State Auto Financial (STFC) 10-Q 1 May 14 2014 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2014	Apr. 25, 2014
Document Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	31-Mar-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q1	'
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)	'	40,860,803

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Assets	'	'
Fixed maturities, available-for-sale, at fair value (amortized cost $1,799.9 and $1,804.0, respectively)	$1,840.40	$1,830.10
Equity securities, available-for-sale, at fair value (cost $210.4 and $196.6, respectively)	274.6	265.3
Other invested assets, available-for-sale, at fair value (cost $49.6 and $49.5, respectively)	83.4	80.9
Other invested assets	5	5
Notes receivable from affiliate	70	70
Total investments	2,273.40	2,251.30
Cash and cash equivalents	78.3	80.3
Accrued investment income and other assets	35.1	33.6
Deferred policy acquisition costs	99.7	96.8
Reinsurance recoverable on losses and loss expenses payable	9.5	9.1
Prepaid reinsurance premiums	4.9	4.7
Current federal income taxes, receivable	0	0.3
Net deferred federal income taxes	7.7	11.9
Property and equipment, at cost	8.3	8.4
Total assets	2,516.90	2,496.40
Liabilities and Stockholders’ Equity	'	'
Losses and loss expenses payable (affiliates $425.9 and $438.0, respectively)	951.7	959.9
Unearned premiums (affiliates $85.4 and $78.4, respectively)	493.9	491
Notes payable (affiliates $15.5 and $15.5, respectively)	100.8	100.8
Postretirement and pension benefits	73.1	71.6
Due to affiliate	8.3	1.3
Current federal income taxes	0.4	0
Other liabilities	71.2	86.8
Total liabilities	1,699.40	1,711.40
Stockholders’ equity:	'	'
Common stock, without par value. Authorized 100.0 shares; 47.6 and 47.5 shares issued, respectively, at stated value of $2.50 per share	119	118.8
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	-116	-115.9
Additional paid-in capital	138.7	137.5
Accumulated other comprehensive income	89.1	80.8
Retained earnings	586.7	563.8
Total stockholders’ equity	817.5	785
Total liabilities and stockholders’ equity	2,516.90	2,496.40
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 $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, except Per Share data, unless otherwise specified
Fixed maturities, available-for-sale, amortized cost	$1,799.90	$1,804
Equity securities, available-for-sale, cost	210.4	196.6
Other invested assets, available-for-sale, cost	49.6	49.5
Losses and loss expenses payable, affiliates	425.9	438
Unearned premiums, affiliates	85.4	78.4
Notes payable, affiliates	$15.50	$15.50
Common stock, shares authorized (shares)	100	100
Common stock, shares issued (shares)	47.6	47.5
Common stock, stated value per share (usd per share)	$2.50	$2.50
Treasury stock (shares)	6.8	6.8
Class A Preferred Stock [Member]	'	'
Preferred stock, no par value (usd per share)	$0	$0
Preferred stock, shares authorized (shares)	2.5	2.5
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)	2.5	2.5
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
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]	'	'
Earned premiums (ceded to affiliates $215.4 and $210.7 for the three months ended, respectively)	$262.50	$261.30
Net investment income (affiliate $1.2 and $1.2 for the three months ended, respectively)	17.6	16.9
Net realized gains on investments:	'	'
Total other-than-temporary impairment losses	-0.8	-0.3
Portion of loss recognized in other comprehensive income	0	0
Other net realized investment gains	11.5	7
Total net realized gains on investments	10.7	6.7
Other income from affiliates	0.5	0.4
Total revenues	291.3	285.3
Losses and loss expenses (ceded to affiliates $142.2 and $131.8 for the three months ended, respectively)	171.8	173
Acquisition and operating expenses	88.5	88.8
Interest expense (affiliates $0.2 and $0.2 for the three months ended, respectively)	1.3	1.7
Other expenses	2	1.8
Total expenses	263.6	265.3
Income before federal income taxes	27.7	20
Federal income tax expense	0.6	0.3
Net income	$27.10	$19.70
Earnings per common share:	'	'
Basic (usd per share)	$0.67	$0.49
Diluted (usd per share)	$0.66	$0.49
Dividends paid per common share (usd per share)	$0.10	$0.10

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Income (Parenthetical) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Statement [Abstract]	'	'
Earned premiums, ceded to affiliates	$215.40	$210.70
Net investment income, from affiliate	1.2	1.2
Losses and loss expenses, ceded to affiliates	142.2	131.8
Interest expense, affiliates	$0.20	$0.20

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]	'	'
Net income	$27.10	$19.70
Net unrealized holding gains on investments:	'	'
Unrealized holding gains arising during the period ended	23	19.4
Reclassification adjustments for gains realized in net income	-10.7	-6.7
Income tax (expense) benefit	-4.2	0.3
Total net unrealized holding gains on investments	8.1	13
Reclassification adjustments for amortization to statements of income:	'	'
Prior service benefits	-1.4	-1.4
Net actuarial loss	1.6	2.3
Total net unrecognized benefit plan obligations	0.2	0.9
Other comprehensive income, net of tax	8.3	13.9
Comprehensive income, net of tax	$35.40	$33.60

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Cash flows from operating activities:	'	'
Net income	$27.10	$19.70
Adjustments to reconcile net income to net cash provided by operating activities:	'	'
Depreciation and amortization, net	3.4	4.3
Share-based compensation	1	1.2
Net realized gains on investments	-10.7	-6.7
Changes in operating assets and liabilities:	'	'
Deferred policy acquisition costs	-2.9	-2.8
Accrued investment income and other assets	-1.5	-3.4
Postretirement and pension benefits	1.7	-1.7
Other liabilities and due to/from affiliates, net	-10.6	-9
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	-0.6	1.2
Losses and loss expenses payable	-8.2	2.1
Unearned premiums	2.9	2.8
Federal income taxes	0.7	0.3
Net cash provided by operating activities	2.3	8
Cash flows from investing activities:	'	'
Purchases of fixed maturities, available-for-sale	-137.2	-81.7
Purchases of equity securities, available-for-sale	-38	-24
Purchases of other invested assets	-0.4	-0.2
Maturities, calls and pay downs of fixed maturities, available-for-sale	68.9	40.2
Sales of fixed maturities, available-for-sale	71.3	38.5
Sales of equity securities, available-for-sale	34.6	28.1
Sales of other invested assets	0.2	0.2
Net additions of property and equipment	0	0.2
Net cash (used in) provided by investing activities	-0.6	1.3
Cash flows from financing activities:	'	'
Proceeds from issuance of common stock	0.6	0.1
Payments to acquire treasury stock	-0.1	-0.1
Payment of dividends	-4.2	-4
Net cash used in financing activities	-3.7	-4
Net (decrease) increase in cash and cash equivalents	-2	5.3
Cash and cash equivalents at beginning of period	80.3	59
Cash and cash equivalents at end of period	78.3	64.3
Supplemental disclosures:	'	'
Interest paid (affiliates $0.2 and $0.2, respectively)	$1.30	$0.20

Condensed_Consolidated_Stateme3

Condensed Consolidated Statements of Cash Flows (Parenthetical) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Statement of Cash Flows [Abstract]	'	'
Interest paid, affiliates	$0.20	$0.20

Basis_of_Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2014
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 month period ended March 31, 2014 are not necessarily indicative of the results that may be expected for the year ending December 31, 2014. The balance sheet at December 31, 2013 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, 2013 (the “2013 Form 10-K”). Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the 2013 Form 10-K.
	Adoption of Accounting Pronouncements
	None.

