State Auto Financial (STFC) 10-Q 6 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Nov. 01, 2013
Document Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Entity Registrant Name	'State Auto Financial CORP	'
Entity Central Index Key	'0000874977	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Accelerated Filer	'
Entity Common Stock, Shares Outstanding (shares)	'	40,663,415

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Assets	'	'
Fixed maturities, available-for-sale, at fair value (amortized cost $1,802.1 and $1,776.2, respectively)	$1,844.80	$1,905.10
Equity securities, available-for-sale, at fair value (cost $195.6 and $196.2, respectively)	251.1	228.4
Other invested assets, available-for-sale, at fair value (cost $49.3 and $49.0, respectively)	76.3	64.4
Other invested assets	5	0.5
Notes receivable from affiliate	70	70
Total investments	2,247.20	2,268.40
Cash and cash equivalents	54.3	59
Accrued investment income and other assets	35.5	31.5
Deferred policy acquisition costs	98.6	91.7
Reinsurance recoverable on losses and loss expenses payable	10.8	13.5
Prepaid reinsurance premiums	4.7	3.9
Due from affiliate	12.6	0
Net deferred federal income taxes	10	1
Property and equipment, at cost	8.4	8.8
Total assets	2,482.10	2,477.80
Liabilities and Stockholders’ Equity	'	'
Losses and loss expenses payable (affiliates $425.6 and $435.1, respectively)	952	942.2
Unearned premiums (affiliates $77.7 and $81.9, respectively)	501.5	481.6
Notes payable (affiliates $15.5 and $15.5, respectively)	100.8	115.9
Postretirement and pension benefits	102.7	113
Due to affiliate	0	8.6
Current federal income taxes	0.1	0
Other liabilities	91	79.3
Total liabilities	1,748.10	1,740.60
Stockholders’ equity:	'	'
Common stock, without par value. Authorized 100.0 shares; 47.4 and 47.3 shares issued, respectively, at stated value of $2.50 per share	118.6	118.1
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	-115.9	-115.8
Additional paid-in capital	135.4	131.6
Accumulated other comprehensive income	44.5	84.2
Retained earnings	551.4	519.1
Total stockholders’ equity	734	737.2
Total liabilities and stockholders’ equity	2,482.10	2,477.80
Class A Preferred Stock [Member]	'	'
Stockholders’ equity:	'	'
Preferred stock value	0	0
Class B Preferred Stock [Member]	'	'
Stockholders’ equity:	'	'
Preferred stock value	$0	$0

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, except Share data, unless otherwise specified
Fixed maturities, available-for-sale, amortized cost	$1,802.10	$1,776.20
Equity securities, available-for-sale, cost	195.6	196.2
Other invested assets, available-for-sale, cost	49.3	49
Losses and loss expenses payable, affiliates	425.6	435.1
Unearned premiums, affiliates	77.7	81.9
Notes payable, affiliates	$15.50	$15.50
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	47,400,000	47,300,000
Common stock, stated value per share (usd per share)	$2.50	$2.50
Treasury stock (shares)	6,800,000	6,800,000
Class A Preferred Stock [Member]	'	'
Preferred stock, no par value (usd per share)	$0	$0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Class B Preferred Stock [Member]	'	'
Preferred stock, no par value (usd per share)	$0	$0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Common Stock [Member]	'	'
Common stock, no par value (usd per share)	$0	$0

Condensed_Consolidated_Stateme

Condensed 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
Income Statement [Abstract]	'	'	'	'
Earned premiums (ceded to affiliates $215.2 and $203.9 for the three months ended and $639.0 and $600.6 for the nine months ended, respectively)	$266	$261.40	$790.80	$774.70
Net investment income (affiliate $1.3 and $1.3 for the three months ended and $3.7 and $3.7 for the nine months ended, respectively)	18.8	17.2	55.4	55.2
Net realized gain on investments:	'	'	'	'
Total other-than-temporary impairment losses	-0.3	-0.7	-2.6	-2.4
Portion of loss recognized in other comprehensive income	0	0	0	0
Other net realized investment gains	7.8	7.7	18.3	23.2
Total net realized gain on investments	7.5	7	15.7	20.8
Other income from affiliates	0.4	0.8	1.4	2.7
Total revenues	292.7	286.4	863.3	853.4
Losses and loss expenses (ceded to affiliates $137.9 and $133.8 for the three months ended and $434.0 and $457.6 for the nine months ended, respectively)	178.8	201.4	537.9	594.4
Acquisition and operating expenses	90.9	86.4	266.7	257.6
Interest expense (affiliates $0.1 and $0.2 for the three months ended and $0.5 and $0.6 for the nine months ended, respectively)	1.6	1.8	7.1	5.3
Other expenses	2.7	2.4	6.6	6.4
Total expenses	274	292	818.3	863.7
Income (loss) before federal income taxes	18.7	-5.6	45	-10.3
Federal income tax expense (benefit)	0.2	-0.1	0.6	-0.1
Net income (loss)	$18.50	($5.50)	$44.40	($10.20)
Earnings (loss) per common share:	'	'	'	'
Basic (usd per share)	$0.46	($0.14)	$1.09	($0.25)
Diluted (usd per share)	$0.45	($0.14)	$1.09	($0.25)
Dividends paid per common share (usd per share)	$0.10	$0.15	$0.30	$0.45

Condensed_Consolidated_Stateme1

Condensed 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
Income Statement [Abstract]	'	'	'	'
Earned premiums, ceded to affiliates	$215.20	$203.90	$639	$600.60
Net investment income, from affiliate	1.3	1.3	3.7	3.7
Losses and loss expenses, ceded to affiliates	137.9	133.8	434	457.6
Interest expense, affiliates	$0.10	$0.20	$0.50	$0.60

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
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]	'	'	'	'
Net income (loss)	$18.50	($5.50)	$44.40	($10.20)
Net unrealized holding gain (loss) on investments:	'	'	'	'
Unrealized holding gain arising during the period ended	12.1	36.7	-35.6	59.3
Reclassification adjustments for gain realized in net income (loss)	-7.5	-7	-15.7	-20.8
Income tax benefit (expense)	1.4	-0.3	9	0
Total net unrealized holding gain on investments	6	29.4	-42.3	38.5
Amortization of gain on derivative used in cash flow hedge	'	'	-0.1	-0.1
Reclassification adjustments for amortization to statements of income:	'	'	'	'
Negative prior service costs	-1.3	-1.3	-4.1	-3.9
Transition assets	0	0.1	0.2	0.2
Net loss	2.2	2	6.6	5.8
Total net unrecognized benefit plan obligations	0.9	0.8	2.7	2.1
Other comprehensive income, net of tax	6.9	30.2	-39.7	40.5
Comprehensive income, net of tax	$25.40	$24.70	$4.70	$30.30

Condensed_Consolidated_Stateme2

Condensed 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 (loss)	$44.40	($10.20)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:	'	'
Depreciation and amortization, net	9.5	6
Share-based compensation	3.6	2.7
Net realized gain on investments	-15.7	-20.8
Changes in operating assets and liabilities:	'	'
Deferred policy acquisition costs	-6.9	-2.2
Accrued investment income and other assets	-2.7	0
Postretirement and pension benefits	-7.4	-6.8
Other liabilities and due to/from affiliates, net	-18.8	-17.5
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	1.9	10.7
Losses and loss expenses payable	9.8	38.8
Unearned premiums	19.9	21.3
Federal income taxes	0.1	6.4
Cash used in Homeowners’ Quota Share initial net unearned premium transfer	0	-75.5
Cash used in pooling changes, December 31, 2011	0	-261.4
Net cash provided by (used in) operating activities	37.7	-308.5
Cash flows from investing activities:	'	'
Purchases of fixed maturities – available-for-sale	-226.6	-429.3
Purchases of equity securities – available-for-sale	-60.5	-112.1
Purchases of other invested assets	-5.2	-0.7
Maturities, calls and pay downs of fixed maturities – available-for-sale	122.7	217.5
Sales of fixed maturities – available-for-sale	77.8	293.9
Sales of equity securities – available-for-sale	74.1	64.4
Sales of other invested assets	0.5	0.5
Net additions (reductions) of property and equipment	0.2	-0.2
Net cash (used in) provided by investing activities	-17	34
Cash flows from financing activities:	'	'
Proceeds from issuance of common stock	2.3	0.8
Payments to acquire treasury stock	-0.1	0
Payment of dividends	-12.1	-18.1
Payments of credit facility issue costs	-0.5	0
Proceeds from long-term debt	85	0
Redemption of long-term debt	-100	0
Net cash used in financing activities	-25.4	-17.3
Net decrease in cash and cash equivalents	-4.7	-291.8
Cash and cash equivalents at beginning of period	59	356
Cash and cash equivalents at end of period	54.3	64.2
Supplemental disclosures:	'	'
Interest paid (affiliates $0.5 and $0.6, respectively)	7.3	3.7
Federal income taxes paid (refunds received)	$0.60	($6.50)

Condensed_Consolidated_Stateme3

Condensed Consolidated Statements of Cash Flows (Parenthetical) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Statement of Cash Flows [Abstract]	'	'
Interest paid, affiliates	$0.50	$0.60

Basis_of_Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]	'
Basis of Presentation	'
	Basis of Presentation
	The accompanying unaudited condensed consolidated financial statements of State Auto Financial Corporation and Subsidiaries (“State Auto Financial” or the “Company”) have been prepared in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by U.S. generally accepted accounting principles (“GAAP”) for complete financial statements. In the opinion of the Company, all adjustments (consisting of normal, recurring accruals) considered necessary for a fair presentation have been included. Operating results for the three and nine month periods ended September 30, 2013 are not necessarily indicative of the results that may be expected for the year ending December 31, 2013. The balance sheet at December 31, 2012 has been derived from the audited financial statements at that date, but does not include all of the information and footnotes required by GAAP for complete financial statements.
	For further information, refer to the consolidated financial statements and footnotes thereto included in the Company’s annual report on Form 10-K for the year ended December 31, 2012 (the “2012 Form 10-K”). Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the 2012 Form 10-K.
	Adoption of Accounting Pronouncements
	Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income
	The amendments in this guidance result in additional disclosure requirements under GAAP and do not change the current requirements for reporting net income or other comprehensive income in financial statements. The new guidance requires an entity to present, either in a single note, or, parenthetically on the face of the statement where net income is presented, the effects of significant amounts reclassified from each component of accumulated other comprehensive income by the respective line items of net income, only, if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. If a component is not required to be reclassified to net income in its entirety, the entity would, instead, cross-reference it to the related disclosure required under GAAP. This guidance was effective prospectively for fiscal years and interim periods beginning after December 15, 2012. The Company adopted this guidance at January 1, 2013 and it did not have a material impact on the consolidated financial statements, see Note 8 - Other Comprehensive Income and Accumulated Other Comprehensive Income.

