State Auto Financial (STFC) 10-K 2 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Feb. 26, 2016	Jun. 30, 2015
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Trading Symbol	STFC
Entity Registrant Name	State Auto Financial CORP
Entity Central Index Key	874,977
Current Fiscal Year End Date	--12-31
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)		41,442,395
Entity Public Float			$ 366,985,130

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $1,972.9 and $1,831.3, respectively)	$ 2,000.7	$ 1,891.9
Equity securities, available-for-sale, at fair value (cost $265.2 and $235.5, respectively)	310.6	310.4
Other invested assets, available-for-sale, at fair value (cost $56.9 and $50.5, respectively)	85.1	80.3
Other invested assets	5.3	5.3
Notes receivable from affiliate	70	70
Total investments	2,471.7	2,357.9
Cash and cash equivalents	58.1	86.3
Accrued investment income and other assets	36	33.8
Deferred policy acquisition costs (affiliated net ceded $15.0 and affiliated net assumed $46.8, respectively)	129.1	126.5
Reinsurance recoverable on losses and loss expenses payable	5.9	9.6
Prepaid reinsurance premiums	6.8	6.1
Due from affiliates	5.9	40.1
Current federal income taxes	4.9	1.1
Net deferred federal income taxes	102.5	97.4
Property and equipment, at cost (net of accumulated depreciation of $6.1 and $5.8, respectively)	7.6	8.1
Total assets	2,828.5	2,766.9
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $532.4 and $494.3, respectively)	1,053	983.2
Unearned premiums (affiliated net assumed $214.2 and $201.7, respectively)	616.3	612.4
Notes payable (affiliates $15.5 and $15.5, respectively)	100.8	100.8
Postretirement and pension benefits (affiliated net ceded $56.0 and $63.2, respectively)	104	117.3
Other liabilities (affiliated net ceded $8.4 and $5.1, respectively)	69.8	80.3
Total liabilities	1,943.9	1,894
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 48.1 and 47.7 shares issued, respectively, at stated value of $2.50 per share	120.4	119.3
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.3)	(116)
Additional paid-in capital	153.5	143.2
Accumulated other comprehensive income (affiliated net ceded $56.7 and $65.1, respectively)	37.6	71.7
Retained earnings	689.4	654.7
Total stockholders’ equity	884.6	872.9
Total liabilities and stockholders’ equity	2,828.5	2,766.9
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 ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fixed maturities, available-for-sale, amortized cost	$ 1,972.9	$ 1,831.3
Equity securities, available-for-sale, cost	265.2	235.5
Other invested assets, available-for-sale, cost	56.9	50.5
Deferred Policy Acquisitions Costs from Affiliates	(15)	46.8
Property and equipment, accumulated depreciation	6.3	6.1
Losses And Loss Expenses Payable From Affiliates	532.4	494.3
Unearned premiums, affiliates	214.2	201.7
Net Postretirement and Pension Benefits	(56)	(63.2)
Net Other Liabilities from Affiliates	(8.4)	(5.1)
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)	48,100,000	47,700,000
Common Stock, Par or Stated Value Per Share	$ 2.50	$ 2.50
Net Accumulated Other Comprehensive Income	$ (56.7)	$ (65.1)
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 Inco

Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Earned premiums (affiliated net assumed $437.6, $212.4 and $200.0, respectively)	$ 1,270.5	$ 1,074.1	$ 1,055
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	71.7	74.7	72.8
Net realized gain on investments:
Total other-than-temporary impairment losses	(7.9)	(2.5)	(4)
Portion of loss recognized in other comprehensive income	0	0	0
Other net realized investment gains	32.2	23.2	27.2
Total net realized gain on investments	24.3	20.7	23.2
Other income (affiliates $2.1, $1.9 and $2.0, respectively)	2.1	3.2	2
Total revenues	1,368.6	1,172.7	1,153
Losses and loss expenses (affiliated net assumed $293.3, $250.8 and $162.5, respectively)	862.8	771.3	719.8
Acquisition and operating expenses (affiliated net assumed $313.0, $156.9 and $172.7, respectively)	426.8	361.9	354.8
Interest expense (affiliates $0.7, $0.7 and $0.7, respectively)	5.4	5.4	8.5
Other expenses	6.3	7.3	8.6
Total expenses	1,301.3	1,145.9	1,091.7
Income before federal income taxes	67.3	26.8	61.3
Current	2.9	0.1	0.5
Deferred	13.2	(80.7)	0
Total federal income tax expense (benefit)	16.1	(80.6)	0.5
Net income	$ 51.2	$ 107.4	$ 60.8
Earnings per common share:
Basic (usd per share)	$ 1.25	$ 2.63	$ 1.50
Diluted (usd per share)	1.23	2.60	1.49
Dividends paid per common share (usd per share)	$ 0.4	$ 0.4	$ 0.40

Consolidated Statements of Inc5

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Earned premiums, net	$ 437.6	$ 212.4	$ 200
Net investment income, affiliates	4.9	4.9	4.9
Other income, affiliates	2.1	1.9	2
Losses and loss expenses, net affiliates	293.3	250.8	162.5
Net Acquisition and Operating Expenses	313	156.9	172.7
Interest Expense Debt To Affiliates	$ 0.7	$ 0.7	$ 0.7

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 51.2	$ 107.4	$ 60.8
Net unrealized holding (losses) gains on investments:
Unrealized holding (losses) gains arising during year	(39.2)	59.8	(27.1)
Reclassification adjustments for gains realized in net income	(24.7)	(20.7)	(23.2)
Income tax benefit (expense)	22.4	(13.7)	10.9
Total net unrealized holding (losses) gains on investments	(41.5)	25.4	(39.4)
Amortization of gain on derivative used in cash flow hedge	0	0	(0.1)
Net unrecognized benefit plan obligations:
Net actuarial gain (loss) arising during period	5.3	(54.4)	32.5
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(5.4)	(5.5)	(5.5)
Net actuarial loss	11.5	6.9	9.1
Income tax (expense) benefit	(4)	18.5	0
Total net unrecognized benefit plan obligations	7.4	(34.5)	36.1
Other comprehensive loss	(34.1)	(9.1)	(3.4)
Comprehensive income	$ 17.1	$ 98.3	$ 57.4

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common shares:	Treasury shares:	Additional paid-in capital:	Accumulated other comprehensive income:	Retained earnings:
Balance at beginning of year (shares) at Dec. 31, 2012		47.3	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Balance at end of year (shares) at Dec. 31, 2013		47.5	6.8
Balance at beginning of year at Dec. 31, 2012		$ 118.1	$ (115.8)	$ 131.6	$ 84.2	$ 519.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		0.7		3.4
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0
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 $10.4, $10.2 and $10.1, respectively)	(16.1)
Balance at end of year at Dec. 31, 2013	785	$ 118.8	$ (115.9)	137.5	80.8	563.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.2
Balance at end of year (shares) at Dec. 31, 2014		47.7	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 0.5		2.9
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0
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 $10.4, $10.2 and $10.1, respectively)	(16.5)
Balance at end of year at Dec. 31, 2014	872.9	$ 119.3	$ (116)	143.2	71.7	654.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.4
Balance at end of year (shares) at Dec. 31, 2015		48.1	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 1.1		5.2
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0.3
Shares acquired on stock option exercises and vested restricted shares			$ (0.3)
Stock options granted				4.8
Change in unrealized holding gains (losses) on investments, net of tax	(41.5)				(41.5)
Amortization of gain on derivative used in cash flow hedge					0
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	7.4				7.4
Net income	51.2					51.2
Cash dividends paid (affiliates $10.4, $10.2 and $10.1, respectively)	(16.5)
Balance at end of year at Dec. 31, 2015	$ 884.6	$ 120.4	$ (116.3)	$ 153.5	$ 37.6	$ 689.4

Consolidated Statements of Sto8

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Retained earnings
Cash dividends paid affiliates	$ 10.4	$ 10.2	$ 10.1

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 51.2	$ 107.4	$ 60.8
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization, net	15.7	11.7	13.5
Share-based compensation	4.5	3.6	4.1
Net realized gain on investments	(24.3)	(20.7)	(23.2)
Changes in operating assets and liabilities:
Deferred policy acquisition benefits	(2.6)	(29.7)	(5.1)
Accrued investment income and other assets	(2.4)	0.2	(2.3)
Postretirement and pension benefits	(6)	(7.3)	(5.3)
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	3	(1.9)	3.6
Other liabilities and due to/from affiliates, net	(39.9)	(50.8)	(0.9)
Losses and loss expenses payable	69.8	23.3	17.7
Unearned premiums	3.9	121.4	9.4
Excess tax (expense) benefit on share-based awards	(0.3)	0	0.1
Federal income taxes	13.7	(81.6)	(0.3)
Cash provided from December 31, 2014 unearned premium transfer related to the homeowners quota-share reinsurance arrangement	63.5	0	0
Net cash provided by operating activities	149.8	75.6	72.1
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(573.9)	(431.4)	(326.7)
Purchases of equity securities available-for-sale	(154)	(119)	(86)
Purchases of other invested assets	(6.9)	(1.8)	(5.5)
Maturities, calls and pay downs of fixed maturities available-for-sale	241	232.4	174.6
Sales of fixed maturities available-for-sale	180.7	163.5	113.3
Sales of equity securities available-for-sale	144.8	99.2	106.4
Sales of other invested assets available-for-sale	0.7	0.6	0.7
Net (disposals) additions of property and equipment	(0.1)	0	0.2
Net cash used in investing activities	(167.7)	(56.5)	(23)
Cash flows from financing activities:
Proceeds from issuance of common stock	6.2	3.5	4
Payments to acquire treasury stock	(0.3)	(0.1)	(0.1)
Payments of dividends (affiliates $10.4, $10.2 and $10.1, respectively)	(16.5)	(16.5)	(16.1)
Payment of credit facility issue costs	0	0	(0.5)
Excess tax expense (benefit) on share-based awards	0.3	0	(0.1)
Proceeds from long-term debt	0	0	85
Redemption of long-term debt	0	0	(100)
Net cash used in financing activities	(10.3)	(13.1)	(27.8)
Net (decrease) increase in cash and cash equivalents	(28.2)	6	21.3
Cash and cash equivalents at beginning of year	86.3	80.3	59
Cash and cash equivalents at end of year	58.1	86.3	80.3
Supplemental disclosures:
Interest paid (affiliates $0.7, $0.7 and $0.7, respectively)	5.3	5.2	8.5
Federal income taxes paid	$ 6.4	$ 1	$ 0.8

Consolidated Statements of Ca10

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Cash Flows [Abstract]
Payments of dividends, affiliates	$ 10.4	$ 10.2	$ 10.1
Interest paid, affiliates	$ 0.7	$ 0.7	$ 0.7

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
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. 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 basis 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 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 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. 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, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Balance, beginning of year $ 126.5 $ 96.8 $ 91.7 Acquisition costs deferred 285.6 251.5 214.6 Acquisition costs amortized to expense (283.0 ) (221.8 ) (209.5 ) Balance, end of year $ 129.1 $ 126.5 $ 96.8 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 $24.7 million and $23.8 million at December 31, 2015 and 2014 , 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 (loss) income. Other comprehensive (loss) 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. k. New Accounting Standards Pending Adoption of Recent Accounting Pronouncements Financial Instrument s - O verall: Recognition and Measurement of Financial Assets and Financial Liabilities In January 2016, the FASB issued guidance to improve certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Specifically the guidance (i) requires equity investments to be measured at fair value with changes in fair value recognized in earnings, (ii) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment, (iii) eliminates the requirement to disclose the methods and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost, (iv) requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes, (v) requires an entity to present separately in other comprehensive income the portion of the total change in fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option, (vi) requires separate presentation of financial assets and liabilities by measurement category and form on the balance sheet or the notes to the financial statements, and (vii) clarifies that the need for a valuation allowance on a deferred tax asset related to an available for sale security should be evaluated with other deferred tax assets. The guidance is effective beginning January 1, 2018 and the Company is currently evaluating the impact on the financial statements.

Investments

Investments	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 219.8 $ 6.6 $ (2.0 ) $ 224.4 Obligations of states and political subdivisions 804.0 22.5 (1.7 ) 824.8 Corporate securities 500.3 5.8 (11.7 ) 494.4 U.S. government agencies mortgage-backed securities 448.8 11.5 (3.2 ) 457.1 Total fixed maturities 1,972.9 46.4 (18.6 ) 2,000.7 Equity securities: Large-cap securities 211.9 34.2 (5.1 ) 241.0 Small-cap securities 53.3 16.5 (0.2 ) 69.6 Total equity securities 265.2 50.7 (5.3 ) 310.6 Other invested assets 56.9 28.3 (0.1 ) 85.1 Total available-for-sale securities $ 2,295.0 $ 125.4 $ (24.0 ) $ 2,396.4 ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 296.7 $ 14.1 $ (1.5 ) $ 309.3 Obligations of states and political subdivisions 742.5 27.4 (0.4 ) 769.5 Corporate securities 333.4 10.2 (3.0 ) 340.6 U.S. government agencies mortgage-backed securities 458.7 15.6 (1.8 ) 472.5 Total fixed maturities 1,831.3 67.3 (6.7 ) 1,891.9 Equity securities: Large-cap securities 185.5 57.3 (0.6 ) 242.2 Small-cap securities 50.0 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 $ 2,117.3 $ 172.6 $ (7.3 ) $ 2,282.6 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, 2015 and 2014 : ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2015 Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 68.6 $ (1.6 ) 15 $ 13.5 $ (0.4 ) 4 $ 82.1 $ (2.0 ) 19 Obligations of states and political subdivisions 137.5 (1.7 ) 16 — — — 137.5 (1.7 ) 16 Corporate securities 246.9 (5.3 ) 36 63.9 (6.4 ) 11 310.8 (11.7 ) 47 U.S. government agencies mortgage-backed securities 132.2 (2.3 ) 18 33.0 (0.9 ) 11 165.2 (3.2 ) 29 Total fixed maturities 585.2 (10.9 ) 85 110.4 (7.7 ) 26 695.6 (18.6 ) 111 Large-cap equity securities 65.8 (5.1 ) 24 — — — 65.8 (5.1 ) 24 Small-cap equity securities 3.4 (0.2 ) 1 — — — 3.4 (0.2 ) 1 Total equity securities 69.2 (5.3 ) 25 — — — 69.2 (5.3 ) 25 Other invested assets 8.1 (0.1 ) 1 — — — 8.1 (0.1 ) 1 Total temporarily impaired securities $ 662.5 $ (16.3 ) 111 $ 110.4 $ (7.7 ) 26 $ 772.9 $ (24.0 ) 137 ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2014 Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 19.9 $ (0.3 ) 4 $ 52.0 $ (1.2 ) 17 $ 71.9 $ (1.5 ) 21 Obligations of states and political subdivisions 6.1 — 5 30.9 (0.4 ) 9 37.0 (0.4 ) 14 Corporate securities 43.5 (0.9 ) 8 56.1 (2.1 ) 11 99.6 (3.0 ) 19 U.S. government agencies mortgage-backed securities 44.0 (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 ) 50 290.0 (6.7 ) 75 Large-cap equity securities 7.2 (0.6 ) 2 — — — 7.2 (0.6 ) 2 Total temporarily impaired securities $ 120.7 $ (2.1 ) 27 $ 176.5 $ (5.2 ) 50 $ 297.2 $ (7.3 ) 77 The 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, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Equity securities: Large-cap securities $ (2.2 ) $ (0.3 ) $ (1.8 ) Small-cap securities (5.7 ) (2.2 ) (2.2 ) Total other-than-temporary impairments $ (7.9 ) $ (2.5 ) $ (4.0 ) The Company reviewed its investments at December 31, 2015 , 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, 2015 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 26.4 $ 26.7 Due after 1 year through 5 years 492.9 499.4 Due after 5 years through 10 years 293.4 293.2 Due after 10 years 711.4 724.3 U.S. government agencies mortgage-backed securities 448.8 457.1 Total $ 1,972.9 $ 2,000.7 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, 2015 , 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 mortgage-backed securities” classification of the Company’s fixed maturity securities portfolio. Fixed maturities with fair values of approximately $8.8 million were on deposit with insurance regulators as required by law at December 31, 2015 and 2014 , respectively. The following table sets forth the components of net investment income for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Fixed maturities $ 61.3 $ 64.3 $ 63.2 Equity securities 6.5 6.2 6.0 Cash and cash equivalents, and other 5.9 6.2 5.7 Investment income 73.7 76.7 74.9 Investment expenses 2.0 2.0 2.1 Net investment income $ 71.7 $ 74.7 $ 72.8 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 2015 , 2014 and 2013 were $326.2 million , $263.3 million and $220.4 million , respectively. The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Realized gains: Fixed maturities $ 4.6 $ 3.1 $ 2.5 Equity securities 29.6 21.3 26.1 Other invested assets 0.2 0.1 0.1 Total realized gains 34.4 24.5 28.7 Realized losses: Equity securities: Sales (1.8 ) (1.3 ) (1.2 ) OTTI (7.9 ) (2.5 ) (4.0 ) Fixed maturities: Sales — — (0.3 ) Total realized losses (9.7 ) (3.8 ) (5.5 ) Net realized gains on investments $ 24.7 $ 20.7 $ 23.2 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ (32.8 ) $ 34.5 $ (102.8 ) Equity securities (29.5 ) 6.2 36.5 Other invested assets (1.6 ) (1.6 ) 16.0 Deferred federal income tax 22.4 (13.7 ) 17.6 Valuation allowance — — (6.7 ) Change in unrealized holding gains (losses), net of tax $ (41.5 ) $ 25.4 $ (39.4 )

