,243.90	1,229	1,201.80	1,182.60	1,180.30	5,067.60	4,793.70	4,590.70
Interest expense									65.2	65.3	61.9
Loss from retirement of debt									0.1	27.7	5.1
Other operating expenses									656.9	667.2	587.6
Total losses and expenses									4,689.60	4,464.60	4,562
Income tax benefit									-4.5	-8	-2.3
Income from continuing operations	90.1	55	82.5	54.7	64.9	61.3	53.1	66.4	282.3	245.7	46.1
Net income	89.9	54.9	82.6	54.6	70.1	61.3	53.4	66.2	282	251	55.9
Net loss from discontinued operations, income tax benefit (expense)									0.1	-4.1	0.5
The Hanover Insurance Group [Member]
Condensed Financial Statements, Captions [Line Items]
Net investment income									3.9	5.5	6.6
Net realized gains from sales and other										0.8	2.5
Interest income from loan to subsidiary									22.5	22.5	22.5
Other income									0.1	0.1	0.1
Total revenues									26.5	28.9	31.7
Interest expense									54.1	55.3	48.6
Loss from retirement of debt									0.1	19.9	0.1
Employee benefit related expenses									11.1	6.2	5.3
Costs (benefit) related to acquired businesses										-6.4	2.6
Other operating expenses									8.1	8	8
Total losses and expenses									73.4	83	64.5
Net loss before income taxes and equity in income of unconsolidated subsidiaries									-46.9	-54.1	-32.8
Income tax benefit									52.1	37.3	40.5
Equity in income of unconsolidated subsidiaries									277.4	262.5	47.5
Income from continuing operations									282.6	245.7	55.2
(Loss) income from discontinued operations (net of income tax (benefit) expense of $(0.2), $4.1 and $(0.1) in 2014, 2013 and 2012)									-0.6	5.3	0.7
Net income									282	251	55.9
Net loss from discontinued operations, income tax benefit (expense)									($0.20)	$4.10	($0.10)

SCHEDULE_II_CONDENSED_FINANCIA2

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Millions, except Share data, unless otherwise specified
Assets
Fixed maturities, at fair value (amortized cost of $7,145.7 and $6,815.2)	$7,378.10	$6,970.60
Equity securities, at fair value (cost of $506.6 and $366.5)	580.8	430.2
Cash and cash equivalents	373.3	486.2	564.8	820.4
Deferred income taxes	131.2	239.7
Other assets	486.6	526.1
Total assets	13,759.70	13,378.70
Liabilities
Debt	903.5	903.9
Total liabilities	10,915.70	10,784.20
Shareholders' Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,830.70	1,830.10
Accumulated other comprehensive income	206.4	177.6
Retained earnings	1,558.70	1,349.10
Treasury stock at cost (16.8 and 16.2 million)	-752.4	-762.9
Total shareholders' equity	2,844	2,594.50	2,595.40
Total liabilities and shareholders' equity	13,759.70	13,378.70
Fixed maturities, amortized cost	7,145.70	6,815.20
Equity securities, cost	506.6	366.5
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	16,600,000	16,800,000
The Hanover Insurance Group [Member]
Assets
Fixed maturities, at fair value (amortized cost of $7,145.7 and $6,815.2)	105.2	101.7
Equity securities, at fair value (cost of $506.6 and $366.5)	1.1	1.1
Cash and cash equivalents	14.5	20.7	23.8	8.9
Investments in unconsolidated subsidiaries	3,129.80	2,876
Net receivable from subsidiaries	20.4	23
Deferred income taxes	62.8	49.1
Current income taxes		8.3
Loan receivable from subsidiary	300	300
Other assets	14	17.6
Total assets	3,647.80	3,397.50
Liabilities
Expenses and state taxes payable	15	16
Current income tax payable	2.2
Interest payable	8.1	8.1
Debt	778.5	778.9
Total liabilities	803.8	803
Shareholders' Equity
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	0.6	0.6
Additional paid-in capital	1,830.70	1,830.10
Accumulated other comprehensive income	206.4	177.6
Retained earnings	1,558.70	1,349.10
Treasury stock at cost (16.8 and 16.2 million)	-752.4	-762.9
Total shareholders' equity	2,844	2,594.50
Total liabilities and shareholders' equity	3,647.80	3,397.50
Fixed maturities, amortized cost	102.1	98.5
Equity securities, cost	$1	$1
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	16,600,000	16,800,000

