State Auto Financial (STFC) 10-K 28 Feb 18 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 23, 2018	Jun. 30, 2017
Document Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
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			$ 416,265,622
Entity Common Stock, Shares Outstanding		42,668,535

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $2,173.1 and $2,095.9, respectively)	$ 2,192.8	$ 2,109.3
Equity securities, available-for-sale, at fair value (cost $318.6 and $323.1, respectively)	365.3	382.8
Other invested assets, available-for-sale, at fair value (cost $25.8 and $25.5, respectively)	56	45.1
Other invested assets	5.6	5.4
Notes receivable from affiliate	70	70
Total investments	2,689.7	2,612.6
Cash and cash equivalents	91.5	51.1
Accrued investment income and other assets	36.5	40
Deferred policy acquisition costs (affiliated net assumed $64.7 and $50.7, respectively)	117.8	129.8
Reinsurance recoverable on losses and loss expenses payable	3.1	3.6
Prepaid reinsurance premiums	6.4	6.1
Current federal income taxes	4.8	6.7
Net deferred federal income taxes	57.2	102.1
Property and equipment, at cost (net of accumulated depreciation of $6.8 and $6.6, respectively)	7.3	7.4
Total assets	3,014.3	2,959.4
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $711.4 and $630.9, respectively)	1,255.6	1,181.6
Unearned premiums (affiliated net assumed $187.9 and $220.9, respectively)	611.8	617.8
Notes payable (affiliates $15.2 and $15.2, respectively)	122.1	122.1
Postretirement and pension benefits (affiliated net ceded $34.8 and $40.1, respectively)	64.5	74.4
Due to affiliate	2.7	2.4
Other liabilities (affiliated net assumed $15.5 and $11.0, respectively)	76.7	69.8
Total liabilities	2,133.4	2,068.1
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 49.2 and 48.6 shares issued, respectively, at stated value of $2.50 per share	123	121.6
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.8)	(116.5)
Additional paid-in capital	171.8	159.9
Accumulated other comprehensive income (affiliated net ceded $50.7 and $53.7, respectively)	36.7	32.5
Retained earnings	666.2	693.8
Total stockholders’ equity	880.9	891.3
Total liabilities and stockholders’ equity	3,014.3	2,959.4
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, 2017	Dec. 31, 2016
Fixed maturities, available-for-sale, amortized cost	$ 2,173.1	$ 2,095.9
Equity securities, available-for-sale, cost	318.6	323.1
Other invested assets, available-for-sale, cost	25.8	25.5
Deferred Policy Acquisitions Costs from Affiliates	64.7	50.7
Property and equipment, accumulated depreciation	6.8	6.6
Losses And Loss Expenses Payable From Affiliates	711.4	630.9
Unearned premiums, affiliates	187.9	220.9
Net Postretirement and Pension Benefits	(34.8)	(40.1)
Net Other Liabilities from Affiliates	15.5	11
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)	49,200,000	48,600,000
Common Stock, Par or Stated Value Per Share	$ 2.50	$ 2.50
Net Accumulated Other Comprehensive Income	$ (50.7)	$ (53.7)
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Earned premiums (affiliated net assumed $458.0, $472.0 and $437.6, respectively)	$ 1,275.1	$ 1,291.9	$ 1,270.5
Net investment income (affiliates $4.9, $4.9 and $4.9, respectively)	78.8	74.7	71.7
Net realized gain on investments:
Total other-than-temporary impairment losses	(3.5)	(6.8)	(7.9)
Portion of loss recognized in other comprehensive income	0	0	0
Other net realized investment gains	68.6	43.3	32.2
Total net realized gain on investments	65.1	36.5	24.3
Other income (affiliates $2.3, $2.3 and $2.1, respectively)	2.3	2.3	2.1
Total revenues	1,421.3	1,405.4	1,368.6
Losses and loss expenses (affiliated net assumed $379.6, $383.8 and $293.3, respectively)	918.3	942.4	862.8
Acquisition and operating expenses (affiliated net assumed $270.7, $285.8 and $248.6, respectively)	455.8	430.4	426.8
Interest expense (affiliates $0.8, $0.8 and $0.7, respectively)	5.9	5.5	5.4
Other expenses	7.9	7.9	6.3
Total expenses	1,387.9	1,386.2	1,301.3
Income before federal income taxes	33.4	19.2	67.3
Current	0.4	(1.7)	2.9
Deferred	43.7	(0.1)	13.2
Total federal income tax expense (benefit)	44.1	(1.8)	16.1
Net (loss) income	$ (10.7)	$ 21	$ 51.2
(Loss) earnings per common share:
Basic (usd per share)	$ (0.25)	$ 0.50	$ 1.25
Diluted (usd per share)	(0.25)	0.50	1.23
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Earned premiums, net	$ 458	$ 472	$ 437.6
Net investment income, affiliates	4.9	4.9	4.9
Other income, affiliates	2.3	2.3	2.1
Losses and loss expenses, net affiliates	379.6	383.8	293.3
Net Acquisition and Operating Expenses	270.7	285.8	248.6
Interest Expense Debt To Affiliates	$ 0.8	$ 0.8	$ 0.7

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (10.7)	$ 21	$ 51.2
Net change in unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during year	69	27.8	(39.2)
Reclassification adjustments for gains realized in net income	(65.1)	(36.5)	(24.7)
Income tax (expense) benefit	(0.7)	3	22.4
Total change in net unrealized holding gains (losses) on investments	3.2	(5.7)	(41.5)
Net unrecognized benefit plan obligations:
Net actuarial (loss) gain arising during period	(1)	(3)	5.3
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(5.5)	(5.5)	(5.4)
Net actuarial gain	8	9.4	11.5
Income tax expense	(0.5)	(0.3)	(4)
Total net unrecognized benefit plan obligations	1	0.6	7.4
Other comprehensive income (loss)	4.2	(5.1)	(34.1)
Comprehensive (loss) income	$ (6.5)	$ 15.9	$ 17.1

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, 2014		47.7	6.8
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
Balance at beginning of year at Dec. 31, 2014		$ 119.3	$ (116)	$ 143.2	$ 71.7	$ 654.7
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 award exercises and vested restricted shares			(0.3)
Stock awards 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 (loss) income	51.2					51.2
Cash dividends paid (affiliates $10.4, $10.4 and $10.4, 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 award exercises and vested restricted shares			$ (0.2)
Stock awards 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 (loss) income	21					21
Cash dividends paid (affiliates $10.4, $10.4 and $10.4, respectively)	(16.6)
Balance at end of year at Dec. 31, 2016	891.3	$ 121.6	$ (116.5)	159.9	32.5	693.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		0.6
Balance at end of year (shares) at Dec. 31, 2017		49.2	6.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 1.4		8.8
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0
Shares acquired on stock award exercises and vested restricted shares			$ (0.3)
Stock awards granted				3.1
Change in unrealized holding gains (losses) on investments, net of tax	3.2				3.2
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	1				1
Net (loss) income	(10.7)					(10.7)
Cash dividends paid (affiliates $10.4, $10.4 and $10.4, respectively)	(16.9)
Balance at end of year at Dec. 31, 2017	$ 880.9	$ 123	$ (116.8)	$ 171.8	$ 36.7	$ 666.2

Consolidated Statements of Sto8

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retained earnings
Cash dividends paid affiliates	$ 10.4	$ 10.4	$ 10.4

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net (loss) income	$ (10.7)	$ 21	$ 51.2
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization, net	12.5	14.6	15.7
Share-based compensation	4.2	3.7	4.5
Net realized gain on investments	(65.1)	(36.5)	(24.3)
Changes in operating assets and liabilities:
Deferred policy acquisition benefits	12	(0.7)	(2.6)
Accrued investment income and other assets	3.5	(4.5)	(2.4)
Postretirement and pension benefits	(9)	(28.8)	(6)
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	0.2	3	3
Other liabilities and due to/from affiliates, net	6	7.3	(39.9)
Losses and loss expenses payable	74	128.6	69.8
Unearned premiums	(6)	1.5	3.9
Excess tax expense on share-based awards	0	(0.2)	(0.3)
Federal income taxes	46.3	4.5	13.7
Cash provided from December 31, 2014 unearned premium transfer related to the homeowners quota-share reinsurance arrangement	0	0	63.5
Net cash provided by operating activities	67.9	113.5	149.8
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(505.4)	(589.3)	(573.9)
Purchases of equity securities available-for-sale	(185.9)	(143.1)	(154)
Purchases of other invested assets	(1.4)	(1.5)	(6.9)
Maturities, calls and pay downs of fixed maturities available-for-sale	233.6	240.9	241
Sales of fixed maturities available-for-sale	184.8	211.6	180.7
Sales of equity securities available-for-sale	252.7	153	144.8
Sales of other invested assets available-for-sale	1.1	0.8	0.7
Net disposals of property and equipment	0	0	(0.1)
Net cash used in investing activities	(20.5)	(127.6)	(167.7)
Cash flows from financing activities:
Proceeds from issuance of common stock	10.2	2.2	6.2
Payments to acquire treasury stock	(0.3)	(0.2)	(0.3)
Payments of dividends (affiliates $10.4, $10.4 and $10.4, respectively)	(16.9)	(16.6)	(16.5)
Excess tax expense on share-based awards	0	0.2	0.3
Proceeds from long-term debt	0	21.5	0
Net cash (used in) provided by financing activities	(7)	7.1	(10.3)
Net increase (decrease) in cash and cash equivalents	40.4	(7)	(28.2)
Cash and cash equivalents at beginning of year	51.1	58.1	86.3
Cash and cash equivalents at end of year	91.5	51.1	58.1
Supplemental disclosures:
Interest paid (affiliates $0.8, $0.8 and $0.7, respectively)	5.7	5.4	5.3
Proceeds from Income Tax Refunds	$ (1.6)
Federal income tax (refund) paid		$ 0	$ 6.4

Consolidated Statements of Ca10

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Cash Flows [Abstract]
Payments of dividends, affiliates	$ 10.4	$ 10.4	$ 10.4
Interest paid, affiliates	$ 0.8	$ 0.8	$ 0.7

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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 income 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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Balance, beginning of year $ 129.8 $ 129.1 $ 126.5 Acquisition costs deferred 245.7 291.0 285.6 Acquisition costs amortized to expense (257.7 ) (290.3 ) (283.0 ) Balance, end of year $ 117.8 $ 129.8 $ 129.1 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 $25.5 million and $27.0 million at December 31, 2017 and 2016 , 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 (loss) 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 (loss) income includes net (loss) income and other comprehensive income (loss). Other comprehensive income (loss) 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 Employee Share-Based Payment Accounting The amendments in this guidance simplify 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 for fiscal years beginning after December 15, 2016, including interim periods within those fiscal years. The Company adopted this guidance prospectively at January 1, 2017, and prior periods were not adjusted. For the year ended December 31, 2017 , the Company recognized $1.5 million of income tax expense as a result of adopting this guidance. There were no other material impacts to the Company’s results of operations or consolidated financial position as a result of adopting this guidance. 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. The Company completed its contract analysis during the third quarter of 2017 and noted that 0.1% of revenue, primarily affiliated investment management fee revenue, is subject to the new revenue recognition guidance. Revenue will not change under the new guidance; thus, upon adoption, the Company does not expect any impact to its' 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. The Company will recognize $60.7 million of net unrealized gains (net of tax) as a cumulative effect adjustment that will increase retained earnings as of January 1, 2018 and decrease accumulated other comprehensive income (“AOCI”) by a corresponding amount. There are no other significant impacts anticipated to the Company’s results of operations, consolidated financial position or cash flows. 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. 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 In August 2016, the FASB issued guidance that addresses eight specific cash flow issues with the objective of reducing existing diversity in practice. The new guidance is effective beginning January 1, 2018 and it is not expected to have a material impact on the Company's cash flows. Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued guidance that addresses certain stranded income tax effects in AOCI resulting from the Tax Cuts and Jobs Act of 2017 (“TCJA”). Current guidance requires the effect of a change in tax laws or rates on deferred tax balances to be reported in income from continuing operations in the accounting period that includes the period of enactment, even if the related income tax effects were originally charged or credited directly to AOCI. The amount of the reclassification would include the effect of the change in the U.S. federal corporate income tax rate on the gross deferred tax amounts and related valuation allowances, if any, at the date of the enactment of TCJA related to items in AOCI. The updated guidance is effective for reporting periods beginning after December 15, 2018 and is to be applied retrospectively to each period in which the effect of the TCJA related to items remaining in AOCI are recognized or at the beginning of the period of adoption. Early adoption is permitted. The Company expects to adopt the updated guidance effective January 1, 2018. The adoption of this guidance will not affect the Company's results of operations, consolidated financial position, or liquidity. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost

Investments

Investments	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 433.8 $ 9.3 $ (6.2 ) $ 436.9 Obligations of states and political subdivisions 507.1 19.1 (0.4 ) 525.8 Corporate securities 527.5 4.5 (2.3 ) 529.7 U.S. government agencies mortgage-backed securities 704.7 7.1 (11.4 ) 700.4 Total fixed maturities 2,173.1 40.0 (20.3 ) 2,192.8 Equity securities: Large-cap securities 62.4 35.1 (0.7 ) 96.8 Mutual and exchange traded funds 256.2 21.6 (9.3 ) 268.5 Total equity securities 318.6 56.7 (10.0 ) 365.3 Other invested assets 25.8 30.2 — 56.0 Total available-for-sale securities $ 2,517.5 $ 126.9 $ (30.3 ) $ 2,614.1 ($ 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 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, 2017 and 2016 : ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2017 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 $ 102.4 $ (0.6 ) 18 $ 196.1 $ (5.6 ) 22 $ 298.5 $ (6.2 ) 40 Obligations of states and political subdivisions 58.6 (0.4 ) 10 — — — 58.6 (0.4 ) 10 Corporate securities 153.2 (1.3 ) 23 67.3 (1.0 ) 10 220.5 (2.3 ) 33 U.S. government agencies mortgage-backed securities 188.6 (2.9 ) 31 252.2 (8.5 ) 41 440.8 (11.4 ) 72 Total fixed maturities 502.8 (5.2 ) 82 515.6 (15.1 ) 73 1,018.4 (20.3 ) 155 Large-cap equity securities 4.4 (0.7 ) 4 — — — 4.4 (0.7 ) 4 Mutual and exchange traded funds 66.9 (9.3 ) 1 — — — 66.9 (9.3 ) 1 Total equity securities 71.3 (10.0 ) 5 — — — 71.3 (10.0 ) 5 Total temporarily impaired securities $ 574.1 $ (15.2 ) 87 $ 515.6 $ (15.1 ) 73 $ 1,089.7 $ (30.3 ) 160 ($ 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 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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Equity securities: Large-cap securities $ (1.5 ) $ (0.6 ) $ (2.2 ) Small-cap securities (2.0 ) (3.9 ) (5.7 ) Fixed maturities — (2.3 ) — Total other-than-temporary impairments $ (3.5 ) $ (6.8 ) $ (7.9 ) The Company reviewed its investments at December 31, 2017 , 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, 2017 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 5.9 $ 5.9 Due after 1 year through 5 years 543.8 545.0 Due after 5 years through 10 years 419.1 420.3 Due after 10 years 499.6 521.2 U.S. government agencies mortgage-backed securities 704.7 700.4 Total $ 2,173.1 $ 2,192.8 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, 2017 , 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 $9.3 million and $9.2 million were on deposit with insurance regulators as required by law at December 31, 2017 and 2016 , respectively. The following table sets forth the components of net investment income for the years ended December 31, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Fixed maturities $ 63.2 $ 63.4 $ 61.3 Equity securities 10.5 7.3 6.5 Cash and cash equivalents, and other 6.3 5.6 5.9 Investment income 80.0 76.3 73.7 Investment expenses 1.2 1.6 2.0 Net investment income $ 78.8 $ 74.7 $ 71.7 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 2017 , 2016 and 2015 were $438.6 million , $365.4 million and $326.2 million , respectively. The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the years ended December 31, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Realized gains: Fixed maturities $ 2.8 $ 2.8 $ 4.6 Equity securities 66.7 29.2 29.6 Other invested assets 0.2 12.1 0.2 Total realized gains 69.7 44.1 34.4 Realized losses: Equity securities: Sales (1.1 ) (0.8 ) (1.8 ) OTTI (3.5 ) (4.5 ) (7.9 ) Fixed maturities: OTTI — (2.3 ) — Total realized losses (4.6 ) (7.6 ) (9.7 ) Net realized gains on investments $ 65.1 $ 36.5 $ 24.7 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ 6.3 $ (14.4 ) $ (32.8 ) Equity securities (13.0 ) 14.3 (29.5 ) Other invested assets 10.6 (8.6 ) (1.6 ) Deferred federal income tax (0.7 ) 3.0 22.4 Change in unrealized holding gains (losses), net of tax $ 3.2 $ (5.7 ) $ (41.5 ) 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, 2017
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, 2017 and 2016 , 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, 2017 and 2016 . 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 held one fixed maturity corporate security included in Level 3 and estimated its fair value using the present value of the future cash flows. This security was sold during the second quarter of 2017. 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 $45.2 million and $35.7 million at December 31, 2017 and 2016 , 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 $43.7 million . In accordance with Accounting Standard Codification 820-10, since the investments are measured at fair value using the net asset value per share practical expedient they have not been classified in the fair value hierarchy. Fair values presented here are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets. The remainder of the Company’s other invested assets consist of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds, based on an observable market price for an identical asset in an active market, reflect their fair values and consequently these securities have been disclosed in Level 1. The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at December 31, 2017 and 2016 : ($ 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, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 436.9 $ — $ 436.9 $ — Obligations of states and political subdivisions 525.8 — 525.8 — Corporate securities 529.7 — 529.7 — U.S. government agencies mortgage-backed securities 700.4 — 700.4 — Total fixed maturities 2,192.8 — 2,192.8 — Equity securities: Large-cap securities 96.8 96.8 — — Small-cap securities — — — — Mutual and exchange traded funds 268.5 268.5 — — Total equity securities 365.3 365.3 — — Other invested assets 10.8 10.8 — — Total available-for-sale investments $ 2,568.9 $ 376.1 $ 2,192.8 $ — ($ 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 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 2017 and 2016 , separately for each major category of assets. This security was sold during the second quarter of 2017. ($ millions) Fixed maturities Balance at January 1, 2017 $ 3.5 Total realized gains (losses)—included in earnings 1.4 Total unrealized gains (losses)—included in other comprehensive income — Purchases 0.1 Sales (5.0 ) Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2017 $ — ($ 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 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, 2017 was $5.1 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, 2017 December 31, 2016 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 72.6 7.00 % $ 70.0 $ 75.7 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, 2017 December 31, 2016 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 $ 20.9 1.73 % $ 21.5 $ 21.0 1.73 % FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest 85.4 85.7 5.03 % 85.4 85.6 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 5.68 % 15.2 15.2 5.13 % Total notes payable $ 122.1 $ 121.8 $ 122.1 $ 121.8