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments Below is the fair value hierarchy that categorizes into three levels the inputs to valuation techniques that are used to measure fair value. • Level 1 includes observable inputs which reflect quoted prices for identical assets or liabilities in active markets at the measurement date. • Level 2 includes observable inputs for assets or liabilities other than quoted prices included in Level 1, and it includes valuation techniques which use prices for similar assets and liabilities. • Level 3 includes unobservable inputs which reflect the reporting entity’s estimates of the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk). The Company utilizes one nationally recognized pricing service to estimate the majority of its available-for-sale investment portfolio’s fair value. The Company obtains one price per security. 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, 2015 and 2014 , 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, 2015 and 2014 . 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 a security discussed below), and U.S. government agencies mortgage-backed securities. All unadjusted estimates of fair value for fixed maturities priced by the pricing service are included in the amounts disclosed in Level 2 of the hierarchy. If market inputs are unavailable, then no fair value is provided by the pricing service. For these securities, fair value is determined either by requesting brokers who are knowledgeable about these securities to provide a quote; or the Company internally determines the fair values by employing widely accepted pricing valuation models, and depending on the level of observable market inputs, renders the fair value estimate as Level 2 or Level 3. The Company holds one fixed maturity corporate security included in Level 3 and estimates its fair value using the present value of the future cash flows. 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 $77.0 million and $72.9 million at December 31, 2015 and 2014 , respectively, which was determined using each fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the funds including obtaining and reviewing each fund’s audited financial statements. There are no unfunded commitments related to the funds. The Company may not sell its investment in the funds; however, the Company may redeem all or a portion of its investment in the funds at net asset value per share with the appropriate prior written notice. The funds are disclosed in Level 2 within the hierarchy due to the Company’s ability to redeem its investment in the funds at net asset value per share at the measurement date. The remainder of the Company’s other invested assets consist of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds, based on an observable market price for an identical asset in an active market, reflect their fair values and consequently these securities have been disclosed in Level 1. The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at December 31, 2015 and 2014 : ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 224.4 $ — $ 224.4 $ — Obligations of states and political subdivisions 824.8 — 824.8 — Corporate securities 494.4 — 491.1 3.3 U.S. government agencies mortgage-backed securities 457.1 — 457.1 — Total fixed maturities 2,000.7 — 1,997.4 3.3 Equity securities: Large-cap securities 241.0 241.0 — — Small-cap securities 69.6 69.6 — — Total equity securities 310.6 310.6 — — Other invested assets 85.1 8.1 77.0 — Total available-for-sale investments $ 2,396.4 $ 318.7 $ 2,074.4 $ 3.3 ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 309.3 $ — $ 309.3 $ — Obligations of states and political subdivisions 769.5 — 769.5 — Corporate securities 340.6 — 331.2 9.4 U.S. government agencies mortgage-backed securities 472.5 — 472.5 — Total fixed maturities 1,891.9 — 1,882.5 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 $ 2,282.6 $ 317.8 $ 1,955.4 $ 9.4 For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for 2015 and 2014 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2015 $ 9.4 Total realized gains (losses)—included in earnings — Total unrealized gains (losses)—included in other comprehensive income (0.2 ) Purchases — Sales (5.9 ) Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2015 $ 3.3 ($ 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 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, 2015 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) December 31, 2015 December 31, 2014 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 74.1 7.00 % $ 70.0 $ 74.6 7.00 % 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) December 31, 2015 December 31, 2014 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest $ 85.3 $ 85.5 5.03 % $ 85.3 $ 86.4 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.5 15.5 4.61 % 15.5 15.5 4.44 % Total notes payable $ 100.8 $ 101.0 $ 100.8 $ 101.9

Losses and Loss Expenses Payabl

Losses and Loss Expenses Payable	12 Months Ended
	Dec. 31, 2015
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) 2015 2014 2013 Losses and loss expenses payable, at beginning of year $ 983.2 $ 959.9 $ 942.2 Less: reinsurance recoverable on losses and loss expenses payable 9.6 9.1 13.5 Net balance at beginning of year 973.6 950.8 928.7 Incurred related to: Current year 852.8 726.2 741.0 Prior years 10.0 45.1 (21.2 ) Total incurred 862.8 771.3 719.8 Paid related to: Current year 421.5 373.2 355.0 Prior years 367.8 375.3 342.7 Total paid 789.3 748.5 697.7 Net balance at end of year 1,047.1 973.6 950.8 Plus: reinsurance recoverable on losses and loss expenses payable 5.9 9.6 9.1 Losses and loss expenses payable, at end of year (affiliates $532.4, $494.3 and $438.0, respectively) $ 1,053.0 $ 983.2 $ 959.9 The Company recorded adverse development related to prior years’ loss and loss expense reserves in 2015 and 2014 of $10.0 million and $45.1 million , respectively, compared to favorable development in 2013 of $21.2 million . Favorable development of unallocated loss adjustment expenses and catastrophe reserves were approximately $6.2 million and $0.7 million , respectively, of the 2015 development. The personal and business insurance segments non-catastrophe loss and ALAE reserves contributed $9.8 million of unfavorable development. The personal insurance segment contributed $9.4 million of the adverse development, driven by personal auto which developed unfavorably by $11.0 million , primarily due to higher than anticipated bodily injury severity from the prior two accident years. The business insurance segment contributed $0.4 million of unfavorable development, driven by commercial auto which developed unfavorably by $10.5 million , also due to higher than anticipated bodily injury severity from the prior two accident years. The unfavorable development was partially offset by favorable development in workers’ compensation and other & product liability of $5.1 million and $3.8 million , respectively. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $7.1 million of adverse development, which was driven by programs and E&S casualty with unfavorable development of $9.7 million and $2.7 million , respectively. Unfavorable development in programs was driven by higher than expected severity in programs with commercial auto exposures. Somewhat offsetting the unfavorable development was favorable development of $5.3 million in the E&S property unit driven by lower than anticipated severity emerging from accident year 2014. 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 $28.7 million of favorable development. The personal insurance segment contributed $6.4 million of favorable development, primarily from accident year 2013. The business insurance segment contributed $22.3 million of favorable development, driven by the other & product liability, workers’ compensation and commercial auto lines with $11.9 million , $5.6 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 2013 and prior. 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 $91.5 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 $3.9 million in the E&S property unit and $3.1 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 $30.5 million of favorable development driven by the workers’ compensation, other & product liability, commercial auto and homeowners lines with $12.3 million , $8.3 million , $8.0 million and $2.9 million of favorable development, respectively. The favorable development in workers’ compensation was driven by better than anticipated severity emerging across all accident years, with approximately one third coming from accident year 2012. The favorable development in the other & product liability, commercial auto and homeowners 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 $22.7 million of adverse development, primarily driven by RED reserve strengthening of $21.3 million related to a large restaurant program and a commercial auto trucking program.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2015
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 years ended December 31, 2015 and 2014, the Company recognized profit commission of $4.2 million and $19.0 million , respectively, 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 $8.1 million of excess ceding commission that had previously been deferred, reflected as a reduction in acquisition and operating expenses on the consolidated statements of income for the year ended December 31, 2014. The following table sets forth the effect of the Company’s external reinsurance on its balance sheets at December 31, 2015 and 2014 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a: ($ millions) 2015 2014 Losses and loss expenses payable: Direct $ 515.6 $ 484.4 Assumed 5.0 4.5 Ceded (5.9 ) (9.6 ) Net losses and loss expenses payable $ 514.7 $ 479.3 Unearned premiums: Direct $ 400.9 $ 409.7 Assumed 1.2 1.0 Ceded (6.8 ) (6.1 ) Net unearned premiums $ 395.3 $ 404.6 The following table sets forth the effect of the Company’s external reinsurance on its income statements for the years ended December 31, 2015 , 2014 and 2013 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a: ($ millions) 2015 2014 2013 Written premiums: Direct $ 854.1 $ 880.6 $ 886.7 Assumed 4.7 4.4 3.9 Ceded (35.4 ) (26.8 ) (24.3 ) Net written premiums $ 823.4 $ 858.2 $ 866.3 Earned premiums: Direct $ 863.1 $ 882.6 $ 874.7 Assumed 4.5 4.4 3.8 Ceded (34.7 ) (25.3 ) (23.5 ) Net earned premiums $ 832.9 $ 861.7 $ 855.0 Losses and loss expenses incurred: Direct $ 569.0 $ 525.6 $ 559.1 Assumed 3.4 2.9 3.0 Ceded (2.9 ) (8.0 ) (4.8 ) Net losses and loss expenses incurred $ 569.5 $ 520.5 $ 557.3

Transactions with Affiliates

Transactions with Affiliates	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 : ($ millions) 2015 2014 Assets Deferred policy acquisition costs: Ceded $ (144.1 ) $ (79.7 ) Assumed 129.1 126.5 Net (ceded) assumed $ (15.0 ) $ 46.8 Liabilities and Stockholders’ Equity Losses and loss expenses payable: Ceded $ (514.7 ) $ (479.3 ) Assumed 1,047.1 973.6 Net assumed $ 532.4 $ 494.3 Unearned premiums: Ceded $ (395.3 ) $ (404.6 ) Assumed 609.5 606.3 Net assumed $ 214.2 $ 201.7 Pension and postretirement benefits: Ceded $ (160.0 ) $ (180.5 ) Assumed 104.0 117.3 Net ceded $ (56.0 ) $ (63.2 ) Other liabilities: Ceded $ (63.8 ) $ (70.7 ) Assumed 55.4 65.6 Net ceded $ (8.4 ) $ (5.1 ) Stockholders’ Equity Accumulated other comprehensive income: Ceded $ (162.0 ) $ (186.0 ) Assumed 105.3 120.9 Net ceded $ (56.7 ) $ (65.1 ) 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, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Written premiums: Ceded $ (823.4 ) $ (858.2 ) $ (866.3 ) Assumed 1,273.5 1,194.2 1,062.1 Net assumed $ 450.1 $ 336.0 $ 195.8 Earned premiums: Ceded $ (832.9 ) $ (861.7 ) $ (855.0 ) Assumed 1,270.5 1,074.1 1,055.0 Net assumed $ 437.6 $ 212.4 $ 200.0 Losses and loss expenses incurred: Ceded $ (570.9 ) $ (523.6 ) $ (560.2 ) Assumed 864.2 774.4 722.7 Net assumed $ 293.3 $ 250.8 $ 162.5 Acquisition and operating expenses: Ceded (119.5 ) (215.4 ) (192.6 ) Assumed 432.5 372.3 365.3 Net assumed $ 313.0 $ 156.9 $ 172.7 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, 2015 and 2014 is approximately $294.7 million and $297.0 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 4.61% at December 31, 2015 ). 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, 2015 , 2014 and 2013 , 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 $1.8 million , $1.5 million and $1.6 million in 2015 , 2014 and 2013 , respectively, and is included in other income (affiliates) on the consolidated statements of income.

Notes Payable and Credit Facili

Notes Payable and Credit Facility	12 Months Ended
	Dec. 31, 2015
Notes Payable [Abstract]
Notes Payable and Credit Facility	Notes Payable and Credit Facility FHLB Loan State Auto Financial’s subsidiary, State Auto P&C, is a member of the Federal Home Loan Bank of Cincinnati (the “FHLB”) and has a term loan with the FHLB in the principal amount of $85.0 million (the “FHLB Loan”). 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 State Auto P&C has a credit facility (the “SPC Credit Facility”) with a syndicate of lenders that 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, 2015 , 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, 2015
Income Tax Disclosure [Abstract]
Federal Income Taxes	Federal Income Taxes The following table sets forth the reconciliation between actual federal income tax expense (benefit) and the amount computed at the indicated statutory rate for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Amount at statutory rate $ 23.5 35.0 % $ 9.4 35.0 % $ 21.5 35.0 % Tax-exempt interest and dividends received deduction (8.7 ) (13.0 ) (8.5 ) (31.5 ) (9.4 ) (15.3 ) Other, net 1.3 1.9 1.1 4.1 0.2 0.4 Valuation allowance — — (82.6 ) (308.1 ) (11.8 ) (19.3 ) Federal income tax expense (benefit) and effective rate $ 16.1 23.9 % $ (80.6 ) (300.5 )% $ 0.5 0.8 % 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, 2015 and 2014 : ($ millions) 2015 2014 Deferred tax assets: Unearned premiums not currently deductible $ 42.5 $ 42.2 Losses and loss expenses payable discounting 19.3 21.0 Postretirement and pension benefits 36.4 41.1 Realized loss on other-than-temporary impairment 12.3 8.4 Other liabilities 21.3 22.2 Net operating loss carryforward 41.8 58.0 Tax credit carryforward 4.8 1.9 Other 4.9 4.7 Total deferred tax assets 183.3 199.5 Deferred tax liabilities: Deferral of policy acquisition costs 45.2 44.3 Net unrealized holding gains on investments 35.6 57.8 Total deferred tax liabilities 80.8 102.1 Net deferred federal income taxes $ 102.5 $ 97.4 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, 2015 , the tax benefit of the net operating loss (“NOL”) carryforward was $41.8 million . The NOL carryforwards do not begin to expire until 2030 and will not fully expire until 2032. At December 31, 2015 , the Company carried no balance for uncertain tax positions. The Company had no accrual for the payment of interest and penalties at December 31, 2015 or 2014 . 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 Bene

Pension and Postretirement Benefit Plans	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 : ($ millions) Pension Postretirement 2015 2014 2015 2014 Change in benefit obligation: Benefit obligation at beginning of year $ 292.5 $ 230.5 $ 23.9 $ 22.3 Service cost 7.7 4.6 — — Interest cost 11.1 10.8 0.9 1.1 Actuarial (gain) loss (17.6 ) 57.3 (3.3 ) 1.7 Benefits paid (12.4 ) (10.7 ) (1.2 ) (1.2 ) The Company’s portion of benefit obligation at end of year $ 281.3 $ 292.5 $ 20.3 $ 23.9 Change in plan assets available for plan benefits: Fair value of plan assets available for plan benefits at beginning of year $ 205.1 $ 185.9 $ 0.3 $ 1.3 Employer contribution 13.0 13.0 — — Actual return on plan assets (1.3 ) 16.9 — — Benefits paid (12.4 ) (10.7 ) (0.3 ) (1.0 ) The Company’s portion of fair value of plan assets at end of year $ 204.4 $ 205.1 $ — $ 0.3 Supplemental executive retirement plan (6.8 ) (6.3 ) — — Funded status at end of year $ (83.7 ) $ (93.7 ) $ (20.3 ) $ (23.6 ) Accumulated benefit obligation end of year $ 263.1 $ 260.1 No assets are expected to be returned during the fiscal year ending December 31, 2016. 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, 2015 and 2014 : ($ millions) 2015 2014 Prior service benefit $ (59.5 ) $ (64.9 ) Net actuarial loss 125.1 141.9 Total $ 65.6 $ 77.0 The following table sets forth the Company’s share of amortization expected to be recognized for the year ending December 31, 2016: ($ millions) 2016 Prior service benefit $ (5.4 ) Net actuarial loss 9.5 Total $ 4.1 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, 2015 , 2014 and 2013 : ($ millions) Pension Postretirement 2015 2014 2013 2015 2014 2013 Components of net periodic cost: Service cost $ 7.9 $ 5.2 $ 6.1 $ — $ — $ 0.4 Interest cost 11.3 11.1 9.6 1.1 1.1 1.2 Expected return on plan assets (13.8 ) (12.6 ) (12.2 ) — — (0.2 ) Amortization of: Prior service benefit — — — (5.4 ) (5.5 ) (5.5 ) Net actuarial loss 10.9 6.3 8.1 0.6 0.6 1.0 Net periodic cost (benefit) $ 16.3 $ 10.0 $ 11.6 $ (3.7 ) $ (3.8 ) $ (3.1 ) 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 2016 $ 10.9 $ 1.4 2017 11.2 1.5 2018 11.8 1.5 2019 12.2 1.4 2020 12.7 1.4 2021-2025 75.2 6.6 The postretirement plan’s gross benefit payments for 2015 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 2015 and estimates future annual subsidies to be approximately $0.3 million . The following table sets forth the weighted average assumptions used to determine the benefit plans’ obligations at December 31, 2015 and 2014 : Pension Postretirement 2015 2014 2015 2014 Benefit obligations weighted-average assumptions: Discount rate 4.20 % 3.85 % 4.20 % 3.85 % Rates of increase in compensation levels 3.50 3.50 — — The following table sets forth the weighted average assumptions used to determine the benefit plans’ net periodic cost for the years ended December 31, 2015 , 2014 and 2013 : Pension Postretirement 2015 2014 2013 2015 2014 2013 Weighted-average assumptions: Discount rate 3.85 % 4.85 % 4.05 % 3.85 % 4.85 % 4.05 % Expected long-term rate of return on assets 7.00 7.00 7.50 — — 7.50 Rates of increase in compensation levels 3.50 3.50 4.00 — — — 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, 2015 , 2014 and 2013 : Postretirement 2015 2014 2013 Assumed health care cost trend rates: Health care cost trend rate assumed for the next year 6.50 % 6.00 % 10.00 % Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 3.80 % 3.80 % 5.00 % Year that the rate reaches the ultimate trend rate 2076 2075 2018 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, 2015 : ($ 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 3.2 2.8 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, 2015 and 2014 . 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, 2015 and 2014 : ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 51.0 $ — $ 51.0 $ — Corporate securities 38.9 — 38.9 — U.S. government agencies mortgage-backed securities 6.4 — 6.4 — Total fixed maturities 96.3 — 96.3 — Equity securities: Large-cap securities 59.0 59.0 — — Small-cap securities 22.3 22.3 — — Total equity securities 81.3 81.3 — — International instruments 18.6 — 18.6 — Total pension plan investments $ 196.2 $ 81.3 $ 114.9 $ — ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2014 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 $ — During 2015, the postretirement plan assets were sold in order to pay retiree medical costs. The following table sets forth the Company’s share of the postretirement plan’s available-for-sale securities within the fair value hierarchy at December 31, 2014: ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2014 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 $ — 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 2016 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 , $5.1 million and $4.6 million for 2015 , 2014 and 2013 , respectively.

