State Auto Financial (STFC) 10-Q 5 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 30, 2018
Document Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
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)		43,146,858

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Document Period End Date	Sep. 30, 2018
Assets
Fixed maturities, available-for-sale, at fair value (amortized cost $2,185.2 and $2,173.1, respectively)	$ 2,141.6	$ 2,192.8
Equity securities	376.3	365.3
Other invested assets	54.5	56
Other invested assets, at cost	5.6	5.6
Notes receivable from affiliate	70	70
Total investments	2,648	2,689.7
Cash and cash equivalents	49.5	91.5
Accrued investment income and other assets	39.5	36.5
Deferred policy acquisition costs (affiliated net assumed $50.1 and $57.2, respectively)	104.5	110.3
Reinsurance recoverable on losses and loss expenses payable	5	3.1
Prepaid reinsurance premiums	6.8	6.4
Due from Affiliates	10.6	0
Current federal income taxes	5.9	4.8
Net deferred federal income taxes	62.3	58.8
Property and equipment, at cost	7.1	7.3
Total assets	2,939.2	3,008.4
Liabilities and Stockholders’ Equity
Losses and loss expenses payable (affiliated net assumed $619.4 and $711.4, respectively)	1,198.7	1,255.6
Unearned premiums (affiliated net assumed $121.7 and $187.9, respectively)	595.6	611.8
Notes payable (affiliates $15.2 and $15.2, respectively)	122	122.1
Pension and postretirement benefits (affiliated $28.0 and $34.8, respectively)	52.1	64.5
Due to Affiliate	0	2.7
Other liabilities (affiliated net assumed $70.0 and $15.5, respectively)	101.4	76.7
Total liabilities	2,069.8	2,133.4
Stockholders’ equity:
Common stock, without par value. Authorized 100.0 shares; 49.8 and 49.2 shares issued, respectively, at stated value of $2.50 per share	124.5	123
Treasury stock, 6.8 and 6.8 shares, respectively, at cost	(117)	(116.8)
Additional paid-in capital	188.6	171.8
Accumulated Other Comprehensive Income (Loss), Net of Tax	(68.3)	36.7
Retained earnings	741.6	660.3
Total stockholders’ equity	869.4	875
Total liabilities and stockholders’ equity	2,939.2	3,008.4
Class A Preferred Stock [Member]
Stockholders’ equity:
Preferred stock value	0	0
Class B Preferred Stock [Member]
Stockholders’ equity:
Preferred stock value	$ 0	$ 0

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fixed maturities, available-for-sale, amortized cost	$ 2,185.2	$ 2,173.1
Deferred policy acquisitions costs from affiliates	50.1	57.2
Losses and loss expenses payable, affiliates	619.4	711.4
Unearned premiums, affiliates	121.7	187.9
Notes payable, affiliates	15.2	15.2
Net Postretirement and Pension Benefits	(28)	(34.8)
Net Other Liabilities from Affiliates	$ 70	$ 15.5
Common stock, no par value (usd per share)	$ 0	$ 0
Common stock, shares authorized (shares)	100,000,000	100,000,000
Common stock, shares issued (shares)	49,800,000	49,200,000
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	$ (47.7)	$ (50.7)
Class A Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$ 0	$ 0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0
Class B Preferred Stock [Member]
Preferred stock, no par value (usd per share)	$ 0	$ 0
Preferred stock, shares authorized (shares)	2,500,000	2,500,000
Preferred stock, shares issued (shares)	0	0

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Debt and Equity Securities, Gain (Loss)	$ 17.8	$ 20.8	$ 18.2	$ 47.3
Earned premiums (affiliated net assumed $71.9 and $115.3, respectively, and $251.1 and $349.0, respectively)	306.8	319	929.2	957.2
Net investment income (affiliate $1.3 and $1.3, respectively, and $3.7 and $3.7, respectively)	20.8	18.7	62.2	56.5
Net realized gains on investments:
Total other-than-temporary impairment losses	0	(0.6)	0	(3.5)
Total net realized gains on investments	17.8	20.2	18.2	43.8
Other income from affiliates	0.6	0.5	1.8	1.7
Total revenues	346	358.4	1,011.4	1,059.2
Losses and loss expenses (affiliated net assumed $34.7 and $128.9, respectively, and $105.4 and $298.8, respectively)	191	253.5	621.1	718.2
Acquisition and operating expenses (affiliated net ceded $26.8 and affiliated net assumed $78.9, respectively, and $52.3 and $231.0, respectively)	110.9	112.7	333	334.1
Interest expense (affiliates $0.2 and $0.2, respectively, and $0.7 and $0.6, respectively)	1.2	1.5	4.5	4.4
Other expenses	1.9	1.9	7.4	5.8
Costs and Expenses	305	369.6	966	1,062.5
Income before federal income taxes	41	(11.2)	45.4	(3.3)
Current Income Tax Expense (Benefit)	(0.1)	(0.1)	(1.1)	0
Deferred Income Tax Expense (Benefit)	7.7	(1.6)	9.2	0.8
Federal income tax expense	7.6	(1.7)	8.1	0.8
Net income	$ 33.4	$ (9.5)	$ 37.3	$ (4.1)
Earnings (loss) per common share:
Basic (usd per share)	$ 0.78	$ (0.23)	$ 0.87	$ (0.10)
Diluted (usd per share)	0.76	(0.23)	0.86	(0.10)
Dividends paid per common share (usd per share)	$ 0.10	$ 0.10	$ 0.30	$ 0.30

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Earned premiums, from affiliates	$ 71.9	$ 115.3	$ 251.1	$ 349
Net investment income, from affiliate	1.3	1.3	3.7	3.7
Losses and loss expenses, from affiliates	34.7	128.9	105.4	298.8
Net Acquisition and Operating Expenses, from affiliates	(26.8)	78.9	52.3	217
Interest Expense Debt To Affiliates	$ 0.2	$ 0.2	$ 0.7	$ 0.6

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 33.4	$ (9.5)	$ 37.3	$ (4.1)
Net unrealized holding losses on fixed maturities:
Unrealized holding losses	(14.7)	11	(61.6)	65.6
Reclassification adjustments for gains realized in net income	0	(20.2)	(1.7)	(43.8)
Income tax benefit	3.1	3.2	13.3	(7.6)
Total net unrealized holding losses on fixed maturities	(11.6)	(6)	(50)	14.2
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.3)	(4.1)	(4.1)
Net actuarial loss	2.2	2.1	6.4	6.1
Income tax expense	(0.2)	(0.3)	(0.5)	(0.8)
Total net unrecognized benefit plan obligations	0.6	0.5	1.8	1.2
Other comprehensive loss	(11)	(5.5)	(48.2)	15.4
Comprehensive loss	$ 22.5	$ (15)	$ (10.9)	$ 11.3

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net income	$ 37.3	$ (4.1)
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Depreciation and amortization, net	6.7	9.9
Share-based compensation	6.7	3
Net investment gain	(18.2)	43.8
Changes in operating assets and liabilities:
Deferred policy acquisition costs	5.8	7.2
Accrued investment income and other assets	(3)	2
Postretirement and pension benefits	(10.6)	(9.1)
Other liabilities and due to/from affiliates, net	11.8	15.1
Reinsurance recoverable on losses and loss expenses payable and prepaid reinsurance premiums	(2.3)	1.9
Losses and loss expenses payable	(56.9)	98.9
Unearned premiums	(16.2)	12.7
Excess Tax Benefit from Share-based Compensation, Operating Activities	0	1.3
Federal income taxes	8.6	(0.3)
Net cash (used in) provided by operating activities	(30.3)	60.7
Cash flows from investing activities:
Purchases of fixed maturities available-for-sale	(267)	(409.2)
Purchases of equity securities	(82.2)	(104.1)
Purchases of other invested assets	(1.1)	(1)
Maturities, calls and pay downs of fixed maturities available-for-sale	180.4	192.1
Sales of fixed maturities available-for-sale	69.7	156.3
Sales of equity securities	89.3	171.3
Sales of other invested assets	0.9	0.8
Net cash (used in) provided by investing activities	(10)	6.2
Cash flows from financing activities:
Proceeds from issuance of common stock	11.7	6
Payments to acquire treasury stock	(0.2)	(0.2)
Payment of dividends	(12.8)	(12.7)
Payment for Debt Extinguishment or Debt Prepayment Cost	(0.4)	0
Net cash provided by (used in) financing activities	(1.7)	(6.9)
Net (decrease) increase in cash and cash equivalents	(42)	60
Cash and cash equivalents at beginning of period	91.5	51.1
Cash and cash equivalents at end of period	49.5	111.1
Supplemental disclosures:
Interest paid (affiliates $0.7 and $0.6, respectively)	$ 4.5	$ 4.3

Condensed Consolidated Statem_4

Condensed Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Cash Flows [Abstract]
Interest Expense Debt To Affiliates	$ 0.2	$ 0.2	$ 0.7	$ 0.6