Losses and Loss Expenses Payabl

Losses and Loss Expenses Payable	12 Months Ended
	Dec. 31, 2017
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) 2017 2016 2015 Losses and loss expenses payable, at beginning of year $ 1,181.6 $ 1,053.0 $ 983.2 Less: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Net balance at beginning of year 1,178.0 1,047.1 973.6 Incurred related to: Current year 964.9 915.4 852.8 Prior years (46.6 ) 27.0 10.0 Total incurred 918.3 942.4 862.8 Paid related to: Current year 445.2 417.8 421.5 Prior years 398.6 393.7 367.8 Total paid 843.8 811.5 789.3 Net balance at end of year 1,252.5 1,178.0 1,047.1 Plus: reinsurance recoverable on losses and loss expenses payable 3.1 3.6 5.9 Losses and loss expenses payable, at end of year (affiliates $711.4, $630.9, and $532.4, respectively) $ 1,255.6 $ 1,181.6 $ 1,053.0 The Company recorded favorable development related to prior years’ loss and loss expense reserves in 2017 of $46.6 million , compared to adverse development in 2016 and 2015 of $27.0 million and $10.0 million , respectively. Unallocated loss adjustment expenses and catastrophe reserves contributed $3.2 million and $2.0 million , respectively, of favorable development in 2017 . Favorable development of prior accident years' non-catastrophe loss and ALAE reserves was primarily driven by $44.0 million of favorable development in the commercial insurance segment. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The personal insurance segment non-catastrophe loss and ALAE reserves accounted for $1.6 million of the favorable development in 2017 . Personal auto contributed $4.4 million of favorable development primarily driven by lower than anticipated severity emerging from accident years 2015 and 2016. Slightly offsetting the favorable development was adverse development in homeowners and other personal of $1.5 million and $1.3 million , respectively. Homeowners adverse development was primarily from accident year 2016. Adverse development in other personal was driven by higher than anticipated severity emerging from accident years 2015 and 2016. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $4.2 million of adverse development, primarily driven by E&S property with adverse development of $3.0 million . E&S property adverse development was driven by higher than anticipated severity for liability coverages on business in run-off. Unallocated loss adjustment expenses contributed $3.7 million of the adverse development and catastrophe reserves contributed $1.4 million of the favorable development in 2016 . The personal insurance segment non-catastrophe loss and ALAE reserves accounted for $9.0 million of the adverse development, 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 commercial insurance segment non-catastrophe loss and ALAE reserves contributed $5.1 million of favorable development, driven by workers’ compensation, other commercial, and farm & ranch of $4.2 million , $1.8 million , and $1.7 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 $3.6 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 insurance segment contributed $10.5 million of the adverse development, driven by personal auto which developed unfavorably by $11.1 million , primarily due to higher than anticipated bodily injury severity from the prior two accident years. The commercial insurance segment contributed $0.7 million of favorable development, driven by favorable development in workers’ compensation and middle market commercial of $5.1 million and $4.3 million , respectively. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The favorable development was offset by commercial auto which developed unfavorably by $9.8 million , due to higher than anticipated bodily injury severity from the prior two 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 $9.6 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 $5.2 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 the Company’s 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 ultimate settlement of the claims. The tables below have been restated to conform to the December 31, 2017 segment presentation. See Note 16 for discussion of the Company’s reportable segments. The personal insurance segment generally only consists of short-tail business, whereas the commercial and specialty insurance segments have both short and long-tail business. The Company determined that these 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, 2017 For the Years Ended December 31, Accident Year 2013* (1) 2014* (1) 2015* 2016 2017 IBNR+ Reported Claims (2) 2013 $ 291.3 $ 286.1 $ 290.3 $ 290.0 $ 289.8 $ 1.5 190,310 2014 273.2 274.9 277.3 276.0 2.7 172,678 2015 339.0 345.5 341.0 7.4 152,698 2016 368.5 370.0 22.8 150,364 2017 372.4 54.0 140,258 Total $ 1,649.2 * Supplementary information and unaudited (1) Allocated claim adjustment expenses, net of reinsurance, for accident years 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 2013* (1) 2014* (1) 2015* 2016 2017 2013 $ 178.1 $ 239.5 $ 267.6 $ 279.4 $ 286.2 2014 174.2 228.4 254.3 266.3 2015 216.5 290.8 318.0 2016 231.9 316.2 2017 236.8 Total $ 1,423.5 All outstanding liabilities before 2013, net of reinsurance $ 3.7 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 229.4 * Supplementary information and unaudited (1) Cumulative paid claims for accident years 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. Commercial Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by lower than expected emergence during 2017 in the commercial auto, small commercial package and middle market commercial products. The unanticipated emergence in these products was driven by lower than expected severity from (i) accident years 2013 and 2014 in commercial auto, (ii) accident years 2013 - 2016 in small commercial package, and (iii) accident years 2014 - 2016 in middle market commercial. These accident years account for approximately 80% of the change in incurred losses and loss adjustment expenses. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2013 $ 214.6 $ 211.9 $ 217.6 $ 217.6 $ 213.9 $ 10.4 39,984 2014 227.0 231.3 233.7 226.3 15.3 38,657 2015 239.4 239.9 232.5 28.4 37,556 2016 237.0 227.6 47.6 34,088 2017 236.8 69.5 28,605 Total $ 1,137.1 * Supplementary information and unaudited (1) Commercial insurance segment - short-tail is an aggregation of the commercial auto, small commercial package, middle market commercial and farm & ranch product lines. Commercial auto reported claims are counted by coverage and small commercial package, middle market commercial and farm & ranch 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 2013* 2014* 2015* 2016 2017 2013 $ 89.9 $ 146.1 $ 168.9 $ 185.4 $ 196.4 2014 103.0 151.9 180.8 195.9 2015 101.5 148.3 169.6 2016 99.2 147.3 2017 104.9 Total $ 814.1 All outstanding liabilities before 2013, net of reinsurance $ 37.2 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 360.2 * Supplementary information and unaudited Commercial Insurance Segment - Long-Tail The change in incurred losses and loss adjustment expenses for prior accident years was due to lower than expected emergence during 2017 in the workers’ compensation product. The unanticipated emergence in the workers’ compensation product was driven by lower than expected severity from accident years 2015 and 2016. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2008 $ 46.6 $ 45.2 $ 42.7 $ 43.6 $ 43.7 $ 43.4 $ 43.3 $ 43.3 $ 43.0 $ 42.4 $ 2.2 4,461 2009 47.3 48.1 46.8 46.7 46.3 46.0 45.8 45.5 45.0 2.4 4,889 2010 38.8 41.3 41.6 40.4 40.3 40.4 40.8 40.2 2.9 5,257 2011 44.6 47.4 45.5 44.8 44.8 44.9 43.9 4.2 12,220 2012 49.7 45.8 43.7 44.1 43.9 42.9 5.0 12,754 2013 48.7 45.4 44.1 43.4 42.9 6.9 11,305 2014 51.6 50.0 47.1 45.7 9.5 11,563 2015 59.7 59.4 56.5 14.3 11,118 2016 62.6 60.6 22.3 12,362 2017 59.0 29.3 11,728 Total $ 479.1 * Supplementary information and unaudited (1) Commercial insurance segment-long-tail consists of the workers' compensation product. 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 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 2008 $ 10.7 $ 26.2 $ 29.9 $ 32.8 $ 34.8 $ 35.8 $ 37.0 $ 37.4 $ 38.0 $ 38.2 2009 11.9 26.4 33.2 37.2 38.7 40.0 40.4 41.1 41.4 2010 9.0 21.6 28.0 31.4 33.2 34.6 35.4 35.8 2011 12.4 25.4 31.9 35.0 36.9 37.7 38.1 2012 12.6 23.5 29.8 32.7 34.7 35.8 2013 12.3 23.3 29.0 32.3 33.7 2014 12.4 24.4 30.0 32.8 2015 13.9 28.2 35.7 2016 12.6 26.8 2017 13.2 $ 331.4 All outstanding liabilities before 2008, net of reinsurance $ 33.1 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 180.8 * Supplementary information and unaudited Specialty Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was attributable to greater than expected emergence during 2017 in E&S property. The unanticipated emergence in E&S property was driven by higher than expected severity from accident years 2013 - 2016 for the liability portion of our Florida package business. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2013 $ 70.9 $ 82.9 $ 85.0 $ 90.9 $ 93.8 $ 2.8 7,689 2014 55.7 56.9 58.5 60.9 4.4 7,805 2015 75.5 85.4 87.8 10.8 7,955 2016 99.3 99.5 28.8 6,811 2017 134.2 48.5 7,917 Total $ 476.2 * 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 2013* 2014* 2015* 2016 2017 2013 $ 18.2 $ 37.7 $ 55.5 $ 72.0 $ 83.1 2014 22.0 34.9 44.0 49.3 2015 24.0 48.3 62.8 2016 25.2 50.2 2017 39.0 Total $ 284.4 All outstanding liabilities before 2012, net of reinsurance $ 18.8 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 210.6 * Supplementary information and unaudited Specialty Insurance Segment - Long-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2008 $ 6.4 $ 6.7 $ 6.7 $ 6.8 $ 6.1 $ 6.0 $ 5.8 $ 6.0 $ 4.9 $ 4.8 $ 1.3 1,652 2009 10.3 11.0 10.0 10.4 11.0 11.0 10.6 12.4 12.9 2.5 1,504 2010 11.2 10.9 9.9 9.7 9.8 9.9 10.3 10.1 1.8 1,224 2011 11.9 10.7 10.7 10.2 10.1 10.9 10.5 1.8 953 2012 16.7 16.7 15.4 17.6 17.4 16.5 1.9 860 2013 20.1 18.9 17.9 16.5 15.9 3.1 1,055 2014 23.3 25.2 27.1 28.3 6.8 1,265 2015 38.6 44.9 44.8 14.7 2,001 2016 60.8 62.9 34.7 4,348 2017 71.6 58.7 4,819 Total $ 278.2 * 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 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 2008 $ 0.1 $ 0.3 $ 0.8 $ 1.3 $ 1.7 $ 2.1 $ 2.4 $ 2.8 $ 3.0 $ 3.3 2009 0.2 2.4 4.9 7.2 8.6 9.4 9.2 10.0 10.4 2010 0.8 2.2 5.2 6.5 7.7 7.6 8.1 8.2 2011 0.9 3.0 5.2 6.6 7.1 7.9 8.1 2012 0.9 4.0 7.3 10.3 12.5 14.0 2013 1.1 3.7 6.7 9.0 10.5 2014 1.2 7.2 12.5 16.7 2015 2.7 10.9 20.6 2016 6.0 15.7 2017 5.5 $ 113.0 All outstanding liabilities before 2008, net of reinsurance $ 2.7 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 167.9 * 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) 2017 2016 2015 Net losses and allocated loss adjustment expenses payable: Personal Insurance Segment Short-tail $ 229.4 $ 229.6 $ 208.3 Other personal 13.2 10.8 12.8 Commercial Insurance Segment Short-tail 360.2 226.4 332.4 Long-tail 180.8 314.2 158.8 Other commercial 26.4 24.6 24.1 Specialty Insurance Segment Short-tail 210.6 178.9 165.8 Long-tail 167.9 129.7 91.1 Net losses and loss expenses payable 1,188.5 1,114.2 993.3 ULAE 64.0 63.8 53.8 Reinsurance recoverable on losses and loss expenses payable 3.1 3.6 5.9 Total losses and loss expenses payable $ 1,255.6 $ 1,181.6 $ 1,053.0 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 62.9 % 21.4 % 9.0 % 4.2 % 2.3 % N/A N/A N/A N/A N/A Commercial Insurance: Short-tail 43.8 % 22.3 % 10.9 % 7.2 % 5.1 % N/A N/A N/A N/A N/A Long-tail 25.5 % 28.4 % 13.5 % 7.4 % 4.1 % 2.6 % 1.7 % 1.1 % 1.1 % 0.4 % Specialty Insurance: Short-tail 27.5 % 23.7 % 16.8 % 13.1 % 11.9 % N/A N/A N/A N/A N/A Long-tail 6.0 % 16.0 % 20.1 % 14.6 % 9.6 % 6.1 % 2.7 % 5.9 % 3.3 % 5.9 % * Supplementary information and unaudited

Short-Duration Insurance Contra

Short-Duration Insurance Contracts Short-Duration Insurance Contracts	12 Months Ended
	Dec. 31, 2017
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) 2017 2016 2015 Losses and loss expenses payable, at beginning of year $ 1,181.6 $ 1,053.0 $ 983.2 Less: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Net balance at beginning of year 1,178.0 1,047.1 973.6 Incurred related to: Current year 964.9 915.4 852.8 Prior years (46.6 ) 27.0 10.0 Total incurred 918.3 942.4 862.8 Paid related to: Current year 445.2 417.8 421.5 Prior years 398.6 393.7 367.8 Total paid 843.8 811.5 789.3 Net balance at end of year 1,252.5 1,178.0 1,047.1 Plus: reinsurance recoverable on losses and loss expenses payable 3.1 3.6 5.9 Losses and loss expenses payable, at end of year (affiliates $711.4, $630.9, and $532.4, respectively) $ 1,255.6 $ 1,181.6 $ 1,053.0 The Company recorded favorable development related to prior years’ loss and loss expense reserves in 2017 of $46.6 million , compared to adverse development in 2016 and 2015 of $27.0 million and $10.0 million , respectively. Unallocated loss adjustment expenses and catastrophe reserves contributed $3.2 million and $2.0 million , respectively, of favorable development in 2017 . Favorable development of prior accident years' non-catastrophe loss and ALAE reserves was primarily driven by $44.0 million of favorable development in the commercial insurance segment. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The personal insurance segment non-catastrophe loss and ALAE reserves accounted for $1.6 million of the favorable development in 2017 . Personal auto contributed $4.4 million of favorable development primarily driven by lower than anticipated severity emerging from accident years 2015 and 2016. Slightly offsetting the favorable development was adverse development in homeowners and other personal of $1.5 million and $1.3 million , respectively. Homeowners adverse development was primarily from accident year 2016. Adverse development in other personal was driven by higher than anticipated severity emerging from accident years 2015 and 2016. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $4.2 million of adverse development, primarily driven by E&S property with adverse development of $3.0 million . E&S property adverse development was driven by higher than anticipated severity for liability coverages on business in run-off. Unallocated loss adjustment expenses contributed $3.7 million of the adverse development and catastrophe reserves contributed $1.4 million of the favorable development in 2016 . The personal insurance segment non-catastrophe loss and ALAE reserves accounted for $9.0 million of the adverse development, 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 commercial insurance segment non-catastrophe loss and ALAE reserves contributed $5.1 million of favorable development, driven by workers’ compensation, other commercial, and farm & ranch of $4.2 million , $1.8 million , and $1.7 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 $3.6 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 insurance segment contributed $10.5 million of the adverse development, driven by personal auto which developed unfavorably by $11.1 million , primarily due to higher than anticipated bodily injury severity from the prior two accident years. The commercial insurance segment contributed $0.7 million of favorable development, driven by favorable development in workers’ compensation and middle market commercial of $5.1 million and $4.3 million , respectively. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. The favorable development was offset by commercial auto which developed unfavorably by $9.8 million , due to higher than anticipated bodily injury severity from the prior two 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 $9.6 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 $5.2 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 the Company’s 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 ultimate settlement of the claims. The tables below have been restated to conform to the December 31, 2017 segment presentation. See Note 16 for discussion of the Company’s reportable segments. The personal insurance segment generally only consists of short-tail business, whereas the commercial and specialty insurance segments have both short and long-tail business. The Company determined that these 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, 2017 For the Years Ended December 31, Accident Year 2013* (1) 2014* (1) 2015* 2016 2017 IBNR+ Reported Claims (2) 2013 $ 291.3 $ 286.1 $ 290.3 $ 290.0 $ 289.8 $ 1.5 190,310 2014 273.2 274.9 277.3 276.0 2.7 172,678 2015 339.0 345.5 341.0 7.4 152,698 2016 368.5 370.0 22.8 150,364 2017 372.4 54.0 140,258 Total $ 1,649.2 * Supplementary information and unaudited (1) Allocated claim adjustment expenses, net of reinsurance, for accident years 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 2013* (1) 2014* (1) 2015* 2016 2017 2013 $ 178.1 $ 239.5 $ 267.6 $ 279.4 $ 286.2 2014 174.2 228.4 254.3 266.3 2015 216.5 290.8 318.0 2016 231.9 316.2 2017 236.8 Total $ 1,423.5 All outstanding liabilities before 2013, net of reinsurance $ 3.7 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 229.4 * Supplementary information and unaudited (1) Cumulative paid claims for accident years 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. Commercial Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by lower than expected emergence during 2017 in the commercial auto, small commercial package and middle market commercial products. The unanticipated emergence in these products was driven by lower than expected severity from (i) accident years 2013 and 2014 in commercial auto, (ii) accident years 2013 - 2016 in small commercial package, and (iii) accident years 2014 - 2016 in middle market commercial. These accident years account for approximately 80% of the change in incurred losses and loss adjustment expenses. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2013 $ 214.6 $ 211.9 $ 217.6 $ 217.6 $ 213.9 $ 10.4 39,984 2014 227.0 231.3 233.7 226.3 15.3 38,657 2015 239.4 239.9 232.5 28.4 37,556 2016 237.0 227.6 47.6 34,088 2017 236.8 69.5 28,605 Total $ 1,137.1 * Supplementary information and unaudited (1) Commercial insurance segment - short-tail is an aggregation of the commercial auto, small commercial package, middle market commercial and farm & ranch product lines. Commercial auto reported claims are counted by coverage and small commercial package, middle market commercial and farm & ranch 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 2013* 2014* 2015* 2016 2017 2013 $ 89.9 $ 146.1 $ 168.9 $ 185.4 $ 196.4 2014 103.0 151.9 180.8 195.9 2015 101.5 148.3 169.6 2016 99.2 147.3 2017 104.9 Total $ 814.1 All outstanding liabilities before 2013, net of reinsurance $ 37.2 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 360.2 * Supplementary information and unaudited Commercial Insurance Segment - Long-Tail The change in incurred losses and loss adjustment expenses for prior accident years was due to lower than expected emergence during 2017 in the workers’ compensation product. The unanticipated emergence in the workers’ compensation product was driven by lower than expected severity from accident years 2015 and 2016. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2008 $ 46.6 $ 45.2 $ 42.7 $ 43.6 $ 43.7 $ 43.4 $ 43.3 $ 43.3 $ 43.0 $ 42.4 $ 2.2 4,461 2009 47.3 48.1 46.8 46.7 46.3 46.0 45.8 45.5 45.0 2.4 4,889 2010 38.8 41.3 41.6 40.4 40.3 40.4 40.8 40.2 2.9 5,257 2011 44.6 47.4 45.5 44.8 44.8 44.9 43.9 4.2 12,220 2012 49.7 45.8 43.7 44.1 43.9 42.9 5.0 12,754 2013 48.7 45.4 44.1 43.4 42.9 6.9 11,305 2014 51.6 50.0 47.1 45.7 9.5 11,563 2015 59.7 59.4 56.5 14.3 11,118 2016 62.6 60.6 22.3 12,362 2017 59.0 29.3 11,728 Total $ 479.1 * Supplementary information and unaudited (1) Commercial insurance segment-long-tail consists of the workers' compensation product. 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 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 2008 $ 10.7 $ 26.2 $ 29.9 $ 32.8 $ 34.8 $ 35.8 $ 37.0 $ 37.4 $ 38.0 $ 38.2 2009 11.9 26.4 33.2 37.2 38.7 40.0 40.4 41.1 41.4 2010 9.0 21.6 28.0 31.4 33.2 34.6 35.4 35.8 2011 12.4 25.4 31.9 35.0 36.9 37.7 38.1 2012 12.6 23.5 29.8 32.7 34.7 35.8 2013 12.3 23.3 29.0 32.3 33.7 2014 12.4 24.4 30.0 32.8 2015 13.9 28.2 35.7 2016 12.6 26.8 2017 13.2 $ 331.4 All outstanding liabilities before 2008, net of reinsurance $ 33.1 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 180.8 * Supplementary information and unaudited Specialty Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was attributable to greater than expected emergence during 2017 in E&S property. The unanticipated emergence in E&S property was driven by higher than expected severity from accident years 2013 - 2016 for the liability portion of our Florida package business. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2013 $ 70.9 $ 82.9 $ 85.0 $ 90.9 $ 93.8 $ 2.8 7,689 2014 55.7 56.9 58.5 60.9 4.4 7,805 2015 75.5 85.4 87.8 10.8 7,955 2016 99.3 99.5 28.8 6,811 2017 134.2 48.5 7,917 Total $ 476.2 * 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 2013* 2014* 2015* 2016 2017 2013 $ 18.2 $ 37.7 $ 55.5 $ 72.0 $ 83.1 2014 22.0 34.9 44.0 49.3 2015 24.0 48.3 62.8 2016 25.2 50.2 2017 39.0 Total $ 284.4 All outstanding liabilities before 2012, net of reinsurance $ 18.8 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 210.6 * Supplementary information and unaudited Specialty Insurance Segment - Long-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2008 $ 6.4 $ 6.7 $ 6.7 $ 6.8 $ 6.1 $ 6.0 $ 5.8 $ 6.0 $ 4.9 $ 4.8 $ 1.3 1,652 2009 10.3 11.0 10.0 10.4 11.0 11.0 10.6 12.4 12.9 2.5 1,504 2010 11.2 10.9 9.9 9.7 9.8 9.9 10.3 10.1 1.8 1,224 2011 11.9 10.7 10.7 10.2 10.1 10.9 10.5 1.8 953 2012 16.7 16.7 15.4 17.6 17.4 16.5 1.9 860 2013 20.1 18.9 17.9 16.5 15.9 3.1 1,055 2014 23.3 25.2 27.1 28.3 6.8 1,265 2015 38.6 44.9 44.8 14.7 2,001 2016 60.8 62.9 34.7 4,348 2017 71.6 58.7 4,819 Total $ 278.2 * 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 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 2008 $ 0.1 $ 0.3 $ 0.8 $ 1.3 $ 1.7 $ 2.1 $ 2.4 $ 2.8 $ 3.0 $ 3.3 2009 0.2 2.4 4.9 7.2 8.6 9.4 9.2 10.0 10.4 2010 0.8 2.2 5.2 6.5 7.7 7.6 8.1 8.2 2011 0.9 3.0 5.2 6.6 7.1 7.9 8.1 2012 0.9 4.0 7.3 10.3 12.5 14.0 2013 1.1 3.7 6.7 9.0 10.5 2014 1.2 7.2 12.5 16.7 2015 2.7 10.9 20.6 2016 6.0 15.7 2017 5.5 $ 113.0 All outstanding liabilities before 2008, net of reinsurance $ 2.7 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 167.9 * 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) 2017 2016 2015 Net losses and allocated loss adjustment expenses payable: Personal Insurance Segment Short-tail $ 229.4 $ 229.6 $ 208.3 Other personal 13.2 10.8 12.8 Commercial Insurance Segment Short-tail 360.2 226.4 332.4 Long-tail 180.8 314.2 158.8 Other commercial 26.4 24.6 24.1 Specialty Insurance Segment Short-tail 210.6 178.9 165.8 Long-tail 167.9 129.7 91.1 Net losses and loss expenses payable 1,188.5 1,114.2 993.3 ULAE 64.0 63.8 53.8 Reinsurance recoverable on losses and loss expenses payable 3.1 3.6 5.9 Total losses and loss expenses payable $ 1,255.6 $ 1,181.6 $ 1,053.0 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 62.9 % 21.4 % 9.0 % 4.2 % 2.3 % N/A N/A N/A N/A N/A Commercial Insurance: Short-tail 43.8 % 22.3 % 10.9 % 7.2 % 5.1 % N/A N/A N/A N/A N/A Long-tail 25.5 % 28.4 % 13.5 % 7.4 % 4.1 % 2.6 % 1.7 % 1.1 % 1.1 % 0.4 % Specialty Insurance: Short-tail 27.5 % 23.7 % 16.8 % 13.1 % 11.9 % N/A N/A N/A N/A N/A Long-tail 6.0 % 16.0 % 20.1 % 14.6 % 9.6 % 6.1 % 2.7 % 5.9 % 3.3 % 5.9 % * Supplementary information and unaudited

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
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, 2017, 2016 and 2015, the Company recognized profit commission of $0.7 million , $1.9 million , and $4.2 million , respectively, reflected as a reduction in acquisition and operating expenses on the consolidated statements of income. The amount of ceding commission earned was limited to the amount of deferred acquisition costs that would have been deferred if not for entering in the arrangement. The following table sets forth the effect of the Company’s external reinsurance on its balance sheets at December 31, 2017 and 2016 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2017 2016 Losses and loss expenses payable: Direct $ 535.0 $ 545.7 Assumed 9.2 5.0 Ceded (3.1 ) (3.6 ) Net losses and loss expenses payable $ 541.1 $ 547.1 Unearned premiums: Direct $ 402.5 $ 395.6 Assumed 21.4 1.3 Ceded (6.4 ) (6.1 ) Net unearned premiums $ 417.5 $ 390.8 The following table sets forth the effect of the Company’s external reinsurance on its income statements for the years ended December 31, 2017 , 2016 and 2015 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2017 2016 2015 Written premiums: Direct $ 835.6 $ 835.4 $ 854.1 Assumed 31.1 5.0 4.7 Ceded (22.9 ) (24.9 ) (35.4 ) Net written premiums $ 843.8 $ 815.5 $ 823.4 Earned premiums: Direct $ 828.7 $ 840.6 $ 863.1 Assumed 11.0 4.9 4.5 Ceded (22.6 ) (25.6 ) (34.7 ) Net earned premiums $ 817.1 $ 819.9 $ 832.9 Losses and loss expenses incurred: Direct $ 530.9 $ 559.1 $ 569.0 Assumed 10.4 3.9 3.4 Ceded (2.5 ) (4.4 ) (2.9 ) Net losses and loss expenses incurred $ 538.8 $ 558.6 $ 569.5

