State Auto Financial (STFC) 10-Q 3 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	Apr. 28, 2017
Document Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Entity Registrant Name	State Auto Financial CORP
Entity Central Index Key	874,977
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding (shares)		42,002,494

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Document Period End Date	Mar. 31, 2017
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $2,104.7 and $2,095.9, respectively)	$ 2,123.5	$ 2,109.3
Equity securities, available-for-sale, at fair value (cost $321.3 and $323.1, respectively)	395.6	382.8
Other invested assets, available-for-sale, at fair value (cost $25.5 and $25.5, respectively)	48.1	45.1
Other invested assets	5.4	5.4
Notes receivable from affiliate	70	70
Total investments	2,642.6	2,612.6
Cash and cash equivalents	44.8	51.1
Accrued investment income and other assets	46.6	40
Deferred policy acquisition costs (affiliated net assumed $47.6 and $50.7, respectively)	124	129.8
Reinsurance recoverable on losses and loss expenses payable	4.8	3.6
Prepaid reinsurance premiums	6	6.1
Due from affiliate	1.1	0
Current federal income taxes	6.8	6.7
Net deferred federal income taxes	94.9	102.1
Property and equipment, at cost	7.3	7.4
Total assets	2,978.9	2,959.4
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $646.8 and $630.9, respectively)	1,217.3	1,181.6
Unearned premiums (affiliated net assumed $222.2 and $220.9, respectively)	606.5	617.8
Notes payable (affiliates $15.2 and $15.2, respectively)	122.1	122.1
Postretirement and pension benefits (affiliated net ceded $38.3 and $40.1, respectively)	71.2	74.4
Due to Affiliate	0	2.4
Other liabilities (affiliated net assumed $16.5 and $11.0, respectively)	60.7	69.8
Total liabilities	2,077.8	2,068.1
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 48.8 and 48.6 shares issued, respectively, at stated value of $2.50 per share	121.9	121.6
Treasury stock, 6.9 and 6.8 shares, respectively, at cost	(116.7)	(116.5)
Additional paid-in capital	162.6	159.9
Accumulated other comprehensive income (affiliated net ceded $52.8 and $53.7, respectively)	47.9	32.5
Retained earnings	685.4	693.8
Total stockholders’ equity	901.1	891.3
Total liabilities and stockholders’ equity	2,978.9	2,959.4
Class A Preferred Stock [Member]
Stockholders’ equity:
Preferred stock value	0	0
Class B Preferred Stock [Member]
Stockholders’ equity:
Preferred stock value	$ 0	$ 0

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Fixed maturities, available-for-sale, amortized cost	$ 2,104.7	$ 2,095.9
Equity securities, available-for-sale, cost	321.3	323.1
Other invested assets, available-for-sale, cost	25.5	25.5
Deferred policy acquisitions costs from affiliates	47.6	50.7
Losses and loss expenses payable, affiliates	(646.8)	(630.9)
Unearned premiums, affiliates	222.2	220.9
Notes payable, affiliates	15.2	15.2
Net Postretirement and Pension Benefits	(38.3)	(40.1)
Net Other Liabilities from Affiliates	$ 16.5	$ 11
Common stock, no par value (usd per share)	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	48,800,000	48,600,000
Common stock, stated value per share (usd per share)	$ 2.5	$ 2.5
Treasury stock (shares)	6,900,000	6,800,000
Net Accumulated Other Comprehensive Income	$ (52.8)	$ (53.7)
Class A Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$ 0	$ 0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Class B Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$ 0	$ 0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Earned premiums (affiliated net assumed $114.8 and $113.4, respectively)	$ 317.3	$ 319.9
Net investment income (affiliate $1.2 and $1.2, respectively)	18.7	17.4
Net realized gains on investments:
Total other-than-temporary impairment losses	(2.1)	(3.7)
Other net realized investment gains	9.9	5
Total net realized gains on investments	7.8	1.3
Other income from affiliates	0.5	0.6
Total revenues	344.3	339.2
Losses and loss expenses (affiliated net assumed $86.2 and $93.7, respectively)	234	225
Acquisition and operating expenses (affiliated net assumed $80.3 and $70.6, respectively)	112.2	107.5
Interest expense (affiliates $0.2 and $0.2, respectively)	1.4	1.3
Other expenses	1.9	1.6
Costs and Expenses	349.5	335.4
Income before federal income taxes	(5.2)	3.8
Current Income Tax Expense (Benefit)	(0.1)	0
Deferred Income Tax Expense (Benefit)	(1)	0.8
Federal income tax expense	(1.1)	0.8
Net income	$ (4.1)	$ 3
(Loss) earnings per common share:
Basic (usd per share)	$ (0.10)	$ 0.07
Diluted (usd per share)	(0.10)	0.07
Dividends paid per common share (usd per share)	$ 0.10	$ 0.10

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Earned premiums, from affiliates	$ 114.8	$ 113.4
Net investment income, from affiliate	1.2	1.2
Losses and loss expenses, from affiliates	86.2	93.7
Net Acquisition and Operating Expenses, from affiliates	80.3	70.6
Interest Expense Debt To Affiliates	$ 0.2	$ 0.2

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (4.1)	$ 3
Net unrealized holding gains on investments:
Unrealized holding gains	30.8	37
Reclassification adjustments for gains realized in net income	(7.8)	(1.3)
Income tax expense	(8)	(12.5)
Total net unrealized holding gains on investments	15	23.2
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.4)
Net actuarial loss	2.1	2.4
Income tax expense	(0.3)	(0.4)
Total net unrecognized benefit plan obligations	0.4	0.6
Other comprehensive income	15.4	23.8
Comprehensive income	$ 11.3	$ 26.8

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net (loss) income	$ (4.1)	$ 3
Adjustments to reconcile net (loss) income to net cash provided by operating activities:
Depreciation and amortization, net	3.1	4.4
Share-based compensation	1.7	1.4
Net realized gains on investments	(7.8)	(1.3)
Changes in operating assets and liabilities:
Deferred policy acquisition benefits	5.8	1.6
Accrued investment income and other assets	(0.5)	(3.7)
Postretirement and pension benefits	(2.9)	(1.8)
Other liabilities and due to/from affiliates, net	(14)	4.9
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	(1.1)	(0.1)
Losses and loss expenses payable	35.7	30.5
Unearned premiums	(11.3)	(15.5)
Federal income taxes	(0.9)	1.1
Net cash provided by operating activities	3.7	24.5
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(120.5)	(160.6)
Purchases of equity securities available-for-sale	(39)	(30.8)
Purchases of other invested assets	(0.2)	(0.4)
Maturities, calls and pay downs of fixed maturities available-for-sale	54.1	60.8
Sales of fixed maturities available-for-sale	50.7	75.8
Sales of equity securities available-for-sale	47.6	23.6
Sales of other invested assets available-for-sale	0.3	0.2
Net cash used in investing activities	(7)	(31.4)
Cash flows from financing activities:
Proceeds from issuance of common stock	1.5	0.8
Payments to acquire treasury stock	(0.2)	(0.2)
Payment of dividends	(4.3)	(4.1)
Net cash used in financing activities	(3)	(3.5)
Net decrease in cash and cash equivalents	(6.3)	(10.4)
Cash and cash equivalents at beginning of period	51.1	58.1
Cash and cash equivalents at end of period	44.8	47.7
Supplemental disclosures:
Interest paid (affiliates $0.2 and $0.2, respectively)	$ 1.4	$ 1.3

Condensed Consolidated Stateme8

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Cash Flows [Abstract]
Interest Expense Debt To Affiliates	$ 0.2	$ 0.2