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2015
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, 2015 , 2014 and 2013 : ($ 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, 2015 $ 110.0 $ — $ (38.3 ) $ 71.7 Other comprehensive income before reclassifications (25.4 ) — 5.3 (20.1 ) Amounts reclassified from AOCI (a) (16.1 ) — 2.1 (14.0 ) Net current period other comprehensive income (41.5 ) — 7.4 (34.1 ) Ending balance at December 31, 2015 $ 68.5 $ — $ (30.9 ) $ 37.6 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 $ 110.0 $ — $ (38.3 ) $ 71.7 Beginning balance at January 1, 2013 $ 124.0 $ 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 (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, 2015 , 2014 and 2013 : ($ millions) Details about Accumulated Other December 31 Affected line item in the Condensed Comprehensive Income Components 2015 2014 2013 Consolidated Statements of Income Unrealized gains on available for sale securities $ 24.7 $ 20.7 $ 23.2 Realized gain on sale of securities 24.7 20.7 23.2 Total before tax (8.6 ) (7.2 ) 0.1 Tax (expense) benefit 16.1 13.5 23.3 Net of tax Amortization of gain on derivative used in cash flow hedge — — 0.1 Realized gain on sale of securities — — 0.1 Total before tax — — — Tax benefit (expense) — — 0.1 Net of tax Amortization of benefit plan items: Negative prior service costs 5.4 5.5 5.5 (a) Net loss (11.5 ) (6.9 ) (9.1 ) (a) (6.1 ) (1.4 ) (3.6 ) Total before tax 4.0 (18.5 ) — Tax expense (2.1 ) (19.9 ) (3.6 ) Net of tax Total reclassifications for the period $ 14.0 $ (6.4 ) $ 19.8 (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2015
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 $81.4 million is available for payment to State Auto Financial from its insurance subsidiaries in 2016 without prior approval. State Auto Financial received dividends from its insurance subsidiaries in the amount of $15.0 million , $20.0 million and $10.0 million in 2015 , 2014 and 2013 , respectively. The Company’s insurance subsidiaries are subject to risk-based capital (“RBC”) requirements that have been adopted by individual states. These requirements subject insurers having statutory capital less than that required by the RBC calculation to varying degrees of regulatory action, depending on the level of capital inadequacy. The RBC formulas specify various weighting factors to be applied to financial balances or various levels of activity based on the perceived degree of risk. Regulatory compliance is determined by a ratio of total adjusted capital to authorized control level RBC. Generally no remedial action is required by an insurance company if its adjusted statutory surplus exceeds 200% of the authorized level RBC. As of December 31, 2015 , each of the Company’s insurance subsidiaries maintained adjusted statutory surplus in excess of 450% of the authorized control level RBC. The following tables set forth reconciliations of statutory capital and surplus and net income, as determined using SAP, to the amounts included in the accompanying consolidated financial statements: ($ millions) 2015 2014 Statutory capital and surplus of insurance subsidiaries $ 814.3 $ 778.4 Net liabilities of non-insurance parent and affiliates (80.2 ) (80.8 ) 734.1 697.6 Increases (decreases): Deferred acquisition costs 129.1 126.5 Postretirement and pension benefits 23.2 27.2 Deferred federal income taxes (41.6 ) (49.7 ) Fixed maturities, at fair value 27.9 60.6 Other, net 11.9 10.7 Stockholders’ equity per accompanying consolidated financial statements $ 884.6 $ 872.9 ($ millions) Year ended December 31 2015 2014 2013 Statutory net income (loss) of insurance subsidiaries $ 65.4 $ (17.5 ) $ 51.1 Net loss of non-insurance parent and affiliates (4.1 ) (5.3 ) (5.1 ) 61.3 (22.8 ) 46.0 Increases (decreases): Deferred acquisition costs 2.5 29.7 5.1 Postretirement and pension benefits 4.2 10.6 10.3 Deferred federal income taxes (12.2 ) 79.3 (1.0 ) Share-based compensation expense (0.1 ) (0.5 ) (1.3 ) Other, net (4.5 ) 11.1 1.7 Net income per accompanying consolidated financial statements $ 51.2 $ 107.4 $ 60.8

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2015
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.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2015
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, 2015 , a total of 0.5 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 2015 , 2014 and 2013 were 0.3 million , 0.2 million and 0.5 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, 2015 , 2014 and 2013 : 2015 2014 2013 Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Outstanding, beginning of year 76,472 $ 19.06 38,233 $ 15.06 55,413 $ 16.21 Granted 74,020 22.83 56,178 21.23 — — Vested (35,859 ) 16.88 (16,892 ) 17.08 (17,180 ) 18.78 Canceled (3,549 ) 21.92 (1,047 ) — 21.23 — — Outstanding, end of year 111,084 $ 22.19 76,472 $ 19.06 38,233 $ 15.06 As of December 31, 2015 , there was $0.9 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 three 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 3.7 million common shares under this plan. As of December 31, 2015 , a total of 3.3 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 terminated on May 31, 2015 . It is the Company’s intent to seek shareholder approval for renewal of this plan with similar terms, in the 2016 Proxy Statement. The Company accounts for the RSU Plan as a liability plan. There were 26,184 RSUs, 25,960 RSUs, and 33,712 RSUs granted in 2015 , 2014 and 2013 , respectively. During 2015 and 2014 , common shares valued at approximately $567,000 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 2015 , 2014 and 2013 . 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, 2015 , 2014 and 2013 : 2015 2014 2013 Fair value per share $ 7.69 $ 7.28 $ 5.15 Expected dividend yield 1.75 % 1.86 % 2.40 % Risk free interest rate 1.60 % 1.65 % 1.26 % Expected volatility factor 36.61 % 39.23 % 37.59 % Expected life in years 6.0 5.7 6.3 The following table sets forth the Company’s total stock option activity and related information for these plans for the years ended December 31, 2015 , 2014 and 2013 : (millions, except per share amounts) 2015 2014 2013 Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Outstanding, beginning of year 3.7 $ 21.29 3.9 $ 22.01 3.9 $ 22.25 Granted 0.4 22.87 0.3 21.20 0.5 16.82 Exercised (0.3 ) 16.56 (0.1 ) 16.43 (0.2 ) 16.82 Canceled (0.4 ) 25.18 (0.4 ) 30.21 (0.3 ) 19.52 Outstanding, end of year 3.4 $ 21.44 3.7 $ 21.29 3.9 $ 22.01 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, 2015 , 2014 and 2013 , the total intrinsic value of stock options exercised was $2.6 million , $0.5 million and $0.5 million , respectively. The tax benefit for tax deductions from share-based awards totaled $0.3 million, $0, and $0 for the years ended December 31, 2015 , 2014 and 2013 , respectively. The following table sets forth information pertaining to the total options outstanding and exercisable at December 31, 2015 : (Options in millions) Options Outstanding Options Exercisable Number Weighted- Average Remaining Contractual Life Weighted- Average Exercise Price Number Weighted- Average Exercise Price Range of Exercise Prices: $10.01 – $20.00 1.7 5.0 $ 16.53 1.5 $ 16.53 $20.01 – $30.00 1.4 4.1 25.01 0.8 26.89 Greater than $30.00 0.3 0.4 33.48 0.3 33.48 3.4 4.2 $ 21.44 2.6 $ 21.67 Aggregate intrinsic value for total options outstanding at December 31, 2015 was $13.5 million . Aggregate intrinsic value for total options exercisable at December 31, 2015 was $6.2 million . Compensation expense recognized during 2015 , 2014 and 2013 was $4.5 million , $3.6 million and $4.1 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, 2015 , there was $2.6 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

Net Earnings (Loss) Per Common Share	12 Months Ended
	Dec. 31, 2015
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, 2015 , 2014 and 2013 : (millions, except per share amounts) 2015 2014 2013 Numerator: Net earnings for basic net earnings per common share $ 51.2 $ 107.4 $ 60.8 Adjusted net earnings for dilutive net earnings per common share $ 51.2 $ 107.4 $ 60.8 Denominator: Weighted average shares for basic net earnings per common share 41.1 40.8 40.6 Effect of dilutive share-based awards 0.5 0.4 0.1 Adjusted weighted average shares for diluted net earnings per common share 41.6 41.2 40.7 Basic net earnings per common share $ 1.25 $ 2.63 $ 1.50 Diluted net earnings per common share $ 1.23 $ 2.60 $ 1.49 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, 2015 , 2014 and 2013 : (millions) 2015 2014 2013 Total number of antidilutive options and awards 1.5 1.8 2.6

Reportable Segments

Reportable Segments	12 Months Ended
	Dec. 31, 2015
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 primarily provides personal automobile and homeowners to the personal insurance market. The business insurance segment primarily provides commercial automobile, commercial multi-peril, fire & allied, general liability and workers’ compensation 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 a result of changes to our reporting structure that occurred during late 2015, effective December 31, 2015, the workers’ compensation unit moved from the specialty insurance segment to the business insurance segment. Prior reporting periods have been restated to conform to the new presentation. The following table sets forth financial information regarding the Company’s reportable segments for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Revenues from external sources: Insurance segments Personal insurance $ 591.8 $ 451.4 $ 464.0 Business insurance 476.0 459.9 433.5 Specialty insurance 202.7 162.8 157.5 Total insurance segments 1,270.5 1,074.1 1,055.0 Investment operations segment Net investment income 71.7 74.7 72.8 Net realized capital gains 24.7 20.7 23.2 Total investment operations segment 96.4 95.4 96.0 Total revenue from reportable segments 1,366.9 1,169.5 1,151.0 All other 1.7 3.2 2.0 Total revenues from external sources 1,368.6 1,172.7 1,153.0 Intersegment revenues 5.6 5.1 5.1 Total revenues 1,374.2 1,177.8 1,158.1 Reconciling items: Eliminate intersegment revenues (5.6 ) (5.1 ) (5.1 ) Total consolidated revenue $ 1,368.6 $ 1,172.7 $ 1,153.0 Segment loss before federal income tax: Insurance segments: Personal insurance SAP underwriting gain (loss) $ 17.5 $ (11.7 ) $ (12.6 ) Business insurance SAP underwriting loss (36.7 ) (4.6 ) (2.7 ) Specialty insurance SAP underwriting loss (5.9 ) (89.5 ) (18.7 ) Total insurance segments (25.1 ) (105.8 ) (34.0 ) Investment operations segment: Net investment income 71.7 74.7 72.8 Net realized capital gains 24.7 20.7 23.2 Total investment operations segment 96.4 95.4 96.0 All other segments income (0.1 ) — 0.9 Reconciling items: GAAP adjustments 4.2 45.8 11.8 Interest expense on corporate debt (5.4 ) (5.4 ) (8.5 ) Corporate expenses (2.7 ) (3.2 ) (4.9 ) Total reconciling items $ (3.9 ) $ 37.2 $ (1.6 ) Total consolidated income before federal income taxes $ 67.3 $ 26.8 $ 61.3 The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2015 and 2014 : ($ millions) 2015 2014 Segment assets: Investment operations segment $ 2,529.8 $ 2,444.2 Total segment assets 2,529.8 2,444.2 Reconciling items: Corporate assets 298.7 322.7 Total consolidated assets $ 2,828.5 $ 2,766.9 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, 2015
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	Quarterly Financial Data (unaudited) The following tables set forth quarterly financial data for 2015 and 2014 : ($ millions, except per share amounts) 2015 For three months ended March 31 June 30 September 30 December 31 Total revenues $ 334.9 $ 337.4 $ 349.7 $ 346.6 Income before federal income taxes 33.2 3.4 29.9 0.8 Net income 24.7 2.7 20.7 3.1 Earnings per common share: Basic $ 0.60 $ 0.06 $ 0.50 $ 0.08 Diluted $ 0.60 $ 0.06 $ 0.50 $ 0.07 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 3.0 11.9 65.4 Earnings per common share: Basic $ 0.67 $ 0.07 $ 0.29 $ 1.60 Diluted $ 0.66 $ 0.07 $ 0.28 $ 1.58

Contingencies

Contingencies	12 Months Ended
	Dec. 31, 2015
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 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 results of operations or have a material adverse effect on its consolidated financial position or cash flows.

SCHEDULE I - SUMMARY OF INVESTM

SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2015
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, 2015 ($ millions) December 31, 2015 Cost or amortized cost (1) Fair value Amount at which shown in the balance sheet Available-for-sale: Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 219.8 $ 224.4 $ 224.4 Obligations of states and political subdivisions 804.0 824.8 824.8 Corporate securities 500.3 494.4 494.4 U.S. government agencies residential mortgage-backed securities 448.8 457.1 457.1 Total fixed maturities 1,972.9 2,000.7 2,000.7 Equity securities: Large-cap securities 211.9 241.0 241.0 Small-cap securities 53.3 69.6 69.6 Total equity securities 265.2 310.6 310.6 Other invested assets 56.9 85.1 85.1 Total available-for-sale securities 2,295.0 2,396.4 2,396.4 Other invested assets 5.3 5.3 5.3 Total investments – other than investments in related parties $ 2,300.3 $ 2,401.7 $ 2,401.7 (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 FINANCI

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT	12 Months Ended
	Dec. 31, 2015
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) December 31 2015 2014 Assets Investments in common stock of subsidiaries (equity method) $ 972.5 $ 960.7 Fixed maturities, available-for-sale, at fair value 0.9 0.9 Other invested assets 3.0 2.8 Cash and cash equivalents 10.5 11.5 Other assets 0.3 0.3 Federal income tax, net 21.9 21.2 Total assets $ 1,009.1 $ 997.4 Liabilities and Stockholders’ Equity Notes payable (affiliates $116.8 and $116.8, respectively) $ 116.8 $ 116.8 Due to affiliates 0.6 0.5 Other liabilities 7.1 7.2 Total liabilities 124.5 124.5 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; 48.1 and 47.7 shares issued, respectively, at stated value of $2.50 per share 120.4 119.3 Treasury stock, 6.8 and 6.8 shares, respectively, at cost (116.3 ) (116.0 ) Additional paid-in capital 153.5 143.2 Accumulated other comprehensive income 37.6 71.7 Retained earnings 689.4 654.7 Total stockholders’ equity 884.6 872.9 Total liabilities and stockholders’ equity $ 1,009.1 $ 997.4 Condensed Statements of Income ($ millions) Year ended December 31 2015 2014 2013 Net investment income $ 0.3 $ 0.3 $ 0.3 Net realized gain on investments — — 0.7 Total revenues 0.3 0.3 1.0 Interest expense (affiliates $6.0, $6.0 and $3.2, respectively) 6.0 6.0 8.8 Other operating expenses 4.9 5.6 6.8 Total expenses 10.9 11.6 15.6 Loss before federal income taxes (10.6 ) (11.3 ) (14.6 ) Federal income tax benefit (2.8 ) (20.6 ) (4.8 ) Net (loss) income before equity in net income of subsidiaries (7.8 ) 9.3 (9.8 ) Equity in net income of subsidiaries 59.0 98.1 70.6 Net income $ 51.2 $ 107.4 $ 60.8 Condensed Statements of Comprehensive Income ($ millions, except per share amounts) Year ended December 31 2015 2014 2013 Net income $ 51.2 $ 107.4 $ 60.8 Other comprehensive loss, net of tax: Net unrealized holding (losses) gains on investments: Unrealized holding (losses) gains arising during the year (0.1 ) (0.2 ) 0.5 Reclassification adjustments for gains realized in net income — — (0.7 ) Income tax benefit — 0.1 — Total net unrealized holding losses on investments (0.1 ) (0.1 ) (0.2 ) Unrealized equity in subsidiaries (34.0 ) (9.0 ) (3.2 ) Other comprehensive loss (34.1 ) (9.1 ) (3.4 ) Comprehensive income $ 17.1 $ 98.3 $ 57.4 Condensed Statements of Cash Flows ($ millions) Year ended December 31 2015 2014 2013 Cash flows from operating activities: Net income $ 51.2 $ 107.4 $ 60.8 Adjustments to reconcile net income to net cash used in operating activities: Depreciation and amortization, net — — 1.3 Share-based compensation (0.3 ) 0.9 1.6 Net realized gain on investments — — (0.7 ) Equity in net income from consolidated subsidiaries (59.0 ) (98.1 ) (70.6 ) Changes in operating assets and liabilities: Other liabilities and due from affiliates 0.9 0.5 0.1 Other assets — — 0.5 Excess tax benefits on share-based awards (0.5 ) — 0.1 Federal income taxes, net (0.7 ) (16.6 ) (3.1 ) Net cash used in operating activities (8.4 ) (5.9 ) (10.0 ) Cash flows from investing activities: Dividends received from consolidated subsidiaries 18.0 24.0 13.8 Purchases of other invested assets (0.3 ) (0.4 ) (0.2 ) Maturities, calls and pay downs of fixed maturities – available-for-sale — 0.5 0.2 Sales of fixed maturities – available-for-sale — 0.3 3.9 Sales of equity securities – available-for-sale — — 3.7 Net cash provided by investing activities 17.7 24.4 21.4 Cash flows from financing activities: Proceeds from issuance of common stock 6.2 3.5 4.0 Payments to acquire treasury stock (0.3 ) (0.1 ) (0.1 ) Payment of dividends (16.5 ) (16.5 ) (16.1 ) Excess tax benefits on share-based awards 0.3 — (0.1 ) Proceeds from long-term debt — — 100.0 Redemption of long-term debt — — (100.0 ) Net cash used in financing activities (10.3 ) (13.1 ) (12.3 ) Net (decrease) increase in cash and cash equivalents (1.0 ) 5.4 (0.9 ) Cash and cash equivalents at beginning of year 11.5 6.1 7.0 Cash and cash equivalents at end of year $ 10.5 $ 11.5 $ 6.1 Supplemental Disclosures: Federal income tax received $ 2.3 $ 4.3 $ 1.5 Interest paid (affiliates $6.0, $6.0 and $3.2, respectively) $ (6.0 ) $ (6.0 ) $ (8.0 ) 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. 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) December 31, 2015 December 31, 2014 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate Affiliate note payable with Milbank, issued $15.0, July 2013 with fixed interest $ 15.0 $ 15.0 5.28 % 15.0 14.8 5.28 % Affiliate note payable with State Auto P&C, issued $85.0, July 2013 with fixed interest 85.0 85.1 5.28 % 85.0 83.9 5.28 % Total notes payable to affiliates $ 100.0 $ 100.1 100.0 98.7

