The Hanover Insurance Group, Inc. (THG) 10-K 24 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
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 Public Float			$ 3,814,120,782
Entity Common Stock, Shares Outstanding		36,387,389
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
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 Annual Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
ICFR Auditor Attestation Flag	true
Documents Incorporated by Reference	Portions of The Hanover Insurance Group, Inc.’s Proxy Statement to be filed pursuant to Regulation 14A relating to the 2021 Annual Meeting of Shareholders to be held May 11, 2021 are incorporated by reference in Part III.
Common Stock, $.01 Par Value [Member]
Document Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 par value
Trading Symbol	THG
Security Exchange Name	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 Symbol	THG
Security Exchange Name	NYSE

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Premiums	$ 4,527.4	$ 4,474.5	$ 4,254.4
Net investment income	265.1	281.3	267.4
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	17.9	4.9	(2.7)
Net change in fair value of equity securities	13.4	106.5	(43.4)
Impairment losses on investments	(26.3)	(2)	(4.6)
Total net realized and unrealized investment gains (losses)	5	109.4	(50.7)
Fees and other income	29.8	25.5	23.2
Total revenues	4,827.3	4,890.7	4,494.3
Losses and expenses
Losses and loss adjustment expenses	2,845.2	2,865.5	2,724.6
Amortization of deferred acquisition costs	951	926.7	891.8
Interest expense	37.1	37.5	45.1
Loss on repayment of debt	6.2		28.2
Other operating expenses	543	538.9	522.1
Total losses and expenses	4,382.5	4,368.6	4,211.8
Income from continuing operations before income taxes	444.8	522.1	282.5
Income tax expense (benefit):
Current	105.9	80.5	46.2
Deferred	(23.1)	12.6	(2.7)
Total income tax expense	82.8	93.1	43.5
Income from continuing operations	362	429	239
Discontinued operations:
Net income	$ 358.7	$ 425.1	$ 391
Basic:
Income from continuing operations	$ 9.61	$ 10.72	$ 5.63
Net income per share	$ 9.52	$ 10.62	$ 9.21
Weighted average shares outstanding	37.7	40	42.4
Diluted:
Income from continuing operations	$ 9.50	$ 10.56	$ 5.56
Net income per share	$ 9.42	$ 10.46	$ 9.09
Weighted average shares outstanding	38.1	40.6	43
Chaucer [Member]
Revenues
Premiums			$ 850
Net investment income			54.9
Net realized and unrealized investment gains (losses):
Total revenues		$ 6.7	912.4
Losses and expenses
Losses and loss adjustment expenses			515.5
Amortization of deferred acquisition costs			252.5
Interest expense			3.8
Other operating expenses			115
Total losses and expenses			883
Discontinued operations:
Sale of business, net of taxes		(1.2)	131.9
Income (loss) from discontinued businesses, net of taxes	$ 0.4	$ 1.6	$ 20
Basic:
Sale of business, net of taxes		$ (0.03)	$ 3.11
Income from Chaucer business, net of taxes	$ 0.01	0.04	0.47
Diluted:
Sale of business, net of taxes		(0.03)	3.07
Income (loss) from discontinued businesses, net of taxes	$ 0.01	$ 0.04	$ 0.46
Life [Member]
Discontinued operations:
Income (loss) from discontinued businesses, net of taxes	$ (3.7)	$ (4.3)	$ 0.1
Basic:
Income from Chaucer business, net of taxes	$ (0.10)	$ (0.11)
Diluted:
Income (loss) from discontinued businesses, net of taxes	$ (0.09)	$ (0.11)

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 358.7	$ 425.1	$ 391
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income	210.6	238.8	(170.1)
Having credit losses recognized in the Consolidated Statements of Income	1.5	2.8
Amount realized with sale of Chaucer business		0.1	19.1
Total available-for-sale securities	212.1	241.7	(151)
Pension and postretirement benefits:
Net actuarial gains (losses) and prior service costs arising in the period	3.1	16.2	(17)
Amortization recognized as net periodic benefit and postretirement cost	4.7	9	7.7
Amount realized with sale of Chaucer business			16.4
Total pension and postretirement benefits	7.8	25.2	7.1
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period			(1.7)
Amount realized with sale of Chaucer business		0.7	23.2
Total cumulative foreign currency translation adjustment		0.7	21.5
Total other comprehensive income (loss), net of tax	219.9	267.6	(122.4)
Comprehensive income	$ 578.6	$ 692.7	$ 268.6

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
Fixed maturities, at fair value (amortized cost of $6,945.6 and $6,452.2)	$ 7,454.4	$ 6,687.1
Equity securities, at fair value	598.5	575.7
Other investments	793.2	733.2
Total investments	8,846.1	7,996
Cash and cash equivalents	120.6	215.7
Accrued investment income	51.2	53
Premiums and accounts receivable, net	1,339.3	1,260.4
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,874.3	1,814
Deferred acquisition costs	477.5	467.4
Goodwill	178.8	178.8
Other assets	445.7	402.4
Assets of discontinued businesses	110.2	102.8
Total assets	13,443.7	12,490.5
Liabilities
Loss and loss adjustment expense reserves	6,024	5,654.4
Unearned premiums	2,482.7	2,416.7
Expenses and taxes payable	687.5	627.7
Deferred income tax liability	97.3	51.8
Reinsurance premiums payable	48.4	53.4
Debt	780.8	653.4
Liabilities of discontinued businesses	120.8	116.9
Total liabilities	10,241.5	9,574.3
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	0.6	0.6
Additional paid-in capital	1,857.4	1,837.3
Accumulated other comprehensive income	372.5	152.6
Retained earnings	2,668	2,410.9
Treasury stock at cost (24.1 and 22.1 million shares)	(1,696.3)	(1,485.2)
Total shareholders’ equity	3,202.2	2,916.2
Total liabilities and shareholders’ equity	$ 13,443.7	$ 12,490.5

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 6,945.6	$ 6,452.2
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	24,100,000	22,100,000

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]	Net Unrealized Appreciation (Depreciation) on Investments [Member]Cumulative Effect, Period of Adoption, Adjustment [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Defined Benefit Pension and Postretirement Plans [Member]Cumulative Effect, Period of Adoption, Adjustment [Member]	Cumulative Foreign Currency Translation Adjustment [Member]	Cumulative Foreign Currency Translation Adjustment [Member]Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings [Member]	Retained Earnings [Member]Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings [Member]Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Treasury Stock	Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Balance at beginning year at Dec. 31, 2017		$ 0.6	$ 1,857	$ 205.4		$ (79.5)		$ (18.3)		$ 1,975	$ 104.3	$ 2,079.3
Balance at beginning year (Accounting Standards Updates No. 2016-01 and No. 2018-02) at Dec. 31, 2017					$ (81.6)		$ (16.2)		$ (3.9)
Balance at beginning of year at Dec. 31, 2017													$ (942.5)
Net income	$ 391									391
Shares purchased at cost													(57.7)
Employee and director stock-based awards and other			14.8
Net appreciation (depreciation) on available-for-sale securities	(151)			(170.1)
Net amount arising in the period	(17)					(17)
Net amount recognized as net periodic benefit cost	7.7					7.7
Amount recognized as cumulative foreign currency translation during the year	21.5							(1.7)
Amount realized with sale of Chaucer business				19.1		16.4		23.2
Dividends to shareholders										(288)
Net shares reissued at cost under employee stock-based compensation plans													16.7
Balance at end of year at Dec. 31, 2018	2,954.7	0.6	1,871.8	(27.2)	$ 1.5	(88.6)		(0.7)		2,182.3	(1.5)	2,180.8		$ (116.5)
Balance at end of year at Dec. 31, 2018													(983.5)
Net income	425.1									425.1
Settlement and prepayment of accelerated share repurchases			(49.5)
Shares purchased at cost													(514.1)
Employee and director stock-based awards and other			15
Net appreciation (depreciation) on available-for-sale securities	241.7			$ 241.6
Net amount arising in the period	16.2					16.2
Net amount recognized as net periodic benefit cost	9					9
Amount recognized as cumulative foreign currency translation during the year	0.7
Accounting Standards Update Extensible List				us-gaap:AccountingStandardsUpdate201708Member
Amount realized with sale of Chaucer business				$ 0.1				$ 0.7
Dividends to shareholders										(195)
Net shares reissued at cost under employee stock-based compensation plans													12.4
Balance at end of year at Dec. 31, 2019	2,916.2	0.6	1,837.3	216		(63.4)				2,410.9	$ (0.9)	$ 2,410		152.6
Balance at end of year at Dec. 31, 2019	(1,485.2)												(1,485.2)
Net income	358.7									358.7
Settlement and prepayment of accelerated share repurchases			4.4
Shares purchased at cost													(217.2)
Employee and director stock-based awards and other			15.7
Net appreciation (depreciation) on available-for-sale securities	212.1			212.1
Net amount arising in the period	3.1					3.1
Net amount recognized as net periodic benefit cost	4.7					4.7
Dividends to shareholders										(100.7)
Net shares reissued at cost under employee stock-based compensation plans													6.1
Balance at end of year at Dec. 31, 2020	3,202.2	$ 0.6	$ 1,857.4	$ 428.1		$ (55.6)				$ 2,668				$ 372.5
Balance at end of year at Dec. 31, 2020	$ (1,696.3)												$ (1,696.3)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows From Operating Activities
Net income	$ 358.7	$ 425.1	$ 391
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on repayment of debt	6.2		28.2
(Gain) loss from sale of Chaucer business		1.2	(131.9)
Net realized investment (gains) losses	(5)	(109.4)	50.7
Net amortization and depreciation	18.2	21.6	24.6
Stock-based compensation expense	20.1	17.4	13.4
Amortization of defined benefit plan costs	6	11.4	9.7
Deferred income tax expense (benefit)	(23)	12.5	(23.4)
Change in deferred acquisition costs	(10.2)	(14.4)	(19.2)
Change in premiums receivable, net of reinsurance premiums payable	(84)	(78.7)	(73.9)
Change in loss, loss adjustment expense and unearned premium reserves	437.9	498.6	584.9
Change in reinsurance recoverable	(60)	(179.1)	(248.1)
Change in expenses and taxes payable	67.5	11	4.6
Other, net	(24.7)	(14.7)	(31.7)
Net cash provided by operating activities	707.6	602.9	551.3
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,317.5	1,336.9	1,094.2
Proceeds from disposals of equity securities and other investments	172.8	250.3	158.4
Purchase of fixed maturities	(1,829.1)	(1,588.3)	(1,425.6)
Purchase of equity securities and other investments	(255.1)	(332.2)	(178.3)
Capital expenditures	(14.9)	(13.3)	(13.1)
Net proceeds from sale of Chaucer business		34.7	635.7
Net cash provided by (used in) investing activities	(608.8)	(311.9)	271.3
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	6.3	14.4	14.8
Proceeds from debt borrowings, net	296.4
Change in cash collateral related to securities lending program		(5)	(19)
Dividends paid to shareholders	(99.5)	(386.2)	(94.3)
Repayment of debt	(175.8)	(151.1)	(11.6)
Repurchases of common stock	(212.8)	(563.6)	(57.7)
Other financing activities	(8.5)	(7.8)	(3.2)
Net cash used in financing activities	(193.9)	(1,099.3)	(171)
Effect of exchange rate changes on cash			(4)
Net change in cash and cash equivalents	(95.1)	(808.3)	647.6
Net change in cash related to discontinued operations		3.3	75.2
Cash and cash equivalents, beginning of year	215.7	1,020.7	297.9
Cash and cash equivalents, end of year	120.6	215.7	1,020.7
Supplemental Cash Flow Information
Interest payments	33.9	37	47.8
Income tax payments, net	97	113.7	57
Continuing Operations [Member]
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized investment (gains) losses	$ (5.1)	$ (109)	$ 51.3

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	1. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES A. Basis of Presentation and Principles of Consolidation The consolidated financial statements of The Hanover Insurance Group, Inc. (“THG” or the “Company”), include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America (“Citizens”), 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 13 – “Segment Information.” The consolidated financial statements also include the Company’s discontinued operations, consisting primarily of the Company’s former accident and health and life insurance businesses and its former Chaucer international business, 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 other international insurance and non-insurance subsidiaries. All intercompany accounts and transactions have been eliminated. 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. Accordingly, for all prior periods presented in this Annual Report on Form 10-K, operations from Chaucer are presented as discontinued operations. The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America (“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 these financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. B. Investments Fixed maturities are classified as available-for-sale and are carried at fair value, with unrealized gains and losses, net of taxes, reported in accumulated other comprehensive income (“AOCI”), a separate component of shareholders’ equity. The amortized cost of fixed maturities is adjusted for amortization of premiums and accretion of discounts to maturity. Equity securities are carried at fair value . Increases and decreases in fair value are reported in net income. Other investments consist primarily of mortgage participations and limited partnerships. Mortgage participations represent interests in commercial mortgage loans originated and serviced by a third-party of which the Company shares, on a pro-rata basis, in all related cash flows of the underlying mortgage loans. Mortgage participations are stated at unpaid principal balances adjusted for deferred fees or expenses, net of an allowance for credit losses. Prior to May 2018, certain reacquisition rights retained by the third-party in the loan participations required that these investments be accounted for as secured borrowings under Accounting Standards Codification (“ASC”) 860, Transfers and Servicing The Company excludes accrued interest receivable from both the estimated fair value and the amortized cost basis of its investment securities and reports such amounts separately on the consolidated balance sheets as accrued investment income. When accrued interest receivable is deemed uncollectible it is written off as a charge to investment income, rather than recorded through an allowance. Net investment income includes interest, dividends and income from limited partnership interests. Interest income is recognized based on the effective yield method, which includes the amortization of premiums and accretion of discounts. The effective yield used to determine the amortization for fixed maturities subject to prepayment risk, such as mortgage-backed and asset-backed securities, is recalculated and adjusted periodically based upon actual historical and projected future cash flows. The adjustment to yields for highly rated prepayable fixed maturities is accounted for using the retrospective method. The adjustment to yields for all other prepayable fixed maturities is accounted for using the prospective method. Fixed maturities and mortgage participations for which payments are delinquent are placed on non-accrual status, and thereafter interest income is recognized only when cash payments are received. Realized investment gains and losses on sales are reported as a component of revenues based upon specific identification of the investment assets sold. Impairments are reported as realized investment losses, and include credit losses (and any subsequent recoveries) on fixed maturities and mortgage participations, and intend-to-sell impairment losses on fixed maturities. Changes in the fair value of equity securities are reported in net realized and unrealized investment gains (losses), including increases and decreases in fair value on securities that are still held and realized gains and losses on securities that have been sold. The Company reviews fixed maturity securities in an unrealized loss position and assesses whether it intends to sell the security or more likely than not will be required to sell the security before the recovery of its amortized cost basis. If the debt security meets either of these two criteria, an intend-to-sell impairment is recognized in earnings equal to the entire difference between the security’s amortized cost basis and its fair value at the impairment measurement date. If neither of the above criteria are met, the credit loss portion of the unrealized loss is recorded through earnings and the non-credit portion remains in other comprehensive income. Credit losses are estimated by comparing the amortized cost of the fixed maturity security with the net present value of the security’s projected future cash flows, discounted at the effective interest rate implicit in the investment prior to impairment. The non-credit portion of the impairment is equal to the difference between the fair value and the net present value of the security’s cash flows at the impairment measurement date. Until January 1, 2020, intend-to-sell impairments and credit losses on fixed maturities resulted in a reduction of the amortized cost basis of the security, and reversal of an impairment was not allowed. Beginning January 1, 2020, credit losses are recorded through an allowance for credit losses rather than an adjustment to amortized cost. Recoveries of impairments on fixed maturities are recognized as reversals of the allowance for credit loss and are no longer accreted as investment income through an adjustment to investment yield. The allowance for credit losses is limited to the amount that fair value is less than amortized cost and therefore, increases in the fair value of investments due to reasons other than credit could result in decreases in the allowance and an increase in net income. Mortgage participations are pooled by similar risk characteristics and evaluated for credit losses. The allowance for credit losses is calculated using expected loss rates, which vary based on risk factors such as property type, geographic market, and loan-to-value and debt service coverage ratios. C. Financial Instruments In the normal course of business, the Company may enter into transactions involving various types of financial instruments, including debt, investments such as fixed maturities, equity securities and mortgage loans, investment and loan commitments, swap contracts, option contracts, forward contracts and futures contracts. These instruments involve credit risk and could also be subject to risk of loss due to interest rate fluctuation. The Company evaluates and monitors each financial instrument individually and, when appropriate, obtains collateral or other security to minimize losses. D . Cash and Cash Equivalents Cash and cash equivalents include cash on hand, amounts due from banks and highly liquid debt instruments purchased with an original maturity of three months or less. E. Deferred Acquisition Costs Acquisition costs consist of commissions, underwriting costs and other costs, which vary with, and are primarily related to, the successful production of premiums. Acquisition costs are deferred and amortized over the terms of the insurance policies. Deferred acquisition costs (“DAC”) for each operating segment are reviewed to determine if the costs are recoverable from future income, including investment income. If such costs are determined to be unrecoverable, they are expensed at the time of determination. Although recoverability of DAC is not assured, the Company believes it is more likely than not that all of these costs will be recovered. The amount of DAC considered recoverable, however, could be reduced in the near term if the estimates of total revenues discussed above are reduced or permanently impaired as a result of a disposition of a line of business. The amount of amortization of DAC could be revised in the near term if any of the estimates discussed above are revised. F. Reinsurance Recoverables The Company shares certain insurance risks it has underwritten, through the use of reinsurance contracts, with various insurance entities. Reinsurance accounting is followed for ceded transactions when the risk transfer provisions of ASC 944, Financial Services – Insurance . G. Property, Equipment, Capitalized Software AND LEASES Property, equipment, leasehold improvements and capitalized software are recorded at cost, less accumulated depreciation and amortization. Depreciation is generally provided using the straight-line method over the estimated useful lives of the related assets, which generally range from 3 to 30 years. The estimated useful life for capitalized software is generally 5 to 7 years. Amortization of leasehold improvements is provided using the straight-line method over the lesser of the term of the lease or the estimated useful life of the improvements. The Company has entered into operating and financing leases through which it uses “right-of-use” assets that are recorded at the present value of future minimum lease payments, less accumulated depreciation. Depreciation is generally provided using the straight-line method over the estimated useful lives of the related assets, which generally range from 4 to 6 years for real estate and fleet leases. The Company tests for the recoverability of long-lived assets whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. The Company recognizes impairment losses only to the extent that the carrying amounts of long-lived assets exceed the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the assets. When an impairment loss occurs, the Company reduces the carrying value of the asset to fair value and no longer depreciates the asset. Fair values are estimated using discounted cash flow analysis. H. GOODWILL AND INTANGIBLE ASSETS In accordance with the provisions of ASC 350, Intangibles Goodwill and Other The Company tests for the recoverability of goodwill and intangible assets with indefinite lives annually, or whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. The Company recognizes impairment losses only to the extent that the carrying amounts of reporting units with goodwill exceed the fair value. The amount of the impairment loss that would be recognized is determined based upon the excess of the carrying value of goodwill compared to the implied fair value of the goodwill, as determined with respect to all assets and liabilities of the reporting unit. The Company has performed its annual review of goodwill and intangible assets with indefinite lives for impairment in the fourth quarters of 2020 and 2019 with no impairments recognized. At December 31, 2020 and 2019, the Company held goodwill of $178.8 million and intangible assets with indefinite lives of $15.0 million. I. LIABILITIES FOR LOSSES, LAE AND UNEARNED PREMIUMS Liabilities for outstanding claims, losses and loss adjustment expenses (“LAE”) are estimates of payments to be made for reported losses and LAE and estimates of losses and LAE incurred but not reported. These liabilities are determined using case basis evaluations and statistical analyses of historical loss patterns and represent estimates of the ultimate cost of all losses incurred but not paid. These estimates are continually reviewed and adjusted as necessary; adjustments are reflected in current operations. Estimated amounts of salvage and subrogation on unpaid losses are deducted from the liability for unpaid claims. Premiums for direct and assumed business are reported as earned on a pro-rata basis over the contract period. The unexpired portion of these premiums is recorded as unearned premiums. All losses, LAE and unearned premium liabilities are based on the various estimates discussed in this note. Although the adequacy of these amounts cannot be assured, the Company believes that it is more likely than not that these liabilities and accruals will be sufficient to meet future obligations of policies in force. The amount of liabilities and accruals, however, could be revised in the near-term if the estimates discussed above are revised. J. Debt The Company’s debt at December 31, 2020 includes senior and subordinated debentures. Debt instruments are carried at principal amount borrowed, net of any applicable unamortized discounts and issuance costs. See Note 6 – “Debt and Credit Arrangements.” K. Premium, Premium Receivable, Fee Revenue and Related Expenses Insurance premiums written are generally recorded at the policy inception and are primarily earned on a pro-rata basis over the terms of the policies for all products. Premiums written may also include estimates that are derived from multiple sources, which include the historical experience of the underlying business, similar businesses and available industry information. These estimates are regularly reviewed and updated, and any resulting adjustments are included in the current year’s results. Unearned premium reserves represent the portion of premiums written that relates to the unexpired terms of the underlying in-force insurance policies and reinsurance contracts. Premium receivables reflect the unpaid balance of premiums written as of the balance sheet date. Premium receivables are generally short-term in nature and are reported net of an allowance for estimated uncollectible premium accounts. The Company reviews its receivables for collectability at the balance sheet date. The allowance for uncollectible accounts was not material as of December 31, 20 20 and 201 9 . Ceded premiums are charged to income over the applicable term of the various reinsurance contracts with third-party reinsurers. Reinsurance reinstatement premiums, when required, are recognized in the same period as the loss event that gave rise to the reinstatement premiums. Losses and related expenses are matched with premiums, resulting in their recognition over the lives of the contracts. This matching is accomplished through estimated and unpaid losses and amortization of deferred acquisition costs. L. Income Taxes The Company is subject to the tax laws and regulations of the U.S. and foreign countries in which it operates or where it operated previously. The Company files a consolidated U.S. federal income tax return that includes the holding company and its U.S. subsidiaries. Generally, taxes are accrued at the U.S. statutory tax rate of 21% for income from the U.S. operations. The Company accrues taxes on certain non-U.S. income that is subject to U.S. tax at the enacted U.S. tax rate. Foreign tax credits, where available, were utilized to offset U.S. tax as permitted. The Company’s accounting for income taxes represents its best estimate of various events and transactions. Deferred income taxes are generally recognized when assets and liabilities have different values for financial statement and tax reporting purposes, and for other temporary taxable and deductible differences as defined by ASC 740, Income Taxes The realization of deferred tax assets depends upon the existence of sufficient taxable income within the carryback or carryforward periods under the tax law in the applicable tax jurisdiction. Consideration is given to all available positive and negative evidence, including reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial operations. Valuation allowances are established if, based on available information, it is determined that it is more likely than not that all or some portion of the deferred tax assets will not be realized. Changes in valuation allowances are generally reflected in income tax expense or as an adjustment to other comprehensive income (loss) depending on the nature of the item for which the valuation allowance is being recorded. M. Stock-Based Compensation The Company recognizes the fair value of compensation costs for all share-based payments, including employee stock options, in the financial statements. Unvested awards are generally expensed on a straight-line basis, by tranche, over the vesting period of the award. The Company’s stock-based compensation plans are discussed further in Note 10 – “Stock-Based Compensation Plans.” N. Earnings Per Share Earnings per share (“EPS”) for the years ended December 31, 2020, 2019 and 2018 is based on a weighted average of the number of shares outstanding during each year. Basic and diluted EPS is computed by dividing income available to common stockholders by the weighted average number of shares outstanding for the period. The weighted average shares outstanding used to calculate basic EPS differ from the weighted average shares outstanding used in the calculation of diluted EPS due to the effect of dilutive employee stock options, nonvested stock grants and other contingently issuable shares. If the effect of such items is antidilutive, the weighted average shares outstanding used to calculate diluted EPS would be equal to those used to calculate basic EPS. Options to purchase shares of common stock whose exercise prices are greater than the average market price of the common shares are not included in the computation of diluted earnings per share because the effect would be antidilutive. O. New Accounting Pronouncements Recently Implemented Standards In August 2018, the Financial Accounting Standards Board (“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. This update removes the requirement for disclosure of the following: 1) the amount and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, 2) the policy for timing of transfers between levels, and 3) the valuation processes for Level 3 fair value measurements. This update also added a requirement to disclose the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, in addition to other fair value disclosure modifications. This update required both prospective and retrospective application to certain disclosures upon implementation. The Company implemented this guidance effective January 1, 2020 and it did not have an impact on the Company’s financial position or results of operations as the update is disclosure related. In February 2018, the FASB issued ASC Update No. 2018-02 (Topic 220) Income Statement – Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In March 2017, the FASB issued ASC Update No. 2017-08, Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities In June 2016, the FASB issued ASC Update No. 2016-13, (Topic 326) Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments (“ASC Update No. 2016-13”). Codification Improvement to Topic 326, Financial Instruments – Credit Losses In 2019 and 2020, the FASB issued several updates to ASC Update 2016-13, including the issuance in April 2019 of ASC Update 2019-04, Codification Improvements to Topic 360, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging and Topic 825, Financial Instruments, Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief, Codification Improvements to Topic 326, Financial Instruments – Credit Losses Financial Instruments – Credit Losses (Topic 326) and Leases (Topic 842) – Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 119 and Update to SEC Section on Effective Date Related to Accounting Standards Update No. 2016-02, Leases (Topic 842). In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASC Update No. 2016-02, Leases (Topic 842) included within operating activities in the statement of cash flows. ASC Update No. 2016-02 requires that implementation of this guidance be through a modified retrospective transition approach. In July 2018, the FASB issued ASC Update No. 2018-11, Leases (Topic 842): Targeted Improvements , which provides entities with an additional transition method to adopt ASC Update No. 2016-02. Under this optional transition method, an entity can initially apply the new guidance at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company implemented this guidance effective January 1, 2019 using the optional transition method provided in ASC Update No. 2018-11 and elected to utilize the practical expedient package available in ASC Update 2016-02. The Company has also elected to utilize the practical expedient which allows an entity not to separate lease and nonlease components for all of its asset classes. The effect of implementing this guidance increased total assets and total liabilities each by approximately $ 35 million on January 1, 2019 and did not have a material effect on the results of operations. Recently Issued Standards In October 2020, the FASB issued ASC Update No. 2020-08, Codification Improvements to Subtopic 310-20, Receivables – Nonrefundable Fees and Other Costs In January 2020, the FASB issued ASC Update No. 2020-01 Investments – Equity Securities (Topic 321), Investments – Equity Method and Joint Ventures (Topic 323), and Derivatives and Hedging (Topic 815) In December 2019, the FASB issued ASC Update No. 2019-12 Income Taxes (Topic 740) – Simplifying the Accounting for Income Taxes Income Taxes Q. Reclassifications Certain prior year amounts have been reclassified to conform to the current year presentation.