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements of State Auto Financial Corporation and Subsidiaries (“State Auto Financial” or the “Company”) have been prepared in accordance with the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by U.S. generally accepted accounting principles (“GAAP”) for complete financial statements. In the opinion of the Company, all adjustments (consisting of normal, recurring accruals) considered necessary for a fair presentation have been included. Operating results for the three month period ended March 31, 2017 , are not necessarily indicative of the results that may be expected for the year ending December 31, 2017 . The balance sheet at December 31, 2016 , has been derived from the audited financial statements at that date, but does not include all of the information and footnotes required by GAAP for complete financial statements. For further information, refer to the consolidated financial statements and footnotes thereto included in the Company’s annual report on Form 10-K for the year ended December 31, 2016 , (the “ 2016 Form 10-K”). Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the 2016 Form 10-K. Adoption of Recent Accounting Pronouncements Improvements to 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. Prior periods were not adjusted and it did not have a material impact on the condensed consolidated financial statements for the thee month period ended March 31, 2017. Pending Adoption of Recent Accounting Pronouncements For information regarding accounting pronouncements that the Company has not yet adopted, see the “Pending Adoption of Recent Accounting Pronouncements” section of Note 1 of the Notes to Consolidated Financial Statements in the 2016

Investments

Investments	3 Months Ended
	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The following tables set forth the cost or amortized cost and fair value of available-for-sale securities by lot at March 31, 2017 and December 31, 2016 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value March 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 441.3 $ 9.9 $ (5.7 ) $ 445.5 Obligations of states and political subdivisions 607.3 14.1 (2.3 ) 619.1 Corporate securities 431.6 6.3 (1.7 ) 436.2 U.S. government agencies mortgage-backed securities 624.5 7.9 (9.7 ) 622.7 Total fixed maturities 2,104.7 38.2 (19.4 ) 2,123.5 Equity securities: Large-cap securities 97.4 39.3 (0.2 ) 136.5 Small-cap securities 56.0 20.5 — 76.5 Mutual and exchange traded funds 167.9 14.8 (0.1 ) 182.6 Total equity securities 321.3 74.6 (0.3 ) 395.6 Other invested assets 25.5 22.6 — 48.1 Total available-for-sale securities $ 2,451.5 $ 135.4 $ (19.7 ) $ 2,567.2 ($ 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 March 31, 2017 and December 31, 2016 : ($ millions, except # of positions) Less than 12 months 12 months or more Total Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions March 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 257.0 $ (5.7 ) 35 $ — $ — — $ 257.0 $ (5.7 ) 35 Obligations of states and political subdivisions 126.6 (2.3 ) 17 — — — 126.6 (2.3 ) 17 Corporate securities 114.0 (1.3 ) 17 15.7 (0.4 ) 3 129.7 (1.7 ) 20 U.S. government agencies mortgage-backed securities 347.5 (8.9 ) 49 20.0 (0.8 ) 9 367.5 (9.7 ) 58 Total fixed maturities 845.1 (18.2 ) 118 35.7 (1.2 ) 12 880.8 (19.4 ) 130 Equity securities: Large-cap equity securities 1.3 — 2 4.6 (0.2 ) 2 5.9 (0.2 ) 4 Mutual and exchange traded funds 2.4 (0.1 ) 1 — — — 2.4 (0.1 ) 1 Total equity securities 3.7 (0.1 ) 3 4.6 (0.2 ) 2 8.3 (0.3 ) 5 Total temporarily impaired securities $ 848.8 $ (18.3 ) 121 $ 40.3 $ (1.4 ) 14 $ 889.1 $ (19.7 ) 135 ($ millions, except # of positions) Less than 12 months 12 months or more Total Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions December 31, 2016 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 Equity securities: 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 Company reviewed its investments at March 31, 2017 , and determined that no additional other-than-temporary impairment existed in the gross unrealized holding losses other than those listed in the table below. The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Equity securities: Large-cap securities $ (1.5 ) $ (0.2 ) Small-cap securities (0.6 ) (1.2 ) Fixed maturities: — (2.3 ) Total other-than-temporary impairments $ (2.1 ) $ (3.7 ) The Company regularly monitors its investments that have fair values less than cost or amortized cost for signs of other-than-temporary impairment, an assessment that requires significant management judgment regarding the evidence known. Such judgments could change in the future as more information becomes known, which could negatively impact the amounts reported. Among the factors that management considers for fixed maturity securities are the financial condition of the issuer including receipt of scheduled principal and interest cash flows, and intent to sell, including if it is more likely than not that the Company will be required to sell the investments before recovery. When a fixed maturity has been determined to have an other-than-temporary impairment, the impairment charge is separated into an amount representing the credit loss, which is recognized in earnings as a realized loss, and the amount related to non-credit factors, which is recognized in accumulated other comprehensive income. Future increases or decreases in fair value, if not other-than-temporary, are included in accumulated other comprehensive income. Among the factors that management considers for equity securities and other invested assets are the length of time and/or the significance of decline below cost, the Company’s ability and intent to hold these securities through their recovery periods, the current financial condition of the issuer and its future business prospects, and the ability of the market value to recover to cost in the near term. When an equity security or other invested asset has been determined to have a decline in fair value that is other-than-temporary, the cost basis of the security is adjusted to fair value. This results in a charge to earnings as a realized loss, which is not reversed for subsequent recoveries in fair value. Future increases or decreases in fair value, if not other-than-temporary, are included in accumulated other comprehensive income. The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at March 31, 2017 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 52.1 $ 52.6 Due after 1 year through 5 years 540.6 546.7 Due after 5 years through 10 years 306.6 310.0 Due after 10 years 580.9 591.5 U.S. government agencies mortgage-backed securities 624.5 622.7 Total $ 2,104.7 $ 2,123.5 Expected maturities may differ from contractual maturities because issuers may have the right to call or prepay the obligations with or without call or prepayment penalties. At March 31, 2017 , State Auto P&C had U.S. government agencies mortgage-backed fixed maturity securities, with a carrying value of approximately $106.5 million , that were pledged as collateral for the FHLB Loans (as defined in Note 3). In accordance with the terms of the FHLB Loans, State Auto P&C retains all rights regarding these pledged securities. Fixed maturities with fair values of $9.3 million and $9.2 million were on deposit with insurance regulators as required by law at March 31, 2017 , and December 31, 2016 , respectively. The Company retains all rights regarding these securities. The following table sets forth the components of net investment income for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Fixed maturities $ 15.8 $ 14.9 Equity securities 1.7 1.5 Cash and cash equivalents, and other 1.5 1.4 Investment income 19.0 17.8 Investment expenses 0.3 0.4 Net investment income $ 18.7 $ 17.4 The Company’s current investment strategy does not rely on the use of derivative financial instruments. Proceeds on sales of available-for-sale securities were $98.6 million and $99.6 million for the three months ended March 31, 2017 , and 2016 , respectively. The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Realized gains: Fixed maturities $ 0.3 $ 1.5 Equity securities 9.7 4.1 Total realized gains 10.0 5.6 Realized losses: Equity securities: Sales (0.1 ) (0.6 ) OTTI (2.1 ) (1.4 ) Fixed maturities: OTTI — (2.3 ) Total realized losses (2.2 ) (4.3 ) Net realized gains on investments $ 7.8 $ 1.3 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ 5.4 $ 36.2 Equity securities 14.6 (1.0 ) Other invested assets 3.0 0.5 Deferred federal income tax liability (8.0 ) (12.5 ) Change in net unrealized holding gains (losses), net of tax $ 15.0 $ 23.2