SCHEDULE III - SUPPLEMENTARY IN

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2015
Supplementary Insurance Information [Abstract]
SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	SCHEDULE III – SUPPLEMENTARY INSURANCE INFORMATION Years Ended December 31, 2015 , 2014 and 2013 ($ millions) Segment Deferred policy acquisition cost Future benefits, claims and losses (1) Unearned premiums Other policy claims and benefits payable Premium revenue Year ended December 31, 2015: Personal insurance segment $ 47.6 $ 227.0 $ 261.9 $ — $ 591.8 Business insurance segment 46.0 553.6 245.5 — 476.0 Specialty insurance segment 35.5 266.5 108.9 — 202.7 Investment operations segment — — — — — Total $ 129.1 $ 1,047.1 $ 616.3 $ — $ 1,270.5 Year ended December 31, 2014: Personal insurance segment $ 48.9 $ 201.9 $ 270.7 $ — $ 451.4 Business insurance segment 44.5 503.4 241.1 — 459.9 Specialty insurance segment 33.1 268.3 100.6 — 162.8 Investment operations segment — — — — — Total $ 126.5 $ 973.6 $ 612.4 $ — $ 1,074.1 Year ended December 31, 2013: Personal insurance segment $ 31.5 $ 223.7 $ 189.9 $ — $ 464.0 Business insurance segment 41.8 511.6 226.6 — 433.5 Specialty insurance segment 23.5 215.5 74.5 — 157.5 Investment operations segment — — — — — Total $ 96.8 $ 950.8 $ 491.0 $ — $ 1,055.0 Segment Net investment income Benefits, losses and settlement expenses (2) Amort. of deferred policy acquisition costs Other operating expenses Premiums written Year ended December 31, 2015: Personal insurance segment $ — $ 403.6 $ 105.4 $ 63.6 $ 583.0 Business insurance segment — 331.3 104.9 74.6 479.5 Specialty insurance segment — 129.4 72.7 5.6 211.0 Investment operations segment 71.7 — — — — Total $ 71.7 $ 864.3 $ 283.0 $ 143.8 $ 1,273.5 Year ended December 31, 2014: Personal insurance segment $ — $ 318.3 $ 75.4 $ 52.0 $ 532.1 Business insurance segment — 280.0 88.0 74.5 473.1 Specialty insurance segment — 176.2 58.4 13.6 189.0 Investment operations segment 74.7 — — — — Total $ 74.7 $ 774.5 $ 221.8 $ 140.1 $ 1,194.2 Year ended December 31, 2013: Personal insurance segment $ — $ 342.2 $ 74.7 $ 55.5 $ 465.4 Business insurance segment — 262.3 81.4 87.1 446.8 Specialty insurance segment — 118.2 53.5 2.6 149.9 Investment operations segment 72.8 — — — — Total $ 72.8 $ 722.7 $ 209.6 $ 145.2 $ 1,062.1 (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, 2015
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
SCHEDULE IV - REINSURANCE	SCHEDULE IV – REINSURANCE Years Ended December 31, 2015 , 2014 and 2013 ($ million, except percentages) Ceded to Assumed from Gross Amount Unaffiliated Companies Affiliated Companies (1) Unaffiliated Companies Affiliated Companies (1) Net Amount Percentage of amount assumed to net (2) Property-casualty earned premiums for year ended December 31, 2015 $ 863.1 $ 34.7 $ 832.9 $ 4.5 $ 1,270.5 $ 1,270.5 0.4 % 2014 882.6 25.3 861.7 4.4 1,074.1 1,074.1 0.4 % 2013 874.7 23.5 855.0 3.8 1,055.0 1,055.0 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 QUAL

SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]
SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	SCHEDULE V – VALUATION AND QUALIFYING ACCOUNTS Years Ended December 31, 2015 , 2014 and 2013 ($ millions) At December 31 2015 2014 2013 Valuation allowance for deferred tax assets: Balance at beginning of period $ — $ 82.6 $ 100.5 Deductions — 82.6 17.9 Balance at end of period $ — $ — $ 82.6

Summary of Significant Accoun33

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
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. 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 basis 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 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 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. 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, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Balance, beginning of year $ 126.5 $ 96.8 $ 91.7 Acquisition costs deferred 285.6 251.5 214.6 Acquisition costs amortized to expense (283.0 ) (221.8 ) (209.5 ) Balance, end of year $ 129.1 $ 126.5 $ 96.8
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 $24.7 million and $23.8 million at December 31, 2015 and 2014 , 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 (loss) income. Other comprehensive (loss) 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 Accoun34

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Schedule of Net Deferred Acquisition Costs	The following table sets forth net deferred acquisition costs for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Balance, beginning of year $ 126.5 $ 96.8 $ 91.7 Acquisition costs deferred 285.6 251.5 214.6 Acquisition costs amortized to expense (283.0 ) (221.8 ) (209.5 ) Balance, end of year $ 129.1 $ 126.5 $ 96.8

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 219.8 $ 6.6 $ (2.0 ) $ 224.4 Obligations of states and political subdivisions 804.0 22.5 (1.7 ) 824.8 Corporate securities 500.3 5.8 (11.7 ) 494.4 U.S. government agencies mortgage-backed securities 448.8 11.5 (3.2 ) 457.1 Total fixed maturities 1,972.9 46.4 (18.6 ) 2,000.7 Equity securities: Large-cap securities 211.9 34.2 (5.1 ) 241.0 Small-cap securities 53.3 16.5 (0.2 ) 69.6 Total equity securities 265.2 50.7 (5.3 ) 310.6 Other invested assets 56.9 28.3 (0.1 ) 85.1 Total available-for-sale securities $ 2,295.0 $ 125.4 $ (24.0 ) $ 2,396.4 ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 296.7 $ 14.1 $ (1.5 ) $ 309.3 Obligations of states and political subdivisions 742.5 27.4 (0.4 ) 769.5 Corporate securities 333.4 10.2 (3.0 ) 340.6 U.S. government agencies mortgage-backed securities 458.7 15.6 (1.8 ) 472.5 Total fixed maturities 1,831.3 67.3 (6.7 ) 1,891.9 Equity securities: Large-cap securities 185.5 57.3 (0.6 ) 242.2 Small-cap securities 50.0 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 $ 2,117.3 $ 172.6 $ (7.3 ) $ 2,282.6
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, 2015 and 2014 : ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2015 Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 68.6 $ (1.6 ) 15 $ 13.5 $ (0.4 ) 4 $ 82.1 $ (2.0 ) 19 Obligations of states and political subdivisions 137.5 (1.7 ) 16 — — — 137.5 (1.7 ) 16 Corporate securities 246.9 (5.3 ) 36 63.9 (6.4 ) 11 310.8 (11.7 ) 47 U.S. government agencies mortgage-backed securities 132.2 (2.3 ) 18 33.0 (0.9 ) 11 165.2 (3.2 ) 29 Total fixed maturities 585.2 (10.9 ) 85 110.4 (7.7 ) 26 695.6 (18.6 ) 111 Large-cap equity securities 65.8 (5.1 ) 24 — — — 65.8 (5.1 ) 24 Small-cap equity securities 3.4 (0.2 ) 1 — — — 3.4 (0.2 ) 1 Total equity securities 69.2 (5.3 ) 25 — — — 69.2 (5.3 ) 25 Other invested assets 8.1 (0.1 ) 1 — — — 8.1 (0.1 ) 1 Total temporarily impaired securities $ 662.5 $ (16.3 ) 111 $ 110.4 $ (7.7 ) 26 $ 772.9 $ (24.0 ) 137 ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2014 Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 19.9 $ (0.3 ) 4 $ 52.0 $ (1.2 ) 17 $ 71.9 $ (1.5 ) 21 Obligations of states and political subdivisions 6.1 — 5 30.9 (0.4 ) 9 37.0 (0.4 ) 14 Corporate securities 43.5 (0.9 ) 8 56.1 (2.1 ) 11 99.6 (3.0 ) 19 U.S. government agencies mortgage-backed securities 44.0 (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 ) 50 290.0 (6.7 ) 75 Large-cap equity securities 7.2 (0.6 ) 2 — — — 7.2 (0.6 ) 2 Total temporarily impaired securities $ 120.7 $ (2.1 ) 27 $ 176.5 $ (5.2 ) 50 $ 297.2 $ (7.3 ) 77
Realized Losses Related to Other-Than-Temporary Impairments	The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Equity securities: Large-cap securities $ (2.2 ) $ (0.3 ) $ (1.8 ) Small-cap securities (5.7 ) (2.2 ) (2.2 ) Total other-than-temporary impairments $ (7.9 ) $ (2.5 ) $ (4.0 )
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, 2015 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 26.4 $ 26.7 Due after 1 year through 5 years 492.9 499.4 Due after 5 years through 10 years 293.4 293.2 Due after 10 years 711.4 724.3 U.S. government agencies mortgage-backed securities 448.8 457.1 Total $ 1,972.9 $ 2,000.7
Components of Net Investment Income	The following table sets forth the components of net investment income for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Fixed maturities $ 61.3 $ 64.3 $ 63.2 Equity securities 6.5 6.2 6.0 Cash and cash equivalents, and other 5.9 6.2 5.7 Investment income 73.7 76.7 74.9 Investment expenses 2.0 2.0 2.1 Net investment income $ 71.7 $ 74.7 $ 72.8
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, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Realized gains: Fixed maturities $ 4.6 $ 3.1 $ 2.5 Equity securities 29.6 21.3 26.1 Other invested assets 0.2 0.1 0.1 Total realized gains 34.4 24.5 28.7 Realized losses: Equity securities: Sales (1.8 ) (1.3 ) (1.2 ) OTTI (7.9 ) (2.5 ) (4.0 ) Fixed maturities: Sales — — (0.3 ) Total realized losses (9.7 ) (3.8 ) (5.5 ) Net realized gains on investments $ 24.7 $ 20.7 $ 23.2 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ (32.8 ) $ 34.5 $ (102.8 ) Equity securities (29.5 ) 6.2 36.5 Other invested assets (1.6 ) (1.6 ) 16.0 Deferred federal income tax 22.4 (13.7 ) 17.6 Valuation allowance — — (6.7 ) Change in unrealized holding gains (losses), net of tax $ (41.5 ) $ 25.4 $ (39.4 )

Fair Value of Financial Instr36

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Company's Available-for-Sale Investments within Fair Value Hierarchy	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at December 31, 2015 and 2014 : ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 224.4 $ — $ 224.4 $ — Obligations of states and political subdivisions 824.8 — 824.8 — Corporate securities 494.4 — 491.1 3.3 U.S. government agencies mortgage-backed securities 457.1 — 457.1 — Total fixed maturities 2,000.7 — 1,997.4 3.3 Equity securities: Large-cap securities 241.0 241.0 — — Small-cap securities 69.6 69.6 — — Total equity securities 310.6 310.6 — — Other invested assets 85.1 8.1 77.0 — Total available-for-sale investments $ 2,396.4 $ 318.7 $ 2,074.4 $ 3.3 ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 309.3 $ — $ 309.3 $ — Obligations of states and political subdivisions 769.5 — 769.5 — Corporate securities 340.6 — 331.2 9.4 U.S. government agencies mortgage-backed securities 472.5 — 472.5 — Total fixed maturities 1,891.9 — 1,882.5 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 $ 2,282.6 $ 317.8 $ 1,955.4 $ 9.4
Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for 2015 and 2014 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2015 $ 9.4 Total realized gains (losses)—included in earnings — Total unrealized gains (losses)—included in other comprehensive income (0.2 ) Purchases — Sales (5.9 ) Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2015 $ 3.3 ($ 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
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) December 31, 2015 December 31, 2014 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 74.1 7.00 % $ 70.0 $ 74.6 7.00 %
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) December 31, 2015 December 31, 2014 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest $ 85.3 $ 85.5 5.03 % $ 85.3 $ 86.4 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.5 15.5 4.61 % 15.5 15.5 4.44 % Total notes payable $ 100.8 $ 101.0 $ 100.8 $ 101.9
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) December 31, 2015 December 31, 2014 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest $ 85.3 $ 85.5 5.03 % $ 85.3 $ 86.4 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.5 15.5 4.61 % 15.5 15.5 4.44 % Total notes payable $ 100.8 $ 101.0 $ 100.8 $ 101.9

Losses and Loss Expenses Paya37

Losses and Loss Expenses Payable (Tables)	12 Months Ended
	Dec. 31, 2015
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) 2015 2014 2013 Losses and loss expenses payable, at beginning of year $ 983.2 $ 959.9 $ 942.2 Less: reinsurance recoverable on losses and loss expenses payable 9.6 9.1 13.5 Net balance at beginning of year 973.6 950.8 928.7 Incurred related to: Current year 852.8 726.2 741.0 Prior years 10.0 45.1 (21.2 ) Total incurred 862.8 771.3 719.8 Paid related to: Current year 421.5 373.2 355.0 Prior years 367.8 375.3 342.7 Total paid 789.3 748.5 697.7 Net balance at end of year 1,047.1 973.6 950.8 Plus: reinsurance recoverable on losses and loss expenses payable 5.9 9.6 9.1 Losses and loss expenses payable, at end of year (affiliates $532.4, $494.3 and $438.0, respectively) $ 1,053.0 $ 983.2 $ 959.9

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a: ($ millions) 2015 2014 Losses and loss expenses payable: Direct $ 515.6 $ 484.4 Assumed 5.0 4.5 Ceded (5.9 ) (9.6 ) Net losses and loss expenses payable $ 514.7 $ 479.3 Unearned premiums: Direct $ 400.9 $ 409.7 Assumed 1.2 1.0 Ceded (6.8 ) (6.1 ) Net unearned premiums $ 395.3 $ 404.6
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, 2015 , 2014 and 2013 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 6a: ($ millions) 2015 2014 2013 Written premiums: Direct $ 854.1 $ 880.6 $ 886.7 Assumed 4.7 4.4 3.9 Ceded (35.4 ) (26.8 ) (24.3 ) Net written premiums $ 823.4 $ 858.2 $ 866.3 Earned premiums: Direct $ 863.1 $ 882.6 $ 874.7 Assumed 4.5 4.4 3.8 Ceded (34.7 ) (25.3 ) (23.5 ) Net earned premiums $ 832.9 $ 861.7 $ 855.0 Losses and loss expenses incurred: Direct $ 569.0 $ 525.6 $ 559.1 Assumed 3.4 2.9 3.0 Ceded (2.9 ) (8.0 ) (4.8 ) Net losses and loss expenses incurred $ 569.5 $ 520.5 $ 557.3

Transactions with Affiliates (T

Transactions with Affiliates (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 : ($ millions) 2015 2014 Assets Deferred policy acquisition costs: Ceded $ (144.1 ) $ (79.7 ) Assumed 129.1 126.5 Net (ceded) assumed $ (15.0 ) $ 46.8 Liabilities and Stockholders’ Equity Losses and loss expenses payable: Ceded $ (514.7 ) $ (479.3 ) Assumed 1,047.1 973.6 Net assumed $ 532.4 $ 494.3 Unearned premiums: Ceded $ (395.3 ) $ (404.6 ) Assumed 609.5 606.3 Net assumed $ 214.2 $ 201.7 Pension and postretirement benefits: Ceded $ (160.0 ) $ (180.5 ) Assumed 104.0 117.3 Net ceded $ (56.0 ) $ (63.2 ) Other liabilities: Ceded $ (63.8 ) $ (70.7 ) Assumed 55.4 65.6 Net ceded $ (8.4 ) $ (5.1 ) Stockholders’ Equity Accumulated other comprehensive income: Ceded $ (162.0 ) $ (186.0 ) Assumed 105.3 120.9 Net ceded $ (56.7 ) $ (65.1 )
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, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Written premiums: Ceded $ (823.4 ) $ (858.2 ) $ (866.3 ) Assumed 1,273.5 1,194.2 1,062.1 Net assumed $ 450.1 $ 336.0 $ 195.8 Earned premiums: Ceded $ (832.9 ) $ (861.7 ) $ (855.0 ) Assumed 1,270.5 1,074.1 1,055.0 Net assumed $ 437.6 $ 212.4 $ 200.0 Losses and loss expenses incurred: Ceded $ (570.9 ) $ (523.6 ) $ (560.2 ) Assumed 864.2 774.4 722.7 Net assumed $ 293.3 $ 250.8 $ 162.5 Acquisition and operating expenses: Ceded (119.5 ) (215.4 ) (192.6 ) Assumed 432.5 372.3 365.3 Net assumed $ 313.0 $ 156.9 $ 172.7

Federal Income Taxes (Tables)

Federal Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
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 expense (benefit) and the amount computed at the indicated statutory rate for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Amount at statutory rate $ 23.5 35.0 % $ 9.4 35.0 % $ 21.5 35.0 % Tax-exempt interest and dividends received deduction (8.7 ) (13.0 ) (8.5 ) (31.5 ) (9.4 ) (15.3 ) Other, net 1.3 1.9 1.1 4.1 0.2 0.4 Valuation allowance — — (82.6 ) (308.1 ) (11.8 ) (19.3 ) Federal income tax expense (benefit) and effective rate $ 16.1 23.9 % $ (80.6 ) (300.5 )% $ 0.5 0.8 %
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, 2015 and 2014 : ($ millions) 2015 2014 Deferred tax assets: Unearned premiums not currently deductible $ 42.5 $ 42.2 Losses and loss expenses payable discounting 19.3 21.0 Postretirement and pension benefits 36.4 41.1 Realized loss on other-than-temporary impairment 12.3 8.4 Other liabilities 21.3 22.2 Net operating loss carryforward 41.8 58.0 Tax credit carryforward 4.8 1.9 Other 4.9 4.7 Total deferred tax assets 183.3 199.5 Deferred tax liabilities: Deferral of policy acquisition costs 45.2 44.3 Net unrealized holding gains on investments 35.6 57.8 Total deferred tax liabilities 80.8 102.1 Net deferred federal income taxes $ 102.5 $ 97.4

