The Hanover Insurance Group, Inc. (THG) 10-Q 31 Oct 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 30, 2019
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Registrant Name	HANOVER INSURANCE GROUP, INC.
Entity Central Index Key	0000944695
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		39,391,552
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	1-13754
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	04-3263626
Entity Address, Address Line One	440 Lincoln Street
Entity Address, City or Town	Worcester
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01653
City Area Code	508
Local Phone Number	855-1000
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Common Stock, $.01 Par Value [Member]
Document Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 par value
Trading Symbols	THG
Name of each exchange on which registered	NYSE
7 5/8% Senior Debentures Due 2025 [Member]
Document Information [Line Items]
Title of 12(b) Security	7 5/8% Senior Debentures due 2025
Trading Symbols	THG
Name of each exchange on which registered	NYSE
6.35% Subordinated Debentures Due 2053 [Member]
Document Information [Line Items]
Title of 12(b) Security	6.35% Subordinated Debentures due 2053
Trading Symbols	THGA
Name of each exchange on which registered	NYSE

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues
Premiums	$ 1,124.1	$ 1,071.7	$ 3,330.2	$ 3,172.4
Net investment income	68.8	66.4	208.6	198
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	0.6	(0.2)	1	(0.1)
Net change in fair value of equity securities	15	23.6	75.3	6.7
Net other–than–temporary impairment losses on investments recognized in earnings	(0.8)	(0.4)	(1.2)	(2.8)
Total net realized and unrealized investment gains	14.8	23	75.1	3.8
Fees and other income	7	5.5	18.9	17.3
Total revenues	1,214.7	1,166.6	3,632.8	3,391.5
Losses and expenses
Losses and loss adjustment expenses	705.3	676.4	2,122.6	2,018.5
Amortization of deferred acquisition costs	232.8	224.4	692.8	664.7
Interest expense	9.4	11.2	28.1	33.9
Other operating expenses	136.9	131.9	400.6	393
Total losses and expenses	1,084.4	1,043.9	3,244.1	3,110.1
Income from continuing operations before income taxes	130.3	122.7	388.7	281.4
Income tax expense:
Current	22.2	16.2	54.8	27.1
Deferred	1.3	2.5	13.9	17.4
Total income tax expense	19.1	18.7	69.9	44.5
Income from continuing operations	111.2	104	318.8	236.9
Discontinued operations (net of taxes):
Net income	$ 118.9	$ 100.4	$ 315.3	$ 267.4
Basic:
Income from continuing operations	$ 2.81	$ 2.45	$ 7.91	$ 5.57
Net income per share	$ 3	$ 2.36	$ 7.82	$ 6.29
Weighted average shares outstanding	39.6	42.5	40.3	42.5
Diluted:
Income from continuing operations	$ 2.77	$ 2.41	$ 7.80	$ 5.50
Net income per share	$ 2.96	$ 2.33	$ 7.71	$ 6.21
Weighted average shares outstanding	40.2	43.1	40.9	43.1
Chaucer [Member]
Revenues
Premiums		$ 212.4		$ 648.2
Net investment income		12.8		41.7
Net realized and unrealized investment gains (losses):
Total revenues	$ 1.7	227.1	$ 6.2	695.3
Losses and expenses
Losses and loss adjustment expenses		136.8		364.3
Amortization of deferred acquisition costs		60.1		196.7
Interest expense		1.5		2.8
Other operating expenses		26.4		85.7
Total losses and expenses		223.3		646.7
Income tax expense:
Effect of new tax regulations on business gain on sale	(4.4)		1.2
Discontinued operations (net of taxes):
Sale of business	6.4		(2.6)
Income (loss) from discontinued operations	$ 2.1	$ (3.6)	$ 1.6	$ 30.5
Basic:
Sale of business	$ 0.16		$ (0.06)
Income (loss) from discontinued operations	0.05	$ (0.09)	0.03	$ 0.72
Diluted:
Sale of business	0.16		(0.06)
Income (loss) from discontinued operations	$ 0.05	$ (0.08)	$ 0.03	$ 0.71
Life [Member]
Discontinued operations (net of taxes):
Income (loss) from discontinued operations	$ (0.8)		$ (2.5)
Basic:
Income (loss) from discontinued operations	$ (0.02)		$ (0.06)
Diluted:
Income (loss) from discontinued operations	$ (0.02)		$ (0.06)

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 118.9	$ 100.4	$ 315.3	$ 267.4
Available-for-sale securities:
Net appreciation (depreciation) during the period	42.3	(25)	258.4	(197.9)
Change in other-than-temporary impairment losses recognized in other comprehensive income	0.7	(0.2)	2.6	0.1
Total available-for-sale securities	43	(25.2)	261	(197.8)
Pension and postretirement benefits:
Net change in net actuarial loss and prior service cost	2.3	1.9	6.8	0.5
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period		1.6	0.7	0.6
Total other comprehensive income (loss), net of tax	45.3	(21.7)	268.5	(196.7)
Comprehensive income	$ 164.2	$ 78.7	$ 583.8	$ 70.7

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Assets
Fixed maturities, at fair value (amortized cost of $6,692.5 and $6,245.9)	$ 6,946,300,000	$ 6,161,500,000
Equity securities, at fair value	570,800,000	464,400,000
Other investments	722,500,000	661,500,000
Total investments	8,239,600,000	7,287,400,000
Cash and cash equivalents	157,600,000	1,020,700,000
Accrued investment income	53,000,000	53,200,000
Premiums and accounts receivable, net	1,298,800,000	1,176,700,000
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,734,200,000	1,648,600,000
Deferred acquisition costs	474,900,000	450,800,000
Deferred income tax asset		50,600,000
Goodwill	178,800,000	178,800,000
Other assets	384,300,000	371,600,000
Assets held-for-sale and discontinued businesses	106,000,000	103,900,000
Total assets	12,627,200,000	12,399,700,000
Liabilities
Loss and loss adjustment expense reserves	5,550,500,000	5,304,100,000
Unearned premiums	2,456,800,000	2,277,800,000
Expenses and taxes payable	649,000,000	909,800,000
Deferred income tax liability	46,300,000
Reinsurance premiums payable	52,800,000	37,300,000
Debt	667,800,000	777,900,000
Liabilities held-for-sale and discontinued businesses	117,200,000	115,900,000
Total liabilities	9,540,400,000	9,445,000,000
Commitments and contingencies
Shareholders’ Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued	600,000	600,000
Additional paid-in capital	1,837,000,000	1,871,800,000
Accumulated other comprehensive income (loss)	153,500,000	(116,500,000)
Retained earnings	2,423,100,000	2,182,300,000
Treasury stock at cost (20.9 million and 18.2 million shares)	(1,327,400,000)	(983,500,000)
Total shareholders’ equity	3,086,800,000	2,954,700,000
Total liabilities and shareholders’ equity	12,627,200,000	12,399,700,000
Assets Held-for-Sale [Member]
Assets
Assets held-for-sale and discontinued businesses	0	57,400,000
Liabilities
Liabilities held-for-sale and discontinued businesses	$ 0	$ 22,200,000

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Statement Of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 6,692.5	$ 6,245.9
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	20,900,000	18,200,000

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Cumulative Foreign Currency Translation Adjustment [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Balance at beginning of period at Dec. 31, 2017		$ 0.6	$ 1,857	$ 205.4	$ (79.5)	$ (18.3)	$ 1,975
Balance at beginning of period at Dec. 31, 2017								$ (942.5)
Cumulative effect of accounting change, net of taxes at Dec. 31, 2017							104.3
Balance at beginning of period, as adjusted at Dec. 31, 2017							2,079.3
Net income	$ 267.4						267.4
Shares purchased at cost								(44.2)
Prepaid repurchase of common stock and other			11.9
Net appreciation (depreciation) on available-for-sale securities	(197.8)			(197.8)
Net amount recognized as net periodic benefit	0.5
Amount recognized as cumulative foreign currency translation during the period	0.6					0.6
Adoption of Accounting Standards Update | Accounting Standards Update No. 2017-08 and No. 2016-01 and No. 2018-02 [Member]				(81.6)
Adoption of Accounting Standards Update | Accounting Standards Update No. 2018-02 [Member]					(16.2)	(3.9)
Dividends to shareholders							(69.1)
Net shares reissued at cost under employee stock-based compensation plans								12.8
Balance at end of period at Sep. 30, 2018	2,982.4	0.6	1,868.9	(74)	(95.2)	(21.6)	2,277.6		$ (190.8)
Balance at end of period at Sep. 30, 2018								(973.9)
Balance at beginning of period at Jun. 30, 2018		0.6	1,864.2	(48.8)	(97.1)	(23.2)	2,200.4
Balance at beginning of period at Jun. 30, 2018								(956.3)
Balance at beginning of period, as adjusted at Jun. 30, 2018							2,200.4
Net income	100.4						100.4
Shares purchased at cost								(18.9)
Prepaid repurchase of common stock and other			4.7
Net appreciation (depreciation) on available-for-sale securities	(25.2)			(25.2)
Net amount recognized as net periodic benefit	1.9
Amount recognized as cumulative foreign currency translation during the period	1.6					1.6
Dividends to shareholders							(23.2)
Net shares reissued at cost under employee stock-based compensation plans								1.3
Balance at end of period at Sep. 30, 2018	2,982.4	0.6	1,868.9	(74)	(95.2)	(21.6)	2,277.6		(190.8)
Balance at end of period at Sep. 30, 2018								(973.9)
Balance at beginning of period at Dec. 31, 2018	2,954.7	0.6	1,871.8	(27.2)	(88.6)	(0.7)	2,182.3
Balance at beginning of period at Dec. 31, 2018	(983.5)							(983.5)
Cumulative effect of accounting change, net of taxes at Dec. 31, 2018							(1.5)
Balance at beginning of period, as adjusted at Dec. 31, 2018							2,180.8
Net income	315.3						315.3
Shares purchased at cost								(355.1)
Prepaid repurchase of common stock and other			(34.8)
Net appreciation (depreciation) on available-for-sale securities	261			261
Net amount recognized as net periodic benefit	6.8
Amount recognized as cumulative foreign currency translation during the period	0.7					$ 0.7
Adoption of Accounting Standards Update | Accounting Standards Update No. 2017-08 and No. 2016-01 and No. 2018-02 [Member]				1.5
Dividends to shareholders							(73)
Net shares reissued at cost under employee stock-based compensation plans								11.2
Balance at end of period at Sep. 30, 2019	3,086.8	0.6	1,837	235.3	(81.8)		2,423.1		153.5
Balance at end of period at Sep. 30, 2019	(1,327.4)							(1,327.4)
Balance at beginning of period at Jun. 30, 2019		0.6	1,833.1	192.3	(84.1)		2,328.1
Balance at beginning of period at Jun. 30, 2019								(1,328.9)
Balance at beginning of period, as adjusted at Jun. 30, 2019							2,328.1
Net income	118.9						118.9
Prepaid repurchase of common stock and other			3.9
Net appreciation (depreciation) on available-for-sale securities	43			43
Net amount recognized as net periodic benefit	2.3
Dividends to shareholders							(23.9)
Net shares reissued at cost under employee stock-based compensation plans								1.5
Balance at end of period at Sep. 30, 2019	3,086.8	$ 0.6	$ 1,837	$ 235.3	$ (81.8)		$ 2,423.1		$ 153.5
Balance at end of period at Sep. 30, 2019	$ (1,327.4)							$ (1,327.4)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash Flows From Operating Activities
Net income	$ 315.3	$ 267.4
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized and unrealized investment gains	(74.9)	(3.8)
Sale of Chaucer business	2.6
Net amortization and depreciation	16.8	21
Stock-based compensation expense	12.7	12.1
Amortization of defined benefit plan costs	8.5	7.3
Deferred income tax expense	13.9	5.8
Change in deferred acquisition costs	(22.1)	(28)
Change in premiums receivable, net of reinsurance premiums payable	(117.7)	(134.2)
Change in loss, loss adjustment expense and unearned premium reserves	436.4	473.7
Change in reinsurance recoverable	(99.1)	(88.6)
Change in expenses and taxes payable	(55.1)	(49.7)
Other, net	(13.4)	(16.6)
Net cash provided by operating activities	423.9	466.4
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	850.1	871
Proceeds from disposals of equity securities and other investments	65.8	77.3
Purchase of fixed maturities	(1,281.9)	(1,273.6)
Purchase of equity securities and other investments	(163.7)	(127.8)
Capital expenditures	(10.2)	(12.4)
Net cash proceeds from sale of Chaucer-related Irish and Australian entities, partially offset by cash transferred	34.7
Net cash used in investing activities	(505.2)	(465.5)
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	12.4	12.7
Proceeds from debt borrowings, net	14.5
Change in cash collateral related to securities lending program	11.1	(8.3)
Dividends paid to shareholders	(265.3)	(68.9)
Repurchases of common stock	(400)	(44.2)
Repayment of debt	(151.1)	(11.6)
Other financing activities	(6.7)	(3.2)
Net cash used in financing activities	(785.1)	(123.5)
Effect of exchange rate changes on cash		(2.5)
Net change in cash and cash equivalents	(866.4)	(125.1)
Net change in cash related to discontinued operations	3.3	(54.9)
Cash and cash equivalents, beginning of period	1,020.7	297.9
Cash and cash equivalents, end of period	$ 157.6	$ 117.9

Basis of Presentation and Princ

Basis of Presentation and Principles of Consolidation	9 Months Ended
	Sep. 30, 2019
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	1. Basis of Presentation and Principles of Consolidation The accompanying unaudited consolidated financial statements of The Hanover Insurance Group, Inc. and subsidiaries (“THG” or the “Company”) have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and with the requirements of Form 10-Q. Certain financial information that is provided in annual financial statements, but is not required in interim reports, has been omitted. The interim consolidated financial statements of THG include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America, THG’s principal property and casualty companies; and other insurance and non-insurance subsidiaries. These legal entities conduct their operations through several business segments discussed in Note 10 – “Segment Information”. The interim consolidated financial statements also include the Company’s discontinued operations consisting of Chaucer Holdings Limited (“Chaucer”), a United Kingdom (“U.K.”) domiciled specialist insurance underwriting group, which operates through the Society and Corporation of Lloyd’s (“Lloyd’s”), and the international insurance and non-insurance subsidiaries, which collectively constituted the former Chaucer segment. On December 28, 2018, the Company completed the sale of Chaucer to China Reinsurance (Group) Corporation (“China Re”), and subsequently completed the sales of the Chaucer-related Irish and Australian entities on February 14, 2019 and April 10, 2019, respectively. Discontinued operations also include the Company’s accident and health and former life insurance businesses. All intercompany accounts and transactions have been eliminated. The preparation of financial statements in conformity with U.S. GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amount of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of the Company’s management, the accompanying interim consolidated financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. The results of operations for the three and nine months ended September 30, 2019 are not necessarily indicative of the results to be expected for the full year. These financial statements should be read in conjunction with the Company’s Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on February 22, 2019.

New Accounting Pronouncements

New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2019
Accounting Changes And Error Corrections [Abstract]
New Accounting Pronouncements	2. New Accounting Pronouncements Recently Implemented Standards In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) Update No. 2016-02, (Topic 842) Leases Leases Targeted Improvements In March 2019, the FASB issued ASC Update No. 2019-01, Leases (Topic 842) – Codification Improvements Accounting Changes and Error Corrections In March 2017, the FASB issued ASC Update No. 2017-08, (Subtopic 310-20) Receivables – Nonrefundable Fees and Other Costs: Premium Amortization on Purchased Callable Debt Securities . This guidance shortens the amortization period of premiums on certain purchased callable debt securities to the earliest call date. The updated guidance is effective for annual and interim periods beginning after December 15, 2018, and should be applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company implemented this guidance effective January 1, 2019. The effect of implementing this guidance resulted in a cumulative effect adjustment reclassifying unrealized losses, net of tax, of $ 1.5 million from accumulated other comprehensive income to retained earnings. Recently Issued Standards In August 2018, the FASB issued ASC Update No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASC Update No. 2018-14 (Topic 715-20) Compensation – Retirement Benefits – Defined Benefit Plans – General – Disclosure Framework – Changes to the Disclosure Requirements for the Defined Benefit Plans. In August 2018, the FASB issued ASC Update No. 2018-13 (Topic 820) Fair Value Measurement, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In January 2017, the FASB issued ASC Update No. 2017-04, (Topic 350) Intangibles – Goodwill and Other: Simplifying the Test for Goodwill Impairment In June 2016, the FASB issued ASC Update No. 2016-13, (Topic 326) Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments . This ASC update introduces new guidance for the accounting for credit losses on financial instruments within its scope. A new model, referred to as the current expected credit losses model, requires an entity to determine credit-related impairment losses for financial instruments held at amortized cost and to estimate these expected credit losses over the life of an exposure (or pool of exposures). The estimate of expected credit losses should consider both historical and current information, reasonable and supportable forecasts, as well as estimates of prepayments. The estimated credit losses and subsequent adjustment to such loss estimates, will be recorded through an allowance account which is deducted from the amortized cost of the financial instrument, with the offset recorded in current earnings. ASC No. 2016-13 also modifies the impairment model for available-for-sale debt securities. The new model will require an estimate of expected credit losses only when the fair value is below the amortized cost of the asset, thus the length of time the fair value of an available-for-sale debt security has been below the amortized cost will no longer affect the determination of whether a credit loss exists. In addition, credit losses on available-for-sale debt securities will be limited to the difference between the security’s amortized cost basis and its fair value. In November 2018, the FASB issued ASC Update No. 2018-19, Codification Improvements to Topic 326, Financial Instruments – Credit Losses , which explicitly states that receivables arising from operating leases are not within the scope of Subtopic 326-20. ASC Update 2016-13 and related guidance is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted for periods beginning after December 15, 2018. The Company does not expect t he adopt ion of ASC Update No. 2016-13 to have a material impact on its financial position or results of operations .