Investments

Investments	9 Months Ended
	Sep. 30, 2013
Investments, Debt and Equity Securities [Abstract]	'
Investments	'
	Investments
	The following tables set forth the cost or amortized cost and fair value of available-for-sale securities by lot at September 30, 2013 and December 31, 2012:
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	30-Sep-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	348.9		$	18.4		$	(3.9	)		$	363.4
	Obligations of states and political subdivisions	791.9			29.8			(16.7		)		805
	Corporate securities	326.7			11.6			(5.3		)		333
	U.S. government agencies residential mortgage-backed securities	334.6			11.4			(2.6		)		343.4
	Total fixed maturities	1,802.10			71.2			(28.5		)		1,844.80
	Equity securities:
	Large-cap securities	150.5			36.2			(2.1		)		184.6
	Small-cap securities	45.1			21.4			—				66.5
	Total equity securities	195.6			57.6			(2.1		)		251.1
	Other invested assets	49.3			27			—				76.3
	Total available-for-sale securities	$	2,047.00		$	155.8		$	(30.6	)		$	2,172.20
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	31-Dec-12
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	328.2		$	38.3		$	—			$	366.5
	Obligations of states and political subdivisions	750.4			50.3			(0.4		)		800.3
	Corporate securities	320.5			19.2			(1.1		)		338.6
	U.S. government agencies residential mortgage-backed securities	377.1			24			(1.4		)		399.7
	Total fixed maturities	1,776.20			131.8			(2.9		)		1,905.10
	Equity securities:
	Large-cap securities	152.6			25			(3.4		)		174.2
	Small-cap securities	43.6			10.6			—				54.2
	Total equity securities	196.2			35.6			(3.4		)		228.4
	Other invested assets	49			15.4			—				64.4
	Total available-for-sale securities	$	2,021.40		$	182.8		$	(6.3	)		$	2,197.90
	The following tables set forth the Company’s gross unrealized losses and fair value on its investments by lot, aggregated by investment category and length of time for individual securities that have been in a continuous unrealized loss position at September 30, 2013 and December 31, 2012:
		Less than 12 months								12 months or more								Total
	($ millions, except # of positions)	Fair value			Unrealized losses			Number of positions		Fair value				Unrealized losses			Number of positions	Fair value		Unrealized losses			Number of positions
	30-Sep-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	86		$	(3.9	)	24		$	—			$	—		—	$	86	$	(3.9	)	24
	Obligations of states and political subdivisions	230.9			(16.7		)	81		—				—			—	230.9		(16.7		)	81
	Corporate securities	102			(4.8		)	20		5.5				(0.5		)	1	107.5		(5.3		)	21
	U.S. government agencies residential mortgage-backed securities	54.2			(1.6		)	16		27.6				(1.0		)	11	81.8		(2.6		)	27
	Total fixed maturities	473.1			(27.0		)	141		33.1				(1.5		)	12	506.2		(28.5		)	153
	Large-cap equity securities	16.2			(1.3		)	3		8.9				(0.8		)	3	25.1		(2.1		)	6
	Total temporarily impaired securities	$	489.3		$	(28.3	)	144		$	42			$	(2.3	)	15	$	531.3	$	(30.6	)	159
		Less than 12 months								12 months or more								Total
	($ millions, except # of positions)	Fair value			Unrealized losses			Number of positions		Fair value				Unrealized losses			Number of positions	Fair value		Unrealized losses			Number of positions
	31-Dec-12
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	7		$	—		2		$	—			$	—		—	$	7	$	—		2
	Obligations of states and political subdivisions	47.4			(0.4		)	12		—				—			—	47.4		(0.4		)	12
	Corporate securities	80.4			(1.1		)	17		—				—			—	80.4		(1.1		)	17
	U.S. government agencies residential mortgage-backed securities	23.3			(0.3		)	6		34.8				(1.1		)	13	58.1		(1.4		)	19
	Total fixed maturities	158.1			(1.8		)	37		34.8				(1.1		)	13	192.9		(2.9		)	50
	Large-cap equity securities	23.7			(2.1		)	4		8.9				(1.3		)	5	32.6		(3.4		)	9
	Total temporarily impaired securities	$	181.8		$	(3.9	)	41		$	43.7			$	(2.4	)	18	$	225.5	$	(6.3	)	59
	The Company reviewed its investments at September 30, 2013, and determined no additional other-than-temporary impairment existed in the gross unrealized holding losses other than those listed in the table below. The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013				2012
	Equity securities:
	Large-cap securities	$	—		$	—		$	(1.3	)		$	—
	Small-cap securities	(0.3		)	(0.7		)	(1.3		)		(2.2		)
	Fixed maturities:	—			—			—				(0.2		)
	Total other-than-temporary impairments	$	(0.3	)	$	(0.7	)	$	(2.6	)		$	(2.4	)
	The Company regularly monitors its investments that have fair values less than cost or amortized cost for signs of other-than-temporary impairment, an assessment that requires significant management judgment regarding the evidence known. Such judgments could change in the future as more information becomes known, which could negatively impact the amounts reported. Among the factors that management considers for fixed maturity securities are the financial condition of the issuer including receipt of scheduled principal and interest cash flows, and intent to sell including if it is more likely than not that the Company will be required to sell the investments before recovery. When a fixed maturity has been determined to have an other-than-temporary impairment, the impairment charge is separated into an amount representing the credit loss, which is recognized in earnings as a realized loss, and the amount related to non-credit factors, which is recognized in accumulated other comprehensive income. Future increases or decreases in fair value, if not other-than-temporary, are included in accumulated other comprehensive income.
	Among the factors that management considers for equity securities and other invested assets are the length of time and/or the significance of decline below cost, the Company’s ability and intent to hold these securities through their recovery periods, the current financial condition of the issuer and its future business prospects, and the ability of the market value to recover to cost in the near term. When an equity security or other invested asset has been determined to have a decline in fair value that is other-than-temporary, the cost basis of the security is adjusted to fair value. This results in a charge to earnings as a realized loss, which is not reversed for subsequent recoveries in fair value. Future increases or decreases in fair value, if not other-than-temporary, are included in accumulated other comprehensive income.
The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at September 30, 2013:
($ millions)	Amortized cost			Fair
				value
Due in 1 year or less	$	57.2		$	57.9
Due after 1 year through 5 years	367.1			385.8
Due after 5 years through 10 years	374.6			391.6
Due after 10 years	668.6			666.1
U.S. government agencies residential mortgage-backed securities	334.6			343.4
Total	$	1,802.10		$	1,844.80
Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay the obligations with or without call or prepayment penalties.
At September 30, 2013, State Auto P&C had fixed maturity securities, with a carrying value of $118.0 million, that were pledged as collateral in connection with an outstanding borrowing of $85.0 million with the Federal Home Loan Bank of Cincinnati ("FHLB"). In accordance with the terms of the borrowing, State Auto P&C retains all rights regarding these securities, which are are included in the "U.S. government agencies residential mortgage-backed securities" classification of the Company's fixed maturity securities portfolio.
Fixed maturities with fair values of $8.6 and $10.0 million, respectively, were on deposit with insurance regulators as required by law at September 30, 2013 and December 31, 2012. The Company retains all rights regarding these securities.
The following table sets forth the components of net investment income for the three and nine months ended September 30, 2013 and 2012:
($ millions)	Three months ended September 30						Nine months ended September 30
	2013			2012			2013				2012
Fixed maturities	$	16.2		$	14.7		$	48.2			$	49.1
Equity securities	1.9			1.5			4.7				3.5
Cash and cash equivalents, and other	1.3			1.4			4.1				4.1
Investment income	19.4			17.6			57				56.7
Investment expenses	0.6			0.4			1.6				1.5
Net investment income	$	18.8		$	17.2		$	55.4			$	55.2
The Company’s current investment strategy does not rely on the use of derivative financial instruments.
The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three and nine months ended September 30, 2013 and 2012:
($ millions)	Three months ended September 30						Nine months ended September 30
	2013			2012			2013				2012
Realized gains:
Fixed maturities	$	0.4		$	3.3		$	1.6			$	14
Equity securities	7.9			4.8			17.9				11.5
Other invested assets	0.1			—			0.1				—
Total realized gains	8.4			8.1			19.6				25.5
Realized losses:
Equity securities:
Sales	(0.3		)	(0.4		)	(1.0		)		(2.3		)
OTTI	(0.3		)	(0.7		)	(2.6		)		(2.2		)
Fixed maturities:
Sales	(0.3		)	—			(0.3		)		—
OTTI	—			—			—				(0.2		)
Total realized losses	(0.9		)	(1.1		)	(3.9		)		(4.7		)
Net realized gain on investments	$	7.5		$	7		$	15.7			$	20.8
Change in unrealized holding gains (losses), net of tax:
Fixed maturities	$	(7.7	)	$	17.2		$	(86.2	)		$	20.7
Equity securities	5.3			8.5			23.3				13.4
Other invested assets	7			4			11.6				4.4
Deferred federal income tax (liability) benefit	(1.6		)	(10.4		)	18				(13.5		)
Valuation allowance	3			10.1			(9.0		)		13.5
Change in net unrealized holding gains (losses), net of tax	$	6		$	29.4		$	(42.3	)		$	38.5
There was a deferred federal income tax liability, net of a valuation allowance, on the net unrealized holding gains at September 30, 2013 and December 31, 2012 of $43.5 million and $52.5 million, respectively.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]	'
Fair Value of Financial Instruments	'
	Fair Value of Financial Instruments
	Below is the fair value hierarchy that categorizes into three levels the inputs to valuation techniques that are used to measure fair value:
	•	Level 1 includes observable inputs which reflect quoted prices for identical assets or liabilities in active markets at the measurement date.
	•	Level 2 includes observable inputs for assets or liabilities other than quoted prices included in Level 1, and it includes valuation techniques which use prices for similar assets and liabilities.
	•	Level 3 includes unobservable inputs which reflect the reporting entity’s estimates of the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk).
	The Company utilizes one nationally recognized pricing service to estimate the majority of its available-for-sale investment portfolio’s fair value. The Company obtains one price per security and the processes and control procedures employed by the Company are designed to ensure the value is accurately recorded on an unadjusted basis. Through discussions with the pricing service, the Company gains an understanding of the methodologies used to price the different types of securities, that the data and the valuation methods utilized are appropriate and consistently applied, and that the assumptions are reasonable and representative of fair value. To validate the reasonableness of the valuations obtained from the pricing service, the Company compares to other fair value pricing information gathered from other independent pricing sources. At September 30, 2013 and December 31, 2012, the Company did not adjust any of the prices received from the pricing service.
	Transfers between level categorizations may occur due to changes in the availability of market observable inputs. Transfers in and out of level categorizations are reported as having occurred at the beginning of the quarter in which the transfer occurred. There were no transfers between level categorizations during the three and nine months ended September 30, 2013 and 2012.
	The following sections describe the valuation methods used by the Company for each type of financial instrument it holds that are carried at fair value:
	Fixed Maturities
	The Company utilizes a third party pricing service to estimate fair value measurements for the majority of its fixed maturities. The fair value estimate of the Company’s fixed maturity investments are determined by evaluations that are based on observable market information rather than market quotes. Inputs to the evaluations include, but are not limited to, market prices from recently completed transactions and transactions of comparable securities, interest rate yield curves, credit spreads, and other market-observable information. The fixed maturity portfolio pricing obtained from the pricing service is reviewed for reasonableness. Regularly, a sample of security prices’ are referred back to the pricing service for more detailed explanation as to how the pricing service arrived at that particular price. The explanations are reviewed for reasonableness by the portfolio manager and investment officer. Additionally, the prices and assumptions are verified against an alternative pricing source for reasonableness and accuracy. Any discrepancies with the pricing are returned to the pricing service for further explanation and if necessary, adjustments are made. To date, the Company has not identified any significant discrepancies in the pricing provided by its third party pricing service. Investments valued using these inputs include U.S. treasury securities and obligations of U.S. government agencies, obligations of states and political subdivisions, corporate securities (except for one security discussed below), and U.S. government agencies residential mortgage-backed securities. All unadjusted estimates of fair value for fixed maturities priced by the pricing service are included in the amounts disclosed in Level 2 of the hierarchy. If market inputs are unavailable, then no fair value is provided by the pricing service. For these securities, fair value is determined either by requesting brokers who are knowledgeable about these securities to provide a quote; or the Company internally determines the fair values by employing widely accepted pricing valuation models, and depending on the level of observable market inputs, renders the fair value estimate as Level 2 or Level 3. The Company holds two fixed maturity corporate securities included in Level 3. The Company estimates the fair value of one security using the present value of the future cash flows and the Company obtains a broker quote of the other security’s fair value. Due to the limited amount of observable market information for both of these securities, the Company includes the fair value estimates in Level 3.
	Equities
	The fair value of each equity security is based on an observable market price for an identical asset in an active market and is priced by the same pricing service discussed above. All equity securities are recorded using unadjusted market prices and have been disclosed in Level 1.
	Other Invested Assets
	Included in other invested assets are two international funds (“the funds”) that invest in equity securities of foreign issuers and are managed by third party investment managers. The funds had a fair value of $70.0 million and $59.0 million at September 30, 2013 and December 31, 2012, respectively, which was determined using each fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the funds including obtaining and reviewing each fund’s audited financial statements. There are no unfunded commitments related to the funds. The Company may not sell its investment in the funds; however, the Company may redeem all or a portion of its investment in the funds at net asset value per share with the appropriate prior written notice. Due to the Company’s ability to redeem its investment in the funds at net asset value per share at the measurement date, the funds have been disclosed in Level 2.
	The remainder of the Company’s other invested assets consist primarily of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds based on an observable market price for an identical asset in an active market reflect their fair values and consequently these securities have been disclosed in Level 1.
	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at September 30, 2013 and December 31, 2012:
	($ millions)	Total			Quoted prices in active markets for identical assets		Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
	30-Sep-13				(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	363.4		$	—	$	363.4		$	—
	Obligations of states and political subdivisions	805			—		805			—
	Corporate securities	333			—		324.1			8.9
	U.S. government agencies residential mortgage-backed securities	343.4			—		343.4			—
	Total fixed maturities	1,844.80			—		1,835.90			8.9
	Equity securities:
	Large-cap securities	184.6			184.6		—			—
	Small-cap securities	66.5			66.5		—			—
	Total equity securities	251.1			251.1		—			—
	Other invested assets	76.3			6.3		70			—
	Total available-for-sale investments	$	2,172.20		$	257.4	$	1,905.90		$	8.9
	($ millions)	Total			Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
	31-Dec-12
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	366.5		$	—	$	366.5		$	—
	Obligations of states and political subdivisions	800.3			—		800.3			—
	Corporate securities	338.6			—		330.1			8.5
	U.S. government agencies residential mortgage-backed securities	399.7			—		399.7			—
	Total fixed maturities	1,905.10			—		1,896.60			8.5
	Equity securities:
	Large-cap securities	174.2			174.2		—			—
	Small-cap securities	54.2			54.2		—			—
	Total equity securities	228.4			228.4		—			—
	Other invested assets	64.4			5.4		59			—
	Total available-for-sale investments	$	2,197.90		$	233.8	$	1,955.60		$	8.5
	For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for the three and nine months ended September 30, 2013 and the year ended December 31, 2012, separately for each major category of assets:
	($ millions)	Fixed maturities
	Balance at January 1, 2013	$	8.5
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains (losses) – included in other comprehensive income	—
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at March 31, 2013	$	8.6
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains (losses) – included in other comprehensive income	0.1
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at June 30, 2013	$	8.8
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains (losses) – included in other comprehensive income	—
Purchases	0.1
Sales	—
Transfers into Level 3	—
Transfers out of Level 3	—
Balance at September 30, 2013	$	8.9
($ millions)	Fixed maturities
Balance at January 1, 2012	$	2.9
Total realized (losses) gains – included in earnings	(0.2		)
Total unrealized gains (losses) – included in other comprehensive income	—
Purchases	5.8
Sales	—
Transfers into Level 3	—
Transfers out of Level 3	—
Balance at December 31, 2012	$	8.5
The following sections describe the valuation methods used by the Company for each type of financial instrument it holds that is not measured at fair value but for which fair value is disclosed:
Financial Instruments Disclosed, But Not Carried, At Fair Value
Other Invested Assets
Included in other invested assets are the Federal Home Loan Bank of Cincinnati (“FHLB”) common stock and the Trust securities. The Trust securities and FHLB common stock are carried at cost, which approximates fair value. The fair value of the FHLB common stock at September 30, 2013 was $4.5 million and the fair value of the Trust common securities was $0.5 million. Both investments have been placed in Level 3 of the fair value hierarchy.
Notes Receivable from Affiliates
In May 2009, the Company entered into two separate credit agreements with State Auto Mutual pursuant to which it loaned State Auto Mutual a total of $70.0 million. The Company estimates the fair value of the notes receivable from affiliate using market quotations for U.S. treasury securities with similar maturity dates and applies an appropriate credit spread. Consequently this has been placed in Level 2 of the fair value hierarchy.
($ millions, except interest rates)	September 30, 2013							December 31, 2012
	Carrying value			Fair value		Interest rate		Carrying value			Fair		Interest rate
											value
Notes receivable from affiliate	$	70		$	75.7	7	%	$	70		$	78.3	7	%
Notes Payable
Included in notes payable are the FHLB Loan and Subordinated Debentures. The Company estimates the fair value of the FHLB Loan by discounting cash flows using a borrowing rate currently available to the company for a loan with similar terms. This has been placed in Level 3 of the fair value hierarchy. The carrying amount of the Subordinated Debentures approximates its fair value as the interest rate adjusts quarterly and has been disclosed in Level 3.
($ millions, except interest rates)	30-Sep-13							31-Dec-12
	Carrying value			Fair Value		Interest rate		Carrying value			Fair value		Interest rate
FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest	$	85.3		$	91.1	4.59	%	N/A			N/A		N/A
Senior Notes due 2013: issued $100.0, November 2003 with fixed interest	N/A			N/A		N/A		100.4			100.3		6.25	%
Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5			15.5		4.46	%	15.5			15.5		4.51	%
Total notes payable	$	100.8		$	106.6			$	115.9		$	115.8