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2018
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, 2018 , are not necessarily indicative of the results that may be expected for the year ending December 31, 2018 . The balance sheet at December 31, 2017 , 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, 2017 , (the “ 2017 Form 10-K”). Capitalized terms used herein and not otherwise defined shall have the meaning ascribed to them in the 2017 Form 10-K. Adoption of Recent Accounting Pronouncements Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities In January 2016, the FASB issued ASU-2016-01 to improve certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. Specifically the guidance (i) requires equity investments, including equity securities and limited partnership interests, that are not accounted for under the equity method of accounting or result in consolidation to be measured at fair value with changes in fair value recognized in earnings, (ii) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment, (iii) eliminates the requirement to disclose the methods and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost, (iv) requires the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes, (v) requires an entity to present separately in other comprehensive income the portion of the total change in fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option, (vi) requires separate presentation of financial assets and liabilities by measurement category and form on the balance sheet or the notes to the financial statements, and (vii) clarifies that the need for a valuation allowance on a deferred tax asset related to an available for sale security should be evaluated with other deferred tax assets. The guidance was effective beginning January 1, 2018. The adoption of this guidance resulted in the recognition of $60.8 million of net unrealized gains (net of tax) as a cumulative effect adjustment that increased retained earnings as of January 1, 2018 and decreased accumulated other comprehensive income (“AOCI”) by the same amount. Changes in the fair value of equity securities and other invested assets previously identified as available-for-sale are reported in "net investment (loss) gain" in the condensed consolidated statements of income. At December 31, 2017, equity securities and other invested assets were classified as available-for-sale on the Company's balance sheet; however, upon adoption, the guidance eliminated the available-for-sale balance sheet classification for equity securities and other invested assets. Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In February 2018, the FASB issued ASU 2018-02 that addresses certain stranded income tax effects in AOCI resulting from the Tax Cuts and Jobs Act of 2017 (“TCJA”). Current guidance requires the effect of a change in tax laws or rates on deferred tax balances to be reported in income from continuing operations in the accounting period that includes the period of enactment, even if the related income tax effects were originally charged or credited directly to AOCI. The amount of the reclassification would include the effect of the change in the U.S. federal corporate income tax rate on the gross deferred tax amounts and related valuation allowances, if any, at the date of the enactment of TCJA related to items in AOCI. The updated guidance is effective for reporting periods beginning after December 15, 2018 and is to be applied retrospectively to each period in which the effect of the TCJA related to items remaining in AOCI are recognized or at the beginning of the period of adoption. Early adoption is permitted. The Company adopted the updated guidance effective January 1, 2018 and elected to reclassify the income tax effects of the TCJA from AOCI to retained earnings as of January 1, 2018. This reclassification resulted in a decrease in retained earnings of $4.0 million as of January 1, 2018 and an increase in AOCI by the same amount. Revenue from Contracts with Customers In May 2014, the FASB issued ASU 2014-09 that requires an entity to recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Insurance contracts do not fall within the scope of this new guidance. The guidance was effective for annual reporting periods beginning after December 15, 2017, with early adoption permitted. The Company adopted this guidance effective January 1, 2018. The adoption of the guidance did not impact how the Company recognizes revenue; thus, there was no impact to its results of operations or consolidated financial position. Statement of Cash Flows - Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued 2016-15 that addresses eight specific cash flow issues with the objective of reducing existing diversity in practice. The new guidance is effective beginning January 1, 2018. The Company adopted this guidance effective January 1, 2018 and it did not impact the Company's cash flows. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued ASU 2017-07 on how to present the components of net periodic benefit costs in the income statement for pension plans and other post-retirement benefit plans. The new guidance is effective for interim and annual reporting periods beginning after December 15, 2017. The Company adopted this guidance effective beginning January 1, 2018 and it did not have a material impact on the Company's results of operations, consolidated financial position, or cash flows. Pending Adoption of Recent Accounting Pronouncements Leases In February 2016, the FASB issued guidance that amended previous guidance on lease accounting. The new guidance requires the recognition of lease assets and lease liabilities by lessees for those leases classified as operating leases under previous GAAP. The guidance is effective beginning January 1, 2019 and it is not expected to have a material impact on the Company’s results of operations, consolidated financial position or cash flows. For information regarding other accounting pronouncements that the Company has not yet adopted, see the “Pending Adoption of Recent Accounting Pronouncements” section of Note 1 of the Notes to Consolidated Financial Statements in the 2017

Investments

Investments	9 Months Ended
	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The following tables set forth the cost or amortized cost and fair value of investments by lot at September 30, 2018 and December 31, 2017 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value September 30, 2018 Available-for-sale fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 409.4 $ 4.3 $ (11.7 ) $ 402.0 Obligations of states and political subdivisions 434.8 6.0 (2.0 ) 438.8 Corporate securities 535.6 1.7 (11.4 ) 525.9 U.S. government agencies mortgage-backed securities 805.4 1.7 (32.2 ) 774.9 Total available-for-sale fixed maturities $ 2,185.2 $ 13.7 $ (57.3 ) $ 2,141.6 ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 433.8 $ 9.3 $ (6.2 ) $ 436.9 Obligations of states and political subdivisions 507.1 19.1 (0.4 ) 525.8 Corporate securities 527.5 4.5 (2.3 ) 529.7 U.S. government agencies mortgage-backed securities 704.7 7.1 (11.4 ) 700.4 Total fixed maturities 2,173.1 40.0 (20.3 ) 2,192.8 Equity securities: Large-cap securities 62.4 35.1 (0.7 ) 96.8 Mutual and exchange traded funds 256.2 21.6 (9.3 ) 268.5 Total equity securities 318.6 56.7 (10.0 ) 365.3 Other invested assets 25.8 30.2 — 56.0 Total available-for-sale securities $ 2,517.5 $ 126.9 $ (30.3 ) $ 2,614.1 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, 2018 and December 31, 2017 : ($ 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, 2018 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 106.1 $ (2.1 ) 15 $ 221.6 $ (9.6 ) 29 $ 327.7 $ (11.7 ) 44 Obligations of states and political subdivisions 130.7 (1.8 ) 19 23.3 (0.2 ) 3 154.0 (2.0 ) 22 Corporate securities 352.9 (5.7 ) 46 114.3 $ (5.7 ) 19 467.2 $ (11.4 ) 65 U.S. government agencies mortgage-backed securities 390.1 (11.8 ) 54 303.1 (20.4 ) 52 693.2 (32.2 ) 106 Total temporarily impaired securities $ 979.8 $ (21.4 ) 134 $ 662.3 $ (35.9 ) 103 $ 1,642.1 $ (57.3 ) 237 ($ 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, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 102.4 $ (0.6 ) 18 $ 196.1 $ (5.6 ) 22 $ 298.5 $ (6.2 ) 40 Obligations of states and political subdivisions 58.6 (0.4 ) 10 — — — 58.6 (0.4 ) 10 Corporate securities 153.2 (1.3 ) 23 67.3 (1.0 ) 10 220.5 (2.3 ) 33 U.S. government agencies mortgage-backed securities 188.6 (2.9 ) 31 252.2 (8.5 ) 41 440.8 (11.4 ) 72 Total fixed maturities 502.8 (5.2 ) 82 515.6 (15.1 ) 73 1,018.4 (20.3 ) 155 Equity securities: Large-cap equity securities 4.4 (0.7 ) 4 — — — 4.4 (0.7 ) 4 Mutual and exchange traded funds 66.9 (9.3 ) 1 — — — 66.9 (9.3 ) 1 Total equity securities 71.3 (10.0 ) 5 — — — 71.3 (10.0 ) 5 Total temporarily impaired securities $ 574.1 $ (15.2 ) 87 $ 515.6 $ (15.1 ) 73 $ 1,089.7 $ (30.3 ) 160 The Company reviewed its investments at September 30, 2018 , and determined that no other-than-temporary impairment ("OTTI") existed in the gross unrealized holding losses. The Company regularly monitors its available-for-sale 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. The following table sets forth the amortized cost and fair value of available-for-sale fixed maturities by contractual maturity at September 30, 2018 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 28.1 $ 28.0 Due after 1 year through 5 years 587.1 576.4 Due after 5 years through 10 years 369.9 364.2 Due after 10 years 394.7 398.1 U.S. government agencies mortgage-backed securities 805.4 774.9 Total $ 2,185.2 $ 2,141.6 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, 2018 , State Auto P&C had U.S. government agencies mortgage-backed fixed maturity securities, with a carrying value of approximately $106.5 million , that were pledged as collateral for the FHLB Loans (as defined in Note 3). In accordance with the terms of the FHLB Loans, State Auto P&C retains all rights regarding these pledged securities. Fixed maturities with fair values of $8.9 million and $9.3 million were on deposit with insurance regulators as required by law at September 30, 2018 , and December 31, 2017 , respectively. The Company retains all rights regarding these securities. The following table sets forth the components of net investment income for the three and nine months ended September 30, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Fixed maturities $ 16.4 $ 15.4 $ 49.4 $ 47.1 Equity securities 3.0 2.0 8.6 5.7 Cash and cash equivalents, and other 1.7 1.6 5.2 4.6 Investment income 21.1 19.0 63.2 57.4 Investment expenses 0.3 0.3 1.0 0.9 Net investment income $ 20.8 $ 18.7 $ 62.2 $ 56.5 The Company’s current investment strategy does not rely on the use of derivative financial instruments. Proceeds on sales of investments were $159.9 million and $328.4 million for the nine months ended September 30, 2018 , and 2017 , respectively. The following table sets forth the realized and unrealized holding gains (losses) on the Company’s investment portfolio for the three and nine months ended September 30, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Realized gains: Fixed maturities $ — $ 0.4 $ 1.7 $ 2.7 Equity securities 1.0 20.9 6.0 45.2 Other invested assets — — — 0.1 Total realized gains 1.0 21.3 7.7 48.0 Realized losses on securities: Sales of equity securities — (0.5 ) (0.6 ) (0.7 ) OTTI — (0.6 ) — (3.5 ) Total equity securities realized losses — (1.1 ) (0.6 ) (4.2 ) Net realized gain on investments $ 1.0 $ 20.2 $ 7.1 $ 43.8 Net unrealized gain (loss) on investments (1) Equity securities 16.4 $ — $ 12.8 $ — Other invested assets 0.4 — (1.7 ) — Net unrealized gain on investments 16.8 — 11.1 — Net investment gain $ 17.8 $ 20.2 $ 18.2 $ 43.8 Change in unrealized holding (losses) gains, net of tax: Fixed maturities $ (14.7 ) $ 0.4 $ (63.3 ) $ 15.8 Equity securities — (12.1 ) — (1.9 ) Other invested assets — 2.5 — 7.9 Deferred federal income tax liability 3.1 3.2 13.3 (7.6 ) Change in net unrealized holding (losses) gains, net of tax $ (11.6 ) $ (6.0 ) $ (50.0 ) $ 14.2 (1) Unrealized holding gains (losses) recognized during the period on securities still held at the reporting date