Investments

Investments	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013:
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	31-Mar-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	314.3		$	15.8		$	(5.0	)		$	325.1
	Obligations of states and political subdivisions	691.4			24.8			(8.2		)		708
	Corporate securities	355.7			11.9			(4.9		)		362.7
	U.S. government agencies residential mortgage-backed securities	438.5			10.2			(4.1		)		444.6
	Total fixed maturities	1,799.90			62.7			(22.2		)		1,840.40
	Equity securities:
	Large-cap securities	163			44.9			—				207.9
	Small-cap securities	47.4			19.3			—				66.7
	Total equity securities	210.4			64.2			—				274.6
	Other invested assets	49.6			33.8			—				83.4
	Total available-for-sale securities	$	2,059.90		$	160.7		$	(22.2	)		$	2,198.40
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	31-Dec-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	345.5		$	13.4		$	(6.5	)		$	352.4
	Obligations of states and political subdivisions	765.3			25.8			(16.9		)		774.2
	Corporate securities	345			11.4			(6.7		)		349.7
	U.S. government agencies residential mortgage-backed securities	348.2			9.7			(4.1		)		353.8
	Total fixed maturities	1,804.00			60.3			(34.2		)		1,830.10
	Equity securities:
	Large-cap securities	148.2			46.5			(0.3		)		194.4
	Small-cap securities	48.4			22.5			—				70.9
	Total equity securities	196.6			69			(0.3		)		265.3
	Other invested assets	49.5			31.4			—				80.9
	Total available-for-sale securities	$	2,050.10		$	160.7		$	(34.5	)		$	2,176.30
	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 March 31, 2014 and December 31, 2013:
		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-Mar-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	77.1		$	(5.0	)	24		$	—			$	—		—	$	77.1	$	(5.0	)	24
	Obligations of states and political subdivisions	178.2			(5.0		)	60		64				(3.2		)	21	242.2		(8.2		)	81
	Corporate securities	91			(2.2		)	18		42.6				(2.7		)	9	133.6		(4.9		)	27
	U.S. government agencies residential mortgage-backed securities	187.4			(3.4		)	37		14.6				(0.7		)	8	202		(4.1		)	45
	Total fixed maturities	533.7			(15.6		)	139		121.2				(6.6		)	38	654.9		(22.2		)	177
	Total temporarily impaired securities	$	533.7		$	(15.6	)	139		$	121.2			$	(6.6	)	38	$	654.9	$	(22.2	)	177
		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-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	109.6		$	(6.5	)	29		$	—			$	—		—	$	109.6	$	(6.5	)	29
	Obligations of states and political subdivisions	206.4			(14.7		)	76		25.6				(2.2		)	7	232		(16.9		)	83
	Corporate securities	105.6			(3.2		)	22		40.9				(3.5		)	8	146.5		(6.7		)	30
	U.S. government agencies residential mortgage-backed securities	103.6			(3.3		)	25		19.3				(0.8		)	10	122.9		(4.1		)	35
	Total fixed maturities	525.2			(27.7		)	152		85.8				(6.5		)	25	611		(34.2		)	177
	Large-cap equity securities	5.3			(0.3		)	2		—				—			—	5.3		(0.3		)	2
	Total temporarily impaired securities	$	530.5		$	(28.0	)	154		$	85.8			$	(6.5	)	25	$	616.3	$	(34.5	)	179
	The Company reviewed its investments at March 31, 2014, and determined that 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 months ended March 31, 2014 and 2013:
	($ millions)	2014			2013
	Equity securities:
	Large-cap securities	$	(0.2	)	$	—
	Small-cap securities	(0.6		)	(0.3		)
	Total other-than-temporary impairments	$	(0.8	)	$	(0.3	)
	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 March 31, 2014:
	($ millions)	Amortized cost			Fair
					value
Due in 1 year or less	$	54.8		$	55.5
Due after 1 year through 5 years	376.2			392.9
Due after 5 years through 10 years	359.7			376.4
Due after 10 years	570.7			571
U.S. government agencies residential mortgage-backed securities	438.5			444.6
Total	$	1,799.90		$	1,840.40
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 March 31, 2014, State Auto P&C had fixed maturity securities, with a carrying value of $85.0 million, that were pledged as collateral for the FHLB Loan. In accordance with the terms of the FHLB Loan, 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 $8.7 million were on deposit with insurance regulators as required by law at March 31, 2014 and December 31, 2013, respectively. The Company retains all rights regarding these securities.
The following table sets forth the components of net investment income for the three months ended March 31, 2014 and 2013:
($ millions)	2014			2013
Fixed maturities	$	15.4		$	14.8
Equity securities	1.4			1.2
Cash and cash equivalents, and other	1.3			1.4
Investment income	18.1			17.4
Investment expenses	0.5			0.5
Net investment income	$	17.6		$	16.9
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 months ended March 31, 2014 and 2013:
($ millions)	2014			2013
Realized gains:
Fixed maturities	$	2.1		$	0.5
Equity securities	9.4			6.6
Other invested assets	0.1			—
Total realized gains	11.6			7.1
Realized losses:
Equity securities:
Sales	(0.1		)	(0.1		)
OTTI	(0.8		)	(0.3		)
Total realized losses	(0.9		)	(0.4		)
Net realized gains on investments	$	10.7		$	6.7
Change in unrealized holding gains (losses), net of tax:
Fixed maturities	$	14.4		$	(9.2	)
Equity securities	(4.5		)	18.3
Other invested assets	2.4			3.6
Deferred federal income tax liability	(4.3		)	(4.5		)
Valuation allowance	0.1			4.8
Change in net unrealized holding gains, net of tax	$	8.1		$	13
There was a deferred federal income tax liability, net of a valuation allowance, on the net unrealized holding gains at March 31, 2014 and December 31, 2013 of $45.8 million and $41.6 million, respectively.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013, 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 months ended March 31, 2014 and 2013.
	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 two securities 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 $76.5 million and $74.2 million at March 31, 2014 and December 31, 2013, respectively, which was determined using each fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the funds including obtaining and reviewing each fund’s audited financial statements. There are no unfunded commitments related to the funds. The Company may not sell its investment in the funds; however, the Company may redeem all or a portion of its investment in the funds at net asset value per share with the appropriate prior written notice. Due to the Company’s ability to redeem its investment in the funds at net asset value per share at the measurement date, the funds have been disclosed in Level 2.
	The remainder of the Company’s other invested assets consist primarily of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds based on an observable market price for an identical asset in an active market reflect their fair values and consequently these securities have been disclosed in Level 1.
	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at March 31, 2014 and December 31, 2013:
	($ millions)	Total		Quoted prices in active markets for identical assets		Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
	31-Mar-14			(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	325.1	$	—	$	325.1		$	—
	Obligations of states and political subdivisions	708		—		708			—
	Corporate securities	362.7		—		353.6			9.1
	U.S. government agencies residential mortgage-backed securities	444.6		—		444.6			—
	Total fixed maturities	1,840.40		—		1,831.30			9.1
	Equity securities:
	Large-cap securities	207.9		207.9		—			—
	Small-cap securities	66.7		66.7		—			—
	Total equity securities	274.6		274.6		—			—
	Other invested assets	83.4		6.9		76.5			—
	Total available-for-sale investments	$	2,198.40	$	281.5	$	1,907.80		$	9.1
	($ 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-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	352.4	$	—	$	352.4		$	—
	Obligations of states and political subdivisions	774.2		—		774.2			—
	Corporate securities	349.7		—		340.8			8.9
	U.S. government agencies residential mortgage-backed securities	353.8		—		353.8			—
	Total fixed maturities	1,830.10		—		1,821.20			8.9
	Equity securities:
	Large-cap securities	194.4		194.4		—			—
	Small-cap securities	70.9		70.9		—			—
	Total equity securities	265.3		265.3		—			—
	Other invested assets	80.9		6.7		74.2			—
	Total available-for-sale investments	$	2,176.30	$	272	$	1,895.40		$	8.9
	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 months ended March 31, 2014 and the year ended December 31, 2013, separately for each major category of assets:
	($ millions)	Fixed maturities
	Balance at January 1, 2014	$	8.9
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains – included in other comprehensive income	0.1
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at March 31, 2014	$	9.1
	($ millions)	Fixed maturities
	Balance at January 1, 2013	$	8.5
	Total realized (losses) gains – included in earnings	—
	Total unrealized gains – included in other comprehensive income	0.2
	Purchases	0.2
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at December 31, 2013	$	8.9
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 common stock of the Federal Home Loan Bank of Cincinnati (“FHLB”) 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 March 31, 2014 was $4.5 million and the fair value of the Trust Securities was $0.5 million. Both investments have been placed in Level 3 of the fair value hierarchy.
Notes Receivable from Affiliate
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)	March 31, 2014						December 31, 2013
	Carrying value		Fair value		Interest rate		Carrying value			Fair		Interest rate
										value
Notes receivable from affiliate	$	70	$	75.8	7	%	$	70		$	74.6	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)	31-Mar-14						31-Dec-13
	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	$	86.5	5.03	%	$	85.3		$	85.7	5.03	%
Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5		15.5		4.44	%	15.5			15.5		4.44	%
Total notes payable	$	100.8	$	102			$	100.8		$	101.2