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2013
Insurance [Abstract]	'
Reinsurance	'
	Reinsurance
	The insurance subsidiaries of State Auto Financial, including State Auto Property & Casualty Insurance Company (“State Auto P&C”), Milbank Insurance Company and State Auto Insurance Company of Ohio (collectively referred to as the “STFC Pooled Companies”) participate in a quota share reinsurance pooling arrangement (“the Pooling Arrangement”) with State Automobile Mutual Insurance Company (“State Auto Mutual”) and its subsidiaries and affiliates, State Auto Insurance Company of Wisconsin, Meridian Citizens Mutual Insurance Company, Meridian Security Insurance Company, Patrons Mutual Insurance Company of Connecticut (which includes Litchfield Mutual Fire Insurance Company as it was merged with Patrons Mutual at the close of business March 31, 2013), Rockhill Insurance Company (“RIC”), Plaza Insurance Company (“Plaza”), American Compensation Insurance Company (“American Compensation”) and Bloomington Compensation Insurance Company (“Bloomington Compensation”), (collectively referred to as the “Mutual Pooled Companies”).
	The following table sets forth a summary of the Company’s external reinsurance transactions, as well as reinsurance transactions with State Auto Mutual under the Pooling Arrangement, for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013			2012
	Premiums earned:
	Assumed from external insurers and reinsurers	$	1.2		$	1.1		$	2.7		$	3.1
	Assumed under Pooling Arrangement	266			261.4			790.8			774.7
	Ceded to external insurers and reinsurers	(5.4		)	(6.6		)	(17.6		)	(22.1		)
	Ceded under Pooling Arrangement	(215.2		)	(203.9		)	(639.0		)	(600.6		)
	Net assumed premiums earned	$	46.6		$	52		$	136.9		$	155.1
	Losses and loss expenses incurred:
	Assumed from external insurers and reinsurers	$	0.9		$	1.2		$	1.3		$	3.1
	Assumed under Pooling Arrangement	179.5			201.7			540.1			595.3
	Ceded to external insurers and reinsurers	(0.8		)	(4.5		)	(4.9		)	(9.1		)
	Ceded under Pooling Arrangement	(137.8		)	(133.8		)	(434.0		)	(457.6		)
	Net assumed losses and loss expenses incurred	$	41.8		$	64.6		$	102.5		$	131.7

Notes_Payable_and_Credit_Facil

Notes Payable and Credit Facility Notes Payable and Credit Facility	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Notes Payable and Credit Facility	'
	Notes Payable and Credit Facility
	FHLB Loan
	On July 11, 2013, State Auto Financial's subsidiary, State Auto P&C, borrowed $85.0 million (the "FHLB Loan") from the Federal Home Loan Bank of Cincinnati (the “FHLB”). The FHLB Loan is a 20-year term loan and is callable after three years with no prepayment penalty thereafter. The FHLB Loan provides for interest-only payments during its term, with principal due in full at maturity. The interest rate is fixed over the term of the loan at 5.03%. The FHLB Loan is fully secured by a pledge of specific investment securities of State Auto P&C. The total loan interest incurred through September 30, 2013 was $0.9 million.
	Senior Note Redemption
	On July 15, 2013, the Company redeemed all $100.0 million of its outstanding Senior Notes. The redemption price of $103.0 million was calculated in accordance with the terms of the Senior Notes and included principal plus a make-whole amount. The redemption price was funded by proceeds from the FHLB Loan and cash on hand.
	Credit Facility
	On July 26, 2013, State Auto Financial terminated its then-current credit agreement with a syndicate of lenders, as further described below. Concurrent with the termination of this credit agreement, State Auto P&C entered into a new credit facility (the “SPC Credit Facility”) with a syndicate of lenders. The SPC Credit Facility provides State Auto P&C with a $100.0 million five-year revolving credit facility maturing in July 2018. During the term of the SPC Credit Facility, State Auto P&C has the right to increase the total facility to a maximum amount of $150.0 million, provided that no event of default has occurred and is continuing. The SPC Credit Facility is available for general corporate purposes and provides for interest-only payments during its term, with principal and interest due in full at maturity. Interest is based on the London Interbank Offered Rate ("LIBOR") or a base rate plus a calculated margin amount. All advances under the SPC Credit Facility are to be fully secured by a pledge of specific investment securities of State Auto P&C. The SPC Credit Facility includes certain requirements, including financial requirements that State Auto Financial maintain a minimum net worth and a certain debt to capitalization ratio.
	As of September 30, 2013, State Auto P&C had not made any borrowings and both State Auto P&C and the Company were in compliance with all covenants and requirements of the SPC Credit Facility.

Income_Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Income Taxes	'
	Income Taxes
	The following table sets forth the reconciliation between actual federal income tax expense and the amount computed at the indicated statutory rate for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended											Nine months ended
		30-Sep											30-Sep
		2013						2012					2013					2012
					%						%					%					%
	Amount at statutory rate	$	6.6		35			$	(1.9	)	35		$	15.8		35		$	(3.6	)	35
	Tax-exempt interest and dividends received deduction	(2.5		)	(13	)		(2.3		)	41		(7.2		)	(16	)	(6.7		)	65
	Other, net	(0.3		)	(1	)		(0.1		)	2		—			—		0.3			(3	)
	Valuation allowance	(3.6		)	(19	)		4.2			(75	)	(8.0		)	(18	)	9.9			(96	)
	Federal income tax expense (benefit) and effective rate	$	0.2		2			$	(0.1	)	3		$	0.6		1		$	(0.1	)	1
	The following table sets forth the tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities at September 30, 2013 and December 31, 2012:
	($ millions)	September 30, 2013			December 31, 2012
	Deferred tax assets:
	Unearned premiums not currently deductible	$	34.8		$	33.4
	Losses and loss expenses payable discounting	25.5			25.2
	Postretirement and pension benefits	35.8			39.4
	Realized loss on other-than-temporary impairment	9.3			7.5
	Other liabilities	17.1			14.5
	Net operating loss carryforward	56.9			66.5
	Tax credit carryforward	1.8			0.9
	Other	7.6			7.9
	Total deferred tax assets	188.8			195.3
	Deferred tax liabilities:
	Deferral of policy acquisition costs	34.5			32.1
	Net unrealized holding gains on investments	43.8			61.7
	Total deferred tax liabilities	78.3			93.8
	Total net deferred tax assets before valuation allowance	110.5			101.5
	Less valuation allowance	100.5			100.5
	Net deferred federal income taxes	$	10		$	1
	Deferred income tax assets and liabilities represent the tax effect of the differences between the financial statement carrying value of existing assets and liabilities and their respective tax bases. In accordance with the Financial Accounting Standards Board’s Accounting Standards Codification 740, Income Taxes (ASC 740), the Company periodically evaluates its deferred tax assets, which requires significant judgment, to determine if they are realizable based upon weighing all available evidence, both positive and negative, including loss carryback potential, past operating results, existence of cumulative losses in the most recent years, projected performance of the business, future taxable income, including the ability to generate capital gains, and prudent and feasible tax planning strategies. In making such judgments, significant weight is given to evidence that can be objectively verified. At both September 30, 2013 and December 31, 2012, the Company recorded a valuation allowance of $100.5 million, respectively. The deferred income tax asset remaining after recognition of the valuation allowance represents a deferred tax asset on the gross unrealized fixed maturity losses where management determined this portion of the asset to be realizable due to management’s assertion that it has both the ability and intent to hold these securities through recovery or maturity.
	Based on ASC 740 intraperiod tax allocation guidelines, the following sets forth the change in valuation allowance attributable to continuing operations and other comprehensive income for the three and nine months ended September 30, 2013 is as follows:
	($ millions)	Three months ended September 30, 2013			Nine months ended September 30, 2013
	Continuing operations	$	(3.6	)	$	(8.0	)
	Other comprehensive income	(3.4		)	8
	Change in valuation allowance	$	(7.0	)	$	—
	In future periods the Company will re-assess its judgments and assumptions regarding the realization of its net deferred tax assets, but until such time the positive evidence exceeds the negative evidence the Company will maintain a valuation allowance against its net deferred tax assets.

Pension_and_Postretirement_Ben

Pension and Postretirement Benefit Plans	9 Months Ended
	Sep. 30, 2013
Compensation and Retirement Disclosure [Abstract]	'
Pension and Postretirement Benefit Plans	'
	Pension and Postretirement Benefit Plans
	The following table sets forth the components of net periodic cost for the Company’s pension and postretirement benefit plans for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Pension						Postretirement						Pension						Postretirement
		Three months ended												Nine months ended
		30-Sep												30-Sep
		2013			2012			2013			2012			2013			2012			2013			2012
	Service cost	$	1.7		$	2		$	0.1		$	—		$	4.7		$	5.8		$	0.2		$	—
	Interest cost	2.4			2.5			0.3			0.3			7.2			7.5			0.9			0.9
	Expected return on plan assets	(3.2		)	(3.0		)	0.4			—			(9.2		)	(8.8		)	0.4			(0.2		)
	Amortization of:
	Prior service (benefits) costs	—			0.1			(1.3		)	(1.4		)	—			0.2			(4.1		)	(4.1		)
	Transition assets	—			0.1			—			—			—			—			0.2			0.2
	Net loss	2			1.8			0.2			0.2			6			5.2			0.6			0.6
	Net periodic cost	$	2.9		$	3.5		$	(0.3	)	$	(0.9	)	$	8.7		$	9.9		$	(1.8	)	$	(2.6	)
	The Company contributed $13.0 million for the nine months ended September 30, 2013 to its pension plan.