Transactions with Affiliates

Transactions with Affiliates	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
Transactions with Affiliates	Transactions with Affiliates a. Reinsurance The insurance subsidiaries of State Auto Financial, including State Auto P&C, Milbank and SA 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”). 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, 2017 and 2016 : ($ millions) 2017 2016 Assets Deferred policy acquisition costs: Ceded $ (53.1 ) $ (79.1 ) Assumed 117.8 129.8 Net assumed $ 64.7 $ 50.7 Liabilities and Stockholders’ Equity Losses and loss expenses payable: Ceded $ (541.1 ) $ (547.1 ) Assumed 1,252.5 1,178.0 Net assumed $ 711.4 $ 630.9 Unearned premiums: Ceded $ (417.5 ) $ (390.8 ) Assumed 605.4 611.7 Net assumed $ 187.9 $ 220.9 Pension and postretirement benefits: Ceded $ (99.3 ) $ (114.5 ) Assumed 64.5 74.4 Net ceded $ (34.8 ) $ (40.1 ) Other liabilities: Ceded $ (43.0 ) $ (42.9 ) Assumed 58.5 53.9 Net assumed $ 15.5 $ 11.0 Stockholders’ Equity Accumulated other comprehensive income: Ceded $ (144.7 ) $ (153.5 ) Assumed 94.0 99.8 Net ceded $ (50.7 ) $ (53.7 ) 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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Written premiums: Ceded $ (843.8 ) $ (815.5 ) $ (823.4 ) Assumed 1,269.3 1,293.3 1,273.5 Net assumed $ 425.5 $ 477.8 $ 450.1 Earned premiums: Ceded $ (817.1 ) $ (819.9 ) $ (832.9 ) Assumed 1,275.1 1,291.9 1,270.5 Net assumed $ 458.0 $ 472.0 $ 437.6 Losses and loss expenses incurred: Ceded $ (540.4 ) $ (560.3 ) $ (570.9 ) Assumed 920.0 944.1 864.2 Net assumed $ 379.6 $ 383.8 $ 293.3 Acquisition and operating expenses: Ceded (189.7 ) (148.5 ) (183.9 ) Assumed 460.4 434.3 432.5 Net assumed $ 270.7 $ 285.8 $ 248.6 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, 2017 and 2016 is approximately $323.1 million and $310.5 million , respectively. b. Notes Payable In May 2003, State Auto Financial formed a Delaware business trust (the “Capital Trust”) to issue $15.0 million of mandatorily redeemable preferred capital securities to a third party and $0.5 million of common securities to State Auto Financial (the capital and common securities are collectively referred to as the “Trust Securities”). The Capital Trust loaned $15.5 million , the proceeds from the issuance of its Trust Securities, to State Auto Financial in the form of Floating Rate Junior Subordinated Debt Securities due in 2033 (the “Subordinated Debentures”). The Subordinated Debentures and interest accrued thereon are the Capital Trust’s only assets. Interest on the Trust Securities is payable quarterly at a rate equal to the three-month LIBOR rate plus 4.20% adjusted quarterly (total 5.68% at December 31, 2017 ). 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, 2017 , 2016 and 2015 , 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 $2.0 million , $1.9 million and $1.8 million in 2017 , 2016 and 2015

Notes Payable and Credit Facili

Notes Payable and Credit Facility	12 Months Ended
	Dec. 31, 2017
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, 2017

Federal Income Taxes

Federal Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal Income Taxes	Federal Income Taxes On December 22, 2017, the United States enacted the TCJA which, among other changes, reduced the U.S. federal tax rate from 35.0% to 21.0% beginning on January 1, 2018. The estimated effects of enactment of TCJA are reflected in the net deferred tax asset that is reported on the Company's balance sheet at December 31, 2017. 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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Amount at statutory rate $ 11.7 35.0 % $ 6.7 35.0 % $ 23.5 35.0 % Tax-exempt interest and dividends received deduction (5.7 ) (17.1 ) (7.1 ) (37.2 ) (8.7 ) (13.0 ) Other, net 1.7 5.4 (1.4 ) (7.2 ) 1.3 1.9 Federal income tax expense (benefit) 7.7 23.3 (1.8) (9.4 )% 16.1 23.9 % Impact of TCJA at enactment 36.4 108.7 — — — — Federal income tax expense (benefit) $ 44.1 132.0 % $ (1.8 ) (9.4 )% $ 16.1 23.9 % Total income tax expense for 2017 included a provisional net charge of $36.4 million to reflect the change in tax laws and tax rates included in TCJA at the date of enactment, resulting primarily from revaluing the Company's deferred tax assets and liabilities. In computing taxable income, property and casualty insurers are required to discount their unpaid loss reserves. TCJA changes the prescribed interest rates to rates based on corporate bond yield curves, requires the use of IRS-prescribed claims payment patterns and extends the applicable time periods for the claims payment patterns. These changes are effective for tax years beginning after 2017 and are subject to a transition rule that spreads the additional tax payment from the amount determined by applying these changes over the subsequent eight years beginning in 2018. The pre-tax provisional change in discounted loss reserves attributable to the TCJA changes to loss reserve discounting is determined to be $50.9 million . As a result of changes to the IRS Code, deferred tax assets were increased by that amount, with an offsetting deferred tax liability. The deferred tax liability will be amortized over a period of eight years beginning in 2018. This item is a taxable temporary difference and has no direct impact on total tax expense for 2017 and future years. The provisional amount will be adjusted once the IRS publishes the official discount factors that should be used. 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 $1.6 million , respectively. The following table sets forth the tax effects of temporary differences that give rise to significant portions of deferred tax assets and deferred tax liabilities at December 31, 2017 and 2016 : ($ millions) 2017 2016 Deferred tax assets: Unearned premiums not currently deductible $ 25.5 $ 42.6 Losses and loss expenses payable discounting 22.0 19.7 Postretirement and pension benefits 13.6 26.1 Realized loss on other-than-temporary impairment 2.1 5.8 Other liabilities 9.2 16.6 Net operating loss carryforward 34.3 58.9 Tax credit carryforward 3.7 3.3 Other 2.5 7.0 Total deferred tax assets 112.9 180.0 Deferred tax liabilities: Deferral of policy acquisition costs 24.7 45.4 Net unrealized holding gains on investments 20.3 32.5 Losses and loss expenses payable discounting (transition rule) 10.7 — Total deferred tax liabilities 55.7 77.9 Net deferred federal income taxes $ 57.2 $ 102.1 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. At December 31, 2017 , the tax benefit of the net operating loss (“NOL”) carryforward was $34.3 million . The NOL carryforwards do not begin to expire until 2030 and will not fully expire until 2036. At December 31, 2017 , the Company carried no balance for uncertain tax positions. The Company had no accrual for the payment of interest and penalties at December 31, 2017 or 2016 .

Pension and Postretirement Bene

Pension and Postretirement Benefit Plans	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan [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, 2017 and 2016 : ($ millions) Pension Postretirement 2017 2016 2017 2016 Change in benefit obligation: Benefit obligation at beginning of year $ 284.5 $ 281.3 $ 18.8 $ 20.3 Service cost 5.6 6.1 — — Interest cost 11.1 11.6 0.7 0.8 Actuarial loss (gain) 17.0 4.5 0.6 (0.9 ) Benefits paid (14.0 ) (19.0 ) (1.8 ) (1.4 ) The Company’s portion of benefit obligation at end of year $ 304.2 $ 284.5 $ 18.3 $ 18.8 Change in plan assets available for plan benefits: Fair value of plan assets available for plan benefits at beginning of year $ 235.7 $ 204.4 $ — $ — Employer contribution 9.8 34.5 — — Actual return on plan assets 33.5 15.8 — — Benefits paid (14.0 ) (19.0 ) — — The Company’s portion of fair value of plan assets at end of year $ 265.0 $ 235.7 $ — $ — Supplemental executive retirement plan (7.0 ) (6.8 ) — — Funded status at end of year $ (46.2 ) $ (55.6 ) $ (18.3 ) $ (18.8 ) Accumulated benefit obligation end of year $ 287.9 $ 267.5 No assets are expected to be returned during the fiscal year ending December 31, 2018 . 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, 2017 and 2016 : ($ millions) 2017 2016 Prior service benefit $ (48.6 ) $ (54.1 ) Net actuarial loss 111.6 118.5 Total $ 63.0 $ 64.4 The following table sets forth the Company’s share of amortization expected to be recognized for the year ending December 31, 2018: ($ millions) 2018 Prior service benefit $ (5.5 ) Net actuarial loss 8.4 Total $ 2.9 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, 2017 , 2016 and 2015 : ($ millions) Pension Postretirement 2017 2016 2015 2017 2016 2015 Components of net periodic cost: Service cost $ 5.7 $ 6.2 $ 7.9 $ — $ — $ — Interest cost 11.4 11.9 11.3 0.8 0.9 1.1 Expected return on plan assets (16.7 ) (15.1 ) (13.8 ) — — — Amortization of: Prior service benefit — — — (5.5 ) (5.5 ) (5.4 ) Net actuarial loss 7.8 9.2 10.9 0.2 0.2 0.6 Net periodic cost (benefit) $ 8.2 $ 12.2 $ 16.3 $ (4.5 ) $ (4.4 ) $ (3.7 ) 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 2018 $ 11.2 $ 1.5 2019 11.5 1.5 2020 12.0 1.4 2021 12.5 1.4 2022 12.9 1.3 2023-2027 76.2 5.9 The postretirement plan’s gross benefit payments for 2017 were $1.8 million , including the prescription drug benefits. The postretirement plan’s subsidy related to Medicare Prescription Drug Improvement and Modernization Act of 2003 was $0.2 million for 2017 and estimates future annual subsidies to be approximately $0.1 million . The following table sets forth the weighted average assumptions used to determine the benefit plans’ obligations at December 31, 2017 and 2016 : Pension Postretirement 2017 2016 2017 2016 Benefit obligations weighted-average assumptions: Discount rate 3.50 % 4.00 % 3.50 % 4.00 % Rates of increase in compensation levels 3.25 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, 2017 , 2016 and 2015 : Pension Postretirement 2017 2016 2015 2017 2016 2015 Weighted-average assumptions: Discount rate 4.00 % 4.20 % 3.85 % 4.00 % 4.20 % 3.85 % 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, 2017 , 2016 and 2015 : Postretirement 2017 2016 2015 Assumed health care cost trend rates: Health care cost trend rate assumed for the next year 6.00 % 6.50 % 6.50 % Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 3.80 % 3.90 % 3.80 % Year that the rate reaches the ultimate trend rate 2075 2075 2076 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, 2017 : ($ 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 1.7 (1.5 ) 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 52.0 % U.S. large-cap equity 24.2 U.S. small-cap equity 10.3 International equity 9.4 Emerging market equity 4.1 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, 2017 and 2016 . Included in the pension plan’s available-for-sale securities are two international funds (“the funds”) that invest in equity securities of foreign issuers and are managed by third party investment managers. The funds had a fair value of $26.6 million and $20.0 million at December 31, 2017 and 2016, respectively, which was determined using the funds’ net asset value. In accordance with Accounting Standard Codification 820-10, since these investments are measured at fair value using the net asset value per share practical expedient these have not been classified in the table below. At December 31, 2016, the Company held an international fund (classified as available-for-sale) that was managed by third party investment managers. During the fourth quarter of 2017, the Company redeemed its investment in the fund. At December 31, 2016, the fund had a fair value of $11.3 million . The following tables set forth the Company’s share of pension plan’s available-for-sale securities within the fair value hierarchy at December 31, 2017 and 2016 : ($ 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, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 70.1 $ — $ 70.1 $ — Corporate securities 60.7 — 60.7 — U.S. government agencies mortgage-backed securities 4.4 — 4.4 — Total fixed maturities 135.2 — 135.2 — Equity securities: Large-cap securities 71.6 71.6 — — Mutual and exchange traded funds 25.1 25.1 — — Total equity securities 96.7 96.7 — — Total pension plan investments $ 231.9 $ 96.7 $ 135.2 $ — ($ 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 $ — 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 2018 is approximately $10.0 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 $6.4 million , $6.3 million and $5.1 million for 2017 , 2016 and 2015

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at January 1, 2017 $ 62.8 $ (30.3 ) $ 32.5 Other comprehensive income before reclassifications 45.5 (1.0 ) 44.5 Amounts reclassified from AOCI (a) (42.3 ) 2.0 (40.3 ) Net current period other comprehensive income 3.2 1.0 4.2 Ending balance at December 31, 2017 $ 66.0 $ (29.3 ) $ 36.7 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 (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, 2017 , 2016 and 2015 : ($ millions) Details about Accumulated Other December 31 Affected line item in the Condensed Comprehensive Income Components 2017 2016 2015 Consolidated Statements of Income Unrealized gains on available for sale securities $ 65.1 $ 36.5 $ 24.7 Realized gain on sale of securities 65.1 36.5 24.7 Total before tax (22.8 ) (12.8 ) (8.6 ) Tax expense 42.3 23.7 16.1 Net of tax Amortization of benefit plan items: Negative prior service costs 5.5 5.5 5.4 (a) Net loss (8.0 ) (9.4 ) (11.5 ) (a) (2.5 ) (3.9 ) (6.1 ) Total before tax 0.5 0.3 4.0 Tax benefit (2.0 ) (3.6 ) (2.1 ) Net of tax Total reclassifications for the period $ 40.3 $ 20.1 $ 14.0 (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, 2017
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 $85.8 million is available for payment to State Auto Financial from its insurance subsidiaries in 2016 without prior approval. State Auto Financial received dividends from its insurance subsidiaries in the amount of $15.0 million , $10.0 million and $15.0 million in 2017 , 2016 and 2015 , respectively. The Company’s insurance subsidiaries are subject to risk-based capital (“RBC”) requirements that have been adopted by individual states. These requirements subject insurers having statutory capital less than that required by the RBC calculation to varying degrees of regulatory action, depending on the level of capital inadequacy. The RBC formulas specify various weighting factors to be applied to financial balances or various levels of activity based on the perceived degree of risk. Regulatory compliance is determined by a ratio of total adjusted capital to authorized control level RBC. Generally no remedial action is required by an insurance company if its adjusted statutory surplus exceeds 200% of the authorized level RBC. As of December 31, 2017 , each of the Company’s insurance subsidiaries maintained adjusted statutory surplus in excess of 400% of the authorized control level RBC. The following tables set forth reconciliations of statutory capital and surplus and net income, as determined using SAP, to the amounts included in the accompanying consolidated financial statements: ($ millions) 2017 2016 Statutory capital and surplus of insurance subsidiaries $ 858.4 $ 844.4 Net liabilities of non-insurance parent and affiliates (89.8 ) (84.9 ) 768.6 759.5 Increases (decreases): Deferred acquisition costs 117.8 129.8 Postretirement and pension benefits 22.3 22.7 Deferred federal income taxes (61.2 ) (49.1 ) Fixed maturities, at fair value 19.6 13.5 Other, net 13.8 14.9 Stockholders’ equity per accompanying consolidated financial statements $ 880.9 $ 891.3 ($ millions) Year ended December 31 2017 2016 2015 Statutory net income of insurance subsidiaries $ 42.8 $ 22.7 $ 65.4 Net loss of non-insurance parent and affiliates (13.1 ) (3.0 ) (4.1 ) 29.7 19.7 61.3 Increases (decreases): Deferred acquisition costs (12.0 ) 0.7 2.5 Postretirement and pension benefits 4.3 4.4 4.2 Deferred federal income taxes (34.6 ) (5.4 ) (12.2 ) Share-based compensation expense — — (0.1 ) Other, net 1.9 1.6 (4.5 ) Net (loss) income per accompanying consolidated financial statements $ (10.7 ) $ 21.0 $ 51.2

Preferred Stock

Preferred Stock	12 Months Ended
	Dec. 31, 2017
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, 2017
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. Effective May 4, 2017, the share-based compensation plan for key employees is the State Auto Financial Corporation 2017 Long-Term Incentive Plan (the “2017 LTIP”). The 2017 LTIP replaced the State Auto Financial Corporation 2009 Equity Compensation Plan (the “2009 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 performance stock awards, performance unit awards, restricted stock, and other stock based awards 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 Company has reserved 2.4 million common shares under the 2017 LTIP. As of December 31, 2017 , a total of 2.2 million common shares are available for issuance under the 2017 LTIP. The 2017 LTIP provides that (i) the maximum value of performance stock awards or performance unit awards settled in cash that may be granted in any calendar year is equal to the excess of the number of awards multiplied by the fair market value of the company’s common shares on the applicable payment or settlement date of the award multiplied by 5.0 and (ii) the maximum number of common shares subject to awards of performance stock and performance units that may be granted in any calendar year to any one individual is 250,000 shares. The 2017 LTIP automatically terminates on May 4, 2027 . Restricted Stock and Performance Unit Awards Service-based restricted stock awards granted to employees are 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. Awards with both service and performance conditions granted to employees are subject to (i) the Restriction Period, and (ii) the achievement of predetermined performance goals within specified time periods. All performance-based awards include a specified number of units that will vest. The number of performance based awards that are ultimately earned for each grant is dependent upon meeting specified target and performance goals that range from 0% to 500% of the target number of performance units awarded based on the extent to which the Company achieves the performance goals for the performance measures as set forth in a performance matrix established by the Compensation Committee. Generally, service-based and performance-based equity awards are expensed pro-rata over their respective vesting periods based on the market value of the awards at the time of grant. Performance-based equity awards that contain variable vesting criteria are expensed based on management’s expectation of the percentage of the award that will ultimately be earned. These estimates can change periodically throughout the measurement period. The following table set forth the Company’s total activity and related information for employee restricted stock and performance unit award activity for the years ended December 31, 2017 , 2016 and 2015 : 2017 2016 2015 Shares Weighted Shares Weighted Shares Weighted Outstanding, beginning of year 107,297 $ 22.04 111,084 $ 22.19 76,472 $ 19.06 Granted 177,846 27.20 45,252 21.55 74,020 22.83 Vested (26,271 ) 21.39 (33,414 ) 21.93 (35,859 ) 16.88 Forfeited (39,425 ) 25.68 (15,625 ) 21.91 (3,549 ) 21.92 Outstanding, end of year 219,447 $ 25.65 107,297 $ 22.04 111,084 $ 22.19 As of December 31, 2017 , there was $2.3 million of total unrecognized compensation cost related to restricted stock and performance unit awards. The remaining cost is expected to be recognized over a period of 2.25 years. Stock Options In accordance with the terms of the 2009 Equity Plan, stock options were 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. 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. The following tables present the weighted-average assumptions used in the option pricing model for options granted to employees during 2016 and 2015. 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 and 2015 : 2017 2016 2015 Fair value per share N/A (1) $ 6.75 $ 7.69 Expected dividend yield N/A (1) 1.87 % 1.75 % Risk free interest rate N/A (1) 1.30 % 1.60 % Expected volatility factor N/A (1) 36.27 % 36.61 % Expected life in years N/A (1) 5.3 6.0 (1) No stock options granted during 2017 The following table sets forth the Company’s stock option activity and related information for the years ended December 31, 2017 , 2016 and 2015 : (millions, except per share amounts) 2017 2016 2015 Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Outstanding, beginning of year 2.5 $ 20.63 3.1 $ 21.45 3.4 $ 21.37 Granted — — 0.2 21.55 0.3 22.86 Exercised (0.4 ) 19.38 (0.4 ) 17.18 (0.3 ) 16.54 Canceled (0.4 ) 28.96 (0.4 ) 30.70 (0.3 ) 25.63 Outstanding, end of year 1.7 $ 19.22 2.5 $ 20.63 3.1 $ 21.45 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, 2017 , 2016 and 2015 , the total intrinsic value of stock options exercised was $3.5 million , $3.0 million and $2.6 million , respectively. The tax benefit for tax deductions from share-based awards totaled $1.4 million and $0.3 million for the years ended December 31, 2016 and 2015 , respectively. In addition, income taxes for the year ended December 31, 2016 reflected 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, 2017 : (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 1.0 3.6 $ 16.53 1.0 $ 16.53 $20.01 – $30.00 0.7 4.7 23.07 0.6 23.33 1.7 4.0 $ 19.22 1.6 $ 18.98 Aggregate intrinsic value for total options outstanding at December 31, 2017 was $16.1 million . Aggregate intrinsic value for total options exercisable at December 31, 2017 was $17.1 million . 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, 2017 , a total of 3.5 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 26,323 RSUs, 22,832 RSUs, and 26,184 RSUs granted in 2017 , 2016 and 2015 , respectively. During 2017 and 2016 , common shares valued at approximately $106,581 and $753,000 , respectively, were distributed by the Company under the RSU Plan. Share-based compensation expense recognized during 2017 , 2016 and 2015 was $4.2 million , $3.7 million and $4.5 million , respectively. Share-based compensation is recognized as a component of loss and loss adjustment expense and acquisition and operating expense in a manner consistent with other employee compensation. As of December 31, 2017 , there was $2.7 million of total unrecognized compensation cost related to 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, 2017
Earnings Per Share [Abstract]
Net Earnings (Loss) Per Common Share	Net (Loss) Earnings Per Common Share The following table sets forth the compilation of basic and diluted net (loss) earnings per common share for the years ended December 31, 2017 , 2016 and 2015 : (millions, except per share amounts) 2017 2016 2015 Numerator: Net (loss) earnings for basic net earnings per common share $ (10.7 ) $ 21.0 $ 51.2 Adjusted net (loss) earnings for dilutive net (loss) earnings per common share $ (10.7 ) $ 21.0 $ 51.2 Denominator: Weighted average shares for basic net (loss) earnings per common share 42.1 41.6 41.1 Effect of dilutive share-based awards — 0.4 0.5 Adjusted weighted average shares for diluted net (loss) earnings per common share 42.1 42.0 41.6 Basic net (loss) earnings per common share $ (0.25 ) $ 0.50 $ 1.25 Diluted net (loss) earnings per common share $ (0.25 ) $ 0.50 $ 1.23 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, 2017 , 2016 and 2015 : (millions) 2017 2016 2015 Total number of antidilutive options and awards 0.5 1.2 1.5