Reinsurance

Reinsurance	3 Months Ended
	Mar. 31, 2014
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, 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 months ended March 31, 2014 and 2013:
		($ millions)	2014			2013
		Premiums earned:
		Assumed from external insurers and reinsurers	$	1		$	0.4
		Assumed under Pooling Arrangement	262.5			261.3
		Ceded to external insurers and reinsurers	(6.0		)	(5.5		)
		Ceded under Pooling Arrangement	(215.4		)	(210.7		)
		Net assumed premiums earned	$	42.1		$	45.5
		Losses and loss expenses incurred:
		Assumed from external insurers and reinsurers	$	0.7		$	(0.2	)
		Assumed under Pooling Arrangement (a)	171.8			173
		Ceded to external insurers and reinsurers	(2.5		)	(1.0		)
		Ceded under Pooling Arrangement (a)	(142.2		)	(131.8		)
		Net assumed losses and loss expenses incurred	$	27.8		$	40
	(a)	Includes adjustments for accounting differences between SAP and GAAP of $0.8 million and $0.7 million for the three months ended March 31, 2014 and 2013, respectively.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013:
	($ millions)	2014						2013
	Amount at statutory rate	$	9.7		35	%		$	7		35	%
	Tax-exempt interest and dividends received deduction	(2.1		)	(8	)		(2.3		)	(12	)
	Other, net	0.2			1			0.2			1
	Valuation allowance	(7.2		)	(26	)		(4.6		)	(23	)
	Federal income tax expense and effective rate	$	0.6		2	%		$	0.3		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 March 31, 2014 and December 31, 2013:
	($ millions)	March 31, 2014			December 31, 2013
	Deferred tax assets:
	Unearned premiums not currently deductible	$	34		$	33.8
	Losses and loss expenses payable discounting	21.4			21.6
	Postretirement and pension benefits	25.4			24.9
	Realized loss on other-than-temporary impairment	8.3			7.5
	Other liabilities	15.7			17.3
	Net operating loss carryforward	50.9			56.5
	Tax credit carryforward	2.3			1.7
	Other	8.3			9.2
	Total deferred tax assets	166.3			172.5
	Deferred tax liabilities:
	Deferral of policy acquisition costs	34.9			33.9
	Net unrealized holding gains on investments	48.5			44.1
	Total deferred tax liabilities	83.4			78
	Total net deferred tax assets before valuation allowance	82.9			94.5
	Less valuation allowance	75.2			82.6
	Net deferred federal income taxes	$	7.7		$	11.9
	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 March 31, 2014 and December 31, 2013, the Company recorded a valuation allowance of $75.2 million and $82.6 million, respectively. The deferred income tax asset remaining after recognition of the valuation allowance represents deferred tax benefits on gross unrealized fixed maturity losses where management determined these benefits to be realizable due to management’s assertion it has both the ability and intent to hold these securities through recovery or maturity.
	Based on ASC 740 intraperiod tax allocation guidelines, the following sets forth the change in valuation allowance attributable to continuing operations and other comprehensive income for the three months ended March 31, 2014 and 2013 is as follows:
	($ millions)	2014			2013
	Continuing operations	$	(7.2	)	$	(4.6	)
	Other comprehensive income	(0.2		)	(5.1		)
	Change in valuation allowance	$	(7.4	)	$	(9.7	)
	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	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013:
	($ millions)	Pension						Postretirement
		2014			2013			2014			2013
	Service cost	$	1.3		$	1.5		$	—		$	—
	Interest cost	2.8			2.4			0.3			0.2
	Expected return on plan assets	(3.2		)	(3.0		)	—			—
	Amortization of:
	Prior service benefits	—			—			(1.4		)	(1.4		)
	Net actuarial loss	1.4			2			0.2			0.3
	Net periodic cost (benefit)	$	2.3		$	2.9		$	(0.9	)	$	(0.9	)
	The Company's share of expected contributions to its pension plan during 2014 is $13.1 million.

Other_Comprehensive_Income_and

Other Comprehensive Income and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Other Comprehensive Income and Accumulated Other Comprehensive Income	'
	Other Comprehensive Income and Accumulated Other Comprehensive Income
	The following table sets forth the changes in the Company’s accumulated other comprehensive income component (AOCI), net of tax, for the three months ended March 31, 2014 and 2013:
	($ millions)		Unrealized Gains and Losses on Available-for-Sale Securities				Benefit Plan Items				Gains and Losses on Cash Flow Hedges		Total
	Beginning balance at January 1, 2014		$	84.6			$	(3.8	)		$	—	$	80.8
	Other comprehensive income before reclassifications		18.8				—				—		18.8
	Amounts reclassified from AOCI (a)		(10.7		)		0.2				—		(10.5		)
	Net current period other comprehensive income		8.1				0.2				—		8.3
	Ending balance at March 31, 2014		$	92.7			$	(3.6	)		$	—	$	89.1
	Beginning balance at January 1, 2013		$	124			$	(39.9	)		$	0.1	$	84.2
	Other comprehensive income before reclassifications		19.7				—				—		19.7
	Amounts reclassified from AOCI (a)		(6.7		)		0.9				—		(5.8		)
	Net current period other comprehensive income		13				0.9				—		13.9
	Ending balance at March 31, 2013		$	137			$	(39.0	)		$	0.1	$	98.1
	(a)	See separate table below for details about these reclassifications
	The following table sets forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s consolidated statement of income for the three months ended March 31, 2014 and 2013:
	($ millions)
	Details about Accumulated Other			Three months ended								Affected line item in the Condensed
	Comprehensive Income Components			31-Mar								Consolidated Statements of Income
				2014				2013
	Unrealized gains on available for sale securities			$	10.7			$	6.7			Realized gain on sale of securities
				10.7				6.7				Total before tax
				—				—				Tax benefit (expense)
				10.7				6.7				Net of tax
	Amortization of benefit plan items
	Prior service benefits			1.4				1.4				(a)
	Net actuarial loss			(1.6		)		(2.3		)		(a)
				(0.2		)		(0.9		)		Total before tax
				—				—				Tax benefit (expense)
				(0.2		)		(0.9		)		Net of tax
	Total reclassifications for the period			$	10.5			$	5.8
	(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_per_Common_Share

Earnings per Common Share	3 Months Ended
	Mar. 31, 2014
Earnings Per Share [Abstract]	'
Earnings (Loss) per Common Share	'
	Earnings per Common Share
	The following table sets forth the compilation of basic and diluted earnings per common share for the three months ended March 31, 2014 and 2013:
	($ and shares in millions, except per share amounts)	2014			2013
	Numerator:
	Net income for basic earnings per common share	$	27.1		$	19.7
	Denominator:
	Weighted average shares for basic net earnings per common share	40.8			40.5
	Effect of dilutive share-based awards	0.5			0.2
	Adjusted weighted average shares for diluted net earnings per common share	41.3			40.7
	Basic net earnings per common share	$	0.67		$	0.49
	Diluted net earnings per common share	$	0.66		$	0.49
	The following table sets forth common stock options and 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 months ended March 31, 2014 and 2013.
	( millions)	2014		2013
	Total number of antidilutive options and awards	1.7		3.2