Discontinued Operations Chaucer

Discontinued Operations Chaucer Business	9 Months Ended
	Sep. 30, 2019
Chaucer [Member]
Discontinued Operations Chaucer Business	3. Discontinued Operations – Chaucer Business Sale of Chaucer Business On December 28, 2018, the Company completed the sale of the U.K. entities of its former subsidiary, Chaucer, to China Re. On February 14, 2019, the Company completed the sale of its Chaucer-related Irish entity and on April 10, 2019, finalized the sale of the Australian entities, completing the transfer of all Chaucer-related companies. Collectively, these entities constituted the Chaucer business in 2018 and prior, whereas just the Irish and Australian Chaucer-related entities comprised the Chaucer business reported in the Company’s results in 2019, until their respective sales to China Re. In December 2018, the Company recognized a pre-tax gain on the sale to China Re of $174.4 million and an income tax expense of $42.5 million. As discussed below, both the pre-tax gain and income tax expense were updated in 2019, including in the third quarter. Included in the previously recorded $174.4 million gain was $31.7 million of contingent proceeds, which were subject to change, based on development of Chaucer’s 2018 catastrophe losses. During the first half of 2019, Chaucer experienced unfavorable development on its 2018 catastrophe losses, primarily due to higher than expected losses for Hurricane Michael, Typhoon Jebi, and a Colombian dam construction loss. Accordingly, the Company updated and reduced its best estimate of pre-tax contingent proceeds by $13.5 million during the second quarter of 2019. In the third quarter of 2019, the Company received payment of $22.0 million in final settlement of the contingent proceeds, an increase of $3.8 million from the estimate in the second quarter. In addition, the Company recognized an income tax charge of $1.2 million in 2019 related to new tax regulations that were issued on June 14, 2019 by the U.S. Department of the Treasury, with an effective date retroactive to January 1, 2018. These new regulations retroactively changed the taxation of certain non-U.S. income. Although the impact of these regulations relates to the calculation of the income tax expense related to the sale of Chaucer, unlike the $42.5 million of income tax expense, noted above, that was reflected in discontinued operations, ASC 740, Income Taxes During 2019, the sale of the Irish entity provided total proceeds of $28 million and resulted in a pre-tax gain of $0.4 million, with a related income tax benefit of $0.5 million. The sale of the Australian entities for total proceeds of $13 million resulted in a pre-tax gain of $1.2 million and a related income tax expense of $0.1 million. Income from Discontinued Chaucer Business Three and nine months ended September 30, 2019 Revenues from the portion of the Chaucer business remaining after the sale of the U.K. entities in 2018 were $1.7 million and $6.2 million for the three and nine months ended September 30, 2019, respectively, and operating income in this business totaled $0.1 million for the three months ended September 30, 2019, and operating losses totaled $0.4 million for the nine months ended September 30, 2019. Income from the Chaucer business in both the three and nine months ended September 30, 2019 includes a $2.0 million benefit related to a decrease in uncertain tax positions due to the expiration of the statute of limitations. Three and nine months ended September 30, 2018 The following table summarizes the results of Chaucer’s operations for the three and nine months ended September 30, 2018: Three Months Ended Nine Months Ended (in millions) September 30, 2018 (1) Revenues Net premiums earned $ 212.4 $ 648.2 Net investment income 12.8 41.7 Other income 1.9 5.4 227.1 695.3 Losses and operating expenses Losses and LAE 136.8 364.3 Amortization of deferred acquisition costs 60.1 196.7 Other expenses 26.4 85.7 223.3 646.7 Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income) 3.8 48.6 Other items: Interest expense (1.5 ) (2.8 ) Net realized and unrealized investment gains (losses) 0.1 (0.4 ) Other expenses (2) (7.1 ) (8.3 ) Income (loss) from Chaucer business before income taxes (4.7 ) 37.1 Income tax benefit (expense) 1.1 (6.6 ) Income (loss) from Chaucer business, net of income taxes $ (3.6 ) $ 30.5 (1) 2019 information is omitted as it is not material. (2) Other expenses primarily reflects those costs associated with the then pending sale of Chaucer. There were no assets or liabilities held-for-sale at September 30, 2019. Total assets and liabilities held-for-sale were $57.4 million and $22.2 million, respectively, at December 31, 2018. Assets and liabilities held-for-sale decreased during the nine months ended September 30, 2019 due to the aforementioned sales of the Chaucer-related Irish and Australian entities. These balances are reflected in the Consolidated Balance Sheets under the captions “Assets held-for-sale” and “Liabilities held-for-sale”, respectively. The following table details the cash flows associated with the Chaucer business for the nine months ended September 30, 2018: Nine Months Ended September 30, (in millions) 2018 Net cash provided by operating activities $ 46.2 Net cash provided by investing activities $ 64.3

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	4. Income Taxes Income tax expense for the nine months ended September 30, 2019 and 2018 has been computed using estimated annual effective tax rates. The prior year’s rate reflects the recomputed income to exclude the Chaucer business. These rates are revised, if necessary, at the end of each successive interim period to reflect current estimates of the annual effective tax rates. The tax provision was comprised of U.S. federal income tax expense of $69.9 million and $44.5 million for the nine months ended September 30, 2019 and 2018, respectively. The tax provision recorded in discontinued operations related to the Chaucer business for the nine months ended September 30, 2018 was comprised of a $6.8 million foreign income tax expense and a $0.2 million U.S. federal income tax benefit. As noted above in Note 3 – “Discontinued Operations – Chaucer Business”, on June 14, 2019, the U.S. Department of the Treasury issued regulations that change the taxation of certain non-U.S. income. These regulations apply retroactively to January 1, 2018. As a result, the Company incurred additional federal income tax of $1.2 million from the 2018 sale of Chaucer. In accordance with ASC 740, the Company has recorded a provision for this amount as a component of income tax expense in continuing operations in its Consolidated Statements of Income for the nine months ended September 30, 2019. During the first nine months of 2019, we recorded a decrease in our uncertain tax positions of $2.0 million due to the expiration of a statute of limitations. The release of uncertain tax position was recorded as a benefit in Chaucer discontinued business. The Company or its subsidiaries files income tax returns in the U.S. federal jurisdiction and various state jurisdictions, as well as foreign jurisdictions. The Company and its subsidiaries are subject to U.S. federal and state income tax examinations and foreign examinations for years after 2015.

Debt

Debt	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Debt	5. Debt Debt consists of the following: (in millions) September 30, 2019 December 31, 2018 Senior debentures maturing April 15, 2026 $ 375.0 $ 375.0 Senior debentures maturing October 15, 2025 62.6 62.6 Subordinated debentures maturing March 30, 2053 175.0 175.0 Subordinated debentures maturing February 3, 2027 50.1 50.1 FHLB borrowings (secured) — 125.0 Short-term borrowings 14.5 — Total principal debt 677.2 787.7 Unamortized debt issuance costs (9.4 ) (9.8 ) Total $ 667.8 $ 777.9 On January 2, 2019, the Company repaid $125 million of its FHLB advances that were due 2029 and had an interest rate of 5.5%, along with related prepayment fees of $26 million; such fees were recognized in 2018. The Company had $14.5 million of short-term advances from the Federal Home Loan Bank (“FHLB”) outstanding at September 30, 2019, all of which were repaid in October 2019. At September 30, 2019, the Company was in compliance with the covenants associated with its debt indentures and credit arrangements.

Investments

Investments	9 Months Ended
	Sep. 30, 2019
Investments Debt And Equity Securities [Abstract]
Investments	6. Investments A. Fixed maturities The amortized cost and fair value of available-for-sale fixed maturities were as follows: September 30, 2019 Gross Gross OTTI Amortized Unrealized Unrealized Unrealized (in millions) Cost Gains Losses Fair Value Losses U.S. Treasury and government agencies $ 350.2 $ 12.0 $ 0.4 $ 361.8 $ — Foreign government 5.0 0.1 — 5.1 — Municipal 848.3 34.4 0.4 882.3 — Corporate 3,758.4 159.1 5.3 3,912.2 3.3 Residential mortgage-backed 959.8 20.3 0.9 979.2 — Commercial mortgage-backed 685.2 33.4 — 718.6 — Asset-backed 85.6 1.5 — 87.1 — Total fixed maturities $ 6,692.5 $ 260.8 $ 7.0 $ 6,946.3 $ 3.3 December 31, 2018 Gross Gross OTTI Amortized Unrealized Unrealized Unrealized (in millions) Cost Gains Losses Fair Value Losses U.S. Treasury and government agencies $ 414.7 $ 2.4 $ 7.2 $ 409.9 $ — Foreign government 7.3 0.1 — 7.4 — Municipal 879.0 16.6 9.8 885.8 — Corporate 3,476.6 26.1 92.0 3,410.7 6.6 Residential mortgage-backed 728.4 2.7 14.7 716.4 — Commercial mortgage-backed 648.4 1.7 9.8 640.3 — Asset-backed 91.5 0.2 0.7 91.0 — Fixed maturities, excluding held-for-sale (Chaucer) 6,245.9 49.8 134.2 6,161.5 6.6 Fixed maturities, held-for-sale 24.9 — 0.4 24.5 — Total fixed maturities $ 6,270.8 $ 49.8 $ 134.6 $ 6,186.0 $ 6.6 Other-than-temporary impairments (“OTTI”) unrealized losses in the tables above represent OTTI recognized in accumulated other comprehensive income (“AOCI”). This amount excludes net unrealized gains on impaired securities relating to changes in the value of such securities subsequent to the impairment measurement date of $4.5 million and $7.4 million as of September 30, 2019 and December 31, 2018, respectively. The Company deposits funds with various state and governmental authorities. For a discussion of the Company’s deposits with state and governmental authorities, see also Note 3 – “Investments” of the Notes to Consolidated Financial Statements in the Company’s 2018 Annual Report on Form 10-K. The amortized cost and fair value by maturity periods for fixed maturities are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers. September 30, 2019 Amortized Fair (in millions) Cost Value Due in one year or less $ 266.2 $ 268.7 Due after one year through five years 1,944.8 2,009.9 Due after five years through ten years 2,358.3 2,473.7 Due after ten years 392.6 409.1 4,961.9 5,161.4 Mortgage-backed and asset-backed securities 1,730.6 1,784.9 Total fixed maturities $ 6,692.5 $ 6,946.3 B. Fixed maturity securities in an unrealized loss position The following tables provide information about the Company’s available-for-sale fixed maturity securities that were in an unrealized loss position at September 30, 2019 and December 31, 2018 including the length of time the securities have been in an unrealized loss position: September 30, 2019 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 0.3 $ 37.1 $ 0.1 $ 17.2 $ 0.4 $ 54.3 Municipal 0.2 29.3 0.2 15.5 0.4 44.8 Corporate 0.9 79.8 0.4 24.0 1.3 103.8 Residential mortgage-backed 0.4 76.8 0.5 58.4 0.9 135.2 Commercial mortgage-backed — 1.8 — 25.0 — 26.8 Asset-backed — 0.5 — 3.3 — 3.8 Total investment grade 1.8 225.3 1.2 143.4 3.0 368.7 Below investment grade: Corporate 3.2 40.8 0.8 9.0 4.0 49.8 Total fixed maturities $ 5.0 $ 266.1 $ 2.0 $ 152.4 $ 7.0 $ 418.5 December 31, 2018 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 1.1 $ 66.0 $ 6.1 $ 210.9 $ 7.2 $ 276.9 Foreign governments — 2.0 — 0.8 — 2.8 Municipal 0.8 110.0 9.0 248.0 9.8 358.0 Corporate 30.0 1,277.9 43.9 781.6 73.9 2,059.5 Residential mortgage-backed 2.6 201.2 12.1 323.7 14.7 524.9 Commercial mortgage-backed 3.4 293.0 6.4 175.5 9.8 468.5 Asset-backed 0.4 42.3 0.3 18.0 0.7 60.3 Total investment grade 38.3 1,992.4 77.8 1,758.5 116.1 3,750.9 Below investment grade: Municipal — — — 0.9 — 0.9 Corporate 8.1 185.6 10.0 54.0 18.1 239.6 Total below investment grade 8.1 185.6 10.0 54.9 18.1 240.5 Fixed maturities, excluding held-for-sale (Chaucer) 46.4 2,178.0 87.8 1,813.4 134.2 3,991.4 Fixed maturities, held-for-sale 0.1 4.0 0.3 18.0 0.4 22.0 Total fixed maturities $ 46.5 $ 2,182.0 $ 88.1 $ 1,831.4 $ 134.6 $ 4,013.4 The Company views gross unrealized losses on fixed maturities as being temporary since it is its assessment that these securities will recover in the near term, allowing the Company to realize the anticipated long-term economic value. The Company employs a systematic methodology to evaluate declines in fair value below amortized cost for fixed maturity securities. In determining OTTI, the Company evaluates several factors and circumstances, including the issuer’s overall financial condition; the issuer’s credit and financial strength ratings; the issuer’s financial performance, including earnings trends and asset quality; any specific events which may influence the operations of the issuer; the general outlook for market conditions in the industry or geographic region in which the issuer operates; and the length of time and the degree to which the fair value of an issuer’s securities remains below the Company’s amortized cost. The Company also considers any factors that might raise doubt about the issuer’s ability to make contractual payments as they come due and whether the Company expects to recover the entire amortized cost basis of the security. C. Proceeds from sales The proceeds from sales of available-for-sale securities and gross realized gains and gross realized losses on those sales were as follows: Three Months Ended September 30, 2019 2018 Proceeds from Gross Gross Proceeds from Gross Gross (in millions) Sales Gains Losses Sales Gains Losses Fixed maturities, excluding held-for-sale (Chaucer) $ 86.9 $ 1.6 $ 2.4 $ 83.8 $ 0.8 $ 1.5 Fixed maturities, held-for-sale — — — 21.4 0.2 0.1 Total fixed maturities $ 86.9 $ 1.6 $ 2.4 $ 105.2 $ 1.0 $ 1.6 Nine Months Ended September 30, 2019 2018 Proceeds from Gross Gross Proceeds from Gross Gross (in millions) Sales Gains Losses Sales Gains Losses Fixed maturities, excluding held-for-sale (Chaucer) $ 309.5 $ 4.2 $ 5.0 $ 225.1 $ 2.0 $ 5.3 Fixed maturities, held-for-sale 0.3 — — 172.5 1.3 1.1 Total fixed maturities $ 309.8 $ 4.2 $ 5.0 $ 397.6 $ 3.3 $ 6.4 D. Other-than-temporary impairments For the three months ended September 30, 2019, total OTTI on fixed maturities was $2.1 million, of which $0.8 million was recognized in earnings and the remaining $1.3 million was recorded as unrealized losses in AOCI. For the nine months ended September 30, 2019, total OTTI on fixed maturities was $2.8 million, of which $1.2 million was recognized in earnings and the remaining $1.6 million was recorded as unrealized losses in AOCI. For the three months ended September 30, 2018, total OTTI from continuing operations was $0.7 million, which consisted entirely of fixed maturities. Of this amount, $0.4 million was recognized in earnings and the remaining $0.3 million was recorded as unrealized losses in AOCI. The methodology and significant inputs used to measure the amount of credit losses on fixed maturities in 2019 and 2018 were as follows: Corporate bonds – the Company utilized a financial model that derives expected cash flows based on probability-of-default factors by credit rating and asset duration and loss-given-default factors based on security type. These factors are based on historical data provided by an independent third-party rating agency. In addition, other market data relevant to the realizability of contractual cash flows may be considered. The following table provides rollforwards of the cumulative amounts related to the Company’s credit loss portion of the OTTI losses on fixed maturity securities from continuing operations for which the non-credit portion of the loss is included in other comprehensive income. Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2019 2018 2019 2018 Credit losses as of the beginning of the period $ 2.9 $ 3.7 $ 3.8 $ 3.6 Credit losses on securities for which an OTTI was not previously recognized 0.6 0.2 1.0 1.0 Additional credit losses on securities for which an OTTI was previously recognized — — — 0.1 Reductions for securities sold, matured or called (1.3 ) — (2.6 ) (0.8 ) Reductions for securities reclassified as intended to sell (0.3 ) — (0.3 ) — Credit losses as of the end of the period $ 1.9 $ 3.9 $ 1.9 $ 3.9 E. Equity securities Equity securities are carried at fair value and all increases or decreases in fair value are reported in net realized and unrealized investment gains (losses) on the Consolidated Statements of Income. The following table provides pre-tax net realized and unrealized gains (losses) on equity securities from continuing operations: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2019 2018 2019 2018 Net gains recognized during the period $ 15.0 $ 23.6 $ 75.3 $ 6.7 Less: net gains (losses) recognized on equity securities sold during the period (0.2 ) (0.2 ) 1.0 (1.0 ) Net unrealized gains recognized during the period on equity securities still held $ 15.2 $ 23.8 $ 74.3 $ 7.7

