State Auto Financial (STFC) 10-K 1 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Feb. 24, 2017	Jun. 30, 2016
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
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 Public Float			$ 344,055,419
Entity Common Stock, Shares Outstanding		41,976,998

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $2,095.9 and $1,972.9, respectively)	$ 2,109.3	$ 2,000.7
Equity securities, available-for-sale, at fair value (cost $323.1 and $265.2, respectively)	382.8	310.6
Other invested assets, available-for-sale, at fair value (cost $25.5 and $56.9, respectively)	45.1	85.1
Other invested assets	5.4	5.3
Notes receivable from affiliate	70	70
Total investments	2,612.6	2,471.7
Cash and cash equivalents	51.1	58.1
Accrued investment income and other assets	40	35.7
Deferred policy acquisition costs (affiliated net assumed $50.7 and $49.3, respectively)	129.8	129.1
Reinsurance recoverable on losses and loss expenses payable	3.6	5.9
Prepaid reinsurance premiums	6.1	6.8
Due from affiliates	0	5.9
Current federal income taxes	6.7	4.9
Net deferred federal income taxes	102.1	102.5
Property and equipment, at cost (net of accumulated depreciation of $6.6 and $6.3, respectively)	7.4	7.6
Total assets	2,959.4	2,828.2
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $630.9 and $532.4, respectively)	1,181.6	1,053
Unearned premiums (affiliated net assumed $220.9 and $214.2, respectively)	617.8	616.3
Notes payable (affiliates $15.2 and $15.2, respectively)	122.1	100.5
Postretirement and pension benefits (affiliated net ceded $40.1 and $56.0, respectively)	74.4	104
Due to affiliate	2.4	0
Other liabilities (affiliated net assumed $11.0 and affiliated net ceded $8.4, respectively)	69.8	69.8
Total liabilities	2,068.1	1,943.6
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 48.6 and 48.1 shares issued, respectively, at stated value of $2.50 per share	121.6	120.4
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.5)	(116.3)
Additional paid-in capital	159.9	153.5
Accumulated other comprehensive income (affiliated net ceded $53.7 and $56.7, respectively)	32.5	37.6
Retained earnings	693.8	689.4
Total stockholders’ equity	891.3	884.6
Total liabilities and stockholders’ equity	2,959.4	2,828.2
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, 2016	Dec. 31, 2015
Fixed maturities, available-for-sale, amortized cost	$ 2,095.9	$ 1,972.9
Equity securities, available-for-sale, cost	323.1	265.2
Other invested assets, available-for-sale, cost	25.5	56.9
Deferred Policy Acquisitions Costs from Affiliates	50.7	49.3
Property and equipment, accumulated depreciation	6.6	6.3
Losses And Loss Expenses Payable From Affiliates	630.9	532.4
Unearned premiums, affiliates	220.9	214.2
Net Postretirement and Pension Benefits	(40.1)	(56)
Net Other Liabilities from Affiliates	11	(8.4)
Notes payable, affiliates	$ 15.2	$ 15.2
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)	48,600,000	48,100,000
Common Stock, Par or Stated Value Per Share	$ 2.50	$ 2.50
Net Accumulated Other Comprehensive Income	$ (53.7)	$ (56.7)
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Earned premiums (affiliated net assumed $472.0, $437.6 and $212.4, respectively)	$ 1,291.9	$ 1,270.5	$ 1,074.1
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	74.7	71.7	74.7
Net realized gain on investments:
Total other-than-temporary impairment losses	(6.8)	(7.9)	(2.5)
Portion of loss recognized in other comprehensive income	0	0	0
Other net realized investment gains	43.3	32.2	23.2
Total net realized gain on investments	36.5	24.3	20.7
Other income (affiliates $2.3, $2.1 and $1.9, respectively)	2.3	2.1	3.2
Total revenues	1,405.4	1,368.6	1,172.7
Losses and loss expenses (affiliated net assumed $383.8, $293.3 and $250.8, respectively)	942.4	862.8	771.3
Acquisition and operating expenses (affiliated net assumed $285.8, $248.6 and $156.9, respectively)	430.4	426.8	361.9
Interest expense (affiliates $0.8, $0.7 and $0.7, respectively)	5.5	5.4	5.4
Other expenses	7.9	6.3	7.3
Total expenses	1,386.2	1,301.3	1,145.9
Income before federal income taxes	19.2	67.3	26.8
Current	(1.7)	2.9	0.1
Deferred	(0.1)	13.2	(80.7)
Total federal income tax (benefit) expense	(1.8)	16.1	(80.6)
Net income	$ 21	$ 51.2	$ 107.4
Earnings per common share:
Basic (usd per share)	$ 0.50	$ 1.25	$ 2.63
Diluted (usd per share)	0.50	1.23	2.60
Dividends paid per common share (usd per share)	$ 0.40	$ 0.40	$ 0.40

Consolidated Statements of Inc5

Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Earned premiums, net	$ 472	$ 437.6	$ 212.4
Net investment income, affiliates	4.9	4.9	4.9
Other income, affiliates	2.3	2.1	1.9
Losses and loss expenses, net affiliates	383.8	293.3	250.8
Net Acquisition and Operating Expenses	285.8	248.6	156.9
Interest Expense Debt To Affiliates	$ 0.8	$ 0.7	$ 0.7

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 21	$ 51.2	$ 107.4
Net change in unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during year	27.8	(39.2)	59.8
Reclassification adjustments for gains realized in net income	(36.5)	(24.7)	(20.7)
Income tax benefit (expense)	3	22.4	(13.7)
Total change in net unrealized holding gains (losses) on investments	(5.7)	(41.5)	25.4
Net unrecognized benefit plan obligations:
Net actuarial (loss) gain arising during period	(3)	5.3	(54.4)
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(5.5)	(5.4)	(5.5)
Net actuarial loss	9.4	11.5	6.9
Income tax (expense) benefit	(0.3)	(4)	18.5
Total net unrecognized benefit plan obligations	0.6	7.4	(34.5)
Other comprehensive loss	(5.1)	(34.1)	(9.1)
Comprehensive income	$ 15.9	$ 17.1	$ 98.3

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, 2013		47.5	6.8
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
Balance at beginning of year at Dec. 31, 2013		$ 118.8	$ (115.9)	$ 137.5	$ 80.8	$ 563.8
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
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, $10.4 and $10.2, 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)
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, $10.4 and $10.2, respectively)	(16.5)
Balance at end of year at Dec. 31, 2015	884.6	$ 120.4	$ (116.3)	153.5	37.6	689.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.5
Balance at end of year (shares) at Dec. 31, 2016		48.6	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 1.2		6.8
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				(3)
Shares acquired on stock option exercises and vested restricted shares			$ (0.2)
Stock options granted				2.6
Change in unrealized holding gains (losses) on investments, net of tax	(5.7)				(5.7)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	0.6				0.6
Net income	21					21
Cash dividends paid (affiliates $10.4, $10.4 and $10.2, respectively)	(16.6)
Balance at end of year at Dec. 31, 2016	$ 891.3	$ 121.6	$ (116.5)	$ 159.9	$ 32.5	$ 693.8

Consolidated Statements of Sto8

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Retained earnings
Cash dividends paid affiliates	$ 10.4	$ 10.4	$ 10.2

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 21	$ 51.2	$ 107.4
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization, net	14.6	15.7	11.7
Share-based compensation	3.7	4.5	3.6
Net realized gain on investments	(36.5)	(24.3)	(20.7)
Changes in operating assets and liabilities:
Deferred policy acquisition benefits	(0.7)	(2.6)	(29.7)
Accrued investment income and other assets	(4.5)	(2.4)	0.2
Postretirement and pension benefits	(28.8)	(6)	(7.3)
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	3	3	(1.9)
Other liabilities and due to/from affiliates, net	7.3	(39.9)	(50.8)
Losses and loss expenses payable	128.6	69.8	23.3
Unearned premiums	1.5	3.9	121.4
Excess tax expense on share-based awards	(0.2)	(0.3)	0
Federal income taxes	4.5	13.7	(81.6)
Cash provided from December 31, 2014 unearned premium transfer related to the homeowners quota-share reinsurance arrangement	0	63.5	0
Net cash provided by operating activities	113.5	149.8	75.6
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(589.3)	(573.9)	(431.4)
Purchases of equity securities available-for-sale	(143.1)	(154)	(119)
Purchases of other invested assets	(1.5)	(6.9)	(1.8)
Maturities, calls and pay downs of fixed maturities available-for-sale	240.9	241	232.4
Sales of fixed maturities available-for-sale	211.6	180.7	163.5
Sales of equity securities available-for-sale	153	144.8	99.2
Sales of other invested assets available-for-sale	0.8	0.7	0.6
Net disposals of property and equipment	0	(0.1)	0
Net cash used in investing activities	(127.6)	(167.7)	(56.5)
Cash flows from financing activities:
Proceeds from issuance of common stock	2.2	6.2	3.5
Payments to acquire treasury stock	(0.2)	(0.3)	(0.1)
Payments of dividends (affiliates $10.4, $10.4 and $10.2, respectively)	(16.6)	(16.5)	(16.5)
Excess tax expense on share-based awards	0.2	0.3	0
Proceeds from long-term debt	21.5	0	0
Net cash provided by (used in) financing activities	7.1	(10.3)	(13.1)
Net (decrease) increase in cash and cash equivalents	(7)	(28.2)	6
Cash and cash equivalents at beginning of year	58.1	86.3	80.3
Cash and cash equivalents at end of year	51.1	58.1	86.3
Supplemental disclosures:
Interest paid (affiliates $0.8, $0.7 and $0.7, respectively)	5.4	5.3	5.2
Federal income taxes paid	$ 0	$ 6.4	$ 1

Consolidated Statements of Ca10

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Cash Flows [Abstract]
Payments of dividends, affiliates	$ 10.4	$ 10.4	$ 10.2
Interest paid, affiliates	$ 0.8	$ 0.7	$ 0.7

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Balance, beginning of year $ 129.1 $ 126.5 $ 96.8 Acquisition costs deferred 291.0 285.6 251.5 Acquisition costs amortized to expense (290.3 ) (283.0 ) (221.8 ) Balance, end of year $ 129.8 $ 129.1 $ 126.5 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 $27.0 million and $24.7 million at December 31, 2016 and 2015 , 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 and unrecognized benefit plan obligations, adjusted for deferred federal income taxes. k. New Accounting Standards Adoption of Recent Accounting Pronouncements Simplifying the Presentation of Debt Issuance Costs The amendments in this guidance simplify the presentation of debt issuance costs by requiring them to be presented in the balance sheet as a direct deduction from the carrying amount of the related recognized debt liability, consistent with debt discounts. The Company adopted this guidance at January 1, 2016 on a retrospective basis and it resulted in a $0.3 million decrease to notes payable and accrued investment income and other assets at December 31, 2015 and 2014, respectively. Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent) The amendments in this guidance remove the requirement to categorize within the fair value hierarchy all investments for which fair value is measured using the net asset value per share practical expedient. The amendments also remove the requirement to make certain disclosures for all investments that are eligible to be measured at fair value using the net asset value per share practical expedient. Rather, those disclosures are limited to investments for which the entity has elected to measure the fair value using that practical expedient. The amendments in this update are effective for public business entities for fiscal years beginning after December 15, 2015, and interim periods within those fiscal years and should be applied retrospectively to all periods presented. The Company adopted this guidance at January 1, 2016. The new guidance affects disclosures only and it did not impact the Company’s results of operations or consolidated financial position. Disclosures about Short-Duration Contracts The amendments in this guidance require expanded disclosures for insurance entities that issue short-duration contracts. The expanded disclosures are designed to provide additional insight into an insurance entity’s significant estimates made in measuring the liability for unpaid claims and claim adjustment expenses. The disclosures include information about incurred and paid claims development by accident year, on a net basis after reinsurance, for the number of years claims incurred typically remain outstanding, not to exceed ten years. Each period presented in the disclosure about claims development that precedes the current reporting period is considered required supplementary information. The expanded disclosures also include information about significant changes in methodologies and assumptions, a reconciliation of incurred and paid claims development to the carrying amount of the liability for unpaid claims and claim adjustment expenses, the total amount of incurred but not reported liabilities plus expected development, the incidence of claims including the methodology used to determine the incidence of claims, and claim duration. The guidance is effective for annual periods beginning after December 15, 2015, and interim periods beginning after December 15, 2016, and is to be applied retrospectively. The Company adopted this guidance at December 31, 2016. The new guidance affects disclosures only and it did not impact the Company’s results of operations or consolidated financial position. Pending Adoption of Recent Accounting Pronouncements Revenue from Contracts with Customers In May 2014, the FASB issued guidance that requires an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. Insurance contracts do not fall within the scope of this new guidance. The guidance is effective for annual reporting periods beginning after December 15, 2017, with early adoption permitted. The guidance has not yet been adopted; however, there is not expected to be a material impact on the Company’s results of operations and consolidated financial position. 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, including equity securities and limited partnership interests, that are not accounted for under the equity method of accounting or result in consolidation 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 its results of operations and consolidated financial position. Leases In February 2016, the FASB issued guidance that amended previous guidance on lease accounting. The new guidance requires the recognition of lease assets and lease liabilities by lessees for those leases classified as operating leases under previous GAAP. The guidance is effective beginning January 1, 2019 and the Company is currently evaluating the impact on its results of operations and consolidated financial position; however, it is not expected to have a material impact on the Company’s results of operations, consolidated financial position or cash flows. Improvements to Employee Share-Based Payment Accounting In March 2016, the FASB issued guidance that simplifies the accounting for share-based payment award transactions including: income tax consequences, classification of awards as either equity or liabilities, classification of excess tax benefits, and classification on the statement of cash flows. The guidance is effective beginning January 1, 2017 and it is not expected to have a material impact on the Company’s results of operations, consolidated financial position or cash flows. Measurement of Credit Losses on Financial Instruments In June 2016, the FASB issued guidance that amended previous guidance on the impairment of financial instruments by adding an impairment model that allows an entity to recognize expected credit losses as an allowance rather than impairing as they are incurred. The new guidance is intended to reduce complexity of credit impairment models and result in a more timely recognition of expected credit losses. The measurement of credit losses for available-for-sale debt securities measured at fair value is not affected except that credit losses recognized are limited to the amount by which fair value is below amortized cost and the carrying value adjustment is recognized through an allowance and not as a direct write-down. The guidance is effective beginning January 1, 2020 and the Company is currently evaluating the impact on its results of operations and consolidated financial position. Statement of Cash Flows - Classification of Certain Cash Receipts and Cash Payments

Investments

Investments	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2016 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 401.9 $ 8.9 $ (6.1 ) $ 404.7 Obligations of states and political subdivisions 634.6 12.3 (3.2 ) 643.7 Corporate securities 445.7 6.1 (2.2 ) 449.6 U.S. government agencies mortgage-backed securities 613.7 8.5 (10.9 ) 611.3 Total fixed maturities 2,095.9 35.8 (22.4 ) 2,109.3 Equity securities: Large-cap securities 108.9 32.7 (2.6 ) 139.0 Small-cap securities 57.2 21.9 — 79.1 Mutual and exchange traded funds 157.0 8.5 (0.8 ) 164.7 Total equity securities 323.1 63.1 (3.4 ) 382.8 Other invested assets 25.5 19.6 — 45.1 Total available-for-sale securities $ 2,444.5 $ 118.5 $ (25.8 ) $ 2,537.2 ($ 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 136.3 29.4 (4.0 ) 161.7 Small-cap securities 53.3 16.5 (0.2 ) 69.6 Mutual and exchange traded funds 75.6 4.8 (1.1 ) 79.3 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 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, 2016 and 2015 : ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2016 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 $ 229.1 $ (6.1 ) 30 $ — $ — — $ 229.1 $ (6.1 ) 30 Obligations of states and political subdivisions 178.9 (3.2 ) 26 — — — 178.9 (3.2 ) 26 Corporate securities 102.9 (1.4 ) 16 29.4 (0.8 ) 4 132.3 (2.2 ) 20 U.S. government agencies mortgage-backed securities 341.7 (10.1 ) 43 20.5 (0.8 ) 11 362.2 (10.9 ) 54 Total fixed maturities 852.6 (20.8 ) 115 49.9 (1.6 ) 15 902.5 (22.4 ) 130 Large-cap equity securities 9.1 (0.9 ) 7 8.8 (1.7 ) 5 17.9 (2.6 ) 12 Mutual and exchange traded funds 29.9 (0.8 ) 2 — — — 29.9 (0.8 ) 2 Total equity securities 39.0 (1.7 ) 9 8.8 (1.7 ) 5 47.8 (3.4 ) 14 Total temporarily impaired securities $ 891.6 $ (22.5 ) 124 $ 58.7 $ (3.3 ) 20 $ 950.3 $ (25.8 ) 144 ($ 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 46.0 (4.0 ) 20 — — — 46.0 (4.0 ) 20 Small-cap equity securities 3.4 (0.2 ) 1 — — — 3.4 (0.2 ) 1 Mutual and exchange traded funds 19.8 (1.1 ) 4 — — — 19.8 (1.1 ) 4 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 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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Equity securities: Large-cap securities $ (0.6 ) $ (2.2 ) $ (0.3 ) Small-cap securities (3.9 ) (5.7 ) (2.2 ) Fixed maturities (2.3 ) — — Total other-than-temporary impairments $ (6.8 ) $ (7.9 ) $ (2.5 ) The Company reviewed its investments at December 31, 2016 , and determined that 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, 2016 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 74.4 $ 75.2 Due after 1 year through 5 years 552.7 557.8 Due after 5 years through 10 years 280.2 282.9 Due after 10 years 574.9 582.1 U.S. government agencies mortgage-backed securities 613.7 611.3 Total $ 2,095.9 $ 2,109.3 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, 2016 , State Auto P&C had fixed maturity securities, with a carrying value of approximately $106.5 million , that were pledged as collateral for the FHLB Loan (as defined in Note 8). 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 $9.2 million and $8.8 million were on deposit with insurance regulators as required by law at December 31, 2016 and 2015 , respectively. The following table sets forth the components of net investment income for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Fixed maturities $ 63.4 $ 61.3 $ 64.3 Equity securities 7.3 6.5 6.2 Cash and cash equivalents, and other 5.6 5.9 6.2 Investment income 76.3 73.7 76.7 Investment expenses 1.6 2.0 2.0 Net investment income $ 74.7 $ 71.7 $ 74.7 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 2016 , 2015 and 2014 were $365.4 million , $326.2 million and $263.3 million , respectively. The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Realized gains: Fixed maturities $ 2.8 $ 4.6 $ 3.1 Equity securities 29.2 29.6 21.3 Other invested assets 12.1 0.2 0.1 Total realized gains 44.1 34.4 24.5 Realized losses: Equity securities: Sales (0.8 ) (1.8 ) (1.3 ) OTTI (4.5 ) (7.9 ) (2.5 ) Fixed maturities: OTTI (2.3 ) — — Total realized losses (7.6 ) (9.7 ) (3.8 ) Net realized gains on investments $ 36.5 $ 24.7 $ 20.7 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ (14.4 ) $ (32.8 ) $ 34.5 Equity securities 14.3 (29.5 ) 6.2 Other invested assets (8.6 ) (1.6 ) (1.6 ) Deferred federal income tax 3.0 22.4 (13.7 ) Change in unrealized holding gains (losses), net of tax $ (5.7 ) $ (41.5 ) $ 25.4 During the fourth quarter of 2016, the Company redeemed a limited partnership investment in international equities that had been classified as other invested assets. The redemption proceeds were reinvested in an international equity mutual fund classified as an equity security. The Company recognized a gain of $12.0 million on the redemption. Out of the total proceeds related to this redemption and reinvestment, $44.0 million

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 , 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, 2016 and 2015 . 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 is one international fund (“the fund”) that invests in equity securities of foreign issuers and is managed by third party investment managers. The fund had a fair value of $35.7 million and $33.3 million at December 31, 2016 and 2015 , respectively, which was determined using the fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the fund including obtaining and reviewing the fund’s audited financial statements. There is no unfunded commitment related to the fund. The Company may not sell its investment in the fund; however, the Company may redeem all or a portion of its investment in the fund at net asset value per share with the appropriate prior written notice. At December 31, 2015, the Company held another international fund that was managed by third party investment managers. During the fourth quarter of 2016, the Company redeemed its investment in the fund. At December 31, 2015, the fund had a fair value of $43.7 million . In accordance with Accounting Standard Codification 820-10, since the investments are measured at fair value using the net asset value per share practical expedient they have not been classified in the fair value hierarchy. Fair values presented here are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. The remainder of the Company’s other invested assets consist of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds, based on an observable market price for an identical asset in an active market, reflect their fair values and consequently these securities have been disclosed in Level 1. The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at December 31, 2016 and 2015 : ($ 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, 2016 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 404.7 $ — $ 404.7 $ — Obligations of states and political subdivisions 643.7 — 643.7 — Corporate securities 449.6 — 446.1 3.5 U.S. government agencies mortgage-backed securities 611.3 — 611.3 — Total fixed maturities 2,109.3 — 2,105.8 3.5 Equity securities: Large-cap securities 139.0 139.0 — — Small-cap securities 79.1 79.1 — — Mutual and exchange traded funds 164.7 164.7 — — Total equity securities 382.8 382.8 — — Other invested assets 9.4 9.4 — — Total available-for-sale investments $ 2,501.5 $ 392.2 $ 2,105.8 $ 3.5 ($ 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 161.7 161.7 — — Small-cap securities 69.6 69.6 — — Mutual and exchange traded funds 79.3 79.3 — — Total equity securities 310.6 310.6 — — Other invested assets 8.1 8.1 — — Total available-for-sale investments $ 2,319.4 $ 318.7 $ 1,997.4 $ 3.3 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 2016 and 2015 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2016 $ 3.3 Total realized gains (losses)—included in earnings — Total unrealized gains (losses)—included in other comprehensive income — Purchases 0.2 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2016 $ 3.5 ($ 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 Financial Instruments Disclosed, But Not Carried, At Fair Value Other Invested Assets Included in other invested assets are common stock of the Federal Home Loan Bank of Cincinnati (the “FHLB”), and the Trust Securities (as defined in Note 7b). 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, 2016 was $4.9 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, 2016 December 31, 2015 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 75.7 7.00 % $ 70.0 $ 74.1 7.00 % Notes Payable Included in notes payable are the FHLB Loans, consisting of the 2013 FHLB Loan and the 2016 FHLB Loan (as defined in Note 8) and Subordinated Debentures. The Company estimates the fair value of the FHLB Loans by discounting cash flows using a borrowing rate currently available to the Company for a loan with similar terms. The FHLB Loans have 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, 2016 December 31, 2015 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2021:, issued $21.5, September 2016 with fixed interest $ 21.5 $ 21.0 1.73 % $ — $ — — % FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest 85.4 85.6 5.03 % 85.3 85.5 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 5.13 % 15.2 15.2 4.61 % Total notes payable $ 122.1 $ 121.8 $ 100.5 $ 100.7