SCHEDULE_II_CONDENSED_FINANCIA3

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash Flows From Operating Activities
Net income	$282	$251	$55.90
Adjustments to reconcile net income to net cash provided by operating activities:
Loss from retirement of debt	0.1	27.7	5.1
Net realized investment gains	-50.2	-33.5	-22.8
Deferred income taxes expense	68.6	74.9	-35.5
Change in expenses and taxes payable	12.5	-62.9	87.6
Net cash provided by operating activities	564.7	383.9	408.2
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,323.10	1,394.30	1,684.80
Purchase of fixed maturities	-1,710.10	-1,698.80	-2,170.40
Net cash (used in) investing activities	-600.7	-358.7	-562.8
Cash Flows From Financing Activities
Proceeds from debt borrowings, net		168.6	7.4
Repurchases of debt	-0.7	-139.9	-73.1
Dividends paid to shareholders	-67	-60	-55.1
Repurchases of common stock	-20.4	-78.2	-20
Proceeds from exercise of employee stock options	12.6	25	2.6
Other financing activities	-3.6	-5.1	-0.4
Net cash used in financing activities	-71.7	-105.5	-133.4
Net change in cash and cash equivalents	-112.9	-78.7	-255.9
Cash and cash equivalents, beginning of period	486.2	564.8	820.4
Cash and cash equivalents, end of period	373.3	486.2	564.8
The Hanover Insurance Group [Member]
Cash Flows From Operating Activities
Net income	282	251	55.9
Adjustments to reconcile net income to net cash provided by operating activities:
Loss (gain) on discontinued operations	0.6	2.5	-0.7
Loss from retirement of debt	0.1	19.9	0.1
Equity in net income of unconsolidated subsidiaries	-277.4	-262.5	-47.5
Net realized investment gains		-0.8	-2.5
Dividends received from unconsolidated subsidiaries	70.2	12.5	1
Deferred income taxes expense	-29.9	-32.3	3.9
Change in expenses and taxes payable	11.4	0.1	6.2
Change in net payable from subsidiaries	16.9	16.5	30.7
Other, net	4.3	3.4	2.8
Net cash provided by operating activities	78.2	10.3	49.9
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	25.6	122.9	44.2
Purchase of fixed maturities	-28.6	-90.8	-2.2
Net cash used for business acquisitions	-2.3	-2.2	-3.4
Net cash (used in) investing activities	-5.3	29.9	38.6
Cash Flows From Financing Activities
Proceeds from debt borrowings, net		168.6
Repurchases of debt	-0.7	-93.6	-0.7
Dividends paid to shareholders	-67	-60	-55.1
Repurchases of common stock	-20.4	-78.2	-20
Proceeds from exercise of employee stock options	12.6	25	2.6
Other financing activities	-3.6	-5.1	-0.4
Net cash used in financing activities	-79.1	-43.3	-73.6
Net change in cash and cash equivalents	-6.2	-3.1	14.9
Cash and cash equivalents, beginning of period	20.7	23.8	8.9
Cash and cash equivalents, end of period	$14.50	$20.70	$23.80

SCHEDULE_II_CONDENSED_FINANCIA4

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (Narrative) (Details) (The Hanover Insurance Group [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
The Hanover Insurance Group [Member]
Supplementary Insurance Information, by Segment [Line Items]
Cash portion of dividends received from unconsolidated subsidiaries	$70.20	$12.50	$1
Investment assets transferred to the parent company to settle dividend balances	$1	$0	$17.90

SCHEDULE_III_SUPPLEMENTARY_INS1

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	$525.70	$506	$489.50
Future policy benefits, losses, claims and loss expenses	6,386.60	6,227.40	6,192.90
Unearned premiums	2,583.90	2,515.80	2,474.80
Other policy claims and benefits payable	5.1	4.1	4.1
Premium revenue	4,710.30	4,450.50	4,239.10
Net investment income	270.3	269	276.6
Benefits, claims, losses and settlement expenses	2,927.50	2,761.10	2,974.40
Amortization of deferred acquisition costs	1,040	971	938.1
Other operating expenses	722.1	732.5	649.5
Premiums written	4,810.10	4,552.70	4,368.40
Commercial, Personal and Other
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	370.3	354.2	346.6
Future policy benefits, losses, claims and loss expenses	3,960.80	3,845.90	3,784.90
Unearned premiums	1,815.80	1,727.10	1,711.10
Other policy claims and benefits payable	5.1	4.1	4.1
Premium revenue	3,488.50	3,412.60	3,272.30
Net investment income	226.1	226.3	236.1
Benefits, claims, losses and settlement expenses	2,293.80	2,222.90	2,467.20
Amortization of deferred acquisition costs	748.5	729.8	704.2
Other operating expenses	481	503	436.5
Premiums written	3,578.70	3,435.20	3,377.90
Chaucer [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred acquisition costs	155.4	151.8	142.9
Future policy benefits, losses, claims and loss expenses	2,425.80	2,381.50	2,408
Unearned premiums	768.1	788.7	763.7
Premium revenue	1,221.80	1,037.90	966.8
Net investment income	44.2	42.7	40.2
Benefits, claims, losses and settlement expenses	633.7	538.2	507.2
Amortization of deferred acquisition costs	291.5	241.2	233.9
Other operating expenses	182.9	170.8	157.6
Premiums written	1,231.40	1,117.50	990.5
Interest On Debt
Supplementary Insurance Information, by Segment [Line Items]
Other operating expenses	65.2	65.3	61.9
Eliminations
Supplementary Insurance Information, by Segment [Line Items]
Net investment income			0.3
Other operating expenses	($7)	($6.60)	($6.50)

