State Auto Financial (STFC) 10-Q 6 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	1-May-15
Document Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Entity Registrant Name	State Auto Financial CORP
Entity Central Index Key	874977
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding (shares)		41,121,315

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $1,894.0 and $1,831.3, respectively)	$1,964	$1,891.90
Equity securities, available-for-sale, at fair value (cost $236.7 and $235.5, respectively)	310.6	310.4
Other invested assets, available-for-sale, at fair value (cost $54.4 and $50.5, respectively)	88.6	80.3
Other invested assets	5.3	5.3
Notes receivable from affiliate	70	70
Total investments	2,438.50	2,357.90
Cash and cash equivalents	68.4	86.3
Accrued investment income and other assets	36.1	33.8
Deferred policy acquisition costs (affiliated net assumed $49.6 and $46.8, respectively)	127.4	126.5
Reinsurance recoverable on losses and loss expenses payable	7.9	9.6
Prepaid reinsurance premiums	6.2	6.1
Due from affiliate	0	40.1
Current federal income taxes	0.7	1.1
Net deferred federal income taxes	84.3	97.4
Property and equipment, at cost	7.6	8.1
Total assets	2,777.10	2,766.90
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $504.8 and $494.3, respectively)	997.7	983.2
Unearned premiums (affiliated net assumed $205.4 and $201.7, respectively)	604.2	612.4
Notes payable (affiliates $15.5 and $15.5, respectively)	100.8	100.8
Postretirement and pension benefits (affiliated net ceded $62.3 and $63.2, respectively)	115.6	117.3
Due to affiliate	6.2	0
Other liabilities (affiliated net ceded $2.8 and $5.1, respectively)	48.4	80.3
Total liabilities	1,872.90	1,894
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 47.8 and 47.7 shares issued, respectively, at stated value of $2.50 per share	119.5	119.3
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	-116.1	-116
Additional paid-in capital	144.5	143.2
Accumulated other comprehensive income (affiliated net ceded $63.7 and $65.1, respectively)	81	71.7
Retained earnings	675.3	654.7
Total stockholders’ equity	904.2	872.9
Total liabilities and stockholders’ equity	2,777.10	2,766.90
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, 2015	Dec. 31, 2014
In Millions, except Share data, unless otherwise specified
Fixed maturities, available-for-sale, amortized cost	$1,894	$1,831.30
Equity securities, available-for-sale, cost	236.7	235.5
Other invested assets, available-for-sale, cost	54.4	50.5
Deferred policy acquisitions costs from affiliates	49.6	46.8
Losses and loss expenses payable, affiliates	504.8	494.3
Unearned premiums, affiliates	205.4	201.7
Notes payable, affiliates	15.5	15.5
Net Postretirement and Pension Benefits	-62.3	-63.2
Net Other Liabilities from Affiliates	-2.8	-5.1
Common stock, no par value (usd per share)	$0	$0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	47,800,000	47,700,000
Common stock, stated value per share (usd per share)	$2.50	$2.50
Treasury stock (shares)	6,800,000	6,800,000
Net Accumulated Other Comprehensive Income	($63.70)	($65.10)
Class A Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$0	$0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Class B Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$0	$0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Earned premiums (affiliated net assumed $105.4 and $47.1, respectively)	$315.30	$262.50
Net investment income (affiliate $1.2 and $1.2, respectively)	15.4	17.6
Net realized gains on investments:
Total other-than-temporary impairment losses	-0.5	-0.8
Portion of loss recognized in other comprehensive income	0	0
Other net realized investment gains	4.3	11.5
Total net realized gains on investments	3.8	10.7
Other income from affiliates	0.4	0.5
Total revenues	334.9	291.3
Losses and loss expenses (affiliated net assumed $74.1 and $30.4, respectively)	195	171.8
Acquisition and operating expenses	103.4	88.5
Interest expense (affiliates $0.2 and $0.2, respectively)	1.3	1.3
Other expenses	2	2
Total expenses	301.7	263.6
Income before federal income taxes	33.2	27.7
Federal income tax expense	8.5	0.6
Net income	$24.70	$27.10
Earnings per common share:
Basic (usd per share)	$0.60	$0.67
Diluted (usd per share)	$0.60	$0.66
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, 2015	Mar. 31, 2014
Income Statement [Abstract]
Earned premiums, ceded to affiliates	$105.40	$47.10
Net investment income, from affiliate	1.2	1.2
Losses and loss expenses, ceded to affiliates	74.1	30.4
Net Acquisition and Operating Expenses	77.3	38.9
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, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$24.70	$27.10
Net unrealized holding gains on investments:
Unrealized holding gains	17	23
Reclassification adjustments for gains realized in net income	-4.2	-10.7
Income tax expense	-4.5	-4.2
Total net unrealized holding gains on investments	8.3	8.1
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	-1.4	-1.4
Net actuarial loss	2.9	1.6
Income tax expense	-0.5	0
Total net unrecognized benefit plan obligations	1	0.2
Other comprehensive income	9.3	8.3
Comprehensive income	$34	$35.40

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net income	$24.70	$27.10
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization, net	5.5	3.4
Share-based compensation	1	1
Net realized gains on investments	-3.8	-10.7
Changes in operating assets and liabilities:
Deferred policy acquisition costs	-0.9	-2.9
Accrued investment income and other assets	-2	-1.5
Postretirement and pension benefits	-0.7	1.7
Other liabilities and due to/from affiliates, net	-49.6	-10.6
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	1.6	-0.6
Losses and loss expenses payable	14.5	-8.2
Unearned premiums	-8.2	2.9
Federal income taxes	9	0.7
Cash provided from December 31, 2014 unearned premium transfer related to the homeowners quota share arrangement	63.5	0
Net cash provided by operating activities	54.6	2.3
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	-212.8	-137.2
Purchases of equity securities available-for-sale	-19.7	-38
Purchases of other invested assets	-3.9	-0.4
Maturities, calls and pay downs of fixed maturities available-for-sale	83.2	68.9
Sales of fixed maturities available-for-sale	62.5	71.3
Sales of equity securities available-for-sale	21.5	34.6
Sales of other invested assets available-for-sale	0.2	0.2
Net cash used in investing activities	-69	-0.6
Cash flows from financing activities:
Proceeds from issuance of common stock	0.7	0.6
Payments to acquire treasury stock	-0.1	-0.1
Payment of dividends	-4.1	-4.2
Net cash used in financing activities	-3.5	-3.7
Net decrease in cash and cash equivalents	-17.9	-2
Cash and cash equivalents at beginning of period	86.3	80.3
Cash and cash equivalents at end of period	68.4	78.3
Supplemental disclosures:
Interest paid (affiliates $0.2 and $0.2, respectively)	$1.30	$1.30

Condensed_Consolidated_Stateme3

Condensed Consolidated Statements of Cash Flows (Parenthetical) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement of Cash Flows [Abstract]
Interest paid, affiliates	$0.20	$0.20

Basis_of_Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2015
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, 2015 are not necessarily indicative of the results that may be expected for the year ending December 31, 2015. The balance sheet at December 31, 2014 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, 2014 (the “2014 Form 10-K”). Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the 2014 Form 10-K.