Pension and Postretirement Be41

Pension and Postretirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 : ($ millions) Pension Postretirement 2015 2014 2015 2014 Change in benefit obligation: Benefit obligation at beginning of year $ 292.5 $ 230.5 $ 23.9 $ 22.3 Service cost 7.7 4.6 — — Interest cost 11.1 10.8 0.9 1.1 Actuarial (gain) loss (17.6 ) 57.3 (3.3 ) 1.7 Benefits paid (12.4 ) (10.7 ) (1.2 ) (1.2 ) The Company’s portion of benefit obligation at end of year $ 281.3 $ 292.5 $ 20.3 $ 23.9 Change in plan assets available for plan benefits: Fair value of plan assets available for plan benefits at beginning of year $ 205.1 $ 185.9 $ 0.3 $ 1.3 Employer contribution 13.0 13.0 — — Actual return on plan assets (1.3 ) 16.9 — — Benefits paid (12.4 ) (10.7 ) (0.3 ) (1.0 ) The Company’s portion of fair value of plan assets at end of year $ 204.4 $ 205.1 $ — $ 0.3 Supplemental executive retirement plan (6.8 ) (6.3 ) — — Funded status at end of year $ (83.7 ) $ (93.7 ) $ (20.3 ) $ (23.6 ) Accumulated benefit obligation end of year $ 263.1 $ 260.1
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, 2015 and 2014 : ($ millions) 2015 2014 Prior service benefit $ (59.5 ) $ (64.9 ) Net actuarial loss 125.1 141.9 Total $ 65.6 $ 77.0
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, 2016: ($ millions) 2016 Prior service benefit $ (5.4 ) Net actuarial loss 9.5 Total $ 4.1
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, 2015 , 2014 and 2013 : ($ millions) Pension Postretirement 2015 2014 2013 2015 2014 2013 Components of net periodic cost: Service cost $ 7.9 $ 5.2 $ 6.1 $ — $ — $ 0.4 Interest cost 11.3 11.1 9.6 1.1 1.1 1.2 Expected return on plan assets (13.8 ) (12.6 ) (12.2 ) — — (0.2 ) Amortization of: Prior service benefit — — — (5.4 ) (5.5 ) (5.5 ) Net actuarial loss 10.9 6.3 8.1 0.6 0.6 1.0 Net periodic cost (benefit) $ 16.3 $ 10.0 $ 11.6 $ (3.7 ) $ (3.8 ) $ (3.1 )
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 2016 $ 10.9 $ 1.4 2017 11.2 1.5 2018 11.8 1.5 2019 12.2 1.4 2020 12.7 1.4 2021-2025 75.2 6.6
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, 2015 and 2014 : Pension Postretirement 2015 2014 2015 2014 Benefit obligations weighted-average assumptions: Discount rate 4.20 % 3.85 % 4.20 % 3.85 % Rates of increase in compensation levels 3.50 3.50 — —
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, 2015 , 2014 and 2013 : Pension Postretirement 2015 2014 2013 2015 2014 2013 Weighted-average assumptions: Discount rate 3.85 % 4.85 % 4.05 % 3.85 % 4.85 % 4.05 % Expected long-term rate of return on assets 7.00 7.00 7.50 — — 7.50 Rates of increase in compensation levels 3.50 3.50 4.00 — — —
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, 2015 , 2014 and 2013 : Postretirement 2015 2014 2013 Assumed health care cost trend rates: Health care cost trend rate assumed for the next year 6.50 % 6.00 % 10.00 % Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 3.80 % 3.80 % 5.00 % Year that the rate reaches the ultimate trend rate 2076 2075 2018
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, 2015 : ($ 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 3.2 2.8
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, 2015 and 2014 : ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 51.0 $ — $ 51.0 $ — Corporate securities 38.9 — 38.9 — U.S. government agencies mortgage-backed securities 6.4 — 6.4 — Total fixed maturities 96.3 — 96.3 — Equity securities: Large-cap securities 59.0 59.0 — — Small-cap securities 22.3 22.3 — — Total equity securities 81.3 81.3 — — International instruments 18.6 — 18.6 — Total pension plan investments $ 196.2 $ 81.3 $ 114.9 $ — ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2014 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 $ — During 2015, the postretirement plan assets were sold in order to pay retiree medical costs. The following table sets forth the Company’s share of the postretirement plan’s available-for-sale securities within the fair value hierarchy at December 31, 2014: ($ millions) Total Quoted prices in active markets for identical assets (Level 1) Significant other observable inputs (Level 2) Significant unobservable inputs (Level 3) December 31, 2014 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 $ —

Other Comprehensive Income an42

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 , 2014 and 2013 : ($ 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, 2015 $ 110.0 $ — $ (38.3 ) $ 71.7 Other comprehensive income before reclassifications (25.4 ) — 5.3 (20.1 ) Amounts reclassified from AOCI (a) (16.1 ) — 2.1 (14.0 ) Net current period other comprehensive income (41.5 ) — 7.4 (34.1 ) Ending balance at December 31, 2015 $ 68.5 $ — $ (30.9 ) $ 37.6 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 $ 110.0 $ — $ (38.3 ) $ 71.7 Beginning balance at January 1, 2013 $ 124.0 $ 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 (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, 2015 , 2014 and 2013 : ($ millions) Details about Accumulated Other December 31 Affected line item in the Condensed Comprehensive Income Components 2015 2014 2013 Consolidated Statements of Income Unrealized gains on available for sale securities $ 24.7 $ 20.7 $ 23.2 Realized gain on sale of securities 24.7 20.7 23.2 Total before tax (8.6 ) (7.2 ) 0.1 Tax (expense) benefit 16.1 13.5 23.3 Net of tax Amortization of gain on derivative used in cash flow hedge — — 0.1 Realized gain on sale of securities — — 0.1 Total before tax — — — Tax benefit (expense) — — 0.1 Net of tax Amortization of benefit plan items: Negative prior service costs 5.4 5.5 5.5 (a) Net loss (11.5 ) (6.9 ) (9.1 ) (a) (6.1 ) (1.4 ) (3.6 ) Total before tax 4.0 (18.5 ) — Tax expense (2.1 ) (19.9 ) (3.6 ) Net of tax Total reclassifications for the period $ 14.0 $ (6.4 ) $ 19.8 (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2015
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) 2015 2014 Statutory capital and surplus of insurance subsidiaries $ 814.3 $ 778.4 Net liabilities of non-insurance parent and affiliates (80.2 ) (80.8 ) 734.1 697.6 Increases (decreases): Deferred acquisition costs 129.1 126.5 Postretirement and pension benefits 23.2 27.2 Deferred federal income taxes (41.6 ) (49.7 ) Fixed maturities, at fair value 27.9 60.6 Other, net 11.9 10.7 Stockholders’ equity per accompanying consolidated financial statements $ 884.6 $ 872.9 ($ millions) Year ended December 31 2015 2014 2013 Statutory net income (loss) of insurance subsidiaries $ 65.4 $ (17.5 ) $ 51.1 Net loss of non-insurance parent and affiliates (4.1 ) (5.3 ) (5.1 ) 61.3 (22.8 ) 46.0 Increases (decreases): Deferred acquisition costs 2.5 29.7 5.1 Postretirement and pension benefits 4.2 10.6 10.3 Deferred federal income taxes (12.2 ) 79.3 (1.0 ) Share-based compensation expense (0.1 ) (0.5 ) (1.3 ) Other, net (4.5 ) 11.1 1.7 Net income per accompanying consolidated financial statements $ 51.2 $ 107.4 $ 60.8

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 , 2014 and 2013 : 2015 2014 2013 Fair value per share $ 7.69 $ 7.28 $ 5.15 Expected dividend yield 1.75 % 1.86 % 2.40 % Risk free interest rate 1.60 % 1.65 % 1.26 % Expected volatility factor 36.61 % 39.23 % 37.59 % Expected life in years 6.0 5.7 6.3
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, 2015 , 2014 and 2013 : (millions, except per share amounts) 2015 2014 2013 Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Outstanding, beginning of year 3.7 $ 21.29 3.9 $ 22.01 3.9 $ 22.25 Granted 0.4 22.87 0.3 21.20 0.5 16.82 Exercised (0.3 ) 16.56 (0.1 ) 16.43 (0.2 ) 16.82 Canceled (0.4 ) 25.18 (0.4 ) 30.21 (0.3 ) 19.52 Outstanding, end of year 3.4 $ 21.44 3.7 $ 21.29 3.9 $ 22.01
Summary of Stock Options Outstanding and Exercisable	The following table sets forth information pertaining to the total options outstanding and exercisable at December 31, 2015 : (Options in millions) Options Outstanding Options Exercisable Number Weighted- Average Remaining Contractual Life Weighted- Average Exercise Price Number Weighted- Average Exercise Price Range of Exercise Prices: $10.01 – $20.00 1.7 5.0 $ 16.53 1.5 $ 16.53 $20.01 – $30.00 1.4 4.1 25.01 0.8 26.89 Greater than $30.00 0.3 0.4 33.48 0.3 33.48 3.4 4.2 $ 21.44 2.6 $ 21.67
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, 2015 , 2014 and 2013 : 2015 2014 2013 Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Outstanding, beginning of year 76,472 $ 19.06 38,233 $ 15.06 55,413 $ 16.21 Granted 74,020 22.83 56,178 21.23 — — Vested (35,859 ) 16.88 (16,892 ) 17.08 (17,180 ) 18.78 Canceled (3,549 ) 21.92 (1,047 ) — 21.23 — — Outstanding, end of year 111,084 $ 22.19 76,472 $ 19.06 38,233 $ 15.06

Net Earnings (Loss) Per Commo45

Net Earnings (Loss) Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 , 2014 and 2013 : (millions, except per share amounts) 2015 2014 2013 Numerator: Net earnings for basic net earnings per common share $ 51.2 $ 107.4 $ 60.8 Adjusted net earnings for dilutive net earnings per common share $ 51.2 $ 107.4 $ 60.8 Denominator: Weighted average shares for basic net earnings per common share 41.1 40.8 40.6 Effect of dilutive share-based awards 0.5 0.4 0.1 Adjusted weighted average shares for diluted net earnings per common share 41.6 41.2 40.7 Basic net earnings per common share $ 1.25 $ 2.63 $ 1.50 Diluted net earnings per common share $ 1.23 $ 2.60 $ 1.49
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, 2015 , 2014 and 2013 : (millions) 2015 2014 2013 Total number of antidilutive options and awards 1.5 1.8 2.6

Reportable Segments (Tables)

Reportable Segments (Tables)	12 Months Ended
	Dec. 31, 2015
Segment Reporting [Abstract]
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments for the years ended December 31, 2015 , 2014 and 2013 : ($ millions) 2015 2014 2013 Revenues from external sources: Insurance segments Personal insurance $ 591.8 $ 451.4 $ 464.0 Business insurance 476.0 459.9 433.5 Specialty insurance 202.7 162.8 157.5 Total insurance segments 1,270.5 1,074.1 1,055.0 Investment operations segment Net investment income 71.7 74.7 72.8 Net realized capital gains 24.7 20.7 23.2 Total investment operations segment 96.4 95.4 96.0 Total revenue from reportable segments 1,366.9 1,169.5 1,151.0 All other 1.7 3.2 2.0 Total revenues from external sources 1,368.6 1,172.7 1,153.0 Intersegment revenues 5.6 5.1 5.1 Total revenues 1,374.2 1,177.8 1,158.1 Reconciling items: Eliminate intersegment revenues (5.6 ) (5.1 ) (5.1 ) Total consolidated revenue $ 1,368.6 $ 1,172.7 $ 1,153.0 Segment loss before federal income tax: Insurance segments: Personal insurance SAP underwriting gain (loss) $ 17.5 $ (11.7 ) $ (12.6 ) Business insurance SAP underwriting loss (36.7 ) (4.6 ) (2.7 ) Specialty insurance SAP underwriting loss (5.9 ) (89.5 ) (18.7 ) Total insurance segments (25.1 ) (105.8 ) (34.0 ) Investment operations segment: Net investment income 71.7 74.7 72.8 Net realized capital gains 24.7 20.7 23.2 Total investment operations segment 96.4 95.4 96.0 All other segments income (0.1 ) — 0.9 Reconciling items: GAAP adjustments 4.2 45.8 11.8 Interest expense on corporate debt (5.4 ) (5.4 ) (8.5 ) Corporate expenses (2.7 ) (3.2 ) (4.9 ) Total reconciling items $ (3.9 ) $ 37.2 $ (1.6 ) Total consolidated income before federal income taxes $ 67.3 $ 26.8 $ 61.3
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2015 and 2014 : ($ millions) 2015 2014 Segment assets: Investment operations segment $ 2,529.8 $ 2,444.2 Total segment assets 2,529.8 2,444.2 Reconciling items: Corporate assets 298.7 322.7 Total consolidated assets $ 2,828.5 $ 2,766.9

Quarterly Financial Data (una47

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following tables set forth quarterly financial data for 2015 and 2014 : ($ millions, except per share amounts) 2015 For three months ended March 31 June 30 September 30 December 31 Total revenues $ 334.9 $ 337.4 $ 349.7 $ 346.6 Income before federal income taxes 33.2 3.4 29.9 0.8 Net income 24.7 2.7 20.7 3.1 Earnings per common share: Basic $ 0.60 $ 0.06 $ 0.50 $ 0.08 Diluted $ 0.60 $ 0.06 $ 0.50 $ 0.07 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 3.0 11.9 65.4 Earnings per common share: Basic $ 0.67 $ 0.07 $ 0.29 $ 1.60 Diluted $ 0.66 $ 0.07 $ 0.28 $ 1.58

Summary of Significant Accoun48

Summary of Significant Accounting Policies - Schedule of Net Deferred Acquisition Costs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of year	$ 126.5	$ 96.8	$ 91.7
Acquisition costs deferred	285.6	251.5	214.6
Acquisition costs amortized to expense	(283)	(221.8)	(209.5)
Balance, end of year	$ 129.1	$ 126.5	$ 96.8

Summary of Significant Accoun49

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Liability for unpaid losses and loss expenses	$ 24.7	$ 23.8

Investments - Cost or Amortized

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,295	$ 2,117.3
Gross unrealized holding gains	125.4	172.6
Gross unrealized holding losses	(24)	(7.3)
Fair value	2,396.4	2,282.6
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,972.9	1,831.3
Gross unrealized holding gains	46.4	67.3
Gross unrealized holding losses	(18.6)	(6.7)
Fair value	2,000.7	1,891.9
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	265.2	235.5
Gross unrealized holding gains	50.7	75.5
Gross unrealized holding losses	(5.3)	(0.6)
Fair value	310.6	310.4
Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	56.9	50.5
Gross unrealized holding gains	28.3	29.8
Gross unrealized holding losses	(0.1)	0
Fair value	85.1	80.3
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	219.8	296.7
Gross unrealized holding gains	6.6	14.1
Gross unrealized holding losses	(2)	(1.5)
Fair value	224.4	309.3
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	804	742.5
Gross unrealized holding gains	22.5	27.4
Gross unrealized holding losses	(1.7)	(0.4)
Fair value	824.8	769.5
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	500.3	333.4
Gross unrealized holding gains	5.8	10.2
Gross unrealized holding losses	(11.7)	(3)
Fair value	494.4	340.6
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	448.8	458.7
Gross unrealized holding gains	11.5	15.6
Gross unrealized holding losses	(3.2)	(1.8)
Fair value	457.1	472.5
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	211.9	185.5
Gross unrealized holding gains	34.2	57.3
Gross unrealized holding losses	(5.1)	(0.6)
Fair value	241	242.2
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	53.3	50
Gross unrealized holding gains	16.5	18.2
Gross unrealized holding losses	(0.2)	0
Fair value	$ 69.6	$ 68.2

Investments - Gross Unrealized

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Dec. 31, 2015USD ($)Positions	Dec. 31, 2014USD ($)Positions
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 662.5	$ 120.7
Less than 12 months, Unrealized losses	$ (16.3)	$ (2.1)
Less than 12 months, Number of positions (positions) | Positions	111	27
12 months or more, Fair value	$ 110.4	$ 176.5
12 months or more, Unrealized losses	$ (7.7)	$ (5.2)
12 months or more, Number of positions (positions) | Positions	26	50
Total, Fair value	$ 772.9	$ 297.2
Total, Unrealized losses	$ (24)	$ (7.3)
Total, Number of positions (positions) | Positions	137	77
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 585.2	$ 113.5
Less than 12 months, Unrealized losses	$ (10.9)	$ (1.5)
Less than 12 months, Number of positions (positions) | Positions	85	25
12 months or more, Fair value	$ 110.4	$ 176.5
12 months or more, Unrealized losses	$ (7.7)	$ (5.2)
12 months or more, Number of positions (positions) | Positions	26	50
Total, Fair value	$ 695.6	$ 290
Total, Unrealized losses	$ (18.6)	$ (6.7)
Total, Number of positions (positions) | Positions	111	75
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 69.2
Less than 12 months, Unrealized losses	$ (5.3)
Less than 12 months, Number of positions (positions) | Positions	25
12 months or more, Fair value	$ 0
12 months or more, Unrealized losses	$ 0
12 months or more, Number of positions (positions) | Positions	0
Total, Fair value	$ 69.2
Total, Unrealized losses	$ (5.3)
Total, Number of positions (positions) | Positions	25
Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 8.1
Less than 12 months, Unrealized losses	$ (0.1)
Less than 12 months, Number of positions (positions) | Positions	1
12 months or more, Fair value	$ 0
12 months or more, Unrealized losses	$ 0
12 months or more, Number of positions (positions) | Positions	0
Total, Fair value	$ 8.1
Total, Unrealized losses	$ (0.1)
Total, Number of positions (positions) | Positions	1
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	$ 68.6	$ 19.9
Less than 12 months, Unrealized losses	$ (1.6)	$ (0.3)
Less than 12 months, Number of positions (positions) | Positions	15	4
12 months or more, Fair value	$ 13.5	$ 52
12 months or more, Unrealized losses	$ (0.4)	$ (1.2)
12 months or more, Number of positions (positions) | Positions	4	17
Total, Fair value	$ 82.1	$ 71.9
Total, Unrealized losses	$ (2)	$ (1.5)
Total, Number of positions (positions) | Positions	19	21
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 137.5	$ 6.1
Less than 12 months, Unrealized losses	$ (1.7)	$ 0
Less than 12 months, Number of positions (positions) | Positions	16	5
12 months or more, Fair value	$ 0	$ 30.9
12 months or more, Unrealized losses	$ 0	$ (0.4)
12 months or more, Number of positions (positions) | Positions	0	9
Total, Fair value	$ 137.5	$ 37
Total, Unrealized losses	$ (1.7)	$ (0.4)
Total, Number of positions (positions) | Positions	16	14
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 246.9	$ 43.5
Less than 12 months, Unrealized losses	$ (5.3)	$ (0.9)
Less than 12 months, Number of positions (positions) | Positions	36	8
12 months or more, Fair value	$ 63.9	$ 56.1
12 months or more, Unrealized losses	$ (6.4)	$ (2.1)
12 months or more, Number of positions (positions) | Positions	11	11
Total, Fair value	$ 310.8	$ 99.6
Total, Unrealized losses	$ (11.7)	$ (3)
Total, Number of positions (positions) | Positions	47	19
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 132.2	$ 44
Less than 12 months, Unrealized losses	$ (2.3)	$ (0.3)
Less than 12 months, Number of positions (positions) | Positions	18	8
12 months or more, Fair value	$ 33	$ 37.5
12 months or more, Unrealized losses	$ (0.9)	$ (1.5)
12 months or more, Number of positions (positions) | Positions	11	13
Total, Fair value	$ 165.2	$ 81.5
Total, Unrealized losses	$ (3.2)	$ (1.8)
Total, Number of positions (positions) | Positions	29	21
Large-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 65.8	$ 7.2
Less than 12 months, Unrealized losses	$ (5.1)	$ (0.6)
Less than 12 months, Number of positions (positions) | Positions	24	2
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions (positions) | Positions	0	0
Total, Fair value	$ 65.8	$ 7.2
Total, Unrealized losses	$ (5.1)	$ (0.6)
Total, Number of positions (positions) | Positions	24	2
Small-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 3.4
Less than 12 months, Unrealized losses	$ (0.2)
Less than 12 months, Number of positions (positions) | Positions	1
12 months or more, Fair value	$ 0
12 months or more, Unrealized losses	$ 0
12 months or more, Number of positions (positions) | Positions	0
Total, Fair value	$ 3.4
Total, Unrealized losses	$ (0.2)
Total, Number of positions (positions) | Positions	1

