State Auto Financial (STFC) 10-Q 4 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Oct. 30, 2015
Document Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
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)		41,332,382

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $1,938.9 and $1,831.3, respectively)	$ 1,979.6	$ 1,891.9
Equity securities, available-for-sale, at fair value (cost $266.1 and $235.5, respectively)	298	310.4
Other invested assets, available-for-sale, at fair value (cost $56.6 and $50.5, respectively)	82.8	80.3
Other invested assets	5.3	5.3
Notes receivable from affiliate	70	70
Total investments	2,435.7	2,357.9
Cash and cash equivalents	50.9	86.3
Accrued investment income and other assets	38.7	33.8
Deferred policy acquisition costs (affiliated net assumed $51.8 and $46.8, respectively)	134	126.5
Reinsurance recoverable on losses and loss expenses payable	7.7	9.6
Prepaid reinsurance premiums	6.8	6.1
Due from affiliate	21.9	40.1
Current federal income taxes	0.3	1.1
Net deferred federal income taxes	104	97.4
Property and equipment, at cost	7.7	8.1
Total assets	2,807.7	2,766.9
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $506.5 and $494.3, respectively)	1,021.5	983.2
Unearned premiums (affiliated net assumed $220.4 and $201.7, respectively)	637.5	612.4
Notes payable (affiliates $15.5 and $15.5, respectively)	100.8	100.8
Postretirement and pension benefits (affiliated net ceded $58.4 and $63.2, respectively)	108.6	117.3
Other liabilities (affiliated net ceded $6.8 and $5.1, respectively)	64	80.3
Total liabilities	1,932.4	1,894
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 47.9 and 47.7 shares issued, respectively, at stated value of $2.50 per share	119.9	119.3
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(116.3)	(116)
Additional paid-in capital	149.8	143.2
Accumulated other comprehensive income (affiliated net ceded $61.0 and $65.1, respectively)	31.4	71.7
Retained earnings	690.5	654.7
Total stockholders’ equity	875.3	872.9
Total liabilities and stockholders’ equity	2,807.7	2,766.9
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	Sep. 30, 2015	Dec. 31, 2014
Fixed maturities, available-for-sale, amortized cost	$ 1,938.9	$ 1,831.3
Equity securities, available-for-sale, cost	266.1	235.5
Other invested assets, available-for-sale, cost	56.6	50.5
Deferred policy acquisitions costs from affiliates	51.8	46.8
Losses and loss expenses payable, affiliates	506.5	494.3
Unearned premiums, affiliates	220.4	201.7
Notes payable, affiliates	15.5	15.5
Net Postretirement and Pension Benefits	(58.4)	(63.2)
Net Other Liabilities from Affiliates	$ (6.8)	$ (5.1)
Common stock, no par value (usd per share)	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	47,900,000	47,700,000
Common stock, stated value per share (usd per share)	$ 2.5	$ 2.5
Treasury stock (shares)	6,800,000	6,800,000
Net Accumulated Other Comprehensive Income	$ (61)	$ (65.1)
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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Earned premiums (affiliated net assumed $112.2 and $55.0, respectively, and $320.5 and $153.6, respectively)	$ 319.7	$ 270.2	$ 946.5	$ 801
Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively)	18.7	18.9	53.9	57
Net realized gains on investments:
Total other-than-temporary impairment losses	(3.5)	(1.1)	(4.8)	(2.3)
Portion of loss recognized in other comprehensive income	0	0	0	0
Other net realized investment gains	14.3	2.7	24.8	19.9
Total net realized gains on investments	10.8	1.6	20	17.6
Other income from affiliates	0.5	0.6	1.6	1.4
Total revenues	349.7	291.3	1,022	877
Losses and loss expenses (affiliated net assumed $67.4 and $53.6, respectively, and $200.9 and $131.6, respectively)	207.5	184.4	627.1	546.6
Acquisition and operating expenses (affiliated net assumed $52.8 and $36.3, respectively, and $188.9 and $117.5, respectively)	109.4	91.4	318.7	277.2
Interest expense (affiliates $0.1 and $0.2, respectively, and $0.5 and $0.5, respectively)	1.3	1.4	4	4
Other expenses	1.6	1	5.7	5.3
Total expenses	319.8	278.2	955.5	833.1
Income before federal income taxes	29.9	13.1	66.5	43.9
Current Income Tax Expense (Benefit)	2.7	1.2	3.3	1.9
Deferred Income Tax Expense (Benefit)	6.5	0	15.1	0
Federal income tax expense	9.2	1.2	18.4	1.9
Net income	$ 20.7	$ 11.9	$ 48.1	$ 42
Earnings per common share:
Basic (usd per share)	$ 0.50	$ 0.29	$ 1.17	$ 1.03
Diluted (usd per share)	0.50	0.28	1.16	1.02
Dividends paid per common share (usd per share)	$ 0.1	$ 0.1	$ 0.3	$ 0.3

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Earned premiums, from affiliates	$ 112.2	$ 55	$ 320.5	$ 153.6
Net investment income, from affiliate	1.3	1.3	3.7	3.7
Losses and loss expenses, from affiliates	67.4	53.6	200.9	131.6
Net Acquisition and Operating Expenses, from affiliates	52.8	36.3	188.9	117.5
Interest Expense Debt To Affiliates			$ 0.5	$ 0.5
Interest Paid To Affiliates	$ 0.1	$ 0.2

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 20.7	$ 11.9	$ 48.1	$ 42
Net unrealized holding (losses) gains on investments:
Unrealized holding (losses) gains	(24.5)	(16.3)	(46.1)	41.6
Reclassification adjustments for gains realized in net income	(10.8)	(1.6)	(20.4)	(17.6)
Income tax benefit (expense)	12.4	(0.6)	23.3	(8.3)
Total net unrealized holding (losses) gains on investments	(22.9)	(18.5)	(43.2)	15.7
Net actuarial loss arising during period			0	(1.6)
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.3)	(1.3)	(4.1)	(4.1)
Net actuarial loss	2.8	1.6	8.6	5.4
Income tax expense	(0.6)	0	(1.6)	0
Total net unrecognized benefit plan obligations	0.9	0.3	2.9	(0.3)
Other comprehensive (loss) income	(22)	(18.2)	(40.3)	15.4
Comprehensive income	$ (1.3)	$ (6.3)	$ 7.8	$ 57.4

Condensed Consolidated Stateme7

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net income	$ 48.1	$ 42
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization, net	10.8	6.6
Share-based compensation	5.1	2.7
Net realized gains on investments	(20)	(17.6)
Changes in operating assets and liabilities:
Deferred policy acquisition costs	(7.5)	(12.6)
Accrued investment income and other assets	(4.5)	0.7
Postretirement and pension benefits	(5.8)	(8)
Other liabilities and due to/from affiliates, net	(63.5)	(11.1)
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	1.2	(1.3)
Losses and loss expenses payable	38.3	1.4
Unearned premiums	25.1	37.8
Excess Tax Benefit from Share-based Compensation, Operating Activities	0.1	0
Federal income taxes	17.6	1
Cash provided from December 31, 2014 unearned premium transfer related to the homeowners quota share arrangement	63.5	0
Net cash provided by operating activities	108.5	41.6
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(459.8)	(325.8)
Purchases of equity securities available-for-sale	(131.8)	(95.7)
Purchases of other invested assets	(6.5)	(1.5)
Maturities, calls and pay downs of fixed maturities available-for-sale	203.4	180.8
Sales of fixed maturities available-for-sale	142.7	135.4
Sales of equity securities available-for-sale	117.4	76.6
Sales of other invested assets available-for-sale	0.5	0.5
Payments to Acquire Property, Plant, and Equipment	(0.1)	0
Net cash used in investing activities	(134.2)	(29.7)
Cash flows from financing activities:
Proceeds from issuance of common stock	2.9	2.4
Payments to acquire treasury stock	(0.3)	(0.1)
Payment of dividends	(12.3)	(12.3)
Net cash used in financing activities	(9.7)	(10)
Net decrease in cash and cash equivalents	(35.4)	1.9
Cash and cash equivalents at beginning of period	86.3	80.3
Cash and cash equivalents at end of period	50.9	82.2
Supplemental disclosures:
Interest paid (affiliates $0.5 and $0.5, respectively)	3.9	3.9
Income Taxes Paid	$ 2.4	$ 1

Condensed Consolidated Stateme8

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Statement of Cash Flows [Abstract]
Interest Expense Debt To Affiliates	$ 0.5	$ 0.5

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2015
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 and nine month period ended September 30, 2015 are not necessarily indicative of the results that may be expected for the year ending December 31, 2015 . The balance sheet at December 31, 2014 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, 2014 (the “ 2014 Form 10-K”). Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the 2014 Form 10-K.