Fair Value of Financial Instrum

Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2018
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 a nationally recognized pricing service to estimate the majority of its 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, 2018 , and December 31, 2017 , the Company did not adjust any of the prices received from the pricing service. 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. 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, 2018, and 2017. Equities The fair value of each equity security is based on an observable market price for an identical asset in an active market and is priced by the same pricing service discussed above. All equity securities are recorded using unadjusted market prices and have been disclosed in Level 1. Other Invested Assets Included in other invested assets is one international fund (“the fund”) that invests in equity securities of foreign issuers and is managed by a third party investment manager. The fund had a fair value of $43.3 million and $45.2 million at September 30, 2018 , and December 31, 2017 , respectively, which was determined using the fund’s net asset value. The Company employs procedures to assess the reasonableness of the fair value of the fund, including obtaining and reviewing the fund’s audited financial statements. There are no unfunded commitments related to the fund. The Company may not sell its investment in the fund; however, the Company may redeem all or a portion of its investment in the fund at net asset value per share with the appropriate prior written notice. In accordance with Accounting Standard Codification 820-10, this investment is measured at fair value using the net asset value per share practical expedient and has not been classified in the fair value hierarchy. Fair values presented here are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated balance sheets. The remainder of the Company’s other invested assets consist primarily of holdings in publicly-traded mutual funds. The Company believes that its prices for these publicly-traded mutual funds based on an observable market price for an identical asset in an active market reflect their fair values and consequently these securities have been disclosed in Level 1. The following tables set forth the Company’s investments within the fair value hierarchy at September 30, 2018 and December 31, 2017 : ($ millions) Total Level 1 Level 2 September 30, 2018 Available-for-sale fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 402.0 $ — $ 402.0 Obligations of states and political subdivisions 438.8 — 438.8 Corporate securities 525.9 — 525.9 U.S. government agencies mortgage-backed securities 774.9 — 774.9 Total available-for-sale fixed maturities 2,141.6 — 2,141.6 Equity securities: Large-cap securities 95.6 95.6 — Mutual and exchange traded funds 280.7 280.7 — Total equity securities 376.3 376.3 — Other invested assets 11.2 11.2 — Total investments $ 2,529.1 $ 387.5 $ 2,141.6 ($ millions) Total Level 1 Level 2 December 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 436.9 $ — $ 436.9 Obligations of states and political subdivisions 525.8 — 525.8 Corporate securities 529.7 — 529.7 U.S. government agencies mortgage-backed securities 700.4 — 700.4 Total fixed maturities 2,192.8 — 2,192.8 Equity securities: Large-cap securities 96.8 96.8 — Mutual and exchange traded funds 268.5 268.5 — Total equity securities 365.3 365.3 — Other invested assets 10.8 10.8 — Total available-for-sale investments $ 2,568.9 $ 376.1 $ 2,192.8 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, at Cost Included in other invested assets, at cost are common stock of the Federal Home Loan Bank of Cincinnati (the "FHLB") and the Trust Securities. The Trust Securities and FHLB common stock are carried at cost, which approximates fair value. The fair value of the FHLB common stock at September 30, 2018 , was $5.1 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, 2018 December 31, 2017 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 72.5 7.00 % $ 70.0 $ 72.6 7.00 % Notes Payable Included in notes payable are the FHLB Loans and Subordinated Debentures. The Company estimates the fair value of the FHLB Loans by discounting cash flows using a borrowing rate currently available to the Company for loans with similar terms. The FHLB Loans have been placed in Level 3 of the fair value hierarchy. The carrying amount of the Subordinated Debentures approximates its fair value as the interest rate adjusts quarterly and has been disclosed in Level 3. ($ millions, except interest rates) September 30, 2018 December 31, 2017 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2021: issued $21.5, September 2016 with fixed interest $ 21.5 $ 20.6 1.73 % $ 21.5 $ 20.9 1.73 % FHLB Loan due 2033: issued $85.0, July 2013 with fixed interest — — — % 85.4 85.7 5.03 % FHLB Loan due 2033: issued $85.0, May 2018 with fixed interest 85.3 86.4 3.96 % — — — % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 6.52 % 15.2 15.2 5.68 % Total notes payable $ 122.0 $ 122.2 $ 122.1 $ 121.8

Losses and Loss Expenses Payabl

Losses and Loss Expenses Payable Losses and Loss Expenses Payable	9 Months Ended
	Sep. 30, 2018
Losses and Loss Expenses Payable [Abstract]
Liability for Future Policy Benefits and Unpaid Claims Disclosure [Text Block]	. Losses and Loss Expenses Payable The following table sets forth the activity in the liability for losses and loss expenses for the nine months ended September 30, 2018 and 2017 : ($ millions) 2018 2017 Losses and loss expenses payable, at beginning of period $ 1,255.6 $ 1,181.6 Less: reinsurance recoverable on losses and loss expenses payable 3.1 3.6 Net balance at beginning of period 1,252.5 1,178.0 Incurred related to: Current year 678.7 752.8 Prior years (57.6 ) (34.6 ) Total incurred 621.1 718.2 Paid related to: Current year 314.1 302.0 Prior years 365.8 318.3 Total paid 679.9 620.3 Net balance at end of period 1,193.7 1,275.9 Plus: reinsurance recoverable on losses and loss expenses payable 5.0 4.9 Losses and loss expenses payable, at end of period $ 1,198.7 $ 1,280.8 The Company recorded favorable development related to prior years’ loss and loss expense reserves for the nine months ended September 30, 2018 , of $57.6 million compared to favorable development of $34.6 million for the same 2017 period. Favorable development of prior years' unallocated loss adjustment expenses was approximately $6.7 million of the 2018 development. Partially offsetting the favorable development was $1.3 million of unfavorable development in catastrophe reserves. Favorable development of prior accident years' non-catastrophe loss and ALAE reserves was primarily due to $56.4 million of favorable development in the personal and commercial insurance segments. In the personal insurance segment, personal auto contributed $18.5 million of favorable development, primarily attributable to lower than anticipated severity from the 2016 and 2017 accident years, and homeowners contributed $7.2 million of favorable development, spread across several accident years. In the commercial insurance segment, workers' compensation, small commercial package, commercial auto, and middle market commercial contributed $8.4 million , $6.5 million , $5.7 million , $5.3 million , and respectively, of favorable development. Slightly offsetting the favorable development was adverse development in the specialty insurance segment of $4.2 million . The specialty insurance segment was impacted by $3.4 million and $0.9 million of adverse development in E&S casualty and programs, respectively. The E&S casualty adverse development for prior accident years was due primarily to development within our healthcare and general liability books of business. The programs quarter and year to date 2018 adverse development for prior accident years was driven by increased ultimate loss estimates for commercial auto coverages. The Company recorded favorable development related to prior years’ loss and loss expense reserves for the nine months ended September 30, 2017 of $34.6 million compared to adverse development of $33.4 million for the same 2016 period. Favorable development of prior years' unallocated loss adjustment expenses and catastrophe reserves was approximately $4.4 million and $1.8 million , respectively, for the nine months ended September 30, 2017 . The favorable development of prior accident years' non-catastrophe loss and ALAE reserves was primarily due to $33.1 million of favorable development in the commercial insurance segment. Slightly offsetting the favorable development was adverse development in the specialty and personal insurance segments of $2.6 million and $2.1 million , respectively. The specialty insurance segment was impacted by $3.0 million of adverse development in E&S property, driven by higher than anticipated severity for liability coverages on business in run-off. The personal insurance segment, homeowners contributed $1.9 million of adverse development, primarily from accident year 2016, and other personal contributed $2.5 million of adverse development, driven by higher than anticipated severity emerging from accident years 2015 and 2016. Slightly offsetting the adverse development was $2.3 million

Reinsurance

Reinsurance	9 Months Ended
	Sep. 30, 2018
Insurance [Abstract]
Reinsurance	Reinsurance The insurance subsidiaries of State Auto Financial, including State Auto P&C, Milbank and SA Ohio (collectively referred to as the “STFC Pooled Companies”) participate in a quota share reinsurance pooling arrangement (“the Pooling Arrangement”) with State Auto Mutual and its subsidiaries and affiliates (collectively referred to as the “Mutual Pooled Companies”). Effective June 1, 2017, State Auto P&C entered into a quota share agreement with Home State County Mutual Insurance Company ("Home State") to assume 100% of the business issued under this agreement defined as Texas Private Passenger Automobile. Home State receives a variable fee (capped at 2%) in consideration for this arrangement. Effective March 1, 2018, the State Auto Group entered into a quota share agreement ("Oil & Gas Quota Share") covering its gas & propane distribution book of business. The Oil & Gas Quota Share expires February 28, 2019. In accordance with this agreement, the State Auto Group ceded to external reinsurers (i) 100% of unearned premiums for this book of business as of the effective date and (ii) 100% of new and renewal policies issued on or after the effective date. In addition, the State Auto Group will receive a minimum ceding commission of 23.0% on premiums ceded under the agreement. On March 1, 2018, the Company ceded unearned premiums of $9.7 million to the reinsurers under the terms of this agreement. The following table sets forth a summary of the Company’s external reinsurance transactions, as well as reinsurance transactions with State Auto Mutual under the Pooling Arrangement, for the three and nine months ended September 30, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Premiums earned: Assumed from external insurers and reinsurers $ 17.3 $ 2.3 $ 41.9 $ 4.7 Assumed under Pooling Arrangement 306.8 319.0 929.2 957.2 Ceded to external insurers and reinsurers (5.9 ) (4.9 ) (16.7 ) (17.1 ) Ceded under Pooling Arrangement (234.9 ) (203.7 ) (678.1 ) (608.2 ) Net assumed premiums earned $ 83.3 $ 112.7 $ 276.3 $ 336.6 Losses and loss expenses incurred: Assumed from external insurers and reinsurers $ 10.9 $ 2.2 $ 34.2 $ 4.7 Assumed under Pooling Arrangement 183.1 254.0 591.4 719.5 Ceded to external insurers and reinsurers (1.9 ) (2.7 ) (3.4 ) (3.7 ) Ceded under Pooling Arrangement (148.4 ) (125.1 ) (486.0 ) (420.7 ) Net assumed losses and loss expenses incurred $ 43.7 $ 128.4 $ 136.3 $ 299.8

Notes Payable and Open Line of

Notes Payable and Open Line of Credit Notes Payable and Open Line of Credit	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Notes Payable and Open Line of Credit	Notes Payable and Open Line of Credit FHLB Line of Credit On March 22, 2018, State Auto P&C entered into an Open Line of Credit Commitment (the "OLC") with the FHLB. The OLC provides State Auto P&C with a $100.0 million one-year open line of credit available for general corporate purposes. Draws under the OLC are to be funded at a daily variable rate advance with a term of no more than 180 days with interest payable monthly. All advances under the OLC are to be fully secured by a pledge of specific investment securities of State Auto P&C. As of September 30, 2018 , no advances had been made under the OLC. On March 30, 2018, State Auto P&C terminated its credit facility (the “SPC Credit Facility”) with a syndicate of lenders. The SPC Credit Facility, which was maturing in July 2018, provided State Auto P&C with a $100.0 million revolving credit facility. The SPC Credit Facility was available for general corporate purposes and provided for interest-only payments during its term, with principal and interest due in full at maturity. Interest was based on LIBOR or a base rate plus a calculated margin amount. All advances under the SPC Credit Facility were to be fully secured by a pledge of specific investment securities of State Auto P&C. Prior to its termination, State Auto P&C had not made any borrowings under the SPC Credit Facility. FHLB Loans On May 17, 2018, State Auto P&C refinanced its $85.0 million loan (the "2013 FHLB loan") for a period of fifteen years at a fixed rate of 3.96%. The new loan (the "2018 FHLB loan") provides for interest-only payments during its term, with principal due in full at maturity. The 2018 FHLB loan is fully secured by a pledge of specific investment securities of State Auto P&C. State Auto P&C incurred a $0.4 million

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	. 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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Amount at statutory rate $ 8.6 21.0 % $ (3.9 ) 35.0 % $ 9.5 21.0 % $ (1.2 ) 35.0 % Tax-exempt interest and dividends received deduction (0.5 ) (1.3 ) (1.4 ) 12.4 (1.7 ) (3.8 ) (4.4 ) 131.8 Other, net (0.5 ) (1.1 ) 3.6 (32.6 ) 0.3 0.6 5.1 (151.7 ) Federal income tax expense (benefit) 7.6 18.6 % (1.7 ) 14.8 % 8.1 17.8 % $ (0.5 ) 15.1 % Deferred tax expense on share-based awards — — — 1.3 Federal income tax expense (benefit) $ 7.6 $ (1.7 ) $ 8.1 $ 0.8