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2014
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. The specialty insurance segment provides commercial coverages, including workers’ compensation for both the legacy State Auto Group and RTW'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.
	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.
	The following table sets forth financial information regarding the Company’s reportable segments for the three months ended March 31, 2014 and 2013:
	($ millions)	Three months ended March 31
		2014			2013
	Revenues from external sources:
	Insurance segments
	Personal insurance	$	114.3		$	114.7
	Business insurance	93.5			88.5
	Specialty insurance	54.7			58.1
	Total insurance segments	262.5			261.3
	Investment operations segment
	Net investment income	17.6			16.9
	Net realized capital gains	10.7			6.7
	Total investment operations segment	28.3			23.6
	All other	0.5			0.4
	Total revenues from external sources	291.3			285.3
	Intersegment revenues:	1.3			1.3
	Total revenues	292.6			286.6
	Reconciling items:
	Eliminate intersegment revenues	(1.3		)	(1.3		)
	Total consolidated revenues	$	291.3		$	285.3
	Segment income before federal income tax:
	Insurance segments SAP underwriting (loss) income
	Personal insurance	$	5.1		$	1.3
	Business insurance	(8.5		)	(5.3		)
	Specialty insurance	(0.5		)	(1.7		)
	Total insurance segments	(3.9		)	(5.7		)
	Investment operations segment
	Net investment income	17.6			16.9
	Net realized capital gains	10.7			6.7
	Total investment operations segment	28.3			23.6
	All other segments income	0.1			0.4
	Total segment income before tax expense	24.5			18.3
	Reconciling items:
	GAAP expense adjustments	5.4			4.5
	Interest expense on corporate debt	(1.3		)	(1.7		)
	Corporate expenses	(0.9		)	(1.1		)
	Total reconciling items	3.2			1.7
	Total consolidated income before federal income tax expense	$	27.7		$	20
	Investable assets attributable to the Company’s investment operations segment totaled $2,351.7 million and $2,331.6 million at March 31, 2014 and December 31, 2013, respectively.

Contingencies_and_Litigation

Contingencies and Litigation	3 Months Ended
	Mar. 31, 2014
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 position.
	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.

Investments_Tables

Investments (Tables)	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013:
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	31-Mar-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	314.3		$	15.8		$	(5.0	)		$	325.1
	Obligations of states and political subdivisions	691.4			24.8			(8.2		)		708
	Corporate securities	355.7			11.9			(4.9		)		362.7
	U.S. government agencies residential mortgage-backed securities	438.5			10.2			(4.1		)		444.6
	Total fixed maturities	1,799.90			62.7			(22.2		)		1,840.40
	Equity securities:
	Large-cap securities	163			44.9			—				207.9
	Small-cap securities	47.4			19.3			—				66.7
	Total equity securities	210.4			64.2			—				274.6
	Other invested assets	49.6			33.8			—				83.4
	Total available-for-sale securities	$	2,059.90		$	160.7		$	(22.2	)		$	2,198.40
	($ millions)	Cost or amortized cost			Gross unrealized holding gains			Gross unrealized holding losses				Fair value
	31-Dec-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	345.5		$	13.4		$	(6.5	)		$	352.4
	Obligations of states and political subdivisions	765.3			25.8			(16.9		)		774.2
	Corporate securities	345			11.4			(6.7		)		349.7
	U.S. government agencies residential mortgage-backed securities	348.2			9.7			(4.1		)		353.8
	Total fixed maturities	1,804.00			60.3			(34.2		)		1,830.10
	Equity securities:
	Large-cap securities	148.2			46.5			(0.3		)		194.4
	Small-cap securities	48.4			22.5			—				70.9
	Total equity securities	196.6			69			(0.3		)		265.3
	Other invested assets	49.5			31.4			—				80.9
	Total available-for-sale securities	$	2,050.10		$	160.7		$	(34.5	)		$	2,176.30
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 March 31, 2014 and December 31, 2013:
		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-Mar-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	77.1		$	(5.0	)	24		$	—			$	—		—	$	77.1	$	(5.0	)	24
	Obligations of states and political subdivisions	178.2			(5.0		)	60		64				(3.2		)	21	242.2		(8.2		)	81
	Corporate securities	91			(2.2		)	18		42.6				(2.7		)	9	133.6		(4.9		)	27
	U.S. government agencies residential mortgage-backed securities	187.4			(3.4		)	37		14.6				(0.7		)	8	202		(4.1		)	45
	Total fixed maturities	533.7			(15.6		)	139		121.2				(6.6		)	38	654.9		(22.2		)	177
	Total temporarily impaired securities	$	533.7		$	(15.6	)	139		$	121.2			$	(6.6	)	38	$	654.9	$	(22.2	)	177
		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-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	109.6		$	(6.5	)	29		$	—			$	—		—	$	109.6	$	(6.5	)	29
	Obligations of states and political subdivisions	206.4			(14.7		)	76		25.6				(2.2		)	7	232		(16.9		)	83
	Corporate securities	105.6			(3.2		)	22		40.9				(3.5		)	8	146.5		(6.7		)	30
	U.S. government agencies residential mortgage-backed securities	103.6			(3.3		)	25		19.3				(0.8		)	10	122.9		(4.1		)	35
	Total fixed maturities	525.2			(27.7		)	152		85.8				(6.5		)	25	611		(34.2		)	177
	Large-cap equity securities	5.3			(0.3		)	2		—				—			—	5.3		(0.3		)	2
	Total temporarily impaired securities	$	530.5		$	(28.0	)	154		$	85.8			$	(6.5	)	25	$	616.3	$	(34.5	)	179
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 months ended March 31, 2014 and 2013:
	($ millions)	2014			2013
	Equity securities:
	Large-cap securities	$	(0.2	)	$	—
	Small-cap securities	(0.6		)	(0.3		)
	Total other-than-temporary impairments	$	(0.8	)	$	(0.3	)
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 March 31, 2014:
	($ millions)	Amortized cost			Fair
					value
	Due in 1 year or less	$	54.8		$	55.5
	Due after 1 year through 5 years	376.2			392.9
	Due after 5 years through 10 years	359.7			376.4
	Due after 10 years	570.7			571
	U.S. government agencies residential mortgage-backed securities	438.5			444.6
	Total	$	1,799.90		$	1,840.40
Components of Net Investment Income	'
	The following table sets forth the components of net investment income for the three months ended March 31, 2014 and 2013:
	($ millions)	2014			2013
	Fixed maturities	$	15.4		$	14.8
	Equity securities	1.4			1.2
	Cash and cash equivalents, and other	1.3			1.4
	Investment income	18.1			17.4
	Investment expenses	0.5			0.5
	Net investment income	$	17.6		$	16.9
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 months ended March 31, 2014 and 2013:
	($ millions)	2014			2013
	Realized gains:
	Fixed maturities	$	2.1		$	0.5
	Equity securities	9.4			6.6
	Other invested assets	0.1			—
	Total realized gains	11.6			7.1
	Realized losses:
	Equity securities:
	Sales	(0.1		)	(0.1		)
	OTTI	(0.8		)	(0.3		)
	Total realized losses	(0.9		)	(0.4		)
	Net realized gains on investments	$	10.7		$	6.7
	Change in unrealized holding gains (losses), net of tax:
	Fixed maturities	$	14.4		$	(9.2	)
	Equity securities	(4.5		)	18.3
	Other invested assets	2.4			3.6
	Deferred federal income tax liability	(4.3		)	(4.5		)
	Valuation allowance	0.1			4.8
	Change in net unrealized holding gains, net of tax	$	8.1		$	13
	There was a deferred federal income tax liability, net of a valuation allowance, on the net unrealized holding gains at March 31, 2014 and December 31, 2013 of $45.8 million and $41.6 million, respectively.