SCHEDULE_V_VALUATION_AND_QUALI1

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	$23.80	$19.70	$18.70
Charged to costs and expenses	10.3	11.7	8.5
Charged to other accounts	-0.3	0.5	0.9
Deductions	-14.7	-8.1	-8.4
Balance at end of period	19.1	23.8	19.7
Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	3.1	2.8	2.3
Charged to costs and expenses	9.4	8.1	8.1
Deductions	-9.3	-7.8	-7.6
Balance at end of period	3.2	3.1	2.8
Allowance for uncollectible reinsurance recoverables
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	20.7	16.9	16.4
Charged to costs and expenses	0.9	3.6	0.4
Charged to other accounts	-0.3	0.5	0.9
Deductions	-5.4	-0.3	-0.8
Balance at end of period	$15.90	$20.70	$16.90

SCHEDULE_VI_SUPPLEMENTAL_INFOR1

SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERAT (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS [Abstract]
Deferred acquisition costs	$525.70		$506		$489.50
Reserves for unpaid claims and claim adjustment expenses	6,391.70	[1]	6,231.50	[1]	6,197	[1]
Discount, if any, deducted from previous column		[2]		[2]		[2]
Unearned premiums	2,583.90	[1]	2,515.80	[1]	2,474.80	[1]
Earned premiums	4,710.30		4,450.50		4,239.10
Net investment income	270.3		269		276.6
Net incurred losses and LAE in respect of losses occurring in Current year	3,026.60		2,837.40		2,990.20
Net incurred losses and LAE in respect of losses occurring in prior year	-99.1		-76.3		-15.8
Amortization of deferred acquisition costs	1,040		971		938.1
Paid claims and claim adjustment expenses	2,727.60		2,683.30		2,714.10
Premiums written	4,810.10		4,552.70		4,368.40
Loss and loss adjustment expense reserves	6,391.70		6,231.50		6,197		5,760.30
Reinsurance Recoverables On Unpaid Losses Property Casualty Liability	1,983		2,030.40		2,074.30		1,931.80
Prepaid premiums, gross	$197.80		$219		$275.40
[1]	YEARS ENDED DECEMBER 31(in millions)Affiliation with RegistrantDeferred acquisition costsReserves for unpaid claims and claim adjustment expenses (1)Discount, if any, deducted from previous column (2)Unearned premiums (1)Earned premiumsNet investment incomeConsolidated Property and Casualty Subsidiaries2014$ 525.7$ 6,391.7$ -$ 2,583.9$ 4,710.3$ 270.32013$ 506.0$ 6,231.5$ -$ 2,515.8$ 4,450.5$ 269.02012$ 489.5$ 6,197.0$ -$ 2,474.8$ 4,239.1$ 276.6Claims and claim adjustment expenses incurred related toCurrent yearPrior yearsAmortization of deferred acquisition costsPaid claims and claim adjustment expensesPremiums written2014$ 3,026.6$ (99.1)$ 1,040.0$ 2,727.6$ 4,810.12013$ 2,837.4$ (76.3)$ 971.0$ 2,683.3$ 4,552.72012$ 2,990.2$ (15.8)$ 938.1$ 2,714.1$ 4,368.4Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,983.0 million, $2,030.4 million and $2,074.3 million of reinsurance recoverable on unpaid losses in 2014, 2013 and 2012, respectively. Unearned premiums are shown gross of prepaid premiums of $197.8 million, $219.0 million and $275.4 million in 2014, 2013 and 2012, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.
[2]	The Company does not use discounting techniques.