The Hanover Insurance Group, Inc. (THG) 10-Q 1 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 28, 2013
Document Information [Line Items]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'THG	'
Entity Registrant Name	'HANOVER INSURANCE GROUP, INC.	'
Entity Central Index Key	'0000944695	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	43,858,956

CONSOLIDATED_STATEMENTS_OF_INC

CONSOLIDATED STATEMENTS OF INCOME (USD $)	3 Months Ended		9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues	'	'	'	'
Premiums	$1,124.70	$1,071	$3,309.80	$3,156.60
Net investment income	65.7	69.2	200.9	206.5
Net realized investment gains (losses):	'	'	'	'
Net realized gains from sales and other	3.1	7.2	26.5	10.4
Net other-than-temporary impairment losses on investments recognized in earnings	-2.1	-2.2	-3.7	-5.7
Total net realized investment gains	1	5	22.8	4.7
Fees and other income	10.4	12.5	31.2	38.9
Total revenues	1,201.80	1,157.70	3,564.70	3,406.70
Losses and expenses	'	'	'	'
Losses and loss adjustment expenses	695.9	708.4	2,071.40	2,101.90
Amortization of deferred acquisition costs	243.9	235.8	719.1	697
Interest expense	16.6	14.6	48.7	46.7
Other operating expenses	159.3	145.7	484.6	426
Total losses and expenses	1,115.70	1,104.50	3,323.80	3,271.60
Income before income taxes	86.1	53.2	240.9	135.1
Income tax expense (benefit):	'	'	'	'
Current	2.4	-16.9	5.9	11.2
Deferred	22.4	29.2	54.2	22.5
Total income tax expense	24.8	12.3	60.1	33.7
Income from continuing operations	61.3	40.9	180.8	101.4
Net gain (loss) from other discontinued operations (net of tax benefit of $0.1 and $0.3 for the three months ended September 30, 2013 and September 30, 2012 and $0.1 and $0.7 for the nine months ended September 30, 2013 and September 30, 2012)	'	-0.5	0.1	9.5
Net income	$61.30	$40.40	$180.90	$110.90
Basic:	'	'	'	'
Income from continuing operations	$1.40	$0.92	$4.10	$2.26
Net gain (loss) from discontinued operations	'	($0.01)	'	$0.21
Net income per share	$1.40	$0.91	$4.10	$2.47
Weighted average shares outstanding	43.8	44.6	44.1	44.8
Diluted:	'	'	'	'
Income from continuing operations	$1.37	$0.90	$4.03	$2.23
Net gain (loss) from discontinued operations	'	($0.01)	'	$0.21
Net income per share	$1.37	$0.89	$4.03	$2.44
Weighted average shares outstanding	44.6	45.2	44.9	45.4

CONSOLIDATED_STATEMENTS_OF_INC1

CONSOLIDATED STATEMENTS OF INCOME (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Gain (loss) from other discontinued operations, tax benefit (expense)	$0.10	$0.30	$0.10	$0.70

CONSOLIDATED_STATEMENTS_OF_COM

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Comprehensive Income (Loss) [Line Items]	'	'	'	'
Net income	$61.30	$40.40	$180.90	$110.90
Available-for-sale securities and derivative instruments:	'	'	'	'
Net appreciation (depreciation) during the period	-9.3	49.4	-142	133.5
Portion of other-than-temporary impairment losses transferred from (to) other comprehensive income	-0.2	2.4	0.5	7.1
Total available-for-sale securities and derivative instruments	-9.5	51.8	-141.5	140.6
Pension and postretirement benefits:	'	'	'	'
Amortization recognized as net periodic benefit and postretirement cost	1.9	1.6	7.9	4.6
Cumulative foreign currency translation adjustment:	'	'	'	'
Amount recognized as cumulative foreign currency translation during the period	5.7	7.5	-4.4	8.4
Other comprehensive income (loss), Net of Tax	-1.9	60.9	-138	153.6
Comprehensive income	$59.40	$101.30	$42.90	$264.50

CONSOLIDATED_BALANCE_SHEETS

CONSOLIDATED BALANCE SHEETS (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Investments:	'	'
Fixed maturities, at fair value (amortized cost of $6,473.3 and $6,529.5)	$6,675.50	$6,952.20
Equity securities, at fair value (cost of $449.0 and $299.0)	494.1	315.8
Other investments	185.3	210.3
Total investments	7,354.90	7,478.30
Cash and cash equivalents	602.2	564.8
Accrued investment income	70	69
Premiums and accounts receivable, net	1,410.70	1,308.80
Reinsurance recoverable on paid and unpaid losses and unearned premiums	2,390.30	2,479.70
Deferred acquisition costs	521.7	489.5
Deferred income taxes	263.2	267.6
Goodwill	184.8	184.9
Other assets	544.7	511.8
Assets of discontinued operations	120.8	130.5
Total assets	13,463.30	13,484.90
Liabilities	'	'
Loss and loss adjustment expense reserves	6,162.80	6,197
Unearned premiums	2,645.50	2,474.80
Expenses and taxes payable	638.4	775.8
Reinsurance premiums payable	412.2	466.2
Debt	938.7	849.4
Liabilities of discontinued operations	121.5	126.3
Total liabilities	10,919.10	10,889.50
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	1,829.60	1,787.10
Accumulated other comprehensive income	187.8	325.8
Retained earnings	1,295.20	1,211.60
Treasury stock at cost (16.9 and 16.2 million shares)	-769	-729.7
Total shareholders' equity	2,544.20	2,595.40
Total liabilities and shareholders' equity	$13,463.30	$13,484.90

CONSOLIDATED_BALANCE_SHEETS_Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, except Per Share data, unless otherwise specified
Fixed maturities, amortized cost	$6,473.30	$6,529.50
Equity securities, cost	$449	$299
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	20	20
Preferred stock, issued	'	'
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	300	300
Common stock, shares issued	60.5	60.5
Treasury stock, shares	16.9	16.2

CONSOLIDATED_STATEMENTS_OF_SHA

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (USD $)	Total	Common Stock	Additional Paid-in Capital	Net Unrealized Appreciation (Depreciation) on Investments and Derivative Instruments:	Defined Benefit Pension and Postretirement Plans:	Cumulative Foreign Currency Translation Adjustment:	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
In Millions, unless otherwise specified
Balance at beginning of period at Dec. 31, 2011	'	$0.60	$1,784.80	$308.70	($86.80)	($11.50)	'	$1,211.30	($723.10)
Net income	110.9	'	'	'	'	'	'	110.9	'
Net appreciation during the period:	'	'	'	'	'	'	'	'	'
Net appreciation (depreciation) on available-for-sale securities and derivative instruments	133.5	'	'	140.6	'	'	'	'	'
Shares purchased at cost	'	'	'	'	'	'	'	'	-20
Employee and director stock-based awards and other	'	'	0.2	'	'	'	'	'	'
Net amount recognized as net periodic benefit cost	4.6	'	'	'	4.6	'	'	'	'
Amount recognized as cumulative foreign currency translation during the period	8.4	'	'	'	'	8.4	'	'	'
Dividends to shareholders	'	'	'	'	'	'	'	-40.3	'
Net shares reissued at cost under employee stock-based compensation plans	'	'	'	'	'	'	'	'	10.8
Stock-based compensation	'	'	'	'	'	'	'	0.2	'
Balance at end of period at Sep. 30, 2012	2,699.40	0.6	1,785	449.3	-82.2	-3.1	364	1,282.10	-732.3
Balance at beginning of period at Dec. 31, 2012	2,595.40	0.6	1,787.10	426	-96.6	-3.6	'	1,211.60	-729.7
Net income	180.9	'	'	'	'	'	'	180.9	'
Net appreciation during the period:	'	'	'	'	'	'	'	'	'
Net appreciation (depreciation) on available-for-sale securities and derivative instruments	-142	'	'	-141.5	'	'	'	'	'
Shares purchased at cost	'	'	'	'	'	'	'	'	-78.2
Employee and director stock-based awards and other	'	'	42.5	'	'	'	'	'	'
Net amount recognized as net periodic benefit cost	7.9	'	'	'	7.9	'	'	'	'
Amount recognized as cumulative foreign currency translation during the period	-4.4	'	'	'	'	-4.4	'	'	'
Dividends to shareholders	'	'	'	'	'	'	'	-43.8	'
Net shares reissued at cost under employee stock-based compensation plans	'	'	'	'	'	'	'	'	38.9
Stock-based compensation	'	'	'	'	'	'	'	-53.5	'
Balance at end of period at Sep. 30, 2013	$2,544.20	$0.60	$1,829.60	$284.50	($88.70)	($8)	$187.80	$1,295.20	($769)

CONSOLIDATED_STATEMENTS_OF_CAS

CONSOLIDATED STATEMENTS OF CASH FLOWS (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Cash Flows From Operating Activities	'	'
Net income	$180.90	$110.90
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Loss on repurchase of debt	11.3	'
Net realized investment gains	-22.8	-3.8
Net amortization and depreciation	25.4	26
Gain on sale of Citizens Management, Inc.	'	-10.8
Stock-based compensation expense	8.9	9.8
Amortization of defined benefit plan costs	12.1	7
Deferred income taxes expense	54.1	21.9
Change in deferred acquisition costs	-32.2	-42.6
Change in premiums receivable, net of reinsurance premiums payable	-155.9	-87.5
Change in loss, loss adjustment expense and unearned premium reserves	146.5	408
Change in reinsurance recoverable	83.9	-170
Change in expenses and taxes payable	-102.8	-23.8
Other, net	-27.5	-40.5
Net cash provided by operating activities	181.9	204.6
Cash Flows From Investing Activities	'	'
Proceeds from disposals and maturities of fixed maturities	1,130.10	1,267.40
Proceeds from disposals of equity securities and other investments	111.5	21.7
Purchase of fixed maturities	-1,090.90	-1,601.10
Purchase of equity securities and other investments	-228.3	-206.1
Proceeds from disposal of Citizens Management, Inc., net of cash transferred	'	5.2
Capital expenditures	-17	-11.2
Net payments related to derivative agreements	'	-4.4
Net cash used in investing activities	-94.6	-528.5
Cash Flows From Financing Activities	'	'
Proceeds from exercise of employee stock options	21.7	1.9
Proceeds from debt borrowings, net	169.5	7.4
Decrease in cash collateral related to securities lending program	-18.1	-6.6
Dividends paid to shareholders	-43.8	-40.3
Repurchases of debt	-96.8	-0.8
Repurchases of common stock	-78.2	-20
Other financing activities	-3.8	-0.2
Net cash used in financing activities	-49.5	-58.6
Effect of exchange rate changes on cash	-0.5	24
Net change in cash and cash equivalents	37.3	-358.5
Net change in cash related to discontinued operations	0.1	0.1
Cash and cash equivalents, beginning of period	564.8	820.4
Cash and cash equivalents, end of period	$602.20	$462

Basis_of_Presentation_and_Prin

Basis of Presentation and Principles of Consolidation	9 Months Ended
	Sep. 30, 2013
Basis of Presentation and Principles of Consolidation	'
	1. Basis of Presentation and Principles of Consolidation
	The accompanying unaudited consolidated financial statements of The Hanover Insurance Group, Inc. and subsidiaries (“THG” or the “Company”) have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and with the requirements of Form 10-Q. Certain financial information that is provided in annual financial statements, but is not required in interim reports, has been omitted.
	The interim consolidated financial statements of THG include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America, THG’s principal U.S. domiciled property and casualty companies; Chaucer Holdings plc (“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 9 – “Segment Information”. Additionally, the interim consolidated financial statements include the Company’s discontinued operations, consisting primarily of the Company’s former life insurance businesses, its accident and health business and prior to April 30, 2012, its third party administrator. All intercompany accounts and transactions have been eliminated. During the quarter, 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 for the current or any prior period.
	The preparation of financial statements in conformity with 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, the accompanying interim consolidated financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. The results of operations for the three and nine months ended September 30, 2013 are not necessarily indicative of the results to be expected for the full year. These financial statements should be read in conjunction with the Company’s 2012 Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on February 26, 2013.

New_Accounting_Pronouncements

New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2013
New Accounting Pronouncements	'
	2. New Accounting Pronouncements
	Recently Implemented Standards
	In February 2013, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) Update No. 2013-02 (Topic 220) Comprehensive Income Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income. 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 8 – Other Comprehensive Income.)
	In July 2012, the FASB issued ASC Update No. 2012-02 (Topic 350) Testing Indefinite – Lived Intangible Assets for Impairment. This ASC update allows an entity to first assess qualitative factors to determine whether the existence of events and circumstances indicates that it is more likely than not that the indefinite – lived intangible asset is impaired. This assessment should be used as a basis for determining whether it is necessary to perform the quantitative impairment test. An entity would not be required to calculate the fair value of the intangible asset and perform the quantitative test unless the entity determines, based upon its qualitative assessment, that it is more likely than not that its fair value is less than its carrying value. The update further improves previous guidance by expanding upon the examples of events and circumstances that an entity should consider in determining whether it is more likely than not that the fair value of an indefinite – lived intangible asset is less than its carrying amount. The update also allows an entity the option to bypass the qualitative assessment for any indefinite-lived intangible asset in any period and proceed directly to performing the quantitative impairment test. An entity will be able to resume performing the qualitative assessment in any subsequent period. This ASC update was effective for annual and interim periods beginning after September 15, 2012, with early adoption permitted. The Company implemented this guidance effective October 1, 2012. The effect of implementing this guidance was not material to the Company’s financial position or results of operations.
	Recently Issued Standards
	In July 2013, the 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 ASC update is effective for annual and interim periods beginning after December 15, 2013, with early adoption permitted, and is to be applied prospectively to all unrecognized tax benefits that exist at the effective date. Retrospective application is permitted. The Company does not expect the adoption of ASC Update 2013-11 to have a material impact on its 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 ASC update is effective for annual and interim periods beginning after December 15, 2013, with early adoption permitted, and is to be applied prospectively to derecognition events occurring after the effective date. The Company does not expect the adoption of ASC Update 2013-05 to have a material impact on its financial position or results of operations.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Taxes	'
	3. Income Taxes
	Income tax expense for the nine months ended September 30, 2013 and 2012 has been computed using estimated annual effective tax rates. These rates are revised, if necessary, at the end of each successive interim period to reflect current estimates of the annual effective tax rates.
	For the nine months ended September 30, 2013, the tax provision is comprised of a $41.8 million U.S. federal income tax expense and $18.3 million foreign income tax expense. For the nine months ended September 30, 2012, the tax provision was comprised of a $3.4 million U.S. federal income tax expense and $30.3 million in foreign income tax expense.
	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 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 $14.7 million of non-U.S. income for the nine months ended September 30, 2013. All of the Company’s non-U.S. income was subject to U.S. taxes during the same period in 2012.
	The Company or its subsidiaries files income tax returns in the U.S. federal jurisdiction and various state jurisdictions, as well as foreign jurisdictions. With few exceptions, the Company and its subsidiaries are no longer subject to U.S. federal income tax examinations by tax authorities for years before 2007. The IRS audits of the years 2009 and 2010 commenced in June 2012. The Company and its subsidiaries are still subject to U.S. state income tax examinations by tax authorities for years after 2006 and foreign examinations for years after 2010.