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2014
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 March 31, 2014 and December 31, 2013:
	($ millions)	Total		Quoted prices in active markets for identical assets		Significant other observable inputs (Level 2)			Significant unobservable inputs (Level 3)
	31-Mar-14			(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	325.1	$	—	$	325.1		$	—
	Obligations of states and political subdivisions	708		—		708			—
	Corporate securities	362.7		—		353.6			9.1
	U.S. government agencies residential mortgage-backed securities	444.6		—		444.6			—
	Total fixed maturities	1,840.40		—		1,831.30			9.1
	Equity securities:
	Large-cap securities	207.9		207.9		—			—
	Small-cap securities	66.7		66.7		—			—
	Total equity securities	274.6		274.6		—			—
	Other invested assets	83.4		6.9		76.5			—
	Total available-for-sale investments	$	2,198.40	$	281.5	$	1,907.80		$	9.1
	($ 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-13
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	352.4	$	—	$	352.4		$	—
	Obligations of states and political subdivisions	774.2		—		774.2			—
	Corporate securities	349.7		—		340.8			8.9
	U.S. government agencies residential mortgage-backed securities	353.8		—		353.8			—
	Total fixed maturities	1,830.10		—		1,821.20			8.9
	Equity securities:
	Large-cap securities	194.4		194.4		—			—
	Small-cap securities	70.9		70.9		—			—
	Total equity securities	265.3		265.3		—			—
	Other invested assets	80.9		6.7		74.2			—
	Total available-for-sale investments	$	2,176.30	$	272	$	1,895.40		$	8.9
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 months ended March 31, 2014 and the year ended December 31, 2013, separately for each major category of assets:
	($ millions)	Fixed maturities
	Balance at January 1, 2014	$	8.9
	Total realized gains (losses) – included in earnings	—
	Total unrealized gains – included in other comprehensive income	0.1
	Purchases	0.1
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at March 31, 2014	$	9.1
	($ millions)	Fixed maturities
	Balance at January 1, 2013	$	8.5
	Total realized (losses) gains – included in earnings	—
	Total unrealized gains – included in other comprehensive income	0.2
	Purchases	0.2
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at December 31, 2013	$	8.9
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)	March 31, 2014						December 31, 2013
		Carrying value		Fair value		Interest rate		Carrying value			Fair		Interest rate
											value
	Notes receivable from affiliate	$	70	$	75.8	7	%	$	70		$	74.6	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)	31-Mar-14						31-Dec-13
		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	$	86.5	5.03	%	$	85.3		$	85.7	5.03	%
	Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5		15.5		4.44	%	15.5			15.5		4.44	%
	Total notes payable	$	100.8	$	102			$	100.8		$	101.2

Reinsurance_Tables

Reinsurance (Tables)	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013:
		($ millions)	2014			2013
		Premiums earned:
		Assumed from external insurers and reinsurers	$	1		$	0.4
		Assumed under Pooling Arrangement	262.5			261.3
		Ceded to external insurers and reinsurers	(6.0		)	(5.5		)
		Ceded under Pooling Arrangement	(215.4		)	(210.7		)
		Net assumed premiums earned	$	42.1		$	45.5
		Losses and loss expenses incurred:
		Assumed from external insurers and reinsurers	$	0.7		$	(0.2	)
		Assumed under Pooling Arrangement (a)	171.8			173
		Ceded to external insurers and reinsurers	(2.5		)	(1.0		)
		Ceded under Pooling Arrangement (a)	(142.2		)	(131.8		)
		Net assumed losses and loss expenses incurred	$	27.8		$	40
	(a)	Includes adjustments for accounting differences between SAP and GAAP of $0.8 million and $0.7 million for the three months ended March 31, 2014 and 2013, respectively.

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013:
	($ millions)	2014						2013
	Amount at statutory rate	$	9.7		35	%		$	7		35	%
	Tax-exempt interest and dividends received deduction	(2.1		)	(8	)		(2.3		)	(12	)
	Other, net	0.2			1			0.2			1
	Valuation allowance	(7.2		)	(26	)		(4.6		)	(23	)
	Federal income tax expense and effective rate	$	0.6		2	%		$	0.3		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 March 31, 2014 and December 31, 2013:
	($ millions)	March 31, 2014			December 31, 2013
	Deferred tax assets:
	Unearned premiums not currently deductible	$	34		$	33.8
	Losses and loss expenses payable discounting	21.4			21.6
	Postretirement and pension benefits	25.4			24.9
	Realized loss on other-than-temporary impairment	8.3			7.5
	Other liabilities	15.7			17.3
	Net operating loss carryforward	50.9			56.5
	Tax credit carryforward	2.3			1.7
	Other	8.3			9.2
	Total deferred tax assets	166.3			172.5
	Deferred tax liabilities:
	Deferral of policy acquisition costs	34.9			33.9
	Net unrealized holding gains on investments	48.5			44.1
	Total deferred tax liabilities	83.4			78
	Total net deferred tax assets before valuation allowance	82.9			94.5
	Less valuation allowance	75.2			82.6
	Net deferred federal income taxes	$	7.7		$	11.9
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 months ended March 31, 2014 and 2013 is as follows:
	($ millions)	2014			2013
	Continuing operations	$	(7.2	)	$	(4.6	)
	Other comprehensive income	(0.2		)	(5.1		)
	Change in valuation allowance	$	(7.4	)	$	(9.7	)

Pension_and_Postretirement_Ben1

Pension and Postretirement Benefit Plans (Tables)	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013:
	($ millions)	Pension						Postretirement
		2014			2013			2014			2013
	Service cost	$	1.3		$	1.5		$	—		$	—
	Interest cost	2.8			2.4			0.3			0.2
	Expected return on plan assets	(3.2		)	(3.0		)	—			—
	Amortization of:
	Prior service benefits	—			—			(1.4		)	(1.4		)
	Net actuarial loss	1.4			2			0.2			0.3
	Net periodic cost (benefit)	$	2.3		$	2.9		$	(0.9	)	$	(0.9	)

Other_Comprehensive_Income_and1

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Changes in Accumulated Other Comprehensive Income by Component, Net of Tax	'
	The following table sets forth the changes in the Company’s accumulated other comprehensive income component (AOCI), net of tax, for the three months ended March 31, 2014 and 2013:
	($ millions)		Unrealized Gains and Losses on Available-for-Sale Securities				Benefit Plan Items				Gains and Losses on Cash Flow Hedges		Total
	Beginning balance at January 1, 2014		$	84.6			$	(3.8	)		$	—	$	80.8
	Other comprehensive income before reclassifications		18.8				—				—		18.8
	Amounts reclassified from AOCI (a)		(10.7		)		0.2				—		(10.5		)
	Net current period other comprehensive income		8.1				0.2				—		8.3
	Ending balance at March 31, 2014		$	92.7			$	(3.6	)		$	—	$	89.1
	Beginning balance at January 1, 2013		$	124			$	(39.9	)		$	0.1	$	84.2
	Other comprehensive income before reclassifications		19.7				—				—		19.7
	Amounts reclassified from AOCI (a)		(6.7		)		0.9				—		(5.8		)
	Net current period other comprehensive income		13				0.9				—		13.9
	Ending balance at March 31, 2013		$	137			$	(39.0	)		$	0.1	$	98.1
	(a)	See separate table below for details about these reclassifications
Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component	'
	The following table sets forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s consolidated statement of income for the three months ended March 31, 2014 and 2013:
	($ millions)
	Details about Accumulated Other			Three months ended								Affected line item in the Condensed
	Comprehensive Income Components			31-Mar								Consolidated Statements of Income
				2014				2013
	Unrealized gains on available for sale securities			$	10.7			$	6.7			Realized gain on sale of securities
				10.7				6.7				Total before tax
				—				—				Tax benefit (expense)
				10.7				6.7				Net of tax
	Amortization of benefit plan items
	Prior service benefits			1.4				1.4				(a)
	Net actuarial loss			(1.6		)		(2.3		)		(a)
				(0.2		)		(0.9		)		Total before tax
				—				—				Tax benefit (expense)
				(0.2		)		(0.9		)		Net of tax
	Total reclassifications for the period			$	10.5			$	5.8
	(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_per_Common_Share_Tabl