Investments - Realized Losses R

Investments - Realized Losses Related to Other-Than-Temporary Impairments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (7.9)	$ (2.5)	$ (4)
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	(2.2)	(0.3)	(1.8)
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (5.7)	$ (2.2)	$ (2.2)

Investments - Amortized Cost an

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Amortized cost
Due in 1 year or less	$ 26.4
Due after 1 year through 5 years	492.9
Due after 5 years through 10 years	293.4
Due after 10 years	711.4
U.S. government agencies mortgage-backed securities	448.8
Total	1,972.9	$ 1,831.3
Fair value
Due in 1 year or less	26.7
Due after 1 year through 5 years	499.4
Due after 5 years through 10 years	293.2
Due after 10 years	724.3
U.S. government agencies mortgage-backed securities	457.1
Total	$ 2,000.7	$ 1,891.9

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 8.8
Proceeds on sales of available-for-sale securities	$ 326.2	$ 263.3	$ 220.4

Investments - Components of Net

Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 73.7	$ 76.7	$ 74.9
Investment expenses	2	2	2.1
Net investment income	71.7	74.7	72.8
Fixed maturities:
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	61.3	64.3	63.2
Equity securities:
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	6.5	6.2	6
Cash and cash equivalents, and other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 5.9	$ 6.2	$ 5.7

Investments - Realized and Unre

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Realized gains:
Total realized gains	$ 34.4	$ 24.5	$ 28.7
Realized losses:
Total realized losses	(9.7)	(3.8)	(5.5)
Net realized gains on investments	24.7	20.7	23.2
Change in unrealized holding gains (losses), net of tax:
Deferred federal income liability thereon	22.4	(13.7)	17.6
Valuation allowance	0	0	(6.7)
Change in net unrealized holding gains, net of tax	(41.5)	25.4	(39.4)
Fixed maturities:
Realized gains:
Total realized gains	4.6	3.1	2.5
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	(32.8)	34.5	(102.8)
Fixed maturities: | Sales
Realized losses:
Total realized losses	0	0	0.3
Equity securities:
Realized gains:
Total realized gains	29.6	21.3	26.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	(29.5)	6.2	36.5
Equity securities: | Sales
Realized losses:
Total realized losses	(1.8)	(1.3)	(1.2)
Equity securities: | OTTI
Realized losses:
Total realized losses	(7.9)	(2.5)	(4)
Other invested assets
Realized gains:
Total realized gains	0.2	0.1	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	$ (1.6)	$ (1.6)	$ 16

Fair Value of Financial Instr57

Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May. 31, 2009USD ($)Agreement	Dec. 31, 2015USD ($)Securities	Dec. 31, 2014USD ($)
Fair Value Measurements Disclosure [Line Items]
Other invested assets fair value		$ 77	$ 72.9
Other investments and securities, at cost		5.3	5.3
Number of credit agreements (agreements) | Agreement	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		$ 0.5
Corporate securities | Fair Value, Inputs, Level 3
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities (securities) | Securities		1
International funds
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities (securities) | Securities		2

Fair Value of Financial Instr58

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	$ 2,396.4	$ 2,282.6
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,000.7	1,891.9
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	310.6	310.4
Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	85.1	80.3
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	224.4	309.3
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	824.8	769.5
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	494.4	340.6
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	457.1	472.5
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	241	242.2
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	69.6	68.2
Available-for-sale:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,396.4	2,282.6
Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,000.7	1,891.9
Available-for-sale: | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	310.6	310.4
Available-for-sale: | Other invested assets
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	85.1	80.3
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	224.4	309.3
Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	824.8	769.5
Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	494.4	340.6
Available-for-sale: | U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	457.1	472.5
Available-for-sale: | Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	241	242.2
Available-for-sale: | Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	69.6	68.2
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	318.7	317.8
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	310.6	310.4
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	8.1	7.4
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 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	241	242.2
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	69.6	68.2
Significant other observable inputs (Level 2) | Available-for-sale:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,074.4	1,955.4
Significant other observable inputs (Level 2) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	1,997.4	1,882.5
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	77	72.9
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	224.4	309.3
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	824.8	769.5
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	491.1	331.2
Significant other observable inputs (Level 2) | Available-for-sale: | U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	457.1	472.5
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	3.3	9.4
Significant unobservable inputs (Level 3) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	3.3	9.4
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	3.3	9.4
Significant unobservable inputs (Level 3) | Available-for-sale: | U.S. government agencies 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 Instr59

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 ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 9.4	$ 8.9
Total realized gains (losses)-included in earnings	0	0
Total unrealized gains (losses)-included in other comprehensive income	(0.2)	0.2
Purchases	0	0.3
Sales	(5.9)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 3.3	$ 9.4

Fair Value of Financial Instr60

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 ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
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	$ 74.1	$ 74.6

Fair Value of Financial Instr61

Fair Value of Financial Instruments - Included in Notes Payable are Senior Notes and Subordinated Debentures (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
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)	4.61%	4.44%
Carrying value
Debt Instrument [Line Items]
Total notes payable	$ 100.8	$ 100.8
Carrying value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	85.3	85.3
Debt instrument, issued	85
Carrying value | Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Total notes payable	15.5	15.5
Debt instrument, issued	15.5
Fair value
Debt Instrument [Line Items]
Total notes payable	101	101.9
Fair value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	85.5	86.4
Fair value | Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Total notes payable	$ 15.5	$ 15.5

Losses and Loss Expenses Paya62

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$ 983.2	$ 959.9	$ 942.2
Less: reinsurance recoverable on losses and loss expenses payable	9.6	9.1	13.5
Net balance at beginning of year	973.6	950.8	928.7
Incurred related to:
Current year	852.8	726.2	741
Prior years	10	45.1	(21.2)
Total incurred	862.8	771.3	719.8
Paid related to:
Current year	421.5	373.2	355
Prior years	367.8	375.3	342.7
Total paid	789.3	748.5	697.7
Net balance at end of year	1,047.1	973.6	950.8
Plus: reinsurance recoverable on losses and loss expenses payable	5.9	9.6	9.1
Losses and loss expenses payable, at end of year (affiliates $532.4, $494.3 and $438.0, respectively)	$ 1,053	$ 983.2	$ 959.9

Losses and Loss Expenses Paya63

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Insurance [Abstract]
Losses and loss expenses payable, affiliates	$ 532.4	$ 494.3	$ 438

Losses and Loss Expenses Paya64

Losses and Loss Expenses Payable - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 10,000,000	$ 45,100,000	$ (21,200,000)
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(6,200,000)	(12,500,000)	(8,000,000)
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(700,000)	(5,200,000)	(5,400,000)
Personal And Business Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(9,800,000)	(28,700,000)	(30,500,000)
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(9,400,000)	(6,400,000)
Personal Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(11,000,000)
Business insurance segment
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(400,000)	(22,300,000)
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(10,500,000)	(5,300,000)	(8,000,000)
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(5,100,000)	(5,600,000)	(12,300,000)
Product Liability [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(3,800,000)	(11,900,000)	(8,300,000)
Specialty Insurance Segment [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(7,100,000)	(91,500,000)	(22,700,000)
Commercial Multi-Peril [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(2,100,000)
Programs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(9,700,000)
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(5,300,000)	(3,900,000)
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ (2,700,000)	(3,100,000)
RED [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		$ (96,700,000)	(21,300,000)
Homeowners [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			(2,900,000)
Specialty Insurance Segment [Member] | Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			$ (0.3333)

Reinsurance - Additional Inform

Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended		36 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2014
Reinsurance [Line Items]
Duration of quota share agreement			3 years
Percentage ceded to reinsurers (percent)			75.00%
Unearned premium	$ 616.3	$ 612.4	$ 612.4
Reinsurance Arrangement, Participating Reinsurers' Margin	9.00%
Profit Commission	$ 4.2	19
Excess Ceding Commission		8.1
Homeowners Quota Share Arrangement
Reinsurance [Line Items]
Unearned premium		$ 89.5	$ 89.5

Reinsurance - External Reinsura

Reinsurance - External Reinsurance on Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Losses and loss expenses payable:
Assumed	$ 1,047.1	$ 973.6
Ceded	(514.7)	(479.3)
Unearned premiums:
Assumed	609.5	606.3
Ceded	(395.3)	(404.6)
Unearned Premiums From Affiliates	214.2	201.7
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	515.6	484.4
Assumed	5	4.5
Ceded	(5.9)	(9.6)
Net losses and loss expenses payable	514.7	479.3
Unearned premiums:
Direct	400.9	409.7
Assumed	1.2	1
Ceded	(6.8)	(6.1)
Unearned Premiums From Affiliates	$ 395.3	$ 404.6

Reinsurance - External Reinsu67

Reinsurance - External Reinsurance on Income Statements (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Written premiums:
Assumed	$ 1,273.5	$ 1,194.2	$ 1,062.1
Ceded	(823.4)	(858.2)	(866.3)
Net written premiums	450.1	336	195.8
Earned premiums:
Direct	863.1	882.6	874.7
Assumed	1,270.5	1,074.1	1,055
Ceded	(832.9)	(861.7)	(855)
Losses and loss expenses incurred:
Assumed	864.2	774.4	722.7
Ceded	(570.9)	(523.6)	(560.2)
Net losses and loss expenses incurred	293.3	250.8	162.5
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	854.1	880.6	886.7
Assumed	4.7	4.4	3.9
Ceded	(35.4)	(26.8)	(24.3)
Net written premiums	823.4	858.2	866.3
Earned premiums:
Direct	863.1	882.6	874.7
Assumed	4.5	4.4	3.8
Ceded	(34.7)	(25.3)	(23.5)
Net earned premiums	832.9	861.7	855
Losses and loss expenses incurred:
Direct	569	525.6	559.1
Assumed	3.4	2.9	3
Ceded	(2.9)	(8)	(4.8)
Net losses and loss expenses incurred	$ 569.5	$ 520.5	$ 557.3

Transactions with Affiliates -

Transactions with Affiliates - Additional Information (Detail) $ in Millions	1 Months Ended		12 Months Ended
	May. 31, 2009USD ($)Agreement	May. 31, 2003USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
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			$ 294.7	$ 297
Mandatorily redeemable preferred capital securities issued to third party		$ 15
Common securities issued to state auto financial		0.5	$ 6.2	$ 3.5	$ 4
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		Dec. 31, 2033
Interest rate (percent)			5.28%	5.28%
Number of credit agreements (agreements) | Agreement	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			$ 1.8	$ 1.5	$ 1.6
Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate (percent)			4.61%	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 69

Transactions with Affiliates - Schedule of Reinsurance Transactions on Balance Sheets for Pooling Arrangement (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	$ (144.1)	$ (79.7)
Deferred Policy Acquisition Costs Assumed	129.1	126.5
Deferred Policy Acquisitions Costs from Affiliates	(15)	46.8
Losses and loss expenses payable:
Ceded	(514.7)	(479.3)
Assumed	1,047.1	973.6
Losses And Loss Expenses Payable From Affiliates	532.4	494.3
Unearned premiums:
Ceded	(395.3)	(404.6)
Assumed	609.5	606.3
Unearned Premiums From Affiliates	214.2	201.7
Pension and Other Postretirement Benefits:
Postretirement and Pension Benefits Ceded	(160)	(180.5)
Postretirement and Pension Benefits Assumed	104	117.3
Postretirement and Pension Benefits Ceded to Affiliates	(56)	(63.2)
Other Liabilities Ceded	(63.8)	(70.7)
Other Liabilities Assumed	55.4	65.6
Other Liabilities	(8.4)	(5.1)
Accumulated Other Comprehensive Income Ceded	(162)	(186)
Accumulated Other Comprehensive Income Assumed	105.3	120.9
Net Accumulated Other Comprehensive Income	$ (56.7)	$ (65.1)

Transactions with Affiliates 70

Transactions with Affiliates - Schedule of Reinsurance Transactions on Income Statements for Pooling Arrangement (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Written premiums:
Ceded	$ (823.4)	$ (858.2)	$ (866.3)
Assumed	1,273.5	1,194.2	1,062.1
Premiums Written, Net	450.1	336	195.8
Earned premiums:
Ceded	(832.9)	(861.7)	(855)
Assumed	1,270.5	1,074.1	1,055
Premiums Earned, Net	437.6	212.4	200
Losses and loss expenses incurred:
Ceded	(570.9)	(523.6)	(560.2)
Assumed	864.2	774.4	722.7
Losses and loss expenses incurred, Net	293.3	250.8	162.5
Acquisition and Operating Expenses [Abstract]
Ceded	(119.5)	(215.4)	(192.6)
Assumed	432.5	372.3	365.3
Acquisition and Operating Expenses, Net	$ 313	$ 156.9	$ 172.7

Notes Payable and Credit Faci71

Notes Payable and Credit Facility - Additional Information (Detail) - USD ($)	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 - Reconcil

Federal Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 23.5	$ 9.4	$ 21.5
Tax-exempt interest and dividends received deduction	(8.7)	(8.5)	(9.4)
Other, net	1.3	1.1	0.2
Valuation allowance	0	(82.6)	(11.8)
Federal income tax expense (benefit) and effective rate	$ 16.1	$ (80.6)	$ 0.5
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)	(13.00%)	(31.50%)	(15.30%)
Other, net (percent)	1.90%	4.10%	0.40%
Valuation allowance (percent)	0.00%	(308.10%)	(19.30%)
Federal income tax expense (benefit) and effective rate (percent)	23.90%	(300.50%)	0.80%

Federal Income Taxes - Tax Effe

Federal Income Taxes - Tax Effects of Temporary Differences of Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Unearned premiums not currently deductible	$ 42.5	$ 42.2
Losses and loss expenses payable discounting	19.3	21
Postretirement and pension benefits	36.4	41.1
Realized loss on other-than-temporary impairment	12.3	8.4
Other liabilities	21.3	22.2
Net operating loss carryforward	41.8	58
Tax credit carryforward	4.8	1.9
Other	4.9	4.7
Total deferred tax assets	183.3	199.5
Deferred tax liabilities:
Deferral of policy acquisition costs	45.2	44.3
Net unrealized holding gains on investments	35.6	57.8
Total deferred tax liabilities	80.8	102.1
Net deferred federal income taxes	$ 102.5	$ 97.4

Federal Income Taxes - Addition

Federal Income Taxes - Additional Information (Detail)	12 Months Ended			36 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2014USD ($)
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	$ 67,300,000	$ 26,800,000	$ 61,300,000		$ 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
Deferred tax asset related to net operating loss carryforwards	41,800,000	58,000,000		$ 58,000,000	$ 58,000,000
Balance for uncertain tax positions	0
Interest and penalties payable	$ 0	$ 0		$ 0	$ 0

Pension and Postretirement Be75

Pension and Postretirement Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	300,000
Employer contribution	$ 13,000,000
Contributions of participant salary (percent)	1.00%
Matching contribution on participant's salary	$ 1
Matching contribution by the employer	0.50
Company's share of expense under the plan	$ 5,100,000	$ 5,100,000	$ 4,600,000
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 Be76

Pension and Postretirement Benefit Plans - Summary of Pension and Postretirement Benefit Plans' Change in Benefit Obligation, Plan Assets and Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in benefit obligation:
Actuarial (gain) loss	$ 5.3	$ (54.4)	$ 32.5
Change in plan assets available for plan benefits:
Employer contribution	13
Pension Costs
Change in benefit obligation:
Benefit obligation at beginning of year	292.5	230.5
Service cost	7.7	4.6
Interest cost	11.1	10.8
Actuarial (gain) loss	(17.6)	57.3
Benefits paid	(12.4)	(10.7)
The Company’s portion of benefit obligation at end of year	281.3	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	205.1	185.9
Employer contribution	13	13
Actual return on plan assets	(1.3)	16.9
Benefits paid	(12.4)	(10.7)
The Company’s portion of fair value of plan assets at end of year	204.4	205.1	185.9
Supplemental executive retirement plan	(6.8)	(6.3)
Funded status at end of year	(83.7)	(93.7)
Accumulated benefit obligation end of year	263.1	260.1
Postretirement Benefit Costs
Change in benefit obligation:
Benefit obligation at beginning of year	23.9	22.3
Service cost	0	0
Interest cost	0.9	1.1
Actuarial (gain) loss	(3.3)	1.7
Benefits paid	(1.2)	(1.2)
The Company’s portion of benefit obligation at end of year	20.3	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	0.3	1.3
Employer contribution	0	0
Actual return on plan assets	0	0
Benefits paid	(0.3)	(1)
The Company’s portion of fair value of plan assets at end of year	0	0.3	$ 1.3
Supplemental executive retirement plan	0	0
Funded status at end of year	$ (20.3)	$ (23.6)