Investments

Investments	9 Months Ended
	Sep. 30, 2015
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 September 30, 2015 and December 31, 2014 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value September 30, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 235.0 $ 7.8 $ (1.7 ) $ 241.1 Obligations of states and political subdivisions 840.6 20.8 (4.3 ) 857.1 Corporate securities 409.0 8.4 (5.7 ) 411.7 U.S. government agencies mortgage-backed securities 454.3 16.9 (1.5 ) 469.7 Total fixed maturities 1,938.9 53.9 (13.2 ) 1,979.6 Equity securities: Large-cap securities 213.0 27.0 (9.1 ) 230.9 Small-cap securities 53.1 14.3 (0.3 ) 67.1 Total equity securities 266.1 41.3 (9.4 ) 298.0 Other invested assets 56.6 26.2 — 82.8 Total available-for-sale securities $ 2,261.6 $ 121.4 $ (22.6 ) $ 2,360.4 ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 296.7 $ 14.1 $ (1.5 ) $ 309.3 Obligations of states and political subdivisions 742.5 27.4 (0.4 ) 769.5 Corporate securities 333.4 10.2 (3.0 ) 340.6 U.S. government agencies mortgage-backed securities 458.7 15.6 (1.8 ) 472.5 Total fixed maturities 1,831.3 67.3 (6.7 ) 1,891.9 Equity securities: Large-cap securities 185.5 57.3 (0.6 ) 242.2 Small-cap securities 50.0 18.2 — 68.2 Total equity securities 235.5 75.5 (0.6 ) 310.4 Other invested assets 50.5 29.8 — 80.3 Total available-for-sale securities $ 2,117.3 $ 172.6 $ (7.3 ) $ 2,282.6 The following tables set forth the Company’s gross unrealized losses and fair value on its investments by lot, aggregated by investment category and length of time for individual securities that have been in a continuous unrealized loss position at September 30, 2015 and December 31, 2014 : ($ 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 September 30, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 62.8 $ (1.1 ) 12 $ 18.7 $ (0.6 ) 4 $ 81.5 $ (1.7 ) 16 Obligations of states and political subdivisions 195.1 (4.3 ) 27 — — — 195.1 (4.3 ) 27 Corporate securities 70.3 (4.3 ) 12 49.6 (1.4 ) 9 119.9 (5.7 ) 21 U.S. government agencies mortgage-backed securities 60.1 (0.9 ) 7 34.2 (0.6 ) 13 94.3 (1.5 ) 20 Total fixed maturities 388.3 (10.6 ) 58 102.5 (2.6 ) 26 490.8 (13.2 ) 84 Equity securities: Large-cap equity securities 91.7 (9.1 ) 35 — — — 91.7 (9.1 ) 35 Small-cap equity securities 4.3 (0.3 ) 1 — — — 4.3 (0.3 ) 1 Total equity securities 96.0 (9.4 ) 36 — — — 96.0 (9.4 ) 36 Total temporarily impaired securities $ 484.3 $ (20.0 ) 94 $ 102.5 $ (2.6 ) 26 $ 586.8 $ (22.6 ) 120 ($ 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, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 19.9 $ (0.3 ) 4 $ 52.0 $ (1.2 ) 17 $ 71.9 $ (1.5 ) 21 Obligations of states and political subdivisions 6.1 — 5 30.9 (0.4 ) 9 37.0 (0.4 ) 14 Corporate securities 43.5 (0.9 ) 8 56.1 (2.1 ) 11 99.6 (3.0 ) 19 U.S. government agencies mortgage-backed securities 44.0 (0.3 ) 8 37.5 (1.5 ) 13 81.5 (1.8 ) 21 Total fixed maturities 113.5 (1.5 ) 25 176.5 (5.2 ) 50 290.0 (6.7 ) 75 Large-cap equity securities 7.2 (0.6 ) 2 — — — 7.2 (0.6 ) 2 Total temporarily impaired securities $ 120.7 $ (2.1 ) 27 $ 176.5 $ (5.2 ) 50 $ 297.2 $ (7.3 ) 77 The Company reviewed its investments at September 30, 2015 , 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 and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Equity securities: Large-cap securities $ — $ — $ — $ (0.3 ) Small-cap securities (3.5 ) (1.1 ) (4.8 ) (2.0 ) Total other-than-temporary impairments $ (3.5 ) $ (1.1 ) $ (4.8 ) $ (2.3 ) 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 September 30, 2015 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 34.1 $ 34.5 Due after 1 year through 5 years 427.7 439.2 Due after 5 years through 10 years 288.1 294.4 Due after 10 years 734.7 741.8 U.S. government agencies mortgage-backed securities 454.3 469.7 Total $ 1,938.9 $ 1,979.6 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 September 30, 2015 , State Auto P&C had U.S. government agencies mortgage-backed fixed maturity securities, with a carrying value of approximately $85.0 million , that were pledged as collateral for the Federal Home Loan Bank of Cincinnati ("FHLB") Loan. In accordance with the terms of the FHLB Loan, State Auto P&C retains all rights regarding these securities. Fixed maturities with fair values of $8.9 million and $8.8 million were on deposit with insurance regulators as required by law at September 30, 2015 and December 31, 2014 , respectively. The Company retains all rights regarding these securities. The following table sets forth the components of net investment income for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Fixed maturities $ 16.1 $ 16.1 $ 46.4 $ 49.4 Equity securities 1.4 1.6 4.7 4.7 Cash and cash equivalents, and other 1.6 1.7 4.3 4.4 Investment income 19.1 19.4 55.4 58.5 Investment expenses 0.4 0.5 1.5 1.5 Net investment income $ 18.7 $ 18.9 $ 53.9 $ 57.0 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 $260.6 million and $212.5 million for the nine months ended September 30, 2015 and 2014 , respectively. The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Realized gains: Fixed maturities $ 2.9 $ 0.1 $ 4.4 $ 2.6 Equity securities 11.8 3.2 21.8 17.9 Other invested assets — — 0.1 0.1 Total realized gains 14.7 3.3 26.3 20.6 Realized losses: Equity securities: Sales (0.4 ) (0.6 ) (1.1 ) (0.7 ) OTTI (3.5 ) (1.1 ) (4.8 ) (2.3 ) Total realized losses (3.9 ) (1.7 ) (5.9 ) (3.0 ) Net realized gains on investments $ 10.8 $ 1.6 $ 20.4 $ 17.6 Change in unrealized holding gains, net of tax: Fixed maturities $ 8.7 $ (8.6 ) $ (19.9 ) $ 25.4 Equity securities (35.5 ) (4.7 ) (43.0 ) (3.0 ) Other invested assets (8.5 ) (4.6 ) (3.6 ) 1.6 Deferred federal income tax liability 12.4 6.2 23.3 (8.4 ) Valuation allowance — (6.8 ) — 0.1 Change in net unrealized holding gains, net of tax $ (22.9 ) $ (18.5 ) $ (43.2 ) $ 15.7