Discontinued Operations

Discontinued Operations	12 Months Ended
	Dec. 31, 2020
Chaucer [Member]
Discontinued Operations	2. DISCONTINUED OPERATIONS Gain on Sale of Chaucer Business On December 28, 2018, the Company completed the sale of the Chaucer U.K. companies (“Chaucer”) to China Re and recognized a pre-tax gain on the sale of $174.4 million and an income tax expense of $42.5 million. THG paid customary transaction costs along with providing certain representations and warranties and agreeing to indemnify China Re for certain pre-sale contingent liabilities. The following table summarizes the components of the estimated gain in 2018 related to the sale of the Chaucer business as of December 28, 2018. As discussed below the table, both the pre-tax gain and income tax expense were updated in 2019. YEAR ENDED DECEMBER 31 2018 (in millions) Initial consideration received from sale (1) $ 779.0 Adjustment (1) (17.0 ) Contingent proceeds (1) (2) 31.7 Total cash proceeds expected from sale of Chaucer Holdings Limited (1) 793.7 Less: Carrying value of Chaucer business (3) 530.0 Transaction and other sale related costs (4) 30.6 Net realized losses on securities, pension and currency translation obligations related to Chaucer business (5) 58.7 Total pre-tax reductions 619.3 Pre-tax gain on sale 174.4 Income tax expense (6) 42.5 Gain on sale $ 131.9 (1) Initial consideration for Chaucer as determined in the purchase and sales agreement was $779 million. This amount, along with $28 million in cash proceeds received from the sale of the Irish entity on February 14, 2019, $13 million from the sale of the Australian entities on April 10, 2019, estimated contingent consideration of $31.7 million, and an $85 million pre-signing dividend from Chaucer that was received in the second quarter of 2018, resulted in expected total proceeds from the entire transaction of $936.7 million. These amounts were partially offset by $17.0 million paid to China Re to adjust the purchase price for amounts received by the Company from Chaucer prior to December 28, 2018. (2) Contingent proceeds, as reflected in the sales and purchase agreement, could have been up to $45 million and was determined based upon 2018 catastrophe losses. In 2018, the Company’s best estimate of contingent consideration was $31.7 million. (3) The carrying value of the Chaucer business reflects its U.S. GAAP book value at December 28, 2018, excluding $7.9 million of U.S.-related deferred tax assets that are no longer likely to be realized and therefore are reflected in the income tax expense category. (4) Transaction and other sale related costs primarily include brokerage, legal, actuarial, tax and other professional fees, employee retention costs, costs for the purchase of aggregate excess of loss catastrophe coverage in consideration of the contingent proceeds provision, along with certain other miscellaneous charges related to the execution of the transaction. (5) As part of the transaction, investments held by Chaucer were transferred to China Re resulting in the recognition of net realized investment losses that were previously reflected in accumulated other comprehensive income. Additionally, Chaucer’s deferred pension obligations and currency translation obligations previously recognized in accumulated other comprehensive income were recognized as losses associated with the transaction. (6) The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that were no longer likely to be realized. 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 Columbian dam construction loss. Accordingly, the Company updated and reduced its best estimate of pre-tax contingent proceeds by $9.7 million. The Company received a cash payment of $22.0 million in final settlement of the contingent proceeds. 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 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 entities comprised the Chaucer business reported in its results in 2019, until their respective sales to China Re. The sale of the Irish entity provided total proceeds of $ 28 0.4 0.5 13 1.2 0.1 Income (loss) from Chaucer Business Year ended December 31, 2019 Revenues from the portion of the Chaucer business remaining after the sale of the U.K. entities in 2018 was $6.7 million during the year ended December 31, 2019. Pre-tax operating losses in this business totaled $0.5 million during the same period, whereas this business provided after-tax income of $1.6 million for the year ended December 31, 2019. Income from the Chaucer business in 2019 included a $2.0 million tax benefit related to a decrease in an uncertain tax position due to the expiration of the statute of limitations. Year ended December 31, 2018 The following table summarizes the results of operations for Chaucer for 2018, reflecting the results of operations for the period in which THG owned the Chaucer U.K. entities, which concluded with their sale on December 28, 2018: (in millions) Revenues Net premiums earned $ 850.0 Net investment income 54.9 Other income 7.5 912.4 Losses and operating expenses Losses and LAE 515.5 Amortization of deferred acquisition costs 252.5 Other expenses 115.0 883.0 Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income) 29.4 Other items: Interest expense (3.8 ) Net realized and unrealized investment losses (1.3 ) Other income 0.4 Income from Chaucer business before income taxes 24.7 Income tax expense (4.7 ) Income from Chaucer business, net of taxes $ 20.0 The Company recognized $0.8 million in foreign currency transaction losses in net income in 2018. The following table provides the effects of reinsurance related to the Chaucer business for the year ended 2018. (in millions) Premiums written: Direct $ 625.3 Assumed 685.3 Ceded (459.9 ) Net premiums written $ 850.7 Premiums earned: Direct $ 611.9 Assumed 667.2 Ceded (429.1 ) Net premiums earned $ 850.0 Losses and LAE: Direct $ 362.2 Assumed 568.7 Ceded (415.4 ) Net losses and LAE $ 515.5 Cash Flows The sales of the Chaucer businesses were completed in 2019, therefore there were no material cash flows associated with the Chaucer business in 2020. The following table details the cash flows associated with the Chaucer business during the years indicated: DECEMBER 31 2019 2018 (in millions) Net cash provided by (used in) operating activities 3.5 (22.8 ) Net cash (used in) provided by investing activities (6.8 ) 131.1 Discontinued accident and health and life businesses During 1999, the Company exited its accident and health insurance business, consisting of its Employee Benefit Services business, its Affinity Group Underwriters business and its accident and health assumed reinsurance pool business. Prior to 1999, these businesses comprised substantially all of the former Corporate Risk Management Services segment. On January 2, 2009, Hanover Insurance directly assumed a portion of the accident and health business and the remainder of the discontinued First Allmerica Financial Life Insurance Company (“FAFLIC”) accident and health business was reinsured by Hanover Insurance in connection with the sale of FAFLIC to Commonwealth Annuity. At December 31, 2020 and 2019, the portion of the discontinued accident and health business that was directly assumed had assets of $90.3 million and $82.4 million, respectively, consisting primarily of invested assets, and liabilities of $89.8 million and $84.4 million, respectively, consisting primarily of policy liabilities. At December 31, 2020 and 2019, the assets and liabilities of this business, as well as those of the reinsured portion of the accident and health business are classified as assets and liabilities of discontinued operations in the Consolidated Balance Sheets. The Company’s former life insurance businesses include indemnity obligations and other activities. Discontinued accident and health and life operations for the year ended December 31, 2020 resulted in losses of $3.7 million, including $ 1.7

Investments

Investments	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Investments	3. INVESTMENTS A. FIXED MATURITIES The amortized cost and fair value of available-for-sale fixed maturities were as follows: DECEMBER 31, 2020 (in millions) Amortized Cost Allowance for Credit Losses Amortized Cost, Net of Allowance for Credit Losses Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury and government agencies $ 376.4 $ — $ 376.4 $ 17.6 $ 2.0 $ 392.0 Foreign government 2.2 — 2.2 0.5 — 2.7 Municipal 1,042.1 — 1,042.1 61.7 0.1 1,103.7 Corporate 3,770.0 (0.1 ) 3,769.9 341.8 0.4 4,111.3 Residential mortgage-backed 978.2 — 978.2 33.1 0.6 1,010.7 Commercial mortgage-backed 705.4 — 705.4 54.8 0.2 760.0 Asset-backed 71.4 — 71.4 2.6 — 74.0 Total fixed maturities $ 6,945.7 $ (0.1 ) $ 6,945.6 $ 512.1 $ 3.3 $ 7,454.4 DECEMBER 31, 2019 (in millions) Gross Gross OTTI Amortized Unrealized Unrealized Unrealized Cost Gains Losses Fair Value Losses (1) U.S. Treasury and government agencies $ 342.0 $ 9.1 $ 1.3 $ 349.8 $ — Foreign government 15.7 0.4 — 16.1 — Municipal 807.1 27.6 1.2 833.5 — Corporate 3,653.5 161.6 3.9 3,811.2 3.0 Residential mortgage-backed 905.4 17.1 1.1 921.4 — Commercial mortgage-backed 666.4 25.6 0.1 691.9 — Asset-backed 62.1 1.1 — 63.2 — Total fixed maturities $ 6,452.2 $ 242.5 $ 7.6 $ 6,687.1 $ 3.0 (1) Other-than-temporary impairments (“OTTI”) unrealized losses represents OTTI recognized in accumulated other comprehensive income. This amount excludes net unrealized gains on impaired securities relating to changes in the value of such securities subsequent to the impairment measurement date through December 31, 2019 of $4.2 million. The Company enters into various agreements that may require its fixed maturities to be held as collateral by others. At December 31, 2020 and 2019, fixed maturities with fair values of $103.9 million and $94.0 million, respectively, were held as collateral for the FHLB collateralized borrowing program. See Note 6 — “Debt and Credit Arrangements” for additional information related to the Company’s FHLB program. Additionally, at December 31, 2020 and 2019, fixed maturities with fair values of $315.2 million and $308.7 million, respectively, were on deposit with various state governmental authorities or trustees. 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. DECEMBER 31 2020 (in millions) Amortized Cost, net of Allowance for Credit Losses Fair Value Due in one year or less $ 334.1 $ 339.1 Due after one year through five years 1,774.6 1,913.3 Due after five years through ten years 2,306.3 2,548.2 Due after ten years 775.6 809.1 5,190.6 5,609.7 Mortgage-backed and asset-backed securities 1,755.0 1,844.7 Total fixed maturities $ 6,945.6 $ 7,454.4 B. UNREALIZED GAINS AND LOSSES Unrealized gains and losses on available-for-sale and other securities are summarized in the following table. YEARS ENDED DECEMBER 31 (in millions) Equity Fixed Securities and 2020 Maturities Other Total Net appreciation, beginning of year $ 216.0 $ — $ 216.0 Net appreciation on available-for-sale securities 280.2 — 280.2 Provision for deferred income taxes (68.1 ) — (68.1 ) 212.1 — 212.1 Net appreciation, end of year $ 428.1 $ — $ 428.1 2019 Net depreciation, beginning of year $ (27.2 ) $ — $ (27.2 ) Net appreciation on available-for-sale securities 324.7 — 324.7 Provision for deferred income taxes (83.0 ) — (83.0 ) Cumulative effect adjustment for ASU 2017-08, net of tax 1.5 — 1.5 243.2 — 243.2 Net appreciation, end of year $ 216.0 $ — $ 216.0 2018 Net appreciation, beginning of year $ 110.1 $ 95.3 $ 205.4 Net depreciation on available-for-sale securities (203.7 ) — (203.7 ) Provision for deferred income taxes 33.6 — 33.6 Amount realized with sale of Chaucer, net of tax 19.1 — 19.1 Cumulative effect adjustment for ASUs 2016-01 and 2018-02, net of tax 13.7 (95.3 ) (81.6 ) (137.3 ) (95.3 ) (232.6 ) Net depreciation, end of year $ (27.2 ) $ — $ (27.2 ) Effective January 1, 2018, increases or decreases in fair value of equity securities are no longer reported as unrealized gains and losses in accumulated other comprehensive income. Instead, they are reported in net realized and unrealized investment gains (losses) in the Consolidated Statements of Income. C. 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 December 31, 2020 and 2019 including the length of time the securities have been in an unrealized loss position: DECEMBER 31, 2020 12 months or less Greater than 12 months Total (in millions) Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 2.0 $ 140.7 $ — $ — $ 2.0 $ 140.7 Municipal 0.1 19.3 — 0.7 0.1 20.0 Corporate 0.4 53.4 — — 0.4 53.4 Residential mortgage-backed 0.6 94.4 — — 0.6 94.4 Commercial mortgage-backed 0.2 33.2 — — 0.2 33.2 Total investment grade 3.3 341.0 — 0.7 3.3 341.7 Below investment grade: Corporate — 4.6 — — — 4.6 Total fixed maturities $ 3.3 $ 345.6 $ — $ 0.7 $ 3.3 $ 346.3 DECEMBER 31, 2019 12 months or less Greater than 12 months Total (in millions) Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 1.3 $ 73.0 $ — $ 9.3 $ 1.3 $ 82.3 Municipal 1.1 72.5 0.1 5.6 1.2 78.1 Corporate 0.7 86.5 0.1 4.7 0.8 91.2 Residential mortgage-backed 0.7 69.2 0.4 34.4 1.1 103.6 Commercial mortgage-backed 0.1 40.6 — 0.9 0.1 41.5 Asset-backed — — — 1.7 — 1.7 Total investment grade 3.9 341.8 0.6 56.6 4.5 398.4 Below investment grade: Corporate 2.2 27.1 0.9 9.0 3.1 36.1 Total fixed maturities $ 6.1 $ 368.9 $ 1.5 $ 65.6 $ 7.6 $ 434.5 The Company views gross unrealized losses on fixed maturities as non-credit related and through its assessment of unrealized losses has determined that these securities will recover, allowing the Company to realize the anticipated long-term economic value. The Company currently does not intend to sell, nor does it expect to be required to sell these securities before recovery of their amortized cost. The Company employs a systematic methodology to evaluate declines in fair value below amortized cost for fixed maturity securities. In determining impairments, 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 degree to which the fair value of an issuer’s securities is 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. D . OTHER INVESTMENTS The Company’s mortgage participations and other mortgage loans were $467.6 million and $441.2 million at December 31, 2020 and 2019, respectively. Participating interests in commercial mortgage loans are originated and serviced by a third-party. For these investments, the Company shares, on a pro-rata basis, in all related cash flows of the underlying mortgages. Mortgage participations and other mortgage loans were comprised of the following property types and geographic locations. DECEMBER 31 2020 2019 (in millions) Property Type: Office $ 160.4 $ 138.2 Apartments 128.3 125.1 Retail 68.5 66.5 Hotel 62.7 62.1 Industrial 55.6 50.6 Valuation allowance (7.9 ) (1.3 ) Total $ 467.6 $ 441.2 DECEMBER 31 2020 2019 (in millions) Geographic Region: Pacific $ 117.0 $ 102.3 South Atlantic 108.3 95.9 Mid-Atlantic 73.8 53.3 West South Central 70.8 71.2 New England 43.5 42.5 East North Central 27.8 27.8 Mountain 12.5 12.5 Other 21.8 37.0 Valuation allowance (7.9 ) (1.3 ) Total $ 467.6 $ 441.2 At December 31, 2020, scheduled maturities of mortgage participations and other loans were as follows: due in 2021 - $57.5 million; in 2022 - $27.7 million; 2023 - $16.5 million; 2024 - $77.1 million and thereafter - $288.8 million. Actual maturities could differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties and loans may be refinanced. During 2020, the Company did not refinance any loans based on terms that differed from current market rates. Other investments also include interests in limited partnerships of $312.5 million and $278.5 million at December 31, 2020 and December 31, 2019, respectively. E. OTHER At December 31, 2020 and 2019, the Company’s exposure to concentration of investments in a single investee that exceeded 10% of shareholders’ equity included securities of U.S. government-sponsored agencies, as well as mortgage participations with a highly rated single third-party of $459.3 million and $432.3 million, respectively. At December 31, 2020, there were contractual investment commitments of up to $205.3 million.