Reportable Segments

Reportable Segments	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Reportable Segments	Reportable Segments The Company has four reportable segments: personal insurance, commercial 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 commercial insurance segment primarily provides commercial automobile, commercial multi-peril, fire & allied, general liability, workers’ compensation insurance covering small-to-medium sized commercial exposures in the commercial insurance market. In addition, the commercial insurance segment provides farm & ranch insurance. 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 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 investment operations segment, managed by Stateco, provides investment services and is evaluated based on investment returns of assets. Asset information by segment is not reported for the insurance segments because the Company does not produce such information internally. The Company’s reportable insurance segments, and the products within each, are 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 After a review of strategic alternatives for our excess and surplus lines business in the specialty insurance segment, during the second half of 2017 management made the decision to begin exiting the excess and surplus lines business either through a series of renewal right transactions or by placing lines of business into run-off since this business is not core to the Company’s ongoing business strategy. The impact of this decision, along with the 2016 decision to exit programs business, on future results will result in the elimination of the specialty insurance segment and its related underwriting results from the State Auto Group. The following table sets forth financial information regarding the Company’s reportable segments for the years ended December 31, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Revenues from external sources: Insurance segments Personal insurance $ 580.3 $ 578.5 $ 591.3 Commercial insurance 455.7 472.6 476.5 Specialty insurance 239.1 240.8 202.7 Total insurance segments 1,275.1 1,291.9 1,270.5 Investment operations segment Net investment income 78.8 74.7 71.7 Net realized capital gains 65.1 36.5 24.7 Total investment operations segment 143.9 111.2 96.4 Total revenue from reportable segments 1,419.0 1,403.1 1,366.9 All other 2.3 2.3 1.7 Total revenues from external sources 1,421.3 1,405.4 1,368.6 Intersegment revenues 6.0 5.8 5.6 Total revenues 1,427.3 1,411.2 1,374.2 Reconciling items: Eliminate intersegment revenues (6.0 ) (5.8 ) (5.6 ) Total consolidated revenue $ 1,421.3 $ 1,405.4 $ 1,368.6 Segment loss before federal income taxes: Insurance segments: Personal insurance SAP underwriting (loss) gain $ (23.3 ) $ (10.3 ) $ 27.5 Commercial insurance SAP underwriting loss (10.6 ) (31.6 ) (46.7 ) Specialty insurance SAP underwriting loss (57.0 ) (42.7 ) (5.9 ) Total insurance segments (90.9 ) (84.6 ) (25.1 ) Investment operations segment: Net investment income 78.8 74.7 71.7 Net realized capital gains 65.1 36.5 24.7 Total investment operations segment 143.9 111.2 96.4 All other segments income 0.6 0.4 (0.1 ) Reconciling items: GAAP adjustments (10.2 ) 2.0 4.2 Interest expense on corporate debt (5.9 ) (5.5 ) (5.4 ) Corporate expenses (4.1 ) (4.3 ) (2.7 ) Total reconciling items $ (20.2 ) $ (7.8 ) $ (3.9 ) Total consolidated income before federal income taxes $ 33.4 $ 19.2 $ 67.3 The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2017 and 2016 : ($ millions) 2017 2016 Segment assets: Investment operations segment $ 2,781.2 $ 2,663.7 Total segment assets 2,781.2 2,663.7 Reconciling items: Corporate assets 233.1 295.7 Total consolidated assets $ 3,014.3 $ 2,959.4

Quarterly Financial Data (unaud

Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data (unaudited)	Quarterly Financial Data (unaudited) The following tables set forth quarterly financial data for 2017 and 2016 : ($ millions, except per share amounts) 2017 For three months ended March 31 June 30 September 30 December 31 Total revenues $ 344.3 $ 355.7 $ 358.4 $ 362.9 (Loss) income before federal income taxes (5.2 ) 11.9 (11.2 ) 37.9 Net (loss) income (4.1 ) 8.7 (9.5 ) (5.8 ) (Loss) earnings per common share: Basic $ (0.10 ) $ 0.21 $ (0.23 ) $ (0.14 ) Diluted $ (0.10 ) $ 0.21 $ (0.23 ) $ (0.14 ) 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

Contingencies

Contingencies	12 Months Ended
	Dec. 31, 2017
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, 2017
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, 2017 ($ millions) December 31, 2017 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 $ 433.8 $ 436.9 $ 436.9 Obligations of states and political subdivisions 507.1 525.8 525.8 Corporate securities 527.5 529.7 529.7 U.S. government agencies residential mortgage-backed securities 704.7 700.4 700.4 Total fixed maturities 2,173.1 2,192.8 2,192.8 Equity securities: Large-cap securities 62.4 96.8 96.8 Mutual and exchange traded funds 256.2 268.5 268.5 Total equity securities 318.6 365.3 365.3 Other invested assets 25.8 56.0 56.0 Total available-for-sale securities 2,517.5 2,614.1 2,614.1 Other invested assets 5.6 5.6 5.6 Total investments – other than investments in related parties $ 2,523.1 $ 2,619.7 $ 2,619.7 (1) Original cost of equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts.

SCHEDULE II - CONDENSED FINANCI

SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF REGISTRANT	12 Months Ended
	Dec. 31, 2017
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 2017 2016 Assets Investments in common stock of subsidiaries (equity method) $ 979.5 $ 983.6 Fixed maturities, available-for-sale, at fair value — 0.9 Other invested assets 4.2 3.5 Cash and cash equivalents 8.8 8.1 Other assets 0.1 0.2 Federal income tax, net 15.8 21.5 Total assets $ 1,008.4 $ 1,017.8 Liabilities and Stockholders’ Equity Notes payable (affiliates $116.5 and $116.5, respectively) $ 116.5 $ 116.5 Due to affiliates 0.5 1.3 Other liabilities 10.5 8.7 Total liabilities 127.5 126.5 Stockholders’ equity: Class A Preferred stock (nonvoting), without par value. Authorized 2.5 shares; none issued — — Class B Preferred stock, without par value. Authorized 2.5 shares; none issued — — Common stock, without par value. Authorized 100.0 shares; 49.2 and 48.6 shares issued, respectively, at stated value of $2.50 per share 123.0 121.6 Treasury stock, 6.8 and 6.8 shares, respectively, at cost (116.8 ) (116.5 ) Additional paid-in capital 171.8 159.9 Accumulated other comprehensive income 36.7 32.5 Retained earnings 666.2 693.8 Total stockholders’ equity 880.9 891.3 Total liabilities and stockholders’ equity $ 1,008.4 $ 1,017.8 ($ millions) Year ended December 31 2017 2016 2015 Net investment income $ 0.4 $ 0.2 $ 0.3 Net realized gain on investments 0.3 — — Total revenues 0.7 0.2 0.3 Interest expense (affiliates $6.1, $6.1 and $6.0, respectively) 6.1 6.1 6.0 Other operating expenses 6.7 7.0 4.9 Total expenses 12.8 13.1 10.9 Loss before federal income taxes (12.1 ) (12.9 ) (10.6 ) Federal income tax expense (benefit) 5.1 (6.0 ) (2.8 ) Net (loss) income before equity in net income of subsidiaries (17.2 ) (6.9 ) (7.8 ) Equity in net income of subsidiaries 6.5 27.9 59.0 Net (loss) income $ (10.7 ) $ 21.0 $ 51.2 ($ millions, except per share amounts) Year ended December 31 2017 2016 2015 Net (loss) income $ (10.7 ) $ 21.0 $ 51.2 Other comprehensive income (loss), net of tax: Net unrealized holding gains (losses) on investments: Unrealized holding gains (losses) arising during the year 0.3 0.2 (0.1 ) Reclassification adjustments for gains realized in net income 0.3 — — Income tax expense (0.1 ) (0.1 ) — Total net unrealized holding gain (losses) on investments 0.5 0.1 (0.1 ) Unrealized equity in subsidiaries 3.7 (5.2 ) (34.0 ) Other comprehensive income (loss) 4.2 (5.1 ) (34.1 ) Comprehensive (loss) income $ (6.5 ) $ 15.9 $ 17.1 ($ millions) Year ended December 31 2017 2016 2015 Cash flows used in operating activities: Net (loss) income $ (10.7 ) $ 21.0 $ 51.2 Adjustments to reconcile net income to net cash used in operating activities: Depreciation and amortization, net — (0.3 ) — Share-based compensation 1.1 1.2 (0.3 ) Net realized gain on investments (0.3 ) — — Equity in net income from consolidated subsidiaries (6.5 ) (27.9 ) (59.0 ) Changes in operating assets and liabilities: Other liabilities and due from affiliates — 0.9 0.9 Other assets — 0.1 — Excess tax benefits on share-based awards — (0.2 ) (0.5 ) Federal income taxes, net 5.6 3.3 (0.7 ) Net cash used in operating activities (10.8 ) (1.9 ) (8.4 ) Cash flows from investing activities: Dividends received from consolidated subsidiaries 17.7 14.2 18.0 Purchases of other invested assets (0.4 ) (0.3 ) (0.3 ) Maturities, calls and pay downs of fixed maturities – available-for-sale 1.2 — — Net cash provided by investing activities 18.5 13.9 17.7 Cash flows from financing activities: Proceeds from issuance of common stock 10.2 2.2 6.2 Payments to acquire treasury stock (0.3 ) (0.2 ) (0.3 ) Payment of dividends (16.9 ) (16.6 ) (16.5 ) Excess tax benefits on share-based awards — 0.2 0.3 Net cash used in financing activities (7.0 ) (14.4 ) (10.3 ) Net increase (decrease) in cash and cash equivalents 0.7 (2.4 ) (1.0 ) Cash and cash equivalents at beginning of year 8.1 10.5 11.5 Cash and cash equivalents at end of year $ 8.8 $ 8.1 $ 10.5 Supplemental Disclosures: Federal income tax received $ 0.5 $ 3.4 $ 2.3 Interest paid (affiliates $6.1, $6.1 and $6.0, respectively) $ (6.1 ) $ (6.1 ) $ (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, 2017 December 31, 2016 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 $ 15.8 5.28 % $ 15.0 $ 15.4 5.28 % Affiliate note payable with State Auto P&C, issued $85.0, July 2013 with fixed interest 85.0 89.6 5.28 % 85.0 87.1 5.28 % Total notes payable to affiliates $ 100.0 $ 105.4 $ 100.0 $ 102.5

SCHEDULE III - SUPPLEMENTARY IN

SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2017
Supplementary Insurance Information [Abstract]
SCHEDULE III - SUPPLEMENTARY INSURANCE INFORMATION	SCHEDULE III – SUPPLEMENTARY INSURANCE INFORMATION Years Ended December 31, 2017 , 2016 and 2015 ($ 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, 2017: Personal insurance segment $ 50.9 $ 256.1 $ 288.1 $ — $ 580.3 Commercial insurance segment 47.9 604.3 225.6 — 455.7 Specialty insurance segment 19.0 392.1 91.7 — 239.1 Investment operations segment — — — — — Total $ 117.8 $ 1,252.5 $ 605.4 $ — $ 1,275.1 Year ended December 31, 2016: Personal insurance segment $ 47.0 $ 254.5 $ 258.7 $ — $ 578.5 Commercial insurance segment 47.4 602.5 227.9 — 472.6 Specialty insurance segment 35.4 321.0 124.8 — 240.8 Investment operations segment — — — — — Total $ 129.8 $ 1,178.0 $ 611.4 $ — $ 1,291.9 Year ended December 31, 2015: Personal insurance segment $ 47.1 $ 227.0 $ 260.1 $ — $ 591.3 Commercial insurance segment 49.9 553.6 240.1 — 476.5 Specialty insurance segment 32.1 266.5 108.9 — 202.7 Investment operations segment — — — — — Total $ 129.1 $ 1,047.1 $ 609.1 $ — $ 1,270.5 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, 2017: Personal insurance segment $ — $ 416.3 $ 102.1 $ 89.3 $ 609.7 Commercial insurance segment — 284.2 95.7 90.4 453.6 Specialty insurance segment — 219.5 59.9 18.4 206.0 Investment operations segment 78.8 — — — — Total $ 78.8 $ 920.0 $ 257.7 $ 198.1 $ 1,269.3 Year ended December 31, 2016: Personal insurance segment $ — $ 422.0 $ 103.3 $ 62.7 $ 577.2 Commercial insurance segment — 331.5 110.4 61.6 459.4 Specialty insurance segment — 190.6 76.6 15.7 256.7 Investment operations segment 74.7 — — — — Total $ 74.7 $ 944.1 $ 290.3 $ 140.0 $ 1,293.3 Year ended December 31, 2015: Personal insurance segment $ — $ 396.2 $ 104.6 $ 61.2 $ 581.0 Commercial insurance segment — 338.6 108.6 74.2 481.5 Specialty insurance segment — 129.4 69.8 8.4 211.0 Investment operations segment 71.7 — — — — Total $ 71.7 $ 864.2 $ 283.0 $ 143.8 $ 1,273.5 (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, 2017
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract]
SCHEDULE IV - REINSURANCE	SCHEDULE IV – REINSURANCE Years Ended December 31, 2017 , 2016 and 2015 ($ 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, 2017 $ 828.7 $ 22.6 $ 817.1 $ 11.0 $ 1,275.1 $ 1,275.1 0.9 % 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 % (1) These columns include the effect of intercompany pooling. (2) Calculated as earned premiums assumed from outside companies to net amount.

Summary of Significant Accoun33

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
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 income 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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Balance, beginning of year $ 129.8 $ 129.1 $ 126.5 Acquisition costs deferred 245.7 291.0 285.6 Acquisition costs amortized to expense (257.7 ) (290.3 ) (283.0 ) Balance, end of year $ 117.8 $ 129.8 $ 129.1
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 $25.5 million and $27.0 million at December 31, 2017 and 2016
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 (loss) 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 (loss) income includes net (loss) income and other comprehensive income (loss). Other comprehensive income (loss) 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 Accoun34

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of Net Deferred Acquisition Costs	The following table sets forth net deferred acquisition costs for the years ended December 31, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Balance, beginning of year $ 129.8 $ 129.1 $ 126.5 Acquisition costs deferred 245.7 291.0 285.6 Acquisition costs amortized to expense (257.7 ) (290.3 ) (283.0 ) Balance, end of year $ 117.8 $ 129.8 $ 129.1

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 433.8 $ 9.3 $ (6.2 ) $ 436.9 Obligations of states and political subdivisions 507.1 19.1 (0.4 ) 525.8 Corporate securities 527.5 4.5 (2.3 ) 529.7 U.S. government agencies mortgage-backed securities 704.7 7.1 (11.4 ) 700.4 Total fixed maturities 2,173.1 40.0 (20.3 ) 2,192.8 Equity securities: Large-cap securities 62.4 35.1 (0.7 ) 96.8 Mutual and exchange traded funds 256.2 21.6 (9.3 ) 268.5 Total equity securities 318.6 56.7 (10.0 ) 365.3 Other invested assets 25.8 30.2 — 56.0 Total available-for-sale securities $ 2,517.5 $ 126.9 $ (30.3 ) $ 2,614.1 ($ 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
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, 2017 and 2016 : ($ millions, except # of positions) Less than 12 months 12 months or more Total December 31, 2017 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 $ 102.4 $ (0.6 ) 18 $ 196.1 $ (5.6 ) 22 $ 298.5 $ (6.2 ) 40 Obligations of states and political subdivisions 58.6 (0.4 ) 10 — — — 58.6 (0.4 ) 10 Corporate securities 153.2 (1.3 ) 23 67.3 (1.0 ) 10 220.5 (2.3 ) 33 U.S. government agencies mortgage-backed securities 188.6 (2.9 ) 31 252.2 (8.5 ) 41 440.8 (11.4 ) 72 Total fixed maturities 502.8 (5.2 ) 82 515.6 (15.1 ) 73 1,018.4 (20.3 ) 155 Large-cap equity securities 4.4 (0.7 ) 4 — — — 4.4 (0.7 ) 4 Mutual and exchange traded funds 66.9 (9.3 ) 1 — — — 66.9 (9.3 ) 1 Total equity securities 71.3 (10.0 ) 5 — — — 71.3 (10.0 ) 5 Total temporarily impaired securities $ 574.1 $ (15.2 ) 87 $ 515.6 $ (15.1 ) 73 $ 1,089.7 $ (30.3 ) 160 ($ 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
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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Equity securities: Large-cap securities $ (1.5 ) $ (0.6 ) $ (2.2 ) Small-cap securities (2.0 ) (3.9 ) (5.7 ) Fixed maturities — (2.3 ) — Total other-than-temporary impairments $ (3.5 ) $ (6.8 ) $ (7.9 )
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, 2017 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 5.9 $ 5.9 Due after 1 year through 5 years 543.8 545.0 Due after 5 years through 10 years 419.1 420.3 Due after 10 years 499.6 521.2 U.S. government agencies mortgage-backed securities 704.7 700.4 Total $ 2,173.1 $ 2,192.8
Components of Net Investment Income	The following table sets forth the components of net investment income for the years ended December 31, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Fixed maturities $ 63.2 $ 63.4 $ 61.3 Equity securities 10.5 7.3 6.5 Cash and cash equivalents, and other 6.3 5.6 5.9 Investment income 80.0 76.3 73.7 Investment expenses 1.2 1.6 2.0 Net investment income $ 78.8 $ 74.7 $ 71.7
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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Realized gains: Fixed maturities $ 2.8 $ 2.8 $ 4.6 Equity securities 66.7 29.2 29.6 Other invested assets 0.2 12.1 0.2 Total realized gains 69.7 44.1 34.4 Realized losses: Equity securities: Sales (1.1 ) (0.8 ) (1.8 ) OTTI (3.5 ) (4.5 ) (7.9 ) Fixed maturities: OTTI — (2.3 ) — Total realized losses (4.6 ) (7.6 ) (9.7 ) Net realized gains on investments $ 65.1 $ 36.5 $ 24.7 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ 6.3 $ (14.4 ) $ (32.8 ) Equity securities (13.0 ) 14.3 (29.5 ) Other invested assets 10.6 (8.6 ) (1.6 ) Deferred federal income tax (0.7 ) 3.0 22.4 Change in unrealized holding gains (losses), net of tax $ 3.2 $ (5.7 ) $ (41.5 )

Fair Value of Financial Instr36

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : ($ 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, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 436.9 $ — $ 436.9 $ — Obligations of states and political subdivisions 525.8 — 525.8 — Corporate securities 529.7 — 529.7 — U.S. government agencies mortgage-backed securities 700.4 — 700.4 — Total fixed maturities 2,192.8 — 2,192.8 — Equity securities: Large-cap securities 96.8 96.8 — — Small-cap securities — — — — Mutual and exchange traded funds 268.5 268.5 — — Total equity securities 365.3 365.3 — — Other invested assets 10.8 10.8 — — Total available-for-sale investments $ 2,568.9 $ 376.1 $ 2,192.8 $ — ($ 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
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 2017 and 2016 , separately for each major category of assets. This security was sold during the second quarter of 2017. ($ millions) Fixed maturities Balance at January 1, 2017 $ 3.5 Total realized gains (losses)—included in earnings 1.4 Total unrealized gains (losses)—included in other comprehensive income — Purchases 0.1 Sales (5.0 ) Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2017 $ — ($ 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
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, 2017 December 31, 2016 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 72.6 7.00 % $ 70.0 $ 75.7 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, 2017 December 31, 2016 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 $ 20.9 1.73 % $ 21.5 $ 21.0 1.73 % FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest 85.4 85.7 5.03 % 85.4 85.6 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 5.68 % 15.2 15.2 5.13 % Total notes payable $ 122.1 $ 121.8 $ 122.1 $ 121.8
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, 2017 December 31, 2016 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 $ 20.9 1.73 % $ 21.5 $ 21.0 1.73 % FHLB Loan due 2033:, issued $85.0, July 2013 with fixed interest 85.4 85.7 5.03 % 85.4 85.6 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 5.68 % 15.2 15.2 5.13 % Total notes payable $ 122.1 $ 121.8 $ 122.1 $ 121.8

Losses and Loss Expenses Paya37

Losses and Loss Expenses Payable (Tables)	12 Months Ended
	Dec. 31, 2017
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) 2017 2016 2015 Losses and loss expenses payable, at beginning of year $ 1,181.6 $ 1,053.0 $ 983.2 Less: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 9.6 Net balance at beginning of year 1,178.0 1,047.1 973.6 Incurred related to: Current year 964.9 915.4 852.8 Prior years (46.6 ) 27.0 10.0 Total incurred 918.3 942.4 862.8 Paid related to: Current year 445.2 417.8 421.5 Prior years 398.6 393.7 367.8 Total paid 843.8 811.5 789.3 Net balance at end of year 1,252.5 1,178.0 1,047.1 Plus: reinsurance recoverable on losses and loss expenses payable 3.1 3.6 5.9 Losses and loss expenses payable, at end of year (affiliates $711.4, $630.9, and $532.4, respectively) $ 1,255.6 $ 1,181.6 $ 1,053.0