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments Below is the fair value hierarchy that categorizes into three levels the inputs to valuation techniques that are used to measure fair value: • Level 1 includes observable inputs which reflect quoted prices for identical assets or liabilities in active markets at the measurement date. • Level 2 includes observable inputs for assets or liabilities other than quoted prices included in Level 1, and it includes valuation techniques which use prices for similar assets and liabilities. • Level 3 includes unobservable inputs which reflect the reporting entity’s estimates of the assumptions that market participants would use in pricing the asset or liability (including assumptions about risk). The Company utilizes one nationally recognized pricing service to estimate the majority of its available-for-sale investment portfolio’s fair value. The Company obtains one price per security 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 September 30, 2015 and December 31, 2014 , 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 and nine months ended September 30, 2015 and 2014 . 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 fixed maturity corporate 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 are two international funds (“the funds”) that invest in equity securities of foreign issuers and are managed by third party investment managers. The funds had a fair value of $74.9 million and $72.9 million at September 30, 2015 and December 31, 2014 , respectively, which was determined using each fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the funds including obtaining and reviewing each fund’s audited financial statements. There are no unfunded commitments related to the funds. The Company may not sell its investment in the funds; however, the Company may redeem all or a portion of its investment in the funds at net asset value per share with the appropriate prior written notice. Due to the Company’s ability to redeem its investment in the funds at net asset value per share at the measurement date, the funds have been disclosed in Level 2. The remainder of the Company’s other invested assets consist primarily of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds based on an observable market price for an identical asset in an active market reflect their fair values and consequently these securities have been disclosed in Level 1. The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at September 30, 2015 and December 31, 2014 : ($ millions) Total Level 1 Level 2 Level 3 September 30, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 241.1 $ — $ 241.1 $ — Obligations of states and political subdivisions 857.1 — 857.1 — Corporate securities 411.7 — 408.1 3.6 U.S. government agencies mortgage-backed securities 469.7 — 469.7 — Total fixed maturities 1,979.6 — 1,976.0 3.6 Equity securities: Large-cap securities 230.9 230.9 — — Small-cap securities 67.1 67.1 — — Total equity securities 298.0 298.0 — — Other invested assets 82.8 7.9 74.9 — Total available-for-sale investments $ 2,360.4 $ 305.9 $ 2,050.9 $ 3.6 ($ millions) Total Level 1 Level 2 Level 3 December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 309.3 $ — $ 309.3 $ — Obligations of states and political subdivisions 769.5 — 769.5 — Corporate securities 340.6 — 331.2 9.4 U.S. government agencies mortgage-backed securities 472.5 — 472.5 — Total fixed maturities 1,891.9 — 1,882.5 9.4 Equity securities: Large-cap securities 242.2 242.2 — — Small-cap securities 68.2 68.2 — — Total equity securities 310.4 310.4 — — Other invested assets 80.3 7.4 72.9 — Total available-for-sale investments $ 2,282.6 $ 317.8 $ 1,955.4 $ 9.4 For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for the three and nine months ended September 30, 2015 and the year ended December 31, 2014 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2015 $ 9.4 Total realized gains – included in earnings 0.2 Total unrealized losses – included in other comprehensive income (0.2 ) Purchases — Sales (5.9 ) Transfers into Level 3 — Transfers out of Level 3 — Balance at March 31, 2015 $ 3.5 Total realized gains – included in earnings — Total unrealized losses – included in other comprehensive income (0.1 ) Purchases 0.1 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at June 30, 2015 $ 3.5 Total realized gains – included in earnings — Total unrealized gains – included in other comprehensive income — Purchases 0.1 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at September 30, 2015 $ 3.6 ($ millions) Fixed maturities Balance at January 1, 2014 $ 8.9 Total realized gains – included in earnings — Total unrealized gains – included in other comprehensive income 0.2 Purchases 0.3 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2014 $ 9.4 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 ("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 September 30, 2015 was $4.8 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) September 30, 2015 December 31, 2014 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 76.2 7.00 % $ 70.0 $ 74.6 7.00 % Notes Payable Included in notes payable are the FHLB Loan and Subordinated Debentures. The Company estimates the fair value of the FHLB Loan by discounting cash flows using a borrowing rate currently available to the Company for a loan with similar terms. This has been placed in Level 3 of the fair value hierarchy. The carrying amount of the Subordinated Debentures approximates its fair value as the interest rate adjusts quarterly and has been disclosed in Level 3. ($ millions, except interest rates) September 30, 2015 December 31, 2014 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2033: issued $85.0, July 2013 with fixed interest $ 85.3 $ 85.8 5.03 % $ 85.3 $ 86.4 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.5 15.5 4.53 % 15.5 15.5 4.44 % Total notes payable $ 100.8 $ 101.3 $ 100.8 $ 101.9

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2015
Insurance [Abstract]
Reinsurance	Reinsurance The insurance subsidiaries of State Auto Financial, including State Auto Property & Casualty Insurance Company (“State Auto P&C”), Milbank Insurance Company and State Auto Insurance Company of Ohio (collectively referred to as the “STFC Pooled Companies”) participate in a quota share reinsurance pooling arrangement (“the Pooling Arrangement”) with State Auto Mutual and its subsidiaries and affiliates, State Auto Insurance Company of Wisconsin, Meridian Security Insurance Company, Patrons Mutual Insurance Company of Connecticut, Rockhill Insurance Company, Plaza Insurance Company, American Compensation Insurance Company and Bloomington Compensation Insurance Company, (collectively referred to as the “Mutual Pooled Companies”). 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 and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Premiums earned: Assumed from external insurers and reinsurers $ 1.1 $ 1.2 $ 3.2 $ 3.3 Assumed under Pooling Arrangement 319.7 270.2 946.5 801.0 Ceded to external insurers and reinsurers (8.7 ) (6.5 ) (26.1 ) (18.5 ) Ceded under Pooling Arrangement (207.5 ) (215.2 ) (626.0 ) (647.4 ) Net assumed premiums earned $ 104.6 $ 49.7 $ 297.6 $ 138.4 Losses and loss expenses incurred: Assumed from external insurers and reinsurers $ 0.8 $ 0.8 $ 2.2 $ 2.3 Assumed under Pooling Arrangement 207.8 185.2 628.1 548.9 Ceded to external insurers and reinsurers (1.2 ) (2.7 ) (3.6 ) (6.4 ) Ceded under Pooling Arrangement (140.4 ) (131.6 ) (427.2 ) (417.3 ) Net assumed losses and loss expenses incurred $ 67.0 $ 51.7 $ 199.5 $ 127.5

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Income Taxes	5. Income Taxes The following table sets forth the reconciliation between actual federal income tax expense and the amount computed at the indicated statutory rate for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Amount at statutory rate $ 10.5 35.0 % $ 4.6 35.0 % $ 23.3 35.0 % $ 15.4 35.0 % Tax-exempt interest and dividends received deduction (2.1 ) (7.0 ) (2.1 ) (16.2 ) (6.6 ) (9.9 ) (6.3 ) (14.4 ) Other, net 0.8 2.8 0.1 0.3 1.7 2.6 1.0 2.3 Valuation allowance — — (1.4 ) (10.1 ) — — (8.2 ) (18.6 ) Federal income tax expense and effective rate $ 9.2 30.8 % $ 1.2 9.0 % $ 18.4 27.7 % $ 1.9 4.3 %

Pension and Postretirement Bene

Pension and Postretirement Benefit Plans	9 Months Ended
	Sep. 30, 2015
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 and nine months ended September 30, 2015 and 2014 : ($ millions) Pension Postretirement Pension Postretirement Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 2015 2014 2015 2014 Service cost $ 2.0 $ 1.3 $ — $ — $ 5.9 $ 3.9 $ — $ — Interest cost 2.8 2.8 0.3 0.3 8.5 8.4 0.7 0.8 Expected return on plan assets (3.5 ) (3.2 ) — — (10.4 ) (9.5 ) — — Amortization of: Negative prior service cost — — (1.3 ) (1.3 ) — — (4.1 ) (4.1 ) Net actuarial loss 2.7 1.4 0.1 0.2 8.2 4.9 0.4 0.5 Net periodic cost (benefit) $ 4.0 $ 2.3 $ (0.9 ) $ (0.8 ) $ 12.2 $ 7.7 $ (3.0 ) $ (2.8 ) The Company contributed $13.0 million to its pension plan for the nine months ended September 30, 2015 .

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2015
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 and nine months ended September 30, 2015 and 2014 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at July 1, 2015 $ 89.7 $ (36.3 ) $ 53.4 Other comprehensive income before reclassifications (15.8 ) — (15.8 ) Amounts reclassified from AOCI (a) (7.1 ) 0.9 (6.2 ) Net current period other comprehensive (loss) income (22.9 ) 0.9 (22.0 ) Ending balance at September 30, 2015 $ 66.8 $ (35.4 ) $ 31.4 Beginning balance at July 1, 2014 $ 118.8 $ (4.4 ) $ 114.4 Other comprehensive income before reclassifications (16.9 ) — (16.9 ) Amounts reclassified from AOCI (a) (1.6 ) 0.3 (1.3 ) Net current period other comprehensive (loss) income (18.5 ) 0.3 (18.2 ) Ending balance at September 30, 2014 $ 100.3 $ (4.1 ) $ 96.2 (a) See separate table below for details about these reclassifications ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at January 1, 2015 $ 110.0 $ (38.3 ) $ 71.7 Other comprehensive income before reclassifications (29.9 ) — (29.9 ) Amounts reclassified from AOCI (a) (13.3 ) 2.9 (10.4 ) Net current period other comprehensive (loss) income (43.2 ) 2.9 (40.3 ) Ending balance at September 30, 2015 $ 66.8 $ (35.4 ) $ 31.4 Beginning balance at January 1, 2014 $ 84.6 $ (3.8 ) $ 80.8 Other comprehensive income before reclassifications 33.3 (1.6 ) 31.7 Amounts reclassified from AOCI (a) (17.6 ) 1.3 (16.3 ) Net current period other comprehensive income (loss) 15.7 (0.3 ) 15.4 Ending balance at September 30, 2014 $ 100.3 $ (4.1 ) $ 96.2 (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 and nine months ended September 30, 2015 and 2014 : ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2015 2014 Unrealized gains on available for sale securities $ 10.8 $ 1.6 Realized gain on sale of securities 10.8 1.6 Total before tax (3.7 ) — Tax expense 7.1 1.6 Net of tax Amortization of benefit plan items Negative prior service cost 1.3 1.3 (a) Net actuarial loss (2.8 ) (1.6 ) (a) (1.5 ) (0.3 ) Total before tax 0.6 — Tax benefit (0.9 ) (0.3 ) Net of tax Total reclassifications for the period $ 6.2 $ 1.3 (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Nine months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2015 2014 Unrealized gains on available for sale securities $ 20.4 $ 17.6 Realized gain on sale of securities 20.4 17.6 Total before tax (7.1 ) — Tax expense 13.3 17.6 Net of tax Amortization of benefit plan items Negative prior service cost 4.1 4.1 (a) Net actuarial loss (8.6 ) (5.4 ) (a) (4.5 ) (1.3 ) Total before tax 1.6 — Tax benefit (2.9 ) (1.3 ) Net of tax Total reclassifications for the period $ 10.4 $ 16.3 (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings per Common Share

