State Auto Financial (STFC) 10-K 3 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 27, 2015	Jun. 30, 2014
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	STFC
Entity Registrant Name	State Auto Financial CORP
Entity Central Index Key	874977
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Filer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filer	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding (shares)		41,050,931
Entity Public Float			$361,818,623

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $1,831.3 and $1,804.0, respectively)	$1,891,900,000	$1,830,100,000
Equity securities, available-for-sale, at fair value (cost $235.5 and $196.6, respectively)	310,400,000	265,300,000
Other invested assets, available-for-sale, at fair value (cost $50.5 and $49.5, respectively)	80,300,000	80,900,000
Other invested assets	5,300,000	5,000,000
Notes receivable from affiliate	70,000,000	70,000,000
Total investments	2,357,900,000	2,251,300,000
Cash and cash equivalents	86,300,000	80,300,000
Accrued investment income and other assets	33,800,000	33,600,000
Deferred policy acquisition costs (affiliated net assumed $46.8 and $19.2, respectively)	126,500,000	96,800,000
Reinsurance recoverable on losses and loss expenses payable	9,600,000	9,100,000
Prepaid reinsurance premiums	6,100,000	4,700,000
Due from affiliates	40,100,000	0
Current federal income taxes	1,100,000	300,000
Net deferred federal income taxes	97,400,000	11,900,000
Property and equipment, at cost (net of accumulated depreciation of $6.1 and $5.8, respectively)	8,100,000	8,400,000
Total assets	2,766,900,000	2,496,400,000
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $494.3 and $438.0, respectively)	983,200,000	959,900,000
Unearned premiums (affiliated net assumed $201.7 and $78.4, respectively)	612,400,000	491,000,000
Notes payable (affiliates $15.5 and $15.5, respectively)	100,800,000	100,800,000
Postretirement and pension benefits (affiliated net ceded $63.2 and $37.3, respectively)	117,300,000	71,600,000
Due to affiliate	0	1,300,000
Other liabilities (affiliated net ceded $5.1 and affiliated net assumed $20.0, respectively)	80,300,000	86,800,000
Total liabilities	1,894,000,000	1,711,400,000
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 47.7 and 47.5 shares issued, respectively, at stated value of $2.50 per share	119,300,000	118,800,000
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	-116,000,000	-115,900,000
Additional paid-in capital	143,200,000	137,500,000
Accumulated other comprehensive income (affiliated net ceded $65.1 and $41.0, respectively)	71,700,000	80,800,000
Retained earnings	654,700,000	563,800,000
Total stockholders’ equity	872,900,000	785,000,000
Total liabilities and stockholders’ equity	2,766,900,000	2,496,400,000
Class A Preferred stock
Stockholders’ equity:
Preferred stock value	0	0
Class B Preferred stock
Stockholders’ equity:
Preferred stock value	$0	$0

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Fixed maturities, available-for-sale, amortized cost	$1,831.30	$1,804
Equity securities, available-for-sale, cost	235.5	196.6
Other invested assets, available-for-sale, cost	50.5	49.5
Deferred Policy Acquisitions Costs from Affiliates	46.8	19.2
Property and equipment, accumulated depreciation	6.1	5.8
Losses And Loss Expenses Payable From Affiliates	494.3	438
Unearned premiums, affiliates	201.7	78.4
Net Postretirement and Pension Benefits	-63.2	-37.3
Net Other Liabilities from Affiliates	-5.1	20
Notes payable, affiliates	15.5	15.5
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)	47,700,000	47,500,000
Common Stock, Par or Stated Value Per Share	$2.50	$2.50
Net Accumulated Other Comprehensive Income	($65.10)	($41)
Treasury Stock, Shares	6,800,000	6,800,000
Class A Preferred stock
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
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

Consolidated_Statements_of_Inc

Consolidated Statements of Income (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Earned premiums (affiliated net assumed $212.4, $200.0 and $232.9, respectively)	$1,074.10	$1,055	$1,042.10
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	74.7	72.8	75.4
Net realized gain on investments:
Total other-than-temporary impairment losses	-2.5	-4	-3.4
Portion of loss recognized in other comprehensive income	0	0	0
Other net realized investment gains	23.2	27.2	32.4
Total net realized gain on investments	20.7	23.2	29
Other income (affiliates $1.9, $2.0 and $3.6, respectively)	3.2	2	3.6
Total revenues	1,172.70	1,153	1,150.10
Losses and loss expenses (affiliated net assumed $250.8, $162.5 and $203.7, respectively)	771.3	719.8	778.3
Acquisition and operating expenses (affiliated net assumed $156.9, $172.7 and $190.8, respectively)	361.9	354.8	345.9
Interest expense (affiliates $0.7, $0.7 and $0.7, respectively)	5.4	8.5	7
Other expenses	7.3	8.6	8.3
Total expenses	1,145.90	1,091.70	1,139.50
Income before federal income taxes	26.8	61.3	10.6
Current	0.1	0.5	-0.1
Deferred	-80.7	0	0
Total federal income tax (benefit) expense	-80.6	0.5	-0.1
Net income	$107.40	$60.80	$10.70
Earnings per common share:
Basic (usd per share)	$2.63	$1.50	$0.26
Diluted (usd per share)	$2.60	$1.49	$0.26
Dividends paid per common share (usd per share)	$0.40	$0.40	$0.55

Consolidated_Statements_of_Inc1

Consolidated Statements of Income (Parenthetical) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Earned premiums, net	$212.40	$200	$232.90
Net investment income, affiliates	4.9	4.9	4.9
Other income, affiliates	1.9	2	3.6
Losses and loss expenses, net affiliates	250.8	162.5	203.7
Net Acquisition and Operating Expenses	156.9	172.7	190.8
Interest Expense Debt To Affiliates	$0.70	$0.70	$0.70

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$107.40	$60.80	$10.70
Net unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during year	59.8	-27.1	53.5
Reclassification adjustments for gains realized in net income	-20.7	-23.2	-28.8
Income tax (expense) benefit	-13.7	10.9	0.6
Total net unrealized holding gains (losses) on investments	25.4	-39.4	25.3
Amortization of gain on derivative used in cash flow hedge	0	-0.1	-0.1
Net unrecognized benefit plan obligations:
Net actuarial (loss) gain arising during period	-54.4	32.5	-7.4
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	-5.5	-5.5	-5.2
Net actuarial loss	6.9	9.1	7.8
Income tax benefit	18.5	0	0
Total net unrecognized benefit plan obligations	-34.5	36.1	-4.8
Other comprehensive (loss) income	-9.1	-3.4	20.4
Comprehensive income	$98.30	$57.40	$31.10

Consolidated_Statements_of_Sto

Consolidated Statements of Stockholders' Equity (USD $)	Total	Common shares:	Treasury shares:	Additional paid-in capital:	Accumulated other comprehensive income:	Retained earnings:
In Millions, unless otherwise specified
Balance at beginning of year at Dec. 31, 2011		$117.80	($115.80)	$127.30	$63.80	$530.70
Balance at beginning of year (shares) at Dec. 31, 2011		47.1	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Issuance of shares		0.3		1.3
Shares acquired on stock option exercises and vested restricted shares			0
Stock options granted				3
Change in unrealized holding gains (losses) on investments, net of tax	25.3				25.3
Amortization of gain on derivative used in cash flow hedge					-0.1
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	-4.8				-4.8
Net income	10.7					10.7
Cash dividends paid (affiliates $10.2, $10.1 and $13.9, respectively)	-22.3
Balance at end of year at Dec. 31, 2012	737.2	118.1	-115.8	131.6	84.2	519.1
Balance at end of year (shares) at Dec. 31, 2012		47.3	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Issuance of shares		0.7		3.4
Shares acquired on stock option exercises and vested restricted shares			-0.1
Stock options granted				2.5
Change in unrealized holding gains (losses) on investments, net of tax	-39.4				-39.4
Amortization of gain on derivative used in cash flow hedge					-0.1
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	36.1				36.1
Net income	60.8					60.8
Cash dividends paid (affiliates $10.2, $10.1 and $13.9, respectively)	-16.1
Balance at end of year at Dec. 31, 2013	785	118.8	-115.9	137.5	80.8	563.8
Balance at end of year (shares) at Dec. 31, 2013		47.5	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Issuance of shares		0.5		2.9
Shares acquired on stock option exercises and vested restricted shares			-0.1
Stock options granted				2.8
Change in unrealized holding gains (losses) on investments, net of tax	25.4				25.4
Amortization of gain on derivative used in cash flow hedge					0
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	-34.5				-34.5
Net income	107.4					107.4
Cash dividends paid (affiliates $10.2, $10.1 and $13.9, respectively)	-16.5
Balance at end of year at Dec. 31, 2014	$872.90	$119.30	($116)	$143.20	$71.70	$654.70
Balance at end of year (shares) at Dec. 31, 2014		47.7	6.8

Consolidated_Statements_of_Sto1

Consolidated Statements of Stockholders' Equity (Parenthetical) (Retained earnings, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Retained earnings
Cash dividends paid affiliates	$10.20	$10.10	$13.90

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$107.40	$60.80	$10.70
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Depreciation and amortization, net	11.7	13.5	7.1
Share-based compensation	3.6	4.1	3.5
Net realized gain on investments	-20.7	-23.2	-29
Changes in operating assets and liabilities:
Deferred policy acquisition (benefits) costs	-29.7	-5.1	0
Accrued investment income and other assets	0.2	-2.3	-1.4
Postretirement and pension benefits	-7.3	-5.3	-4.6
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	-1.9	3.6	16
Other liabilities and due to/from affiliates, net	-50.8	-0.9	-9.8
Losses and loss expenses payable	23.3	17.7	35.1
Unearned premiums	121.4	9.4	11.4
Excess tax benefits on share-based awards	0	0.1	0
Federal income taxes	-81.6	-0.3	12.3
Cash used in December 31, 2011 unearned premium transfer related to the homeowners quota-share reinsurance arrangement	0	0	-75.5
Cash used in pooling changes, December 31, 2011 and January 1, 2011	0	0	-261.4
Net cash provided by (used in) operating activities	75.6	72.1	-285.6
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	-431.4	-326.7	-540.4
Purchases of equity securities available-for-sale	-119	-86	-143
Purchases of other invested assets	-1.8	-5.5	-1.1
Maturities, calls and pay downs of fixed maturities available-for-sale	232.4	174.6	257
Sales of fixed maturities available-for-sale	163.5	113.3	332.8
Sales of equity securities available-for-sale	99.2	106.4	101.8
Sales of other invested assets available-for-sale	0.6	0.7	0.7
Net additions of property and equipment	0	0.2	1.5
Net cash (used in) provided by investing activities	-56.5	-23	9.3
Cash flows from financing activities:
Proceeds from issuance of common stock	3.5	4	1.6
Payments to acquire treasury stock	-0.1	-0.1	0
Payments of dividends (affiliates $10.2, $10.1 and $13.9, respectively)	-16.5	-16.1	-22.3
Payment of credit facility issue costs	0	-0.5	0
Excess tax benefits on share-based awards	0	-0.1	0
Proceeds from long-term debt	0	85	0
Redemption of long-term debt	0	-100	0
Net cash used in financing activities	-13.1	-27.8	-20.7
Net increase (decrease) in cash and cash equivalents	6	21.3	-297
Cash and cash equivalents at beginning of year	80.3	59	356
Cash and cash equivalents at end of year	86.3	80.3	59
Supplemental disclosures:
Interest paid (affiliates $0.7, $0.7 and $0.7, respectively)	5.2	8.5	7
Federal income taxes paid (received)	$1	$0.80	($12.40)

Consolidated_Statements_of_Cas1

Consolidated Statements of Cash Flows (Parenthetical) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Cash Flows [Abstract]
Payments of dividends, affiliates	$10.20	$10.10	$13.90
Interest paid, affiliates	$0.70	$0.70	$0.70

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies
	a. Principles of Consolidation
	The consolidated financial statements include State Auto Financial Corporation (“State Auto Financial”), an Ohio corporation, and the following wholly owned subsidiaries of State Auto Financial:
	•	State Auto Property and Casualty Insurance Company (“State Auto P&C”), an Iowa corporation
	•	Milbank Insurance Company (“Milbank”), an Iowa corporation
	•	State Auto Insurance Company of Ohio (“SA Ohio”), an Ohio corporation
	•	Stateco Financial Services, Inc. (“Stateco”), an Ohio corporation
	The consolidated financial statements also include the operations and financial position of 518 Property Management and Leasing, LLC (“518 PML”), an Ohio limited liability company whose only members are State Auto P&C and Stateco. Farmers Casualty Insurance Company ("Farmers"), a former wholly owned subsidiary of State Auto Financial, was merged with State Auto P&C at the close of business on December 31, 2012.
	State Auto Financial is a majority-owned subsidiary of State Automobile Mutual Insurance Company (“State Auto Mutual”), an Ohio corporation. State Auto Financial and its subsidiaries are referred to herein as the “Company.” All intercompany balances and transactions have been eliminated in consolidation.
	b. Description of Business
	The Company markets its insurance products throughout the United States primarily through independent agencies, which include retail agencies and wholesale brokers. The Company’s principal lines of insurance include personal and commercial automobile, homeowners, commercial multi-peril, workers’ compensation, general liability and fire insurance. State Auto P&C, Milbank and SA Ohio are chartered and licensed property and casualty insurers. As such, they are subject to the regulations of the applicable Departments of Insurance of their respective states of domicile (the “Departments”) and the regulations of each state in which they operate. These property and casualty insurance companies undergo periodic financial examination by the Departments and insurance regulatory agencies of the states that choose to participate. A large portion of the Company’s revenues are derived from a reinsurance pooling agreement with State Auto Mutual and its affiliates. The underwriting activity and geographic distribution of State Auto Mutual and its affiliates is generally the same as the underwriting activity and geographic distribution of the Company.
	Through the employees of State Auto P&C, the Company provides management and operation services under management agreements for all of its insurance and non-insurance affiliates.
	Through Stateco, the Company provides investment management services to affiliated companies.
	518 PML owns and leases property to the Company’s affiliates.
	c. Basis of Presentation
	The consolidated financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“GAAP”), which vary in certain respects from statutory accounting practices (“SAP”) followed by State Auto P&C, Milbank and SA Ohio that are prescribed or permitted by the Departments.
	The Company's insurance subsidiaries, domiciled in Ohio and Iowa, are required to prepare statutory financial statements in accordance with the accounting practices prescribed or permitted by the insurance departments of the states of domicile. Prescribed statutory accounting practices are those practices that are incorporated directly or by reference in state laws, regulations, and general administrative rules applicable to all insurance enterprises domiciled in a particular state. The Ohio and Iowa Departments of Insurance require insurers domiciled in their respective states to prepare statutory financial statements in accordance with National Association of Insurance Commissioners' ("NAIC") statutory accounting practices. Permitted statutory accounting practices are those practices that differ either from state-prescribed statutory accounting practices or NAIC statutory accounting practices. The Company's insurance subsidiaries do not apply any statutory accounting practices that would be considered a prescribed statutory accounting practice that differs from NAIC statutory accounting practices.
	In preparing the consolidated financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheet, revenues and expenses for the periods then ended, and the accompanying notes to the financial statements. Such estimates and assumptions could change in the future as more information becomes known which could impact the amounts reported and disclosed herein.
	Material estimates that are particularly susceptible to significant change in the near term relate to the determination of losses and loss expenses payable and the realization of deferred tax assets. In connection with the determination of losses and loss expenses payable, management uses historical data, current business conditions and assumptions about future conditions to formulate estimates of the ultimate cost to settle claims. Deferred tax assets are evaluated periodically by management to determine if they are realizable, requiring management to make certain judgments and assumptions. In evaluating the ability to recover deferred tax assets, management considers all available evidence, including loss carryback potential, past operating results, existence of cumulative losses in the most recent years, projected performance of the business, future taxable income, including the ability to generate capital gains, and prudent and feasible tax planning strategies. If, based on available information, it is more likely than not that the deferred income tax assets will not be realized, then a valuation allowance must be established with a corresponding charge to net income and/or other comprehensive income (loss). These estimates by their nature are subject to uncertainties for various reasons.
	d. Investments
	Investments in fixed maturities, equity securities and certain other invested assets are classified as available-for-sale and are carried at fair value. The unrealized holding gains and losses, net of applicable deferred income taxes, are shown as a separate component of stockholders’ equity as a part of accumulated other comprehensive income (loss) and, as such, are not included in the determination of net income. Realized gains and losses on the sales of investments are computed using the first-in, first-out method.
	The Company views gross unrealized losses on fixed maturities and equity securities as being temporary since it is its assessment that these securities will recover in the near term, allowing the Company to realize the anticipated long-term economic value. The Company 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 security 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 other comprehensive income. Future increases or decreases in fair value, if not other-than-temporary, are included in 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 other comprehensive income.
	e. Cash Equivalents
	The Company considers all liquid debt instruments with a maturity of three months or less to be cash equivalents. The carrying amounts reported approximate their fair value.
	f. Deferred Acquisition Costs
	Acquisition costs, consisting of net commissions (including ceding commissions), premium taxes and certain underwriting expenses related to the successful acquisition or renewal of property and casualty business, are deferred and amortized over the same period in which the related premiums are earned. Ceding commissions relating to reinsurance agreements reimburse us for both deferrable and non-deferrable acquisition costs. To the extent these ceding commissions exceed the deferrable amount of acquisition costs, the excess is reported as a deferred liability and is included in other liabilities in our consolidated balance sheet. Excess ceding commissions are amortized in proportion to net revenue recognized on the underlying policies resulting in excess ceding commissions being recognized as a reduction of acquisition and operating expenses.
	The method followed for computing the acquisition costs limits the amount of such deferred costs to their estimated realizable value. In determining estimated realizable value, the computation gives effect to the premium to be earned, losses and loss expenses expected to be incurred, and certain other costs expected to be incurred as premium is earned. Future changes in estimates, the most significant of which is expected losses and loss adjustment expenses, that indicate a reduction in expected future profitability may result in unrecoverable deferred acquisition costs. Anticipated investment income is considered in determining whether a premium deficiency exists.
	The following table sets forth net deferred acquisition costs for the years ended December 31, 2014, 2013 and 2012:
	($ millions)		2014			2013			2012
	Balance, beginning of year		$	96.8		$	91.7		$	91.7
	Acquisition costs deferred		251.5			214.6			213.1
	Acquisition costs amortized to expense		(221.8		)	(209.5		)	(213.1		)
	Balance, end of year		$	126.5		$	96.8		$	91.7
	g. Federal Income Taxes
	The Company files a consolidated federal income tax return. Pursuant to a written tax sharing agreement, each entity within the consolidated group pays or receives its share of federal income taxes based on separate return calculations.
	The Company recognizes deferred income tax assets and liabilities for the expected future tax effects attributable to temporary differences between the financial statement and tax return bases of assets and liabilities, based on enacted tax rates and other provisions of the tax law. The effect of a change in tax laws or rates on deferred tax assets and liabilities is recognized in income in the period in which such change is enacted. Deferred tax assets and liabilities include provisions for unrealized investment gains and losses as well as the net funded status of pension and other postretirement benefit obligations with the changes for each period included in the respective components of other comprehensive income. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that all or some portion of the deferred tax assets will not be realized.
	Interest and penalties related to uncertain tax positions are recorded in the balance sheet as other liabilities, and recognized in the income statement as other expenses.
	h. Losses and Loss Expenses Payable
	Losses and loss expenses payable are based on formula and case-basis estimates for reported claims and on estimates, based on experience and perceived trends, for unreported claims and loss expenses. The liability for unpaid losses and loss expenses, net of estimated salvage and subrogation recoverable of $23.8 million and $24.7 million at December 31, 2014 and 2013, respectively, has been established to cover the estimated ultimate cost to settle insured losses. The amounts are based on estimates of future rates of inflation and other factors, and accordingly, there can be no assurance that the ultimate liability will not vary materially from such estimates. The estimates are continually reviewed and adjusted as necessary; such adjustments are included in current operations (see Note 4). Anticipated salvage and subrogation is estimated using historical experience. As such, losses and loss expenses payable represent management’s best estimate of the ultimate liability related to reported and unreported claims.
	i. Premiums
	Premiums are recognized as earned prorata over the policy period. Unearned premiums represent the portion of premiums written relative to the unexpired terms of coverage.
	j. Comprehensive Income
	Comprehensive income is defined as all changes in an enterprise’s equity during a period other than those resulting from investments by owners and distributions to owners. Comprehensive income includes net income and other comprehensive income. Other comprehensive income includes all other non-owner related changes to equity and includes net unrealized gains and losses on available-for-sale investments, derivative instruments and unrecognized benefit plan obligations, adjusted for deferred federal income taxes.

Investments

Investments	12 Months Ended
	Dec. 31, 2014
Investments [Abstract]
Investments	Investments
	The following tables set forth the cost or amortized cost and fair value of available-for-sale securities by lot at December 31, 2014 and 2013:
	31-Dec-14		Cost or			Gross			Gross				Fair
			amortized			unrealized			unrealized				value
			cost			holding			holding
						gains			losses
	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 residential 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
	31-Dec-13		Cost or			Gross			Gross				Fair
			amortized			unrealized			unrealized				value
			cost			holding			holding
						gains			losses
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies		$	345.5		$	13.4		$	(6.5	)		$	352.4
	Obligations of states and political subdivisions		765.3			25.8			(16.9		)		774.2
	Corporate securities		345			11.4			(6.7		)		349.7
	U.S. government agencies residential mortgage-backed securities		348.2			9.7			(4.1		)		353.8
	Total fixed maturities		1,804.00			60.3			(34.2		)		1,830.10
	Equity securities:
	Large-cap securities		148.2			46.5			(0.3		)		194.4
	Small-cap securities		48.4			22.5			—				70.9
	Total equity securities		196.6			69			(0.3		)		265.3
	Other invested assets		49.5			31.4			—				80.9
	Total available-for-sale securities		$	2,050.10		$	160.7		$	(34.5	)		$	2,176.30
	The following tables set forth the Company’s gross unrealized losses and fair value on its investments by lot, aggregated by investment category and length of time for individual securities that have been in a continuous unrealized loss position at December 31, 2014 and 2013:
		Less than 12 months									12 months or more								Total
	31-Dec-14	Fair			Unrealized			Number			Fair				Unrealized			Number	Fair		Unrealized			Number
		value			losses			of			value				losses			of	value		losses			of
								positions										positions						positions
	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 residential 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
		Less than 12 months									12 months or more								Total
	31-Dec-13	Fair			Unrealized			Number			Fair				Unrealized			Number	Fair		Unrealized			Number
		value			losses			of			value				losses			of	value		losses			of
								positions										positions						positions
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	109.6		$	(6.5	)	29			$	—			$	—		—	$	109.6	$	(6.5	)	29
	Obligations of states and political subdivisions	206.4			(14.7		)	76			25.6				(2.2		)	7	232		(16.9		)	83
	Corporate securities	105.6			(3.2		)	22			40.9				(3.5		)	8	146.5		(6.7		)	30
	U.S. government agencies residential mortgage-backed securities	103.6			(3.3		)	25			19.3				(0.8		)	10	122.9		(4.1		)	35
	Total fixed maturities	525.2			(27.7		)	152			85.8				(6.5		)	25	611		(34.2		)	177
	Large-cap equity securities	5.3			(0.3		)	2			—				—			—	5.3		(0.3		)	2
	Total temporarily impaired securities	$	530.5		$	(28.0	)	154			$	85.8			$	(6.5	)	25	$	616.3	$	(34.5	)	179
	The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Equity securities:
	Large-cap securities	$	(0.3	)	$	(1.8	)	$	—
	Small-cap securities	(2.2		)	(2.2		)	(3.2		)
Fixed maturities	—			—			(0.2		)
Total other-than-temporary impairments	$	(2.5	)	$	(4.0	)	$	(3.4	)
The Company reviewed its investments at December 31, 2014, and determined no additional other-than-temporary impairment exists in the gross unrealized holding losses.
The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at December 31, 2014:
($ millions)	Amortized			Fair
	cost			value
Due in 1 year or less	$	55.7		$	56.5
Due after 1 year through 5 years	369.8			382.4
Due after 5 years through 10 years	299.9			312.4
Due after 10 years	647.2			668.1
U.S. government agencies residential mortgage-backed securities	458.7			472.5
Total	$	1,831.30		$	1,891.90
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 December 31, 2014, State Auto P&C had fixed maturity securities, with a carrying value of approximately $85.0 million, that were pledged as collateral for the FHLB Loan (as defined in Note 7). In accordance with the terms of the FHLB Loan, State Auto P&C retains all rights regarding these securities, which are included in the "U.S. government agencies residential mortgage-backed securities" classification of the Company's fixed maturity securities portfolio.
Fixed maturities with fair values of approximately $8.8 million and $8.7 million were on deposit with insurance regulators as required by law at December 31, 2014 and 2013, respectively.
The following table sets forth the components of net investment income for the years ended December 31, 2014, 2013 and 2012:
($ millions)	2014			2013			2012
Fixed maturities	$	64.3		$	63.2		$	66.9
Equity securities	6.2			6			4.9
Cash and cash equivalents, and other	6.2			5.7			5.6
Investment income	76.7			74.9			77.4
Investment expenses	2			2.1			2
Net investment income	$	74.7		$	72.8		$	75.4
The Company’s current investment strategy does not rely on the use of derivative financial instruments.
Proceeds on sales of available-for-sale securities in 2014, 2013 and 2012 were $263.3 million, $220.4 million and $435.3 million, respectively.
The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the years ended December 31, 2014, 2013 and 2012:
($ millions)	2014			2013			2012
Realized gains:
Fixed maturities	$	3.1		$	2.5		$	15.7
Equity securities	21.3			26.1			19
Other invested assets	0.1			0.1			0.1
Total realized gains	24.5			28.7			34.8
Realized losses:
Equity securities:
Sales	(1.3		)	(1.2		)	(2.6		)
OTTI	(2.5		)	(4.0		)	(3.2		)
Fixed maturities:
Sales	—			(0.3		)	—
OTTI	—			—			(0.2		)
Total realized losses	(3.8		)	(5.5		)	(6.0		)
Net realized gains on investments	$	20.7		$	23.2		$	28.8
Change in unrealized holding gains (losses), net of tax:
Fixed maturities	$	34.5		$	(102.8	)	$	11.3
Equity securities	6.2			36.5			6.6
Other invested assets	(1.6		)	16			6.8
Deferred federal income tax	(13.7		)	17.6			(8.6		)
Valuation allowance	—			(6.7		)	9.2
Change in unrealized holding gains (losses), net of tax	$	25.4		$	(39.4	)	$	25.3
There was a deferred federal income tax liability on the net unrealized holding gains at December 31, 2014 of $55.3 million and a deferred federal income tax liability, net of a valuation allowance, on the net unrealized holding gains at December 31, 2013 of $41.6 million.