Short-Duration Insurance Cont38

Short-Duration Insurance Contracts Short-Duration Insurance Contracts (Tables)	12 Months Ended
	Dec. 31, 2017
Incurred Losses and Loss Expenses Payable, Net of Reinsurance [Abstract]
Short-duration Insurance Contracts, Claims Development [Table Text Block]	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, 2017 For the Years Ended December 31, Accident Year 2013* (1) 2014* (1) 2015* 2016 2017 IBNR+ Reported Claims (2) 2013 $ 291.3 $ 286.1 $ 290.3 $ 290.0 $ 289.8 $ 1.5 190,310 2014 273.2 274.9 277.3 276.0 2.7 172,678 2015 339.0 345.5 341.0 7.4 152,698 2016 368.5 370.0 22.8 150,364 2017 372.4 54.0 140,258 Total $ 1,649.2 * Supplementary information and unaudited (1) Allocated claim adjustment expenses, net of reinsurance, for accident years 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 2013* (1) 2014* (1) 2015* 2016 2017 2013 $ 178.1 $ 239.5 $ 267.6 $ 279.4 $ 286.2 2014 174.2 228.4 254.3 266.3 2015 216.5 290.8 318.0 2016 231.9 316.2 2017 236.8 Total $ 1,423.5 All outstanding liabilities before 2013, net of reinsurance $ 3.7 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 229.4 * Supplementary information and unaudited (1) Cumulative paid claims for accident years 2013 and 2014 were impacted by the HO QS Arrangement. See Note 6 for a more detailed discussion of the HO QS Arrangement. Commercial Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was caused by lower than expected emergence during 2017 in the commercial auto, small commercial package and middle market commercial products. The unanticipated emergence in these products was driven by lower than expected severity from (i) accident years 2013 and 2014 in commercial auto, (ii) accident years 2013 - 2016 in small commercial package, and (iii) accident years 2014 - 2016 in middle market commercial. These accident years account for approximately 80% of the change in incurred losses and loss adjustment expenses. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2013 $ 214.6 $ 211.9 $ 217.6 $ 217.6 $ 213.9 $ 10.4 39,984 2014 227.0 231.3 233.7 226.3 15.3 38,657 2015 239.4 239.9 232.5 28.4 37,556 2016 237.0 227.6 47.6 34,088 2017 236.8 69.5 28,605 Total $ 1,137.1 * Supplementary information and unaudited (1) Commercial insurance segment - short-tail is an aggregation of the commercial auto, small commercial package, middle market commercial and farm & ranch product lines. Commercial auto reported claims are counted by coverage and small commercial package, middle market commercial and farm & ranch 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 2013* 2014* 2015* 2016 2017 2013 $ 89.9 $ 146.1 $ 168.9 $ 185.4 $ 196.4 2014 103.0 151.9 180.8 195.9 2015 101.5 148.3 169.6 2016 99.2 147.3 2017 104.9 Total $ 814.1 All outstanding liabilities before 2013, net of reinsurance $ 37.2 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 360.2 * Supplementary information and unaudited Commercial Insurance Segment - Long-Tail The change in incurred losses and loss adjustment expenses for prior accident years was due to lower than expected emergence during 2017 in the workers’ compensation product. The unanticipated emergence in the workers’ compensation product was driven by lower than expected severity from accident years 2015 and 2016. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2008 $ 46.6 $ 45.2 $ 42.7 $ 43.6 $ 43.7 $ 43.4 $ 43.3 $ 43.3 $ 43.0 $ 42.4 $ 2.2 4,461 2009 47.3 48.1 46.8 46.7 46.3 46.0 45.8 45.5 45.0 2.4 4,889 2010 38.8 41.3 41.6 40.4 40.3 40.4 40.8 40.2 2.9 5,257 2011 44.6 47.4 45.5 44.8 44.8 44.9 43.9 4.2 12,220 2012 49.7 45.8 43.7 44.1 43.9 42.9 5.0 12,754 2013 48.7 45.4 44.1 43.4 42.9 6.9 11,305 2014 51.6 50.0 47.1 45.7 9.5 11,563 2015 59.7 59.4 56.5 14.3 11,118 2016 62.6 60.6 22.3 12,362 2017 59.0 29.3 11,728 Total $ 479.1 * Supplementary information and unaudited (1) Commercial insurance segment-long-tail consists of the workers' compensation product. 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 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 2008 $ 10.7 $ 26.2 $ 29.9 $ 32.8 $ 34.8 $ 35.8 $ 37.0 $ 37.4 $ 38.0 $ 38.2 2009 11.9 26.4 33.2 37.2 38.7 40.0 40.4 41.1 41.4 2010 9.0 21.6 28.0 31.4 33.2 34.6 35.4 35.8 2011 12.4 25.4 31.9 35.0 36.9 37.7 38.1 2012 12.6 23.5 29.8 32.7 34.7 35.8 2013 12.3 23.3 29.0 32.3 33.7 2014 12.4 24.4 30.0 32.8 2015 13.9 28.2 35.7 2016 12.6 26.8 2017 13.2 $ 331.4 All outstanding liabilities before 2008, net of reinsurance $ 33.1 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 180.8 * Supplementary information and unaudited Specialty Insurance Segment - Short-Tail The change in incurred losses and loss adjustment expenses for prior accident years was attributable to greater than expected emergence during 2017 in E&S property. The unanticipated emergence in E&S property was driven by higher than expected severity from accident years 2013 - 2016 for the liability portion of our Florida package business. ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2013 $ 70.9 $ 82.9 $ 85.0 $ 90.9 $ 93.8 $ 2.8 7,689 2014 55.7 56.9 58.5 60.9 4.4 7,805 2015 75.5 85.4 87.8 10.8 7,955 2016 99.3 99.5 28.8 6,811 2017 134.2 48.5 7,917 Total $ 476.2 * 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 2013* 2014* 2015* 2016 2017 2013 $ 18.2 $ 37.7 $ 55.5 $ 72.0 $ 83.1 2014 22.0 34.9 44.0 49.3 2015 24.0 48.3 62.8 2016 25.2 50.2 2017 39.0 Total $ 284.4 All outstanding liabilities before 2012, net of reinsurance $ 18.8 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 210.6 * Supplementary information and unaudited Specialty Insurance Segment - Long-Tail ($ in millions except number of claims) Incurred Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance As of December 31, 2017 For the Years Ended December 31, Accident Year 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 IBNR+ Reported Claims (1) 2008 $ 6.4 $ 6.7 $ 6.7 $ 6.8 $ 6.1 $ 6.0 $ 5.8 $ 6.0 $ 4.9 $ 4.8 $ 1.3 1,652 2009 10.3 11.0 10.0 10.4 11.0 11.0 10.6 12.4 12.9 2.5 1,504 2010 11.2 10.9 9.9 9.7 9.8 9.9 10.3 10.1 1.8 1,224 2011 11.9 10.7 10.7 10.2 10.1 10.9 10.5 1.8 953 2012 16.7 16.7 15.4 17.6 17.4 16.5 1.9 860 2013 20.1 18.9 17.9 16.5 15.9 3.1 1,055 2014 23.3 25.2 27.1 28.3 6.8 1,265 2015 38.6 44.9 44.8 14.7 2,001 2016 60.8 62.9 34.7 4,348 2017 71.6 58.7 4,819 Total $ 278.2 * 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 2008* 2009* 2010* 2011* 2012* 2013* 2014* 2015* 2016 2017 2008 $ 0.1 $ 0.3 $ 0.8 $ 1.3 $ 1.7 $ 2.1 $ 2.4 $ 2.8 $ 3.0 $ 3.3 2009 0.2 2.4 4.9 7.2 8.6 9.4 9.2 10.0 10.4 2010 0.8 2.2 5.2 6.5 7.7 7.6 8.1 8.2 2011 0.9 3.0 5.2 6.6 7.1 7.9 8.1 2012 0.9 4.0 7.3 10.3 12.5 14.0 2013 1.1 3.7 6.7 9.0 10.5 2014 1.2 7.2 12.5 16.7 2015 2.7 10.9 20.6 2016 6.0 15.7 2017 5.5 $ 113.0 All outstanding liabilities before 2008, net of reinsurance $ 2.7 Losses and allocated loss adjustment expenses payable, net of reinsurance $ 167.9 * 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) 2017 2016 2015 Net losses and allocated loss adjustment expenses payable: Personal Insurance Segment Short-tail $ 229.4 $ 229.6 $ 208.3 Other personal 13.2 10.8 12.8 Commercial Insurance Segment Short-tail 360.2 226.4 332.4 Long-tail 180.8 314.2 158.8 Other commercial 26.4 24.6 24.1 Specialty Insurance Segment Short-tail 210.6 178.9 165.8 Long-tail 167.9 129.7 91.1 Net losses and loss expenses payable 1,188.5 1,114.2 993.3 ULAE 64.0 63.8 53.8 Reinsurance recoverable on losses and loss expenses payable 3.1 3.6 5.9 Total losses and loss expenses payable $ 1,255.6 $ 1,181.6 $ 1,053.0
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 62.9 % 21.4 % 9.0 % 4.2 % 2.3 % N/A N/A N/A N/A N/A Commercial Insurance: Short-tail 43.8 % 22.3 % 10.9 % 7.2 % 5.1 % N/A N/A N/A N/A N/A Long-tail 25.5 % 28.4 % 13.5 % 7.4 % 4.1 % 2.6 % 1.7 % 1.1 % 1.1 % 0.4 % Specialty Insurance: Short-tail 27.5 % 23.7 % 16.8 % 13.1 % 11.9 % N/A N/A N/A N/A N/A Long-tail 6.0 % 16.0 % 20.1 % 14.6 % 9.6 % 6.1 % 2.7 % 5.9 % 3.3 % 5.9 % * Supplementary information and unaudited

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2017 2016 Losses and loss expenses payable: Direct $ 535.0 $ 545.7 Assumed 9.2 5.0 Ceded (3.1 ) (3.6 ) Net losses and loss expenses payable $ 541.1 $ 547.1 Unearned premiums: Direct $ 402.5 $ 395.6 Assumed 21.4 1.3 Ceded (6.4 ) (6.1 ) Net unearned premiums $ 417.5 $ 390.8
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, 2017 , 2016 and 2015 , prior to the reinsurance transaction with State Auto Mutual under the Pooling Arrangement, as discussed in Note 7a: ($ millions) 2017 2016 2015 Written premiums: Direct $ 835.6 $ 835.4 $ 854.1 Assumed 31.1 5.0 4.7 Ceded (22.9 ) (24.9 ) (35.4 ) Net written premiums $ 843.8 $ 815.5 $ 823.4 Earned premiums: Direct $ 828.7 $ 840.6 $ 863.1 Assumed 11.0 4.9 4.5 Ceded (22.6 ) (25.6 ) (34.7 ) Net earned premiums $ 817.1 $ 819.9 $ 832.9 Losses and loss expenses incurred: Direct $ 530.9 $ 559.1 $ 569.0 Assumed 10.4 3.9 3.4 Ceded (2.5 ) (4.4 ) (2.9 ) Net losses and loss expenses incurred $ 538.8 $ 558.6 $ 569.5

Transactions with Affiliates (T

Transactions with Affiliates (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : ($ millions) 2017 2016 Assets Deferred policy acquisition costs: Ceded $ (53.1 ) $ (79.1 ) Assumed 117.8 129.8 Net assumed $ 64.7 $ 50.7 Liabilities and Stockholders’ Equity Losses and loss expenses payable: Ceded $ (541.1 ) $ (547.1 ) Assumed 1,252.5 1,178.0 Net assumed $ 711.4 $ 630.9 Unearned premiums: Ceded $ (417.5 ) $ (390.8 ) Assumed 605.4 611.7 Net assumed $ 187.9 $ 220.9 Pension and postretirement benefits: Ceded $ (99.3 ) $ (114.5 ) Assumed 64.5 74.4 Net ceded $ (34.8 ) $ (40.1 ) Other liabilities: Ceded $ (43.0 ) $ (42.9 ) Assumed 58.5 53.9 Net assumed $ 15.5 $ 11.0 Stockholders’ Equity Accumulated other comprehensive income: Ceded $ (144.7 ) $ (153.5 ) Assumed 94.0 99.8 Net ceded $ (50.7 ) $ (53.7 )
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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Written premiums: Ceded $ (843.8 ) $ (815.5 ) $ (823.4 ) Assumed 1,269.3 1,293.3 1,273.5 Net assumed $ 425.5 $ 477.8 $ 450.1 Earned premiums: Ceded $ (817.1 ) $ (819.9 ) $ (832.9 ) Assumed 1,275.1 1,291.9 1,270.5 Net assumed $ 458.0 $ 472.0 $ 437.6 Losses and loss expenses incurred: Ceded $ (540.4 ) $ (560.3 ) $ (570.9 ) Assumed 920.0 944.1 864.2 Net assumed $ 379.6 $ 383.8 $ 293.3 Acquisition and operating expenses: Ceded (189.7 ) (148.5 ) (183.9 ) Assumed 460.4 434.3 432.5 Net assumed $ 270.7 $ 285.8 $ 248.6

Federal Income Taxes (Tables)

Federal Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Amount at statutory rate $ 11.7 35.0 % $ 6.7 35.0 % $ 23.5 35.0 % Tax-exempt interest and dividends received deduction (5.7 ) (17.1 ) (7.1 ) (37.2 ) (8.7 ) (13.0 ) Other, net 1.7 5.4 (1.4 ) (7.2 ) 1.3 1.9 Federal income tax expense (benefit) 7.7 23.3 (1.8) (9.4 )% 16.1 23.9 % Impact of TCJA at enactment 36.4 108.7 — — — — Federal income tax expense (benefit) $ 44.1 132.0 % $ (1.8 ) (9.4 )% $ 16.1 23.9 %
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, 2017 and 2016 : ($ millions) 2017 2016 Deferred tax assets: Unearned premiums not currently deductible $ 25.5 $ 42.6 Losses and loss expenses payable discounting 22.0 19.7 Postretirement and pension benefits 13.6 26.1 Realized loss on other-than-temporary impairment 2.1 5.8 Other liabilities 9.2 16.6 Net operating loss carryforward 34.3 58.9 Tax credit carryforward 3.7 3.3 Other 2.5 7.0 Total deferred tax assets 112.9 180.0 Deferred tax liabilities: Deferral of policy acquisition costs 24.7 45.4 Net unrealized holding gains on investments 20.3 32.5 Losses and loss expenses payable discounting (transition rule) 10.7 — Total deferred tax liabilities 55.7 77.9 Net deferred federal income taxes $ 57.2 $ 102.1

Pension and Postretirement Be42

Pension and Postretirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Defined Benefit Plan [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, 2017 and 2016 : ($ millions) Pension Postretirement 2017 2016 2017 2016 Change in benefit obligation: Benefit obligation at beginning of year $ 284.5 $ 281.3 $ 18.8 $ 20.3 Service cost 5.6 6.1 — — Interest cost 11.1 11.6 0.7 0.8 Actuarial loss (gain) 17.0 4.5 0.6 (0.9 ) Benefits paid (14.0 ) (19.0 ) (1.8 ) (1.4 ) The Company’s portion of benefit obligation at end of year $ 304.2 $ 284.5 $ 18.3 $ 18.8 Change in plan assets available for plan benefits: Fair value of plan assets available for plan benefits at beginning of year $ 235.7 $ 204.4 $ — $ — Employer contribution 9.8 34.5 — — Actual return on plan assets 33.5 15.8 — — Benefits paid (14.0 ) (19.0 ) — — The Company’s portion of fair value of plan assets at end of year $ 265.0 $ 235.7 $ — $ — Supplemental executive retirement plan (7.0 ) (6.8 ) — — Funded status at end of year $ (46.2 ) $ (55.6 ) $ (18.3 ) $ (18.8 ) Accumulated benefit obligation end of year $ 287.9 $ 267.5
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, 2017 and 2016 : ($ millions) 2017 2016 Prior service benefit $ (48.6 ) $ (54.1 ) Net actuarial loss 111.6 118.5 Total $ 63.0 $ 64.4
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, 2018: ($ millions) 2018 Prior service benefit $ (5.5 ) Net actuarial loss 8.4 Total $ 2.9
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, 2017 , 2016 and 2015 : ($ millions) Pension Postretirement 2017 2016 2015 2017 2016 2015 Components of net periodic cost: Service cost $ 5.7 $ 6.2 $ 7.9 $ — $ — $ — Interest cost 11.4 11.9 11.3 0.8 0.9 1.1 Expected return on plan assets (16.7 ) (15.1 ) (13.8 ) — — — Amortization of: Prior service benefit — — — (5.5 ) (5.5 ) (5.4 ) Net actuarial loss 7.8 9.2 10.9 0.2 0.2 0.6 Net periodic cost (benefit) $ 8.2 $ 12.2 $ 16.3 $ (4.5 ) $ (4.4 ) $ (3.7 )
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 2018 $ 11.2 $ 1.5 2019 11.5 1.5 2020 12.0 1.4 2021 12.5 1.4 2022 12.9 1.3 2023-2027 76.2 5.9
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, 2017 and 2016 : Pension Postretirement 2017 2016 2017 2016 Benefit obligations weighted-average assumptions: Discount rate 3.50 % 4.00 % 3.50 % 4.00 % Rates of increase in compensation levels 3.25 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, 2017 , 2016 and 2015 : Pension Postretirement 2017 2016 2015 2017 2016 2015 Weighted-average assumptions: Discount rate 4.00 % 4.20 % 3.85 % 4.00 % 4.20 % 3.85 % 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, 2017 , 2016 and 2015 : Postretirement 2017 2016 2015 Assumed health care cost trend rates: Health care cost trend rate assumed for the next year 6.00 % 6.50 % 6.50 % Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 3.80 % 3.90 % 3.80 % Year that the rate reaches the ultimate trend rate 2075 2075 2076
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, 2017 : ($ 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 1.7 (1.5 )
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 52.0 % U.S. large-cap equity 24.2 U.S. small-cap equity 10.3 International equity 9.4 Emerging market equity 4.1 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, 2017 and 2016 : ($ 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, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 70.1 $ — $ 70.1 $ — Corporate securities 60.7 — 60.7 — U.S. government agencies mortgage-backed securities 4.4 — 4.4 — Total fixed maturities 135.2 — 135.2 — Equity securities: Large-cap securities 71.6 71.6 — — Mutual and exchange traded funds 25.1 25.1 — — Total equity securities 96.7 96.7 — — Total pension plan investments $ 231.9 $ 96.7 $ 135.2 $ — ($ 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 $ —

Other Comprehensive Income an43

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at January 1, 2017 $ 62.8 $ (30.3 ) $ 32.5 Other comprehensive income before reclassifications 45.5 (1.0 ) 44.5 Amounts reclassified from AOCI (a) (42.3 ) 2.0 (40.3 ) Net current period other comprehensive income 3.2 1.0 4.2 Ending balance at December 31, 2017 $ 66.0 $ (29.3 ) $ 36.7 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 (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, 2017 , 2016 and 2015 : ($ millions) Details about Accumulated Other December 31 Affected line item in the Condensed Comprehensive Income Components 2017 2016 2015 Consolidated Statements of Income Unrealized gains on available for sale securities $ 65.1 $ 36.5 $ 24.7 Realized gain on sale of securities 65.1 36.5 24.7 Total before tax (22.8 ) (12.8 ) (8.6 ) Tax expense 42.3 23.7 16.1 Net of tax Amortization of benefit plan items: Negative prior service costs 5.5 5.5 5.4 (a) Net loss (8.0 ) (9.4 ) (11.5 ) (a) (2.5 ) (3.9 ) (6.1 ) Total before tax 0.5 0.3 4.0 Tax benefit (2.0 ) (3.6 ) (2.1 ) Net of tax Total reclassifications for the period $ 40.3 $ 20.1 $ 14.0 (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, 2017
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) 2017 2016 Statutory capital and surplus of insurance subsidiaries $ 858.4 $ 844.4 Net liabilities of non-insurance parent and affiliates (89.8 ) (84.9 ) 768.6 759.5 Increases (decreases): Deferred acquisition costs 117.8 129.8 Postretirement and pension benefits 22.3 22.7 Deferred federal income taxes (61.2 ) (49.1 ) Fixed maturities, at fair value 19.6 13.5 Other, net 13.8 14.9 Stockholders’ equity per accompanying consolidated financial statements $ 880.9 $ 891.3 ($ millions) Year ended December 31 2017 2016 2015 Statutory net income of insurance subsidiaries $ 42.8 $ 22.7 $ 65.4 Net loss of non-insurance parent and affiliates (13.1 ) (3.0 ) (4.1 ) 29.7 19.7 61.3 Increases (decreases): Deferred acquisition costs (12.0 ) 0.7 2.5 Postretirement and pension benefits 4.3 4.4 4.2 Deferred federal income taxes (34.6 ) (5.4 ) (12.2 ) Share-based compensation expense — — (0.1 ) Other, net 1.9 1.6 (4.5 ) Net (loss) income per accompanying consolidated financial statements $ (10.7 ) $ 21.0 $ 51.2

Share-Based Compensation (Table

Share-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Share-based Compensation [Abstract]
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]	The following table set forth the Company’s total activity and related information for employee restricted stock and performance unit award activity for the years ended December 31, 2017 , 2016 and 2015 : 2017 2016 2015 Shares Weighted Shares Weighted Shares Weighted Outstanding, beginning of year 107,297 $ 22.04 111,084 $ 22.19 76,472 $ 19.06 Granted 177,846 27.20 45,252 21.55 74,020 22.83 Vested (26,271 ) 21.39 (33,414 ) 21.93 (35,859 ) 16.88 Forfeited (39,425 ) 25.68 (15,625 ) 21.91 (3,549 ) 21.92 Outstanding, end of year 219,447 $ 25.65 107,297 $ 22.04 111,084 $ 22.19
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]	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 and 2015 : 2017 2016 2015 Fair value per share N/A (1) $ 6.75 $ 7.69 Expected dividend yield N/A (1) 1.87 % 1.75 % Risk free interest rate N/A (1) 1.30 % 1.60 % Expected volatility factor N/A (1) 36.27 % 36.61 % Expected life in years N/A (1) 5.3 6.0 (1) No stock options granted during 2017
Share-based Compensation, Stock Options, Activity [Table Text Block]	The following table sets forth the Company’s stock option activity and related information for the years ended December 31, 2017 , 2016 and 2015 : (millions, except per share amounts) 2017 2016 2015 Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Options Weighted- Average Exercise Price Outstanding, beginning of year 2.5 $ 20.63 3.1 $ 21.45 3.4 $ 21.37 Granted — — 0.2 21.55 0.3 22.86 Exercised (0.4 ) 19.38 (0.4 ) 17.18 (0.3 ) 16.54 Canceled (0.4 ) 28.96 (0.4 ) 30.70 (0.3 ) 25.63 Outstanding, end of year 1.7 $ 19.22 2.5 $ 20.63 3.1 $ 21.45
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, 2017 : (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 1.0 3.6 $ 16.53 1.0 $ 16.53 $20.01 – $30.00 0.7 4.7 23.07 0.6 23.33 1.7 4.0 $ 19.22 1.6 $ 18.98