Losses and Loss Expenses Payabl

Losses and Loss Expenses Payable	12 Months Ended
	Dec. 31, 2016
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) 2016 2015 2014 Losses and loss expenses payable, at beginning of year $ 1,053.0 $ 983.2 $ 959.9 Less: reinsurance recoverable on losses and loss expenses payable 5.9 9.6 9.1 Net balance at beginning of year 1,047.1 973.6 950.8 Incurred related to: Current year 915.4 852.8 726.2 Prior years 27.0 10.0 45.1 Total incurred 942.4 862.8 771.3 Paid related to: Current year 417.8 421.5 373.2 Prior years 393.7 367.8 375.3 Total paid 811.5 789.3 748.5 Net balance at end of year 1,178.0 1,047.1 973.6 Plus: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Losses and loss expenses payable, at end of year (affiliates $630.9, $532.4, and $494.3, respectively) $ 1,181.6 $ 1,053.0 $ 983.2 The Company recorded adverse development related to prior years’ loss and loss expense reserves in 2016 , 2015 and 2014 of $27.0 million and $10.0 million , and $45.1 million , respectively. Unallocated loss adjustment expenses contributed $3.7 million of the adverse development and catastrophe reserves contributed $1.4 million of favorable development in 2016 . The personal insurance segment non-catastrophe loss and ALAE reserves accounted for $7.8 million of the adverse development in 2016 , driven by personal auto which contributed $7.9 million of adverse development, primarily due to higher than anticipated bodily injury severity from the prior two accident years. The business insurance segment non-catastrophe loss and ALAE reserves contributed $3.9 million of favorable development in 2016 , driven by other & product liability and workers’ compensation of $4.5 million and $4.2 million , respectively. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The favorable development was partially offset by adverse development in commercial auto of $3.3 million , which was driven by higher than anticipated bodily injury severity from the prior two accident years. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $20.8 million of adverse development in 2016 , which was driven by programs and E&S casualty with adverse development of $14.3 million and $4.6 million , respectively. Programs adverse development was driven by higher than expected severity in programs with commercial auto exposures. E&S casualty adverse development was driven by increased severity from the healthcare line, which was placed in run-off in the first quarter of 2016. 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 adverse 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 adverse 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 adverse 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 adverse development of $9.7 million and $2.7 million , respectively. Adverse development in programs was driven by higher than expected severity in programs with commercial auto exposures. Somewhat offsetting the adverse 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 adverse development was favorable development of $3.9 million in the E&S property unit and $3.1 million The following tables set forth: • undiscounted, incurred and paid claims and allocated claim adjustment expenses by accident year, on a net basis after reinsurance; • the sum of IBNR claims liabilities + expected development on reported claims ("IBNR+") included within the incurred claims development tables, for each accident year, for the most recent reporting period; and • cumulative claim frequency information for each accident year; • average annual percentage payout of incurred claims by age, net of reinsurance. The above information is provided for each of our reportable insurance segments and then is further disaggregated between short and long-tail where "tail" refers to the time period between the occurrence of a loss and the settlement of the claims. See Note 16 for discussion of our reportable segments. The personal insurance segment generally only consists of short-tail business, whereas the business and specialty insurance segments have both short and long-tail business. We believe our disaggregated groupings have homogeneous risk characteristics with similar development patterns and are generally subject to similar trends. Short-Tail Business: For short-tail business, claims are typically settled within five years, and the most common actuarial estimates are based on techniques using link ratio projections of incurred losses, paid losses, claim counts and claim severities. Each of these methods is described in the "Losses and Loss Expenses Payable" section of "Critical Accounting Policies" included in Item 7 of this Form10-K. Separate projections are made for catastrophes that are in the very early stages of development based on specific information known through the reporting date. Long-Tail Business: For long-tail business, a material portion of claims may not be settled within five years. Reserve estimates for long-tail business use the same methods listed above along with several other methods as determined by the actuary. For example, premium-based methods may be used in developing ultimate loss estimates, including the Expected Loss Ratio, Bornhuetter-Ferguson, and Least-Squares techniques as described in the "Losses and Loss Expenses Payable" section of "Critical Accounting Policies" included in Item 7 of this Form10-K. Statistical models may also be used when the historical patterns can be reasonably approximated. Cumulative Claim Frequency: Cumulative claim frequency is defined as the cumulative number of reported claims ("reported claims"). Reported claims include claims that do not result in a liability (e.g. those below a deductible). Reported claims disclosed in the tables below are for the State Auto Group and have not been pooled (see Note 7a for additional information regarding the Pooling Arrangement). Personal Insurance Segment - Short-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* (1) 2013* (1) 2014* (1) 2015* 2016 IBNR+ Reported Claims (2) 2012 $ 305.3 $ 307.6 $ 306.7 $ 308.9 $ 308.6 $ 0.9 212,774 2013 288.6 283.3 287.9 287.6 2.8 189,392 2014 271.2 272.8 275.2 4.0 171,772 2015 337.2 343.6 20.3 151,130 2016 366.4 44.9 137,561 Total $ 1,581.4 * Supplementary information and unaudited (1) Allocated claim adjustment expenses, net of reinsurance, for accident years 2012, 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. (2) Personal insurance segment - short-tail is an aggregation of the homeowners and personal auto product lines. Homeowners reported claims are counted by claimant and personal auto claims are counted by coverage. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* (1) 2013* (1) 2014* (1) 2015* 2016 2012 $ 193.8 $ 260.2 $ 285.1 $ 298.1 $ 304.5 2013 176.0 236.9 265.4 277.2 2014 172.5 226.3 252.1 2015 214.9 288.9 2016 230.8 Total $ 1,353.5 All outstanding liabilities before 2012, net of reinsurance 0.5 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 228.4 * Supplementary information and unaudited (1) Cumulative paid claims for accident years 2012, 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. Business Insurance Segment - Short-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2012 $ 170.3 $ 170.5 $ 168.1 $ 170.9 $ 169.1 $ 3.7 33,897 2013 170.0 168.7 175.2 176.8 6.4 33,131 2014 181.0 183.9 185.6 15.0 32,253 2015 189.1 191.0 31.2 30,445 2016 186.8 49.8 25,340 Total $ 909.3 * Supplementary information and unaudited (1) Business insurance segment - short-tail is an aggregation of the commercial auto, commercial-multi peril and fire & allied product lines. Commercial auto reported claims are counted by coverage, and commercial multi-peril and fire & allied claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 2012 $ 91.9 $ 128.0 $ 145.8 $ 156.8 $ 161.8 2013 79.4 126.2 146.0 158.4 2014 92.9 131.3 150.5 2015 90.4 130.0 2016 87.7 Total $ 688.4 All outstanding liabilities before 2012, net of reinsurance 14.0 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 234.9 * Supplementary information and unaudited Business Insurance Segment - Long-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by lower than expected loss emergence during 2016 in our other & product liability and workers’ compensation lines of business. The unanticipated emergence in the other & product liability line was driven by much lower than expected severity in accident years 2013 and 2015. The unanticipated emergence in the workers’ compensation line was driven by lower than expected severity in accident year 2014. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2007 $ 73.4 $ 73.3 $ 74.1 $ 73.2 $ 71.9 $ 71.9 $ 72.5 $ 72.4 $ 73.0 $ 73.3 $ 2.7 8,072 2008 86.1 79.1 75.6 75.0 75.4 75.3 74.9 74.2 73.4 4.0 9,577 2009 90.2 92.0 87.4 86.0 85.0 83.1 82.4 82.3 5.8 10,086 2010 85.3 93.5 86.7 82.2 82.0 79.5 78.9 7.2 10,644 2011 86.9 86.4 80.9 77.5 76.1 75.3 9.0 17,390 2012 92.8 85.5 80.2 78.0 77.6 12.2 17,321 2013 92.6 88.2 88.2 84.9 19.7 16,646 2014 96.1 96.6 95.2 27.3 16,501 2015 108.9 106.2 45.5 15,783 2016 109.5 65.6 8,548 Total $ 856.6 * Supplementary information and unaudited (1) Business insurance segment-long-tail is an aggregation of the other & product liability and workers' compensation product lines. Other & product liability reported claims are counted by claimant and workers' compensation reported claims are counted by a combination of claimant and coverage. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2007 $ 12.7 $ 28.7 $ 43.0 $ 50.3 $ 57.6 $ 61.4 $ 64.0 $ 65.7 $ 68.1 $ 68.6 2008 14.3 34.7 43.9 51.6 56.4 61.3 64.0 65.0 66.0 2009 15.2 34.9 48.2 60.2 66.2 70.3 72.1 74.1 2010 12.9 33.0 46.3 54.2 63.3 65.8 67.6 2011 16.4 33.9 46.4 53.9 60.7 63.6 2012 15.3 32.6 43.7 51.8 57.3 2013 16.3 36.7 50.1 57.2 2014 15.8 37.6 53.9 2015 17.3 38.6 2016 15.4 $ 562.3 All outstanding liabilities before 2007, net of reinsurance 36.6 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 330.9 * Supplementary information and unaudited Specialty Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by greater than expected loss emergence during 2016 in our Programs line of business. The unanticipated emergence in the Programs line was driven by higher than expected severity in accident year 2015 for programs with commercial auto exposure. The changes in incurred losses and loss adjustment expenses for accident years 2012 and 2013 for the years ended December 31, 2013, 2014 and 2015 are primarily attributable to our Programs line of business, including RED. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2012 $ 108.6 $ 115.6 $ 164.8 $ 162.8 $ 163.7 $ 9.0 9,697 2013 70.9 82.9 85.0 90.9 6.7 7,680 2014 55.7 56.9 58.6 7.8 7,657 2015 75.5 85.4 21.6 7,666 2016 99.3 48.8 5,754 Total $ 497.9 * Supplementary information and unaudited (1) Specialty insurance segment-short-tail is an aggregation of the E&S property and Programs product lines. E&S property and Programs reported claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 2012 $ 25.4 $ 59.8 $ 94.7 $ 122.9 $ 142.5 2013 18.2 37.7 55.5 72.0 2014 22.0 34.9 44.0 2015 24.0 48.2 2016 25.2 Total $ 331.9 All outstanding liabilities before 2012, net of reinsurance 12.9 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 178.9 * Supplementary information and unaudited Specialty Insurance Segment - Long-Tail The change in incurred loss and loss adjustment expenses for prior accident years was caused by greater than expected loss emergence during 2016, driven by higher than anticipated severity in accident years 2014 and 2015 for our healthcare product. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2007 $ 9.7 $ 9.2 $ 9.4 $ 9.0 $ 8.8 $ 8.5 $ 8.4 $ 8.3 $ 8.3 $ 4.1 $ 2.4 1,101 2008 6.4 6.7 6.7 6.8 6.1 6.0 5.8 6.0 4.9 1.6 1,502 2009 10.3 11.0 10.0 10.4 11.0 11.0 10.6 12.4 2.0 1,429 2010 11.2 10.9 9.9 9.7 9.8 9.9 10.3 2.2 1,169 2011 11.9 10.7 10.7 10.2 10.1 10.9 2.2 911 2012 16.7 16.7 15.4 17.6 17.4 3.0 862 2013 20.1 18.9 17.9 16.5 4.9 936 2014 23.3 25.2 27.1 9.5 1,044 2015 38.6 45.0 22.8 1,409 2016 60.8 47.6 1,401 Total $ 209.4 * Supplementary information and unaudited (1) Specialty insurance segment-long-tail consists of the E&S casualty product line. E&S casualty reported claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2007 $ — $ 0.1 $ 0.6 $ 0.1 $ 0.5 $ 1.3 $ 1.3 $ 1.5 $ 1.5 $ 1.6 2008 0.1 0.3 0.8 1.3 1.7 2.1 2.4 2.8 3.0 2009 0.2 2.4 4.9 7.2 8.6 9.4 9.2 10.0 2010 0.8 2.2 5.2 6.5 7.7 7.6 8.1 2011 0.9 3.0 5.2 6.6 7.1 7.9 2012 0.9 4.0 7.3 10.3 12.5 2013 1.1 3.7 6.7 9.1 2014 1.2 7.2 12.5 2015 2.7 11.0 2016 6.0 $ 81.7 All outstanding liabilities before 2007, net of reinsurance 2.0 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 129.7 * Supplementary information and unaudited The following table sets forth the reconciliation of the claims development tables to the balance sheet losses and loss expenses payable, with separate disclosure of unallocated loss adjustment expenses, net of reinsurance ("ULAE") and reinsurance recoverable on losses and loss expenses payable, for the years ended December 31: ($ in millions) 2016 2015 2014 Net losses and allocated loss adjustment expenses payable: Personal Insurance Segment Short-tail $ 228.4 $ 207.4 $ 183.1 Other personal 8.9 7.5 7.5 Business Insurance Segment Short-tail 234.9 213.5 183.9 Long-tail 330.9 306.6 285.6 Other business 2.5 1.4 2.4 Specialty Insurance Segment Short-tail 178.9 165.8 195.3 Long-tail 129.7 91.1 65.4 Net losses and loss expenses payable 1,114.2 993.3 923.2 ULAE 63.8 53.8 50.4 Reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Total losses and loss expenses payable $ 1,181.6 $ 1,053.0 $ 983.2 The following table sets forth the historical average annual payout of incurred losses and allocated loss adjustment expenses (claims duration) for short-duration contracts, based on the disaggregated information in the paid loss development tables, net of reinsurance: Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years Segment 1* 2* 3* 4* 5* 6* 7* 8* 9* 10* Personal Insurance: Short-tail 62.4 % 21.0 % 9.1 % 4.2 % 2.1 % N/A N/A N/A N/A N/A Business Insurance: Short-tail 48.7 % 22.3 % 10.7 % 6.8 % 3.0 % N/A N/A N/A N/A N/A Long-tail 17.9 % 23.5 % 16.1 % 10.5 % 8.6 % 4.8 % 2.9 % 2.0 % 2.4 % 0.7 % Specialty Insurance: Short-tail 25.3 % 23.2 % 18.8 % 17.7 % 11.9 % N/A N/A N/A N/A N/A Long-tail 5.3 % 14.5 % 18.5 % 10.6 % 9.6 % 8.1 % 2.4 % 6.3 % 2.8 % 2.2 % * Supplementary information and unaudited

Short-Duration Insurance Contra

Short-Duration Insurance Contracts Short-Duration Insurance Contracts	12 Months Ended
	Dec. 31, 2016
Incurred Losses and Loss Expenses Payable, Net of Reinsurance [Abstract]
Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block]	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) 2016 2015 2014 Losses and loss expenses payable, at beginning of year $ 1,053.0 $ 983.2 $ 959.9 Less: reinsurance recoverable on losses and loss expenses payable 5.9 9.6 9.1 Net balance at beginning of year 1,047.1 973.6 950.8 Incurred related to: Current year 915.4 852.8 726.2 Prior years 27.0 10.0 45.1 Total incurred 942.4 862.8 771.3 Paid related to: Current year 417.8 421.5 373.2 Prior years 393.7 367.8 375.3 Total paid 811.5 789.3 748.5 Net balance at end of year 1,178.0 1,047.1 973.6 Plus: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Losses and loss expenses payable, at end of year (affiliates $630.9, $532.4, and $494.3, respectively) $ 1,181.6 $ 1,053.0 $ 983.2 The Company recorded adverse development related to prior years’ loss and loss expense reserves in 2016 , 2015 and 2014 of $27.0 million and $10.0 million , and $45.1 million , respectively. Unallocated loss adjustment expenses contributed $3.7 million of the adverse development and catastrophe reserves contributed $1.4 million of favorable development in 2016 . The personal insurance segment non-catastrophe loss and ALAE reserves accounted for $7.8 million of the adverse development in 2016 , driven by personal auto which contributed $7.9 million of adverse development, primarily due to higher than anticipated bodily injury severity from the prior two accident years. The business insurance segment non-catastrophe loss and ALAE reserves contributed $3.9 million of favorable development in 2016 , driven by other & product liability and workers’ compensation of $4.5 million and $4.2 million , respectively. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The favorable development was partially offset by adverse development in commercial auto of $3.3 million , which was driven by higher than anticipated bodily injury severity from the prior two accident years. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $20.8 million of adverse development in 2016 , which was driven by programs and E&S casualty with adverse development of $14.3 million and $4.6 million , respectively. Programs adverse development was driven by higher than expected severity in programs with commercial auto exposures. E&S casualty adverse development was driven by increased severity from the healthcare line, which was placed in run-off in the first quarter of 2016. 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 adverse 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 adverse 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 adverse 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 adverse development of $9.7 million and $2.7 million , respectively. Adverse development in programs was driven by higher than expected severity in programs with commercial auto exposures. Somewhat offsetting the adverse 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 adverse development was favorable development of $3.9 million in the E&S property unit and $3.1 million The following tables set forth: • undiscounted, incurred and paid claims and allocated claim adjustment expenses by accident year, on a net basis after reinsurance; • the sum of IBNR claims liabilities + expected development on reported claims ("IBNR+") included within the incurred claims development tables, for each accident year, for the most recent reporting period; and • cumulative claim frequency information for each accident year; • average annual percentage payout of incurred claims by age, net of reinsurance. The above information is provided for each of our reportable insurance segments and then is further disaggregated between short and long-tail where "tail" refers to the time period between the occurrence of a loss and the settlement of the claims. See Note 16 for discussion of our reportable segments. The personal insurance segment generally only consists of short-tail business, whereas the business and specialty insurance segments have both short and long-tail business. We believe our disaggregated groupings have homogeneous risk characteristics with similar development patterns and are generally subject to similar trends. Short-Tail Business: For short-tail business, claims are typically settled within five years, and the most common actuarial estimates are based on techniques using link ratio projections of incurred losses, paid losses, claim counts and claim severities. Each of these methods is described in the "Losses and Loss Expenses Payable" section of "Critical Accounting Policies" included in Item 7 of this Form10-K. Separate projections are made for catastrophes that are in the very early stages of development based on specific information known through the reporting date. Long-Tail Business: For long-tail business, a material portion of claims may not be settled within five years. Reserve estimates for long-tail business use the same methods listed above along with several other methods as determined by the actuary. For example, premium-based methods may be used in developing ultimate loss estimates, including the Expected Loss Ratio, Bornhuetter-Ferguson, and Least-Squares techniques as described in the "Losses and Loss Expenses Payable" section of "Critical Accounting Policies" included in Item 7 of this Form10-K. Statistical models may also be used when the historical patterns can be reasonably approximated. Cumulative Claim Frequency: Cumulative claim frequency is defined as the cumulative number of reported claims ("reported claims"). Reported claims include claims that do not result in a liability (e.g. those below a deductible). Reported claims disclosed in the tables below are for the State Auto Group and have not been pooled (see Note 7a for additional information regarding the Pooling Arrangement). Personal Insurance Segment - Short-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* (1) 2013* (1) 2014* (1) 2015* 2016 IBNR+ Reported Claims (2) 2012 $ 305.3 $ 307.6 $ 306.7 $ 308.9 $ 308.6 $ 0.9 212,774 2013 288.6 283.3 287.9 287.6 2.8 189,392 2014 271.2 272.8 275.2 4.0 171,772 2015 337.2 343.6 20.3 151,130 2016 366.4 44.9 137,561 Total $ 1,581.4 * Supplementary information and unaudited (1) Allocated claim adjustment expenses, net of reinsurance, for accident years 2012, 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. (2) Personal insurance segment - short-tail is an aggregation of the homeowners and personal auto product lines. Homeowners reported claims are counted by claimant and personal auto claims are counted by coverage. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* (1) 2013* (1) 2014* (1) 2015* 2016 2012 $ 193.8 $ 260.2 $ 285.1 $ 298.1 $ 304.5 2013 176.0 236.9 265.4 277.2 2014 172.5 226.3 252.1 2015 214.9 288.9 2016 230.8 Total $ 1,353.5 All outstanding liabilities before 2012, net of reinsurance 0.5 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 228.4 * Supplementary information and unaudited (1) Cumulative paid claims for accident years 2012, 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. Business Insurance Segment - Short-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2012 $ 170.3 $ 170.5 $ 168.1 $ 170.9 $ 169.1 $ 3.7 33,897 2013 170.0 168.7 175.2 176.8 6.4 33,131 2014 181.0 183.9 185.6 15.0 32,253 2015 189.1 191.0 31.2 30,445 2016 186.8 49.8 25,340 Total $ 909.3 * Supplementary information and unaudited (1) Business insurance segment - short-tail is an aggregation of the commercial auto, commercial-multi peril and fire & allied product lines. Commercial auto reported claims are counted by coverage, and commercial multi-peril and fire & allied claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 2012 $ 91.9 $ 128.0 $ 145.8 $ 156.8 $ 161.8 2013 79.4 126.2 146.0 158.4 2014 92.9 131.3 150.5 2015 90.4 130.0 2016 87.7 Total $ 688.4 All outstanding liabilities before 2012, net of reinsurance 14.0 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 234.9 * Supplementary information and unaudited Business Insurance Segment - Long-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by lower than expected loss emergence during 2016 in our other & product liability and workers’ compensation lines of business. The unanticipated emergence in the other & product liability line was driven by much lower than expected severity in accident years 2013 and 2015. The unanticipated emergence in the workers’ compensation line was driven by lower than expected severity in accident year 2014. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2007 $ 73.4 $ 73.3 $ 74.1 $ 73.2 $ 71.9 $ 71.9 $ 72.5 $ 72.4 $ 73.0 $ 73.3 $ 2.7 8,072 2008 86.1 79.1 75.6 75.0 75.4 75.3 74.9 74.2 73.4 4.0 9,577 2009 90.2 92.0 87.4 86.0 85.0 83.1 82.4 82.3 5.8 10,086 2010 85.3 93.5 86.7 82.2 82.0 79.5 78.9 7.2 10,644 2011 86.9 86.4 80.9 77.5 76.1 75.3 9.0 17,390 2012 92.8 85.5 80.2 78.0 77.6 12.2 17,321 2013 92.6 88.2 88.2 84.9 19.7 16,646 2014 96.1 96.6 95.2 27.3 16,501 2015 108.9 106.2 45.5 15,783 2016 109.5 65.6 8,548 Total $ 856.6 * Supplementary information and unaudited (1) Business insurance segment-long-tail is an aggregation of the other & product liability and workers' compensation product lines. Other & product liability reported claims are counted by claimant and workers' compensation reported claims are counted by a combination of claimant and coverage. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2007 $ 12.7 $ 28.7 $ 43.0 $ 50.3 $ 57.6 $ 61.4 $ 64.0 $ 65.7 $ 68.1 $ 68.6 2008 14.3 34.7 43.9 51.6 56.4 61.3 64.0 65.0 66.0 2009 15.2 34.9 48.2 60.2 66.2 70.3 72.1 74.1 2010 12.9 33.0 46.3 54.2 63.3 65.8 67.6 2011 16.4 33.9 46.4 53.9 60.7 63.6 2012 15.3 32.6 43.7 51.8 57.3 2013 16.3 36.7 50.1 57.2 2014 15.8 37.6 53.9 2015 17.3 38.6 2016 15.4 $ 562.3 All outstanding liabilities before 2007, net of reinsurance 36.6 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 330.9 * Supplementary information and unaudited Specialty Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by greater than expected loss emergence during 2016 in our Programs line of business. The unanticipated emergence in the Programs line was driven by higher than expected severity in accident year 2015 for programs with commercial auto exposure. The changes in incurred losses and loss adjustment expenses for accident years 2012 and 2013 for the years ended December 31, 2013, 2014 and 2015 are primarily attributable to our Programs line of business, including RED. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2012 $ 108.6 $ 115.6 $ 164.8 $ 162.8 $ 163.7 $ 9.0 9,697 2013 70.9 82.9 85.0 90.9 6.7 7,680 2014 55.7 56.9 58.6 7.8 7,657 2015 75.5 85.4 21.6 7,666 2016 99.3 48.8 5,754 Total $ 497.9 * Supplementary information and unaudited (1) Specialty insurance segment-short-tail is an aggregation of the E&S property and Programs product lines. E&S property and Programs reported claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 2012 $ 25.4 $ 59.8 $ 94.7 $ 122.9 $ 142.5 2013 18.2 37.7 55.5 72.0 2014 22.0 34.9 44.0 2015 24.0 48.2 2016 25.2 Total $ 331.9 All outstanding liabilities before 2012, net of reinsurance 12.9 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 178.9 * Supplementary information and unaudited Specialty Insurance Segment - Long-Tail The change in incurred loss and loss adjustment expenses for prior accident years was caused by greater than expected loss emergence during 2016, driven by higher than anticipated severity in accident years 2014 and 2015 for our healthcare product. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2007 $ 9.7 $ 9.2 $ 9.4 $ 9.0 $ 8.8 $ 8.5 $ 8.4 $ 8.3 $ 8.3 $ 4.1 $ 2.4 1,101 2008 6.4 6.7 6.7 6.8 6.1 6.0 5.8 6.0 4.9 1.6 1,502 2009 10.3 11.0 10.0 10.4 11.0 11.0 10.6 12.4 2.0 1,429 2010 11.2 10.9 9.9 9.7 9.8 9.9 10.3 2.2 1,169 2011 11.9 10.7 10.7 10.2 10.1 10.9 2.2 911 2012 16.7 16.7 15.4 17.6 17.4 3.0 862 2013 20.1 18.9 17.9 16.5 4.9 936 2014 23.3 25.2 27.1 9.5 1,044 2015 38.6 45.0 22.8 1,409 2016 60.8 47.6 1,401 Total $ 209.4 * Supplementary information and unaudited (1) Specialty insurance segment-long-tail consists of the E&S casualty product line. E&S casualty reported claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2007 $ — $ 0.1 $ 0.6 $ 0.1 $ 0.5 $ 1.3 $ 1.3 $ 1.5 $ 1.5 $ 1.6 2008 0.1 0.3 0.8 1.3 1.7 2.1 2.4 2.8 3.0 2009 0.2 2.4 4.9 7.2 8.6 9.4 9.2 10.0 2010 0.8 2.2 5.2 6.5 7.7 7.6 8.1 2011 0.9 3.0 5.2 6.6 7.1 7.9 2012 0.9 4.0 7.3 10.3 12.5 2013 1.1 3.7 6.7 9.1 2014 1.2 7.2 12.5 2015 2.7 11.0 2016 6.0 $ 81.7 All outstanding liabilities before 2007, net of reinsurance 2.0 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 129.7 * Supplementary information and unaudited The following table sets forth the reconciliation of the claims development tables to the balance sheet losses and loss expenses payable, with separate disclosure of unallocated loss adjustment expenses, net of reinsurance ("ULAE") and reinsurance recoverable on losses and loss expenses payable, for the years ended December 31: ($ in millions) 2016 2015 2014 Net losses and allocated loss adjustment expenses payable: Personal Insurance Segment Short-tail $ 228.4 $ 207.4 $ 183.1 Other personal 8.9 7.5 7.5 Business Insurance Segment Short-tail 234.9 213.5 183.9 Long-tail 330.9 306.6 285.6 Other business 2.5 1.4 2.4 Specialty Insurance Segment Short-tail 178.9 165.8 195.3 Long-tail 129.7 91.1 65.4 Net losses and loss expenses payable 1,114.2 993.3 923.2 ULAE 63.8 53.8 50.4 Reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Total losses and loss expenses payable $ 1,181.6 $ 1,053.0 $ 983.2 The following table sets forth the historical average annual payout of incurred losses and allocated loss adjustment expenses (claims duration) for short-duration contracts, based on the disaggregated information in the paid loss development tables, net of reinsurance: Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years Segment 1* 2* 3* 4* 5* 6* 7* 8* 9* 10* Personal Insurance: Short-tail 62.4 % 21.0 % 9.1 % 4.2 % 2.1 % N/A N/A N/A N/A N/A Business Insurance: Short-tail 48.7 % 22.3 % 10.7 % 6.8 % 3.0 % N/A N/A N/A N/A N/A Long-tail 17.9 % 23.5 % 16.1 % 10.5 % 8.6 % 4.8 % 2.9 % 2.0 % 2.4 % 0.7 % Specialty Insurance: Short-tail 25.3 % 23.2 % 18.8 % 17.7 % 11.9 % N/A N/A N/A N/A N/A Long-tail 5.3 % 14.5 % 18.5 % 10.6 % 9.6 % 8.1 % 2.4 % 6.3 % 2.8 % 2.2 % * Supplementary information and unaudited