Investment Income and Gains and

Investment Income and Gains and Losses	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Investment Income and Gains and Losses	4. INVESTMENT INCOME AND GAINS AND LOSSES A. NET INVESTMENT INCOME The components of net investment income were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Fixed maturities $ 222.5 $ 232.4 $ 217.7 Mortgage loans 17.5 16.3 14.0 Limited partnerships 16.7 19.7 24.1 Equity securities 14.8 16.3 17.0 Other investments 3.2 5.3 4.8 Gross investment income 274.7 290.0 277.6 Less: investment expenses (9.6 ) (8.7 ) (10.2 ) Net investment income $ 265.1 $ 281.3 $ 267.4 The change in fair value of limited partnerships measured using NAV is reported in net investment income, of which, $6.9 million and $4.6 million of holding losses related to securities still owned at December 21, 2020 and 2019, respectively, and $5.3 million of holding gains related to securities still owned at December 31, 2018. The carrying values of fixed maturity securities on non-accrual status at December 31, 2020 and 2019 were not material. The effects of non-accruals for the years ended December 31, 2020, 2019 and 2018, compared with amounts of net investment income that would have been recognized in accordance with the original terms of the fixed maturities were also not material. B. NET REALIZED AND UNREALIZED INVESTMENT GAINS AND LOSSES Net realized and unrealized gains (losses) on investments, including impairments, were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Equity securities $ 13.4 $ 106.5 $ (43.4 ) Other investments 2.5 — (1.3 ) Mortgage loans (6.7 ) (0.2 ) (0.2 ) Fixed maturities (4.2 ) 3.1 (5.8 ) Net realized and unrealized investment gains (losses) $ 5.0 $ 109.4 $ (50.7 ) The following table provides pre-tax net realized and unrealized gains (losses) on equity securities: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Net gains (losses) recognized during the period $ 13.4 $ 106.5 $ (43.4 ) Less: net (losses) gains recognized on equity securities sold during the period (19.8 ) 2.8 (3.9 ) Net unrealized gains (losses) recognized during the period on equity securities still held $ 33.2 $ 103.7 $ (39.5 ) Impairments Included in net realized and unrealized investment gains (losses) for the years ended December 31, 2020, 2019 and 2018, were impairments of investment securities totaling $26.3 million, $2.0 million and $4.6 million, respectively. In 2020, impairments primarily consisted of $17.6 million on fixed maturities and $6.7 million of estimated credit losses on mortgage loans. Impairments on fixed maturities included $16.5 million relating to intend-to-sell securities and $1.1 million of estimated credit losses. In 2019, impairments consisted entirely of corporate fixed maturity securities. In 2018, impairments consisted of $2.6 million on fixed maturities and $2.0 million on other invested assets. See Note 1, “Summary of Significant Accounting Policies — New Accounting Pronouncements: Recently Implemented Standards,” for a discussion of new guidance effective January 1, 2020, which affects the accounting for expected credit losses on fixed maturity securities and mortgage loans. Under the new guidance, credit losses on fixed maturities continue to be measured based on the present value of expected future cash flows compared to amortized cost; however, credit losses on available-for-sale fixed maturities are now recognized through an allowance instead of a direct write down of amortized cost. The new guidance stipulates that recoveries of previously recorded credit losses are recorded immediately as a reversal of the allowance. In addition, the allowance is limited to the amount that fair value is less than amortized cost and therefore, increases in the fair value of investments due to reasons other than credit could result in decreases in the allowance. Changes in the allowance for credit losses are recorded in net realized and unrealized investment gains (losses). At December 31, 2020 and 2019, the allowance for credit losses on mortgage loans was $7.9 million and $1.3 million, respectively. The allowance for credit losses on available-for-sale securities was $0.1 million at December 31, 2020. There was no allowance for credit losses on available-for-sale securities at December 31, 2019. The methodology and significant inputs used to measure the amount of credit losses were as follows: Fixed maturities, 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 qualitative market data relevant to the realizability of contractual cash flows may be considered, including current conditions and reasonable and supportable forecasts. Mortgage loans – the Company estimated losses by applying expected loss rates, which are based on historical data. Embedded in expected loss rates are mortgage risk ratings and risk factors associated with property type such as office, retail, lodging, multi-family and industrial. Risk ratings, based on property characteristics and metrics including the geographic market, are predominantly driven by estimates of loan-to-value and debt service coverage ratios. Ratings may be adjusted to reflect current conditions and to incorporate reasonable and supportable forecasts, such as volatility of cash flows and valuation. The proceeds from sales of available-for-sale fixed maturities and the gross realized gains and gross realized losses on those sales, were as follows: YEARS ENDED DECEMBER 31 (in millions) Proceeds Gross Gross 2020 from Sales Gains Losses Fixed maturities $ 264.1 $ 9.9 $ 2.9 2019 Fixed maturities, excluding held-for-sale (Chaucer) $ 541.2 $ 6.9 $ 5.7 Fixed maturities, held-for-sale 0.3 — — Total fixed maturities $ 541.5 $ 6.9 $ 5.7 2018 Fixed maturities, excluding held-for-sale (Chaucer) $ 296.5 $ 2.1 $ 8.4 Fixed maturities, held-for-sale 276.8 1.7 1.7 Total fixed maturities $ 573.3 $ 3.8 $ 10.1

Fair Value

Fair Value	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value	5. 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 is 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 and delinquency/default trends. • 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, automobile 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, credit stress, 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 (“NAV”) provided by the general partner adjusted for recent financial information and are excluded from the fair value hierarchy. The estimated fair values of the financial instruments were as follows: December 31, 2020 December 31, 2019 Carrying Fair Carrying Fair (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 7,454.4 $ 7,454.4 $ 6,687.1 $ 6,687.1 Fair Value through Net Income: Equity securities 598.5 598.5 575.7 575.7 Other investments 176.8 176.8 187.1 187.1 Amortized Cost/Cost: Other investments 470.3 504.8 443.3 463.7 Cash and cash equivalents 120.6 120.6 215.7 215.7 Total financial instruments $ 8,820.6 $ 8,855.1 $ 8,108.9 $ 8,129.3 Financial Liabilities carried at: Amortized Cost: Debt $ 780.8 $ 887.7 $ 653.4 $ 722.1 The Company has processes designed to ensure that the values received from its third-party pricing services 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 reviews the pricing services’ policies describing its methodology, processes, practices and inputs, including various financial models used to value securities. For assets carried at 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. 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 2020 and 2019, 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. DECEMBER 31, 2020 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 392.0 $ 155.8 $ 236.2 $ — Foreign government 2.7 — 2.7 — Municipal 1,103.7 — 1,096.7 7.0 Corporate 4,111.3 — 4,110.8 0.5 Residential mortgage-backed 1,010.7 — 1,010.7 — Commercial mortgage-backed 760.0 — 747.6 12.4 Asset-backed 74.0 — 74.0 — Total fixed maturities 7,454.4 155.8 7,278.7 19.9 Equity securities 598.5 593.0 — 5.5 Other investments 4.0 — — 4.0 Total investment assets at fair value $ 8,056.9 $ 748.8 $ 7,278.7 $ 29.4 DECEMBER 31, 2019 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 349.8 $ 157.9 $ 191.9 $ — Foreign government 16.1 — 16.1 — Municipal 833.5 — 821.4 12.1 Corporate 3,811.2 — 3,810.6 0.6 Residential mortgage-backed 921.4 — 921.4 — Commercial mortgage-backed 691.9 — 679.2 12.7 Asset-backed 63.2 — 63.2 — Total fixed maturities 6,687.1 157.9 6,503.8 25.4 Equity securities 575.7 573.6 — 2.1 Other investments 3.5 — — 3.5 Total investment assets at fair value $ 7,266.3 $ 731.5 $ 6,503.8 $ 31.0 Limited partnerships measured at fair value using the NAV based on an ownership interest in partners’ capital have not been included in the hierarchy tables. At December 31, 2020 and 2019, the fair values of these investments were $172.8 million and $183.6 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. DECEMBER 31, 2020 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 120.6 $ 120.6 $ — $ — Other investments 504.8 — 2.7 502.1 Total financial instruments $ 625.4 $ 120.6 $ 2.7 $ 502.1 Liabilities: Debt $ 887.7 $ — $ 887.7 $ — DECEMBER 31, 2019 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 215.7 $ 215.7 $ — $ — Other investments 463.7 — 2.1 461.6 Total financial instruments $ 679.4 $ 215.7 $ 2.1 $ 461.6 Liabilities: Debt $ 722.1 $ — $ 722.1 $ — The following tables provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). YEAR ENDED DECEMBER 31, 2020 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year $ 12.1 $ 0.6 $ 12.7 $ 25.4 $ 5.6 $ 31.0 Transfers out of Level 3 (3.5 ) — — (3.5 ) — (3.5 ) Total gains: Included in total net realized and unrealized investment gains — — — — 3.9 3.9 Included in other comprehensive income - net appreciation on available-for-sale securities 0.1 — 0.7 0.8 — 0.8 Sales (1.7 ) (0.1 ) (1.0 ) (2.8 ) — (2.8 ) Balance at end of year $ 7.0 $ 0.5 $ 12.4 $ 19.9 $ 9.5 $ 29.4 Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year $ 0.3 $ — $ 0.7 $ 1.0 $ — $ 1.0 YEAR ENDED DECEMBER 31, 2019 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year $ 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 — 0.5 1.4 — 1.4 Purchases and sales: Purchases — — — — 1.0 1.0 Sales (9.9 ) (0.2 ) (0.9 ) (11.0 ) — (11.0 ) Balance at end of year $ 12.1 $ 0.6 $ 12.7 $ 25.4 $ 5.6 $ 31.0 During the year ended December 31, 2020, a fixed maturity security was transferred from Level 3 to Level 2, primarily as a result of assessing the significance of unobservable inputs on the fair value measurement. For the year ended December 31, 2019, there were no transfers between Level 2 and Level 3. There were no Level 3 liabilities held by the Company for the years ended December 31, 2020 and 2019. 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. DECEMBER 31, 2020 2019 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted Discount for: $ 7.0 $ 12.1 cash flow Small issue size 0.7 - 6.8% (5.0%) 0.7 - 6.8% (4.3%) Credit stress 0.2% (0.2%) — Above-market coupon — 0.5% (0.5%) Corporate Discounted Discount for: 0.5 0.6 cash flow Small issue size 2.5% (2.5%) 2.5% (2.5%) Above-market coupon 0.3% (0.3%) 0.3% (0.3%) Commercial Discounted Discount for: 12.4 12.7 mortgage-backed cash flow Small issue size 1.9 - 3.1% (2.7%) 1.9 - 3.1% (2.7%) Above-market coupon 0.5% (0.5%) 0.5% (0.5%) Lease structure 0.3% (0.3%) 0.3% (0.3%) Credit stress 0.2% (0.2%) — Equity securities Market comparables Net tangible asset market multiples 1.1 1.0X (1.0X) 1.1 1.0X (1.0X) Internal price Unadjusted price from financing round 4.4 5.60 (5.60) 1.0 1.27 (1.27) Other Discounted cash flow Discount rate 4.0 16.2% (16.2%) 3.5 18.0% (18.0%) The weighted average of the unobservable inputs was weighted by the relative fair value of the fixed maturity securities to which the inputs were applied. Each unobservable input is based on the Company’s subjective opinion and therefore inherently contains a degree of uncertainty. 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.

Debt and Credit Arrangements

Debt and Credit Arrangements	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt and Credit Arrangements	6. DEBT AND CREDIT ARRANGEMENTS Debt consists of the following: DECEMBER 31 2020 2019 (in millions) Senior debentures maturing April 15, 2026 $ 375.0 $ 375.0 Senior debentures maturing September 1, 2030 300.0 — Senior debentures maturing October 15, 2025 61.8 62.6 Subordinated debentures maturing February 3, 2027 50.1 50.1 Subordinated debentures maturing March 30, 2053 — 175.0 Total principal debt 786.9 662.7 Unamortized debt issuance costs (6.1 ) (9.3 ) Total $ 780.8 $ 653.4 On August 24, 2020, the Company issued $300.0 million aggregate principal amount of 2.50% unsecured senior debentures maturing September 1, 2030. Net proceeds from the issuance were $296.4 million. On September 19, 2020, the Company used a portion of these net proceeds to redeem its outstanding $175.0 million principal amount of 6.35% subordinated unsecured debentures due March 30, 2053. The Company recognized $6.1 million of unamortized debt issuance costs associated with this debt. The Company also held $375.0 million par value of unsecured senior debentures at December 31, 2020 and 2019, that were issued on April 8, 2016, and mature on April 15, 2026. Additionally, the Company had outstanding 7.625% unsecured senior debentures with a par value of $61.8 million and $62.6 million as of December 31, 2020 and 2019, respectively, maturing on October 15, 2025. All of the Company’s outstanding senior debentures are subject to certain restrictive covenants, including limitations on the issuance or disposition of stock of restricted subsidiaries and limitations on liens, and pay interest semi-annually. The Company also held subordinated debentures maturing February 3, 2027 which had a par value of $50.1 million as of December 31, 2020 and 2019, and pay cumulative dividends semi-annually at 8.207%. On January 2, 2019, the Company repaid $ 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. Membership in FHLB provides the Company with access to additional short-term liquidity based on the level of investment in FHLB stock and pledged collateral. Total holdings of FHLB s tock were $ 2.6 million and $ 2.1 million at December 31, 20 20 and 201 9 , respectively. At December 31, 2020 and 2019, the Company had pledged government agency securities with a fair value of $ 103.9 million and $ 94.0 million , respectively, as collateral for periodic short-term borrowings with the FHLB. There were no borrowings outstanding with the FHLB at December 31, 20 20 or 201 9 . At December 31, 2020, the Company had a $200.0 million credit agreement which expires in April 2024. The Company had no borrowings under this agreement as of December 31, 2020 Interest expense was $ 37.1

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	7. INCOME TAXES Provisions for income taxes have been calculated in accordance with the provisions of ASC 740. Income from continuing operations before income taxes and a summary of the components of income tax expense in the Consolidated Statements of Income are shown below: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Income from continuing operations before income taxes $ 444.8 $ 522.1 $ 282.5 Income tax expense (benefit): Current $ 105.9 $ 80.5 $ 46.2 Deferred (23.1 ) 12.6 (2.7 ) Total income tax expense $ 82.8 $ 93.1 $ 43.5 As noted above, in Note 2 – “Discontinued Operations,” on June 14, 2019, the U.S. Department of the Treasury issued regulations that changed the taxation of certain non-U.S. income. These regulations were applied 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 year ended December 31, 2019. The income tax expense attributable to the consolidated results of continuing operations is different from the amount determined by multiplying income from continuing operations before income taxes by the U.S. statutory federal income tax rate of 21% . The sources of the difference and the tax effects of each were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Expected income tax expense $ 93.4 $ 109.6 $ 59.3 Tax difference related to investment disposals and maturities (9.2 ) (14.8 ) (9.2 ) Stock-based compensation windfall benefit (2.1 ) (3.0 ) (2.3 ) Dividend received deduction (1.1 ) (1.3 ) (1.2 ) Tax-exempt interest (0.2 ) (0.3 ) (0.4 ) Nondeductible expenses 1.8 1.7 1.6 Effect of changes in the tax law and rates — 1.2 (4.3 ) Other, net 0.2 — — Income tax expense $ 82.8 $ 93.1 $ 43.5 Effective tax rate 18.6 % 17.8 % 15.4 % The following are the components of the Company’s deferred tax assets and liabilities, (excluding those associated with its discontinued operations). DECEMBER 31 2020 2019 (in millions) Deferred tax assets: Loss, LAE and unearned premium reserves, net $ 150.8 $ 139.9 Employee benefit plans 6.0 7.6 Other 16.9 8.3 Total deferred tax assets 173.7 155.8 Deferred tax liabilities: Investments, net 146.7 87.9 Deferred acquisition costs 100.3 98.1 Software capitalization 24.0 21.0 Other — 0.6 Total deferred tax liabilities 271.0 207.6 Net deferred tax liability $ (97.3 ) $ (51.8 ) Deferred tax assets are reduced by a valuation allowance if it is more likely than not that all or some portion of the deferred tax assets will not be realized. The Company believes it is more likely than not that the deferred tax assets will be realized; therefore there was no valuation allowance required at December 31, 2020 or 2019. In prior years, the Company completed several transactions which resulted in, for tax purposes only, realized gains in its investment portfolio. As a result of these transactions, the Company was able to utilize capital losses carried forward and to release the valuation allowance recorded against the deferred tax asset related to these losses. The releases of valuation allowances were recorded as a benefit in accumulated other comprehensive income. Previously unrealized benefits of $9.2 million, $14.8 million and $9.2 million, were recognized as part of income from continuing operations during 2020, 2019 and 2018, respectively. The remaining amount of $11.6 million in accumulated other comprehensive income will be released into income from continuing operations in future years, as the investment securities subject to these transactions are sold or mature. The table below provides a reconciliation of the beginning and ending liability for uncertain tax positions as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Liability at beginning of year, net $ 1.3 $ 3.0 $ 3.0 Additions for tax positions of current year — — — Subtractions as a result of a lapse of the applicable statute of limitations (0.4 ) (1.7 ) — Liability at end of year, net $ 0.9 $ 1.3 $ 3.0 There were no tax positions at December 31, 2020, 2019 and 2018 for which the ultimate deductibility was highly certain, but for which there was uncertainty about the timing of such deductibility. Because of the impact of deferred tax accounting, other than interest and penalties, a change in the timing of deductions would not impact the annual effective tax rate. The Company recognizes interest and penalties related to unrecognized tax benefits in federal income tax expense. For the years ended December 31, 2020, 2019 and 2018 the Company recognized a de minimis amount of net interest and has not recognized any penalties associated with unrecognized tax benefits. During 2020 and 2019, the Company released accrued interest of $0.1 million and $0.3 million, respectively, due to the expirations of a statute of limitations. In 2021, the Company is expecting to release the remaining $0.9 million of liability due to the expiration of a statute of limitations. The Company and its subsidiaries file 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 2016.

Pension Plans

Pension Plans	12 Months Ended
	Dec. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Pension Plans	8. PENSION PLANS DEFINED BENEFIT PLANS The Company recognizes the funded status of its defined benefit plans in its Consolidated Balance Sheets. The funded status is measured as the difference between the fair value of plan assets and the projected benefit obligation of the Company’s defined benefit plans. The Company provides information for its overfunded plan separate from its underfunded plan. Defined Benefit Plans Prior to 2005, THG provided retirement benefits to substantially all of its employees under defined benefit pension plans. These plans were based on a defined benefit cash balance formula, whereby the Company annually provided an allocation to each covered employee based on a percentage of that employee’s eligible salary, similar to a defined contribution plan arrangement. In addition to the cash balance allocation, certain transition group employees who had met specified age and service requirements as of December 31, 1994 were eligible for a grandfathered benefit based primarily on each employee’s years of service and compensation during their highest five consecutive plan years of employment. The Company’s policy for the plans is to fund at least the minimum amount required by the Employee Retirement Income Security Act of 1974 (“ERISA”). As of January 1, 2005, the defined benefit pension plans were frozen and since that date, no further cash balance allocations have been credited to participants. Participants’ accounts are credited with interest daily, based upon the General Agreement of Trades and Tariffs rate (the 30-year Treasury Bond interest rate). In addition, the grandfathered benefits for the transition group were also frozen at January 1, 2005 levels with an annual transition pension adjustment calculated at an interest rate equal to 5% per year up to 35 years of completed service, and 3% thereafter. As of December 31, 2020, based on current estimates of plan liabilities and other assumptions, the assets of the qualified defined benefit pension plan exceeded the projected benefit obligation by approximately $27.4 million. Assumptions Defined Benefit Plans In order to measure the expense associated with these plans, management must make various estimates and assumptions, including discount rates used to value liabilities, assumed rates of return on plan assets, employee turnover rates and anticipated mortality rates, for example. The estimates used by management are based on the Company’s historical experience, as well as current facts and circumstances. In addition, the Company uses outside actuaries to assist in measuring the expense and liability associated with these plans. The Company measures the funded status of its plans as of the date of its year-end statement of financial position. The Company utilizes a measurement date of December 31 st Weighted average assumptions used to determine pension benefit obligations are as follows: DECEMBER 31 2020 2019 2018 Discount rate - qualified plan 3.00 % 3.75 % 4.50 % Discount rate - non-qualified plan 2.88 % 4.00 % 4.50 % Cash balance interest crediting rate 3.00 % 3.50 % 3.50 % The Company utilizes a measurement date of January 1 st YEARS ENDED DECEMBER 31 2020 2019 2018 Qualified plan Discount rate 3.75 % 4.50 % 3.88 % Expected return on plan assets 4.75 % 5.50 % 4.75 % Cash balance interest crediting rate 3.50 % 3.50 % 3.50 % Non-qualified plan Discount rate 4.00 % 4.50 % 3.88 % The expected rates of return were determined by using historical mean returns for each asset class, adjusted for certain factors believed to have an impact on future returns. These returns are generally weighted to the plan’s actual asset allocation, and are net of administrative expenses. For the qualified defined benefit plan, the 2020 expected return on plan assets of 4.75% reflects long-term expectations and decreased modestly based upon long-term market expectations and expense management efforts. The Company reviews and updates, at least annually, its expected return on plan assets based on changes in the actual assets held by the plan and market conditions. Plan Assets Qualified Defined Benefit Plan For the qualified defined benefit plan, a target allocation approach is utilized, which focuses on creating a mix of assets that will generate modest growth from equity securities while minimizing volatility from changes in the markets and economic environment. Various factors are taken into consideration in determining the appropriate asset mix, such as census data, actuarial valuation information and capital market assumptions. Target allocations are reviewed and updated at least annually. Changes are made periodically. The following table provides its year-end 2020 target allocations and actual invested asset allocations at December 31, 2020 and 2019. DECEMBER 31 2020 TARGET LEVELS 2020 2019 Fixed income securities: Fixed maturities 88 % 87 % 88 % Money market funds 2 % 3 % 2 % Total fixed income securities 90 % 90 % 90 % Equity securities 10 % 10 % 10 % Total plan assets 100 % 100 % 100 % The following table presents, for each hierarchy level, the qualified defined benefit plan’s investment assets that are measured at fair value at December 31, 2020 and 2019. Refer to Note 5 – “Fair Value” for a description of the different levels in the Fair Value Hierarchy. DECEMBER 31 2020 2019 (in millions) Total Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Fixed income securities: Fixed maturities $ 29.5 $ 10.4 $ — $ 19.1 $ 33.6 $ 10.7 $ — $ 22.9 Money market mutual funds 11.3 11.3 — — 7.8 7.8 — — Total investments at fair value $ 40.8 $ 21.7 $ — $ 19.1 $ 41.4 $ 18.5 $ — $ 22.9 Fixed Income Securities and Mutual Funds Securities classified as Level 1 at December 31, 2020 and 2019 include actively traded mutual funds and publicly traded securities, which are valued at quoted market prices. Securities classified as Level 3 at December 31, 2020 and 2019 include assets held in a fixed account of an insurance company. The fair value of the investment is estimated using a comparable public market financial institution derived fair value curve that uses non-observable inputs for market liquidity and unique credit characteristics of its underlying securities. The Plan also holds investments measured at fair value using NAV based on the value of the underlying investments, which is determined independently by the investment manager and have not been included in the table above. These include investments in commingled pools and investment-grade fixed income securities held in a custom fund, and other commingled pools that primarily invest in publicly traded common stocks. The daily NAV, which is not published as a quoted market price for these investments, is used as the basis for transactions. Redemption of these funds is not subject to restriction. The fair values of these investments are as follows: DECEMBER 31 2020 2019 Fixed maturities $ 414.4 $ 394.3 Equity securities 52.7 51.7 Total investments carried at NAV $ 467.1 $ 446.0 The table below provides a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). YEARS ENDED DECEMBER 31 2020 2019 (in millions) Balance at beginning of period $ 22.9 $ 25.8 Less: Assets transferred to Level 1 investments (4.3 ) (3.5 ) Actual return on plan assets related to assets still held 0.5 0.6 Balance at end of year $ 19.1 $ 22.9 Obligations and Funded Status The Company recognizes the current funded status of its plans in its Consolidated Balance Sheets. Changes in the funded status of the plans are reflected as components of either net income or accumulated other comprehensive income or loss. The components of accumulated other comprehensive income or loss are reflected as a net actuarial gain or loss. The following table reflects the benefit obligations, fair value of plan assets and funded status of the plans at December 31, 2020 and 2019. DECEMBER 31 Qualified Pension Plan Non-Qualified Pension Plan (in millions) 2020 2019 2020 2019 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 472.5 $ 469.5 $ 31.7 $ 33.8 Interest cost 17.1 19.9 1.2 1.4 Actuarial losses (gains) 25.9 20.4 3.0 (0.6 ) Benefits paid (32.2 ) (37.3 ) (2.9 ) (2.9 ) Benefit obligation, end of year (1) 483.3 472.5 33.0 31.7 Change in plan assets: Fair value of plan assets, beginning of period 487.6 450.7 — — Actual return on plan assets 55.3 64.2 — — Contributions — 10.0 2.9 2.9 Benefits paid (32.2 ) (37.3 ) (2.9 ) (2.9 ) Fair value of plan assets, end of year 510.7 487.6 — — Funded status of the plans $ 27.4 $ 15.1 $ (33.0 ) $ (31.7 ) (1) The accumulated benefit obligation for these plans is equal to the projected benefit obligation. Actuarial losses related to the change in the benefit obligation for the Company’s qualified benefit plan were $25.9 million and $20.4 million for the years ended December 31, 2020 and 2019, respectively. Actuarial losses related to the change in the benefit obligation for the Company’s non-qualified benefit plan were $3.0 million for the year ended December 31, 2020, compared to actuarial gains of $0.6 million for the year ended December 31, 2019. For both plans, the actuarial losses in 2020 primarily reflect reductions in the discount rate driven by decreases in corporate bond interest rates, partially offset by favorable mortality experience. In 2019, the actuarial gains or losses in both plans also reflect the effect of lower discount rates and favorable mortality experience. The non-qualified plan also had favorable demographic experience ultimately producing actuarial gains in 2019. Components of Net Periodic Pension Cost The components of total net periodic pension cost are as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Interest cost $ 18.3 $ 21.3 $ 19.8 Expected return on plan assets (22.2 ) (23.4 ) (20.6 ) Recognized net actuarial loss 5.8 11.3 9.6 Net periodic pension cost $ 1.9 $ 9.2 $ 8.8 The following table reflects the total amounts recognized in accumulated other comprehensive income relating to the defined benefit pension plans as of December 31, 2020 and 2019. DECEMBER 31 2020 2019 (in millions) Net actuarial loss $ 67.4 $ 77.3 The unrecognized net actuarial gains or losses which exceed 10% of the greater of the projected benefit obligations or the fair value of plan assets are amortized as a component of net periodic pension cost over the next five years. Contributions In accordance with ERISA guidelines, the Company is not required to fund its qualified benefit plan in 2021. The Company expects to contribute $2.9 million to its non-qualified pension plan to fund 2021 benefit payments. During 2019, the Company made a discretionary contribution of $10.0 million to its qualified benefit plan. At this time, no additional discretionary contributions are expected to be made into any of the plans during 2021. Benefit Payments YEARS ENDED DECEMBER 31 2021 2022 2023 2024 2025 2026-2030 (in millions) Qualified pension plan $ 39.4 $ 38.1 $ 37.5 $ 35.8 $ 34.2 $ 152.4 Non-qualified pension plan $ 2.9 $ 2.9 $ 2.7 $ 2.6 $ 2.5 $ 11.1 The benefit payments are based on the same assumptions used to measure the Company’s benefit obligations at the end of 2020. Benefit payments related to the qualified plan will be made from plan assets held in trust and not included with Company assets, whereas those payments related to the non-qualified plan will be provided for by the Company. DEFINED CONTRIBUTION PLAN In addition to the defined benefit plans, THG provides a qualified defined contribution 401(k) plan for all of its employees, whereby the Company matches employee elective 401(k) contributions, up to a maximum of 6% of eligible compensation in 2020, 2019 and 2018. The Company’s expense for this matching provision was $24.1 million, $22.2 million and $21.7 million for 2020, 2019 and 2018, respectively. In addition to this matching provision, the Company can elect to make an annual contribution to employees’ accounts.