Net Earnings (Loss) Per Commo46

Net Earnings (Loss) Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2017
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 (loss) earnings per common share for the years ended December 31, 2017 , 2016 and 2015 : (millions, except per share amounts) 2017 2016 2015 Numerator: Net (loss) earnings for basic net earnings per common share $ (10.7 ) $ 21.0 $ 51.2 Adjusted net (loss) earnings for dilutive net (loss) earnings per common share $ (10.7 ) $ 21.0 $ 51.2 Denominator: Weighted average shares for basic net (loss) earnings per common share 42.1 41.6 41.1 Effect of dilutive share-based awards — 0.4 0.5 Adjusted weighted average shares for diluted net (loss) earnings per common share 42.1 42.0 41.6 Basic net (loss) earnings per common share $ (0.25 ) $ 0.50 $ 1.25 Diluted net (loss) earnings per common share $ (0.25 ) $ 0.50 $ 1.23
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, 2017 , 2016 and 2015 : (millions) 2017 2016 2015 Total number of antidilutive options and awards 0.5 1.2 1.5

Reportable Segments (Tables)

Reportable Segments (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , 2016 and 2015 : ($ millions) 2017 2016 2015 Revenues from external sources: Insurance segments Personal insurance $ 580.3 $ 578.5 $ 591.3 Commercial insurance 455.7 472.6 476.5 Specialty insurance 239.1 240.8 202.7 Total insurance segments 1,275.1 1,291.9 1,270.5 Investment operations segment Net investment income 78.8 74.7 71.7 Net realized capital gains 65.1 36.5 24.7 Total investment operations segment 143.9 111.2 96.4 Total revenue from reportable segments 1,419.0 1,403.1 1,366.9 All other 2.3 2.3 1.7 Total revenues from external sources 1,421.3 1,405.4 1,368.6 Intersegment revenues 6.0 5.8 5.6 Total revenues 1,427.3 1,411.2 1,374.2 Reconciling items: Eliminate intersegment revenues (6.0 ) (5.8 ) (5.6 ) Total consolidated revenue $ 1,421.3 $ 1,405.4 $ 1,368.6 Segment loss before federal income taxes: Insurance segments: Personal insurance SAP underwriting (loss) gain $ (23.3 ) $ (10.3 ) $ 27.5 Commercial insurance SAP underwriting loss (10.6 ) (31.6 ) (46.7 ) Specialty insurance SAP underwriting loss (57.0 ) (42.7 ) (5.9 ) Total insurance segments (90.9 ) (84.6 ) (25.1 ) Investment operations segment: Net investment income 78.8 74.7 71.7 Net realized capital gains 65.1 36.5 24.7 Total investment operations segment 143.9 111.2 96.4 All other segments income 0.6 0.4 (0.1 ) Reconciling items: GAAP adjustments (10.2 ) 2.0 4.2 Interest expense on corporate debt (5.9 ) (5.5 ) (5.4 ) Corporate expenses (4.1 ) (4.3 ) (2.7 ) Total reconciling items $ (20.2 ) $ (7.8 ) $ (3.9 ) Total consolidated income before federal income taxes $ 33.4 $ 19.2 $ 67.3
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments at December 31, 2017 and 2016 : ($ millions) 2017 2016 Segment assets: Investment operations segment $ 2,781.2 $ 2,663.7 Total segment assets 2,781.2 2,663.7 Reconciling items: Corporate assets 233.1 295.7 Total consolidated assets $ 3,014.3 $ 2,959.4

Quarterly Financial Data (una48

Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following tables set forth quarterly financial data for 2017 and 2016 : ($ millions, except per share amounts) 2017 For three months ended March 31 June 30 September 30 December 31 Total revenues $ 344.3 $ 355.7 $ 358.4 $ 362.9 (Loss) income before federal income taxes (5.2 ) 11.9 (11.2 ) 37.9 Net (loss) income (4.1 ) 8.7 (9.5 ) (5.8 ) (Loss) earnings per common share: Basic $ (0.10 ) $ 0.21 $ (0.23 ) $ (0.14 ) Diluted $ (0.10 ) $ 0.21 $ (0.23 ) $ (0.14 ) 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

Summary of Significant Accoun49

Summary of Significant Accounting Policies - Schedule of Net Deferred Acquisition Costs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of year	$ 129.8	$ 129.1	$ 126.5
Acquisition costs deferred	245.7	291	285.6
Acquisition costs amortized to expense	(257.7)	(290.3)	(283)
Balance, end of year	$ 117.8	$ 129.8	$ 129.1

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	$ 1.5
Liability for unpaid losses and loss expenses	$ 25.5	$ 27
Qualitative Assessment of Revenue Subject to the new Revenue Recognition Standard	0.10%
Unrealized gains on equity securities reclassified from accumulated other comprehensive income to retained earnings, after tax upon adoption of ASU 2016-01	$ (60.7)

Investments - Cost or Amortized

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,517.5	$ 2,444.5
Gross unrealized holding gains	126.9	118.5
Gross unrealized holding losses	(30.3)	(25.8)
Fair value	2,614.1	2,537.2
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,173.1	2,095.9
Gross unrealized holding gains	40	35.8
Gross unrealized holding losses	(20.3)	(22.4)
Fair value	2,192.8	2,109.3
Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	318.6	323.1
Gross unrealized holding gains	56.7	63.1
Gross unrealized holding losses	(10)	(3.4)
Fair value	365.3	382.8
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	25.8	25.5
Gross unrealized holding gains	30.2	19.6
Gross unrealized holding losses	0	0
Fair value	56	45.1
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	433.8	401.9
Gross unrealized holding gains	9.3	8.9
Gross unrealized holding losses	(6.2)	(6.1)
Fair value	436.9	404.7
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	507.1	634.6
Gross unrealized holding gains	19.1	12.3
Gross unrealized holding losses	(0.4)	(3.2)
Fair value	525.8	643.7
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	527.5	445.7
Gross unrealized holding gains	4.5	6.1
Gross unrealized holding losses	(2.3)	(2.2)
Fair value	529.7	449.6
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	704.7	613.7
Gross unrealized holding gains	7.1	8.5
Gross unrealized holding losses	(11.4)	(10.9)
Fair value	700.4	611.3
Large-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	62.4	108.9
Gross unrealized holding gains	35.1	32.7
Gross unrealized holding losses	(0.7)	(2.6)
Fair value	96.8	139
Small-cap securities | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost		57.2
Gross unrealized holding gains		21.9
Gross unrealized holding losses		0
Fair value		79.1
Mutual and exchange traded funds [Domain] [Member] | Equity securities:
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	256.2	157
Gross unrealized holding gains	21.6	8.5
Gross unrealized holding losses	(9.3)	(0.8)
Fair value	$ 268.5	$ 164.7

Investments - Gross Unrealized

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Dec. 31, 2017USD ($)position	Dec. 31, 2016USD ($)position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 574.1	$ 891.6
Less than 12 months, Unrealized losses	$ (15.2)	$ (22.5)
Less than 12 months, Number of positions (positions) | position	87	124
12 months or more, Fair value	$ 515.6	$ 58.7
12 months or more, Unrealized losses	$ (15.1)	$ (3.3)
12 months or more, Number of positions (positions) | position	73	20
Total, Fair value	$ 1,089.7	$ 950.3
Total, Unrealized losses	$ (30.3)	$ (25.8)
Total, Number of positions (positions) | position	160	144
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 502.8	$ 852.6
Less than 12 months, Unrealized losses	$ (5.2)	$ (20.8)
Less than 12 months, Number of positions (positions) | position	82	115
12 months or more, Fair value	$ 515.6	$ 49.9
12 months or more, Unrealized losses	$ (15.1)	$ (1.6)
12 months or more, Number of positions (positions) | position	73	15
Total, Fair value	$ 1,018.4	$ 902.5
Total, Unrealized losses	$ (20.3)	$ (22.4)
Total, Number of positions (positions) | position	155	130
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 71.3	$ 39
Less than 12 months, Unrealized losses	$ (10)	$ (1.7)
Less than 12 months, Number of positions (positions) | position	5	9
12 months or more, Fair value	$ 0	$ 8.8
12 months or more, Unrealized losses	$ 0	$ (1.7)
12 months or more, Number of positions (positions) | position	0	5
Total, Fair value	$ 71.3	$ 47.8
Total, Unrealized losses	$ (10)	$ (3.4)
Total, Number of positions (positions) | position	5	14
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	$ 102.4	$ 229.1
Less than 12 months, Unrealized losses	$ (0.6)	$ (6.1)
Less than 12 months, Number of positions (positions) | position	18	30
12 months or more, Fair value	$ 196.1	$ 0
12 months or more, Unrealized losses	$ (5.6)	$ 0
12 months or more, Number of positions (positions) | position	22	0
Total, Fair value	$ 298.5	$ 229.1
Total, Unrealized losses	$ (6.2)	$ (6.1)
Total, Number of positions (positions) | position	40	30
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 58.6	$ 178.9
Less than 12 months, Unrealized losses	$ (0.4)	$ (3.2)
Less than 12 months, Number of positions (positions) | position	10	26
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	$ 58.6	$ 178.9
Total, Unrealized losses	$ (0.4)	$ (3.2)
Total, Number of positions (positions) | position	10	26
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 153.2	$ 102.9
Less than 12 months, Unrealized losses	$ (1.3)	$ (1.4)
Less than 12 months, Number of positions (positions) | position	23	16
12 months or more, Fair value	$ 67.3	$ 29.4
12 months or more, Unrealized losses	$ (1)	$ (0.8)
12 months or more, Number of positions (positions) | position	10	4
Total, Fair value	$ 220.5	$ 132.3
Total, Unrealized losses	$ (2.3)	$ (2.2)
Total, Number of positions (positions) | position	33	20
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 188.6	$ 341.7
Less than 12 months, Unrealized losses	$ (2.9)	$ (10.1)
Less than 12 months, Number of positions (positions) | position	31	43
12 months or more, Fair value	$ 252.2	$ 20.5
12 months or more, Unrealized losses	$ (8.5)	$ (0.8)
12 months or more, Number of positions (positions) | position	41	11
Total, Fair value	$ 440.8	$ 362.2
Total, Unrealized losses	$ (11.4)	$ (10.9)
Total, Number of positions (positions) | position	72	54
Large-cap securities
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 4.4	$ 9.1
Less than 12 months, Unrealized losses	$ (0.7)	$ (0.9)
Less than 12 months, Number of positions (positions) | position	4	7
12 months or more, Fair value	$ 0	$ 8.8
12 months or more, Unrealized losses	$ 0	$ (1.7)
12 months or more, Number of positions (positions) | position	0	5
Total, Fair value	$ 4.4	$ 17.9
Total, Unrealized losses	$ (0.7)	$ (2.6)
Total, Number of positions (positions) | position	4	12
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 66.9	$ 29.9
Less than 12 months, Unrealized losses	$ (9.3)	$ (0.8)
Less than 12 months, Number of positions (positions) | position	1	2
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions (positions) | position	0	0
Total, Fair value	$ 66.9	$ 29.9
Total, Unrealized losses	$ (9.3)	$ (0.8)
Total, Number of positions (positions) | position	1	2

Investments - Realized Losses R

Investments - Realized Losses Related to Other-Than-Temporary Impairments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (3.5)	$ (6.8)	$ (7.9)
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0	(2.3)	0
Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	(1.5)	(0.6)	(2.2)
Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (2)	$ (3.9)	$ (5.7)

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, 2017	Dec. 31, 2016
Amortized cost
Due in 1 year or less	$ 5.9
Due after 1 year through 5 years	543.8
Due after 5 years through 10 years	419.1
Due after 10 years	499.6
U.S. government agencies mortgage-backed securities	704.7
Total	2,173.1	$ 2,095.9
Fair value
Due in 1 year or less	5.9
Due after 1 year through 5 years	545
Due after 5 years through 10 years	420.3
Due after 10 years	521.2
U.S. government agencies mortgage-backed securities	700.4
Total	$ 2,192.8	$ 2,109.3

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized Gains	$ 69,700,000	$ 44,100,000	$ 34,400,000
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	9,300,000	9,200,000
Proceeds on sales of available-for-sale securities	438,600,000	365,400,000	326,200,000
Other Invested Assets [Member]
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized Gains	200,000	$ 12,100,000	$ 200,000
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 80	$ 76.3	$ 73.7
Investment expenses	1.2	1.6	2
Net investment income	78.8	74.7	71.7
Fixed maturities:
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	63.2	63.4	61.3
Equity Securities [Member]
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	10.5	7.3	6.5
Cash and cash equivalents, and other
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 6.3	$ 5.6	$ 5.9

Investments - Realized and Unre

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Realized gains:
Total realized gains	$ 69.7	$ 44.1	$ 34.4
Realized losses:
Total realized losses	(4.6)	(7.6)	(9.7)
Net realized gains on investments	65.1	36.5	24.7
Change in unrealized holding gains (losses), net of tax:
Deferred federal income liability thereon	(0.7)	3	22.4
Change in net unrealized holding gains, net of tax	3.2	(5.7)	(41.5)
Fixed maturities:
Realized gains:
Total realized gains	2.8	2.8	4.6
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	6.3	(14.4)	(32.8)
Fixed maturities: | OTTI
Realized losses:
Total realized losses	0	(2.3)	0
Equity Securities [Member]
Realized gains:
Total realized gains	66.7	29.2	29.6
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	(13)	14.3	(29.5)
Equity Securities [Member] | Sales
Realized losses:
Total realized losses	(1.1)	(0.8)	(1.8)
Equity Securities [Member] | OTTI
Realized losses:
Total realized losses	(3.5)	(4.5)	(7.9)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0.2	12.1	0.2
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding gains, net of tax	$ 10.6	$ (8.6)	$ (1.6)

Fair Value of Financial Instr58

Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May 31, 2009USD ($)Agreement	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)
Fair Value Measurements Disclosure [Line Items]
Other invested assets fair value		$ 45.2	$ 35.7
Other investments and securities, at cost		5.6	5.4
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		5.1
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 Instr59

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	$ 2,614.1	$ 2,537.2
Investments, Fair Value Disclosure	2,568.9	2,501.5
Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,192.8	2,109.3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	365.3	382.8
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	56	45.1
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	436.9	404.7
Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	525.8	643.7
Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	529.7	449.6
U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	700.4	611.3
Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	96.8	139
Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments		79.1
Mutual and exchange traded funds [Domain] [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	268.5	164.7
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	10.8	9.4
Available-for-sale: | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	2,192.8	2,109.3
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	436.9	404.7
Available-for-sale: | Obligations of states and political subdivisions | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	525.8	643.7
Available-for-sale: | Corporate securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	529.7	449.6
Available-for-sale: | U.S. government agencies mortgage-backed securities | Fixed maturities:
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	700.4	611.3
Available-for-sale: | Large-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	96.8
Available-for-sale: | Small-cap securities | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total available-for-sale investments	0
Quoted prices in active markets for identical assets (Level 1)
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	376.1	392.2
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	365.3	382.8
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		139
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		79.1
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	268.5	164.7
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	10.8	9.4
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	96.8
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	0
Significant other observable inputs (Level 2)
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	2,192.8	2,105.8
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	2,192.8	2,105.8
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	436.9	404.7
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	525.8	643.7
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	529.7	446.1
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	700.4	611.3
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	0	3.5
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	0	3.5
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	0	3.5
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 Instr60

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, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 3.5	$ 3.3
Total realized gains (losses)-included in earnings	1.4	0
Total unrealized gains (losses)-included in other comprehensive income	0	0
Purchases	0.1	0.2
Sales	(5)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 0	$ 3.5

Fair Value of Financial Instr61

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, 2017	Dec. 31, 2016
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	$ 72.6	$ 75.7

Fair Value of Financial Instr62

Fair Value of Financial Instruments - Included in Notes Payable are Senior Notes and Subordinated Debentures (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
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%	1.73%
Debt instrument, issued	$ 21,500,000
Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Interest rate (percent)	5.03%	5.03%
Debt instrument, issued	$ 85,000,000
Affiliate Subordinated Debentures due 2033
Debt Instrument [Line Items]
Interest rate (percent)	5.68%	5.13%
Debt instrument, issued	$ 15,500,000
Carrying value
Debt Instrument [Line Items]
Total notes payable	122,100,000	$ 122,100,000
Carrying value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	21,500,000	21,500,000
Carrying value | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	85,400,000	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	121,800,000
Fair value | Federal Home Loan Bank Advances
Debt Instrument [Line Items]
Total notes payable	20,900,000	21,000,000
Fair value | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	85,700,000	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 Paya63

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Losses and loss expenses payable, at beginning of year	$ 1,181.6	$ 1,053	$ 983.2
Less: reinsurance recoverable on losses and loss expenses payable	3.6	5.9	9.6
Net balance at beginning of year	1,178	1,047.1	973.6
Incurred related to:
Current year	964.9	915.4	852.8
Prior years	(46.6)	27	10
Total incurred	918.3	942.4	862.8
Paid related to:
Current year	445.2	417.8	421.5
Prior years	398.6	393.7	367.8
Total paid	843.8	811.5	789.3
Net balance at end of year	1,252.5	1,178	1,047.1
Plus: reinsurance recoverable on losses and loss expenses payable	3.1	3.6	5.9
Losses and loss expenses payable, at end of year (affiliates $711.4, $630.9, and $532.4, respectively)	$ 1,255.6	$ 1,181.6	$ 1,053

Losses and Loss Expenses Paya64

Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses(Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Losses and loss expenses payable, affiliates	$ 711.4	$ 630.9	$ 532.4

Losses and Loss Expenses Paya65

Losses and Loss Expenses Payable - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 46.6	$ (27)	$ (10)
Unallocated loss adjustment expense [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	3.2	(3.7)	6.2
Personal Insurance Segments [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.6	(9)	10.5
Personal Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	4.4	(7.9)	11.1
Homeowners [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.5
Other Personal [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	1.3
Commercial insurance segment
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	44	5.1	0.7
Commercial Auto [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(3.6)	(9.8)
Workers' compensation [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		4.2	5.1
Farm & Ranch [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		1.7
Other Commercial [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		1.8
Specialty Insurance Segment [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(4.2)	(20.8)	7.1
Programs [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(14.3)	9.6
Excess & Surplus Property [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	(3)		5.2
Excess & Surplus Casualty [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years		(4.6)	2.7
Middle Market Commercial [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years			4.3
Catastrophe [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Prior years	$ 2	$ 1.4	$ 0.7

Short-Duration Insurance Cont66

Short-Duration Insurance Contracts Claims Development (Details) $ in Millions	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)	Dec. 31, 2008USD ($)
Claims Development [Line Items]
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	$ 1,188.5	$ 1,114.2	$ 993.3
Personal Insurance Segments [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	1,649.2
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	1,423.5
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	3.7
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	229.4	229.6	208.3
Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	1,137.1
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	814.1
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	37.2
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	360.2	226.4	332.4
Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	476.2
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	284.4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	18.8
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	210.6	178.9	165.8
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	289.8	290	290.3	$ 286.1	$ 291.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1.5
Short-duration Insurance Contracts, Number of Reported Claims	190,310
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 286.2	279.4	267.6	239.5	178.1
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	213.9	217.6	217.6	211.9	214.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10.4
Short-duration Insurance Contracts, Number of Reported Claims	39,984
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 196.4	185.4	168.9	146.1	89.9
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	93.8	90.9	85	82.9	70.9
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.8
Short-duration Insurance Contracts, Number of Reported Claims	7,689
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 83.1	72	55.5	37.7	18.2
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	276	277.3	274.9	273.2
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.7
Short-duration Insurance Contracts, Number of Reported Claims	172,678
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 266.3	254.3	228.4	174.2
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	226.3	233.7	231.3	227
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 15.3
Short-duration Insurance Contracts, Number of Reported Claims	38,657
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 195.9	180.8	151.9	103
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	60.9	58.5	56.9	55.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.4
Short-duration Insurance Contracts, Number of Reported Claims	7,805
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 49.3	44	34.9	22
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	341	345.5	339
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7.4
Short-duration Insurance Contracts, Number of Reported Claims	152,698
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 318	290.8	216.5
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	232.5	239.9	239.4
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 28.4
Short-duration Insurance Contracts, Number of Reported Claims	37,556
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 169.6	148.3	101.5
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	87.8	85.4	75.5
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 10.8
Short-duration Insurance Contracts, Number of Reported Claims	7,955
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 62.8	48.3	24
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	370	368.5
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22.8
Short-duration Insurance Contracts, Number of Reported Claims	150,364
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 316.2	231.9
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	227.6	237
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 47.6
Short-duration Insurance Contracts, Number of Reported Claims	34,088
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 147.3	99.2
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	99.5	99.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 28.8
Short-duration Insurance Contracts, Number of Reported Claims	6,811
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 50.2	25.2
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	372.4
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 54
Short-duration Insurance Contracts, Number of Reported Claims	140,258
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 236.8
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	236.8
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 69.5
Short-duration Insurance Contracts, Number of Reported Claims	28,605
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 104.9
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	134.2
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 48.5
Short-duration Insurance Contracts, Number of Reported Claims	7,917
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 39
Long-tail [Domain] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	479.1
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	331.4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	33.1
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	180.8	314.2	158.8
Long-tail [Domain] | Specialty Insurance Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	278.2
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	113
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net, Not Separately Presented	2.7
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	167.9	129.7	91.1
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2012 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	42.9	43.4	44.1	45.4	48.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 6.9
Short-duration Insurance Contracts, Number of Reported Claims	11,305
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 33.7	32.3	29	23.3	12.3
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	15.9	16.5	17.9	18.9	20.1
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3.1
Short-duration Insurance Contracts, Number of Reported Claims	1,055
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 10.5	9	6.7	3.7	1.1
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2013 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	45.7	47.1	50	51.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 9.5
Short-duration Insurance Contracts, Number of Reported Claims	11,563
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 32.8	30	24.4	12.4
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	28.3	27.1	25.2	23.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 6.8
Short-duration Insurance Contracts, Number of Reported Claims	1,265
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 16.7	12.5	7.2	1.2
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2014 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	56.5	59.4	59.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 14.3
Short-duration Insurance Contracts, Number of Reported Claims	11,118
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 35.7	28.2	13.9
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	44.8	44.9	38.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 14.7
Short-duration Insurance Contracts, Number of Reported Claims	2,001
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 20.6	10.9	2.7
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2015 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	60.6	62.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 22.3
Short-duration Insurance Contracts, Number of Reported Claims	12,362
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 26.8	12.6
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	62.9	60.8
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 34.7
Short-duration Insurance Contracts, Number of Reported Claims	4,348
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 15.7	6
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2016 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	59
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 29.3
Short-duration Insurance Contracts, Number of Reported Claims	11,728
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 13.2
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	71.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 58.7
Short-duration Insurance Contracts, Number of Reported Claims	4,819
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 5.5
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2007 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	42.4	43	43.3	43.3	43.4	$ 43.7	$ 43.6	$ 42.7	$ 45.2	$ 46.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.2
Short-duration Insurance Contracts, Number of Reported Claims	4,461
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 38.2	38	37.4	37	35.8	34.8	32.8	29.9	26.2	10.7
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	4.8	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	$ 1.3
Short-duration Insurance Contracts, Number of Reported Claims	1,652
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 3.3	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 2008 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	45	45.5	45.8	46	46.3	46.7	46.8	48.1	47.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.4
Short-duration Insurance Contracts, Number of Reported Claims	4,889
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 41.4	41.1	40.4	40	38.7	37.2	33.2	26.4	11.9
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	12.9	12.4	10.6	11	11	10.4	10	11	10.3
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.5
Short-duration Insurance Contracts, Number of Reported Claims	1,504
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 10.4	10	9.2	9.4	8.6	7.2	4.9	2.4	$ 0.2
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2009 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	40.2	40.8	40.4	40.3	40.4	41.6	41.3	38.8
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 2.9
Short-duration Insurance Contracts, Number of Reported Claims	5,257
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 35.8	35.4	34.6	33.2	31.4	28	21.6	9
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	10.1	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	$ 1.8
Short-duration Insurance Contracts, Number of Reported Claims	1,224
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 8.2	8.1	7.6	7.7	6.5	5.2	2.2	$ 0.8
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2010 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	43.9	44.9	44.8	44.8	45.5	47.4	44.6
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 4.2
Short-duration Insurance Contracts, Number of Reported Claims	12,220
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 38.1	37.7	36.9	35	31.9	25.4	12.4
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	10.5	10.9	10.1	10.2	10.7	10.7	11.9
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1.8
Short-duration Insurance Contracts, Number of Reported Claims	953
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 8.1	7.9	7.1	6.6	5.2	3	$ 0.9
Long-tail [Domain] | Short-duration Insurance Contracts, Accident Year 2011 [Member] | Commercial insurance
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred Claims and Allocated Claim Adjustment Expense, Net	42.9	43.9	44.1	43.7	45.8	49.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5
Short-duration Insurance Contracts, Number of Reported Claims	12,754
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 35.8	34.7	32.7	29.8	23.5	12.6
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	16.5	17.4	17.6	15.4	16.7	16.7
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 1.9
Short-duration Insurance Contracts, Number of Reported Claims	860
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 14	$ 12.5	$ 10.3	$ 7.3	$ 4	$ 0.9