Fair_Value_of_Financial_Instru

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments
	Below is the fair value hierarchy that categorizes into three levels the inputs to valuation techniques that are used to measure fair value.
	•	Level 1 includes observable inputs which reflect quoted prices for identical assets or liabilities in active markets at the measurement date.
	•	Level 2 includes observable inputs for assets or liabilities other than quoted prices included in Level 1, and it includes valuation techniques which use prices for similar assets and liabilities.
	•	Level 3 includes unobservable inputs which reflect the reporting entity’s estimates of the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk).
	The Company utilizes one nationally recognized pricing service to estimate the majority of its available-for-sale investment portfolio’s fair value. The Company obtains one price per security. The Company’s processes and control procedures are designed to ensure the price is accurately recorded on an unadjusted basis. Through discussions with the pricing service, the Company obtains 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 the valuations received to other fair value pricing from other independent pricing sources. At December 31, 2014 and 2013, the Company did not adjust any of the prices received from the pricing service.
	Transfers between levels may occur due to changes in the availability of market observable inputs. Transfers in and out of levels are reported as having occurred at the beginning of the quarter in which the transfer occurred. There were no transfers between levels during the years ended December 31, 2014 and 2013.
	The following sections describe the valuation methods used by the Company for each type of financial instrument carried at fair value.
	Fixed Maturities
	The fair value estimate of the Company’s fixed maturity investments are determined by evaluations that are based on observable market information rather than market quotes. Inputs to the evaluations include, but are not limited to, market prices from recently completed transactions and transactions of comparable securities, interest rate yield curves, credit spreads, and other market-observable information. The fixed maturity portfolio pricing obtained from the pricing service is reviewed for reasonableness. Regularly, samples of security prices are referred back to the pricing service for more detailed explanation as to how the pricing service arrived at that particular price. The explanations are reviewed for reasonableness by the portfolio manager or investment officer. Additionally, the prices and assumptions are verified against an alternative pricing source for reasonableness and accuracy. Any discrepancies with the pricing are returned to the pricing service for further explanation and if necessary, adjustments are made. To date, the Company has not identified any significant discrepancies in the pricing provided by its third party pricing service. Investments valued using these inputs include U.S. treasury securities and obligations of U.S. government agencies, obligations of states and political subdivisions, corporate securities (except for two securities discussed below), and U.S. government agencies residential mortgage-backed securities. All unadjusted estimates of fair value for fixed maturities priced by the pricing service are included in the amounts disclosed in Level 2 of the hierarchy. If market inputs are unavailable, then no fair value is provided by the pricing service. For these securities, fair value is determined either by requesting brokers who are knowledgeable about these securities to provide a quote; or the Company internally determines the fair values by employing widely accepted pricing valuation models, and depending on the level of observable market inputs, renders the fair value estimate as Level 2 or Level 3. The Company holds two fixed maturity corporate securities included in Level 3. The Company estimates the fair value of one security using the present value of the future cash flows and the Company obtains a broker quote of the other security’s fair value. Due to the limited amount of observable market information for both of these securities, the Company includes the fair value estimates in Level 3.
	Equities
	The fair value of each equity security is based on an observable market price for an identical asset in an active market and is priced by the same pricing service discussed above. All equity securities are recorded using unadjusted market prices and have been disclosed in Level 1.
	Other Invested Assets
	Included in other invested assets are two international funds (“the funds”) that invest in equity securities of foreign issuers and are managed by third party investment managers. The funds had a fair value of $72.9 million and $74.2 million at December 31, 2014 and 2013, respectively, which was determined using each fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the funds including obtaining and reviewing each fund’s audited financial statements. There are no unfunded commitments related to the funds. The Company may not sell its investment in the funds; however, the Company may redeem all or a portion of its investment in the funds at net asset value per share with the appropriate prior written notice. The funds are disclosed in Level 2 within the hierarchy due to the Company’s ability to redeem its investment in the funds at net asset value per share at the measurement date.
	The remainder of the Company’s other invested assets consist of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds, based on an observable market price for an identical asset in an active market, reflect their fair values and consequently these securities have been disclosed in Level 1.
	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at December 31, 2014 and 2013:
	($ millions)	Total		Quoted prices		Significant			Significant
				in active		other			unobservable
				markets for		observable			inputs
				identical		inputs			(Level 3)
				assets		(Level 2)
	31-Dec-14			(Level 1)
	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 residential 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
	($ millions)	Total		Quoted prices		Significant			Significant
				in active		other			unobservable
				markets for		observable			inputs
				identical		inputs			(Level 3)
				assets		(Level 2)
	31-Dec-13			(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	352.4	$	—	$	352.4		$	—
	Obligations of states and political subdivisions	774.2		—		774.2			—
	Corporate securities	349.7		—		340.8			8.9
	U.S. government agencies residential mortgage-backed securities	353.8		—		353.8			—
	Total fixed maturities	1,830.10		—		1,821.20			8.9
	Equity securities:
	Large-cap securities	194.4		194.4		—			—
	Small-cap securities	70.9		70.9		—			—
	Total equity securities	265.3		265.3		—			—
	Other invested assets	80.9		6.7		74.2			—
	Total available-for-sale investments	$	2,176.30	$	272	$	1,895.40		$	8.9
	For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for 2014 and 2013, separately for each major category of assets:
	($ millions)	Fixed
		maturities
	Balance at January 1, 2014	$	8.9
	Total realized gains (losses)—included in earnings	—
	Total unrealized gains (losses)—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
	($ millions)	Fixed
maturities
Balance at January 1, 2013	$	8.5
Total realized gains (losses)—included in earnings	—
Total unrealized gains (losses)—included in other comprehensive income	0.2
Purchases	0.2
Sales	—
Transfers into Level 3	—
Transfers out of Level 3	—
Balance at December 31, 2013	$	8.9
Financial Instruments Disclosed, But Not Carried, At Fair Value
Other Invested Assets
Included in other invested assets are the FHLB membership common stock ("FHLB common stock"), purchased when State Auto P&C joined the FHLB in 2013, and the Trust Securities (as defined in Note 6b). The Trust Securities and FHLB common stock are carried at cost, which approximates fair value. The fair value of the FHLB common stock at December 31, 2014 was $4.8 million and the fair value of the Trust Securities were $0.5 million. Both investments have been placed in Level 3 of the fair value hierarchy.
Notes Receivable from Affiliates
In May 2009, the Company entered into two separate credit agreements with State Auto Mutual pursuant to which it loaned State Auto Mutual a total of $70.0 million. The Company estimates the fair value of the notes receivable from affiliates using market quotations for U.S. treasury securities with similar maturity dates and applies an appropriate credit spread. This has been placed in Level 2 of the fair value hierarchy.
($ millions, except interest rates)	31-Dec-14						31-Dec-13
	Carrying		Fair		Interest		Carrying			Fair		Interest
	value		value		rate		value			value		rate
Notes receivable from affiliate	$	70	$	74.6	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-Dec-14						31-Dec-13
	Carrying		Fair		Interest		Carrying			Fair		Interest
	value		Value		rate		value			value		rate
FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest	$	85.3	$	86.4	5.03	%	$	85.3		$	85.7	5.03	%
Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5		15.5		4.44	%	15.5			15.5		4.44	%
Total notes payable	$	100.8	$	101.9			$	100.8		$	101.2

Losses_and_Loss_Expenses_Payab

Losses and Loss Expenses Payable	12 Months Ended
	Dec. 31, 2014
Insurance [Abstract]
Losses and Loss Expenses Payable	Losses and Loss Expenses Payable
	The following table sets forth the activity in the liability for losses and loss expenses for the years ended December 31:
	($ millions)	2014		2013			2012
	Losses and loss expenses payable, at beginning of year	$	959.9	$	942.2		$	907.1
	Less: reinsurance recoverable on losses and loss expenses payable	9.1		13.5			25.5
	Net balance at beginning of year	950.8		928.7			881.6
	Incurred related to:
	Current year	726.2		741			795.2
	Prior years	45.1		(21.2		)	(16.9		)
	Total incurred	771.3		719.8			778.3
	Paid related to:
	Current year	373.2		355			397.2
	Prior years	375.3		342.7			334
	Total paid	748.5		697.7			731.2
	Net balance at end of year	973.6		950.8			928.7
	Plus: reinsurance recoverable on losses and loss expenses payable	9.6		9.1			13.5
	Losses and loss expenses payable, at end of year (affiliates $494.3, $438.0 and $435.1, respectively)	$	983.2	$	959.9		$	942.2
	The Company recorded adverse development related to prior years' loss and loss expense reserves in 2014 of $45.1 million, compared to favorable development for 2013 and 2012 of $21.2 million and $16.9 million, respectively. Favorable development of unallocated loss adjustment expenses contributed approximately $12.5 million of the 2014 development, while favorable development of catastrophe reserves was approximately $5.2 million, which was slightly lower than 2013. The personal and business insurance segments non-catastrophe loss and ALAE reserves accounted for $23.1 million of favorable development. The business insurance segment contributed $16.7 million of favorable development, driven by the other & product liability and commercial auto lines with $11.9 million and $5.3 million of favorable development, respectively. The favorable development in these lines was driven by the emergence of lower than anticipated claim severity from accident years 2012 and prior. The personal insurance segment contributed $6.4 million of favorable development, primarily from accident year 2013. Somewhat offsetting the favorable development was adverse development of $2.1 million in the commercial multi-peril line, driven by third party liability coverage. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $85.9 million of adverse development, driven by RED reserve strengthening of $96.7 million related to a large restaurant program and a commercial auto trucking program. Somewhat offsetting the unfavorable development was favorable development of $5.7 million in the Workers' compensation unit, $3.9 million in the E&S property unit and $2.1 million in the E&S casualty unit. These results were driven by better than anticipated severity emerging from the 2012 and 2013 accident years.
	Favorable development of unallocated loss adjustment expenses contributed approximately $8.0 million of the 2013 development, while $5.4 million was attributable to favorable development on catastrophe reserves, which is lower than 2012, but, in line with the Company's historical favorable development. The personal and business insurance segments non-catastrophe loss and ALAE reserves accounted for $18.3 million of favorable development driven by the other & product liability, commercial auto and homeowners lines with $8.3 million, $8.0 million and $2.9 million of favorable development, respectively. The favorable development in these lines was driven by the emergence of lower than anticipated claim severity. Somewhat offsetting the favorable development was adverse development in the personal auto line, specifically the no-fault coverage. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $10.5 million of adverse development, driven by RED reserve strengthening of $21.3 million related to a large restaurant program and a commercial auto trucking program. Somewhat offsetting the unfavorable development was favorable development of workers' compensation reserves of $12.3 million driven by better than anticipated severity emerging across all accident years, with approximately one third coming from accident year 2012.
	Favorable development on prior years loss adjustment expense reserves contributed approximately $6.3 million of the 2012 development, while $10.4 million was attributable to favorable development on catastrophe reserves from accident year 2011. The personal and business insurance segments' non-catastrophe loss and ALAE reserves accounted for $28.0 million of favorable development related to the latest three accident years, primarily in the personal auto liability, other & product liability, and fire & allied lines with $10.5 million, $9.4 million and $5.1 million of favorable development, respectively. The favorable development in these lines was driven by emergence of lower than anticipated claim severity. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $27.8 million of adverse development related to the latest two accident years, which was driven by RED reserve strengthening.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
Reinsurance	Reinsurance
	In the ordinary course of business, the Company assumes and cedes reinsurance with other insurers and reinsurers and is a member in various pools and associations. See Note 6a for discussion of reinsurance with affiliates. The voluntary arrangements provide greater diversification of business and limit the maximum net loss potential arising from large risks and catastrophes. Most of the ceded reinsurance is effected under reinsurance contracts known as treaties; the remainder is by negotiation on individual risks. Although the ceding of reinsurance does not discharge the original insurer from its primary liability to its policyholder, the insurance company that assumes the coverage assumes the related liability.
	Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured business. The recoverability of these assets depends on the reinsurers’ ability to perform under the reinsurance agreements. The Company evaluates and monitors the financial condition and concentrations of credit risk associated with its reinsurers under voluntary reinsurance arrangements to minimize its exposure to significant losses from reinsurer insolvencies. The Company has reported ceded losses and loss expenses payable and prepaid reinsurance premiums with other insurers and reinsurers as assets. All reinsurance contracts provide for indemnification against loss or liability relating to insurance risk and have been accounted for as reinsurance.
	On December 31, 2011, the State Auto Group entered into the Homeowners Quota Share Arrangement, which was a three-year quota share agreement covering its homeowners line of business. Under the arrangement, the State Auto Group ceded to reinsurers 75% of its homeowners business under policies in force at the effective date and new and renewal policies thereafter issued during the term of the agreement. The arrangement expired on December 31, 2014. The Company received $89.5 million of unearned premiums related to the expiration of this arrangement. Subject to the terms and conditions of the arrangement, the participating reinsurers’ margin was capped at 9.0%, with any excess returned to the Company in the form of profit commission. For the year ended December 31, 2014, the Company recognized $19.0 million of profit commission, reflected as a reduction in acquisition and operating expenses on the consolidated statements of income. The amount of ceding commission earned was limited to the amount of deferred acquisition costs that would have been deferred if not for entering in the arrangement. As a result of the expiration of the HO QS Arrangement, the Company recognized $8.1 million of excess ceding commission that had previously been deferred, reflected as a reduction in acquisition and operating expenses on the consolidated statements of income for the year ended December 31, 2014. For the years ended December 31, 2013 and 2012, the Company recognized $8.6 million and $8.4 million, respectively, of excess ceding commission as a deferred liability on the consolidated balance sheet.
	The following table sets forth the effect of the Company’s external reinsurance on its balance sheets at December 31, 2014 and 2013, prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a:
	($ millions)	2014			2013
	Losses and loss expenses payable:
	Direct	$	484.4		$	516.9
	Assumed	4.5			5
	Ceded	(9.6		)	(9.1		)
	Net losses and loss expenses payable	$	479.3		$	512.8
	Unearned premiums:
	Direct	$	409.7		$	411.6
	Assumed	1			1
	Ceded	(6.1		)	(4.7		)
	Net unearned premiums	$	404.6		$	407.9
	The following table sets forth the effect of the Company’s external reinsurance on its income statements for the years ended December 31, 2014, 2013 and 2012, prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a:
	($ millions)	2014			2013			2012
	Written premiums:
	Direct	$	880.6		$	886.7		$	860.1
	Assumed	4.4			3.9			4
	Ceded	(26.8		)	(24.3		)	(24.3		)
	Net written premiums	$	858.2		$	866.3		$	839.8
	Earned premiums:
	Direct	$	882.6		$	874.7		$	833.3
	Assumed	4.4			3.8			4.1
	Ceded	(25.3		)	(23.5		)	(28.2		)
	Net earned premiums	$	861.7		$	855		$	809.2
	Losses and loss expenses incurred:
	Direct	$	525.6		$	559.1		$	577.4
	Assumed	2.9			3			3.6
	Ceded	(8.0		)	(4.8		)	(6.4		)
	Net losses and loss expenses incurred	$	520.5		$	557.3		$	574.6

Transactions_with_Affiliates

Transactions with Affiliates	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Transactions with Affiliates	Transactions with Affiliates
	a. 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 which includes Meridian Citizens Mutual Insurance Company (merged with State Auto Mutual at the close of business on July 2, 2014), 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”). State Auto P&C, Milbank and SA Ohio are referred to as the “STFC Pooled Companies,” State Auto Mutual, SA Wisconsin, Meridian Security and Patrons Mutual are referred to as the “Mutual Pooled Companies,” and RIC, Plaza, American Compensation and Bloomington Compensation are referred to as the "Rockhill Insurers." The STFC Pooled Companies, the Mutual Pooled Companies and the Rockhill Insurers are collectively referred to as the “State Auto Group.”
	In general, under the Pooling Arrangement, the STFC Pooled Companies and the Mutual Pooled Companies other than State Auto Mutual cede to State Auto Mutual all of their insurance business and assume from State Auto Mutual an amount equal to their respective participation percentages in the Pooling Arrangement. All premiums, losses and loss expenses and underwriting expenses are allocated among the participants on the basis of each Company’s participation percentage in the Pooling Arrangement. The Pooling Arrangement provides indemnification against loss or liability relating to insurance risk and has been accounted for as reinsurance.
	The Pooling Arrangement does not relieve each individual pooled subsidiary of its primary liability as the originating insurer; consequently, there is a concentration of credit risk arising from business ceded to State Auto Mutual. As the Pooling Arrangement provides for the right of offset, the Company has reported losses and loss expenses payable and prepaid reinsurance premiums to State Auto Mutual as assets only in situations when net amounts ceded to State Auto Mutual exceed net amounts assumed. All parties that participate in the Pooling Arrangement have an A.M. Best rating of A (Excellent).
	The following table sets forth the reinsurance transactions on the Company’s balance sheets for the Pooling Arrangement between the STFC Pooled Companies and State Auto Mutual at December 31, 2014 and 2013:
	($ millions)	2014				2013
	Assets
	Deferred policy acquisition costs:
	Ceded	$	(79.7	)		$	(77.6	)
	Assumed	126.5				96.8
	Net assumed	$	46.8			$	19.2
	Liabilities and Stockholders' Equity
	Losses and loss expenses payable:
	Ceded	$	(479.3	)		$	(512.8	)
	Assumed	973.6				950.8
	Net assumed	$	494.3			$	438
	Unearned premiums:
	Ceded	$	(404.6	)		$	(407.9	)
	Assumed	606.3				486.3
	Net assumed	$	201.7			$	78.4
	Pension and postretirement benefits:
	Ceded	$	(180.5	)		$	(106.5	)
	Assumed	117.3				69.2
	Net ceded	$	(63.2	)		$	(37.3	)
	Other liabilities:
	Ceded	$	(70.7	)		$	(47.2	)
	Assumed	65.6				67.2
	Net ceded	$	(5.1	)		$	20
	Stockholders' Equity
	Accumulated other comprehensive income:
	Ceded	$	(186.0	)		$	(117.1	)
	Assumed	120.9				76.1
	Net ceded	$	(65.1	)		$	(41.0	)
	The following table sets forth the reinsurance transactions on the Company’s income statements for the Pooling Arrangement between the STFC Pooled Companies and State Auto Mutual for the years ended December 31, 2014, 2013 and 2012:
	($ millions)		2014				2013			2012
	Written premiums:
	Ceded		$	(858.2	)		$	(866.3	)	$	(839.9	)
	Assumed		1,194.20				1,062.10			1,055.30
	Net assumed		$	336			$	195.8		$	215.4
	Earned premiums:
	Ceded		$	(861.7	)		$	(855.0	)	$	(809.2	)
	Assumed		1,074.10				1,055.00			1,042.10
	Net assumed		$	212.4			$	200		$	232.9
	Losses and loss expenses incurred:
	Ceded		$	(523.6	)		$	(560.2	)	$	(575.7	)
	Assumed		774.4				722.7			779.4
	Net assumed		$	250.8			$	162.5		$	203.7
	Acquisition and operating expenses:
	Ceded		(215.4		)		(192.6		)	(161.8		)
	Assumed		372.3				365.3			352.6
	Net assumed		$	156.9			$	172.7		$	190.8
	Intercompany Balances
	Pursuant to the Pooling Arrangement, State Auto Mutual receives all premiums and pays all losses and expenses associated with the insurance business produced by the pool participants and then settles the intercompany balances generated by these transactions with the participating companies on a quarterly basis within 60 days following each quarter end. No interest is paid on this balance. When settling the intercompany balances, State Auto Mutual provides the pool participants with full credit for the premiums written and net losses paid during the quarter and retains all receivable amounts from insureds and agents and reinsurance recoverable on paid losses from unaffiliated reinsurers. Any receivable amounts that are ultimately deemed to be uncollectible are charged-off by State Auto Mutual and allocated to the pool members on the basis of pool participation. As a result, the Company has an off-balance sheet credit risk related to the balances due to State Auto Mutual from insurers, agents and reinsurers, which are offset by the unearned premium from the respective policies. The Company’s share of the premium balances due to State Auto Mutual from agents and insureds at December 31, 2014 and 2013 is approximately $297.0 million and $295.5 million, respectively.
	b. Notes Payable
	In May 2003, State Auto Financial formed a Delaware business trust (the “Capital Trust”) to issue $15.0 million of mandatorily redeemable preferred capital securities to a third party and $0.5 million of common securities to State Auto Financial (the capital and common securities are collectively referred to as the “Trust Securities”). The Capital Trust loaned $15.5 million, the proceeds from the issuance of its Trust Securities, to State Auto Financial in the form of Floating Rate Junior Subordinated Debt Securities due in 2033 (the “Subordinated Debentures”). The Subordinated Debentures and interest accrued thereon are the Capital Trust’s only assets. Interest on the Trust Securities is payable quarterly at a rate equal to the three-month LIBOR rate plus 4.20% adjusted quarterly (total 4.44% at December 31, 2014). Because the interest rate and interest payment dates on the Subordinated Debentures are the same as the interest rate and interest payment dates on the Trust Securities, payments from the Subordinated Debentures finance the distributions paid on the Trust Securities. State Auto Financial has the right to redeem the Subordinated Debentures, in whole or in part, on or after May 2008. State Auto Financial has unconditionally and irrevocably guaranteed payment of any required distributions on the capital securities, the redemption price when the capital securities are to be redeemed, and any amounts due if the Capital Trust is liquidated or terminated. State Auto Financial’s equity interest in the Capital Trust is included in other invested assets. In accordance with the Consolidation Topic of the FASB ASC 810, State Auto Financial determined that the business trust is a variable interest entity for which it is not the primary beneficiary and therefore, does not consolidate the Capital Trust with the Company.
	c. Notes Receivable
	In May 2009, the Company entered into two separate credit agreements with State Auto Mutual pursuant to which it loaned State Auto Mutual a total of $70.0 million. Interest is payable semi-annually at a fixed annual interest rate of 7.00%, with the principal payable in May 2019. There is no prepayment penalty, and no collateral was given as security for the payment of this loan.
	Under these agreements, the Company earned interest of $4.9 million for each of the three years ended December 31, 2014, 2013 and 2012, respectively. See Note 3 for the notes receivable fair value discussion.
	d. Management Services
	Stateco provides State Auto Mutual and its affiliates investment management services. Investment management income is recognized quarterly based on a percentage of the average fair value of investable assets and the equity portfolio performance of each company managed. Revenue related to these services amounted to $1.5 million, $1.6 million and $2.9 million in 2014, 2013 and 2012, respectively, and is included in other income (affiliates) on the consolidated statements of income.

Notes_Payable_and_Credit_Facil

Notes Payable and Credit Facility	12 Months Ended
	Dec. 31, 2014
Notes Payable [Abstract]
Notes Payable and Credit Facility	Notes Payable and Credit Facility
	FHLB Loan
	On July 11, 2013, State Auto Financial's subsidiary, State Auto P&C, borrowed $85.0 million (the "FHLB Loan") from the FHLB. The FHLB Loan is a 20-year term loan and is callable after three years with no prepayment penalty thereafter. The FHLB Loan provides for interest-only payments during its term, with principal due in full at maturity. The interest rate is fixed over the term of the loan at 5.03%. The FHLB Loan is fully secured by a pledge of specific investment securities of State Auto P&C.
	Senior Note Redemption
	On July 15, 2013, State Auto Financial redeemed all $100.0 million of its outstanding Senior Notes. The redemption price of $103.0 million was calculated in accordance with the terms of the Senior Notes and included principal plus a make-whole amount. The redemption price was funded by proceeds from the FHLB Loan and cash on hand.
	Credit Facility
	On July 26, 2013, State Auto Financial terminated its then-current credit agreement with a syndicate of lenders, as further described below. Concurrent with the termination of this credit agreement, State Auto P&C entered into a new credit facility (the “SPC Credit Facility”) with a syndicate of lenders. The SPC Credit Facility provides State Auto P&C with a $100.0 million five-year revolving credit facility maturing in July 2018. During the term of the SPC Credit Facility, State Auto P&C has the right to increase the total facility to a maximum amount of $150.0 million, provided that no event of default has occurred and is continuing. The SPC Credit Facility is available for general corporate purposes and provides for interest-only payments during its term, with principal and interest due in full at maturity. Interest is based on the London Interbank Offered Rate ("LIBOR") or a base rate plus a calculated margin amount. All advances under the SPC Credit Facility are to be fully secured by a pledge of specific investment securities of State Auto P&C. The SPC Credit Facility includes certain requirements, including financial requirements that State Auto Financial maintain a minimum net worth and a certain debt to capitalization ratio.
	As of December 31, 2014, State Auto P&C had not made any borrowings and both State Auto P&C and State Auto Financial were in compliance with all covenants and requirements of the SPC Credit Facility.