Debt

Debt	9 Months Ended
	Sep. 30, 2013
Debt	'
	4. Debt
	Debt consists of the following:
	(in millions)	September 30,			December 31,
		2013			2012
	Senior debentures maturing June 15, 2021	$	300		$	300
	Senior debentures maturing March 1, 2020		200			200
	Senior debentures maturing October 15, 2025		81.2			120.9
	Subordinated debentures maturing March 30, 2053		175			—
	Subordinated debentures maturing February 3, 2027		59.7			59.7
	FHLBB borrowings (secured)		125			171.3
	Total principal debt	$	940.9		$	851.9
	Unamortized debt discount		(2.2	)		(2.5	)
	Total	$	938.7		$	849.4
	During the second quarter of 2013, the Company repurchased senior debentures maturing October 15, 2025, with a carrying value of $39.2 million at a cost of $50.5 million, resulting in a loss of $11.3 million.
	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. If the debentures are not redeemed in whole, at least $25.0 million aggregate principal amount of the debentures must remain outstanding.
	In 2009, Hanover Insurance received a $125.0 million Federal Home Loan Bank of Boston (“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 $145.2 million and $200.8 million, for the aggregate borrowings of $125.0 million and $171.3 million as of September 30, 2013 and December 31, 2012, respectively. The amount of required collateral decreased in conjunction with the repayment of the $46.3 million of FHLBB advances. 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 and $9.7 million at September 30, 2013 and December 31, 2012, respectively.
	At September 30, 2013, the Company was in compliance with the covenants associated with all of its debt indentures and credit arrangements.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
Investments	'
	5. 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:
		September 30, 2013
	(in millions)	Amortized			Gross			Gross			Fair Value			OTTI
		Cost or			Unrealized			Unrealized						Unrealized
		Cost			Gains			Losses						Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	376.3		$	4.6		$	10.4		$	370.5		$	—
	Foreign government		286.6			2.6			0.8			288.4			—
	Municipal		1,062.00			47.4			12.5			1,096.90			—
	Corporate		3,532.70			183.6			34.4			3,681.90			8.6
	Residential mortgage-backed		694			22.7			9.6			707.1			1.7
	Commercial mortgage-backed		349.1			11.2			4.4			355.9			—
	Asset-backed		172.6			2.5			0.3			174.8			—
	Total fixed maturities	$	6,473.30		$	274.6		$	72.4		$	6,675.50		$	10.3
	Equity securities	$	449		$	50.5		$	5.4		$	494.1		$	—
		December 31, 2012
	(in millions)	Amortized			Gross			Gross			Fair Value			OTTI
		Cost or			Unrealized			Unrealized						Unrealized
		Cost			Gains			Losses						Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	317.2		$	8.8		$	0.4		$	325.6		$	—
	Foreign government		348.5			4.6			0.2			352.9			—
	Municipal		1,010.20			87.2			1.1			1,096.30			—
	Corporate		3,512.80			275.4			14.8			3,773.40			9.3
	Residential mortgage-backed		769			39.4			3.2			805.2			1.7
	Commercial mortgage-backed		373.3			23.2			0.3			396.2			—
	Asset-backed		198.5			4.1			—			202.6			—
	Total fixed maturities	$	6,529.50		$	442.7		$	20		$	6,952.20		$	11
	Equity securities	$	299		$	21.6		$	4.8		$	315.8		$	—
	Other-than-temporary impairments (“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 $16.4 million and $20.5 million as of September 30, 2013 and December 31, 2012, respectively.
	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.
	(in millions)	September 30, 2013
		Amortized			Fair
		Cost			Value
	Due in one year or less	$	444.7		$	450.6
	Due after one year through five years		2,216.90			2,321.50
	Due after five years through ten years		1,904.40			1,959.00
	Due after ten years		691.6			706.6
			5,257.60			5,437.70
	Mortgage-backed and asset-backed securities		1,215.70			1,237.80
	Total fixed maturities	$	6,473.30		$	6,675.50
	B. 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 September 30, 2013 and December 31, 2012.
		September 30, 2013
		12 months or less						Greater than 12 months						Total
	(in millions)	Gross			Fair			Gross			Fair			Gross		Fair
		Unrealized			Value			Unrealized			Value			Unrealized		Value
		Losses						Losses						Losses
	Fixed maturities:
	Investment grade:
	U.S. Treasury and government agencies	$	10.3		$	178.4		$	0.1		$	4.6		$	10.4	$	183
	Foreign governments		0.7			73.1			0.1			15.5			0.8		88.6
	Municipal		11.5			277.1			1			17.8			12.5		294.9
	Corporate		20.1			666.2			6.8			37.5			26.9		703.7
	Residential mortgage-backed		7.5			218.5			1.6			10.9			9.1		229.4
	Commercial mortgage-backed		4.4			113.8			—			0.9			4.4		114.7
	Asset-backed		0.3			41.3			—			0.2			0.3		41.5
	Total investment grade		54.8			1,568.40			9.6			87.4			64.4		1,655.80
	Below investment grade:
	Corporate		4.4			91.9			3.1			27.8			7.5		119.7
	Residential mortgage-backed		0.1			3			0.4			1.6			0.5		4.6
Total below investment grade		4.5			94.9			3.5			29.4			8		124.3
Total fixed maturities		59.3			1,663.30			13.1			116.8			72.4		1,780.10
Equity securities		3.6			128.7			1.8			3.9			5.4		132.6
Total	$	62.9		$	1,792.00		$	14.9		$	120.7		$	77.8	$	1,912.70
	December 31, 2012
	12 months or less						Greater than 12 months						Total
(in millions)	Gross			Fair			Gross			Fair			Gross		Fair Value
	Unrealized			Value			Unrealized			Value			Unrealized
	Losses						Losses						Losses
Fixed maturities:
Investment grade:
U.S. Treasury and government agencies	$	0.2		$	89.5		$	0.2		$	8.5		$	0.4	$	98
Foreign governments		0.2			81.2			—			0.4			0.2		81.6
Municipal		0.5			61.9			0.6			24			1.1		85.9
Corporate		1.8			224.8			6.6			59			8.4		283.8
Residential mortgage-backed		0.5			47.3			2			9.4			2.5		56.7
Commercial mortgage-backed		0.2			29.9			0.1			4.9			0.3		34.8
Asset-backed		—			11.4			—			0.3			—		11.7
Total investment grade		3.4			546			9.5			106.5			12.9		652.5
Below investment grade:
Municipal		—			—			—			2			—		2
Corporate		1.1			26.6			5.3			50.6			6.4		77.2
Residential mortgage-backed		0.1			1.6			0.6			2.5			0.7		4.1
Total below investment grade		1.2			28.2			5.9			55.1			7.1		83.3
Total fixed maturities		4.6			574.2			15.4			161.6			20		735.8
Equity securities		4.8			74.4			—			—			4.8		74.4
Total	$	9.4		$	648.6		$	15.4		$	161.6		$	24.8	$	810.2
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.
C. Proceeds from sales
The proceeds from sales of available-for-sale securities and gross realized gains and losses on those sales, were as follows:
	Three Months Ended September 30,
	2013									2012
(in millions)	Proceeds from			Gross			Gross			Proceeds from			Gross		Gross
	Sales			Gains			Losses			Sales			Gains		Losses
Fixed maturities	$	83.3		$	1		$	0.9		$	78.8		$	2.3	$	0.3
Equity securities	$	10.8		$	1.5		$	—		$	13.6		$	2.4	$	—
	Nine Months Ended September 30,
	2013									2012
(in millions)	Proceeds from			Gross			Gross			Proceeds from			Gross		Gross
	Sales			Gains			Losses			Sales			Gains		Losses
Fixed maturities	$	369		$	5.6		$	1.9		$	486.8		$	8.9	$	1.9
Equity securities	$	89.7		$	18.5		$	0.3		$	17.1		$	2.9	$	0.2
D. Other-than-temporary impairments
For the three months ended September 30, 2013, total OTTI of fixed maturities and equity securities were $2.5 million. Of this amount, $2.1 million was recognized in earnings and the remaining $0.4 million was recorded as unrealized losses in accumulated other comprehensive income. For the first nine months of 2013, total OTTI of fixed maturities and equity securities were $4.5 million. Of this amount, $3.7 million was recognized in earnings and the remaining $0.8 million was recorded as unrealized losses in accumulated other comprehensive income.
For the three months ended September 30, 2012, total OTTI of fixed maturities and equity securities were $1.4 million. Of this amount, $2.2 million was recognized in earnings, including $0.8 million that was transferred from accumulated other comprehensive income. For the first nine months of 2012, total OTTI of fixed maturities and equity securities were $5.2 million. Of this amount, $5.7 million was recognized in earnings, including $0.5 million that was transferred from accumulated other comprehensive income.
The methodology and significant inputs used to measure the amount of credit losses on fixed maturities in 2013 and 2012 were as follows:
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.
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.
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.
	Three Months Ended						Nine Months Ended
	September 30,						September 30,
(in millions)	2013			2012			2013			2012
Credit losses at beginning of period	$	7.3		$	13.8		$	8.6		$	14.5
Credit losses for which an OTTI was not previously recognized		0.1			0.1			0.4			0.5
Additional credit losses on securities for which an OTTI was previously recognized		—			—			0.2			0.6
Reductions for securities sold, matured or called		(0.1	)		(2.5	)		(1.9	)		(4.2	)
Reductions for securities reclassified as intended to sell		—			(0.2	)		—			(0.2	)
Credit losses at the end of period	$	7.3		$	11.2		$	7.3		$	11.2
E. Funds at Lloyd’s
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. These assets consisted of approximately $341 million of fixed maturities and $28 million of cash and cash equivalents as of September 30, 2013. The Company also deposits funds with various state and governmental authorities in the U.S. For a discussion of the Company’s deposits with state and governmental authorities, see also Note 3 – “Investments” of the Notes to Consolidated Financial Statements in our Annual Report on Form 10-K for the year ended December 31, 2012.