Earnings per Common Share	9 Months Ended
	Sep. 30, 2015
Earnings Per Share [Abstract]
Earnings (Loss) per Common Share	Net Earnings per Common Share The following table sets forth the compilation of basic and diluted earnings per common share for the three and nine months ended September 30, 2015 and 2014 : ($ and shares in millions, except per share amounts) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Numerator: Net income for basic earnings per common share $ 20.7 $ 11.9 $ 48.1 $ 42.0 Denominator: Weighted average shares for basic net earnings per common share 41.1 40.9 41.1 40.8 Effect of dilutive share-based awards 0.8 0.6 0.4 0.4 Adjusted weighted average shares for diluted net earnings per common share 41.9 41.5 41.5 41.2 Basic net earnings per common share $ 0.50 $ 0.29 $ 1.17 $ 1.03 Diluted net earnings per common share $ 0.50 $ 0.28 $ 1.16 $ 1.02 The following table sets forth common stock options and restricted share units ("RSU award") provided to each outside director of the Company that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three and nine months ended September 30, 2015 and 2014 . (shares in millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Total number of antidilutive options and awards 1.1 1.5 1.7 1.8

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2015
Segment Reporting [Abstract]
Segment Information	Segment Information The Company has four reportable segments: personal insurance, business 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 personal insurance segment provides primarily personal automobile and homeowners to the personal insurance market. The business insurance segment provides primarily commercial automobile, commercial multi-peril, fire & allied and general liability insurance covering small-to-medium sized commercial exposures. The specialty insurance segment provides commercial coverages, including workers’ compensation for both the legacy State Auto Group and RTW's insurance subsidiaries, that require specialized product underwriting, claims handling or risk management services through a distribution channel of retail agents and wholesale brokers, which may include program administrators and other specialty sources. The investment operations segment, managed by Stateco, provides investment services. The Company evaluates the performance of its insurance segments using industry financial measurements based on 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 and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Revenue from external sources: Insurance segments Personal insurance $ 147.2 $ 112.3 $ 445.4 $ 340.3 Business insurance 96.8 96.0 289.4 285.4 Specialty insurance 75.7 61.9 211.7 175.3 Total insurance segments 319.7 270.2 946.5 801.0 Investment operations segment Net investment income 18.7 18.9 53.9 57.0 Net realized capital gains 10.8 1.6 20.4 17.6 Total investment operations segment 29.5 20.5 74.3 74.6 All other 0.5 0.6 1.2 1.4 Total revenue from external sources 349.7 291.3 1,022.0 877.0 Intersegment revenue 1.4 1.3 4.2 3.9 Total revenue 351.1 292.6 1,026.2 880.9 Reconciling items: Eliminate intersegment revenues (1.4 ) (1.3 ) (4.2 ) (3.9 ) Total consolidated revenues $ 349.7 $ 291.3 $ 1,022.0 $ 877.0 Segment income before federal income tax: Insurance segments SAP underwriting income (loss) Personal insurance $ 6.9 $ 4.5 $ 14.1 $ (0.1 ) Business insurance (6.8 ) (3.3 ) (23.0 ) (18.6 ) Specialty insurance 1.2 (11.4 ) (0.6 ) (24.1 ) Total insurance segments 1.3 (10.2 ) (9.5 ) (42.8 ) Investment operations segment Net investment income 18.7 18.9 53.9 57.0 Net realized capital gains 10.8 1.6 20.4 17.6 Total investment operations segment 29.5 20.5 74.3 74.6 All other 0.1 — (0.2 ) — Total segment income before tax expense 30.9 10.3 64.6 31.8 Reconciling items: GAAP expense adjustments 0.9 4.2 8.6 18.4 Interest expense on corporate debt (1.3 ) (1.4 ) (4.0 ) (4.0 ) Corporate expenses (0.6 ) — (2.7 ) (2.3 ) Total reconciling items (1.0 ) 2.8 1.9 12.1 Total consolidated income before federal income tax expense $ 29.9 $ 13.1 $ 66.5 $ 43.9 Investable assets attributable to the Company’s investment operations segment totaled $2,486.6 million and $2,444.2 million at September 30, 2015 and December 31, 2014 , respectively.

Contingencies and Litigation

Contingencies and Litigation	9 Months Ended
	Sep. 30, 2015
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 or cash flows. Additionally, the Company may be impacted by adverse regulatory actions and adverse court decisions where insurance coverages are expanded beyond the scope originally contemplated in its insurance policies. The Company believes that the effects, if any, of such regulatory actions and published court decisions are not likely to have a material adverse effect on its financial or cash flow position.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2015
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 September 30, 2015 and December 31, 2014 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value September 30, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 235.0 $ 7.8 $ (1.7 ) $ 241.1 Obligations of states and political subdivisions 840.6 20.8 (4.3 ) 857.1 Corporate securities 409.0 8.4 (5.7 ) 411.7 U.S. government agencies mortgage-backed securities 454.3 16.9 (1.5 ) 469.7 Total fixed maturities 1,938.9 53.9 (13.2 ) 1,979.6 Equity securities: Large-cap securities 213.0 27.0 (9.1 ) 230.9 Small-cap securities 53.1 14.3 (0.3 ) 67.1 Total equity securities 266.1 41.3 (9.4 ) 298.0 Other invested assets 56.6 26.2 — 82.8 Total available-for-sale securities $ 2,261.6 $ 121.4 $ (22.6 ) $ 2,360.4 ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 296.7 $ 14.1 $ (1.5 ) $ 309.3 Obligations of states and political subdivisions 742.5 27.4 (0.4 ) 769.5 Corporate securities 333.4 10.2 (3.0 ) 340.6 U.S. government agencies mortgage-backed securities 458.7 15.6 (1.8 ) 472.5 Total fixed maturities 1,831.3 67.3 (6.7 ) 1,891.9 Equity securities: Large-cap securities 185.5 57.3 (0.6 ) 242.2 Small-cap securities 50.0 18.2 — 68.2 Total equity securities 235.5 75.5 (0.6 ) 310.4 Other invested assets 50.5 29.8 — 80.3 Total available-for-sale securities $ 2,117.3 $ 172.6 $ (7.3 ) $ 2,282.6
Gross Unrealized Losses and Fair Value on its Investments	The following tables set forth the Company’s gross unrealized losses and fair value on its investments by lot, aggregated by investment category and length of time for individual securities that have been in a continuous unrealized loss position at September 30, 2015 and December 31, 2014 : ($ 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 September 30, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 62.8 $ (1.1 ) 12 $ 18.7 $ (0.6 ) 4 $ 81.5 $ (1.7 ) 16 Obligations of states and political subdivisions 195.1 (4.3 ) 27 — — — 195.1 (4.3 ) 27 Corporate securities 70.3 (4.3 ) 12 49.6 (1.4 ) 9 119.9 (5.7 ) 21 U.S. government agencies mortgage-backed securities 60.1 (0.9 ) 7 34.2 (0.6 ) 13 94.3 (1.5 ) 20 Total fixed maturities 388.3 (10.6 ) 58 102.5 (2.6 ) 26 490.8 (13.2 ) 84 Equity securities: Large-cap equity securities 91.7 (9.1 ) 35 — — — 91.7 (9.1 ) 35 Small-cap equity securities 4.3 (0.3 ) 1 — — — 4.3 (0.3 ) 1 Total equity securities 96.0 (9.4 ) 36 — — — 96.0 (9.4 ) 36 Total temporarily impaired securities $ 484.3 $ (20.0 ) 94 $ 102.5 $ (2.6 ) 26 $ 586.8 $ (22.6 ) 120 ($ 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, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 19.9 $ (0.3 ) 4 $ 52.0 $ (1.2 ) 17 $ 71.9 $ (1.5 ) 21 Obligations of states and political subdivisions 6.1 — 5 30.9 (0.4 ) 9 37.0 (0.4 ) 14 Corporate securities 43.5 (0.9 ) 8 56.1 (2.1 ) 11 99.6 (3.0 ) 19 U.S. government agencies mortgage-backed securities 44.0 (0.3 ) 8 37.5 (1.5 ) 13 81.5 (1.8 ) 21 Total fixed maturities 113.5 (1.5 ) 25 176.5 (5.2 ) 50 290.0 (6.7 ) 75 Large-cap equity securities 7.2 (0.6 ) 2 — — — 7.2 (0.6 ) 2 Total temporarily impaired securities $ 120.7 $ (2.1 ) 27 $ 176.5 $ (5.2 ) 50 $ 297.2 $ (7.3 ) 77
Realized Losses Related to Other-than-Temporary Impairments	The following table sets forth the realized losses related to other-than-temporary impairments on the Company’s investment portfolio recognized for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Equity securities: Large-cap securities $ — $ — $ — $ (0.3 ) Small-cap securities (3.5 ) (1.1 ) (4.8 ) (2.0 ) Total other-than-temporary impairments $ (3.5 ) $ (1.1 ) $ (4.8 ) $ (2.3 )
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 September 30, 2015 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 34.1 $ 34.5 Due after 1 year through 5 years 427.7 439.2 Due after 5 years through 10 years 288.1 294.4 Due after 10 years 734.7 741.8 U.S. government agencies mortgage-backed securities 454.3 469.7 Total $ 1,938.9 $ 1,979.6
Components of Net Investment Income	The following table sets forth the components of net investment income for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Fixed maturities $ 16.1 $ 16.1 $ 46.4 $ 49.4 Equity securities 1.4 1.6 4.7 4.7 Cash and cash equivalents, and other 1.6 1.7 4.3 4.4 Investment income 19.1 19.4 55.4 58.5 Investment expenses 0.4 0.5 1.5 1.5 Net investment income $ 18.7 $ 18.9 $ 53.9 $ 57.0
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 and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Realized gains: Fixed maturities $ 2.9 $ 0.1 $ 4.4 $ 2.6 Equity securities 11.8 3.2 21.8 17.9 Other invested assets — — 0.1 0.1 Total realized gains 14.7 3.3 26.3 20.6 Realized losses: Equity securities: Sales (0.4 ) (0.6 ) (1.1 ) (0.7 ) OTTI (3.5 ) (1.1 ) (4.8 ) (2.3 ) Total realized losses (3.9 ) (1.7 ) (5.9 ) (3.0 ) Net realized gains on investments $ 10.8 $ 1.6 $ 20.4 $ 17.6 Change in unrealized holding gains, net of tax: Fixed maturities $ 8.7 $ (8.6 ) $ (19.9 ) $ 25.4 Equity securities (35.5 ) (4.7 ) (43.0 ) (3.0 ) Other invested assets (8.5 ) (4.6 ) (3.6 ) 1.6 Deferred federal income tax liability 12.4 6.2 23.3 (8.4 ) Valuation allowance — (6.8 ) — 0.1 Change in net unrealized holding gains, net of tax $ (22.9 ) $ (18.5 ) $ (43.2 ) $ 15.7