Federal_Income_Taxes

Federal Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Federal Income Taxes	Federal Income Taxes
	The following table sets forth the reconciliation between actual federal income tax (benefit) expense and the amount computed at the indicated statutory rate for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014					2013					2012
	Amount at statutory rate	$	9.4		35	%	$	21.5		35	%	$	3.7		35	%
	Tax-exempt interest and dividends received deduction	(8.5		)	(32.0	)	(9.4		)	(15.0	)	(9.1		)	(85.0	)
	Other, net	1.1			4		0.2			—		0.5			4
	Valuation allowance	(82.6		)	(308.0	)	(11.8		)	(19.0	)	4.8			45
	Federal income tax (benefit) expense and effective rate	$	(80.6	)	(301.0	)%	$	0.5		1	%	$	(0.1	)	(1.0	)%
	The following table sets forth the tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities at December 31, 2014 and 2013:
	($ millions)	2014			2013
	Deferred tax assets:
	Unearned premiums not currently deductible	$	42.2		$	33.8
	Losses and loss expenses payable discounting	21			21.6
	Postretirement and pension benefits	41.1			24.9
	Realized loss on other-than-temporary impairment	8.4			7.5
	Other liabilities	22.2			17.3
	Net operating loss carryforward	58			56.5
	Tax credit carryforward	1.9			1.7
	Other	4.7			9.2
	Total deferred tax assets	199.5			172.5
	Deferred tax liabilities:
	Deferral of policy acquisition costs	44.3			33.9
	Net unrealized holding gains on investments	57.8			44.1
	Total deferred tax liabilities	102.1			78
	Total net deferred tax assets before valuation allowance	97.4			94.5
	Less valuation allowance	—			82.6
	Net deferred federal income taxes	$	97.4		$	11.9
	Deferred income tax assets and liabilities represent the tax effect of the differences between the financial statement carrying value of existing assets and liabilities and their respective tax bases. The Company periodically evaluates its deferred tax assets, which requires significant judgment, to determine if they are realizable based upon weighing all available evidence, both positive and negative, including loss carryback potential, past operating results, existence of cumulative losses in the most recent years, projected performance of the business, future taxable income, including the ability to generate capital gains, and prudent and feasible tax planning strategies. In making such judgments, significant weight is given to evidence that can be objectively verified.
	During 2011, the Company determined that a valuation allowance should be established due to the magnitude of the catastrophe losses from unprecedented storms experienced industry wide. As part of its assessment of the appropriateness of the deferred tax asset valuation allowance at December 31, 2014, management considered the following positive evidence: (i) three-year cumulative pre-tax income of $98.7 million, (ii) three consecutive years of pre-tax income, (iii) the expiration of the HO QS Arrangement under which we ceded 75% of our homeowners line underwriting results to the participating reinsurers, (iv) the actions the Company took (including reserve strengthening and entering into an ADC reinsurance agreement protecting against the risk of further adverse development for one of the programs) with respect to the terminated RED program business, which significantly reduced the Company's reported financial results since 2011, (v) more adequate pricing, which has contributed to improved underwriting margins, and (vi) profitable growth within the specialty insurance segment, excluding the terminated RED program business. Management concluded that this positive evidence outweighed available negative evidence and further concluded that a valuation allowance against the Company's net deferred tax assets was no longer appropriate. As a result, the valuation allowance was reversed at December 31, 2014. With the reversal of the valuation allowance at December 31, 2014, the Company recorded an income tax benefit of $82.6 million. At December 31, 2013, the valuation allowance was $82.6 million, with $11.8 million recorded as a reduction of income tax expense in the income statement.
	At December 31, 2014, the tax benefit of the net operating loss carryforward was $58.0 million; if not used, it will expire in 2032.
	At December 31, 2014, the Company carried no balance for uncertain tax positions. The Company had no accrual for the payment of interest and penalties at December 31, 2014 or 2013.
	State Auto Financial and its subsidiaries file a consolidated U.S. federal income tax return. State Auto Financial and its subsidiaries also file in various state jurisdictions. The Company is no longer subject to U.S. federal or state and local income tax examinations by tax authorities for years before 2012. The Company has no current U.S. federal or state and local income tax examinations on-going at this time.

Pension_and_Postretirement_Ben

Pension and Postretirement Benefit Plans	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Pension and Postretirement Benefit Plans	Pension and Postretirement Benefit Plans
	The Company, through the employees of State Auto P&C, provides management and operation services under management agreements for all insurance and non-insurance affiliates. The annual periodic costs related to the Company’s benefit plans are allocated to affiliated companies based on allocations pursuant to intercompany management agreements including the Pooling Arrangement for insurance subsidiaries and affiliates party to this agreement.
	The Company provides a defined benefit pension plan for its eligible employees. Substantially all Company employees hired prior to January 1, 2010 become eligible to participate the year after becoming 20 years of age and vest with 5 years of credited service or attaining age 65. The Company’s policy is to fund pension costs in accordance with the requirements of the Employee Retirement Income Security Act of 1974. Benefits are determined by applying factors specified in the plan to a participant’s defined average annual compensation.
	The defined benefit pension and postretirement benefit plans are referred to herein as “the benefit plans.”
	The following table sets forth information regarding the pension and postretirement benefit plans’ change in benefit obligation, plan assets and funded status at December 31, 2014 and 2013:
	($ millions)	Pension								Postretirement
		2014				2013				2014				2013
	Change in benefit obligation:
	Benefit obligation at beginning of year	$	230.5			$	246.1			$	22.3			$	25.1
	Service cost	4.6				6.1				—				—
	Interest cost	10.8				9.6				1.1				1
	Actuarial loss (gain)	57.3				(19.6		)		1.7				(2.6		)
	Benefits paid	(10.7		)		(11.7		)		(1.2		)		(1.2		)
	The Company’s portion of benefit obligation at end of year	$	292.5			$	230.5			$	23.9			$	22.3
	Change in plan assets available for plan benefits:
	Fair value of plan assets available for plan benefits at beginning of year	$	185.9			$	162.2			$	1.3			$	1.8
	Employer contribution	13				13				—				—
	Actual return on plan assets	16.9				22.4				—				0.3
	Benefits paid	(10.7		)		(11.7		)		(1.0		)		(0.8		)
	The Company’s portion of fair value of plan assets at end of year	$	205.1			$	185.9			$	0.3			$	1.3
	Supplemental executive retirement plan	(6.3		)		(6.0		)		—				—
	Funded status at end of year	$	(93.7	)		$	(50.6	)		$	(23.6	)		$	(21.0	)
	Accumulated benefit obligation end of year	$	260.1			$	211.2
	No assets are expected to be returned during the fiscal year ending December 31, 2014.
	The following table sets forth the Company’s share of the amounts included in accumulated other comprehensive income (loss) that have not been recognized in net periodic cost at December 31, 2014 and 2013:
	($ millions)	2014				2013
	Prior service benefit	$	(64.9	)		$	(70.3	)
	Net actuarial loss	141.9				94.3
	Total	$	77			$	24
	The following table sets forth the Company’s share of amortization expected to be recognized for the year ending December 31, 2015:
	($ millions)	2015
	Prior service benefit	$	(5.4	)
	Net actuarial loss	11.5
	Total	$	6.1
	The following table sets forth information regarding the Company’s share of pension and postretirement benefit plans’ components of net periodic cost for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	Pension												Postretirement
		2014				2013				2012				2014				2013			2012
	Components of net periodic cost:
	Service cost	$	5.2			$	6.1			$	7.8			$	—			$	0.4		$	—
	Interest cost	11.1				9.6				9.9				1.1				1.2			1.1
	Expected return on plan assets	(12.6		)		(12.2		)		(11.7		)		—				(0.2		)	(0.1		)
	Amortization of:
	Prior service cost (benefit)	—				—				0.3				(5.5		)		(5.5		)	(5.5		)
	Net actuarial loss	6.3				8.1				6.7				0.6				1			1.1
	Net periodic cost (benefit)	$	10			$	11.6			$	13			$	(3.8	)		$	(3.1	)	$	(3.4	)
	The following table sets forth the Company’s share of the benefit payments, which reflect expected future service, expected to be paid:
	($ millions)	Pension				Postretirement
	2015	$	9.6			$	1.7
	2016	10				1.7
	2017	10.5				1.7
	2018	11.2				1.6
	2019	11.8				1.6
	2020-2024	72				7.3
	The postretirement plan’s gross benefit payments for 2014 were $1.2 million, including the prescription drug benefits. The postretirement plan’s subsidy related to Medicare Prescription Drug Improvement and Modernization Act of 2003 was $0.3 million for 2014 and estimates future annual subsidies to be approximately $0.2 million.
	The following table sets forth the weighted average assumptions used to determine the benefit plans’ obligations at December 31, 2014 and 2013:
		Pension						Postretirement
		2014			2013			2014			2013
	Benefit obligations weighted-average assumptions:
	Discount rate	3.85	%		4.85	%		3.85	%		4.85	%
	Rates of increase in compensation levels	3.5			3.5			—			—
	The following table sets forth the weighted average assumptions used to determine the benefit plans’ net periodic cost for the years ended December 31, 2014, 2013 and 2012:
				Pension									Postretirement
				2014			2013			2012			2014			2013			2012
	Weighted-average assumptions:
Discount rate			4.85	%		4.05	%		4.4	%		4.85	%		4.05	%		4.4	%
Expected long-term rate of return on assets			7			7.5			7.5			—			7.5			7.5
Rates of increase in compensation levels			3.5			4			4			—			—			—
The benefit plans’ obligations are long-term in nature and consequently the investment strategies have a long-term time horizon. In establishing the long-term rate of return assumption on plan assets, management, along with its pension consulting actuary, reviews the historical performance of the plan assets and the stability in the mix of the investment portfolio. The expected inflation rate and expected real rates of return of applicable asset classes are then determined to assist in setting appropriate assumptions.
The following table sets forth the assumed health care cost trend rates used for the years ended December 31, 2014, 2013 and 2012:
	Postretirement
	2014			2013			2012
Assumed health care cost trend rates:
Health care cost trend rate assumed for the next year	6	%		10	%		10	%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	3.8	%		5	%		5	%
Year that the rate reaches the ultimate trend rate	2075			2018			2017
The assumed health care cost trend rates have a significant effect on the amounts reported for the postretirement plan. The following table sets forth the effects of a one percentage point change in assumed health care cost trend rates for the year ended December 31, 2014:
($ millions)	Postretirement
	Increase				(Decrease)
One percentage point change:
Effect on total service and interest cost	$	0.1			$	0.1
Effect on accumulated postretirement benefit obligation	3.8				3.3
The pension plan's investment policy objective is to preserve the investment principal while generating income and appreciation in fair value to meet the pension plan's obligations. The pension plan's investment strategy and risk tolerance is balanced between meeting cash obligation requirements and a long-term relatively high risk tolerance takes into account the predictable cash requirements, nature of the plan’s liabilities and the plan’s long term time horizon. Since the nature and timing of the benefit plans’ liabilities and cash requirements are predictable, the liquidity requirements are somewhat moderate. One of the goals of diversifying the benefit plans’ portfolio among different asset classes is the elimination of concentration of risk in one asset class. Management also has investment policy guidelines with respect to limiting the ownership in any single debt or equity issuer. The international fund investments are also composed of numerous securities to reduce our exposure to a single issuer. The following table sets forth the asset allocation targets, as a percentage of total fair value, which are used as a guide by management when allocating funds as they become available.
	Asset
	allocation
	target
	(0 to 100%)
Asset Category:
Fixed maturity	49	%
U.S. large-cap equity	26
U.S. small-cap equity	11
International equity	10
Emerging market equity	4
Total	100	%
Effective January 1, 2014, the Investment Committee approved a change to a liability driven investment (LDI) for the pension plan assets.  The primary goal of the LDI strategy is to shift the asset allocation to more closely align with the plan liability, thereby reducing the volatility of the funded status.  The implementation of the LDI strategy will occur over a period of time and the actual asset allocation at any point in time is dependent upon the funded status and the level of interest rates.  This glide path helps to balance interest rate risk, curve steepness risk, and credit spread risk, as incremental changes are made to the allocation over time.  The new allocation strategy reduces exposure to equity holdings and increases exposure to long duration fixed income.  This change will result in lower volatility for the plan assets.  By moving more of the plan's assets to long duration fixed income, the duration of the assets will increase to more closely match the duration of the plan's liabilities.  While the LDI approach is new for the pension plan, the investment policy objective is unchanged.
See Note 3 for the valuation methods used by the Company for each type of financial instrument the plans hold that are carried at fair value. There were no transfers between level categorizations during the years ended December 31, 2014 and 2013.
The following tables set forth the Company’s share of pension plan’s available-for-sale securities within the fair value hierarchy at December 31, 2014 and 2013:
($ millions)	Total				Quoted prices				Significant				Significant
					in active				other				unobservable
					markets for				observable				inputs
					identical				inputs				(Level 3)
					assets				(Level 2)
31-Dec-14					(Level 1)
Fixed maturities:
U.S. treasury securities and obligations of U.S. government agencies	$	56.4			$	—			$	56.4			$	—
Corporate securities	39.7				—				39.7				—
U.S. government agencies mortgage-backed securities	9.4				—				9.4				—
Total fixed maturities	105.5				—				105.5				—
Equity securities:
Large-cap securities	53.7				53.7				—				—
Small-cap securities	21.8				21.8				—				—
Total equity securities	75.5				75.5				—				—
International instruments	18.7				—				18.7				—
Total pension plan investments	$	199.7			$	75.5			$	124.2			$	—
($ millions)	Total				Quoted prices				Significant				Significant
					in active				other				unobservable
					markets for				observable				inputs
					identical				inputs				(Level 3)
					assets				(Level 2)
31-Dec-13					(Level 1)
Fixed maturities:
U.S. treasury securities and obligations of U.S. government agencies	$	30			$	—			$	30			$	—
Corporate securities	24.6				—				24.6				—
U.S. government agencies mortgage-backed securities	11.3				—				11.3				—
Total fixed maturities	65.9				—				65.9				—
Equity securities:
Large-cap securities	58				58				—				—
Small-cap securities	26.3				26.3				—				—
Total equity securities	84.3				84.3				—				—
International instruments	25.4				—				25.4				—
Total pension plan investments	$	175.6			$	84.3			$	91.3			$	—
The following tables set forth the Company’s share of the postretirement plan’s available-for-sale securities within the fair value hierarchy at December 31, 2014 and 2013:
($ millions)	Total				Quoted prices				Significant				Significant
					in active				other				unobservable
					markets for				observable				inputs
					identical				inputs				(Level 3)
					assets				(Level 2)
31-Dec-14					(Level 1)
Fixed maturities:
Corporate securities	$	0.1			$	—			$	0.1			$	—
Total fixed maturities	0.1				—				0.1				—
Short-term money market funds	0.2				0.2				—				—
Total postretirement plan investments	$	0.3			$	0.2			$	0.1			$	—
	Total				Quoted prices				Significant				Significant
					in active				other				unobservable
					markets for				observable				inputs
					identical				inputs				(Level 3)
					assets				(Level 2)
31-Dec-13					(Level 1)
Fixed maturities:
Corporate securities	$	0.2			$	—			$	0.2			$	—
Total fixed maturities	0.2				—				0.2				—
Equity securities:
Large-cap securities	0.8				0.8				—				—
Total equity securities	0.8				0.8				—				—
Total postretirement plan investments	$	1			$	0.8			$	0.2			$	—
The actuarially prepared funding amount to the pension plan ranges from the minimum amount the Company would be required to contribute to the maximum amount that would be deductible for tax purposes. Contributed amounts in excess of the minimum amounts are deemed voluntary. Amounts in excess of the maximum amount would be subject to an excise tax and may not be deductible for tax purposes. The Company’s share of expected contributions during 2015 is approximately $13.0 million.
The Company maintains a defined contribution plan that covers substantially all employees of the Company. The Company matches the first 1% of contributions of participants’ salary at the rate of one dollar for each dollar contributed. Participant contributions of 2% to 6% are matched at a rate of 50 cents for each dollar contributed. In addition, the Company contributes a percentage of the employee’s annual income for those employees hired on or after January 1, 2010, and for those employees hired prior to January 1, 2010 who chose to freeze their existing accrued pension benefit effective June 30, 2010. The Company’s share of the expense under the plan totaled $5.1 million, $4.6 million and $4.5 million for 2014, 2013 and 2012, respectively.

Other_Comprehensive_Income_and

Other Comprehensive Income and Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	Other Comprehensive Income and Accumulated Other Comprehensive Income
	The following tables set forth the changes in the Company’s accumulated other comprehensive income component (AOCI), net of tax, for the years ended December 31, 2014, 2013 and 2012:
	($ millions)		Unrealized Gains and Losses on Available-for-Sale Securities				Gains and Losses on Cash Flow Hedges				Benefit Plan Items				Total
	Beginning balance at January 1, 2014		$	84.6			$	—			$	(3.8	)		$	80.8
	Other comprehensive income before reclassifications		38.9				—				(54.4		)		(15.5		)
	Amounts reclassified from AOCI (a)		(13.5		)		—				19.9				6.4
	Net current period other comprehensive income		25.4				—				(34.5		)		(9.1		)
	Ending balance at December 31, 2014		$	110			$	—			$	(38.3	)		$	71.7
	Beginning balance at January 1, 2013		$	124			$	0.1			$	(39.9	)		$	84.2
	Other comprehensive income before reclassifications		(16.1		)		—				32.5				16.4
	Amounts reclassified from AOCI (a)		(23.3		)		(0.1		)		3.6				(19.8		)
	Net current period other comprehensive income		(39.4		)		(0.1		)		36.1				(3.4		)
	Ending balance at December 31, 2013		$	84.6			$	—			$	(3.8	)		$	80.8
	Beginning balance at January 1, 2012		$	98.7			$	0.2			$	(35.1	)		$	63.8
	Other comprehensive income before reclassifications		54.1				—				(7.4		)		46.7
	Amounts reclassified from AOCI (a)		(28.8		)		(0.1		)		2.6				(26.3		)
	Net current period other comprehensive income		25.3				(0.1		)		(4.8		)		20.4
	Ending balance at December 31, 2012		$	124			$	0.1			$	(39.9	)		$	84.2
	(a)	See separate table below for details about these reclassifications
	The following tables set forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s consolidated statement of income for the years ended December 31, 2014, 2013 and 2012:
	($ millions)
	Details about Accumulated Other			31-Dec												Affected line item in the Condensed
	Comprehensive Income Components			2014				2013				2012				Consolidated Statements of Income
	Unrealized gains on available for sale securities			$	20.7			$	23.2			$	28.8			Realized gain on sale of securities
				20.7				23.2				28.8				Total before tax
				(7.2		)		0.1				—				Tax (expense) benefit
				13.5				23.3				28.8				Net of tax
	Amortization of gain on derivative used in cash flow hedge			—				0.1				0.1				Realized gain on sale of securities
				—				0.1				0.1				Total before tax
				—				—				—				Tax benefit (expense)
				—				0.1				0.1				Net of tax
	Amortization of benefit plan items:
	Negative prior service costs			5.5				5.5				5.2				(a)
	Net loss			(6.9		)		(9.1		)		(7.8		)		(a)
				(1.4		)		(3.6		)		(2.6		)		Total before tax
				(18.5		)		—				—				Tax expense
				(19.9		)		(3.6		)		(2.6		)		Net of tax
	Total reclassifications for the period			$	(6.4	)		$	19.8			$	26.3
	(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).

Stockholders_Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Stockholders' Equity	Stockholders’ Equity
	a. Dividend Restrictions and Statutory Financial Information
	State Auto P&C, Milbank and SA Ohio are subject to regulations and restrictions under which payment of dividends from statutory surplus can be made to State Auto Financial during the year without prior approval of regulatory authorities. Under the insurance regulations of Iowa and Ohio (the states of domicile), the maximum amount of dividends that the Company may pay out of earned surplus to shareholders within a twelve month period without prior approval of the Department is limited to the greater of 10% of the most recent year-end policyholders' surplus or net income for the twelve month period ending the 31st day of December of the previous year-end. Pursuant to these rules, approximately $77.8 million is available for payment to State Auto Financial from its insurance subsidiaries in 2015 without prior approval. State Auto Financial received dividends from its insurance subsidiaries in the amount of $20.0 million, $10.0 million and $20.0 million in 2014, 2013 and 2012, respectively.
	The Company's insurance subsidiaries are subject to risk-based capital ("RBC") requirements that have been adopted by individual states. These requirements subject insurers having statutory capital less than that required by the RBC calculation to varying degrees of regulatory action, depending on the level of capital inadequacy. The RBC formulas specify various weighting factors to be applied to financial balances or various levels of activity based on the perceived degree of risk. Regulatory compliance is determined by a ratio of total adjusted capital to authorized control level RBC. Generally no remedial action is required by an insurance company if its adjusted statutory surplus exceeds 200% of the authorized level RBC. As of December 31, 2014, each of the Company's insurance subsidiaries maintained adjusted statutory surplus in excess of 450% of the authorized control level RBC.
	The following tables set forth reconciliations of statutory capital and surplus and net income, as determined using SAP, to the amounts included in the accompanying consolidated financial statements:
	($ millions)	2014			2013
	Statutory capital and surplus of insurance subsidiaries	$	778.4		$	753.2
	Net liabilities of non-insurance parent and affiliates	(80.8		)	(82.4		)
		697.6			670.8
	Increases (decreases):
	Deferred acquisition costs	126.5			96.8
	Postretirement and pension benefits	27.2			6.3
	Deferred federal income taxes	(49.7		)	(15.8		)
	Fixed maturities, at fair value	60.6			26.1
	Other, net	10.7			0.8
	Stockholders’ equity per accompanying consolidated financial statements	$	872.9		$	785
	($ millions)	Year ended December 31
		2014			2013			2012
	Statutory net (loss) income of insurance subsidiaries	$	(17.5	)	$	51.1		$	5.5
	Net (loss) income of non-insurance parent and affiliates	(5.3		)	(5.1		)	0.2
		(22.8		)	46			5.7
	Increases (decreases):
	Deferred acquisition costs	29.7			5.1			—
	Postretirement and pension benefits	10.6			10.3			5.3
	Deferred federal income taxes	79.3			(1.0		)	(3.3		)
	Share-based compensation expense	(0.5		)	(1.3		)	(2.7		)
	Other, net	11.1			1.7			5.7
	Net income per accompanying consolidated financial statements	$	107.4		$	60.8		$	10.7

Preferred_Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2014
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock	Preferred Stock
	State Auto Financial has two authorized classes of preferred stock. For both classes, upon issuance, the Board of Directors has authority to fix and determine the significant features of the shares issued, including, among other things, the dividend rate, redemption price, redemption rights, conversion features and liquidation price payable in the event of any liquidation, dissolution, or winding up of the affairs of State Auto Financial.
	The Class A preferred stock is not entitled to voting rights until, for any period, dividends are in arrears in the amount of six or more quarterly dividends.

ShareBased_Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2014
Share-based Compensation [Abstract]
Share-Based Compensation	Share-Based Compensation
	The Company maintains share-based compensation plans for key employees and outside, or non-employee, directors. The share-based compensation plan for key employees is the State Auto Financial Corporation 2009 Equity Compensation Plan (the “Equity Plan”). The stock-based compensation plan for outside directors is the Outside Directors Restricted Share Unit Plan (the “RSU Plan”).
	The Company’s share-based compensation plans authorize the granting of various equity-based incentives including stock options, restricted stock and restricted share units to employees and non-employee directors and agents. The expense for these equity-based incentives is based on their fair value at the date of grant and amortized over their vesting period. The fair value of each stock option is estimated on the date of grant using the Black-Scholes closed-form pricing model. The pricing model requires assumptions such as the expected life of the option and expected volatility of the Company’s stock over the expected life of the option, which significantly impacts the assumed fair value. The Company uses historical data to determine these assumptions and if these assumptions change significantly for future grants, share-based compensation expense will fluctuate in future periods.
	Equity Plan
	The Equity Plan provides for the award of qualified and nonqualified stock options, restricted shares, performance shares, performance units and other stock-based awards. The Company has reserved 3.0 million common shares under the Equity Plan. As of December 31, 2014, a total of 0.8 million common shares are available for issuance under the Equity Plan. The Equity Plan provides that (i) no more than 33% of the common shares authorized for issuance under the Equity Plan may be granted in the form of awards other than stock options, (ii) the maximum number of common shares subject to awards of stock options, restricted shares and performance shares that may be granted in any calendar year is equal to 1.5% of the total number of common shares of the Company outstanding as of December 31 of the prior year, and (iii) the maximum number of common shares subject to awards of stock options, restricted shares and performance shares that may be granted in any calendar year to any one individual is 250,000 shares. The Equity Plan automatically terminates on May 8, 2019.
	The Equity Plan provides that qualified stock options may be granted at an option price not less than the fair market value of the common shares at the date of grant and that nonqualified stock options may be granted at any price determined by the Compensation Committee of the Board of Directors. Options granted generally vest over a three-year period, with one-third of the options vesting on each anniversary of the grant date, and must be exercised no later than ten years from the date of grant. Stock options granted under the Equity Plan for 2014, 2013 and 2012 were 0.2 million, 0.5 million and 0.3 million, respectively.
	The Equity Plan provides for the granting of restricted shares subject to a vesting schedule based on the employee’s continued employment (“Restriction Period”), for which vesting is generally on the third anniversary after the date of grant. The Company recognizes compensation expense based on the number of restricted shares granted at the then grant date fair value over the Restriction Period.
	The following table sets forth the status of the Company’s non-vested and vested restricted shares and changes for the years ended December 31, 2014, 2013 and 2012:
		2014							2013								2012
		Shares			Weighted				Shares				Weighted				Shares		Weighted
					Average								Average						Average
					Grant								Grant						Grant
					Date Fair								Date Fair						Date Fair
					Value								Value						Value
	Outstanding, beginning of year	38,233			$	15.06			55,413				$	16.21			33,887		$	17.92
	Granted	56,178			21.23				—				—				21,526		13.53
	Vested	(16,892	)		17.08				(17,180	)			18.78				—		—
	Canceled	(1,047	)		21.23				—		—		—				—		—
	Outstanding, end of year	76,472			$	19.06			38,233				$	15.06			55,413		$	16.21
	As of December 31, 2014, there was $0.8 million of total unrecognized compensation cost related to non-vested restricted share compensation arrangements. The remaining cost is expected to be recognized over a period of 2.25 years.
	Employee Stock Purchase Plan
	The Company also has a broad-based employee stock purchase plan under which employees of the Company may choose at two different specified time intervals each year to have up to 6% of their annual base earnings withheld to purchase the Company’s common shares. The purchase price of the common shares is 85% of the lower of its beginning-of-interval or end-of-interval market price. The Company has reserved 3.4 million common shares under this plan. As of December 31, 2014, a total of 3.2 million common shares have been purchased under this plan. This plan remains in effect until terminated by the Board of Directors.
	Outside Directors Plan
	The RSU Plan is an unfunded deferred compensation plan which currently provides each outside director with an award of 1,400 restricted share units (the “RSU award”) following each annual meeting of shareholders. The amount of the award may change from year to year, based on the provision described below. The RSU awards are fully vested six months after the date of grant. RSU awards are not common shares of the Company and, as such, no participant has any rights as a holder of common shares under the RSU Plan. RSU awards represent the right to receive an amount, payable in cash or common shares of the Company, as previously elected by the outside director, equal to the value of a specified number of common shares of the Company at the end of the restricted period. Such election may be changed within the constraints set forth in the RSU Plan. The restricted period for the RSU awards begins on the date of grant and expires on the date the outside director retires from or otherwise terminates service as a director of the Company. During the restricted period, outside directors are credited with dividends, equivalent in value to those declared and paid on the Company’s common shares, on all RSU awards granted to them. At the end of the restricted period, outside directors receive distributions of their RSU awards either (i) in a single lump sum payment, or (ii) in annual installment payments over a five- or ten-year period, as previously elected by the outside director. The administrative committee for the RSU Plan (currently the Company’s Compensation Committee) retains the right to increase the annual number of RSU awards granted to each outside director to as many as 5,000 or to decrease such annual number to not less than 500, without seeking shareholder approval, if such increase or decrease is deemed appropriate by the administrative committee to maintain director compensation at appropriate levels. The RSU Plan automatically terminates on May 31, 2015. The Company accounts for the RSU Plan as a liability plan. There were 25,960 RSUs, 33,712 RSUs, and 26,480 RSUs granted in 2014, 2013 and 2012, respectively.
	During 2014 and 2013, common shares valued at approximately $51,000 and $51,000, respectively, were distributed by the Company under the RSU Plan.
	Stock Options
	The fair value of each stock option granted is estimated on the date of grant using the Black-Scholes closed-form pricing model. The following tables present the weighted-average assumptions used in the option pricing model for options granted to employees during 2014, 2013 and 2012. The expected life of the options for employees represents the period of time the options are expected to be outstanding and is based on historical trends. For non-employees the expected life of the option approximates the remaining contractual term of the option. The expected stock price volatility is based on the historical volatility of the Company’s stock for a period approximating the expected life and the expected dividend yield is based on the Company’s most recent period’s dividend payout. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant and has a term approximating the expected life of the option.
	The fair value of share-based awards granted to employees was estimated at the date of grant using the Black-Scholes option-pricing model. The following table sets forth the weighted average fair values and related assumptions for options granted for the years ended December 31, 2014, 2013 and 2012:
		2014				2013				2012
	Fair value per share	$	7.28			$	5.15			$	3.46
	Expected dividend yield	1.86		%		2.4		%		4.41		%
	Risk free interest rate	1.65		%		1.26		%		1.1		%
	Expected volatility factor	39.23		%		37.59		%		41.5		%
	Expected life in years	5.7				6.3				5.4
	The following table sets forth the Company’s total stock option activity and related information for these plans for the years ended December 31, 2014, 2013 and 2012:
	(millions, except per share amounts)	2014							2013							2012
		Options			Weighted-				Options			Weighted-				Options		Weighted-
					Average							Average						Average
					Exercise							Exercise						Exercise
					Price							Price						Price
	Outstanding, beginning of year	3.9			$	22.01			3.9			$	22.25			3.8		$	22.79
	Granted	0.3			21.2				0.5			16.82				0.4		13.54
	Exercised	(0.1	)		16.43				(0.2	)		16.82				—		14.49
	Canceled	(0.4	)		30.21				(0.3	)		19.52				(0.3	)	17.95
	Outstanding, end of year	3.7			$	21.29			3.9			$	22.01			3.9		$	22.25
	Intrinsic value for stock options is defined as the difference between the current market value and the grant price. For the years ended December 31, 2014, 2013 and 2012, the total intrinsic value of stock options exercised was $0.5 million, $0.5 million and $0 million, respectively. The tax benefit for tax deductions from share-based awards totaled $0 for the years ended December 31, 2014, 2013 and 2012, respectively.
	The following table sets forth information pertaining to the total options outstanding and exercisable at December 31, 2014:
	(Options in millions)	Options Outstanding									Options Exercisable
		Number			Weighted-		Weighted-				Number			Weighted-
					Average		Average							Average
					Remaining		Exercise							Exercise
					Contractual Life		Price							Price
	Range of Exercise Prices:
	$10.01 – $20.00	2.1			5.8		$	16.47			1.5			$	16.65
	$20.01 – $30.00	1.3			3.1		25.84				1			27.32
	Greater than $30.00	0.3			1.4		33.45				0.3			33.45
		3.7			4.5		$	21.29			2.8			$	22.3
	Aggregate intrinsic value for total options outstanding at December 31, 2014 was $17.2 million. Aggregate intrinsic value for total options exercisable at December 31, 2014 was $8.4 million.
	Compensation expense recognized during 2014, 2013 and 2012 was $3.6 million, $4.1 million and $3.5 million, respectively. Share-based compensation is recognized as a component of loss and loss adjustment expense and acquisition and operating expense in a manner consistent with other employee compensation. As of December 31, 2014, there was $3.2 million of total unrecognized compensation cost related to option-based compensation arrangements granted under the plans. The remaining cost is expected to be recognized over a period of three years.