Fair_Value

Fair Value	9 Months Ended
	Sep. 30, 2013
Fair Value	'
	6. 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 2 also includes fair values obtained from net asset values provided by mutual fund investment managers, upon which subscriptions and redemptions can be executed. 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. Generally, all prices provided by the pricing service, except quoted market prices, are reported as Level 2. 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 3 as market multiples represent significant unobservable inputs.
	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 cost basis limited partnerships and mortgage loans. Cost basis limited partnerships’ fair values are based on the net asset value provided by the general partner and recent financial information and are reported as Level 3. Mortgage loans’ fair values 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 2.
	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:
	(in millions)		September 30, 2013						December 31, 2012
			Carrying			Fair			Carrying				Fair
			Value			Value			Value				Value
	Financial Assets
	Cash and cash equivalents		$	602.2		$	602.2		$	564.8			$	564.8
	Fixed maturities			6,675.50			6,675.50			6,952.20				6,952.20
	Equity securities			494.1			494.1			315.8				315.8
	Other investments			165.5			165.9			188.9				189.4
	Total financial assets		$	7,937.30		$	7,937.70		$	8,021.70			$	8,022.20
	Financial Liabilities
	Debt		$	938.7		$	1,005.30		$	849.4			$	995.2
	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 2013 and 2012, 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.
			September 30, 2013
	(in millions)		Total			Level 1			Level 2				Level 3
	Fixed maturities:
	U.S. Treasury and government agencies		$	370.5		$	167.1		$	203.4			$	—
	Foreign government			288.4			39.2			249.2				—
	Municipal			1,096.90			—			1,070.00				26.9
	Corporate			3,681.90			—			3,662.30				19.6
	Residential mortgage-backed, U.S. agency backed			541.5			—			541.5				—
	Residential mortgage-backed, non-agency			165.6			—			165.1				0.5
	Commercial mortgage-backed			355.9			—			332.7				23.2
	Asset-backed			174.8			—			174.8				—
	Total fixed maturities			6,675.50			206.3			6,399.00				70.2
	Equity securities			484.8			453.3			—				31.5
	Other investments			153.8			—			150.2				3.6
	Total investment assets at fair value		$	7,314.10		$	659.6		$	6,549.20			$	105.3
			December 31, 2012
	(in millions)		Total			Level 1			Level 2				Level 3
	Fixed maturities:
	U.S. Treasury and government agencies		$	325.6		$	144.2		$	181.4			$	—
	Foreign government			352.9			60.9			292				—
Municipal			1,096.30			—			1,076.90				19.4
Corporate			3,773.40			—			3,747.00				26.4
Residential mortgage-backed, U.S. agency backed			610.8			—			610.8				—
Residential mortgage-backed, non-agency			194.4			—			193.7				0.7
Commercial mortgage-backed			396.2			—			369.5				26.7
Asset-backed			202.6			—			201.1				1.5
Total fixed maturities			6,952.20			205.1			6,672.40				74.7
Equity securities			306.1			226.9			54.8				24.4
Other investments			172.8			—			169.2				3.6
Total investment assets at fair value		$	7,431.10		$	432		$	6,896.4			$	102.7
The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value:
		September 30, 2013
(in millions)		Total			Level 1			Level 2				Level 3
Assets:
Cash and cash equivalents		$	602.2		$	602.2		$	—			$	—
Equity securities			9.3			—			9.3				—
Other investments			12.1			—			2.9				9.2
Liabilities:
Debt		$	1,005.30		$	—		$	1,005.30			$	—
		December 31, 2012
(in millions)		Total			Level 1			Level 2				Level 3
Assets:
Cash and cash equivalents		$	564.8		$	564.8		$	—			$	—
Equity securities			9.7			—			9.7				—
Other investments			16.6			—			4.8				11.8
Liabilities:
Debt		$	995.2		$	—		$	995.2			$	—
The tables below provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).
		Fixed Maturities
(in millions)		Municipal			Corporate			Residential				Commercial			Asset-			Total			Equity and			Total Assets
								mortgage-				mortgage-			backed						Other
								backed, non-				backed
								agency
Three Months Ended September 30, 2013
Balance July 1, 2013		$	27.9		$	19.5		$	0.6			$	23.6		$	—		$	71.6		$	32.3		$	103.9
Total gains (losses):
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities			(0.1	)		0.1			—				—			—			—			2.8			2.8
Sales			(0.9	)		—			(0.1	)			(0.4	)		—			(1.4	)		—			(1.4	)
Balance September 30, 2013		$	26.9		$	19.6		$	0.5			$	23.2		$	—		$	70.2		$	35.1		$	105.3
Three Months Ended September 30, 2012
Balance July 1, 2012		$	16.9		$	27.4		$	0.6			$	27.4		$	1.1		$	73.4		$	29.2		$	102.6
Transfers into Level 3			2.6			—			—				—			—			2.6			—			2.6
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			0.3			—			—				0.6			—			0.9			(1.7	)		(0.8	)
Purchases and sales:
Purchases			—			—			0.1				—			—			0.1			—			0.1
Sales			(0.2	)		(0.1	)		—				(0.6	)		—			(0.9	)		—			(0.9	)
Balance September 30, 2012		$	19.6		$	27.4		$	0.7			$	27.4		$	1.1		$	76.2		$	27.5		$	103.7
		Fixed Maturities
(in millions)		Municipal			Corporate			Residential				Commercial			Asset-			Total			Equity and			Total Assets
								mortgage-				mortgage-			backed						Other
								backed, non-				backed
								agency
Nine Months Ended September 30, 2013
Balance January 1, 2013		$	19.4		$	26.4		$	0.7			$	26.7		$	1.5		$	74.7		$	28		$	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			—			0.4			—				—			—			0.4			—			0.4	/B>
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities			(0.4	)		(0.9	)		—				(1.4	)		—			(2.7	)		8			5.3
Sales			(1.8	)		(4.3	)		(0.2	)			(2.1	)		—			(8.4	)		—			(8.4	)
Balance September 30, 2013		$	26.9		$	19.6		$	0.5			$	23.2		$	—		$	70.2		$	35.1		$	105.3
Nine Months Ended September 30, 2012
Balance January 1, 2012		$	13.6		$	23.8		$	5.2			$	23.7		$	9.2		$	75.5		$	27		$	102.5
Transfers into Level 3			2.6			4.3			—				—			—			6.9			0.1			7
Transfers out of Level 3			—			(2.1	)		—				—			(8.7	)		(10.8	)		—			(10.8	)
Total gains (losses):
Included in earnings			—			0.4			—				—			—			0.4			(0.2	)		0.2
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities			1			0.5			0.1				(0.1	)		—			1.5			1.3			2.8
Purchases and sales:
Purchases			3			1.5			0.1				5.2			1			10.8			—			10.8
Sales			(0.6	)		(1.0	)		(4.7	)			(1.4	)		(0.4	)		(8.1	)		(0.7	)		(8.8	)
Balance September 30, 2012		$	19.6		$	27.4		$	0.7			$	27.4		$	1.1		$	76.2		$	27.5		$	103.7
There were no transfers between Level 2 and Level 3 during the three months ended September 30, 2013. During the three months ended September 30, 2012 and nine months ended September 30, 2013 and 2012, 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 the three months or the nine months ended September 30, 2013 or 2012.
For the three months ended September 30, 2013, there were no gains or losses due to changes in fair value that were recorded in net income for Level 3 assets. The following table summarizes gains and losses due to changes in fair value that were recorded in net income for Level 3 assets:
		Three Months Ended			Nine Months Ended
		September 30,			September 30,
		2012			2013			2012
(in millions)		Net Realized			Net Realized Investment
		Investment Gains			Gains (Losses)
Level 3 Assets:
Fixed maturities:
Corporate		$	0.1		$	0.4		$	0.4
Equity securities			—			—			(0.2	)
Total assets		$	0.1		$	0.4		$	0.2
There were no Level 3 liabilities held by the Company for the three and nine months ended September 30, 2013 and 2012.
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.
					September 30, 2013					December 31, 2012
(in millions)		Valuation		Significant Unobservable Inputs	Fair			Range		Fair				Range
		Technique			Value			(Wtd Average)		Value				(Wtd Average)
Fixed maturities:
Municipal		Discounted cash flow		Discount for:	$	26.9				$	19.4
				Small issue size				1.0-4.0% (2.3%)						1.0-4.0% (3.1%)
				Above-market coupon				0.3-1.0% (0.5%)						0.3-1.0% (0.5%)
				Long maturity				NA						0.5% (0.5%)
Corporate		Discounted cash flow		Discount for:		19.4					26.4
				Credit stress				1.0-2.0% (1.0%)						1.0-3.0% (1.1%)
				Above-market coupon				0.3-1.0% (0.6%)						0.3-1.0% (0.7%)
				Small issue size				0.3-2.0% (0.5%)						0.3-3.0% (0.5%)
				Long maturity				NA						0.5% (0.5%)
Residential mortgage-backed, non-agency		Discounted cash flow		Discount for:		0.5					0.7
				Small issue size				0.5% (0.5%)						0.5% (0.5%)
Commercial mortgage-backed		Discounted cash flow		Discount for:		23.2					26.7
				Credit stress				0.5% (0.5%)						1.0% (1.0%)
				Small issue size				0.5% (0.5%)						0.5% (0.5%)
				Above-market coupon				0.3-0.8% (0.4%)						0.3-0.8% (0.4%)
				Lease structure				0.3% (0.3%)						0.3% (0.3%)
				Long maturity				NA						0.5-0.8% (0.7%)
Asset backed		Discounted cash flow		Discount for:		—					1.5
				Small issue size				NA						0.7-2.0% (1.6%)
Equity securities		Market comparables		Net tangible asset		31.4					24.3
				market multiples				1.1X (1.1X)						0.9X (0.9X)
Other		Discounted cash flow		Discount rate		3.6		18.0% (18.0%)			3.6			18.0% (18.0%)
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.

Pension_and_Other_Postretireme

Pension and Other Postretirement Benefit Plans	9 Months Ended
	Sep. 30, 2013
Pension and Other Postretirement Benefit Plans	'
	7. Pension and Other Postretirement Benefit Plans
	The components of net periodic pension cost for defined benefit pension and other postretirement benefit plans included in the Company’s results of operations are as follows:
		Three Months Ended September 30,
	(in millions)	2013			2012			2013			2012
		Pension Plans						Postretirement Plans
	Service cost - benefits earned during the period	$	0.4		$	0.4		$	—		$	—
	Interest cost		8			8.8			0.2			0.6
	Expected return on plan assets		(8.9	)		(9.6	)		—			—
	Recognized net actuarial loss		3.7			3.2			0.1			0.1
	Amortization of prior service cost		—			0.1			(0.9	)		(1.0	)
	Net periodic pension cost (benefit)	$	3.2		$	2.9		$	(0.6	)	$	(0.3	)
		Nine Months Ended September 30,
	(in millions)	2013			2012			2013			2012
		Pension Plans						Postretirement Plans
	Service cost - benefits earned during the period	$	1.3		$	1.2		$	0.1		$	0.1
	Interest cost		23.9			26.4			1			1.6
	Expected return on plan assets		(26.7	)		(29.0	)		—			—
	Recognized net actuarial loss		11			9.6			0.2			0.2
	Amortization of prior service cost		—			0.1			(2.8	)		(2.9	)
	Net settlement gain		—			—			(1.6	)		—
	Net periodic pension cost (benefit)	$	9.5		$	8.3		$	(3.1	)	$	(1.0	)
	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 guaranteeing 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.

Other_Comprehensive_Income

Other Comprehensive Income	9 Months Ended
	Sep. 30, 2013
Other Comprehensive Income	'
	8. Other Comprehensive Income
	The following table provides changes in other comprehensive income.
		Three Months Ended September 30,
		2013									2012
	(in millions)	Pre-Tax			Tax			Net of			Pre-Tax			Tax			Net of
					Benefit			Tax						Benefit			Tax
					(Expense)									(Expense)
	Unrealized gains (losses) on available-for-sale securities and derivative instruments:
	Unrealized gains (losses) arising during period	$	(7.1	)	$	2.2		$	(4.9	)	$	90.2		$	(31.9	)	$	58.3
	Less:
	Amount of realized gains from sales and other		3.9			2.1			6			5.9			2.1			8
	Portion of other-than-temporary impairment losses recognized in earnings		(2.1	)		0.7			(1.4	)		(2.3	)		0.8			(1.5	)
	Net unrealized gains (losses)		(8.9	)		(0.6	)		(9.5	)		86.6			(34.8	)		51.8
	Pension and postretirement benefits:
	Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost		2.9			(1.0	)		1.9			2.4			(0.8	)		1.6
	Cumulative foreign currency translation adjustment:
	Foreign currency translation recognized during the period		8.7			(3.0	)		5.7			11.6			(4.1	)		7.5
	Other comprehensive income (loss)	$	2.7		$	(4.6	)	$	(1.9	)	$	100.6		$	(39.7	)	$	60.9
		Nine Months Ended September 30,
		2013									2012
	(in millions)	Pre-Tax			Tax			Net of			Pre-Tax			Tax			Net of
					Benefit			Tax						Benefit			Tax
					(Expense)									(Expense)
	Unrealized gains (losses) on available-for-sale securities and derivative instruments:
	Unrealized gains (losses) arising during period (net of pre-tax, ceded unrealized losses of $0.8 million for the nine months ended September 30, 2013)	$	(172.1	)	$	60.1		$	(112.0	)	$	191.1		$	(35.6	)	$	155.5
	Less:
	Amount of realized gains from sales and other		27.7			4.2			31.9			13.8			5.9			19.7
	Portion of other-than-temporary impairment losses recognized in earnings		(3.7	)		1.3			(2.4	)		(6.8	)		2			(4.8	)
	Net unrealized gains (losses)		(196.1	)		54.6			(141.5	)		184.1			(43.5	)		140.6
	Pension and postretirement benefits:
	Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost		12.1			(4.2	)		7.9			7			(2.4	)		4.6
	Cumulative foreign currency translation adjustment:
	Foreign currency translation recognized during the period		(6.8	)		2.4			(4.4	)		13			(4.6	)		8.4
	Other comprehensive income (loss)	$	(190.8	)	$	52.8		$	(138.0	)	$	204.1		$	(50.5	)	$	153.6
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Details about Accumulated Other Comprehensive Income	Amount Reclassified from												Affected Line Item in the Statement
	Components	Accumulated Other												Where Net Income is Presented
		Comprehensive Income
	Unrealized gains (losses) on available-for-sale securities and derivative instruments	$	3.9		$	5.9		$	27.7		$	13.9		Net realized gains from sales and other
			(2.1	)		(2.2	)		(3.7	)		(5.7	)	Net other-than-temporary impairment losses on investments recognized in earnings
	Other		—			(0.1	)		—			(0.3	)
			1.8			3.6			24			7.9		Total before tax
			2.8			2.9			5.5			7.9		Tax benefit (expense)
			4.6			6.5			29.5			15.8
			—			—			—			(0.9	)	Discontinued operations, net of tax
			4.6			6.5			29.5			14.9		Net of tax
	Amortization of defined benefit pension and postretirement plans		2.9			2.4			12.1			7		Loss adjustment expenses and other operating expenses
			(1.0	)		(0.8	)		(4.2	)		(2.4	)	Tax benefit (expense)
			1.9			1.6			7.9			4.6		Net of tax
	Total reclassifications for the period	$	6.5		$	8.1		$	37.4		$	19.5		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 both the three and nine months ended September 30, 2013 and 2012.

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2013
Segment Information	'
	9. 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 voluntary pools business which is in run-off. 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 (formerly referred to as segment income) which also excludes interest expense on debt. Operating income (loss) before taxes excludes certain items which are included in net income (loss), such as net realized investment gains and losses (including 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 (loss) before taxes excludes net gains and losses on disposals of businesses, discontinued operations, costs to acquire businesses, restructuring costs, extraordinary items, the cumulative effect of accounting changes and certain other items. Although the items excluded from operating income (loss) before taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income (loss) before taxes enhances an investor’s understanding of the Company’s results of operations by highlighting net income (loss) attributable to the core operations of the business. However, operating income (loss) before taxes should not be construed as a substitute for income (loss) before income taxes and operating income (loss) should not be construed as a substitute for net income (loss).
	Summarized below is financial information with respect to the Company’s business segments.
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Operating revenues:
	Commercial Lines	$	530		$	498.9		$	1,568.40		$	1,461.90
	Personal Lines		385.1			391.6			1,165.10			1,168.80
	Chaucer		283.4			259.2			800.8			762.7
	Other		2.3			3			7.6			8.6
	Total		1,200.80			1,152.70			3,541.90			3,402.00
	Net realized investment gains		1			5			22.8			4.7
	Total revenues	$	1,201.80		$	1,157.70		$	3,564.70		$	3,406.70
	Operating income (loss) before income taxes:
	Commercial Lines:
	GAAP underwriting gain (loss)	$	2.8		$	(27.8	)	$	(9.8	)	$	(75.0	)
	Net investment income		35.3			35.6			107.4			106.3
	Other income		0.4			0.3			0.1			1.3
	Commercial Lines operating income		38.5			8.1			97.7			32.6
	Personal Lines:
	GAAP underwriting income (loss)		18			(8.4	)		27.8			(14.6	)
	Net investment income		18.6			21.6			56.7			64.5
	Other income		1.1			1.5			3.6			4.6
	Personal Lines operating income		37.7			14.7			88.1			54.5
	Chaucer:
	GAAP underwriting income		20.8			30.7			78.5			64.6
	Net investment income		10.2			10.2			31.4			29.8
	Other income		1.4			1.1			0.3			2.9
	Chaucer operating income		32.4			42			110.2			97.3
	Other:
	GAAP underwriting loss		(0.2	)		(1.1	)		(2.3	)		(2.3	)
	Net investment income		1.6			1.7			5.4			5.7
	Other net expenses		(2.8	)		(2.5	)		(8.6	)		(8.2	)
	Other operating loss		(1.4	)		(1.9	)		(5.5	)		(4.8	)
	Operating income before interest expense and income taxes		107.2			62.9			290.5			179.6
	Interest on debt		(16.6	)		(14.6	)		(48.7	)		(46.7	)
	Operating income before income taxes		90.6			48.3			241.8			132.9
	Non-operating income items:
	Net realized investment gains		1			5			22.8			4.7
	Loss on real estate		(4.7	)		—			(4.7	)		—
	Net loss from repayment of debt		—			—			(19.1	)		—
	Net benefit (costs) related to acquired businesses		(0.2	)		0.2			—			(2.2	)
	Net foreign exchange gains (losses)		(0.6	)		(0.3	)		0.1			(0.3	)
	Income before income taxes	$	86.1		$	53.2		$	240.9		$	135.1
	The Company recognized $7.5 million in net foreign currency transaction gains in the Statement of Income during the three months ended September 30, 2013 compared to $1.0 million in net foreign currency losses during the three months ended September 30, 2012. The Company recognized $0.2 million in net foreign currency transaction losses in the Statement of Income during the nine months ended September 30, 2013 compared to $3.0 million in net foreign currency gains during the nine months ended September 30, 2012.
	In September 2013, the Company decided to consolidate 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. Accordingly, the Company recognized a loss of $4.7 million. This is included in other operating expenses in the Consolidated Statements of Income during the three and nine months ended September 30, 2013.
	The following table provides identifiable assets for the Company’s business segments and discontinued operations:
	(in millions)	September 30,			December 31,
		2013			2012
		Identifiable Assets
	U.S. Companies	$	8,918.40		$	8,909.60
	Chaucer		4,424.10			4,444.80
	Discontinued operations		120.8			130.5
	Total	$	13,463.30		$	13,484.90
	The Company reviews the assets of its U.S. Companies collectively and does not allocate them between the Commercial Lines, Personal Lines and Other segments.