Other_Comprehensive_Income_and

Other Comprehensive Income and Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Other Comprehensive Income and Accumulated Other Comprehensive Income	'
	Other Comprehensive Income and Accumulated Other Comprehensive Income
	The following tables set forth the changes in the Company’s accumulated other comprehensive income component (AOCI), net of tax, for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Unrealized Gains and Losses on Available-for-Sale Securities				Gains and Losses on Cash Flow Hedges			Benefit Plan Items			Total
	The three months ended September 30, 2013 and 2012:
	Beginning balance at July 1, 2013	$	75.7			$	—		$	(38.1	)	$	37.6
	Other comprehensive income before reclassifications	13.6				—			—			13.6
	Amounts reclassified from AOCI (a)	(7.6		)		—			0.9			(6.7		)
	Net current period other comprehensive income	6				—			0.9			6.9
	Ending balance at September 30, 2013	$	81.7			$	—		$	(37.2	)	$	44.5
	Beginning balance at July 1, 2012	$	107.8			$	0.1		$	(33.8	)	$	74.1
	Other comprehensive income before reclassifications	36.4				—			—			36.4
	Amounts reclassified from AOCI (a)	(7.0		)		—			0.8			(6.2		)
	Net current period other comprehensive income	29.4				—			0.8			30.2
	Ending balance at September 30, 2012	$	137.2			$	0.1		$	(33.0	)	$	104.3
	(a)	See separate table below for details about these reclassifications
	($ millions)	Unrealized Gains and Losses on Available-for-Sale Securities				Gains and Losses on Cash Flow Hedges			Benefit Plan Items			Total
	The nine months ended September 30, 2013 and 2012:
	Beginning balance at January 1, 2013	$	124			$	0.1		$	(39.9	)	$	84.2
	Other comprehensive income before reclassifications	(26.5		)		—			—			(26.5		)
	Amounts reclassified from AOCI (a)	(15.8		)		(0.1		)	2.7			(13.2		)
	Net current period other comprehensive income	(42.3		)		(0.1		)	2.7			(39.7		)
	Ending balance at September 30, 2013	$	81.7			$	—		$	(37.2	)	$	44.5
	Beginning balance at January 1, 2012	$	98.7			$	0.2		$	(35.1	)	$	63.8
	Other comprehensive income before reclassifications	59.3				—			—			59.3
	Amounts reclassified from AOCI (a)	(20.8		)		(0.1		)	2.1			(18.8		)
	Net current period other comprehensive income	38.5				(0.1		)	2.1			40.5
	Ending balance at September 30, 2012	$	137.2			$	0.1		$	(33.0	)	$	104.3
	(a)	See separate table below for details about these reclassifications
	The following tables set forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s consolidated statement of income for the three and nine months ended September 30, 2013 and 2012:
	($ millions)
	Details about Accumulated Other	Three months ended						Affected line item in the Condensed
	Comprehensive Income Components	30-Sep						Consolidated Statements of Income
		2013			2012
	Unrealized gains (losses) on available for sale securities	7.5			7			Realized gain (loss) on sale of securities
		7.5			7			Total before tax
		0.1			—			Tax benefit (expense)
		7.6			7			Net of tax
	Amortization of benefit plan items
	Negative prior service costs	1.3			1.3			(a)
	Transition assets	—			(0.1	)		(a)
	Net loss	(2.2	)		(2.0	)		(a)
		(0.9	)		(0.8	)		Total before tax
		—			—			Tax benefit (expense)
		(0.9	)		(0.8	)		Net of tax
	Total reclassifications for the period	6.7			6.2
	(a)	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).
	($ millions)
	Details about Accumulated Other	Nine months ended							Affected line item in the Condensed
	Comprehensive Income Components	30-Sep							Consolidated Statements of Income
		2013				2012
	Unrealized gains (losses) on available for sale securities	$	15.7			$	20.8		Realized gain (loss) on sale of securities
		15.7				20.8			Total before tax
		0.1				—			Tax benefit (expense)
		15.8				20.8			Net of tax
	Amortization of gain on derivative used in cash flow hedge	0.1				0.1			Realized gain (loss) on sale of securities
		0.1				0.1			Total before tax
		—				—			Tax benefit (expense)
		0.1				0.1			Net of tax
	Amortization of benefit plan items
	Negative prior service costs	4.1				3.9			(a)
	Transition assets	(0.2		)		(0.2		)	(a)
	Net loss	(6.6		)		(5.8		)	(a)
		(2.7		)		(2.1		)	Total before tax
	—				—			Tax benefit (expense)
	(2.7		)		(2.1		)	Net of tax
Total reclassifications for the period	$	13.2			$	18.8
(a)	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings_Loss_per_Common_Share

Earnings (Loss) per Common Share	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Earnings (Loss) per Common Share	'
	Earnings (Loss) per Common Share
	The following table sets forth the compilation of basic and diluted earnings (loss) per common share for the three and nine months ended September 30, 2013 and 2012:
	($ and shares in millions, except per share amounts)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013		2012
	Numerator:
	Net income (loss) for basic earnings (loss) per common share	$	18.5		$	(5.5	)	$	44.4	$	(10.2	)
	Denominator:
	Weighted average shares for basic net earnings (loss) per common share	40.6			40.4			40.5		40.4
	Effect of dilutive share-based awards	0.3			—			0.2		—
	Adjusted weighted average shares for diluted net earnings (loss) per common share	40.9			40.4			40.7		40.4
	Basic net earnings (loss) per common share	$	0.46		$	(0.14	)	$	1.09	$	(0.25	)
	Diluted net earnings (loss) per common share	$	0.45		$	(0.14	)	$	1.09	$	(0.25	)
	The following table sets forth the options to purchase shares of common stock and the restricted share units ("RSU award") provided to each outside director of the Company, that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three and nine months ended September 30, 2013 and 2012.
	(number of options and awards in millions)	Three months ended September 30					Nine months ended September 30
		2013		2012			2013		2012
	Number of options	2.1		3.7			2.7		3.7
	Number of RSU awards	0.2		—			0.1		—
	Total number of antidilutive shares and awards	2.3		3.7			2.8		3.7

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Segment Information	'
	Segment Information
	The Company has four reportable segments: personal insurance, business insurance, specialty insurance and investment operations. The reportable insurance segments are business units managed separately because of the differences in the type of customers they serve, the products they provide or services they offer. The insurance segments market a broad line of property and casualty insurance products throughout the United States through independent insurance agencies, which include retail agents and wholesale brokers. The personal insurance segment provides primarily personal automobile and homeowners to the personal insurance market. The business insurance segment provides primarily commercial automobile, commercial multi-peril, fire & allied and general liability insurance covering small-to-medium sized commercial exposures in the business insurance market. The specialty insurance segment provides commercial coverages, including workers’ compensation for both legacy State Auto Group and RTW, Inc.’s insurance subsidiaries, that require specialized product underwriting, claims handling or risk management services through a distribution channel of retail agents and wholesale brokers, which may include program administrators and other specialty sources. The investment operations segment, managed by Stateco, provides investment services.
	The Company evaluates the performance of its insurance segments using industry financial measurements based on Statutory Accounting Practices (“SAP”), which include loss and loss adjustment expense ratios, underwriting expense ratios, combined ratios, statutory underwriting gain (loss), net premiums earned and net written premiums. One of the most significant differences between SAP and GAAP is that SAP requires all underwriting expenses to be expensed immediately and not deferred and amortized over the same period the premium is earned.
	The accounting for pension benefits also contributes to the difference between our GAAP loss and expense ratios and our SAP loss and expense ratios. At January 1, 2013, we adopted new SAP pension guidance, which required the recognition of service costs for non-vested participants. In accordance with GAAP, service costs related to non-vested participants was recognized over the vesting period.
	The investment operations segment is evaluated based on investment returns of assets managed by Stateco. Asset information by segment is not reported for the insurance segments because the Company does not produce such information internally.
	As of January 1, 2013 our units within the specialty insurance segment changed from RED, Rockhill and Workers’ Compensation to Excess & Surplus Property, Excess & Surplus Casualty, Programs and Workers’ Compensation. This change did not have any impact on segment reporting.
	The following table sets forth financial information regarding the Company’s reportable segments for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013			2012
	Revenues from external sources:
	Insurance segments
	Personal insurance	$	116.4		$	117.4		$	348.3		$	352.1
	Business insurance	92.9			83.4			271			240.1
	Specialty insurance	56.7			60.6			171.5			182.5
	Total insurance segments	266			261.4			790.8			774.7
	Investment operations segment
	Net investment income	18.8			17.2			55.4			55.2
	Net realized capital gains	7.5			7			15.7			20.8
	Total investment operations segment	26.3			24.2			71.1			76
	All other	0.4			0.8			1.4			2.7
	Total revenues from external sources	292.7			286.4			863.3			853.4
	Intersegment revenues:	1.4			5.6			4			10.3
	Total revenues	294.1			292			867.3			863.7
	Reconciling items:
	Eliminate intersegment revenues	(1.4		)	(5.6		)	(4.0		)	(10.3		)
	Total consolidated revenues	$	292.7		$	286.4		$	863.3		$	853.4
	Segment (loss) income before federal income tax:
	Insurance segments SAP underwriting (loss) income
	Personal insurance	$	(6.0	)	$	2.9		$	(11.5	)	$	(8.8	)
	Business insurance	(1.1		)	(7.4		)	(11.0		)	(40.8		)
	Specialty insurance	3.1			(25.1		)	(4.6		)	(37.4		)
	Total insurance segments	(4.0		)	(29.6		)	(27.1		)	(87.0		)
	Investment operations segment
	Net investment income	18.8			17.2			55.4			55.2
	Net realized capital gains	7.5			7			15.7			20.8
	Total investment operations segment	26.3			24.2			71.1			76
	All other	—			0.3			0.8			1.4
	Total segment income (loss) before tax expense (benefit)	22.3			(5.1		)	44.8			(9.6		)
	Reconciling items:
	GAAP expense adjustments	(0.4		)	2.4			11.4			7.7
	Interest expense on corporate debt	(1.6		)	(1.8		)	(7.1		)	(5.3		)
	Corporate expenses	(1.6		)	(1.1		)	(4.1		)	(3.1		)
	Total reconciling items	(3.6		)	(0.5		)	0.2			(0.7		)
	Total consolidated income (loss) before federal income tax expense (benefit)	$	18.7		$	(5.6	)	$	45		$	(10.3	)
	Investable assets attributable to the Company’s investment operations segment totaled $2,301.5 million and $2,327.4 million at September 30, 2013 and December 31, 2012, respectively.

Contingencies_and_Litigation

Contingencies and Litigation	9 Months Ended
	Sep. 30, 2013
Commitments and Contingencies Disclosure [Abstract]	'
Contingencies and Litigation	'
	Contingencies and Litigation
	In accordance with the Contingencies Topic of the Financial Accounting Standards Board's Accounting Standards Codification, the Company accrues for a litigation-related liability when it is probable that such a liability has been incurred and the amount can be reasonably estimated. The Company reviews all litigation on an ongoing basis when making accrual and disclosure decisions. For certain legal proceedings, the Company cannot reasonably estimate losses or a range of loss, if any, particularly for proceedings that are in their early stages of development or where the plaintiffs seek indeterminate damages. Various factors, including, but not limited to, the outcome of potentially lengthy discovery and the resolution of important factual questions, may need to be determined before probability can be established or before a loss or range of loss can be reasonably estimated. If the loss contingency in question is not both probable and reasonably estimable, the Company does not establish an accrual and the matter will continue to be monitored for any developments that would make the loss contingency both probable and reasonably estimable. Based on currently available information known to the Company, it believes that its reserves for litigation-related liabilities are reasonable. However, in the event that a legal proceeding results in a substantial judgment against, or settlement by, the Company, there can be no assurance that any resulting liability or financial commitment would not have a material adverse effect on the financial condition, results of operations or cash flows of the consolidated financial statements of State Auto Financial Corporation.
	The following describes a pending legal proceeding, other than routine litigation incidental to our business, to which State Auto Financial or any of its subsidiaries is a party or to which any of its or their property is subject:
	In April 2013, a putative class action lawsuit (Schumacher vs. State Automobile Mutual Insurance Company, et al.) was filed against State Auto Mutual, State Auto Financial and State Auto P&C in Federal District Court in Ohio. Plaintiffs claim that in connection with the homeowners policies of various State Auto companies, the coverage limits and premiums were improperly increased as a result of an insurance to value (“ITV”) program and Plaintiffs allege that they purchased coverage in excess of that which was necessary to insure them in the event of loss. Plaintiffs’ claims include breach of good faith and fair dealing, negligent misrepresentation and fraud, violation of the Ohio Deceptive Trade Practices Act, and fraudulent inducement. Plaintiffs are seeking class certification and compensatory and punitive damages to be determined by the court. The Company intends to deny any and all liability to plaintiffs or the alleged class and to vigorously defend this lawsuit.
	The Company is involved in other lawsuits arising in the ordinary course of our business operations arising out of or otherwise related to our insurance policies. Additionally, from time to time the Company may be involved in lawsuits arising in the ordinary course of business but not arising out of or otherwise related to its insurance policies. These lawsuits are in various stages of development. The Company generally will contest these matters vigorously but may pursue settlement if appropriate. Based on currently available information, the Company does not believe it is reasonably possible that any such lawsuit or related lawsuits will be material to its results of operations or have a material adverse effect on its consolidated financial or cash flow positions.
	Additionally, the Company may be impacted by adverse regulatory actions and adverse court decisions where insurance coverages are expanded beyond the scope originally contemplated in its insurance policies. The Company believes that the effects, if any, of such regulatory actions and published court decisions are not likely to have a material adverse effect on its financial or cash flow position.

Basis_of_Presentation_Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]	'
Reporting of Amounts Reclassified Out of Accumulated Other Comprehensive Income	'
	The amendments in this guidance result in additional disclosure requirements under GAAP and do not change the current requirements for reporting net income or other comprehensive income in financial statements. The new guidance requires an entity to present, either in a single note, or, parenthetically on the face of the statement where net income is presented, the effects of significant amounts reclassified from each component of accumulated other comprehensive income by the respective line items of net income, only, if the amount reclassified is required under GAAP to be reclassified to net income in its entirety in the same reporting period. If a component is not required to be reclassified to net income in its entirety, the entity would, instead, cross-reference it to the related disclosure required under GAAP. This guidance was effective prospectively for fiscal years and interim periods beginning after December 15, 2012. The Company adopted this guidance at January 1, 2013 and it did not have a material impact on the consolidated financial statements, see Note 8 - Other Comprehensive Income and Accumulated Other Comprehensive Income.