Net_Earnings_Loss_Per_Common_S

Net Earnings (Loss) Per Common Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Net Earnings (Loss) Per Common Share	Net Earnings Per Common Share
	The following table sets forth the compilation of basic and diluted net earnings per common share for the years ended December 31, 2014, 2013 and 2012:
	(millions, except per share amounts)	2014			2013			2012
	Numerator:
	Net earnings for basic net earnings per common share	$	107.4		$	60.8		$	10.7
	Adjusted net earnings for dilutive net earnings per common share	$	107.4		$	60.8		$	10.7
	Denominator:
	Weighted average shares for basic net earnings per common share	40.8			40.6			40.4
	Effect of dilutive share-based awards	0.4			0.1			0.1
	Adjusted weighted average shares for diluted net earnings per common share	41.2			40.7			40.5
	Basic net earnings per common share	$	2.63		$	1.5		$	0.26
	Diluted net earnings per common share	$	2.6		$	1.49		$	0.26
	The following table sets forth the options to purchase shares of common stock and the restricted share units ("RSU award") provided to each outside director of the Company, that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the years ended December 31, 2014, 2013 and 2012:
	(millions)	2014		2013			2012
	Total number of antidilutive options and awards	1.8		2.6			3.7

Reportable_Segments

Reportable Segments	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Reportable Segments	Reportable Segments
	The Company has four reportable segments: personal insurance, business insurance, specialty insurance (the “insurance segments”) and investment operations. The insurance segments are business units managed separately because of the differences in the type of customers they serve or products they provide or services they offer. The personal insurance segment provides primarily personal automobile and homeowners to the personal insurance market. The business insurance segment provides primarily commercial automobile, commercial multi-peril, fire & allied and general liability insurance covering small-to-medium sized commercial exposures in the business insurance market. The specialty insurance segment provides commercial coverages 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 SAP, which include loss and loss adjustment expense ratios, underwriting expense ratios, combined ratios, statutory underwriting gain (loss), net premiums earned and net written premiums. One of the most significant differences between SAP and GAAP is that SAP requires all underwriting expenses to be expensed immediately and not deferred and amortized over the same period the premium is earned.
	The accounting for pension benefits also contributes to the difference between our GAAP loss and expense ratios and our SAP loss and expense ratios. At January 1, 2013, we adopted new SAP pension guidance, which required the recognition of service costs for non-vested participants. In accordance with GAAP, service costs related to non-vested participants was recognized over the two year vesting period ended December 31, 2014.
	The investment operations segment is evaluated based on investment returns of assets managed by Stateco. Asset information by segment is not reported for the insurance segments because the Company does not produce such information internally.
	As of January 1, 2013 our units within the specialty insurance segment changed from RED, Rockhill and Workers’ Compensation to E&S property, E&S casualty, Programs and Workers’ compensation. This change did not have any impact on segment reporting.
	The following table sets forth financial information regarding the Company’s reportable segments for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Revenues from external sources:
	Insurance segments
	Personal insurance	$	451.4		$	464		$	469.8
	Business insurance	381.8			364.2			327.2
	Specialty insurance	240.9			226.8			245.1
	Total insurance segments	1,074.10			1,055.00			1,042.10
	Investment operations segment
	Net investment income	74.7			72.8			75.4
	Net realized capital gains	20.7			23.2			28.8
	Total investment operations segment	95.4			96			104.2
	Total revenue from reportable segments	1,169.50			1,151.00			1,146.30
	All other	3.2			2			3.8
	Total revenues from external sources	1,172.70			1,153.00			1,150.10
	Intersegment revenues	5.1			5.1			9.5
	Total revenues	1,177.80			1,158.10			1,159.60
	Reconciling items:
	Eliminate intersegment revenues	(5.1		)	(5.1		)	(9.5		)
	Total consolidated revenue	$	1,172.70		$	1,153.00		$	1,150.10
	Segment loss before federal income tax:
	Insurance segments:
	Personal insurance SAP underwriting loss	$	(11.7	)	$	(12.6	)	$	(1.6	)
	Business insurance SAP underwriting loss	(8.5		)	(10.2		)	(42.3		)
	Specialty insurance SAP underwriting loss	(85.6		)	(11.2		)	(48.1		)
	Total insurance segments	(105.8		)	(34.0		)	(92.0		)
	Investment operations segment:
	Net investment income	74.7			72.8			75.4
	Net realized capital gains	20.7			23.2			28.8
	Total investment operations segment	95.4			96			104.2
	All other segments income	—			0.9			2
	Reconciling items:
	GAAP adjustments	45.8			11.8			7.2
	Interest expense on corporate debt	(5.4		)	(8.5		)	(7.0		)
	Corporate expenses	(3.2		)	(4.9		)	(3.8		)
	Total reconciling items	$	37.2		$	(1.6	)	$	(3.6	)
	Total consolidated income before federal income taxes	$	26.8		$	61.3		$	10.6
	The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2014 and 2013:
	($ millions)	2014			2013
	Segment assets:
	Investment operations segment	$	2,444.20		$	2,331.60
	Total segment assets	2,444.20			2,331.60
	Reconciling items:
	Corporate assets	322.7			164.8
	Total consolidated assets	$	2,766.90		$	2,496.40
	Assets attributed to the investment operations segment include the total investments and cash and cash equivalent categories from the balance sheet. All other assets are corporate assets and are not assigned to a segment.

Quarterly_Financial_Data_unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	Quarterly Financial Data (unaudited)
	The following tables set forth quarterly financial data for 2014 and 2013:
	($ millions, except per share amounts)	2014
		For three months ended
		March 31		June 30		September 30		December 31
	Total revenues	$	291.3	$	294.4	$	291.3	$	295.7
	Income (loss) before federal income taxes	27.7		3.1		13.1		(17.1		)
	Net income	27.1		3		11.9		65.4
	Earnings per common share:
	Basic	$	0.67	$	0.07	$	0.29	$	1.6
	Diluted	$	0.66	$	0.07	$	0.28	$	1.58
		2013
		For three months ended
		March 31		June 30		September 30		December 31
	Total revenues	$	285.3	$	285.3	$	292.7	$	289.7
	Income before federal income taxes	20		6.3		18.7		16.3
	Net income	19.7		6.2		18.5		16.4
	Earnings per common share:
	Basic	$	0.49	$	0.15	$	0.46	$	0.4
	Diluted	$	0.49	$	0.15	$	0.45	$	0.4

Contingencies

Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies
	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.
	The following describes a pending legal proceeding in which the Company is a party:
	In April 2013, a putative class action lawsuit (Schumacher vs. State Automobile Mutual Insurance Company, et al.) was filed against State Auto Mutual, State Auto Financial and State Auto P&C in Federal District Court in Ohio. Plaintiffs claim that in connection with the homeowners policies of various State Auto companies, the coverage limits and premiums were improperly increased as a result of an insurance to value (“ITV”) program and Plaintiffs allege that they purchased coverage in excess of that which was necessary to insure them in the event of loss. Plaintiffs’ claims include breach of good faith and fair dealing, negligent misrepresentation and fraud, violation of the Ohio Deceptive Trade Practices Act, and fraudulent inducement. Plaintiffs 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 February 13, 2015, the Court stayed the proceedings to allow the parties to engage in settlement discussions.
	The Company is involved in other 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.

SCHEDULE_I_SUMMARY_OF_INVESTME

SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2014
Summary of Investments, Other than Investments in Related Parties [Abstract]
SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	SCHEDULE I – SUMMARY OF INVESTMENTS - OTHER THAN
	INVESTMENTS IN RELATED PARTIES
	December 31, 2014
	($ millions)
	31-Dec-14		Cost or		Fair		Amount at
			amortized		value		which shown
			cost (1)				in the
							balance sheet
	Available-for-sale:
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies		$	296.7	$	309.3	$	309.3
	Obligations of states and political subdivisions		742.5		769.5		769.5
	Corporate securities		333.4		340.6		340.6
	U.S. government agencies residential mortgage-backed securities		458.7		472.5		472.5
	Total fixed maturities		1,831.30		1,891.90		1,891.90
	Equity securities:
	Large-cap securities		185.5		242.2		242.2
	Small-cap securities		50		68.2		68.2
	Total equity securities		235.5		310.4		310.4
	Other invested assets		50.5		80.3		80.3
	Total available-for-sale securities		2,117.30		2,282.60		2,282.60
	Other invested assets		5.3		5.3		5.3
	Total investments – other than investments in related parties		$	2,122.60	$	2,287.90	$	2,287.90
	-1	Original cost of equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts.

SCHEDULE_II_CONDENSED_FINANCIA

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT	12 Months Ended
	Dec. 31, 2014
Schedule Schedule I I Condensed Financial Information Of Registrant Additional Information [Abstract]
SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT	Condensed Balance Sheets
	(in millions, except per share amounts)	31-Dec
		2014			2013
	Assets
	Investments in common stock of subsidiaries (equity method)	$	960.7		$	891.1
	Fixed maturities, available-for-sale, at fair value	0.9			1.6
	Other invested assets	2.8			2.6
	Cash and cash equivalents	11.5			6.1
	Other assets	0.3			0.4
	Due from affiliates	—			1.5
	Federal income tax, net	21.2			4.6
	Total assets	$	997.4		$	907.9
	Liabilities and Stockholders’ Equity
	Notes payable (affiliates $116.8 and $116.8, respectively)	$	116.8		$	116.8
	Due to affiliates	0.5			—
	Other liabilities	7.2			6.1
	Total liabilities	124.5			122.9
	Stockholders’ equity:
	Class A Preferred stock (nonvoting), without par value. Authorized 2.5 shares; none issued	—			—
	Class B Preferred stock, without par value. Authorized 2.5 shares; none issued	—			—
	Common stock, without par value. Authorized 100.0 shares; 47.7 and 47.5 shares issued, respectively, at stated value of $2.50 per share	119.3			118.8
	Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.0		)	(115.9		)
	Additional paid-in capital	143.2			137.5
	Accumulated other comprehensive income	71.7			80.8
	Retained earnings	654.7			563.8
	Total stockholders’ equity	872.9			785
	Total liabilities and stockholders’ equity	$	997.4		$	907.9
	Condensed Statements of Income
	($ millions)	Year ended December 31
		2014			2013			2012
	Net investment income	$	0.3		$	0.3		$	0.2
	Net realized gain on investments	—			0.7			0.4
	Total revenues	0.3			1			0.6
	Interest expense (affiliates $6.0, $3.2 and $0.7, respectively)	6			8.8			7
	Other operating expenses	5.6			6.8			5.8
	Total expenses	11.6			15.6			12.8
	Loss before federal income taxes	(11.3		)	(14.6		)	(12.2		)
	Federal income tax benefit	(20.6		)	(4.8		)	(1.7		)
	Net income (loss) before equity in net income of subsidiaries	9.3			(9.8		)	(10.5		)
	Equity in net income of subsidiaries	98.1			70.6			21.2
	Net income	$	107.4		$	60.8		$	10.7
	Condensed Statements of Comprehensive Income
	($ millions, except per share amounts)	Year ended December 31
		2014			2013			2012
	Net income	$	107.4		$	60.8		$	10.7
	Other comprehensive income, net of tax:
	Net unrealized holding (losses) gains on investments:
	Unrealized holding (losses) gains arising during the year	(0.2		)	0.5			0.7
	Reclassification adjustments for gains realized in net income	—			(0.7		)	(0.4		)
	Income tax benefit	0.1			—			—
	Total net unrealized holding (losses) gains on investments	(0.1		)	(0.2		)	0.3
	Unrealized equity in subsidiaries	(9.0		)	(3.2		)	20.1
	Other comprehensive (loss) income	(9.1		)	(3.4		)	20.4
	Comprehensive income	$	98.3		$	57.4		$	31.1
	Condensed Statements of Cash Flows
	($ millions)	Year ended December 31
		2014			2013			2012
	Cash flows from operating activities:
	Net income	$	107.4		$	60.8		$	10.7
	Adjustments to reconcile net income to net cash used in operating activities:
	Depreciation and amortization, net	—			1.3			(0.3		)
	Share-based compensation	0.9			1.6			0.4
	Net realized gain on investments	—			(0.7		)	(0.4		)
	Equity in net income from consolidated subsidiaries	(98.1		)	(70.6		)	(21.2		)
Changes in operating assets and liabilities:
Other liabilities and due from affiliates	0.5			0.1			1.5
Other assets	—			0.5			0.3
Excess tax benefits on share-based awards	—			0.1			—
Federal income taxes, net	(16.6		)	(3.1		)	(1.2		)
Net cash used in operating activities	(5.9		)	(10.0		)	(10.2		)
Cash flows from investing activities:
Dividends received from consolidated subsidiaries	24			13.8			27
Purchases of equity securities – available-for-sale	—			—			(3.2		)
Purchases of other invested assets	(0.4		)	(0.2		)	(0.3		)
Maturities, calls and pay downs of fixed maturities – available-for-sale	0.5			0.2			2.5
Sales of fixed maturities – available-for-sale	0.3			3.9			0.3
Sales of equity securities – available-for-sale	—			3.7			3.2
Net cash provided by investing activities	24.4			21.4			29.5
Cash flows from financing activities:
Proceeds from issuance of common stock	3.5			4			1.6
Payments to acquire treasury stock	(0.1		)	(0.1		)	—
Payment of dividends	(16.5		)	(16.1		)	(22.3		)
Excess tax benefits on share-based awards	—			(0.1		)	—
Proceeds from long-term debt	—			100			—
Redemption of long-term debt	—			(100.0		)	—
Net cash used in financing activities	(13.1		)	(12.3		)	(20.7		)
Net increase (decrease) in cash and cash equivalents	5.4			(0.9		)	(1.4		)
Cash and cash equivalents at beginning of year	6.1			7			8.4
Cash and cash equivalents at end of year	$	11.5		$	6.1		$	7
Supplemental Disclosures:
Federal income tax received	$	4.3		$	1.5		$	0.4
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)	$	(6.0	)	$	(8.0	)	$	(7.0	)
STFC’s investment in subsidiaries is stated at cost plus equity in net income from consolidated subsidiaries since the date of acquisition. STFC’s share of net income of its unconsolidated subsidiaries is included in consolidated income using the equity method.
During 2012, STFC received dividends from Stateco in the amount of $9.2 million in investments.
These financial statements should be read in conjunction with the consolidated financial statements of State Auto Financial Corporation.
On July 11, 2013, STFC entered into two separate credit agreements with two of its subsidiaries, State Auto P&C and Milbank. Under the terms of the credit agreements, STFC borrowed $85.0 million and $15.0 million, from State Auto P&C and Milbank, respectively. Under the terms of each credit agreement, interest is payable semi-annually at a fixed annual interest rate of 5.28%, with the principal due at the maturity date. There are no prepayment penalties and no collateral was given as security for the payment of either of these loans.
Financial Instruments Disclosed, But Not Carried, At Fair Value
Notes Payable
Included in notes payable are the credit agreements described above with State Auto P&C and Milbank. STFC estimates the fair value of each note payable by obtaining market quotations for U.S. treasury securities with similar maturity dates and applies an appropriate credit spread. These have been placed in Level 3 of the fair value hierarchy.
($ millions, except interest rates)	31-Dec-14									31-Dec-13
	Carrying			Fair			Interest			Carrying	Fair	Interest
	value			Value			rate			value	value	rate
Affiliate note payable with Milbank, issued $15.0, July 2013 with fixed interest	$	15		$	14.8		5.28	%		15	14.4	5.28	%
Affiliate note payable with State Auto P&C, issued $85.0, July 2013 with fixed interest	85			83.9			5.28	%		85	81.4	5.28	%
Total notes payable to affiliates	$	100		$	98.7					100	95.8

SCHEDULE_III_SUPPLEMENTARY_INS

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2014
Supplementary Insurance Information [Abstract]
SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	SCHEDULE III – SUPPLEMENTARY INSURANCE INFORMATION
	Years Ended December 31, 2014, 2013 and 2012
	($ millions)	Deferred			Future			Unearned			Other policy			Premium
		policy			benefits,			premiums			claims and			revenue
		acquisition			claims and						benefits
	Segment	cost			losses(1)						payable
	Year ended December 31, 2014:
	Personal insurance segment	$	48.9		$	201.9		$	270.7		$	—		$	451.4
	Business insurance segment	41.4			349.8			201.9			—			381.8
	Specialty insurance segment	36.2			421.9			139.8			—			240.9
	Investment operations segment	—			—			—			—			—
	Total	$	126.5		$	973.6		$	612.4		$	—		$	1,074.10
	Year ended December 31, 2013:
	Personal insurance segment	$	31.5		$	223.7		$	189.9		$	—		$	464
	Business insurance segment	38.8			359.6			193.1			—			364.2
	Specialty insurance segment	26.5			367.5			108			—			226.8
	Investment operations segment	—			—			—			—			—
	Total	$	96.8		$	950.8		$	491		$	—		$	1,055.00
	Year ended December 31, 2012:
	Personal insurance segment	$	30.1		$	231		$	188.4		$	—		$	469.8
	Business insurance segment	35			341.8			180.9			—			327.2
	Specialty insurance segment	26.6			355.9			112.3			—			245.1
	Investment operations segment	—			—			—			—			—
	Total	$	91.7		$	928.7		$	481.6		$	—		$	1,042.10
	Segment		Net			Benefits,			Amort.			Other			Premiums
			investment			losses and			of deferred			operating			written
			income			settlement			policy			expenses
						expenses(2)			acquisition
									costs
	Year ended December 31, 2014:
	Personal insurance segment		$	—		$	318.3		$	75.4		$	52		$	532.1
	Business insurance segment		—			231.3			81.9			58.1			389.2
	Specialty insurance segment		—			219			64.5			30			272.9
	Investment operations segment		74.7			—			—			—			—
	Total		$	74.7		$	768.6		$	221.8		$	140.1		$	1,194.20
	Year ended December 31, 2013:
	Personal insurance segment		$	—		$	342.2		$	74.7		$	55.5		$	465.4
	Business insurance segment		—			221.8			74.3			73.5			374.8
	Specialty insurance segment		—			158.7			60.6			16.2			221.9
	Investment operations segment		72.8			—			—			—			—
	Total		$	72.8		$	722.7		$	209.6		$	145.2		$	1,062.10
	Year ended December 31, 2012:
	Personal insurance segment		$	—		$	344.8		$	76.6		$	46.6		$	469.5
	Business insurance segment		—			222.5			63.1			80.6			349.4
	Specialty insurance segment		—			211.7			73.4			5.6			236.4
	Investment operations segment		75.4			—			—			—			—
	Total		$	75.4		$	779		$	213.1		$	132.8		$	1,055.30
	-1	Segmented balances are net of reinsurance recoverable on losses and loss expenses payable.
	-2	Benefits, losses and settlement expenses are monitored on a statutory basis.

SCHEDULE_IV_REINSURANCE

SCHEDULE IV - REINSURANCE	12 Months Ended
	Dec. 31, 2014
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
SCHEDULE IV - REINSURANCE	SCHEDULE IV – REINSURANCE
	Years Ended December 31, 2014, 2013 and 2012
	($ million, except percentages)				Ceded to				Assumed from
			Gross		Unaffiliated		Affiliated		Unaffiliated		Affiliated		Net		Percentage
			Amount		Companies		Companies(1)		Companies		Companies(1)		Amount		of  amount
															assumed
															to net (2)
	Property-casualty
	earned premiums for
	year ended December 31,
	2014		$	882.6	$	25.3	$	861.7	$	4.4	$	1,074.10	$	1,074.10	0.4	%
	2013		874.7		23.5		855		3.8		1,055.00		1,055.00		0.4	%
	2012		833.3		28.2		809.2		4.1		1,042.10		1,042.10		0.4	%
	-1	These columns include the effect of intercompany pooling.
	-2	Calculated as earned premiums assumed from outside companies to net amount.