Stockbased_Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2013
Stock-based Compensation	'
	10. Stock-based Compensation
	Compensation cost and the related tax benefits were as follows:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Stock-based compensation expense	$	3.2		$	3.5		$	8.9		$	9.8
	Tax benefit		(1.1	)		(1.2	)		(3.1	)		(3.4	)
	Stock-based compensation expense, net of taxes	$	2.1		$	2.3		$	5.8		$	6.4
	Stock Options
	Information on the Company’s stock option plans is summarized below.
		Nine Months Ended September 30,
		2013						2012
	(in whole shares and dollars)	Shares			Weighted Average			Shares			Weighted Average
					Exercise Price						Exercise Price
	Outstanding, beginning of period		2,892,882		$	38.28			2,715,430		$	38.57
	Granted		537,800			42.53			517,500			36.81
	Exercised		(965,861	)		33.8			(56,874	)		29.33
	Forfeited or cancelled		(183,250	)		41.6			(90,574	)		41.84
	Expired		—			—			(159,850	)		44.05
	Outstanding, end of period		2,281,571			40.91			2,925,632			38.06
	Restricted Stock Units
	The following tables summarize activity information about employee restricted stock units:
		Nine Months Ended September 30,
		2013						2012
	(in whole shares and dollars)	Shares			Weighted			Shares			Weighted
					Average Grant						Average Grant
					Date Fair						Date Fair
					Value						Value
	Time-based restricted stock units:
	Outstanding, beginning of period		750,837		$	40.15			768,529		$	40.17
	Granted		139,682			43.21			172,691			36.99
	Vested		(265,209	)		39.59			(127,966	)		35.66
	Forfeited		(60,042	)		41.25			(43,105	)		40.35
	Outstanding, end of period		565,268			41.05			770,149			40.19
	Performance-based and market-based restricted stock units:
	Outstanding, beginning of period		132,775		$	39.97			69,500		$	45.37
	Granted		82,795			41.85			94,000			36.75
	Forfeited		(22,700	)		40.83			(1,125	)		41.16
	Outstanding, end of period		192,870			40.68			162,375			40.34
	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 award. 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 the first three months of 2013 and 2012, the Company granted market-based awards totaling 76,175 and 90,250, 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 a three-year average relative TSR 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.

Earnings_Per_Share_and_Shareho

Earnings Per Share and Shareholders' Equity Transactions	9 Months Ended
	Sep. 30, 2013
Earnings Per Share and Shareholders' Equity Transactions	'
	11. 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:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions, except per share data)	2013			2012			2013			2012
	Basic shares used in the calculation of earnings per share		43.8			44.6			44.1			44.8
	Dilutive effect of securities:
	Employee stock options		0.4			0.2			0.3			0.2
	Non-vested stock grants		0.4			0.4			0.5			0.4
	Diluted shares used in the calculation of earnings per share		44.6			45.2			44.9			45.4
	Per share effect of dilutive securities on income (loss) from continuing operations and net income (loss)	$	(0.03	)	$	(0.02	)	$	(0.07	)	$	(0.03	)
	Diluted earnings per share for the three months ended September 30, 2012 excludes 2.1 million of common shares issuable under the Company’s stock compensation plans, because their effect would be antidilutive. All of the common shares issuable under the Company’s stock compensation plan were included in the diluted earnings per share for the three months ended September 30, 2013. Diluted earnings per share for the nine months ended September 30, 2013 and 2012 excludes 0.7 million and 1.8 million, respectively, of common shares issuable under the Company’s stock compensation plans, because their effect would be antidilutive.
	Since October 2007 and through September 2013, the Company’s Board of Directors has authorized aggregate repurchases of the Company’s common stock of up to $600 million, including a $100 million increase in the program in May 2013. As of September 30, 2013, the Company has $137.0 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 the first nine months of 2013, the Company purchased 1.6 million shares of the Company’s common stock at a cost of $78.2 million.

Commitments_and_Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies	'
	12. 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 understated the accrued benefit in the calculation. The plaintiff claims that the Plan understated 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 discounted by applicable IRS rates.
	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. The District Court granted the Company’s Motion to Dismiss the additional claims on statute of limitations grounds by a Memorandum Opinion dated March 31, 2011, leaving the claims substantially as set forth in the original March 12, 2007 complaint. Plaintiffs filed a Motion for Reconsideration of the District Court’s decision to dismiss the additional claims, which was denied with respect to the claims set forth in (a) and (b) above; however, the Court did allow the fiduciary duty claim regarding plaintiffs’ “whipsaw” claim to stand. On June 22, 2012, the Company and the Plan filed a Partial Motion for Summary Judgment to dismiss the claims of one of the plaintiffs who received his lump sum distribution after December 31, 2003, on the basis that certain amendments to the Plan effective January 1, 2004 eliminated any basis for payment of an additional “whipsaw” amount to participants who received lump sum distributions after December 31, 2003. On December 13, 2012, the Court held this motion in abeyance pending a ruling on Plaintiffs’ Motion for Class Certification. Plaintiffs filed their Motion for Class Certification on January 14, 2013. On February 8, 2013, the Company and the Plan informed the Court that they did not oppose the certification of a class. On October 2, 2013, the Court granted the Company and the Plan’s Partial Motion for Summary Judgment and dismissed with prejudice the claims of the named plaintiff and putative class members who received lump sum distributions after December 31, 2003. The Court also granted the Plaintiffs’ unopposed motion for class certification as to the remaining members of the putative class. In a hearing on October 23, 2013, Plaintiffs argued that the Court’s October 2, 2013 dismissal order only applied to the claims of the named plaintiff and not the claims of similarly situated putative class members. The Company opposed this argument. The Court said it would consider the parties’ positions and issue an order clarifying the scope of the dismissal order.
	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 alleged 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.

Subsequent_Events

Subsequent Events	9 Months Ended
	Sep. 30, 2013
Subsequent Events	'
	13. Subsequent Events
	There were no subsequent events requiring adjustment to the financial statements and no additional disclosures required in the notes to the interim consolidated financial statements.

Debt_Tables

Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Schedule of Long-Term Debt	'
	Debt consists of the following:
	(in millions)	September 30,			December 31,
		2013			2012
	Senior debentures maturing June 15, 2021	$	300		$	300
	Senior debentures maturing March 1, 2020		200			200
	Senior debentures maturing October 15, 2025		81.2			120.9
	Subordinated debentures maturing March 30, 2053		175			—
	Subordinated debentures maturing February 3, 2027		59.7			59.7
	FHLBB borrowings (secured)		125			171.3
	Total principal debt	$	940.9		$	851.9
	Unamortized debt discount		(2.2	)		(2.5	)
	Total	$	938.7		$	849.4

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities and Cost and Fair Value of 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:
		September 30, 2013
	(in millions)	Amortized			Gross			Gross			Fair Value			OTTI
		Cost or			Unrealized			Unrealized						Unrealized
		Cost			Gains			Losses						Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	376.3		$	4.6		$	10.4		$	370.5		$	—
	Foreign government		286.6			2.6			0.8			288.4			—
	Municipal		1,062.00			47.4			12.5			1,096.90			—
	Corporate		3,532.70			183.6			34.4			3,681.90			8.6
	Residential mortgage-backed		694			22.7			9.6			707.1			1.7
	Commercial mortgage-backed		349.1			11.2			4.4			355.9			—
	Asset-backed		172.6			2.5			0.3			174.8			—
	Total fixed maturities	$	6,473.30		$	274.6		$	72.4		$	6,675.50		$	10.3
	Equity securities	$	449		$	50.5		$	5.4		$	494.1		$	—
		December 31, 2012
	(in millions)	Amortized			Gross			Gross			Fair Value			OTTI
		Cost or			Unrealized			Unrealized						Unrealized
		Cost			Gains			Losses						Losses
	Fixed maturities:
	U.S. Treasury and government agencies	$	317.2		$	8.8		$	0.4		$	325.6		$	—
	Foreign government		348.5			4.6			0.2			352.9			—
	Municipal		1,010.20			87.2			1.1			1,096.30			—
	Corporate		3,512.80			275.4			14.8			3,773.40			9.3
	Residential mortgage-backed		769			39.4			3.2			805.2			1.7
	Commercial mortgage-backed		373.3			23.2			0.3			396.2			—
	Asset-backed		198.5			4.1			—			202.6			—
	Total fixed maturities	$	6,529.50		$	442.7		$	20		$	6,952.20		$	11
	Equity securities	$	299		$	21.6		$	4.8		$	315.8		$	—
Maturities of Available-For-Sale Debt Securities	'
	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.
	(in millions)	September 30, 2013
		Amortized			Fair
		Cost			Value
	Due in one year or less	$	444.7		$	450.6
	Due after one year through five years		2,216.90			2,321.50
	Due after five years through ten years		1,904.40			1,959.00
	Due after ten years		691.6			706.6
			5,257.60			5,437.70
	Mortgage-backed and asset-backed securities		1,215.70			1,237.80
	Total fixed maturities	$	6,473.30		$	6,675.50
Fixed Maturities and Equity Securities in 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 September 30, 2013 and December 31, 2012.
		September 30, 2013
		12 months or less						Greater than 12 months						Total
	(in millions)	Gross			Fair			Gross			Fair			Gross		Fair
		Unrealized			Value			Unrealized			Value			Unrealized		Value
		Losses						Losses						Losses
	Fixed maturities:
	Investment grade:
	U.S. Treasury and government agencies	$	10.3		$	178.4		$	0.1		$	4.6		$	10.4	$	183
	Foreign governments		0.7			73.1			0.1			15.5			0.8		88.6
	Municipal		11.5			277.1			1			17.8			12.5		294.9
	Corporate		20.1			666.2			6.8			37.5			26.9		703.7
	Residential mortgage-backed		7.5			218.5			1.6			10.9			9.1		229.4
	Commercial mortgage-backed		4.4			113.8			—			0.9			4.4		114.7
	Asset-backed		0.3			41.3			—			0.2			0.3		41.5
	Total investment grade		54.8			1,568.40			9.6			87.4			64.4		1,655.80
	Below investment grade:
	Corporate		4.4			91.9			3.1			27.8			7.5		119.7
	Residential mortgage-backed		0.1			3			0.4			1.6			0.5		4.6
	Total below investment grade		4.5			94.9			3.5			29.4			8		124.3
	Total fixed maturities		59.3			1,663.30			13.1			116.8			72.4		1,780.10
	Equity securities		3.6			128.7			1.8			3.9			5.4		132.6
	Total	$	62.9		$	1,792.00		$	14.9		$	120.7		$	77.8	$	1,912.70
		December 31, 2012
		12 months or less						Greater than 12 months						Total
	(in millions)	Gross			Fair			Gross			Fair			Gross		Fair Value
		Unrealized			Value			Unrealized			Value			Unrealized
		Losses						Losses						Losses
	Fixed maturities:
	Investment grade:
	U.S. Treasury and government agencies	$	0.2		$	89.5		$	0.2		$	8.5		$	0.4	$	98
	Foreign governments		0.2			81.2			—			0.4			0.2		81.6
	Municipal		0.5			61.9			0.6			24			1.1		85.9
	Corporate		1.8			224.8			6.6			59			8.4		283.8
	Residential mortgage-backed		0.5			47.3			2			9.4			2.5		56.7
	Commercial mortgage-backed		0.2			29.9			0.1			4.9			0.3		34.8
	Asset-backed		—			11.4			—			0.3			—		11.7
	Total investment grade		3.4			546			9.5			106.5			12.9		652.5
	Below investment grade:
	Municipal		—			—			—			2			—		2
	Corporate		1.1			26.6			5.3			50.6			6.4		77.2
	Residential mortgage-backed		0.1			1.6			0.6			2.5			0.7		4.1
	Total below investment grade		1.2			28.2			5.9			55.1			7.1		83.3
	Total fixed maturities		4.6			574.2			15.4			161.6			20		735.8
	Equity securities		4.8			74.4			—			—			4.8		74.4
	Total	$	9.4		$	648.6		$	15.4		$	161.6		$	24.8	$	810.2
Proceeds from Sale of Available-for-Sale Securities	'
	The proceeds from sales of available-for-sale securities and gross realized gains and losses on those sales, were as follows:
		Three Months Ended September 30,
		2013									2012
	(in millions)	Proceeds from			Gross			Gross			Proceeds from			Gross		Gross
		Sales			Gains			Losses			Sales			Gains		Losses
	Fixed maturities	$	83.3		$	1		$	0.9		$	78.8		$	2.3	$	0.3
	Equity securities	$	10.8		$	1.5		$	—		$	13.6		$	2.4	$	—
		Nine Months Ended September 30,
		2013									2012
	(in millions)	Proceeds from			Gross			Gross			Proceeds from			Gross		Gross
		Sales			Gains			Losses			Sales			Gains		Losses
	Fixed maturities	$	369		$	5.6		$	1.9		$	486.8		$	8.9	$	1.9
	Equity securities	$	89.7		$	18.5		$	0.3		$	17.1		$	2.9	$	0.2
Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses	'
	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.
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Credit losses at beginning of period	$	7.3		$	13.8		$	8.6		$	14.5
	Credit losses for which an OTTI was not previously recognized		0.1			0.1			0.4			0.5
	Additional credit losses on securities for which an OTTI was previously recognized		—			—			0.2			0.6
	Reductions for securities sold, matured or called		(0.1	)		(2.5	)		(1.9	)		(4.2	)
	Reductions for securities reclassified as intended to sell		—			(0.2	)		—			(0.2	)
	Credit losses at the end of period	$	7.3		$	11.2		$	7.3		$	11.2