Investments_Tables

Investments (Tables)	9 Months Ended
	Sep. 30, 2013
Investments, Debt and Equity Securities [Abstract]	'
Cost or Amortized Cost and Fair Value of Available-for-Sale Securities	'
	The following tables set forth the cost or amortized cost and fair value of available-for-sale securities by lot at September 30, 2013 and December 31, 2012:
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	30-Sep-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	348.9		$	18.4		$	(3.9	)		$	363.4
	Obligations of states and political subdivisions	791.9			29.8			(16.7		)		805
	Corporate securities	326.7			11.6			(5.3		)		333
	U.S. government agencies residential mortgage-backed securities	334.6			11.4			(2.6		)		343.4
	Total fixed maturities	1,802.10			71.2			(28.5		)		1,844.80
	Equity securities:
	Large-cap securities	150.5			36.2			(2.1		)		184.6
	Small-cap securities	45.1			21.4			—				66.5
	Total equity securities	195.6			57.6			(2.1		)		251.1
	Other invested assets	49.3			27			—				76.3
	Total available-for-sale securities	$	2,047.00		$	155.8		$	(30.6	)		$	2,172.20
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	31-Dec-12
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	328.2		$	38.3		$	—			$	366.5
	Obligations of states and political subdivisions	750.4			50.3			(0.4		)		800.3
	Corporate securities	320.5			19.2			(1.1		)		338.6
	U.S. government agencies residential mortgage-backed securities	377.1			24			(1.4		)		399.7
	Total fixed maturities	1,776.20			131.8			(2.9		)		1,905.10
	Equity securities:
	Large-cap securities	152.6			25			(3.4		)		174.2
	Small-cap securities	43.6			10.6			—				54.2
	Total equity securities	196.2			35.6			(3.4		)		228.4
	Other invested assets	49			15.4			—				64.4
	Total available-for-sale securities	$	2,021.40		$	182.8		$	(6.3	)		$	2,197.90
Gross Unrealized Losses and Fair Value on its Investments	'
	The following tables set forth the Company’s gross unrealized losses and fair value on its investments by lot, aggregated by investment category and length of time for individual securities that have been in a continuous unrealized loss position at September 30, 2013 and December 31, 2012:
		Less than 12 months								12 months or more								Total
	($ millions, except # of positions)	Fair value			Unrealized losses			Number of positions		Fair value				Unrealized losses			Number of positions	Fair value		Unrealized losses			Number of positions
	30-Sep-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	86		$	(3.9	)	24		$	—			$	—		—	$	86	$	(3.9	)	24
	Obligations of states and political subdivisions	230.9			(16.7		)	81		—				—			—	230.9		(16.7		)	81
	Corporate securities	102			(4.8		)	20		5.5				(0.5		)	1	107.5		(5.3		)	21
	U.S. government agencies residential mortgage-backed securities	54.2			(1.6		)	16		27.6				(1.0		)	11	81.8		(2.6		)	27
	Total fixed maturities	473.1			(27.0		)	141		33.1				(1.5		)	12	506.2		(28.5		)	153
	Large-cap equity securities	16.2			(1.3		)	3		8.9				(0.8		)	3	25.1		(2.1		)	6
	Total temporarily impaired securities	$	489.3		$	(28.3	)	144		$	42			$	(2.3	)	15	$	531.3	$	(30.6	)	159
		Less than 12 months								12 months or more								Total
	($ millions, except # of positions)	Fair value			Unrealized losses			Number of positions		Fair value				Unrealized losses			Number of positions	Fair value		Unrealized losses			Number of positions
	31-Dec-12
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	7		$	—		2		$	—			$	—		—	$	7	$	—		2
	Obligations of states and political subdivisions	47.4			(0.4		)	12		—				—			—	47.4		(0.4		)	12
	Corporate securities	80.4			(1.1		)	17		—				—			—	80.4		(1.1		)	17
	U.S. government agencies residential mortgage-backed securities	23.3			(0.3		)	6		34.8				(1.1		)	13	58.1		(1.4		)	19
	Total fixed maturities	158.1			(1.8		)	37		34.8				(1.1		)	13	192.9		(2.9		)	50
	Large-cap equity securities	23.7			(2.1		)	4		8.9				(1.3		)	5	32.6		(3.4		)	9
	Total temporarily impaired securities	$	181.8		$	(3.9	)	41		$	43.7			$	(2.4	)	18	$	225.5	$	(6.3	)	59
Realized Losses Related to Other-than-Temporary Impairments	'
	The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013				2012
	Equity securities:
	Large-cap securities	$	—		$	—		$	(1.3	)		$	—
	Small-cap securities	(0.3		)	(0.7		)	(1.3		)		(2.2		)
	Fixed maturities:	—			—			—				(0.2		)
	Total other-than-temporary impairments	$	(0.3	)	$	(0.7	)	$	(2.6	)		$	(2.4	)
Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity	'
	The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at September 30, 2013:
	($ millions)	Amortized cost			Fair
					value
	Due in 1 year or less	$	57.2		$	57.9
	Due after 1 year through 5 years	367.1			385.8
	Due after 5 years through 10 years	374.6			391.6
	Due after 10 years	668.6			666.1
	U.S. government agencies residential mortgage-backed securities	334.6			343.4
	Total	$	1,802.10		$	1,844.80
Components of Net Investment Income	'
	The following table sets forth the components of net investment income for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013				2012
	Fixed maturities	$	16.2		$	14.7		$	48.2			$	49.1
	Equity securities	1.9			1.5			4.7				3.5
	Cash and cash equivalents, and other	1.3			1.4			4.1				4.1
	Investment income	19.4			17.6			57				56.7
	Investment expenses	0.6			0.4			1.6				1.5
	Net investment income	$	18.8		$	17.2		$	55.4			$	55.2
Realized and Unrealized Holding Gains (Losses)	'
	The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013				2012
	Realized gains:
	Fixed maturities	$	0.4		$	3.3		$	1.6			$	14
	Equity securities	7.9			4.8			17.9				11.5
	Other invested assets	0.1			—			0.1				—
	Total realized gains	8.4			8.1			19.6				25.5
	Realized losses:
	Equity securities:
	Sales	(0.3		)	(0.4		)	(1.0		)		(2.3		)
	OTTI	(0.3		)	(0.7		)	(2.6		)		(2.2		)
	Fixed maturities:
	Sales	(0.3		)	—			(0.3		)		—
	OTTI	—			—			—				(0.2		)
	Total realized losses	(0.9		)	(1.1		)	(3.9		)		(4.7		)
	Net realized gain on investments	$	7.5		$	7		$	15.7			$	20.8
	Change in unrealized holding gains (losses), net of tax:
	Fixed maturities	$	(7.7	)	$	17.2		$	(86.2	)		$	20.7
	Equity securities	5.3			8.5			23.3				13.4
	Other invested assets	7			4			11.6				4.4
	Deferred federal income tax (liability) benefit	(1.6		)	(10.4		)	18				(13.5		)
	Valuation allowance	3			10.1			(9.0		)		13.5
	Change in net unrealized holding gains (losses), net of tax	$	6		$	29.4		$	(42.3	)		$	38.5
	There was a deferred federal income tax liability, net of a valuation allowance, on the net unrealized holding gains at September 30, 2013 and December 31, 2012 of $43.5 million and $52.5 million, respectively.

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2013
Fair Value Disclosures [Abstract]	'
Company's Available-for-Sale Investments within Fair Value Hierarchy	'
	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at September 30, 2013 and December 31, 2012:
	($ millions)	Total			Quoted prices in active markets for identical assets		Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
	30-Sep-13				(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	363.4		$	—	$	363.4		$	—
	Obligations of states and political subdivisions	805			—		805			—
	Corporate securities	333			—		324.1			8.9
	U.S. government agencies residential mortgage-backed securities	343.4			—		343.4			—
	Total fixed maturities	1,844.80			—		1,835.90			8.9
	Equity securities:
	Large-cap securities	184.6			184.6		—			—
	Small-cap securities	66.5			66.5		—			—
	Total equity securities	251.1			251.1		—			—
	Other invested assets	76.3			6.3		70			—
	Total available-for-sale investments	$	2,172.20		$	257.4	$	1,905.90		$	8.9
	($ millions)	Total			Quoted prices in active markets for identical assets (Level 1)		Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
	31-Dec-12
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	366.5		$	—	$	366.5		$	—
	Obligations of states and political subdivisions	800.3			—		800.3			—
	Corporate securities	338.6			—		330.1			8.5
	U.S. government agencies residential mortgage-backed securities	399.7			—		399.7			—
	Total fixed maturities	1,905.10			—		1,896.60			8.5
	Equity securities:
	Large-cap securities	174.2			174.2		—			—
	Small-cap securities	54.2			54.2		—			—
	Total equity securities	228.4			228.4		—			—
	Other invested assets	64.4			5.4		59			—
	Total available-for-sale investments	$	2,197.90		$	233.8	$	1,955.60		$	8.5
Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	'
	For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for the three and nine months ended September 30, 2013 and the year ended December 31, 2012, separately for each major category of assets:
	($ millions)	Fixed maturities
	Balance at January 1, 2013	$	8.5
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains (losses) – included in other comprehensive income	—
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at March 31, 2013	$	8.6
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains (losses) – included in other comprehensive income	0.1
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at June 30, 2013	$	8.8
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains (losses) – included in other comprehensive income	—
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at September 30, 2013	$	8.9
	($ millions)	Fixed maturities
	Balance at January 1, 2012	$	2.9
	Total realized (losses) gains – included in earnings	(0.2		)
	Total unrealized gains (losses) – included in other comprehensive income	—
	Purchases	5.8
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at December 31, 2012	$	8.5
Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities	'
	Consequently this has been placed in Level 2 of the fair value hierarchy.
	($ millions, except interest rates)	September 30, 2013							December 31, 2012
		Carrying value			Fair value		Interest rate		Carrying value			Fair		Interest rate
												value
	Notes receivable from affiliate	$	70		$	75.7	7	%	$	70		$	78.3	7	%
Included in Notes Payable are Senior Notes and Subordinated Debentures	'
	The carrying amount of the Subordinated Debentures approximates its fair value as the interest rate adjusts quarterly and has been disclosed in Level 3.
	($ millions, except interest rates)	30-Sep-13							31-Dec-12
		Carrying value			Fair Value		Interest rate		Carrying value			Fair value		Interest rate
	FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest	$	85.3		$	91.1	4.59	%	N/A			N/A		N/A
	Senior Notes due 2013: issued $100.0, November 2003 with fixed interest	N/A			N/A		N/A		100.4			100.3		6.25	%
	Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5			15.5		4.46	%	15.5			15.5		4.51	%
	Total notes payable	$	100.8		$	106.6			$	115.9		$	115.8

Reinsurance_Tables

Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2013
Insurance [Abstract]	'
Summary of External Reinsurance Transactions with State Auto Mutual Under Pooling Arrangement	'
	The following table sets forth a summary of the Company’s external reinsurance transactions, as well as reinsurance transactions with State Auto Mutual under the Pooling Arrangement, for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013			2012
	Premiums earned:
	Assumed from external insurers and reinsurers	$	1.2		$	1.1		$	2.7		$	3.1
	Assumed under Pooling Arrangement	266			261.4			790.8			774.7
	Ceded to external insurers and reinsurers	(5.4		)	(6.6		)	(17.6		)	(22.1		)
	Ceded under Pooling Arrangement	(215.2		)	(203.9		)	(639.0		)	(600.6		)
	Net assumed premiums earned	$	46.6		$	52		$	136.9		$	155.1
	Losses and loss expenses incurred:
	Assumed from external insurers and reinsurers	$	0.9		$	1.2		$	1.3		$	3.1
	Assumed under Pooling Arrangement	179.5			201.7			540.1			595.3
	Ceded to external insurers and reinsurers	(0.8		)	(4.5		)	(4.9		)	(9.1		)
	Ceded under Pooling Arrangement	(137.8		)	(133.8		)	(434.0		)	(457.6		)
	Net assumed losses and loss expenses incurred	$	41.8		$	64.6		$	102.5		$	131.7