Fair Value of Financial Instrum

Fair Value of Financial Instruments	3 Months Ended
	Mar. 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 and the processes and control procedures employed by the Company are designed to ensure the value is accurately recorded on an unadjusted basis. Through discussions with the pricing service, the Company gains an understanding of the methodologies used to price the different types of securities, that the data and the valuation methods utilized are appropriate and consistently applied, and that the assumptions are reasonable and representative of fair value. To validate the reasonableness of the valuations obtained from the pricing service, the Company compares to other fair value pricing information gathered from other independent pricing sources. At March 31, 2017 , and December 31, 2016 , the Company did not adjust any of the prices received from the pricing service. Transfers between level categorizations may occur due to changes in the availability of market observable inputs. Transfers in and out of level categorizations are reported as having occurred at the beginning of the quarter in which the transfer occurred. There were no transfers between level categorizations during the three months ended March 31, 2017 , and 2016 . The following sections describe the valuation methods used by the Company for each type of financial instrument it holds that are carried at fair value. Fixed Maturities The Company utilizes a third party pricing service to estimate fair value measurements for the majority of its fixed maturities. The fair value estimate of the Company’s fixed maturity investments are determined by evaluations that are based on observable market information rather than market quotes. Inputs to the evaluations include, but are not limited to, market prices from recently completed transactions and transactions of comparable securities, interest rate yield curves, credit spreads, and other market-observable information. The fixed maturity portfolio pricing obtained from the pricing service is reviewed for reasonableness. The Company regularly selects a random sample of security prices which are compared to one or more alternative pricing sources for reasonableness. Any discrepancies with the pricing are returned to the pricing service for further explanation and, if necessary, adjustments are made. To date, the Company has not identified any significant discrepancies in the pricing provided by its third party pricing service. Investments valued using these inputs include U.S. treasury securities and obligations of U.S. government agencies, obligations of states and political subdivisions, corporate securities (except for a security discussed below), and U.S. government agencies mortgage-backed securities. All unadjusted estimates of fair value for fixed maturities priced by the pricing service are included in the amounts disclosed in Level 2 of the hierarchy. If market inputs are unavailable, then no fair value is provided by the pricing service. For these securities, fair value is determined either by requesting brokers who are knowledgeable about these securities to provide a quote; or the Company internally determines the fair values by employing widely accepted pricing valuation models, and depending on the level of observable market inputs, renders the fair value estimate as Level 2 or Level 3. The Company holds one corporate fixed maturity security included in Level 3 and estimates its fair value using the present value of the future cash flows. Due to the limited amount of observable market information for this security, the Company includes the fair value estimate in Level 3. Equities The fair value of each equity security is based on an observable market price for an identical asset in an active market and is priced by the same pricing service discussed above. All equity securities are recorded using unadjusted market prices and have been disclosed in Level 1. Other Invested Assets Included in other invested assets is one international fund (“the fund”) that invests in equity securities of foreign issuers and is managed by a third party investment manager. The fund had a fair value of $38.4 million and $35.7 million at March 31, 2017 , and December 31, 2016 , respectively, which was determined using the fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the fund including obtaining and reviewing the fund’s audited financial statements. There are no unfunded commitments related to the fund. The Company may not sell its investment in the fund; however, the Company may redeem all or a portion of its investment in the fund at net asset value per share with the appropriate prior written notice. In accordance with Accounting Standard Codification 820-10, this investment is measured at fair value using the net asset value per share practical expedient and has not been classified in the fair value hierarchy. Fair values presented here are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated balance sheets. The remainder of the Company’s other invested assets consist primarily of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds based on an observable market price for an identical asset in an active market reflect their fair values and consequently these securities have been disclosed in Level 1. The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at March 31, 2017 and December 31, 2016 : ($ millions) Total Level 1 Level 2 Level 3 March 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 445.5 $ — $ 445.5 $ — Obligations of states and political subdivisions 619.1 — 619.1 — Corporate securities 436.2 — 432.6 3.6 U.S. government agencies mortgage-backed securities 622.7 — 622.7 — Total fixed maturities 2,123.5 — 2,119.9 3.6 Equity securities: Large-cap securities 136.5 136.5 — — Small-cap securities 76.5 76.5 — — Mutual and exchange traded funds 182.6 182.6 — — Total equity securities 395.6 395.6 — — Other invested assets 9.7 9.7 — — Total available-for-sale investments $ 2,528.8 $ 405.3 $ 2,119.9 $ 3.6 ($ millions) Total Level 1 Level 2 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 the three months ended March 31, 2017 , and the year ended December 31, 2016 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2017 $ 3.5 Total realized gains – included in earnings — Total unrealized losses – included in other comprehensive income — Purchases 0.1 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at March 31, 2017 $ 3.6 ($ millions) Fixed maturities Balance at January 1, 2016 $ 3.3 Total realized gains – included in earnings — Total unrealized gains – 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 The following sections describe the valuation methods used by the Company for each type of financial instrument it holds that is not measured at fair value but for which fair value is disclosed: Financial Instruments Disclosed, But Not Carried, At Fair Value Other Invested Assets Included in other invested assets are common stock of the Federal Home Loan Bank of Cincinnati (the "FHLB") and the Trust Securities. The Trust Securities and FHLB common stock are carried at cost, which approximates fair value. The fair value of the FHLB common stock at March 31, 2017 , was $4.9 million and the fair value of the Trust Securities was $0.5 million . The investments have been placed in Level 3 of the fair value hierarchy. Notes Receivable from Affiliate In May 2009, the Company entered into two separate credit agreements with State Automobile Mutual Insurance Company (“State Auto Mutual") pursuant to which it loaned State Auto Mutual a total of $70.0 million . The Company estimates the fair value of the notes receivable from affiliate using market quotations for U.S. treasury securities with similar maturity dates and applies an appropriate credit spread. Consequently this has been placed in Level 2 of the fair value hierarchy. ($ millions, except interest rates) March 31, 2017 December 31, 2016 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 74.3 7.00 % $ 70.0 $ 75.7 7.00 % Notes Payable Included in notes payable are the FHLB Loans 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 loans 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) March 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 $ 21.0 1.73 % $ 21.5 $ 21.0 1.73 % FHLB Loan due 2033: issued $85.0, July 2013 with fixed interest 85.4 85.3 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 5.25 % 15.2 15.2 5.13 % Total notes payable $ 122.1 $ 121.5 $ 122.1 $ 121.8

Losses and Loss Expenses Payabl

Losses and Loss Expenses Payable Losses and Loss Expenses Payable	3 Months Ended
	Mar. 31, 2017
Losses and Loss Expenses Payable [Abstract]
Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block]	4. Losses and Loss Expenses Payable The following table sets forth the activity in the liability for losses and loss expenses for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Losses and loss expenses payable, at beginning of period $ 1,181.6 $ 1,053.0 Less: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 Net balance at beginning of period 1,178.0 1,047.1 Incurred related to: Current year 243.7 215.4 Prior years (9.7 ) 9.6 Total incurred 234.0 225.0 Paid related to: Current year 60.1 52.3 Prior years 139.4 142.8 Total paid 199.5 195.1 Net balance at end of period 1,212.5 1,077.0 Plus: reinsurance recoverable on losses and loss expenses payable 4.8 6.5 Losses and loss expenses payable, at end of period $ 1,217.3 $ 1,083.5 The Company recorded favorable development related to prior years’ loss and loss expense reserves for the three months ended March 31, 2017 , of $9.7 million compared to adverse development of $9.6 million for the same 2016 period. Favorable development of prior years' unallocated loss adjustment expenses and catastrophe reserves were approximately $2.7 million and $2.2 million , respectively, of the 2017 development. Favorable development of prior accident years' non-catastrophe loss and ALAE reserves was primarily driven by the commercial insurance segment. The commercial insurance segment non-catastrophe loss and ALAE reserves contributed $7.6 million of favorable development in 2017 , driven by other commercial, small commercial package and workers’ compensation of $3.2 million , $2.0 million and $1.9 million , respectively. Favorable development in these lines was driven by lower than anticipated severity emerging from multiple accident years. Slightly offsetting the favorable development was adverse development in the personal insurance segment of $2.3 million , driven by homeowners' which contributed $2.8 million of adverse development, primarily from accident year 2016, which included three large losses that occurred in late 2016. Favorable development of catastrophe reserves was approximately $0.7 million for the three months ended March 31, 2016 , and adverse development of unallocated loss adjustment expenses was $1.6 million for the same 2016 period. The specialty insurance segment non-catastrophe loss and ALAE reserves accounted for $4.9 million of adverse development, which was driven by programs with adverse development of $3.9 million . Adverse development in programs was primarily due to two programs with commercial auto exposures, both of which are in run-off. The personal insurance segment contributed $3.4 million of the adverse development, driven by personal auto which developed unfavorably by $3.2 million

Reinsurance

Reinsurance	3 Months Ended
	Mar. 31, 2017
Insurance [Abstract]
Reinsurance	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 (collectively referred to as the “Mutual Pooled Companies”). The following table sets forth a summary of the Company’s external reinsurance transactions, as well as reinsurance transactions with State Auto Mutual under the Pooling Arrangement, for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Premiums earned: Assumed from external insurers and reinsurers $ 1.1 $ 1.4 Assumed under Pooling Arrangement 317.3 319.9 Ceded to external insurers and reinsurers (6.1 ) (6.8 ) Ceded under Pooling Arrangement (202.5 ) (206.5 ) Net assumed premiums earned $ 109.8 $ 108.0 Losses and loss expenses incurred: Assumed from external insurers and reinsurers $ 1.6 $ 1.1 Assumed under Pooling Arrangement 234.4 225.4 Ceded to external insurers and reinsurers (2.0 ) (1.9 ) Ceded under Pooling Arrangement (148.2 ) (131.7 ) Net assumed losses and loss expenses incurred $ 85.8 $ 92.9