Other Comprehensive Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Other Comprehensive Income	9. OTHER COMPREHENSIVE INCOME The following table provides changes in other comprehensive income (loss). YEARS ENDED DECEMBER 31 2020 2019 2018 Tax Tax Tax Benefit Net of Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on investment securities: Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income $ 274.6 $ (57.7 ) $ 216.9 $ 326.0 $ (68.5 ) $ 257.5 $ (206.2 ) $ 43.3 $ (162.9 ) Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income 1.3 (0.2 ) 1.1 1.5 (0.4 ) 1.1 (3.5 ) 0.7 (2.8 ) Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income (13.5 ) (6.4 ) (19.9 ) (5.0 ) (13.7 ) (18.7 ) 2.5 (9.7 ) (7.2 ) Amount of credit losses recognized in Consolidated Statements of Income 1.2 (0.3 ) 0.9 2.1 (0.4 ) 1.7 3.5 (0.7 ) 2.8 Amount of additional impairment losses recognized in Consolidated Statements of Income 16.6 (3.5 ) 13.1 — — — — — — Unrealized losses (gains) realized with sale of Chaucer business — — — 0.1 — 0.1 24.2 (5.1 ) 19.1 Net unrealized gains (losses) 280.2 (68.1 ) 212.1 324.7 (83.0 ) 241.7 (179.5 ) 28.5 (151.0 ) Pension and postretirement benefits: Net gains (losses) arising in the period from net actuarial (gains) losses 3.9 (0.8 ) 3.1 20.5 (4.3 ) 16.2 (21.3 ) 4.3 (17.0 ) Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost 6.0 (1.3 ) 4.7 11.4 (2.4 ) 9.0 9.7 (2.0 ) 7.7 Pension obligations recognized with sale of Chaucer business — — — — — — 21.2 (4.8 ) 16.4 Total pension and postretirement benefits 9.9 (2.1 ) 7.8 31.9 (6.7 ) 25.2 9.6 (2.5 ) 7.1 Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period — — — — — — (2.2 ) 0.5 (1.7 ) Currency translation obligation recognized with sale of Chaucer business — — — 0.9 (0.2 ) 0.7 29.4 (6.2 ) 23.2 Total cumulative foreign currency translation adjustment — — — 0.9 (0.2 ) 0.7 27.2 (5.7 ) 21.5 Other comprehensive income (loss) $ 290.1 $ (70.2 ) $ 219.9 $ 357.5 $ (89.9 ) $ 267.6 $ (142.7 ) $ 20.3 $ (122.4 ) Reclassifications out of accumulated other comprehensive income were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Amount Reclassified from Details about Accumulated Other Accumulated Affected Line Item in the Statement Comprehensive Income Components Other Comprehensive Income Where Net Income is Presented Net unrealized gains (losses) on investment securities $ 13.4 $ 5.0 $ (3.2 ) Net realized gains (losses) from sales and other (17.6 ) (2.0 ) (2.6 ) Impairment losses on investments (4.2 ) 3.0 (5.8 ) Total before tax 10.2 14.1 10.3 Tax benefit 6.0 17.1 4.5 Continuing operations; net of tax — (0.1 ) (19.1 ) Gain on sale of Chaucer business — — (0.2 ) Discontinued operations - Chaucer business (0.1 ) (0.1 ) 0.1 Discontinued life businesses 5.9 16.9 (14.7 ) Net of tax Amortization of defined benefit pension and postretirement actuarial losses (6.0 ) (11.4 ) (9.5 ) Loss adjustment expenses and other operating expenses (1) 1.3 2.4 1.9 Tax benefit (4.7 ) (9.0 ) (7.6 ) Continuing operations; net of tax — — (16.4 ) Gain on sale of Chaucer business — — (0.1 ) Discontinued operations - Chaucer business (4.7 ) (9.0 ) (24.1 ) Net of tax Currency translation obligation recognized with sale of Chaucer business — (0.7 ) (23.2 ) Gain on sale of Chaucer business Total reclassifications for the period $ 1.2 $ 7.2 $ (62.0 ) Benefit (expense) 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 each of the years ended December 31, 2020, 2019 and 2018.

Stock-based Compensation Plans

Stock-based Compensation Plans	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation Plans	10. STOCK-BASED COMPENSATION PLANS On May 20, 2014, shareholders approved The Hanover Insurance Group 2014 Long-Term Incentive Plan (the “2014 Stock Plan”). With respect to new share-based award issuances, the 2014 Stock Plan replaced The Hanover Insurance Group, Inc. 2006 Long-Term Incentive Plan (the “2006 Stock Plan”) and authorized the issuance of 6,100,000 shares in a new share pool, plus any shares subject to outstanding awards under the 2006 Stock Plan that may become available for reissuance as a result of the cash settlement, forfeiture, expiration or cancellation of such awards. The 2014 Stock Plan provides for the granting of the same types of awards as the 2006 Stock Plan, including stock options and stock appreciation rights (“SARS”), restricted and unrestricted stock, stock units, performance and market-based stock awards and cash awards. In accordance with the 2014 Stock Plan, the issuance of one share of common stock in the form of an option or SAR will reduce the share pool by one share, whereas the issuance of one share of common stock for the other types of stock awards provided by the Plan will reduce the pool by 3.8 shares. As of December 31, 2020, there were 2,790,071 shares available for grants under the 2014 Stock Plan. Additionally, on May 20, 2014, shareholders approved The Hanover Insurance Group 2014 Employee Stock Purchase Plan (the “ESPP Plan”) authorizing the issuance of 2,500,000 shares under such plan. As of December 31, 2020, 2,332,094 shares were available for grant under the ESPP Plan. Compensation cost, excluding the discontinued operations related to the Chaucer business, for the years ended December 31, 2020, 2019 and 2018 totaled $20.1 million, $17.4 million and $14.9 million, respectively. Related tax benefits were $4.2 million, $3.7 million and $3.1 million, respectively. STOCK OPTIONS Under the 2014 Stock Plan, options may be granted to eligible employees, directors or consultants at an exercise price equal to the market price of the Company’s common stock on the date of grant. Option shares may be exercised subject to the terms prescribed by the Compensation and Human Capital Committee of the Board of Directors (the “Committee”) at the time of grant. Options granted in 2020, 2019 and 2018 generally vest over 3 years with 33 1/3 Information on the Company’s stock options is summarized below. YEARS ENDED DECEMBER 31 2020 2019 2018 (in whole shares and dollars) Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Outstanding, beginning of year 1,121,559 $ 87.88 1,099,076 $ 85.75 1,062,177 $ 75.53 Granted (1) 242,598 118.34 252,813 119.36 301,152 110.98 Exercised (54,721 ) 75.86 (191,601 ) 71.20 (210,190 ) 69.41 Forfeited or cancelled (27,158 ) 112.18 (38,729 ) 107.34 (54,063 ) 89.09 Outstanding, end of year 1,282,278 $ 93.64 1,121,559 $ 87.88 1,099,076 $ 85.75 Exercisable, end of year 852,804 $ 82.76 662,555 $ 75.63 515,286 $ 69.83 (1) In accordance with plan provisions, 2019 includes 67,605 options related to special dividends paid by the Company in January 2019 and December 2019, in order to retain the intrinsic value of outstanding awards. The remaining 185,208 option awards were granted at an exercise price of $119.36. Cash received for options exercised for the years ended December 31, 2020, 2019 and 2018 was $3.9 million, $12.3 million and $14.3 million, respectively. The intrinsic value of options exercised for the years ended December 31, 2020, 2019 and 2018 was $2.7 million, $10.1 million and $9.6 million, respectively. The excess tax benefit realized from options exercised for the years ended December 31, 2020, 2019 and 2018 was $2.1 million, $1.7 million and $1.6 million, respectively. The aggregate intrinsic value at December 31, 2020 for shares outstanding and shares exercisable was $29.9 million and $29.1 million, respectively. At December 31, 2020, the weighted average remaining contractual life for shares outstanding and shares exercisable was 6.6 years and 5.7 years, respectively. Additional information about employee options outstanding and exercisable at December 31, 2020 is included in the following table: Options Outstanding Options Currently Exercisable Range of Exercise Prices Number Weighted Average Remaining Contractual Lives Weighted Average Exercise Price Number Weighted Average Exercise Price $ 34.66 to $40.01 42,573 1.81 $ 38.33 42,573 $ 38.33 $ 54.61 66,107 3.14 54.61 66,107 54.61 $ 66.14 to $68.41 102,996 4.16 66.15 102,996 66.15 $ 70.51 to $77.91 154,014 5.35 75.45 154,014 75.45 $ 82.39 to $92.68 246,844 6.22 85.39 244,951 85.33 $ 104.11 to $105.53 266,274 7.16 104.12 181,613 104.12 $ 113.32 to $118.54 403,470 8.75 117.96 60,550 117.07 The fair value of each option is estimated on the date of grant using the Black-Scholes option pricing model. For all options granted through December 31, 2020, the exercise price equaled the market price on the grant date. Compensation cost related to options is based upon the grant date fair value and expensed on a straight-line basis over the service period for each separately vesting portion of the option as if the option was, in substance, multiple awards. The weighted average grant date fair value of options granted during the years ended December 31, 2020, 2019 and 2018 was $14.45 , $18.12 and $16.30, respectively. The following significant assumptions were used to determine the fair value for options granted in the years indicated. 2020 2019 2018 Dividend yield 2.193% to 2.805 % 2.011 % 1.740% to 1.954 % Expected volatility 17.671% to 24.495 % 18.495% to 19.100 % 17.943% to 18.500 % Weighted average expected volatility 18.10 % 18.86 % 18.20 % Risk-free interest rate 0.262 to 1.042% % 2.527% to 2.617 % 2.365% to 2.969 % Expected term, in years 2.5 to 6.5 2.5 to 6.5 2.5 to 6.0 The expected dividend yield is based on the Company’s dividend payout rate(s), in the year noted, excluding the effect of any special dividends provided. Expected volatility is based generally on the Company’s historical daily stock price volatility. The risk-free rate for periods within the contractual life of the option is based on the U.S. Treasury yield curve in effect at the time of grant. The expected term of options granted represents the period of time that options are expected to be outstanding and is derived primarily using historical exercise, forfeit and cancellation behavior, along with certain other factors expected to differ from historical data. The fair value of shares that vested during the years ended December 31, 20 20 and 201 9 was $ 4.6 million and $ million, respectively. As of December 31, 20 20 , the Company had unrecognized compensation expense of $ million related to unvested stock options that is expected to be recognized over a weighted average period of 1.4 years . RESTRICTED STOCK UNITS Stock grants may be awarded to eligible employees at a price established by the Committee (which may be zero). Under the 2014 Stock Plan, the Company may award shares of restricted stock, restricted stock units, as well as shares of unrestricted stock. Restricted stock grants may vest based upon performance criteria, market criteria or continued employment and be in the form of shares or units. Vesting periods are established by the Committee. The Company granted market-based restricted share units in 2020, 2019 and 2018. Additionally, in 2020 and 2019 the Company granted performance-based restricted share units. These units generally vest after three years of continued employment and after the achievement of certain performance targets. The Company also granted time-based restricted stock units to eligible employees in 2020, 2019 and 2018 that generally vest after three years of continued employment. The following table summarizes information about employee restricted stock units: YEARS ENDED DECEMBER 31 2020 2019 2018 Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Time-based restricted stock units: Outstanding, beginning of year 335,166 $ 109.55 332,481 $ 97.28 298,528 $ 83.45 Granted 143,128 117.72 149,698 117.60 152,529 111.66 Vested (94,993 ) 94.54 (109,493 ) 84.57 (72,710 ) 72.69 Forfeited (1) (32,821 ) 115.74 (37,520 ) 106.03 (45,866 ) 94.04 Outstanding, end of year 350,480 $ 116.37 335,166 $ 109.55 332,481 $ 97.28 Performance and market-based restricted stock units: Outstanding, beginning of year 86,252 $ 110.70 69,838 $ 95.58 102,586 $ 81.21 Granted 54,415 105.10 42,605 116.67 35,063 118.60 Vested (40,601 ) 80.92 (23,521 ) 79.57 (14,032 ) 70.24 Forfeited (1) (3,023 ) 124.78 (2,670 ) 135.92 (53,779 ) 89.79 Outstanding, end of year 97,043 $ 119.59 86,252 $ 110.70 69,838 $ 95.58 (1) As a result of the sale of Chaucer and included in forfeitures in 2018 are 19,655 shares of time-based restricted stock units, 43,449 shares of performance-based restricted stock units and 2,705 shares of market-based restricted stock units that were awarded to Chaucer employees. In 2020, 2019 and 2018, the Company granted market-based awards totaling 21,379, 24,410, and 31,866, 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 In 2020, 13,532 market-based restricted stock units were included as granted due to completion levels in excess of 100% for units granted in 2017. The weighted average grant date fair value of these awards was $80.92. In 2019, 5,820 market-based restricted stock units were included as granted due to completion levels in excess of 100% for units granted in both 2016 and 2017. The weighted average grant date fair value of these awards was $75.95. In 2018, 3,115 market-based restricted stock units were included as granted due to completion levels in excess of 100% for units granted in 2015. The weighted average grant date fair value of these awards was $70.24. In 2020 and 2019, the Company also granted performance-based restricted stock units totaling 19,504 and 18,195, 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 determined through the use of a performance-based metric (return on equity) and has the potential to range from 0% to 150% of the shares disclosed. The increase in intrinsic value from grant date of restricted stock and restricted stock units that vested during the years ended December 31, 2020, 2019 and 2018 was $4.5 million, $3.7 million and $2.9 million, respectively. The intrinsic value for performance and market-based restricted stock units that vested in 2020, 2019 and 2018 was $2.0 million, $1.1 million, $0.6 million, respectively. There were 1,949 shares, 1,674 shares and 10,330 shares of market-based awards that forfeited in 2020, 2019 and 2018, respectively. Also, during 20 20 , 201 9 and 201 8 , there were 1,074 shares , 996 shares and 43,449 shares , respectively, of performance-based awards that forfeited. At December 31, 2020, the aggregate intrinsic value of restricted stock units was $40.8 million and the weighted average remaining contractual life was 1.3 years. The aggregate intrinsic value of performance and market-based restricted stock units was $11.6 million and the weighted average remaining contractual life was 1.3 years. As of December 31, 2020, there was $25.1 million of total unrecognized compensation cost related to unvested restricted stock units and performance and market-based restricted stock units. The cost is expected to be recognized over a weighted average period of 1.8 years. Compensation cost associated with restricted stock, restricted stock units and performance and market-based restricted stock units is generally calculated based upon grant date fair value, which is determined using current market prices.