Investments

Investments	3 Months Ended
	Mar. 31, 2015
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, 2015 and December 31, 2014:
	($ millions)	Cost or amortized cost				Gross unrealized holding gains				Gross unrealized holding losses				Fair value
	31-Mar-15
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	287			$	19			$	(1.0	)		$	305
	Obligations of states and political subdivisions	834.2				26.6				(1.5		)		859.3
	Corporate securities	302.9				10.6				(2.5		)		311
	U.S. government agencies mortgage-backed securities	469.9				20.5				(1.7		)		488.7
	Total fixed maturities	1,894.00				76.7				(6.7		)		1,964.00
	Equity securities:
	Large-cap securities	181.6				54.3				(1.3		)		234.6
	Small-cap securities	55.1				20.9				—				76
	Total equity securities	236.7				75.2				(1.3		)		310.6
	Other invested assets	54.4				34.2				—				88.6
	Total available-for-sale securities	$	2,185.10			$	186.1			$	(8.0	)		$	2,363.20
	($ millions)	Cost or amortized cost				Gross unrealized holding gains				Gross unrealized holding losses				Fair value
	31-Dec-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	296.7			$	14.1			$	(1.5	)		$	309.3
	Obligations of states and political subdivisions	742.5				27.4				(0.4		)		769.5
	Corporate securities	333.4				10.2				(3.0		)		340.6
	U.S. government agencies mortgage-backed securities	458.7				15.6				(1.8		)		472.5
	Total fixed maturities	1,831.30				67.3				(6.7		)		1,891.90
	Equity securities:
	Large-cap securities	185.5				57.3				(0.6		)		242.2
	Small-cap securities	50				18.2				—				68.2
	Total equity securities	235.5				75.5				(0.6		)		310.4
	Other invested assets	50.5				29.8				—				80.3
	Total available-for-sale securities	$	2,117.30			$	172.6			$	(7.3	)		$	2,282.60
	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, 2015 and December 31, 2014:
		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-15
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	41.1			$	(0.3	)		7		$	24.1			$	(0.7	)	7	$	65.2	$	(1.0	)	14
	Obligations of states and political subdivisions	117.7				(1.5		)		23		6.8				—			3	124.5		(1.5		)	26
	Corporate securities	33.3				(1.2		)		5		50.6				(1.3		)	10	83.9		(2.5		)	15
	U.S. government agencies mortgage-backed securities	48.3				(0.7		)		5		39.4				(1.0		)	13	87.7		(1.7		)	18
	Total fixed maturities	240.4				(3.7		)		40		120.9				(3.0		)	33	361.3		(6.7		)	73
	Large-cap equity securities	14				(1.3		)		4		—				—			—	14		(1.3		)	4
	Total temporarily impaired securities	$	254.4			$	(5.0	)		44		$	120.9			$	(3.0	)	33	$	375.3	$	(8.0	)	77
		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-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	19.9			$	(0.3	)		4		$	52			$	(1.2	)	17	$	71.9	$	(1.5	)	21
	Obligations of states and political subdivisions	6.1				—				5		30.9				(0.4		)	9	37		(0.4		)	14
	Corporate securities	43.5				(0.9		)		8		56.1				(2.1		)	11	99.6		(3.0		)	19
	U.S. government agencies mortgage-backed securities	44				(0.3		)		8		37.5				(1.5		)	13	81.5		(1.8		)	21
	Total fixed maturities	113.5				(1.5		)		25		176.5				(5.2		)	50	290		(6.7		)	75
	Large-cap equity securities	7.2				(0.6		)		2		—				—			—	7.2		(0.6		)	2
	Total temporarily impaired securities	$	120.7			$	(2.1	)		27		$	176.5			$	(5.2	)	50	$	297.2	$	(7.3	)	77
	The Company reviewed its investments at March 31, 2015, 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, 2015 and 2014:
	($ millions)		2015				2014
	Equity securities:
	Large-cap securities		$	—			$	(0.2	)
	Small-cap securities		(0.5		)		(0.6		)
	Total other-than-temporary impairments		$	(0.5	)		$	(0.8	)
	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, 2015:
	($ millions)	Amortized cost				Fair
						value
Due in 1 year or less	$	38.6			$	39.4
Due after 1 year through 5 years	329.3				343.4
Due after 5 years through 10 years	278.9				293.1
Due after 10 years	777.3				799.4
U.S. government agencies mortgage-backed securities	469.9				488.7
Total	$	1,894.00			$	1,964.00
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, 2015, State Auto P&C had U.S. government agencies mortgage-backed fixed maturity securities, with a carrying value of approximately $85.0 million, that were pledged as collateral for the Federal Home Loan Bank of Cincinnati ("FHLB") Loan. In accordance with the terms of the FHLB Loan, State Auto P&C retains all rights regarding these securities.
Fixed maturities with fair values of $8.8 million were on deposit with insurance regulators as required by law at March 31, 2015 and December 31, 2014. The Company retains all rights regarding these securities.
The following table sets forth the components of net investment income for the three months ended March 31, 2015 and 2014:
($ millions)	2015				2014
Fixed maturities	$	12.9			$	15.4
Equity securities	1.6				1.4
Cash and cash equivalents, and other	1.4				1.3
Investment income	15.9				18.1
Investment expenses	0.5				0.5
Net investment income	$	15.4			$	17.6
The Company’s current investment strategy does not rely on the use of derivative financial instruments.
Proceeds on sales of available-for-sale securities were $84.2 million and $106.1 million for the three months ended March 31, 2015 and 2014.
The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three months ended March 31, 2015 and 2014:
($ millions)	2015				2014
Realized gains:
Fixed maturities	$	1.1			$	2.1
Equity securities	4.1				9.4
Other invested assets	—				0.1
Total realized gains	5.2				11.6
Realized losses:
Equity securities:
Sales	(0.5		)		(0.1		)
OTTI	(0.5		)		(0.8		)
Total realized losses	(1.0		)		(0.9		)
Net realized gains on investments	$	4.2			$	10.7
Change in unrealized holding gains, net of tax:
Fixed maturities	$	9.4			$	14.4
Equity securities	(1.0		)		(4.5		)
Other invested assets	4.4				2.4
Deferred federal income tax liability	(4.5		)		(4.3		)
Valuation allowance	—				0.1
Change in net unrealized holding gains, net of tax	$	8.3			$	8.1