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	. Income Taxes The following table sets forth the reconciliation between actual federal income tax (benefit) expense and the amount computed at the indicated statutory rate for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Amount at statutory rate $ (1.8 ) 35.0 % $ 1.3 35.0 % Tax-exempt interest and dividends received deduction (1.5 ) 29.1 (2.0 ) (52.8 ) Other, net 2.2 (42.4 ) 1.5 37.8 Federal income tax (benefit) expense and effective rate $ (1.1 ) 21.7 % $ 0.8 20.0 %

Pension and Postretirement Bene

Pension and Postretirement Benefit Plans	3 Months Ended
	Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]
Pension and Postretirement Benefit Plans	Pension and Postretirement Benefit Plans The following table sets forth the components of net periodic cost for the Company’s pension and postretirement benefit plans for the three months ended March 31, 2017 and 2016 : ($ millions) Pension Postretirement 2017 2016 2017 2016 Service cost $ 1.7 $ 1.6 $ — $ — Interest cost 2.9 2.9 0.2 0.2 Expected return on plan assets (4.2 ) (3.7 ) — — Amortization of: Negative prior service cost — — (1.4 ) (1.4 ) Net actuarial loss 2.0 2.3 0.1 0.1 Net periodic cost (benefit) $ 2.4 $ 3.1 $ (1.1 ) $ (1.1 ) The Company contributed $3.3 million to its pension plan for the three months ended March 31, 2017 , and expects to contribute an additional $6.5 million

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	Other Comprehensive Income and Accumulated Other Comprehensive Income The following table sets forth the changes in the Company’s accumulated other comprehensive income component ("AOCI"), net of tax, for the three months ended March 31, 2017 and 2016 : ($ 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 20.1 — 20.1 Amounts reclassified from AOCI (a) (5.1 ) 0.4 (4.7 ) Net current period other comprehensive income 15.0 0.4 15.4 Ending balance at March 31, 2017 $ 77.8 $ (29.9 ) $ 47.9 Beginning balance at January 1, 2016 $ 68.5 $ (30.9 ) $ 37.6 Other comprehensive loss before reclassifications 24.0 — 24.0 Amounts reclassified from AOCI (a) (0.8 ) 0.6 (0.2 ) Net current period other comprehensive income 23.2 0.6 23.8 Ending balance at March 31, 2016 $ 91.7 $ (30.3 ) $ 61.4 (a) See separate table below for details about these reclassifications The following table sets forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s condensed consolidated statement of income for the three months ended March 31, 2017 and 2016 : ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components March 31 Consolidated Statements of Income 2017 2016 Unrealized gains on available for sale securities $ 7.8 $ 1.3 Realized gain on sale of securities 7.8 1.3 Total before tax (2.7 ) (0.5 ) Tax expense 5.1 0.8 Net of tax Amortization of benefit plan items Negative prior service cost 1.4 1.4 (b) Net actuarial loss (2.1 ) (2.4 ) (b) (0.7 ) (1.0 ) Total before tax 0.3 0.4 Tax benefit (0.4 ) (0.6 ) Net of tax Total reclassifications for the period $ 4.7 $ 0.2 (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings per Common Share

Earnings per Common Share	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Earnings (Loss) per Common Share	Net (Loss) Earnings per Common Share The following table sets forth the compilation of basic and diluted earnings per common share for the three months ended March 31, 2017 and 2016 : ($ and shares in millions, except per share amounts) 2017 2016 Numerator: Net (loss) income for basic (loss) earnings per common share $ (4.1 ) $ 3.0 Denominator: Weighted average shares for basic net (loss) earnings per common share 41.9 41.4 Effect of dilutive share-based awards — 0.4 Adjusted weighted average shares for diluted net (loss) earnings per common share 41.9 41.8 Basic net (loss) earnings per common share $ (0.10 ) $ 0.07 Diluted net (loss) earnings per common share $ (0.10 ) $ 0.07 The following table sets forth common stock options, stock awards and restricted share units ("RSU award") of the Company that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three months ended March 31, 2017 and 2016 : (shares in millions) 2017 2016 Total number of antidilutive options and awards 0.6 1.7

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2017
Segment Reporting [Abstract]
Segment Information	Segment Information The Company has four reportable segments: personal insurance, commercial insurance, specialty insurance and investment operations. The reportable insurance segments are business units managed separately because of the differences in the type of customers they serve, the products they provide or services they offer. The insurance segments market a broad line of property and casualty insurance products throughout the United States through independent insurance agencies, which include retail agents and wholesale brokers. The investment operations segment, managed by Stateco, provides investment services. Effective January 1, 2017, the Company completed the transition of its internal decision-making process to a product management structure. Under this structure, the Company’s principal operating decision makers assess performance based on the underwriting results of individual products. The products within each insurance segment, 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 Certain amounts previously reported within the commercial insurance segment due to immateriality and convenience, are now reported in the personal insurance segment. Prior reporting periods have been restated to conform to the new presentation. The Company evaluates the performance of its insurance segments using industry financial measurements based on Statutory Accounting Practices (“SAP”), which include loss and loss adjustment expense ratios, underwriting expense ratios, combined ratios, statutory underwriting gain (loss), net premiums earned and net written premiums. One of the most significant differences between SAP and GAAP is that SAP requires all underwriting expenses to be expensed immediately and not deferred and amortized over the same period the premium is earned. The investment operations segment is evaluated based on investment returns of assets managed by Stateco. Asset information by segment is not reported for the insurance segments because the Company does not produce such information internally. The following table sets forth financial information regarding the Company’s reportable segments for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Revenue from external sources: Insurance segments Personal insurance $ 152.7 $ 153.5 Commercial insurance 103.6 111.4 Specialty insurance 61.0 55.0 Total insurance segments 317.3 319.9 Investment operations segment Net investment income 18.7 17.4 Net realized capital gains 7.8 1.3 Total investment operations segment 26.5 18.7 All other 0.5 0.6 Total revenue from external sources 344.3 339.2 Intersegment revenue 1.5 1.4 Total revenue 345.8 340.6 Reconciling items: Eliminate intersegment revenue (1.5 ) (1.4 ) Total consolidated revenues $ 344.3 $ 339.2 Segment income before federal income tax: Insurance segments SAP underwriting (loss) gain Personal insurance $ (10.6 ) $ 4.4 Commercial insurance (4.9 ) (7.4 ) Specialty insurance (8.6 ) (8.7 ) Total insurance segments (loss) gain (24.1 ) (11.7 ) Investment operations segment Net investment income 18.7 17.4 Net realized capital gains 7.8 1.3 Total investment operations segment 26.5 18.7 All other 0.1 0.1 Total segment income before tax expense 2.5 7.1 Reconciling items: GAAP expense adjustments (5.4 ) (1.2 ) Interest expense on corporate debt (1.4 ) (1.3 ) Corporate expenses (0.9 ) (0.8 ) Total reconciling items (7.7 ) (3.3 ) Total consolidated (loss) income before federal income tax expense $ (5.2 ) $ 3.8 Investable assets attributable to the Company’s investment operations segment totaled $2,687.4 million and $2,663.7 million at March 31, 2017 , and December 31, 2016