Fair_Value_Tables

Fair Value (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value of Financial Instruments	'
	The estimated fair value of the financial instruments were as follows:
	(in millions)	September 30, 2013						December 31, 2012
		Carrying			Fair			Carrying				Fair
		Value			Value			Value				Value
	Financial Assets
	Cash and cash equivalents	$	602.2		$	602.2		$	564.8			$	564.8
	Fixed maturities		6,675.50			6,675.50			6,952.20				6,952.20
	Equity securities		494.1			494.1			315.8				315.8
	Other investments		165.5			165.9			188.9				189.4
	Total financial assets	$	7,937.30		$	7,937.70		$	8,021.70			$	8,022.20
	Financial Liabilities
	Debt	$	938.7		$	1,005.30		$	849.4			$	995.2
Fair Value, Assets Measured on Recurring Basis	'
	The following tables provide, for each hierarchy level, the Company’s assets that were measured at fair value on a recurring basis.
		September 30, 2013
	(in millions)	Total			Level 1			Level 2				Level 3
	Fixed maturities:
	U.S. Treasury and government agencies	$	370.5		$	167.1		$	203.4			$	—
	Foreign government		288.4			39.2			249.2				—
	Municipal		1,096.90			—			1,070.00				26.9
	Corporate		3,681.90			—			3,662.30				19.6
	Residential mortgage-backed, U.S. agency backed		541.5			—			541.5				—
	Residential mortgage-backed, non-agency		165.6			—			165.1				0.5
	Commercial mortgage-backed		355.9			—			332.7				23.2
	Asset-backed		174.8			—			174.8				—
	Total fixed maturities		6,675.50			206.3			6,399.00				70.2
	Equity securities		484.8			453.3			—				31.5
	Other investments		153.8			—			150.2				3.6
	Total investment assets at fair value	$	7,314.10		$	659.6		$	6,549.20			$	105.3
		December 31, 2012
	(in millions)	Total			Level 1			Level 2				Level 3
	Fixed maturities:
	U.S. Treasury and government agencies	$	325.6		$	144.2		$	181.4			$	—
	Foreign government		352.9			60.9			292				—
	Municipal		1,096.30			—			1,076.90				19.4
	Corporate		3,773.40			—			3,747.00				26.4
	Residential mortgage-backed, U.S. agency backed		610.8			—			610.8				—
	Residential mortgage-backed, non-agency		194.4			—			193.7				0.7
	Commercial mortgage-backed		396.2			—			369.5				26.7
	Asset-backed		202.6			—			201.1				1.5
	Total fixed maturities		6,952.20			205.1			6,672.40				74.7
	Equity securities		306.1			226.9			54.8				24.4
	Other investments		172.8			—			169.2				3.6
	Total investment assets at fair value	$	7,431.10		$	432		$	6,896.4			$	102.7
Estimated Fair Values of Financial Instruments Not Carried at Fair Value	'
	The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value:
		September 30, 2013
	(in millions)	Total			Level 1			Level 2				Level 3
	Assets:
	Cash and cash equivalents	$	602.2		$	602.2		$	—			$	—
	Equity securities		9.3			—			9.3				—
	Other investments		12.1			—			2.9				9.2
	Liabilities:
	Debt	$	1,005.30		$	—		$	1,005.30			$	—
		December 31, 2012
	(in millions)	Total			Level 1			Level 2				Level 3
	Assets:
	Cash and cash equivalents	$	564.8		$	564.8		$	—			$	—
	Equity securities		9.7			—			9.7				—
	Other investments		16.6			—			4.8				11.8
	Liabilities:
	Debt	$	995.2		$	—		$	995.2			$	—
Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	'
	The tables below provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3).
		Fixed Maturities
	(in millions)	Municipal			Corporate			Residential				Commercial			Asset-			Total			Equity and			Total Assets
								mortgage-				mortgage-			backed						Other
								backed, non-				backed
								agency
	Three Months Ended September 30, 2013
	Balance July 1, 2013	$	27.9		$	19.5		$	0.6			$	23.6		$	—		$	71.6		$	32.3		$	103.9
	Total gains (losses):
	Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities		(0.1	)		0.1			—				—			—			—			2.8			2.8
	Sales		(0.9	)		—			(0.1	)			(0.4	)		—			(1.4	)		—			(1.4	)
	Balance September 30, 2013	$	26.9		$	19.6		$	0.5			$	23.2		$	—		$	70.2		$	35.1		$	105.3
	Three Months Ended September 30, 2012
	Balance July 1, 2012	$	16.9		$	27.4		$	0.6			$	27.4		$	1.1		$	73.4		$	29.2		$	102.6
	Transfers into Level 3		2.6			—			—				—			—			2.6			—			2.6
	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		0.3			—			—				0.6			—			0.9			(1.7	)		(0.8	)
	Purchases and sales:
	Purchases		—			—			0.1				—			—			0.1			—			0.1
	Sales		(0.2	)		(0.1	)		—				(0.6	)		—			(0.9	)		—			(0.9	)
	Balance September 30, 2012	$	19.6		$	27.4		$	0.7			$	27.4		$	1.1		$	76.2		$	27.5		$	103.7
		Fixed Maturities
	(in millions)	Municipal			Corporate			Residential				Commercial			Asset-			Total			Equity and			Total Assets
								mortgage-				mortgage-			backed						Other
								backed, non-				backed
								agency
	Nine Months Ended September 30, 2013
	Balance January 1, 2013	$	19.4		$	26.4		$	0.7			$	26.7		$	1.5		$	74.7		$	28		$	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		—			0.4			—				—			—			0.4			—			0.4
	Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities		(0.4	)		(0.9	)		—				(1.4	)		—			(2.7	)		8			5.3
	Sales		(1.8	)		(4.3	)		(0.2	)			(2.1	)		—			(8.4	)		—			(8.4	)
	Balance September 30, 2013	$	26.9		$	19.6		$	0.5			$	23.2		$	—		$	70.2		$	35.1		$	105.3
	Nine Months Ended September 30, 2012
	Balance January 1, 2012	$	13.6		$	23.8		$	5.2			$	23.7		$	9.2		$	75.5		$	27		$	102.5
	Transfers into Level 3		2.6			4.3			—				—			—			6.9			0.1			7
	Transfers out of Level 3		—			(2.1	)		—				—			(8.7	)		(10.8	)		—			(10.8	)
	Total gains (losses):
	Included in earnings		—			0.4			—				—			—			0.4			(0.2	)		0.2
	Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities		1			0.5			0.1				(0.1	)		—			1.5			1.3			2.8
	Purchases and sales:
	Purchases		3			1.5			0.1				5.2			1			10.8			—			10.8
	Sales		(0.6	)		(1.0	)		(4.7	)			(1.4	)		(0.4	)		(8.1	)		(0.7	)		(8.8	)
	Balance September 30, 2012	$	19.6		$	27.4		$	0.7			$	27.4		$	1.1		$	76.2		$	27.5		$	103.7
Schedule of Gain And Losses Due to Change in Fair Value Level 3 Assets	'
	The following table summarizes gains and losses due to changes in fair value that were recorded in net income for Level 3 assets:
		Three Months Ended			Nine Months Ended
		September 30,			September 30,
		2012			2013			2012
	(in millions)	Net Realized			Net Realized Investment
		Investment Gains			Gains (Losses)
	Level 3 Assets:
	Fixed maturities:
	Corporate	$	0.1		$	0.4		$	0.4
	Equity securities		—			—			(0.2	)
	Total assets	$	0.1		$	0.4		$	0.2
Schedule of Additional Information about Significant Unobservable Inputs Used in Fair Valuations of Level 3	'
	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.
					September 30, 2013					December 31, 2012
	(in millions)	Valuation		Significant Unobservable Inputs	Fair			Range		Fair				Range
		Technique			Value			(Wtd Average)		Value				(Wtd Average)
	Fixed maturities:
	Municipal	Discounted cash flow		Discount for:	$	26.9				$	19.4
				Small issue size				1.0-4.0% (2.3%)						1.0-4.0% (3.1%)
				Above-market coupon				0.3-1.0% (0.5%)						0.3-1.0% (0.5%)
				Long maturity				NA						0.5% (0.5%)
	Corporate	Discounted cash flow		Discount for:		19.4					26.4
				Credit stress				1.0-2.0% (1.0%)						1.0-3.0% (1.1%)
				Above-market coupon				0.3-1.0% (0.6%)						0.3-1.0% (0.7%)
				Small issue size				0.3-2.0% (0.5%)						0.3-3.0% (0.5%)
				Long maturity				NA						0.5% (0.5%)
	Residential mortgage-backed, non-agency	Discounted cash flow		Discount for:		0.5					0.7
				Small issue size				0.5% (0.5%)						0.5% (0.5%)
	Commercial mortgage-backed	Discounted cash flow		Discount for:		23.2					26.7
				Credit stress				0.5% (0.5%)						1.0% (1.0%)
				Small issue size				0.5% (0.5%)						0.5% (0.5%)
				Above-market coupon				0.3-0.8% (0.4%)						0.3-0.8% (0.4%)
				Lease structure				0.3% (0.3%)						0.3% (0.3%)
				Long maturity				NA						0.5-0.8% (0.7%)
	Asset backed	Discounted cash flow		Discount for:		—					1.5
				Small issue size				NA						0.7-2.0% (1.6%)
	Equity securities	Market comparables		Net tangible asset		31.4					24.3
				market multiples				1.1X (1.1X)						0.9X (0.9X)
	Other	Discounted cash flow		Discount rate		3.6		18.0% (18.0%)			3.6			18.0% (18.0%)

Pension_and_Other_Postretireme1

Pension and Other Postretirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2013
Components of Net Periodic Pension Cost	'
	The components of net periodic pension cost for defined benefit pension and other postretirement benefit plans included in the Company’s results of operations are as follows:
		Three Months Ended September 30,
	(in millions)	2013			2012			2013			2012
		Pension Plans						Postretirement Plans
	Service cost - benefits earned during the period	$	0.4		$	0.4		$	—		$	—
	Interest cost		8			8.8			0.2			0.6
	Expected return on plan assets		(8.9	)		(9.6	)		—			—
	Recognized net actuarial loss		3.7			3.2			0.1			0.1
	Amortization of prior service cost		—			0.1			(0.9	)		(1.0	)
	Net periodic pension cost (benefit)	$	3.2		$	2.9		$	(0.6	)	$	(0.3	)
		Nine Months Ended September 30,
	(in millions)	2013			2012			2013			2012
		Pension Plans						Postretirement Plans
	Service cost - benefits earned during the period	$	1.3		$	1.2		$	0.1		$	0.1
	Interest cost		23.9			26.4			1			1.6
	Expected return on plan assets		(26.7	)		(29.0	)		—			—
	Recognized net actuarial loss		11			9.6			0.2			0.2
	Amortization of prior service cost		—			0.1			(2.8	)		(2.9	)
	Net settlement gain		—			—			(1.6	)		—
	Net periodic pension cost (benefit)	$	9.5		$	8.3		$	(3.1	)	$	(1.0	)

Other_Comprehensive_Income_Tab

Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2013
Changes in Other Comprehensive Income	'
	The following table provides changes in other comprehensive income.
		Three Months Ended September 30,
		2013									2012
	(in millions)	Pre-Tax			Tax			Net of			Pre-Tax			Tax			Net of
					Benefit			Tax						Benefit			Tax
					(Expense)									(Expense)
	Unrealized gains (losses) on available-for-sale securities and derivative instruments:
	Unrealized gains (losses) arising during period	$	(7.1	)	$	2.2		$	(4.9	)	$	90.2		$	(31.9	)	$	58.3
	Less:
	Amount of realized gains from sales and other		3.9			2.1			6			5.9			2.1			8
	Portion of other-than-temporary impairment losses recognized in earnings		(2.1	)		0.7			(1.4	)		(2.3	)		0.8			(1.5	)
	Net unrealized gains (losses)		(8.9	)		(0.6	)		(9.5	)		86.6			(34.8	)		51.8
	Pension and postretirement benefits:
	Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost		2.9			(1.0	)		1.9			2.4			(0.8	)		1.6
	Cumulative foreign currency translation adjustment:
	Foreign currency translation recognized during the period		8.7			(3.0	)		5.7			11.6			(4.1	)		7.5
	Other comprehensive income (loss)	$	2.7		$	(4.6	)	$	(1.9	)	$	100.6		$	(39.7	)	$	60.9
		Nine Months Ended September 30,
		2013									2012
	(in millions)	Pre-Tax			Tax			Net of			Pre-Tax			Tax			Net of
					Benefit			Tax						Benefit			Tax
					(Expense)									(Expense)
	Unrealized gains (losses) on available-for-sale securities and derivative instruments:
	Unrealized gains (losses) arising during period (net of pre-tax, ceded unrealized losses of $0.8 million for the nine months ended September 30, 2013)	$	(172.1	)	$	60.1		$	(112.0	)	$	191.1		$	(35.6	)	$	155.5
	Less:
	Amount of realized gains from sales and other		27.7			4.2			31.9			13.8			5.9			19.7
	Portion of other-than-temporary impairment losses recognized in earnings		(3.7	)		1.3			(2.4	)		(6.8	)		2			(4.8	)
	Net unrealized gains (losses)		(196.1	)		54.6			(141.5	)		184.1			(43.5	)		140.6
	Pension and postretirement benefits:
	Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost		12.1			(4.2	)		7.9			7			(2.4	)		4.6
	Cumulative foreign currency translation adjustment:
	Foreign currency translation recognized during the period		(6.8	)		2.4			(4.4	)		13			(4.6	)		8.4
	Other comprehensive income (loss)	$	(190.8	)	$	52.8		$	(138.0	)	$	204.1		$	(50.5	)	$	153.6
Reclassifications Out of Accumulated Other Comprehensive Income	'
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Details about Accumulated Other Comprehensive Income	Amount Reclassified from												Affected Line Item in the Statement
	Components	Accumulated Other												Where Net Income is Presented
		Comprehensive Income
	Unrealized gains (losses) on available-for-sale securities and derivative instruments	$	3.9		$	5.9		$	27.7		$	13.9		Net realized gains from sales and other
			(2.1	)		(2.2	)		(3.7	)		(5.7	)	Net other-than-temporary impairment losses on investments recognized in earnings
	Other		—			(0.1	)		—			(0.3	)
			1.8			3.6			24			7.9		Total before tax
			2.8			2.9			5.5			7.9		Tax benefit (expense)
			4.6			6.5			29.5			15.8
			—			—			—			(0.9	)	Discontinued operations, net of tax
			4.6			6.5			29.5			14.9		Net of tax
	Amortization of defined benefit pension and postretirement plans		2.9			2.4			12.1			7		Loss adjustment expenses and other operating expenses
			(1.0	)		(0.8	)		(4.2	)		(2.4	)	Tax benefit (expense)
			1.9			1.6			7.9			4.6		Net of tax
	Total reclassifications for the period	$	6.5		$	8.1		$	37.4		$	19.5		Net of tax