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2015
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, 2015 and December 31, 2014, 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, 2015 and 2014.
	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. The Company regularly selects a random sample of security prices which are compared to one or more alternative pricing sources for reasonableness. 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 a security discussed below), and U.S. government agencies 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 one fixed maturity corporate security included in Level 3 and estimates its fair value using the present value of the future cash flows. Due to the limited amount of observable market information for this security, the Company includes the fair value estimate 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 $80.5 million and $72.9 million at March 31, 2015 and December 31, 2014, respectively, which was determined using each fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the funds including obtaining and reviewing each fund’s audited financial statements. There are no unfunded commitments related to the funds. The Company may not sell its investment in the funds; however, the Company may redeem all or a portion of its investment in the funds at net asset value per share with the appropriate prior written notice. Due to the Company’s ability to redeem its investment in the funds at net asset value per share at the measurement date, the funds have been disclosed in Level 2.
	The remainder of the Company’s other invested assets consist primarily of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds based on an observable market price for an identical asset in an active market reflect their fair values and consequently these securities have been disclosed in Level 1.
	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at March 31, 2015 and December 31, 2014:
	($ millions)	Total			Level 1		Level 2			Level 3
	31-Mar-15
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	305		$	—	$	305		$	—
	Obligations of states and political subdivisions	859.3			—		859.3			—
	Corporate securities	311			—		307.5			3.5
	U.S. government agencies mortgage-backed securities	488.7			—		488.7			—
	Total fixed maturities	1,964.00			—		1,960.50			3.5
	Equity securities:
	Large-cap securities	234.6			234.6		—			—
	Small-cap securities	76			76		—			—
	Total equity securities	310.6			310.6		—			—
	Other invested assets	88.6			8.1		80.5			—
	Total available-for-sale investments	$	2,363.20		$	318.7	$	2,041.00		$	3.5
	($ millions)	Total			Level 1		Level 2			Level 3
	31-Dec-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	309.3		$	—	$	309.3		$	—
	Obligations of states and political subdivisions	769.5			—		769.5			—
	Corporate securities	340.6			—		331.2			9.4
	U.S. government agencies mortgage-backed securities	472.5			—		472.5			—
	Total fixed maturities	1,891.90			—		1,882.50			9.4
	Equity securities:
	Large-cap securities	242.2			242.2		—			—
	Small-cap securities	68.2			68.2		—			—
	Total equity securities	310.4			310.4		—			—
	Other invested assets	80.3			7.4		72.9			—
	Total available-for-sale investments	$	2,282.60		$	317.8	$	1,955.40		$	9.4
	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, 2015 and the year ended December 31, 2014, separately for each major category of assets:
	($ millions)	Fixed maturities
	Balance at January 1, 2015	$	9.4
	Total realized gains – included in earnings	0.2
	Total unrealized losses – included in other comprehensive income	(0.2		)
	Purchases	—
	Sales	(5.9		)
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at March 31, 2015	$	3.5
	($ millions)	Fixed maturities
	Balance at January 1, 2014	$	8.9
	Total realized gains – included in earnings	—
	Total unrealized gains – included in other comprehensive income	0.2
	Purchases	0.3
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at December 31, 2014	$	9.4
	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, 2015 was $4.8 million and the fair value of the Trust Securities was $0.5 million. The 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 Automobile Mutual Insurance Company (“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, 2015							December 31, 2014
	Carrying value			Fair value		Interest rate		Carrying value			Fair		Interest rate
											value
Notes receivable from affiliate	$	70		$	76.4	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-15							31-Dec-14
	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		$	86.4	5.03	%
Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5			15.5		4.46	%	15.5			15.5		4.44	%
Total notes payable	$	100.8		$	102			$	100.8		$	101.9

Reinsurance

Reinsurance	3 Months Ended
	Mar. 31, 2015
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 Auto Mutual and its subsidiaries and affiliates, State Auto Insurance Company of Wisconsin, Meridian Security Insurance Company, Patrons Mutual Insurance Company of Connecticut, Rockhill Insurance Company, Plaza Insurance Company, American Compensation Insurance Company and Bloomington Compensation Insurance Company, (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, 2015 and 2014:
	($ millions)	2015			2014
	Premiums earned:
	Assumed from external insurers and reinsurers	$	1.1		$	1
	Assumed under Pooling Arrangement	315.3			262.5
	Ceded to external insurers and reinsurers	(8.9		)	(6.0		)
	Ceded under Pooling Arrangement	(209.9		)	(215.4		)
	Net assumed premiums earned	$	97.6		$	42.1
	Losses and loss expenses incurred:
	Assumed from external insurers and reinsurers	$	0.8		$	0.7
	Assumed under Pooling Arrangement	195.3			172.6
	Ceded to external insurers and reinsurers	0.1			(2.4		)
	Ceded under Pooling Arrangement	(121.2		)	(142.2		)
	Net assumed losses and loss expenses incurred	$	75		$	28.7
	Subject to the terms and conditions of the expired Homeowners Quota Share Arrangement ("HO QS Arrangement"), the participating reinsurers’ margin was capped at 9.0%, with any excess returned to the State Auto Group in the form of profit commission. For the three months ended March 31, 2015, the Company recognized $1.3 million of profit commission, which was reflected as a reduction in acquisition and operating expenses on our consolidated statements of income.

Income_Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	5. 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, 2015 and 2014:
	($ millions)	2015					2014
	Amount at statutory rate	$	11.6		35	%	$	9.7		35	%
	Tax-exempt interest and dividends received deduction	(2.2		)	(7	)	(2.1		)	(8	)
	Other, net	(0.9		)	(3	)	0.2			1
	Valuation allowance	—			—		(7.2		)	(26	)
	Federal income tax expense and effective rate	$	8.5		25	%	$	0.6		2	%

Pension_and_Postretirement_Ben

Pension and Postretirement Benefit Plans	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)	Pension						Postretirement
		Three months ended March 31
		2015			2014			2015			2014
	Service cost	$	2		$	1.3		$	—		$	—
	Interest cost	2.8			2.8			0.2			0.3
	Expected return on plan assets	(3.4		)	(3.2		)	—			—
	Amortization of:
	Negative prior service cost	—			—			(1.4		)	(1.4		)
	Net actuarial loss	2.7			1.4			0.2			0.2
	Net periodic cost (benefit)	$	4.1		$	2.3		$	(1.0	)	$	(0.9	)
	The Company's share of contributions were $3.3 million for the three months ended March 31, 2015 and expects its remaining share of contributions to be an additional $9.7 million to the pension plan during 2015.