Pension and Postretirement Bene

Pension and Postretirement Benefit Plans	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [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, 2018 and 2017 : ($ millions) Pension Postretirement Pension Postretirement Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 2018 2017 2018 2017 Service cost $ 1.2 $ 1.4 $ — $ — $ 3.8 $ 4.4 $ — $ — Interest cost 2.7 2.9 0.2 0.1 8.0 8.6 0.5 0.5 Expected return on plan assets (4.5 ) (4.2 ) — — (13.5 ) (12.6 ) — — Amortization of: Negative prior service cost — — (1.4 ) (1.3 ) — — (4.1 ) (4.1 ) Net actuarial loss 2.1 2.0 0.1 0.1 6.2 5.9 0.2 0.2 Net periodic cost (benefit) $ 1.5 $ 2.1 $ (1.1 ) $ (1.1 ) $ 4.5 $ 6.3 $ (3.4 ) $ (3.4 ) The Company contributed $9.8 million

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income	Other Comprehensive Loss and Accumulated Other Comprehensive Loss The following tables set forth the changes in the Company’s accumulated other comprehensive (loss) income ("AOC(L)I"), net of tax, for the three and nine months ended September 30, 2018 and 2017 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at July 1, 2018 $ (20.3 ) $ (37.0 ) $ (57.3 ) Other comprehensive loss before reclassifications (11.6 ) — (11.6 ) Amounts reclassified from AOCI (a) — 0.6 0.6 Net current period other comprehensive (loss) income (11.6 ) 0.6 (11.0 ) Ending balance at September 30, 2018 $ (31.9 ) $ (36.4 ) $ (68.3 ) Beginning balance at July 1, 2017 $ 83.0 $ (29.6 ) $ 53.4 Other comprehensive income before reclassifications 7.2 — 7.2 Amounts reclassified from AOCI (a) (13.2 ) 0.5 (12.7 ) Net current period other comprehensive (loss) income (6.0 ) 0.5 (5.5 ) Ending balance at September 30, 2017 $ 77.0 $ (29.1 ) $ 47.9 (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, 2018 $ 66.0 $ (29.3 ) $ 36.7 Cumulative effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018 (47.9 ) (8.9 ) (56.8 ) Adjusted beginning balance at January 1, 2018 18.1 (38.2 ) (20.1 ) Other comprehensive loss before reclassifications (48.7 ) — (48.7 ) Amounts reclassified from AOCI (a) (1.3 ) 1.8 0.5 Net current period other comprehensive (loss) income (50.0 ) 1.8 (48.2 ) Ending balance at September 30, 2018 $ (31.9 ) $ (36.4 ) $ (68.3 ) Beginning balance at January 1, 2017 $ 62.8 $ (30.3 ) $ 32.5 Other comprehensive income before reclassifications 42.7 — 42.7 Amounts reclassified from AOCI (a) (28.5 ) 1.2 (27.3 ) Net current period other comprehensive income 14.2 1.2 15.4 Ending balance at September 30, 2017 $ 77.0 $ (29.1 ) $ 47.9 (a) See separate table below for details about these reclassifications The following tables set forth the reclassifications out of accumulated other comprehensive income, by component, to the Company’s condensed consolidated statement of income for the three and nine months ended September 30, 2018 and 2017 : ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2018 Unrealized gains on available-for-sale fixed maturity investments $ — Realized gain on sale of securities — Total before tax — Tax expense — Net of tax Amortization of benefit plan items Negative prior service cost 1.4 (b) Net actuarial loss (2.2 ) (b) (0.8 ) Total before tax 0.2 Tax benefit (0.6 ) Net of tax Total reclassifications for the period $ (0.6 ) (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Nine months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2018 Unrealized gains on available-for-sale fixed maturity investments $ 1.7 Realized gains on sale of securities 1.7 Total before tax (0.4 ) Tax expense 1.3 Net of tax Amortization of benefit plan items Negative prior service cost 4.1 (b) Net actuarial loss (6.4 ) (b) (2.3 ) Total before tax 0.5 Tax benefit (1.8 ) Net of tax Total reclassifications for the period $ (0.5 ) (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2017 Unrealized gains on available for sale securities $ 20.2 Realized gain on sale of securities 20.2 Total before tax (7.0 ) Tax expense 13.2 Net of tax Amortization of benefit plan items Negative prior service cost 1.3 (b) Net actuarial loss (2.1 ) (b) (0.8 ) Total before tax 0.3 Tax benefit (0.5 ) Net of tax Total reclassifications for the period $ 12.7 (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Nine Months Ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2017 Unrealized gains on available for sale securities $ 43.8 Realized gain on sale of securities 43.8 Total before tax (15.3 ) Tax expense 28.5 Net of tax Amortization of benefit plan items Negative prior service cost 4.1 (b) Net actuarial loss (6.1 ) (b) (2.0 ) Total before tax 0.8 Tax benefit (1.2 ) Net of tax Total reclassifications for the period $ 27.3 (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings per Common Share

Earnings per Common Share	9 Months Ended
	Sep. 30, 2018
Earnings Per Share [Abstract]
Earnings (Loss) per Common Share	Net Earnings (Loss) 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, 2018 and 2017 : ($ and shares in millions, except per share amounts) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Numerator: Net income (loss) for basic earnings per common share $ 33.4 $ (9.5 ) $ 37.3 $ (4.1 ) Denominator: Weighted average shares for basic earnings (loss) per common share 43.0 42.2 42.8 42.0 Effect of dilutive share-based awards 0.6 — 0.5 — Adjusted weighted average shares for diluted net earnings (loss) per common share 43.6 42.2 43.3 42.0 Basic net earnings (loss) per common share $ 0.78 $ (0.23 ) $ 0.87 $ (0.10 ) Diluted net earnings (loss) per common share $ 0.76 $ (0.23 ) $ 0.86 $ (0.10 ) The following table sets forth common stock options, stock awards and restricted share units ("RSU award") of the Company that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three and nine months ended September 30, 2018 and 2017 : (shares in millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Total number of antidilutive options and awards — 0.5 0.1 0.7

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information The Company has four reportable segments: personal insurance, commercial insurance, specialty insurance and investment operations. The reportable insurance segments are business units managed separately because of the differences in the type of customers they serve, the products they provide or services they offer. The insurance segments market a broad line of property and casualty insurance products throughout the United States through independent insurance agencies, which include retail agents and wholesale brokers. The investment operations segment, managed by Stateco, provides investment services. 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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Revenue from external sources: Insurance segments Personal insurance $ 173.0 $ 144.1 $ 494.2 $ 430.6 Commercial insurance 115.2 114.3 345.7 341.8 Specialty insurance 18.6 60.6 89.3 184.8 Total insurance segments 306.8 319.0 929.2 957.2 Investment operations segment Net investment income 20.8 18.7 62.2 56.5 Net investment gain 17.8 20.2 18.2 43.8 Total investment operations segment 38.6 38.9 80.4 100.3 All other 0.6 0.5 1.8 1.7 Total revenue from external sources 346.0 358.4 1,011.4 1,059.2 Intersegment revenue 1.6 1.6 4.8 3.0 Total revenue 347.6 360.0 1,016.2 1,062.2 Reconciling items: Eliminate intersegment revenue (1.6 ) (1.6 ) (4.8 ) (3.0 ) Total consolidated revenues $ 346.0 $ 358.4 $ 1,011.4 $ 1,059.2 Segment income before federal income tax: Insurance segments SAP underwriting gain (loss) Personal insurance $ 6.9 $ (7.4 ) $ (4.6 ) $ (31.4 ) Commercial insurance (5.2 ) 1.3 (22.3 ) (7.0 ) Specialty insurance 1.9 (39.1 ) 4.5 (52.1 ) Total insurance segments gain (loss) 3.6 (45.2 ) (22.4 ) (90.5 ) Investment operations segment Net investment income 20.8 18.7 62.2 56.5 Net investment gain 17.8 20.2 18.2 43.8 Total investment operations segment 38.6 38.9 80.4 100.3 All other 0.1 0.1 0.3 0.2 Total segment income (loss) before reconciling items 42.3 (6.2 ) 58.3 10.0 Reconciling items: GAAP expense adjustments 0.8 (2.8 ) (2.6 ) (6.3 ) Interest expense on corporate debt (1.2 ) (1.5 ) (4.5 ) (4.4 ) Corporate expenses (0.9 ) (0.7 ) (5.8 ) (2.6 ) Total reconciling items (1.3 ) (5.0 ) (12.9 ) (13.3 ) Total consolidated income (loss) before federal income tax expense $ 41.0 $ (11.2 ) $ 45.4 $ (3.3 ) Investable assets attributable to the Company’s investment operations segment totaled $2,697.5 million and $2,781.2 million at September 30, 2018 , and December 31, 2017