Short-Duration Insurance Cont67

Short-Duration Insurance Contracts Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 1,188.5	$ 1,114.2	$ 993.3
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	64	63.8	53.8
Reinsurance recoverable on losses and loss expenses payable	3.1	3.6	5.9	$ 9.6
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	1,255.6	1,181.6	1,053	$ 983.2
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	229.4	229.6	208.3
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	13.2	10.8	12.8
Commercial insurance
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	360.2	226.4	332.4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	26.4	24.6	24.1
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	210.6	178.9	165.8
Long-tail [Domain] | Commercial insurance
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	180.8	314.2	158.8
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	$ 167.9	$ 129.7	$ 91.1

Short-Duration Insurance Cont68

Short-Duration Insurance Contracts Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance (Details)	Dec. 31, 2017
Personal Insurance Segments [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	62.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	9.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.30%
Commercial insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	43.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	22.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.10%
Specialty Insurance Segment [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	27.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	16.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	13.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	11.90%
Long-tail [Domain] | Commercial insurance
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	25.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	28.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	2.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.40%
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	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	16.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	20.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	14.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	6.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	5.90%

Short-Duration Insurance Cont69

Short-Duration Insurance Contracts Additional Information (Details)	12 Months Ended
	Dec. 31, 2017
Short-Duration Insurance Contracts Additional Information [Abstract]
Unanticipated emergence from prior accident years	80.00%

Reinsurance - Additional Inform

Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended			36 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015
Reinsurance [Line Items]
Duration of quota share agreement				3 years
Percentage ceded to reinsurers (percent)				75.00%
Unearned premium	$ 611.8	$ 617.8
Reinsurance Arrangement, Participating Reinsurers' Margin	9.00%
Profit Commission	$ 0.7	1.9	$ 4.2
Homeowners Quota Share Arrangement
Reinsurance [Line Items]
Unearned premium		$ 89.5

Reinsurance - External Reinsura

Reinsurance - External Reinsurance on Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Losses and loss expenses payable:
Assumed	$ 1,252.5	$ 1,178
Ceded	(541.1)	(547.1)
Unearned premiums:
Assumed	605.4	611.7
Ceded	(417.5)	(390.8)
Unearned Premiums From Affiliates	187.9	220.9
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	535	545.7
Assumed	9.2	5
Ceded	(3.1)	(3.6)
Net losses and loss expenses payable	541.1	547.1
Unearned premiums:
Direct	402.5	395.6
Assumed	21.4	1.3
Ceded	(6.4)	(6.1)
Unearned Premiums From Affiliates	$ 417.5	$ 390.8

Reinsurance - External Reinsu72

Reinsurance - External Reinsurance on Income Statements (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Written premiums:
Assumed	$ 1,269.3	$ 1,293.3	$ 1,273.5
Ceded	(843.8)	(815.5)	(823.4)
Net written premiums	425.5	477.8	450.1
Earned premiums:
Direct	828.7	840.6	863.1
Assumed	1,275.1	1,291.9	1,270.5
Ceded	(817.1)	(819.9)	(832.9)
Losses and loss expenses incurred:
Assumed	920	944.1	864.2
Ceded	(540.4)	(560.3)	(570.9)
Net losses and loss expenses incurred	379.6	383.8	293.3
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	835.6	835.4	854.1
Assumed	31.1	5	4.7
Ceded	(22.9)	(24.9)	(35.4)
Net written premiums	843.8	815.5	823.4
Earned premiums:
Direct	828.7	840.6	863.1
Assumed	11	4.9	4.5
Ceded	(22.6)	(25.6)	(34.7)
Net earned premiums	817.1	819.9	832.9
Losses and loss expenses incurred:
Direct	530.9	559.1	569
Assumed	10.4	3.9	3.4
Ceded	(2.5)	(4.4)	(2.9)
Net losses and loss expenses incurred	$ 538.8	$ 558.6	$ 569.5

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, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
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			$ 323.1	$ 310.5
Mandatorily redeemable preferred capital securities issued to third party		$ 15
Common securities issued to state auto financial		0.5	$ 10.2	$ 2.2	$ 6.2
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			$ 2	$ 1.9	$ 1.8
Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate (percent)			5.68%	5.13%
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, 2017	Dec. 31, 2016
Deferred Policy Acquisition Costs:
Deferred Policy Acquisition Costs Ceded	$ (53.1)	$ (79.1)
Deferred Policy Acquisition Costs Assumed	117.8	129.8
Deferred Policy Acquisitions Costs from Affiliates	64.7	50.7
Losses and loss expenses payable:
Ceded	(541.1)	(547.1)
Assumed	1,252.5	1,178
Losses And Loss Expenses Payable From Affiliates	711.4	630.9
Unearned premiums:
Ceded	(417.5)	(390.8)
Assumed	605.4	611.7
Unearned Premiums From Affiliates	187.9	220.9
Pension and Other Postretirement Benefits:
Postretirement and Pension Benefits Ceded	(99.3)	(114.5)
Postretirement and Pension Benefits Assumed	64.5	74.4
Postretirement and Pension Benefits Ceded to Affiliates	(34.8)	(40.1)
Other Liabilities Ceded	(43)	(42.9)
Other Liabilities Assumed	58.5	53.9
Other Liabilities	15.5	11
Accumulated Other Comprehensive Income Ceded	(144.7)	(153.5)
Accumulated Other Comprehensive Income Assumed	94	99.8
Net Accumulated Other Comprehensive Income	$ (50.7)	$ (53.7)

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Written premiums:
Ceded	$ (843.8)	$ (815.5)	$ (823.4)
Assumed	1,269.3	1,293.3	1,273.5
Premiums Written, Net	425.5	477.8	450.1
Earned premiums:
Ceded	(817.1)	(819.9)	(832.9)
Assumed	1,275.1	1,291.9	1,270.5
Premiums Earned, Net	458	472	437.6
Losses and loss expenses incurred:
Ceded	(540.4)	(560.3)	(570.9)
Assumed	920	944.1	864.2
Losses and loss expenses incurred, Net	379.6	383.8	293.3
Acquisition and Operating Expenses [Abstract]
Ceded	(189.7)	(148.5)	(183.9)
Assumed	460.4	434.3	432.5
Acquisition and Operating Expenses, Net	$ 270.7	$ 285.8	$ 248.6

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 11.7	$ 6.7	$ 23.5
Tax-exempt interest and dividends received deduction	(5.7)	(7.1)	(8.7)
Other, net	1.7	(1.4)	1.3
Federal income tax expense (benefit)	7.7	(1.8)	16.1
Impact of TCJA at enactment	36.4	0	0
Total federal income tax expense (benefit)	$ 44.1	$ (1.8)	$ 16.1
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	(17.10%)	(37.20%)	(13.00%)
Other, net (percent)	5.40%	(7.20%)	1.90%
Effective Income Tax Rate Reconciliation, Deduction, Percent	23.30%	(9.40%)	23.90%
Impact of TCJA at enactment, Percent	108.70%	0.00%	0.00%
Federal income tax expense (benefit) and effective rate (percent)	132.00%	(9.40%)	23.90%

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, 2017	Dec. 31, 2016
Deferred tax assets:
Unearned premiums not currently deductible	$ 25.5	$ 42.6
Losses and loss expenses payable discounting	22	19.7
Postretirement and pension benefits	13.6	26.1
Realized loss on other-than-temporary impairment	2.1	5.8
Other liabilities	9.2	16.6
Net operating loss carryforward	34.3	58.9
Tax credit carryforward	3.7	3.3
Other	2.5	7
Total deferred tax assets	112.9	180
Deferred tax liabilities:
Deferral of policy acquisition costs	24.7	45.4
Net unrealized holding gains on investments	20.3	32.5
Losses and loss expenses payable discounting (transition rule)	10.7	0
Total deferred tax liabilities	55.7	77.9
Net deferred federal income taxes	$ 57.2	$ 102.1

Federal Income Taxes - Addition

Federal Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%	35.00%
Corporate income tax rate due to enactment of TCJA	21.00%
Document Period End Date	Dec. 31, 2017
Impact of TCJA at enactment	$ 36,400,000	$ 0	$ 0
Pre-tax provisional change in discounted loss reserves attributable to TCJA	50,900,000
Tax Adjustments, Settlements, and Unusual Provisions	1,600,000
Deferred tax asset related to net operating loss carryforwards	34,300,000	58,900,000
Balance for uncertain tax positions	0
Interest and penalties payable	$ 0	$ 0

Pension and Postretirement Be80

Pension and Postretirement Benefit Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Document Period End Date		Dec. 31, 2017
Other Invested Assets Fair Value Disclosure		$ 45,200,000	$ 35,700,000
Available-for-sale Securities		$ 2,614,100,000	2,537,200,000
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	$ 200,000
Postretirement plan's estimated future subsidies		$ 100,000
Employer contribution		$ 10,000,000
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		$ 6,400,000	6,300,000	$ 5,100,000
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Participant's contribution (percent)		2.00%
Private Equity Funds, Foreign [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure			11,300,000
Postretirement Benefit Costs
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement plan's gross benefit payments		$ 1,800,000
Employer contribution		0	0
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Postretirement plan's gross benefit payments		11,200,000
Pension | Private Equity Funds, Foreign [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Other Invested Assets Fair Value Disclosure		$ 26,600,000	$ 20,000,000
Other Invested Assets Fair Value Disclosure		2

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Actuarial loss (gain)	$ 1	$ 3	$ (5.3)
Change in plan assets available for plan benefits:
Employer contribution	10
Pension Costs
Change in benefit obligation:
Benefit obligation at beginning of year	284.5	281.3
Service cost	5.6	6.1
Interest cost	11.1	11.6
Actuarial loss (gain)	17	4.5
Benefits paid	(14)	(19)
The Company’s portion of benefit obligation at end of year	304.2	284.5	281.3
Change in plan assets available for plan benefits:
Fair value of plan assets available for plan benefits at beginning of year	235.7	204.4
Employer contribution	9.8	34.5
Actual return on plan assets	33.5	15.8
Benefits paid	(14)	(19)
The Company’s portion of fair value of plan assets at end of year	265	235.7	204.4
Supplemental executive retirement plan	(7)	(6.8)
Funded status at end of year	(46.2)	(55.6)
Accumulated benefit obligation end of year	287.9	267.5
Postretirement Benefit Costs
Change in benefit obligation:
Benefit obligation at beginning of year	18.8	20.3
Service cost	0	0
Interest cost	0.7	0.8
Actuarial loss (gain)	0.6	(0.9)
Benefits paid	(1.8)	(1.4)
The Company’s portion of benefit obligation at end of year	18.3	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	0	0
Employer contribution	0	0
Actual return on plan assets	0	0
Benefits paid	0	0
The Company’s portion of fair value of plan assets at end of year	0	0	$ 0
Supplemental executive retirement plan	0	0
Funded status at end of year	$ (18.3)	$ (18.8)

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	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan [Abstract]
Prior service benefit	$ (48.6)	$ (54.1)
Net actuarial loss	111.6	118.5
Total	$ 63	$ 64.4

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, 2018USD ($)
Defined Benefit Plan [Abstract]
Prior service benefit	$ (5.5)
Net actuarial loss	8.4
Total	$ 2.9

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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amortization of:
Prior service benefit	$ (5.5)
Pension
Components of net periodic cost:
Service cost		$ 5.7	$ 6.2	$ 7.9
Interest cost		11.4	11.9	11.3
Expected return on plan assets		(16.7)	(15.1)	(13.8)
Amortization of:
Prior service benefit		0	0	0
Net actuarial loss		7.8	9.2	10.9
Net periodic cost (benefit)		8.2	12.2	16.3
Postretirement
Components of net periodic cost:
Service cost		0	0	0
Interest cost		0.8	0.9	1.1
Expected return on plan assets		0	0	0
Amortization of:
Prior service benefit		(5.5)	(5.5)	(5.4)
Net actuarial loss		0.2	0.2	0.6
Net periodic cost (benefit)		$ (4.5)	$ (4.4)	$ (3.7)

Pension and Postretirement Be85

Pension and Postretirement Benefit Plans-Company's Share of Expected Future Benefits Payments Schedule (Detail) $ in Millions	Dec. 31, 2017USD ($)
Pension
Schedule of Expected Future Pension Benefit Payment [Line Items]
2,018	$ 11.2
2,019	11.5
2,020	12
2,021	12.5
2,022	12.9
2023 - 2027	76.2
Postretirement
Schedule of Expected Future Pension Benefit Payment [Line Items]
2,018	1.5
2,019	1.5
2,020	1.4
2,021	1.4
2,022	1.3
2023 - 2027	$ 5.9

Pension and Postretirement Be86

Pension and Postretirement Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Plans' Obligations (Detail)	Dec. 31, 2017	Dec. 31, 2016
Pension
Benefit obligations weighted-average assumptions:
Discount rate (percent)	3.50%	4.00%
Rates of increase in compensation levels (percent)	3.25%	3.50%
Postretirement
Benefit obligations weighted-average assumptions:
Discount rate (percent)	3.50%	4.00%
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate (percent)	4.00%	4.20%	3.85%
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)	4.00%	4.20%	3.85%
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Assumed health care cost trend rates:
Health care cost trend rate assumed for the next year (percent)	6.00%	6.50%	6.50%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) (percent)	3.80%	3.90%	3.80%
Year that the rate reaches the ultimate trend rate	2,075	2,075	2,076

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, 2017USD ($)
One percentage point change:
Effect on total service and interest cost	$ 0.1
Effect on accumulated postretirement benefit obligation	1.7
Postretirement
One percentage point change:
Effect on total service and interest cost	(0.1)
Effect on accumulated postretirement benefit obligation	$ (1.5)

Pension and Postretirement Be90

Pension and Postretirement Benefit Plans - Summary of Asset Allocation Targets (Detail)	Dec. 31, 2018
Asset Category:
Asset allocation target (percent)	100.00%
Fixed maturity
Asset Category:
Asset allocation target (percent)	52.00%
Large-cap securities
Asset Category:
Asset allocation target (percent)	24.20%
Small-cap securities
Asset Category:
Asset allocation target (percent)	10.30%
International equity
Asset Category:
Asset allocation target (percent)	9.40%
Emerging market equity
Asset Category:
Asset allocation target (percent)	4.10%

Pension and Postretirement Be91

Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Document Period End Date	Dec. 31, 2017
Available-for-sale Securities	$ 2,614.1	$ 2,537.2
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	231.9	200.9
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	135.2	113
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	70.1	60.4
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	60.7	48
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	4.4	4.6
Pension | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	96.7	85.9
Pension | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	71.6	59.6
Pension | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities		26.3
Pension | Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	25.1
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	96.7	85.9
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	96.7	85.9
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	71.6	59.6
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
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	25.1
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	135.2	115
Pension | Significant other observable inputs (Level 2) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	135.2	113
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	70.1	60.4
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	60.7	48
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	4.4	4.6
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
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 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, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 2,614.1	$ 2,537.2

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ 32.5	$ 37.6	$ 71.7
Other comprehensive income before reclassifications	44.5	15	(20.1)
Amounts reclassified from AOCI	40.3	20.1	14
Net current period other comprehensive income	4.2	(5.1)	(34.1)
Ending balance	36.7	32.5	37.6
Unrealized Gains and Losses on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	62.8	68.5	110
Other comprehensive income before reclassifications	45.5	18	(25.4)
Amounts reclassified from AOCI	42.3	23.7	16.1
Net current period other comprehensive income	3.2	(5.7)	(41.5)
Ending balance	66	62.8	68.5
Benefit Plan Items
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(30.3)	(30.9)	(38.3)
Other comprehensive income before reclassifications	(1)	(3)	5.3
Amounts reclassified from AOCI	(2)	(3.6)	(2.1)
Net current period other comprehensive income	1	0.6	7.4
Ending balance	$ (29.3)	$ (30.3)	$ (30.9)

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	$ 65.1	$ 36.5	$ 24.3
Tax expense	(44.1)	1.8	(16.1)
Total reclassifications for the period	40.3	20.1	14
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	40.3	20.1	14
Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	42.3	23.7	16.1
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	65.1	36.5	24.7
Total before tax	65.1	36.5	24.7
Tax expense	(22.8)	(12.8)	(8.6)
Total reclassifications for the period	(42.3)	(23.7)	(16.1)
Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(2)	(3.6)	(2.1)
Benefit Plan Items | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(2.5)	(3.9)	(6.1)
Tax expense	0.5	0.3	4
Negative prior service costs	5.5	5.5	5.4
Net loss	(8)	(9.4)	(11.5)
Total reclassifications for the period	$ 2	$ 3.6	$ 2.1

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 85.8
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	$ 15	$ 10	$ 15

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stockholders' Equity Note [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 858.4				$ 844.4				$ 858.4	$ 844.4
Net liabilities of non-insurance parent and affiliates	(89.8)				(84.9)				(89.8)	(84.9)
Total	768.6				759.5				768.6	759.5
Increases (decreases):
Deferred acquisition costs	117.8				129.8				117.8	129.8	$ 129.1	$ 126.5
Postretirement and pension benefits	22.3				22.7				22.3	22.7
Deferred federal income taxes	(61.2)				(49.1)				(61.2)	(49.1)
Fixed maturities, at fair value	19.6				13.5				19.6	13.5
Other, net	(13.8)				(14.9)				(13.8)	(14.9)
Stockholders’ equity per accompanying consolidated financial statements	880.9				891.3				880.9	891.3	884.6
Statutory net (loss) income of insurance subsidiaries									42.8	22.7	65.4
Net Income (Loss) Attributable to Parent									(13.1)	(3)	(4.1)
Total									(29.7)	(19.7)	(61.3)
Deferred acquisition costs									(12)	0.7	2.5
Postretirement and pension benefits									4.3	4.4	4.2
Deferred federal income taxes									(34.6)	(5.4)	(12.2)
Share-based compensation expense									0	0	(0.1)
Other, net									1.9	1.6	(4.5)
Net (loss) income	$ (5.8)	$ (9.5)	$ 8.7	$ (4.1)	$ 32.5	$ 10.1	$ (24.6)	$ 3	$ (10.7)	$ 21	$ 51.2

Preferred Stock Preferred Stock

Preferred Stock Preferred Stock (Details)	12 Months Ended
	Dec. 31, 2017class_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)	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value	$ 16,100,000
Share Based Compensation Arrangement By Share Based Payment Award Options Exercisable Outstanding Aggregate Intrinsic Value	17,100,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	3,500,000	$ 3,000,000	$ 2,600,000
Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits, Share-based Compensation Cost	$ 1,500,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense		1,400,000	300,000
Common Stock, Capital Shares Reserved for Future Issuance | shares	3,700,000
Document Period End Date	Dec. 31, 2017
Common shares valued (shares)	$ 123,000,000	121,600,000
Share-based compensation	4,200,000	$ 3,700,000	$ 4,500,000
Tax Adjustments, Settlements, and Unusual Provisions	$ 1,600,000
Share-based compensation performance unit award minimum award earned	0.00%
Share-based compensation performance unit award maximum award earned	500.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross | shares	0	200,000	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 | shares	3,500,000
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	$ 2,300,000
Common Stock, Capital Shares Reserved for Future Issuance | shares	2,400,000
Number Of Securities Available For Issuance Under Equity Compensation Plans | shares	2,200,000
Maximum value of awards that may be granted in any calendar year	5
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized | shares	250,000
Plan Termination Date	May 4, 2027
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	$ 2,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	3 years
Share Based Compensation Stock Options Expiration Period	10 years
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)	$ 106,581	$ 753,000
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 | shares	1,400
Restricted Stock Plan Termination Date	May 31, 2026
Stock Issued During Period, Shares, Restricted Stock Award, Gross | shares	26,323	22,832	26,184
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 | shares	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 | shares	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 | $ / shares	$ 10.01
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit | $ / shares	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 | $ / shares	20.01
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, Upper Range Limit | $ / shares	$ 30