Other_Comprehensive_Income_and

Other Comprehensive Income and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)		Unrealized Gains				Benefit Plan Items				Total
			and Losses on
			Available-for-Sale
			Securities
	Beginning balance at January 1, 2015		$	110			$	(38.3	)		$	71.7
	Other comprehensive income before reclassifications		12.5				—				12.5
	Amounts reclassified from AOCI (a)		(4.2		)		1				(3.2		)
	Net current period other comprehensive income		8.3				1				9.3
	Ending balance at March 31, 2015		$	118.3			$	(37.3	)		$	81
	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
	(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 condensed consolidated statement of income for the three months ended March 31, 2015 and 2014:
	($ millions)
	Details about Accumulated Other			Three months ended								Affected line item in the Condensed
	Comprehensive Income Components			31-Mar								Consolidated Statements of Income
				2015				2014
	Unrealized gains on available for sale securities			$	4.2			$	10.7			Realized gain on sale of securities
				4.2				10.7				Total before tax
				(1.5		)		—				Tax expense
				2.7				10.7				Net of tax
	Amortization of benefit plan items
	Negative prior service cost			1.4				1.4				(a)
	Net actuarial loss			(2.9		)		(1.6		)		(a)
				(1.5		)		(0.2		)		Total before tax
				0.5				—				Tax benefit
				(1.0		)		(0.2		)		Net of tax
	Total reclassifications for the period			$	1.7			$	10.5
	(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, 2015
Earnings Per Share [Abstract]
Earnings (Loss) per Common Share	Net 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, 2015 and 2014:
	($ and shares in millions, except per share amounts)	2015			2014
	Numerator:
	Net income for basic earnings per common share	$	24.7		$	27.1
	Denominator:
	Weighted average shares for basic net earnings per common share	41			40.8
	Effect of dilutive share-based awards	0.4			0.3
	Adjusted weighted average shares for diluted net earnings per common share	41.4			41.1
	Basic net earnings per common share	$	0.6		$	0.67
	Diluted net earnings per common share	$	0.6		$	0.66
	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, 2015 and 2014.
	(shares in millions)	2015		2014
	Total number of antidilutive options and awards	1.7		1.7

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2015
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 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, 2015 and 2014:
	($ millions)	2015			2014
	Revenue from external sources:
	Insurance segments
	Personal insurance	$	150		$	114.3
	Business insurance	96.1			93.5
	Specialty insurance	69.2			54.7
	Total insurance segments	315.3			262.5
	Investment operations segment
	Net investment income	15.4			17.6
	Net realized capital gains	4.2			10.7
	Total investment operations segment	19.6			28.3
	All other	—			0.5
	Total revenue from external sources	334.9			291.3
	Intersegment revenue	1.4			1.3
	Total revenue	336.3			292.6
	Reconciling items:
	Eliminate intersegment revenues	(1.4		)	(1.3		)
	Total consolidated revenues	$	334.9		$	291.3
	Segment income before federal income tax:
	Insurance segments SAP underwriting income (loss)
	Personal insurance	$	17.4		$	5.1
	Business insurance	(1.4		)	(8.5		)
	Specialty insurance	(0.8		)	(0.5		)
	Total insurance segments	15.2			(3.9		)
	Investment operations segment
	Net investment income	15.4			17.6
	Net realized capital gains	4.2			10.7
	Total investment operations segment	19.6			28.3
	All other	(0.4		)	0.1
	Total segment income before tax expense	34.4			24.5
	Reconciling items:
	GAAP expense adjustments	1.2			5.4
	Interest expense on corporate debt	(1.3		)	(1.3		)
	Corporate expenses	(1.1		)	(0.9		)
	Total reconciling items	(1.2		)	3.2
	Total consolidated income before federal income tax expense	$	33.2		$	27.7
	Investable assets attributable to the Company’s investment operations segment totaled $2,506.9 million and $2,444.2 million at March 31, 2015 and December 31, 2014, respectively.