Contingencies and Litigation

Contingencies and Litigation	3 Months Ended
	Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Contingencies and Litigation	Contingencies and Litigation In accordance with the Contingencies Topic of the Financial Accounting Standards Board's Accounting Standards Codification, the Company accrues for a litigation-related liability when it is probable that such a liability has been incurred and the amount can be reasonably estimated. The Company reviews all litigation on an ongoing basis when making accrual and disclosure decisions. For certain legal proceedings, the Company cannot reasonably estimate losses or a range of loss, if any, particularly for proceedings that are in their early stages of development or where the plaintiffs seek indeterminate damages. Various factors, including, but not limited to, the outcome of potentially lengthy discovery and the resolution of important factual questions, may need to be determined before probability can be established or before a loss or range of loss can be reasonably estimated. If the loss contingency in question is not both probable and reasonably estimable, the Company does not establish an accrual and the matter will continue to be monitored for any developments that would make the loss contingency both probable and reasonably estimable. Based on currently available information known to the Company, it believes that its reserves for litigation-related liabilities are reasonable. However, in the event that a legal proceeding results in a substantial judgment against, or settlement by, the Company, there can be no assurance that any resulting liability or financial commitment would not have a material adverse effect on the financial condition, results of operations or cash flows of the consolidated financial statements of the Company. 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, results of operations or cash flows.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Cost or Amortized Cost and Fair Value of Available-for-Sale Securities	The following tables set forth the cost or amortized cost and fair value of available-for-sale securities by lot at March 31, 2017 and December 31, 2016 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value March 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 441.3 $ 9.9 $ (5.7 ) $ 445.5 Obligations of states and political subdivisions 607.3 14.1 (2.3 ) 619.1 Corporate securities 431.6 6.3 (1.7 ) 436.2 U.S. government agencies mortgage-backed securities 624.5 7.9 (9.7 ) 622.7 Total fixed maturities 2,104.7 38.2 (19.4 ) 2,123.5 Equity securities: Large-cap securities 97.4 39.3 (0.2 ) 136.5 Small-cap securities 56.0 20.5 — 76.5 Mutual and exchange traded funds 167.9 14.8 (0.1 ) 182.6 Total equity securities 321.3 74.6 (0.3 ) 395.6 Other invested assets 25.5 22.6 — 48.1 Total available-for-sale securities $ 2,451.5 $ 135.4 $ (19.7 ) $ 2,567.2 ($ 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 March 31, 2017 and December 31, 2016 : ($ millions, except # of positions) Less than 12 months 12 months or more Total Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions March 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 257.0 $ (5.7 ) 35 $ — $ — — $ 257.0 $ (5.7 ) 35 Obligations of states and political subdivisions 126.6 (2.3 ) 17 — — — 126.6 (2.3 ) 17 Corporate securities 114.0 (1.3 ) 17 15.7 (0.4 ) 3 129.7 (1.7 ) 20 U.S. government agencies mortgage-backed securities 347.5 (8.9 ) 49 20.0 (0.8 ) 9 367.5 (9.7 ) 58 Total fixed maturities 845.1 (18.2 ) 118 35.7 (1.2 ) 12 880.8 (19.4 ) 130 Equity securities: Large-cap equity securities 1.3 — 2 4.6 (0.2 ) 2 5.9 (0.2 ) 4 Mutual and exchange traded funds 2.4 (0.1 ) 1 — — — 2.4 (0.1 ) 1 Total equity securities 3.7 (0.1 ) 3 4.6 (0.2 ) 2 8.3 (0.3 ) 5 Total temporarily impaired securities $ 848.8 $ (18.3 ) 121 $ 40.3 $ (1.4 ) 14 $ 889.1 $ (19.7 ) 135 ($ millions, except # of positions) Less than 12 months 12 months or more Total Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions Fair value Unrealized losses Number of positions December 31, 2016 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 Equity securities: 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 three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Equity securities: Large-cap securities $ (1.5 ) $ (0.2 ) Small-cap securities (0.6 ) (1.2 ) Fixed maturities: — (2.3 ) Total other-than-temporary impairments $ (2.1 ) $ (3.7 )
Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity	The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at March 31, 2017 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 52.1 $ 52.6 Due after 1 year through 5 years 540.6 546.7 Due after 5 years through 10 years 306.6 310.0 Due after 10 years 580.9 591.5 U.S. government agencies mortgage-backed securities 624.5 622.7 Total $ 2,104.7 $ 2,123.5
Components of Net Investment Income	The following table sets forth the components of net investment income for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Fixed maturities $ 15.8 $ 14.9 Equity securities 1.7 1.5 Cash and cash equivalents, and other 1.5 1.4 Investment income 19.0 17.8 Investment expenses 0.3 0.4 Net investment income $ 18.7 $ 17.4
Realized and Unrealized Holding Gains (Losses)	The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Realized gains: Fixed maturities $ 0.3 $ 1.5 Equity securities 9.7 4.1 Total realized gains 10.0 5.6 Realized losses: Equity securities: Sales (0.1 ) (0.6 ) OTTI (2.1 ) (1.4 ) Fixed maturities: OTTI — (2.3 ) Total realized losses (2.2 ) (4.3 ) Net realized gains on investments $ 7.8 $ 1.3 Change in unrealized holding gains (losses), net of tax: Fixed maturities $ 5.4 $ 36.2 Equity securities 14.6 (1.0 ) Other invested assets 3.0 0.5 Deferred federal income tax liability (8.0 ) (12.5 ) Change in net unrealized holding gains (losses), net of tax $ 15.0 $ 23.2

Fair Value of Financial Instr21

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 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 March 31, 2017 and December 31, 2016 : ($ millions) Total Level 1 Level 2 Level 3 March 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 445.5 $ — $ 445.5 $ — Obligations of states and political subdivisions 619.1 — 619.1 — Corporate securities 436.2 — 432.6 3.6 U.S. government agencies mortgage-backed securities 622.7 — 622.7 — Total fixed maturities 2,123.5 — 2,119.9 3.6 Equity securities: Large-cap securities 136.5 136.5 — — Small-cap securities 76.5 76.5 — — Mutual and exchange traded funds 182.6 182.6 — — Total equity securities 395.6 395.6 — — Other invested assets 9.7 9.7 — — Total available-for-sale investments $ 2,528.8 $ 405.3 $ 2,119.9 $ 3.6 ($ millions) Total Level 1 Level 2 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 the three months ended March 31, 2017 , and the year ended December 31, 2016 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2017 $ 3.5 Total realized gains – included in earnings — Total unrealized losses – included in other comprehensive income — Purchases 0.1 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at March 31, 2017 $ 3.6 ($ millions) Fixed maturities Balance at January 1, 2016 $ 3.3 Total realized gains – included in earnings — Total unrealized gains – 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	Consequently this has been placed in Level 2 of the fair value hierarchy. ($ millions, except interest rates) March 31, 2017 December 31, 2016 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 74.3 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) March 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 $ 21.0 1.73 % $ 21.5 $ 21.0 1.73 % FHLB Loan due 2033: issued $85.0, July 2013 with fixed interest 85.4 85.3 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 5.25 % 15.2 15.2 5.13 % Total notes payable $ 122.1 $ 121.5 $ 122.1 $ 121.8

Losses and Loss Expenses Paya22

Losses and Loss Expenses Payable Losses and Loss Expenses Payable (Tables)	3 Months Ended
	Mar. 31, 2017
Losses and Loss Expenses Payable [Abstract]
Schedule Of Activity In Liability For Unpaid Losses And Loss Adjustment Expenses Table [Table Text Block]	($ millions) 2017 2016 Losses and loss expenses payable, at beginning of period $ 1,181.6 $ 1,053.0 Less: reinsurance recoverable on losses and loss expenses payable 3.6 5.9 Net balance at beginning of period 1,178.0 1,047.1 Incurred related to: Current year 243.7 215.4 Prior years (9.7 ) 9.6 Total incurred 234.0 225.0 Paid related to: Current year 60.1 52.3 Prior years 139.4 142.8 Total paid 199.5 195.1 Net balance at end of period 1,212.5 1,077.0 Plus: reinsurance recoverable on losses and loss expenses payable 4.8 6.5 Losses and loss expenses payable, at end of period $ 1,217.3 $ 1,083.5

Reinsurance (Tables)