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2016
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 7a 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, 2016, 2015 and 2014, the Company recognized profit commission of $1.9 million , $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 $8.1 million of excess ceding commission that had previously been deferred, reflected as a reduction in acquisition and operating expenses on the consolidated statements of income for the year ended December 31, 2014. The following table sets forth the effect of the Company’s external reinsurance on its balance sheets at December 31, 2016 and 2015 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2016 2015 Losses and loss expenses payable: Direct $ 545.7 $ 515.6 Assumed 5.0 5.0 Ceded (3.6 ) (5.9 ) Net losses and loss expenses payable $ 547.1 $ 514.7 Unearned premiums: Direct $ 395.6 $ 400.9 Assumed 1.3 1.2 Ceded (6.1 ) (6.8 ) Net unearned premiums $ 390.8 $ 395.3 The following table sets forth the effect of the Company’s external reinsurance on its income statements for the years ended December 31, 2016 , 2015 and 2014 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2016 2015 2014 Written premiums: Direct $ 835.4 $ 854.1 $ 880.6 Assumed 5.0 4.7 4.4 Ceded (24.9 ) (35.4 ) (26.8 ) Net written premiums $ 815.5 $ 823.4 $ 858.2 Earned premiums: Direct $ 840.6 $ 863.1 $ 882.6 Assumed 4.9 4.5 4.4 Ceded (25.6 ) (34.7 ) (25.3 ) Net earned premiums $ 819.9 $ 832.9 $ 861.7 Losses and loss expenses incurred: Direct $ 559.1 $ 569.0 $ 525.6 Assumed 3.9 3.4 2.9 Ceded (4.4 ) (2.9 ) (8.0 ) Net losses and loss expenses incurred $ 558.6 $ 569.5 $ 520.5

Transactions with Affiliates

Transactions with Affiliates	12 Months Ended
	Dec. 31, 2016
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 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, 2016 and 2015 : ($ millions) 2016 2015 Assets Deferred policy acquisition costs: Ceded $ (79.1 ) $ (79.8 ) Assumed 129.8 129.1 Net assumed $ 50.7 $ 49.3 Liabilities and Stockholders’ Equity Losses and loss expenses payable: Ceded $ (547.1 ) $ (514.7 ) Assumed 1,178.0 1,047.1 Net assumed $ 630.9 $ 532.4 Unearned premiums: Ceded $ (390.8 ) $ (395.3 ) Assumed 611.7 609.5 Net assumed $ 220.9 $ 214.2 Pension and postretirement benefits: Ceded $ (114.5 ) $ (160.0 ) Assumed 74.4 104.0 Net ceded $ (40.1 ) $ (56.0 ) Other liabilities: Ceded $ (42.9 ) $ (63.8 ) Assumed 53.9 55.4 Net assumed (ceded) $ 11.0 $ (8.4 ) Stockholders’ Equity Accumulated other comprehensive income: Ceded $ (153.5 ) $ (162.0 ) Assumed 99.8 105.3 Net ceded $ (53.7 ) $ (56.7 ) 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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Written premiums: Ceded $ (815.5 ) $ (823.4 ) $ (858.2 ) Assumed 1,293.3 1,273.5 1,194.2 Net assumed $ 477.8 $ 450.1 $ 336.0 Earned premiums: Ceded $ (819.9 ) $ (832.9 ) $ (861.7 ) Assumed 1,291.9 1,270.5 1,074.1 Net assumed $ 472.0 $ 437.6 $ 212.4 Losses and loss expenses incurred: Ceded $ (560.3 ) $ (570.9 ) $ (523.6 ) Assumed 944.1 864.2 774.4 Net assumed $ 383.8 $ 293.3 $ 250.8 Acquisition and operating expenses: Ceded (148.5 ) (183.9 ) (215.4 ) Assumed 434.3 432.5 372.3 Net assumed $ 285.8 $ 248.6 $ 156.9 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, 2016 and 2015 is approximately $310.5 million and $294.7 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 5.13% at December 31, 2016 ). 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, 2016 , 2015 and 2014 , 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 $1.9 million , $1.8 million and $1.5 million in 2016 , 2015 and 2014

Notes Payable and Credit Facili

Notes Payable and Credit Facility	12 Months Ended
	Dec. 31, 2016
Notes Payable [Abstract]
Notes Payable and Credit Facility	Notes Payable and Credit Facility FHLB Loans State Auto Financial’s subsidiary, State Auto P&C, is a member of the Federal Home Loan Bank of Cincinnati (the “FHLB”) and in September 2016 entered into a new term loan with the FHLB in the principal amount of $21.5 million (the “2016 FHLB Loan”). The 2016 FHLB Loan is a five -year term loan and may be called (prepaid) after three years with no prepayment penalty. The 2016 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 1.73% . The 2016 FHLB Loan is fully secured by a pledge of specific investment securities of State Auto P&C. State Auto P&C has a term loan with the FHLB in the principal amount of $85.0 million (the “2013 FHLB Loan”). The 2013 FHLB Loan is a 20 -year term loan and is callable after three years with no prepayment penalty thereafter. The 2013 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 2013 FHLB Loan is fully secured by a pledge of specific investment securities of State Auto P&C. 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, 2016

Federal Income Taxes

Federal Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal Income Taxes	Federal Income Taxes The following table sets forth the reconciliation between actual federal income tax (benefit) expense and the amount computed at the indicated statutory rate for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Amount at statutory rate $ 6.7 35.0 % $ 23.5 35.0 % $ 9.4 35.0 % Tax-exempt interest and dividends received deduction (7.1 ) (37.2 ) (8.7 ) (13.0 ) (8.5 ) (31.5 ) Other, net (1.4 ) (7.2 ) 1.3 1.9 1.1 4.1 Valuation allowance — — — — (82.6 ) (308.1 ) Federal income tax (benefit) expense and effective rate $ (1.8 ) (9.4 )% $ 16.1 23.9 % $ (80.6 ) (300.5 )% Income taxes for the year ended December 31, 2016 reflect the impact of a correction of prior period deferred tax expense related to expired stock options. As a result of the correction, deferred federal income tax expense (“other, net” above) and additional paid-in-capital were reduced by $1.6 million , respectively. The following table sets forth the tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities at December 31, 2016 and 2015 : ($ millions) 2016 2015 Deferred tax assets: Unearned premiums not currently deductible $ 42.6 $ 42.5 Losses and loss expenses payable discounting 19.7 19.3 Postretirement and pension benefits 26.1 36.4 Realized loss on other-than-temporary impairment 4.0 12.3 Other liabilities 16.6 21.3 Net operating loss carryforward 58.9 41.8 Tax credit carryforward 3.3 4.8 Other 8.8 4.9 Total deferred tax assets 180.0 183.3 Deferred tax liabilities: Deferral of policy acquisition costs 45.4 45.2 Net unrealized holding gains on investments 32.5 35.6 Total deferred tax liabilities 77.9 80.8 Net deferred federal income taxes $ 102.1 $ 102.5 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, 2016 , the tax benefit of the net operating loss (“NOL”) carryforward was $58.9 million . The NOL carryforwards do not begin to expire until 2030 and will not fully expire until 2036. At December 31, 2016 , the Company carried no balance for uncertain tax positions. The Company had no accrual for the payment of interest and penalties at December 31, 2016 or 2015 .

Pension and Postretirement Bene

Pension and Postretirement Benefit Plans	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 : ($ millions) Pension Postretirement 2016 2015 2016 2015 Change in benefit obligation: Benefit obligation at beginning of year $ 281.3 $ 292.5 $ 20.3 $ 23.9 Service cost 6.1 7.7 — — Interest cost 11.6 11.1 0.8 0.9 Actuarial loss (gain) 4.5 (17.6 ) (0.9 ) (3.3 ) Benefits paid (19.0 ) (12.4 ) (1.4 ) (1.2 ) The Company’s portion of benefit obligation at end of year $ 284.5 $ 281.3 $ 18.8 $ 20.3 Change in plan assets available for plan benefits: Fair value of plan assets available for plan benefits at beginning of year $ 204.4 $ 205.1 $ — $ 0.3 Employer contribution 34.5 13.0 — — Actual return on plan assets 15.8 (1.3 ) — — Benefits paid (19.0 ) (12.4 ) — (0.3 ) The Company’s portion of fair value of plan assets at end of year $ 235.7 $ 204.4 $ — $ — Supplemental executive retirement plan (6.8 ) (6.8 ) — — Funded status at end of year $ (55.6 ) $ (83.7 ) $ (18.8 ) $ (20.3 ) Accumulated benefit obligation end of year $ 267.5 $ 263.1 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, 2016 and 2015 : ($ millions) 2016 2015 Prior service benefit $ (54.1 ) $ (59.5 ) Net actuarial loss 118.5 125.1 Total $ 64.4 $ 65.6 The following table sets forth the Company’s share of amortization expected to be recognized for the year ending December 31, 2017: ($ millions) 2017 Prior service benefit $ (5.5 ) Net actuarial loss 8.0 Total $ 2.5 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, 2016 , 2015 and 2014 : ($ millions) Pension Postretirement 2016 2015 2014 2016 2015 2014 Components of net periodic cost: Service cost $ 6.2 $ 7.9 $ 5.2 $ — $ — $ — Interest cost 11.9 11.3 11.1 0.9 1.1 1.1 Expected return on plan assets (15.1 ) (13.8 ) (12.6 ) — — — Amortization of: Prior service benefit — — — (5.5 ) (5.4 ) (5.5 ) Net actuarial loss 9.2 10.9 6.3 0.2 0.6 0.6 Net periodic cost (benefit) $ 12.2 $ 16.3 $ 10.0 $ (4.4 ) $ (3.7 ) $ (3.8 ) 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 2017 $ 11.3 $ 1.5 2018 11.8 1.4 2019 12.0 1.4 2020 12.5 1.4 2021 13.1 1.4 2022-2026 76.8 6.0 The postretirement plan’s gross benefit payments for 2016 were $1.4 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 2016 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, 2016 and 2015 : Pension Postretirement 2016 2015 2016 2015 Benefit obligations weighted-average assumptions: Discount rate 4.00 % 4.20 % 4.00 % 4.20 % 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, 2016 , 2015 and 2014 : Pension Postretirement 2016 2015 2014 2016 2015 2014 Weighted-average assumptions: Discount rate 4.20 % 3.85 % 4.85 % 4.20 % 3.85 % 4.85 % Expected long-term rate of return on assets 7.00 7.00 7.00 — — — Rates of increase in compensation levels 3.50 3.50 3.50 — — — 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, 2016 , 2015 and 2014 : Postretirement 2016 2015 2014 Assumed health care cost trend rates: Health care cost trend rate assumed for the next year 6.50 % 6.50 % 6.00 % Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 3.90 % 3.80 % 3.80 % Year that the rate reaches the ultimate trend rate 2075 2076 2075 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, 2016 : ($ 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 2.8 2.4 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 49.0 % U.S. large-cap equity 26.0 U.S. small-cap equity 11.0 International equity 10.0 Emerging market equity 4.0 Total 100.0 % 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 maturity holdings. 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. 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, 2016 and 2015 . Included in the pension plan’s available-for-sale securities is an international fund (“the fund”) that invests in equity securities of foreign issuers and is managed by third party investment managers. The fund had a fair value of $31.3 million and $18.6 million at December 31, 2016 and 2015, respectively, which was determined using the fund’s net asset value. In accordance with Accounting Standard Codification 820-10, since this investment is measured at fair value using the net asset value per share practical expedient this has not been classified in the table below. The following tables set forth the Company’s share of pension plan’s available-for-sale securities within the fair value hierarchy at December 31, 2016 and 2015 : ($ 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, 2016 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 60.4 $ — $ 60.4 $ — Corporate securities 48.0 — 48.0 — U.S. government agencies mortgage-backed securities 4.6 — 4.6 — Total fixed maturities 113.0 — 113.0 — Equity securities: Large-cap securities 59.6 59.6 — — Small-cap securities 26.3 26.3 — — Total equity securities 85.9 85.9 — — Short-term fixed maturities 2.0 — 2.0 — Total pension plan investments $ 200.9 $ 85.9 $ 115.0 $ — ($ 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 — — Total pension plan investments $ 177.6 $ 81.3 $ 96.3 $ — During 2015, the postretirement plan assets were sold in order to pay retiree medical costs. 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 2017 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 $6.3 million , $5.1 million and $5.1 million for 2016 , 2015 and 2014

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2016
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, 2016 , 2015 and 2014 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at January 1, 2016 $ 68.5 $ (30.9 ) $ 37.6 Other comprehensive income before reclassifications 18.0 (3.0 ) 15.0 Amounts reclassified from AOCI (a) (23.7 ) 3.6 (20.1 ) Net current period other comprehensive income (5.7 ) 0.6 (5.1 ) Ending balance at December 31, 2016 $ 62.8 $ (30.3 ) $ 32.5 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 (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, 2016 , 2015 and 2014 : ($ millions) Details about Accumulated Other December 31 Affected line item in the Condensed Comprehensive Income Components 2016 2015 2014 Consolidated Statements of Income Unrealized gains on available for sale securities $ 36.5 $ 24.7 $ 20.7 Realized gain on sale of securities 36.5 24.7 20.7 Total before tax (12.8 ) (8.6 ) (7.2 ) Tax expense 23.7 16.1 13.5 Net of tax Amortization of benefit plan items: Negative prior service costs 5.5 5.4 5.5 (a) Net loss (9.4 ) (11.5 ) (6.9 ) (a) (3.9 ) (6.1 ) (1.4 ) Total before tax 0.3 4.0 (18.5 ) Tax benefit (expense) (3.6 ) (2.1 ) (19.9 ) Net of tax Total reclassifications for the period $ 20.1 $ 14.0 $ (6.4 ) (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, 2016
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 $84.4 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 $10.0 million , $15.0 million and $20.0 million in 2016 , 2015 and 2014 , respectively. The Company’s insurance subsidiaries are subject to risk-based capital (“RBC”) requirements that have been adopted by individual states. These requirements subject insurers having statutory capital less than that required by the RBC calculation to varying degrees of regulatory action, depending on the level of capital inadequacy. The RBC formulas specify various weighting factors to be applied to financial balances or various levels of activity based on the perceived degree of risk. Regulatory compliance is determined by a ratio of total adjusted capital to authorized control level RBC. Generally no remedial action is required by an insurance company if its adjusted statutory surplus exceeds 200% of the authorized level RBC. As of December 31, 2016 , each of the Company’s insurance subsidiaries maintained adjusted statutory surplus in excess of 400% of the authorized control level RBC. The following tables set forth reconciliations of statutory capital and surplus and net income, as determined using SAP, to the amounts included in the accompanying consolidated financial statements: ($ millions) 2016 2015 Statutory capital and surplus of insurance subsidiaries $ 844.4 $ 814.3 Net liabilities of non-insurance parent and affiliates (84.9 ) (80.2 ) 759.5 734.1 Increases (decreases): Deferred acquisition costs 129.8 129.1 Postretirement and pension benefits 22.7 23.2 Deferred federal income taxes (49.1 ) (41.6 ) Fixed maturities, at fair value 13.5 27.9 Other, net 14.9 11.9 Stockholders’ equity per accompanying consolidated financial statements $ 891.3 $ 884.6 ($ millions) Year ended December 31 2016 2015 2014 Statutory net income (loss) of insurance subsidiaries $ 22.7 $ 65.4 $ (17.5 ) Net loss of non-insurance parent and affiliates (3.0 ) (4.1 ) (5.3 ) 19.7 61.3 (22.8 ) Increases (decreases): Deferred acquisition costs 0.7 2.5 29.7 Postretirement and pension benefits 4.4 4.2 10.6 Deferred federal income taxes (5.4 ) (12.2 ) 79.3 Share-based compensation expense — (0.1 ) (0.5 ) Other, net 1.6 (4.5 ) 11.1 Net income per accompanying consolidated financial statements $ 21.0 $ 51.2 $ 107.4

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2016
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

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Dec. 31, 2016
Share-based Compensation [Abstract]
Disclosure of Compensation Related Costs, Share-based Payments [Text Block]	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 5.0 million common shares under the Equity Plan. As of December 31, 2016 , a total of 2.4 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 2016 , 2015 and 2014 were 0.2 million , 0.3 million and 0.2 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, 2016 , 2015 and 2014 : 2016 2015 2014 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 111,084 $ 22.19 76,472 $ 19.06 38,233 $ 15.06 Granted 45,252 21.55 74,020 22.83 56,178 21.23 Vested (33,414 ) 21.93 (35,859 ) 16.88 (16,892 ) 17.08 Canceled (15,625 ) 21.91 (3,549 ) — 21.92 (1,047 ) 21.23 Outstanding, end of year 107,297 $ 22.04 111,084 $ 22.19 76,472 $ 19.06 As of December 31, 2016 , 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 2.25 years. Employee Stock Purchase Plan The Company also has a broad-based employee stock purchase plan under which employees of the Company may choose, at two different specified time intervals each year, to have up to 6% of their annual base earnings withheld to purchase the Company’s common shares. The purchase price of the common shares is 85% of the lower of its beginning-of-interval or end-of-interval market price. The Company has reserved 3.7 million common shares under this plan. As of December 31, 2016 , a total of 3.4 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 cash or common share 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 10,000 or to decrease such annual number to not less than 500 , without seeking shareholder approval, if such increase or decrease is deemed appropriate by the administrative committee to maintain director compensation at appropriate levels. The RSU Plan automatically terminates on May 31, 2026 . The Company accounts for the RSU Plan as a liability plan. There were 22,832 RSUs, 26,184 RSUs, and 25,960 RSUs granted in 2016 , 2015 and 2014 , respectively. During 2016 and 2015 , common shares valued at approximately $753,000 and $567,000 , 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 2016 , 2015 and 2014 . 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, 2016 , 2015 and 2014 : 2016 2015 2014 Fair value per share $ 6.75 $ 7.69 $ 7.28 Expected dividend yield 1.87 % 1.75 % 1.86 % Risk free interest rate 1.30 % 1.60 % 1.65 % Expected volatility factor 36.27 % 36.61 % 39.23 % Expected life in years 5.3 6.0 5.7 The following table sets forth the Company’s total activity and related information for these plans for the years ended December 31, 2016 , 2015 and 2014 : (millions, except per share amounts) 2016 2015 2014 Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Outstanding, beginning of year 3.4 $ 21.44 3.7 $ 21.29 3.9 $ 22.01 Granted 0.3 21.47 0.4 22.87 0.3 21.20 Exercised (0.4 ) 17.55 (0.3 ) 16.56 (0.1 ) 16.43 Canceled (0.5 ) 29.81 (0.4 ) 25.18 (0.4 ) 30.21 Outstanding, end of year 2.8 $ 20.67 3.4 $ 21.44 3.7 $ 21.29 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, 2016 , 2015 and 2014 , the total intrinsic value of stock options exercised was $3.0 million , $2.6 million and $0.5 million , respectively. The tax benefit for tax deductions from share-based awards totaled $1.4 million , $0.3 million and $0 for the years ended December 31, 2016 , 2015 and 2014 , respectively. In addition, income taxes for the year ended December 31, 2016 reflect the impact of a $1.6 million correction of prior period deferred tax expense related to expired stock options. See Note 9 for additional information. The following table sets forth information pertaining to the total options outstanding and exercisable at December 31, 2016 : (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 1.4 4.1 $ 16.44 1.3 $ 16.46 $20.01 – $30.00 1.4 3.6 24.57 0.9 25.96 2.8 3.8 $ 20.67 2.2 $ 20.53 Aggregate intrinsic value for total options outstanding at December 31, 2016 was $25.3 million . Aggregate intrinsic value for total options exercisable at December 31, 2016 was $14.9 million . Compensation expense recognized during 2016 , 2015 and 2014 was $3.7 million , $4.5 million and $3.6 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, 2016 , there was $2.2 million of total unrecognized compensation cost related to option-based compensation arrangements granted under the plans. The remaining cost is expected to be recognized over a period of 2.25