Contingencies_and_Litigation

Contingencies and Litigation	3 Months Ended
	Mar. 31, 2015
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 the Company.
	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 ("State Auto Defendants") in Federal District Court in Ohio. Plaintiffs claimed 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 alleged that they purchased coverage in excess of that which was necessary to insure them in the event of loss. Plaintiffs’ claims included breach of good faith and fair dealing, negligent misrepresentation and fraud, violation of the Ohio Deceptive Trade Practices Act, and fraudulent inducement. Plaintiffs sought compensatory and punitive damages to be determined by the court, as well as class certification. On February 2, 2015, the Court struck the class allegations, and on March 13, 2015, Plaintiffs settled with the State Auto Defendants for a nominal amount and dismissed their remaining individual claims with prejudice, thereby terminating the litigation.
	The Company is involved in lawsuits in the ordinary course of its business arising out of or otherwise related to its insurance policies. Additionally, from time to time the Company may be involved in lawsuits, including class actions, 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 position or cash flows.
	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, 2015
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, 2015 and December 31, 2014:
	($ millions)	Cost or amortized cost				Gross unrealized holding gains				Gross unrealized holding losses				Fair value
	31-Mar-15
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	287			$	19			$	(1.0	)		$	305
	Obligations of states and political subdivisions	834.2				26.6				(1.5		)		859.3
	Corporate securities	302.9				10.6				(2.5		)		311
	U.S. government agencies mortgage-backed securities	469.9				20.5				(1.7		)		488.7
	Total fixed maturities	1,894.00				76.7				(6.7		)		1,964.00
	Equity securities:
	Large-cap securities	181.6				54.3				(1.3		)		234.6
	Small-cap securities	55.1				20.9				—				76
	Total equity securities	236.7				75.2				(1.3		)		310.6
	Other invested assets	54.4				34.2				—				88.6
	Total available-for-sale securities	$	2,185.10			$	186.1			$	(8.0	)		$	2,363.20
	($ millions)	Cost or amortized cost				Gross unrealized holding gains				Gross unrealized holding losses				Fair value
	31-Dec-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	296.7			$	14.1			$	(1.5	)		$	309.3
	Obligations of states and political subdivisions	742.5				27.4				(0.4		)		769.5
	Corporate securities	333.4				10.2				(3.0		)		340.6
	U.S. government agencies mortgage-backed securities	458.7				15.6				(1.8		)		472.5
	Total fixed maturities	1,831.30				67.3				(6.7		)		1,891.90
	Equity securities:
	Large-cap securities	185.5				57.3				(0.6		)		242.2
	Small-cap securities	50				18.2				—				68.2
	Total equity securities	235.5				75.5				(0.6		)		310.4
	Other invested assets	50.5				29.8				—				80.3
	Total available-for-sale securities	$	2,117.30			$	172.6			$	(7.3	)		$	2,282.60
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, 2015 and December 31, 2014:
		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-15
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	41.1			$	(0.3	)		7		$	24.1			$	(0.7	)	7	$	65.2	$	(1.0	)	14
	Obligations of states and political subdivisions	117.7				(1.5		)		23		6.8				—			3	124.5		(1.5		)	26
	Corporate securities	33.3				(1.2		)		5		50.6				(1.3		)	10	83.9		(2.5		)	15
	U.S. government agencies mortgage-backed securities	48.3				(0.7		)		5		39.4				(1.0		)	13	87.7		(1.7		)	18
	Total fixed maturities	240.4				(3.7		)		40		120.9				(3.0		)	33	361.3		(6.7		)	73
	Large-cap equity securities	14				(1.3		)		4		—				—			—	14		(1.3		)	4
	Total temporarily impaired securities	$	254.4			$	(5.0	)		44		$	120.9			$	(3.0	)	33	$	375.3	$	(8.0	)	77
		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-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	19.9			$	(0.3	)		4		$	52			$	(1.2	)	17	$	71.9	$	(1.5	)	21
	Obligations of states and political subdivisions	6.1				—				5		30.9				(0.4		)	9	37		(0.4		)	14
	Corporate securities	43.5				(0.9		)		8		56.1				(2.1		)	11	99.6		(3.0		)	19
	U.S. government agencies mortgage-backed securities	44				(0.3		)		8		37.5				(1.5		)	13	81.5		(1.8		)	21
	Total fixed maturities	113.5				(1.5		)		25		176.5				(5.2		)	50	290		(6.7		)	75
	Large-cap equity securities	7.2				(0.6		)		2		—				—			—	7.2		(0.6		)	2
	Total temporarily impaired securities	$	120.7			$	(2.1	)		27		$	176.5			$	(5.2	)	50	$	297.2	$	(7.3	)	77
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, 2015 and 2014:
	($ millions)		2015				2014
	Equity securities:
	Large-cap securities		$	—			$	(0.2	)
	Small-cap securities		(0.5		)		(0.6		)
	Total other-than-temporary impairments		$	(0.5	)		$	(0.8	)
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, 2015:
	($ millions)	Amortized cost				Fair
						value
	Due in 1 year or less	$	38.6			$	39.4
	Due after 1 year through 5 years	329.3				343.4
	Due after 5 years through 10 years	278.9				293.1
	Due after 10 years	777.3				799.4
	U.S. government agencies mortgage-backed securities	469.9				488.7
	Total	$	1,894.00			$	1,964.00
Components of Net Investment Income	The following table sets forth the components of net investment income for the three months ended March 31, 2015 and 2014:
	($ millions)	2015				2014
	Fixed maturities	$	12.9			$	15.4
	Equity securities	1.6				1.4
	Cash and cash equivalents, and other	1.4				1.3
	Investment income	15.9				18.1
	Investment expenses	0.5				0.5
	Net investment income	$	15.4			$	17.6
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, 2015 and 2014:
	($ millions)	2015				2014
	Realized gains:
	Fixed maturities	$	1.1			$	2.1
	Equity securities	4.1				9.4
	Other invested assets	—				0.1
	Total realized gains	5.2				11.6
	Realized losses:
	Equity securities:
	Sales	(0.5		)		(0.1		)
	OTTI	(0.5		)		(0.8		)
	Total realized losses	(1.0		)		(0.9		)
	Net realized gains on investments	$	4.2			$	10.7
	Change in unrealized holding gains, net of tax:
	Fixed maturities	$	9.4			$	14.4
	Equity securities	(1.0		)		(4.5		)
	Other invested assets	4.4				2.4
	Deferred federal income tax liability	(4.5		)		(4.3		)
	Valuation allowance	—				0.1
	Change in net unrealized holding gains, net of tax	$	8.3			$	8.1

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and December 31, 2014:
	($ millions)	Total			Level 1		Level 2			Level 3
	31-Mar-15
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	305		$	—	$	305		$	—
	Obligations of states and political subdivisions	859.3			—		859.3			—
	Corporate securities	311			—		307.5			3.5
	U.S. government agencies mortgage-backed securities	488.7			—		488.7			—
	Total fixed maturities	1,964.00			—		1,960.50			3.5
	Equity securities:
	Large-cap securities	234.6			234.6		—			—
	Small-cap securities	76			76		—			—
	Total equity securities	310.6			310.6		—			—
	Other invested assets	88.6			8.1		80.5			—
	Total available-for-sale investments	$	2,363.20		$	318.7	$	2,041.00		$	3.5
	($ millions)	Total			Level 1		Level 2			Level 3
	31-Dec-14
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	309.3		$	—	$	309.3		$	—
	Obligations of states and political subdivisions	769.5			—		769.5			—
	Corporate securities	340.6			—		331.2			9.4
	U.S. government agencies mortgage-backed securities	472.5			—		472.5			—
	Total fixed maturities	1,891.90			—		1,882.50			9.4
	Equity securities:
	Large-cap securities	242.2			242.2		—			—
	Small-cap securities	68.2			68.2		—			—
	Total equity securities	310.4			310.4		—			—
	Other invested assets	80.3			7.4		72.9			—
	Total available-for-sale investments	$	2,282.60		$	317.8	$	1,955.40		$	9.4
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, 2015 and the year ended December 31, 2014, separately for each major category of assets:
	($ millions)	Fixed maturities
	Balance at January 1, 2015	$	9.4
	Total realized gains – included in earnings	0.2
	Total unrealized losses – included in other comprehensive income	(0.2		)
	Purchases	—
	Sales	(5.9		)
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at March 31, 2015	$	3.5
	($ millions)	Fixed maturities
	Balance at January 1, 2014	$	8.9
	Total realized gains – included in earnings	—
	Total unrealized gains – included in other comprehensive income	0.2
	Purchases	0.3
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at December 31, 2014	$	9.4
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, 2015							December 31, 2014
		Carrying value			Fair value		Interest rate		Carrying value			Fair		Interest rate
												value
	Notes receivable from affiliate	$	70		$	76.4	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-15							31-Dec-14
		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		$	86.4	5.03	%
	Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5			15.5		4.46	%	15.5			15.5		4.44	%
	Total notes payable	$	100.8		$	102			$	100.8		$	101.9

Reinsurance_Tables

Reinsurance (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)	2015			2014
	Premiums earned:
	Assumed from external insurers and reinsurers	$	1.1		$	1
	Assumed under Pooling Arrangement	315.3			262.5
	Ceded to external insurers and reinsurers	(8.9		)	(6.0		)
	Ceded under Pooling Arrangement	(209.9		)	(215.4		)
	Net assumed premiums earned	$	97.6		$	42.1
	Losses and loss expenses incurred:
	Assumed from external insurers and reinsurers	$	0.8		$	0.7
	Assumed under Pooling Arrangement	195.3			172.6
	Ceded to external insurers and reinsurers	0.1			(2.4		)
	Ceded under Pooling Arrangement	(121.2		)	(142.2		)
	Net assumed losses and loss expenses incurred	$	75		$	28.7