Reinsurance (Tables)	3 Months Ended
	Mar. 31, 2017
Insurance [Abstract]
Summary of External Reinsurance Transactions with State Auto Mutual Under Pooling Arrangement	The following table sets forth a summary of the Company’s external reinsurance transactions, as well as reinsurance transactions with State Auto Mutual under the Pooling Arrangement, for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Premiums earned: Assumed from external insurers and reinsurers $ 1.1 $ 1.4 Assumed under Pooling Arrangement 317.3 319.9 Ceded to external insurers and reinsurers (6.1 ) (6.8 ) Ceded under Pooling Arrangement (202.5 ) (206.5 ) Net assumed premiums earned $ 109.8 $ 108.0 Losses and loss expenses incurred: Assumed from external insurers and reinsurers $ 1.6 $ 1.1 Assumed under Pooling Arrangement 234.4 225.4 Ceded to external insurers and reinsurers (2.0 ) (1.9 ) Ceded under Pooling Arrangement (148.2 ) (131.7 ) Net assumed losses and loss expenses incurred $ 85.8 $ 92.9

Income Taxes (Tables)

Income Taxes (Tables)	3 Months Ended
	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Reconciliation Between Actual Federal Income Tax Benefit and Amount Computed at Indicated Statutory Rate	The following table sets forth the reconciliation between actual federal income tax (benefit) expense and the amount computed at the indicated statutory rate for the three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Amount at statutory rate $ (1.8 ) 35.0 % $ 1.3 35.0 % Tax-exempt interest and dividends received deduction (1.5 ) 29.1 (2.0 ) (52.8 ) Other, net 2.2 (42.4 ) 1.5 37.8 Federal income tax (benefit) expense and effective rate $ (1.1 ) 21.7 % $ 0.8 20.0 %

Pension and Postretirement Be25

Pension and Postretirement Benefit Plans (Tables)	3 Months Ended
	Mar. 31, 2017
Compensation and Retirement Disclosure [Abstract]
Company's Share of Net Periodic Cost Components	The following table sets forth the components of net periodic cost for the Company’s pension and postretirement benefit plans for the three months ended March 31, 2017 and 2016 : ($ millions) Pension Postretirement 2017 2016 2017 2016 Service cost $ 1.7 $ 1.6 $ — $ — Interest cost 2.9 2.9 0.2 0.2 Expected return on plan assets (4.2 ) (3.7 ) — — Amortization of: Negative prior service cost — — (1.4 ) (1.4 ) Net actuarial loss 2.0 2.3 0.1 0.1 Net periodic cost (benefit) $ 2.4 $ 3.1 $ (1.1 ) $ (1.1 )

Other Comprehensive Income an26

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2017
Equity [Abstract]
Changes in Accumulated Other Comprehensive Income by Component, Net of Tax	The following table sets forth the changes in the Company’s accumulated other comprehensive income component ("AOCI"), net of tax, for the three months ended March 31, 2017 and 2016 : ($ 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 20.1 — 20.1 Amounts reclassified from AOCI (a) (5.1 ) 0.4 (4.7 ) Net current period other comprehensive income 15.0 0.4 15.4 Ending balance at March 31, 2017 $ 77.8 $ (29.9 ) $ 47.9 Beginning balance at January 1, 2016 $ 68.5 $ (30.9 ) $ 37.6 Other comprehensive loss before reclassifications 24.0 — 24.0 Amounts reclassified from AOCI (a) (0.8 ) 0.6 (0.2 ) Net current period other comprehensive income 23.2 0.6 23.8 Ending balance at March 31, 2016 $ 91.7 $ (30.3 ) $ 61.4 (a) See separate table below for details about these reclassifications
Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component	The following table sets forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s condensed consolidated statement of income for the three months ended March 31, 2017 and 2016 : ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components March 31 Consolidated Statements of Income 2017 2016 Unrealized gains on available for sale securities $ 7.8 $ 1.3 Realized gain on sale of securities 7.8 1.3 Total before tax (2.7 ) (0.5 ) Tax expense 5.1 0.8 Net of tax Amortization of benefit plan items Negative prior service cost 1.4 1.4 (b) Net actuarial loss (2.1 ) (2.4 ) (b) (0.7 ) (1.0 ) Total before tax 0.3 0.4 Tax benefit (0.4 ) (0.6 ) Net of tax Total reclassifications for the period $ 4.7 $ 0.2 (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings per Common Share (Tabl

Earnings per Common Share (Tables)	3 Months Ended
	Mar. 31, 2017
Earnings Per Share [Abstract]
Compilation of Basic and Diluted Earnings Per Common Share	The following table sets forth the compilation of basic and diluted earnings per common share for the three months ended March 31, 2017 and 2016 : ($ and shares in millions, except per share amounts) 2017 2016 Numerator: Net (loss) income for basic (loss) earnings per common share $ (4.1 ) $ 3.0 Denominator: Weighted average shares for basic net (loss) earnings per common share 41.9 41.4 Effect of dilutive share-based awards — 0.4 Adjusted weighted average shares for diluted net (loss) earnings per common share 41.9 41.8 Basic net (loss) earnings per common share $ (0.10 ) $ 0.07 Diluted net (loss) earnings per common share $ (0.10 ) $ 0.07
Options to Purchase Shares of Common Stock	The following table sets forth common stock options, stock awards and restricted share units ("RSU award") of the Company that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three months ended March 31, 2017 and 2016 : (shares in millions) 2017 2016 Total number of antidilutive options and awards 0.6 1.7

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 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 three months ended March 31, 2017 and 2016 : ($ millions) 2017 2016 Revenue from external sources: Insurance segments Personal insurance $ 152.7 $ 153.5 Commercial insurance 103.6 111.4 Specialty insurance 61.0 55.0 Total insurance segments 317.3 319.9 Investment operations segment Net investment income 18.7 17.4 Net realized capital gains 7.8 1.3 Total investment operations segment 26.5 18.7 All other 0.5 0.6 Total revenue from external sources 344.3 339.2 Intersegment revenue 1.5 1.4 Total revenue 345.8 340.6 Reconciling items: Eliminate intersegment revenue (1.5 ) (1.4 ) Total consolidated revenues $ 344.3 $ 339.2 Segment income before federal income tax: Insurance segments SAP underwriting (loss) gain Personal insurance $ (10.6 ) $ 4.4 Commercial insurance (4.9 ) (7.4 ) Specialty insurance (8.6 ) (8.7 ) Total insurance segments (loss) gain (24.1 ) (11.7 ) Investment operations segment Net investment income 18.7 17.4 Net realized capital gains 7.8 1.3 Total investment operations segment 26.5 18.7 All other 0.1 0.1 Total segment income before tax expense 2.5 7.1 Reconciling items: GAAP expense adjustments (5.4 ) (1.2 ) Interest expense on corporate debt (1.4 ) (1.3 ) Corporate expenses (0.9 ) (0.8 ) Total reconciling items (7.7 ) (3.3 ) Total consolidated (loss) income before federal income tax expense $ (5.2 ) $ 3.8

Investments - Cost or Amortized

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,451.5	$ 2,444.5
Gross unrealized holding gains	135.4	118.5
Gross unrealized holding losses	(19.7)	(25.8)
Fair value	2,567.2	2,537.2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,104.7	2,095.9
Gross unrealized holding gains	38.2	35.8
Gross unrealized holding losses	(19.4)	(22.4)
Fair value	2,123.5	2,109.3
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	441.3	401.9
Gross unrealized holding gains	9.9	8.9
Gross unrealized holding losses	(5.7)	(6.1)
Fair value	445.5	404.7
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	607.3	634.6
Gross unrealized holding gains	14.1	12.3
Gross unrealized holding losses	(2.3)	(3.2)
Fair value	619.1	643.7
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	431.6	445.7
Gross unrealized holding gains	6.3	6.1
Gross unrealized holding losses	(1.7)	(2.2)
Fair value	436.2	449.6
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	624.5	613.7
Gross unrealized holding gains	7.9	8.5
Gross unrealized holding losses	(9.7)	(10.9)
Fair value	622.7	611.3
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	321.3	323.1
Gross unrealized holding gains	74.6	63.1
Gross unrealized holding losses	(0.3)	(3.4)
Fair value	395.6	382.8
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	97.4	108.9
Gross unrealized holding gains	39.3	32.7
Gross unrealized holding losses	(0.2)	(2.6)
Fair value	136.5	139
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	56	57.2
Gross unrealized holding gains	20.5	21.9
Gross unrealized holding losses	0	0
Fair value	76.5	79.1
Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	167.9	157
Gross unrealized holding gains	14.8	8.5
Gross unrealized holding losses	(0.1)	(0.8)
Fair value	182.6	164.7
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	25.5	25.5
Gross unrealized holding gains	22.6	19.6
Gross unrealized holding losses	0	0
Fair value	$ 48.1	$ 45.1