Income_Taxes_Tables

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'
Reconciliation Between Actual Federal Income Tax Benefit and Amount Computed at Indicated Statutory Rate	'
	The following table sets forth the reconciliation between actual federal income tax expense and the amount computed at the indicated statutory rate for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended											Nine months ended
		30-Sep											30-Sep
		2013						2012					2013					2012
					%						%					%					%
	Amount at statutory rate	$	6.6		35			$	(1.9	)	35		$	15.8		35		$	(3.6	)	35
	Tax-exempt interest and dividends received deduction	(2.5		)	(13	)		(2.3		)	41		(7.2		)	(16	)	(6.7		)	65
	Other, net	(0.3		)	(1	)		(0.1		)	2		—			—		0.3			(3	)
	Valuation allowance	(3.6		)	(19	)		4.2			(75	)	(8.0		)	(18	)	9.9			(96	)
	Federal income tax expense (benefit) and effective rate	$	0.2		2			$	(0.1	)	3		$	0.6		1		$	(0.1	)	1
Tax Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities	'
	The following table sets forth the tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities at September 30, 2013 and December 31, 2012:
	($ millions)	September 30, 2013			December 31, 2012
	Deferred tax assets:
	Unearned premiums not currently deductible	$	34.8		$	33.4
	Losses and loss expenses payable discounting	25.5			25.2
	Postretirement and pension benefits	35.8			39.4
	Realized loss on other-than-temporary impairment	9.3			7.5
	Other liabilities	17.1			14.5
	Net operating loss carryforward	56.9			66.5
	Tax credit carryforward	1.8			0.9
	Other	7.6			7.9
	Total deferred tax assets	188.8			195.3
	Deferred tax liabilities:
	Deferral of policy acquisition costs	34.5			32.1
	Net unrealized holding gains on investments	43.8			61.7
	Total deferred tax liabilities	78.3			93.8
	Total net deferred tax assets before valuation allowance	110.5			101.5
	Less valuation allowance	100.5			100.5
	Net deferred federal income taxes	$	10		$	1
Change in Valuation Allowance Attributable to Continuing Operations and Other Comprehensive Income	'
	Based on ASC 740 intraperiod tax allocation guidelines, the following sets forth the change in valuation allowance attributable to continuing operations and other comprehensive income for the three and nine months ended September 30, 2013 is as follows:
	($ millions)	Three months ended September 30, 2013			Nine months ended September 30, 2013
	Continuing operations	$	(3.6	)	$	(8.0	)
	Other comprehensive income	(3.4		)	8
	Change in valuation allowance	$	(7.0	)	$	—

Pension_and_Postretirement_Ben1

Pension and Postretirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2013
Compensation and Retirement Disclosure [Abstract]	'
Company's Share of Net Periodic Cost Components	'
	The following table sets forth the components of net periodic cost for the Company’s pension and postretirement benefit plans for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Pension						Postretirement						Pension						Postretirement
		Three months ended												Nine months ended
		30-Sep												30-Sep
		2013			2012			2013			2012			2013			2012			2013			2012
	Service cost	$	1.7		$	2		$	0.1		$	—		$	4.7		$	5.8		$	0.2		$	—
	Interest cost	2.4			2.5			0.3			0.3			7.2			7.5			0.9			0.9
	Expected return on plan assets	(3.2		)	(3.0		)	0.4			—			(9.2		)	(8.8		)	0.4			(0.2		)
	Amortization of:
	Prior service (benefits) costs	—			0.1			(1.3		)	(1.4		)	—			0.2			(4.1		)	(4.1		)
	Transition assets	—			0.1			—			—			—			—			0.2			0.2
	Net loss	2			1.8			0.2			0.2			6			5.2			0.6			0.6
	Net periodic cost	$	2.9		$	3.5		$	(0.3	)	$	(0.9	)	$	8.7		$	9.9		$	(1.8	)	$	(2.6	)

Other_Comprehensive_Income_and1

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2013
Equity [Abstract]	'
Changes in Accumulated Other Comprehensive Income by Component, Net of Tax	'
	The following tables set forth the changes in the Company’s accumulated other comprehensive income component (AOCI), net of tax, for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Unrealized Gains and Losses on Available-for-Sale Securities				Gains and Losses on Cash Flow Hedges			Benefit Plan Items			Total
	The three months ended September 30, 2013 and 2012:
	Beginning balance at July 1, 2013	$	75.7			$	—		$	(38.1	)	$	37.6
	Other comprehensive income before reclassifications	13.6				—			—			13.6
	Amounts reclassified from AOCI (a)	(7.6		)		—			0.9			(6.7		)
	Net current period other comprehensive income	6				—			0.9			6.9
	Ending balance at September 30, 2013	$	81.7			$	—		$	(37.2	)	$	44.5
	Beginning balance at July 1, 2012	$	107.8			$	0.1		$	(33.8	)	$	74.1
	Other comprehensive income before reclassifications	36.4				—			—			36.4
	Amounts reclassified from AOCI (a)	(7.0		)		—			0.8			(6.2		)
	Net current period other comprehensive income	29.4				—			0.8			30.2
	Ending balance at September 30, 2012	$	137.2			$	0.1		$	(33.0	)	$	104.3
	(a)	See separate table below for details about these reclassifications
	($ millions)	Unrealized Gains and Losses on Available-for-Sale Securities				Gains and Losses on Cash Flow Hedges			Benefit Plan Items			Total
	The nine months ended September 30, 2013 and 2012:
	Beginning balance at January 1, 2013	$	124			$	0.1		$	(39.9	)	$	84.2
	Other comprehensive income before reclassifications	(26.5		)		—			—			(26.5		)
	Amounts reclassified from AOCI (a)	(15.8		)		(0.1		)	2.7			(13.2		)
	Net current period other comprehensive income	(42.3		)		(0.1		)	2.7			(39.7		)
	Ending balance at September 30, 2013	$	81.7			$	—		$	(37.2	)	$	44.5
	Beginning balance at January 1, 2012	$	98.7			$	0.2		$	(35.1	)	$	63.8
	Other comprehensive income before reclassifications	59.3				—			—			59.3
	Amounts reclassified from AOCI (a)	(20.8		)		(0.1		)	2.1			(18.8		)
	Net current period other comprehensive income	38.5				(0.1		)	2.1			40.5
	Ending balance at September 30, 2012	$	137.2			$	0.1		$	(33.0	)	$	104.3
	(a)	See separate table below for details about these reclassifications
Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component	'
	The following tables set forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s consolidated statement of income for the three and nine months ended September 30, 2013 and 2012:
	($ millions)
	Details about Accumulated Other	Three months ended						Affected line item in the Condensed
	Comprehensive Income Components	30-Sep						Consolidated Statements of Income
		2013			2012
	Unrealized gains (losses) on available for sale securities	7.5			7			Realized gain (loss) on sale of securities
		7.5			7			Total before tax
		0.1			—			Tax benefit (expense)
		7.6			7			Net of tax
	Amortization of benefit plan items
	Negative prior service costs	1.3			1.3			(a)
	Transition assets	—			(0.1	)		(a)
	Net loss	(2.2	)		(2.0	)		(a)
		(0.9	)		(0.8	)		Total before tax
		—			—			Tax benefit (expense)
		(0.9	)		(0.8	)		Net of tax
	Total reclassifications for the period	6.7			6.2
	(a)	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).
	($ millions)
	Details about Accumulated Other	Nine months ended							Affected line item in the Condensed
	Comprehensive Income Components	30-Sep							Consolidated Statements of Income
		2013				2012
	Unrealized gains (losses) on available for sale securities	$	15.7			$	20.8		Realized gain (loss) on sale of securities
		15.7				20.8			Total before tax
		0.1				—			Tax benefit (expense)
		15.8				20.8			Net of tax
	Amortization of gain on derivative used in cash flow hedge	0.1				0.1			Realized gain (loss) on sale of securities
		0.1				0.1			Total before tax
		—				—			Tax benefit (expense)
		0.1				0.1			Net of tax
	Amortization of benefit plan items
	Negative prior service costs	4.1				3.9			(a)
	Transition assets	(0.2		)		(0.2		)	(a)
	Net loss	(6.6		)		(5.8		)	(a)
		(2.7		)		(2.1		)	Total before tax
		—				—			Tax benefit (expense)
		(2.7		)		(2.1		)	Net of tax
	Total reclassifications for the period	$	13.2			$	18.8
	(a)	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings_Loss_per_Common_Share1

Earnings (Loss) per Common Share (Tables)	9 Months Ended
	Sep. 30, 2013
Earnings Per Share [Abstract]	'
Compilation of Basic and Diluted Earnings (Loss) Per Common Share	'
	The following table sets forth the compilation of basic and diluted earnings (loss) per common share for the three and nine months ended September 30, 2013 and 2012:
	($ and shares in millions, except per share amounts)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013		2012
	Numerator:
	Net income (loss) for basic earnings (loss) per common share	$	18.5		$	(5.5	)	$	44.4	$	(10.2	)
	Denominator:
	Weighted average shares for basic net earnings (loss) per common share	40.6			40.4			40.5		40.4
	Effect of dilutive share-based awards	0.3			—			0.2		—
	Adjusted weighted average shares for diluted net earnings (loss) per common share	40.9			40.4			40.7		40.4
	Basic net earnings (loss) per common share	$	0.46		$	(0.14	)	$	1.09	$	(0.25	)
	Diluted net earnings (loss) per common share	$	0.45		$	(0.14	)	$	1.09	$	(0.25	)
Options to Purchase Shares of Common Stock	'
	The following table sets forth the options to purchase shares of common stock and the restricted share units ("RSU award") provided to each outside director of the Company, that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three and nine months ended September 30, 2013 and 2012.
	(number of options and awards in millions)	Three months ended September 30					Nine months ended September 30
		2013		2012			2013		2012
	Number of options	2.1		3.7			2.7		3.7
	Number of RSU awards	0.2		—			0.1		—
	Total number of antidilutive shares and awards	2.3		3.7			2.8		3.7

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2013
Segment Reporting [Abstract]	'
Financial Information Regarding Company's Reportable Segments	'
	The following table sets forth financial information regarding the Company’s reportable segments for the three and nine months ended September 30, 2013 and 2012:
	($ millions)	Three months ended September 30						Nine months ended September 30
		2013			2012			2013			2012
	Revenues from external sources:
	Insurance segments
	Personal insurance	$	116.4		$	117.4		$	348.3		$	352.1
	Business insurance	92.9			83.4			271			240.1
	Specialty insurance	56.7			60.6			171.5			182.5
	Total insurance segments	266			261.4			790.8			774.7
	Investment operations segment
	Net investment income	18.8			17.2			55.4			55.2
	Net realized capital gains	7.5			7			15.7			20.8
	Total investment operations segment	26.3			24.2			71.1			76
	All other	0.4			0.8			1.4			2.7
	Total revenues from external sources	292.7			286.4			863.3			853.4
	Intersegment revenues:	1.4			5.6			4			10.3
	Total revenues	294.1			292			867.3			863.7
	Reconciling items:
	Eliminate intersegment revenues	(1.4		)	(5.6		)	(4.0		)	(10.3		)
	Total consolidated revenues	$	292.7		$	286.4		$	863.3		$	853.4
	Segment (loss) income before federal income tax:
	Insurance segments SAP underwriting (loss) income
	Personal insurance	$	(6.0	)	$	2.9		$	(11.5	)	$	(8.8	)
	Business insurance	(1.1		)	(7.4		)	(11.0		)	(40.8		)
	Specialty insurance	3.1			(25.1		)	(4.6		)	(37.4		)
	Total insurance segments	(4.0		)	(29.6		)	(27.1		)	(87.0		)
	Investment operations segment
	Net investment income	18.8			17.2			55.4			55.2
	Net realized capital gains	7.5			7			15.7			20.8
	Total investment operations segment	26.3			24.2			71.1			76
	All other	—			0.3			0.8			1.4
	Total segment income (loss) before tax expense (benefit)	22.3			(5.1		)	44.8			(9.6		)
	Reconciling items:
	GAAP expense adjustments	(0.4		)	2.4			11.4			7.7
	Interest expense on corporate debt	(1.6		)	(1.8		)	(7.1		)	(5.3		)
	Corporate expenses	(1.6		)	(1.1		)	(4.1		)	(3.1		)
	Total reconciling items	(3.6		)	(0.5		)	0.2			(0.7		)
	Total consolidated income (loss) before federal income tax expense (benefit)	$	18.7		$	(5.6	)	$	45		$	(10.3	)

Investments_Cost_or_Amortized_

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	$2,047	$2,021.40
Gross unrealized holding gains	155.8	182.8
Gross unrealized holding losses	-30.6	-6.3
Fair value	2,172.20	2,197.90
Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	1,802.10	1,776.20
Gross unrealized holding gains	71.2	131.8
Gross unrealized holding losses	-28.5	-2.9
Fair value	1,844.80	1,905.10
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	348.9	328.2
Gross unrealized holding gains	18.4	38.3
Gross unrealized holding losses	-3.9	0
Fair value	363.4	366.5
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	791.9	750.4
Gross unrealized holding gains	29.8	50.3
Gross unrealized holding losses	-16.7	-0.4
Fair value	805	800.3
Fixed Maturities [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	326.7	320.5
Gross unrealized holding gains	11.6	19.2
Gross unrealized holding losses	-5.3	-1.1
Fair value	333	338.6
Fixed Maturities [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	334.6	377.1
Gross unrealized holding gains	11.4	24
Gross unrealized holding losses	-2.6	-1.4
Fair value	343.4	399.7
Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	195.6	196.2
Gross unrealized holding gains	57.6	35.6
Gross unrealized holding losses	-2.1	-3.4
Fair value	251.1	228.4
Equity Securities [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	150.5	152.6
Gross unrealized holding gains	36.2	25
Gross unrealized holding losses	-2.1	-3.4
Fair value	184.6	174.2
Equity Securities [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	45.1	43.6
Gross unrealized holding gains	21.4	10.6
Gross unrealized holding losses	0	0
Fair value	66.5	54.2
Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	49.3	49
Gross unrealized holding gains	27	15.4
Gross unrealized holding losses	0	0
Fair value	$76.30	$64.40