Income_Taxes_Tables

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)	2015					2014
	Amount at statutory rate	$	11.6		35	%	$	9.7		35	%
	Tax-exempt interest and dividends received deduction	(2.2		)	(7	)	(2.1		)	(8	)
	Other, net	(0.9		)	(3	)	0.2			1
	Valuation allowance	—			—		(7.2		)	(26	)
	Federal income tax expense and effective rate	$	8.5		25	%	$	0.6		2	%

Pension_and_Postretirement_Ben1

Pension and Postretirement Benefit Plans (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)	Pension						Postretirement
		Three months ended March 31
		2015			2014			2015			2014
	Service cost	$	2		$	1.3		$	—		$	—
	Interest cost	2.8			2.8			0.2			0.3
	Expected return on plan assets	(3.4		)	(3.2		)	—			—
	Amortization of:
	Negative prior service cost	—			—			(1.4		)	(1.4		)
	Net actuarial loss	2.7			1.4			0.2			0.2
	Net periodic cost (benefit)	$	4.1		$	2.3		$	(1.0	)	$	(0.9	)

Other_Comprehensive_Income_and1

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)		Unrealized Gains				Benefit Plan Items				Total
			and Losses on
			Available-for-Sale
			Securities
	Beginning balance at January 1, 2015		$	110			$	(38.3	)		$	71.7
	Other comprehensive income before reclassifications		12.5				—				12.5
	Amounts reclassified from AOCI (a)		(4.2		)		1				(3.2		)
	Net current period other comprehensive income		8.3				1				9.3
	Ending balance at March 31, 2015		$	118.3			$	(37.3	)		$	81
	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
	(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 condensed consolidated statement of income for the three months ended March 31, 2015 and 2014:
	($ millions)
	Details about Accumulated Other			Three months ended								Affected line item in the Condensed
	Comprehensive Income Components			31-Mar								Consolidated Statements of Income
				2015				2014
	Unrealized gains on available for sale securities			$	4.2			$	10.7			Realized gain on sale of securities
				4.2				10.7				Total before tax
				(1.5		)		—				Tax expense
				2.7				10.7				Net of tax
	Amortization of benefit plan items
	Negative prior service cost			1.4				1.4				(a)
	Net actuarial loss			(2.9		)		(1.6		)		(a)
				(1.5		)		(0.2		)		Total before tax
				0.5				—				Tax benefit
				(1.0		)		(0.2		)		Net of tax
	Total reclassifications for the period			$	1.7			$	10.5
	(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, 2015
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, 2015 and 2014:
	($ and shares in millions, except per share amounts)	2015			2014
	Numerator:
	Net income for basic earnings per common share	$	24.7		$	27.1
	Denominator:
	Weighted average shares for basic net earnings per common share	41			40.8
	Effect of dilutive share-based awards	0.4			0.3
	Adjusted weighted average shares for diluted net earnings per common share	41.4			41.1
	Basic net earnings per common share	$	0.6		$	0.67
	Diluted net earnings per common share	$	0.6		$	0.66
Options to Purchase Shares of Common Stock	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, 2015 and 2014.
	(shares in millions)	2015		2014
	Total number of antidilutive options and awards	1.7		1.7

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2015
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, 2015 and 2014:
	($ millions)	2015			2014
	Revenue from external sources:
	Insurance segments
	Personal insurance	$	150		$	114.3
	Business insurance	96.1			93.5
	Specialty insurance	69.2			54.7
	Total insurance segments	315.3			262.5
	Investment operations segment
	Net investment income	15.4			17.6
	Net realized capital gains	4.2			10.7
	Total investment operations segment	19.6			28.3
	All other	—			0.5
	Total revenue from external sources	334.9			291.3
	Intersegment revenue	1.4			1.3
	Total revenue	336.3			292.6
	Reconciling items:
	Eliminate intersegment revenues	(1.4		)	(1.3		)
	Total consolidated revenues	$	334.9		$	291.3
	Segment income before federal income tax:
	Insurance segments SAP underwriting income (loss)
	Personal insurance	$	17.4		$	5.1
	Business insurance	(1.4		)	(8.5		)
	Specialty insurance	(0.8		)	(0.5		)
	Total insurance segments	15.2			(3.9		)
	Investment operations segment
	Net investment income	15.4			17.6
	Net realized capital gains	4.2			10.7
	Total investment operations segment	19.6			28.3
	All other	(0.4		)	0.1
	Total segment income before tax expense	34.4			24.5
	Reconciling items:
	GAAP expense adjustments	1.2			5.4
	Interest expense on corporate debt	(1.3		)	(1.3		)
	Corporate expenses	(1.1		)	(0.9		)
	Total reconciling items	(1.2		)	3.2
	Total consolidated income before federal income tax expense	$	33.2		$	27.7

Investments_Cost_or_Amortized_

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$2,185.10	$2,117.30
Gross unrealized holding gains	186.1	172.6
Gross unrealized holding losses	-8	-7.3
Fair value	2,363.20	2,282.60
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	305	309.3
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	859.3	769.5
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	311	340.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	488.7	472.5
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	234.6	242.2
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	76	68.2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,894	1,831.30
Gross unrealized holding gains	76.7	67.3
Gross unrealized holding losses	-6.7	-6.7
Fair value	1,964	1,891.90
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	287	296.7
Gross unrealized holding gains	19	14.1
Gross unrealized holding losses	-1	-1.5
Fair value	305	309.3
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	834.2	742.5
Gross unrealized holding gains	26.6	27.4
Gross unrealized holding losses	-1.5	-0.4
Fair value	859.3	769.5
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	302.9	333.4
Gross unrealized holding gains	10.6	10.2
Gross unrealized holding losses	-2.5	-3
Fair value	311	340.6
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	469.9	458.7
Gross unrealized holding gains	20.5	15.6
Gross unrealized holding losses	-1.7	-1.8
Fair value	488.7	472.5
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	236.7	235.5
Gross unrealized holding gains	75.2	75.5
Gross unrealized holding losses	-1.3	-0.6
Fair value	310.6	310.4
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	181.6	185.5
Gross unrealized holding gains	54.3	57.3
Gross unrealized holding losses	-1.3	-0.6
Fair value	234.6	242.2
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	55.1	50
Gross unrealized holding gains	20.9	18.2
Gross unrealized holding losses	0	0
Fair value	76	68.2
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	54.4	50.5
Gross unrealized holding gains	34.2	29.8
Gross unrealized holding losses	0	0
Fair value	$88.60	$80.30