Net Earnings (Loss) Per Common

Net Earnings (Loss) Per Common Share	12 Months Ended
	Dec. 31, 2016
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, 2016 , 2015 and 2014 : (millions, except per share amounts) 2016 2015 2014 Numerator: Net earnings for basic net earnings per common share $ 21.0 $ 51.2 $ 107.4 Adjusted net earnings for dilutive net earnings per common share $ 21.0 $ 51.2 $ 107.4 Denominator: Weighted average shares for basic net earnings per common share 41.6 41.1 40.8 Effect of dilutive share-based awards 0.4 0.5 0.4 Adjusted weighted average shares for diluted net earnings per common share 42.0 41.6 41.2 Basic net earnings per common share $ 0.50 $ 1.25 $ 2.63 Diluted net earnings per common share $ 0.50 $ 1.23 $ 2.60 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, 2016 , 2015 and 2014 : (millions) 2016 2015 2014 Total number of antidilutive options and awards 1.2 1.5 1.8

Reportable Segments

Reportable Segments	12 Months Ended
	Dec. 31, 2016
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. Effective January 1, 2017, the Company completed the transition of its internal decision-making process to a product management structure under which its principal operating decision makers assess performance and allocate resources based on the underwriting results of individual products. Accordingly, beginning January 1, 2017, the Company’s reportable segments, and the products within each, will be as follows: • Personal Insurance Segment - personal auto, homeowners, farm & ranch and other personal • Commercial Insurance Segment - commercial auto, small commercial package, middle market commercial, workers’ compensation and other commercial • Specialty Insurance Segment - E&S property, E&S casualty and programs The following table sets forth financial information regarding the Company’s reportable segments for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Revenues from external sources: Insurance segments Personal insurance $ 582.2 $ 591.8 $ 451.4 Business insurance 468.9 476.0 459.9 Specialty insurance 240.8 202.7 162.8 Total insurance segments 1,291.9 1,270.5 1,074.1 Investment operations segment Net investment income 74.7 71.7 74.7 Net realized capital gains 36.5 24.7 20.7 Total investment operations segment 111.2 96.4 95.4 Total revenue from reportable segments 1,403.1 1,366.9 1,169.5 All other 2.3 1.7 3.2 Total revenues from external sources 1,405.4 1,368.6 1,172.7 Intersegment revenues 5.8 5.6 5.1 Total revenues 1,411.2 1,374.2 1,177.8 Reconciling items: Eliminate intersegment revenues (5.8 ) (5.6 ) (5.1 ) Total consolidated revenue $ 1,405.4 $ 1,368.6 $ 1,172.7 Segment loss before federal income tax: Insurance segments: Personal insurance SAP underwriting (loss) gain $ (15.2 ) $ 17.5 $ (11.7 ) Business insurance SAP underwriting loss (26.7 ) (36.7 ) (4.6 ) Specialty insurance SAP underwriting loss (42.7 ) (5.9 ) (89.5 ) Total insurance segments (84.6 ) (25.1 ) (105.8 ) Investment operations segment: Net investment income 74.7 71.7 74.7 Net realized capital gains 36.5 24.7 20.7 Total investment operations segment 111.2 96.4 95.4 All other segments income 0.4 (0.1 ) — Reconciling items: GAAP adjustments 2.0 4.2 45.8 Interest expense on corporate debt (5.5 ) (5.4 ) (5.4 ) Corporate expenses (4.3 ) (2.7 ) (3.2 ) Total reconciling items $ (7.8 ) $ (3.9 ) $ 37.2 Total consolidated income before federal income taxes $ 19.2 $ 67.3 $ 26.8 The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2016 and 2015 : ($ millions) 2016 2015 Segment assets: Investment operations segment $ 2,663.7 $ 2,529.8 Total segment assets 2,663.7 2,529.8 Reconciling items: Corporate assets 295.7 298.4 Total consolidated assets $ 2,959.4 $ 2,828.2

Quarterly Financial Data (unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	Quarterly Financial Data (unaudited) The following tables set forth quarterly financial data for 2016 and 2015 : ($ millions, except per share amounts) 2016 For three months ended March 31 June 30 September 30 December 31 Total revenues $ 339.2 $ 348.5 $ 352.8 $ 364.9 Income (loss) before federal income taxes 3.8 (25.1 ) 7.4 33.1 Net income (loss) 3.0 (24.6 ) 10.1 32.5 Earnings (loss) per common share: Basic $ 0.07 $ (0.59 ) $ 0.24 $ 0.78 Diluted $ 0.07 $ (0.59 ) $ 0.24 $ 0.77 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

Contingencies

Contingencies	12 Months Ended
	Dec. 31, 2016
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 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.

SCHEDULE I - SUMMARY OF INVESTM

SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2016
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, 2016 ($ millions) December 31, 2016 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 $ 401.9 $ 404.7 $ 404.7 Obligations of states and political subdivisions 634.6 643.7 643.7 Corporate securities 445.7 449.6 449.6 U.S. government agencies residential mortgage-backed securities 613.7 611.3 611.3 Total fixed maturities 2,095.9 2,109.3 2,109.3 Equity securities: Large-cap securities 108.9 139.0 139.0 Small-cap securities 57.2 79.1 79.1 Mutual and exchange traded funds 157.0 164.7 164.7 Total equity securities 323.1 382.8 382.8 Other invested assets 25.5 45.1 45.1 Total available-for-sale securities 2,444.5 2,537.2 2,537.2 Other invested assets 5.4 5.4 5.4 Total investments – other than investments in related parties $ 2,449.9 $ 2,542.6 $ 2,542.6 (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, 2016
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 2016 2015 Assets Investments in common stock of subsidiaries (equity method) $ 983.6 $ 972.5 Fixed maturities, available-for-sale, at fair value 0.9 0.9 Other invested assets 3.5 3.0 Cash and cash equivalents 8.1 10.5 Other assets 0.2 — Federal income tax, net 21.5 21.9 Total assets $ 1,017.8 $ 1,008.8 Liabilities and Stockholders’ Equity Notes payable (affiliates $116.5 and $116.5, respectively) $ 116.5 $ 116.5 Due to affiliates 1.3 0.6 Other liabilities 8.7 7.1 Total liabilities 126.5 124.2 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; 48.6 and 48.1 shares issued, respectively, at stated value of $2.50 per share 121.6 120.4 Treasury stock, 6.8 and 6.8 shares, respectively, at cost (116.5 ) (116.3 ) Additional paid-in capital 159.9 153.5 Accumulated other comprehensive income 32.5 37.6 Retained earnings 693.8 689.4 Total stockholders’ equity 891.3 884.6 Total liabilities and stockholders’ equity $ 1,017.8 $ 1,008.8 ($ millions) Year ended December 31 2016 2015 2014 Net investment income $ 0.2 $ 0.3 $ 0.3 Total revenues 0.2 0.3 0.3 Interest expense (affiliates $6.1, $6.0 and $6.0, respectively) 6.1 6.0 6.0 Other operating expenses 7.0 4.9 5.6 Total expenses 13.1 10.9 11.6 Loss before federal income taxes (12.9 ) (10.6 ) (11.3 ) Federal income tax benefit (6.0 ) (2.8 ) (20.6 ) Net (loss) income before equity in net income of subsidiaries (6.9 ) (7.8 ) 9.3 Equity in net income of subsidiaries 27.9 59.0 98.1 Net income $ 21.0 $ 51.2 $ 107.4 ($ millions, except per share amounts) Year ended December 31 2016 2015 2014 Net income $ 21.0 $ 51.2 $ 107.4 Other comprehensive loss, net of tax: Net unrealized holding gains (losses) on investments: Unrealized holding gains (losses) arising during the year 0.2 (0.1 ) (0.2 ) Reclassification adjustments for gains realized in net income — — — Income tax (expense) benefit (0.1 ) — 0.1 Total net unrealized holding gain (losses) on investments 0.1 (0.1 ) (0.1 ) Unrealized equity in subsidiaries (5.2 ) (34.0 ) (9.0 ) Other comprehensive loss (5.1 ) (34.1 ) (9.1 ) Comprehensive income $ 15.9 $ 17.1 $ 98.3 ($ millions) Year ended December 31 2016 2015 2014 Cash flows from operating activities: Net income $ 21.0 $ 51.2 $ 107.4 Adjustments to reconcile net income to net cash used in operating activities: Depreciation and amortization, net (0.3 ) — — Share-based compensation 1.2 (0.3 ) 0.9 Equity in net income from consolidated subsidiaries (27.9 ) (59.0 ) (98.1 ) Changes in operating assets and liabilities: Other liabilities and due from affiliates 0.9 0.9 0.5 Other assets 0.1 — — Excess tax benefits on share-based awards (0.2 ) (0.5 ) — Federal income taxes, net 3.3 (0.7 ) (16.6 ) Net cash used in operating activities (1.9 ) (8.4 ) (5.9 ) Cash flows from investing activities: Dividends received from consolidated subsidiaries 14.2 18.0 24.0 Purchases of other invested assets (0.3 ) (0.3 ) (0.4 ) Maturities, calls and pay downs of fixed maturities – available-for-sale — — 0.5 Sales of fixed maturities – available-for-sale — — 0.3 Net cash provided by investing activities 13.9 17.7 24.4 Cash flows from financing activities: Proceeds from issuance of common stock 2.2 6.2 3.5 Payments to acquire treasury stock (0.2 ) (0.3 ) (0.1 ) Payment of dividends (16.6 ) (16.5 ) (16.5 ) Excess tax benefits on share-based awards 0.2 0.3 — Net cash used in financing activities (14.4 ) (10.3 ) (13.1 ) Net (decrease) increase in cash and cash equivalents (2.4 ) (1.0 ) 5.4 Cash and cash equivalents at beginning of year 10.5 11.5 6.1 Cash and cash equivalents at end of year $ 8.1 $ 10.5 $ 11.5 Supplemental Disclosures: Federal income tax received $ — $ 2.3 $ 4.3 Interest paid (affiliates $6.1, $6.0 and $6.0, respectively) $ (6.1 ) $ (6.0 ) $ (6.0 ) 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, 2016 December 31, 2015 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 $ 15.4 5.28 % 15.0 15.0 5.28 % Affiliate note payable with State Auto P&C, issued $85.0, July 2013 with fixed interest 85.0 87.1 5.28 % 85.0 85.1 5.28 % Total notes payable to affiliates $ 100.0 $ 102.5 100.0 100.1

SCHEDULE III - SUPPLEMENTARY IN

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2016
Supplementary Insurance Information [Abstract]
SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	SCHEDULE III – SUPPLEMENTARY INSURANCE INFORMATION Years Ended December 31, 2016 , 2015 and 2014 ($ 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, 2016: Personal insurance segment $ 47.5 $ 251.1 $ 262.5 $ — $ 582.2 Business insurance segment 43.7 605.9 230.4 — 468.9 Specialty insurance segment 38.6 321.0 124.8 — 240.8 Investment operations segment — — — — — Total $ 129.8 $ 1,178.0 $ 617.8 $ — $ 1,291.9 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 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, 2016: Personal insurance segment $ — $ 425.5 $ 104.0 $ 67.1 $ 582.8 Business insurance segment — 327.9 103.4 63.5 453.8 Specialty insurance segment — 190.7 82.9 9.5 256.7 Investment operations segment 74.7 — — — — Total $ 74.7 $ 944.1 $ 290.3 $ 140.1 $ 1,293.3 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 (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, 2016
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
SCHEDULE IV - REINSURANCE	SCHEDULE IV – REINSURANCE Years Ended December 31, 2016 , 2015 and 2014 ($ 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, 2016 $ 840.6 $ 25.6 $ 819.9 $ 4.9 $ 1,291.9 $ 1,291.9 0.4 % 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 % (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, 2016
Valuation and Qualifying Accounts [Abstract]
SCHEDULE V - VALUATION AND QUALIFYING ACCOUNTS	SCHEDULE V – VALUATION AND QUALIFYING ACCOUNTS Years Ended December 31, 2016 , 2015 and 2014 ($ millions) At December 31 2016 2015 2014 Valuation allowance for deferred tax assets: Balance at beginning of period $ — $ — $ 82.6 Deductions — — 82.6 Balance at end of period $ — $ — $ —

Summary of Significant Accoun34

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
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.
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.
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.
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.
Cash Equivalents	Cash EquivalentsThe 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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Balance, beginning of year $ 129.1 $ 126.5 $ 96.8 Acquisition costs deferred 291.0 285.6 251.5 Acquisition costs amortized to expense (290.3 ) (283.0 ) (221.8 ) Balance, end of year $ 129.8 $ 129.1 $ 126.5
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.
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 $27.0 million and $24.7 million at December 31, 2016 and 2015
Premiums	PremiumsPremiums 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 IncomeComprehensive 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 and unrecognized benefit plan obligations, adjusted for deferred federal income taxes.

Summary of Significant Accoun35

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Schedule of Net Deferred Acquisition Costs	The following table sets forth net deferred acquisition costs for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Balance, beginning of year $ 129.1 $ 126.5 $ 96.8 Acquisition costs deferred 291.0 285.6 251.5 Acquisition costs amortized to expense (290.3 ) (283.0 ) (221.8 ) Balance, end of year $ 129.8 $ 129.1 $ 126.5

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2016 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 401.9 $ 8.9 $ (6.1 ) $ 404.7 Obligations of states and political subdivisions 634.6 12.3 (3.2 ) 643.7 Corporate securities 445.7 6.1 (2.2 ) 449.6 U.S. government agencies mortgage-backed securities 613.7 8.5 (10.9 ) 611.3 Total fixed maturities 2,095.9 35.8 (22.4 ) 2,109.3 Equity securities: Large-cap securities 108.9 32.7 (2.6 ) 139.0 Small-cap securities 57.2 21.9 — 79.1 Mutual and exchange traded funds 157.0 8.5 (0.8 ) 164.7 Total equity securities 323.1 63.1 (3.4 ) 382.8 Other invested assets 25.5 19.6 — 45.1 Total available-for-sale securities $ 2,444.5 $ 118.5 $ (25.8 ) $ 2,537.2 ($ 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 136.3 29.4 (4.0 ) 161.7 Small-cap securities 53.3 16.5 (0.2 ) 69.6 Mutual and exchange traded funds 75.6 4.8 (1.1 ) 79.3 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
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, 2016 and 2015 : ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2016 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 $ 229.1 $ (6.1 ) 30 $ — $ — — $ 229.1 $ (6.1 ) 30 Obligations of states and political subdivisions 178.9 (3.2 ) 26 — — — 178.9 (3.2 ) 26 Corporate securities 102.9 (1.4 ) 16 29.4 (0.8 ) 4 132.3 (2.2 ) 20 U.S. government agencies mortgage-backed securities 341.7 (10.1 ) 43 20.5 (0.8 ) 11 362.2 (10.9 ) 54 Total fixed maturities 852.6 (20.8 ) 115 49.9 (1.6 ) 15 902.5 (22.4 ) 130 Large-cap equity securities 9.1 (0.9 ) 7 8.8 (1.7 ) 5 17.9 (2.6 ) 12 Mutual and exchange traded funds 29.9 (0.8 ) 2 — — — 29.9 (0.8 ) 2 Total equity securities 39.0 (1.7 ) 9 8.8 (1.7 ) 5 47.8 (3.4 ) 14 Total temporarily impaired securities $ 891.6 $ (22.5 ) 124 $ 58.7 $ (3.3 ) 20 $ 950.3 $ (25.8 ) 144 ($ 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 46.0 (4.0 ) 20 — — — 46.0 (4.0 ) 20 Small-cap equity securities 3.4 (0.2 ) 1 — — — 3.4 (0.2 ) 1 Mutual and exchange traded funds 19.8 (1.1 ) 4 — — — 19.8 (1.1 ) 4 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
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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Equity securities: Large-cap securities $ (0.6 ) $ (2.2 ) $ (0.3 ) Small-cap securities (3.9 ) (5.7 ) (2.2 ) Fixed maturities (2.3 ) — — Total other-than-temporary impairments $ (6.8 ) $ (7.9 ) $ (2.5 )
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, 2016 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 74.4 $ 75.2 Due after 1 year through 5 years 552.7 557.8 Due after 5 years through 10 years 280.2 282.9 Due after 10 years 574.9 582.1 U.S. government agencies mortgage-backed securities 613.7 611.3 Total $ 2,095.9 $ 2,109.3
Components of Net Investment Income	The following table sets forth the components of net investment income for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Fixed maturities $ 63.4 $ 61.3 $ 64.3 Equity securities 7.3 6.5 6.2 Cash and cash equivalents, and other 5.6 5.9 6.2 Investment income 76.3 73.7 76.7 Investment expenses 1.6 2.0 2.0 Net investment income $ 74.7 $ 71.7 $ 74.7
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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Realized gains: Fixed maturities $ 2.8 $ 4.6 $ 3.1 Equity securities 29.2 29.6 21.3 Other invested assets 12.1 0.2 0.1 Total realized gains 44.1 34.4 24.5 Realized losses: Equity securities: Sales (0.8 ) (1.8 ) (1.3 ) OTTI (4.5 ) (7.9 ) (2.5 ) Fixed maturities: OTTI (2.3 ) — — Total realized losses (7.6 ) (9.7 ) (3.8 ) Net realized gains on investments $ 36.5 $ 24.7 $ 20.7 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ (14.4 ) $ (32.8 ) $ 34.5 Equity securities 14.3 (29.5 ) 6.2 Other invested assets (8.6 ) (1.6 ) (1.6 ) Deferred federal income tax 3.0 22.4 (13.7 ) Change in unrealized holding gains (losses), net of tax $ (5.7 ) $ (41.5 ) $ 25.4

Fair Value of Financial Instr37

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 : ($ 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, 2016 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 404.7 $ — $ 404.7 $ — Obligations of states and political subdivisions 643.7 — 643.7 — Corporate securities 449.6 — 446.1 3.5 U.S. government agencies mortgage-backed securities 611.3 — 611.3 — Total fixed maturities 2,109.3 — 2,105.8 3.5 Equity securities: Large-cap securities 139.0 139.0 — — Small-cap securities 79.1 79.1 — — Mutual and exchange traded funds 164.7 164.7 — — Total equity securities 382.8 382.8 — — Other invested assets 9.4 9.4 — — Total available-for-sale investments $ 2,501.5 $ 392.2 $ 2,105.8 $ 3.5 ($ 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 161.7 161.7 — — Small-cap securities 69.6 69.6 — — Mutual and exchange traded funds 79.3 79.3 — — Total equity securities 310.6 310.6 — — Other invested assets 8.1 8.1 — — Total available-for-sale investments $ 2,319.4 $ 318.7 $ 1,997.4 $ 3.3
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 2016 and 2015 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2016 $ 3.3 Total realized gains (losses)—included in earnings — Total unrealized gains (losses)—included in other comprehensive income — Purchases 0.2 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2016 $ 3.5 ($ 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
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, 2016 December 31, 2015 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 75.7 7.00 % $ 70.0 $ 74.1 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, 2016 December 31, 2015 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2021:, issued $21.5, September 2016 with fixed interest $ 21.5 $ 21.0 1.73 % $ — $ — — % FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest 85.4 85.6 5.03 % 85.3 85.5 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 5.13 % 15.2 15.2 4.61 % Total notes payable $ 122.1 $ 121.8 $ 100.5 $ 100.7
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, 2016 December 31, 2015 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2021:, issued $21.5, September 2016 with fixed interest $ 21.5 $ 21.0 1.73 % $ — $ — — % FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest 85.4 85.6 5.03 % 85.3 85.5 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 5.13 % 15.2 15.2 4.61 % Total notes payable $ 122.1 $ 121.8 $ 100.5 $ 100.7

Losses and Loss Expenses Paya38

Losses and Loss Expenses Payable (Tables)	12 Months Ended
	Dec. 31, 2016
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) 2016 2015 2014 Losses and loss expenses payable, at beginning of year $ 1,053.0 $ 983.2 $ 959.9 Less: reinsurance recoverable on losses and loss expenses payable 5.9 9.6 9.1 Net balance at beginning of year 1,047.1 973.6 950.8 Incurred related to: Current year 915.4 852.8 726.2 Prior years 27.0 10.0 45.1 Total incurred 942.4 862.8 771.3 Paid related to: Current year 417.8 421.5 373.2 Prior years 393.7 367.8 375.3 Total paid 811.5 789.3 748.5 Net balance at end of year 1,178.0 1,047.1 973.6 Plus: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Losses and loss expenses payable, at end of year (affiliates $630.9, $532.4, and $494.3, respectively) $ 1,181.6 $ 1,053.0 $ 983.2