Investments_Gross_Unrealized_L

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified	Positions	Positions
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	$489.30	$181.80
Less than 12 months, Unrealized losses	-28.3	-3.9
Less than 12 months, Number of positions (positions)	144	41
12 months or more, Fair value	42	43.7
12 months or more, Unrealized losses	-2.3	-2.4
12 months or more, Number of positions (positions)	15	18
Total, Fair value	531.3	225.5
Total, Unrealized losses	-30.6	-6.3
Total, Number of positions	159	59
Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	473.1	158.1
Less than 12 months, Unrealized losses	-27	-1.8
Less than 12 months, Number of positions (positions)	141	37
12 months or more, Fair value	33.1	34.8
12 months or more, Unrealized losses	-1.5	-1.1
12 months or more, Number of positions (positions)	12	13
Total, Fair value	506.2	192.9
Total, Unrealized losses	-28.5	-2.9
Total, Number of positions	153	50
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	86	7
Less than 12 months, Unrealized losses	-3.9	0
Less than 12 months, Number of positions (positions)	24	2
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
12 months or more, Number of positions (positions)	0	0
Total, Fair value	86	7
Total, Unrealized losses	-3.9	0
Total, Number of positions	24	2
Obligations of States and Political Subdivisions [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	230.9	47.4
Less than 12 months, Unrealized losses	-16.7	-0.4
Less than 12 months, Number of positions (positions)	81	12
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
12 months or more, Number of positions (positions)	0	0
Total, Fair value	230.9	47.4
Total, Unrealized losses	-16.7	-0.4
Total, Number of positions	81	12
Corporate Securities [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	102	80.4
Less than 12 months, Unrealized losses	-4.8	-1.1
Less than 12 months, Number of positions (positions)	20	17
12 months or more, Fair value	5.5	0
12 months or more, Unrealized losses	-0.5	0
12 months or more, Number of positions (positions)	1	0
Total, Fair value	107.5	80.4
Total, Unrealized losses	-5.3	-1.1
Total, Number of positions	21	17
U.S. Government Agencies Residential Mortgage-Backed Securities [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	54.2	23.3
Less than 12 months, Unrealized losses	-1.6	-0.3
Less than 12 months, Number of positions (positions)	16	6
12 months or more, Fair value	27.6	34.8
12 months or more, Unrealized losses	-1	-1.1
12 months or more, Number of positions (positions)	11	13
Total, Fair value	81.8	58.1
Total, Unrealized losses	-2.6	-1.4
Total, Number of positions	27	19
Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	16.2	23.7
Less than 12 months, Unrealized losses	-1.3	-2.1
Less than 12 months, Number of positions (positions)	3	4
12 months or more, Fair value	8.9	8.9
12 months or more, Unrealized losses	-0.8	-1.3
12 months or more, Number of positions (positions)	3	5
Total, Fair value	25.1	32.6
Total, Unrealized losses	($2.10)	($3.40)
Total, Number of positions	6	9

Investments_Realized_Losses_Re

Investments - Realized Losses Related to Other-than-Temporary Impairments (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 Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairments	($0.30)	($0.70)	($2.60)	($2.40)
Fixed Maturities [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairments	0	0	0	-0.2
Large-Cap Equity Securities [Member] | Equity Securities [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairments	0	0	-1.3	0
Small-Cap Securities [Member] | Equity Securities [Member]	'	'	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Other-than-temporary impairments	($0.30)	($0.70)	($1.30)	($2.20)

Investments_Amortized_Cost_and

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Amortized cost	'	'
Due in 1 year or less, Amortized cost	$57.20	'
Due after 1 year through 5 years, Amortized cost	367.1	'
Due after 5 years through 10 years, Amortized cost	374.6	'
Due after 10 years, Amortized cost	668.6	'
U.S. government agencies residential mortgage-backed securities, Amortized cost	334.6	'
Total, Amortized cost	1,802.10	1,776.20
Fair value	'	'
Due in 1 year or less, Fair value	57.9	'
Due after 1 year through 5 years, Fair value	385.8	'
Due after 5 years through 10 years, Fair value	391.6	'
Due after 10 years, Fair value	666.1	'
U.S. government agencies residential mortgage-backed securities, Fair value	343.4	'
Total, Fair value	$1,844.80	$1,905.10

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Jul. 11, 2013
In Millions, unless otherwise specified			Federal Home Loan Bank of Cincinnati [Member]	Federal Home Loan Bank of Cincinnati [Member]
Schedule of Available-for-sale Securities [Line Items]	'	'	'	'
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	'	'	$118	'
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	'	'	'	85
Fixed maturities fair value of deposit with insurance regulators	8.6	10	'	'
Deferred federal income tax liability on the net unrealized holding gains	$43.50	$52.50	'	'

Investments_Components_of_Net_

Investments - Components of Net Investment 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
Net Investment Income [Line Items]	'	'	'	'
Investment income	$19.40	$17.60	$57	$56.70
Investment expenses	0.6	0.4	1.6	1.5
Net investment income	18.8	17.2	55.4	55.2
Fixed Maturities [Member]	'	'	'	'
Net Investment Income [Line Items]	'	'	'	'
Investment income	16.2	14.7	48.2	49.1
Equity Securities [Member]	'	'	'	'
Net Investment Income [Line Items]	'	'	'	'
Investment income	1.9	1.5	4.7	3.5
Cash and Cash Equivalents, and Other [Member]	'	'	'	'
Net Investment Income [Line Items]	'	'	'	'
Investment income	$1.30	$1.40	$4.10	$4.10

Investments_Realized_and_Unrea

Investments - Realized and Unrealized Holding Gains (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
Realized gains:	'	'	'	'
Total realized gains	$8.40	$8.10	$19.60	$25.50
Realized losses:	'	'	'	'
Total realized losses	-0.9	-1.1	-3.9	-4.7
Net realized gain on investments	7.5	7	15.7	20.8
Change in unrealized holding gains (losses), net of tax:	'	'	'	'
Deferred federal income tax (liability) benefit	-1.6	-10.4	18	-13.5
Valuation allowance	3	10.1	-9	13.5
Change in net unrealized holding gains (losses), net of tax	6	29.4	-42.3	38.5
Fixed Maturities [Member]	'	'	'	'
Realized gains:	'	'	'	'
Total realized gains	0.4	3.3	1.6	14
Change in unrealized holding gains (losses), net of tax:	'	'	'	'
Change in net unrealized holding gains, net of tax	-7.7	17.2	-86.2	20.7
Fixed Maturities [Member] | Sales [Member]	'	'	'	'
Realized losses:	'	'	'	'
Total realized losses	-0.3	0	-0.3	0
Fixed Maturities [Member] | OTTI [Member]	'	'	'	'
Realized losses:	'	'	'	'
Total realized losses	0	0	0	-0.2
Equity Securities [Member]	'	'	'	'
Realized gains:	'	'	'	'
Total realized gains	7.9	4.8	17.9	11.5
Change in unrealized holding gains (losses), net of tax:	'	'	'	'
Change in net unrealized holding gains, net of tax	5.3	8.5	23.3	13.4
Equity Securities [Member] | Sales [Member]	'	'	'	'
Realized losses:	'	'	'	'
Total realized losses	-0.3	-0.4	-1	-2.3
Equity Securities [Member] | OTTI [Member]	'	'	'	'
Realized losses:	'	'	'	'
Total realized losses	-0.3	-0.7	-2.6	-2.2
Other Invested Assets [Member]	'	'	'	'
Realized gains:	'	'	'	'
Total realized gains	0.1	0	0.1	0
Change in unrealized holding gains (losses), net of tax:	'	'	'	'
Change in net unrealized holding gains, net of tax	$7	$4	$11.60	$4.40

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	1 Months Ended
In Millions, unless otherwise specified	31-May-09	Sep. 30, 2013	Dec. 31, 2012
Fair Value Measurements Disclosure [Line Items]	'	'	'
Total available-for-sale investments	'	$2,172.20	$2,197.90
Other invested assets	'	5	0.5
Number of Credit Agreements	2	'	'
Notes receivable from affiliate	'	70	70
Significant Unobservable Inputs (Level 3) [Member]	'	'	'
Fair Value Measurements Disclosure [Line Items]	'	'	'
Total available-for-sale investments	'	8.9	8.5
Federal Home Loan Bank Stock	'	4.5	'
Other invested assets	'	0.5	'
Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]	'	'	'
Fair Value Measurements Disclosure [Line Items]	'	'	'
Number of fixed maturity corporate securities included in Level 3 (securities)	'	2	'
Total available-for-sale investments	'	8.9	8.5
International funds [Member]	'	'	'
Fair Value Measurements Disclosure [Line Items]	'	'	'
Number of fixed maturity corporate securities included in Level 3 (securities)	'	2	'
Total available-for-sale investments	'	70	59
Affiliated Entity [Member]	'	'	'
Fair Value Measurements Disclosure [Line Items]	'	'	'
Notes receivable from affiliate	'	$70	'

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	$2,172.20	$2,197.90
Fair Value [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	2,172.20	2,197.90
Fair Value [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	363.4	366.5
Fair Value [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	805	800.3
Fair Value [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	333	338.6
Fair Value [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	343.4	399.7
Fair Value [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	1,844.80	1,905.10
Fair Value [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	184.6	174.2
Fair Value [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	66.5	54.2
Fair Value [Member] | Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	76.3	64.4
Fair Value [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	251.1	228.4
Quoted prices in active markets for identical assets (Level 1) [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	257.4	233.8
Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	184.6	174.2
Quoted prices in active markets for identical assets (Level 1) [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	66.5	54.2
Quoted prices in active markets for identical assets (Level 1) [Member] | Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	6.3	5.4
Quoted prices in active markets for identical assets (Level 1) [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	251.1	228.4
Significant Other Observable Inputs (Level 2) [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	1,905.90	1,955.60
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	363.4	366.5
Significant Other Observable Inputs (Level 2) [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	805	800.3
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	324.1	330.1
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	343.4	399.7
Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	1,835.90	1,896.60
Significant Other Observable Inputs (Level 2) [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Other Observable Inputs (Level 2) [Member] | Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	70	59
Significant Other Observable Inputs (Level 2) [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	8.9	8.5
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	8.9	8.5
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	8.9	8.5
Significant Unobservable Inputs (Level 3) [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	$0	$0

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (Available-for-Sale Securities [Member], Fair Value, Measurements, Recurring [Member], USD $)	3 Months Ended			12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Available-for-Sale Securities [Member] | Fair Value, Measurements, Recurring [Member]	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning Balance	$8.80	$8.60	$8.50	$2.90
Total realized gains (losses) - included in earnings	0	0	0	-0.2
Total unrealized gains (losses) - included in other comprehensive income	0	0.1	0	0
Purchases	0.1	0.1	0.1	5.8
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending Balance	$8.90	$8.80	$8.60	$8.50

Fair_Value_of_Financial_Instru5

Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) (Affiliated Entity [Member], Significant Other Observable Inputs (Level 2) [Member], USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Notes receivable interest rate (percent)	7.00%	7.00%
Carrying Value [Member]	'	'
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Notes receivable from affiliate	70	70
Fair Value [Member]	'	'
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Notes receivable from affiliate	75.7	78.3

Fair_Value_of_Financial_Instru6

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Carrying Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	$100.80	$115.90
Fair Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	106.6	115.8
Federal Home Loan Bank Advances [Member]	'	'
Debt Instrument [Line Items]	'	'
Interest rate (percent)	4.59%	'
Federal Home Loan Bank Advances [Member] | Carrying Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	85.3	'
Federal Home Loan Bank Advances [Member] | Fair Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	91.1	'
Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
Interest rate (percent)	'	6.25%
Senior Notes [Member] | Carrying Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	'	100.4
Senior Notes [Member] | Fair Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	'	100.3
Affiliate Subordinated Debentures Due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Interest rate (percent)	4.46%	4.51%
Affiliate Subordinated Debentures Due 2033 [Member] | Carrying Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	15.5	15.5
Affiliate Subordinated Debentures Due 2033 [Member] | Fair Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	$15.50	$15.50

Fair_Value_of_Financial_Instru7

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Parenthetical) (Detail) (USD $)	Sep. 30, 2013	Jul. 15, 2013
In Millions, unless otherwise specified
Federal Home Loan Bank Advances [Member]	'	'
Debt Instrument [Line Items]	'	'
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$85	'
Senior Notes [Member]	'	'
Debt Instrument [Line Items]	'	'
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	100	100
Affiliate Subordinated Debentures Due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$15.50	'

Reinsurance_Summary_of_Externa

Reinsurance - Summary of External Reinsurance Transactions, as well as reinsurance transactions with State Auto Mutual Under the Pooling Arrangement (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
Premiums earned:	'	'	'	'
Assumed from external insurers and reinsurers	$1.20	$1.10	$2.70	$3.10
Assumed under Pooling Arrangement	266	261.4	790.8	774.7
Ceded to external insurers and reinsurers	-5.4	-6.6	-17.6	-22.1
Ceded under Pooling Arrangement	-215.2	-203.9	-639	-600.6
Net assumed premiums earned	46.6	52	136.9	155.1
Losses and loss expenses incurred:	'	'	'	'
Assumed from external insurers and reinsurers	0.9	1.2	1.3	3.1
Assumed under Pooling Arrangement	179.5	201.7	540.1	595.3
Ceded to external insurers and reinsurers	-0.8	-4.5	-4.9	-9.1
Ceded under Pooling Arrangement	-137.8	-133.8	-434	-457.6
Net assumed losses and loss expenses incurred	$41.80	$64.60	$102.50	$131.70