SCHEDULE_V_VALUATION_AND_QUALI

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	SCHEDULE V – VALUATION AND QUALIFYING ACCOUNTS
	Years Ended December 31, 2014, 2013 and 2012
	($ millions)	At December 31
		2014		2013		2012
	Valuation allowance for deferred tax assets:
	Balance at beginning of period	$	82.6	$	100.5	$	103.3
	Deductions	82.6		17.9		2.8
	Balance at end of period	$	—	$	82.6	$	100.5

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation
	The consolidated financial statements include State Auto Financial Corporation (“State Auto Financial”), an Ohio corporation, and the following wholly owned subsidiaries of State Auto Financial:
	•	State Auto Property and Casualty Insurance Company (“State Auto P&C”), an Iowa corporation
	•	Milbank Insurance Company (“Milbank”), an Iowa corporation
	•	State Auto Insurance Company of Ohio (“SA Ohio”), an Ohio corporation
	•	Stateco Financial Services, Inc. (“Stateco”), an Ohio corporation
	The consolidated financial statements also include the operations and financial position of 518 Property Management and Leasing, LLC (“518 PML”), an Ohio limited liability company whose only members are State Auto P&C and Stateco. Farmers Casualty Insurance Company ("Farmers"), a former wholly owned subsidiary of State Auto Financial, was merged with State Auto P&C at the close of business on December 31, 2012.
	State Auto Financial is a majority-owned subsidiary of State Automobile Mutual Insurance Company (“State Auto Mutual”), an Ohio corporation. State Auto Financial and its subsidiaries are referred to herein as the “Company.” All intercompany balances and transactions have been eliminated in consolidation.
Description of Business	Description of Business
	The Company markets its insurance products throughout the United States primarily through independent agencies, which include retail agencies and wholesale brokers. The Company’s principal lines of insurance include personal and commercial automobile, homeowners, commercial multi-peril, workers’ compensation, general liability and fire insurance. State Auto P&C, Milbank and SA Ohio are chartered and licensed property and casualty insurers. As such, they are subject to the regulations of the applicable Departments of Insurance of their respective states of domicile (the “Departments”) and the regulations of each state in which they operate. These property and casualty insurance companies undergo periodic financial examination by the Departments and insurance regulatory agencies of the states that choose to participate. A large portion of the Company’s revenues are derived from a reinsurance pooling agreement with State Auto Mutual and its affiliates. The underwriting activity and geographic distribution of State Auto Mutual and its affiliates is generally the same as the underwriting activity and geographic distribution of the Company.
	Through the employees of State Auto P&C, the Company provides management and operation services under management agreements for all of its insurance and non-insurance affiliates.
	Through Stateco, the Company provides investment management services to affiliated companies.
	518 PML owns and leases property to the Company’s affiliates.
Basis of Presentation	Basis of Presentation
	The consolidated financial statements have been prepared in conformity with U.S. generally accepted accounting principles (“GAAP”), which vary in certain respects from statutory accounting practices (“SAP”) followed by State Auto P&C, Milbank and SA Ohio that are prescribed or permitted by the Departments.
	The Company's insurance subsidiaries, domiciled in Ohio and Iowa, are required to prepare statutory financial statements in accordance with the accounting practices prescribed or permitted by the insurance departments of the states of domicile. Prescribed statutory accounting practices are those practices that are incorporated directly or by reference in state laws, regulations, and general administrative rules applicable to all insurance enterprises domiciled in a particular state. The Ohio and Iowa Departments of Insurance require insurers domiciled in their respective states to prepare statutory financial statements in accordance with National Association of Insurance Commissioners' ("NAIC") statutory accounting practices. Permitted statutory accounting practices are those practices that differ either from state-prescribed statutory accounting practices or NAIC statutory accounting practices. The Company's insurance subsidiaries do not apply any statutory accounting practices that would be considered a prescribed statutory accounting practice that differs from NAIC statutory accounting practices.
	In preparing the consolidated financial statements, management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities as of the date of the balance sheet, revenues and expenses for the periods then ended, and the accompanying notes to the financial statements. Such estimates and assumptions could change in the future as more information becomes known which could impact the amounts reported and disclosed herein.
	Material estimates that are particularly susceptible to significant change in the near term relate to the determination of losses and loss expenses payable and the realization of deferred tax assets. In connection with the determination of losses and loss expenses payable, management uses historical data, current business conditions and assumptions about future conditions to formulate estimates of the ultimate cost to settle claims. Deferred tax assets are evaluated periodically by management to determine if they are realizable, requiring management to make certain judgments and assumptions. In evaluating the ability to recover deferred tax assets, management considers all available evidence, including loss carryback potential, past operating results, existence of cumulative losses in the most recent years, projected performance of the business, future taxable income, including the ability to generate capital gains, and prudent and feasible tax planning strategies. If, based on available information, it is more likely than not that the deferred income tax assets will not be realized, then a valuation allowance must be established with a corresponding charge to net income and/or other comprehensive income (loss). These estimates by their nature are subject to uncertainties for various reasons.
Investments	Investments
	Investments in fixed maturities, equity securities and certain other invested assets are classified as available-for-sale and are carried at fair value. The unrealized holding gains and losses, net of applicable deferred income taxes, are shown as a separate component of stockholders’ equity as a part of accumulated other comprehensive income (loss) and, as such, are not included in the determination of net income. Realized gains and losses on the sales of investments are computed using the first-in, first-out method.
	The Company views gross unrealized losses on fixed maturities and equity securities as being temporary since it is its assessment that these securities will recover in the near term, allowing the Company to realize the anticipated long-term economic value. The Company 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 security 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 other comprehensive income. Future increases or decreases in fair value, if not other-than-temporary, are included in 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 other comprehensive income.
Cash Equivalents	Cash Equivalents
	The Company considers all liquid debt instruments with a maturity of three months or less to be cash equivalents. The carrying amounts reported approximate their fair value.
Deferred Acquisition Costs	Deferred Acquisition Costs
	Acquisition costs, consisting of net commissions (including ceding commissions), premium taxes and certain underwriting expenses related to the successful acquisition or renewal of property and casualty business, are deferred and amortized over the same period in which the related premiums are earned. Ceding commissions relating to reinsurance agreements reimburse us for both deferrable and non-deferrable acquisition costs. To the extent these ceding commissions exceed the deferrable amount of acquisition costs, the excess is reported as a deferred liability and is included in other liabilities in our consolidated balance sheet. Excess ceding commissions are amortized in proportion to net revenue recognized on the underlying policies resulting in excess ceding commissions being recognized as a reduction of acquisition and operating expenses.
	The method followed for computing the acquisition costs limits the amount of such deferred costs to their estimated realizable value. In determining estimated realizable value, the computation gives effect to the premium to be earned, losses and loss expenses expected to be incurred, and certain other costs expected to be incurred as premium is earned. Future changes in estimates, the most significant of which is expected losses and loss adjustment expenses, that indicate a reduction in expected future profitability may result in unrecoverable deferred acquisition costs. Anticipated investment income is considered in determining whether a premium deficiency exists.
	The following table sets forth net deferred acquisition costs for the years ended December 31, 2014, 2013 and 2012:
	($ millions)		2014			2013			2012
	Balance, beginning of year		$	96.8		$	91.7		$	91.7
	Acquisition costs deferred		251.5			214.6			213.1
	Acquisition costs amortized to expense		(221.8		)	(209.5		)	(213.1		)
	Balance, end of year		$	126.5		$	96.8		$	91.7
Federal Income Taxes	Federal Income Taxes
	The Company files a consolidated federal income tax return. Pursuant to a written tax sharing agreement, each entity within the consolidated group pays or receives its share of federal income taxes based on separate return calculations.
	The Company recognizes deferred income tax assets and liabilities for the expected future tax effects attributable to temporary differences between the financial statement and tax return bases of assets and liabilities, based on enacted tax rates and other provisions of the tax law. The effect of a change in tax laws or rates on deferred tax assets and liabilities is recognized in income in the period in which such change is enacted. Deferred tax assets and liabilities include provisions for unrealized investment gains and losses as well as the net funded status of pension and other postretirement benefit obligations with the changes for each period included in the respective components of other comprehensive income. Deferred tax assets are reduced by a valuation allowance if it is more likely than not that all or some portion of the deferred tax assets will not be realized.
	Interest and penalties related to uncertain tax positions are recorded in the balance sheet as other liabilities, and recognized in the income statement as other expenses.
Losses and Loss Expenses Payable	Losses and Loss Expenses Payable
	Losses and loss expenses payable are based on formula and case-basis estimates for reported claims and on estimates, based on experience and perceived trends, for unreported claims and loss expenses. The liability for unpaid losses and loss expenses, net of estimated salvage and subrogation recoverable of $23.8 million and $24.7 million at December 31, 2014 and 2013, respectively, has been established to cover the estimated ultimate cost to settle insured losses. The amounts are based on estimates of future rates of inflation and other factors, and accordingly, there can be no assurance that the ultimate liability will not vary materially from such estimates. The estimates are continually reviewed and adjusted as necessary; such adjustments are included in current operations (see Note 4). Anticipated salvage and subrogation is estimated using historical experience. As such, losses and loss expenses payable represent management’s best estimate of the ultimate liability related to reported and unreported claims.
Premiums	Premiums
	Premiums are recognized as earned prorata over the policy period. Unearned premiums represent the portion of premiums written relative to the unexpired terms of coverage.
Comprehensive Income (Loss)	Comprehensive Income
	Comprehensive income is defined as all changes in an enterprise’s equity during a period other than those resulting from investments by owners and distributions to owners. Comprehensive income includes net income and other comprehensive income. Other comprehensive income includes all other non-owner related changes to equity and includes net unrealized gains and losses on available-for-sale investments, derivative instruments and unrecognized benefit plan obligations, adjusted for deferred federal income taxes.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Schedule of Net Deferred Acquisition Costs	The following table sets forth net deferred acquisition costs for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Balance, beginning of year	$	96.8		$	91.7		$	91.7
	Acquisition costs deferred	251.5			214.6			213.1
	Acquisition costs amortized to expense	(221.8		)	(209.5		)	(213.1		)
	Balance, end of year	$	126.5		$	96.8		$	91.7

Investments_Tables

Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Investments [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 December 31, 2014 and 2013:
	31-Dec-14		Cost or			Gross			Gross				Fair
			amortized			unrealized			unrealized				value
			cost			holding			holding
						gains			losses
	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 residential 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
	31-Dec-13		Cost or			Gross			Gross				Fair
			amortized			unrealized			unrealized				value
			cost			holding			holding
						gains			losses
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies		$	345.5		$	13.4		$	(6.5	)		$	352.4
	Obligations of states and political subdivisions		765.3			25.8			(16.9		)		774.2
	Corporate securities		345			11.4			(6.7		)		349.7
	U.S. government agencies residential mortgage-backed securities		348.2			9.7			(4.1		)		353.8
	Total fixed maturities		1,804.00			60.3			(34.2		)		1,830.10
	Equity securities:
	Large-cap securities		148.2			46.5			(0.3		)		194.4
	Small-cap securities		48.4			22.5			—				70.9
	Total equity securities		196.6			69			(0.3		)		265.3
	Other invested assets		49.5			31.4			—				80.9
	Total available-for-sale securities		$	2,050.10		$	160.7		$	(34.5	)		$	2,176.30
Gross Unrealized Losses and Fair Value on its Investments	The following tables set forth the Company’s gross unrealized losses and fair value on its investments by lot, aggregated by investment category and length of time for individual securities that have been in a continuous unrealized loss position at December 31, 2014 and 2013:
		Less than 12 months									12 months or more								Total
	31-Dec-14	Fair			Unrealized			Number			Fair				Unrealized			Number	Fair		Unrealized			Number
		value			losses			of			value				losses			of	value		losses			of
								positions										positions						positions
	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 residential 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
		Less than 12 months									12 months or more								Total
	31-Dec-13	Fair			Unrealized			Number			Fair				Unrealized			Number	Fair		Unrealized			Number
		value			losses			of			value				losses			of	value		losses			of
								positions										positions						positions
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	109.6		$	(6.5	)	29			$	—			$	—		—	$	109.6	$	(6.5	)	29
	Obligations of states and political subdivisions	206.4			(14.7		)	76			25.6				(2.2		)	7	232		(16.9		)	83
	Corporate securities	105.6			(3.2		)	22			40.9				(3.5		)	8	146.5		(6.7		)	30
	U.S. government agencies residential mortgage-backed securities	103.6			(3.3		)	25			19.3				(0.8		)	10	122.9		(4.1		)	35
	Total fixed maturities	525.2			(27.7		)	152			85.8				(6.5		)	25	611		(34.2		)	177
	Large-cap equity securities	5.3			(0.3		)	2			—				—			—	5.3		(0.3		)	2
	Total temporarily impaired securities	$	530.5		$	(28.0	)	154			$	85.8			$	(6.5	)	25	$	616.3	$	(34.5	)	179
Realized Losses Related to Other-Than-Temporary Impairments	The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Equity securities:
	Large-cap securities	$	(0.3	)	$	(1.8	)	$	—
	Small-cap securities	(2.2		)	(2.2		)	(3.2		)
	Fixed maturities	—			—			(0.2		)
	Total other-than-temporary impairments	$	(2.5	)	$	(4.0	)	$	(3.4	)
Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity	The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at December 31, 2014:
	($ millions)	Amortized			Fair
		cost			value
	Due in 1 year or less	$	55.7		$	56.5
	Due after 1 year through 5 years	369.8			382.4
	Due after 5 years through 10 years	299.9			312.4
	Due after 10 years	647.2			668.1
	U.S. government agencies residential mortgage-backed securities	458.7			472.5
	Total	$	1,831.30		$	1,891.90
Components of Net Investment Income	The following table sets forth the components of net investment income for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Fixed maturities	$	64.3		$	63.2		$	66.9
	Equity securities	6.2			6			4.9
	Cash and cash equivalents, and other	6.2			5.7			5.6
	Investment income	76.7			74.9			77.4
	Investment expenses	2			2.1			2
	Net investment income	$	74.7		$	72.8		$	75.4
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 years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Realized gains:
	Fixed maturities	$	3.1		$	2.5		$	15.7
	Equity securities	21.3			26.1			19
	Other invested assets	0.1			0.1			0.1
	Total realized gains	24.5			28.7			34.8
	Realized losses:
	Equity securities:
	Sales	(1.3		)	(1.2		)	(2.6		)
	OTTI	(2.5		)	(4.0		)	(3.2		)
	Fixed maturities:
	Sales	—			(0.3		)	—
	OTTI	—			—			(0.2		)
	Total realized losses	(3.8		)	(5.5		)	(6.0		)
	Net realized gains on investments	$	20.7		$	23.2		$	28.8
	Change in unrealized holding gains (losses), net of tax:
	Fixed maturities	$	34.5		$	(102.8	)	$	11.3
	Equity securities	6.2			36.5			6.6
	Other invested assets	(1.6		)	16			6.8
	Deferred federal income tax	(13.7		)	17.6			(8.6		)
	Valuation allowance	—			(6.7		)	9.2
	Change in unrealized holding gains (losses), net of tax	$	25.4		$	(39.4	)	$	25.3

Fair_Value_of_Financial_Instru1

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Company's Available-for-Sale Investments within Fair Value Hierarchy	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at December 31, 2014 and 2013:
	($ millions)	Total		Quoted prices		Significant			Significant
				in active		other			unobservable
				markets for		observable			inputs
				identical		inputs			(Level 3)
				assets		(Level 2)
	31-Dec-14			(Level 1)
	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 residential 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
	($ millions)	Total		Quoted prices		Significant			Significant
				in active		other			unobservable
				markets for		observable			inputs
				identical		inputs			(Level 3)
				assets		(Level 2)
	31-Dec-13			(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	352.4	$	—	$	352.4		$	—
	Obligations of states and political subdivisions	774.2		—		774.2			—
	Corporate securities	349.7		—		340.8			8.9
	U.S. government agencies residential mortgage-backed securities	353.8		—		353.8			—
	Total fixed maturities	1,830.10		—		1,821.20			8.9
	Equity securities:
	Large-cap securities	194.4		194.4		—			—
	Small-cap securities	70.9		70.9		—			—
	Total equity securities	265.3		265.3		—			—
	Other invested assets	80.9		6.7		74.2			—
	Total available-for-sale investments	$	2,176.30	$	272	$	1,895.40		$	8.9
Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for 2014 and 2013, separately for each major category of assets:
	($ millions)	Fixed
		maturities
	Balance at January 1, 2014	$	8.9
	Total realized gains (losses)—included in earnings	—
	Total unrealized gains (losses)—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
	($ millions)	Fixed
		maturities
	Balance at January 1, 2013	$	8.5
	Total realized gains (losses)—included in earnings	—
	Total unrealized gains (losses)—included in other comprehensive income	0.2
	Purchases	0.2
	Sales	—
	Transfers into Level 3	—
	Transfers out of Level 3	—
	Balance at December 31, 2013	$	8.9
Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities	The Company estimates the fair value of the notes receivable from affiliates using market quotations for U.S. treasury securities with similar maturity dates and applies an appropriate credit spread. This has been placed in Level 2 of the fair value hierarchy.
	($ millions, except interest rates)	31-Dec-14						31-Dec-13
		Carrying		Fair		Interest		Carrying			Fair		Interest
		value		value		rate		value			value		rate
	Notes receivable from affiliate	$	70	$	74.6	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-Dec-14						31-Dec-13
		Carrying		Fair		Interest		Carrying			Fair		Interest
		value		Value		rate		value			value		rate
	FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest	$	85.3	$	86.4	5.03	%	$	85.3		$	85.7	5.03	%
	Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5		15.5		4.44	%	15.5			15.5		4.44	%
	Total notes payable	$	100.8	$	101.9			$	100.8		$	101.2
Company's Assets and Liabilities within 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-Dec-14						31-Dec-13
		Carrying		Fair		Interest		Carrying			Fair		Interest
		value		Value		rate		value			value		rate
	FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest	$	85.3	$	86.4	5.03	%	$	85.3		$	85.7	5.03	%
	Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest	15.5		15.5		4.44	%	15.5			15.5		4.44	%
	Total notes payable	$	100.8	$	101.9			$	100.8		$	101.2

Losses_and_Loss_Expenses_Payab1

Losses and Loss Expenses Payable (Tables)	12 Months Ended
	Dec. 31, 2014
Insurance [Abstract]
Schedule of Activity in Liability for Losses and Loss Expenses	The following table sets forth the activity in the liability for losses and loss expenses for the years ended December 31:
	($ millions)	2014		2013			2012
	Losses and loss expenses payable, at beginning of year	$	959.9	$	942.2		$	907.1
	Less: reinsurance recoverable on losses and loss expenses payable	9.1		13.5			25.5
	Net balance at beginning of year	950.8		928.7			881.6
	Incurred related to:
	Current year	726.2		741			795.2
	Prior years	45.1		(21.2		)	(16.9		)
	Total incurred	771.3		719.8			778.3
	Paid related to:
	Current year	373.2		355			397.2
	Prior years	375.3		342.7			334
	Total paid	748.5		697.7			731.2
	Net balance at end of year	973.6		950.8			928.7
	Plus: reinsurance recoverable on losses and loss expenses payable	9.6		9.1			13.5
	Losses and loss expenses payable, at end of year (affiliates $494.3, $438.0 and $435.1, respectively)	$	983.2	$	959.9		$	942.2

Reinsurance_Tables

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2014
Reinsurance Disclosures [Abstract]
External Reinsurance on Balance Sheets	The following table sets forth the effect of the Company’s external reinsurance on its balance sheets at December 31, 2014 and 2013, prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a:
	($ millions)	2014			2013
	Losses and loss expenses payable:
	Direct	$	484.4		$	516.9
	Assumed	4.5			5
	Ceded	(9.6		)	(9.1		)
	Net losses and loss expenses payable	$	479.3		$	512.8
	Unearned premiums:
	Direct	$	409.7		$	411.6
	Assumed	1			1
	Ceded	(6.1		)	(4.7		)
	Net unearned premiums	$	404.6		$	407.9
External Reinsurance on Income Statements	The following table sets forth the effect of the Company’s external reinsurance on its income statements for the years ended December 31, 2014, 2013 and 2012, prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a:
	($ millions)	2014			2013			2012
	Written premiums:
	Direct	$	880.6		$	886.7		$	860.1
	Assumed	4.4			3.9			4
	Ceded	(26.8		)	(24.3		)	(24.3		)
	Net written premiums	$	858.2		$	866.3		$	839.8
	Earned premiums:
	Direct	$	882.6		$	874.7		$	833.3
	Assumed	4.4			3.8			4.1
	Ceded	(25.3		)	(23.5		)	(28.2		)
	Net earned premiums	$	861.7		$	855		$	809.2
	Losses and loss expenses incurred:
	Direct	$	525.6		$	559.1		$	577.4
	Assumed	2.9			3			3.6
	Ceded	(8.0		)	(4.8		)	(6.4		)
	Net losses and loss expenses incurred	$	520.5		$	557.3		$	574.6

Transactions_with_Affiliates_T

Transactions with Affiliates (Tables)	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Schedule of Reinsurance Transactions on Balance Sheets for Pooling Arrangement	The following table sets forth the reinsurance transactions on the Company’s balance sheets for the Pooling Arrangement between the STFC Pooled Companies and State Auto Mutual at December 31, 2014 and 2013:
	($ millions)	2014				2013
	Assets
	Deferred policy acquisition costs:
	Ceded	$	(79.7	)		$	(77.6	)
	Assumed	126.5				96.8
	Net assumed	$	46.8			$	19.2
	Liabilities and Stockholders' Equity
	Losses and loss expenses payable:
	Ceded	$	(479.3	)		$	(512.8	)
	Assumed	973.6				950.8
	Net assumed	$	494.3			$	438
	Unearned premiums:
	Ceded	$	(404.6	)		$	(407.9	)
	Assumed	606.3				486.3
	Net assumed	$	201.7			$	78.4
	Pension and postretirement benefits:
	Ceded	$	(180.5	)		$	(106.5	)
	Assumed	117.3				69.2
	Net ceded	$	(63.2	)		$	(37.3	)
	Other liabilities:
	Ceded	$	(70.7	)		$	(47.2	)
	Assumed	65.6				67.2
	Net ceded	$	(5.1	)		$	20
	Stockholders' Equity
	Accumulated other comprehensive income:
	Ceded	$	(186.0	)		$	(117.1	)
	Assumed	120.9				76.1
	Net ceded	$	(65.1	)		$	(41.0	)
Schedule of Reinsurance Transactions on Income Statements for Pooling Arrangement	The following table sets forth the reinsurance transactions on the Company’s income statements for the Pooling Arrangement between the STFC Pooled Companies and State Auto Mutual for the years ended December 31, 2014, 2013 and 2012:
	($ millions)		2014				2013			2012
	Written premiums:
	Ceded		$	(858.2	)		$	(866.3	)	$	(839.9	)
	Assumed		1,194.20				1,062.10			1,055.30
	Net assumed		$	336			$	195.8		$	215.4
	Earned premiums:
	Ceded		$	(861.7	)		$	(855.0	)	$	(809.2	)
	Assumed		1,074.10				1,055.00			1,042.10
	Net assumed		$	212.4			$	200		$	232.9
	Losses and loss expenses incurred:
	Ceded		$	(523.6	)		$	(560.2	)	$	(575.7	)
	Assumed		774.4				722.7			779.4
	Net assumed		$	250.8			$	162.5		$	203.7
	Acquisition and operating expenses:
	Ceded		(215.4		)		(192.6		)	(161.8		)
	Assumed		372.3				365.3			352.6
	Net assumed		$	156.9			$	172.7		$	190.8

Federal_Income_Taxes_Tables

Federal Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate	The following table sets forth the reconciliation between actual federal income tax (benefit) expense and the amount computed at the indicated statutory rate for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014					2013					2012
	Amount at statutory rate	$	9.4		35	%	$	21.5		35	%	$	3.7		35	%
	Tax-exempt interest and dividends received deduction	(8.5		)	(32.0	)	(9.4		)	(15.0	)	(9.1		)	(85.0	)
	Other, net	1.1			4		0.2			—		0.5			4
	Valuation allowance	(82.6		)	(308.0	)	(11.8		)	(19.0	)	4.8			45
	Federal income tax (benefit) expense and effective rate	$	(80.6	)	(301.0	)%	$	0.5		1	%	$	(0.1	)	(1.0	)%
Tax Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities	The following table sets forth the tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities at December 31, 2014 and 2013:
	($ millions)	2014			2013
	Deferred tax assets:
	Unearned premiums not currently deductible	$	42.2		$	33.8
	Losses and loss expenses payable discounting	21			21.6
	Postretirement and pension benefits	41.1			24.9
	Realized loss on other-than-temporary impairment	8.4			7.5
	Other liabilities	22.2			17.3
	Net operating loss carryforward	58			56.5
	Tax credit carryforward	1.9			1.7
	Other	4.7			9.2
	Total deferred tax assets	199.5			172.5
	Deferred tax liabilities:
	Deferral of policy acquisition costs	44.3			33.9
	Net unrealized holding gains on investments	57.8			44.1
	Total deferred tax liabilities	102.1			78
	Total net deferred tax assets before valuation allowance	97.4			94.5
	Less valuation allowance	—			82.6
	Net deferred federal income taxes	$	97.4		$	11.9

Pension_and_Postretirement_Ben1

Pension and Postretirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Summary of Pension and Postretirement Benefit Plans' Change in Benefit Obligation, Plan Assets and Funded Status	The following table sets forth information regarding the pension and postretirement benefit plans’ change in benefit obligation, plan assets and funded status at December 31, 2014 and 2013:
	($ millions)	Pension								Postretirement
		2014				2013				2014				2013
	Change in benefit obligation:
	Benefit obligation at beginning of year	$	230.5			$	246.1			$	22.3			$	25.1
	Service cost	4.6				6.1				—				—
	Interest cost	10.8				9.6				1.1				1
	Actuarial loss (gain)	57.3				(19.6		)		1.7				(2.6		)
	Benefits paid	(10.7		)		(11.7		)		(1.2		)		(1.2		)
	The Company’s portion of benefit obligation at end of year	$	292.5			$	230.5			$	23.9			$	22.3
	Change in plan assets available for plan benefits:
	Fair value of plan assets available for plan benefits at beginning of year	$	185.9			$	162.2			$	1.3			$	1.8
	Employer contribution	13				13				—				—
	Actual return on plan assets	16.9				22.4				—				0.3
	Benefits paid	(10.7		)		(11.7		)		(1.0		)		(0.8		)
	The Company’s portion of fair value of plan assets at end of year	$	205.1			$	185.9			$	0.3			$	1.3
	Supplemental executive retirement plan	(6.3		)		(6.0		)		—				—
	Funded status at end of year	$	(93.7	)		$	(50.6	)		$	(23.6	)		$	(21.0	)
	Accumulated benefit obligation end of year	$	260.1			$	211.2
Company's Share of Amounts Included in Accumulated Other Comprehensive Income (Loss)	The following table sets forth the Company’s share of the amounts included in accumulated other comprehensive income (loss) that have not been recognized in net periodic cost at December 31, 2014 and 2013:
	($ millions)	2014				2013
	Prior service benefit	$	(64.9	)		$	(70.3	)
	Net actuarial loss	141.9				94.3
	Total	$	77			$	24
Company's Share of Amortization Expected to be Recognized	The following table sets forth the Company’s share of amortization expected to be recognized for the year ending December 31, 2015:
	($ millions)	2015
	Prior service benefit	$	(5.4	)
	Net actuarial loss	11.5
	Total	$	6.1
Company's Share of Net Periodic Cost Components	The following table sets forth information regarding the Company’s share of pension and postretirement benefit plans’ components of net periodic cost for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	Pension												Postretirement
		2014				2013				2012				2014				2013			2012
	Components of net periodic cost:
	Service cost	$	5.2			$	6.1			$	7.8			$	—			$	0.4		$	—
	Interest cost	11.1				9.6				9.9				1.1				1.2			1.1
	Expected return on plan assets	(12.6		)		(12.2		)		(11.7		)		—				(0.2		)	(0.1		)
	Amortization of:
	Prior service cost (benefit)	—				—				0.3				(5.5		)		(5.5		)	(5.5		)
	Net actuarial loss	6.3				8.1				6.7				0.6				1			1.1
	Net periodic cost (benefit)	$	10			$	11.6			$	13			$	(3.8	)		$	(3.1	)	$	(3.4	)
Company's Share of Expected Future Benefits Payments Schedule	The following table sets forth the Company’s share of the benefit payments, which reflect expected future service, expected to be paid:
	($ millions)	Pension				Postretirement
	2015	$	9.6			$	1.7
	2016	10				1.7
	2017	10.5				1.7
	2018	11.2				1.6
	2019	11.8				1.6
	2020-2024	72				7.3
Weighted Average Assumptions Used to Determine Benefit Plans' Obligations	The following table sets forth the weighted average assumptions used to determine the benefit plans’ obligations at December 31, 2014 and 2013:
		Pension						Postretirement
		2014			2013			2014			2013
	Benefit obligations weighted-average assumptions:
	Discount rate	3.85	%		4.85	%		3.85	%		4.85	%
	Rates of increase in compensation levels	3.5			3.5			—			—
Weighted Average Assumptions Used to Determine Benefit Plans' Net Periodic Cost	The following table sets forth the weighted average assumptions used to determine the benefit plans’ net periodic cost for the years ended December 31, 2014, 2013 and 2012:
				Pension									Postretirement
				2014			2013			2012			2014			2013			2012
	Weighted-average assumptions:
	Discount rate			4.85	%		4.05	%		4.4	%		4.85	%		4.05	%		4.4	%
	Expected long-term rate of return on assets			7			7.5			7.5			—			7.5			7.5
	Rates of increase in compensation levels			3.5			4			4			—			—			—
Schedule of Assumed Health Care Cost Trend Rates Used	The following table sets forth the assumed health care cost trend rates used for the years ended December 31, 2014, 2013 and 2012:
		Postretirement
		2014			2013			2012
	Assumed health care cost trend rates:
	Health care cost trend rate assumed for the next year	6	%		10	%		10	%
	Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	3.8	%		5	%		5	%
	Year that the rate reaches the ultimate trend rate	2075			2018			2017
Summary on Effective Impact of Assumed Health Care Cost Trend Rates on Amounts Reported for Postretirement Plan	The following table sets forth the effects of a one percentage point change in assumed health care cost trend rates for the year ended December 31, 2014:
	($ millions)	Postretirement
		Increase				(Decrease)
	One percentage point change:
	Effect on total service and interest cost	$	0.1			$	0.1
	Effect on accumulated postretirement benefit obligation	3.8				3.3
Summary of Asset Allocation Targets	The following table sets forth the asset allocation targets, as a percentage of total fair value, which are used as a guide by management when allocating funds as they become available.
		Asset
		allocation
		target
		(0 to 100%)
	Asset Category:
	Fixed maturity	49	%
	U.S. large-cap equity	26
	U.S. small-cap equity	11
	International equity	10
	Emerging market equity	4
	Total	100	%
Company's Share of Pension Plan's Available-for-Sale Securities	The following tables set forth the Company’s share of pension plan’s available-for-sale securities within the fair value hierarchy at December 31, 2014 and 2013:
	($ millions)	Total				Quoted prices				Significant				Significant
						in active				other				unobservable
						markets for				observable				inputs
						identical				inputs				(Level 3)
						assets				(Level 2)
	31-Dec-14					(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	56.4			$	—			$	56.4			$	—
	Corporate securities	39.7				—				39.7				—
	U.S. government agencies mortgage-backed securities	9.4				—				9.4				—
	Total fixed maturities	105.5				—				105.5				—
	Equity securities:
	Large-cap securities	53.7				53.7				—				—
	Small-cap securities	21.8				21.8				—				—
	Total equity securities	75.5				75.5				—				—
	International instruments	18.7				—				18.7				—
	Total pension plan investments	$	199.7			$	75.5			$	124.2			$	—
	($ millions)	Total				Quoted prices				Significant				Significant
						in active				other				unobservable
						markets for				observable				inputs
						identical				inputs				(Level 3)
						assets				(Level 2)
	31-Dec-13					(Level 1)
	Fixed maturities:
	U.S. treasury securities and obligations of U.S. government agencies	$	30			$	—			$	30			$	—
	Corporate securities	24.6				—				24.6				—
	U.S. government agencies mortgage-backed securities	11.3				—				11.3				—
	Total fixed maturities	65.9				—				65.9				—
	Equity securities:
	Large-cap securities	58				58				—				—
	Small-cap securities	26.3				26.3				—				—
	Total equity securities	84.3				84.3				—				—
	International instruments	25.4				—				25.4				—
	Total pension plan investments	$	175.6			$	84.3			$	91.3			$	—
	The following tables set forth the Company’s share of the postretirement plan’s available-for-sale securities within the fair value hierarchy at December 31, 2014 and 2013:
	($ millions)	Total				Quoted prices				Significant				Significant
						in active				other				unobservable
						markets for				observable				inputs
						identical				inputs				(Level 3)
						assets				(Level 2)
	31-Dec-14					(Level 1)
	Fixed maturities:
	Corporate securities	$	0.1			$	—			$	0.1			$	—
	Total fixed maturities	0.1				—				0.1				—
	Short-term money market funds	0.2				0.2				—				—
	Total postretirement plan investments	$	0.3			$	0.2			$	0.1			$	—
		Total				Quoted prices				Significant				Significant
						in active				other				unobservable
						markets for				observable				inputs
						identical				inputs				(Level 3)
						assets				(Level 2)
	31-Dec-13					(Level 1)
	Fixed maturities:
	Corporate securities	$	0.2			$	—			$	0.2			$	—
	Total fixed maturities	0.2				—				0.2				—
	Equity securities:
	Large-cap securities	0.8				0.8				—				—
	Total equity securities	0.8				0.8				—				—
	Total postretirement plan investments	$	1			$	0.8			$	0.2			$	—