Short-Duration Insurance Cont39

Short-Duration Insurance Contracts Short-Duration Insurance Contracts (Tables)	12 Months Ended
	Dec. 31, 2016
Incurred Losses and Loss Expenses Payable, Net of Reinsurance [Abstract]
Short-duration Insurance Contracts, Claims Development [Table Text Block]	($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* (1) 2013* (1) 2014* (1) 2015* 2016 IBNR+ Reported Claims (2) 2012 $ 305.3 $ 307.6 $ 306.7 $ 308.9 $ 308.6 $ 0.9 212,774 2013 288.6 283.3 287.9 287.6 2.8 189,392 2014 271.2 272.8 275.2 4.0 171,772 2015 337.2 343.6 20.3 151,130 2016 366.4 44.9 137,561 Total $ 1,581.4 * Supplementary information and unaudited (1) Allocated claim adjustment expenses, net of reinsurance, for accident years 2012, 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. (2) Personal insurance segment - short-tail is an aggregation of the homeowners and personal auto product lines. Homeowners reported claims are counted by claimant and personal auto claims are counted by coverage. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* (1) 2013* (1) 2014* (1) 2015* 2016 2012 $ 193.8 $ 260.2 $ 285.1 $ 298.1 $ 304.5 2013 176.0 236.9 265.4 277.2 2014 172.5 226.3 252.1 2015 214.9 288.9 2016 230.8 Total $ 1,353.5 All outstanding liabilities before 2012, net of reinsurance 0.5 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 228.4 * Supplementary information and unaudited (1) Cumulative paid claims for accident years 2012, 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. Business Insurance Segment - Short-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2012 $ 170.3 $ 170.5 $ 168.1 $ 170.9 $ 169.1 $ 3.7 33,897 2013 170.0 168.7 175.2 176.8 6.4 33,131 2014 181.0 183.9 185.6 15.0 32,253 2015 189.1 191.0 31.2 30,445 2016 186.8 49.8 25,340 Total $ 909.3 * Supplementary information and unaudited (1) Business insurance segment - short-tail is an aggregation of the commercial auto, commercial-multi peril and fire & allied product lines. Commercial auto reported claims are counted by coverage, and commercial multi-peril and fire & allied claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 2012 $ 91.9 $ 128.0 $ 145.8 $ 156.8 $ 161.8 2013 79.4 126.2 146.0 158.4 2014 92.9 131.3 150.5 2015 90.4 130.0 2016 87.7 Total $ 688.4 All outstanding liabilities before 2012, net of reinsurance 14.0 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 234.9 * Supplementary information and unaudited Business Insurance Segment - Long-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by lower than expected loss emergence during 2016 in our other & product liability and workers’ compensation lines of business. The unanticipated emergence in the other & product liability line was driven by much lower than expected severity in accident years 2013 and 2015. The unanticipated emergence in the workers’ compensation line was driven by lower than expected severity in accident year 2014. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2007 $ 73.4 $ 73.3 $ 74.1 $ 73.2 $ 71.9 $ 71.9 $ 72.5 $ 72.4 $ 73.0 $ 73.3 $ 2.7 8,072 2008 86.1 79.1 75.6 75.0 75.4 75.3 74.9 74.2 73.4 4.0 9,577 2009 90.2 92.0 87.4 86.0 85.0 83.1 82.4 82.3 5.8 10,086 2010 85.3 93.5 86.7 82.2 82.0 79.5 78.9 7.2 10,644 2011 86.9 86.4 80.9 77.5 76.1 75.3 9.0 17,390 2012 92.8 85.5 80.2 78.0 77.6 12.2 17,321 2013 92.6 88.2 88.2 84.9 19.7 16,646 2014 96.1 96.6 95.2 27.3 16,501 2015 108.9 106.2 45.5 15,783 2016 109.5 65.6 8,548 Total $ 856.6 * Supplementary information and unaudited (1) Business insurance segment-long-tail is an aggregation of the other & product liability and workers' compensation product lines. Other & product liability reported claims are counted by claimant and workers' compensation reported claims are counted by a combination of claimant and coverage. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2007 $ 12.7 $ 28.7 $ 43.0 $ 50.3 $ 57.6 $ 61.4 $ 64.0 $ 65.7 $ 68.1 $ 68.6 2008 14.3 34.7 43.9 51.6 56.4 61.3 64.0 65.0 66.0 2009 15.2 34.9 48.2 60.2 66.2 70.3 72.1 74.1 2010 12.9 33.0 46.3 54.2 63.3 65.8 67.6 2011 16.4 33.9 46.4 53.9 60.7 63.6 2012 15.3 32.6 43.7 51.8 57.3 2013 16.3 36.7 50.1 57.2 2014 15.8 37.6 53.9 2015 17.3 38.6 2016 15.4 $ 562.3 All outstanding liabilities before 2007, net of reinsurance 36.6 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 330.9 * Supplementary information and unaudited Specialty Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by greater than expected loss emergence during 2016 in our Programs line of business. The unanticipated emergence in the Programs line was driven by higher than expected severity in accident year 2015 for programs with commercial auto exposure. The changes in incurred losses and loss adjustment expenses for accident years 2012 and 2013 for the years ended December 31, 2013, 2014 and 2015 are primarily attributable to our Programs line of business, including RED. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2012 $ 108.6 $ 115.6 $ 164.8 $ 162.8 $ 163.7 $ 9.0 9,697 2013 70.9 82.9 85.0 90.9 6.7 7,680 2014 55.7 56.9 58.6 7.8 7,657 2015 75.5 85.4 21.6 7,666 2016 99.3 48.8 5,754 Total $ 497.9 * Supplementary information and unaudited (1) Specialty insurance segment-short-tail is an aggregation of the E&S property and Programs product lines. E&S property and Programs reported claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2012* 2013* 2014* 2015* 2016 2012 $ 25.4 $ 59.8 $ 94.7 $ 122.9 $ 142.5 2013 18.2 37.7 55.5 72.0 2014 22.0 34.9 44.0 2015 24.0 48.2 2016 25.2 Total $ 331.9 All outstanding liabilities before 2012, net of reinsurance 12.9 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 178.9 * Supplementary information and unaudited Specialty Insurance Segment - Long-Tail The change in incurred loss and loss adjustment expenses for prior accident years was caused by greater than expected loss emergence during 2016, driven by higher than anticipated severity in accident years 2014 and 2015 for our healthcare product. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2016 For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 IBNR+ Reported Claims (1) 2007 $ 9.7 $ 9.2 $ 9.4 $ 9.0 $ 8.8 $ 8.5 $ 8.4 $ 8.3 $ 8.3 $ 4.1 $ 2.4 1,101 2008 6.4 6.7 6.7 6.8 6.1 6.0 5.8 6.0 4.9 1.6 1,502 2009 10.3 11.0 10.0 10.4 11.0 11.0 10.6 12.4 2.0 1,429 2010 11.2 10.9 9.9 9.7 9.8 9.9 10.3 2.2 1,169 2011 11.9 10.7 10.7 10.2 10.1 10.9 2.2 911 2012 16.7 16.7 15.4 17.6 17.4 3.0 862 2013 20.1 18.9 17.9 16.5 4.9 936 2014 23.3 25.2 27.1 9.5 1,044 2015 38.6 45.0 22.8 1,409 2016 60.8 47.6 1,401 Total $ 209.4 * Supplementary information and unaudited (1) Specialty insurance segment-long-tail consists of the E&S casualty product line. E&S casualty reported claims are counted by claimant. ($ in millions) Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance For the Years Ended December 31, Accident Year 2007* 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2007 $ — $ 0.1 $ 0.6 $ 0.1 $ 0.5 $ 1.3 $ 1.3 $ 1.5 $ 1.5 $ 1.6 2008 0.1 0.3 0.8 1.3 1.7 2.1 2.4 2.8 3.0 2009 0.2 2.4 4.9 7.2 8.6 9.4 9.2 10.0 2010 0.8 2.2 5.2 6.5 7.7 7.6 8.1 2011 0.9 3.0 5.2 6.6 7.1 7.9 2012 0.9 4.0 7.3 10.3 12.5 2013 1.1 3.7 6.7 9.1 2014 1.2 7.2 12.5 2015 2.7 11.0 2016 6.0 $ 81.7 All outstanding liabilities before 2007, net of reinsurance 2.0 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 129.7 * Supplementary information and unaudited
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Table Text Block]	The following table sets forth the reconciliation of the claims development tables to the balance sheet losses and loss expenses payable, with separate disclosure of unallocated loss adjustment expenses, net of reinsurance ("ULAE") and reinsurance recoverable on losses and loss expenses payable, for the years ended December 31: ($ in millions) 2016 2015 2014 Net losses and allocated loss adjustment expenses payable: Personal Insurance Segment Short-tail $ 228.4 $ 207.4 $ 183.1 Other personal 8.9 7.5 7.5 Business Insurance Segment Short-tail 234.9 213.5 183.9 Long-tail 330.9 306.6 285.6 Other business 2.5 1.4 2.4 Specialty Insurance Segment Short-tail 178.9 165.8 195.3 Long-tail 129.7 91.1 65.4 Net losses and loss expenses payable 1,114.2 993.3 923.2 ULAE 63.8 53.8 50.4 Reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Total losses and loss expenses payable $ 1,181.6 $ 1,053.0 $ 983.2
Short-duration Insurance Contracts, Schedule of Historical Claims Duration [Table Text Block]	The following table sets forth the historical average annual payout of incurred losses and allocated loss adjustment expenses (claims duration) for short-duration contracts, based on the disaggregated information in the paid loss development tables, net of reinsurance: Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years Segment 1* 2* 3* 4* 5* 6* 7* 8* 9* 10* Personal Insurance: Short-tail 62.4 % 21.0 % 9.1 % 4.2 % 2.1 % N/A N/A N/A N/A N/A Business Insurance: Short-tail 48.7 % 22.3 % 10.7 % 6.8 % 3.0 % N/A N/A N/A N/A N/A Long-tail 17.9 % 23.5 % 16.1 % 10.5 % 8.6 % 4.8 % 2.9 % 2.0 % 2.4 % 0.7 % Specialty Insurance: Short-tail 25.3 % 23.2 % 18.8 % 17.7 % 11.9 % N/A N/A N/A N/A N/A Long-tail 5.3 % 14.5 % 18.5 % 10.6 % 9.6 % 8.1 % 2.4 % 6.3 % 2.8 % 2.2 % * Supplementary information and unaudited

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2016 2015 Losses and loss expenses payable: Direct $ 545.7 $ 515.6 Assumed 5.0 5.0 Ceded (3.6 ) (5.9 ) Net losses and loss expenses payable $ 547.1 $ 514.7 Unearned premiums: Direct $ 395.6 $ 400.9 Assumed 1.3 1.2 Ceded (6.1 ) (6.8 ) Net unearned premiums $ 390.8 $ 395.3
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, 2016 , 2015 and 2014 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2016 2015 2014 Written premiums: Direct $ 835.4 $ 854.1 $ 880.6 Assumed 5.0 4.7 4.4 Ceded (24.9 ) (35.4 ) (26.8 ) Net written premiums $ 815.5 $ 823.4 $ 858.2 Earned premiums: Direct $ 840.6 $ 863.1 $ 882.6 Assumed 4.9 4.5 4.4 Ceded (25.6 ) (34.7 ) (25.3 ) Net earned premiums $ 819.9 $ 832.9 $ 861.7 Losses and loss expenses incurred: Direct $ 559.1 $ 569.0 $ 525.6 Assumed 3.9 3.4 2.9 Ceded (4.4 ) (2.9 ) (8.0 ) Net losses and loss expenses incurred $ 558.6 $ 569.5 $ 520.5

Transactions with Affiliates (T

Transactions with Affiliates (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 : ($ millions) 2016 2015 Assets Deferred policy acquisition costs: Ceded $ (79.1 ) $ (79.8 ) Assumed 129.8 129.1 Net assumed $ 50.7 $ 49.3 Liabilities and Stockholders’ Equity Losses and loss expenses payable: Ceded $ (547.1 ) $ (514.7 ) Assumed 1,178.0 1,047.1 Net assumed $ 630.9 $ 532.4 Unearned premiums: Ceded $ (390.8 ) $ (395.3 ) Assumed 611.7 609.5 Net assumed $ 220.9 $ 214.2 Pension and postretirement benefits: Ceded $ (114.5 ) $ (160.0 ) Assumed 74.4 104.0 Net ceded $ (40.1 ) $ (56.0 ) Other liabilities: Ceded $ (42.9 ) $ (63.8 ) Assumed 53.9 55.4 Net assumed (ceded) $ 11.0 $ (8.4 ) Stockholders’ Equity Accumulated other comprehensive income: Ceded $ (153.5 ) $ (162.0 ) Assumed 99.8 105.3 Net ceded $ (53.7 ) $ (56.7 )
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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Written premiums: Ceded $ (815.5 ) $ (823.4 ) $ (858.2 ) Assumed 1,293.3 1,273.5 1,194.2 Net assumed $ 477.8 $ 450.1 $ 336.0 Earned premiums: Ceded $ (819.9 ) $ (832.9 ) $ (861.7 ) Assumed 1,291.9 1,270.5 1,074.1 Net assumed $ 472.0 $ 437.6 $ 212.4 Losses and loss expenses incurred: Ceded $ (560.3 ) $ (570.9 ) $ (523.6 ) Assumed 944.1 864.2 774.4 Net assumed $ 383.8 $ 293.3 $ 250.8 Acquisition and operating expenses: Ceded (148.5 ) (183.9 ) (215.4 ) Assumed 434.3 432.5 372.3 Net assumed $ 285.8 $ 248.6 $ 156.9

Federal Income Taxes (Tables)

Federal Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate	The following table sets forth the reconciliation between actual federal income tax (benefit) expense and the amount computed at the indicated statutory rate for the years ended December 31, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Amount at statutory rate $ 6.7 35.0 % $ 23.5 35.0 % $ 9.4 35.0 % Tax-exempt interest and dividends received deduction (7.1 ) (37.2 ) (8.7 ) (13.0 ) (8.5 ) (31.5 ) Other, net (1.4 ) (7.2 ) 1.3 1.9 1.1 4.1 Valuation allowance — — — — (82.6 ) (308.1 ) Federal income tax (benefit) expense and effective rate $ (1.8 ) (9.4 )% $ 16.1 23.9 % $ (80.6 ) (300.5 )%
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, 2016 and 2015 : ($ millions) 2016 2015 Deferred tax assets: Unearned premiums not currently deductible $ 42.6 $ 42.5 Losses and loss expenses payable discounting 19.7 19.3 Postretirement and pension benefits 26.1 36.4 Realized loss on other-than-temporary impairment 4.0 12.3 Other liabilities 16.6 21.3 Net operating loss carryforward 58.9 41.8 Tax credit carryforward 3.3 4.8 Other 8.8 4.9 Total deferred tax assets 180.0 183.3 Deferred tax liabilities: Deferral of policy acquisition costs 45.4 45.2 Net unrealized holding gains on investments 32.5 35.6 Total deferred tax liabilities 77.9 80.8 Net deferred federal income taxes $ 102.1 $ 102.5

Pension and Postretirement Be43

Pension and Postretirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 and 2015 : ($ millions) Pension Postretirement 2016 2015 2016 2015 Change in benefit obligation: Benefit obligation at beginning of year $ 281.3 $ 292.5 $ 20.3 $ 23.9 Service cost 6.1 7.7 — — Interest cost 11.6 11.1 0.8 0.9 Actuarial loss (gain) 4.5 (17.6 ) (0.9 ) (3.3 ) Benefits paid (19.0 ) (12.4 ) (1.4 ) (1.2 ) The Company’s portion of benefit obligation at end of year $ 284.5 $ 281.3 $ 18.8 $ 20.3 Change in plan assets available for plan benefits: Fair value of plan assets available for plan benefits at beginning of year $ 204.4 $ 205.1 $ — $ 0.3 Employer contribution 34.5 13.0 — — Actual return on plan assets 15.8 (1.3 ) — — Benefits paid (19.0 ) (12.4 ) — (0.3 ) The Company’s portion of fair value of plan assets at end of year $ 235.7 $ 204.4 $ — $ — Supplemental executive retirement plan (6.8 ) (6.8 ) — — Funded status at end of year $ (55.6 ) $ (83.7 ) $ (18.8 ) $ (20.3 ) Accumulated benefit obligation end of year $ 267.5 $ 263.1
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, 2016 and 2015 : ($ millions) 2016 2015 Prior service benefit $ (54.1 ) $ (59.5 ) Net actuarial loss 118.5 125.1 Total $ 64.4 $ 65.6
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, 2017: ($ millions) 2017 Prior service benefit $ (5.5 ) Net actuarial loss 8.0 Total $ 2.5
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, 2016 , 2015 and 2014 : ($ millions) Pension Postretirement 2016 2015 2014 2016 2015 2014 Components of net periodic cost: Service cost $ 6.2 $ 7.9 $ 5.2 $ — $ — $ — Interest cost 11.9 11.3 11.1 0.9 1.1 1.1 Expected return on plan assets (15.1 ) (13.8 ) (12.6 ) — — — Amortization of: Prior service benefit — — — (5.5 ) (5.4 ) (5.5 ) Net actuarial loss 9.2 10.9 6.3 0.2 0.6 0.6 Net periodic cost (benefit) $ 12.2 $ 16.3 $ 10.0 $ (4.4 ) $ (3.7 ) $ (3.8 )
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 2017 $ 11.3 $ 1.5 2018 11.8 1.4 2019 12.0 1.4 2020 12.5 1.4 2021 13.1 1.4 2022-2026 76.8 6.0
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, 2016 and 2015 : Pension Postretirement 2016 2015 2016 2015 Benefit obligations weighted-average assumptions: Discount rate 4.00 % 4.20 % 4.00 % 4.20 % 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, 2016 , 2015 and 2014 : Pension Postretirement 2016 2015 2014 2016 2015 2014 Weighted-average assumptions: Discount rate 4.20 % 3.85 % 4.85 % 4.20 % 3.85 % 4.85 % Expected long-term rate of return on assets 7.00 7.00 7.00 — — — Rates of increase in compensation levels 3.50 3.50 3.50 — — —
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, 2016 , 2015 and 2014 : Postretirement 2016 2015 2014 Assumed health care cost trend rates: Health care cost trend rate assumed for the next year 6.50 % 6.50 % 6.00 % Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 3.90 % 3.80 % 3.80 % Year that the rate reaches the ultimate trend rate 2075 2076 2075
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, 2016 : ($ 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 2.8 2.4
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 49.0 % U.S. large-cap equity 26.0 U.S. small-cap equity 11.0 International equity 10.0 Emerging market equity 4.0 Total 100.0 %
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, 2016 and 2015 : ($ 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, 2016 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 60.4 $ — $ 60.4 $ — Corporate securities 48.0 — 48.0 — U.S. government agencies mortgage-backed securities 4.6 — 4.6 — Total fixed maturities 113.0 — 113.0 — Equity securities: Large-cap securities 59.6 59.6 — — Small-cap securities 26.3 26.3 — — Total equity securities 85.9 85.9 — — Short-term fixed maturities 2.0 — 2.0 — Total pension plan investments $ 200.9 $ 85.9 $ 115.0 $ — ($ 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 — — Total pension plan investments $ 177.6 $ 81.3 $ 96.3 $ — During 2015, the postretirement plan assets were sold in order to pay retiree medical costs.

Other Comprehensive Income an44

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 , 2015 and 2014 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at January 1, 2016 $ 68.5 $ (30.9 ) $ 37.6 Other comprehensive income before reclassifications 18.0 (3.0 ) 15.0 Amounts reclassified from AOCI (a) (23.7 ) 3.6 (20.1 ) Net current period other comprehensive income (5.7 ) 0.6 (5.1 ) Ending balance at December 31, 2016 $ 62.8 $ (30.3 ) $ 32.5 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 (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, 2016 , 2015 and 2014 : ($ millions) Details about Accumulated Other December 31 Affected line item in the Condensed Comprehensive Income Components 2016 2015 2014 Consolidated Statements of Income Unrealized gains on available for sale securities $ 36.5 $ 24.7 $ 20.7 Realized gain on sale of securities 36.5 24.7 20.7 Total before tax (12.8 ) (8.6 ) (7.2 ) Tax expense 23.7 16.1 13.5 Net of tax Amortization of benefit plan items: Negative prior service costs 5.5 5.4 5.5 (a) Net loss (9.4 ) (11.5 ) (6.9 ) (a) (3.9 ) (6.1 ) (1.4 ) Total before tax 0.3 4.0 (18.5 ) Tax benefit (expense) (3.6 ) (2.1 ) (19.9 ) Net of tax Total reclassifications for the period $ 20.1 $ 14.0 $ (6.4 ) (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, 2016
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) 2016 2015 Statutory capital and surplus of insurance subsidiaries $ 844.4 $ 814.3 Net liabilities of non-insurance parent and affiliates (84.9 ) (80.2 ) 759.5 734.1 Increases (decreases): Deferred acquisition costs 129.8 129.1 Postretirement and pension benefits 22.7 23.2 Deferred federal income taxes (49.1 ) (41.6 ) Fixed maturities, at fair value 13.5 27.9 Other, net 14.9 11.9 Stockholders’ equity per accompanying consolidated financial statements $ 891.3 $ 884.6 ($ millions) Year ended December 31 2016 2015 2014 Statutory net income (loss) of insurance subsidiaries $ 22.7 $ 65.4 $ (17.5 ) Net loss of non-insurance parent and affiliates (3.0 ) (4.1 ) (5.3 ) 19.7 61.3 (22.8 ) Increases (decreases): Deferred acquisition costs 0.7 2.5 29.7 Postretirement and pension benefits 4.4 4.2 10.6 Deferred federal income taxes (5.4 ) (12.2 ) 79.3 Share-based compensation expense — (0.1 ) (0.5 ) Other, net 1.6 (4.5 ) 11.1 Net income per accompanying consolidated financial statements $ 21.0 $ 51.2 $ 107.4

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Share-based Compensation [Abstract]
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]	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, 2016 , 2015 and 2014 : 2016 2015 2014 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 111,084 $ 22.19 76,472 $ 19.06 38,233 $ 15.06 Granted 45,252 21.55 74,020 22.83 56,178 21.23 Vested (33,414 ) 21.93 (35,859 ) 16.88 (16,892 ) 17.08 Canceled (15,625 ) 21.91 (3,549 ) — 21.92 (1,047 ) 21.23 Outstanding, end of year 107,297 $ 22.04 111,084 $ 22.19 76,472 $ 19.06
Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions [Table Text Block]	The following table sets forth the weighted average fair values and related assumptions for options granted for the years ended December 31, 2016 , 2015 and 2014 : 2016 2015 2014 Fair value per share $ 6.75 $ 7.69 $ 7.28 Expected dividend yield 1.87 % 1.75 % 1.86 % Risk free interest rate 1.30 % 1.60 % 1.65 % Expected volatility factor 36.27 % 36.61 % 39.23 % Expected life in years 5.3 6.0 5.7
Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]	The following table sets forth the Company’s total activity and related information for these plans for the years ended December 31, 2016 , 2015 and 2014 : (millions, except per share amounts) 2016 2015 2014 Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Outstanding, beginning of year 3.4 $ 21.44 3.7 $ 21.29 3.9 $ 22.01 Granted 0.3 21.47 0.4 22.87 0.3 21.20 Exercised (0.4 ) 17.55 (0.3 ) 16.56 (0.1 ) 16.43 Canceled (0.5 ) 29.81 (0.4 ) 25.18 (0.4 ) 30.21 Outstanding, end of year 2.8 $ 20.67 3.4 $ 21.44 3.7 $ 21.29
Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block]	The following table sets forth information pertaining to the total options outstanding and exercisable at December 31, 2016 : (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 1.4 4.1 $ 16.44 1.3 $ 16.46 $20.01 – $30.00 1.4 3.6 24.57 0.9 25.96 2.8 3.8 $ 20.67 2.2 $ 20.53

Net Earnings (Loss) Per Commo47

Net Earnings (Loss) Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 , 2015 and 2014 : (millions, except per share amounts) 2016 2015 2014 Numerator: Net earnings for basic net earnings per common share $ 21.0 $ 51.2 $ 107.4 Adjusted net earnings for dilutive net earnings per common share $ 21.0 $ 51.2 $ 107.4 Denominator: Weighted average shares for basic net earnings per common share 41.6 41.1 40.8 Effect of dilutive share-based awards 0.4 0.5 0.4 Adjusted weighted average shares for diluted net earnings per common share 42.0 41.6 41.2 Basic net earnings per common share $ 0.50 $ 1.25 $ 2.63 Diluted net earnings per common share $ 0.50 $ 1.23 $ 2.60
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, 2016 , 2015 and 2014 : (millions) 2016 2015 2014 Total number of antidilutive options and awards 1.2 1.5 1.8

Reportable Segments (Tables)