Fair Value

Fair Value	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value	7. Fair Value Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability, i.e., exit price, in an orderly transaction between market participants. The Company emphasizes the use of observable market data whenever available in determining fair value. Fair values presented for certain financial instruments are estimates which, in many cases, may differ significantly from the amounts that could be realized upon immediate liquidation. A hierarchy of the three broad levels of fair value are as follows, with the highest priority given to Level 1 as these are the most observable, and the lowest priority given to Level 3: Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities. Level 2 – Quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data, including model-derived valuations. Level 3 – Unobservable inputs that are supported by little or no market activity. When more than one level of input is used to determine fair value, the financial instrument is classified as Level 2 or 3 according to the lowest level input that has a significant impact on the fair value measurement. The following methods and assumptions were used to estimate the fair value of each class of financial instruments and have not changed since last year. Fixed Maturities Level 1 securities generally include U.S. Treasury issues and other securities that are highly liquid and for which quoted market prices are available. Level 2 securities are valued using pricing for similar securities and pricing models that incorporate observable inputs including, but not limited to yield curves and issuer spreads. Level 3 securities include issues for which little observable data can be obtained, primarily due to the illiquid nature of the securities, and for which significant inputs used to determine fair value are based on the Company’s own assumptions. The Company utilizes a third party pricing service for the valuation of the majority of its fixed maturity securities and receives one quote per security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. Since fixed maturities other than U.S. Treasury securities generally do not trade on a daily basis, the pricing service prepares estimates of fair value for those securities using pricing techniques based on a market approach. Inputs into the fair value pricing common to all asset classes include: benchmark U.S. Treasury security yield curves; reported trades of identical or similar fixed maturity securities; broker/dealer quotes of identical or similar fixed maturity securities and structural characteristics such as maturity date, coupon, mandatory principal payment dates, frequency of interest and principal payments, and optional redemption features. Inputs into the fair value applications that are unique by asset class include, but are not limited to: • U.S. government agencies – determination of direct versus indirect government support and whether any contingencies exist with respect to the timely payment of principal and interest. • Foreign government – estimates of appropriate market spread versus underlying related sovereign treasury curve(s) dependent on liquidity and direct or contingent support. • Municipals – overall credit quality, including assessments of the level and variability of: sources of payment such as income, sales or property taxes, levies or user fees; credit support such as insurance; state or local economic and political base; natural resource availability; and susceptibility to natural or man-made catastrophic events such as hurricanes, earthquakes or acts of terrorism. • Corporate fixed maturities – overall credit quality, including assessments of the level and variability of: economic sensitivity; liquidity; corporate financial policies; management quality; regulatory environment; competitive position; ownership; restrictive covenants; and security or collateral. • Residential mortgage-backed securities – estimates of prepayment speeds based upon: historical prepayment rate trends; underlying collateral interest rates; geographic concentration; vintage year; borrower credit quality characteristics; interest rate and yield curve forecasts; government or monetary authority support programs; tax policies; delinquency/default trends; and, in the case of non-agency collateralized mortgage obligations, severity of loss upon default and length of time to recover proceeds following default. • Commercial mortgage-backed securities – overall credit quality, including assessments of the value and supply/demand characteristics of: collateral type such as office, retail, residential, lodging, or other; geographic concentration by region, state, metropolitan statistical area and locale; vintage year; historical collateral performance including defeasance, delinquency, default and special servicer trends; and capital structure support features. • Asset-backed securities – overall credit quality, including assessments of the underlying collateral type such as credit card receivables, auto loan receivables and equipment lease receivables; geographic diversification; vintage year; historical collateral performance including delinquency, default and casualty trends; economic conditions influencing use rates and resale values; and contract structural support features. Generally, all prices provided by the pricing service, except actively traded securities with quoted market prices, are reported as Level 2. The Company holds privately placed fixed maturity securities and certain other fixed maturity securities that do not have an active market and for which the pricing service cannot provide fair values. The Company determines fair values for these securities using either matrix pricing utilizing the market approach or broker quotes. The Company will use observable market data as inputs into the fair value techniques, as discussed in the determination of Level 2 fair values, to the extent it is available, but is also required to use a certain amount of unobservable judgment due to the illiquid nature of the securities involved. Unobservable judgment reflected in the Company’s matrix model accounts for estimates of additional spread required by market participants for factors such as issue size, structural complexity, high bond coupon or other unique features. These matrix-priced securities are reported as Level 2 or Level 3, depending on the significance of the impact of unobservable judgment on the security’s value. Additionally, the Company may obtain non-binding broker quotes which are reported as Level 3. Equity Securities Level 1 consists of publicly traded securities, including exchange traded funds, valued at quoted market prices. Level 2 includes securities that are valued using pricing for similar securities and pricing models that incorporate observable inputs. Level 3 consists of common or preferred stock of private companies for which observable inputs are not available. The Company utilizes a third party pricing service for the valuation of the majority of its equity securities and receives one quote for each equity security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. The Company holds certain equity securities that have been issued by privately-held entities that do not have an active market and for which the pricing service cannot provide fair values. Generally, the Company estimates fair value for these securities based on the issuer’s book value and market multiples and reports them as Level 3. Additionally, the Company may obtain non-binding broker quotes which are reported as Level 3. Other Investments Other investments primarily include mortgage participations and limited partnerships not subject to the equity method of accounting. The fair values of limited partnerships not subject to the equity method of accounting are based on the net asset value provided by the general partner adjusted for recent financial information and are excluded from the fair value hierarchy. The estimated fair value of the financial instruments were as follows: September 30, 2019 December 31, 2018 Carrying Fair Carrying Fair (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 6,946.3 $ 6,946.3 $ 6,161.5 $ 6,161.5 Fair Value through Net Income: Equity securities 570.8 570.8 464.4 464.4 Other investments 180.0 180.0 175.0 175.0 Amortized Cost/Cost: Other investments 449.3 473.4 414.4 418.9 Cash and cash equivalents 157.6 157.6 1,020.7 1,020.7 Total financial instruments, excluding held-for-sale 8,304.0 8,328.1 8,236.0 8,240.5 Financial instruments, held-for-sale (Chaucer) — — 27.8 27.8 Total financial instruments $ 8,304.0 $ 8,328.1 $ 8,263.8 $ 8,268.3 Financial Liabilities carried at: Amortized Cost: Debt $ 667.8 $ 732.0 $ 777.9 $ 825.0 The Company has processes designed to ensure that the values received from its third party pricing service are accurately recorded, that the data inputs and valuation approaches and techniques utilized are appropriate and consistently applied, and that the assumptions are reasonable and consistent with the objective of determining fair value. The Company performs a review of the fair value hierarchy classifications and of prices received from its pricing service on a quarterly basis. The Company reviews the pricing services’ policies describing its methodology, processes, practices and inputs, including various financial models used to value securities. Also, the Company reviews the portfolio pricing, including a process for which securities with changes in prices that exceed a defined threshold are verified to independent sources, if available. If upon review, the Company is not satisfied with the validity of a given price, a pricing challenge would be submitted to the pricing service along with supporting documentation for its review. The Company does not adjust quotes or prices obtained from the pricing service unless the pricing service agrees with the Company’s challenge. During 2019 and 2018, the Company did not adjust any prices received from its pricing service. Changes in the observability of valuation inputs may result in a reclassification of certain financial assets or liabilities within the fair value hierarchy. As previously discussed, the Company utilizes a third-party pricing service for the valuation of the majority of its fixed maturities and equity securities. The pricing service has indicated that it will only produce an estimate of fair value if there is objectively verifiable information to produce a valuation. If the pricing service discontinues pricing an investment, the Company will use observable market data to the extent it is available, but may also be required to make assumptions for market based inputs that are unavailable due to market conditions. The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis. September 30, 2019 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 361.8 $ 162.0 $ 199.8 $ — Foreign government 5.1 — 5.1 — Municipal 882.3 — 869.7 12.6 Corporate 3,912.2 — 3,911.5 0.7 Residential mortgage-backed, U.S. agency backed 976.6 — 976.6 — Residential mortgage-backed, non-agency 2.6 — 2.6 — Commercial mortgage-backed 718.6 — 705.6 13.0 Asset-backed 87.1 — 87.1 — Total fixed maturities 6,946.3 162.0 6,758.0 26.3 Equity securities 570.8 568.7 — 2.1 Other investments 3.5 — — 3.5 Total investment assets at fair value $ 7,520.6 $ 730.7 $ 6,758.0 $ 31.9 December 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 409.9 $ 154.9 $ 255.0 $ — Foreign government 7.4 — 7.4 — Municipal 885.8 — 864.7 21.1 Corporate 3,410.7 — 3,409.9 0.8 Residential mortgage-backed, U.S. agency backed 713.7 — 713.7 — Residential mortgage-backed, non-agency 2.7 — 2.7 — Commercial mortgage-backed 640.3 — 627.2 13.1 Asset-backed 91.0 — 91.0 — Total fixed maturities 6,161.5 154.9 5,971.6 35.0 Equity securities 464.4 463.3 — 1.1 Other investments 3.5 — — 3.5 Total investment assets at fair value, excluding held-for-sale (Chaucer) 6,629.4 618.2 5,971.6 39.6 Investment assets, held-for-sale 24.5 5.7 18.8 — Total investment assets at fair value $ 6,653.9 $ 623.9 $ 5,990.4 $ 39.6 Limited partnerships measured at fair value using NAV based on an ownership interest in partners’ capital have not been included in the hierarchy tables. At September 30, 2019 and December 31, 2018, the fair values of these investments were $176.5 million and $171.5 million, respectively, approximately 2% of total investment assets. The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value: September 30, 2019 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 157.6 $ 157.6 $ — $ — Other investments 473.4 — 2.6 470.8 Total financial instruments $ 631.0 $ 157.6 $ 2.6 $ 470.8 Liabilities: Debt $ 732.0 $ — $ 732.0 $ — December 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 1,020.7 $ 1,020.7 $ — $ — Other investments 418.9 — 8.7 410.2 Total financial instruments, excluding held-for-sale 1,439.6 1,020.7 8.7 410.2 Financial instruments, held-for-sale (Chaucer) 3.3 3.3 — — Total financial instruments $ 1,442.9 $ 1,024.0 $ 8.7 $ 410.2 Liabilities: Debt $ 825.0 $ — $ 825.0 $ — The tables below provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Three Months Ended September 30, 2019 Balance July 1, 2019 $ 12.9 $ 0.7 $ 13.0 $ 26.6 $ 5.6 $ 32.2 Total gains: Included in other comprehensive income-net appreciation on available-for-sale securities 0.1 — 0.2 0.3 — 0.3 Sales (0.4 ) — (0.2 ) (0.6 ) — (0.6 ) Balance September 30, 2019 $ 12.6 $ 0.7 $ 13.0 $ 26.3 $ 5.6 $ 31.9 Three Months Ended September 30, 2018 Balance July 1, 2018, excluding held-for-sale (Chaucer) $ 22.2 $ 0.8 $ 13.2 $ 36.2 $ 4.7 $ 40.9 Transfers out of Level 3 (0.5 ) — — (0.5 ) — (0.5 ) Total losses: Included in other comprehensive income-net depreciation on available-for-sale securities — — (0.1 ) (0.1 ) — (0.1 ) Sales (0.4 ) (0.1 ) (0.1 ) (0.6 ) — (0.6 ) Balance September 30, 2018, excluding held-for-sale (Chaucer) $ 21.3 $ 0.7 $ 13.0 $ 35.0 $ 4.7 $ 39.7 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Nine Months Ended September 30, 2019 Balance January 1, 2019 $ 21.1 $ 0.8 $ 13.1 $ 35.0 $ 4.6 $ 39.6 Total gains: Included in other comprehensive income - net appreciation on available-for-sale securities 0.9 — 0.7 1.6 — 1.6 Purchases and sales: Purchases — — — — 1.0 1.0 Sales (9.4 ) (0.1 ) (0.8 ) (10.3 ) — (10.3 ) Balance September 30, 2019 $ 12.6 $ 0.7 $ 13.0 $ 26.3 $ 5.6 $ 31.9 Nine Months Ended September 30, 2018 Balance January 1, 2018, excluding held-for-sale (Chaucer) $ 24.6 $ 0.9 $ 14.2 $ 39.7 $ 4.7 $ 44.4 Transfers out of Level 3 (0.5 ) — — (0.5 ) — (0.5 ) Total gains (losses): Included in total net realized and unrealized investment gains 0.1 — — 0.1 — 0.1 Included in other comprehensive income-net depreciation on available-for-sale securities (0.6 ) — (0.5 ) (1.1 ) — (1.1 ) Sales (2.3 ) (0.2 ) (0.7 ) (3.2 ) — (3.2 ) Balance September 30, 2018, excluding held-for-sale (Chaucer) $ 21.3 $ 0.7 $ 13.0 $ 35.0 $ 4.7 $ 39.7 There were no transfers between Level 2 and Level 3, and there were no Level 3 liabilities held by the Company for the three and nine months ended September 30, 2019 and 2018. The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets. Where discounted cash flows were used in the valuation of fixed maturities, the internally-developed discount rate was adjusted by the significant unobservable inputs shown in the table. September 30, 2019 December 31, 2018 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted cash flow Discount for: Small issue size Above-market coupon Credit stress $ 12.6 0.7 - 6.8% (4.3%) 0.3 - 0.5% (0.5%) — $ 21.1 0.7 - 6.8% (3.4%) 0.3 - 0.5% (0.5%) 1.3% (1.3%) Corporate Discounted cash flow Discount for: Small issue size Above-market coupon 0.7 2.5% (2.5%) 0.3% (0.3%) 0.8 2.5% (2.5%) 0.3% (0.3%) Commercial mortgage-backed Discounted cash flow Discount for: Small issue size Above-market coupon Lease structure 13.0 1.9 - 3.1% (2.7%) 0.5% (0.5%) 0.3% (0.3%) 13.1 1.9 - 3.1% (2.7%) 0.5% (0.5%) 0.3% (0.3%) Equity securities Market comparables Net tangible asset market multiples 2.1 1.0X (1.0X) 1.1 1.0X (1.0X) Other Discounted cash flow Discount rate 3.5 18.0% (18.0%) 3.5 18.0% (18.0%) Significant increases (decreases) in any of the above inputs in isolation would result in a significantly lower (higher) fair value measurement. There were no interrelationships between these inputs which might magnify or mitigate the effect of changes in unobservable inputs on the fair value measurement.