Earnings Per Share and Sharehol

Earnings Per Share and Shareholders' Equity Transactions	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Earnings Per Share and Shareholders' Equity Transactions	11. 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: DECEMBER 31 2020 2019 2018 (in millions, except per share data) Basic shares used in the calculation of earnings per share 37.7 40.0 42.4 Dilutive effect of securities: Employee stock options 0.2 0.3 0.3 Non-vested stock grants 0.2 0.3 0.3 Diluted shares used in the calculation of earnings per share 38.1 40.6 43.0 Per share effect of dilutive securities on income from continuing operations $ (0.11 ) $ (0.16 ) $ (0.07 ) Per share effect of dilutive securities on net income $ (0.10 ) $ (0.16 ) $ (0.12 ) Diluted earnings per share during 2020, 2019 and 2018 excludes 0.4 million, 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. The Board of Directors has authorized a stock repurchase program which provides for aggregate repurchases of the Company’s common stock of up to $900 million, and under which the Company had approximately $124 million available at December 31, 2020. Under the repurchase authorization, the Company may repurchase, from time to time, common stock 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. During 2020, 2019 and 2018, the Company had executed several accelerated share repurchase (“ASR”) agreements, through which it repurchased a total of 5.2 million shares. Pursuant to the terms of ASR agreements executed on December 30, 2018 and on June 28, 2019, the Company repurchased approximately 3.2 million shares of its common stock for $400.0 million. On December 9, 2019 (the "December 2019 ASR"), the Company repurchased approximately 0.9 million shares of its common stock for $150.0 million pursuant to an additional ASR, which represented approximately 80% of the total number of shares expected to be repurchased under this agreement. On February 26, 2020, the Company received approximately 0.2 million of its common stock shares as a final settlement of shares repurchased under the December 2019 ASR. On October 29, 2020, pursuant to the terms of another ASR agreement (the "October 2020 ASR") the Company paid $100.0 million and received an initial delivery of approximately 0.8 million shares of its common stock, which was approximately 80% of the total number of shares expected to be repurchased under the October 2020 ASR. On January 29, 2021, the Company received approximately 45,000 shares of its common stock shares as final settlement of shares repurchased under the October 2020 ASR

Dividend Restrictions

Dividend Restrictions	12 Months Ended
	Dec. 31, 2020
Disclosure Of Restrictions On Dividends Loans And Advances Disclosure [Abstract]
Dividend Restrictions	12. DIVIDEND RESTRICTIONS INSURANCE SUBSIDIARIES The individual law of all states, including New Hampshire and Michigan, where Hanover Insurance and Citizens are domiciled, respectively, restrict the payment of dividends to stockholders by insurers. These laws affect the dividend paying ability of Hanover Insurance and Citizens. Pursuant to New Hampshire’s statute, the maximum dividends and other distributions that an insurer may pay in any twelve month period, without prior approval of the New Hampshire Insurance Commissioner, is limited to the lesser of 10% of such insurer’s statutory policyholder surplus as of the preceding December 31, or statutory net income less net realized gains. Hanover Insurance declared and paid dividends to its parent totaling $245.0 million in 2020, and $140.0 million in both 2019 and 2018. At January 1, 2021, the maximum dividend payable without prior approval was $13.3 million. In May 2021, the maximum dividend declared payable without prior approval will increase by $245.0 million to a total amount of $258.3 million. Pursuant to Michigan’s statute, the maximum dividends and other distributions that an insurer may pay in any twelve month period, without prior approval of the Michigan Insurance Commissioner, is limited to the greater of 10% of policyholders’ surplus as of December 31 of the immediately preceding year or the statutory net income less net realized gains, for the immediately preceding calendar year. Citizens declared and paid an extraordinary dividend to its parent, Hanover Insurance, totaling $82.0 million in the fourth quarter of 2020. In 2019 and 2018, ordinary dividends of $106.0 million and $87.9 million, respectively, were declared and paid by Citizens. At January 1, 2021, the maximum dividend payable without prior approval was $26.7 million. In October 2021, the maximum dividend declared payable without prior approval will increase by $82.0 million to a total amount of $108.7 million. The statutes in both New Hampshire and Michigan require that prior notice to the respective Insurance Commissioner of any proposed dividend be provided and such Commissioner may, in certain circumstances, prohibit the payment of the proposed dividend.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Segment Information	13. 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 management and professional liability, marine, Hanover Programs, specialty industrial and commercial property, monoline general liability and surety. 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 property and casualty voluntary pools business. In 2018 and 2019, the Company completed the sales of its former Chaucer segment to China Re. Accordingly, as of December 31, 2018, and for all periods presented, Chaucer’s results of operations are classified as Discontinued Operations in the Consolidated Statements of Income (see Note 2 – “Discontinued Operations”). The separate financial information is presented consistent with the way results are regularly evaluated by the chief operating decision maker in deciding how to allocate resources and in assessing performance. The Company reports interest expense related to debt separately from the earnings of its operating segments. For 2020, this consisted of interest on the Company’s senior and subordinated debentures. Management evaluates the results of the aforementioned segments based on operating income before income taxes, excluding interest expense on debt. Operating income before income 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 income 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 income taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income before income 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 income taxes should not be construed as a substitute for income before income taxes or income from continuing operations 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. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Operating revenues: Commercial Lines $ 2,870.8 $ 2,843.5 $ 2,739.5 Personal Lines 1,931.4 1,911.8 1,791.3 Other 20.1 26.0 14.2 Total 4,822.3 4,781.3 4,545.0 Net realized and unrealized investment gains (losses) 5.0 109.4 (50.7 ) Total revenues $ 4,827.3 $ 4,890.7 $ 4,494.3 Operating income (loss) before interest expense and income taxes: Commercial Lines: Underwriting income $ 106.3 $ 121.5 $ 85.0 Net investment income 175.3 180.1 182.2 Other expense (6.2 ) (1.5 ) (1.5 ) Commercial Lines operating income 275.4 300.1 265.7 Personal Lines: Underwriting income 132.9 59.1 66.6 Net investment income 76.7 80.1 73.7 Other income 2.9 5.7 5.9 Personal Lines operating income 212.5 144.9 146.2 Other: Underwriting loss (4.3 ) (1.3 ) (4.2 ) Net investment income 13.1 21.1 11.5 Other expense (12.0 ) (11.2 ) (12.7 ) Other operating income (loss) (3.2 ) 8.6 (5.4 ) Operating income before interest expense and income taxes 484.7 453.6 406.5 Interest on debt (37.1 ) (37.5 ) (45.1 ) Operating income before income taxes 447.6 416.1 361.4 Non-operating income (loss) items: Net realized and unrealized investment gains (losses) 5.0 109.4 (50.7 ) Net loss from repayment of debt (6.2 ) — (28.2 ) Other non-operating items (1.6 ) (3.4 ) — Income from continuing operations before income taxes $ 444.8 $ 522.1 $ 282.5 The following table provides identifiable assets for the Company’s business segments and discontinued operations: DECEMBER 31 2020 2019 (in millions) Identifiable Assets Property and Casualty $ 13,333.5 $ 12,387.7 Assets of discontinued businesses 110.2 102.8 Total $ 13,443.7 $ 12,490.5 The Company reviews the assets of its Property and Casualty Insurance Companies collectively and does not allocate them among the Commercial Lines, Personal Lines and Other segments.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2020
Reinsurance Disclosures [Abstract]
Reinsurance	14. REINSURANCE In the normal course of business, the Company seeks to reduce the losses that may arise from catastrophes or other events that cause unfavorable underwriting results by reinsuring certain levels of risk in various areas of exposure with other insurance enterprises or reinsurers. Reinsurance transactions are accounted for in accordance with the provisions of ASC 944. Amounts recoverable from reinsurers are estimated in a manner consistent with the claim liability associated with the reinsured policy. Reinsurance contracts do not relieve the Company from its obligations to policyholders. Failure of reinsurers to honor their obligations could result in losses to the Company; consequently, allowances are established for amounts deemed uncollectible. The Company determines the appropriate amount of reinsurance based on evaluations of the risks accepted and analyses prepared by consultants and on market conditions (including the availability and pricing of reinsurance). The Company also believes that the terms of its reinsurance contracts are consistent with industry practice in that they contain standard terms with respect to lines of business covered, limit and retention, arbitration and occurrence. The Company believes that its reinsurers are financially sound, based upon an ongoing review of financial strength ratings assigned to them by rating agencies, their reputations in the reinsurance marketplace, collections history, advice from third parties and the analysis, and guidance of the Company’s reinsurance advisors. As a condition to conduct certain business in various states, the Company is required to participate in residual market mechanisms, facilities, and pooling arrangements such as the Michigan Catastrophic Claims Association (“MCCA”). The Company is subject to concentration of risk with respect to reinsurance ceded to the MCCA. Funding for MCCA comes from assessments against automobile insurers based upon their share of insured automobiles in the state for which the policyholders have elected unlimited personal injury protection (“PIP”) benefits. Insurers are allowed to pass along this cost to Michigan automobile policyholders that have elected unlimited PIP benefits. The Company ceded to the MCCA premiums earned and losses and LAE incurred of $84.6 million and $99.2 million, respectively, in 2020, $84.3 million and $108.5 million, respectively, in 2019, and $70.9 million and $108.8 million, respectively, in 2018. The MCCA represented 54.7% of the total reinsurance receivable balance at December 31, 2020. Reinsurance recoverables related to MCCA were $1,024.7 million and $1,023.7 million at December 31, 2020 and 2019, respectively. Because the MCCA is supported by assessments permitted by statute, and there have been no significant uncollectible balances from MCCA identified during the three years ending December 31, 2020, the Company believes that it has no significant exposure to uncollectible reinsurance balances from this entity. The following table provides the effects of reinsurance. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Premiums written: Direct $ 5,131.4 $ 5,090.8 $ 4,816.0 Assumed 23.1 27.7 27.6 Ceded (556.0 ) (536.8 ) (458.8 ) Net premiums written $ 4,598.5 $ 4,581.7 $ 4,384.8 Premiums earned: Direct $ 5,063.8 $ 4,953.3 $ 4,673.6 Assumed 24.6 26.3 26.6 Ceded (561.0 ) (505.1 ) (445.8 ) Net premiums earned $ 4,527.4 $ 4,474.5 $ 4,254.4 Percentage of assumed to net premiums earned 0.5 % 0.6 % 0.6 % Losses and LAE: Direct $ 3,200.5 $ 3,223.3 $ 2,986.2 Assumed 15.9 21.4 25.2 Ceded (371.2 ) (379.2 ) (286.8 ) Net losses and LAE $ 2,845.2 $ 2,865.5 $ 2,724.6

Liabilities for Outstanding Cla

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2020
Insurance [Abstract]
Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	15. LIABILITIES FOR OUTSTANDING CLAIMS, LOSSES AND LOSS ADJUSTMENT EXPENSES Reserving Process Overview Management’s process for establishing loss reserves is a comprehensive process that involves input from multiple functions throughout the organization, including actuarial, finance, claims, legal, underwriting, distribution and business operations management. A review of loss reserves for each of the classes of business that the Company writes is conducted regularly, generally quarterly. This review process takes into consideration a variety of trends that impact the ultimate settlement of claims. Where appropriate, the loss reserving process includes a review of overall payment patterns and the emergence of paid and reported losses relative to expectations. The loss reserve estimation process relies on the basic assumption that past experience, adjusted for the effects of current developments and likely trends, is an appropriate basis for predicting future outcomes. As part of this process, the Company uses a variety of analytical methods that consider experience, trends and other relevant factors. Incurred but not reported (“IBNR”) reserves are generally calculated by first projecting the ultimate cost of all claims that have been reported or expected to be reported in the future and then subtracting reported losses and LAE . IBNR reserves include both incurred-but-not-reported liabilities plus expected development on reported claims included in the liability for unpaid claims and claim adjustment expenses. Reported losses include cumulative paid losses and LAE plus outstanding case reserves. The Company’s ultimate IBNR reserves are estimated by management and reserving actuaries on an aggregate basis for each line of business or coverage for loss and LAE liabilities not reflected within the case reserves. Case reserves are established by claim personnel individually on a claim by claim basis and based on information specific to the occurrence and terms of the underlying policy. Case reserves are periodically reviewed and modified based on new or additional information pertaining to the claim. For events designated as catastrophes, the Company generally calculates IBNR reserves directly as a result of an estimated IBNR claim count and an estimated average claim amount for each event. Such an assessment involves a comprehensive analysis of the nature of the event, of policyholder exposures within the affected geographic area and of available claims intelligence. Carried reserves for each line of business and coverage are determined based on the quarterly loss reserving process. In making the determination, the Company considers numerous quantitative and qualitative factors. Quantitative factors include actual payments made and changes in case reserve estimates in the period, as compared to previously experienced patterns, the maturity of the accident year, trends observed over the recent past, the level of volatility within a particular class of business, the estimated effects of reinsurance, including reinstatement premiums, general economic trends and other factors. Qualitative factors may include legal and regulatory developments, changes in claim handling and case reserving practices, recent entry into new markets or products, changes in underwriting practices, concerns that the Company does not have sufficient or quality historical reported and paid loss and LAE information with respect to a particular line of business and coverage, effects of the economy and political outlook, perceived anomalies in the historical results, evolving trends or other factors, such as the impact of the Pandemic. 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 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 loss and LAE “development.” 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. The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Gross reserve for losses and LAE, beginning of year $ 5,654.4 $ 5,304.1 $ 5,058.5 Reinsurance recoverable on unpaid losses 1,574.8 1,472.6 1,455.0 Net loss and LAE reserves, beginning of year 4,079.6 3,831.5 3,603.5 Net incurred losses and LAE in respect of losses occurring in: Current year 2,877.8 2,893.0 2,733.5 Prior years (32.6 ) (28.4 ) (8.9 ) Total incurred losses and LAE 2,845.2 2,864.6 2,724.6 Net payments of losses and LAE in respect of losses occurring in: Current year 1,347.7 1,315.4 1,232.3 Prior years 1,194.7 1,301.1 1,264.3 Total payments 2,542.4 2,616.5 2,496.6 Net reserve for losses and LAE, end of year 4,382.4 4,079.6 3,831.5 Reinsurance recoverable on unpaid losses 1,641.6 1,574.8 1,472.6 Gross reserve for losses and LAE, end of year $ 6,024.0 $ 5,654.4 $ 5,304.1 The following table provides a summary of (favorable)/unfavorable loss and LAE reserve development. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Commercial multiple peril $ (0.2 ) $ (19.5 ) $ (4.0 ) Workers’ compensation (36.0 ) (32.6 ) (31.0 ) Commercial automobile 15.4 5.8 23.2 Other commercial lines: Hanover Programs 10.6 19.8 17.0 General liability 0.3 (7.8 ) (27.9 ) Marine (17.6 ) (13.3 ) (10.7 ) Surety (3.5 ) (4.1 ) (9.0 ) Umbrella (1.5 ) 2.8 2.3 Other lines (5.3 ) (4.4 ) (0.8 ) Total other commercial lines (17.0 ) (7.0 ) (29.1 ) Total Commercial Lines (37.8 ) (53.3 ) (40.9 ) Personal automobile 5.2 21.6 14.3 Homeowners and other personal lines (4.2 ) 2.1 16.5 Total Personal Lines 1.0 23.7 30.8 Total Other Segment 4.2 1.2 1.2 Total loss and LAE reserve development, including catastrophes $ (32.6 ) $ (28.4 ) $ (8.9 ) Within other commercial lines, general liability is comprised of both monoline and claims-made general liability. Claims-made general liability is primarily comprised of certain management and professional liability coverages. Other lines is primarily comprised of specialty industrial property, miscellaneous commercial property, fidelity and healthcare lines. Loss and LAE reserve development in the Other Segment is related to run-off voluntary assumed property and casualty reinsurance pools business. 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 $32.6 million, $28.4 million and $8.9 million in 2020, 2019 and 2018, respectively. 2020 In 2020, net favorable loss and LAE development was $32.6 million, primarily as a result of net favorable Commercial Lines development of $37.8 million, partially offset by unfavorable development in the Other Segment. Commercial Lines favorable development was primarily due to lower than expected losses of $36.0 million within the workers’ compensation line in accident years 2016 through 2019 and within other commercial lines. These were partially offset by higher than expected losses in the commercial automobile line driven by higher bodily injury and personal injury protection losses, primarily in accident years 2017 through 2019. In addition, Commercial Lines favorable development includes lower than expected losses related to certain 2017, 2018 and 2019 wind storms, winter storms and hurricanes, and the 2017 and 2018 California wildfires. Within other commercial lines, lower than expected losses in the marine line, primarily in accident years 2017 through 2019, and the specialty industrial property lines were partially offset by higher than expected losses 2019 In 2019, net favorable loss and LAE development was $28.4 million, primarily as a result of net favorable Commercial Lines development of $53.3 million, partially offset by unfavorable Personal Lines development of $23.7 million. Commercial Lines favorable development was primarily due to lower than expected losses of $32.6 million within the workers’ compensation line in accident years 2015 through 2018, as well as within the commercial multiple peril line in accident years 2015 and 2016, and within other commercial lines. These were partially offset by higher than expected losses in the commercial automobile line driven by higher bodily injury severity and personal injury protection losses. In addition, Commercial Lines favorable development includes lower than expected losses related to the 2017 and 2018 California wildfires, including the sale of subrogation rights on certain California wildfire losses, and hurricane Florence in 2018. Within other commercial lines, lower than expected losses in the marine line, primarily in accident years 2015 through 2018, general liability lines and surety line were partially offset by higher than expected losses in Hanover Programs, primarily in accident years 2011, 2013, 2015 and 2017. Personal Lines unfavorable development was primarily due to higher than expected losses of $21.6 million in the personal automobile line, driven by bodily injury severity and personal injury protection in accident years 2016 through 2017. In addition, Other Segment unfavorable development of $1.2 million was due to the Company’s run-off voluntary assumed property and casualty reinsurance pools business , which includes asbestos and environmental reserves. 2018 In 2018, net favorable loss and LAE development was $8.9 million, primarily as a result of net favorable Commercial Lines development of $40.9 million, partially offset by unfavorable Personal Lines development of $30.8 million. Commercial Lines favorable development was primarily due to lower than expected losses of $31.0 million within the workers’ compensation line in accident years 2015 through 2017 and $29.1 million in other commercial lines, partially offset by higher than expected losses of $23.2 million in the commercial automobile line, driven by higher bodily injury severity in the 2014, 2016 and 2017 accident years. Within other commercial lines, lower than expected losses in the general liability lines, related to the 2014 through 2016 accident years, marine line in accident years 2015 through 2017, and surety line, were partially offset by higher than expected losses in Hanover Programs, primarily in accident years 2010 through 2014. Personal Lines unfavorable development was primarily due to higher than expected losses of $ 14.3 million in the personal automobile line, driven by bodily injury severity in the 2015 and 2016 accident years, and in the homeowners line in accident years 2015 through 2017. In addition, Other Segment unfavorable development of $1.2 million was due to the Company’s run-off voluntary assumed property and casualty reinsurance pools business, which includes asbestos and environmental reserves. Carried Reserves The table below summarizes the gross, ceded and net reserve for losses and LAE and reconciles to the incurred claims development in the following section. Accordingly, the commercial multiple peril, workers’ compensation, commercial automobile liability and general liability and umbrella - occurrence lines presentation includes Hanover Programs business. YEAR ENDED DECEMBER 31, 2020 2019 (in millions) Gross Ceded Net Gross Ceded Net Commercial multiple peril $ 1,429.4 $ (163.9 ) $ 1,265.5 $ 1,298.5 $ (126.5 ) $ 1,172.0 Workers’ compensation 795.4 (182.6 ) 612.8 795.3 (186.0 ) 609.3 Commercial automobile liability 504.1 (30.6 ) 473.5 467.7 (27.3 ) 440.4 General liability and umbrella - occurrence 612.0 (176.0 ) 436.0 575.9 (155.5 ) 420.4 General liability - claims made 323.7 (30.5 ) 293.2 268.1 (20.1 ) 248.0 Other lines 451.6 (85.1 ) 366.5 409.5 (89.5 ) 320.0 Total Commercial Lines and other 4,116.2 (668.7 ) 3,447.5 3,815.0 (604.9 ) 3,210.1 Personal automobile liability 1,649.9 (968.5 ) 681.4 1,626.4 (963.2 ) 663.2 Homeowners 210.6 (3.0 ) 207.6 172.4 (4.8 ) 167.6 Other personal lines 47.3 (1.4 ) 45.9 40.6 (1.9 ) 38.7 Total Personal Lines 1,907.8 (972.9 ) 934.9 1,839.4 (969.9 ) 869.5 Total loss and LAE reserves $ 6,024.0 $ (1,641.6 ) $ 4,382.4 $ 5,654.4 $ (1,574.8 ) $ 4,079.6 General liability and umbrella - occurrence is primarily comprised of the Company’s commercial monoline general liability and umbrella coverages. General liability - claims made is primarily comprised of the Company’s commercial professional and management liability lines. Within total Commercial Lines and other, other lines is primarily comprised of marine, surety, specialty industrial and commercial property, product liability, voluntary pools, healthcare and fidelity lines. Included in the above table, primarily in other lines, are $57.7 million and $55.5 million of gross asbestos and environmental reserves as of December 31, 2020 and 2019, respectively. Incurred claims development tables For the following net reserve components, commercial multiple peril, workers’ compensation, commercial automobile liability, general liability and umbrella - occurrence, general liability - claims made, personal automobile liability and homeowners, the Company is presenting incurred claims development tables by accident year. The commercial multiple peril, workers’ compensation, commercial automobile liability, general liability and umbrella - occurrence lines presentation includes Hanover Programs business. Incurred claim count information presented represents claim frequency by individual claimant and measures the frequency of direct claim settlements that have resulted in or are expected to result in claim payments. Claim count information is presented in a manner consistent with that used in the quarterly loss reserving process. A single claim event, particularly in automobile lines, may result in multiple individual claimants and, therefore, multiple claim counts. Incurred claim counts are comprised of outstanding claims and those that are closed with a loss payment and exclude those that are closed without a loss payment. A single claim event may result in multiple claims closed with a payment when a claim is subsequently reopened with further payment. In this case, a reopened claim payment is counted as an incremental claim settlement. Claim count information is not available for direct and assumed participations in various involuntary pools and residual market mechanisms, which represent approximately 4 % or less of the total gross earned premium and gross incurred claims for the lines presented. Incurred claim counts are also not adjusted for the effect of claims ceded as part of reinsurance programs, although the incurred losses and cumulative paid losses presented in the following tables are presented net of reinsurance ceded. Commercial multiple peril ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2013 $ 380.0 $ 362.5 $ 367.9 $ 391.8 $ 392.1 $ 393.0 $ 393.7 $ 393.6 $ 7.8 14,801 2014 443.9 439.6 464.8 459.4 449.1 444.4 441.0 13.1 15,881 2015 446.0 456.3 463.7 467.4 465.2 460.9 16.9 15,583 2016 447.1 449.6 448.7 447.4 449.6 25.8 15,750 2017 538.7 544.8 551.6 555.6 38.5 16,638 2018 578.2 560.9 556.1 77.8 17,123 2019 583.8 593.0 138.0 16,012 2020 628.9 245.5 13,136 Total $ 4,078.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2013 2014 2015 Year Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 2013 $ 137.6 $ 221.5 $ 262.3 $ 306.4 $ 334.6 $ 355.9 $ 366.3 $ 373.1 2014 171.7 267.8 316.0 363.4 395.2 407.7 411.7 2015 161.9 260.1 315.6 363.2 397.0 413.6 2016 140.3 237.9 290.2 342.2 369.6 2017 170.9 296.4 370.6 422.9 2018 178.8 306.2 363.7 2019 169.3 317.2 2020 217.4 Total 2,889.2 Total reserves for 2013 – 2020 accident years (incurred - paid) 1,189.5 Total reserves for 2012 and prior accident years 51.9 Unallocated loss adjustment expense 24.1 Net reserves at December 31, 2020 $ 1,265.5 Workers' compensation ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2011 $ 141.4 $ 144.2 $ 144.3 $ 145.5 $ 143.8 $ 146.7 $ 148.3 $ 147.9 $ 147.7 $ 146.9 $ 6.3 12,484 2012 176.3 171.1 165.2 157.2 162.9 163.7 163.7 161.9 161.4 8.4 13,083 2013 179.3 167.4 160.1 154.4 155.3 155.4 154.9 153.3 8.8 11,734 2014 182.1 172.9 154.7 152.1 150.3 148.2 145.8 11.8 10,943 2015 189.6 164.2 157.5 150.3 146.0 142.7 16.0 11,485 2016 189.6 180.5 164.6 158.2 152.8 16.1 15,894 2017 186.1 172.1 160.8 156.6 20.0 16,722 2018 187.2 182.1 172.1 24.2 17,454 2019 187.9 182.0 30.1 17,163 2020 182.5 76.9 12,549 Total $ 1,596.1 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2011 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 2011 $ 30.0 $ 71.3 $ 97.3 $ 114.0 $ 122.9 $ 128.0 $ 130.9 $ 132.8 $ 134.2 $ 135.3 2012 30.4 74.8 102.6 120.1 130.7 136.0 141.4 143.7 146.6 2013 30.9 74.6 101.2 114.0 122.8 127.4 131.4 133.8 2014 30.6 70.5 92.3 105.6 112.5 117.8 121.0 2015 28.0 65.7 87.2 99.4 105.8 109.6 2016 33.9 78.1 99.5 111.1 117.1 2017 32.8 73.0 94.1 106.6 2018 35.6 80.7 103.4 2019 33.3 83.1 2020 30.7 Total 1,087.2 Total reserves for 2011 – 2020 accident years (incurred - paid) 508.9 Total reserves for 2010 and prior accident years 84.6 Unallocated loss adjustment expense and other 19.3 Net reserves at December 31, 2020 $ 612.8 Commercial automobile liability ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2014 2015 Claim Year Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2014 168.5 $ 163.3 $ 177.3 $ 181.7 $ 185.1 $ 185.0 $ 183.5 $ 1.7 13,477 2015 163.4 168.3 166.9 167.8 167.6 170.3 1.2 12,829 2016 157.0 157.7 163.0 174.3 172.5 4.3 11,660 2017 159.5 170.1 182.1 188.5 7.6 11,504 2018 182.8 176.1 182.9 25.5 11,181 2019 180.5 192.0 57.0 10,210 2020 171.6 115.9 5,772 Total $ 1,261.3 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2014 2015 Year Unaudited Unaudited 2016 2017 2018 2019 2020 2014 33.1 $ 70.8 $ 102.7 $ 137.1 $ 168.2 $ 176.3 $ 178.2 2015 32.2 63.8 96.4 129.3 144.5 152.5 2016 27.8 60.7 98.3 134.0 147.2 2017 26.9 71.2 105.9 137.7 2018 29.2 59.7 92.5 2019 29.5 75.8 2020 23.4 Total 807.3 Total reserves for 2014 – 2020 accident years (incurred - paid) 454.0 Total reserves for 2013 and prior accident years 13.7 Unallocated loss adjustment expense 5.8 Net reserves at December 31, 2020 $ 473.5 General liability and umbrella - occurrence ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2011 $ 47.6 $ 43.8 $ 38.2 $ 42.2 $ 51.0 $ 55.6 $ 55.4 $ 57.5 $ 58.8 $ 59.3 $ 2.4 1,393 2012 77.2 59.3 62.2 64.3 71.1 74.2 69.1 71.2 72.8 2.5 1,774 2013 84.1 67.4 71.5 88.6 84.4 81.2 80.4 79.1 4.1 2,001 2014 100.9 82.3 98.6 101.4 98.3 99.8 98.4 6.4 2,108 2015 104.2 99.3 99.6 97.0 98.3 99.7 9.9 2,448 2016 95.6 100.7 101.5 100.8 98.7 15.7 1,913 2017 97.7 107.6 109.2 110.7 24.9 1,806 2018 99.8 106.4 104.0 35.2 1,823 2019 104.2 105.7 55.8 1,605 2020 99.8 81.0 1,213 Total $ 928.2 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2011 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 2011 1.6 $ 6.7 $ 19.4 $ 31.1 $ 39.8 $ 45.8 $ 47.8 $ 50.6 $ 52.9 $ 54.8 2012 2.2 12.6 29.8 43.8 52.1 60.6 63.0 64.5 65.0 2013 2.4 11.0 26.8 43.1 56.1 63.5 67.6 70.1 2014 3.1 14.5 31.4 52.8 70.4 83.6 87.3 2015 3.3 15.2 30.8 48.5 65.7 77.3 2016 3.1 15.0 31.6 52.5 63.4 2017 4.4 17.0 34.5 49.8 2018 4.1 15.6 35.7 2019 7.5 16.7 2020 3.9 Total 524.0 Total reserves for 2011 – 2020 accident years (incurred - paid) 404.2 Total reserves for 2010 and prior accident years 18.1 Unallocated loss adjustment expense and other 13.7 Net reserves at December 31, 2020 $ 436.0 General liability - claims made ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2014 2015 Claim Year Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2014 $ 61.7 $ 69.9 $ 83.9 $ 86.2 $ 82.5 $ 82.9 $ 82.4 $ 0.9 906 2015 93.2 98.8 98.4 90.2 90.3 91.0 1.9 1,002 2016 103.6 101.7 97.1 89.7 95.2 4.7 1,011 2017 103.3 104.6 98.1 96.0 8.8 1,122 2018 120.9 127.8 132.5 14.3 1,386 2019 126.8 127.1 31.7 1,554 2020 142.8 76.3 2,514 Total $ 767.0 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2014 2015 Year Unaudited Unaudited 2016 2017 2018 2019 2020 2014 $ 11.0 $ 38.8 $ 59.1 $ 71.5 $ 75.4 $ 79.1 $ 79.9 2015 10.0 43.6 64.2 76.0 81.8 83.7 2016 11.1 42.9 66.1 75.5 80.4 2017 12.3 42.3 67.2 76.5 2018 17.3 56.1 87.5 2019 17.8 56.7 2020 18.1 Total 482.8 Total reserves for 2014 – 2020 accident years (incurred - paid) 284.2 Total reserves for 2013 and prior accident years 3.8 Unallocated loss adjustment expense 5.1 Net reserves at December 31, 2020 $ 293.1 Personal automobile liability ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident Claim Year 2016 2017 2018 2019 2020 IBNR Count 2016 337.9 344.5 355.3 365.8 $ 367.7 $ 4.2 42,375 2017 363.6 362.3 383.4 392.2 9.7 42,993 2018 394.2 395.3 400.3 23.9 42,841 2019 431.2 431.8 67.0 41,703 2020 378.0 199.2 24,339 Total $ 1,970.0 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident Year 2016 2017 2018 2019 2020 2016 $ 112.8 $ 213.1 $ 288.0 $ 329.4 $ 346.3 2017 115.0 229.2 302.7 343.8 2018 121.7 237.2 307.4 2019 131.0 262.2 2020 95.9 Total 1,355.6 Total reserves for 2016 – 2020 accident years (incurred - paid) 614.4 Total reserves for 2015 and prior accident years 51.7 Unallocated loss adjustment expense 15.3 Net reserves at December 31, 2020 $ 681.4 Homeowners ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident Claim Year 2017 2018 2019 2020 IBNR Count 2017 $ 291.4 $ 296.5 $ 297.3 $ 295.2 0.6 33,474 2018 312.9 316.2 317.3 6.1 32,725 2019 344.5 345.3 14.3 33,582 2020 418.8 77.2 31,392 Total $ 1,376.6 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident Year 2017 2018 2019 2020 2017 $ 204.5 $ 271.6 $ 283.5 $ 289.0 2018 213.7 290.1 300.8 2019 234.7 311.7 2020 281.5 Total 1,183.0 Total reserves for 2017 – 2020 accident years (incurred - paid) 193.6 Total reserves for 2016 and prior accident years 10.5 Unallocated loss adjustment expense 3.4 Net reserves at December 31, 2020 $ 207.5 The following table is information about average historical claims duration as of December 31, 2020. The table is computed based on the paid and incurred claims data, net of reinsurance, for the accident years presented in the preceding claims development tables. Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance: Unaudited 1 2 3 4 5 6 7 8 9 10 Commercial multiple peril 33.3 % 22.4 % 11.4 % 10.7 % 7.0 % 4.0 % 1.8 % 1.7 % Workers' compensation 19.9 % 27.4 % 15.4 % 9.1 % 5.2 % 3.2 % 2.5 % 1.4 % 1.3 % 0.7 % Commercial automobile liability 16.0 % 20.4 % 18.9 % 18.9 % 11.2 % 4.5 % 1.0 % General liability and umbrella - occurrence 3.7 % 11.1 % 18.7 % 19.1 % 14.8 % 11.3 % 3.9 % 3.3 % 2.3 % 3.3 % General liability - claims made 12.7 % 32.5 % 24.3 % 11.9 % 5.4 % 3.3 % 0.9 % Personal automobile liability 29.2 % 28.9 % 18.9 % 10.9 % 4.6 % Homeowners 68.0 % 23.0 % 3.7 % 1.8 %