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2013
Financial Information with Respect to Business Segments	'
	Summarized below is financial information with respect to the Company’s business segments.
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Operating revenues:
	Commercial Lines	$	530		$	498.9		$	1,568.40		$	1,461.90
	Personal Lines		385.1			391.6			1,165.10			1,168.80
	Chaucer		283.4			259.2			800.8			762.7
	Other		2.3			3			7.6			8.6
	Total		1,200.80			1,152.70			3,541.90			3,402.00
	Net realized investment gains		1			5			22.8			4.7
	Total revenues	$	1,201.80		$	1,157.70		$	3,564.70		$	3,406.70
	Operating income (loss) before income taxes:
	Commercial Lines:
	GAAP underwriting gain (loss)	$	2.8		$	(27.8	)	$	(9.8	)	$	(75.0	)
	Net investment income		35.3			35.6			107.4			106.3
	Other income		0.4			0.3			0.1			1.3
	Commercial Lines operating income		38.5			8.1			97.7			32.6
	Personal Lines:
	GAAP underwriting income (loss)		18			(8.4	)		27.8			(14.6	)
	Net investment income		18.6			21.6			56.7			64.5
	Other income		1.1			1.5			3.6			4.6
	Personal Lines operating income		37.7			14.7			88.1			54.5
	Chaucer:
	GAAP underwriting income		20.8			30.7			78.5			64.6
	Net investment income		10.2			10.2			31.4			29.8
	Other income		1.4			1.1			0.3			2.9
	Chaucer operating income		32.4			42			110.2			97.3
	Other:
	GAAP underwriting loss		(0.2	)		(1.1	)		(2.3	)		(2.3	)
	Net investment income		1.6			1.7			5.4			5.7
	Other net expenses		(2.8	)		(2.5	)		(8.6	)		(8.2	)
	Other operating loss		(1.4	)		(1.9	)		(5.5	)		(4.8	)
	Operating income before interest expense and income taxes		107.2			62.9			290.5			179.6
	Interest on debt		(16.6	)		(14.6	)		(48.7	)		(46.7	)
	Operating income before income taxes		90.6			48.3			241.8			132.9
	Non-operating income items:
	Net realized investment gains		1			5			22.8			4.7
	Loss on real estate		(4.7	)		—			(4.7	)		—
	Net loss from repayment of debt		—			—			(19.1	)		—
	Net benefit (costs) related to acquired businesses		(0.2	)		0.2			—			(2.2	)
	Net foreign exchange gains (losses)		(0.6	)		(0.3	)		0.1			(0.3	)
	Income before income taxes	$	86.1		$	53.2		$	240.9		$	135.1
Business Segments and Discontinued Operations Related to Identifiable Assets	'
	The following table provides identifiable assets for the Company’s business segments and discontinued operations:
	(in millions)	September 30,			December 31,
		2013			2012
		Identifiable Assets
	U.S. Companies	$	8,918.40		$	8,909.60
	Chaucer		4,424.10			4,444.80
	Discontinued operations		120.8			130.5
	Total	$	13,463.30		$	13,484.90

Stockbased_Compensation_Tables

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2013
Compensation Cost and Related Tax Benefits	'
	Compensation cost and the related tax benefits were as follows:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions)	2013			2012			2013			2012
	Stock-based compensation expense	$	3.2		$	3.5		$	8.9		$	9.8
	Tax benefit		(1.1	)		(1.2	)		(3.1	)		(3.4	)
	Stock-based compensation expense, net of taxes	$	2.1		$	2.3		$	5.8		$	6.4
Summary of Stock Option Plan Activity	'
	Information on the Company’s stock option plans is summarized below.
		Nine Months Ended September 30,
		2013						2012
	(in whole shares and dollars)	Shares			Weighted Average			Shares			Weighted Average
					Exercise Price						Exercise Price
	Outstanding, beginning of period		2,892,882		$	38.28			2,715,430		$	38.57
	Granted		537,800			42.53			517,500			36.81
	Exercised		(965,861	)		33.8			(56,874	)		29.33
	Forfeited or cancelled		(183,250	)		41.6			(90,574	)		41.84
	Expired		—			—			(159,850	)		44.05
	Outstanding, end of period		2,281,571			40.91			2,925,632			38.06
Summary of Restricted Stock Activity	'
	The following tables summarize activity information about employee restricted stock units:
		Nine Months Ended September 30,
		2013						2012
	(in whole shares and dollars)	Shares			Weighted			Shares			Weighted
					Average Grant						Average Grant
					Date Fair						Date Fair
					Value						Value
	Time-based restricted stock units:
	Outstanding, beginning of period		750,837		$	40.15			768,529		$	40.17
	Granted		139,682			43.21			172,691			36.99
	Vested		(265,209	)		39.59			(127,966	)		35.66
	Forfeited		(60,042	)		41.25			(43,105	)		40.35
	Outstanding, end of period		565,268			41.05			770,149			40.19
	Performance-based and market-based restricted stock units:
	Outstanding, beginning of period		132,775		$	39.97			69,500		$	45.37
	Granted		82,795			41.85			94,000			36.75
	Forfeited		(22,700	)		40.83			(1,125	)		41.16
	Outstanding, end of period		192,870			40.68			162,375			40.34

Earnings_Per_Share_and_Shareho1

Earnings Per Share and Shareholders' Equity Transactions (Tables)	9 Months Ended
	Sep. 30, 2013
Information Regarding Basic and Diluted Earnings Per Share	'
	The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share:
		Three Months Ended						Nine Months Ended
		September 30,						September 30,
	(in millions, except per share data)	2013			2012			2013			2012
	Basic shares used in the calculation of earnings per share		43.8			44.6			44.1			44.8
	Dilutive effect of securities:
	Employee stock options		0.4			0.2			0.3			0.2
	Non-vested stock grants		0.4			0.4			0.5			0.4
	Diluted shares used in the calculation of earnings per share		44.6			45.2			44.9			45.4
	Per share effect of dilutive securities on income (loss) from continuing operations and net income (loss)	$	(0.03	)	$	(0.02	)	$	(0.07	)	$	(0.03	)

Basis_of_Presentation_and_Prin1

Basis of Presentation and Principles of Consolidation - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Additional Paid-in Capital	'
Organization and Summary of Significant Accounting Policies Disclosure [Line Items]	'
Prior period reclassification adjustment	$34
Retained Earnings	'
Organization and Summary of Significant Accounting Policies Disclosure [Line Items]	'
Prior period reclassification adjustment	($34)

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Income Taxes [Line Items]	'	'
Federal income tax expense	$41.80	$3.40
Foreign income tax expense	18.3	30.3
Foreign income permanently reinvested	$14.70	'

Long_Term_Debt_Detail

Long Term Debt (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Debt Instrument [Line Items]	'	'
Total principal debt	$940.90	$851.90
Unamortized debt discount	-2.2	-2.5
Total	938.7	849.4
Senior debentures maturing June 15, 2021	'	'
Debt Instrument [Line Items]	'	'
Total principal debt	300	300
Senior debentures maturing March 1, 2020	'	'
Debt Instrument [Line Items]	'	'
Total principal debt	200	200
Senior debentures maturing October 15, 2025	'	'
Debt Instrument [Line Items]	'	'
Total principal debt	81.2	120.9
Subordinated debentures maturing March 30, 2053	'	'
Debt Instrument [Line Items]	'	'
Total principal debt	175	'
Subordinated debentures maturing February 3, 2027	'	'
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	$171.30

Debt_Additional_Information_De

Debt - Additional Information (Detail) (USD $)	9 Months Ended		3 Months Ended	1 Months Ended		9 Months Ended	12 Months Ended		1 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Jun. 30, 2013	Mar. 20, 2013	Jan. 31, 2013	Sep. 30, 2013	Dec. 31, 2009	Jul. 31, 2010	Jan. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
			Senior debentures maturing October 15, 2025	Subordinated debentures (unsecured) maturing March 30, 2053	Federal Home Loan Bank of Boston	Federal Home Loan Bank of Boston	Federal Home Loan Bank of Boston	Federal Home Loan Bank of Boston	Collateralized Borrowing Program	Collateralized Borrowing Program	Collateralized Borrowing Program
Debt Instrument [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Repurchase of senior debentures	'	'	$39.20	'	'	'	'	'	'	'	'
Maturity date	'	'	15-Oct-25	30-Mar-53	'	30-Jul-20	'	'	'	'	'
Repurchase of senior debentures at cost	'	'	50.5	'	'	'	'	'	'	'	'
Gain (loss) on the repurchase of debt	-19.1	'	-11.3	'	'	'	'	'	'	'	'
Aggregate principal amount	'	'	'	175	'	'	'	'	'	'	'
Debt instrument interest rate	'	'	'	6.35%	'	'	5.50%	'	'	'	'
Debt Instrument Redemption Date	'	'	'	30-Mar-18	'	'	'	'	'	'	'
Required Amount of Outstanding Principal Amount of Debentures after redemption in part	'	'	'	25	'	'	'	'	'	'	'
Advance received	'	'	'	'	'	'	125	'	'	'	'
Length of debt instrument	'	'	'	'	'	'	'20 years	'	'	'	'
Securities borrowed	'	'	'	'	'	'	'	46.3	'	'	'
Weighted average fixed interest rate	'	'	'	'	'	3.88%	'	'	'	'	'
Repayment of FHLBB community development advances	'	'	'	'	46.3	'	'	'	46.3	'	'
Prepayment fees on FHLBB borrowings	'	'	'	'	7.8	'	'	'	'	'	'
Total payments of FHLBB borrowings	'	'	'	'	54.1	'	'	'	'	'	'
Securities held as collateral, at fair value	'	'	'	'	'	'	'	'	'	145.2	200.8
Aggregate amount of borrowings	'	'	'	'	'	'	'	'	'	125	171.3
Total holdings of FHLBB stock	$9.30	$9.70	'	'	'	'	'	'	'	'	'

Amortized_Cost_and_Fair_Value_

Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities and Cost and Fair Value of Equity Securities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	$6,473.30	$6,529.50
Fixed maturities, Gross Unrealized Gains	274.6	442.7
Fixed maturities, Gross Unrealized Losses	72.4	20
Fixed maturities, Fair Value	6,675.50	6,952.20
Fixed maturities, OTTI Unrealized Losses	10.3	11
Equity securities, Amortized Cost or Cost	449	299
Equity securities, Gross Unrealized Gains	50.5	21.6
Equity securities, Gross Unrealized Losses	5.4	4.8
Equity securities, Fair Value	494.1	315.8
Equity securities, OTTI Unrealized Losses	'	'
U.S. Treasury and government agencies	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	376.3	317.2
Fixed maturities, Gross Unrealized Gains	4.6	8.8
Fixed maturities, Gross Unrealized Losses	10.4	0.4
Fixed maturities, Fair Value	370.5	325.6
Foreign government	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	286.6	348.5
Fixed maturities, Gross Unrealized Gains	2.6	4.6
Fixed maturities, Gross Unrealized Losses	0.8	0.2
Fixed maturities, Fair Value	288.4	352.9
Municipal	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	1,062	1,010.20
Fixed maturities, Gross Unrealized Gains	47.4	87.2
Fixed maturities, Gross Unrealized Losses	12.5	1.1
Fixed maturities, Fair Value	1,096.90	1,096.30
Corporate	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	3,532.70	3,512.80
Fixed maturities, Gross Unrealized Gains	183.6	275.4
Fixed maturities, Gross Unrealized Losses	34.4	14.8
Fixed maturities, Fair Value	3,681.90	3,773.40
Fixed maturities, OTTI Unrealized Losses	8.6	9.3
Residential mortgage-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	694	769
Fixed maturities, Gross Unrealized Gains	22.7	39.4
Fixed maturities, Gross Unrealized Losses	9.6	3.2
Fixed maturities, Fair Value	707.1	805.2
Fixed maturities, OTTI Unrealized Losses	1.7	1.7
Commercial mortgage-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	349.1	373.3
Fixed maturities, Gross Unrealized Gains	11.2	23.2
Fixed maturities, Gross Unrealized Losses	4.4	0.3
Fixed maturities, Fair Value	355.9	396.2
Asset-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities, Amortized Cost or Cost	172.6	198.5
Fixed maturities, Gross Unrealized Gains	2.5	4.1
Fixed maturities, Gross Unrealized Losses	0.3	'
Fixed maturities, Fair Value	$174.80	$202.60

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Investments [Line Items]	'	'	'	'	'
Net unrealized gains on impaired securities	'	'	$16.40	'	$20.50
Other-than-temporary impairments	2.5	1.4	4.5	5.2	'
Net other-than-temporary impairment losses on securities recognized in earnings	2.1	2.2	3.7	5.7	'
Amount transferred from unrealized losses in accumulated other comprehensive income	0.4	0.8	0.8	0.5	'
Restricted assets, fixed maturities	341	'	341	'	'
Restricted assets, cash and cash equivalents	$28	'	$28	'	'

Amortized_Cost_and_Fair_Value_1

Amortized Cost and Fair Value by Maturity Periods for Fixed Maturities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Investments [Line Items]	'	'
Due in one year or less, Amortized Cost	$444.70	'
Due after one year through five years, Amortized Cost	2,216.90	'
Due after five years through ten years, Amortized Cost	1,904.40	'
Due after ten years, Amortized Cost	691.6	'
Total fixed maturities, Amortized Cost	5,257.60	'
Mortgage-backed and asset-backed securities, Amortized Cost	1,215.70	'
Total fixed maturities, Amortized Cost	6,473.30	6,529.50
Due in one year or less, Fair Value	450.6	'
Due after one year through five years, Fair Value	2,321.50	'
Due after five years through ten years, Fair Value	1,959	'
Due after ten years, Fair Value	706.6	'
Total fixed maturities, Fair Value	5,437.70	'
Mortgage-backed and asset-backed securities, Fair Value	1,237.80	'
Fixed maturities, Fair Value	$6,675.50	$6,952.20