Share-Based Compensation - Summ

Share-Based Compensation - Summary of Non-Vested and Vested Restricted Shares and Changes (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding, Number	219,447	107,297	111,084	76,472
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value, Outstanding	$ 25.65	$ 22.04	$ 22.19	$ 19.06
Weighted Average Grant Date Fair Value
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	177,846	45,252	74,020
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Intrinsic Value, Amount Per Share	$ 27.20	$ 21.55	$ 22.83
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period	26,271	33,414	35,859
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Intrinsic Value, Amount Per Share	$ 21.39	$ 21.93	$ 16.88
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period	(39,425)	(15,625)	(3,549)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value	$ 25.68	$ 21.91	$ 21.92

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
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
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Dividend Rate	1.87%	1.75%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Risk Free Interest Rate	1.30%	1.60%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate	36.27%	36.61%
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Term	5 years 3 months 18 days	6 years

Share-Based Compensation - S101

Share-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number	1.7	2.5	3.1	3.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	0	0.2	0.3
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(0.4)	(0.4)	(0.3)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 19.22	$ 20.63	$ 21.45	$ 21.37
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 19.38	17.18	16.54
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	1.7
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 0	$ 21.55	$ 22.86
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period	(0.4)	(0.4)	(0.3)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price	$ 28.96	$ 30.70	$ 25.63

Share-Based Compensation - S102

Share-Based Compensation - Summary of Stock Options Outstanding and Exercisable (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Share Based Compensation Arrangement [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number	1.7
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	4 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 19.22	$ 20.63	$ 21.45	$ 21.37
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	1.6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 18.98
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
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	$ 16.53
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 16.53
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	0.7
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	4 years 8 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price	$ 23.07
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number	0.6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 23.33

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net (loss) earnings for basic net earnings per common share	$ (5.8)	$ (9.5)	$ 8.7	$ (4.1)	$ 32.5	$ 10.1	$ (24.6)	$ 3	$ (10.7)	$ 21	$ 51.2
Adjusted net (loss) earnings for dilutive net (loss) earnings per common share									$ (10.7)	$ 21	$ 51.2
Denominator:
Weighted average shares for basic net earnings (loss) per common share (shares)									42.1	41.6	41.1
Effect of dilutive share-based awards (shares)									0	0.4	0.5
Adjusted weighted average shares for diluted net earnings (loss) per common share (shares)									42.1	42	41.6
Basic net earnings (loss) per common share (usd per share)	$ (0.14)	$ (0.23)	$ 0.21	$ (0.10)	$ 0.78	$ 0.24	$ (0.59)	$ 0.07	$ (0.25)	$ 0.50	$ 1.25
Diluted net earnings (loss) per common share (usd per share)	$ (0.14)	$ (0.23)	$ 0.21	$ (0.10)	$ 0.77	$ 0.24	$ (0.59)	$ 0.07	$ (0.25)	$ 0.50	$ 1.23

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Number of options and awards (shares)	0.5	1.2	1.5

Reportable Segments - Additiona

Reportable Segments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017Segment
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
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from external sources:
Earned premiums, net									$ 458	$ 472	$ 437.6
Supplementary Insurance Information, Premium Revenue									1,275.1	1,291.9	1,270.5
Net investment income									78.8	74.7	71.7
Income (loss) before federal income tax									33.4	19.2	67.3
Intersegment revenues	$ 362.9	$ 358.4	$ 355.7	$ 344.3	$ 364.9	$ 352.8	$ 348.5	$ 339.2	1,421.3	1,405.4	1,368.6
Segment loss before federal income tax:
Income (loss) before federal income tax									33.4	19.2	67.3
Net investment income									78.8	74.7	71.7
Income (loss) before federal income tax									33.4	19.2	67.3
Segment assets:
Total segment assets	2,781.2				2,663.7				2,781.2	2,663.7
Reconciling items:
Total consolidated assets	3,014.3				2,959.4				3,014.3	2,959.4
Interest expense (affiliates $0.8, $0.8 and $0.7, respectively)									5.9	5.5	5.4
Personal Insurance Segments [Member]
Revenues from external sources:
Supplementary Insurance Information, Premium Revenue									580.3	578.5	591.3
Commercial insurance
Revenues from external sources:
Supplementary Insurance Information, Premium Revenue									455.7	472.6	476.5
Specialty insurance
Revenues from external sources:
Supplementary Insurance Information, Premium Revenue									239.1	240.8	202.7
Total investment operations segment
Revenues from external sources:
Supplementary Insurance Information, Premium Revenue									0	0	0
Segment assets:
Total segment assets	2,781.2				2,663.7				2,781.2	2,663.7
Total internal and external revenue [Member]
Revenues from external sources:
Intersegment revenues									1,427.3	1,411.2	1,374.2
Segment Reconciling Items [Member]
Revenues from external sources:
Income (loss) before federal income tax									(20.2)	(7.8)	(3.9)
Intersegment revenues									6	5.8	5.6
Segment loss before federal income tax:
Income (loss) before federal income tax									(20.2)	(7.8)	(3.9)
Income (loss) before federal income tax									(20.2)	(7.8)	(3.9)
Reconciling items:
Gaap Expense Adjustments									(10.2)	2	4.2
Interest expense (affiliates $0.8, $0.8 and $0.7, respectively)									(5.9)	(5.5)	(5.4)
Corporate Expenses									(4.1)	(4.3)	(2.7)
Corporate, Non-Segment [Member]
Reconciling items:
Total consolidated assets	$ 233.1				$ 295.7				233.1	295.7
Intersegment Eliminations [Member]
Revenues from external sources:
Intersegment revenues									(6)	(5.8)	(5.6)
Reportable segments
Revenues from external sources:
Intersegment revenues									1,419	1,403.1	1,366.9
Reportable segments | Personal Insurance Segments [Member]
Revenues from external sources:
Earned premiums, net											591.3
Income (loss) before federal income tax									(23.3)	(10.3)	27.5
Segment loss before federal income tax:
Income (loss) before federal income tax									(23.3)	(10.3)	27.5
Income (loss) before federal income tax									(23.3)	(10.3)	27.5
Reportable segments | Commercial insurance
Revenues from external sources:
Earned premiums, net											476.5
Income (loss) before federal income tax									(10.6)	(31.6)	(46.7)
Segment loss before federal income tax:
Income (loss) before federal income tax									(10.6)	(31.6)	(46.7)
Income (loss) before federal income tax									(10.6)	(31.6)	(46.7)
Reportable segments | Specialty insurance
Revenues from external sources:
Earned premiums, net											202.7
Income (loss) before federal income tax									(57)	(42.7)	(5.9)
Segment loss before federal income tax:
Income (loss) before federal income tax									(57)	(42.7)	(5.9)
Income (loss) before federal income tax									(57)	(42.7)	(5.9)
Reportable segments | Total insurance segments
Revenues from external sources:
Earned premiums, net										1,291.9	1,270.5
Income (loss) before federal income tax									(90.9)	(84.6)	(25.1)
Segment loss before federal income tax:
Income (loss) before federal income tax									(90.9)	(84.6)	(25.1)
Income (loss) before federal income tax									(90.9)	(84.6)	(25.1)
Reportable segments | Net investment income
Revenues from external sources:
Net investment income									78.8	74.7	71.7
Segment loss before federal income tax:
Net investment income									78.8	74.7	71.7
Reportable segments | Net realized capital gains
Revenues from external sources:
Income (loss) before federal income tax									65.1	36.5	24.7
Segment loss before federal income tax:
Income (loss) before federal income tax									65.1	36.5	24.7
Income (loss) before federal income tax									65.1	36.5	24.7
Reportable segments | Total investment operations segment
Revenues from external sources:
Total investment operations segment									143.9	111.2	96.4
Segment loss before federal income tax:
Total investment operations segment									143.9	111.2	96.4
Reportable segments | All other
Revenues from external sources:
Income (loss) before federal income tax									0.6	0.4	(0.1)
Intersegment revenues									2.3	2.3	1.7
Segment loss before federal income tax:
Income (loss) before federal income tax									0.6	0.4	(0.1)
Income (loss) before federal income tax									$ 0.6	$ 0.4	$ (0.1)

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 362.9	$ 358.4	$ 355.7	$ 344.3	$ 364.9	$ 352.8	$ 348.5	$ 339.2	$ 1,421.3	$ 1,405.4	$ 1,368.6
(Loss) income before federal income taxes	37.9	(11.2)	11.9	(5.2)	33.1	7.4	(25.1)	3.8
Net (loss) income	$ (5.8)	$ (9.5)	$ 8.7	$ (4.1)	$ 32.5	$ 10.1	$ (24.6)	$ 3	$ (10.7)	$ 21	$ 51.2
(Loss) earnings per common share:
Basic (usd per share)	$ (0.14)	$ (0.23)	$ 0.21	$ (0.10)	$ 0.78	$ 0.24	$ (0.59)	$ 0.07	$ (0.25)	$ 0.50	$ 1.25
Diluted (usd per share)	$ (0.14)	$ (0.23)	$ 0.21	$ (0.10)	$ 0.77	$ 0.24	$ (0.59)	$ 0.07	$ (0.25)	$ 0.50	$ 1.23

Schedule I - Summary of Inve108

Schedule I - Summary of Investments - Other Than Investments in Related Parties (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	$ 2,517.5
Fair value	2,614.1
Amount at which shown in the balance sheet	2,614.1
Available-for-sale Securities, Amortized Cost Basis	2,517.5	$ 2,444.5
Available-for-sale Securities	2,614.1	$ 2,537.2
Equity securities: | Available-for-sale:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	318.6
Fair value	365.3
Amount at which shown in the balance sheet	365.3
Equity securities: | Large-cap securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	62.4
Fair value	96.8
Amount at which shown in the balance sheet	96.8
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	256.2
Fair value	268.5
Amount at which shown in the balance sheet	268.5
Other Invested Assets [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	25.8
Fair value	56
Amount at which shown in the balance sheet	56
Other invested assets
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	5.6
Fair value	5.6
Amount at which shown in the balance sheet	5.6
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	2,523.1
Other than investments in related parties, fair value [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair value	2,619.7
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	2,619.7
Available-for-sale: | Fixed maturities:
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	2,173.1
Fair value	2,192.8
Amount at which shown in the balance sheet	2,192.8
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	433.8
Fair value	436.9
Amount at which shown in the balance sheet	436.9
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	507.1
Fair value	525.8
Amount at which shown in the balance sheet	525.8
Available-for-sale: | Fixed maturities: | Corporate securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost or amortized cost	527.5
Fair value	529.7
Amount at which shown in the balance sheet	529.7
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	704.7
Fair value	700.4
Amount at which shown in the balance sheet	$ 700.4

Schedule II - Condensed Fina109

Schedule II - Condensed Financial Information of Registrant - Condensed Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Fixed maturities, available-for-sale, at fair value	$ 2,192.8	$ 2,109.3
Other invested assets	5.6	5.4
Cash and cash equivalents	91.5	51.1
Other assets	13.8	14.9
Federal income tax, net	57.2	102.1
Total assets	3,014.3	2,959.4
Liabilities and Stockholders’ Equity
Notes payable (affiliates $116.5 and $116.5, respectively)	122.1	122.1
Due to affiliate	2.7	2.4
Other liabilities	76.7	69.8
Total liabilities	2,133.4	2,068.1
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 49.2 and 48.6 issued, respectively, at stated value of $2.50 per share	123	121.6
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.8)	(116.5)
Additional paid-in capital	171.8	159.9
Accumulated other comprehensive income	36.7	32.5	$ 37.6	$ 71.7
Retained earnings	666.2	693.8
Stockholders’ equity per accompanying consolidated financial statements	880.9	891.3	884.6
Total liabilities and stockholders' equity	3,014.3	2,959.4
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)	979.5	983.6
Fixed maturities, available-for-sale, at fair value	0	0.9
Other invested assets	4.2	3.5
Cash and cash equivalents	8.8	8.1	$ 10.5	$ 11.5
Other assets	0.1	0.2
Federal income tax, net	15.8	21.5
Total assets	1,008.4	1,017.8
Liabilities and Stockholders’ Equity
Notes payable (affiliates $116.5 and $116.5, respectively)	116.5	116.5
Due to affiliate	0.5	1.3
Other liabilities	10.5	8.7
Total liabilities	127.5	126.5
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 49.2 and 48.6 issued, respectively, at stated value of $2.50 per share	123	121.6
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.8)	(116.5)
Additional paid-in capital	171.8	159.9
Accumulated other comprehensive income	36.7	32.5
Retained earnings	666.2	693.8
Stockholders’ equity per accompanying consolidated financial statements	880.9	891.3
Total liabilities and stockholders' equity	1,008.4	1,017.8
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, 2017	Dec. 31, 2016
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)	49,200,000	48,600,000
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)	49,200,000	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
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net investment income									$ 78.8	$ 74.7	$ 71.7
Net realized gain on investments									65.1	36.5	24.3
Total revenues	$ 362.9	$ 358.4	$ 355.7	$ 344.3	$ 364.9	$ 352.8	$ 348.5	$ 339.2	1,421.3	1,405.4	1,368.6
Interest expense (affiliates $6.1, $6.1 and $6.0, respectively)									5.9	5.5	5.4
Other operating expenses									7.9	7.9	6.3
Total expenses									1,387.9	1,386.2	1,301.3
Income before federal income taxes									33.4	19.2	67.3
Federal income tax expense (benefit)									(44.1)	1.8	(16.1)
Net Income (Loss) Attributable to Parent									(13.1)	(3)	(4.1)
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net investment income									0.4	0.2	0.3
Net realized gain on investments									0.3	0	0
Total revenues									0.7	0.2	0.3
Interest expense (affiliates $6.1, $6.1 and $6.0, respectively)									6.1	6.1	6
Other operating expenses									6.7	7	4.9
Total expenses									12.8	13.1	10.9
Income before federal income taxes									(12.1)	(12.9)	(10.6)
Federal income tax expense (benefit)									5.1	(6)	(2.8)
Net (loss) income before equity in net income of subsidiaries									(17.2)	(6.9)	(7.8)
Equity in net income of subsidiaries									6.5	27.9	59
Net Income (Loss) Attributable to Parent									$ (10.7)	$ 21	$ 51.2

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, 2017	Dec. 31, 2016	Dec. 31, 2015
State Auto Financial
Condensed Income Statements, Captions [Line Items]
Interest Expense Debt To Affiliates, Parent Company	$ 6.1	$ 6.1	$ 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, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	$ (13.1)	$ (3)	$ (4.1)
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	69	27.8	(39.2)
Reclassification adjustments for gains realized in net income	(65.1)	(36.5)	(24.7)
Income tax expense	(0.7)	3	22.4
Total change in net unrealized holding gains (losses) on investments	3.2	(5.7)	(41.5)
Other comprehensive income (loss)	4.2	(5.1)	(34.1)
Comprehensive Income (loss)	(6.5)	15.9	17.1
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent	(10.7)	21	51.2
Net unrealized holding gains on investments:
Unrealized holding gain arising during the year	0.3	0.2	(0.1)
Reclassification adjustments for gains realized in net income	0.3	0	0
Income tax expense	(0.1)	(0.1)	0
Total change in net unrealized holding gains (losses) on investments	0.5	0.1	(0.1)
Unrealized equity in subsidiaries	3.7	(5.2)	(34)
Other comprehensive income (loss)	4.2	(5.1)	(34.1)
Comprehensive Income (loss)	$ (6.5)	$ 15.9	$ 17.1

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent		$ (13.1)	$ (3)	$ (4.1)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net		12.5	14.6	15.7
Share-based compensation		4.2	3.7	4.5
Gain (Loss) on Sale of Investments		65.1	36.5	24.3
Changes in operating assets and liabilities:
Other liabilities and due from affiliates		6	7.3	(39.9)
Other assets		1.9	1.6	(4.5)
Excess tax expense on share-based awards		0	(0.2)	(0.3)
Federal income taxes, net		46.3	4.5	13.7
Net cash provided by operating activities		67.9	113.5	149.8
Cash flows from investing activities:
Purchases of equity securities - available-for-sale		(185.9)	(143.1)	(154)
Purchases of other invested assets		(1.4)	(1.5)	(6.9)
Maturities, calls and pay downs of fixed maturities - available-for-sale		233.6	240.9	241
Sales of fixed maturities - available-for-sale		184.8	211.6	180.7
Sales of equity securities - available-for-sale		252.7	153	144.8
Net cash used in investing activities		(20.5)	(127.6)	(167.7)
Cash flows from financing activities:
Proceeds from issuance of common stock	$ 0.5	10.2	2.2	6.2
Payments to acquire treasury stock		(0.3)	(0.2)	(0.3)
Payment of dividends		(16.9)	(16.6)	(16.5)
Excess tax expense on share-based awards		0	0.2	0.3
Proceeds from long-term debt		0	21.5	0
Net cash (used in) provided by financing activities		(7)	7.1	(10.3)
Net increase (decrease) in cash and cash equivalents		40.4	(7)	(28.2)
Cash and cash equivalents at beginning of year		51.1
Cash and cash equivalents at end of year		91.5	51.1
Supplemental disclosures:
Federal income tax received			0	6.4
Interest paid (affiliates $6.1, $6.1 and $6.0, respectively)		5.7	5.4	5.3
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Net Income (Loss) Attributable to Parent		(10.7)	21	51.2
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization, net		0	(0.3)	0
Share-based compensation		1.1	1.2	(0.3)
Gain (Loss) on Sale of Investments		(0.3)	0	0
Equity in net income from consolidated subsidiaries		(6.5)	(27.9)	(59)
Changes in operating assets and liabilities:
Other liabilities and due from affiliates		0	0.9	0.9
Other assets		0	(0.1)	0
Excess tax expense on share-based awards		0	(0.2)	(0.5)
Federal income taxes, net		5.6	3.3	(0.7)
Net cash provided by operating activities		(10.8)	(1.9)	(8.4)
Cash flows from investing activities:
Dividends received from consolidated subsidiaries		17.7	14.2	18
Purchases of other invested assets		(0.4)	(0.3)	(0.3)
Maturities, calls and pay downs of fixed maturities - available-for-sale		1.2	0	0
Net cash used in investing activities		18.5	13.9	17.7
Cash flows from financing activities:
Proceeds from issuance of common stock		10.2	2.2	6.2
Payments to acquire treasury stock		(0.3)	(0.2)	(0.3)
Payment of dividends		(16.9)	(16.6)	(16.5)
Excess tax expense on share-based awards		0	0.2	0.3
Net cash (used in) provided by financing activities		(7)	(14.4)	(10.3)
Net increase (decrease) in cash and cash equivalents		0.7	(2.4)	(1)
Cash and cash equivalents at beginning of year		8.1	10.5	11.5
Cash and cash equivalents at end of year		8.8	8.1	10.5
Supplemental disclosures:
Federal income tax received		0.5	3.4	2.3
Interest paid (affiliates $6.1, $6.1 and $6.0, respectively)		$ (6.1)	$ (6.1)	$ (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, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$ 0.8	$ 0.8	$ 0.7
State Auto Financial
Condensed Financial Statements, Captions [Line Items]
Interest paid, affiliates	$ 6.1	$ 6.1	$ 6

Schedule II - Condensed Fina116

Schedule II - Condensed Financial Information of Registrant - Additional Information (Detail)	Jul. 11, 2013USD ($)Agreementsubsidiary	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
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		105,400,000	102,500,000
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		89,600,000	87,100,000
Notes Payable | Milbank
Condensed Financial Statements, Captions [Line Items]
Debt instrument, issued	$ 15,000,000	15,000,000	15,000,000
Fair Value		$ 15,800,000	$ 15,400,000

Schedule III - Supplementary117

Schedule III - Supplementary Insurance Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	$ 117.8	$ 129.8	$ 129.1
Future benefits, claims and losses	1,252.5	1,178	1,047.1
Unearned premiums	605.4	611.4	609.1
Other policy claims and benefits payable	0	0	0
Premium revenue	1,275.1	1,291.9	1,270.5
Net investment income	78.8	74.7	71.7
Benefits, losses and settlement expenses	920	944.1	864.2
Amortization of deferred policy acquisition costs	257.7	290.3	283
Other operating expenses	198.1	140	143.8
Premiums written	1,269.3	1,293.3	1,273.5
Personal insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	50.9	47	47.1
Future benefits, claims and losses	256.1	254.5	227
Unearned premiums	288.1	258.7	260.1
Other policy claims and benefits payable	0	0	0
Premium revenue	580.3	578.5	591.3
Net investment income	0	0	0
Benefits, losses and settlement expenses	416.3	422	396.2
Amortization of deferred policy acquisition costs	102.1	103.3	104.6
Other operating expenses	89.3	62.7	61.2
Premiums written	609.7	577.2	581
Commercial insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	47.9	47.4	49.9
Future benefits, claims and losses	604.3	602.5	553.6
Unearned premiums	225.6	227.9	240.1
Other policy claims and benefits payable	0	0	0
Premium revenue	455.7	472.6	476.5
Net investment income	0	0	0
Benefits, losses and settlement expenses	284.2	331.5	338.6
Amortization of deferred policy acquisition costs	95.7	110.4	108.6
Other operating expenses	90.4	61.6	74.2
Premiums written	453.6	459.4	481.5
Specialty insurance segment
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition cost	19	35.4	32.1
Future benefits, claims and losses	392.1	321	266.5
Unearned premiums	91.7	124.8	108.9
Other policy claims and benefits payable	0	0	0
Premium revenue	239.1	240.8	202.7
Net investment income	0	0	0
Benefits, losses and settlement expenses	219.5	190.6	129.4
Amortization of deferred policy acquisition costs	59.9	76.6	69.8
Other operating expenses	18.4	15.7	8.4
Premiums written	206	256.7	211
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	78.8	74.7	71.7
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 828.7	$ 840.6	$ 863.1
Ceded to	817.1	819.9	832.9
Assumed from	1,275.1	1,291.9	1,270.5
Net Amount	$ 1,275.1	$ 1,291.9	$ 1,270.5
Percentage of amount assumed to net (percent)	0.90%	0.40%	0.40%
Affiliated entity
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	$ 817.1	$ 819.9	$ 832.9
Assumed from	1,275.1	1,291.9	1,270.5
Net Amount	1,275.1	1,291.9	1,270.5
Unaffiliated Companies
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded to	22.6	25.6	34.7
Assumed from	$ 11	$ 4.9	$ 4.5