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	16. 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 quarterly 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.

Statutory Financial Information

Statutory Financial Information	12 Months Ended
	Dec. 31, 2020
Statutory Accounting Practices [Abstract]
Statutory Financial Information	17. STATUTORY FINANCIAL INFORMATION The Company’s insurance subsidiaries are required to file annual statements with state regulatory authorities prepared on an accounting basis prescribed or permitted by such authorities (statutory basis), as codified by the National Association of Insurance Commissioners (“NAIC”). Permitted statutory accounting practices encompass all accounting practices that are not prescribed; such practices differ from state to state, may differ from company to company within a state, and may change in the future. The Company’s insurance subsidiaries did not have any permitted practices as of or for the years ended December 31, 2020, 2019 and 2018. Statutory capital and surplus differs from shareholders’ equity reported in accordance with U.S. GAAP primarily because under the statutory basis of accounting, deferred acquisition costs are expensed when incurred and the recognition of deferred tax assets is based on different recoverability assumptions. The following table provides statutory net income for the years ended December 31 and statutory capital and surplus for the insurance subsidiaries as of December 31 for the periods indicated: (in millions) 2020 2019 2018 Statutory Net Income $ 360.9 $ 348.4 $ 296.1 Statutory Capital and Surplus 2,588.5 2,470.2 2,172.5 The minimum statutory capital and surplus necessary to satisfy the Company’s regulatory requirements was $552.6 million, $531.8 million and $518.4 million, which equals the Authorized Control Level at December 31, 2020, 2019 and 2018, respectively.

Quarterly Results of Operations

Quarterly Results of Operations	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations	18. QUARTERLY RESULTS OF OPERATIONS (UNAUDITED) The quarterly results of operations for 2020 and 2019 are summarized below. THREE MONTHS ENDED (in millions, except per share data) 2020 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,056.2 $ 1,224.3 $ 1,249.8 $ 1,297.0 Income (loss) from continuing operations (38.7 ) 115.3 119.5 165.9 Net income (loss) (40.0 ) 115.2 118.9 164.6 Income (loss) from continuing operations per share: Basic (1.01 ) 3.03 3.17 4.52 Diluted (1.01 ) 3.01 3.14 4.47 Net income (loss) per share: Basic (1.04 ) 3.03 3.15 4.49 Diluted (1.04 ) 3.01 3.13 4.43 Dividends declared per share 0.65 0.65 0.65 0.70 THREE MONTHS ENDED (in millions, except per share data) 2019 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,219.5 $ 1,198.6 $ 1,214.7 $ 1,257.9 Income from continuing operations 122.6 85.0 111.2 110.2 Net income 122.4 74.0 118.9 109.8 Income from continuing operations per share: Basic 3.02 2.09 2.81 2.81 Diluted 2.98 2.06 2.77 2.77 Net income per share: Basic 3.01 1.82 3.00 2.80 Diluted 2.97 1.79 2.96 2.76 Dividends declared per share (1) 0.60 0.60 0.60 3.15 (1) On December 5, 2019 the Board of Directors declared a special cash dividend of $2.50 per share. During the fourth quarter of 2020, pursuant to the terms of the October 2020 ASR, the Company paid $100.0 million, on October 29, 2020, in exchange for an initial delivery of approximately 0.8 million shares of common stock, which was approximately 80% of the total number of shares expected to be repurchased under this agreement. On January 29, 2021, the Company received approximately 45,000 shares of common stock as final settlement of shares repurchased under the October 2020 ASR. Due to the use of weighted average shares outstanding when calculating earnings per common share, the sum of the quarterly per common share data may not equal the per common share data for the year.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	19. SUBSEQUENT EVENTS There were no subsequent events requiring adjustment to the financial statements and no additional disclosures required in the notes to the consolidated financial statements.

SCHEDULE I SUMMARY OF INVESTMEN

SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2020
Summary Of Investments Other Than Investments In Related Parties [Abstract]
SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	SCHEDULE I THE HANOVER INSURANCE GROUP, INC. SUMMARY OF INVESTMENTS – OTHER THAN INVESTMENTS IN RELATED PARTIES DECEMBER 31, 2020 (in millions) Type of investment Cost (1) Fair Value Amount at which shown in the balance sheet (2) Fixed maturities: Bonds: United States Government and government agencies and authorities $ 1,353.1 $ 1,401.1 $ 1,401.1 States, municipalities and political subdivisions 1,042.1 1,103.7 1,103.7 Foreign governments 2.2 2.7 2.7 Public utilities 392.3 430.1 430.1 All other corporate bonds 4,156.0 4,516.8 4,516.8 Total fixed maturities 6,945.7 7,454.4 7,454.4 Equity securities: Common stocks: Public utilities 56.8 89.6 89.6 Banks, trusts and insurance companies 44.1 47.6 47.6 Industrial, miscellaneous and all other 302.0 456.9 456.9 Nonredeemable preferred stock 1.0 4.4 4.4 Total equity securities 403.9 598.5 598.5 Mortgage loans on real estate 475.5 502.1 467.6 Real estate 6.4 6.4 6.4 Other long-term investments 300.4 319.2 319.2 Total investments $ 8,131.9 $ 8,880.6 $ 8,846.1 (1) Original cost of equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts. (2) Mortgage loans on real estate are shown on the balance sheet net of the allowance for credit losses.

SCHEDULE II CONDENSED FINANCIAL

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]
SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY STATEMENTS OF INCOME	SCHEDULE II THE HANOVER INSURANCE GROUP, INC. CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY STATEMENTS OF INCOME YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Revenues Net investment income $ 10.7 $ 19.8 $ 11.8 Net realized gains (losses) from sales and other 0.4 5.3 (0.5 ) Other income — 0.4 0.8 Total revenues 11.1 25.5 12.1 Expenses Interest expense 37.0 37.4 37.8 Employee benefit related expenses 2.3 5.0 5.1 Interest expense on loan from subsidiary 7.0 6.9 6.8 Loss from repayment of debt 6.2 — 1.9 Other operating expenses 9.8 7.1 12.1 Total expenses 62.3 56.4 63.7 Net loss before income taxes and equity in income of subsidiaries (51.2 ) (30.9 ) (51.6 ) Income tax benefit 22.1 25.6 21.4 Equity in income of subsidiaries 387.2 432.4 288.4 Income from continuing operations 358.1 427.1 258.2 Sale of Chaucer business (net of income tax benefit (expense) of $5.3 and $(42.5) in 2019 and 2018) — (2.1 ) 131.9 Income from discontinued life business (net of income tax benefit (expense) of $0.5 and $(0.2) in 2020 and 2018) 0.6 0.1 0.9 Net income 358.7 425.1 391.0 Other comprehensive income (loss), net of tax 219.9 267.6 (122.4 ) Comprehensive income $ 578.6 $ 692.7 $ 268.6 The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto. SCHEDULE II (CONTINUED) THE HANOVER INSURANCE GROUP, INC. CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY BALANCE SHEETS DECEMBER 31 2020 2019 (in millions, except per share data) Assets Fixed maturities - at fair value (amortized cost of $369.5 and $321.2) $ 381.3 $ 328.5 Equity securities - at fair value 1.1 1.1 Cash and cash equivalents 15.6 12.2 Investments in subsidiaries 3,718.3 3,352.2 Net receivable from subsidiaries 25.2 25.1 Other assets 2.9 3.2 Total assets $ 4,144.4 $ 3,722.3 Liabilities Expenses and state taxes payable $ 23.2 $ 16.4 Current income tax payable 2.7 3.5 Interest payable 10.5 7.8 Debt (1) 905.8 778.4 Total liabilities 942.2 806.1 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 0.6 0.6 Additional paid-in capital 1,857.4 1,837.3 Accumulated other comprehensive income 372.5 152.6 Retained earnings 2,668.0 2,410.9 Treasury stock at cost (24.1 and 22.1 million shares) (1,696.3 ) (1,485.2 ) Total shareholders' equity 3,202.2 2,916.2 Total liabilities and shareholders' equity $ 4,144.4 $ 3,722.3 ( 1 ) In 2020, the Company issued $300.0 million aggregate principal amount of 2.50% unsecured senior debentures. With the proceeds, THG retired its outstanding $175.0 million principal amount of 6.35% subordinated unsecured debentures. (See Note 6 – “Debt and Credit Arrangements” in Notes to Consolidated Financial Statements). The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto. SCHEDULE II (CONTINUED) THE HANOVER INSURANCE GROUP, INC. CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY STATEMENTS OF CASH FLOWS YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Cash flows from operating activities Net income $ 358.7 $ 425.1 $ 391.0 Adjustments to reconcile net income to net cash provided by (used in) operating activities: Loss (gain) from sale of Chaucer business — 2.1 (131.9 ) Net realized investment (gains) losses (0.4 ) (5.3 ) 0.6 Net loss from repayment of debt 6.2 — 1.9 Equity in net income of subsidiaries (387.2 ) (432.3 ) (288.4 ) Dividends received from subsidiaries 60.3 69.9 86.4 Deferred income tax benefit (8.5 ) (13.9 ) (5.3 ) Change in expenses and taxes payable 5.6 (107.8 ) 59.7 Change in net receivable from subsidiaries 15.5 11.6 8.9 Other, net 4.4 3.9 (0.4 ) Net cash provided by (used in) operating activities 54.6 (46.7 ) 122.5 Cash flows from investing activities Proceeds from disposals and maturities of fixed maturities 166.6 447.9 82.8 Proceeds from disposals of equity securities — 121.9 — Purchase of fixed maturities (32.4 ) (316.1 ) (4.0 ) Purchase of equity securities — (121.9 ) — Net cash received from sale of Chaucer business — 35.1 762.0 Net cash provided by investing activities 134.2 166.9 840.8 Cash flows from financing activities Proceeds from exercise of employee stock options 6.3 14.4 14.8 Proceeds from debt borrowings, net 296.4 — — Dividends paid to shareholders (99.5 ) (386.2 ) (94.3 ) Repayment of debt (175.8 ) — (11.6 ) Repurchases of common stock (212.8 ) (563.6 ) (57.7 ) Net cash related to short-term intercompany borrowings — — (19.8 ) Net cash used in financing activities (185.4 ) (935.4 ) (168.6 ) Net change in cash and cash equivalents 3.4 (815.2 ) 794.7 Cash and cash equivalents, beginning of year 12.2 827.4 32.7 Cash and cash equivalents, end of year $ 15.6 $ 12.2 $ 827.4 Included in other operating cash flows was the cash portion of dividends received from unconsolidated subsidiaries. Additionally, investment assets of $185.6 million, $70.1 million and $179.8 million were transferred to the parent company in 2020, 2019 and 2018, respectively, to settle dividend obligations and other intercompany borrowings and balances. The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto.

SCHEDULE III SUPPLEMENTARY INSU

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	12 Months Ended
	Dec. 31, 2020
Supplementary Insurance Information [Abstract]
SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	SCHEDULE III THE HANOVER INSURANCE GROUP, INC. SUPPLEMENTARY INSURANCE INFORMATION DECEMBER 31, 2020 (in millions) Segments Deferred acquisition costs Future policy benefits, losses, claims and loss expenses Unearned premiums Other policy claims and benefits payable Premium revenue Net investment income (1) Benefits, claims, losses and settlement expenses Amortization of deferred acquisition costs Other operating expenses (2) (3) Premiums written Commercial Lines $ 312.1 $ 4,067.4 $ 1,513.9 $ 8.6 $ 2,683.3 $ 175.3 $ 1,647.6 $ 618.4 $ 330.3 $ 2,733.1 Personal Lines 165.4 1,907.6 968.8 — 1,844.1 76.7 1,193.3 332.6 193.7 1,865.4 Other — 40.4 — — — 13.1 4.3 — 31.4 — Interest on Debt — — — — — — — — 37.1 — Eliminations — — — — — — — — (6.2 ) — Total (4) $ 477.5 $ 6,015.4 $ 2,482.7 $ 8.6 $ 4,527.4 $ 265.1 $ 2,845.2 $ 951.0 $ 586.3 $ 4,598.5 DECEMBER 31, 2019 (in millions) Segments Deferred acquisition costs Future policy benefits, losses, claims and loss expenses Unearned premiums Other policy claims and benefits payable Premium revenue Net investment income (1) Benefits, claims, losses and settlement expenses Amortization of deferred acquisition costs Other operating expenses (2) Premiums written Commercial Lines $ 307.4 $ 3,768.9 $ 1,446.7 $ 8.4 $ 2,654.2 $ 180.1 $ 1,610.0 $ 604.4 $ 330.3 $ 2,707.2 Personal Lines 160.0 1,839.4 970.0 — 1,820.3 80.1 1,254.2 322.3 191.6 1,874.5 Other — 37.7 — — — 21.1 1.3 — 23.1 — Interest on Debt — — — — — — — — 37.5 — Eliminations — — — — — — — — (6.1 ) — Total (4) $ 467.4 $ 5,646.0 $ 2,416.7 $ 8.4 $ 4,474.5 $ 281.3 $ 2,865.5 $ 926.7 $ 576.4 $ 4,581.7 DECEMBER 31, 2018 (in millions) Segments Deferred acquisition costs Future policy benefits, losses, claims and loss expenses Unearned premiums Other policy claims and benefits payable Premium revenue Net investment income (1) Benefits, claims, losses and settlement expenses Amortization of deferred acquisition costs Other operating expenses (2), (3) Premiums written Commercial Lines $ 296.9 $ 3,550.6 $ 1,369.5 $ 8.0 $ 2,548.4 $ 182.2 $ 1,568.3 $ 587.5 $ 318.0 $ 2,610.7 Personal Lines 153.9 1,707.6 908.3 — 1,706.0 73.7 1,155.0 304.3 185.8 1,774.1 Other — 37.9 — — — 11.5 1.3 — 54.3 — Interest on Debt — — — — — — — — 45.1 — Eliminations — — — — — — — — (7.8 ) — Total (4) $ 450.8 $ 5,296.1 $ 2,277.8 $ 8.0 $ 4,254.4 $ 267.4 $ 2,724.6 $ 891.8 $ 595.4 $ 4,384.8 (1) The Company manages investment assets for its Commercial Lines, Personal Lines and Other segments on a combined basis, based on the requirements of its combined property and casualty insurance companies. Net investment income is allocated to these segments based on actuarial information related to the underlying businesses. (2) For other operating expenses that are not directly attributable to a single segment for combined property and casualty insurance companies, expenses are generally allocated based upon either net premiums written or net premiums earned. (3) In 2020 and 2018, the Other segment includes $6.2 million and $28.2 million, respectively, of losses from the repayment of debt. (4) Information for all periods excludes results and balances related to the former Chaucer business (See Note 2 – “Discontinued Operations” in the Notes to Consolidated Financial Statements).