Investments - Gross Unrealized

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Mar. 31, 2017USD ($)position	Dec. 31, 2016USD ($)position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 848.8	$ 891.6
Less than 12 months, Unrealized losses	$ (18.3)	$ (22.5)
Less than 12 months, Number of positions | position	121	124
12 months or more, Fair value	$ 40.3	$ 58.7
12 months or more, Unrealized losses	$ (1.4)	$ (3.3)
12 months or more, Number of positions | position	14	20
Total, Fair value	$ 889.1	$ 950.3
Total, Unrealized losses	$ (19.7)	$ (25.8)
Total, Number of positions | position	135	144
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 1.3	$ 9.1
Less than 12 months, Unrealized losses	$ 0	$ (0.9)
Less than 12 months, Number of positions | position	2	7
12 months or more, Fair value	$ 4.6	$ 8.8
12 months or more, Unrealized losses	$ (0.2)	$ (1.7)
12 months or more, Number of positions | position	2	5
Total, Fair value	$ 5.9	$ 17.9
Total, Unrealized losses	$ (0.2)	$ (2.6)
Total, Number of 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	$ 2.4	$ 29.9
Less than 12 months, Unrealized losses	$ (0.1)	$ (0.8)
Less than 12 months, Number of 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 | position	0	0
Total, Fair value	$ 2.4	$ 29.9
Total, Unrealized losses	$ (0.1)	$ (0.8)
Total, Number of positions | position	1	2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 845.1	$ 852.6
Less than 12 months, Unrealized losses	$ (18.2)	$ (20.8)
Less than 12 months, Number of positions | position	118	115
12 months or more, Fair value	$ 35.7	$ 49.9
12 months or more, Unrealized losses	$ (1.2)	$ (1.6)
12 months or more, Number of positions | position	12	15
Total, Fair value	$ 880.8	$ 902.5
Total, Unrealized losses	$ (19.4)	$ (22.4)
Total, Number of positions | position	130	130
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 257	$ 229.1
Less than 12 months, Unrealized losses	$ (5.7)	$ (6.1)
Less than 12 months, Number of positions | position	35	30
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions | position	0	0
Total, Fair value	$ 257	$ 229.1
Total, Unrealized losses	$ (5.7)	$ (6.1)
Total, Number of positions | position	35	30
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 126.6	$ 178.9
Less than 12 months, Unrealized losses	$ (2.3)	$ (3.2)
Less than 12 months, Number of positions | position	17	26
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions | position	0	0
Total, Fair value	$ 126.6	$ 178.9
Total, Unrealized losses	$ (2.3)	$ (3.2)
Total, Number of positions | position	17	26
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 114	$ 102.9
Less than 12 months, Unrealized losses	$ (1.3)	$ (1.4)
Less than 12 months, Number of positions | position	17	16
12 months or more, Fair value	$ 15.7	$ 29.4
12 months or more, Unrealized losses	$ (0.4)	$ (0.8)
12 months or more, Number of positions | position	3	4
Total, Fair value	$ 129.7	$ 132.3
Total, Unrealized losses	$ (1.7)	$ (2.2)
Total, Number of positions | position	20	20
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 347.5	$ 341.7
Less than 12 months, Unrealized losses	$ (8.9)	$ (10.1)
Less than 12 months, Number of positions | position	49	43
12 months or more, Fair value	$ 20	$ 20.5
12 months or more, Unrealized losses	$ (0.8)	$ (0.8)
12 months or more, Number of positions | position	9	11
Total, Fair value	$ 367.5	$ 362.2
Total, Unrealized losses	$ (9.7)	$ (10.9)
Total, Number of positions | position	58	54
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 3.7	$ 39
Less than 12 months, Unrealized losses	$ (0.1)	$ (1.7)
Less than 12 months, Number of positions | position	3	9
12 months or more, Fair value	$ 4.6	$ 8.8
12 months or more, Unrealized losses	$ (0.2)	$ (1.7)
12 months or more, Number of positions | position	2	5
Total, Fair value	$ 8.3	$ 47.8
Total, Unrealized losses	$ (0.3)	$ (3.4)
Total, Number of positions | position	5	14

Investments - Realized Losses R

Investments - Realized Losses Related to Other-than-Temporary Impairments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (2.1)	$ (3.7)
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0	(2.3)
Large-Cap Equity Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	(1.5)	(0.2)
Small-Cap Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (0.6)	$ (1.2)

Investments - Amortized Cost an

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Amortized cost
Due in 1 year or less, Amortized cost	$ 52.1
Due after 1 year through 5 years, Amortized cost	540.6
Due after 5 years through 10 years, Amortized cost	306.6
Due after 10 years, Amortized cost	580.9
U.S. government agencies mortgage-backed securities, Amortized cost	624.5
Total, Amortized cost	2,104.7	$ 2,095.9
Fair value
Due in 1 year or less, Fair value	52.6
Due after 1 year through 5 years, Fair value	546.7
Due after 5 years through 10 years, Fair value	310
Due after 10 years, Fair value	591.5
U.S. government agencies mortgage-backed securities, Fair value	622.7
Total, Fair value	$ 2,123.5	$ 2,109.3

Investments - Components of Net

Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Net Investment Income [Line Items]
Investment income	$ 19	$ 17.8
Investment expenses	0.3	0.4
Net investment income	18.7	17.4
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	15.8	14.9
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	1.7	1.5
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.5	$ 1.4

Investments - Realized and Unre

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Realized gains:
Total realized gains	$ 10	$ 5.6
Realized losses:
Total realized losses	(2.2)	(4.3)
Net realized gain on investments	7.8	1.3
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	(8)	(12.5)
Change in net unrealized holding (losses) gains, net of tax	15	23.2
Fixed Maturities [Member]
Realized gains:
Total realized gains	0.3	1.5
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	5.4	36.2
Fixed Maturities [Member] | OTTI [Member]
Realized losses:
Total realized losses	0	(2.3)
Equity Securities [Member]
Realized gains:
Total realized gains	9.7	4.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	14.6	(1)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.1)	(0.6)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(2.1)	(1.4)
Other Invested Assets [Member]
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 3	$ 0.5

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities fair value of deposit with insurance regulators	$ 9.3		$ 9.2
Proceeds on sale of available-for-sale securities	98.6	$ 99.6
Federal Home Loan Bank of Cincinnati [Member]
Schedule of Available-for-sale Securities [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 106.5

Fair Value of Financial Instr36

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 2,567.2		$ 2,537.2
Investments, Fair Value Disclosure	2,528.8		2,501.5
Sales of other invested assets available-for-sale	0.3	$ 0.2
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	445.5		404.7
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	619.1		643.7
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	436.2		449.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	622.7		611.3
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	2,123.5		2,109.3
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	136.5		139
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	76.5		79.1
Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	182.6		164.7
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	395.6		382.8
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Fair Value Disclosure	9.7		9.4
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	405.3		392.2
Level 1 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 1 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 1 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 1 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 1 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	136.5		139
Level 1 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	76.5		79.1
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	182.6		164.7
Level 1 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	395.6		382.8
Level 1 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	9.7		9.4
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,119.9		2,105.8
Level 2 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	445.5		404.7
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	619.1		643.7
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	432.6		446.1
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	622.7		611.3
Level 2 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,119.9		2,105.8
Level 2 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 2 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 2 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 2 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 2 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.6		3.5
Level 3 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.6		3.5
Level 3 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.6		3.5
Level 3 [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Mutual and exchange traded funds [Domain] [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	0		0
Level 3 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 0		$ 0

Fair Value of Financial Instr37

Fair Value of Financial Instruments - Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - Available-for-Sale Securities [Member] - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 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	0	0
Total unrealized gains (losses) - included in other comprehensive income	0	0
Purchases	0.1	0.2
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 3.6	$ 3.5

Fair Value of Financial Instr38

Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) - Affiliated Entity [Member] - Level 2 [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable 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	$ 74.3	$ 75.7