Earnings per Common Share (Tables)	3 Months Ended
	Mar. 31, 2014
Earnings Per Share [Abstract]	'
Compilation of Basic and Diluted Earnings Per Common Share	'
	The following table sets forth the compilation of basic and diluted earnings per common share for the three months ended March 31, 2014 and 2013:
	($ and shares in millions, except per share amounts)	2014			2013
	Numerator:
	Net income for basic earnings per common share	$	27.1		$	19.7
	Denominator:
	Weighted average shares for basic net earnings per common share	40.8			40.5
	Effect of dilutive share-based awards	0.5			0.2
	Adjusted weighted average shares for diluted net earnings per common share	41.3			40.7
	Basic net earnings per common share	$	0.67		$	0.49
	Diluted net earnings per common share	$	0.66		$	0.49
Options to Purchase Shares of Common Stock	'
	antidilutive for the three months ended March 31, 2014 and 2013.
	( millions)	2014		2013
	Total number of antidilutive options and awards	1.7		3.2

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2014
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 months ended March 31, 2014 and 2013:
	($ millions)	Three months ended March 31
		2014			2013
	Revenues from external sources:
	Insurance segments
	Personal insurance	$	114.3		$	114.7
	Business insurance	93.5			88.5
	Specialty insurance	54.7			58.1
	Total insurance segments	262.5			261.3
	Investment operations segment
	Net investment income	17.6			16.9
	Net realized capital gains	10.7			6.7
	Total investment operations segment	28.3			23.6
	All other	0.5			0.4
	Total revenues from external sources	291.3			285.3
	Intersegment revenues:	1.3			1.3
	Total revenues	292.6			286.6
	Reconciling items:
	Eliminate intersegment revenues	(1.3		)	(1.3		)
	Total consolidated revenues	$	291.3		$	285.3
	Segment income before federal income tax:
	Insurance segments SAP underwriting (loss) income
	Personal insurance	$	5.1		$	1.3
	Business insurance	(8.5		)	(5.3		)
	Specialty insurance	(0.5		)	(1.7		)
	Total insurance segments	(3.9		)	(5.7		)
	Investment operations segment
	Net investment income	17.6			16.9
	Net realized capital gains	10.7			6.7
	Total investment operations segment	28.3			23.6
	All other segments income	0.1			0.4
	Total segment income before tax expense	24.5			18.3
	Reconciling items:
	GAAP expense adjustments	5.4			4.5
	Interest expense on corporate debt	(1.3		)	(1.7		)
	Corporate expenses	(0.9		)	(1.1		)
	Total reconciling items	3.2			1.7
	Total consolidated income before federal income tax expense	$	27.7		$	20

Investments_Cost_or_Amortized_

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	$2,059.90	$2,050.10
Gross unrealized holding gains	160.7	160.7
Gross unrealized holding losses	-22.2	-34.5
Fair value	2,198.40	2,176.30
Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	1,799.90	1,804
Gross unrealized holding gains	62.7	60.3
Gross unrealized holding losses	-22.2	-34.2
Fair value	1,840.40	1,830.10
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	314.3	345.5
Gross unrealized holding gains	15.8	13.4
Gross unrealized holding losses	-5	-6.5
Fair value	325.1	352.4
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	691.4	765.3
Gross unrealized holding gains	24.8	25.8
Gross unrealized holding losses	-8.2	-16.9
Fair value	708	774.2
Fixed Maturities [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	355.7	345
Gross unrealized holding gains	11.9	11.4
Gross unrealized holding losses	-4.9	-6.7
Fair value	362.7	349.7
Fixed Maturities [Member] | U.S. Government Agencies Residential Mortgage-Backed Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	438.5	348.2
Gross unrealized holding gains	10.2	9.7
Gross unrealized holding losses	-4.1	-4.1
Fair value	444.6	353.8
Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	210.4	196.6
Gross unrealized holding gains	64.2	69
Gross unrealized holding losses	0	-0.3
Fair value	274.6	265.3
Equity Securities [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	163	148.2
Gross unrealized holding gains	44.9	46.5
Gross unrealized holding losses	0	-0.3
Fair value	207.9	194.4
Equity Securities [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	47.4	48.4
Gross unrealized holding gains	19.3	22.5
Gross unrealized holding losses	0	0
Fair value	66.7	70.9
Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Cost or amortized cost	49.6	49.5
Gross unrealized holding gains	33.8	31.4
Gross unrealized holding losses	0	0
Fair value	$83.40	$80.90

Investments_Gross_Unrealized_L

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified	Positions	Positions
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	$533.70	$530.50
Less than 12 months, Unrealized losses	-15.6	-28
Less than 12 months, Number of positions (positions)	139	154
12 months or more, Fair value	121.2	85.8
12 months or more, Unrealized losses	-6.6	-6.5
12 months or more, Number of positions (positions)	38	25
Total, Fair value	654.9	616.3
Total, Unrealized losses	-22.2	-34.5
Total, Number of positions	177	179
Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	533.7	525.2
Less than 12 months, Unrealized losses	-15.6	-27.7
Less than 12 months, Number of positions (positions)	139	152
12 months or more, Fair value	121.2	85.8
12 months or more, Unrealized losses	-6.6	-6.5
12 months or more, Number of positions (positions)	38	25
Total, Fair value	654.9	611
Total, Unrealized losses	-22.2	-34.2
Total, Number of positions	177	177
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	77.1	109.6
Less than 12 months, Unrealized losses	-5	-6.5
Less than 12 months, Number of positions (positions)	24	29
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	77.1	109.6
Total, Unrealized losses	-5	-6.5
Total, Number of positions	24	29
Obligations of States and Political Subdivisions [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	178.2	206.4
Less than 12 months, Unrealized losses	-5	-14.7
Less than 12 months, Number of positions (positions)	60	76
12 months or more, Fair value	64	25.6
12 months or more, Unrealized losses	-3.2	-2.2
12 months or more, Number of positions (positions)	21	7
Total, Fair value	242.2	232
Total, Unrealized losses	-8.2	-16.9
Total, Number of positions	81	83
Corporate Securities [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	91	105.6
Less than 12 months, Unrealized losses	-2.2	-3.2
Less than 12 months, Number of positions (positions)	18	22
12 months or more, Fair value	42.6	40.9
12 months or more, Unrealized losses	-2.7	-3.5
12 months or more, Number of positions (positions)	9	8
Total, Fair value	133.6	146.5
Total, Unrealized losses	-4.9	-6.7
Total, Number of positions	27	30
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	187.4	103.6
Less than 12 months, Unrealized losses	-3.4	-3.3
Less than 12 months, Number of positions (positions)	37	25
12 months or more, Fair value	14.6	19.3
12 months or more, Unrealized losses	-0.7	-0.8
12 months or more, Number of positions (positions)	8	10
Total, Fair value	202	122.9
Total, Unrealized losses	-4.1	-4.1
Total, Number of positions	45	35
Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Less than 12 months, Fair value	'	5.3
Less than 12 months, Unrealized losses	'	-0.3
Less than 12 months, Number of positions (positions)	'	2
12 months or more, Fair value	'	0
12 months or more, Unrealized losses	'	0
12 months or more, Number of positions (positions)	'	0
Total, Fair value	'	5.3
Total, Unrealized losses	'	($0.30)
Total, Number of positions	'	2

Investments_Realized_Losses_Re

Investments - Realized Losses Related to Other-than-Temporary Impairments (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Schedule of Available-for-sale Securities [Line Items]	'	'
Other-than-temporary impairments	($0.80)	($0.30)
Large-Cap Equity Securities [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Other-than-temporary impairments	-0.2	0
Small-Cap Securities [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Other-than-temporary impairments	($0.60)	($0.30)