Pension and Other Postretiremen

Pension and Other Postretirement Benefit Plans	9 Months Ended
	Sep. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Pension and Other Postretirement Benefit Plans	8. Pension and Other Postretirement Benefit Plans The components of net periodic pension cost for defined benefit pension and other postretirement benefit plans included in the Company’s results of operations are as follows: Three Months Ended September 30, 2019 2018 2019 2018 (in millions) Pension Plans Postretirement Plans Service cost - benefits earned during the period $ — $ — $ — $ — Interest cost 5.3 5.0 0.1 0.1 Expected return on plan assets (5.8 ) (5.2 ) — — Recognized net actuarial loss 2.8 2.4 — — Amortization of prior service cost — — — — Net periodic pension cost $ 2.3 $ 2.2 $ 0.1 $ 0.1 Nine Months Ended September 30, 2019 2018 2019 2018 (in millions) Pension Plans Postretirement Plans Service cost - benefits earned during the period $ — $ — $ — $ — Interest cost 15.9 14.9 0.3 0.3 Expected return on plan assets (17.5 ) (15.5 ) — — Recognized net actuarial loss 8.5 7.2 0.1 0.1 Amortization of prior service cost — — (0.1 ) (0.2 ) Net periodic pension cost $ 6.9 $ 6.6 $ 0.3 $ 0.2

Other Comprehensive Income

Other Comprehensive Income	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Other Comprehensive Income	9. Other Comprehensive Income The following tables provide changes in other comprehensive income (loss). Three Months Ended September 30, 2019 2018 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Unrealized gains (losses) on available-for-sale securities: Unrealized gains (losses) arising during period $ 58.4 $ (12.3 ) $ 46.1 $ (29.2 ) $ 6.2 $ (23.0 ) Amount of realized gains from sales and other (0.6 ) (3.1 ) (3.7 ) (0.2 ) (2.4 ) (2.6 ) Portion of other-than-temporary impairment losses recognized in earnings 0.8 (0.2 ) 0.6 0.5 (0.1 ) 0.4 Net unrealized gains (losses) 58.6 (15.6 ) 43.0 (28.9 ) 3.7 (25.2 ) Pension and postretirement benefits: Net change in net actuarial loss and prior service cost 2.8 (0.5 ) 2.3 2.4 (0.5 ) 1.9 Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period — — — 2.0 (0.4 ) 1.6 Other comprehensive income (loss) $ 61.4 $ (16.1 ) $ 45.3 $ (24.5 ) $ 2.8 $ (21.7 ) Nine Months Ended September 30, 2019 2018 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Unrealized gains (losses) on available-for-sale securities: Unrealized gains (losses) arising during period $ 343.6 $ (72.2 ) $ 271.4 $ (242.1 ) $ 50.9 $ (191.2 ) Amount of realized gains from sales and other (1.1 ) (10.2 ) (11.3 ) — (8.2 ) (8.2 ) Portion of other-than-temporary impairment losses recognized in earnings 1.2 (0.3 ) 0.9 2.0 (0.4 ) 1.6 Net unrealized gains (losses) 343.7 (82.7 ) 261.0 (240.1 ) 42.3 (197.8 ) Pension and postretirement benefits: Net change in net actuarial loss and prior service cost 8.5 (1.7 ) 6.8 0.6 (0.1 ) 0.5 Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period 0.9 (0.2 ) 0.7 0.8 (0.2 ) 0.6 Other comprehensive income (loss) $ 353.1 $ (84.6 ) $ 268.5 $ (238.7 ) $ 42.0 $ (196.7 ) Reclassifications out of accumulated other comprehensive income were as follows: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2019 2018 2019 2018 Amount Reclassified from Details about Accumulated Other Accumulated Other Affected Line Item in the Statement Comprehensive Income Components Comprehensive Income Where Net Income is Presented Unrealized gains on available-for- sale securities $ 0.6 $ (0.2 ) $ 1.1 $ 1.6 Net realized gains (losses) from sales and other (0.8 ) (0.4 ) (1.2 ) (1.7 ) Net other-than-temporary impairment losses on investments recognized in earnings (0.2 ) (0.6 ) (0.1 ) (0.1 ) Total before tax 3.3 2.5 10.5 8.2 Tax benefit 3.1 1.9 10.4 8.1 Continued operations; net of tax — 0.3 — (1.5 ) Discontinued operations - Chaucer business 3.1 2.2 10.4 6.6 Net of tax Amortization of defined benefit pension and postretirement plans (2.8 ) (2.5 ) (8.5 ) (7.2 ) Loss adjustment expenses and other operating expenses (1) 0.5 0.5 1.7 1.5 Tax benefit (2.3 ) (2.0 ) (6.8 ) (5.7 ) Continued operations; net of tax — — — (0.1 ) Discontinued Operations - Chaucer Business (2.3 ) (2.0 ) (6.8 ) (5.8 ) Net of tax Total reclassifications for the period $ 0.8 $ 0.2 $ 3.6 $ 0.8 Benefit reflected in income, net of tax (1) The amount reclassified from accumulated other comprehensive income for the pension and postretirement benefits was allocated approximately 40% to loss adjustment expenses and 60% to other operating expenses for the three and nine months ended September 30, 2019 and 2018.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Segment Information	10. Segment Information The Company’s primary business operations include insurance products and services provided through three operating segments: Commercial Lines, Personal Lines and Other. Commercial Lines includes commercial multiple peril, commercial automobile, workers’ compensation, and other commercial coverages, such as inland marine, specialty program business, management and professional liability, surety and specialty industrial property. Personal Lines includes personal automobile, homeowners and other personal coverages. Included in the Other segment are Opus Investment Management, Inc., which markets investment management services to institutions, pension funds and other organizations; earnings on holding company assets; holding company and other expenses, including certain costs associated with retirement benefits due to the Company’s former life insurance employees and agents; and, a run-off voluntary pools business. On December 28, 2018, the Company completed the sale of Chaucer to China Re and the sales of the Chaucer-related Irish and Australian entities were subsequently completed on February 14, 2019 and April 10, 2019, respectively. Accordingly, as of December 31, 2018, and for all prior periods, Chaucer’s results of operations have been classified as Discontinued Operations in the Consolidated Statements of Income and assets and liabilities related to the Chaucer business were classified as held-for-sale in the Consolidated Balance Sheet at December 31, 2018 The Company reports interest expense related to debt separately from the earnings of its operating segments. This consists of interest on the Company’s senior and subordinated debentures. Management evaluates the results of the aforementioned segments based on operating income before taxes, excluding interest expense on debt. Operating income before taxes excludes certain items which are included in net income, such as net realized and unrealized investment gains and losses. Such gains and losses are excluded since they are determined by interest rates, financial markets and the timing of sales. Also, operating income before taxes excludes net gains and losses on disposals of businesses, gains and losses related to the repayment of debt, discontinued operations, costs to acquire businesses, restructuring costs, the cumulative effect of accounting changes and certain other items. Although the items excluded from operating income before taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income before taxes enhances an investor’s understanding of the Company’s results of operations by highlighting net income attributable to the core operations of the business. However, operating income before taxes should not be construed as a substitute for income before income taxes and operating income should not be construed as a substitute for net income. Summarized below is financial information with respect to the Company’s business segments. Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2019 2018 2019 2018 Operating revenues: Commercial Lines $ 710.3 $ 687.4 $ 2,114.2 $ 2,045.7 Personal Lines 483.3 452.4 1,423.1 1,331.8 Other 6.3 3.8 20.4 10.2 Total 1,199.9 1,143.6 3,557.7 3,387.7 Net realized and unrealized investment gains 14.8 23.0 75.1 3.8 Total revenues $ 1,214.7 $ 1,166.6 $ 3,632.8 $ 3,391.5 Operating income (loss) before interest expense and income taxes: Commercial Lines: Underwriting income $ 30.4 $ 20.8 $ 96.2 $ 74.6 Net investment income 44.1 45.0 132.8 135.1 Other expense (0.4 ) (0.7 ) (1.9 ) (1.4 ) Commercial Lines operating income 74.1 65.1 227.1 208.3 Personal Lines: Underwriting income 28.8 28.8 45.4 51.3 Net investment income 19.6 18.2 59.0 54.7 Other income 1.8 1.2 4.3 3.7 Personal Lines operating income 50.2 48.2 108.7 109.7 Other: Underwriting loss (0.4 ) (1.0 ) (1.0 ) (3.1 ) Net investment income 5.1 3.2 16.8 8.2 Other expense (2.7 ) (2.7 ) (8.5 ) (9.7 ) Other operating income (loss) 2.0 (0.5 ) 7.3 (4.6 ) Operating income before interest expense and income taxes 126.3 112.8 343.1 313.4 Interest on debt (9.4 ) (11.2 ) (28.1 ) (33.9 ) Operating income before income taxes 116.9 101.6 315.0 279.5 Non-operating income items: Net realized and unrealized investment gains 14.8 23.0 75.1 3.8 Other non-operating items (1.4 ) (1.9 ) (1.4 ) (1.9 ) Income from continuing operations before income taxes $ 130.3 $ 122.7 $ 388.7 $ 281.4 The following table provides identifiable assets for the Company’s business segments and discontinued operations: September 30, 2019 December 31, 2018 (in millions) Identifiable Assets U.S. Companies $ 12,521.2 $ 12,238.4 Assets held-for-sale (1) — 57.4 Assets of discontinued operations 106.0 103.9 Total $ 12,627.2 $ 12,399.7 (1) See also Note 3 – “Discontinued Operations – Chaucer Business”. The Company reviews the assets of its U.S. Companies collectively and does not allocate them between the Commercial Lines, Personal Lines and Other segments.

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	11. Stock-based Compensation As of September 30, 2019, there were 3,645,690 shares and 2,361,547 shares available for grant under The Hanover Insurance Group 2014 Long-Term Incentive Plan and The Hanover Insurance Group 2014 Employee Stock Purchase Plan, respectively. Compensation cost for the Company’s stock-based awards and the related tax benefits were as follows: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2019 2018 2019 2018 Stock-based compensation expense $ 4.1 $ 4.0 $ 12.7 $ 10.6 Tax benefit (0.8 ) (0.8 ) (2.6 ) (2.2 ) Stock-based compensation expense, net of taxes $ 3.3 $ 3.2 $ 10.1 $ 8.4 Stock-based compensation expense for Chaucer, net of taxes, of $1.2 million for the nine months ended September 30, 2018 is reported in discontinued operations and is excluded from the amounts disclosed above. Stock Options Information on the Company’s stock option activity for the nine months ended September 30, 2019 and 2018 is summarized below. Nine Months Ended September 30, 2019 2018 (in whole shares and dollars) Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Outstanding, beginning of period 1,099,076 $ 85.75 1,062,177 $ 75.53 Granted 232,568 95.05 301,152 110.98 Exercised (172,551 ) 70.40 (169,196 ) 70.94 Forfeited or cancelled (38,729 ) 107.34 (54,063 ) 89.09 Outstanding, end of period 1,120,364 89.30 1,140,070 84.93 Restricted Stock Units The Company currently issues time-based, market-based and performance-based restricted stock units to eligible employees, all of which generally vest after 3 years of continued employment. The following tables summarize activity information about employee restricted stock units: Nine Months Ended September 30, 2019 2018 (in whole shares and dollars) Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Time-based restricted stock units: Outstanding, beginning of period 332,481 $ 97.28 298,528 $ 83.45 Granted 146,715 117.71 148,619 111.67 Vested (107,829 ) 84.17 (72,546 ) 72.62 Forfeited (33,156 ) 104.80 (23,130 ) 91.20 Outstanding, end of period 338,211 109.59 351,471 97.10 Performance-based and market-based restricted stock units: Outstanding, beginning of period 69,838 $ 95.58 102,586 $ 81.21 Granted 42,119 117.09 35,063 118.60 Vested (14,118 ) 84.39 (14,032 ) 70.24 Forfeited (2,670 ) 135.92 (7,625 ) 81.91 Outstanding, end of period 95,169 105.63 115,992 93.79 In the first nine months of 2019 and 2018, the Company granted market-based awards totaling 23,924 and 31,688, respectively, to certain members of senior management, which are included in the table above as performance and market-based restricted stock activity. The vesting of these stock units is based on the relative total shareholder return (“TSR”) of the Company. This metric is generally based on relative TSR for a three-year period as compared to a pre-selected group of property and casualty companies. The fair value of market-based awards was estimated at the date of grant using a valuation model. These units have the potential to range from 0% to 150% of the shares disclosed. Included in the amount granted above in 2019 and 2018 are 5,820 shares and 3,115 shares, respectively, related to market-based awards that achieved a payout in excess of 100%. These awards vested in the first quarters of 2019 and 2018, respectively. The Company also granted performance-based restricted stock units in 2019 which are based upon the Company’s achievement of return on equity objectives. These units have the potential to range from 0% to 150% of the shares disclosed, which varies based on grant year and individual participation level. Increases above the 100% target level are reflected as granted in the period in which performance-based stock unit goals are achieved. Decreases below the 100% target level are reflected as forfeited.

Earnings Per Share and Sharehol

Earnings Per Share and Shareholders' Equity Transactions	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Earnings Per Share and Shareholders' Equity Transactions	12. Earnings Per Share and Shareholders’ Equity Transactions The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: Three Months Ended Nine Months Ended September 30, September 30, (in millions, except per share data) 2019 2018 2019 2018 Basic shares used in the calculation of earnings per share 39.6 42.5 40.3 42.5 Dilutive effect of securities: Employee stock options 0.3 0.3 0.3 0.3 Non-vested stock grants 0.3 0.3 0.3 0.3 Diluted shares used in the calculation of earnings per share 40.2 43.1 40.9 43.1 Per share effect of dilutive securities on income from continuing operations $ (0.04 ) $ (0.04 ) $ (0.11 ) $ (0.07 ) Per share effect of dilutive securities on net income $ (0.04 ) $ (0.03 ) $ (0.11 ) $ (0.08 ) Diluted earnings per share for the three months ended September 30, 2019 excludes 0.2 million of common shares issuable under the Company’s stock compensation plans because their effect would be antidilutive. Diluted earnings per share for the nine months ended September 30, 2019 and 2018 excludes 0.2 million and 0.3 million, respectively, of common shares issuable under the Company’s stock compensation plans because their effect would be antidilutive. On December 30, 2018, the Board of Directors authorized a new share repurchase program which provides for aggregate repurchases of up to $600 million. Under the repurchase authorization, the Company may repurchase, from time to time, common shares in amounts, at prices and at such times as the Company deems appropriate, subject to market conditions and other considerations. Repurchases may be executed using open market purchases, privately negotiated transactions, accelerated repurchase programs or other transactions. The Company is not required to purchase any specific number of shares or to make purchases by any certain date under this program. received

Liabilities for Outstanding Cla

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2019
Insurance [Abstract]
Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	1 3 . Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses Reserve Rollforward and Prior Year Development The Company regularly updates its reserve estimates as new information becomes available and further events occur which may impact the resolution of unsettled claims. Reserve adjustments are reflected in results of operations as adjustments to losses and loss adjustment expenses (“LAE”). Often these adjustments are recognized in periods subsequent to the period in which the underlying policy was written and loss event occurred. These types of subsequent adjustments are described as “prior years’ loss reserves”. Such development can be either favorable or unfavorable to the Company’s financial results and may vary by line of business. In this section, all amounts presented include catastrophe losses and LAE, unless otherwise indicated. The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. Nine Months Ended September 30, (in millions) 2019 2018 Gross loss and LAE reserves, beginning of period $ 5,304.1 $ 5,058.5 Reinsurance recoverable on unpaid losses 1,472.6 1,455.0 Net loss and LAE reserves, beginning of period 3,831.5 3,603.5 Net incurred losses and LAE in respect of losses occurring in: Current year 2,142.9 2,025.6 Prior years (20.3 ) (7.1 ) Total incurred losses and LAE 2,122.6 2,018.5 Net payments of losses and LAE in respect of losses occurring in: Current year 889.2 848.6 Prior years 1,043.4 997.3 Total payments 1,932.6 1,845.9 Net reserve for losses and LAE, end of period 4,021.5 3,776.1 Reinsurance recoverable on unpaid losses 1,529.0 1,412.8 Gross reserve for losses and LAE, end of period $ 5,550.5 $ 5,188.9 As a result of continuing trends in the Company’s business, reserves including catastrophes have been re-estimated for all prior accident years and were decreased by $20.3 million and $7.1 million in 2019 and 2018, respectively. 2019 For the nine months ended September 30, 2019, net favorable loss and LAE development was $20.3 million, primarily as a result of favorable catastrophe development due to the sale of subrogation rights on the 2017 and 2018 California wildfires and lower than expected losses related to the 2018 hurricanes. For non-catastrophe loss and LAE development, higher than expected losses in the personal and commercial automobile lines were substantially offset by lower than expected losses in the workers’ compensation line. Within other commercial lines, lower than expected losses in the marine, miscellaneous property, monoline general liability and professional and management liability lines were partially offset by higher than expected losses in Hanover Programs, primarily in business classes which have since been terminated. 2018 For the nine months ended September 30, 2018, net favorable loss and LAE development was $7.1 million, primarily as a result of favorable catastrophe development related to the 2017 hurricanes and California wildfires. In addition, lower than expected losses in workers’ compensation of $14.2 million and other commercial lines of $14.0 million was partially offset by higher than expected losses in our commercial and personal automobile lines of $22.8 million and, to a lesser extent, in our homeowners line. Within other commercial lines, lower than expected losses in our professional and management liability and monoline general liability lines was partially offset by higher than expected losses in Hanover Programs in business classes which have since been terminated.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2019
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	1 4 . Commitments and Contingencies Legal Proceedings The Company has been named a defendant in various legal proceedings arising in the normal course of business. In addition, the Company is involved, from time to time, in examinations, investigations and proceedings by governmental and self-regulatory agencies. The potential outcome of any such action or regulatory proceedings in which the Company has been named a defendant or the subject of an inquiry or investigation, and its ultimate liability, if any, from such action or regulatory proceedings, is difficult to predict at this time. The ultimate resolutions of such proceedings are not expected to have a material effect on its financial position, although they could have a material effect on the results of operations for a particular quarter or annual period. Residual Markets The Company is required to participate in residual markets in various states, which generally pertain to high risk insureds, disrupted markets or lines of business or geographic areas where rates are regarded as excessive. The results of the residual markets are not subject to the predictability associated with the Company’s own managed business, and are significant to both the personal and commercial automobile lines of business .