Other_Comprehensive_Income_and1

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	The following tables set forth the changes in the Company’s accumulated other comprehensive income component (AOCI), net of tax, for the years ended December 31, 2014, 2013 and 2012:
	($ millions)		Unrealized Gains and Losses on Available-for-Sale Securities				Gains and Losses on Cash Flow Hedges				Benefit Plan Items				Total
	Beginning balance at January 1, 2014		$	84.6			$	—			$	(3.8	)		$	80.8
	Other comprehensive income before reclassifications		38.9				—				(54.4		)		(15.5		)
	Amounts reclassified from AOCI (a)		(13.5		)		—				19.9				6.4
	Net current period other comprehensive income		25.4				—				(34.5		)		(9.1		)
	Ending balance at December 31, 2014		$	110			$	—			$	(38.3	)		$	71.7
	Beginning balance at January 1, 2013		$	124			$	0.1			$	(39.9	)		$	84.2
	Other comprehensive income before reclassifications		(16.1		)		—				32.5				16.4
	Amounts reclassified from AOCI (a)		(23.3		)		(0.1		)		3.6				(19.8		)
	Net current period other comprehensive income		(39.4		)		(0.1		)		36.1				(3.4		)
	Ending balance at December 31, 2013		$	84.6			$	—			$	(3.8	)		$	80.8
	Beginning balance at January 1, 2012		$	98.7			$	0.2			$	(35.1	)		$	63.8
	Other comprehensive income before reclassifications		54.1				—				(7.4		)		46.7
	Amounts reclassified from AOCI (a)		(28.8		)		(0.1		)		2.6				(26.3		)
	Net current period other comprehensive income		25.3				(0.1		)		(4.8		)		20.4
	Ending balance at December 31, 2012		$	124			$	0.1			$	(39.9	)		$	84.2
	(a)	See separate table below for details about these reclassifications
Reclassification out of Accumulated Other Comprehensive Income	The following tables set forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s consolidated statement of income for the years ended December 31, 2014, 2013 and 2012:
	($ millions)
	Details about Accumulated Other			31-Dec												Affected line item in the Condensed
	Comprehensive Income Components			2014				2013				2012				Consolidated Statements of Income
	Unrealized gains on available for sale securities			$	20.7			$	23.2			$	28.8			Realized gain on sale of securities
				20.7				23.2				28.8				Total before tax
				(7.2		)		0.1				—				Tax (expense) benefit
				13.5				23.3				28.8				Net of tax
	Amortization of gain on derivative used in cash flow hedge			—				0.1				0.1				Realized gain on sale of securities
				—				0.1				0.1				Total before tax
				—				—				—				Tax benefit (expense)
				—				0.1				0.1				Net of tax
	Amortization of benefit plan items:
	Negative prior service costs			5.5				5.5				5.2				(a)
	Net loss			(6.9		)		(9.1		)		(7.8		)		(a)
				(1.4		)		(3.6		)		(2.6		)		Total before tax
				(18.5		)		—				—				Tax expense
				(19.9		)		(3.6		)		(2.6		)		Net of tax
	Total reclassifications for the period			$	(6.4	)		$	19.8			$	26.3
	(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).

Stockholders_Equity_Tables

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Reconciliations of Statutory Capital and Surplus and Net Income	The following tables set forth reconciliations of statutory capital and surplus and net income, as determined using SAP, to the amounts included in the accompanying consolidated financial statements:
	($ millions)	2014			2013
	Statutory capital and surplus of insurance subsidiaries	$	778.4		$	753.2
	Net liabilities of non-insurance parent and affiliates	(80.8		)	(82.4		)
		697.6			670.8
	Increases (decreases):
	Deferred acquisition costs	126.5			96.8
	Postretirement and pension benefits	27.2			6.3
	Deferred federal income taxes	(49.7		)	(15.8		)
	Fixed maturities, at fair value	60.6			26.1
	Other, net	10.7			0.8
	Stockholders’ equity per accompanying consolidated financial statements	$	872.9		$	785
	($ millions)	Year ended December 31
		2014			2013			2012
	Statutory net (loss) income of insurance subsidiaries	$	(17.5	)	$	51.1		$	5.5
	Net (loss) income of non-insurance parent and affiliates	(5.3		)	(5.1		)	0.2
		(22.8		)	46			5.7
	Increases (decreases):
	Deferred acquisition costs	29.7			5.1			—
	Postretirement and pension benefits	10.6			10.3			5.3
	Deferred federal income taxes	79.3			(1.0		)	(3.3		)
	Share-based compensation expense	(0.5		)	(1.3		)	(2.7		)
	Other, net	11.1			1.7			5.7
	Net income per accompanying consolidated financial statements	$	107.4		$	60.8		$	10.7

ShareBased_Compensation_Tables

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Average Fair Values and Related Assumptions for Options Granted	The following table sets forth the weighted average fair values and related assumptions for options granted for the years ended December 31, 2014, 2013 and 2012:
		2014				2013				2012
	Fair value per share	$	7.28			$	5.15			$	3.46
	Expected dividend yield	1.86		%		2.4		%		4.41		%
	Risk free interest rate	1.65		%		1.26		%		1.1		%
	Expected volatility factor	39.23		%		37.59		%		41.5		%
	Expected life in years	5.7				6.3				5.4
Summary of Stock Option Activity	The following table sets forth the Company’s total stock option activity and related information for these plans for the years ended December 31, 2014, 2013 and 2012:
	(millions, except per share amounts)	2014							2013							2012
		Options			Weighted-				Options			Weighted-				Options		Weighted-
					Average							Average						Average
					Exercise							Exercise						Exercise
					Price							Price						Price
	Outstanding, beginning of year	3.9			$	22.01			3.9			$	22.25			3.8		$	22.79
	Granted	0.3			21.2				0.5			16.82				0.4		13.54
	Exercised	(0.1	)		16.43				(0.2	)		16.82				—		14.49
	Canceled	(0.4	)		30.21				(0.3	)		19.52				(0.3	)	17.95
	Outstanding, end of year	3.7			$	21.29			3.9			$	22.01			3.9		$	22.25
Summary of Stock Options Outstanding and Exercisable	The following table sets forth information pertaining to the total options outstanding and exercisable at December 31, 2014:
	(Options in millions)	Options Outstanding									Options Exercisable
		Number			Weighted-		Weighted-				Number			Weighted-
					Average		Average							Average
					Remaining		Exercise							Exercise
					Contractual Life		Price							Price
	Range of Exercise Prices:
	$10.01 – $20.00	2.1			5.8		$	16.47			1.5			$	16.65
	$20.01 – $30.00	1.3			3.1		25.84				1			27.32
	Greater than $30.00	0.3			1.4		33.45				0.3			33.45
		3.7			4.5		$	21.29			2.8			$	22.3
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Summary of Non-Vested and Vested Restricted Shares and Changes	The following table sets forth the status of the Company’s non-vested and vested restricted shares and changes for the years ended December 31, 2014, 2013 and 2012:
		2014							2013								2012
		Shares			Weighted				Shares				Weighted				Shares		Weighted
					Average								Average						Average
					Grant								Grant						Grant
					Date Fair								Date Fair						Date Fair
					Value								Value						Value
	Outstanding, beginning of year	38,233			$	15.06			55,413				$	16.21			33,887		$	17.92
	Granted	56,178			21.23				—				—				21,526		13.53
	Vested	(16,892	)		17.08				(17,180	)			18.78				—		—
	Canceled	(1,047	)		21.23				—		—		—				—		—
	Outstanding, end of year	76,472			$	19.06			38,233				$	15.06			55,413		$	16.21

Net_Earnings_Loss_Per_Common_S1

Net Earnings (Loss) Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Compilation of Basic and Diluted Net (Loss) Per Common Share	The following table sets forth the compilation of basic and diluted net earnings per common share for the years ended December 31, 2014, 2013 and 2012:
	(millions, except per share amounts)	2014			2013			2012
	Numerator:
	Net earnings for basic net earnings per common share	$	107.4		$	60.8		$	10.7
	Adjusted net earnings for dilutive net earnings per common share	$	107.4		$	60.8		$	10.7
	Denominator:
	Weighted average shares for basic net earnings per common share	40.8			40.6			40.4
	Effect of dilutive share-based awards	0.4			0.1			0.1
	Adjusted weighted average shares for diluted net earnings per common share	41.2			40.7			40.5
	Basic net earnings per common share	$	2.63		$	1.5		$	0.26
	Diluted net earnings per common share	$	2.6		$	1.49		$	0.26
Options to Purchase Shares of Common Stock	The following table sets forth the options to purchase shares of common stock and the restricted share units ("RSU award") provided to each outside director of the Company, that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the years ended December 31, 2014, 2013 and 2012:
	(millions)	2014		2013			2012
	Total number of antidilutive options and awards	1.8		2.6			3.7

Reportable_Segments_Tables

Reportable Segments (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments for the years ended December 31, 2014, 2013 and 2012:
	($ millions)	2014			2013			2012
	Revenues from external sources:
	Insurance segments
	Personal insurance	$	451.4		$	464		$	469.8
	Business insurance	381.8			364.2			327.2
	Specialty insurance	240.9			226.8			245.1
	Total insurance segments	1,074.10			1,055.00			1,042.10
	Investment operations segment
	Net investment income	74.7			72.8			75.4
	Net realized capital gains	20.7			23.2			28.8
	Total investment operations segment	95.4			96			104.2
	Total revenue from reportable segments	1,169.50			1,151.00			1,146.30
	All other	3.2			2			3.8
	Total revenues from external sources	1,172.70			1,153.00			1,150.10
	Intersegment revenues	5.1			5.1			9.5
	Total revenues	1,177.80			1,158.10			1,159.60
	Reconciling items:
	Eliminate intersegment revenues	(5.1		)	(5.1		)	(9.5		)
	Total consolidated revenue	$	1,172.70		$	1,153.00		$	1,150.10
	Segment loss before federal income tax:
	Insurance segments:
	Personal insurance SAP underwriting loss	$	(11.7	)	$	(12.6	)	$	(1.6	)
	Business insurance SAP underwriting loss	(8.5		)	(10.2		)	(42.3		)
	Specialty insurance SAP underwriting loss	(85.6		)	(11.2		)	(48.1		)
	Total insurance segments	(105.8		)	(34.0		)	(92.0		)
	Investment operations segment:
	Net investment income	74.7			72.8			75.4
	Net realized capital gains	20.7			23.2			28.8
	Total investment operations segment	95.4			96			104.2
	All other segments income	—			0.9			2
	Reconciling items:
	GAAP adjustments	45.8			11.8			7.2
	Interest expense on corporate debt	(5.4		)	(8.5		)	(7.0		)
	Corporate expenses	(3.2		)	(4.9		)	(3.8		)
	Total reconciling items	$	37.2		$	(1.6	)	$	(3.6	)
	Total consolidated income before federal income taxes	$	26.8		$	61.3		$	10.6
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2014 and 2013:
	($ millions)	2014			2013
	Segment assets:
	Investment operations segment	$	2,444.20		$	2,331.60
	Total segment assets	2,444.20			2,331.60
	Reconciling items:
	Corporate assets	322.7			164.8
	Total consolidated assets	$	2,766.90		$	2,496.40

Quarterly_Financial_Data_unaud1

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following tables set forth quarterly financial data for 2014 and 2013:
	($ millions, except per share amounts)	2014
		For three months ended
		March 31		June 30		September 30		December 31
	Total revenues	$	291.3	$	294.4	$	291.3	$	295.7
	Income (loss) before federal income taxes	27.7		3.1		13.1		(17.1		)
	Net income	27.1		3		11.9		65.4
	Earnings per common share:
	Basic	$	0.67	$	0.07	$	0.29	$	1.6
	Diluted	$	0.66	$	0.07	$	0.28	$	1.58
		2013
		For three months ended
		March 31		June 30		September 30		December 31
	Total revenues	$	285.3	$	285.3	$	292.7	$	289.7
	Income before federal income taxes	20		6.3		18.7		16.3
	Net income	19.7		6.2		18.5		16.4
	Earnings per common share:
	Basic	$	0.49	$	0.15	$	0.46	$	0.4
	Diluted	$	0.49	$	0.15	$	0.45	$	0.4

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Schedule of Net Deferred Acquisition Costs (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of year	$96.80	$91.70	$91.70
Acquisition costs deferred	251.5	214.6	213.1
Acquisition costs amortized to expense	-221.8	-209.5	-213.1
Balance, end of year	$126.50	$96.80	$91.70

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Accounting Policies [Abstract]
Liability for unpaid losses and loss expenses	$23.80	$24.70

Investments_Cost_or_Amortized_

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$2,117.30	$2,050.10
Gross unrealized holding gains	172.6	160.7
Gross unrealized holding losses	7.3	34.5
Fair value	2,282.60	2,176.30
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,831.30	1,804
Gross unrealized holding gains	67.3	60.3
Gross unrealized holding losses	6.7	34.2
Fair value	1,891.90	1,830.10
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	235.5	196.6
Gross unrealized holding gains	75.5	69
Gross unrealized holding losses	0.6	0.3
Fair value	310.4	265.3
Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	50.5	49.5
Gross unrealized holding gains	29.8	31.4
Gross unrealized holding losses	0	0
Fair value	80.3	80.9
U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	296.7	345.5
Gross unrealized holding gains	14.1	13.4
Gross unrealized holding losses	1.5	6.5
Fair value	309.3	352.4
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	742.5	765.3
Gross unrealized holding gains	27.4	25.8
Gross unrealized holding losses	0.4	16.9
Fair value	769.5	774.2
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	333.4	345
Gross unrealized holding gains	10.2	11.4
Gross unrealized holding losses	3	6.7
Fair value	340.6	349.7
U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	458.7	348.2
Gross unrealized holding gains	15.6	9.7
Gross unrealized holding losses	1.8	4.1
Fair value	472.5	353.8
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	185.5	148.2
Gross unrealized holding gains	57.3	46.5
Gross unrealized holding losses	0.6	0.3
Fair value	242.2	194.4
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	50	48.4
Gross unrealized holding gains	18.2	22.5
Gross unrealized holding losses	0	0
Fair value	$68.20	$70.90

Investments_Gross_Unrealized_L

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified	Positions	Positions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$120.70	$530.50
Less than 12 months, Unrealized losses	-2.1	-28
Less than 12 months, Number of positions (positions)	27	154
12 months or more, Fair value	176.5	85.8
12 months or more, Unrealized losses	-5.2	-6.5
12 months or more, Number of positions (positions)	50	25
Total, Fair value	297.2	616.3
Total, Unrealized losses	-7.3	-34.5
Total, Number of positions (positions)	77	179
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	113.5	525.2
Less than 12 months, Unrealized losses	-1.5	-27.7
Less than 12 months, Number of positions (positions)	25	152
12 months or more, Fair value	176.5	85.8
12 months or more, Unrealized losses	-5.2	-6.5
12 months or more, Number of positions (positions)	50	25
Total, Fair value	290	611
Total, Unrealized losses	-6.7	-34.2
Total, Number of positions (positions)	75	177
U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	19.9	109.6
Less than 12 months, Unrealized losses	-0.3	-6.5
Less than 12 months, Number of positions (positions)	4	29
12 months or more, Fair value	52	0
12 months or more, Unrealized losses	-1.2	0
12 months or more, Number of positions (positions)	17	0
Total, Fair value	71.9	109.6
Total, Unrealized losses	-1.5	-6.5
Total, Number of positions (positions)	21	29
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	6.1	206.4
Less than 12 months, Unrealized losses	0	-14.7
Less than 12 months, Number of positions (positions)	5	76
12 months or more, Fair value	30.9	25.6
12 months or more, Unrealized losses	-0.4	-2.2
12 months or more, Number of positions (positions)	9	7
Total, Fair value	37	232
Total, Unrealized losses	-0.4	-16.9
Total, Number of positions (positions)	14	83
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	43.5	105.6
Less than 12 months, Unrealized losses	-0.9	-3.2
Less than 12 months, Number of positions (positions)	8	22
12 months or more, Fair value	56.1	40.9
12 months or more, Unrealized losses	-2.1	-3.5
12 months or more, Number of positions (positions)	11	8
Total, Fair value	99.6	146.5
Total, Unrealized losses	-3	-6.7
Total, Number of positions (positions)	19	30
U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	44	103.6
Less than 12 months, Unrealized losses	-0.3	-3.3
Less than 12 months, Number of positions (positions)	8	25
12 months or more, Fair value	37.5	19.3
12 months or more, Unrealized losses	-1.5	-0.8
12 months or more, Number of positions (positions)	13	10
Total, Fair value	81.5	122.9
Total, Unrealized losses	-1.8	-4.1
Total, Number of positions (positions)	21	35
Large-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	7.2	5.3
Less than 12 months, Unrealized losses	-0.6	-0.3
Less than 12 months, Number of positions (positions)	2	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	7.2	5.3
Total, Unrealized losses	($0.60)	($0.30)
Total, Number of positions (positions)	2	2

Investments_Realized_Losses_Re

Investments - Realized Losses Related to Other-Than-Temporary Impairments (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	($2.50)	($4)	($3.40)
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0	0	-0.2
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	-0.3	-1.8	0
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	($2.20)	($2.20)	($3.20)

Investments_Amortized_Cost_and

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Amortized cost
Due in 1 year or less	$55.70
Due after 1 year through 5 years	369.8
Due after 5 years through 10 years	299.9
Due after 10 years	647.2
U.S. government agencies residential mortgage-backed securities	458.7
Total	1,831.30	1,804
Fair value
Due in 1 year or less	56.5
Due after 1 year through 5 years	382.4
Due after 5 years through 10 years	312.4
Due after 10 years	668.1
U.S. government agencies residential mortgage-backed securities	472.5
Total	$1,891.90	$1,830.10

Investments_Additional_Informa

Investments - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments [Abstract]
Carrying value of securities pledged as collateral for the FHLB Loan	$85
Fixed maturities fair value of deposit with insurance regulators	8.8	8.7
Proceeds on sales of available-for-sale securities	263.3	220.4	435.3
Deferred federal income tax liability on the net unrealized holding gains	$55.30	$41.60

Investments_Components_of_Net_

Investments - Components of Net Investment Income (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$76.70	$74.90	$77.40
Investment expenses	2	2.1	2
Net investment income	74.7	72.8	75.4
Fixed maturities:
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	64.3	63.2	66.9
Equity securities:
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	6.2	6	4.9
Cash and cash equivalents, and other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$6.20	$5.70	$5.60

Investments_Realized_and_Unrea

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Realized gains:
Total realized gains	$24.50	$28.70	$34.80
Realized losses:
Total realized losses	-3.8	-5.5	-6
Net realized gains on investments	20.7	23.2	28.8
Change in unrealized holding gains (losses), net of tax:
Deferred federal income liability thereon	-13.7	17.6	-8.6
Valuation allowance	0	-6.7	9.2
Change in net unrealized holding gains, net of tax	25.4	-39.4	25.3
Fixed maturities:
Realized gains:
Total realized gains	3.1	2.5	15.7
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	34.5	-102.8	11.3
Fixed maturities: | Sales
Realized losses:
Total realized losses	0	0.3	0
Fixed maturities: | OTTI
Realized losses:
Total realized losses	0	0	-0.2
Equity securities:
Realized gains:
Total realized gains	21.3	26.1	19
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	6.2	36.5	6.6
Equity securities: | Sales
Realized losses:
Total realized losses	-1.3	-1.2	-2.6
Equity securities: | OTTI
Realized losses:
Total realized losses	-2.5	-4	-3.2
Other invested assets
Realized gains:
Total realized gains	0.1	0.1	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	($1.60)	$16	$6.80

Fair_Value_of_Financial_Instru2

Fair Value of Financial Instruments - Additional Information (Detail) (USD $)	1 Months Ended
In Millions, unless otherwise specified	31-May-09	Dec. 31, 2014	Dec. 31, 2013
	Agreement
Fair Value Measurements Disclosure [Line Items]
Other invested assets fair value		$72.90	$74.20
Other investments and securities, at cost		5.3	5
Number of credit agreements (agreements)	2
Notes receivable from affiliate	70	70	70
Fair Value, Inputs, Level 3
Fair Value Measurements Disclosure [Line Items]
Federal home loan bank stock		4.8
Other investments and securities, at cost		$0.50
Corporate securities | Fair Value, Inputs, Level 3
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities (securities)		2
Securities with fair value estimated by present value of future cash flows (securities)		1
International funds
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities (securities)		2

Fair_Value_of_Financial_Instru3

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	$2,282.60	$2,176.30
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	1,891.90	1,830.10
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	310.4	265.3
Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	80.3	80.9
U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	309.3	352.4
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	769.5	774.2
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	340.6	349.7
U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	472.5	353.8
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	242.2	194.4
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	68.2	70.9
Available-for-sale:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,282.60	2,176.30
Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	1,891.90	1,830.10
Available-for-sale: | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	310.4	265.3
Available-for-sale: | Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	80.3	80.9
Available-for-sale: | U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	309.3	352.4
Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	769.5	774.2
Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	340.6	349.7
Available-for-sale: | U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	472.5	353.8
Available-for-sale: | Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	242.2	194.4
Available-for-sale: | Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	68.2	70.9
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	317.8	272
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	310.4	265.3
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	7.4	6.7
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	242.2	194.4
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	68.2	70.9
Significant other observable inputs (Level 2) | Available-for-sale:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	1,955.40	1,895.40
Significant other observable inputs (Level 2) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	1,882.50	1,821.20
Significant other observable inputs (Level 2) | Available-for-sale: | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant other observable inputs (Level 2) | Available-for-sale: | Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	72.9	74.2
Significant other observable inputs (Level 2) | Available-for-sale: | U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	309.3	352.4
Significant other observable inputs (Level 2) | Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	769.5	774.2
Significant other observable inputs (Level 2) | Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	331.2	340.8
Significant other observable inputs (Level 2) | Available-for-sale: | U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	472.5	353.8
Significant other observable inputs (Level 2) | Available-for-sale: | Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant other observable inputs (Level 2) | Available-for-sale: | Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	9.4	8.9
Significant unobservable inputs (Level 3) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	9.4	8.9
Significant unobservable inputs (Level 3) | Available-for-sale: | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale: | Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale: | U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	9.4	8.9
Significant unobservable inputs (Level 3) | Available-for-sale: | U.S. government agencies residential mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale: | Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Available-for-sale: | Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	$0	$0

Fair_Value_of_Financial_Instru4

Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (Recurring, Available-for-sale:, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Recurring | Available-for-sale:
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$8.90	$8.50
Total realized gains (losses)-included in earnings	0	0
Total unrealized gains (losses)-included in other comprehensive income	0.2	0.2
Purchases	0.3	0.2
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$9.40	$8.90

Fair_Value_of_Financial_Instru5

Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) (Affiliated entity, Significant other observable inputs (Level 2), USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate (percent)	7.00%	7.00%
Carrying value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	70	70
Fair value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	74.6	74.6

Fair_Value_of_Financial_Instru6

Fair Value of Financial Instruments - Included in Notes Payable are Senior Notes and Subordinated Debentures (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Interest rate (percent)	5.28%	5.28%
Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Interest rate (percent)	5.03%	5.03%
Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Interest rate (percent)	4.44%	4.44%
Carrying value
Debt Instrument [Line Items]
Total notes payable	100,800,000	100,800,000
Carrying value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	85,300,000	85,300,000
Debt instrument, issued	85,000,000
Carrying value | Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Total notes payable	15,500,000	15,500,000
Debt instrument, issued	15,500,000
Fair value
Debt Instrument [Line Items]
Total notes payable	101,900,000	101,200,000
Fair value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	86,400,000	85,700,000
Fair value | Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Total notes payable	15,500,000	15,500,000