Investments_Gross_Unrealized_L

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	position	position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$254.40	$120.70
Less than 12 months, Unrealized losses	-5	-2.1
Less than 12 months, Number of positions (positions)	44	27
12 months or more, Fair value	120.9	176.5
12 months or more, Unrealized losses	-3	-5.2
12 months or more, Number of positions (positions)	33	50
Total, Fair value	375.3	297.2
Total, Unrealized losses	-8	-7.3
Total, Number of positions	77	77
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	14	7.2
Less than 12 months, Unrealized losses	-1.3	-0.6
Less than 12 months, Number of positions (positions)	4	2
12 months or more, Fair value	0	0
12 months or more, Unrealized losses	0	0
12 months or more, Number of positions (positions)	0	0
Total, Fair value	14	7.2
Total, Unrealized losses	-1.3	-0.6
Total, Number of positions	4	2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	240.4	113.5
Less than 12 months, Unrealized losses	-3.7	-1.5
Less than 12 months, Number of positions (positions)	40	25
12 months or more, Fair value	120.9	176.5
12 months or more, Unrealized losses	-3	-5.2
12 months or more, Number of positions (positions)	33	50
Total, Fair value	361.3	290
Total, Unrealized losses	-6.7	-6.7
Total, Number of positions	73	75
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	41.1	19.9
Less than 12 months, Unrealized losses	-0.3	-0.3
Less than 12 months, Number of positions (positions)	7	4
12 months or more, Fair value	24.1	52
12 months or more, Unrealized losses	-0.7	-1.2
12 months or more, Number of positions (positions)	7	17
Total, Fair value	65.2	71.9
Total, Unrealized losses	-1	-1.5
Total, Number of positions	14	21
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	117.7	6.1
Less than 12 months, Unrealized losses	-1.5	0
Less than 12 months, Number of positions (positions)	23	5
12 months or more, Fair value	6.8	30.9
12 months or more, Unrealized losses	0	-0.4
12 months or more, Number of positions (positions)	3	9
Total, Fair value	124.5	37
Total, Unrealized losses	-1.5	-0.4
Total, Number of positions	26	14
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	33.3	43.5
Less than 12 months, Unrealized losses	-1.2	-0.9
Less than 12 months, Number of positions (positions)	5	8
12 months or more, Fair value	50.6	56.1
12 months or more, Unrealized losses	-1.3	-2.1
12 months or more, Number of positions (positions)	10	11
Total, Fair value	83.9	99.6
Total, Unrealized losses	-2.5	-3
Total, Number of positions	15	19
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	48.3	44
Less than 12 months, Unrealized losses	-0.7	-0.3
Less than 12 months, Number of positions (positions)	5	8
12 months or more, Fair value	39.4	37.5
12 months or more, Unrealized losses	-1	-1.5
12 months or more, Number of positions (positions)	13	13
Total, Fair value	87.7	81.5
Total, Unrealized losses	($1.70)	($1.80)
Total, Number of positions	18	21

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, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	($0.50)	($0.80)
Large-Cap Equity Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0	-0.2
Small-Cap Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	($0.50)	($0.60)

Investments_Amortized_Cost_and

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Amortized cost
Due in 1 year or less, Amortized cost	$38.60
Due after 1 year through 5 years, Amortized cost	329.3
Due after 5 years through 10 years, Amortized cost	278.9
Due after 10 years, Amortized cost	777.3
U.S. government agencies mortgage-backed securities, Amortized cost	469.9
Total, Amortized cost	1,894	1,831.30
Fair value
Due in 1 year or less, Fair value	39.4
Due after 1 year through 5 years, Fair value	343.4
Due after 5 years through 10 years, Fair value	293.1
Due after 10 years, Fair value	799.4
U.S. government agencies mortgage-backed securities, Fair value	488.7
Total, Fair value	$1,964	$1,891.90

Investments_Components_of_Net_

Investments - Components of Net Investment Income (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Net Investment Income [Line Items]
Investment income	$15.90	$18.10
Investment expenses	0.5	0.5
Net investment income	15.4	17.6
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	12.9	15.4
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	1.6	1.4
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$1.40	$1.30

Investments_Realized_and_Unrea

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Realized gains:
Total realized gains	$5.20	$11.60
Realized losses:
Total realized losses	-1	-0.9
Net realized gain on investments	4.2	10.7
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	-4.5	-4.3
Valuation allowance	0	0.1
Change in net unrealized holding (losses) gains, net of tax	8.3	8.1
Fixed Maturities [Member]
Realized gains:
Total realized gains	1.1	2.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	9.4	14.4
Equity Securities [Member]
Realized gains:
Total realized gains	4.1	9.4
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	-1	-4.5
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	-0.5	-0.1
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	-0.5	-0.8
Other Invested Assets [Member]
Realized gains:
Total realized gains	0	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$4.40	$2.40

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities fair value of deposit with insurance regulators	$8.80		$8.80
Proceeds on sale of available-for-sale securities	84.2	106.1
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

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Fair value	$2,363.20	$2,282.60
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	305	309.3
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	859.3	769.5
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	311	340.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	488.7	472.5
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	1,964	1,891.90
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	234.6	242.2
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	76	68.2
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	310.6	310.4
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	88.6	80.3
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	318.7	317.8
Level 1 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 1 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 1 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 1 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 1 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	234.6	242.2
Level 1 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	76	68.2
Level 1 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	310.6	310.4
Level 1 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	8.1	7.4
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,041	1,955.40
Level 2 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	305	309.3
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	859.3	769.5
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	307.5	331.2
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	488.7	472.5
Level 2 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	1,960.50	1,882.50
Level 2 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 2 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 2 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 2 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	80.5	72.9
Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.5	9.4
Level 3 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 3 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 3 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.5	9.4
Level 3 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 3 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.5	9.4
Level 3 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 3 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 3 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0	0
Level 3 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$0	$0

Fair_Value_of_Financial_Instru3

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, 2015	Dec. 31, 2014
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	$9.40	$8.90
Total realized gains (losses) - included in earnings	0.2	0
Total unrealized gains (losses) - included in other comprehensive income	-0.2	0.2
Purchases	0	0.3
Sales	-5.9	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$3.50	$9.40

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) (Affiliated Entity [Member], Level 2 [Member], USD $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
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	76.4	74.6

Fair_Value_of_Financial_Instru5

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Carrying Value [Member]
Debt Instrument [Line Items]
Total notes payable	$100,800,000	$100,800,000
Fair Value [Member]
Debt Instrument [Line Items]
Total notes payable	102,000,000	101,900,000
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Interest rate (percent)	5.03%	5.03%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	85,000,000
Federal Home Loan Bank Advances [Member] | Carrying Value [Member]
Debt Instrument [Line Items]
Total notes payable	85,300,000	85,300,000
Federal Home Loan Bank Advances [Member] | Fair Value [Member]
Debt Instrument [Line Items]
Total notes payable	86,500,000	86,400,000
Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Interest rate (percent)	4.46%	4.44%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	15,500,000
Affiliate Subordinated Debentures Due 2033 [Member] | Carrying Value [Member]
Debt Instrument [Line Items]
Total notes payable	15,500,000	15,500,000
Affiliate Subordinated Debentures Due 2033 [Member] | Fair Value [Member]
Debt Instrument [Line Items]
Total notes payable	$15,500,000	$15,500,000