Reportable Segments (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 , 2015 and 2014 : ($ millions) 2016 2015 2014 Revenues from external sources: Insurance segments Personal insurance $ 582.2 $ 591.8 $ 451.4 Business insurance 468.9 476.0 459.9 Specialty insurance 240.8 202.7 162.8 Total insurance segments 1,291.9 1,270.5 1,074.1 Investment operations segment Net investment income 74.7 71.7 74.7 Net realized capital gains 36.5 24.7 20.7 Total investment operations segment 111.2 96.4 95.4 Total revenue from reportable segments 1,403.1 1,366.9 1,169.5 All other 2.3 1.7 3.2 Total revenues from external sources 1,405.4 1,368.6 1,172.7 Intersegment revenues 5.8 5.6 5.1 Total revenues 1,411.2 1,374.2 1,177.8 Reconciling items: Eliminate intersegment revenues (5.8 ) (5.6 ) (5.1 ) Total consolidated revenue $ 1,405.4 $ 1,368.6 $ 1,172.7 Segment loss before federal income tax: Insurance segments: Personal insurance SAP underwriting (loss) gain $ (15.2 ) $ 17.5 $ (11.7 ) Business insurance SAP underwriting loss (26.7 ) (36.7 ) (4.6 ) Specialty insurance SAP underwriting loss (42.7 ) (5.9 ) (89.5 ) Total insurance segments (84.6 ) (25.1 ) (105.8 ) Investment operations segment: Net investment income 74.7 71.7 74.7 Net realized capital gains 36.5 24.7 20.7 Total investment operations segment 111.2 96.4 95.4 All other segments income 0.4 (0.1 ) — Reconciling items: GAAP adjustments 2.0 4.2 45.8 Interest expense on corporate debt (5.5 ) (5.4 ) (5.4 ) Corporate expenses (4.3 ) (2.7 ) (3.2 ) Total reconciling items $ (7.8 ) $ (3.9 ) $ 37.2 Total consolidated income before federal income taxes $ 19.2 $ 67.3 $ 26.8
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2016 and 2015 : ($ millions) 2016 2015 Segment assets: Investment operations segment $ 2,663.7 $ 2,529.8 Total segment assets 2,663.7 2,529.8 Reconciling items: Corporate assets 295.7 298.4 Total consolidated assets $ 2,959.4 $ 2,828.2

Quarterly Financial Data (una49

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following tables set forth quarterly financial data for 2016 and 2015 : ($ millions, except per share amounts) 2016 For three months ended March 31 June 30 September 30 December 31 Total revenues $ 339.2 $ 348.5 $ 352.8 $ 364.9 Income (loss) before federal income taxes 3.8 (25.1 ) 7.4 33.1 Net income (loss) 3.0 (24.6 ) 10.1 32.5 Earnings (loss) per common share: Basic $ 0.07 $ (0.59 ) $ 0.24 $ 0.78 Diluted $ 0.07 $ (0.59 ) $ 0.24 $ 0.77 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

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Schedule of Net Deferred Acquisition Costs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of year	$ 129.1	$ 126.5	$ 96.8
Acquisition costs deferred	291	285.6	251.5
Acquisition costs amortized to expense	(290.3)	(283)	(221.8)
Balance, end of year	$ 129.8	$ 129.1	$ 126.5

Summary of Significant Accoun51

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Amortization of Debt Issuance Costs	$ 0.3
Liability for unpaid losses and loss expenses	$ 27	$ 24.7

Investments - Cost or Amortized

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,444.5	$ 2,295
Gross unrealized holding gains	118.5	125.4
Gross unrealized holding losses	(25.8)	(24)
Fair value	2,537.2	2,396.4
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,095.9	1,972.9
Gross unrealized holding gains	35.8	46.4
Gross unrealized holding losses	(22.4)	(18.6)
Fair value	2,109.3	2,000.7
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	323.1	265.2
Gross unrealized holding gains	63.1	50.7
Gross unrealized holding losses	(3.4)	(5.3)
Fair value	382.8	310.6
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	25.5	56.9
Gross unrealized holding gains	19.6	28.3
Gross unrealized holding losses	0	(0.1)
Fair value	45.1	85.1
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	401.9	219.8
Gross unrealized holding gains	8.9	6.6
Gross unrealized holding losses	(6.1)	(2)
Fair value	404.7	224.4
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	634.6	804
Gross unrealized holding gains	12.3	22.5
Gross unrealized holding losses	(3.2)	(1.7)
Fair value	643.7	824.8
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	445.7	500.3
Gross unrealized holding gains	6.1	5.8
Gross unrealized holding losses	(2.2)	(11.7)
Fair value	449.6	494.4
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	613.7	448.8
Gross unrealized holding gains	8.5	11.5
Gross unrealized holding losses	(10.9)	(3.2)
Fair value	611.3	457.1
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	108.9	136.3
Gross unrealized holding gains	32.7	29.4
Gross unrealized holding losses	(2.6)	(4)
Fair value	139	161.7
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	57.2	53.3
Gross unrealized holding gains	21.9	16.5
Gross unrealized holding losses	0	(0.2)
Fair value	79.1	69.6
Mutual and exchange traded funds [Domain] [Member] | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	157	75.6
Gross unrealized holding gains	8.5	4.8
Gross unrealized holding losses	(0.8)	(1.1)
Fair value	$ 164.7	$ 79.3

Investments - Gross Unrealized

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Dec. 31, 2016USD ($)position	Dec. 31, 2015USD ($)position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 891.6	$ 662.5
Less than 12 months, Unrealized losses	$ (22.5)	$ (16.3)
Less than 12 months, Number of positions (positions) | position	124	111
12 months or more, Fair value	$ 58.7	$ 110.4
12 months or more, Unrealized losses	$ (3.3)	$ (7.7)
12 months or more, Number of positions (positions) | position	20	26
Total, Fair value	$ 950.3	$ 772.9
Total, Unrealized losses	$ (25.8)	$ (24)
Total, Number of positions (positions) | position	144	137
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 852.6	$ 585.2
Less than 12 months, Unrealized losses	$ (20.8)	$ (10.9)
Less than 12 months, Number of positions (positions) | position	115	85
12 months or more, Fair value	$ 49.9	$ 110.4
12 months or more, Unrealized losses	$ (1.6)	$ (7.7)
12 months or more, Number of positions (positions) | position	15	26
Total, Fair value	$ 902.5	$ 695.6
Total, Unrealized losses	$ (22.4)	$ (18.6)
Total, Number of positions (positions) | position	130	111
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 39	$ 69.2
Less than 12 months, Unrealized losses	$ (1.7)	$ (5.3)
Less than 12 months, Number of positions (positions) | position	9	25
12 months or more, Fair value	$ 8.8	$ 0
12 months or more, Unrealized losses	$ (1.7)	$ 0
12 months or more, Number of positions (positions) | position	5	0
Total, Fair value	$ 47.8	$ 69.2
Total, Unrealized losses	$ (3.4)	$ (5.3)
Total, Number of positions (positions) | position	14	25
Other Invested Assets [Member]
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) | position		1
12 months or more, Fair value		$ 0
12 months or more, Unrealized losses		$ 0
12 months or more, Number of positions (positions) | position		0
Total, Fair value		$ 8.1
Total, Unrealized losses		$ (0.1)
Total, Number of positions (positions) | position		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	$ 229.1	$ 68.6
Less than 12 months, Unrealized losses	$ (6.1)	$ (1.6)
Less than 12 months, Number of positions (positions) | position	30	15
12 months or more, Fair value	$ 0	$ 13.5
12 months or more, Unrealized losses	$ 0	$ (0.4)
12 months or more, Number of positions (positions) | position	0	4
Total, Fair value	$ 229.1	$ 82.1
Total, Unrealized losses	$ (6.1)	$ (2)
Total, Number of positions (positions) | position	30	19
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 178.9	$ 137.5
Less than 12 months, Unrealized losses	$ (3.2)	$ (1.7)
Less than 12 months, Number of positions (positions) | position	26	16
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions (positions) | position	0	0
Total, Fair value	$ 178.9	$ 137.5
Total, Unrealized losses	$ (3.2)	$ (1.7)
Total, Number of positions (positions) | position	26	16
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 102.9	$ 246.9
Less than 12 months, Unrealized losses	$ (1.4)	$ (5.3)
Less than 12 months, Number of positions (positions) | position	16	36
12 months or more, Fair value	$ 29.4	$ 63.9
12 months or more, Unrealized losses	$ (0.8)	$ (6.4)
12 months or more, Number of positions (positions) | position	4	11
Total, Fair value	$ 132.3	$ 310.8
Total, Unrealized losses	$ (2.2)	$ (11.7)
Total, Number of positions (positions) | position	20	47
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 341.7	$ 132.2
Less than 12 months, Unrealized losses	$ (10.1)	$ (2.3)
Less than 12 months, Number of positions (positions) | position	43	18
12 months or more, Fair value	$ 20.5	$ 33
12 months or more, Unrealized losses	$ (0.8)	$ (0.9)
12 months or more, Number of positions (positions) | position	11	11
Total, Fair value	$ 362.2	$ 165.2
Total, Unrealized losses	$ (10.9)	$ (3.2)
Total, Number of positions (positions) | position	54	29
Large-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 9.1	$ 46
Less than 12 months, Unrealized losses	$ (0.9)	$ (4)
Less than 12 months, Number of positions (positions) | position	7	20
12 months or more, Fair value	$ 8.8	$ 0
12 months or more, Unrealized losses	$ (1.7)	$ 0
12 months or more, Number of positions (positions) | position	5	0
Total, Fair value	$ 17.9	$ 46
Total, Unrealized losses	$ (2.6)	$ (4)
Total, Number of positions (positions) | position	12	20
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) | position		1
12 months or more, Fair value		$ 0
12 months or more, Unrealized losses		$ 0
12 months or more, Number of positions (positions) | position		0
Total, Fair value		$ 3.4
Total, Unrealized losses		$ (0.2)
Total, Number of positions (positions) | position		1
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 29.9	$ 19.8
Less than 12 months, Unrealized losses	$ (0.8)	$ (1.1)
Less than 12 months, Number of positions (positions) | position	2	4
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions (positions) | position	0	0
Total, Fair value	$ 29.9	$ 19.8
Total, Unrealized losses	$ (0.8)	$ (1.1)
Total, Number of positions (positions) | position	2	4

Investments - Realized Losses R

Investments - Realized Losses Related to Other-Than-Temporary Impairments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (6.8)	$ (7.9)	$ (2.5)
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	(2.3)	0	0
Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	(0.6)	(2.2)	(0.3)
Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (3.9)	$ (5.7)	$ (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, 2016	Dec. 31, 2015
Amortized cost
Due in 1 year or less	$ 74.4
Due after 1 year through 5 years	552.7
Due after 5 years through 10 years	280.2
Due after 10 years	574.9
U.S. government agencies mortgage-backed securities	613.7
Total	2,095.9	$ 1,972.9
Fair value
Due in 1 year or less	75.2
Due after 1 year through 5 years	557.8
Due after 5 years through 10 years	282.9
Due after 10 years	582.1
U.S. government agencies mortgage-backed securities	611.3
Total	$ 2,109.3	$ 2,000.7

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 44,100,000	$ 34,400,000	$ 24,500,000
Noncash or Part Noncash Acquisition, Investments Acquired	44,000,000
Carrying value of securities pledged as collateral for the FHLB Loan	106,500,000
Fixed maturities fair value of deposit with insurance regulators	9,200,000	8,800,000
Proceeds on sales of available-for-sale securities	365,400,000	326,200,000	263,300,000
Other Invested Assets [Member]
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized Gains	12,100,000	$ 200,000	$ 100,000
Other Invested Assets [Member] | Partnership Interest [Member]
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 12,000,000

Investments - Components of Net

Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 76.3	$ 73.7	$ 76.7
Investment expenses	1.6	2	2
Net investment income	74.7	71.7	74.7
Fixed maturities:
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	63.4	61.3	64.3
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	7.3	6.5	6.2
Cash and cash equivalents, and other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 5.6	$ 5.9	$ 6.2

Investments - Realized and Unre

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Realized gains:
Total realized gains	$ 44.1	$ 34.4	$ 24.5
Realized losses:
Total realized losses	(7.6)	(9.7)	(3.8)
Net realized gains on investments	36.5	24.7	20.7
Change in unrealized holding gains (losses), net of tax:
Deferred federal income liability thereon	3	22.4	(13.7)
Change in net unrealized holding gains, net of tax	(5.7)	(41.5)	25.4
Fixed maturities:
Realized gains:
Total realized gains	2.8	4.6	3.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	(14.4)	(32.8)	34.5
Fixed maturities: | OTTI
Realized losses:
Total realized losses	2.3	0	0
Equity Securities [Member]
Realized gains:
Total realized gains	29.2	29.6	21.3
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	14.3	(29.5)	6.2
Equity Securities [Member] | Sales
Realized losses:
Total realized losses	(0.8)	(1.8)	(1.3)
Equity Securities [Member] | OTTI
Realized losses:
Total realized losses	4.5	(7.9)	(2.5)
Other Invested Assets [Member]
Realized gains:
Total realized gains	12.1	0.2	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	$ (8.6)	$ (1.6)	$ (1.6)

Fair Value of Financial Instr59

Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May 31, 2009USD ($)Agreement	Dec. 31, 2016USD ($)security	Dec. 31, 2015USD ($)
Fair Value Measurements Disclosure [Line Items]
Other invested assets fair value		$ 35.7	$ 33.3
Other investments and securities, at cost		5.4	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		4.9
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) | security		1
International funds
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities (securities) | security		1
Other invested assets fair value			$ 43.7

Fair Value of Financial Instr60

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	$ 2,537.2	$ 2,396.4
Investments, Fair Value Disclosure	2,501.5	2,319.4
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,109.3	2,000.7
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	382.8	310.6
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	45.1	85.1
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	404.7	224.4
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	643.7	824.8
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	449.6	494.4
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	611.3	457.1
Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	139	161.7
Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	79.1	69.6
Mutual and exchange traded funds [Domain] [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	164.7	79.3
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	9.4	8.1
Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,109.3	2,000.7
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	404.7	224.4
Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	643.7	824.8
Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	449.6	494.4
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	611.3	457.1
Available-for-sale: | Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	139
Available-for-sale: | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	79.1
Quoted prices in active markets for identical assets (Level 1)
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	392.2	318.7
Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	382.8	310.6
Quoted prices in active markets for identical assets (Level 1) | Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		161.7
Quoted prices in active markets for identical assets (Level 1) | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		69.6
Quoted prices in active markets for identical assets (Level 1) | Mutual and exchange traded funds [Domain] [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	164.7	79.3
Quoted prices in active markets for identical assets (Level 1) | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	9.4	8.1
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: | 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 [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	139
Quoted prices in active markets for identical assets (Level 1) | Available-for-sale: | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	79.1
Significant other observable inputs (Level 2)
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	2,105.8	1,997.4
Significant other observable inputs (Level 2) | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant other observable inputs (Level 2) | Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		0
Significant other observable inputs (Level 2) | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		0
Significant other observable inputs (Level 2) | Mutual and exchange traded funds [Domain] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant other observable inputs (Level 2) | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		0
Significant other observable inputs (Level 2) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,105.8	1,997.4
Significant other observable inputs (Level 2) | Available-for-sale: | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0
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	404.7	224.4
Significant other observable inputs (Level 2) | Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	643.7	824.8
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	446.1	491.1
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	611.3	457.1
Significant other observable inputs (Level 2) | Available-for-sale: | Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0
Significant other observable inputs (Level 2) | Available-for-sale: | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0
Significant unobservable inputs (Level 3)
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	3.5	3.3
Significant unobservable inputs (Level 3) | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		0
Significant unobservable inputs (Level 3) | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		0
Significant unobservable inputs (Level 3) | Mutual and exchange traded funds [Domain] [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0	0
Significant unobservable inputs (Level 3) | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		0
Significant unobservable inputs (Level 3) | Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	3.5	3.3
Significant unobservable inputs (Level 3) | Available-for-sale: | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	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	3.5	3.3
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 [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0
Significant unobservable inputs (Level 3) | Available-for-sale: | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	$ 0

Fair Value of Financial Instr61

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, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 3.3	$ 9.4
Total realized gains (losses)-included in earnings	0	0
Total unrealized gains (losses)-included in other comprehensive income	0	(0.2)
Purchases	0.2	0
Sales	0	(5.9)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 3.5	$ 3.3

Fair Value of Financial Instr62

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, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate (percent)	7.00%	7.00%
Reported Value Measurement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 70	$ 70
Estimate of Fair Value Measurement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 75.7	$ 74.1

Fair Value of Financial Instr63

Fair Value of Financial Instruments - Included in Notes Payable are Senior Notes and Subordinated Debentures (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest rate (percent)	5.28%	5.28%
Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Interest rate (percent)	1.73%	5.03%
Debt instrument, issued	$ 21,500,000
Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Interest rate (percent)	5.03%
Debt instrument, issued	$ 85,000,000
Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Interest rate (percent)	5.13%	4.61%
Debt instrument, issued	$ 15,500,000
Carrying value
Debt Instrument [Line Items]
Total notes payable	122,100,000	$ 100,500,000
Carrying value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	21,500,000	85,300,000
Carrying value | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	85,400,000
Carrying value | Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Total notes payable	15,200,000	15,200,000
Fair value
Debt Instrument [Line Items]
Total notes payable	121,800,000	100,700,000
Fair value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	21,000,000	85,500,000
Fair value | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	85,600,000
Fair value | Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Total notes payable	$ 15,200,000	$ 15,200,000

Losses and Loss Expenses Paya64

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$ 1,053	$ 983.2	$ 959.9
Less: reinsurance recoverable on losses and loss expenses payable	5.9	9.6	9.1
Net balance at beginning of year	1,047.1	973.6	950.8
Incurred related to:
Current year	915.4	852.8	726.2
Prior years	27	10	45.1
Total incurred	942.4	862.8	771.3
Paid related to:
Current year	417.8	421.5	373.2
Prior years	393.7	367.8	375.3
Total paid	811.5	789.3	748.5
Net balance at end of year	1,178	1,047.1	973.6
Plus: reinsurance recoverable on losses and loss expenses payable	3.6	5.9	9.6
Losses and loss expenses payable, at end of year (affiliates $630.9, $532.4, and $494.3, respectively)	$ 1,181.6	$ 1,053	$ 983.2

Losses and Loss Expenses Paya65

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance [Abstract]
Losses and loss expenses payable, affiliates	$ 630.9	$ 532.4	$ 494.3

Losses and Loss Expenses Paya66

Losses and Loss Expenses Payable - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ (27)	$ (10)	$ (45.1)
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(3.7)	6.2	12.5
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.4	0.7	5.2
Personal And Business Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		9.8	28.7
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(7.8)	9.4	6.4
Personal Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(7.9)	11
Business insurance segment
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	3.9	0.4	22.3
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(3.3)	10.5	5.3
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	4.2	5.1	5.6
Product Liability [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	4.5	3.8	11.9
Specialty Insurance Segment [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(20.8)	7.1	91.5
Commercial Multi-Peril [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			2.1
Programs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(14.3)	9.7
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		5.3	3.9
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ (4.6)	$ 2.7	3.1
RED [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			$ 96.7

Short-Duration Insurance Cont67

Short-Duration Insurance Contracts Claims Development (Details) $ in Millions	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)	Dec. 31, 2007USD ($)
Claims Development [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 1,114.2	$ 993.3	$ 923.2
Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	1,581.4
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,353.5
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	0.5
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	228.4	207.4	183.1
Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	909.3
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	688.4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	14
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	234.9	213.5	183.9
Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	497.9
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	331.9
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	12.9
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	178.9	165.8	195.3
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	308.6	308.9	306.7	$ 307.6	$ 305.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0.9
Short-duration Insurance Contracts, Number of Reported Claims	212,774
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 304.5	298.1	285.1	260.2	193.8
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	169.1	170.9	168.1	170.5	170.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3.7
Short-duration Insurance Contracts, Number of Reported Claims	33,897
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 161.8	156.8	145.8	128	91.9
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	163.7	162.8	164.8	115.6	108.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9
Short-duration Insurance Contracts, Number of Reported Claims	9,697
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 142.5	122.9	94.7	59.8	25.4
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	287.6	287.9	283.3	288.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.8
Short-duration Insurance Contracts, Number of Reported Claims	189,392
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 277.2	265.4	236.9	176
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	176.8	175.2	168.7	170
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 6.4
Short-duration Insurance Contracts, Number of Reported Claims	33,131
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 158.4	146	126.2	79.4
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	90.9	85	82.9	70.9
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 6.7
Short-duration Insurance Contracts, Number of Reported Claims	7,680
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 72	55.5	37.7	18.2
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	275.2	272.8	271.2
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4
Short-duration Insurance Contracts, Number of Reported Claims	171,772
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 252.1	226.3	172.5
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	185.6	183.9	181
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15
Short-duration Insurance Contracts, Number of Reported Claims	32,253
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 150.5	131.3	92.9
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	58.6	56.9	55.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7.8
Short-duration Insurance Contracts, Number of Reported Claims	7,657
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 44	34.9	22
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	343.6	337.2
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 20.3
Short-duration Insurance Contracts, Number of Reported Claims	151,130
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 288.9	214.9
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	191	189.1
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 31.2
Short-duration Insurance Contracts, Number of Reported Claims	30,445
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 130	90.4
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	85.4	75.5
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 21.6
Short-duration Insurance Contracts, Number of Reported Claims	7,666
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 48.2	24
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	366.4
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 44.9
Short-duration Insurance Contracts, Number of Reported Claims	137,561
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 230.8
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	186.8
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 49.8
Short-duration Insurance Contracts, Number of Reported Claims	25,340
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 87.7
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	99.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 48.8
Short-duration Insurance Contracts, Number of Reported Claims	5,754
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 25.2
Long-tail [Domain] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	856.6
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	562.3
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	36.6
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	330.9	306.6	285.6
Long-tail [Domain] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	209.4
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	81.7
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	129.7	91.1	65.4
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	77.6	78	80.2	85.5	92.8
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 12.2
Short-duration Insurance Contracts, Number of Reported Claims	17,321
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 57.3	51.8	43.7	32.6	15.3
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	17.4	17.6	15.4	16.7	16.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3
Short-duration Insurance Contracts, Number of Reported Claims	862
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 12.5	10.3	7.3	4	0.9
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	84.9	88.2	88.2	92.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 19.7
Short-duration Insurance Contracts, Number of Reported Claims	16,646
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 57.2	50.1	36.7	16.3
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	16.5	17.9	18.9	20.1
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.9
Short-duration Insurance Contracts, Number of Reported Claims	936
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 9.1	6.7	3.7	1.1
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	95.2	96.6	96.1
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 27.3
Short-duration Insurance Contracts, Number of Reported Claims	16,501
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 53.9	37.6	15.8
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	27.1	25.2	23.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9.5
Short-duration Insurance Contracts, Number of Reported Claims	1,044
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 12.5	7.2	1.2
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	106.2	108.9
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 45.5
Short-duration Insurance Contracts, Number of Reported Claims	15,783
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 38.6	17.3
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	45	38.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22.8
Short-duration Insurance Contracts, Number of Reported Claims	1,409
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 11	2.7
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	109.5
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 65.6
Short-duration Insurance Contracts, Number of Reported Claims	8,548
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 15.4
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	60.8
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 47.6
Short-duration Insurance Contracts, Number of Reported Claims	1,401
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 6
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	73.3	73	72.4	72.5	71.9	$ 71.9	$ 73.2	$ 74.1	$ 73.3	$ 73.4
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.7
Short-duration Insurance Contracts, Number of Reported Claims	8,072
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 68.6	68.1	65.7	64	61.4	57.6	50.3	43	28.7	12.7
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	4.1	8.3	8.3	8.4	8.5	8.8	9	9.4	9.2	9.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.4
Short-duration Insurance Contracts, Number of Reported Claims	1,101
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 1.6	1.5	1.5	1.3	1.3	0.5	0.1	0.6	0.1	$ 0
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	73.4	74.2	74.9	75.3	75.4	75	75.6	79.1	86.1
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4
Short-duration Insurance Contracts, Number of Reported Claims	9,577
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 66	65	64	61.3	56.4	51.6	43.9	34.7	14.3
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2008 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	4.9	6	5.8	6	6.1	6.8	6.7	6.7	6.4
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1.6
Short-duration Insurance Contracts, Number of Reported Claims	1,502
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 3	2.8	2.4	2.1	1.7	1.3	0.8	0.3	$ 0.1
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	82.3	82.4	83.1	85	86	87.4	92	90.2
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5.8
Short-duration Insurance Contracts, Number of Reported Claims	10,086
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 74.1	72.1	70.3	66.2	60.2	48.2	34.9	15.2
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	12.4	10.6	11	11	10.4	10	11	10.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2
Short-duration Insurance Contracts, Number of Reported Claims	1,429
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 10	9.2	9.4	8.6	7.2	4.9	2.4	$ 0.2
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	78.9	79.5	82	82.2	86.7	93.5	85.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7.2
Short-duration Insurance Contracts, Number of Reported Claims	10,644
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 67.6	65.8	63.3	54.2	46.3	33	12.9
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	10.3	9.9	9.8	9.7	9.9	10.9	11.2
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.2
Short-duration Insurance Contracts, Number of Reported Claims	1,169
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 8.1	7.6	7.7	6.5	5.2	2.2	$ 0.8
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Business insurance segment
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	75.3	76.1	77.5	80.9	86.4	86.9
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9
Short-duration Insurance Contracts, Number of Reported Claims	17,390
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 63.6	60.7	53.9	46.4	33.9	16.4
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	10.9	10.1	10.2	10.7	10.7	11.9
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.2
Short-duration Insurance Contracts, Number of Reported Claims	911
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 7.9	$ 7.1	$ 6.6	$ 5.2	$ 3	$ 0.9