Contingencies and Litigation

Contingencies and Litigation	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies and Litigation	Contingencies and Litigation In accordance with the Contingencies Topic of the FASB'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 a loss or a range of loss, if any, particularly for proceedings that are in their early stages of development or where the plaintiffs seek indeterminate damages. Various factors, including, but not limited to, the outcome of potentially lengthy discovery and the resolution of important factual questions, may need to be determined before probability can be established or before a loss or range of loss can be reasonably estimated. If the loss contingency in question is not both probable and reasonably estimable, the Company does not establish an accrual and the matter will continue to be monitored for any developments that would make the loss contingency both probable and reasonably estimable. Based on currently available information known to the Company, it believes that its reserves for litigation-related liabilities are reasonable. However, in the event that a legal proceeding results in a substantial judgment against, or settlement by, the Company, there can be no assurance that any resulting liability or financial commitment would not have a material adverse effect on the financial condition, results of operations or cash flows of the consolidated financial statements of the Company. The Company is involved in lawsuits in the ordinary course of its business arising out of or otherwise related to its insurance policies. Additionally, from time to time the Company may be involved in lawsuits, including class actions, in the ordinary course of business but not arising out of or otherwise related to its insurance policies. These lawsuits are in various stages of development. The Company generally will contest these matters vigorously but may pursue settlement if appropriate. Based on currently available information, the Company does not believe it is reasonably possible that any such lawsuit or related lawsuits will be material to its results of operations or have a material adverse effect on its consolidated financial position, results of operations or cash flows.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2018
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 investments by lot at September 30, 2018 and December 31, 2017 : ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value September 30, 2018 Available-for-sale fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 409.4 $ 4.3 $ (11.7 ) $ 402.0 Obligations of states and political subdivisions 434.8 6.0 (2.0 ) 438.8 Corporate securities 535.6 1.7 (11.4 ) 525.9 U.S. government agencies mortgage-backed securities 805.4 1.7 (32.2 ) 774.9 Total available-for-sale fixed maturities $ 2,185.2 $ 13.7 $ (57.3 ) $ 2,141.6 ($ millions) Cost or amortized cost Gross unrealized holding gains Gross unrealized holding losses Fair value December 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 433.8 $ 9.3 $ (6.2 ) $ 436.9 Obligations of states and political subdivisions 507.1 19.1 (0.4 ) 525.8 Corporate securities 527.5 4.5 (2.3 ) 529.7 U.S. government agencies mortgage-backed securities 704.7 7.1 (11.4 ) 700.4 Total fixed maturities 2,173.1 40.0 (20.3 ) 2,192.8 Equity securities: Large-cap securities 62.4 35.1 (0.7 ) 96.8 Mutual and exchange traded funds 256.2 21.6 (9.3 ) 268.5 Total equity securities 318.6 56.7 (10.0 ) 365.3 Other invested assets 25.8 30.2 — 56.0 Total available-for-sale securities $ 2,517.5 $ 126.9 $ (30.3 ) $ 2,614.1
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, 2018 and December 31, 2017 : ($ 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, 2018 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 106.1 $ (2.1 ) 15 $ 221.6 $ (9.6 ) 29 $ 327.7 $ (11.7 ) 44 Obligations of states and political subdivisions 130.7 (1.8 ) 19 23.3 (0.2 ) 3 154.0 (2.0 ) 22 Corporate securities 352.9 (5.7 ) 46 114.3 $ (5.7 ) 19 467.2 $ (11.4 ) 65 U.S. government agencies mortgage-backed securities 390.1 (11.8 ) 54 303.1 (20.4 ) 52 693.2 (32.2 ) 106 Total temporarily impaired securities $ 979.8 $ (21.4 ) 134 $ 662.3 $ (35.9 ) 103 $ 1,642.1 $ (57.3 ) 237 ($ 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, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 102.4 $ (0.6 ) 18 $ 196.1 $ (5.6 ) 22 $ 298.5 $ (6.2 ) 40 Obligations of states and political subdivisions 58.6 (0.4 ) 10 — — — 58.6 (0.4 ) 10 Corporate securities 153.2 (1.3 ) 23 67.3 (1.0 ) 10 220.5 (2.3 ) 33 U.S. government agencies mortgage-backed securities 188.6 (2.9 ) 31 252.2 (8.5 ) 41 440.8 (11.4 ) 72 Total fixed maturities 502.8 (5.2 ) 82 515.6 (15.1 ) 73 1,018.4 (20.3 ) 155 Equity securities: Large-cap equity securities 4.4 (0.7 ) 4 — — — 4.4 (0.7 ) 4 Mutual and exchange traded funds 66.9 (9.3 ) 1 — — — 66.9 (9.3 ) 1 Total equity securities 71.3 (10.0 ) 5 — — — 71.3 (10.0 ) 5 Total temporarily impaired securities $ 574.1 $ (15.2 ) 87 $ 515.6 $ (15.1 ) 73 $ 1,089.7 $ (30.3 ) 160
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, 2018 : ($ millions) Amortized cost Fair value Due in 1 year or less $ 28.1 $ 28.0 Due after 1 year through 5 years 587.1 576.4 Due after 5 years through 10 years 369.9 364.2 Due after 10 years 394.7 398.1 U.S. government agencies mortgage-backed securities 805.4 774.9 Total $ 2,185.2 $ 2,141.6
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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Fixed maturities $ 16.4 $ 15.4 $ 49.4 $ 47.1 Equity securities 3.0 2.0 8.6 5.7 Cash and cash equivalents, and other 1.7 1.6 5.2 4.6 Investment income 21.1 19.0 63.2 57.4 Investment expenses 0.3 0.3 1.0 0.9 Net investment income $ 20.8 $ 18.7 $ 62.2 $ 56.5
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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Realized gains: Fixed maturities $ — $ 0.4 $ 1.7 $ 2.7 Equity securities 1.0 20.9 6.0 45.2 Other invested assets — — — 0.1 Total realized gains 1.0 21.3 7.7 48.0 Realized losses on securities: Sales of equity securities — (0.5 ) (0.6 ) (0.7 ) OTTI — (0.6 ) — (3.5 ) Total equity securities realized losses — (1.1 ) (0.6 ) (4.2 ) Net realized gain on investments $ 1.0 $ 20.2 $ 7.1 $ 43.8 Net unrealized gain (loss) on investments (1) Equity securities 16.4 $ — $ 12.8 $ — Other invested assets 0.4 — (1.7 ) — Net unrealized gain on investments 16.8 — 11.1 — Net investment gain $ 17.8 $ 20.2 $ 18.2 $ 43.8 Change in unrealized holding (losses) gains, net of tax: Fixed maturities $ (14.7 ) $ 0.4 $ (63.3 ) $ 15.8 Equity securities — (12.1 ) — (1.9 ) Other invested assets — 2.5 — 7.9 Deferred federal income tax liability 3.1 3.2 13.3 (7.6 ) Change in net unrealized holding (losses) gains, net of tax $ (11.6 ) $ (6.0 ) $ (50.0 ) $ 14.2 (1) Unrealized holding gains (losses) recognized during the period on securities still held at the reporting date

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Company's Available-for-Sale Investments within Fair Value Hierarchy	The following tables set forth the Company’s investments within the fair value hierarchy at September 30, 2018 and December 31, 2017 : ($ millions) Total Level 1 Level 2 September 30, 2018 Available-for-sale fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 402.0 $ — $ 402.0 Obligations of states and political subdivisions 438.8 — 438.8 Corporate securities 525.9 — 525.9 U.S. government agencies mortgage-backed securities 774.9 — 774.9 Total available-for-sale fixed maturities 2,141.6 — 2,141.6 Equity securities: Large-cap securities 95.6 95.6 — Mutual and exchange traded funds 280.7 280.7 — Total equity securities 376.3 376.3 — Other invested assets 11.2 11.2 — Total investments $ 2,529.1 $ 387.5 $ 2,141.6 ($ millions) Total Level 1 Level 2 December 31, 2017 Fixed maturities: U.S. treasury securities and obligations of U.S. government agencies $ 436.9 $ — $ 436.9 Obligations of states and political subdivisions 525.8 — 525.8 Corporate securities 529.7 — 529.7 U.S. government agencies mortgage-backed securities 700.4 — 700.4 Total fixed maturities 2,192.8 — 2,192.8 Equity securities: Large-cap securities 96.8 96.8 — Mutual and exchange traded funds 268.5 268.5 — Total equity securities 365.3 365.3 — Other invested assets 10.8 10.8 — Total available-for-sale investments $ 2,568.9 $ 376.1 $ 2,192.8
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, 2018 December 31, 2017 Carrying value Fair value Interest rate Carrying value Fair value Interest rate Notes receivable from affiliate $ 70.0 $ 72.5 7.00 % $ 70.0 $ 72.6 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, 2018 December 31, 2017 Carrying value Fair Value Interest rate Carrying value Fair value Interest rate FHLB Loan due 2021: issued $21.5, September 2016 with fixed interest $ 21.5 $ 20.6 1.73 % $ 21.5 $ 20.9 1.73 % FHLB Loan due 2033: issued $85.0, July 2013 with fixed interest — — — % 85.4 85.7 5.03 % FHLB Loan due 2033: issued $85.0, May 2018 with fixed interest 85.3 86.4 3.96 % — — — % Affiliate Subordinated Debentures due 2033: issued $15.5, May 2003 with variable interest 15.2 15.2 6.52 % 15.2 15.2 5.68 % Total notes payable $ 122.0 $ 122.2 $ 122.1 $ 121.8

Losses and Loss Expenses Paya_2

Losses and Loss Expenses Payable Losses and Loss Expenses Payable (Tables)	9 Months Ended
	Sep. 30, 2018
Losses and Loss Expenses Payable [Abstract]
Schedule Of Activity In Liability For Unpaid Losses And Loss Adjustment Expenses Table [Table Text Block]	($ millions) 2018 2017 Losses and loss expenses payable, at beginning of period $ 1,255.6 $ 1,181.6 Less: reinsurance recoverable on losses and loss expenses payable 3.1 3.6 Net balance at beginning of period 1,252.5 1,178.0 Incurred related to: Current year 678.7 752.8 Prior years (57.6 ) (34.6 ) Total incurred 621.1 718.2 Paid related to: Current year 314.1 302.0 Prior years 365.8 318.3 Total paid 679.9 620.3 Net balance at end of period 1,193.7 1,275.9 Plus: reinsurance recoverable on losses and loss expenses payable 5.0 4.9 Losses and loss expenses payable, at end of period $ 1,198.7 $ 1,280.8

Reinsurance (Tables)

Reinsurance (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Premiums earned: Assumed from external insurers and reinsurers $ 17.3 $ 2.3 $ 41.9 $ 4.7 Assumed under Pooling Arrangement 306.8 319.0 929.2 957.2 Ceded to external insurers and reinsurers (5.9 ) (4.9 ) (16.7 ) (17.1 ) Ceded under Pooling Arrangement (234.9 ) (203.7 ) (678.1 ) (608.2 ) Net assumed premiums earned $ 83.3 $ 112.7 $ 276.3 $ 336.6 Losses and loss expenses incurred: Assumed from external insurers and reinsurers $ 10.9 $ 2.2 $ 34.2 $ 4.7 Assumed under Pooling Arrangement 183.1 254.0 591.4 719.5 Ceded to external insurers and reinsurers (1.9 ) (2.7 ) (3.4 ) (3.7 ) Ceded under Pooling Arrangement (148.4 ) (125.1 ) (486.0 ) (420.7 ) Net assumed losses and loss expenses incurred $ 43.7 $ 128.4 $ 136.3 $ 299.8

Income Taxes (Tables)

Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Amount at statutory rate $ 8.6 21.0 % $ (3.9 ) 35.0 % $ 9.5 21.0 % $ (1.2 ) 35.0 % Tax-exempt interest and dividends received deduction (0.5 ) (1.3 ) (1.4 ) 12.4 (1.7 ) (3.8 ) (4.4 ) 131.8 Other, net (0.5 ) (1.1 ) 3.6 (32.6 ) 0.3 0.6 5.1 (151.7 ) Federal income tax expense (benefit) 7.6 18.6 % (1.7 ) 14.8 % 8.1 17.8 % $ (0.5 ) 15.1 % Deferred tax expense on share-based awards — — — 1.3 Federal income tax expense (benefit) $ 7.6 $ (1.7 ) $ 8.1 $ 0.8

Pension and Postretirement Be_2

Pension and Postretirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2018
Retirement Benefits [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, 2018 and 2017 : ($ millions) Pension Postretirement Pension Postretirement Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 2018 2017 2018 2017 Service cost $ 1.2 $ 1.4 $ — $ — $ 3.8 $ 4.4 $ — $ — Interest cost 2.7 2.9 0.2 0.1 8.0 8.6 0.5 0.5 Expected return on plan assets (4.5 ) (4.2 ) — — (13.5 ) (12.6 ) — — Amortization of: Negative prior service cost — — (1.4 ) (1.3 ) — — (4.1 ) (4.1 ) Net actuarial loss 2.1 2.0 0.1 0.1 6.2 5.9 0.2 0.2 Net periodic cost (benefit) $ 1.5 $ 2.1 $ (1.1 ) $ (1.1 ) $ 4.5 $ 6.3 $ (3.4 ) $ (3.4 )