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2019
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events There were no subsequent events requiring adjustment to the financial statements and no additional disclosures required in the notes to the interim consolidated financial statements.

New Accounting Pronouncements (

New Accounting Pronouncements (Policies)	9 Months Ended
	Sep. 30, 2019
Accounting Changes And Error Corrections [Abstract]
New Accounting Pronouncements	New Accounting Pronouncements Recently Implemented Standards In February 2016, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) Update No. 2016-02, (Topic 842) Leases Leases Targeted Improvements In March 2019, the FASB issued ASC Update No. 2019-01, Leases (Topic 842) – Codification Improvements Accounting Changes and Error Corrections In March 2017, the FASB issued ASC Update No. 2017-08, (Subtopic 310-20) Receivables – Nonrefundable Fees and Other Costs: Premium Amortization on Purchased Callable Debt Securities . This guidance shortens the amortization period of premiums on certain purchased callable debt securities to the earliest call date. The updated guidance is effective for annual and interim periods beginning after December 15, 2018, and should be applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company implemented this guidance effective January 1, 2019. The effect of implementing this guidance resulted in a cumulative effect adjustment reclassifying unrealized losses, net of tax, of $ 1.5 million from accumulated other comprehensive income to retained earnings. Recently Issued Standards In August 2018, the FASB issued ASC Update No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASC Update No. 2018-14 (Topic 715-20) Compensation – Retirement Benefits – Defined Benefit Plans – General – Disclosure Framework – Changes to the Disclosure Requirements for the Defined Benefit Plans. In August 2018, the FASB issued ASC Update No. 2018-13 (Topic 820) Fair Value Measurement, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In January 2017, the FASB issued ASC Update No. 2017-04, (Topic 350) Intangibles – Goodwill and Other: Simplifying the Test for Goodwill Impairment In June 2016, the FASB issued ASC Update No. 2016-13, (Topic 326) Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments . This ASC update introduces new guidance for the accounting for credit losses on financial instruments within its scope. A new model, referred to as the current expected credit losses model, requires an entity to determine credit-related impairment losses for financial instruments held at amortized cost and to estimate these expected credit losses over the life of an exposure (or pool of exposures). The estimate of expected credit losses should consider both historical and current information, reasonable and supportable forecasts, as well as estimates of prepayments. The estimated credit losses and subsequent adjustment to such loss estimates, will be recorded through an allowance account which is deducted from the amortized cost of the financial instrument, with the offset recorded in current earnings. ASC No. 2016-13 also modifies the impairment model for available-for-sale debt securities. The new model will require an estimate of expected credit losses only when the fair value is below the amortized cost of the asset, thus the length of time the fair value of an available-for-sale debt security has been below the amortized cost will no longer affect the determination of whether a credit loss exists. In addition, credit losses on available-for-sale debt securities will be limited to the difference between the security’s amortized cost basis and its fair value. In November 2018, the FASB issued ASC Update No. 2018-19, Codification Improvements to Topic 326, Financial Instruments – Credit Losses , which explicitly states that receivables arising from operating leases are not within the scope of Subtopic 326-20. ASC Update 2016-13 and related guidance is effective for interim and annual periods beginning after December 15, 2019. Early adoption is permitted for periods beginning after December 15, 2018. The Company does not expect t he adopt ion of ASC Update No. 2016-13 to have a material impact on its financial position or results of operations .

Discontinued Operations Chauc_2

Discontinued Operations Chaucer Business (Tables) - Chaucer [Member]	9 Months Ended
	Sep. 30, 2019
Summary of Discontinued Chaucer Business	The following table summarizes the results of Chaucer’s operations for the three and nine months ended September 30, 2018: Three Months Ended Nine Months Ended (in millions) September 30, 2018 (1) Revenues Net premiums earned $ 212.4 $ 648.2 Net investment income 12.8 41.7 Other income 1.9 5.4 227.1 695.3 Losses and operating expenses Losses and LAE 136.8 364.3 Amortization of deferred acquisition costs 60.1 196.7 Other expenses 26.4 85.7 223.3 646.7 Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income) 3.8 48.6 Other items: Interest expense (1.5 ) (2.8 ) Net realized and unrealized investment gains (losses) 0.1 (0.4 ) Other expenses (2) (7.1 ) (8.3 ) Income (loss) from Chaucer business before income taxes (4.7 ) 37.1 Income tax benefit (expense) 1.1 (6.6 ) Income (loss) from Chaucer business, net of income taxes $ (3.6 ) $ 30.5 (1) 2019 information is omitted as it is not material. (2) Other expenses primarily reflects those costs associated with the then pending sale of Chaucer.
Summary of Cash Flows Associated with the Discontinued Chaucer Business	The following table details the cash flows associated with the Chaucer business for the nine months ended September 30, 2018: Nine Months Ended September 30, (in millions) 2018 Net cash provided by operating activities $ 46.2 Net cash provided by investing activities $ 64.3

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2019
Debt Disclosure [Abstract]
Schedule of Debt	Debt consists of the following: (in millions) September 30, 2019 December 31, 2018 Senior debentures maturing April 15, 2026 $ 375.0 $ 375.0 Senior debentures maturing October 15, 2025 62.6 62.6 Subordinated debentures maturing March 30, 2053 175.0 175.0 Subordinated debentures maturing February 3, 2027 50.1 50.1 FHLB borrowings (secured) — 125.0 Short-term borrowings 14.5 — Total principal debt 677.2 787.7 Unamortized debt issuance costs (9.4 ) (9.8 ) Total $ 667.8 $ 777.9

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2019
Investments Debt And Equity Securities [Abstract]
Schedule of Available-for-sale Fixed Maturities Reconciliation	The amortized cost and fair value of available-for-sale fixed maturities were as follows: September 30, 2019 Gross Gross OTTI Amortized Unrealized Unrealized Unrealized (in millions) Cost Gains Losses Fair Value Losses U.S. Treasury and government agencies $ 350.2 $ 12.0 $ 0.4 $ 361.8 $ — Foreign government 5.0 0.1 — 5.1 — Municipal 848.3 34.4 0.4 882.3 — Corporate 3,758.4 159.1 5.3 3,912.2 3.3 Residential mortgage-backed 959.8 20.3 0.9 979.2 — Commercial mortgage-backed 685.2 33.4 — 718.6 — Asset-backed 85.6 1.5 — 87.1 — Total fixed maturities $ 6,692.5 $ 260.8 $ 7.0 $ 6,946.3 $ 3.3 December 31, 2018 Gross Gross OTTI Amortized Unrealized Unrealized Unrealized (in millions) Cost Gains Losses Fair Value Losses U.S. Treasury and government agencies $ 414.7 $ 2.4 $ 7.2 $ 409.9 $ — Foreign government 7.3 0.1 — 7.4 — Municipal 879.0 16.6 9.8 885.8 — Corporate 3,476.6 26.1 92.0 3,410.7 6.6 Residential mortgage-backed 728.4 2.7 14.7 716.4 — Commercial mortgage-backed 648.4 1.7 9.8 640.3 — Asset-backed 91.5 0.2 0.7 91.0 — Fixed maturities, excluding held-for-sale (Chaucer) 6,245.9 49.8 134.2 6,161.5 6.6 Fixed maturities, held-for-sale 24.9 — 0.4 24.5 — Total fixed maturities $ 6,270.8 $ 49.8 $ 134.6 $ 6,186.0 $ 6.6
Investments Classified by Contractual Maturity Date	The amortized cost and fair value by maturity periods for fixed maturities are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers. September 30, 2019 Amortized Fair (in millions) Cost Value Due in one year or less $ 266.2 $ 268.7 Due after one year through five years 1,944.8 2,009.9 Due after five years through ten years 2,358.3 2,473.7 Due after ten years 392.6 409.1 4,961.9 5,161.4 Mortgage-backed and asset-backed securities 1,730.6 1,784.9 Total fixed maturities $ 6,692.5 $ 6,946.3
Schedule of Unrealized Loss on Investments	The following tables provide information about the Company’s available-for-sale fixed maturity securities that were in an unrealized loss position at September 30, 2019 and December 31, 2018 including the length of time the securities have been in an unrealized loss position: September 30, 2019 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 0.3 $ 37.1 $ 0.1 $ 17.2 $ 0.4 $ 54.3 Municipal 0.2 29.3 0.2 15.5 0.4 44.8 Corporate 0.9 79.8 0.4 24.0 1.3 103.8 Residential mortgage-backed 0.4 76.8 0.5 58.4 0.9 135.2 Commercial mortgage-backed — 1.8 — 25.0 — 26.8 Asset-backed — 0.5 — 3.3 — 3.8 Total investment grade 1.8 225.3 1.2 143.4 3.0 368.7 Below investment grade: Corporate 3.2 40.8 0.8 9.0 4.0 49.8 Total fixed maturities $ 5.0 $ 266.1 $ 2.0 $ 152.4 $ 7.0 $ 418.5 December 31, 2018 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 1.1 $ 66.0 $ 6.1 $ 210.9 $ 7.2 $ 276.9 Foreign governments — 2.0 — 0.8 — 2.8 Municipal 0.8 110.0 9.0 248.0 9.8 358.0 Corporate 30.0 1,277.9 43.9 781.6 73.9 2,059.5 Residential mortgage-backed 2.6 201.2 12.1 323.7 14.7 524.9 Commercial mortgage-backed 3.4 293.0 6.4 175.5 9.8 468.5 Asset-backed 0.4 42.3 0.3 18.0 0.7 60.3 Total investment grade 38.3 1,992.4 77.8 1,758.5 116.1 3,750.9 Below investment grade: Municipal — — — 0.9 — 0.9 Corporate 8.1 185.6 10.0 54.0 18.1 239.6 Total below investment grade 8.1 185.6 10.0 54.9 18.1 240.5 Fixed maturities, excluding held-for-sale (Chaucer) 46.4 2,178.0 87.8 1,813.4 134.2 3,991.4 Fixed maturities, held-for-sale 0.1 4.0 0.3 18.0 0.4 22.0 Total fixed maturities $ 46.5 $ 2,182.0 $ 88.1 $ 1,831.4 $ 134.6 $ 4,013.4
Schedule of Realized Gain (Loss)	The proceeds from sales of available-for-sale securities and gross realized gains and gross realized losses on those sales were as follows: Three Months Ended September 30, 2019 2018 Proceeds from Gross Gross Proceeds from Gross Gross (in millions) Sales Gains Losses Sales Gains Losses Fixed maturities, excluding held-for-sale (Chaucer) $ 86.9 $ 1.6 $ 2.4 $ 83.8 $ 0.8 $ 1.5 Fixed maturities, held-for-sale — — — 21.4 0.2 0.1 Total fixed maturities $ 86.9 $ 1.6 $ 2.4 $ 105.2 $ 1.0 $ 1.6 Nine Months Ended September 30, 2019 2018 Proceeds from Gross Gross Proceeds from Gross Gross (in millions) Sales Gains Losses Sales Gains Losses Fixed maturities, excluding held-for-sale (Chaucer) $ 309.5 $ 4.2 $ 5.0 $ 225.1 $ 2.0 $ 5.3 Fixed maturities, held-for-sale 0.3 — — 172.5 1.3 1.1 Total fixed maturities $ 309.8 $ 4.2 $ 5.0 $ 397.6 $ 3.3 $ 6.4
Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses	The following table provides rollforwards of the cumulative amounts related to the Company’s credit loss portion of the OTTI losses on fixed maturity securities from continuing operations for which the non-credit portion of the loss is included in other comprehensive income. Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2019 2018 2019 2018 Credit losses as of the beginning of the period $ 2.9 $ 3.7 $ 3.8 $ 3.6 Credit losses on securities for which an OTTI was not previously recognized 0.6 0.2 1.0 1.0 Additional credit losses on securities for which an OTTI was previously recognized — — — 0.1 Reductions for securities sold, matured or called (1.3 ) — (2.6 ) (0.8 ) Reductions for securities reclassified as intended to sell (0.3 ) — (0.3 ) — Credit losses as of the end of the period $ 1.9 $ 3.9 $ 1.9 $ 3.9
Schedule of Pre-tax Realized and Unrealized Gains and Losses on Equity Securities From Continuing Operations	Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2019 2018 2019 2018 Net gains recognized during the period $ 15.0 $ 23.6 $ 75.3 $ 6.7 Less: net gains (losses) recognized on equity securities sold during the period (0.2 ) (0.2 ) 1.0 (1.0 ) Net unrealized gains recognized during the period on equity securities still held $ 15.2 $ 23.8 $ 74.3 $ 7.7

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
	Sep. 30, 2019
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	The estimated fair value of the financial instruments were as follows: September 30, 2019 December 31, 2018 Carrying Fair Carrying Fair (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 6,946.3 $ 6,946.3 $ 6,161.5 $ 6,161.5 Fair Value through Net Income: Equity securities 570.8 570.8 464.4 464.4 Other investments 180.0 180.0 175.0 175.0 Amortized Cost/Cost: Other investments 449.3 473.4 414.4 418.9 Cash and cash equivalents 157.6 157.6 1,020.7 1,020.7 Total financial instruments, excluding held-for-sale 8,304.0 8,328.1 8,236.0 8,240.5 Financial instruments, held-for-sale (Chaucer) — — 27.8 27.8 Total financial instruments $ 8,304.0 $ 8,328.1 $ 8,263.8 $ 8,268.3 Financial Liabilities carried at: Amortized Cost: Debt $ 667.8 $ 732.0 $ 777.9 $ 825.0
Fair Value, Investment Assets Measured on Recurring Basis	The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis. September 30, 2019 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 361.8 $ 162.0 $ 199.8 $ — Foreign government 5.1 — 5.1 — Municipal 882.3 — 869.7 12.6 Corporate 3,912.2 — 3,911.5 0.7 Residential mortgage-backed, U.S. agency backed 976.6 — 976.6 — Residential mortgage-backed, non-agency 2.6 — 2.6 — Commercial mortgage-backed 718.6 — 705.6 13.0 Asset-backed 87.1 — 87.1 — Total fixed maturities 6,946.3 162.0 6,758.0 26.3 Equity securities 570.8 568.7 — 2.1 Other investments 3.5 — — 3.5 Total investment assets at fair value $ 7,520.6 $ 730.7 $ 6,758.0 $ 31.9 December 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 409.9 $ 154.9 $ 255.0 $ — Foreign government 7.4 — 7.4 — Municipal 885.8 — 864.7 21.1 Corporate 3,410.7 — 3,409.9 0.8 Residential mortgage-backed, U.S. agency backed 713.7 — 713.7 — Residential mortgage-backed, non-agency 2.7 — 2.7 — Commercial mortgage-backed 640.3 — 627.2 13.1 Asset-backed 91.0 — 91.0 — Total fixed maturities 6,161.5 154.9 5,971.6 35.0 Equity securities 464.4 463.3 — 1.1 Other investments 3.5 — — 3.5 Total investment assets at fair value, excluding held-for-sale (Chaucer) 6,629.4 618.2 5,971.6 39.6 Investment assets, held-for-sale 24.5 5.7 18.8 — Total investment assets at fair value $ 6,653.9 $ 623.9 $ 5,990.4 $ 39.6
Estimated Fair Values of Financial Instruments Not Carried at Fair Value	The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value: September 30, 2019 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 157.6 $ 157.6 $ — $ — Other investments 473.4 — 2.6 470.8 Total financial instruments $ 631.0 $ 157.6 $ 2.6 $ 470.8 Liabilities: Debt $ 732.0 $ — $ 732.0 $ — December 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 1,020.7 $ 1,020.7 $ — $ — Other investments 418.9 — 8.7 410.2 Total financial instruments, excluding held-for-sale 1,439.6 1,020.7 8.7 410.2 Financial instruments, held-for-sale (Chaucer) 3.3 3.3 — — Total financial instruments $ 1,442.9 $ 1,024.0 $ 8.7 $ 410.2 Liabilities: Debt $ 825.0 $ — $ 825.0 $ —
Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	The tables below provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Three Months Ended September 30, 2019 Balance July 1, 2019 $ 12.9 $ 0.7 $ 13.0 $ 26.6 $ 5.6 $ 32.2 Total gains: Included in other comprehensive income-net appreciation on available-for-sale securities 0.1 — 0.2 0.3 — 0.3 Sales (0.4 ) — (0.2 ) (0.6 ) — (0.6 ) Balance September 30, 2019 $ 12.6 $ 0.7 $ 13.0 $ 26.3 $ 5.6 $ 31.9 Three Months Ended September 30, 2018 Balance July 1, 2018, excluding held-for-sale (Chaucer) $ 22.2 $ 0.8 $ 13.2 $ 36.2 $ 4.7 $ 40.9 Transfers out of Level 3 (0.5 ) — — (0.5 ) — (0.5 ) Total losses: Included in other comprehensive income-net depreciation on available-for-sale securities — — (0.1 ) (0.1 ) — (0.1 ) Sales (0.4 ) (0.1 ) (0.1 ) (0.6 ) — (0.6 ) Balance September 30, 2018, excluding held-for-sale (Chaucer) $ 21.3 $ 0.7 $ 13.0 $ 35.0 $ 4.7 $ 39.7 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Nine Months Ended September 30, 2019 Balance January 1, 2019 $ 21.1 $ 0.8 $ 13.1 $ 35.0 $ 4.6 $ 39.6 Total gains: Included in other comprehensive income - net appreciation on available-for-sale securities 0.9 — 0.7 1.6 — 1.6 Purchases and sales: Purchases — — — — 1.0 1.0 Sales (9.4 ) (0.1 ) (0.8 ) (10.3 ) — (10.3 ) Balance September 30, 2019 $ 12.6 $ 0.7 $ 13.0 $ 26.3 $ 5.6 $ 31.9 Nine Months Ended September 30, 2018 Balance January 1, 2018, excluding held-for-sale (Chaucer) $ 24.6 $ 0.9 $ 14.2 $ 39.7 $ 4.7 $ 44.4 Transfers out of Level 3 (0.5 ) — — (0.5 ) — (0.5 ) Total gains (losses): Included in total net realized and unrealized investment gains 0.1 — — 0.1 — 0.1 Included in other comprehensive income-net depreciation on available-for-sale securities (0.6 ) — (0.5 ) (1.1 ) — (1.1 ) Sales (2.3 ) (0.2 ) (0.7 ) (3.2 ) — (3.2 ) Balance September 30, 2018, excluding held-for-sale (Chaucer) $ 21.3 $ 0.7 $ 13.0 $ 35.0 $ 4.7 $ 39.7
Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3 Assets	The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets. Where discounted cash flows were used in the valuation of fixed maturities, the internally-developed discount rate was adjusted by the significant unobservable inputs shown in the table. September 30, 2019 December 31, 2018 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted cash flow Discount for: Small issue size Above-market coupon Credit stress $ 12.6 0.7 - 6.8% (4.3%) 0.3 - 0.5% (0.5%) — $ 21.1 0.7 - 6.8% (3.4%) 0.3 - 0.5% (0.5%) 1.3% (1.3%) Corporate Discounted cash flow Discount for: Small issue size Above-market coupon 0.7 2.5% (2.5%) 0.3% (0.3%) 0.8 2.5% (2.5%) 0.3% (0.3%) Commercial mortgage-backed Discounted cash flow Discount for: Small issue size Above-market coupon Lease structure 13.0 1.9 - 3.1% (2.7%) 0.5% (0.5%) 0.3% (0.3%) 13.1 1.9 - 3.1% (2.7%) 0.5% (0.5%) 0.3% (0.3%) Equity securities Market comparables Net tangible asset market multiples 2.1 1.0X (1.0X) 1.1 1.0X (1.0X) Other Discounted cash flow Discount rate 3.5 18.0% (18.0%) 3.5 18.0% (18.0%)