Short-Duration Insurance Cont68

Short-Duration Insurance Contracts Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 1,114.2	$ 993.3	$ 923.2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	63.8	53.8	50.4
Reinsurance recoverable on losses and loss expenses payable	3.6	5.9	9.6	$ 9.1
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	1,181.6	1,053	983.2	$ 959.9
Personal Insurance Segments [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	228.4	207.4	183.1
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	8.9	7.5	7.5
Business insurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	234.9	213.5	183.9
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	2.5	1.4	2.4
Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	178.9	165.8	195.3
Long-tail [Domain] | Business insurance segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	330.9	306.6	285.6
Long-tail [Domain] | Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 129.7	$ 91.1	$ 65.4

Short-Duration Insurance Cont69

Short-Duration Insurance Contracts Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (Details)	Dec. 31, 2016
Personal Insurance Segments [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	62.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	9.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.10%
Business insurance segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	48.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	22.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	3.00%
Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	25.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	11.90%
Long-tail [Domain] | Business insurance segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	17.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	16.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	10.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	8.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Long-tail [Domain] | Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	5.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	14.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	10.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	6.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.20%

Reinsurance - Additional Inform

Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015
Reinsurance [Line Items]
Duration of quota share agreement				3 years
Percentage ceded to reinsurers (percent)				75.00%
Unearned premium	$ 617.8	$ 616.3		$ 616.3
Reinsurance Arrangement, Participating Reinsurers' Margin	9.00%
Profit Commission	$ 1.9	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, 2016	Dec. 31, 2015
Losses and loss expenses payable:
Assumed	$ 1,178	$ 1,047.1
Ceded	(547.1)	(514.7)
Unearned premiums:
Assumed	611.7	609.5
Ceded	(390.8)	(395.3)
Unearned Premiums From Affiliates	220.9	214.2
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	545.7	515.6
Assumed	5	5
Ceded	(3.6)	(5.9)
Net losses and loss expenses payable	547.1	514.7
Unearned premiums:
Direct	395.6	400.9
Assumed	1.3	1.2
Ceded	(6.1)	(6.8)
Unearned Premiums From Affiliates	$ 390.8	$ 395.3

Reinsurance - External Reinsu72

Reinsurance - External Reinsurance on Income Statements (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Written premiums:
Assumed	$ 1,293.3	$ 1,273.5	$ 1,194.2
Ceded	(815.5)	(823.4)	(858.2)
Net written premiums	477.8	450.1	336
Earned premiums:
Direct	840.6	863.1	882.6
Assumed	1,291.9	1,270.5	1,074.1
Ceded	(819.9)	(832.9)	(861.7)
Losses and loss expenses incurred:
Assumed	944.1	864.2	774.4
Ceded	(560.3)	(570.9)	(523.6)
Net losses and loss expenses incurred	383.8	293.3	250.8
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	835.4	854.1	880.6
Assumed	5	4.7	4.4
Ceded	(24.9)	(35.4)	(26.8)
Net written premiums	815.5	823.4	858.2
Earned premiums:
Direct	840.6	863.1	882.6
Assumed	4.9	4.5	4.4
Ceded	(25.6)	(34.7)	(25.3)
Net earned premiums	819.9	832.9	861.7
Losses and loss expenses incurred:
Direct	559.1	569	525.6
Assumed	3.9	3.4	2.9
Ceded	(4.4)	(2.9)	(8)
Net losses and loss expenses incurred	$ 558.6	$ 569.5	$ 520.5

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, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
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			$ 310.5	$ 294.7
Mandatorily redeemable preferred capital securities issued to third party		$ 15
Common securities issued to state auto financial		0.5	$ 2.2	$ 6.2	$ 3.5
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			$ 1.9	$ 1.8	$ 1.5
Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate (percent)			5.13%	4.61%
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 74

Transactions with Affiliates - Schedule of Reinsurance Transactions on Balance Sheets for Pooling Arrangement (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	$ (79.1)	$ (79.8)
Deferred Policy Acquisition Costs Assumed	129.8	129.1
Deferred Policy Acquisitions Costs from Affiliates	50.7	49.3
Losses and loss expenses payable:
Ceded	(547.1)	(514.7)
Assumed	1,178	1,047.1
Losses And Loss Expenses Payable From Affiliates	630.9	532.4
Unearned premiums:
Ceded	(390.8)	(395.3)
Assumed	611.7	609.5
Unearned Premiums From Affiliates	220.9	214.2
Pension and Other Postretirement Benefits:
Postretirement and Pension Benefits Ceded	(114.5)	(160)
Postretirement and Pension Benefits Assumed	74.4	104
Postretirement and Pension Benefits Ceded to Affiliates	(40.1)	(56)
Other Liabilities Ceded	(42.9)	(63.8)
Other Liabilities Assumed	53.9	55.4
Other Liabilities	11	(8.4)
Accumulated Other Comprehensive Income Ceded	(153.5)	(162)
Accumulated Other Comprehensive Income Assumed	99.8	105.3
Net Accumulated Other Comprehensive Income	$ (53.7)	$ (56.7)

Transactions with Affiliates 75

Transactions with Affiliates - Schedule of Reinsurance Transactions on Income Statements for Pooling Arrangement (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Written premiums:
Ceded	$ (815.5)	$ (823.4)	$ (858.2)
Assumed	1,293.3	1,273.5	1,194.2
Premiums Written, Net	477.8	450.1	336
Earned premiums:
Ceded	(819.9)	(832.9)	(861.7)
Assumed	1,291.9	1,270.5	1,074.1
Premiums Earned, Net	472	437.6	212.4
Losses and loss expenses incurred:
Ceded	(560.3)	(570.9)	(523.6)
Assumed	944.1	864.2	774.4
Losses and loss expenses incurred, Net	383.8	293.3	250.8
Acquisition and Operating Expenses [Abstract]
Ceded	(148.5)	(183.9)	(215.4)
Assumed	434.3	432.5	372.3
Acquisition and Operating Expenses, Net	$ 285.8	$ 248.6	$ 156.9

Notes Payable and Credit Faci76

Notes Payable and Credit Facility - Additional Information (Detail) - USD ($)	Sep. 03, 2016	Jul. 26, 2013	Jul. 11, 2013	Sep. 02, 2016
Debt Instrument [Line Items]
Long-term Federal Home Loan Bank Advances				$ 21,500,000
Long-term Purchase Commitment, Period	5 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)				1.73%
Current borrowing capacity		$ 100,000,000
Expiration period		5 years
Maximum borrowing capacity under credit facility		$ 150,000,000
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 6.7	$ 23.5	$ 9.4
Tax-exempt interest and dividends received deduction	(7.1)	(8.7)	(8.5)
Other, net	(1.4)	1.3	1.1
Valuation allowance	0	0	(82.6)
Federal income tax (benefit) expense and effective rate	$ (1.8)	$ 16.1	$ (80.6)
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)	(37.20%)	(13.00%)	(31.50%)
Other, net (percent)	(7.20%)	1.90%	4.10%
Valuation allowance (percent)	0.00%	0.00%	(308.10%)
Federal income tax expense (benefit) and effective rate (percent)	(9.40%)	23.90%	(300.50%)

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, 2016	Dec. 31, 2015
Deferred tax assets:
Unearned premiums not currently deductible	$ 42.6	$ 42.5
Losses and loss expenses payable discounting	19.7	19.3
Postretirement and pension benefits	26.1	36.4
Realized loss on other-than-temporary impairment	4	12.3
Other liabilities	16.6	21.3
Net operating loss carryforward	58.9	41.8
Tax credit carryforward	3.3	4.8
Other	8.8	4.9
Total deferred tax assets	180	183.3
Deferred tax liabilities:
Deferral of policy acquisition costs	45.4	45.2
Net unrealized holding gains on investments	32.5	35.6
Total deferred tax liabilities	77.9	80.8
Net deferred federal income taxes	$ 102.1	$ 102.5

Federal Income Taxes - Addition

Federal Income Taxes - Additional Information (Detail)	12 Months Ended			36 Months Ended
	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2015USD ($)
Income Tax Disclosure [Abstract]
Tax Adjustments, Settlements, and Unusual Provisions	$ 1,600,000
Cumulative Pre-Tax Income				3 years
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	19,200,000	$ 67,300,000	$ 26,800,000	$ 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	58,900,000	41,800,000		$ 41,800,000	$ 41,800,000
Balance for uncertain tax positions	0
Interest and penalties payable	$ 0	$ 0		$ 0	$ 0

Pension and Postretirement Be80

Pension and Postretirement Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure	$ 35,700,000	$ 33,300,000
Available-for-sale Securities	$ 2,537,200,000	2,396,400,000
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
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	6.00%
Matching contribution by the employer	$ 0.50
Company's share of expense under the plan	$ 6,300,000	5,100,000	$ 5,100,000
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Participant's contribution (percent)	2.00%
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	1,400,000
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Available-for-sale Securities	200,900,000	177,600,000
Postretirement plan's gross benefit payments	11,300,000
Pension | International funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure	$ 31,300,000	$ 18,600,000

Pension and Postretirement Be81

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in benefit obligation:
Actuarial loss (gain)	$ (3)	$ 5.3	$ (54.4)
Change in plan assets available for plan benefits:
Employer contribution	13
Pension Costs
Change in benefit obligation:
Benefit obligation at beginning of year	281.3	292.5
Service cost	6.1	7.7
Interest cost	11.6	11.1
Actuarial loss (gain)	4.5	(17.6)
Benefits paid	(19)	(12.4)
The Company’s portion of benefit obligation at end of year	284.5	281.3	292.5
Change in plan assets available for plan benefits:
Fair value of plan assets available for plan benefits at beginning of year	204.4	205.1
Employer contribution	34.5	13
Actual return on plan assets	15.8	(1.3)
Benefits paid	(19)	(12.4)
The Company’s portion of fair value of plan assets at end of year	235.7	204.4	205.1
Supplemental executive retirement plan	(6.8)	(6.8)
Funded status at end of year	(55.6)	(83.7)
Accumulated benefit obligation end of year	267.5	263.1
Postretirement Benefit Costs
Change in benefit obligation:
Benefit obligation at beginning of year	20.3	23.9
Service cost	0	0
Interest cost	0.8	0.9
Actuarial loss (gain)	(0.9)	(3.3)
Benefits paid	(1.4)	(1.2)
The Company’s portion of benefit obligation at end of year	18.8	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	0	0.3
Employer contribution	0	0
Actual return on plan assets	0	0
Benefits paid	0	(0.3)
The Company’s portion of fair value of plan assets at end of year	0	0	$ 0.3
Supplemental executive retirement plan	0	0
Funded status at end of year	$ (18.8)	$ (20.3)

Pension and Postretirement Be82

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, 2016	Dec. 31, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Prior service benefit	$ (54.1)	$ (59.5)
Net actuarial loss	118.5	125.1
Total	$ 64.4	$ 65.6

Pension and Postretirement Be83

Pension and Postretirement Benefit Plans - Company's Share of Amortization Expected to be Recognized (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Prior service benefit	$ (5.5)
Net actuarial loss	8
Total	$ 2.5

Pension and Postretirement Be84

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amortization of:
Prior service benefit	$ (54.1)	$ (59.5)
Pension
Components of net periodic cost:
Service cost	6.2	7.9	$ 5.2
Interest cost	11.9	11.3	11.1
Expected return on plan assets	(15.1)	(13.8)	(12.6)
Amortization of:
Prior service benefit	0	0	0
Net actuarial loss	9.2	10.9	6.3
Net periodic cost (benefit)	12.2	16.3	10
Postretirement
Components of net periodic cost:
Service cost	0	0	0
Interest cost	0.9	1.1	1.1
Expected return on plan assets	0	0	0
Amortization of:
Prior service benefit	(5.5)	(5.4)	(5.5)
Net actuarial loss	0.2	0.6	0.6
Net periodic cost (benefit)	$ (4.4)	$ (3.7)	$ (3.8)

Pension and Postretirement Be85

Pension and Postretirement Benefit Plans-Company's Share of Expected Future Benefits Payments Schedule (Detail) $ in Millions	Dec. 31, 2016USD ($)
Pension
Schedule of Expected Future Pension Benefit Payment [Line Items]
2,016	$ 11.3
2,017	11.8
2,018	12
2,019	12.5
2,020	13.1
2021 - 2025	76.8
Postretirement
Schedule of Expected Future Pension Benefit Payment [Line Items]
2,016	1.5
2,017	1.4
2,018	1.4
2,019	1.4
2,020	1.4
2021 - 2025	$ 6

Pension and Postretirement Be86

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plans' Obligations (Detail)	Dec. 31, 2016	Dec. 31, 2015
Pension
Benefit obligations weighted-average assumptions:
Discount rate (percent)	4.00%	4.20%
Rates of increase in compensation levels (percent)	3.50%	3.50%
Postretirement
Benefit obligations weighted-average assumptions:
Discount rate (percent)	4.00%	4.20%
Rates of increase in compensation levels (percent)	0.00%	0.00%

Pension and Postretirement Be87

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plan's Net Periodic Cost (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	4.20%	3.85%	4.85%
Expected long-term rate of return on assets (percent)	7.00%	7.00%	7.00%
Rates of increase in compensation levels (percent)	3.50%	3.50%	3.50%
Postretirement
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	4.20%	3.85%	4.85%
Expected long-term rate of return on assets (percent)	0.00%	0.00%	0.00%
Rates of increase in compensation levels (percent)	0.00%	0.00%	0.00%

Pension and Postretirement Be88

Pension and Postretirement Benefit Plans - Schedule of Assumed Health Care Cost Trend Rates Used (Detail) - Postretirement	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assumed health care cost trend rates:
Health care cost trend rate assumed for the next year (percent)	6.50%	6.50%	6.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (percent)	3.90%	3.80%	3.80%
Year that the rate reaches the ultimate trend rate	2,075	2,076	2,075

Pension and Postretirement Be89

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, 2016USD ($)
One percentage point change:
Effect on total service and interest cost	$ 0.1
Effect on accumulated postretirement benefit obligation	2.8
Postretirement
One percentage point change:
Effect on total service and interest cost	0.1
Effect on accumulated postretirement benefit obligation	$ 2.4

Pension and Postretirement Be90

Pension and Postretirement Benefit Plans - Summary of Asset Allocation Targets (Detail)	12 Months Ended
	Dec. 31, 2016
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 Be91

Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 2,537.2	$ 2,396.4
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	200.9	177.6
Pension | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	2
Pension | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	113	96.3
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]
Available-for-sale Securities	60.4	51
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	48	38.9
Pension | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	4.6	6.4
Pension | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	85.9	81.3
Pension | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	59.6	59
Pension | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	26.3	22.3
Pension | Quoted prices in active markets for identical assets (Level 1)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	85.9	81.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	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]
Available-for-sale Securities	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]
Available-for-sale Securities	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]
Available-for-sale Securities	0	0
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	85.9	81.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	59.6	59
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	26.3	22.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0
Pension | Significant other observable inputs (Level 2)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	115	96.3
Pension | Significant other observable inputs (Level 2) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	113	96.3
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]
Available-for-sale Securities	60.4	51
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	48	38.9
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]
Available-for-sale Securities	4.6	6.4
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0	0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0	0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0	$ 0
Pension | Significant other observable inputs (Level 2) | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 2

Pension and Postretirement Be92

Pension and Postretirement Benefit Plans - Company's Share of Postretirement Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 2,537.2	$ 2,396.4

Other Comprehensive Income an93

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 37.6	$ 71.7	$ 80.8
Other comprehensive income before reclassifications	15	(20.1)	(15.5)
Amounts reclassified from AOCI	20.1	14	(6.4)
Net current period other comprehensive income	(5.1)	(34.1)	(9.1)
Ending balance	32.5	37.6	71.7
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	68.5	110	84.6
Other comprehensive income before reclassifications	18	(25.4)	38.9
Amounts reclassified from AOCI	23.7	16.1	13.5
Net current period other comprehensive income	(5.7)	(41.5)	25.4
Ending balance	62.8	68.5	110
Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(30.9)	(38.3)	(3.8)
Other comprehensive income before reclassifications	(3)	5.3	(54.4)
Amounts reclassified from AOCI	(3.6)	(2.1)	(19.9)
Net current period other comprehensive income	0.6	7.4	(34.5)
Ending balance	$ (30.3)	$ (30.9)	$ (38.3)

Other Comprehensive Income an94

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	$ 36.5	$ 24.3	$ 20.7
Tax expense	1.8	(16.1)	80.6
Total reclassifications for the period	20.1	14	(6.4)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	20.1	14	(6.4)
Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	23.7	16.1	13.5
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	36.5	24.7	20.7
Total before tax	36.5	24.7	20.7
Tax expense	(12.8)	(8.6)	(7.2)
Total reclassifications for the period	(23.7)	(16.1)	(13.5)
Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(3.6)	(2.1)	(19.9)
Benefit Plan Items | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(3.9)	(6.1)	(1.4)
Tax expense	0.3	4	(18.5)
Negative prior service costs	5.5	5.4	5.5
Net loss	(9.4)	(11.5)	(6.9)
Total reclassifications for the period	$ 3.6	$ 2.1	$ 19.9

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 84.4
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)	400.00%
State Auto Financial
Equity, Class of Treasury Stock [Line Items]
Dividends received from insurance subsidiaries	$ 10	$ 15	$ 20

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stockholders' Equity Note [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 844.4				$ 814.3				$ 844.4	$ 814.3
Net liabilities of non-insurance parent and affiliates	(84.9)				(80.2)				(84.9)	(80.2)
Total	759.5				734.1				759.5	734.1
Increases (decreases):
Deferred acquisition costs	129.8				129.1				129.8	129.1	$ 126.5	$ 96.8
Postretirement and pension benefits	22.7				23.2				22.7	23.2
Deferred federal income taxes	(49.1)				(41.6)				(49.1)	(41.6)
Fixed maturities, at fair value	13.5				27.9				13.5	27.9
Other, net	(14.9)				(11.9)				(14.9)	(11.9)
Stockholders’ equity per accompanying consolidated financial statements	891.3				884.6				891.3	884.6	872.9
Statutory net (loss) income of insurance subsidiaries									22.7	65.4	(17.5)
Net Income (Loss) Attributable to Parent									(3)	(4.1)	(5.3)
Total									(19.7)	(61.3)	22.8
Deferred acquisition costs									0.7	2.5	29.7
Postretirement and pension benefits									4.4	4.2	10.6
Deferred federal income taxes									(5.4)	(12.2)	79.3
Share-based compensation expense									0	(0.1)	(0.5)
Other, net									1.6	(4.5)	11.1
Net income	$ 32.5	$ 10.1	$ (24.6)	$ 3	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 21	$ 51.2	$ 107.4

Preferred Stock Preferred Stock

Preferred Stock Preferred Stock (Details)	12 Months Ended
	Dec. 31, 2016class_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, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 25,300,000
Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Outstanding Aggregate Intrinsic Value	14,900,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	3,000,000	$ 2,600,000	$ 500,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 1,400,000	300,000	0
Common Stock, Capital Shares Reserved for Future Issuance	3,700,000
Common shares valued (shares)	$ 121,600,000	120,400,000
Share-based compensation	3,700,000	$ 4,500,000	$ 3,600,000
Tax Adjustments, Settlements, and Unusual Provisions	$ 1,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	300,000	400,000	300,000
Percentage Of Annual Base Earnings	6.00%
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent	85.00%
Share-based Compensation Arrangement by Share-based Payment Award, Shares Purchased for Award	3,400,000
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 900,000
Common Stock, Capital Shares Reserved for Future Issuance	5,000,000
Number Of Securities Available For Issuance Under Equity Compensation Plans	2,400,000
Percentage Of Common Shares Authorized For Issuance	33.00%
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Other, 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
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	250,000
Plan Termination Date	May 8, 2019
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 3 months
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Annual Vesting Percentage	33.33%
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized	$ 2,200,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Share Based Compensation Stock Options Expiration Period	10 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	200,000	300,000	200,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 3 months
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Vesting Percentage Three Years After Grant Date	100.00%
Outside Directors Award Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares valued (shares)	$ 753,000	$ 567,000
Share Based Compensation Arrangement, Restricted Stock Units Awarded to Directors, Award Vesting Period	6 months
Number Of Shares Of Restricted Stock Issued To Each Non Employee Director	1,400
Restricted Stock Plan Termination Date	May 31, 2026
Stock Issued During Period, Shares, Restricted Stock Award, Gross	22,832	26,184	25,960
Outside Directors Award Plan [Member] | Restricted Stock Units (RSUs) [Member] | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement, Restricted Stock Units, Annual Installment Payments	10 years
Share Based Compensation Restricted Stock Units Awarded To Directors	10,000
Outside Directors Award Plan [Member] | Restricted Stock Units (RSUs) [Member] | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement, Restricted Stock Units, Annual Installment Payments	5 years
Share Based Compensation Restricted Stock Units Awarded To Directors	500
Exercise Prices Range One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit	$ 10.01
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit	20
Exercise Prices Range Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit	20.01
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit	30
Exercise Prices Range Three [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Lower Range Limit	$ 30.01