Fair Value of Financial Instr39

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Detail) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Interest rate (percent)	1.73%	1.73%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$ 21,500,000
Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Interest rate (percent)	5.03%	5.03%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$ 85,000,000
Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Interest rate (percent)	5.25%	5.13%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$ 15,500,000
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	122,100,000	$ 122,100,000
Reported Value Measurement [Member] | Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Total notes payable	21,500,000	21,500,000
Reported Value Measurement [Member] | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	85,400,000	85,400,000
Reported Value Measurement [Member] | Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Total notes payable	15,200,000	15,200,000
Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	121,500,000	121,800,000
Estimate of Fair Value Measurement [Member] | Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Total notes payable	21,000,000	21,000,000
Estimate of Fair Value Measurement [Member] | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	85,300,000	85,600,000
Estimate of Fair Value Measurement [Member] | Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Total notes payable	$ 15,200,000	$ 15,200,000

Fair Value of Financial Instr40

Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May 31, 2009Agreement	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 2,567.2	$ 2,537.2
Other invested assets		5.4	5.4
Number of Credit Agreements | Agreement	2
Notes receivable from affiliate		70	70
Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		3.6	3.5
Federal home loan bank stock		4.9
Other invested assets		0.5
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 2,119.9	2,105.8
Corporate Securities [Member] | Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities		1
Fair value		$ 3.6	3.5
Corporate Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 432.6	446.1
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities		1
Fair value		$ 38.4	$ 35.7
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate		$ 70

Losses and Loss Expenses Paya41

Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Schedule of Activity in Liability for Unpaid Losses and Loss Expenses Payable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,217.3	$ 1,083.5	$ 1,181.6	$ 1,053
Reinsurance Recoverables, Including Reinsurance Premium Paid	4.8	6.5	3.6	5.9
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,212.5	1,077	$ 1,178	$ 1,047.1
Current Year Claims and Claims Adjustment Expense	243.7	215.4
Prior Year Claims and Claims Adjustment Expense	(9.7)	9.6
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	234	225
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	60.1	52.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	139.4	142.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 199.5	$ 195.1

Losses and Loss Expenses Paya42

Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 9.7	$ (9.6)
Unallocated loss adjustment expense [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(2.7)	1.6
Business Insurance Segments [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(7.6)
Other Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(3.2)
Small Commercial Package [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(2)
Workers' compensation [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(1.9)
Personal Insurance Segments [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 2.3	3.4
Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Number of claims	3
Prior Year Claims and Claims Adjustment Expense	$ 2.8
Specialty Insurance Segment [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		4.9
Programs [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 3.9
Number of programs		2
Personal Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 3.2
Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ (2.2)	$ (0.7)

Reinsurance - Summary of Extern

Reinsurance - Summary of External Reinsurance Transactions, as well as reinsurance transactions with State Auto Mutual Under the Pooling Arrangement (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Premiums earned:
Assumed from external insurers and reinsurers	$ 1.1	$ 1.4
Assumed under Pooling Arrangement	317.3	319.9
Ceded to external insurers and reinsurers	(6.1)	(6.8)
Ceded under Pooling Arrangement	(202.5)	(206.5)
Net assumed premiums earned	109.8	108
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	1.6	1.1
Assumed under Pooling Arrangement	234.4	225.4
Ceded to external insurers and reinsurers	(2)	(1.9)
Ceded under Pooling Arrangement	(148.2)	(131.7)
Net assumed losses and loss expenses incurred	$ 85.8	$ 92.9

Income Taxes - Reconciliation B

Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ (1.8)	$ 1.3
Tax-exempt interest and dividends received deduction	(1.5)	(2)
Other, net	2.2	1.5
Federal income tax expense and effective rate	$ (1.1)	$ 0.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	29.10%	(52.80%)
Other, net (percent)	(42.40%)	37.80%
Federal income tax expense (benefit) and effective rate (percent)	21.70%	20.00%

Pension and Postretirement Be45

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1.7	$ 1.6
Interest cost	2.9	2.9
Expected return on plan assets	(4.2)	(3.7)
Amortization of:
Negative prior service cost	0	0
Net actuarial loss	2	2.3
Net periodic cost (benefit)	2.4	3.1
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0
Interest cost	0.2	0.2
Expected return on plan assets	0	0
Amortization of:
Negative prior service cost	(1.4)	(1.4)
Net actuarial loss	0.1	0.1
Net periodic cost (benefit)	$ (1.1)	$ (1.1)

Pension and Postretirement Be46

Pension and Postretirement Benefit Plans - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)
Compensation and Retirement Disclosure [Abstract]
Contributions by employer	$ 3.3
Defined Benefit Plan, Expected Contributions in Current Fiscal Year	$ 6.5

Other Comprehensive Income an47

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	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 32.5	$ 37.6
Other comprehensive income before reclassifications	20.1	24
Amounts reclassified from AOCI	(4.7)	(0.2)
Other comprehensive income	15.4	23.8
Ending balance	47.9	61.4
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	62.8	68.5
Other comprehensive income before reclassifications	20.1	24
Amounts reclassified from AOCI	(5.1)	(0.8)
Other comprehensive income	15	23.2
Ending balance	77.8	91.7
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(30.3)	(30.9)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from AOCI	0.4	0.6
Other comprehensive income	0.4	0.6
Ending balance	$ (29.9)	$ (30.3)

Other Comprehensive Income an48

Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	$ 7.8	$ 1.3
Tax (expense) benefit	1.1	(0.8)
Total reclassifications for the period	4.7	0.2
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	5.1	0.8
Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(0.4)	(0.6)
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	4.7	0.2
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	7.8	1.3
Total before tax	7.8	1.3
Tax (expense) benefit	(2.7)	(0.5)
Net of tax	5.1	0.8
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive income components/Negative prior-service costs	1.4	1.4
Accumulated other comprehensive income components/Net actuarial loss	(2.1)	(2.4)
Total before tax	(0.7)	(1)
Tax (expense) benefit	0.3	0.4
Net of tax	$ (0.4)	$ (0.6)

Earnings Per Common Share - Com

Earnings Per Common Share - Compilation of Basic and Diluted Earnings (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net (loss) income basic earnings per common share	$ (4.1)	$ 3
Denominator:
Weighted average shares for basic net earnings per common share (shares)	41.9	41.4
Effect of dilutive share-based awards (shares)	0	0.4
Adjusted weighted average shares for diluted net earnings per common share (shares)	41.9	41.8
Basic net (loss) earnings per common share (usd per share)	$ (0.10)	$ 0.07
Diluted net (loss) earnings per common share (usd per share)	$ (0.10)	$ 0.07

Earnings Per Common Share - Opt

Earnings Per Common Share - Options to Purchase Shares of Common Stock (Detail) - shares shares in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share [Abstract]
Number of antidilutive options and awards	0.6	1.7

Segment Information - Financial

Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Revenues from external sources:
Revenue	$ 344.3	$ 339.2
Net Investment Income	18.7	17.4
Segment income before federal income tax:
Interest expense on corporate debt	(1.4)	(1.3)
Income before federal income taxes	(5.2)	3.8
Operating Segments [Member]
Revenues from external sources:
Revenue	344.3	339.2
Segment income before federal income tax:
Income before federal income taxes	2.5	7.1
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	152.7	153.5
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	103.6	111.4
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	61	55
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	317.3	319.9
Segment income before federal income tax:
Income before federal income taxes	(24.1)	(11.7)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	18.7	17.4
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	26.5	18.7
Segment income before federal income tax:
Income before federal income taxes	26.5	18.7
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	7.8	1.3
Segment income before federal income tax:
Income before federal income taxes	7.8	1.3
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0.5	0.6
Segment income before federal income tax:
Income before federal income taxes	0.1	0.1
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(10.6)	4.4
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(4.9)	(7.4)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(8.6)	(8.7)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.5	1.4
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.5)	(1.4)
Segment income before federal income tax:
Corporate Expenses	(0.9)	(0.8)
Income before federal income taxes	(7.7)	(3.3)
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	(5.4)	(1.2)
Interest expense on corporate debt	1.4	1.3
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 345.8	$ 340.6

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)Segment	Mar. 31, 2016USD ($)	Dec. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Acquisition And Operating Expenses	$ 112.2	$ 107.5
Number of reportable segments (segments) | Segment	4
Investable assets	$ 2,687.4		$ 2,663.7