Pension and Other Postretirem_2

Pension and Other Postretirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2019
Compensation And Retirement Disclosure [Abstract]
Components of Net Periodic Pension Cost	The components of net periodic pension cost for defined benefit pension and other postretirement benefit plans included in the Company’s results of operations are as follows: Three Months Ended September 30, 2019 2018 2019 2018 (in millions) Pension Plans Postretirement Plans Service cost - benefits earned during the period $ — $ — $ — $ — Interest cost 5.3 5.0 0.1 0.1 Expected return on plan assets (5.8 ) (5.2 ) — — Recognized net actuarial loss 2.8 2.4 — — Amortization of prior service cost — — — — Net periodic pension cost $ 2.3 $ 2.2 $ 0.1 $ 0.1 Nine Months Ended September 30, 2019 2018 2019 2018 (in millions) Pension Plans Postretirement Plans Service cost - benefits earned during the period $ — $ — $ — $ — Interest cost 15.9 14.9 0.3 0.3 Expected return on plan assets (17.5 ) (15.5 ) — — Recognized net actuarial loss 8.5 7.2 0.1 0.1 Amortization of prior service cost — — (0.1 ) (0.2 ) Net periodic pension cost $ 6.9 $ 6.6 $ 0.3 $ 0.2

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2019
Equity [Abstract]
Changes in Other Comprehensive Income (Loss)	The following tables provide changes in other comprehensive income (loss). Three Months Ended September 30, 2019 2018 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Unrealized gains (losses) on available-for-sale securities: Unrealized gains (losses) arising during period $ 58.4 $ (12.3 ) $ 46.1 $ (29.2 ) $ 6.2 $ (23.0 ) Amount of realized gains from sales and other (0.6 ) (3.1 ) (3.7 ) (0.2 ) (2.4 ) (2.6 ) Portion of other-than-temporary impairment losses recognized in earnings 0.8 (0.2 ) 0.6 0.5 (0.1 ) 0.4 Net unrealized gains (losses) 58.6 (15.6 ) 43.0 (28.9 ) 3.7 (25.2 ) Pension and postretirement benefits: Net change in net actuarial loss and prior service cost 2.8 (0.5 ) 2.3 2.4 (0.5 ) 1.9 Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period — — — 2.0 (0.4 ) 1.6 Other comprehensive income (loss) $ 61.4 $ (16.1 ) $ 45.3 $ (24.5 ) $ 2.8 $ (21.7 ) Nine Months Ended September 30, 2019 2018 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Unrealized gains (losses) on available-for-sale securities: Unrealized gains (losses) arising during period $ 343.6 $ (72.2 ) $ 271.4 $ (242.1 ) $ 50.9 $ (191.2 ) Amount of realized gains from sales and other (1.1 ) (10.2 ) (11.3 ) — (8.2 ) (8.2 ) Portion of other-than-temporary impairment losses recognized in earnings 1.2 (0.3 ) 0.9 2.0 (0.4 ) 1.6 Net unrealized gains (losses) 343.7 (82.7 ) 261.0 (240.1 ) 42.3 (197.8 ) Pension and postretirement benefits: Net change in net actuarial loss and prior service cost 8.5 (1.7 ) 6.8 0.6 (0.1 ) 0.5 Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period 0.9 (0.2 ) 0.7 0.8 (0.2 ) 0.6 Other comprehensive income (loss) $ 353.1 $ (84.6 ) $ 268.5 $ (238.7 ) $ 42.0 $ (196.7 )
Reclassifications Out of Accumulated Other Comprehensive Income	Reclassifications out of accumulated other comprehensive income were as follows: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2019 2018 2019 2018 Amount Reclassified from Details about Accumulated Other Accumulated Other Affected Line Item in the Statement Comprehensive Income Components Comprehensive Income Where Net Income is Presented Unrealized gains on available-for- sale securities $ 0.6 $ (0.2 ) $ 1.1 $ 1.6 Net realized gains (losses) from sales and other (0.8 ) (0.4 ) (1.2 ) (1.7 ) Net other-than-temporary impairment losses on investments recognized in earnings (0.2 ) (0.6 ) (0.1 ) (0.1 ) Total before tax 3.3 2.5 10.5 8.2 Tax benefit 3.1 1.9 10.4 8.1 Continued operations; net of tax — 0.3 — (1.5 ) Discontinued operations - Chaucer business 3.1 2.2 10.4 6.6 Net of tax Amortization of defined benefit pension and postretirement plans (2.8 ) (2.5 ) (8.5 ) (7.2 ) Loss adjustment expenses and other operating expenses (1) 0.5 0.5 1.7 1.5 Tax benefit (2.3 ) (2.0 ) (6.8 ) (5.7 ) Continued operations; net of tax — — — (0.1 ) Discontinued Operations - Chaucer Business (2.3 ) (2.0 ) (6.8 ) (5.8 ) Net of tax Total reclassifications for the period $ 0.8 $ 0.2 $ 3.6 $ 0.8 Benefit reflected in income, net of tax (1) The amount reclassified from accumulated other comprehensive income for the pension and postretirement benefits was allocated approximately 40% to loss adjustment expenses and 60% to other operating expenses for the three and nine months ended September 30, 2019 and 2018.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2019
Segment Reporting [Abstract]
Financial Information with Respect to Business Segments	Summarized below is financial information with respect to the Company’s business segments. Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2019 2018 2019 2018 Operating revenues: Commercial Lines $ 710.3 $ 687.4 $ 2,114.2 $ 2,045.7 Personal Lines 483.3 452.4 1,423.1 1,331.8 Other 6.3 3.8 20.4 10.2 Total 1,199.9 1,143.6 3,557.7 3,387.7 Net realized and unrealized investment gains 14.8 23.0 75.1 3.8 Total revenues $ 1,214.7 $ 1,166.6 $ 3,632.8 $ 3,391.5 Operating income (loss) before interest expense and income taxes: Commercial Lines: Underwriting income $ 30.4 $ 20.8 $ 96.2 $ 74.6 Net investment income 44.1 45.0 132.8 135.1 Other expense (0.4 ) (0.7 ) (1.9 ) (1.4 ) Commercial Lines operating income 74.1 65.1 227.1 208.3 Personal Lines: Underwriting income 28.8 28.8 45.4 51.3 Net investment income 19.6 18.2 59.0 54.7 Other income 1.8 1.2 4.3 3.7 Personal Lines operating income 50.2 48.2 108.7 109.7 Other: Underwriting loss (0.4 ) (1.0 ) (1.0 ) (3.1 ) Net investment income 5.1 3.2 16.8 8.2 Other expense (2.7 ) (2.7 ) (8.5 ) (9.7 ) Other operating income (loss) 2.0 (0.5 ) 7.3 (4.6 ) Operating income before interest expense and income taxes 126.3 112.8 343.1 313.4 Interest on debt (9.4 ) (11.2 ) (28.1 ) (33.9 ) Operating income before income taxes 116.9 101.6 315.0 279.5 Non-operating income items: Net realized and unrealized investment gains 14.8 23.0 75.1 3.8 Other non-operating items (1.4 ) (1.9 ) (1.4 ) (1.9 ) Income from continuing operations before income taxes $ 130.3 $ 122.7 $ 388.7 $ 281.4
Identifiable Assets by Business Segment	The following table provides identifiable assets for the Company’s business segments and discontinued operations: September 30, 2019 December 31, 2018 (in millions) Identifiable Assets U.S. Companies $ 12,521.2 $ 12,238.4 Assets held-for-sale (1) — 57.4 Assets of discontinued operations 106.0 103.9 Total $ 12,627.2 $ 12,399.7 (1) See also Note 3 – “Discontinued Operations – Chaucer Business”.

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2019
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Compensation Cost and Related Tax Benefits	Compensation cost for the Company’s stock-based awards and the related tax benefits were as follows: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2019 2018 2019 2018 Stock-based compensation expense $ 4.1 $ 4.0 $ 12.7 $ 10.6 Tax benefit (0.8 ) (0.8 ) (2.6 ) (2.2 ) Stock-based compensation expense, net of taxes $ 3.3 $ 3.2 $ 10.1 $ 8.4
Summary of Stock Option Plan Activity	Information on the Company’s stock option activity for the nine months ended September 30, 2019 and 2018 is summarized below. Nine Months Ended September 30, 2019 2018 (in whole shares and dollars) Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Outstanding, beginning of period 1,099,076 $ 85.75 1,062,177 $ 75.53 Granted 232,568 95.05 301,152 110.98 Exercised (172,551 ) 70.40 (169,196 ) 70.94 Forfeited or cancelled (38,729 ) 107.34 (54,063 ) 89.09 Outstanding, end of period 1,120,364 89.30 1,140,070 84.93
Summary of Restricted Stock Activity	The following tables summarize activity information about employee restricted stock units: Nine Months Ended September 30, 2019 2018 (in whole shares and dollars) Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Time-based restricted stock units: Outstanding, beginning of period 332,481 $ 97.28 298,528 $ 83.45 Granted 146,715 117.71 148,619 111.67 Vested (107,829 ) 84.17 (72,546 ) 72.62 Forfeited (33,156 ) 104.80 (23,130 ) 91.20 Outstanding, end of period 338,211 109.59 351,471 97.10 Performance-based and market-based restricted stock units: Outstanding, beginning of period 69,838 $ 95.58 102,586 $ 81.21 Granted 42,119 117.09 35,063 118.60 Vested (14,118 ) 84.39 (14,032 ) 70.24 Forfeited (2,670 ) 135.92 (7,625 ) 81.91 Outstanding, end of period 95,169 105.63 115,992 93.79

Earnings Per Share and Shareh_2

Earnings Per Share and Shareholders' Equity Transactions (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share [Abstract]
Information Regarding Basic and Diluted Earnings Per Share	The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: Three Months Ended Nine Months Ended September 30, September 30, (in millions, except per share data) 2019 2018 2019 2018 Basic shares used in the calculation of earnings per share 39.6 42.5 40.3 42.5 Dilutive effect of securities: Employee stock options 0.3 0.3 0.3 0.3 Non-vested stock grants 0.3 0.3 0.3 0.3 Diluted shares used in the calculation of earnings per share 40.2 43.1 40.9 43.1 Per share effect of dilutive securities on income from continuing operations $ (0.04 ) $ (0.04 ) $ (0.11 ) $ (0.07 ) Per share effect of dilutive securities on net income $ (0.04 ) $ (0.03 ) $ (0.11 ) $ (0.08 )

Liabilities for Outstanding C_2

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2019
Insurance [Abstract]
Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses	The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. Nine Months Ended September 30, (in millions) 2019 2018 Gross loss and LAE reserves, beginning of period $ 5,304.1 $ 5,058.5 Reinsurance recoverable on unpaid losses 1,472.6 1,455.0 Net loss and LAE reserves, beginning of period 3,831.5 3,603.5 Net incurred losses and LAE in respect of losses occurring in: Current year 2,142.9 2,025.6 Prior years (20.3 ) (7.1 ) Total incurred losses and LAE 2,122.6 2,018.5 Net payments of losses and LAE in respect of losses occurring in: Current year 889.2 848.6 Prior years 1,043.4 997.3 Total payments 1,932.6 1,845.9 Net reserve for losses and LAE, end of period 4,021.5 3,776.1 Reinsurance recoverable on unpaid losses 1,529.0 1,412.8 Gross reserve for losses and LAE, end of period $ 5,550.5 $ 5,188.9

New Accounting Pronouncements_2

New Accounting Pronouncements (Narrative) (Details) $ in Millions	Jan. 01, 2019USD ($)
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative effect to increase assets and liabilities	$ 35
Accounting Standards Update 2017-08 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Cumulative effect on retained earnings, net of tax	$ 1.5

Discontinued Operations Chauc_3

Discontinued Operations Chaucer Business (Narrative) (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Revenues from the Chaucer business	$ 1,214,700,000		$ 1,166,600,000	$ 3,632,800,000	$ 3,391,500,000
Operating income (losses)	126,300,000		112,800,000	343,100,000	313,400,000
Assets held-for-sale	106,000,000			106,000,000		$ 103,900,000
Liabilities held-for-sale	117,200,000			117,200,000		115,900,000
Assets Held-for-Sale [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held-for-sale	0			0		57,400,000
Liabilities held-for-sale	0			0		22,200,000
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale						174,400,000
Income tax expense (benefit)						42,500,000
Contingent proceeds	22,000,000					$ 31,700,000
Increase (decrease) in estimate of pre-tax contingent proceeds	3,800,000	$ (13,500,000)
Recognized income tax charge related to new tax regulations	(4,400,000)			1,200,000
Recognized tax benefits related to prior years	3,400,000
Revenues from the Chaucer business	1,700,000		$ 227,100,000	6,200,000	$ 695,300,000
Operating income (losses)	100,000			(400,000)
Decrease in uncertain tax positions due to expiration of statute of limitations	$ 2,000,000			2,000,000
Chaucer [Member] | Federal [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Recognized income tax charge related to new tax regulations				1,200,000
Chaucer Irish Entity[Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale				400,000
Income tax expense (benefit)				(500,000)
Proceeds from sale of businesses segment				28,000,000
Australian Entities [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale				1,200,000
Income tax expense (benefit)				100,000
Cash proceeds to be received				$ 13,000,000