Other Comprehensive Income an_2

Other Comprehensive Income and Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2018
Equity [Abstract]
Changes in Accumulated Other Comprehensive Income by Component, Net of Tax	The following tables set forth the changes in the Company’s accumulated other comprehensive (loss) income ("AOC(L)I"), net of tax, for the three and nine months ended September 30, 2018 and 2017 : ($ millions) Unrealized Gains and Losses on Available-for-Sale Securities Benefit Plan Items Total Beginning balance at July 1, 2018 $ (20.3 ) $ (37.0 ) $ (57.3 ) Other comprehensive loss before reclassifications (11.6 ) — (11.6 ) Amounts reclassified from AOCI (a) — 0.6 0.6 Net current period other comprehensive (loss) income (11.6 ) 0.6 (11.0 ) Ending balance at September 30, 2018 $ (31.9 ) $ (36.4 ) $ (68.3 ) Beginning balance at July 1, 2017 $ 83.0 $ (29.6 ) $ 53.4 Other comprehensive income before reclassifications 7.2 — 7.2 Amounts reclassified from AOCI (a) (13.2 ) 0.5 (12.7 ) Net current period other comprehensive (loss) income (6.0 ) 0.5 (5.5 ) Ending balance at September 30, 2017 $ 77.0 $ (29.1 ) $ 47.9 (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, 2018 $ 66.0 $ (29.3 ) $ 36.7 Cumulative effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018 (47.9 ) (8.9 ) (56.8 ) Adjusted beginning balance at January 1, 2018 18.1 (38.2 ) (20.1 ) Other comprehensive loss before reclassifications (48.7 ) — (48.7 ) Amounts reclassified from AOCI (a) (1.3 ) 1.8 0.5 Net current period other comprehensive (loss) income (50.0 ) 1.8 (48.2 ) Ending balance at September 30, 2018 $ (31.9 ) $ (36.4 ) $ (68.3 ) Beginning balance at January 1, 2017 $ 62.8 $ (30.3 ) $ 32.5 Other comprehensive income before reclassifications 42.7 — 42.7 Amounts reclassified from AOCI (a) (28.5 ) 1.2 (27.3 ) Net current period other comprehensive income 14.2 1.2 15.4 Ending balance at September 30, 2017 $ 77.0 $ (29.1 ) $ 47.9 (a) See separate table below for details about these reclassifications
Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component	The following tables set 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, 2018 and 2017 : ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2018 Unrealized gains on available-for-sale fixed maturity investments $ — Realized gain on sale of securities — Total before tax — Tax expense — Net of tax Amortization of benefit plan items Negative prior service cost 1.4 (b) Net actuarial loss (2.2 ) (b) (0.8 ) Total before tax 0.2 Tax benefit (0.6 ) Net of tax Total reclassifications for the period $ (0.6 ) (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Nine months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2018 Unrealized gains on available-for-sale fixed maturity investments $ 1.7 Realized gains on sale of securities 1.7 Total before tax (0.4 ) Tax expense 1.3 Net of tax Amortization of benefit plan items Negative prior service cost 4.1 (b) Net actuarial loss (6.4 ) (b) (2.3 ) Total before tax 0.5 Tax benefit (1.8 ) Net of tax Total reclassifications for the period $ (0.5 ) (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Three months ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2017 Unrealized gains on available for sale securities $ 20.2 Realized gain on sale of securities 20.2 Total before tax (7.0 ) Tax expense 13.2 Net of tax Amortization of benefit plan items Negative prior service cost 1.3 (b) Net actuarial loss (2.1 ) (b) (0.8 ) Total before tax 0.3 Tax benefit (0.5 ) Net of tax Total reclassifications for the period $ 12.7 (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details). ($ millions) Details about Accumulated Other Nine Months Ended Affected line item in the Condensed Comprehensive Income Components September 30 Consolidated Statements of Income 2017 Unrealized gains on available for sale securities $ 43.8 Realized gain on sale of securities 43.8 Total before tax (15.3 ) Tax expense 28.5 Net of tax Amortization of benefit plan items Negative prior service cost 4.1 (b) Net actuarial loss (6.1 ) (b) (2.0 ) Total before tax 0.8 Tax benefit (1.2 ) Net of tax Total reclassifications for the period $ 27.3 (b) These accumulated other comprehensive income components are included in the computation of net periodic pension cost (see pension and postretirement benefit plans footnote for additional details).

Earnings per Common Share (Tabl

Earnings per Common Share (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 : ($ and shares in millions, except per share amounts) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Numerator: Net income (loss) for basic earnings per common share $ 33.4 $ (9.5 ) $ 37.3 $ (4.1 ) Denominator: Weighted average shares for basic earnings (loss) per common share 43.0 42.2 42.8 42.0 Effect of dilutive share-based awards 0.6 — 0.5 — Adjusted weighted average shares for diluted net earnings (loss) per common share 43.6 42.2 43.3 42.0 Basic net earnings (loss) per common share $ 0.78 $ (0.23 ) $ 0.87 $ (0.10 ) Diluted net earnings (loss) per common share $ 0.76 $ (0.23 ) $ 0.86 $ (0.10 )
Options to Purchase Shares of Common Stock	The following table sets forth common stock options, stock awards and restricted share units ("RSU award") of the Company that were not included in the computation of diluted earnings per common share because the exercise price of the options, or awards, was greater than the average market price or their inclusion would have been antidilutive for the three and nine months ended September 30, 2018 and 2017 : (shares in millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Total number of antidilutive options and awards — 0.5 0.1 0.7

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 : ($ millions) Three months ended September 30 Nine months ended September 30 2018 2017 2018 2017 Revenue from external sources: Insurance segments Personal insurance $ 173.0 $ 144.1 $ 494.2 $ 430.6 Commercial insurance 115.2 114.3 345.7 341.8 Specialty insurance 18.6 60.6 89.3 184.8 Total insurance segments 306.8 319.0 929.2 957.2 Investment operations segment Net investment income 20.8 18.7 62.2 56.5 Net investment gain 17.8 20.2 18.2 43.8 Total investment operations segment 38.6 38.9 80.4 100.3 All other 0.6 0.5 1.8 1.7 Total revenue from external sources 346.0 358.4 1,011.4 1,059.2 Intersegment revenue 1.6 1.6 4.8 3.0 Total revenue 347.6 360.0 1,016.2 1,062.2 Reconciling items: Eliminate intersegment revenue (1.6 ) (1.6 ) (4.8 ) (3.0 ) Total consolidated revenues $ 346.0 $ 358.4 $ 1,011.4 $ 1,059.2 Segment income before federal income tax: Insurance segments SAP underwriting gain (loss) Personal insurance $ 6.9 $ (7.4 ) $ (4.6 ) $ (31.4 ) Commercial insurance (5.2 ) 1.3 (22.3 ) (7.0 ) Specialty insurance 1.9 (39.1 ) 4.5 (52.1 ) Total insurance segments gain (loss) 3.6 (45.2 ) (22.4 ) (90.5 ) Investment operations segment Net investment income 20.8 18.7 62.2 56.5 Net investment gain 17.8 20.2 18.2 43.8 Total investment operations segment 38.6 38.9 80.4 100.3 All other 0.1 0.1 0.3 0.2 Total segment income (loss) before reconciling items 42.3 (6.2 ) 58.3 10.0 Reconciling items: GAAP expense adjustments 0.8 (2.8 ) (2.6 ) (6.3 ) Interest expense on corporate debt (1.2 ) (1.5 ) (4.5 ) (4.4 ) Corporate expenses (0.9 ) (0.7 ) (5.8 ) (2.6 ) Total reconciling items (1.3 ) (5.0 ) (12.9 ) (13.3 ) Total consolidated income (loss) before federal income tax expense $ 41.0 $ (11.2 ) $ 45.4 $ (3.3 )

Basis of Presentation Adoption

Basis of Presentation Adoption of Recent Accounting Pronouncements (Details) $ in Millions	Jan. 01, 2018USD ($)
Adoption of Recent Accounting Pronouncements [Abstract]
Unrealized gains on equity securities reclassified from accumulated other comprehensive income to retained earnings, after tax upon adoption of ASU 2016-01	$ 60.8
Stranded income tax effects resulting from TCJA reclassified from accumulated other comprehensive income to retained earnings upon adoption of ASU 2018-02	$ 4

Investments - Cost or Amortized

Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2018
Cost or amortized cost		$ 2,517.5
Gross unrealized holding gains		126.9
Gross unrealized holding losses		(30.3)
Fair value		2,614.1
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	$ 2,185.2	2,173.1
Gross unrealized holding gains	13.7	40
Gross unrealized holding losses	(57.3)	(20.3)
Fair value	2,141.6	2,192.8
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	409.4	433.8
Gross unrealized holding gains	4.3	9.3
Gross unrealized holding losses	(11.7)	(6.2)
Fair value	402	436.9
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	434.8	507.1
Gross unrealized holding gains	6	19.1
Gross unrealized holding losses	(2)	(0.4)
Fair value	438.8	525.8
Fixed Maturities [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	535.6	527.5
Gross unrealized holding gains	1.7	4.5
Gross unrealized holding losses	(11.4)	(2.3)
Fair value	525.9	529.7
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost	805.4	704.7
Gross unrealized holding gains	1.7	7.1
Gross unrealized holding losses	(32.2)	(11.4)
Fair value	$ 774.9	700.4
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost		318.6
Gross unrealized holding gains		56.7
Gross unrealized holding losses		(10)
Fair value		365.3
Equity Securities [Member] | Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost		62.4
Gross unrealized holding gains		35.1
Gross unrealized holding losses		(0.7)
Fair value		96.8
Equity Securities [Member] | Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost		256.2
Gross unrealized holding gains		21.6
Gross unrealized holding losses		(9.3)
Fair value		268.5
Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Cost or amortized cost		25.8
Gross unrealized holding gains		30.2
Gross unrealized holding losses		0
Fair value		$ 56