SCHEDULE IV REINSURANCE

SCHEDULE IV REINSURANCE	12 Months Ended
	Dec. 31, 2020
Supplemental Schedule Of Reinsurance Premiums For Insurance Companies Abstract
SCHEDULE IV REINSURANCE	SCHEDULE IV THE HANOVER INSURANCE GROUP, INC. REINSURANCE Incorporated herein by reference to Note 14 — “Reinsurance” in the Notes to Consolidated Financial Statements.

SCHEDULE V VALUATION AND QUALIF

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2020
Valuation And Qualifying Accounts [Abstract]
SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	SCHEDULE V THE HANOVER INSURANCE GROUP, INC. VALUATION AND QUALIFYING ACCOUNTS DECEMBER 31 Additions (in millions) Description Balance at beginning of period Charged to costs and expenses Charged to other accounts (1) Deductions Balance at end of period 2020 Allowance for doubtful accounts $ 3.5 $ 16.9 $ — $ (11.1 ) $ 9.3 Allowance for uncollectible reinsurance recoverables 3.9 2.7 — — 6.6 $ 7.4 $ 19.6 $ — $ (11.1 ) $ 15.9 2019 Allowance for doubtful accounts $ 4.2 $ 9.5 $ — $ (10.2 ) $ 3.5 Allowance for uncollectible reinsurance recoverables 3.9 — — — 3.9 $ 8.1 $ 9.5 $ — $ (10.2 ) $ 7.4 2018 Allowance for doubtful accounts $ 4.3 $ 9.5 $ — $ (9.6 ) $ 4.2 Allowance for uncollectible reinsurance recoverables 9.7 0.6 (6.4 ) — 3.9 $ 14.0 $ 10.1 $ (6.4 ) $ (9.6 ) $ 8.1 (1) Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its former Chaucer business; accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.

SCHEDULE VI SUPPLEMENTAL INFORM

SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	12 Months Ended
	Dec. 31, 2020
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	SCHEDULE VI THE HANOVER INSURANCE GROUP, INC. SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS YEARS ENDED DECEMBER 31 (in millions) Affiliation with Registrant Deferred acquisition costs Reserves for unpaid claims and claim adjustment expenses (1) Discount, if any, deducted from previous column (2) Unearned premiums (1) Earned premiums Net investment income Consolidated Property and Casualty Subsidiaries (3) 2020 $ 477.5 $ 6,024.0 $ — $ 2,482.7 $ 4,527.4 $ 265.1 2019 $ 467.4 $ 5,654.4 $ — $ 2,416.7 $ 4,474.5 $ 281.3 2018 $ 450.8 $ 5,304.1 $ — $ 2,277.8 $ 4,254.4 $ 267.4 Claims and claim adjustment expenses incurred related to Current year Prior years Amortization of deferred acquisition costs Paid claims and claim adjustment expenses Premiums written 2020 $ 2,877.8 $ (32.6 ) $ 951.0 $ 2,542.4 $ 4,598.5 2019 $ 2,893.9 $ (28.4 ) $ 926.7 $ 2,616.5 $ 4,581.7 2018 $ 2,733.5 $ (8.9 ) $ 891.8 $ 2,496.6 $ 4,384.8 (1) Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,641.6 million, $1,574.8 million and $1,472.6 million of reinsurance recoverable on unpaid losses in 2020, 2019 and 2018, respectively. Unearned premiums are shown gross of prepaid premiums of $144.2 million, $149.3 million and $117.6 million in 2020, 2019 and 2018, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends. (2) The Company does not use discounting techniques. (3) Information for all periods excludes results and balances related to the discontinued Chaucer business. See Note 2 – “Discontinued Operations” in the Notes to Consolidated Financial Statements for further information on that business.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
BASIS OF PRESENTATION AND PRINCIPLES OF CONSOLIDATION	A. Basis of Presentation and Principles of Consolidation The consolidated financial statements of The Hanover Insurance Group, Inc. (“THG” or the “Company”), include the accounts of The Hanover Insurance Company (“Hanover Insurance”) and Citizens Insurance Company of America (“Citizens”), 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 13 – “Segment Information.” The consolidated financial statements also include the Company’s discontinued operations, consisting primarily of the Company’s former accident and health and life insurance businesses and its former Chaucer international business, 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 other international insurance and non-insurance subsidiaries. All intercompany accounts and transactions have been eliminated. 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. Accordingly, for all prior periods presented in this Annual Report on Form 10-K, operations from Chaucer are presented as discontinued operations. The preparation of financial statements in conformity with generally accepted accounting principles in the United States of America (“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 these financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations.
INVESTMENTS	B. Investments Fixed maturities are classified as available-for-sale and are carried at fair value, with unrealized gains and losses, net of taxes, reported in accumulated other comprehensive income (“AOCI”), a separate component of shareholders’ equity. The amortized cost of fixed maturities is adjusted for amortization of premiums and accretion of discounts to maturity. Equity securities are carried at fair value . Increases and decreases in fair value are reported in net income. Other investments consist primarily of mortgage participations and limited partnerships. Mortgage participations represent interests in commercial mortgage loans originated and serviced by a third-party of which the Company shares, on a pro-rata basis, in all related cash flows of the underlying mortgage loans. Mortgage participations are stated at unpaid principal balances adjusted for deferred fees or expenses, net of an allowance for credit losses. Prior to May 2018, certain reacquisition rights retained by the third-party in the loan participations required that these investments be accounted for as secured borrowings under Accounting Standards Codification (“ASC”) 860, Transfers and Servicing The Company excludes accrued interest receivable from both the estimated fair value and the amortized cost basis of its investment securities and reports such amounts separately on the consolidated balance sheets as accrued investment income. When accrued interest receivable is deemed uncollectible it is written off as a charge to investment income, rather than recorded through an allowance. Net investment income includes interest, dividends and income from limited partnership interests. Interest income is recognized based on the effective yield method, which includes the amortization of premiums and accretion of discounts. The effective yield used to determine the amortization for fixed maturities subject to prepayment risk, such as mortgage-backed and asset-backed securities, is recalculated and adjusted periodically based upon actual historical and projected future cash flows. The adjustment to yields for highly rated prepayable fixed maturities is accounted for using the retrospective method. The adjustment to yields for all other prepayable fixed maturities is accounted for using the prospective method. Fixed maturities and mortgage participations for which payments are delinquent are placed on non-accrual status, and thereafter interest income is recognized only when cash payments are received. Realized investment gains and losses on sales are reported as a component of revenues based upon specific identification of the investment assets sold. Impairments are reported as realized investment losses, and include credit losses (and any subsequent recoveries) on fixed maturities and mortgage participations, and intend-to-sell impairment losses on fixed maturities. Changes in the fair value of equity securities are reported in net realized and unrealized investment gains (losses), including increases and decreases in fair value on securities that are still held and realized gains and losses on securities that have been sold. The Company reviews fixed maturity securities in an unrealized loss position and assesses whether it intends to sell the security or more likely than not will be required to sell the security before the recovery of its amortized cost basis. If the debt security meets either of these two criteria, an intend-to-sell impairment is recognized in earnings equal to the entire difference between the security’s amortized cost basis and its fair value at the impairment measurement date. If neither of the above criteria are met, the credit loss portion of the unrealized loss is recorded through earnings and the non-credit portion remains in other comprehensive income. Credit losses are estimated by comparing the amortized cost of the fixed maturity security with the net present value of the security’s projected future cash flows, discounted at the effective interest rate implicit in the investment prior to impairment. The non-credit portion of the impairment is equal to the difference between the fair value and the net present value of the security’s cash flows at the impairment measurement date. Until January 1, 2020, intend-to-sell impairments and credit losses on fixed maturities resulted in a reduction of the amortized cost basis of the security, and reversal of an impairment was not allowed. Beginning January 1, 2020, credit losses are recorded through an allowance for credit losses rather than an adjustment to amortized cost. Recoveries of impairments on fixed maturities are recognized as reversals of the allowance for credit loss and are no longer accreted as investment income through an adjustment to investment yield. The allowance for credit losses is limited to the amount that fair value is less than amortized cost and therefore, increases in the fair value of investments due to reasons other than credit could result in decreases in the allowance and an increase in net income. Mortgage participations are pooled by similar risk characteristics and evaluated for credit losses. The allowance for credit losses is calculated using expected loss rates, which vary based on risk factors such as property type, geographic market, and loan-to-value and debt service coverage ratios.
FINANCIAL INSTRUMENTS	C. Financial Instruments In the normal course of business, the Company may enter into transactions involving various types of financial instruments, including debt, investments such as fixed maturities, equity securities and mortgage loans, investment and loan commitments, swap contracts, option contracts, forward contracts and futures contracts. These instruments involve credit risk and could also be subject to risk of loss due to interest rate fluctuation. The Company evaluates and monitors each financial instrument individually and, when appropriate, obtains collateral or other security to minimize losses.
CASH AND CASH EQUIVALENTS	D . Cash and Cash Equivalents Cash and cash equivalents include cash on hand, amounts due from banks and highly liquid debt instruments purchased with an original maturity of three months or less.
DEFERRED ACQUISITION COSTS	E. Deferred Acquisition Costs Acquisition costs consist of commissions, underwriting costs and other costs, which vary with, and are primarily related to, the successful production of premiums. Acquisition costs are deferred and amortized over the terms of the insurance policies. Deferred acquisition costs (“DAC”) for each operating segment are reviewed to determine if the costs are recoverable from future income, including investment income. If such costs are determined to be unrecoverable, they are expensed at the time of determination. Although recoverability of DAC is not assured, the Company believes it is more likely than not that all of these costs will be recovered. The amount of DAC considered recoverable, however, could be reduced in the near term if the estimates of total revenues discussed above are reduced or permanently impaired as a result of a disposition of a line of business. The amount of amortization of DAC could be revised in the near term if any of the estimates discussed above are revised.
REINSURANCE RECOVERABLES	F. Reinsurance Recoverables The Company shares certain insurance risks it has underwritten, through the use of reinsurance contracts, with various insurance entities. Reinsurance accounting is followed for ceded transactions when the risk transfer provisions of ASC 944, Financial Services – Insurance .
PROPERTY, EQUIPMENT, CAPITALIZED SOFTWARE AND LEASES	G. Property, Equipment, Capitalized Software AND LEASES Property, equipment, leasehold improvements and capitalized software are recorded at cost, less accumulated depreciation and amortization. Depreciation is generally provided using the straight-line method over the estimated useful lives of the related assets, which generally range from 3 to 30 years. The estimated useful life for capitalized software is generally 5 to 7 years. Amortization of leasehold improvements is provided using the straight-line method over the lesser of the term of the lease or the estimated useful life of the improvements. The Company has entered into operating and financing leases through which it uses “right-of-use” assets that are recorded at the present value of future minimum lease payments, less accumulated depreciation. Depreciation is generally provided using the straight-line method over the estimated useful lives of the related assets, which generally range from 4 to 6 years for real estate and fleet leases. The Company tests for the recoverability of long-lived assets whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. The Company recognizes impairment losses only to the extent that the carrying amounts of long-lived assets exceed the sum of the undiscounted cash flows expected to result from the use and eventual disposition of the assets. When an impairment loss occurs, the Company reduces the carrying value of the asset to fair value and no longer depreciates the asset. Fair values are estimated using discounted cash flow analysis.
GOODWILL AND INTANGIBLE ASSETS	H. GOODWILL AND INTANGIBLE ASSETS In accordance with the provisions of ASC 350, Intangibles Goodwill and Other The Company tests for the recoverability of goodwill and intangible assets with indefinite lives annually, or whenever events or changes in circumstances indicate that the carrying amounts may not be recoverable. The Company recognizes impairment losses only to the extent that the carrying amounts of reporting units with goodwill exceed the fair value. The amount of the impairment loss that would be recognized is determined based upon the excess of the carrying value of goodwill compared to the implied fair value of the goodwill, as determined with respect to all assets and liabilities of the reporting unit. The Company has performed its annual review of goodwill and intangible assets with indefinite lives for impairment in the fourth quarters of 2020 and 2019 with no impairments recognized. At December 31, 2020 and 2019, the Company held goodwill of $178.8 million and intangible assets with indefinite lives of $15.0 million.
LIABILITIES FOR LOSSES, LAE, AND UNEARNED PREMIUMS	I. LIABILITIES FOR LOSSES, LAE AND UNEARNED PREMIUMS Liabilities for outstanding claims, losses and loss adjustment expenses (“LAE”) are estimates of payments to be made for reported losses and LAE and estimates of losses and LAE incurred but not reported. These liabilities are determined using case basis evaluations and statistical analyses of historical loss patterns and represent estimates of the ultimate cost of all losses incurred but not paid. These estimates are continually reviewed and adjusted as necessary; adjustments are reflected in current operations. Estimated amounts of salvage and subrogation on unpaid losses are deducted from the liability for unpaid claims. Premiums for direct and assumed business are reported as earned on a pro-rata basis over the contract period. The unexpired portion of these premiums is recorded as unearned premiums. All losses, LAE and unearned premium liabilities are based on the various estimates discussed in this note. Although the adequacy of these amounts cannot be assured, the Company believes that it is more likely than not that these liabilities and accruals will be sufficient to meet future obligations of policies in force. The amount of liabilities and accruals, however, could be revised in the near-term if the estimates discussed above are revised.
DEBT	J. Debt The Company’s debt at December 31, 2020 includes senior and subordinated debentures. Debt instruments are carried at principal amount borrowed, net of any applicable unamortized discounts and issuance costs. See Note 6 – “Debt and Credit Arrangements.”
PREMIUM, PREMIUM RECEIVABLE, FEE REVENUE AND RELATED EXPENSES	K. Premium, Premium Receivable, Fee Revenue and Related Expenses Insurance premiums written are generally recorded at the policy inception and are primarily earned on a pro-rata basis over the terms of the policies for all products. Premiums written may also include estimates that are derived from multiple sources, which include the historical experience of the underlying business, similar businesses and available industry information. These estimates are regularly reviewed and updated, and any resulting adjustments are included in the current year’s results. Unearned premium reserves represent the portion of premiums written that relates to the unexpired terms of the underlying in-force insurance policies and reinsurance contracts. Premium receivables reflect the unpaid balance of premiums written as of the balance sheet date. Premium receivables are generally short-term in nature and are reported net of an allowance for estimated uncollectible premium accounts. The Company reviews its receivables for collectability at the balance sheet date. The allowance for uncollectible accounts was not material as of December 31, 20 20 and 201 9 . Ceded premiums are charged to income over the applicable term of the various reinsurance contracts with third-party reinsurers. Reinsurance reinstatement premiums, when required, are recognized in the same period as the loss event that gave rise to the reinstatement premiums. Losses and related expenses are matched with premiums, resulting in their recognition over the lives of the contracts. This matching is accomplished through estimated and unpaid losses and amortization of deferred acquisition costs.
INCOME TAXES	L. Income Taxes The Company is subject to the tax laws and regulations of the U.S. and foreign countries in which it operates or where it operated previously. The Company files a consolidated U.S. federal income tax return that includes the holding company and its U.S. subsidiaries. Generally, taxes are accrued at the U.S. statutory tax rate of 21% for income from the U.S. operations. The Company accrues taxes on certain non-U.S. income that is subject to U.S. tax at the enacted U.S. tax rate. Foreign tax credits, where available, were utilized to offset U.S. tax as permitted. The Company’s accounting for income taxes represents its best estimate of various events and transactions. Deferred income taxes are generally recognized when assets and liabilities have different values for financial statement and tax reporting purposes, and for other temporary taxable and deductible differences as defined by ASC 740, Income Taxes The realization of deferred tax assets depends upon the existence of sufficient taxable income within the carryback or carryforward periods under the tax law in the applicable tax jurisdiction. Consideration is given to all available positive and negative evidence, including reversals of deferred tax liabilities, projected future taxable income, tax planning strategies and recent financial operations. Valuation allowances are established if, based on available information, it is determined that it is more likely than not that all or some portion of the deferred tax assets will not be realized. Changes in valuation allowances are generally reflected in income tax expense or as an adjustment to other comprehensive income (loss) depending on the nature of the item for which the valuation allowance is being recorded.
STOCK-BASED COMPENSATION	M. Stock-Based Compensation The Company recognizes the fair value of compensation costs for all share-based payments, including employee stock options, in the financial statements. Unvested awards are generally expensed on a straight-line basis, by tranche, over the vesting period of the award. The Company’s stock-based compensation plans are discussed further in Note 10 – “Stock-Based Compensation Plans.”
EARNINGS PER SHARE	N. Earnings Per Share Earnings per share (“EPS”) for the years ended December 31, 2020, 2019 and 2018 is based on a weighted average of the number of shares outstanding during each year. Basic and diluted EPS is computed by dividing income available to common stockholders by the weighted average number of shares outstanding for the period. The weighted average shares outstanding used to calculate basic EPS differ from the weighted average shares outstanding used in the calculation of diluted EPS due to the effect of dilutive employee stock options, nonvested stock grants and other contingently issuable shares. If the effect of such items is antidilutive, the weighted average shares outstanding used to calculate diluted EPS would be equal to those used to calculate basic EPS. Options to purchase shares of common stock whose exercise prices are greater than the average market price of the common shares are not included in the computation of diluted earnings per share because the effect would be antidilutive.
NEW ACCOUNTING PRONOUNCEMENTS	O. New Accounting Pronouncements Recently Implemented Standards In August 2018, the Financial Accounting Standards Board (“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. This update removes the requirement for disclosure of the following: 1) the amount and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, 2) the policy for timing of transfers between levels, and 3) the valuation processes for Level 3 fair value measurements. This update also added a requirement to disclose the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, in addition to other fair value disclosure modifications. This update required both prospective and retrospective application to certain disclosures upon implementation. The Company implemented this guidance effective January 1, 2020 and it did not have an impact on the Company’s financial position or results of operations as the update is disclosure related. In February 2018, the FASB issued ASC Update No. 2018-02 (Topic 220) Income Statement – Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In March 2017, the FASB issued ASC Update No. 2017-08, Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities In June 2016, the FASB issued ASC Update No. 2016-13, (Topic 326) Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments (“ASC Update No. 2016-13”). Codification Improvement to Topic 326, Financial Instruments – Credit Losses In 2019 and 2020, the FASB issued several updates to ASC Update 2016-13, including the issuance in April 2019 of ASC Update 2019-04, Codification Improvements to Topic 360, Financial Instruments – Credit Losses, Topic 815, Derivatives and Hedging and Topic 825, Financial Instruments, Financial Instruments – Credit Losses (Topic 326): Targeted Transition Relief, Codification Improvements to Topic 326, Financial Instruments – Credit Losses Financial Instruments – Credit Losses (Topic 326) and Leases (Topic 842) – Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 119 and Update to SEC Section on Effective Date Related to Accounting Standards Update No. 2016-02, Leases (Topic 842). In February 2016, the Financial Accounting Standards Board (“FASB”) issued ASC Update No. 2016-02, Leases (Topic 842) included within operating activities in the statement of cash flows. ASC Update No. 2016-02 requires that implementation of this guidance be through a modified retrospective transition approach. In July 2018, the FASB issued ASC Update No. 2018-11, Leases (Topic 842): Targeted Improvements , which provides entities with an additional transition method to adopt ASC Update No. 2016-02. Under this optional transition method, an entity can initially apply the new guidance at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company implemented this guidance effective January 1, 2019 using the optional transition method provided in ASC Update No. 2018-11 and elected to utilize the practical expedient package available in ASC Update 2016-02. The Company has also elected to utilize the practical expedient which allows an entity not to separate lease and nonlease components for all of its asset classes. The effect of implementing this guidance increased total assets and total liabilities each by approximately $ 35 million on January 1, 2019 and did not have a material effect on the results of operations. Recently Issued Standards In October 2020, the FASB issued ASC Update No. 2020-08, Codification Improvements to Subtopic 310-20, Receivables – Nonrefundable Fees and Other Costs In January 2020, the FASB issued ASC Update No. 2020-01 Investments – Equity Securities (Topic 321), Investments – Equity Method and Joint Ventures (Topic 323), and Derivatives and Hedging (Topic 815) In December 2019, the FASB issued ASC Update No. 2019-12 Income Taxes (Topic 740) – Simplifying the Accounting for Income Taxes Income Taxes
RECLASSIFICATIONS	Q. Reclassifications Certain prior year amounts have been reclassified to conform to the current year presentation.