Discontinued Operations Chauc_4

Discontinued Operations Chaucer Business (Summary of Discontinued Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues
Premiums	$ 1,124.1	$ 1,071.7	$ 3,330.2	$ 3,172.4
Net investment income	68.8	66.4	208.6	198
Total revenues	1,214.7	1,166.6	3,632.8	3,391.5
Losses and operating expenses
Losses and loss adjustment expenses	705.3	676.4	2,122.6	2,018.5
Amortization of deferred acquisition costs	232.8	224.4	692.8	664.7
Other operating expenses	136.9	131.9	400.6	393
Total losses and expenses	1,084.4	1,043.9	3,244.1	3,110.1
Other items:
Interest expense	(9.4)	(11.2)	(28.1)	(33.9)
Net realized and unrealized investment gains (losses)			74.9	3.8
Chaucer [Member]
Revenues
Premiums		212.4		648.2
Net investment income		12.8		41.7
Other income		1.9		5.4
Total revenues	1.7	227.1	6.2	695.3
Losses and operating expenses
Losses and loss adjustment expenses		136.8		364.3
Amortization of deferred acquisition costs		60.1		196.7
Other operating expenses		26.4		85.7
Total losses and expenses		223.3		646.7
Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income)		3.8		48.6
Other items:
Interest expense		(1.5)		(2.8)
Net realized and unrealized investment gains (losses)		0.1		(0.4)
Other expenses		(7.1)		(8.3)
Income (loss) from Chaucer business before income taxes		(4.7)		37.1
Income tax benefit (expense)		1.1		(6.6)
Income (loss) from Chaucer business, net of income taxes	$ 2.1	$ (3.6)	$ 1.6	$ 30.5

Discontinued Operations Chauc_5

Discontinued Operations Chaucer Business (Summary of Cash Flows Associated with the Discontinued Chaucer Business) (Details) - Chaucer [Member] $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash provided by operating activities	$ 46.2
Net cash provided by investing activities	$ 64.3

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Taxes [Line Items]
Federal income tax expense			$ 69.9	$ 44.5
Chaucer [Member]
Income Taxes [Line Items]
Income tax expense (benefit), discontinued operations		$ (1.1)		6.6
Additional income tax incurred in sales of business as result from effect of tax regulations	$ (4.4)		1.2
Decrease in uncertain tax positions due to expiration of statute of limitations	$ 2		2
Chaucer [Member] | Federal [Member]
Income Taxes [Line Items]
Income tax expense (benefit), discontinued operations				(0.2)
Additional income tax incurred in sales of business as result from effect of tax regulations			$ 1.2
Chaucer [Member] | Foreign [Member]
Income Taxes [Line Items]
Income tax expense (benefit), discontinued operations				$ 6.8

Debt (Schedule of Debt) (Detail

Debt (Schedule of Debt) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Short-term borrowings	$ 14.5
Total principal debt	677.2	$ 787.7
Unamortized debt discount	(9.4)	(9.8)
Total	667.8	777.9
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Long-term borrowings	$ 375	$ 375
Maturity date	Apr. 15, 2026	Apr. 15, 2026
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Long-term borrowings	$ 62.6	$ 62.6
Maturity date	Oct. 15, 2025	Oct. 15, 2025
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Long-term borrowings	$ 175	$ 175
Maturity date	Mar. 30, 2053	Mar. 30, 2053
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Long-term borrowings	$ 50.1	$ 50.1
Maturity date	Feb. 3, 2027	Feb. 3, 2027
FHLB borrowings (secured) [Member]
Debt Instrument [Line Items]
Long-term borrowings		$ 125

Debt (Narrative) (Details)

Debt (Narrative) (Details) - USD ($) $ in Millions	Jan. 02, 2019	Sep. 30, 2019
Debt Instrument [Line Items]
Repayment of FHLB advance	$ 125
FHLB due date	2029
FHLB Interest rate	5.50%
Prepayment fees	$ 26
Short-term advances		$ 14.5
Federal Home Loan Bank [Member]
Debt Instrument [Line Items]
Short-term advances		$ 14.5
Short-term advances repayment term		2019-10

Investments (Schedule of Availa

Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 6,692.5	$ 6,245.9
Fair Value	6,946.3	6,161.5
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	848.3	879
Gross Unrealized Gains	34.4	16.6
Gross Unrealized Loss	0.4	9.8
Fair Value	882.3	885.8
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6,245.9
Gross Unrealized Gains		49.8
Gross Unrealized Loss		134.2
Fair Value		6,161.5
OTTI Unrealized Losses		6.6
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		24.9
Gross Unrealized Loss		0.4
Fair Value		24.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,692.5	6,270.8
Gross Unrealized Gains	260.8	49.8
Gross Unrealized Loss	7	134.6
Fair Value	6,946.3	6,186
OTTI Unrealized Losses	3.3	6.6
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	350.2	414.7
Gross Unrealized Gains	12	2.4
Gross Unrealized Loss	0.4	7.2
Fair Value	361.8	409.9
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5	7.3
Gross Unrealized Gains	0.1	0.1
Fair Value	5.1	7.4
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,758.4	3,476.6
Gross Unrealized Gains	159.1	26.1
Gross Unrealized Loss	5.3	92
Fair Value	3,912.2	3,410.7
OTTI Unrealized Losses	3.3	6.6
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	959.8	728.4
Gross Unrealized Gains	20.3	2.7
Gross Unrealized Loss	0.9	14.7
Fair Value	979.2	716.4
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	685.2	648.4
Gross Unrealized Gains	33.4	1.7
Gross Unrealized Loss		9.8
Fair Value	718.6	640.3
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	85.6	91.5
Gross Unrealized Gains	1.5	0.2
Gross Unrealized Loss		0.7
Fair Value	$ 87.1	$ 91

Investments (Narrative) (Detail

Investments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities	$ 4.5		$ 4.5		$ 7.4
Other-than-temporary impairments on investments recognized in earnings	0.8	$ 0.4	1.2	$ 2.8
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Other-than-temporary impairments on investments	2.1	0.7	2.8
Other-than-temporary impairments on investments recorded as unrealized losses in AOCI	1.3	0.3	1.6
Other-than-temporary impairments on investments recognized in earnings	$ 0.8	$ 0.4	$ 1.2
Fixed Maturities and Other Invested Assets [Member]
Schedule of Investments [Line Items]
Other-than-temporary impairments on investments				3.4
Other-than-temporary impairments on investments recorded as unrealized losses in AOCI				0.6
Other-than-temporary impairments on investments recognized in earnings				$ 2.8

Investments (Investments Classi

Investments (Investments Classified by Contractual Maturity Date) (Details) $ in Millions	Sep. 30, 2019USD ($)
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost	$ 266.2
Due after one year through five years, Amortized Cost	1,944.8
Due after five years through ten years, Amortized Cost	2,358.3
Due after ten years, Amortized Cost	392.6
Gross fixed maturities, Amortized Cost	4,961.9
Mortgage-backed and asset-backed securities, Amortized Cost	1,730.6
Total fixed maturities, Amortized Cost	6,692.5
Due in one year or less, Fair Value	268.7
Due after one year through five years, Fair Value	2,009.9
Due after five years through ten years, Fair Value	2,473.7
Due after ten years, Fair Value	409.1
Gross fixed maturities, Fair Value	5,161.4
Mortgage-backed and asset-backed securities, Fair Value	1,784.9
Total fixed maturities, Fair Value	$ 6,946.3

Investments (Schedule of Unreal

Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 5
12 months or less, Fair Value	266.1
Greater than 12 months, Gross Unrealized Losses	2
Greater than 12 months, Fair Value	152.4
Total, Gross Unrealized Losses	7
Total, Fair Value	418.5
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.8	$ 38.3
12 months or less, Fair Value	225.3	1,992.4
Greater than 12 months, Gross Unrealized Losses	1.2	77.8
Greater than 12 months, Fair Value	143.4	1,758.5
Total, Gross Unrealized Losses	3	116.1
Total, Fair Value	368.7	3,750.9
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		8.1
12 months or less, Fair Value		185.6
Greater than 12 months, Gross Unrealized Losses		10
Greater than 12 months, Fair Value		54.9
Total, Gross Unrealized Losses		18.1
Total, Fair Value		240.5
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		46.4
12 months or less, Fair Value		2,178
Greater than 12 months, Gross Unrealized Losses		87.8
Greater than 12 months, Fair Value		1,813.4
Total, Gross Unrealized Losses		134.2
Total, Fair Value		3,991.4
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.1
12 months or less, Fair Value		4
Greater than 12 months, Gross Unrealized Losses		0.3
Greater than 12 months, Fair Value		18
Total, Gross Unrealized Losses		0.4
Total, Fair Value		22
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		46.5
12 months or less, Fair Value		2,182
Greater than 12 months, Gross Unrealized Losses		88.1
Greater than 12 months, Fair Value		1,831.4
Total, Gross Unrealized Losses		134.6
Total, Fair Value		4,013.4
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	1.1
12 months or less, Fair Value	37.1	66
Greater than 12 months, Gross Unrealized Losses	0.1	6.1
Greater than 12 months, Fair Value	17.2	210.9
Total, Gross Unrealized Losses	0.4	7.2
Total, Fair Value	54.3	276.9
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value		2
Greater than 12 months, Fair Value		0.8
Total, Fair Value		2.8
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.8
12 months or less, Fair Value	29.3	110
Greater than 12 months, Gross Unrealized Losses	0.2	9
Greater than 12 months, Fair Value	15.5	248
Total, Gross Unrealized Losses	0.4	9.8
Total, Fair Value	44.8	358
Municipal [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value		0.9
Total, Fair Value		0.9
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	30
12 months or less, Fair Value	79.8	1,277.9
Greater than 12 months, Gross Unrealized Losses	0.4	43.9
Greater than 12 months, Fair Value	24	781.6
Total, Gross Unrealized Losses	1.3	73.9
Total, Fair Value	103.8	2,059.5
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.2	8.1
12 months or less, Fair Value	40.8	185.6
Greater than 12 months, Gross Unrealized Losses	0.8	10
Greater than 12 months, Fair Value	9	54
Total, Gross Unrealized Losses	4	18.1
Total, Fair Value	49.8	239.6
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	2.6
12 months or less, Fair Value	76.8	201.2
Greater than 12 months, Gross Unrealized Losses	0.5	12.1
Greater than 12 months, Fair Value	58.4	323.7
Total, Gross Unrealized Losses	0.9	14.7
Total, Fair Value	135.2	524.9
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		3.4
12 months or less, Fair Value	1.8	293
Greater than 12 months, Gross Unrealized Losses		6.4
Greater than 12 months, Fair Value	25	175.5
Total, Gross Unrealized Losses		9.8
Total, Fair Value	26.8	468.5
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.4
12 months or less, Fair Value	0.5	42.3
Greater than 12 months, Gross Unrealized Losses		0.3
Greater than 12 months, Fair Value	3.3	18
Total, Gross Unrealized Losses		0.7
Total, Fair Value	$ 3.8	$ 60.3

Investments (Schedule of Realiz

Investments (Schedule of Realized Gain (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 86.9	$ 83.8	$ 309.5	$ 225.1
Gross Gains	1.6	0.8	4.2	2
Gross Losses	2.4	1.5	5	5.3
Fixed Maturities Held-For-Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		21.4	0.3	172.5
Gross Gains		0.2		1.3
Gross Losses		0.1		1.1
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	86.9	105.2	309.8	397.6
Gross Gains	1.6	1	4.2	3.3
Gross Losses	$ 2.4	$ 1.6	$ 5	$ 6.4

Investments (Rollforward of Cum

Investments (Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Credit losses as of the beginning of the period	$ 2.9	$ 3.7	$ 3.8	$ 3.6
Credit losses on securities for which an OTTI was not previously recognized	0.6	0.2	1	1
Additional credit losses on securities for which an OTTI was previously recognized				0.1
Reductions for securities sold, matured or called	(1.3)		(2.6)	(0.8)
Reductions for securities reclassified as intended to sell	(0.3)		(0.3)
Credit losses as of the end of the period	$ 1.9	$ 3.9	$ 1.9	$ 3.9

Investments (Schedule of Pre-ta

Investments (Schedule of Pre-tax Realized and Unrealized Gains and Losses on Equity Securities From Continuing Operations) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Net gains recognized during the period	$ 15	$ 23.6	$ 75.3	$ 6.7
Less: net gains (losses) recognized on equity securities sold during the period	(0.2)	(0.2)	1	(1)
Net unrealized gains recognized during the period on equity securities still held	$ 15.2	$ 23.8	$ 74.3	$ 7.7

Fair Value (Fair Value of Finan

Fair Value (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 157.6	$ 1,020.7
Total financial instruments, excluding held-for-sale	8,304	8,236
Total financial instruments	8,304	8,263.8
Carrying Value [Member] | Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)		27.8
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	157.6	1,020.7
Total financial instruments, excluding held-for-sale	8,328.1	8,240.5
Total financial instruments	8,328.1	8,268.3
Debt	732	825
Estimated Fair Value [Member] | Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)		27.8
Income Approach Valuation Technique [Member] | Carrying Value [Member] | Fair Value Through AOCI [Member] | Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	6,946.3	6,161.5
Income Approach Valuation Technique [Member] | Carrying Value [Member] | Fair Value Through Net Income [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	180	175
Income Approach Valuation Technique [Member] | Carrying Value [Member] | Fair Value Through Net Income [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	570.8	464.4
Income Approach Valuation Technique [Member] | Estimated Fair Value [Member] | Fair Value Through AOCI [Member] | Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	6,946.3	6,161.5
Income Approach Valuation Technique [Member] | Estimated Fair Value [Member] | Fair Value Through Net Income [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	180	175
Income Approach Valuation Technique [Member] | Estimated Fair Value [Member] | Fair Value Through Net Income [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	570.8	464.4
Cost Approach Valuation Technique [Member] | Carrying Value [Member] | Amortized Cost/Cost [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	157.6	1,020.7
Debt	667.8	777.9
Cost Approach Valuation Technique [Member] | Carrying Value [Member] | Amortized Cost/Cost [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	449.3	414.4
Cost Approach Valuation Technique [Member] | Estimated Fair Value [Member] | Amortized Cost/Cost [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	157.6	1,020.7
Debt	732	825
Cost Approach Valuation Technique [Member] | Estimated Fair Value [Member] | Amortized Cost/Cost [Member] | Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	$ 473.4	$ 418.9

Fair Value (Fair Value,Investme

Fair Value (Fair Value,Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,946.3	$ 6,161.5
Equity securities	570.8	464.4
Other investments	722.5	661.5
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	882.3	885.8
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	2.1	1.1
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,946.3	6,161.5
Equity securities	570.8	464.4
Other investments	3.5	3.5
Total investment assets at fair value, excluding held-for-sale	7,520.6	6,653.9
Total investment assets at fair value, excluding held-for-sale (Chaucer)		6,629.4
Fair Value, Measurements, Recurring [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment assets, held-for-sale		24.5
Fair Value, Measurements, Recurring [Member] | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	361.8	409.9
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	5.1	7.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	882.3	885.8
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,912.2	3,410.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities U S Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	976.6	713.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.6	2.7
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	718.6	640.3
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	87.1	91
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	162	154.9
Equity securities	568.7	463.3
Total investment assets at fair value, excluding held-for-sale	730.7	623.9
Total investment assets at fair value, excluding held-for-sale (Chaucer)		618.2
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment assets, held-for-sale		5.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	162	154.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,758	5,971.6
Total investment assets at fair value, excluding held-for-sale	6,758	5,990.4
Total investment assets at fair value, excluding held-for-sale (Chaucer)		5,971.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment assets, held-for-sale		18.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury And Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	199.8	255
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	5.1	7.4
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	869.7	864.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,911.5	3,409.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage Backed Securities U S Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	976.6	713.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.6	2.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	705.6	627.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	87.1	91
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	26.3	35
Equity securities	2.1	1.1
Other investments	3.5	3.5
Total investment assets at fair value, excluding held-for-sale	31.9	39.6
Total investment assets at fair value, excluding held-for-sale (Chaucer)		39.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	12.6	21.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.7	0.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 13	$ 13.1

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Fair Value Measurements [Line Items]
Investments measured at net asset value based on an ownership interest in partners'	2.00%		2.00%		2.00%
Transfer between Level 2 and Level 3	$ 0	$ 0	$ 0	$ 0
Fair Value Measured Using NAV [Member] | Limited Partnerships Interest [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	176,500,000		176,500,000		$ 171,500,000
Level 3 [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	3,500,000		3,500,000		$ 3,500,000
Liabilities held	$ 0	$ 0	$ 0	$ 0

Fair Value (Estimated Fair Valu

Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	$ 570.8	$ 464.4
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	2.1	1.1
Other investments	3.5	3.5
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	157.6	1,020.7
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	157.6	1,020.7
Total financial instruments, excluding held-for-sale		1,439.6
Total financial instruments	631	1,442.9
Other investments	473.4	418.9
Debt	732	825
Estimated Fair Value [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	157.6	1,020.7
Total financial instruments, excluding held-for-sale		1,020.7
Total financial instruments	157.6	1,024
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial instruments, excluding held-for-sale		8.7
Total financial instruments	2.6	8.7
Other investments	2.6	8.7
Debt	732	825
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial instruments, excluding held-for-sale		410.2
Total financial instruments	470.8	410.2
Other investments	$ 470.8	410.2
Estimated Fair Value [Member] | Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)		3.3
Estimated Fair Value [Member] | Chaucer [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)		$ 3.3