Fair Value of Financial Instr20

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Company's Available-for-Sale Investments within Fair Value Hierarchy	The following tables set forth the Company’s available-for-sale investments within the fair value hierarchy at September 30, 2015 and December 31, 2014 : ($ millions) Total Level 1 Level 2 Level 3 September 30, 2015 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 241.1 $ — $ 241.1 $ — Obligations of states and political subdivisions 857.1 — 857.1 — Corporate securities 411.7 — 408.1 3.6 U.S. government agencies mortgage-backed securities 469.7 — 469.7 — Total fixed maturities 1,979.6 — 1,976.0 3.6 Equity securities: Large-cap securities 230.9 230.9 — — Small-cap securities 67.1 67.1 — — Total equity securities 298.0 298.0 — — Other invested assets 82.8 7.9 74.9 — Total available-for-sale investments $ 2,360.4 $ 305.9 $ 2,050.9 $ 3.6 ($ millions) Total Level 1 Level 2 Level 3 December 31, 2014 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 309.3 $ — $ 309.3 $ — Obligations of states and political subdivisions 769.5 — 769.5 — Corporate securities 340.6 — 331.2 9.4 U.S. government agencies mortgage-backed securities 472.5 — 472.5 — Total fixed maturities 1,891.9 — 1,882.5 9.4 Equity securities: Large-cap securities 242.2 242.2 — — Small-cap securities 68.2 68.2 — — Total equity securities 310.4 310.4 — — Other invested assets 80.3 7.4 72.9 — Total available-for-sale investments $ 2,282.6 $ 317.8 $ 1,955.4 $ 9.4
Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	For assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3), the following tables set forth a reconciliation of the beginning and ending balances for the three and nine months ended September 30, 2015 and the year ended December 31, 2014 , separately for each major category of assets: ($ millions) Fixed maturities Balance at January 1, 2015 $ 9.4 Total realized gains – included in earnings 0.2 Total unrealized losses – included in other comprehensive income (0.2 ) Purchases — Sales (5.9 ) Transfers into Level 3 — Transfers out of Level 3 — Balance at March 31, 2015 $ 3.5 Total realized gains – included in earnings — Total unrealized losses – included in other comprehensive income (0.1 ) Purchases 0.1 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at June 30, 2015 $ 3.5 Total realized gains – included in earnings — Total unrealized gains – included in other comprehensive income — Purchases 0.1 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at September 30, 2015 $ 3.6 ($ millions) Fixed maturities Balance at January 1, 2014 $ 8.9 Total realized gains – included in earnings — Total unrealized gains – included in other comprehensive income 0.2 Purchases 0.3 Sales — Transfers into Level 3 — Transfers out of Level 3 — Balance at December 31, 2014 $ 9.4
Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities	Consequently this has been placed in Level 2 of the fair value hierarchy. ($ millions, except interest rates) September 30, 2015 December 31, 2014 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 76.2 7.00 % $ 70.0 $ 74.6 7.00 %
Included in Notes Payable are Senior Notes and Subordinated Debentures	The carrying amount of the Subordinated Debentures approximates its fair value as the interest rate adjusts quarterly and has been disclosed in Level 3. ($ millions, except interest rates) September 30, 2015 December 31, 2014 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2033: issued $85.0, July 2013 with fixed interest $ 85.3 $ 85.8 5.03 % $ 85.3 $ 86.4 5.03 % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.5 15.5 4.53 % 15.5 15.5 4.44 % Total notes payable $ 100.8 $ 101.3 $ 100.8 $ 101.9

Reinsurance (Tables)

Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2015
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 and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Premiums earned: Assumed from external insurers and reinsurers $ 1.1 $ 1.2 $ 3.2 $ 3.3 Assumed under Pooling Arrangement 319.7 270.2 946.5 801.0 Ceded to external insurers and reinsurers (8.7 ) (6.5 ) (26.1 ) (18.5 ) Ceded under Pooling Arrangement (207.5 ) (215.2 ) (626.0 ) (647.4 ) Net assumed premiums earned $ 104.6 $ 49.7 $ 297.6 $ 138.4 Losses and loss expenses incurred: Assumed from external insurers and reinsurers $ 0.8 $ 0.8 $ 2.2 $ 2.3 Assumed under Pooling Arrangement 207.8 185.2 628.1 548.9 Ceded to external insurers and reinsurers (1.2 ) (2.7 ) (3.6 ) (6.4 ) Ceded under Pooling Arrangement (140.4 ) (131.6 ) (427.2 ) (417.3 ) Net assumed losses and loss expenses incurred $ 67.0 $ 51.7 $ 199.5 $ 127.5

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2015
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 expense and the amount computed at the indicated statutory rate for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Amount at statutory rate $ 10.5 35.0 % $ 4.6 35.0 % $ 23.3 35.0 % $ 15.4 35.0 % Tax-exempt interest and dividends received deduction (2.1 ) (7.0 ) (2.1 ) (16.2 ) (6.6 ) (9.9 ) (6.3 ) (14.4 ) Other, net 0.8 2.8 0.1 0.3 1.7 2.6 1.0 2.3 Valuation allowance — — (1.4 ) (10.1 ) — — (8.2 ) (18.6 ) Federal income tax expense and effective rate $ 9.2 30.8 % $ 1.2 9.0 % $ 18.4 27.7 % $ 1.9 4.3 %

Pension and Postretirement Be23

Pension and Postretirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2015
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 and nine months ended September 30, 2015 and 2014 : ($ millions) Pension Postretirement Pension Postretirement Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 2015 2014 2015 2014 Service cost $ 2.0 $ 1.3 $ — $ — $ 5.9 $ 3.9 $ — $ — Interest cost 2.8 2.8 0.3 0.3 8.5 8.4 0.7 0.8 Expected return on plan assets (3.5 ) (3.2 ) — — (10.4 ) (9.5 ) — — Amortization of: Negative prior service cost — — (1.3 ) (1.3 ) — — (4.1 ) (4.1 ) Net actuarial loss 2.7 1.4 0.1 0.2 8.2 4.9 0.4 0.5 Net periodic cost (benefit) $ 4.0 $ 2.3 $ (0.9 ) $ (0.8 ) $ 12.2 $ 7.7 $ (3.0 ) $ (2.8 )