Pension and Postretirement Be77

Pension and Postretirement Benefit Plans - Company's Share of Amounts Included in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Prior service benefit	$ (59.5)	$ (64.9)
Net actuarial loss	125.1	141.9
Total	$ 65.6	$ 77

Pension and Postretirement Be78

Pension and Postretirement Benefit Plans - Company's Share of Amortization Expected to be Recognized (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Prior service benefit	$ (5.4)
Net actuarial loss	9.5
Total	$ 4.1

Pension and Postretirement Be79

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amortization of:
Prior service benefit	$ (59.5)	$ (64.9)
Pension
Components of net periodic cost:
Service cost	7.9	5.2	$ 6.1
Interest cost	11.3	11.1	9.6
Expected return on plan assets	(13.8)	(12.6)	(12.2)
Amortization of:
Prior service benefit	0	0	0
Net actuarial loss	10.9	6.3	8.1
Net periodic cost (benefit)	16.3	10	11.6
Postretirement
Components of net periodic cost:
Service cost	0	0	0.4
Interest cost	1.1	1.1	1.2
Expected return on plan assets	0	0	(0.2)
Amortization of:
Prior service benefit	(5.4)	(5.5)	(5.5)
Net actuarial loss	0.6	0.6	1
Net periodic cost (benefit)	$ (3.7)	$ (3.8)	$ (3.1)

Pension and Postretirement Be80

Pension and Postretirement Benefit Plans-Company's Share of Expected Future Benefits Payments Schedule (Detail) $ in Millions	Dec. 31, 2015USD ($)
Pension
Schedule of Expected Future Pension Benefit Payment [Line Items]
2,016	$ 10.9
2,017	11.2
2,018	11.8
2,019	12.2
2,020	12.7
2021 - 2025	75.2
Postretirement
Schedule of Expected Future Pension Benefit Payment [Line Items]
2,016	1.4
2,017	1.5
2,018	1.5
2,019	1.4
2,020	1.4
2021 - 2025	$ 6.6

Pension and Postretirement Be81

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plans' Obligations (Detail)	Dec. 31, 2015	Dec. 31, 2014
Pension
Benefit obligations weighted-average assumptions:
Discount rate (percent)	4.20%	3.85%
Rates of increase in compensation levels (percent)	3.50%	3.50%
Postretirement
Benefit obligations weighted-average assumptions:
Discount rate (percent)	4.20%	3.85%
Rates of increase in compensation levels (percent)	0.00%	0.00%

Pension and Postretirement Be82

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plan's Net Periodic Cost (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	3.85%	4.85%	4.05%
Expected long-term rate of return on assets (percent)	7.00%	7.00%	7.50%
Rates of increase in compensation levels (percent)	3.50%	3.50%	4.00%
Postretirement
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	3.85%	4.85%	4.05%
Expected long-term rate of return on assets (percent)	0.00%	0.00%	7.50%
Rates of increase in compensation levels (percent)	0.00%	0.00%	0.00%

Pension and Postretirement Be83

Pension and Postretirement Benefit Plans - Schedule of Assumed Health Care Cost Trend Rates Used (Detail) - Postretirement	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assumed health care cost trend rates:
Health care cost trend rate assumed for the next year (percent)	6.50%	6.00%	10.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (percent)	3.80%	3.80%	5.00%
Year that the rate reaches the ultimate trend rate	2,076	2,075	2,018

Pension and Postretirement Be84

Pension and Postretirement Benefit Plans - Summary on Effective Impact of Assumed Health Care Cost Trend Rates on Amounts Reported for Postretirement Plan (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015USD ($)
One percentage point change:
Effect on total service and interest cost	$ 0.1
Effect on accumulated postretirement benefit obligation	3.2
Postretirement
One percentage point change:
Effect on total service and interest cost	0.1
Effect on accumulated postretirement benefit obligation	$ 2.8

Pension and Postretirement Be85

Pension and Postretirement Benefit Plans - Summary of Asset Allocation Targets (Detail)	12 Months Ended
	Dec. 31, 2015
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%

Pension and Postretirement Be86

Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$ 2,396.4	$ 2,282.6
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	196.2	199.7
Pension | International instruments
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	18.6	18.7
Pension | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	96.3	105.5
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	51	56.4
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	38.9	39.7
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	6.4	9.4
Pension | Equity securities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	81.3	75.5
Pension | Equity securities: | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	59	53.7
Pension | Equity securities: | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	22.3	21.8
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	81.3	75.5
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	81.3	75.5
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	59	53.7
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	22.3	21.8
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	114.9	124.2
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	18.6	18.7
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	96.3	105.5
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	51	56.4
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	38.9	39.7
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	6.4	9.4
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

Pension and Postretirement Be87

Pension and Postretirement Benefit Plans - Company's Share of Postretirement Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value	$ 2,396.4	$ 2,282.6
Postretirement
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.3
Postretirement | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
Postretirement | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Asset Available-for-sale, at fair value		0.1
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
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
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
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
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
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
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
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 an88

Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 71.7	$ 80.8	$ 84.2
Other comprehensive income before reclassifications	(20.1)	(15.5)	16.4
Amounts reclassified from AOCI	14	(6.4)	19.8
Net current period other comprehensive income	(34.1)	(9.1)	(3.4)
Ending balance	37.6	71.7	80.8
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	110	84.6	124
Other comprehensive income before reclassifications	(25.4)	38.9	(16.1)
Amounts reclassified from AOCI	16.1	13.5	23.3
Net current period other comprehensive income	(41.5)	25.4	(39.4)
Ending balance	68.5	110	84.6
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	0	0	0.1
Other comprehensive income before reclassifications	0	0	0
Amounts reclassified from AOCI	0	0	0.1
Net current period other comprehensive income	0	0	(0.1)
Ending balance	0	0	0
Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(38.3)	(3.8)	(39.9)
Other comprehensive income before reclassifications	5.3	(54.4)	32.5
Amounts reclassified from AOCI	(2.1)	(19.9)	(3.6)
Net current period other comprehensive income	7.4	(34.5)	36.1
Ending balance	$ (30.9)	$ (38.3)	$ (3.8)

Other Comprehensive Income an89

Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	$ 24.3	$ 20.7	$ 23.2
Tax (expense) benefit	(16.1)	80.6	(0.5)
Total reclassifications for the period	(14)	6.4	(19.8)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(14)	6.4	(19.8)
Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(16.1)	(13.5)	(23.3)
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	24.7	20.7	23.2
Total before tax	24.7	20.7	23.2
Tax (expense) benefit	(8.6)	(7.2)	0.1
Total reclassifications for the period	16.1	13.5	23.3
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	0	0	(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	0.1
Total before tax	0	0	0.1
Tax (expense) benefit	0	0	0
Total reclassifications for the period	0	0	0.1
Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	2.1	19.9	3.6
Benefit Plan Items | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(6.1)	(1.4)	(3.6)
Tax (expense) benefit	4	(18.5)	0
Negative prior service costs	5.4	5.5	5.5
Net loss	(11.5)	(6.9)	(9.1)
Total reclassifications for the period	$ (2.1)	$ (19.9)	$ (3.6)

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 81.4
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	$ 15	$ 20	$ 10

Stockholders' Equity - Reconcil

Stockholders' Equity - Reconciliations of Statutory Capital and Surplus and Net Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholders' Equity Note [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 814.3				$ 778.4				$ 814.3	$ 778.4
Net liabilities of non-insurance parent and affiliates	(80.2)				(80.8)				(80.2)	(80.8)
Total	734.1				697.6				734.1	697.6
Increases (decreases):
Deferred acquisition costs	129.1				126.5				129.1	126.5	$ 96.8	$ 91.7
Postretirement and pension benefits	23.2				27.2				23.2	27.2
Deferred federal income taxes	(41.6)				(49.7)				(41.6)	(49.7)
Fixed maturities, at fair value	27.9				60.6				27.9	60.6
Other, net	11.9				10.7				11.9	10.7
Stockholders’ equity per accompanying consolidated financial statements	884.6				872.9				884.6	872.9	785
Statutory net (loss) income of insurance subsidiaries									65.4	(17.5)	51.1
Net income (loss) of non-insurance parent and affiliates									(4.1)	(5.3)	(5.1)
Total									(61.3)	22.8	(46)
Deferred acquisition costs									2.5	29.7	5.1
Postretirement and pension benefits									4.2	10.6	10.3
Deferred federal income taxes									(12.2)	79.3	(1)
Share-based compensation expense									(0.1)	(0.5)	(1.3)
Other, net									(4.5)	11.1	1.7
Net income	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 65.4	$ 11.9	$ 3	$ 27.1	$ 51.2	$ 107.4	$ 60.8

Preferred Stock Preferred Stock

Preferred Stock Preferred Stock (Details)	12 Months Ended
	Dec. 31, 2015class_of_stockquarter
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Number of authorized classes of preferred stock (class of stock) | class_of_stock	2
Minimum number of quarters dividends are paid in arrears (minimum of 6 quarters) | quarter	6

Share-Based Compensation - Addi

Share-Based Compensation - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (shares)	400,000	300,000	500,000
Common shares valued (shares)	$ 120,400,000	$ 119,300,000
Total intrinsic value of stock options exercised	2,600,000	500,000	$ 500,000
Tax benefit for tax deductions from share-based awards	300,000	0	0
Aggregate intrinsic value for total options outstanding	13,500,000
Aggregate intrinsic value for total options exercisable	6,200,000
Share-based compensation	$ 4,500,000	$ 3,600,000	$ 4,100,000
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)	500,000
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 December 31 of the prior year
Maximum number of common shares available for awards of stock options (shares)	250,000
Termination date	May 8, 2019
Share based compensation expiration period	10 years
Stock options granted (shares)	300,000	200,000	500,000
Unrecognized compensation cost related to non-vested restricted share	$ 900,000
Remaining unrecognized compensation cost to be recognized in years	3 years
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)	3,700,000
Percentage of annual base earnings (percent)	6.00%
Purchase price of the common shares (percent)	85.00%
Common shares purchased under this plan	3,300,000
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	May 31, 2015
RSUs granted in period (shares)	26,184	25,960	33,712
Common shares valued (shares)	$ 567,000	$ 51,000
Outside Directors Plan | Restricted Stock Units (RSUs) | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement, Restricted Stock Units, Annual Installment Payments	10 years
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]
Share Based Compensation Arrangement, Restricted Stock Units, Annual Installment Payments	5 years
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	$ 2,600,000
Remaining unrecognized compensation cost to be recognized in years	3 years

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Non-Vested and Vested Restricted Shares and Changes (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shares
Outstanding, beginning of year (shares)	76,472	38,233	55,413
Granted (shares)	74,020	56,178	0
Vested (shares)	(35,859)	(16,892)	(17,180)
Forfeited (shares)	(3,549)	(1,047)	0
Outstanding, end of year (shares)	111,084	76,472	38,233
Weighted Average Grant Date Fair Value
Outstanding, beginning of year (usd per share)	$ 19.06	$ 15.06	$ 16.21
Granted (usd per share)	22.83	21.23	0
Vested (usd per share)	16.88	17.08	18.78
Forfeited (usd per share)	21.92	21.23	0
Outstanding, end of year (usd per share)	$ 22.19	$ 19.06	$ 15.06

Share-Based Compensation - Weig

Share-Based Compensation - Weighted Average Fair Values and Related Assumptions for Options Granted (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation [Abstract]
Fair value per share (usd per share)	$ 7.69	$ 7.28	$ 5.15
Expected dividend yield (percent)	1.75%	1.86%	2.40%
Risk free interest rate (percent)	1.60%	1.65%	1.26%
Expected volatility factor (percent)	36.61%	39.23%	37.59%
Expected life in years	6 years	5 years 7 months 30 days	6 years 3 months 30 days

Share-Based Compensation - Su96

Share-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shares
Options Outstanding, beginning of year	3.7	3.9	3.9
Granted	0.4	0.3	0.5
Options, Exercised	(0.3)	(0.1)	(0.2)
Options, Canceled	(0.4)	(0.4)	(0.3)
Options Outstanding, end of year	3.4	3.7	3.9
Weighted- Average Exercise Price
Outstanding, beginning of year (usd per share)	$ 21.29	$ 22.01	$ 22.25
Granted (usd per share)	22.87	21.20	16.82
Exercised (usd per share)	16.56	16.43	16.82
Canceled (usd per share)	25.18	30.21	19.52
Outstanding, end of year (usd per share)	$ 21.44	$ 21.29	$ 22.01

Share-Based Compensation - Su97

Share-Based Compensation - Summary of Stock Options Outstanding and Exercisable (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Share Based Compensation Arrangement [Line Items]
Number (shares)	3.4	3.7	3.9	3.9
Weighted- Average Remaining Contractual Life	4 years 2 months
Weighted- Average Exercise Price (usd per share)	$ 21.44	$ 21.29	$ 22.01	$ 22.25
Number (shares)	2.6
Weighted- Average Exercise Price (usd per share)	$ 21.67
$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)	1.7
Weighted- Average Remaining Contractual Life	5 years
Weighted- Average Exercise Price (usd per share)	$ 16.53
Number (shares)	1.5
Weighted- Average Exercise Price (usd per share)	$ 16.53
$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)	1.4
Weighted- Average Remaining Contractual Life	4 years 1 month 1 day
Weighted- Average Exercise Price (usd per share)	$ 25.01
Number (shares)	0.8
Weighted- Average Exercise Price (usd per share)	$ 26.89
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	4 months 31 days
Weighted- Average Exercise Price (usd per share)	$ 33.48
Number (shares)	0.3
Weighted- Average Exercise Price (usd per share)	$ 33.48

Net Earnings (Loss) per Commo98

Net Earnings (Loss) per Common Share - Compilation of Basic and Diluted Net (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net earnings for basic net earnings per common share	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 65.4	$ 11.9	$ 3	$ 27.1	$ 51.2	$ 107.4	$ 60.8
Adjusted net earnings for dilutive net earnings per common share									$ 51.2	$ 107.4	$ 60.8
Denominator:
Weighted average shares for basic net earnings (loss) per common share (shares)									41.1	40.8	40.6
Effect of dilutive share-based awards (shares)									0.5	0.4	0.1
Adjusted weighted average shares for diluted net earnings (loss) per common share (shares)									41.6	41.2	40.7
Basic net earnings (loss) per common share (usd per share)	$ 0.08	$ 0.50	$ 0.06	$ 0.60	$ 1.60	$ 0.29	$ 0.07	$ 0.67	$ 1.25	$ 2.63	$ 1.50
Diluted net earnings (loss) per common share (usd per share)	$ 0.07	$ 0.50	$ 0.06	$ 0.60	$ 1.58	$ 0.28	$ 0.07	$ 0.66	$ 1.23	$ 2.60	$ 1.49

Net Earnings (Loss) per Commo99

Net Earnings (Loss) per Common Share - Options to Purchase Shares of Common Stock (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Number of options and awards (shares)	1.5	1.8	2.6

Reportable Segments - Additiona

Reportable Segments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014Segment
Segment Reporting [Abstract]
Number of reportable segments (segment)	4