Fixed_Maturities_and_Equity_Se

Fixed Maturities and Equity Securities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	$62.90	$9.40
12 months or less, Fair Value	1,792	648.6
Greater than 12 months, Gross Unrealized Losses	14.9	15.4
Greater than 12 months, Fair Value	120.7	161.6
Total, Gross Unrealized Losses	77.8	24.8
Total, Fair Value	1,912.70	810.2
Fixed Maturities	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	59.3	4.6
12 months or less, Fair Value	1,663.30	574.2
Greater than 12 months, Gross Unrealized Losses	13.1	15.4
Greater than 12 months, Fair Value	116.8	161.6
Total, Gross Unrealized Losses	72.4	20
Total, Fair Value	1,780.10	735.8
Fixed Maturities | Investment grade	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	54.8	3.4
12 months or less, Fair Value	1,568.40	546
Greater than 12 months, Gross Unrealized Losses	9.6	9.5
Greater than 12 months, Fair Value	87.4	106.5
Total, Gross Unrealized Losses	64.4	12.9
Total, Fair Value	1,655.80	652.5
Fixed Maturities | Investment grade | U.S. Treasury And Government Agencies	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	10.3	0.2
12 months or less, Fair Value	178.4	89.5
Greater than 12 months, Gross Unrealized Losses	0.1	0.2
Greater than 12 months, Fair Value	4.6	8.5
Total, Gross Unrealized Losses	10.4	0.4
Total, Fair Value	183	98
Fixed Maturities | Investment grade | Foreign government	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	0.7	0.2
12 months or less, Fair Value	73.1	81.2
Greater than 12 months, Gross Unrealized Losses	0.1	'
Greater than 12 months, Fair Value	15.5	0.4
Total, Gross Unrealized Losses	0.8	0.2
Total, Fair Value	88.6	81.6
Fixed Maturities | Investment grade | Municipal	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	11.5	0.5
12 months or less, Fair Value	277.1	61.9
Greater than 12 months, Gross Unrealized Losses	1	0.6
Greater than 12 months, Fair Value	17.8	24
Total, Gross Unrealized Losses	12.5	1.1
Total, Fair Value	294.9	85.9
Fixed Maturities | Investment grade | Corporate	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	20.1	1.8
12 months or less, Fair Value	666.2	224.8
Greater than 12 months, Gross Unrealized Losses	6.8	6.6
Greater than 12 months, Fair Value	37.5	59
Total, Gross Unrealized Losses	26.9	8.4
Total, Fair Value	703.7	283.8
Fixed Maturities | Investment grade | Residential mortgage-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	7.5	0.5
12 months or less, Fair Value	218.5	47.3
Greater than 12 months, Gross Unrealized Losses	1.6	2
Greater than 12 months, Fair Value	10.9	9.4
Total, Gross Unrealized Losses	9.1	2.5
Total, Fair Value	229.4	56.7
Fixed Maturities | Investment grade | Commercial mortgage-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	4.4	0.2
12 months or less, Fair Value	113.8	29.9
Greater than 12 months, Gross Unrealized Losses	'	0.1
Greater than 12 months, Fair Value	0.9	4.9
Total, Gross Unrealized Losses	4.4	0.3
Total, Fair Value	114.7	34.8
Fixed Maturities | Investment grade | Asset-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	0.3	'
12 months or less, Fair Value	41.3	11.4
Greater than 12 months, Fair Value	0.2	0.3
Total, Gross Unrealized Losses	0.3	'
Total, Fair Value	41.5	11.7
Fixed Maturities | Below Investment Grade	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	4.5	1.2
12 months or less, Fair Value	94.9	28.2
Greater than 12 months, Gross Unrealized Losses	3.5	5.9
Greater than 12 months, Fair Value	29.4	55.1
Total, Gross Unrealized Losses	8	7.1
Total, Fair Value	124.3	83.3
Fixed Maturities | Below Investment Grade | Municipal	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Greater than 12 months, Fair Value	'	2
Total, Fair Value	'	2
Fixed Maturities | Below Investment Grade | Corporate	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	4.4	1.1
12 months or less, Fair Value	91.9	26.6
Greater than 12 months, Gross Unrealized Losses	3.1	5.3
Greater than 12 months, Fair Value	27.8	50.6
Total, Gross Unrealized Losses	7.5	6.4
Total, Fair Value	119.7	77.2
Fixed Maturities | Below Investment Grade | Residential mortgage-backed	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	0.1	0.1
12 months or less, Fair Value	3	1.6
Greater than 12 months, Gross Unrealized Losses	0.4	0.6
Greater than 12 months, Fair Value	1.6	2.5
Total, Gross Unrealized Losses	0.5	0.7
Total, Fair Value	4.6	4.1
Equity Securities	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
12 months or less, Gross Unrealized Losses	3.6	4.8
12 months or less, Fair Value	128.7	74.4
Greater than 12 months, Gross Unrealized Losses	1.8	'
Greater than 12 months, Fair Value	3.9	'
Total, Gross Unrealized Losses	5.4	4.8
Total, Fair Value	$132.60	$74.40

Proceeds_from_Sales_of_Availab

Proceeds from Sales of Available for Sale Securities (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fixed Maturities	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Proceeds from Sales	$83.30	$78.80	$369	$486.80
Gross Gains	1	2.3	5.6	8.9
Gross Losses	0.9	0.3	1.9	1.9
Equity Securities	'	'	'	'
Gain (Loss) on Investments [Line Items]	'	'	'	'
Proceeds from Sales	10.8	13.6	89.7	17.1
Gross Gains	1.5	2.4	18.5	2.9
Gross Losses	'	'	$0.30	$0.20

Rollforward_of_Cumulative_Amou

Rollforward of Cumulative Amounts Related to Credit Loss Portion of Other than Temporary Impairments Losses (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investments [Line Items]	'	'	'	'
Credit losses at beginning of period	$7.30	$13.80	$8.60	$14.50
Credit losses for which an OTTI was not previously recognized	0.1	0.1	0.4	0.5
Additional credit losses on securities for which an OTTI was previously recognized	'	'	0.2	0.6
Reductions for securities sold, matured or called	-0.1	-2.5	-1.9	-4.2
Reductions for securities reclassified as intended to sell	'	-0.2	'	-0.2
Credit losses at the end of period	$7.30	$11.20	$7.30	$11.20

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'	'
Cash and cash equivalents	$602.20	$564.80	$462	$820.40
Fixed maturities	6,675.50	6,952.20	'	'
Equity securities	494.1	315.8	'	'
Other investments	185.3	210.3	'	'
Debt	938.7	849.4	'	'
Carrying Value	'	'	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'	'
Cash and cash equivalents	602.2	564.8	'	'
Fixed maturities	6,675.50	6,952.20	'	'
Equity securities	494.1	315.8	'	'
Other investments	165.5	188.9	'	'
Total financial assets	7,937.30	8,021.70	'	'
Debt	938.7	849.4	'	'
Fair Value	'	'	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'	'	'
Cash and cash equivalents	602.2	564.8	'	'
Fixed maturities	6,675.50	6,952.20	'	'
Equity securities	494.1	315.8	'	'
Other investments	165.9	189.4	'	'
Total financial assets	7,937.70	8,022.20	'	'
Debt	$1,005.30	$995.20	'	'

Assets_and_Liabilities_Measure

Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	$6,675.50	$6,952.20
Equity securities	494.1	315.8
Other investments	185.3	210.3
Fair Value, Measurements, Recurring	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Other investments	153.8	172.8
Total investment assets at fair value	7,314.10	7,431.10
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Total investment assets at fair value	659.6	432
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Other investments	150.2	169.2
Total investment assets at fair value	6,549.20	6,896.40
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Other investments	3.6	3.6
Total investment assets at fair value	105.3	102.7
Fair Value, Measurements, Recurring | Fixed Maturities	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	6,675.50	6,952.20
Fair Value, Measurements, Recurring | Fixed Maturities | Fair Value, Inputs, Level 1	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	206.3	205.1
Fair Value, Measurements, Recurring | Fixed Maturities | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	6,399	6,672.40
Fair Value, Measurements, Recurring | Fixed Maturities | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	70.2	74.7
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	370.5	325.6
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies | Fair Value, Inputs, Level 1	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	167.1	144.2
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	203.4	181.4
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	288.4	352.9
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government | Fair Value, Inputs, Level 1	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	39.2	60.9
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign government | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	249.2	292
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	1,096.90	1,096.30
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	1,070	1,076.90
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	26.9	19.4
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	3,681.90	3,773.40
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	3,662.30	3,747
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	19.6	26.4
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	541.5	610.8
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	541.5	610.8
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	165.6	194.4
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	165.1	193.7
Fair Value, Measurements, Recurring | Fixed Maturities | Residential mortgage-backed | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	0.5	0.7
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	355.9	396.2
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	332.7	369.5
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial mortgage-backed | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	23.2	26.7
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	174.8	202.6
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	174.8	201.1
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Fixed maturities	'	1.5
Fair Value, Measurements, Recurring | Equity Securities	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Equity securities	484.8	306.1
Fair Value, Measurements, Recurring | Equity Securities | Fair Value, Inputs, Level 1	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Equity securities	453.3	226.9
Fair Value, Measurements, Recurring | Equity Securities | Fair Value, Inputs, Level 2	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Equity securities	'	54.8
Fair Value, Measurements, Recurring | Equity Securities | Fair Value, Inputs, Level 3	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Equity securities	$31.50	$24.40

Estimated_Fair_Values_of_Finan

Estimated Fair Values of Financial Instruments Not Carried at Fair Value (Detail) (Estimate of Fair Value, Fair Value Disclosure, USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Cash and cash equivalents	$602.20	$564.80
Equity securities	9.3	9.7
Other investments	12.1	16.6
Debt	1,005.30	995.2
Fair Value, Inputs, Level 1	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Cash and cash equivalents	602.2	564.8
Fair Value, Inputs, Level 2	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Equity securities	9.3	9.7
Other investments	2.9	4.8
Debt	1,005.30	995.2
Fair Value, Inputs, Level 3	'	'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'	'
Other investments	$9.20	$11.80

Reconciliation_for_Assets_Meas

Reconciliation for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	$103.90	$102.60	$102.70	$102.50
Transfers into Level 3	'	2.6	9.9	7
Transfers out of Level 3	'	'	-4.6	-10.8
Included in earnings	'	0.1	0.4	0.2
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	2.8	-0.8	5.3	2.8
Purchases	'	0.1	'	10.8
Sales	-1.4	-0.9	-8.4	-8.8
Ending Balance	105.3	103.7	105.3	103.7
Fixed Maturities	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	71.6	73.4	74.7	75.5
Transfers into Level 3	'	2.6	9.9	6.9
Transfers out of Level 3	'	'	-3.7	-10.8
Included in earnings	'	0.1	0.4	0.4
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	'	0.9	-2.7	1.5
Purchases	'	0.1	'	10.8
Sales	-1.4	-0.9	-8.4	-8.1
Ending Balance	70.2	76.2	70.2	76.2
Fixed Maturities | Municipal	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	27.9	16.9	19.4	13.6
Transfers into Level 3	'	2.6	9.7	2.6
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	-0.1	0.3	-0.4	1
Purchases	'	'	'	3
Sales	-0.9	-0.2	-1.8	-0.6
Ending Balance	26.9	19.6	26.9	19.6
Fixed Maturities | Corporate	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	19.5	27.4	26.4	23.8
Transfers into Level 3	'	'	0.2	4.3
Transfers out of Level 3	'	'	-2.2	-2.1
Included in earnings	'	0.1	0.4	0.4
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	0.1	'	-0.9	0.5
Purchases	'	'	'	1.5
Sales	'	-0.1	-4.3	-1
Ending Balance	19.6	27.4	19.6	27.4
Fixed Maturities | Residential mortgage-backed	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	0.6	0.6	0.7	5.2
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	'	'	'	0.1
Purchases	'	0.1	'	0.1
Sales	-0.1	'	-0.2	-4.7
Ending Balance	0.5	0.7	0.5	0.7
Fixed Maturities | Commercial mortgage-backed	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	23.6	27.4	26.7	23.7
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	'	0.6	-1.4	-0.1
Purchases	'	'	'	5.2
Sales	-0.4	-0.6	-2.1	-1.4
Ending Balance	23.2	27.4	23.2	27.4
Fixed Maturities | Asset-backed	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	'	1.1	1.5	9.2
Transfers into Level 3	'	'	'	'
Transfers out of Level 3	'	'	-1.5	-8.7
Purchases	'	'	'	1
Sales	'	'	'	-0.4
Ending Balance	'	1.1	'	1.1
Equity Securities and Other Investments	'	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'	'
Beginning Balance	32.3	29.2	28	27
Transfers into Level 3	'	'	'	0.1
Transfers out of Level 3	'	'	-0.9	'
Included in earnings	'	'	'	-0.2
Included in other comprehensive income -net appreciation (depreciation) on available-for-sale securities	2.8	-1.7	8	1.3
Sales	'	'	'	-0.7
Ending Balance	$35.10	$27.50	$35.10	$27.50

Summary_of_Gains_and_Losses_Du

Summary of Gains and Losses Due to Changes in Fair Value in Net Income for Level Three Assets (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Net realized investment gains (losses)	($0.10)	($0.40)	($0.20)
Fair Value, Inputs, Level 3	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Net realized investment gains (losses)	0.1	0.4	0.2
Fair Value, Inputs, Level 3 | Fixed Maturities | Corporate	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Net realized investment gains (losses)	0.1	0.4	0.4
Fair Value, Inputs, Level 3 | Equity Securities	'	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'	'
Net realized investment gains (losses)	'	'	($0.20)

Schedule_of_Additional_Informa

Schedule of Additional Information about Significant Unobservable Inputs Used in Fair Valuations of Level Three (Detail) (USD $)

Sep. 30, 2013

Jun. 30, 2013

Dec. 31, 2012

Sep. 30, 2012

Jun. 30, 2012

Dec. 31, 2011

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

Sep. 30, 2013

Dec. 31, 2012

In Millions, unless otherwise specified

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Fixed Maturities

Other securities

Other securities

Other securities

Other securities

Equity Securities

Equity Securities

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Municipal

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Corporate

Residential mortgage-backed

Residential mortgage-backed

Residential mortgage-backed

Residential mortgage-backed

Residential mortgage-backed

Residential mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Commercial mortgage-backed

Asset-backed

Asset-backed

Asset-backed

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Long Maturity

Discount For Long Maturity

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Long Maturity

Discount For Long Maturity

Discount For Credit Stress

Discount For Credit Stress

Discount For Credit Stress

Discount For Credit Stress

Discount For Credit Stress

Discount For Credit Stress

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Above Market Coupon

Discount For Long Maturity

Discount For Long Maturity

Discount For Long Maturity

Discount For Credit Stress

Discount For Credit Stress

Discount For Credit Stress

Discount For Credit Stress

Discount For Lease Structure

Discount For Lease Structure

Discount For Lease Structure

Discount For Lease Structure

Discount For Small Issue Size

Discount For Small Issue Size

Discount For Small Issue Size

Minimum

Minimum

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Discounted Cash Flow

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Minimum

Maximum

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Fair Value, Inputs, Level 3

Minimum

Minimum

Maximum

Maximum

Minimum

Minimum

Maximum

Maximum

Minimum

Minimum

Minimum

Maximum

Maximum

Minimum

Minimum

Maximum

Maximum

Minimum

Minimum

Minimum

Maximum

Maximum

Minimum

Minimum

Minimum

Minimum

Minimum

Minimum

Maximum

Maximum

Minimum

Minimum

Minimum

Minimum

Minimum

Maximum

Fair Value [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Fair value measurement market multiples