Other Comprehensive Income an24

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2015
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 and nine months ended September 30, 2015 and 2014 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at July 1, 2015 $ 89.7 $ (36.3 ) $ 53.4 Other comprehensive income before reclassifications (15.8 ) — (15.8 ) Amounts reclassified from AOCI (a) (7.1 ) 0.9 (6.2 ) Net current period other comprehensive (loss) income (22.9 ) 0.9 (22.0 ) Ending balance at September 30, 2015 $ 66.8 $ (35.4 ) $ 31.4 Beginning balance at July 1, 2014 $ 118.8 $ (4.4 ) $ 114.4 Other comprehensive income before reclassifications (16.9 ) — (16.9 ) Amounts reclassified from AOCI (a) (1.6 ) 0.3 (1.3 ) Net current period other comprehensive (loss) income (18.5 ) 0.3 (18.2 ) Ending balance at September 30, 2014 $ 100.3 $ (4.1 ) $ 96.2 (a) See separate table below for details about these reclassifications ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at January 1, 2015 $ 110.0 $ (38.3 ) $ 71.7 Other comprehensive income before reclassifications (29.9 ) — (29.9 ) Amounts reclassified from AOCI (a) (13.3 ) 2.9 (10.4 ) Net current period other comprehensive (loss) income (43.2 ) 2.9 (40.3 ) Ending balance at September 30, 2015 $ 66.8 $ (35.4 ) $ 31.4 Beginning balance at January 1, 2014 $ 84.6 $ (3.8 ) $ 80.8 Other comprehensive income before reclassifications 33.3 (1.6 ) 31.7 Amounts reclassified from AOCI (a) (17.6 ) 1.3 (16.3 ) Net current period other comprehensive income (loss) 15.7 (0.3 ) 15.4 Ending balance at September 30, 2014 $ 100.3 $ (4.1 ) $ 96.2 (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 and nine months ended September 30, 2015 and 2014 : ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2015 2014 Unrealized gains on available for sale securities $ 10.8 $ 1.6 Realized gain on sale of securities 10.8 1.6 Total before tax (3.7 ) — Tax expense 7.1 1.6 Net of tax Amortization of benefit plan items Negative prior service cost 1.3 1.3 (a) Net actuarial loss (2.8 ) (1.6 ) (a) (1.5 ) (0.3 ) Total before tax 0.6 — Tax benefit (0.9 ) (0.3 ) Net of tax Total reclassifications for the period $ 6.2 $ 1.3 (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Nine months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2015 2014 Unrealized gains on available for sale securities $ 20.4 $ 17.6 Realized gain on sale of securities 20.4 17.6 Total before tax (7.1 ) — Tax expense 13.3 17.6 Net of tax Amortization of benefit plan items Negative prior service cost 4.1 4.1 (a) Net actuarial loss (8.6 ) (5.4 ) (a) (4.5 ) (1.3 ) Total before tax 1.6 — Tax benefit (2.9 ) (1.3 ) Net of tax Total reclassifications for the period $ 10.4 $ 16.3 (a) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings per Common Share (Tabl

Earnings per Common Share (Tables)	9 Months Ended
	Sep. 30, 2015
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 and nine months ended September 30, 2015 and 2014 : ($ and shares in millions, except per share amounts) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Numerator: Net income for basic earnings per common share $ 20.7 $ 11.9 $ 48.1 $ 42.0 Denominator: Weighted average shares for basic net earnings per common share 41.1 40.9 41.1 40.8 Effect of dilutive share-based awards 0.8 0.6 0.4 0.4 Adjusted weighted average shares for diluted net earnings per common share 41.9 41.5 41.5 41.2 Basic net earnings per common share $ 0.50 $ 0.29 $ 1.17 $ 1.03 Diluted net earnings per common share $ 0.50 $ 0.28 $ 1.16 $ 1.02
Options to Purchase Shares of Common Stock	The following table sets forth common stock options and restricted share units ("RSU award") provided to each outside director of the Company that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three and nine months ended September 30, 2015 and 2014 . (shares in millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Total number of antidilutive options and awards 1.1 1.5 1.7 1.8

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2015
Segment Reporting [Abstract]
Financial Information Regarding Company's Reportable Segments	The following table sets forth financial information regarding the Company’s reportable segments for the three and nine months ended September 30, 2015 and 2014 : ($ millions) Three months ended September 30 Nine months ended September 30 2015 2014 2015 2014 Revenue from external sources: Insurance segments Personal insurance $ 147.2 $ 112.3 $ 445.4 $ 340.3 Business insurance 96.8 96.0 289.4 285.4 Specialty insurance 75.7 61.9 211.7 175.3 Total insurance segments 319.7 270.2 946.5 801.0 Investment operations segment Net investment income 18.7 18.9 53.9 57.0 Net realized capital gains 10.8 1.6 20.4 17.6 Total investment operations segment 29.5 20.5 74.3 74.6 All other 0.5 0.6 1.2 1.4 Total revenue from external sources 349.7 291.3 1,022.0 877.0 Intersegment revenue 1.4 1.3 4.2 3.9 Total revenue 351.1 292.6 1,026.2 880.9 Reconciling items: Eliminate intersegment revenues (1.4 ) (1.3 ) (4.2 ) (3.9 ) Total consolidated revenues $ 349.7 $ 291.3 $ 1,022.0 $ 877.0 Segment income before federal income tax: Insurance segments SAP underwriting income (loss) Personal insurance $ 6.9 $ 4.5 $ 14.1 $ (0.1 ) Business insurance (6.8 ) (3.3 ) (23.0 ) (18.6 ) Specialty insurance 1.2 (11.4 ) (0.6 ) (24.1 ) Total insurance segments 1.3 (10.2 ) (9.5 ) (42.8 ) Investment operations segment Net investment income 18.7 18.9 53.9 57.0 Net realized capital gains 10.8 1.6 20.4 17.6 Total investment operations segment 29.5 20.5 74.3 74.6 All other 0.1 — (0.2 ) — Total segment income before tax expense 30.9 10.3 64.6 31.8 Reconciling items: GAAP expense adjustments 0.9 4.2 8.6 18.4 Interest expense on corporate debt (1.3 ) (1.4 ) (4.0 ) (4.0 ) Corporate expenses (0.6 ) — (2.7 ) (2.3 ) Total reconciling items (1.0 ) 2.8 1.9 12.1 Total consolidated income before federal income tax expense $ 29.9 $ 13.1 $ 66.5 $ 43.9

Investments - Cost or Amortized

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 2,261.6	$ 2,117.3
Gross unrealized holding gains	121.4	172.6
Gross unrealized holding losses	22.6	7.3
Fair value	2,360.4	2,282.6
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	241.1	309.3
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	857.1	769.5
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	411.7	340.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	469.7	472.5
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	230.9	242.2
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	67.1	68.2
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	1,938.9	1,831.3
Gross unrealized holding gains	53.9	67.3
Gross unrealized holding losses	13.2	6.7
Fair value	1,979.6	1,891.9
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	235	296.7
Gross unrealized holding gains	7.8	14.1
Gross unrealized holding losses	1.7	1.5
Fair value	241.1	309.3
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	840.6	742.5
Gross unrealized holding gains	20.8	27.4
Gross unrealized holding losses	4.3	0.4
Fair value	857.1	769.5
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	409	333.4
Gross unrealized holding gains	8.4	10.2
Gross unrealized holding losses	5.7	3
Fair value	411.7	340.6
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	454.3	458.7
Gross unrealized holding gains	16.9	15.6
Gross unrealized holding losses	1.5	1.8
Fair value	469.7	472.5
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	266.1	235.5
Gross unrealized holding gains	41.3	75.5
Gross unrealized holding losses	9.4	0.6
Fair value	298	310.4
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	213	185.5
Gross unrealized holding gains	27	57.3
Gross unrealized holding losses	9.1	0.6
Fair value	230.9	242.2
Equity Securities [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	53.1	50
Gross unrealized holding gains	14.3	18.2
Gross unrealized holding losses	0.3	0
Fair value	67.1	68.2
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	56.6	50.5
Gross unrealized holding gains	26.2	29.8
Gross unrealized holding losses	0	0
Fair value	$ 82.8	$ 80.3