Investments_Amortized_Cost_and

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Amortized cost	'	'
Due in 1 year or less, Amortized cost	$54.80	'
Due after 1 year through 5 years, Amortized cost	376.2	'
Due after 5 years through 10 years, Amortized cost	359.7	'
Due after 10 years, Amortized cost	570.7	'
U.S. government agencies residential mortgage-backed securities, Amortized cost	438.5	'
Total, Amortized cost	1,799.90	1,804
Fair value	'	'
Due in 1 year or less, Fair value	55.5	'
Due after 1 year through 5 years, Fair value	392.9	'
Due after 5 years through 10 years, Fair value	376.4	'
Due after 10 years, Fair value	571	'
U.S. government agencies residential mortgage-backed securities, Fair value	444.6	'
Total, Fair value	$1,840.40	$1,830.10

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Fixed maturities fair value of deposit with insurance regulators	$8.70	$8.70
Deferred federal income tax liability on the net unrealized holding gains	45.8	41.6
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	$85	'

Investments_Components_of_Net_

Investments - Components of Net Investment Income (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Net Investment Income [Line Items]	'	'
Investment income	$18.10	$17.40
Investment expenses	0.5	0.5
Net investment income	17.6	16.9
Fixed Maturities [Member]	'	'
Net Investment Income [Line Items]	'	'
Investment income	15.4	14.8
Equity Securities [Member]	'	'
Net Investment Income [Line Items]	'	'
Investment income	1.4	1.2
Cash and Cash Equivalents, and Other [Member]	'	'
Net Investment Income [Line Items]	'	'
Investment income	$1.30	$1.40

Investments_Realized_and_Unrea

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Realized gains:	'	'
Total realized gains	$11.60	$7.10
Realized losses:	'	'
Total realized losses	-0.9	-0.4
Net realized gain on investments	10.7	6.7
Change in unrealized holding gains (losses), net of tax:	'	'
Deferred federal income tax liability	-4.3	-4.5
Valuation allowance	0.1	4.8
Change in net unrealized holding gains, net of tax	8.1	13
Fixed Maturities [Member]	'	'
Realized gains:	'	'
Total realized gains	2.1	0.5
Change in unrealized holding gains (losses), net of tax:	'	'
Change in net unrealized holding gains, net of tax	14.4	-9.2
Equity Securities [Member]	'	'
Realized gains:	'	'
Total realized gains	9.4	6.6
Change in unrealized holding gains (losses), net of tax:	'	'
Change in net unrealized holding gains, net of tax	-4.5	18.3
Equity Securities [Member] | Sales [Member]	'	'
Realized losses:	'	'
Total realized losses	-0.1	-0.1
Equity Securities [Member] | OTTI [Member]	'	'
Realized losses:	'	'
Total realized losses	-0.8	-0.3
Other Invested Assets [Member]	'	'
Realized gains:	'	'
Total realized gains	0.1	0
Change in unrealized holding gains (losses), net of tax:	'	'
Change in net unrealized holding gains, net of tax	$2.40	$3.60

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	Mar. 31, 2014	Dec. 31, 2013
Fair Value Measurements Disclosure [Line Items]	'	'	'
Total available-for-sale investments	'	$2,198.40	$2,176.30
Other invested assets	'	5	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	'	9.1	8.9
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	'
Number of Available for Sale Securities, Fair Value Estimated by Present Value of Future Cash Flows	'	1	'
Total available-for-sale investments	'	9.1	8.9
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	'	76.5	74.2
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 $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	$2,198.40	$2,176.30
Fair Value [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	2,198.40	2,176.30
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	325.1	352.4
Fair Value [Member] | Obligations of States and Political Subdivisions [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	708	774.2
Fair Value [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	362.7	349.7
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	444.6	353.8
Fair Value [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	1,840.40	1,830.10
Fair Value [Member] | Large-Cap Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	207.9	194.4
Fair Value [Member] | Small-Cap Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	66.7	70.9
Fair Value [Member] | Other Invested Assets [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	83.4	80.9
Fair Value [Member] | Equity Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	274.6	265.3
Quoted prices in active markets for identical assets (Level 1) [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	281.5	272
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	207.9	194.4
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	66.7	70.9
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	6.9	6.7
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	274.6	265.3
Significant Other Observable Inputs (Level 2) [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	1,907.80	1,895.40
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	325.1	352.4
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	708	774.2
Significant Other Observable Inputs (Level 2) [Member] | Corporate Securities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	353.6	340.8
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	444.6	353.8
Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member]	'	'
Schedule of Available-for-sale Securities [Line Items]	'	'
Total available-for-sale investments	1,831.30	1,821.20
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	76.5	74.2
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	9.1	8.9
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	9.1	8.9
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	9.1	8.9
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	Mar. 31, 2014	Dec. 31, 2013
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	$8.90	$8.50
Total realized gains (losses) - included in earnings	0	0
Total unrealized gains (losses) - included in other comprehensive income	0.1	0.2
Purchases	0.1	0.2
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$9.10	$8.90

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 $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
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	75.8	74.6

Fair_Value_of_Financial_Instru6

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Carrying Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	$100.80	$100.80
Fair Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	102	101.2
Federal Home Loan Bank Advances [Member]	'	'
Debt Instrument [Line Items]	'	'
Interest rate (percent)	5.03%	5.03%
Federal Home Loan Bank Advances [Member] | Carrying Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	85.3	85.3
Federal Home Loan Bank Advances [Member] | Fair Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Total notes payable	86.5	85.7
Affiliate Subordinated Debentures Due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Interest rate (percent)	4.44%	4.44%
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 $)	Mar. 31, 2014
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
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
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Insurance [Abstract]	'	'
Gaap Expense Adjustments	$0.80	$0.70
Premiums earned:	'	'
Assumed from external insurers and reinsurers	1	0.4
Assumed under Pooling Arrangement	262.5	261.3
Ceded to external insurers and reinsurers	-6	-5.5
Ceded under Pooling Arrangement	-215.4	-210.7
Net assumed premiums earned	42.1	45.5
Losses and loss expenses incurred:	'	'
Assumed from external insurers and reinsurers	0.7	-0.2
Assumed under Pooling Arrangement (a)	171.8	173
Ceded to external insurers and reinsurers	-2.5	-1
Ceded under Pooling Arrangement (a)	-142.2	-131.8
Net assumed losses and loss expenses incurred	$27.80	$40

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
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Effective Income Tax Rate Reconciliation, Amount [Abstract]	'	'
Amount at statutory rate	$9.70	$7
Tax-exempt interest and dividends received deduction	-2.1	-2.3
Other, net	0.2	0.2
Valuation allowance	-7.2	-4.6
Federal income tax expense and effective rate	$0.60	$0.30
Effective Income Tax Rate Reconciliation, Percent [Abstract]	'	'
Amount at statutory rate (percent)	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	-8.00%	-12.00%
Other, net (percent)	1.00%	1.00%
Valuation allowance (percent)	-26.00%	-23.00%
Federal income tax expense (benefit) and effective rate (percent)	2.00%	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 $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Deferred tax assets:	'	'
Unearned premiums not currently deductible	$34	$33.80
Losses and loss expenses payable discounting	21.4	21.6
Postretirement and pension benefits	25.4	24.9
Realized loss on other-than-temporary impairment	8.3	7.5
Other liabilities	15.7	17.3
Net operating loss carryforward	50.9	56.5
Tax credit carryforward	2.3	1.7
Other	8.3	9.2
Total deferred tax assets	166.3	172.5
Deferred tax liabilities:	'	'
Deferral of policy acquisition costs	34.9	33.9
Net unrealized holding gains on investments	48.5	44.1
Total deferred tax liabilities	83.4	78
Total net deferred tax assets before valuation allowance	82.9	94.5
Less valuation allowance	75.2	82.6
Net deferred federal income taxes	$7.70	$11.90

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Income Tax Disclosure [Abstract]	'	'
Net deferred tax asset valuation allowance	$75.20	$82.60