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Non-Vested and Vested Restricted Shares and Changes (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number	107,297	111,084	76,472	38,233
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value	$ 22.04	$ 22.19	$ 19.06	$ 15.06
Weighted Average Grant Date Fair Value
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	45,252	74,020	56,178
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value	$ 21.55	$ 22.83	$ 21.23
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	33,414	35,859	16,892
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value	$ 21.93	$ 16.88	$ 17.08
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	(15,625)	(3,549)	(1,047)
Share-based Compensation Arrangements by Share-based Payment Award, Options, Forfeitures in Period, Weighted Average Exercise Price	$ 21.91	$ 21.92	$ 21.23

Share-Based Compensation - Weig

Share-Based Compensation - Weighted Average Fair Values and Related Assumptions for Options Granted (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-Based Compensation - Weighted Average Fair Values and Related Assumptions for Options Granted (Detail) [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Exercise Price	$ 6.75	$ 7.69	$ 7.28
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate	1.87%	1.75%	1.86%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.30%	1.60%	1.65%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	36.27%	36.61%	39.23%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	5 years 3 months 18 days	6 years	5 years 8 months 12 days

Share-Based Compensation - S101

Share-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation [Abstract]
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 17.55	$ 16.56	$ 16.43
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	0.4	0.3	0.1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	2.8	3.4	3.7	3.9
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 20.67	$ 21.44	$ 21.29	$ 22.01
Weighted- Average Exercise Price
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0.3	0.4	0.3
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 21.47	$ 22.87	$ 21.20
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expirations in Period	0.5	0.4	0.4
Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price	$ 29.81	$ 25.18	$ 30.21

Share-Based Compensation - S102

Share-Based Compensation - Summary of Stock Options Outstanding and Exercisable (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Share Based Compensation Arrangement [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	2.8	3.4	3.7	3.9
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	3 years 9 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 20.67	$ 21.44	$ 21.29	$ 22.01
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	2.2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 20.53
Exercise Prices Range One [Member]
Schedule Of Share Based Compensation Arrangement [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	1.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	4 years 1 month 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 16.44
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	1.3
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 16.46
Exercise Prices Range Two [Member]
Schedule Of Share Based Compensation Arrangement [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	1.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	3 years 7 months 6 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 24.57
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	0.9
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 25.96

Net Earnings (Loss) per Comm103

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net earnings for basic net earnings per common share	$ 32.5	$ 10.1	$ (24.6)	$ 3	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 21	$ 51.2	$ 107.4
Adjusted net earnings for dilutive net earnings per common share									$ 21	$ 51.2	$ 107.4
Denominator:
Weighted average shares for basic net earnings (loss) per common share (shares)									41.6	41.1	40.8
Effect of dilutive share-based awards (shares)									0.4	0.5	0.4
Adjusted weighted average shares for diluted net earnings (loss) per common share (shares)									42	41.6	41.2
Basic net earnings (loss) per common share (usd per share)	$ 0.78	$ 0.24	$ (0.59)	$ 0.07	$ 0.08	$ 0.50	$ 0.06	$ 0.60	$ 0.50	$ 1.25	$ 2.63
Diluted net earnings (loss) per common share (usd per share)	$ 0.77	$ 0.24	$ (0.59)	$ 0.07	$ 0.07	$ 0.50	$ 0.06	$ 0.60	$ 0.50	$ 1.23	$ 2.60

Net Earnings (Loss) per Comm104

Net Earnings (Loss) per Common Share - Options to Purchase Shares of Common Stock (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Number of options and awards (shares)	1.2	1.5	1.8

Reportable Segments - Additiona

Reportable Segments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016Segment
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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015
Revenues from external sources:
Earned premiums, net									$ 472	$ 437.6	$ 212.4
Net investment income									74.7	71.7	74.7
Income (loss) before federal income tax									19.2	67.3	26.8	$ 98.7
Intersegment revenues	$ 364.9	$ 352.8	$ 348.5	$ 339.2	$ 346.6	$ 349.7	$ 337.4	$ 334.9	1,405.4	1,368.6	1,172.7
Segment loss before federal income tax:
Income (loss) before federal income tax									19.2	67.3	26.8	98.7
Net investment income									74.7	71.7	74.7
Income (loss) before federal income tax									19.2	67.3	26.8	98.7
Segment assets:
Total segment assets	2,663.7				2,529.8				2,663.7	2,529.8		2,529.8
Reconciling items:
Total consolidated assets	2,959.4				2,828.2				2,959.4	2,828.2		2,828.2
Interest expense (affiliates $0.8, $0.7 and $0.7, respectively)									5.5	5.4	5.4
Total investment operations segment
Segment assets:
Total segment assets	2,663.7				2,529.8				2,663.7	2,529.8		2,529.8
Total internal and external revenue [Member]
Revenues from external sources:
Intersegment revenues									1,411.2	1,374.2	1,177.8
Segment Reconciling Items [Member]
Revenues from external sources:
Income (loss) before federal income tax									(7.8)	(3.9)	37.2
Intersegment revenues									5.8	5.6	5.1
Segment loss before federal income tax:
Income (loss) before federal income tax									(7.8)	(3.9)	37.2
Income (loss) before federal income tax									(7.8)	(3.9)	37.2
Reconciling items:
Gaap Expense Adjustments									2	4.2	45.8
Interest expense (affiliates $0.8, $0.7 and $0.7, respectively)									(5.5)	(5.4)	(5.4)
Corporate Expenses									(4.3)	(2.7)	(3.2)
Corporate, Non-Segment [Member]
Reconciling items:
Total consolidated assets	$ 295.7				$ 298.4				295.7	298.4		$ 298.4
Intersegment Eliminations [Member]
Revenues from external sources:
Intersegment revenues									(5.8)	(5.6)	(5.1)
Reportable segments
Revenues from external sources:
Intersegment revenues									1,403.1	1,366.9	1,169.5
Reportable segments | Personal Insurance Segments [Member]
Revenues from external sources:
Earned premiums, net									582.2	591.8	451.4
Income (loss) before federal income tax									(15.2)	17.5	(11.7)
Segment loss before federal income tax:
Income (loss) before federal income tax									(15.2)	17.5	(11.7)
Income (loss) before federal income tax									(15.2)	17.5	(11.7)
Reportable segments | Business insurance
Revenues from external sources:
Earned premiums, net									468.9	476	459.9
Income (loss) before federal income tax									(26.7)	(36.7)	(4.6)
Segment loss before federal income tax:
Income (loss) before federal income tax									(26.7)	(36.7)	(4.6)
Income (loss) before federal income tax									(26.7)	(36.7)	(4.6)
Reportable segments | Specialty insurance
Revenues from external sources:
Earned premiums, net									240.8	202.7	162.8
Income (loss) before federal income tax									(42.7)	(5.9)	(89.5)
Segment loss before federal income tax:
Income (loss) before federal income tax									(42.7)	(5.9)	(89.5)
Income (loss) before federal income tax									(42.7)	(5.9)	(89.5)
Reportable segments | Total insurance segments
Revenues from external sources:
Earned premiums, net									1,291.9	1,270.5	1,074.1
Income (loss) before federal income tax									(84.6)	(25.1)	(105.8)
Segment loss before federal income tax:
Income (loss) before federal income tax									(84.6)	(25.1)	(105.8)
Income (loss) before federal income tax									(84.6)	(25.1)	(105.8)
Reportable segments | Net investment income
Revenues from external sources:
Net investment income									74.7	71.7	74.7
Segment loss before federal income tax:
Net investment income									74.7	71.7	74.7
Reportable segments | Net realized capital gains
Revenues from external sources:
Income (loss) before federal income tax									36.5	24.7	20.7
Segment loss before federal income tax:
Income (loss) before federal income tax									36.5	24.7	20.7
Income (loss) before federal income tax									36.5	24.7	20.7
Reportable segments | Total investment operations segment
Revenues from external sources:
Total investment operations segment									111.2	96.4	95.4
Segment loss before federal income tax:
Total investment operations segment									111.2	96.4	95.4
Reportable segments | All other
Revenues from external sources:
Income (loss) before federal income tax									0.4	(0.1)	0
Intersegment revenues									2.3	1.7	3.2
Segment loss before federal income tax:
Income (loss) before federal income tax									0.4	(0.1)	0
Income (loss) before federal income tax									$ 0.4	$ (0.1)	$ 0

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 364.9	$ 352.8	$ 348.5	$ 339.2	$ 346.6	$ 349.7	$ 337.4	$ 334.9	$ 1,405.4	$ 1,368.6	$ 1,172.7
Income (loss) before federal income taxes	33.1	7.4	(25.1)	3.8	0.8	29.9	3.4	33.2
Net income	$ 32.5	$ 10.1	$ (24.6)	$ 3	$ 3.1	$ 20.7	$ 2.7	$ 24.7	$ 21	$ 51.2	$ 107.4
Earnings per common share:
Basic (usd per share)	$ 0.78	$ 0.24	$ (0.59)	$ 0.07	$ 0.08	$ 0.50	$ 0.06	$ 0.60	$ 0.50	$ 1.25	$ 2.63
Diluted (usd per share)	$ 0.77	$ 0.24	$ (0.59)	$ 0.07	$ 0.07	$ 0.50	$ 0.06	$ 0.60	$ 0.50	$ 1.23	$ 2.60

Schedule I - Summary of Inve108

Schedule I - Summary of Investments - Other Than Investments in Related Parties (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	$ 2,444.5
Fair value	2,537.2
Amount at which shown in the balance sheet	2,537.2
Available-for-sale Securities, Amortized Cost Basis	2,444.5	$ 2,295
Available-for-sale Securities	2,537.2	$ 2,396.4
Equity securities: | Available-for-sale:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	323.1
Fair value	382.8
Amount at which shown in the balance sheet	382.8
Equity securities: | Large-cap securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	108.9
Fair value	139
Amount at which shown in the balance sheet	139
Equity securities: | Small-cap securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	57.2
Fair value	79.1
Amount at which shown in the balance sheet	79.1
Equity securities: | Mutual and exchange traded funds [Domain] [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	157
Fair value	164.7
Amount at which shown in the balance sheet	164.7
Other Invested Assets [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	25.5
Fair value	45.1
Amount at which shown in the balance sheet	45.1
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	5.4
Fair value	5.4
Amount at which shown in the balance sheet	5.4
Other than investments in related parties, cost [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,449.9
Other than investments in related parties, fair value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair value	2,542.6
Other than investments in related parties, carrying amount [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amount at which shown in the balance sheet	2,542.6
Available-for-sale: | Fixed maturities:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,095.9
Fair value	2,109.3
Amount at which shown in the balance sheet	2,109.3
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	401.9
Fair value	404.7
Amount at which shown in the balance sheet	404.7
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	634.6
Fair value	643.7
Amount at which shown in the balance sheet	643.7
Available-for-sale: | Fixed maturities: | Corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	445.7
Fair value	449.6
Amount at which shown in the balance sheet	449.6
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	613.7
Fair value	611.3
Amount at which shown in the balance sheet	$ 611.3

Schedule II - Condensed Fina109

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Assets
Fixed maturities, available-for-sale, at fair value	$ 2,109.3	$ 2,000.7
Other invested assets	5.4	5.3
Cash and cash equivalents	51.1	58.1
Other assets	14.9	11.9
Federal income tax, net	102.1	102.5
Total assets	2,959.4	2,828.2
Liabilities and Stockholders’ Equity
Notes payable (affiliates $116.5 and $116.5, respectively)	122.1	100.5
Due to affiliate	2.4	0
Other liabilities (affiliated net assumed $11.0 and affiliated net ceded $8.4, respectively)	69.8	69.8
Total liabilities	2,068.1	1,943.6
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	121.6	120.4
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.5)	(116.3)
Additional paid-in capital	159.9	153.5
Accumulated other comprehensive income (affiliated net ceded $53.7 and $56.7, respectively)	32.5	37.6	$ 71.7	$ 80.8
Retained earnings	693.8	689.4
Stockholders’ equity per accompanying consolidated financial statements	891.3	884.6	872.9
Total liabilities and stockholders' equity	2,959.4	2,828.2
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)	983.6	972.5
Fixed maturities, available-for-sale, at fair value	0.9	0.9
Other invested assets	3.5	3
Cash and cash equivalents	8.1	10.5	$ 11.5	$ 6.1
Other assets	0.2	0
Federal income tax, net	21.5	21.9
Total assets	1,017.8	1,008.8
Liabilities and Stockholders’ Equity
Notes payable (affiliates $116.5 and $116.5, respectively)	116.5	116.5
Due to affiliate	1.3	0.6
Other liabilities (affiliated net assumed $11.0 and affiliated net ceded $8.4, respectively)	8.7	7.1
Total liabilities	126.5	124.2
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	121.6	120.4
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.5)	(116.3)
Additional paid-in capital	159.9	153.5
Accumulated other comprehensive income (affiliated net ceded $53.7 and $56.7, respectively)	32.5	37.6
Retained earnings	693.8	689.4
Stockholders’ equity per accompanying consolidated financial statements	891.3	884.6
Total liabilities and stockholders' equity	1,017.8	1,008.8
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 Fina110

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Parenthetical) (Detail) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Notes payable, affiliates	$ 15.2	$ 15.2
Common Stock, No Par Value	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	48,600,000	48,100,000
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	$ 116.5	$ 116.5
Common Stock, No Par Value	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	48,600,000	48,100,000
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 Fina111

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net investment income									$ 74.7	$ 71.7	$ 74.7
Net realized gain on investments									36.5	24.3	20.7
Total revenues	$ 364.9	$ 352.8	$ 348.5	$ 339.2	$ 346.6	$ 349.7	$ 337.4	$ 334.9	1,405.4	1,368.6	1,172.7
Interest expense (affiliates $6.0, $6.0 and $3.2, respectively)									5.5	5.4	5.4
Other operating expenses									7.9	6.3	7.3
Total expenses									1,386.2	1,301.3	1,145.9
Income before federal income taxes									19.2	67.3	26.8	$ 98.7
Federal income tax (benefit) expense									1.8	(16.1)	80.6
Net Income (Loss) Attributable to Parent									(3)	(4.1)	(5.3)
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net investment income									0.2	0.3	0.3
Total revenues									0.2	0.3	0.3
Interest expense (affiliates $6.0, $6.0 and $3.2, respectively)									6.1	6	6
Other operating expenses									7	4.9	5.6
Total expenses									13.1	10.9	11.6
Income before federal income taxes									(12.9)	(10.6)	(11.3)
Federal income tax (benefit) expense									(6)	(2.8)	(20.6)
Net (loss) income before equity in net income of subsidiaries									(6.9)	(7.8)	9.3
Equity in net income (loss) of subsidiaries									27.9	59	98.1
Net Income (Loss) Attributable to Parent									$ 21	$ 51.2	$ 107.4

Schedule II - Condensed Fina112

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Income (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
State Auto Financial
Condensed Income Statements, Captions [Line Items]
Interest Expense Debt To Affiliates, Parent Company	$ 6.1	$ 6	$ 6

Schedule II - Condensed Fina113

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	$ (3)	$ (4.1)	$ (5.3)
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	27.8	(39.2)	59.8
Reclassification adjustments for gains realized in net income	(36.5)	(24.7)	(20.7)
Income tax expense	3	22.4	(13.7)
Total change in net unrealized holding gains (losses) on investments	(5.7)	(41.5)	25.4
Other comprehensive loss	(5.1)	(34.1)	(9.1)
Comprehensive Income (loss)	15.9	17.1	98.3
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	21	51.2	107.4
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	0.2	(0.1)	(0.2)
Reclassification adjustments for gains realized in net income	0	0	0
Income tax expense	(0.1)	0	0.1
Total change in net unrealized holding gains (losses) on investments	0.1	(0.1)	(0.1)
Unrealized equity in subsidiaries	(5.2)	(34)	(9)
Other comprehensive loss	(5.1)	(34.1)	(9.1)
Comprehensive Income (loss)	$ 15.9	$ 17.1	$ 98.3

Schedule II - Condensed Fina114

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent		$ (3)	$ (4.1)	$ (5.3)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net		14.6	15.7	11.7
Share-based compensation		3.7	4.5	3.6
Net realized gain on investments		(36.5)	(24.3)	(20.7)
Changes in operating assets and liabilities:
Other liabilities and due from affiliates		7.3	(39.9)	(50.8)
Other assets		1.6	(4.5)	11.1
Excess tax expense on share-based awards		(0.2)	(0.3)	0
Federal income taxes, net		4.5	13.7	(81.6)
Net cash provided by operating activities		113.5	149.8	75.6
Cash flows from investing activities:
Purchases of equity securities - available-for-sale		(143.1)	(154)	(119)
Purchases of other invested assets		(1.5)	(6.9)	(1.8)
Maturities, calls and pay downs of fixed maturities - available-for-sale		240.9	241	232.4
Sales of fixed maturities - available-for-sale		211.6	180.7	163.5
Sales of equity securities - available-for-sale		153	144.8	99.2
Net cash used in investing activities		(127.6)	(167.7)	(56.5)
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0.5	2.2	6.2	3.5
Payments to acquire treasury stock		(0.2)	(0.3)	(0.1)
Payment of dividends		(16.6)	(16.5)	(16.5)
Excess tax expense on share-based awards		0.2	0.3	0
Proceeds from long-term debt		21.5	0	0
Net cash provided by (used in) financing activities		7.1	(10.3)	(13.1)
Net (decrease) increase in cash and cash equivalents		(7)	(28.2)	6
Cash and cash equivalents at beginning of year		58.1
Cash and cash equivalents at end of year		51.1	58.1
Supplemental disclosures:
Federal income tax received		0	6.4	1
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)		5.4	5.3	5.2
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent		21	51.2	107.4
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net		(0.3)	0	0
Share-based compensation		1.2	(0.3)	0.9
Equity in net income from consolidated subsidiaries		(27.9)	(59)	(98.1)
Changes in operating assets and liabilities:
Other liabilities and due from affiliates		0.9	0.9	0.5
Other assets		(0.1)	0	0
Excess tax expense on share-based awards		(0.2)	(0.5)	0
Federal income taxes, net		3.3	(0.7)	(16.6)
Net cash provided by operating activities		(1.9)	(8.4)	(5.9)
Cash flows from investing activities:
Dividends received from consolidated subsidiaries		14.2	18	24
Purchases of other invested assets		(0.3)	(0.3)	(0.4)
Maturities, calls and pay downs of fixed maturities - available-for-sale		0	0	0.5
Sales of fixed maturities - available-for-sale		0	0	0.3
Net cash used in investing activities		13.9	17.7	24.4
Cash flows from financing activities:
Proceeds from issuance of common stock		2.2	6.2	3.5
Payments to acquire treasury stock		(0.2)	(0.3)	(0.1)
Payment of dividends		(16.6)	(16.5)	(16.5)
Excess tax expense on share-based awards		0.2	0.3	0
Net cash provided by (used in) financing activities		(14.4)	(10.3)	(13.1)
Net (decrease) increase in cash and cash equivalents		(2.4)	(1)	5.4
Cash and cash equivalents at beginning of year		10.5	11.5	6.1
Cash and cash equivalents at end of year		8.1	10.5	11.5
Supplemental disclosures:
Federal income tax received		0	2.3	4.3
Interest paid (affiliates $6.0, $3.2 and $0.7, respectively)		$ (6.1)	$ (6)	$ (6)

Schedule II - Condensed Fina115

Schedule II - Condensed Financial Information of Registrant - Condensed Statements of Cash Flows (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$ 0.8	$ 0.7	$ 0.7
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$ 6.1	$ 6	$ 6

Schedule II - Condensed Fina116

Schedule II - Condensed Financial Information of Registrant - Additional Information (Detail)	Jul. 11, 2013USD ($)Agreementsubsidiary	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
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		102,500,000	100,100,000
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		87,100,000	85,100,000
Notes Payable | Milbank
Condensed Financial Statements, Captions [Line Items]
Debt instrument, issued	$ 15,000,000	15,000,000	15,000,000
Fair Value		$ 15,400,000	$ 15,000,000

Schedule III - Supplementary117

Schedule III - Supplementary Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	$ 129.8	$ 129.1	$ 126.5
Future benefits, claims and losses	1,178	1,047.1	973.6
Unearned premiums	617.8	616.3	612.4
Other policy claims and benefits payable	0	0	0
Premium revenue	1,291.9	1,270.5	1,074.1
Net investment income	74.7	71.7	74.7
Benefits, losses and settlement expenses	944.1	864.3	774.5
Amortization of deferred policy acquisition costs	290.3	283	221.8
Other operating expenses	140.1	143.8	140.1
Premiums written	1,293.3	1,273.5	1,194.2
Personal insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	47.5	47.6	48.9
Future benefits, claims and losses	251.1	227	201.9
Unearned premiums	262.5	261.9	270.7
Other policy claims and benefits payable	0	0	0
Premium revenue	582.2	591.8	451.4
Net investment income	0	0	0
Benefits, losses and settlement expenses	425.5	403.6	318.3
Amortization of deferred policy acquisition costs	104	105.4	75.4
Other operating expenses	67.1	63.6	52
Premiums written	582.8	583	532.1
Business insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	43.7	46	44.5
Future benefits, claims and losses	605.9	553.6	503.4
Unearned premiums	230.4	245.5	241.1
Other policy claims and benefits payable	0	0	0
Premium revenue	468.9	476	459.9
Net investment income	0	0	0
Benefits, losses and settlement expenses	327.9	331.3	280
Amortization of deferred policy acquisition costs	103.4	104.9	88
Other operating expenses	63.5	74.6	74.5
Premiums written	453.8	479.5	473.1
Specialty insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	38.6	35.5	33.1
Future benefits, claims and losses	321	266.5	268.3
Unearned premiums	124.8	108.9	100.6
Other policy claims and benefits payable	0	0	0
Premium revenue	240.8	202.7	162.8
Net investment income	0	0	0
Benefits, losses and settlement expenses	190.7	129.4	176.2
Amortization of deferred policy acquisition costs	82.9	72.7	58.4
Other operating expenses	9.5	5.6	13.6
Premiums written	256.7	211	189
Investment operations segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	0	0	0
Future benefits, claims and losses	0	0	0
Unearned premiums	0	0	0
Other policy claims and benefits payable	0	0	0
Premium revenue	0	0	0
Net investment income	74.7	71.7	74.7
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 840.6	$ 863.1	$ 882.6
Ceded to	819.9	832.9	861.7
Assumed from	1,291.9	1,270.5	1,074.1
Net Amount	$ 1,291.9	$ 1,270.5	$ 1,074.1
Percentage of amount assumed to net (percent)	0.40%	0.40%	0.40%
Unaffiliated Companies
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	$ 25.6	$ 34.7	$ 25.3
Assumed from	4.9	4.5	4.4
Affiliated entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	819.9	832.9	861.7
Assumed from	1,291.9	1,270.5	1,074.1
Net Amount	$ 1,291.9	$ 1,270.5	$ 1,074.1

Schedule V - Valuation and Q119

Schedule V - Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Valuation allowance for deferred tax assets:
Balance at beginning of period	$ 0	$ 0	$ 82.6
Deductions	0	0	82.6
Balance at end of period	$ 0	$ 0	$ 0

Average Annual Percentage Payou

Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (Details)	Dec. 31, 2016
Personal Insurance Segments [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	62.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	9.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.10%
Business insurance segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	48.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	22.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	3.00%
Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	25.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	11.90%
Long-tail [Domain] | Business insurance segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	17.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	16.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	10.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	8.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Long-tail [Domain] | Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	5.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	14.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	10.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	6.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.20%