Investments - Gross Unrealized

Investments - Gross Unrealized Losses and Fair Value on its Investments (Detail) $ in Millions	Sep. 30, 2015USD ($)position	Dec. 31, 2014USD ($)position
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 484.3	$ 120.7
Less than 12 months, Unrealized losses	$ (20)	$ (2.1)
Less than 12 months, Number of positions (positions) | position	94	27
12 months or more, Fair value	$ 102.5	$ 176.5
12 months or more, Unrealized losses	$ (2.6)	$ (5.2)
12 months or more, Number of positions (positions) | position	26	50
Total, Fair value	$ 586.8	$ 297.2
Total, Unrealized losses	$ (22.6)	$ (7.3)
Total, Number of positions | position	120	77
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 91.7	$ 7.2
Less than 12 months, Unrealized losses	$ (9.1)	$ (0.6)
Less than 12 months, Number of positions (positions) | position	35	2
12 months or more, Fair value	$ 0	$ 0
12 months or more, Unrealized losses	$ 0	$ 0
12 months or more, Number of positions (positions) | position	0	0
Total, Fair value	$ 91.7	$ 7.2
Total, Unrealized losses	$ (9.1)	$ (0.6)
Total, Number of positions | position	35	2
Small Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 4.3
Less than 12 months, Unrealized losses	$ (0.3)
Less than 12 months, Number of positions (positions) | position	1
12 months or more, Fair value	$ 0
12 months or more, Unrealized losses	$ 0
12 months or more, Number of positions (positions) | position	0
Total, Fair value	$ 4.3
Total, Unrealized losses	$ (0.3)
Total, Number of positions | position	1
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 388.3	$ 113.5
Less than 12 months, Unrealized losses	$ (10.6)	$ (1.5)
Less than 12 months, Number of positions (positions) | position	58	25
12 months or more, Fair value	$ 102.5	$ 176.5
12 months or more, Unrealized losses	$ (2.6)	$ (5.2)
12 months or more, Number of positions (positions) | position	26	50
Total, Fair value	$ 490.8	$ 290
Total, Unrealized losses	$ (13.2)	$ (6.7)
Total, Number of positions | position	84	75
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	$ 62.8	$ 19.9
Less than 12 months, Unrealized losses	$ (1.1)	$ (0.3)
Less than 12 months, Number of positions (positions) | position	12	4
12 months or more, Fair value	$ 18.7	$ 52
12 months or more, Unrealized losses	$ (0.6)	$ (1.2)
12 months or more, Number of positions (positions) | position	4	17
Total, Fair value	$ 81.5	$ 71.9
Total, Unrealized losses	$ (1.7)	$ (1.5)
Total, Number of positions | position	16	21
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 195.1	$ 6.1
Less than 12 months, Unrealized losses	$ (4.3)	$ 0
Less than 12 months, Number of positions (positions) | position	27	5
12 months or more, Fair value	$ 0	$ 30.9
12 months or more, Unrealized losses	$ 0	$ (0.4)
12 months or more, Number of positions (positions) | position	0	9
Total, Fair value	$ 195.1	$ 37
Total, Unrealized losses	$ (4.3)	$ (0.4)
Total, Number of positions | position	27	14
Fixed Maturities [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 70.3	$ 43.5
Less than 12 months, Unrealized losses	$ (4.3)	$ (0.9)
Less than 12 months, Number of positions (positions) | position	12	8
12 months or more, Fair value	$ 49.6	$ 56.1
12 months or more, Unrealized losses	$ (1.4)	$ (2.1)
12 months or more, Number of positions (positions) | position	9	11
Total, Fair value	$ 119.9	$ 99.6
Total, Unrealized losses	$ (5.7)	$ (3)
Total, Number of positions | position	21	19
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	$ 60.1	$ 44
Less than 12 months, Unrealized losses	$ (0.9)	$ (0.3)
Less than 12 months, Number of positions (positions) | position	7	8
12 months or more, Fair value	$ 34.2	$ 37.5
12 months or more, Unrealized losses	$ (0.6)	$ (1.5)
12 months or more, Number of positions (positions) | position	13	13
Total, Fair value	$ 94.3	$ 81.5
Total, Unrealized losses	$ (1.5)	$ (1.8)
Total, Number of positions | position	20	21
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair value	$ 96
Less than 12 months, Unrealized losses	$ (9.4)
Less than 12 months, Number of positions (positions) | position	36
12 months or more, Fair value	$ 0
12 months or more, Unrealized losses	$ 0
12 months or more, Number of positions (positions) | position	0
Total, Fair value	$ 96
Total, Unrealized losses	$ (9.4)
Total, Number of positions | position	36

Investments - Realized Losses R

Investments - Realized Losses Related to Other-than-Temporary Impairments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (3.5)	$ (1.1)	$ (4.8)	$ (2.3)
Large-Cap Equity Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	0	0	0	(0.3)
Small-Cap Securities [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other-than-temporary impairments	$ (3.5)	$ (1.1)	$ (4.8)	$ (2)

Investments - Amortized Cost an

Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Amortized cost
Due in 1 year or less, Amortized cost	$ 34.1
Due after 1 year through 5 years, Amortized cost	427.7
Due after 5 years through 10 years, Amortized cost	288.1
Due after 10 years, Amortized cost	734.7
U.S. government agencies mortgage-backed securities, Amortized cost	454.3
Total, Amortized cost	1,938.9	$ 1,831.3
Fair value
Due in 1 year or less, Fair value	34.5
Due after 1 year through 5 years, Fair value	439.2
Due after 5 years through 10 years, Fair value	294.4
Due after 10 years, Fair value	741.8
U.S. government agencies mortgage-backed securities, Fair value	469.7
Total, Fair value	$ 1,979.6	$ 1,891.9

Investments - Components of Net

Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Investment Income [Line Items]
Investment income	$ 19.1	$ 19.4	$ 55.4	$ 58.5
Investment expenses	0.4	0.5	1.5	1.5
Net investment income	18.7	18.9	53.9	57
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	16.1	16.1	46.4	49.4
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	1.4	1.6	4.7	4.7
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.6	$ 1.7	$ 4.3	$ 4.4

Investments - Realized and Unre

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Realized gains:
Total realized gains	$ 14.7	$ 3.3	$ 26.3	$ 20.6
Realized losses:
Total realized losses	(3.9)	(1.7)	(5.9)	(3)
Net realized gain on investments	10.8	1.6	20.4	17.6
Change in unrealized holding gains (losses), net of tax:
Deferred federal income tax liability	12.4	6.2	23.3	(8.4)
Valuation allowance	0	(6.8)	0	0.1
Change in net unrealized holding (losses) gains, net of tax	(22.9)	(18.5)	(43.2)	15.7
Fixed Maturities [Member]
Realized gains:
Total realized gains	2.9	0.1	4.4	2.6
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	8.7	(8.6)	(19.9)	25.4
Equity Securities [Member]
Realized gains:
Total realized gains	11.8	3.2	21.8	17.9
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(35.5)	(4.7)	(43)	(3)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	(0.4)	(0.6)	(1.1)	(0.7)
Equity Securities [Member] | OTTI [Member]
Realized losses:
Total realized losses	(3.5)	(1.1)	(4.8)	(2.3)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0	0	0.1	0.1
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ (8.5)	$ (4.6)	$ (3.6)	$ 1.6

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities fair value of deposit with insurance regulators	$ 8.9		$ 8.8
Proceeds on sale of available-for-sale securities	260.6	$ 212.5
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	$ 85

Fair Value of Financial Instr34

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value	$ 2,360.4	$ 2,282.6
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	241.1	309.3
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	857.1	769.5
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	411.7	340.6
U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	469.7	472.5
Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	1,979.6	1,891.9
Large-Cap Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	230.9	242.2
Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	67.1	68.2
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	298	310.4
Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	82.8	80.3
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	305.9	317.8
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	230.9	242.2
Level 1 [Member] | Small-Cap Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	67.1	68.2
Level 1 [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	298	310.4
Level 1 [Member] | Other Invested Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	7.9	7.4
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	2,050.9	1,955.4
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	241.1	309.3
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	857.1	769.5
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	408.1	331.2
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	469.7	472.5
Level 2 [Member] | Fixed Maturities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	1,976	1,882.5
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] | 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	74.9	72.9
Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	3.6	9.4
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	9.4
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	9.4
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] | 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 Instr35

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
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 3.5	$ 9.4		$ 8.9
Total realized gains (losses) - included in earnings	0	0.2	$ 0	0
Total unrealized gains (losses) - included in other comprehensive income	0	(0.2)	(0.1)	0.2
Purchases	0.1	0	0.1	0.3
Sales	0	(5.9)	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	$ 0	0	0
Ending Balance	$ 3.6		$ 3.5	$ 9.4

Fair Value of Financial Instr36

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	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
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	$ 76.2	$ 74.6

Fair Value of Financial Instr37

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	$ 100,800,000	$ 100,800,000
Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	$ 101,300,000	$ 101,900,000
Federal Home Loan Bank Advances [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
Federal Home Loan Bank Advances [Member] | Reported Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	85,300,000	$ 85,300,000
Federal Home Loan Bank Advances [Member] | Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	$ 85,800,000	$ 86,400,000
Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Interest rate (percent)	4.53%	4.44%
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
Affiliate Subordinated Debentures Due 2033 [Member] | Reported Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	15,500,000	$ 15,500,000
Affiliate Subordinated Debentures Due 2033 [Member] | Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	$ 15,500,000	$ 15,500,000