Losses_and_Loss_Expenses_Payab2

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$959.90	$942.20	$907.10
Less: reinsurance recoverable on losses and loss expenses payable	9.1	13.5	25.5
Net balance at beginning of year	950.8	928.7	881.6
Incurred related to:
Current year	726.2	741	795.2
Prior years	45.1	-21.2	-16.9
Total incurred	771.3	719.8	778.3
Paid related to:
Current year	373.2	355	397.2
Prior years	375.3	342.7	334
Total paid	748.5	697.7	731.2
Net balance at end of year	973.6	950.8	928.7
Plus: reinsurance recoverable on losses and loss expenses payable	9.6	9.1	13.5
Losses and loss expenses payable, at end of year (affiliates $494.3, $438.0 and $435.1, respectively)	$983.20	$959.90	$942.20

Losses_and_Loss_Expenses_Payab3

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Parenthetical) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Millions, unless otherwise specified
Insurance [Abstract]
Losses and loss expenses payable, affiliates	$494.30	$438	$435.10

Losses_and_Loss_Expenses_Payab4

Losses and Loss Expenses Payable - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	($45,100,000)	$21,200,000	$16,900,000
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	12,500,000	8,000,000	6,300,000
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	5,200,000	5,400,000	10,400,000
Personal And Business Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	23,100,000	18,300,000	28,000,000
Loss and Loss Expense Reserve Development, Accident Years			3
Consumer Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			10,500,000
Business insurance segment
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	16,700,000
Product Liability [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	11,900,000	8,300,000	9,400,000
Fire [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			5,100,000
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	5,300,000	8,000,000
Homeowners [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		2,900,000
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	6,400,000
Commercial Multi-Peril [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	2,100,000
Specialty Insurance Segment [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	85,900,000	10,500,000	27,800,000
Loss and Loss Expense Reserve Development, Accident Years			2
RED [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	96,700,000	21,300,000
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	5,700,000	12,300,000
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	3,900,000
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	2,100,000
Specialty Insurance Segment [Member] | Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			$0.33

Reinsurance_Additional_Informa

Reinsurance - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance [Line Items]
Duration of quota share agreement			3 years
Percentage ceded to reinsurers (percent)	75.00%
Unearned premium	$612.40	$491
Excess Ceding Commission	8.1	8.6	8.4
Reinsurance Arrangement, Participating Reinsurers' Margin	9.00%
Profit Commission	19
Homeowners Quota Share Arrangement
Reinsurance [Line Items]
Unearned premium	$89.50

Reinsurance_External_Reinsuran

Reinsurance - External Reinsurance on Balance Sheets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Losses and loss expenses payable:
Assumed	$973.60	$950.80
Ceded	-479.3	-512.8
Unearned premiums:
Assumed	606.3	486.3
Ceded	-404.6	-407.9
Unearned Premiums From Affiliates	201.7	78.4
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	484.4	516.9
Assumed	4.5	5
Ceded	-9.6	-9.1
Net losses and loss expenses payable	479.3	512.8
Unearned premiums:
Direct	409.7	411.6
Assumed	1	1
Ceded	-6.1	-4.7
Unearned Premiums From Affiliates	$404.60	$407.90

Reinsurance_External_Reinsuran1

Reinsurance - External Reinsurance on Income Statements (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Written premiums:
Assumed	$1,194.20	$1,062.10	$1,055.30
Ceded	-858.2	-866.3	-839.9
Net written premiums	336	195.8	215.4
Earned premiums:
Direct	882.6	874.7	833.3
Assumed	1,074.10	1,055	1,042.10
Ceded	-861.7	-855	-809.2
Losses and loss expenses incurred:
Assumed	774.4	722.7	779.4
Ceded	-523.6	-560.2	-575.7
Net losses and loss expenses incurred	250.8	162.5	203.7
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	880.6	886.7	860.1
Assumed	4.4	3.9	4
Ceded	-26.8	-24.3	-24.3
Net written premiums	858.2	866.3	839.8
Earned premiums:
Direct	882.6	874.7	833.3
Assumed	4.4	3.8	4.1
Ceded	-25.3	-23.5	-28.2
Net earned premiums	861.7	855	809.2
Losses and loss expenses incurred:
Direct	525.6	559.1	577.4
Assumed	2.9	3	3.6
Ceded	-8	-4.8	-6.4
Net losses and loss expenses incurred	$520.50	$557.30	$574.60

Transactions_with_Affiliates_A

Transactions with Affiliates - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended
In Millions, unless otherwise specified	31-May-09	31-May-03	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Agreement
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Number of days intercompany balances settled after quarter end			60 days
Company's premium share balances due			$297	$295.50
Mandatorily redeemable preferred capital securities issued to third party		15
Common securities issued to state auto financial		0.5	3.5	4	1.6
Proceeds from the issuance of Trust Securities		15.5
Description of variable rate basis		three-month LIBOR rate plus 4.20%
Subordinated Borrowing, Due Date		31-Dec-33
Interest rate (percent)			5.28%	5.28%
Number of credit agreements (agreements)	2
Credit agreement loan amount	70		70	70
Fixed annual interest rate (percent)	7.00%
Interest income earned			4.9	4.9	4.9
Revenue from investment management services			$1.50	$1.60	$2.90
Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate (percent)			4.44%	4.44%
3 month LIBOR | Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate of Trust Securities payable quarterly (percent)		4.20%

Transactions_with_Affiliates_S

Transactions with Affiliates - Schedule of Reinsurance Transactions on Balance Sheets for Pooling Arrangement (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	($79.70)	($77.60)
Deferred Policy Acquisition Costs Assumed	126.5	96.8
Deferred Policy Acquisitions Costs from Affiliates	46.8	19.2
Losses and loss expenses payable:
Ceded	-479.3	-512.8
Assumed	973.6	950.8
Losses And Loss Expenses Payable From Affiliates	494.3	438
Unearned premiums:
Ceded	-404.6	-407.9
Assumed	606.3	486.3
Unearned Premiums From Affiliates	201.7	78.4
Pension and Other Postretirement Benefits:
Postretirement and Pension Benefits Ceded	-180.5	-106.5
Postretirement and Pension Benefits Assumed	117.3	69.2
Postretirement and Pension Benefits Ceded to Affiliates	-63.2	-37.3
Other Liabilities Ceded	-70.7	-47.2
Other Liabilities Assumed	65.6	67.2
Other Liabilities	-5.1	20
Accumulated Other Comprehensive Income Ceded	-186	-117.1
Accumulated Other Comprehensive Income Assumed	120.9	76.1
Net Accumulated Other Comprehensive Income	($65.10)	($41)

Transactions_with_Affiliates_S1

Transactions with Affiliates - Schedule of Reinsurance Transactions on Income Statements for Pooling Arrangement (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Written premiums:
Ceded	($858.20)	($866.30)	($839.90)
Assumed	1,194.20	1,062.10	1,055.30
Premiums Written, Net	336	195.8	215.4
Earned premiums:
Ceded	-861.7	-855	-809.2
Assumed	1,074.10	1,055	1,042.10
Premiums Earned, Net	212.4	200	232.9
Losses and loss expenses incurred:
Ceded	-523.6	-560.2	-575.7
Assumed	774.4	722.7	779.4
Losses and loss expenses incurred, Net	250.8	162.5	203.7
Acquisition and Operating Expenses [Abstract]
Ceded	-215.4	-192.6	-161.8
Assumed	372.3	365.3	352.6
Acquisition and Operating Expenses, Net	$156.90	$172.70	$190.80

Notes_Payable_and_Credit_Facil1

Notes Payable and Credit Facility - Additional Information (Detail) (USD $)	0 Months Ended
	Jul. 26, 2013	Jul. 15, 2013	Jul. 11, 2013
Debt Instrument [Line Items]
Current borrowing capacity	$100,000,000
Expiration period	5 years
Maximum borrowing capacity under credit facility	150,000,000
Senior Notes
Debt Instrument [Line Items]
Debt instrument, issued		100,000,000
Debt instrument redemption price paid		103,000,000
Federal Home Loan Bank of Cincinnati
Debt Instrument [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances			85,000,000
Federal home loan bank advances long-term, duration			20 years
Federal home loan bank advances, long-term non-callable period			3 years
Federal home loan Bank, advances, general debt obligations, disclosures, repayment and penalties			$0
Federal home loan bank, advances, branch of FHLB Bank, interest rate (percent)			5.03%

Federal_Income_Taxes_Reconcili

Federal Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$9.40	$21.50	$3.70
Tax-exempt interest and dividends received deduction	-8.5	-9.4	-9.1
Other, net	1.1	0.2	0.5
Valuation allowance	-82.6	-11.8	4.8
Federal income tax (benefit) expense and effective rate	($80.60)	$0.50	($0.10)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	-32.00%	-15.00%	-85.00%
Other, net (percent)	4.00%	0.00%	4.00%
Valuation allowance (percent)	-308.00%	-19.00%	45.00%
Federal income tax expense (benefit) and effective rate (percent)	-301.00%	1.00%	-1.00%

Federal_Income_Taxes_Tax_Effec

Federal Income Taxes - Tax Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Deferred tax assets:
Unearned premiums not currently deductible	$42.20	$33.80
Losses and loss expenses payable discounting	21	21.6
Postretirement and pension benefits	41.1	24.9
Realized loss on other-than-temporary impairment	8.4	7.5
Other liabilities	22.2	17.3
Net operating loss carryforward	58	56.5
Tax credit carryforward	1.9	1.7
Other	4.7	9.2
Total deferred tax assets	199.5	172.5
Deferred tax liabilities:
Deferral of policy acquisition costs	44.3	33.9
Net unrealized holding gains on investments	57.8	44.1
Total deferred tax liabilities	102.1	78
Net Deferred Tax Assets Before Valuation Allowance	97.4	94.5
Deferred Tax Assets, Valuation Allowance	0	82.6
Net deferred federal income taxes	$97.40	$11.90

Federal_Income_Taxes_Additiona

Federal Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended			36 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Cumulative Pre-Tax Income	3 years
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	$26,800,000	$61,300,000	$10,600,000	$98,700,000
consecutive years of pre-tax income	3 years
Percentage Ceded To Reinsurers	75.00%
Specialty Insurance Segment, former RED Program Business, Number of Programs	1
Income Tax Benefit, Deferred	82,600,000
Net deferred tax asset valuation allowance	0	82,600,000		0
Valuation Allowance Attributable To Continuing Operations		11,800,000
Deferred tax asset related to net operating loss carryforwards	58,000,000	56,500,000		58,000,000
Balance for uncertain tax positions	0			0
Interest and penalties payable	$0	$0		$0

Pension_and_Postretirement_Ben2

Pension and Postretirement Benefit Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Eligible age of years to participate in benefit plans	20 years
Eligible years of credit service required to participate in benefit plans	5 years
Maximum age of years required to participate in benefit plans	65 years
Postretirement plan's subsidy related to Medicare Prescription Drug Improvement and Modernization Act of 2003	$300,000
Postretirement plan's estimated future subsidies	200,000
Employer contribution	13,000,000
Contributions of participant salary (percent)	1.00%
Matching contribution on participant's salary	1
Matching contribution by the employer	0.5
Company's share of expense under the plan	5,100,000	4,600,000	4,500,000
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Participant's contribution (percent)	2.00%
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Participant's contribution (percent)	6.00%
Postretirement Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	0	0
Postretirement Benefit Costs | Gross Benefit Payments
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement plan's gross benefit payments	$1,200,000

Pension_and_Postretirement_Ben3

Pension and Postretirement Benefit Plans - Summary of Pension and Postretirement Benefit Plans' Change in Benefit Obligation, Plan Assets and Funded Status (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in benefit obligation:
Actuarial loss (gain)	($54.40)	$32.50	($7.40)
Change in plan assets available for plan benefits:
Employer contribution	13
Pension Costs
Change in benefit obligation:
Benefit obligation at beginning of year	230.5	246.1
Service cost	4.6	6.1
Interest cost	10.8	9.6
Actuarial loss (gain)	57.3	-19.6
Benefits paid	-10.7	-11.7
The Companybs portion of benefit obligation at end of year	292.5	230.5
Change in plan assets available for plan benefits:
Fair value of plan assets available for plan benefits at beginning of year	185.9	162.2
Employer contribution	13	13
Actual return on plan assets	16.9	22.4
Benefits paid	-10.7	-11.7
The Companybs portion of fair value of plan assets at end of year	205.1	185.9
Supplemental executive retirement plan	-6.3	-6
Funded status at end of year	-93.7	-50.6
Accumulated benefit obligation end of year	260.1	211.2
Postretirement Benefit Costs
Change in benefit obligation:
Benefit obligation at beginning of year	22.3	25.1
Service cost	0	0
Interest cost	1.1	1
Actuarial loss (gain)	1.7	-2.6
Benefits paid	-1.2	-1.2
The Companybs portion of benefit obligation at end of year	23.9	22.3
Change in plan assets available for plan benefits:
Fair value of plan assets available for plan benefits at beginning of year	1.3	1.8
Employer contribution	0	0
Actual return on plan assets	0	0.3
Benefits paid	-1	-0.8
The Companybs portion of fair value of plan assets at end of year	0.3	1.3
Supplemental executive retirement plan	0	0
Funded status at end of year	($23.60)	($21)

Pension_and_Postretirement_Ben4

Pension and Postretirement Benefit Plans - Company's Share of Amounts Included in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Prior service benefit	($64.90)	($70.30)
Net actuarial loss	141.9	94.3
Total	$77	$24

Pension_and_Postretirement_Ben5

Pension and Postretirement Benefit Plans - Company's Share of Amortization Expected to be Recognized (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Prior service benefit	($5.40)
Net actuarial loss	11.5
Total	$6.10

Pension_and_Postretirement_Ben6

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amortization of:
Prior service cost (benefit)	($64.90)	($70.30)
Pension
Components of net periodic cost:
Service cost	5.2	6.1	7.8
Interest cost	11.1	9.6	9.9
Expected return on plan assets	-12.6	-12.2	-11.7
Amortization of:
Prior service cost (benefit)	0	0	0.3
Net actuarial loss	6.3	8.1	6.7
Net periodic cost (benefit)	10	11.6	13
Postretirement
Components of net periodic cost:
Service cost	0	0.4	0
Interest cost	1.1	1.2	1.1
Expected return on plan assets	0	-0.2	-0.1
Amortization of:
Prior service cost (benefit)	-5.5	-5.5	-5.5
Net actuarial loss	0.6	1	1.1
Net periodic cost (benefit)	($3.80)	($3.10)	($3.40)

Pension_and_Postretirement_Ben7

Pension and Postretirement Benefit Plans-Company's Share of Expected Future Benefits Payments Schedule (Detail) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Pension
Schedule of Expected Future Pension Benefit Payment [Line Items]
2015	$9.60
2016	10
2017	10.5
2018	11.2
2019	11.8
2020 - 2024	72
Postretirement
Schedule of Expected Future Pension Benefit Payment [Line Items]
2015	1.7
2016	1.7
2017	1.7
2018	1.6
2019	1.6
2020 - 2024	$7.30

Pension_and_Postretirement_Ben8

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plans' Obligations (Detail)	Dec. 31, 2014	Dec. 31, 2013
Pension
Benefit obligations weighted-average assumptions:
Discount rate (percent)	3.85%	4.85%
Rates of increase in compensation levels (percent)	3.50%	3.50%
Postretirement
Benefit obligations weighted-average assumptions:
Discount rate (percent)	3.85%	4.85%
Rates of increase in compensation levels (percent)	0.00%	0.00%

Pension_and_Postretirement_Ben9

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plan's Net Periodic Cost (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	4.85%	4.05%	4.40%
Expected long-term rate of return on assets (percent)	7.00%	7.50%	7.50%
Rates of increase in compensation levels (percent)	3.50%	4.00%	4.00%
Postretirement
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	4.85%	4.05%	4.40%
Expected long-term rate of return on assets (percent)	0.00%	7.50%	7.50%
Rates of increase in compensation levels (percent)	0.00%	0.00%	0.00%

Recovered_Sheet1

Pension and Postretirement Benefit Plans - Schedule of Assumed Health Care Cost Trend Rates Used (Detail) (Postretirement)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Postretirement
Assumed health care cost trend rates:
Health care cost trend rate assumed for the next year (percent)	6.00%	10.00%	10.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (percent)	3.80%	5.00%	5.00%
Year that the rate reaches the ultimate trend rate	2075	2018	2017

Recovered_Sheet2

Pension and Postretirement Benefit Plans - Summary on Effective Impact of Assumed Health Care Cost Trend Rates on Amounts Reported for Postretirement Plan (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
One percentage point change:
Effect on total service and interest cost	$0.10
Effect on accumulated postretirement benefit obligation	3.8
Postretirement
One percentage point change:
Effect on total service and interest cost	0.1
Effect on accumulated postretirement benefit obligation	$3.30

Recovered_Sheet3

Pension and Postretirement Benefit Plans - Summary of Asset Allocation Targets (Detail)	12 Months Ended
	Dec. 31, 2014
Asset Category:
Asset allocation target (percent)	100.00%
Fixed maturity
Asset Category:
Asset allocation target (percent)	49.00%
Large-cap securities
Asset Category:
Asset allocation target (percent)	26.00%
Small-cap securities
Asset Category:
Asset allocation target (percent)	11.00%
International equity
Asset Category:
Asset allocation target (percent)	10.00%
Emerging market equity
Asset Category:
Asset allocation target (percent)	4.00%

Recovered_Sheet4

Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$2,282.60	$2,176.30
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	199.7	175.6
Pension | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	18.7	25.4
Pension | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	105.5	65.9
Pension | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	56.4	30
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	39.7	24.6
Pension | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	9.4	11.3
Pension | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	75.5	84.3
Pension | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	53.7	58
Pension | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	21.8	26.3
Pension | Quoted prices in active markets for identical assets (Level 1)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	75.5	84.3
Pension | Quoted prices in active markets for identical assets (Level 1) | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	75.5	84.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	53.7	58
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	21.8	26.3
Pension | Significant other observable inputs (Level 2)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	124.2	91.3
Pension | Significant other observable inputs (Level 2) | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	18.7	25.4
Pension | Significant other observable inputs (Level 2) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	105.5	65.9
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	56.4	30
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	39.7	24.6
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	9.4	11.3
Pension | Significant other observable inputs (Level 2) | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Significant other observable inputs (Level 2) | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Pension | Significant other observable inputs (Level 2) | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$0	$0

Recovered_Sheet5

Pension and Postretirement Benefit Plans - Company's Share of Postretirement Plans' Available-for-Sale Securities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$2,282.60	$2,176.30
Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.3	1
Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.1	0.2
Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.1	0.2
Postretirement | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.8
Postretirement | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.8
Postretirement | Money Market Funds [Member] | Money Market Funds [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.2
Quoted prices in active markets for identical assets (Level 1) | Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.2	0.8
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0	0
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.8
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.8
Quoted prices in active markets for identical assets (Level 1) | Postretirement | Money Market Funds [Member] | Money Market Funds [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.2
Significant other observable inputs (Level 2) | Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.1	0.2
Significant other observable inputs (Level 2) | Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.1	0.2
Significant other observable inputs (Level 2) | Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	0.1	0.2
Significant other observable inputs (Level 2) | Postretirement | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0
Significant other observable inputs (Level 2) | Postretirement | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0
Significant other observable inputs (Level 2) | Postretirement | Money Market Funds [Member] | Money Market Funds [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$0

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 $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$80.80	$84.20	$63.80
Other comprehensive income before reclassifications	-15.5	16.4	46.7
Amounts reclassified from AOCI	-6.4	19.8	26.3
Net current period other comprehensive income	-9.1	-3.4	20.4
Ending balance	71.7	80.8	84.2
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	84.6	124	98.7
Other comprehensive income before reclassifications	38.9	-16.1	54.1
Amounts reclassified from AOCI	13.5	23.3	28.8
Net current period other comprehensive income	25.4	-39.4	25.3
Ending balance	110	84.6	124
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0.1	0.2
Other comprehensive income before reclassifications	0	0	0
Amounts reclassified from AOCI	0	0.1	0.1
Net current period other comprehensive income	0	-0.1	-0.1
Ending balance	0	0	0.1
Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	-3.8	-39.9	-35.1
Other comprehensive income before reclassifications	-54.4	32.5	-7.4
Amounts reclassified from AOCI	-19.9	-3.6	-2.6
Net current period other comprehensive income	-34.5	36.1	-4.8
Ending balance	($38.30)	($3.80)	($39.90)

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 $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	$20.70	$23.20	$29
Tax (expense) benefit	80.6	-0.5	0.1
Total reclassifications for the period	6.4	-19.8	-26.3
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	6.4	-19.8	-26.3
Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	-13.5	-23.3	-28.8
Unrealized Gains and Losses on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	20.7	23.2	28.8
Total before tax	20.7	23.2	28.8
Tax (expense) benefit	-7.2	0.1	0
Total reclassifications for the period	13.5	23.3	28.8
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	0	-0.1	-0.1
Gains and Losses on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	0	0.1	0.1
Total before tax	0	0.1	0.1
Tax (expense) benefit	0	0	0
Total reclassifications for the period	0	0.1	0.1
Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	19.9	3.6	2.6
Benefit Plan Items | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	-1.4	-3.6	-2.6
Tax (expense) benefit	-18.5	0	0
Negative prior service costs	5.5	5.5	5.2
Net loss	-6.9	-9.1	-7.8
Total reclassifications for the period	($19.90)	($3.60)	($2.60)

Stockholders_Equity_Additional

Stockholders' Equity - Additional Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity, Class of Treasury Stock [Line Items]
Statutory Accounting Policies, Statutory Percent Available for Dividend Payments without Regulatory Approval	10.00%
Amount of dividends available for distributions without prior approval	$77.80
Statutory surplus in excess of authorized level, no remedial action required (percent)	200.00%
Statutory surplus in excess of authorized level, level maintained by company and subsidiaries (percent)	450.00%
State Auto Financial
Equity, Class of Treasury Stock [Line Items]
Dividends received from insurance subsidiaries	$20	$10	$20

Stockholders_Equity_Reconcilia

Stockholders' Equity - Reconciliations of Statutory Capital and Surplus and Net Income (Detail) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Stockholders' Equity Note [Abstract]
Statutory capital and surplus of insurance subsidiaries	$778.40				$753.20				$778.40	$753.20
Net liabilities of non-insurance parent and affiliates	-80.8				-82.4				-80.8	-82.4
Total	697.6				670.8				697.6	670.8
Increases (decreases):
Deferred acquisition costs	126.5				96.8				126.5	96.8	91.7	91.7
Postretirement and pension benefits	27.2				6.3				27.2	6.3
Deferred federal income taxes	-49.7				-15.8				-49.7	-15.8
Fixed maturities, at fair value	60.6				26.1				60.6	26.1
Other, net	10.7				0.8				10.7	0.8
Stockholdersb equity per accompanying consolidated financial statements	872.9				785				872.9	785	737.2
Statutory net (loss) income of insurance subsidiaries									-17.5	51.1	5.5
Net income (loss) of non-insurance parent and affiliates									-5.3	-5.1	0.2
Total									22.8	-46	-5.7
Deferred acquisition costs									29.7	5.1	0
Postretirement and pension benefits									10.6	10.3	5.3
Deferred federal income taxes									79.3	-1	-3.3
Share-based compensation expense									-0.5	-1.3	-2.7
Other, net									11.1	1.7	5.7
Net income	$65.40	$11.90	$3	$27.10	$16.40	$18.50	$6.20	$19.70	$107.40	$60.80	$10.70

Preferred_Stock_Preferred_Stoc

Preferred Stock Preferred Stock (Details)	12 Months Ended
	Dec. 31, 2014
	quarter
	class_of_stock
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Number of authorized classes of preferred stock (class of stock)	2
Minimum number of quarters dividends are paid in arrears (minimum of 6 quarters)	6

ShareBased_Compensation_Additi

Share-Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (shares)	300,000	500,000	400,000
Common shares valued (shares)	$119,300,000	$118,800,000
Total intrinsic value of stock options exercised	500,000	500,000	0
Tax benefit for tax deductions from share-based awards	0	0	0
Aggregate intrinsic value for total options outstanding	17,200,000
Aggregate intrinsic value for total options exercisable	8,400,000
Share-based compensation	3,600,000	4,100,000	3,500,000
Equity Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved (shares)	3,000,000
Common shares available for issuance (shares)	800,000
Percentage of common shares authorized for issuance (percent)	33.00%
Stock options awards description	the maximum number of common shares subject to awards of stock options, restricted shares and performance shares that may be granted in any calendar year is equal to 1.5% of the total number of common shares of the Company outstanding as of DecemberB 31 of the prior year
Maximum number of common shares available for awards of stock options (shares)	250,000
Termination date	MayB 8, 2019
Share based compensation expiration period	10 years
Stock options granted (shares)	200,000	500,000	300,000
Unrecognized compensation cost related to non-vested restricted share	800,000
Remaining unrecognized compensation cost to be recognized in years	2 years 2 months 30 days
Equity Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock vesting percentage three years after grant date (percent)	100.00%
Equity Plan | Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period	3 years
Vesting percent (percent)	33.33%
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved (shares)	3,400,000
Percentage of annual base earnings (percent)	6.00%
Purchase price of the common shares (percent)	85.00%
Common shares purchased under this plan	3,200,000
Outside Directors Plan | Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted share units issued to non employee directors (shares)	1,400
Restricted stock units awarded to directors, award vesting period	6 months
RSU Plan automatically termination date	2015-05-31T00:00:00
RSUs granted in period (shares)	25,960	33,712	26,480
Common shares valued (shares)	51,000	51,000
Outside Directors Plan | Restricted Stock Units (RSUs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSU awards granted to each outside director (shares)	5,000
Outside Directors Plan | Restricted Stock Units (RSUs) | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSU awards granted to each outside director (shares)	500
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost related to non-vested restricted share	$3,200,000
Remaining unrecognized compensation cost to be recognized in years	3 years

ShareBased_Compensation_Summar

Share-Based Compensation - Summary of Non-Vested and Vested Restricted Shares and Changes (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shares
Outstanding, beginning of year (shares)	38,233	55,413	33,887
Granted (shares)	56,178	0	21,526
Vested (shares)	-16,892	-17,180	0
Forfeited (shares)	-1,047	0	0
Outstanding, end of year (shares)	76,472	38,233	55,413
Weighted Average Grant Date Fair Value
Outstanding, beginning of year (usd per share)	$15.06	$16.21	$17.92
Granted (usd per share)	$21.23	$0	$13.53
Vested (usd per share)	$17.08	$18.78	$0
Forfeited (usd per share)	$21.23	$0	$0
Outstanding, end of year (usd per share)	$19.06	$15.06	$16.21