Fair Value (Fair Value on Recur

Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 32.2	$ 40.9	$ 39.6	$ 44.4
Transfers out of Level 3		(0.5)		(0.5)
Included in other comprehensive income-net appreciation on available-for-sale securities	0.3	(0.1)	1.6	(1.1)
Purchases and sales:
Purchases			1
Sales	(0.6)	(0.6)	(10.3)	(3.2)
Ending Balance	31.9	39.7	31.9	39.7
Included in total net realized and unrealized investment gains (losses)				0.1
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.9	22.2	21.1	24.6
Transfers out of Level 3		(0.5)		(0.5)
Included in other comprehensive income-net appreciation on available-for-sale securities	0.1		0.9	(0.6)
Purchases and sales:
Sales	(0.4)	(0.4)	(9.4)	(2.3)
Ending Balance	12.6	21.3	12.6	21.3
Included in total net realized and unrealized investment gains (losses)				0.1
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.7	0.8	0.8	0.9
Purchases and sales:
Sales		(0.1)	(0.1)	(0.2)
Ending Balance	0.7	0.7	0.7	0.7
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	13	13.2	13.1	14.2
Included in other comprehensive income-net appreciation on available-for-sale securities	0.2	(0.1)	0.7	(0.5)
Purchases and sales:
Sales	(0.2)	(0.1)	(0.8)	(0.7)
Ending Balance	13	13	13	13
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	26.6	36.2	35	39.7
Transfers out of Level 3		(0.5)		(0.5)
Included in other comprehensive income-net appreciation on available-for-sale securities	0.3	(0.1)	1.6	(1.1)
Purchases and sales:
Sales	(0.6)	(0.6)	(10.3)	(3.2)
Ending Balance	26.3	35	26.3	35
Included in total net realized and unrealized investment gains (losses)				0.1
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	5.6	4.7	4.6	4.7
Purchases and sales:
Purchases			1
Ending Balance	$ 5.6	$ 4.7	$ 5.6	$ 4.7

Fair Value (Schedule of Additio

Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3 Assets) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Fair Value [Line Items]
Fair Value	$ 6,946.3	$ 6,161.5
Equity securities, at fair value	570.8	464.4
Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 2.1	$ 1.1
Equity Securities, FV-NI, Valuation Technique [Extensible List]	us-gaap:MarketApproachValuationTechniqueMember	us-gaap:MarketApproachValuationTechniqueMember
Equity Securities, FV-NI, Measurement Input [Extensible List]	thg:MeasurementInputNetTangibleAssetMarketMultipleMember	thg:MeasurementInputNetTangibleAssetMarketMultipleMember
Other investments	$ 3.5	$ 3.5
Alternative Investment, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Alternative Investment, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Fair value measurement market multiples	1	1
Range	0.180	0.180
Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1	1
Range	0.180	0.180
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 882.3	$ 885.8
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 12.6	$ 21.1
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Small issue size		1.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	0.70%	0.70%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	6.80%	6.80%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	4.30%	3.40%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount For Credit Stress [Member]
Fair Value [Line Items]
Small issue size		1.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.7	$ 0.8
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount Rate [Member]
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 13	$ 13.1
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	1.90%	1.90%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	3.10%	3.10%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	2.70%	2.70%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount For Above-market coupon [Member]
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount For Lease Structure [Member]
Fair Value [Line Items]
Small issue size	0.30%	0.30%

Pension and Other Postretirem_3

Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	$ 5.3	$ 5	$ 15.9	$ 14.9
Expected return on plan assets	(5.8)	(5.2)	(17.5)	(15.5)
Recognized net actuarial loss	2.8	2.4	8.5	7.2
Net periodic pension cost	2.3	2.2	6.9	6.6
Postretirement Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	0.1	0.1	0.3	0.3
Recognized net actuarial loss			0.1	0.1
Amortization of prior service cost			(0.1)	(0.2)
Net periodic pension cost	$ 0.1	$ 0.1	$ 0.3	$ 0.2

Other Comprehensive Income (Cha

Other Comprehensive Income (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Equity [Abstract]
Unrealized gains (losses) arising during period, Pre-Tax	$ 58.4	$ (29.2)	$ 343.6	$ (242.1)
Amount of realized gains from sales and other, Pre-Tax	(0.6)	(0.2)	(1.1)
Portion of other-than-temporary impairment losses recognized in earnings, Pre-Tax	0.8	0.5	1.2	2
Net unrealized gains (losses), Pre-Tax	58.6	(28.9)	343.7	(240.1)
Net change in net actuarial loss and prior service cost, Pre-Tax	2.8	2.4	8.5	0.6
Foreign currency translation recognized during the period, Pre-Tax		2	0.9	0.8
Other comprehensive (loss) income, Pre-Tax	61.4	(24.5)	353.1	(238.7)
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	(12.3)	6.2	(72.2)	50.9
Amount of realized gains from sales and other, Tax Benefit (Expense)	(3.1)	(2.4)	(10.2)	(8.2)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	(0.2)	(0.1)	(0.3)	(0.4)
Net unrealized gains, Tax Benefit (Expense)	(15.6)	3.7	(82.7)	42.3
Net change in net actuarial loss and prior service cost, Tax Benefit (Expense)	(0.5)	(0.5)	(1.7)	(0.1)
Foreign currency translation recognized during the period, Tax Benefit (Expense)		(0.4)	(0.2)	(0.2)
Other comprehensive (loss) income, Tax Benefit (Expense)	(16.1)	2.8	(84.6)	42
Unrealized gains (losses) arising during period, Net of Tax	46.1	(23)	271.4	(191.2)
Amount of realized gains from sales and other, Net of Tax	(3.7)	(2.6)	(11.3)	(8.2)
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	0.6	0.4	0.9	1.6
Total available-for-sale securities	43	(25.2)	261	(197.8)
Net change in net actuarial loss and prior service cost, Net of Tax	2.3	1.9	6.8	0.5
Foreign currency translation recognized during the period, Net of Tax		1.6	0.7	0.6
Total other comprehensive income (loss), net of tax	$ 45.3	$ (21.7)	$ 268.5	$ (196.7)

Other Comprehensive Income (Rec

Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	$ 0.6	$ (0.2)	$ 1	$ (0.1)
Net other–than–temporary impairment losses on investments recognized in earnings	(0.8)	(0.4)	(1.2)	(2.8)
Income from continuing operations before income taxes	130.3	122.7	388.7	281.4
Loss adjustment expenses and other operating expenses	(705.3)	(676.4)	(2,122.6)	(2,018.5)
Tax benefit	(19.1)	(18.7)	(69.9)	(44.5)
Continued operations; net of tax	111.2	104	318.8	236.9
Net of tax	118.9	100.4	315.3	267.4
Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses		(136.8)		(364.3)
Discontinued operations - Chaucer business	2.1	(3.6)	1.6	30.5
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	0.8	0.2	3.6	0.8
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other	0.6	(0.2)	1.1	1.6
Net other–than–temporary impairment losses on investments recognized in earnings	(0.8)	(0.4)	(1.2)	(1.7)
Income from continuing operations before income taxes	(0.2)	(0.6)	(0.1)	(0.1)
Tax benefit	3.3	2.5	10.5	8.2
Continued operations; net of tax	3.1	1.9	10.4	8.1
Net of tax	3.1	2.2	10.4	6.6
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member] | Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Discontinued operations - Chaucer business		0.3		(1.5)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined Benefit Pension and Postretirement Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses	(2.8)	(2.5)	(8.5)	(7.2)
Tax benefit	0.5	0.5	1.7	1.5
Continued operations; net of tax	(2.3)	(2)	(6.8)	(5.7)
Net of tax	$ (2.3)	$ (2)	$ (6.8)	(5.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined Benefit Pension and Postretirement Plans [Member] | Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Discontinued operations - Chaucer business				$ (0.1)

Other Comprehensive Income (R_2

Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Parenthetical) (Details) - Defined Benefit Pension and Postretirement Plans [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Comprehensive Income (Loss) [Line Items]
Loss Adjustment Expense	40.00%	40.00%	40.00%	40.00%
Percentage Of Other Operating Expenses	60.00%	60.00%	60.00%	60.00%

Segment Information (Narrative)

Segment Information (Narrative) (Details)	9 Months Ended
	Sep. 30, 2019segment
Segment Reporting [Abstract]
Operating segments	3

Segment Information (Financial

Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Operating revenues	$ 1,199.9	$ 1,143.6	$ 3,557.7	$ 3,387.7
Net realized and unrealized investment gains	14.8	23	75.1	3.8
Total revenues	1,214.7	1,166.6	3,632.8	3,391.5
Net investment income	68.8	66.4	208.6	198
Operating income (loss) before interest expense and income taxes	126.3	112.8	343.1	313.4
Interest on debt	(9.4)	(11.2)	(28.1)	(33.9)
Operating income before income taxes	116.9	101.6	315	279.5
Other non-operating items	(1.4)	(1.9)	(1.4)	(1.9)
Income from continuing operations before income taxes	130.3	122.7	388.7	281.4
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	710.3	687.4	2,114.2	2,045.7
Underwriting income (loss)	30.4	20.8	96.2	74.6
Net investment income	44.1	45	132.8	135.1
Other income (expense)	(0.4)	(0.7)	(1.9)	(1.4)
Operating income (loss) before interest expense and income taxes	74.1	65.1	227.1	208.3
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	483.3	452.4	1,423.1	1,331.8
Underwriting income (loss)	28.8	28.8	45.4	51.3
Net investment income	19.6	18.2	59	54.7
Other income (expense)	1.8	1.2	4.3	3.7
Operating income (loss) before interest expense and income taxes	50.2	48.2	108.7	109.7
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	6.3	3.8	20.4	10.2
Underwriting income (loss)	(0.4)	(1)	(1)	(3.1)
Net investment income	5.1	3.2	16.8	8.2
Other expense	(2.7)	(2.7)	(8.5)	(9.7)
Operating income (loss) before interest expense and income taxes	$ 2	$ (0.5)	$ 7.3	$ (4.6)

Segment Information (Identifiab

Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($)	Sep. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Identifiable assets	$ 12,627,200,000	$ 12,399,700,000
Identifiable assets	106,000,000	103,900,000
Assets Held-for-Sale [Member]
Segment Reporting Information [Line Items]
Identifiable assets	0	57,400,000
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	106,000,000	103,900,000
U.S. Companies [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 12,521,200,000	$ 12,238,400,000

Stock-based Compensation (Narra

Stock-based Compensation (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense, net of taxes	$ 3.3	$ 3.2	$ 10.1	$ 8.4
Time-based, Market-based and Performance-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			3 years
Market-based awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			3 years
Granted, Shares			23,924	31,688
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares			42,119	35,063
Performance-Based and Market-Based Restricted Stock Units [Member] | Minimum [Member] | Senior management [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range			0.00%
Performance-Based and Market-Based Restricted Stock Units [Member] | Maximum [Member] | Senior management [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range			150.00%
Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares			5,820	3,115
Increase as grant in period, percentage			100.00%
Performance-based restricted stock units [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range			0.00%
Performance-based restricted stock units [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range			150.00%
Chaucer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense, net of taxes				$ 1.2
2014 Stock Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	3,645,690		3,645,690
ESPP Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	2,361,547		2,361,547

Stock-based Compensation (Compe

Stock-based Compensation (Compensation Cost and Related Tax Benefits) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based compensation expense	$ 4.1	$ 4	$ 12.7	$ 10.6
Tax benefit	(0.8)	(0.8)	(2.6)	(2.2)
Stock-based compensation expense, net of taxes	$ 3.3	$ 3.2	$ 10.1	$ 8.4

Stock-based Compensation (Summa

Stock-based Compensation (Summary of Stock Option Activity) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Shares
Outstanding, beginning of period, Shares	1,099,076	1,062,177
Granted, Shares	232,568	301,152
Exercised, Shares	(172,551)	(169,196)
Forfeited or cancelled, Shares	(38,729)	(54,063)
Outstanding, end of period, Shares	1,120,364	1,140,070
Weighted Average Exercise Price
Outstanding, beginning of period, Weighted Average Exercise Price	$ 85.75	$ 75.53
Granted, Weighted Average Exercise Price	95.05	110.98
Exercised, Weighted Average Exercise Price	70.40	70.94
Forfeited or cancelled, Weighted Average Exercise Price	107.34	89.09
Outstanding, end of period, Weighted Average Exercise Price	$ 89.30	$ 84.93

Stock-based Compensation (Sum_2

Stock-based Compensation (Summary of Restricted Stock Activity) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Time-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	332,481	298,528
Granted, Shares	146,715	148,619
Vested, Shares	(107,829)	(72,546)
Forfeited, Shares	(33,156)	(23,130)
Outstanding, end of period, Shares	338,211	351,471
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 97.28	$ 83.45
Granted, Weighted Average Grant Date Fair Value	117.71	111.67
Vested, Weighted Average Grant Date Fair Value	84.17	72.62
Forfeited, Weighted Average Grant Date Fair Value	104.80	91.20
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 109.59	$ 97.10
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	69,838	102,586
Granted, Shares	42,119	35,063
Vested, Shares	(14,118)	(14,032)
Forfeited, Shares	(2,670)	(7,625)
Outstanding, end of period, Shares	95,169	115,992
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 95.58	$ 81.21
Granted, Weighted Average Grant Date Fair Value	117.09	118.60
Vested, Weighted Average Grant Date Fair Value	84.39	70.24
Forfeited, Weighted Average Grant Date Fair Value	135.92	81.91
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 105.63	$ 93.79

Earnings Per Share and Shareh_3

Earnings Per Share and Shareholders' Equity Transactions (Information Regarding Basic and Diluted Earnings Per Share) (Details) - $ / shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic shares used in the calculation of earnings per share	39.6	42.5	40.3	42.5
Diluted shares used in the calculation of earnings per share	40.2	43.1	40.9	43.1
Per share effect of dilutive securities on income from continuing operations	$ (0.04)	$ (0.04)	$ (0.11)	$ (0.07)
Per share effect of dilutive securities on net income	$ (0.04)	$ (0.03)	$ (0.11)	$ (0.08)
Employee Stock Option [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.3	0.3	0.3	0.3
Non-Vested Stock Grants [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.3	0.3	0.3	0.3

Earnings Per Share and Shareh_4

Earnings Per Share and Shareholders' Equity Transactions (Narrative) (Details) - USD ($) shares in Millions	Oct. 30, 2019	Jun. 28, 2019	Jan. 02, 2019	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 30, 2018
Class Of Stock [Line Items]
Antidilutive securities excluded from calculation of earnings per share				0.2	0.2	0.3
December Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Final settlement of repurchase shares of common shares received		0.3
Amount paid by company			$ 250,000,000
Initial delivery of shares of common stock received			1.8
Shares repurchased under repurchase agreement, percentage			80.00%
Termination date of accelerated share repurchase program			Jun. 27, 2019
June Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Amount paid by company		$ 150,000,000
Initial delivery of shares of common stock received		0.9
Shares repurchased under repurchase agreement, percentage		80.00%
Termination date of accelerated share repurchase program			Oct. 28, 2019
June Accelerated Share Repurchase Agreement [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Final settlement of repurchase shares of common shares received	0.2
Maximum [Member] | Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Repurchases common stock, authorized							$ 600,000,000

Liabilities for Outstanding C_3

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Insurance [Abstract]
Gross loss and LAE reserves, beginning of period	$ 5,304.1	$ 5,058.5
Reinsurance recoverable on unpaid losses, beginning of period	1,472.6	1,455
Net loss and LAE reserves, beginning of period	3,831.5	3,603.5
Net incurred losses and LAE in respect of losses occurring in current year	2,142.9	2,025.6
Net incurred losses and LAE in respect of losses occurring in prior years	(20.3)	(7.1)
Total incurred losses and LAE	2,122.6	2,018.5
Net payments of losses and LAE in respect of losses occurring in Current year	889.2	848.6
Net payments of losses and LAE in respect of losses occurring in Prior years	1,043.4	997.3
Total payments	1,932.6	1,845.9
Net reserve for losses and LAE, end of period	4,021.5	3,776.1
Reinsurance recoverable on unpaid losses, end of Period	1,529	1,412.8
Gross reserve for losses and LAE, end of period	$ 5,550.5	$ 5,188.9

Liabilities for Outstanding C_4

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (20.3)	$ (7.1)
Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(14.2)
Other Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(14)
Commercial and Personal Automobile Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		$ (22.8)