Fair Value of Financial Instr38

Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May. 31, 2009Agreement	Sep. 30, 2015USD ($)security	Dec. 31, 2014USD ($)
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 2,360.4	$ 2,282.6
Other invested assets		5.3	5.3
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	9.4
Federal home loan bank stock		4.8
Other invested assets		0.5
Corporate Securities [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 411.7	340.6
Corporate Securities [Member] | Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Number of fixed maturity corporate securities included in Level 3 (securities) | security		1
Fair value		$ 3.6	9.4
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of fixed maturity corporate securities included in Level 3 (securities) | security		2
Fair value		$ 74.9	$ 72.9
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate		$ 70

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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Premiums earned:
Assumed from external insurers and reinsurers	$ 1.1	$ 1.2	$ 3.2	$ 3.3
Assumed under Pooling Arrangement	319.7	270.2	946.5	801
Ceded to external insurers and reinsurers	(8.7)	(6.5)	(26.1)	(18.5)
Ceded under Pooling Arrangement	(207.5)	(215.2)	(626)	(647.4)
Net assumed premiums earned	104.6	49.7	297.6	138.4
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	0.8	0.8	2.2	2.3
Assumed under Pooling Arrangement	207.8	185.2	628.1	548.9
Ceded to external insurers and reinsurers	(1.2)	(2.7)	(3.6)	(6.4)
Ceded under Pooling Arrangement	(140.4)	(131.6)	(427.2)	(417.3)
Net assumed losses and loss expenses incurred	$ 67	$ 51.7	$ 199.5	$ 127.5

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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 10.5	$ 4.6	$ 23.3	$ 15.4
Tax-exempt interest and dividends received deduction	(2.1)	(2.1)	(6.6)	(6.3)
Other, net	0.8	0.1	1.7	1
Valuation allowance	0	(1.4)	0	(8.2)
Federal income tax expense and effective rate	$ 9.2	$ 1.2	$ 18.4	$ 1.9
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	(7.00%)	(16.20%)	(9.90%)	(14.40%)
Other, net (percent)	2.80%	0.30%	2.60%	2.30%
Valuation allowance (percent)	0.00%	(10.10%)	0.00%	(18.60%)
Federal income tax expense (benefit) and effective rate (percent)	30.80%	9.00%	27.70%	4.30%

Pension and Postretirement Be41

Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2	$ 1.3	$ 5.9	$ 3.9
Interest cost	2.8	2.8	8.5	8.4
Expected return on plan assets	(3.5)	(3.2)	(10.4)	(9.5)
Amortization of:
Negative prior service cost	0	0	0	0
Net actuarial loss	2.7	1.4	8.2	4.9
Net periodic cost (benefit)	4	2.3	12.2	7.7
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	0	0
Interest cost	0.3	0.3	0.7	0.8
Expected return on plan assets	0	0	0	0
Amortization of:
Negative prior service cost	(1.3)	(1.3)	(4.1)	(4.1)
Net actuarial loss	0.1	0.2	0.4	0.5
Net periodic cost (benefit)	$ (0.9)	$ (0.8)	$ (3)	$ (2.8)

Pension and Postretirement Be42

Pension and Postretirement Benefit Plans - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)
Compensation and Retirement Disclosure [Abstract]
Contributions by employer	$ 13

Other Comprehensive Income an43

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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 53.4	$ 114.4	$ 71.7	$ 80.8
Other comprehensive income before reclassifications	(15.8)	(16.9)	(29.9)	31.7
Amounts reclassified from AOCI	(6.2)	(1.3)	(10.4)	(16.3)
Other comprehensive (loss) income	(22)	(18.2)	(40.3)	15.4
Ending balance	31.4	96.2	31.4	96.2
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	89.7	118.8	110	84.6
Other comprehensive income before reclassifications	(15.8)	(16.9)	(29.9)	33.3
Amounts reclassified from AOCI	(7.1)	(1.6)	(13.3)	(17.6)
Other comprehensive (loss) income	(22.9)	(18.5)	(43.2)	15.7
Ending balance	66.8	100.3	66.8	100.3
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(36.3)	(4.4)	(38.3)	(3.8)
Other comprehensive income before reclassifications	0	0	0	(1.6)
Amounts reclassified from AOCI	0.9	0.3	2.9	1.3
Other comprehensive (loss) income	0.9	0.3	2.9	(0.3)
Ending balance	$ (35.4)	$ (4.1)	$ (35.4)	$ (4.1)

Other Comprehensive Income an44

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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	$ 10.8	$ 1.6	$ 20	$ 17.6
Tax (expense) benefit	(9.2)	(1.2)	(18.4)	(1.9)
Total reclassifications for the period	6.2	1.3	10.4	16.3
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	7.1	1.6	13.3	17.6
Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(0.9)	(0.3)	(2.9)	(1.3)
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	6.2	1.3	10.4	16.3
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	10.8	1.6	20.4	17.6
Total before tax	10.8	1.6	20.4	17.6
Tax (expense) benefit	(3.7)	0	(7.1)	0
Net of tax	7.1	1.6	13.3	17.6
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	1.3	1.3	4.1	4.1
Accumulated other comprehensive income components/Net actuarial loss	(2.8)	(1.6)	(8.6)	(5.4)
Total before tax	(1.5)	(0.3)	(4.5)	(1.3)
Tax (expense) benefit	0.6	0	1.6	0
Net of tax	$ (0.9)	$ (0.3)	$ (2.9)	$ (1.3)

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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net income for basic earnings per common share	$ 20.7	$ 11.9	$ 48.1	$ 42
Denominator:
Weighted average shares for basic net earnings per common share (shares)	41.1	40.9	41.1	40.8
Effect of dilutive share-based awards (shares)	0.8	0.6	0.4	0.4
Adjusted weighted average shares for diluted net earnings per common share (shares)	41.9	41.5	41.5	41.2
Basic net earnings per common share (usd per share)	$ 0.50	$ 0.29	$ 1.17	$ 1.03
Diluted net earnings per common share (usd per share)	$ 0.50	$ 0.28	$ 1.16	$ 1.02

Earnings Per Common Share - Opt

Earnings Per Common Share - Options to Purchase Shares of Common Stock (Detail) - shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Number of antidilutive options and awards	1.1	1.5	1.7	1.8

Segment Information - Financial

Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues from external sources:
Revenue	$ 349.7	$ 291.3	$ 1,022	$ 877
Net Investment Income	18.7	18.9	53.9	57
Segment income before federal income tax:
Interest expense on corporate debt	(1.3)	(1.4)	(4)	(4)
Income before federal income taxes	29.9	13.1	66.5	43.9
Operating Segments [Member]
Revenues from external sources:
Revenue	349.7	291.3	1,022	877
Segment income before federal income tax:
Income before federal income taxes	30.9	10.3	64.6	31.8
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	147.2	112.3	445.4	340.3
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	96.8	96	289.4	285.4
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	75.7	61.9	211.7	175.3
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	319.7	270.2	946.5	801
Segment income before federal income tax:
Income before federal income taxes	1.3	(10.2)	(9.5)	(42.8)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income	18.7	18.9	53.9	57
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	29.5	20.5	74.3	74.6
Segment income before federal income tax:
Income before federal income taxes	29.5	20.5	74.3	74.6
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Revenues from external sources:
Revenue	10.8	1.6	20.4	17.6
Segment income before federal income tax:
Income before federal income taxes	10.8	1.6	20.4	17.6
Operating Segments [Member] | All Other Segments [Member]
Segment income before federal income tax:
Income before federal income taxes	0.1	0	(0.2)	0
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	6.9	4.5	14.1	(0.1)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(6.8)	(3.3)	(23)	(18.6)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	1.2	(11.4)	(0.6)	(24.1)
Corporate and Other [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue	0.5	0.6	1.2	1.4
Corporate and Other [Member] | Gaap Adjustments [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	0.9	4.2	8.6	18.4
Corporate and Other [Member] | Corporate Segment [Member]
Segment income before federal income tax:
Corporate Expenses	(0.6)	0	(2.7)	(2.3)
Corporate and Other [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Interest expense on corporate debt	1.3	1.4	4	4
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	(1.4)	(1.3)	(4.2)	(3.9)
Eliminate Intersegment Revenues [Member] | Intersegment [Member]
Revenues from external sources:
Revenue	1.4	1.3	4.2	3.9
External and Intersegment Sources [Member]
Revenues from external sources:
Revenue	351.1	292.6	1,026.2	880.9
Total Consolidated [Member]
Revenues from external sources:
Revenue	349.7	291.3	1,022	877
Segment Reconciling Items [Member]
Segment income before federal income tax:
Income before federal income taxes	$ (1)	$ 2.8	$ 1.9	$ 12.1

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)Segment	Dec. 31, 2014USD ($)
Segment Reporting [Abstract]
Number of reportable segments (segments)	4
Investable assets | $	$ 2,486.6	$ 2,444.2