Investments - Available for Sal

Investments - Available for Sale Securities Continuous Unrealized Loss Position Fair Value and Aggregate Losses (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2018
Less than 12 months, Fair value		$ 574.1
Less than 12 months, Unrealized losses		$ (15.2)
Less than 12 months, Number of positions	134	87
12 months or more, Fair value		$ 515.6
12 months or more, Unrealized losses		$ (15.1)
12 months or more, Number of positions	103	73
Total, Fair value		$ 1,089.7
Total, Unrealized losses		$ (30.3)
Total, Number of positions	237	160
Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value		$ 4.4
Less than 12 months, Unrealized losses		$ (0.7)
Less than 12 months, Number of positions		4
12 months or more, Fair value		$ 0
12 months or more, Unrealized losses		$ 0
12 months or more, Number of positions		0
Total, Fair value		$ 4.4
Total, Unrealized losses		$ (0.7)
Total, Number of positions		4
Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value		$ 66.9
Less than 12 months, Unrealized losses		$ (9.3)
Less than 12 months, Number of positions		1
12 months or more, Fair value		$ 0
12 months or more, Unrealized losses		$ 0
12 months or more, Number of positions		0
Total, Fair value		$ 66.9
Total, Unrealized losses		$ (9.3)
Total, Number of positions		1
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 979.8	$ 502.8
Less than 12 months, Unrealized losses	(21.4)	$ (5.2)
Less than 12 months, Number of positions		82
12 months or more, Fair value	662.3	$ 515.6
12 months or more, Unrealized losses	(35.9)	$ (15.1)
12 months or more, Number of positions		73
Total, Fair value	1,642.1	$ 1,018.4
Total, Unrealized losses	(57.3)	$ (20.3)
Total, Number of positions		155
Fixed Maturities [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	106.1	$ 102.4
Less than 12 months, Unrealized losses	$ (2.1)	$ (0.6)
Less than 12 months, Number of positions	15	18
12 months or more, Fair value	$ 221.6	$ 196.1
12 months or more, Unrealized losses	$ (9.6)	$ (5.6)
12 months or more, Number of positions	29	22
Total, Fair value	$ 327.7	$ 298.5
Total, Unrealized losses	$ (11.7)	$ (6.2)
Total, Number of positions	44	40
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 130.7	$ 58.6
Less than 12 months, Unrealized losses	$ (1.8)	$ (0.4)
Less than 12 months, Number of positions	19	10
12 months or more, Fair value	$ 23.3	$ 0
12 months or more, Unrealized losses	$ (0.2)	$ 0
12 months or more, Number of positions	3	0
Total, Fair value	$ 154	$ 58.6
Total, Unrealized losses	$ (2)	$ (0.4)
Total, Number of positions	22	10
Fixed Maturities [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 352.9	$ 153.2
Less than 12 months, Unrealized losses	$ (5.7)	$ (1.3)
Less than 12 months, Number of positions	46	23
12 months or more, Fair value	$ 114.3	$ 67.3
12 months or more, Unrealized losses	$ (5.7)	$ (1)
12 months or more, Number of positions	19	10
Total, Fair value	$ 467.2	$ 220.5
Total, Unrealized losses	$ (11.4)	$ (2.3)
Total, Number of positions	65	33
Fixed Maturities [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value	$ 390.1	$ 188.6
Less than 12 months, Unrealized losses	$ (11.8)	$ (2.9)
Less than 12 months, Number of positions	54	31
12 months or more, Fair value	$ 303.1	$ 252.2
12 months or more, Unrealized losses	$ (20.4)	$ (8.5)
12 months or more, Number of positions	52	41
Total, Fair value	$ 693.2	$ 440.8
Total, Unrealized losses	$ (32.2)	$ (11.4)
Total, Number of positions	106	72
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Less than 12 months, Fair value		$ 71.3
Less than 12 months, Unrealized losses		$ (10)
Less than 12 months, Number of positions		5
12 months or more, Fair value		$ 0
12 months or more, Unrealized losses		$ 0
12 months or more, Number of positions		0
Total, Fair value		$ 71.3
Total, Unrealized losses		$ (10)
Total, Number of positions		5

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, 2018	Dec. 31, 2017
Amortized cost
Due in 1 year or less, Amortized cost	$ 28.1
Due after 1 year through 5 years, Amortized cost	587.1
Due after 5 years through 10 years, Amortized cost	369.9
Due after 10 years, Amortized cost	394.7
U.S. government agencies mortgage-backed securities, Amortized cost	805.4
Total, Amortized cost	2,185.2	$ 2,173.1
Fair value
Due in 1 year or less, Fair value	28
Due after 1 year through 5 years, Fair value	576.4
Due after 5 years through 10 years, Fair value	364.2
Due after 10 years, Fair value	398.1
U.S. government agencies mortgage-backed securities, Fair value	774.9
Total, Fair value	$ 2,141.6	$ 2,192.8

Investments - Components of Net

Investments - Components of Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Investment income	$ 21.1	$ 19	$ 63.2	$ 57.4
Investment expenses	0.3	0.3	1	0.9
Net investment income	20.8	18.7	62.2	56.5
Fixed Maturities [Member]
Net Investment Income [Line Items]
Investment income	16.4	15.4	49.4	47.1
Equity Securities [Member]
Net Investment Income [Line Items]
Investment income	3	2	8.6	5.7
Cash and Cash Equivalents, and Other [Member]
Net Investment Income [Line Items]
Investment income	$ 1.7	$ 1.6	$ 5.2	$ 4.6

Investments - Realized and Unre

Investments - Realized and Unrealized Holding Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Realized gains:
Total realized gains	$ 1	$ 21.3	$ 7.7	$ 48
Realized losses:
Total realized losses	0	(1.1)	(0.6)	(4.2)
Net realized gain on investments	1	20.2	7.1	43.8
Unrealized Gain (Loss) on Investments	16.8	0	11.1	0
Realized Investment Gains (Losses)	(17.8)	(20.2)	(18.2)	(43.8)
Change in unrealized holding gains (losses), net of tax:
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	3.1	3.2	13.3	(7.6)
Change in net unrealized holding (losses) gains, net of tax	(11.6)	(6)	(50)	14.2
Fixed Maturities [Member]
Realized gains:
Total realized gains	0	0.4	1.7	2.7
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	(14.7)	0.4	(63.3)	15.8
Equity Securities [Member]
Realized gains:
Total realized gains	1	20.9	6	45.2
Realized losses:
Unrealized Gain (Loss) on Investments	16.4	0	12.8	0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	0	(12.1)	0	(1.9)
Equity Securities [Member] | Sales [Member]
Realized losses:
Total realized losses	0	(0.5)	(0.6)	(0.7)
Equity Securities [Member] | Other Than Temporary Impairments [Member]
Realized losses:
Total realized losses	0	(0.6)	0	(3.5)
Other Invested Assets [Member]
Realized gains:
Total realized gains	0	0	0	0.1
Realized losses:
Unrealized Gain (Loss) on Investments	0.4	0	(1.7)	0
Change in unrealized holding gains (losses), net of tax:
Change in net unrealized holding (losses) gains, net of tax	$ 0	$ 2.5	$ 0	$ 7.9

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Fixed maturities fair value of deposit with insurance regulators	$ 8.9		$ 9.3
Proceeds on sale of available-for-sale securities	159.9	$ 328.4
Federal Home Loan Bank of Cincinnati [Member]
Debt Securities, Available-for-sale [Line Items]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	$ 106.5

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Company's Available-for-Sale Investments within Fair Value Hierarchy (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Document Period End Date	Sep. 30, 2018
Fair value			$ 2,614.1
Investments, Fair Value Disclosure	$ 2,529.1		2,568.9
Sales of other invested assets	0.9	$ 0.8
U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	402		436.9
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	438.8		525.8
Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	525.9		529.7
U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	774.9		700.4
Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	2,141.6		2,192.8
Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	95.6		96.8
Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	280.7		268.5
Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	376.3		365.3
Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Investments, Fair Value Disclosure	11.2		10.8
Level 1 [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	387.5		376.1
Level 1 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 1 [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 1 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 1 [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 1 [Member] | Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	95.6		96.8
Level 1 [Member] | Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	280.7		268.5
Level 1 [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	376.3		365.3
Level 1 [Member] | Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	11.2		10.8
Level 2 [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	2,141.6		2,192.8
Level 2 [Member] | U.S. Treasury Securities and Obligations of U.S. Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	402		436.9
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	438.8		525.8
Level 2 [Member] | Corporate Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	525.9		529.7
Level 2 [Member] | U.S. Government Agencies Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	774.9		700.4
Level 2 [Member] | Fixed Maturities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	2,141.6		2,192.8
Level 2 [Member] | Large-Cap Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 2 [Member] | Mutual and exchange traded funds [Domain] [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 2 [Member] | Equity Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	0		0
Level 2 [Member] | Other Invested Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Fair value	$ 0		$ 0

Fair Value of Financial Instr_4

Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Sep. 30, 2018
Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable interest rate (percent)	7.00%	7.00%
Reported Value Measurement [Member] | Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 70	$ 70
Estimate of Fair Value Measurement [Member] | Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 72.5	$ 72.6

Fair Value of Financial Instr_5

Fair Value of Financial Instruments - Included in Notes Payable are the FHLB loan and Subordinated Debentures (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Document Period End Date	Sep. 30, 2018
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Interest rate (percent)	1.73%	1.73%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$ 21,500,000
Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Interest rate (percent)	0.00%	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
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Interest rate (percent)	3.96%	0.00%
Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Interest rate (percent)	6.52%	5.68%
FHLB loan due and issued with fixed interest, Senior notes due and issued with fixed interest and Affiliate subordinated debentures due with variable interest	$ 15,500,000
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	122,000,000	$ 122,100,000
Reported Value Measurement [Member] | Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Total notes payable	21,500,000	21,500,000
Reported Value Measurement [Member] | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	0	85,400,000
Reported Value Measurement [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Total notes payable	85,300,000	0
Reported Value Measurement [Member] | Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Total notes payable	15,200,000	15,200,000
Estimate of Fair Value Measurement [Member]
Debt Instrument [Line Items]
Total notes payable	122,200,000	121,800,000
Estimate of Fair Value Measurement [Member] | Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Total notes payable	20,600,000	20,900,000
Estimate of Fair Value Measurement [Member] | Federal Home Loan Bank of Cincinnati [Member]
Debt Instrument [Line Items]
Total notes payable	0	85,700,000
Estimate of Fair Value Measurement [Member] | Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Total notes payable	86,400,000	0
Estimate of Fair Value Measurement [Member] | Affiliate Subordinated Debentures Due 2033 [Member]
Debt Instrument [Line Items]
Total notes payable	$ 15,200,000	$ 15,200,000

Fair Value of Financial Instr_6

Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended
	May 31, 2009Agreement	Sep. 30, 2018USD ($)	Dec. 31, 2017USD ($)
Fair Value Measurements Disclosure [Line Items]
Fair value			$ 2,614.1
Other invested assets, at cost		$ 5.6	5.6
Number of Credit Agreements | Agreement	2
Notes receivable from affiliate		70	70
Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Federal home loan bank stock		5.1
Other invested assets, at cost		0.5
Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		2,141.6	2,192.8
Corporate Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Fair value		$ 525.9	529.7
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities		1
Fair value		$ 43.3	$ 45.2
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate		$ 70

Losses and Loss Expenses Paya_3

Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Schedule of Activity in Liability for Unpaid Losses and Loss Expenses Payable (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Losses and Loss Expenses Payable [Abstract]
Liability for Claims and Claims Adjustment Expense, Property Casualty Liability	$ 1,198.7	$ 1,280.8	$ 1,198.7	$ 1,280.8	$ 1,255.6	$ 1,181.6
Reinsurance Recoverables, Including Reinsurance Premium Paid	5	4.9	5	4.9	3.1	3.6
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,193.7	1,275.9	1,193.7	1,275.9	$ 1,252.5	$ 1,178
Current Year Claims and Claims Adjustment Expense			678.7	752.8
Prior Year Claims and Claims Adjustment Expense			57.6	(34.6)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 191	$ 253.5	621.1	718.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year			314.1	302
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years			365.8	318.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid			$ 679.9	$ 620.3