Income_Taxes_Changes_in_Valuat

Income Taxes - Changes in Valuation Allowance Attributable to Continuing Operations and Other Comprehensive Income (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Income Tax Disclosure [Abstract]	'	'
Continuing operations	($7.20)	($4.60)
Other comprehensive income	-0.2	-5.1
Change in valuation allowance	($7.40)	($9.70)

Pension_and_Postretirement_Ben2

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Pension [Member]	'	'
Pension And Other Employee Benefit Plans [Line Items]	'	'
Service cost	$1.30	$1.50
Interest cost	2.8	2.4
Expected return on plan assets	-3.2	-3
Amortization of:	'	'
Prior service benefits	0	0
Net actuarial loss	1.4	2
Net periodic cost (benefit)	2.3	2.9
Postretirement [Member]	'	'
Pension And Other Employee Benefit Plans [Line Items]	'	'
Service cost	0	0
Interest cost	0.3	0.2
Expected return on plan assets	0	0
Amortization of:	'	'
Prior service benefits	-1.4	-1.4
Net actuarial loss	0.2	0.3
Net periodic cost (benefit)	($0.90)	($0.90)

Pension_and_Postretirement_Ben3

Pension and Postretirement Benefit Plans - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014
Compensation and Retirement Disclosure [Abstract]	'
Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year	$13.10

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
	Mar. 31, 2014	Mar. 31, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'	'
Beginning balance	$80,800,000	$84,200,000
Other comprehensive income before reclassifications	18,800,000	19,700,000
Amounts reclassified from AOCI	-10,500,000	-5,800,000
Other comprehensive income, net of tax	8,300,000	13,900,000
Ending balance	89,100,000	98,100,000
Unrealized Gains and Losses on Available-for-Sale Securities [Member]	'	'
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'	'
Beginning balance	84,600,000	124,000,000
Other comprehensive income before reclassifications	18,800,000	19,700,000
Amounts reclassified from AOCI	-10,700,000	-6,700,000
Other comprehensive income, net of tax	8,100,000	13,000,000
Ending balance	92,700,000	137,000,000
Benefit Plan Items [Member]	'	'
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'	'
Beginning balance	-3,800,000	-39,900,000
Other comprehensive income before reclassifications	0	0
Amounts reclassified from AOCI	200,000	900,000
Other comprehensive income, net of tax	200,000	900,000
Ending balance	-3,600,000	-39,000,000
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member]	'	'
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]	'	'
Beginning balance	'	100,000
Other comprehensive income before reclassifications	'	0
Amounts reclassified from AOCI	'	0
Other comprehensive income, net of tax	'	0
Ending balance	'	$100,000

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
	Mar. 31, 2014	Mar. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'
Realized gain/(loss) on sale of securities	$10,700,000	$6,700,000
Tax (expense) benefit	-600,000	-300,000
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	10,500,000	5,800,000
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	10,700,000	6,700,000
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	-200,000	-900,000
Reclassification Out of Accumulated Other Comprehensive Income [Member]	'	'
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]	'	'
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	10,500,000	5,800,000
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	10,700,000	6,700,000
Total before tax	10,700,000	6,700,000
Tax (expense) benefit	0	0
Net of tax	10,700,000	6,700,000
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/Negative prior-service costs	1,400,000	1,400,000
Accumulated other comprehensive income components/Net actuarial loss	-1,600,000	-2,300,000
Total before tax	-200,000	-900,000
Tax (expense) benefit	0	0
Net of tax	($200,000)	($900,000)

Earnings_Per_Common_Share_Comp

Earnings Per Common Share - Compilation of Basic and Diluted Earnings (Loss) Per Common Share (Detail) (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Numerator:	'	'
Net income	$27.10	$19.70
Denominator:	'	'
Weighted average shares for basic net earnings per common share (shares)	40.8	40.5
Effect of dilutive share-based awards (shares)	0.5	0.2
Adjusted weighted average shares for diluted net earnings per common share (shares)	41.3	40.7
Basic net earnings per common share (usd per share)	$0.67	$0.49
Diluted net earnings per common share (usd per share)	$0.66	$0.49

Earnings_Per_Common_Share_Opti

Earnings Per Common Share - Options to Purchase Shares of Common Stock (Detail)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]	'	'
Number of antidilutive options and awards	1.7	3.2

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
	Segment
Segment Reporting [Abstract]	'	'
Number of reportable segments (segments)	4	'
Investable assets	$2,351.70	$2,331.60

Segment_Information_Financial_

Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013
Revenues from external sources:	'	'
Revenue	$291.30	$285.30
Net Investment Income	17.6	16.9
Segment income before federal income tax:	'	'
Gaap Expense Adjustments	0.8	0.7
Interest expense on corporate debt	-1.3	-1.7
Income before federal income taxes	27.7	20
Operating Segments [Member]	'	'
Revenues from external sources:	'	'
Revenue	291.3	285.3
Segment income before federal income tax:	'	'
Income before federal income taxes	24.5	18.3
Operating Segments [Member] | Personal Insurance Segments [Member]	'	'
Revenues from external sources:	'	'
Revenue	114.3	114.7
Operating Segments [Member] | Business Insurance Segments [Member]	'	'
Revenues from external sources:	'	'
Revenue	93.5	88.5
Operating Segments [Member] | Specialty Insurance Segment [Member]	'	'
Revenues from external sources:	'	'
Revenue	54.7	58.1
Operating Segments [Member] | Total Insurance Segments [Member]	'	'
Revenues from external sources:	'	'
Revenue	262.5	261.3
Segment income before federal income tax:	'	'
Income before federal income taxes	-3.9	-5.7
Operating Segments [Member] | Net Investment Income [Member]	'	'
Revenues from external sources:	'	'
Net Investment Income	17.6	16.9
Operating Segments [Member] | Total Investment Operations [Member]	'	'
Revenues from external sources:	'	'
Revenue	28.3	23.6
Segment income before federal income tax:	'	'
Income before federal income taxes	28.3	23.6
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]	'	'
Revenues from external sources:	'	'
Revenue	10.7	6.7
Segment income before federal income tax:	'	'
Income before federal income taxes	10.7	6.7
Operating Segments [Member] | All Other Segments [Member]	'	'
Segment income before federal income tax:	'	'
Income before federal income taxes	0.1	0.4
Operating Segments [Member] | Personal Insurance [Member]	'	'
Segment income before federal income tax:	'	'
Income before federal income taxes	5.1	1.3
Operating Segments [Member] | Business Insurance [Member]	'	'
Segment income before federal income tax:	'	'
Income before federal income taxes	-8.5	-5.3
Operating Segments [Member] | Specialty Insurance [Member]	'	'
Segment income before federal income tax:	'	'
Income before federal income taxes	-0.5	-1.7
Corporate and Other [Member] | All Other Segments [Member]	'	'
Revenues from external sources:	'	'
Revenue	0.5	0.4
Corporate and Other [Member] | Gaap Adjustments [Member]	'	'
Segment income before federal income tax:	'	'
Gaap Expense Adjustments	5.4	4.5
Corporate and Other [Member] | Corporate Segment [Member]	'	'
Segment income before federal income tax:	'	'
Corporate Expenses	-0.9	-1.1
Corporate and Other [Member] | Segment Reconciling Items [Member]	'	'
Segment income before federal income tax:	'	'
Interest expense on corporate debt	1.3	1.7
Eliminate Intersegment Revenues [Member]	'	'
Revenues from external sources:	'	'
Revenue	-1.3	-1.3
Eliminate Intersegment Revenues [Member] | Intersegment [Member]	'	'
Revenues from external sources:	'	'
Revenue	1.3	1.3
External and Intersegment Sources [Member]	'	'
Revenues from external sources:	'	'
Revenue	292.6	286.6
Total Consolidated [Member]	'	'
Revenues from external sources:	'	'
Revenue	291.3	285.3
Segment Reconciling Items [Member]	'	'
Segment income before federal income tax:	'	'
Income before federal income taxes	$3.20	$1.70