Fair_Value_of_Financial_Instru6

Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	1 Months Ended
In Millions, unless otherwise specified	31-May-09	Mar. 31, 2015	Dec. 31, 2014
	Agreement
Fair Value Measurements Disclosure [Line Items]
Fair value		$2,363.20	$2,282.60
Other invested assets		5.3	5.3
Number of Credit Agreements	2
Notes receivable from affiliate		70	70
Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		3.5	9.4
Federal home loan bank stock		4.8
Other invested assets		0.5
Corporate Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		311	340.6
Corporate Securities [Member] | Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Number of fixed maturity corporate securities included in Level 3 (securities)		1
Fair value		3.5	9.4
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of fixed maturity corporate securities included in Level 3 (securities)		2
Fair value		80.5	72.9
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate		$70

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, 2015	Mar. 31, 2014
Insurance [Abstract]
Reinsurance arrangement, participating reinsurers' margin	9.00%
Profit Commission	$1.30
Premiums earned:
Assumed from external insurers and reinsurers	1.1	1
Assumed under Pooling Arrangement	315.3	262.5
Ceded to external insurers and reinsurers	-8.9	-6
Ceded under Pooling Arrangement	-209.9	-215.4
Net assumed premiums earned	97.6	42.1
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	0.8	0.7
Assumed under Pooling Arrangement	195.3	172.6
Ceded to external insurers and reinsurers	0.1	-2.4
Ceded under Pooling Arrangement	-121.2	-142.2
Net assumed losses and loss expenses incurred	$75	$28.70

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, 2015	Mar. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$11.60	$9.70
Tax-exempt interest and dividends received deduction	-2.2	-2.1
Other, net	-0.9	0.2
Valuation allowance	0	-7.2
Federal income tax expense and effective rate	$8.50	$0.60
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	-7.00%	-8.00%
Other, net (percent)	-3.00%	1.00%
Valuation allowance (percent)	0.00%	-26.00%
Federal income tax expense (benefit) and effective rate (percent)	25.00%	2.00%

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, 2015	Mar. 31, 2014
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$2	$1.30
Interest cost	2.8	2.8
Expected return on plan assets	-3.4	-3.2
Amortization of:
Prior service benefits	0	0
Net actuarial loss	2.7	1.4
Net periodic cost (benefit)	4.1	2.3
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0
Interest cost	0.2	0.3
Expected return on plan assets	0	0
Amortization of:
Prior service benefits	-1.4	-1.4
Net actuarial loss	0.2	0.2
Net periodic cost (benefit)	($1)	($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, 2015
Compensation and Retirement Disclosure [Abstract]
Contributions by employer	$3.30
Contributions by employer for fiscal year	$9.70

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
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$71.70	$80.80
Other comprehensive income before reclassifications	12.5	18.8
Amounts reclassified from AOCI	-3.2	-10.5
Other comprehensive income	9.3	8.3
Ending balance	81	89.1
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	110	84.6
Other comprehensive income before reclassifications	12.5	18.8
Amounts reclassified from AOCI	-4.2	-10.7
Other comprehensive income	8.3	8.1
Ending balance	118.3	92.7
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	-38.3	-3.8
Other comprehensive income before reclassifications	0	0
Amounts reclassified from AOCI	1	0.2
Other comprehensive income	1	0.2
Ending balance	($37.30)	($3.60)

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
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	$3.80	$10.70
Tax (expense) benefit	-8.5	-0.6
Total reclassifications for the period	3.2	10.5
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	4.2	10.7
Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	-1	-0.2
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	1.7	10.5
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	4.2	10.7
Total before tax	4.2	10.7
Tax (expense) benefit	-1.5	0
Net of tax	2.7	10.7
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	1.4	1.4
Accumulated other comprehensive income components/Net actuarial loss	-2.9	-1.6
Total before tax	-1.5	-0.2
Tax (expense) benefit	0.5	0
Net of tax	($1)	($0.20)

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, 2015	Mar. 31, 2014
Numerator:
Net income for basic earnings per common share	$24.70	$27.10
Denominator:
Weighted average shares for basic net earnings per common share (shares)	41	40.8
Effect of dilutive share-based awards (shares)	0.4	0.3
Adjusted weighted average shares for diluted net earnings per common share (shares)	41.4	41.1
Basic net earnings per common share (usd per share)	$0.60	$0.67
Diluted net earnings per common share (usd per share)	$0.60	$0.66

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, 2015	Mar. 31, 2014
Earnings Per Share [Abstract]
Number of antidilutive options and awards	1.7	1.7

Segment_Information_Financial_

Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Revenues from external sources:
Revenue	$334.90	$291.30
Net Investment Income	15.4	17.6
Segment income before federal income tax:
Interest expense on corporate debt	-1.3	-1.3
Income before federal income taxes	33.2	27.7
Operating Segments [Member]
Revenues from external sources:
Revenue	334.9	291.3
Segment income before federal income tax:
Income before federal income taxes	34.4	24.5
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	150	114.3
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	96.1	93.5
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	69.2	54.7
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	315.3	262.5
Segment income before federal income tax:
Income before federal income taxes	15.2	-3.9
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	15.4	17.6
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	19.6	28.3
Segment income before federal income tax:
Income before federal income taxes	19.6	28.3
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	4.2	10.7
Segment income before federal income tax:
Income before federal income taxes	4.2	10.7
Operating Segments [Member] | All Other Segments [Member]
Segment income before federal income tax:
Income before federal income taxes	-0.4	0.1
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	17.4	5.1
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	-1.4	-8.5
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	-0.8	-0.5
Corporate and Other [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0	0.5
Corporate and Other [Member] | Gaap Adjustments [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	1.2	5.4
Corporate and Other [Member] | Corporate Segment [Member]
Segment income before federal income tax:
Corporate Expenses	-1.1	-0.9
Corporate and Other [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Interest expense on corporate debt	-1.3	-1.3
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	-1.4	-1.3
Eliminate Intersegment Revenues [Member] | Intersegment [Member]
Revenues from external sources:
Revenue	1.4	1.3
External and Intersegment Sources [Member]
Revenues from external sources:
Revenue	336.3	292.6
Total Consolidated [Member]
Revenues from external sources:
Revenue	334.9	291.3
Segment Reconciling Items [Member]
Segment income before federal income tax:
Income before federal income taxes	($1.20)	$3.20

Segment_Information_Additional

Segment Information - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
	Segment
Segment Reporting [Abstract]
Number of reportable segments (segments)	4
Investable assets	$2,506.90	$2,444.20