Losses and Loss Expenses Paya_4

Losses and Loss Expenses Payable Losses and Loss Expenses Payable - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ (57.6)	$ 34.6
Unallocated loss adjustment expense [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	6.7	4.4
Business Insurance Segments [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	56.4
Specialty Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(4.2)	(2.6)
E&S Property [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(3)
Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	7.2	(1.9)
Middle Market Commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	5.3
Personal Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	18.5	2.3
Commercial Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	5.7
Workers' compensation [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	8.4
E&S Casualty [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(3.4)
Programs [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(0.9)
Small commercial [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	6.5
Personal Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(2.1)
Business Insurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		33.1
Other Personal [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(2.5)
Catastrophe [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ (1.3)	$ 1.8

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 01, 2018
Insurance [Abstract]
Unearned Premiums Ceded					$ 9.7
Premiums earned:
Assumed from external insurers and reinsurers	$ 17.3	$ 2.3	$ 41.9	$ 4.7
Assumed under Pooling Arrangement	306.8	319	929.2	957.2
Ceded to external insurers and reinsurers	(5.9)	(4.9)	(16.7)	(17.1)
Ceded under Pooling Arrangement	(234.9)	(203.7)	(678.1)	(608.2)
Net assumed premiums earned	83.3	112.7	276.3	336.6
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	10.9	2.2	34.2	4.7
Assumed under Pooling Arrangement	183.1	254	591.4	719.5
Ceded to external insurers and reinsurers	(1.9)	(2.7)	(3.4)	(3.7)
Ceded under Pooling Arrangement	(148.4)	(125.1)	(486)	(420.7)
Net assumed losses and loss expenses incurred	$ 43.7	$ 128.4	$ 136.3	$ 299.8

Notes Payable and Open Line o_2

Notes Payable and Open Line of Credit Notes Payable and Open Line of Credit (Details) - USD ($) $ in Millions	May 17, 2018	Mar. 23, 2018	Sep. 30, 2018	Sep. 30, 2017	Mar. 22, 2018	Jul. 26, 2013
Debt Disclosure [Abstract]
Line of Credit Facility, Current Borrowing Capacity					$ 100	$ 100
Line of Credit Facility, Expiration Period		1 year
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 85
Federal Home Loan Bank, Advances, Maturity Period, Fixed Rate	15 years
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Interest Rate	3.96%
Debt Prepayment Cost			$ 0.4	$ 0

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 8.6	$ (3.9)	$ 9.5	$ (1.2)
Tax-exempt interest and dividends received deduction	(0.5)	(1.4)	(1.7)	(4.4)
Other, net	(0.5)	3.6	0.3	5.1
Federal income tax expense and effective rate	7.6	(1.7)	8.1	(0.5)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	0	0	0	1.3
Income Tax Expense (Benefit)	$ 7.6	$ (1.7)	$ 8.1	$ 0.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	21.00%	35.00%	21.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	(1.30%)	12.40%	(3.80%)	131.80%
Other, net (percent)	(1.10%)	(32.60%)	0.60%	(151.70%)
Effective Income Tax Rate Reconciliation, Deduction, Percent	18.60%	14.80%	17.80%	15.10%

Pension and Postretirement Be_3

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1.2	$ 1.4	$ 3.8	$ 4.4
Interest cost	2.7	2.9	8	8.6
Expected return on plan assets	4.5	4.2	13.5	12.6
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0	0	0	0
Amortization of:
Defined Benefit Plan, Amortization of Gain (Loss)	2.1	2	6.2	5.9
Net periodic cost (benefit)	1.5	2.1	4.5	6.3
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	0	0
Interest cost	0.2	0.1	0.5	0.5
Expected return on plan assets	0	0	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(1.4)	(1.3)	(4.1)	(4.1)
Amortization of:
Defined Benefit Plan, Amortization of Gain (Loss)	0.1	0.1	0.2	0.2
Net periodic cost (benefit)	$ (1.1)	$ (1.1)	$ (3.4)	$ (3.4)

Pension and Postretirement Be_4

Pension and Postretirement Benefit Plans - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Retirement Benefits [Abstract]
Contributions by employer	$ 9.8

Other Comprehensive Income an_3

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ (57.3)	$ 53.4	$ 36.7	$ 32.5
Other comprehensive income before reclassifications	(11.6)	7.2	(48.7)	42.7
Amounts reclassified from AOCI	0.6	(12.7)	0.5	(27.3)
Other comprehensive loss	(11)	(5.5)	(48.2)	15.4
Ending balance	(68.3)	47.9	(68.3)	47.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					$ (56.8)
Amounts reclassified from AOCI	0.6	(12.7)	0.5	(27.3)
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(20.3)	83	66	62.8
Other comprehensive income before reclassifications	(11.6)	7.2	(48.7)	42.7
Amounts reclassified from AOCI	0	(13.2)	(1.3)	(28.5)
Other comprehensive loss	(11.6)	(6)	(50)	14.2
Ending balance	(31.9)	77	(31.9)	77
Unrealized Gains and Losses on Available-for-Sale Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					(47.9)
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(37)	(29.6)	(29.3)	(30.3)
Other comprehensive income before reclassifications	0	0	0	0
Amounts reclassified from AOCI	0.6	0.5	1.8	1.2
Other comprehensive loss	0.6	0.5	1.8	1.2
Ending balance	$ (36.4)	$ (29.1)	$ (36.4)	$ (29.1)
Benefit Plan Items [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Cumulative Effect of change in accounting for equity securities and other invested assets and reclassification of stranded tax effects as of January 1, 2018					$ (8.9)

Other Comprehensive Income an_4

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	$ 17.8	$ 20.2	$ 18.2	$ 43.8
Tax (expense) benefit	(7.6)	1.7	(8.1)	(0.8)
Total reclassifications for the period	(0.6)	12.7	(0.5)	27.3
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	0	13.2	1.3	28.5
Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(0.6)	(0.5)	(1.8)	(1.2)
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(0.6)	12.7	(0.5)	27.3
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	0	20.2	1.7	43.8
Total before tax	0	20.2	1.7	43.8
Tax (expense) benefit	0	(7)	(0.4)	(15.3)
Net of tax	0	13.2	1.3	28.5
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive income components/Negative prior-service costs	1.4	1.3	4.1	4.1
Accumulated other comprehensive income components/Net actuarial loss	(2.2)	(2.1)	(6.4)	(6.1)
Total before tax	(0.8)	(0.8)	(2.3)	(2)
Tax (expense) benefit	0.2	0.3	0.5	0.8
Net of tax	$ (0.6)	$ (0.5)	$ (1.8)	$ (1.2)

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income	$ 33.4	$ (9.5)	$ 37.3	$ (4.1)
Denominator:
Weighted average shares for basic net earnings per common share (shares)	43	42.2	42.8	42
Effect of dilutive share-based awards (shares)	0.6	0	0.5	0
Adjusted weighted average shares for diluted net earnings per common share (shares)	43.6	42.2	43.3	42
Basic net (loss) earnings per common share (usd per share)	$ 0.78	$ (0.23)	$ 0.87	$ (0.10)
Diluted net (loss) earnings per common share (usd per share)	$ 0.76	$ (0.23)	$ 0.86	$ (0.10)

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Number of antidilutive options and awards	0	0.5	0.1	0.7

Segment Information - Financial

Segment Information - Financial Information Regarding Company's Reportable Segments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues from external sources:
Revenue	$ 346	$ 358.4	$ 1,011.4	$ 1,059.2
Other Income	0.6	0.5	1.8	1.7
Net Investment Income	20.8	18.7	62.2	56.5
Realized Investment Gains (Losses)	17.8	20.2	18.2	43.8
Segment income before federal income tax:
Interest expense on corporate debt	(1.2)	(1.5)	(4.5)	(4.4)
Income before federal income taxes	41	(11.2)	45.4	(3.3)
Operating Segments [Member]
Revenues from external sources:
Revenue	346	358.4	1,011.4	1,059.2
Segment income before federal income tax:
Income before federal income taxes	42.3	(6.2)	58.3	10
Operating Segments [Member] | Personal Insurance Segments [Member]
Revenues from external sources:
Revenue	173	144.1	494.2	430.6
Operating Segments [Member] | Business Insurance Segments [Member]
Revenues from external sources:
Revenue	115.2	114.3	345.7	341.8
Operating Segments [Member] | Specialty Insurance Segment [Member]
Revenues from external sources:
Revenue	18.6	60.6	89.3	184.8
Operating Segments [Member] | Total Insurance Segments [Member]
Revenues from external sources:
Revenue	306.8	319	929.2	957.2
Segment income before federal income tax:
Income before federal income taxes	3.6	(45.2)	(22.4)	(90.5)
Operating Segments [Member] | Net Investment Income [Member]
Revenues from external sources:
Net Investment Income		18.7		56.5
Operating Segments [Member] | Total Investment Operations [Member]
Revenues from external sources:
Revenue	38.6	38.9	80.4	100.3
Segment income before federal income tax:
Income before federal income taxes	38.6	38.9	80.4	100.3
Operating Segments [Member] | Total Investment Operations [Member] | Net Realized Capital Gains [Member]
Segment income before federal income tax:
Income before federal income taxes		20.2	18.2	43.8
Operating Segments [Member] | All Other Segments [Member]
Revenues from external sources:
Revenue		0.5		1.7
Segment income before federal income tax:
Income before federal income taxes	0.1	0.1	0.3	0.2
Operating Segments [Member] | Personal Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	6.9	(7.4)	(4.6)	(31.4)
Operating Segments [Member] | Business Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	(5.2)	1.3	(22.3)	(7)
Operating Segments [Member] | Specialty Insurance [Member]
Segment income before federal income tax:
Income before federal income taxes	1.9	(39.1)	4.5	(52.1)
Eliminate Intersegment Revenues [Member]
Revenues from external sources:
Revenue	1.6	1.6	4.8	3
Segment Reconciling Items [Member]
Revenues from external sources:
Revenue	(1.6)	(1.6)	(4.8)	(3)
Segment income before federal income tax:
Corporate Expenses	(0.9)	(0.7)	(5.8)	(2.6)
Income before federal income taxes	(1.3)	(5)	(12.9)	(13.3)
Segment Reconciling Items [Member] | Segment Reconciling Items [Member]
Segment income before federal income tax:
Gaap Expense Adjustments	0.8	(2.8)	(2.6)	(6.3)
Interest expense on corporate debt	1.2	1.5	4.5	4.4
Corporate And Reconciling Items [Member]
Revenues from external sources:
Revenue	$ 347.6	$ 360	$ 1,016.2	$ 1,062.2

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)Segment	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Segment Reporting Information [Line Items]
Acquisition And Operating Expenses	$ 110.9	$ 112.7	$ 333	$ 334.1
Number of reportable segments (segments) | Segment			4
Investable assets	$ 2,697.5		$ 2,697.5		$ 2,781.2

Uncategorized Items - stfc93020

Label	Element	Value
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax	$ (38,200,000)
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax	$ 18,100,000