Notes_Payable_and_Credit_Facil1

Notes Payable and Credit Facility - Additional Information (Detail) (USD $)	1 Months Ended		0 Months Ended		0 Months Ended	9 Months Ended
	Jul. 26, 2013	Sep. 30, 2013	Jul. 15, 2013	Sep. 30, 2013	Jul. 11, 2013	Sep. 30, 2013
			Senior Notes [Member]	Senior Notes [Member]	Federal Home Loan Bank of Cincinnati [Member]	Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]	'	'	'	'	'	'
Term amount loan	'	'	'	'	$85,000,000	'
Federal Home Loan Bank Term	'	'	'	'	'20 years	'
Federal Home Loan Bank Non Callable Period	'	'	'	'	'3 years	'
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Repayment and Penalties	'	'	'	'	0	'
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	'	'	'	'	5.03%	'
Interest Expense	'	'	'	'	'	900,000
Redemption of outstanding	'	'	100,000,000	100,000,000	'	'
Redemption amount paid by company	'	'	103,000,000	'	'	'
Current revolving credit facility	'	100,000,000	'	'	'	'
Credit facility period	'5 years	'	'	'	'	'
Maximum amount of revolving credit facility	'	$150,000,000	'	'	'	'

Income_Taxes_Reconciliation_Be

Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (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
Effective Income Tax Rate Reconciliation, Amount [Abstract]	'	'	'	'
Amount at statutory rate	$6.60	($1.90)	$15.80	($3.60)
Tax-exempt interest and dividends received deduction	-2.5	-2.3	-7.2	-6.7
Other, net	-0.3	-0.1	0	0.3
Valuation allowance	-3.6	4.2	-8	9.9
Federal income tax expense (benefit) and effective rate	$0.20	($0.10)	$0.60	($0.10)
Effective Income Tax Rate Reconciliation, Percent [Abstract]	'	'	'	'
Amount at statutory rate (percent)	35.00%	35.00%	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	-13.00%	41.00%	-16.00%	65.00%
Other, net (percent)	-1.00%	2.00%	0.00%	-3.00%
Valuation allowance (percent)	-19.00%	-75.00%	-18.00%	-96.00%
Federal income tax expense (benefit) and effective rate (percent)	2.00%	3.00%	1.00%	1.00%

Income_Taxes_Tax_Effects_of_Te

Income Taxes - Tax Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Deferred tax assets:	'	'
Unearned premiums not currently deductible	$34.80	$33.40
Losses and loss expenses payable discounting	25.5	25.2
Postretirement and pension benefits	35.8	39.4
Realized loss on other-than-temporary impairment	9.3	7.5
Other liabilities	17.1	14.5
Net operating loss carryforward	56.9	66.5
Tax credit carryforward	1.8	0.9
Other	7.6	7.9
Total deferred tax assets	188.8	195.3
Deferred tax liabilities:	'	'
Deferral of policy acquisition costs	34.5	32.1
Net unrealized holding gains on investments	43.8	61.7
Total deferred tax liabilities	78.3	93.8
Total net deferred tax assets before valuation allowance	110.5	101.5
Less valuation allowance	100.5	100.5
Net deferred federal income taxes	$10	$1

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Income Tax Disclosure [Abstract]	'	'
Net deferred tax asset valuation allowance	$100.50	$100.50

Income_Taxes_Changes_in_Valuat

Income Taxes - Changes in Valuation Allowance Attributable to Continuing Operations and Other Comprehensive Income (Detail) (USD $)	3 Months Ended	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013
Income Tax Disclosure [Abstract]	'	'
Continuing operations	($3.60)	($8)
Other comprehensive income	-3.4	8
Change in valuation allowance	($7)	$0

Pension_and_Postretirement_Ben2

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (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 [Member]	'	'	'	'
Pension And Other Employee Benefit Plans [Line Items]	'	'	'	'
Service cost	$1.70	$2	$4.70	$5.80
Interest cost	2.4	2.5	7.2	7.5
Expected return on plan assets	-3.2	-3	-9.2	-8.8
Amortization of:	'	'	'	'
Prior service (benefits) costs	0	0.1	0	0.2
Transition assets	0	0.1	0	0
Net loss	2	1.8	6	5.2
Net periodic cost	2.9	3.5	8.7	9.9
Postretirement [Member]	'	'	'	'
Pension And Other Employee Benefit Plans [Line Items]	'	'	'	'
Service cost	0.1	0	0.2	0
Interest cost	0.3	0.3	0.9	0.9
Expected return on plan assets	0.4	0	0.4	-0.2
Amortization of:	'	'	'	'
Prior service (benefits) costs	-1.3	-1.4	-4.1	-4.1
Transition assets	0	0	0.2	0.2
Net loss	0.2	0.2	0.6	0.6
Net periodic cost	($0.30)	($0.90)	($1.80)	($2.60)

Pension_and_Postretirement_Ben3

Pension and Postretirement Benefit Plans - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013
Compensation and Retirement Disclosure [Abstract]	'
Employer contribution	$13

Other_Comprehensive_Income_and2

Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (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), Net of Tax [Abstract]	'		'		'		'
Beginning balance	$37.60		$74.10		$84.20		$63.80
Other comprehensive income before reclassifications	13.6		36.4		-26.5		59.3
Amounts reclassified from AOCI	-6.7	[1]	-6.2	[1]	-13.2	[1]	-18.8	[1]
Other comprehensive income, net of tax	6.9		30.2		-39.7		40.5
Ending balance	44.5		104.3		44.5		104.3
Unrealized Gains and Losses on Available-for-Sale Securities [Member]	'		'		'		'
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'		'		'		'
Beginning balance	75.7		107.8		124		98.7
Other comprehensive income before reclassifications	13.6		36.4		-26.5		59.3
Amounts reclassified from AOCI	-7.6	[1]	-7	[1]	-15.8	[1]	-20.8	[1]
Other comprehensive income, net of tax	6		29.4		-42.3		38.5
Ending balance	81.7		137.2		81.7		137.2
Gains and Losses on Cash Flow Hedges [Member]	'		'		'		'
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'		'		'		'
Beginning balance	0		0.1		0.1		0.2
Other comprehensive income before reclassifications	0		0		0		0
Amounts reclassified from AOCI	0	[1]	0	[1]	-0.1	[1]	-0.1	[1]
Other comprehensive income, net of tax	0		0		-0.1		-0.1
Ending balance	0		0.1		0		0.1
Benefit Plan Items [Member]	'		'		'		'
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'		'		'		'
Beginning balance	-38.1		-33.8		-39.9		-35.1
Other comprehensive income before reclassifications	0		0		0		0
Amounts reclassified from AOCI	0.9	[1]	0.8	[1]	2.7	[1]	2.1	[1]
Other comprehensive income, net of tax	0.9		0.8		2.7		2.1
Ending balance	($37.20)		($33)		($37.20)		($33)
[1]	See separate table below for details about these reclassifications

Other_Comprehensive_Income_and3

Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component (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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Realized gain/(loss) on sale of securities	$7.50		$7		$15.70		$20.80
Accumulated other comprehensive income components/Transition asset	-1.3		-1.3		-4.1		-3.9
Tax (expense) benefit	-0.2		0.1		-0.6		0.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	6.7	[1]	6.2	[1]	13.2	[1]	18.8	[1]
Unrealized Gains and Losses on Available-for-Sale Securities [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	7.6	[1]	7	[1]	15.8	[1]	20.8	[1]
Gains and Losses on Cash Flow Hedges [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	0	[1]	0	[1]	0.1	[1]	0.1	[1]
Benefit Plan Items [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	-0.9	[1]	-0.8	[1]	-2.7	[1]	-2.1	[1]
Reclassification Out of Accumulated Other Comprehensive Income [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	6.7		'		13.2		18.8
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	'		6.2		'		'
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Realized gain/(loss) on sale of securities	7.5		7		15.7		20.8
Total before tax	7.5		7		15.7		20.8
Tax (expense) benefit	0.1		0		0.1		0
Net of tax	7.6		7		15.8		20.8
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Gains and Losses on Cash Flow Hedges [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Realized gain/(loss) on sale of securities	'		'		0.1		0.1
Total before tax	'		'		0.1		0.1
Tax (expense) benefit	'		'		0		0
Net of tax	'		'		0.1		0.1
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Benefit Plan Items [Member]	'		'		'		'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'		'		'		'
Accumulated other comprehensive income components/Transition asset	1.3		1.3	[2]	4.1		3.9	[2]
Accumulated other comprehensive income components/Negative prior-service costs	0		-0.1	[2]	-0.2		-0.2	[2]
Accumulated other comprehensive income components/Net actuarial loss	-2.2		-2	[2]	-6.6		-5.8	[2]
Total before tax	-0.9		-0.8		-2.7		-2.1
Tax (expense) benefit	0		0		0		0
Net of tax	($0.90)		($0.80)		($2.70)		($2.10)
[1]	See separate table below for details about these reclassifications
[2]	These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Recovered_Sheet1

Earnings (Loss) Per Common Share - Compilation of Basic and Diluted Earnings (Loss) Per Common Share (Detail) (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
Numerator:	'	'	'	'
Net income (loss)	$18.50	($5.50)	$44.40	($10.20)
Denominator:	'	'	'	'
Weighted average shares for basic net earnings (loss) per common share (shares)	40.6	40.4	40.5	40.4
Effect of dilutive share-based awards (shares)	0.3	0	0.2	0
Adjusted weighted average shares for diluted net earnings (loss) per common share (shares)	40.9	40.4	40.7	40.4
Basic net earnings (loss) per common share (usd per share)	$0.46	($0.14)	$1.09	($0.25)
Diluted net earnings (loss) per common share (usd per share)	$0.45	($0.14)	$1.09	($0.25)

Recovered_Sheet2

Earnings (Loss) Per Common Share - Options to Purchase Shares of Common Stock (Detail)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'	'
Number of options (shares)	2.3	3.7	2.8	3.7
Stock Option [Member]	'	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'	'
Number of options (shares)	2.1	3.7	2.7	3.7
Restricted Stock Units (RSUs) [Member]	'	'	'	'
Schedule of Trading Securities and Other Trading Assets [Line Items]	'	'	'	'
Number of options (shares)	0.2	0	0.1	0

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
	Segment
Segment Reporting [Abstract]	'	'
Number of reportable segments (segments)	4	'
Investable assets	$2,301.50	$2,327.40

Segment_Information_Financial_

Segment Information - Financial Information Regarding Company's Reportable 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
Revenues from external sources:	'	'	'	'
Revenue	$292.70	$286.40	$863.30	$853.40
Intersegment revenues	1.4	5.6	4	10.3
Total revenues	294.1	292	867.3	863.7
Segment loss before federal income tax:	'	'	'	'
Interest expense on corporate debt	-1.6	-1.8	-7.1	-5.3
Income (loss) before federal income taxes	18.7	-5.6	45	-10.3
Operating Segments [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	292.7	286.4	863.3	853.4
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	22.3	-5.1	44.8	-9.6
Operating Segments [Member] | Total Insurance Segments [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	266	261.4	790.8	774.7
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	-4	-29.6	-27.1	-87
Operating Segments [Member] | Total Insurance Segments [Member] | Personal Insurance [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	116.4	117.4	348.3	352.1
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	-6	2.9	-11.5	-8.8
Operating Segments [Member] | Total Insurance Segments [Member] | Business Insurance [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	92.9	83.4	271	240.1
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	-1.1	-7.4	-11	-40.8
Operating Segments [Member] | Total Insurance Segments [Member] | Specialty Insurance [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	56.7	60.6	171.5	182.5
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	3.1	-25.1	-4.6	-37.4
Operating Segments [Member] | Total Investment Operations [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	26.3	24.2	71.1	76
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	26.3	24.2	71.1	76
Operating Segments [Member] | Total Investment Operations [Member] | Net Investment Income [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	18.8	17.2	55.4	55.2
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	18.8	17.2	55.4	55.2
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	7.5	7	15.7	20.8
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	7.5	7	15.7	20.8
Operating Segments [Member] | All Other Segments [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	0.4	0.8	1.4	2.7
Segment loss before federal income tax:	'	'	'	'
Income (loss) before federal income taxes	0	0.3	0.8	1.4
Eliminate Intersegment Revenues [Member]	'	'	'	'
Revenues from external sources:	'	'	'	'
Revenue	-1.4	-5.6	-4	-10.3
Reconciling Items [Member]	'	'	'	'
Segment loss before federal income tax:	'	'	'	'
GAAP expense adjustments	-0.4	2.4	11.4	7.7
Interest expense on corporate debt	-1.6	-1.8	-7.1	-5.3
Corporate expenses	-1.6	-1.1	-4.1	-3.1
Income (loss) before federal income taxes	($3.60)	($0.50)	$0.20	($0.70)