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.1

0.9

Fair value measurement weighted average market multiples

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.1

0.9

Fair Value

$105.30

$103.90

$102.70

$103.70

$102.60

$102.50

$31.40

$24.30

$26.90

$19.40

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$19.40

$26.40

'

'

'

'

$0.50

$0.70

'

'

'

'

$23.20

$26.70

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$1.50

'

'

$3.60

$3.60

'

'

'

'

Weighted average rate

'

'

'

'

'

'

'

'

2.30%

3.10%

1.00%

1.00%

4.00%

4.00%

0.50%

0.50%

0.30%

0.30%

1.00%

1.00%

0.50%

0.50%

0.50%

0.50%

0.30%

0.30%

2.00%

3.00%

0.60%

0.70%

0.30%

0.30%

1.00%

1.00%

0.50%

0.50%

1.00%

1.10%

1.00%

1.00%

2.00%

3.00%

'

'

0.50%

0.50%

0.50%

0.50%

'

'

0.50%

0.50%

0.50%

0.50%

0.40%

0.40%

0.30%

0.30%

0.80%

0.80%

0.70%

0.50%

0.80%

0.50%

1.00%

0.50%

1.00%

0.30%

0.30%

0.30%

0.30%

1.60%

0.70%

2.00%

18.00%

18.00%

18.00%

18.00%

'

'

Components_of_Net_Periodic_Ben

Components of Net Periodic Benefit Cost (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Benefits	'	'	'	'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'	'	'	'
Service cost - benefits earned during the period	$0.40	$0.40	$1.30	$1.20
Interest cost	8	8.8	23.9	26.4
Expected return on plan assets	-8.9	-9.6	-26.7	-29
Recognized net actuarial loss	3.7	3.2	11	9.6
Amortization of prior service cost	'	0.1	'	0.1
Net periodic pension cost (benefit)	3.2	2.9	9.5	8.3
Postretirement Benefits	'	'	'	'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'	'	'	'
Service cost - benefits earned during the period	'	'	0.1	0.1
Interest cost	0.2	0.6	1	1.6
Recognized net actuarial loss	0.1	0.1	0.2	0.2
Amortization of prior service cost	-0.9	-1	-2.8	-2.9
Net settlement gain	'	'	-1.6	'
Net periodic pension cost (benefit)	($0.60)	($0.30)	($3.10)	($1)

Changes_in_Other_Comprehensive

Changes in Other Comprehensive Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'
Unrealized gains arising during period, Pre-tax	($7.10)	$90.20	($172.10)	$191.10
Amount of realized gains from sales and other, Pre-tax	3.9	5.9	27.7	13.8
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	-2.1	-2.3	-3.7	-6.8
Net unrealized gains (losses), Pre-tax	-8.9	86.6	-196.1	184.1
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Pre-tax	2.9	2.4	12.1	7
Foreign currency translation recognized during the period, Pre-tax	8.7	11.6	-6.8	13
Other comprehensive income (loss), Pre-tax	2.7	100.6	-190.8	204.1
Unrealized gains arising during period, Tax Benefit (Expense)	2.2	-31.9	60.1	-35.6
Amount of realized gains from sales and other, Tax Benefit (Expense)	2.1	2.1	4.2	5.9
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	0.7	0.8	1.3	2
Net unrealized gains (losses), Tax Benefit (Expense)	-0.6	-34.8	54.6	-43.5
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Tax Benefit (Expense)	-1	-0.8	-4.2	-2.4
Foreign currency translation recognized during the period, Tax Benefit (Expense)	-3	-4.1	2.4	-4.6
Other comprehensive income (loss), Tax Benefit (Expense)	-4.6	-39.7	52.8	-50.5
Unrealized gains arising during period, Net of Tax	-4.9	58.3	-112	155.5
Amount of realized gains from sales and other, Net of Tax	6	8	31.9	19.7
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	-1.4	-1.5	-2.4	-4.8
Net unrealized gains (losses), Net of Tax	-9.5	51.8	-141.5	140.6
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Net of Tax	1.9	1.6	7.9	4.6
Foreign currency translation recognized during the period, Net of Tax	5.7	7.5	-4.4	8.4
Other comprehensive income (loss), Net of Tax	($1.90)	$60.90	($138)	$153.60

Changes_in_Other_Comprehensive1

Changes in Other Comprehensive Income (Parenthetical) (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]	'
Ceded unrealized losses, net of pre-tax	$0.80

Reclassifications_out_of_Accum

Reclassifications out of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]	'	'	'	'
Net realized gains from sales and other	$3.10	$7.20	$26.50	$10.40
Total losses and expenses	-1,115.70	-1,104.50	-3,323.80	-3,271.60
Net other-than-temporary impairment losses on investments recognized in earnings	-2.1	-2.2	-3.7	-5.7
Income before income taxes	86.1	53.2	240.9	135.1
Tax benefit (expense)	-24.8	-12.3	-60.1	-33.7
Income from continuing operations	61.3	40.9	180.8	101.4
Discontinued operations, net of tax	'	-0.5	0.1	9.5
Net income	61.3	40.4	180.9	110.9
Reclassification out of Accumulated Other Comprehensive Income	'	'	'	'
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]	'	'	'	'
Net income	6.5	8.1	37.4	19.5
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)	'	'	'	'
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]	'	'	'	'
Net realized gains from sales and other	3.9	5.9	27.7	13.9
Net other-than-temporary impairment losses on investments recognized in earnings	-2.1	-2.2	-3.7	-5.7
Other	'	-0.1	'	-0.3
Income before income taxes	1.8	3.6	24	7.9
Tax benefit (expense)	2.8	2.9	5.5	7.9
Income from continuing operations	4.6	6.5	29.5	15.8
Discontinued operations, net of tax	'	'	'	-0.9
Net income	4.6	6.5	29.5	14.9
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment	'	'	'	'
Other Comprehensive Income Loss Reclassification Adjustments [Line Items]	'	'	'	'
Total losses and expenses	2.9	2.4	12.1	7
Tax benefit (expense)	-1	-0.8	-4.2	-2.4
Net income	$1.90	$1.60	$7.90	$4.60

Other_Comprehensive_Income_Add

Other Comprehensive Income - Additional Information (Detail) (Pension And Postretirement Benefit Costs)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension And Postretirement Benefit Costs	'	'	'	'
Comprehensive Income (Loss) [Line Items]	'	'	'	'
Loss Adjustment Expense	40.00%	40.00%	40.00%	40.00%
Percentage Of Other Operating Expenses	60.00%	60.00%	60.00%	60.00%

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
			Segment
Segment Reporting Information [Line Items]	'	'	'	'
Operating segments	'	'	4	'
Net foreign currency transaction gains	$7.50	'	'	$3
Net foreign currency transaction losses	'	1	0.2	'
Loss on real estate	$4.70	'	$4.70	'

Financial_Information_with_Res

Financial Information with Respect to Business Segments (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]	'	'	'	'
Net realized investment gains	$1	$5	$22.80	$4.70
Total revenues	1,201.80	1,157.70	3,564.70	3,406.70
Operating income before interest expense and income taxes	107.2	62.9	290.5	179.6
Interest on debt	-16.6	-14.6	-48.7	-46.7
Operating income before income taxes	90.6	48.3	241.8	132.9
Net realized investment gains	1	5	22.8	4.7
Loss on real estate	-4.7	'	-4.7	'
Net loss from repayment of debt	'	'	-19.1	'
Net benefit (costs) related to acquired businesses	-0.2	0.2	'	-2.2
Net foreign exchange gains (losses)	-0.6	-0.3	0.1	-0.3
Income before income taxes	86.1	53.2	240.9	135.1
Net investment income	65.7	69.2	200.9	206.5
Commercial Lines	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating income before interest expense and income taxes	38.5	8.1	97.7	32.6
GAAP underwriting income (loss)	2.8	-27.8	-9.8	-75
Net investment income	35.3	35.6	107.4	106.3
Other income	0.4	0.3	0.1	1.3
Personal Lines	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating income before interest expense and income taxes	37.7	14.7	88.1	54.5
GAAP underwriting income (loss)	18	-8.4	27.8	-14.6
Net investment income	18.6	21.6	56.7	64.5
Other income	1.1	1.5	3.6	4.6
Chaucer	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating income before interest expense and income taxes	32.4	42	110.2	97.3
GAAP underwriting income (loss)	20.8	30.7	78.5	64.6
Net investment income	10.2	10.2	31.4	29.8
Other income	1.4	1.1	0.3	2.9
Other	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating income before interest expense and income taxes	-1.4	-1.9	-5.5	-4.8
GAAP underwriting income (loss)	-0.2	-1.1	-2.3	-2.3
Net investment income	1.6	1.7	5.4	5.7
Other net expenses	-2.8	-2.5	-8.6	-8.2
Earned Premiums	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating revenues	1,200.80	1,152.70	3,541.90	3,402
Earned Premiums | Commercial Lines	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating revenues	530	498.9	1,568.40	1,461.90
Earned Premiums | Personal Lines	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating revenues	385.1	391.6	1,165.10	1,168.80
Earned Premiums | Chaucer	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating revenues	283.4	259.2	800.8	762.7
Earned Premiums | Other	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Operating revenues	$2.30	$3	$7.60	$8.60

Identifiable_Assets_for_Busine

Identifiable Assets for Business Segments and Discontinued Operations (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Segment Reporting Information [Line Items]	'	'
Discontinued operations	$120.80	$130.50
Identifiable assets	13,463.30	13,484.90
U.S. Companies	'	'
Segment Reporting Information [Line Items]	'	'
Identifiable assets	8,918.40	8,909.60
Chaucer	'	'
Segment Reporting Information [Line Items]	'	'
Identifiable assets	$4,424.10	$4,444.80

Compensation_Cost_and_Related_

Compensation Cost and Related Tax Benefits (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'	'	'
Stock-based compensation expense	$3.20	$3.50	$8.90	$9.80
Tax benefit	-1.1	-1.2	-3.1	-3.4
Stock-based compensation expense, net of taxes	$2.10	$2.30	$5.80	$6.40

Summary_of_Stock_Option_Plan_A

Summary of Stock Option Plan Activity (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'	'
Outstanding, beginning of period, Shares	2,892,882	2,715,430
Granted, Shares	537,800	517,500
Exercised, Shares	-965,861	-56,874
Forfeited or cancelled, Shares	-183,250	-90,574
Expired, Shares	'	-159,850
Outstanding, end of period, Shares	2,281,571	2,925,632
Outstanding, beginning of period, Weighted Average Exercise Price	$38.28	$38.57
Granted, Weighted Average Exercise Price	$42.53	$36.81
Exercised, Weighted Average Exercise Price	$33.80	$29.33
Forfeited or cancelled, Weighted Average Exercise Price	$41.60	$41.84
Expired, Weighted Average Exercise Price	'	$44.05
Outstanding, end of period, Weighted Average Exercise Price	$40.91	$38.06

Summary_of_Restricted_Stock_Ac

Summary of Restricted Stock Activity (Detail) (USD $)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Time-Based Restricted Stock Units	'	'
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]	'	'
Outstanding, beginning of period, Shares	750,837	768,529
Granted, Shares	139,682	172,691
Vested, Shares	-265,209	-127,966
Forfeited, Shares	-60,042	-43,105
Outstanding, end of period, Shares	565,268	770,149
Performance And Market-Based Restricted Stock Units	'	'
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]	'	'
Outstanding, beginning of period, Shares	132,775	69,500
Granted, Shares	82,795	94,000
Forfeited, Shares	-22,700	-1,125
Outstanding, end of period, Shares	192,870	162,375
Weighted Average Grant Date Fair Value	'	'
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]	'	'
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	40.15	40.17
Granted, Weighted Average Grant Date Fair Value	43.21	36.99
Vested, Weighted Average Grant Date Fair Value	39.59	35.66
Forfeited, Weighted Average Grant Date Fair Value	41.25	40.35
Outstanding, end of period, Weighted Average Grant Date Fair Value	41.05	40.19
Performance-Based Weighted Average Grant Date Fair Value	'	'
Schedule of Share based Compensation Arrangements by Share based Payment Award, Equity Instruments, Other Than Options, Restricted Stock Units [Line Items]	'	'
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	39.97	45.37
Granted, Weighted Average Grant Date Fair Value	41.85	36.75
Forfeited, Weighted Average Grant Date Fair Value	40.83	41.16
Outstanding, end of period, Weighted Average Grant Date Fair Value	40.68	40.34

Stock_Based_Compensation_Addit

Stock Based Compensation - Additional Information (Detail)	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013
			Performance And Market-Based Restricted Stock Units	Performance And Market-Based Restricted Stock Units	Performance And Market-Based Restricted Stock Units	Performance And Market-Based Restricted Stock Units	Performance And Market-Based Restricted Stock Units	Performance-Based Shares
						Senior management	Senior management
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%
Average period based on relative TSR	'3 years	'	'	'	'	'	'	'
Market-based awards granted to senior management	'	'	'	82,795	94,000	76,175	90,250	'
Performance metric, minimum	'	'	0.00%	'	'	'	'	'
Performance metric, maximum	'	'	150.00%	'	'	'	'	'

Information_Regarding_Basic_an

Information Regarding Basic and Diluted Earnings Per Share (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'
Basic shares used in the calculation of earnings per share	43.8	44.6	44.1	44.8
Diluted shares used in the calculation of earnings per share	44.6	45.2	44.9	45.4
Employee Stock Option	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'
Dilutive effect of securities	0.4	0.2	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	0.4	0.5	0.4
Segment, Continuing Operations	'	'	'	'
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]	'	'	'	'
Per share effect of dilutive securities on income (loss) from continuing operations and net income (loss)	-0.03	-0.02	-0.07	-0.03

Earnings_Per_Share_and_Shareho2

Earnings Per Share and Shareholders Equity Transactions - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	9 Months Ended		72 Months Ended
In Millions, unless otherwise specified	31-May-13	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]	'	'	'	'	'
Antidilutive effect on earnings per share	'	2.1	0.7	1.8	'
Repurchases common stock, authorized	$100	'	'	'	$600
Repurchases common stock, available for repurchases	'	'	137	'	'
Repurchases common stock, shares	'	'	1.6	'	'
Repurchases common stock, value	'	'	$78.20	'	'

Commitments_and_Contingencies_

Commitments and Contingencies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2013	Dec. 11, 2009
	Y	Plaintiff
Loss Contingencies [Line Items]	'	'
Participant's retirement age	65	'
Treasury rate, years	'30 years	'
Named plaintiffs	'	2