Reportable Segments - Financial

Reportable Segments - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended			36 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Revenues from external sources:
Earned premiums, net									$ 437.6	$ 212.4	$ 200
Net investment income									71.7	74.7	72.8
Income (loss) before federal income tax									67.3	26.8	61.3	$ 98.7
Intersegment revenues	$ 346.6	$ 349.7	$ 337.4	$ 334.9	$ 295.7	$ 291.3	$ 294.4	$ 291.3	1,368.6	1,172.7	1,153
Segment loss before federal income tax:
Income (loss) before federal income tax									67.3	26.8	61.3	98.7
Net investment income									71.7	74.7	72.8
Income (loss) before federal income tax									67.3	26.8	61.3	98.7
Segment assets:
Total segment assets	2,529.8				2,444.2				2,529.8	2,444.2		2,444.2
Reconciling items:
Total consolidated assets	2,828.5				2,766.9				2,828.5	2,766.9		2,766.9
Total investment operations segment
Segment assets:
Total segment assets	2,529.8				2,444.2				2,529.8	2,444.2		2,444.2
Intersegment revenues
Revenues from external sources:
Intersegment revenues									(5.6)	(5.1)	(5.1)
Corporate expenses
Reconciling items:
Total consolidated assets	$ 298.7				$ 322.7				298.7	322.7		$ 322.7
Reportable segments | Personal insurance
Revenues from external sources:
Earned premiums, net									591.8	451.4	464
Reportable segments | Business insurance
Revenues from external sources:
Earned premiums, net									476	459.9	433.5
Reportable segments | Specialty insurance
Revenues from external sources:
Earned premiums, net									202.7	162.8	157.5
Income (loss) before federal income tax									(5.9)	(89.5)	(18.7)
Segment loss before federal income tax:
Income (loss) before federal income tax									(5.9)	(89.5)	(18.7)
Income (loss) before federal income tax									(5.9)	(89.5)	(18.7)
Reportable segments | Total insurance segments
Revenues from external sources:
Earned premiums, net									1,270.5	1,074.1	1,055
Income (loss) before federal income tax									(25.1)	(105.8)	(34)
Segment loss before federal income tax:
Income (loss) before federal income tax									(25.1)	(105.8)	(34)
Income (loss) before federal income tax									(25.1)	(105.8)	(34)
Reportable segments | Net investment income
Revenues from external sources:
Net investment income									71.7	74.7	72.8
Segment loss before federal income tax:
Net investment income									71.7	74.7	72.8
Reportable segments | Net realized capital gains
Revenues from external sources:
Income (loss) before federal income tax									24.7	20.7	23.2
Segment loss before federal income tax:
Income (loss) before federal income tax									24.7	20.7	23.2
Income (loss) before federal income tax									24.7	20.7	23.2
Reportable segments | Total investment operations segment
Revenues from external sources:
Total investment operations segment									96.4	95.4	96
Segment loss before federal income tax:
Total investment operations segment									96.4	95.4	96
Reportable segments | Reportable segments
Revenues from external sources:
Intersegment revenues									1,366.9	1,169.5	1,151
Reportable segments | All other
Revenues from external sources:
Income (loss) before federal income tax									(0.1)	0	0.9
Segment loss before federal income tax:
Income (loss) before federal income tax									(0.1)	0	0.9
Income (loss) before federal income tax									(0.1)	0	0.9
Reportable segments | Personal insurance SAP underwriting gain (loss)
Revenues from external sources:
Income (loss) before federal income tax									17.5	(11.7)	(12.6)
Segment loss before federal income tax:
Income (loss) before federal income tax									17.5	(11.7)	(12.6)
Income (loss) before federal income tax									17.5	(11.7)	(12.6)
Reportable segments | Business insurance SAP underwriting loss
Revenues from external sources:
Income (loss) before federal income tax									(36.7)	(4.6)	(2.7)
Segment loss before federal income tax:
Income (loss) before federal income tax									(36.7)	(4.6)	(2.7)
Income (loss) before federal income tax									(36.7)	(4.6)	(2.7)
All other
Revenues from external sources:
Income (loss) before federal income tax									(3.9)	37.2	(1.6)
Segment loss before federal income tax:
Income (loss) before federal income tax									(3.9)	37.2	(1.6)
Income (loss) before federal income tax									(3.9)	37.2	(1.6)
All other | All other
Revenues from external sources:
Intersegment revenues									1.7	3.2	2
All other | GAAP adjustments
Revenues from external sources:
Income (loss) before federal income tax									4.2	45.8	11.8
Segment loss before federal income tax:
Income (loss) before federal income tax									4.2	45.8	11.8
Income (loss) before federal income tax									4.2	45.8	11.8
All other | Interest expense on corporate debt
Revenues from external sources:
Income (loss) before federal income tax									(5.4)	(5.4)	(8.5)
Segment loss before federal income tax:
Income (loss) before federal income tax									(5.4)	(5.4)	(8.5)
Income (loss) before federal income tax									(5.4)	(5.4)	(8.5)
All other | Corporate expenses
Revenues from external sources:
Income (loss) before federal income tax									(2.7)	(3.2)	(4.9)
Segment loss before federal income tax:
Income (loss) before federal income tax									(2.7)	(3.2)	(4.9)
Income (loss) before federal income tax									(2.7)	(3.2)	(4.9)
Intersegment revenues | Intersegment revenues
Revenues from external sources:
Intersegment revenues									5.6	5.1	5.1
External and Intersegment Sources
Revenues from external sources:
Intersegment revenues									1,374.2	1,177.8	1,158.1
Total consolidated
Revenues from external sources:
Intersegment revenues									$ 1,368.6	$ 1,172.7	$ 1,153

Quarterly Financial Data - Sche

Quarterly Financial Data - Schedule of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 346.6	$ 349.7	$ 337.4	$ 334.9	$ 295.7	$ 291.3	$ 294.4	$ 291.3	$ 1,368.6	$ 1,172.7	$ 1,153
Income before federal income taxes	0.8	29.9	3.4	33.2	(17.1)	13.1	3.1	27.7
Net income	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 65.4	$ 11.9	$ 3	$ 27.1	$ 51.2	$ 107.4	$ 60.8
Earnings per common share:
Basic (usd per share)	$ 0.08	$ 0.50	$ 0.06	$ 0.60	$ 1.60	$ 0.29	$ 0.07	$ 0.67	$ 1.25	$ 2.63	$ 1.50
Diluted (usd per share)	$ 0.07	$ 0.50	$ 0.06	$ 0.60	$ 1.58	$ 0.28	$ 0.07	$ 0.66	$ 1.23	$ 2.60	$ 1.49

Schedule I - Summary of Inve103

Schedule I - Summary of Investments - Other Than Investments in Related Parties (Detail) $ in Millions	Dec. 31, 2015USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	$ 2,300.3
Fair value	2,401.7
Amount at which shown in the balance sheet	2,401.7
Equity securities: | Available-for-sale:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	265.2
Fair value	310.6
Amount at which shown in the balance sheet	310.6
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	2,295
Fair value	2,396.4
Amount at which shown in the balance sheet	2,396.4
Available-for-sale: | Fixed maturities:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	1,972.9
Fair value	2,000.7
Amount at which shown in the balance sheet	2,000.7
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	219.8
Fair value	224.4
Amount at which shown in the balance sheet	224.4
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	804
Fair value	824.8
Amount at which shown in the balance sheet	824.8
Available-for-sale: | Fixed maturities: | Corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	500.3
Fair value	494.4
Amount at which shown in the balance sheet	494.4
Available-for-sale: | Fixed maturities: | U.S. government agencies mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	448.8
Fair value	457.1
Amount at which shown in the balance sheet	457.1
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	211.9
Fair value	241
Amount at which shown in the balance sheet	241
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	53.3
Fair value	69.6
Amount at which shown in the balance sheet	69.6
Available-for-sale: | Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	56.9
Fair value	85.1
Amount at which shown in the balance sheet	$ 85.1

Schedule II - Condensed Fina104

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Assets
Fixed maturities, available-for-sale, at fair value	$ 2,000.7	$ 1,891.9
Other invested assets	5.3	5.3
Cash and cash equivalents	58.1	86.3
Other assets	(11.9)	(10.7)
Due from affiliates	5.9	40.1
Federal income tax, net	102.5	97.4
Total assets	2,828.5	2,766.9
Liabilities and Stockholders’ Equity
Notes payable (affiliates $116.8 and $116.8, respectively)	100.8	100.8
Other liabilities (affiliated net ceded $8.4 and $5.1, respectively)	69.8	80.3
Total liabilities	1,943.9	1,894
Stockholders’ 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	120.4	119.3
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.3)	(116)
Additional paid-in capital	153.5	143.2
Accumulated other comprehensive income (affiliated net ceded $56.7 and $65.1, respectively)	37.6	71.7	$ 80.8	$ 84.2
Retained earnings	689.4	654.7
Stockholders’ equity per accompanying consolidated financial statements	884.6	872.9	785
Total liabilities and stockholders' equity	2,828.5	2,766.9
Class A Preferred stock
Stockholders’ equity:
Preferred stock, value	0	0
Class B Preferred stock
Stockholders’ equity:
Preferred stock, value	0	0
State Auto Financial
Assets
Investments in common stock of subsidiaries (equity method)	972.5	960.7
Fixed maturities, available-for-sale, at fair value	0.9	0.9
Other invested assets	3	2.8
Cash and cash equivalents	10.5	11.5	$ 6.1	$ 7
Other assets	0.3	0.3
Federal income tax, net	21.9	21.2
Total assets	1,009.1	997.4
Liabilities and Stockholders’ Equity
Notes payable (affiliates $116.8 and $116.8, respectively)	116.8	116.8
Due to affiliate	0.6	0.5
Other liabilities (affiliated net ceded $8.4 and $5.1, respectively)	7.1	7.2
Total liabilities	124.5	124.5
Stockholders’ 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	120.4	119.3
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.3)	(116)
Additional paid-in capital	153.5	143.2
Accumulated other comprehensive income (affiliated net ceded $56.7 and $65.1, respectively)	37.6	71.7
Retained earnings	689.4	654.7
Stockholders’ equity per accompanying consolidated financial statements	884.6	872.9
Total liabilities and stockholders' equity	1,009.1	997.4
State Auto Financial | Class A Preferred stock
Stockholders’ equity:
Preferred stock, value	0	0
State Auto Financial | Class B Preferred stock
Stockholders’ equity:
Preferred stock, value	$ 0	$ 0

Schedule II - Condensed Fina105

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Notes payable, affiliates	$ 15.5	$ 15.5
Common Stock, No Par Value	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	48,100,000	47,700,000
Common stock, stated value per share (usd per share)	$ 2.50	$ 2.50
Treasury Stock, Shares	6,800,000	6,800,000
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	$ 116.8	$ 116.8
Common Stock, No Par Value	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	48,100,000	47,500,000
Common stock, stated value per share (usd per share)	$ 2.50	$ 2.5
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 Fina106

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended			36 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net investment income									$ 71.7	$ 74.7	$ 72.8
Net realized gain on investments									24.3	20.7	23.2
Total revenues	$ 346.6	$ 349.7	$ 337.4	$ 334.9	$ 295.7	$ 291.3	$ 294.4	$ 291.3	1,368.6	1,172.7	1,153
Interest expense (affiliates $6.0, $6.0 and $3.2, respectively)									5.4	5.4	8.5
Other operating expenses									6.3	7.3	8.6
Total expenses									1,301.3	1,145.9	1,091.7
Income before federal income taxes									67.3	26.8	61.3	$ 98.7
Federal income tax (benefit) expense									(16.1)	80.6	(0.5)
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net investment income									0.3	0.3	0.3
Net realized gain on investments									0	0	0.7
Total revenues									0.3	0.3	1
Interest expense (affiliates $6.0, $6.0 and $3.2, respectively)									6	6	8.8
Other operating expenses									4.9	5.6	6.8
Total expenses									10.9	11.6	15.6
Income before federal income taxes									(10.6)	(11.3)	(14.6)
Federal income tax (benefit) expense									(2.8)	(20.6)	(4.8)
Net (loss) income before equity in net income of subsidiaries									(7.8)	9.3	(9.8)
Equity in net income (loss) of subsidiaries									59	98.1	70.6
Net Income (Loss) Attributable to Parent									$ 51.2	$ 107.4	$ 60.8

Schedule II - Condensed Fina107

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Income (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Interest Expense Debt To Affiliates, Parent Company	$ 6	$ 6	$ 3.2

Schedule II - Condensed Fina108

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	$ (39.2)	$ 59.8	$ (27.1)
Reclassification adjustments for gains realized in net income	(24.7)	(20.7)	(23.2)
Income tax expense	22.4	(13.7)	10.9
Total net unrealized holding (losses) gains on investments	(41.5)	25.4	(39.4)
Other comprehensive loss	(34.1)	(9.1)	(3.4)
Comprehensive Income (loss)	17.1	98.3	57.4
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	51.2	107.4	60.8
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	(0.1)	(0.2)	0.5
Reclassification adjustments for gains realized in net income	0	0	(0.7)
Income tax expense	0	0.1	0
Total net unrealized holding (losses) gains on investments	(0.1)	(0.1)	(0.2)
Unrealized equity in subsidiaries	(34)	(9)	(3.2)
Other comprehensive loss	(34.1)	(9.1)	(3.4)
Comprehensive Income (loss)	$ 17.1	$ 98.3	$ 57.4

Schedule II - Condensed Fina109

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	May. 31, 2003	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net		$ 15.7	$ 11.7	$ 13.5
Share-based compensation		4.5	3.6	4.1
Net realized gain on investments		(24.3)	(20.7)	(23.2)
Changes in operating assets and liabilities:
Other liabilities and due from affiliates		(39.9)	(50.8)	(0.9)
Other assets		(4.5)	11.1	1.7
Excess tax (expense) benefit on share-based awards		(0.3)	0	0.1
Federal income taxes, net		13.7	(81.6)	(0.3)
Net cash provided by operating activities		149.8	75.6	72.1
Cash flows from investing activities:
Purchases of equity securities - available-for-sale		(154)	(119)	(86)
Purchases of other invested assets		(6.9)	(1.8)	(5.5)
Maturities, calls and pay downs of fixed maturities - available-for-sale		241	232.4	174.6
Sales of fixed maturities - available-for-sale		180.7	163.5	113.3
Sales of equity securities - available-for-sale		144.8	99.2	106.4
Net cash used in investing activities		(167.7)	(56.5)	(23)
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0.5	6.2	3.5	4
Payments to acquire treasury stock		(0.3)	(0.1)	(0.1)
Payment of dividends		(16.5)	(16.5)	(16.1)
Excess tax expense (benefit) on share-based awards		0.3	0	(0.1)
Proceeds from long-term debt		0	0	85
Redemption of long-term debt		0	0	(100)
Net cash used in financing activities		(10.3)	(13.1)	(27.8)
Net (decrease) increase in cash and cash equivalents		(28.2)	6	21.3
Cash and cash equivalents at beginning of year		86.3
Cash and cash equivalents at end of year		58.1	86.3
Supplemental disclosures:
Federal income tax received		6.4	1	0.8
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)		5.3	5.2	8.5
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent		51.2	107.4	60.8
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net		0	0	1.3
Share-based compensation		(0.3)	0.9	1.6
Net realized gain on investments		0	0	(0.7)
Equity in net income from consolidated subsidiaries		(59)	(98.1)	(70.6)
Changes in operating assets and liabilities:
Other liabilities and due from affiliates		0.9	0.5	0.1
Other assets		0	0	(0.5)
Excess tax (expense) benefit on share-based awards		(0.5)	0	0.1
Federal income taxes, net		(0.7)	(16.6)	(3.1)
Net cash provided by operating activities		(8.4)	(5.9)	(10)
Cash flows from investing activities:
Dividends received from consolidated subsidiaries		18	24	13.8
Purchases of other invested assets		(0.3)	(0.4)	(0.2)
Maturities, calls and pay downs of fixed maturities - available-for-sale		0	0.5	0.2
Sales of fixed maturities - available-for-sale		0	0.3	3.9
Sales of equity securities - available-for-sale		0	0	3.7
Net cash used in investing activities		17.7	24.4	21.4
Cash flows from financing activities:
Proceeds from issuance of common stock		6.2	3.5	4
Payments to acquire treasury stock		(0.3)	(0.1)	(0.1)
Payment of dividends		(16.5)	(16.5)	(16.1)
Excess tax expense (benefit) on share-based awards		0.3	0	(0.1)
Proceeds from long-term debt		0	0	100
Redemption of long-term debt		0	0	(100)
Net cash used in financing activities		(10.3)	(13.1)	(12.3)
Net (decrease) increase in cash and cash equivalents		(1)	5.4	(0.9)
Cash and cash equivalents at beginning of year		11.5	6.1	7
Cash and cash equivalents at end of year		10.5	11.5	6.1
Supplemental disclosures:
Federal income tax received		2.3	4.3	1.5
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)		$ (6)	$ (6)	$ (8)

Schedule II - Condensed Fina110

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$ 0.7	$ 0.7	$ 0.7
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$ 6	$ 6	$ 3.2

Schedule II - Condensed Fina111

Schedule II - Condensed Financial Information of Registrant - Additional Information (Detail)	Jul. 11, 2013USD ($)Agreementsubsidiary	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Condensed Financial Statements, Captions [Line Items]
Number of subsidiaries with shared credit agreements (subsidiaries) | subsidiary	2
Interest rate (percent)		5.28%	5.28%
Notes Payable
Condensed Financial Statements, Captions [Line Items]
Number of credit agreements (agreements) | Agreement	2
Debt instrument, issued		$ 100,000,000	$ 100,000,000
Fair Value		100,100,000	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		85,100,000	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		$ 15,000,000	$ 14,800,000

Schedule III - Supplementary112

Schedule III - Supplementary Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	$ 129.1	$ 126.5	$ 96.8
Future benefits, claims and losses	1,047.1	973.6	950.8
Unearned premiums	616.3	612.4	491
Other policy claims and benefits payable	0	0	0
Premium revenue	1,270.5	1,074.1	1,055
Net investment income	71.7	74.7	72.8
Benefits, losses and settlement expenses	864.3	774.5	722.7
Amortization of deferred policy acquisition costs	283	221.8	209.6
Other operating expenses	143.8	140.1	145.2
Premiums written	1,273.5	1,194.2	1,062.1
Personal insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	47.6	48.9	31.5
Future benefits, claims and losses	227	201.9	223.7
Unearned premiums	261.9	270.7	189.9
Other policy claims and benefits payable	0	0	0
Premium revenue	591.8	451.4	464
Net investment income	0	0	0
Benefits, losses and settlement expenses	403.6	318.3	342.2
Amortization of deferred policy acquisition costs	105.4	75.4	74.7
Other operating expenses	63.6	52	55.5
Premiums written	583	532.1	465.4
Business insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	46	44.5	41.8
Future benefits, claims and losses	553.6	503.4	511.6
Unearned premiums	245.5	241.1	226.6
Other policy claims and benefits payable	0	0	0
Premium revenue	476	459.9	433.5
Net investment income	0	0	0
Benefits, losses and settlement expenses	331.3	280	262.3
Amortization of deferred policy acquisition costs	104.9	88	81.4
Other operating expenses	74.6	74.5	87.1
Premiums written	479.5	473.1	446.8
Specialty insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	35.5	33.1	23.5
Future benefits, claims and losses	266.5	268.3	215.5
Unearned premiums	108.9	100.6	74.5
Other policy claims and benefits payable	0	0	0
Premium revenue	202.7	162.8	157.5
Net investment income	0	0	0
Benefits, losses and settlement expenses	129.4	176.2	118.2
Amortization of deferred policy acquisition costs	72.7	58.4	53.5
Other operating expenses	5.6	13.6	2.6
Premiums written	211	189	149.9
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	71.7	74.7	72.8
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 (Deta

Schedule IV - Reinsurance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 863.1	$ 882.6	$ 874.7
Ceded to	832.9	861.7	855
Assumed from	1,270.5	1,074.1	1,055
Net Amount	$ 1,270.5	$ 1,074.1	$ 1,055
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	$ 34.7	$ 25.3	$ 23.5
Assumed from	4.5	4.4	3.8
Affiliated entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	832.9	861.7	855
Assumed from	1,270.5	1,074.1	1,055
Net Amount	$ 1,270.5	$ 1,074.1	$ 1,055

Schedule V - Valuation and Q114

Schedule V - Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Valuation allowance for deferred tax assets:
Balance at beginning of period	$ 0	$ 82.6	$ 100.5
Deductions	0	82.6	17.9
Balance at end of period	$ 0	$ 0	$ 82.6