ShareBased_Compensation_Weight

Share-Based Compensation - Weighted Average Fair Values and Related Assumptions for Options Granted (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation [Abstract]
Fair value per share (usd per share)	$7.28	$5.15	$3.46
Expected dividend yield (percent)	1.86%	2.40%	4.41%
Risk free interest rate (percent)	1.65%	1.26%	1.10%
Expected volatility factor (percent)	39.23%	37.59%	41.50%
Expected life in years	5 years 7 months 30 days	6 years 3 months 30 days	5 years 4 months 30 days

ShareBased_Compensation_Summar1

Share-Based Compensation - Summary of Stock Option Activity (Detail) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shares
Options Outstanding, beginning of year	3.9	3.9	3.8
Granted	0.3	0.5	0.4
Options, Exercised	-0.1	-0.2	0
Options, Canceled	-0.4	-0.3	-0.3
Options Outstanding, end of year	3.7	3.9	3.9
Weighted- Average Exercise Price
Outstanding, beginning of year (usd per share)	$22.01	$22.25	$22.79
Granted (usd per share)	$21.20	$16.82	$13.54
Exercised (usd per share)	$16.43	$16.82	$14.49
Canceled (usd per share)	$30.21	$19.52	$17.95
Outstanding, end of year (usd per share)	$21.29	$22.01	$22.25

ShareBased_Compensation_Summar2

Share-Based Compensation - Summary of Stock Options Outstanding and Exercisable (Detail) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Share Based Compensation Arrangement [Line Items]
Number (shares)	3.7	3.9	3.9	3.8
Weighted- Average Remaining Contractual Life	4 years 5 months 15 days
Weighted- Average Exercise Price (usd per share)	$21.29	$22.01	$22.25	$22.79
Number (shares)	2.8
Weighted- Average Exercise Price (usd per share)	$22.30
$10.01 - $20.00
Schedule Of Share Based Compensation Arrangement [Line Items]
Exercise price range, lower range limit	$10.01
Exercise price range, upper range limit	$20
Number (shares)	2.1
Weighted- Average Remaining Contractual Life	5 years 8 months 31 days
Weighted- Average Exercise Price (usd per share)	$16.47
Number (shares)	1.5
Weighted- Average Exercise Price (usd per share)	$16.65
$20.01 - $30.00
Schedule Of Share Based Compensation Arrangement [Line Items]
Exercise price range, lower range limit	$20.01
Exercise price range, upper range limit	$30
Number (shares)	1.3
Weighted- Average Remaining Contractual Life	3 years 1 month 5 days
Weighted- Average Exercise Price (usd per share)	$25.84
Number (shares)	1
Weighted- Average Exercise Price (usd per share)	$27.32
Greater than $30.00
Schedule Of Share Based Compensation Arrangement [Line Items]
Exercise price range, lower range limit	$30.01
Number (shares)	0.3
Weighted- Average Remaining Contractual Life	1 year 4 months 12 days
Weighted- Average Exercise Price (usd per share)	$33.45
Number (shares)	0.3
Weighted- Average Exercise Price (usd per share)	$33.45

Recovered_Sheet6

Net Earnings (Loss) per Common Share - Compilation of Basic and Diluted Net (Loss) Per Common Share (Detail) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net earnings for basic net earnings per common share	$65.40	$11.90	$3	$27.10	$16.40	$18.50	$6.20	$19.70	$107.40	$60.80	$10.70
Adjusted net earnings for dilutive net earnings per common share									$107.40	$60.80	$10.70
Denominator:
Weighted average shares for basic net earnings (loss) per common share (shares)									40.8	40.6	40.4
Effect of dilutive share-based awards (shares)									0.4	0.1	0.1
Adjusted weighted average shares for diluted net earnings (loss) per common share (shares)									41.2	40.7	40.5
Basic net earnings (loss) per common share (usd per share)	$1.60	$0.29	$0.07	$0.67	$0.40	$0.46	$0.15	$0.49	$2.63	$1.50	$0.26
Diluted net earnings (loss) per common share (usd per share)	$1.58	$0.28	$0.07	$0.66	$0.40	$0.45	$0.15	$0.49	$2.60	$1.49	$0.26

Recovered_Sheet7

Net Earnings (Loss) per Common Share - Options to Purchase Shares of Common Stock (Detail)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Number of options and awards (shares)	1.8	2.6	3.7

Reportable_Segments_Additional

Reportable Segments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2013
	Segment
Segment Reporting [Abstract]
Number of reportable segments (segment)	4

Reportable_Segments_Financial_

Reportable Segments - Financial Information Regarding Company's Reportable Segments (Detail) (USD $)	3 Months Ended								12 Months Ended			36 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Revenues from external sources:
Earned premiums, net									$212.40	$200	$232.90
Net investment income									74.7	72.8	75.4
Income (loss) before federal income tax									26.8	61.3	10.6	98.7
Intersegment revenues	295.7	291.3	294.4	291.3	289.7	292.7	285.3	285.3	1,172.70	1,153	1,150.10
Segment loss before federal income tax:
Income (loss) before federal income tax									26.8	61.3	10.6	98.7
Net investment income									74.7	72.8	75.4
Income (loss) before federal income tax									26.8	61.3	10.6	98.7
Segment assets:
Total segment assets	2,444.20				2,331.60				2,444.20	2,331.60		2,444.20
Reconciling items:
Total consolidated assets	2,766.90				2,496.40				2,766.90	2,496.40		2,766.90
Total investment operations segment
Segment assets:
Total segment assets	2,444.20				2,331.60				2,444.20	2,331.60		2,444.20
Intersegment revenues
Revenues from external sources:
Intersegment revenues									-5.1	-5.1	-9.5
Corporate expenses
Reconciling items:
Total consolidated assets	322.7				164.8				322.7	164.8		322.7
Reportable segments | Personal insurance
Revenues from external sources:
Earned premiums, net									451.4	464	469.8
Reportable segments | Business insurance
Revenues from external sources:
Earned premiums, net									381.8	364.2	327.2
Reportable segments | Specialty insurance
Revenues from external sources:
Earned premiums, net									240.9	226.8	245.1
Income (loss) before federal income tax									-85.6	-11.2	-48.1
Segment loss before federal income tax:
Income (loss) before federal income tax									-85.6	-11.2	-48.1
Income (loss) before federal income tax									-85.6	-11.2	-48.1
Reportable segments | Total insurance segments
Revenues from external sources:
Earned premiums, net									1,074.10	1,055	1,042.10
Income (loss) before federal income tax									-105.8	-34	-92
Segment loss before federal income tax:
Income (loss) before federal income tax									-105.8	-34	-92
Income (loss) before federal income tax									-105.8	-34	-92
Reportable segments | Net investment income
Revenues from external sources:
Net investment income									74.7	72.8	75.4
Segment loss before federal income tax:
Net investment income									74.7	72.8	75.4
Reportable segments | Net realized capital gains
Revenues from external sources:
Income (loss) before federal income tax									20.7	23.2	28.8
Segment loss before federal income tax:
Income (loss) before federal income tax									20.7	23.2	28.8
Income (loss) before federal income tax									20.7	23.2	28.8
Reportable segments | Total investment operations segment
Revenues from external sources:
Total investment operations segment									95.4	96	104.2
Segment loss before federal income tax:
Total investment operations segment									95.4	96	104.2
Reportable segments | Reportable segments
Revenues from external sources:
Intersegment revenues									1,169.50	1,151	1,146.30
Reportable segments | All other
Revenues from external sources:
Income (loss) before federal income tax									0	0.9	2
Segment loss before federal income tax:
Income (loss) before federal income tax									0	0.9	2
Income (loss) before federal income tax									0	0.9	2
Reportable segments | Personal insurance SAP underwriting loss
Revenues from external sources:
Income (loss) before federal income tax									-11.7	-12.6	-1.6
Segment loss before federal income tax:
Income (loss) before federal income tax									-11.7	-12.6	-1.6
Income (loss) before federal income tax									-11.7	-12.6	-1.6
Reportable segments | Business insurance SAP underwriting loss
Revenues from external sources:
Income (loss) before federal income tax									-8.5	-10.2	-42.3
Segment loss before federal income tax:
Income (loss) before federal income tax									-8.5	-10.2	-42.3
Income (loss) before federal income tax									-8.5	-10.2	-42.3
All other
Revenues from external sources:
Income (loss) before federal income tax									37.2	-1.6	-3.6
Segment loss before federal income tax:
Income (loss) before federal income tax									37.2	-1.6	-3.6
Income (loss) before federal income tax									37.2	-1.6	-3.6
All other | All other
Revenues from external sources:
Intersegment revenues									3.2	2	3.8
All other | GAAP adjustments
Revenues from external sources:
Income (loss) before federal income tax									45.8	11.8	7.2
Segment loss before federal income tax:
Income (loss) before federal income tax									45.8	11.8	7.2
Income (loss) before federal income tax									45.8	11.8	7.2
All other | Interest expense on corporate debt
Revenues from external sources:
Income (loss) before federal income tax									-5.4	-8.5	-7
Segment loss before federal income tax:
Income (loss) before federal income tax									-5.4	-8.5	-7
Income (loss) before federal income tax									-5.4	-8.5	-7
All other | Corporate expenses
Revenues from external sources:
Income (loss) before federal income tax									-3.2	-4.9	-3.8
Segment loss before federal income tax:
Income (loss) before federal income tax									-3.2	-4.9	-3.8
Income (loss) before federal income tax									-3.2	-4.9	-3.8
Intersegment revenues | Intersegment revenues
Revenues from external sources:
Intersegment revenues									5.1	5.1	9.5
External and Intersegment Sources
Revenues from external sources:
Intersegment revenues									1,177.80	1,158.10	1,159.60
Total consolidated
Revenues from external sources:
Intersegment revenues									$1,172.70	$1,153	$1,150.10

Quarterly_Financial_Data_Sched

Quarterly Financial Data - Schedule of Quarterly Financial Data (Detail) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$295.70	$291.30	$294.40	$291.30	$289.70	$292.70	$285.30	$285.30	$1,172.70	$1,153	$1,150.10
Income (loss) before federal income taxes	-17.1	13.1	3.1	27.7	16.3	18.7	6.3	20
Net income	$65.40	$11.90	$3	$27.10	$16.40	$18.50	$6.20	$19.70	$107.40	$60.80	$10.70
Earnings per common share:
Basic (usd per share)	$1.60	$0.29	$0.07	$0.67	$0.40	$0.46	$0.15	$0.49	$2.63	$1.50	$0.26
Diluted (usd per share)	$1.58	$0.28	$0.07	$0.66	$0.40	$0.45	$0.15	$0.49	$2.60	$1.49	$0.26

Recovered_Sheet8

Schedule I - Summary of Investments - Other Than Investments in Related Parties (Detail) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	$2,122.60
Fair value	2,287.90
Amount at which shown in the balance sheet	2,287.90
Equity securities: | Available-for-sale:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	235.5
Fair value	310.4
Amount at which shown in the balance sheet	310.4
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	5.3
Fair value	5.3
Amount at which shown in the balance sheet	5.3
Available-for-sale:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,117.30
Fair value	2,282.60
Amount at which shown in the balance sheet	2,282.60
Available-for-sale: | Fixed maturities:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1,831.30
Fair value	1,891.90
Amount at which shown in the balance sheet	1,891.90
Available-for-sale: | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	296.7
Fair value	309.3
Amount at which shown in the balance sheet	309.3
Available-for-sale: | Fixed maturities: | Obligations of states and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	742.5
Fair value	769.5
Amount at which shown in the balance sheet	769.5
Available-for-sale: | Fixed maturities: | Corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	333.4
Fair value	340.6
Amount at which shown in the balance sheet	340.6
Available-for-sale: | Fixed maturities: | U.S. government agencies residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	458.7
Fair value	472.5
Amount at which shown in the balance sheet	472.5
Available-for-sale: | Equity securities: | Large-cap securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	185.5
Fair value	242.2
Amount at which shown in the balance sheet	242.2
Available-for-sale: | Equity securities: | Small-cap securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	50
Fair value	68.2
Amount at which shown in the balance sheet	68.2
Available-for-sale: | Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	50.5
Fair value	80.3
Amount at which shown in the balance sheet	$80.30

Recovered_Sheet9

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Assets
Fixed maturities, available-for-sale, at fair value	$1,891,900,000	$1,830,100,000
Other invested assets	5,300,000	5,000,000
Cash and cash equivalents	86,300,000	80,300,000
Other assets	-10,700,000	-800,000
Due from affiliates	40,100,000	0
Federal income tax, net	97,400,000	11,900,000
Total assets	2,766,900,000	2,496,400,000
Liabilities and Stockholdersb Equity
Notes payable (affiliates $116.8 and $116.8, respectively)	100,800,000	100,800,000
Due to affiliate	0	1,300,000
Other liabilities (affiliated net ceded $5.1 and affiliated net assumed $20.0, respectively)	80,300,000	86,800,000
Total liabilities	1,894,000,000	1,711,400,000
Stockholdersb equity:
Common stock, without par value. Authorized 100.0 shares; 47.7 and 47.5 issued, respectively, at stated value of $2.50 per share	119,300,000	118,800,000
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	-116,000,000	-115,900,000
Additional paid-in capital	143,200,000	137,500,000
Accumulated other comprehensive income (affiliated net ceded $65.1 and $41.0, respectively)	71,700,000	80,800,000	84,200,000	63,800,000
Retained earnings	654,700,000	563,800,000
Stockholdersb equity per accompanying consolidated financial statements	872,900,000	785,000,000	737,200,000
Total liabilities and stockholders' equity	2,766,900,000	2,496,400,000
Class A Preferred stock
Stockholdersb equity:
Preferred stock, value	0	0
Class B Preferred stock
Stockholdersb equity:
Preferred stock, value	0	0
State Auto Financial
Assets
Investments in common stock of subsidiaries (equity method)	960,700,000	891,100,000
Fixed maturities, available-for-sale, at fair value	900,000	1,600,000
Other invested assets	2,800,000	2,600,000
Cash and cash equivalents	11,500,000	6,100,000	7,000,000	8,400,000
Other assets	300,000	400,000
Due from affiliates	0	1,500,000
Federal income tax, net	21,200,000	4,600,000
Total assets	997,400,000	907,900,000
Liabilities and Stockholdersb Equity
Notes payable (affiliates $116.8 and $116.8, respectively)	116,800,000	116,800,000
Due to affiliate	500,000	0
Other liabilities (affiliated net ceded $5.1 and affiliated net assumed $20.0, respectively)	7,200,000	6,100,000
Total liabilities	124,500,000	122,900,000
Stockholdersb equity:
Common stock, without par value. Authorized 100.0 shares; 47.7 and 47.5 issued, respectively, at stated value of $2.50 per share	119,300,000	118,800,000
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	-116,000,000	-115,900,000
Additional paid-in capital	143,200,000	137,500,000
Accumulated other comprehensive income (affiliated net ceded $65.1 and $41.0, respectively)	71,700,000	80,800,000
Retained earnings	654,700,000	563,800,000
Stockholdersb equity per accompanying consolidated financial statements	872,900,000	785,000,000
Total liabilities and stockholders' equity	997,400,000	907,900,000
State Auto Financial | Class A Preferred stock
Stockholdersb equity:
Preferred stock, value	0	0
State Auto Financial | Class B Preferred stock
Stockholdersb equity:
Preferred stock, value	$0	$0

Schedule_II_Condensed_Financia1

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Parenthetical) (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Condensed Financial Statements, Captions [Line Items]
Notes payable, affiliates	$15.50	$15.50
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	47,700,000	47,500,000
Common stock, stated value per share (usd per share)	$2.50	$2.50
Treasury Stock, Shares	6,800,000	6,800,000
Class A Preferred stock
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Class B Preferred stock
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Notes payable, affiliates	$116.80	$116.80
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	47,700,000	47,500,000
Common stock, stated value per share (usd per share)	$2.50	$2.50
Treasury Stock, Shares	6,800,000	6,800,000
State Auto Financial | Class A Preferred stock
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
State Auto Financial | Class B Preferred stock
Condensed Financial Statements, Captions [Line Items]
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0

Schedule_II_Condensed_Financia2

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Income (Detail) (USD $)	3 Months Ended								12 Months Ended			36 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net investment income									$74.70	$72.80	$75.40
Net realized gain on investments									20.7	23.2	29
Total revenues	295.7	291.3	294.4	291.3	289.7	292.7	285.3	285.3	1,172.70	1,153	1,150.10
Interest expense (affiliates $6.0, $3.2 and $0.7, respectively)									5.4	8.5	7
Other operating expenses									7.3	8.6	8.3
Total expenses									1,145.90	1,091.70	1,139.50
Income before federal income taxes									26.8	61.3	10.6	98.7
Federal income tax (benefit) expense									80.6	-0.5	0.1
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net investment income									0.3	0.3	0.2
Net realized gain on investments									0	0.7	0.4
Total revenues									0.3	1	0.6
Interest expense (affiliates $6.0, $3.2 and $0.7, respectively)									6	8.8	7
Other operating expenses									5.6	6.8	5.8
Total expenses									11.6	15.6	12.8
Income before federal income taxes									-11.3	-14.6	-12.2
Federal income tax (benefit) expense									-20.6	-4.8	-1.7
Net income (loss) before equity in net income of subsidiaries									9.3	-9.8	-10.5
Equity in net income (loss) of subsidiaries									98.1	70.6	21.2
Net Income (Loss) Attributable to Parent									$107.40	$60.80	$10.70

Schedule_II_Condensed_Financia3

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Income (Parenthetical) (Detail) (Parent [Member], USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Interest Expense Debt To Affiliates, Parent Company	$6	$3.20	$0.70

Schedule_II_Condensed_Financia4

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Comprehensive Income (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	$59.80	($27.10)	$53.50
Reclassification adjustments for gains realized in net income	-20.7	-23.2	-28.8
Income tax expense	-13.7	10.9	0.6
Total net unrealized holding gains (losses) on investments	25.4	-39.4	25.3
Other comprehensive (loss) income	-9.1	-3.4	20.4
Comprehensive Income (loss)	98.3	57.4	31.1
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	107.4	60.8	10.7
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	-0.2	0.5	0.7
Reclassification adjustments for gains realized in net income	0	-0.7	-0.4
Income tax expense	0.1	0	0
Total net unrealized holding gains (losses) on investments	-0.1	-0.2	0.3
Unrealized equity in subsidiaries	-9	-3.2	20.1
Other comprehensive (loss) income	-9.1	-3.4	20.4
Comprehensive Income (loss)	$98.30	$57.40	$31.10

Schedule_II_Condensed_Financia5

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net	$11.70	$13.50	$7.10
Share-based compensation	3.6	4.1	3.5
Net realized gain on investments	-20.7	-23.2	-29
Changes in operating assets and liabilities:
Other liabilities and due from affiliates	-50.8	-0.9	-9.8
Other assets	11.1	1.7	5.7
Excess tax benefits on share-based awards	0	0.1	0
Federal income taxes, net	-81.6	-0.3	12.3
Net cash provided by (used in) operating activities	75.6	72.1	-285.6
Cash flows from investing activities:
Purchases of equity securities - available-for-sale	-119	-86	-143
Purchases of other invested assets	-1.8	-5.5	-1.1
Maturities, calls and pay downs of fixed maturities - available-for-sale	232.4	174.6	257
Sales of fixed maturities - available-for-sale	163.5	113.3	332.8
Sales of equity securities - available-for-sale	99.2	106.4	101.8
Net cash (used in) provided by investing activities	-56.5	-23	9.3
Cash flows from financing activities:
Proceeds from issuance of common stock	3.5	4	1.6
Payments to acquire treasury stock	-0.1	-0.1	0
Payment of dividends	-16.5	-16.1	-22.3
Excess tax benefits on share-based awards	0	-0.1	0
Proceeds from long-term debt	0	85	0
Redemption of long-term debt	0	-100	0
Net cash used in financing activities	-13.1	-27.8	-20.7
Net increase (decrease) in cash and cash equivalents	6	21.3	-297
Cash and cash equivalents at end of year	86.3	80.3
Cash and cash equivalents at beginning of year	80.3
Supplemental disclosures:
Federal income tax received	1	0.8	-12.4
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)	5.2	8.5	7
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	107.4	60.8	10.7
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net	0	1.3	-0.3
Share-based compensation	0.9	1.6	0.4
Net realized gain on investments	0	-0.7	-0.4
Equity in net income from consolidated subsidiaries	-98.1	-70.6	-21.2
Changes in operating assets and liabilities:
Other liabilities and due from affiliates	0.5	0.1	1.5
Other assets	0	-0.5	-0.3
Excess tax benefits on share-based awards	0	0.1	0
Federal income taxes, net	-16.6	-3.1	-1.2
Net cash provided by (used in) operating activities	-5.9	-10	-10.2
Cash flows from investing activities:
Dividends received from consolidated subsidiaries	24	13.8	27
Purchases of equity securities - available-for-sale	0	0	-3.2
Purchases of other invested assets	-0.4	-0.2	-0.3
Maturities, calls and pay downs of fixed maturities - available-for-sale	0.5	0.2	2.5
Sales of fixed maturities - available-for-sale	0.3	3.9	0.3
Sales of equity securities - available-for-sale	0	3.7	3.2
Net cash (used in) provided by investing activities	24.4	21.4	29.5
Cash flows from financing activities:
Proceeds from issuance of common stock	3.5	4	1.6
Payments to acquire treasury stock	-0.1	-0.1	0
Payment of dividends	-16.5	-16.1	-22.3
Excess tax benefits on share-based awards	0	-0.1	0
Proceeds from long-term debt	0	100	0
Redemption of long-term debt	0	-100	0
Net cash used in financing activities	-13.1	-12.3	-20.7
Net increase (decrease) in cash and cash equivalents	5.4	-0.9	-1.4
Cash and cash equivalents at end of year	11.5	6.1	7
Cash and cash equivalents at beginning of year	6.1	7	8.4
Supplemental disclosures:
Federal income tax received	4.3	1.5	0.4
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)	($6)	($8)	($7)

Schedule_II_Condensed_Financia6

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Parenthetical) (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$0.70	$0.70	$0.70
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$6	$3.20	$0.70

Schedule_II_Condensed_Financia7

Schedule II - Condensed Financial Information of Registrant - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Jul. 11, 2013	Dec. 31, 2013	Dec. 31, 2014
	subsidiary
Condensed Financial Statements, Captions [Line Items]
Number of subsidiaries with shared credit agreements (subsidiaries)	2
Interest rate (percent)		5.28%	5.28%
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Dividends received from Stateco		$9,200,000
Notes Payable
Condensed Financial Statements, Captions [Line Items]
Number of credit agreements (agreements)	2
Debt instrument, issued		100,000,000	100,000,000
Fair Value		95,800,000	98,700,000
Interest rate (percent)	5.28%
Notes Payable | State Auto P&C
Condensed Financial Statements, Captions [Line Items]
Debt instrument, issued	85,000,000	85,000,000	85,000,000
Fair Value		81,400,000	83,900,000
Notes Payable | Milbank
Condensed Financial Statements, Captions [Line Items]
Debt instrument, issued	15,000,000	15,000,000	15,000,000
Fair Value		$14,400,000	14,800,000

Recovered_Sheet10

Schedule III - Supplementary Insurance Information (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	$126.50	$96.80	$91.70
Future benefits, claims and losses	973.6	950.8	928.7
Unearned premiums	612.4	491	481.6
Other policy claims and benefits payable	0	0	0
Premium revenue	1,074.10	1,055	1,042.10
Net investment income	74.7	72.8	75.4
Benefits, losses and settlement expenses	768.6	722.7	779
Amortization of deferred policy acquisition costs	221.8	209.6	213.1
Other operating expenses	140.1	145.2	132.8
Premiums written	1,194.20	1,062.10	1,055.30
Personal insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	48.9	31.5	30.1
Future benefits, claims and losses	201.9	223.7	231
Unearned premiums	270.7	189.9	188.4
Other policy claims and benefits payable	0	0	0
Premium revenue	451.4	464	469.8
Net investment income	0	0	0
Benefits, losses and settlement expenses	318.3	342.2	344.8
Amortization of deferred policy acquisition costs	75.4	74.7	76.6
Other operating expenses	52	55.5	46.6
Premiums written	532.1	465.4	469.5
Business insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	41.4	38.8	35
Future benefits, claims and losses	349.8	359.6	341.8
Unearned premiums	201.9	193.1	180.9
Other policy claims and benefits payable	0	0	0
Premium revenue	381.8	364.2	327.2
Net investment income	0	0	0
Benefits, losses and settlement expenses	231.3	221.8	222.5
Amortization of deferred policy acquisition costs	81.9	74.3	63.1
Other operating expenses	58.1	73.5	80.6
Premiums written	389.2	374.8	349.4
Specialty insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	36.2	26.5	26.6
Future benefits, claims and losses	421.9	367.5	355.9
Unearned premiums	139.8	108	112.3
Other policy claims and benefits payable	0	0	0
Premium revenue	240.9	226.8	245.1
Net investment income	0	0	0
Benefits, losses and settlement expenses	219	158.7	211.7
Amortization of deferred policy acquisition costs	64.5	60.6	73.4
Other operating expenses	30	16.2	5.6
Premiums written	272.9	221.9	236.4
Investment operations segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	0	0	0
Future benefits, claims and losses	0	0	0
Unearned premiums	0	0	0
Other policy claims and benefits payable	0	0	0
Premium revenue	0	0	0
Net investment income	74.7	72.8	75.4
Benefits, losses and settlement expenses	0	0	0
Amortization of deferred policy acquisition costs	0	0	0
Other operating expenses	0	0	0
Premiums written	$0	$0	$0

Schedule_IV_Reinsurance_Detail

Schedule IV - Reinsurance (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$882.60	$874.70	$833.30
Ceded to	861.7	855	809.2
Assumed from	1,074.10	1,055	1,042.10
Net Amount	1,074.10	1,055	1,042.10
Percentage of amount assumed to net (percent)	0.40%	0.40%	0.40%
Unaffiliated Companies
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	25.3	23.5	28.2
Assumed from	4.4	3.8	4.1
Affiliated entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	861.7	855	809.2
Assumed from	1,074.10	1,055	1,042.10
Net Amount	$1,074.10	$1,055	$1,042.10

Recovered_Sheet11

Schedule V - Valuation and Qualifying Accounts (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Valuation allowance for deferred tax assets:
Balance at beginning of period	$82.60	$100.50	$103.30
Deductions	82.6	17.9	2.8
Balance at end of period	$0	$82.60	$100.50