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2020
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Components of Estimated Gain From Disposal of Chaucer Business	The following table summarizes the components of the estimated gain in 2018 related to the sale of the Chaucer business as of December 28, 2018. As discussed below the table, both the pre-tax gain and income tax expense were updated in 2019. YEAR ENDED DECEMBER 31 2018 (in millions) Initial consideration received from sale (1) $ 779.0 Adjustment (1) (17.0 ) Contingent proceeds (1) (2) 31.7 Total cash proceeds expected from sale of Chaucer Holdings Limited (1) 793.7 Less: Carrying value of Chaucer business (3) 530.0 Transaction and other sale related costs (4) 30.6 Net realized losses on securities, pension and currency translation obligations related to Chaucer business (5) 58.7 Total pre-tax reductions 619.3 Pre-tax gain on sale 174.4 Income tax expense (6) 42.5 Gain on sale $ 131.9 (1) Initial consideration for Chaucer as determined in the purchase and sales agreement was $779 million. This amount, along with $28 million in cash proceeds received from the sale of the Irish entity on February 14, 2019, $13 million from the sale of the Australian entities on April 10, 2019, estimated contingent consideration of $31.7 million, and an $85 million pre-signing dividend from Chaucer that was received in the second quarter of 2018, resulted in expected total proceeds from the entire transaction of $936.7 million. These amounts were partially offset by $17.0 million paid to China Re to adjust the purchase price for amounts received by the Company from Chaucer prior to December 28, 2018. (2) Contingent proceeds, as reflected in the sales and purchase agreement, could have been up to $45 million and was determined based upon 2018 catastrophe losses. In 2018, the Company’s best estimate of contingent consideration was $31.7 million. (3) The carrying value of the Chaucer business reflects its U.S. GAAP book value at December 28, 2018, excluding $7.9 million of U.S.-related deferred tax assets that are no longer likely to be realized and therefore are reflected in the income tax expense category. (4) Transaction and other sale related costs primarily include brokerage, legal, actuarial, tax and other professional fees, employee retention costs, costs for the purchase of aggregate excess of loss catastrophe coverage in consideration of the contingent proceeds provision, along with certain other miscellaneous charges related to the execution of the transaction. (5) As part of the transaction, investments held by Chaucer were transferred to China Re resulting in the recognition of net realized investment losses that were previously reflected in accumulated other comprehensive income. Additionally, Chaucer’s deferred pension obligations and currency translation obligations previously recognized in accumulated other comprehensive income were recognized as losses associated with the transaction. (6) The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that were no longer likely to be realized.
Schedule of Effects of Reinsurance	The following table provides the effects of reinsurance. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Premiums written: Direct $ 5,131.4 $ 5,090.8 $ 4,816.0 Assumed 23.1 27.7 27.6 Ceded (556.0 ) (536.8 ) (458.8 ) Net premiums written $ 4,598.5 $ 4,581.7 $ 4,384.8 Premiums earned: Direct $ 5,063.8 $ 4,953.3 $ 4,673.6 Assumed 24.6 26.3 26.6 Ceded (561.0 ) (505.1 ) (445.8 ) Net premiums earned $ 4,527.4 $ 4,474.5 $ 4,254.4 Percentage of assumed to net premiums earned 0.5 % 0.6 % 0.6 % Losses and LAE: Direct $ 3,200.5 $ 3,223.3 $ 2,986.2 Assumed 15.9 21.4 25.2 Ceded (371.2 ) (379.2 ) (286.8 ) Net losses and LAE $ 2,845.2 $ 2,865.5 $ 2,724.6
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Summary of Discontinued Chaucer Business	Year ended December 31, 2018 The following table summarizes the results of operations for Chaucer for 2018, reflecting the results of operations for the period in which THG owned the Chaucer U.K. entities, which concluded with their sale on December 28, 2018: (in millions) Revenues Net premiums earned $ 850.0 Net investment income 54.9 Other income 7.5 912.4 Losses and operating expenses Losses and LAE 515.5 Amortization of deferred acquisition costs 252.5 Other expenses 115.0 883.0 Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income) 29.4 Other items: Interest expense (3.8 ) Net realized and unrealized investment losses (1.3 ) Other income 0.4 Income from Chaucer business before income taxes 24.7 Income tax expense (4.7 ) Income from Chaucer business, net of taxes $ 20.0
Schedule of Effects of Reinsurance	(in millions) Premiums written: Direct $ 625.3 Assumed 685.3 Ceded (459.9 ) Net premiums written $ 850.7 Premiums earned: Direct $ 611.9 Assumed 667.2 Ceded (429.1 ) Net premiums earned $ 850.0 Losses and LAE: Direct $ 362.2 Assumed 568.7 Ceded (415.4 ) Net losses and LAE $ 515.5
Summary of Cash Flows Associated with the Discontinued Chaucer Business	Cash Flows The sales of the Chaucer businesses were completed in 2019, therefore there were no material cash flows associated with the Chaucer business in 2020. The following table details the cash flows associated with the Chaucer business during the years indicated: DECEMBER 31 2019 2018 (in millions) Net cash provided by (used in) operating activities 3.5 (22.8 ) Net cash (used in) provided by investing activities (6.8 ) 131.1

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2020
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: DECEMBER 31, 2020 (in millions) Amortized Cost Allowance for Credit Losses Amortized Cost, Net of Allowance for Credit Losses Gross Unrealized Gains Gross Unrealized Losses Fair Value U.S. Treasury and government agencies $ 376.4 $ — $ 376.4 $ 17.6 $ 2.0 $ 392.0 Foreign government 2.2 — 2.2 0.5 — 2.7 Municipal 1,042.1 — 1,042.1 61.7 0.1 1,103.7 Corporate 3,770.0 (0.1 ) 3,769.9 341.8 0.4 4,111.3 Residential mortgage-backed 978.2 — 978.2 33.1 0.6 1,010.7 Commercial mortgage-backed 705.4 — 705.4 54.8 0.2 760.0 Asset-backed 71.4 — 71.4 2.6 — 74.0 Total fixed maturities $ 6,945.7 $ (0.1 ) $ 6,945.6 $ 512.1 $ 3.3 $ 7,454.4 DECEMBER 31, 2019 (in millions) Gross Gross OTTI Amortized Unrealized Unrealized Unrealized Cost Gains Losses Fair Value Losses (1) U.S. Treasury and government agencies $ 342.0 $ 9.1 $ 1.3 $ 349.8 $ — Foreign government 15.7 0.4 — 16.1 — Municipal 807.1 27.6 1.2 833.5 — Corporate 3,653.5 161.6 3.9 3,811.2 3.0 Residential mortgage-backed 905.4 17.1 1.1 921.4 — Commercial mortgage-backed 666.4 25.6 0.1 691.9 — Asset-backed 62.1 1.1 — 63.2 — Total fixed maturities $ 6,452.2 $ 242.5 $ 7.6 $ 6,687.1 $ 3.0 (1) Other-than-temporary impairments (“OTTI”) unrealized losses represents OTTI recognized in accumulated other comprehensive income. This amount excludes net unrealized gains on impaired securities relating to changes in the value of such securities subsequent to the impairment measurement date through December 31, 2019 of $4.2 million.
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. DECEMBER 31 2020 (in millions) Amortized Cost, net of Allowance for Credit Losses Fair Value Due in one year or less $ 334.1 $ 339.1 Due after one year through five years 1,774.6 1,913.3 Due after five years through ten years 2,306.3 2,548.2 Due after ten years 775.6 809.1 5,190.6 5,609.7 Mortgage-backed and asset-backed securities 1,755.0 1,844.7 Total fixed maturities $ 6,945.6 $ 7,454.4
Unrealized Gains and Losses on Available-for-Sale and Other Securities	Unrealized gains and losses on available-for-sale and other securities are summarized in the following table. YEARS ENDED DECEMBER 31 (in millions) Equity Fixed Securities and 2020 Maturities Other Total Net appreciation, beginning of year $ 216.0 $ — $ 216.0 Net appreciation on available-for-sale securities 280.2 — 280.2 Provision for deferred income taxes (68.1 ) — (68.1 ) 212.1 — 212.1 Net appreciation, end of year $ 428.1 $ — $ 428.1 2019 Net depreciation, beginning of year $ (27.2 ) $ — $ (27.2 ) Net appreciation on available-for-sale securities 324.7 — 324.7 Provision for deferred income taxes (83.0 ) — (83.0 ) Cumulative effect adjustment for ASU 2017-08, net of tax 1.5 — 1.5 243.2 — 243.2 Net appreciation, end of year $ 216.0 $ — $ 216.0 2018 Net appreciation, beginning of year $ 110.1 $ 95.3 $ 205.4 Net depreciation on available-for-sale securities (203.7 ) — (203.7 ) Provision for deferred income taxes 33.6 — 33.6 Amount realized with sale of Chaucer, net of tax 19.1 — 19.1 Cumulative effect adjustment for ASUs 2016-01 and 2018-02, net of tax 13.7 (95.3 ) (81.6 ) (137.3 ) (95.3 ) (232.6 ) Net depreciation, end of year $ (27.2 ) $ — $ (27.2 )
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 December 31, 2020 and 2019 including the length of time the securities have been in an unrealized loss position: DECEMBER 31, 2020 12 months or less Greater than 12 months Total (in millions) Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 2.0 $ 140.7 $ — $ — $ 2.0 $ 140.7 Municipal 0.1 19.3 — 0.7 0.1 20.0 Corporate 0.4 53.4 — — 0.4 53.4 Residential mortgage-backed 0.6 94.4 — — 0.6 94.4 Commercial mortgage-backed 0.2 33.2 — — 0.2 33.2 Total investment grade 3.3 341.0 — 0.7 3.3 341.7 Below investment grade: Corporate — 4.6 — — — 4.6 Total fixed maturities $ 3.3 $ 345.6 $ — $ 0.7 $ 3.3 $ 346.3 DECEMBER 31, 2019 12 months or less Greater than 12 months Total (in millions) Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 1.3 $ 73.0 $ — $ 9.3 $ 1.3 $ 82.3 Municipal 1.1 72.5 0.1 5.6 1.2 78.1 Corporate 0.7 86.5 0.1 4.7 0.8 91.2 Residential mortgage-backed 0.7 69.2 0.4 34.4 1.1 103.6 Commercial mortgage-backed 0.1 40.6 — 0.9 0.1 41.5 Asset-backed — — — 1.7 — 1.7 Total investment grade 3.9 341.8 0.6 56.6 4.5 398.4 Below investment grade: Corporate 2.2 27.1 0.9 9.0 3.1 36.1 Total fixed maturities $ 6.1 $ 368.9 $ 1.5 $ 65.6 $ 7.6 $ 434.5
Schedule of Other Investments	Mortgage participations and other mortgage loans were comprised of the following property types and geographic locations DECEMBER 31 2020 2019 (in millions) Property Type: Office $ 160.4 $ 138.2 Apartments 128.3 125.1 Retail 68.5 66.5 Hotel 62.7 62.1 Industrial 55.6 50.6 Valuation allowance (7.9 ) (1.3 ) Total $ 467.6 $ 441.2 DECEMBER 31 2020 2019 (in millions) Geographic Region: Pacific $ 117.0 $ 102.3 South Atlantic 108.3 95.9 Mid-Atlantic 73.8 53.3 West South Central 70.8 71.2 New England 43.5 42.5 East North Central 27.8 27.8 Mountain 12.5 12.5 Other 21.8 37.0 Valuation allowance (7.9 ) (1.3 ) Total $ 467.6 $ 441.2

Investment Income and Gains a_2

Investment Income and Gains and Losses (Tables)	12 Months Ended
	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Components of Net Investment Income	The components of net investment income were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Fixed maturities $ 222.5 $ 232.4 $ 217.7 Mortgage loans 17.5 16.3 14.0 Limited partnerships 16.7 19.7 24.1 Equity securities 14.8 16.3 17.0 Other investments 3.2 5.3 4.8 Gross investment income 274.7 290.0 277.6 Less: investment expenses (9.6 ) (8.7 ) (10.2 ) Net investment income $ 265.1 $ 281.3 $ 267.4
Schedule of Net Realized and Unrealized Gains (Losses) on Investments Including Impairments	Net realized and unrealized gains (losses) on investments, including impairments, were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Equity securities $ 13.4 $ 106.5 $ (43.4 ) Other investments 2.5 — (1.3 ) Mortgage loans (6.7 ) (0.2 ) (0.2 ) Fixed maturities (4.2 ) 3.1 (5.8 ) Net realized and unrealized investment gains (losses) $ 5.0 $ 109.4 $ (50.7 )
Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities	The following table provides pre-tax net realized and unrealized gains (losses) on equity securities: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Net gains (losses) recognized during the period $ 13.4 $ 106.5 $ (43.4 ) Less: net (losses) gains recognized on equity securities sold during the period (19.8 ) 2.8 (3.9 ) Net unrealized gains (losses) recognized during the period on equity securities still held $ 33.2 $ 103.7 $ (39.5 )
Schedule of Realized Gain (Loss)	The proceeds from sales of available-for-sale fixed maturities and the gross realized gains and gross realized losses on those sales, were as follows: YEARS ENDED DECEMBER 31 (in millions) Proceeds Gross Gross 2020 from Sales Gains Losses Fixed maturities $ 264.1 $ 9.9 $ 2.9 2019 Fixed maturities, excluding held-for-sale (Chaucer) $ 541.2 $ 6.9 $ 5.7 Fixed maturities, held-for-sale 0.3 — — Total fixed maturities $ 541.5 $ 6.9 $ 5.7 2018 Fixed maturities, excluding held-for-sale (Chaucer) $ 296.5 $ 2.1 $ 8.4 Fixed maturities, held-for-sale 276.8 1.7 1.7 Total fixed maturities $ 573.3 $ 3.8 $ 10.1

Fair Value (Tables)

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Values of Financial Instruments	The estimated fair values of the financial instruments were as follows: December 31, 2020 December 31, 2019 Carrying Fair Carrying Fair (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 7,454.4 $ 7,454.4 $ 6,687.1 $ 6,687.1 Fair Value through Net Income: Equity securities 598.5 598.5 575.7 575.7 Other investments 176.8 176.8 187.1 187.1 Amortized Cost/Cost: Other investments 470.3 504.8 443.3 463.7 Cash and cash equivalents 120.6 120.6 215.7 215.7 Total financial instruments $ 8,820.6 $ 8,855.1 $ 8,108.9 $ 8,129.3 Financial Liabilities carried at: Amortized Cost: Debt $ 780.8 $ 887.7 $ 653.4 $ 722.1
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. DECEMBER 31, 2020 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 392.0 $ 155.8 $ 236.2 $ — Foreign government 2.7 — 2.7 — Municipal 1,103.7 — 1,096.7 7.0 Corporate 4,111.3 — 4,110.8 0.5 Residential mortgage-backed 1,010.7 — 1,010.7 — Commercial mortgage-backed 760.0 — 747.6 12.4 Asset-backed 74.0 — 74.0 — Total fixed maturities 7,454.4 155.8 7,278.7 19.9 Equity securities 598.5 593.0 — 5.5 Other investments 4.0 — — 4.0 Total investment assets at fair value $ 8,056.9 $ 748.8 $ 7,278.7 $ 29.4 DECEMBER 31, 2019 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 349.8 $ 157.9 $ 191.9 $ — Foreign government 16.1 — 16.1 — Municipal 833.5 — 821.4 12.1 Corporate 3,811.2 — 3,810.6 0.6 Residential mortgage-backed 921.4 — 921.4 — Commercial mortgage-backed 691.9 — 679.2 12.7 Asset-backed 63.2 — 63.2 — Total fixed maturities 6,687.1 157.9 6,503.8 25.4 Equity securities 575.7 573.6 — 2.1 Other investments 3.5 — — 3.5 Total investment assets at fair value $ 7,266.3 $ 731.5 $ 6,503.8 $ 31.0
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. DECEMBER 31, 2020 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 120.6 $ 120.6 $ — $ — Other investments 504.8 — 2.7 502.1 Total financial instruments $ 625.4 $ 120.6 $ 2.7 $ 502.1 Liabilities: Debt $ 887.7 $ — $ 887.7 $ — DECEMBER 31, 2019 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 215.7 $ 215.7 $ — $ — Other investments 463.7 — 2.1 461.6 Total financial instruments $ 679.4 $ 215.7 $ 2.1 $ 461.6 Liabilities: Debt $ 722.1 $ — $ 722.1 $ —
Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)	The following tables provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). YEAR ENDED DECEMBER 31, 2020 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year $ 12.1 $ 0.6 $ 12.7 $ 25.4 $ 5.6 $ 31.0 Transfers out of Level 3 (3.5 ) — — (3.5 ) — (3.5 ) Total gains: Included in total net realized and unrealized investment gains — — — — 3.9 3.9 Included in other comprehensive income - net appreciation on available-for-sale securities 0.1 — 0.7 0.8 — 0.8 Sales (1.7 ) (0.1 ) (1.0 ) (2.8 ) — (2.8 ) Balance at end of year $ 7.0 $ 0.5 $ 12.4 $ 19.9 $ 9.5 $ 29.4 Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year $ 0.3 $ — $ 0.7 $ 1.0 $ — $ 1.0 YEAR ENDED DECEMBER 31, 2019 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year $ 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 — 0.5 1.4 — 1.4 Purchases and sales: Purchases — — — — 1.0 1.0 Sales (9.9 ) (0.2 ) (0.9 ) (11.0 ) — (11.0 ) Balance at end of year $ 12.1 $ 0.6 $ 12.7 $ 25.4 $ 5.6 $ 31.0
Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3	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. DECEMBER 31, 2020 2019 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted Discount for: $ 7.0 $ 12.1 cash flow Small issue size 0.7 - 6.8% (5.0%) 0.7 - 6.8% (4.3%) Credit stress 0.2% (0.2%) — Above-market coupon — 0.5% (0.5%) Corporate Discounted Discount for: 0.5 0.6 cash flow Small issue size 2.5% (2.5%) 2.5% (2.5%) Above-market coupon 0.3% (0.3%) 0.3% (0.3%) Commercial Discounted Discount for: 12.4 12.7 mortgage-backed cash flow Small issue size 1.9 - 3.1% (2.7%) 1.9 - 3.1% (2.7%) Above-market coupon 0.5% (0.5%) 0.5% (0.5%) Lease structure 0.3% (0.3%) 0.3% (0.3%) Credit stress 0.2% (0.2%) — Equity securities Market comparables Net tangible asset market multiples 1.1 1.0X (1.0X) 1.1 1.0X (1.0X) Internal price Unadjusted price from financing round 4.4 5.60 (5.60) 1.0 1.27 (1.27) Other Discounted cash flow Discount rate 4.0 16.2% (16.2%) 3.5 18.0% (18.0%)

Debt and Credit Arrangements (T

Debt and Credit Arrangements (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of Debt	Debt consists of the following: DECEMBER 31 2020 2019 (in millions) Senior debentures maturing April 15, 2026 $ 375.0 $ 375.0 Senior debentures maturing September 1, 2030 300.0 — Senior debentures maturing October 15, 2025 61.8 62.6 Subordinated debentures maturing February 3, 2027 50.1 50.1 Subordinated debentures maturing March 30, 2053 — 175.0 Total principal debt 786.9 662.7 Unamortized debt issuance costs (6.1 ) (9.3 ) Total $ 780.8 $ 653.4

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Components of Income from Continuing Operations Before Income Taxes and Summary of Components of Income Tax Expense (Benefit)	Income from continuing operations before income taxes and a summary of the components of income tax expense in the Consolidated Statements of Income are shown below: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Income from continuing operations before income taxes $ 444.8 $ 522.1 $ 282.5 Income tax expense (benefit): Current $ 105.9 $ 80.5 $ 46.2 Deferred (23.1 ) 12.6 (2.7 ) Total income tax expense $ 82.8 $ 93.1 $ 43.5
Schedule of Effective Income Tax Rate Reconciliation	The income tax expense attributable to the consolidated results of continuing operations is different from the amount determined by multiplying income from continuing operations before income taxes by the U.S. statutory federal income tax rate of 21% . The sources of the difference and the tax effects of each were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Expected income tax expense $ 93.4 $ 109.6 $ 59.3 Tax difference related to investment disposals and maturities (9.2 ) (14.8 ) (9.2 ) Stock-based compensation windfall benefit (2.1 ) (3.0 ) (2.3 ) Dividend received deduction (1.1 ) (1.3 ) (1.2 ) Tax-exempt interest (0.2 ) (0.3 ) (0.4 ) Nondeductible expenses 1.8 1.7 1.6 Effect of changes in the tax law and rates — 1.2 (4.3 ) Other, net 0.2 — — Income tax expense $ 82.8 $ 93.1 $ 43.5 Effective tax rate 18.6 % 17.8 % 15.4 %
Schedule of Deferred Tax Assets and Liabilities	The following are the components of the Company’s deferred tax assets and liabilities, (excluding those associated with its discontinued operations). DECEMBER 31 2020 2019 (in millions) Deferred tax assets: Loss, LAE and unearned premium reserves, net $ 150.8 $ 139.9 Employee benefit plans 6.0 7.6 Other 16.9 8.3 Total deferred tax assets 173.7 155.8 Deferred tax liabilities: Investments, net 146.7 87.9 Deferred acquisition costs 100.3 98.1 Software capitalization 24.0 21.0 Other — 0.6 Total deferred tax liabilities 271.0 207.6 Net deferred tax liability $ (97.3 ) $ (51.8 )
Summary of Income Tax Uncertainties	The table below provides a reconciliation of the beginning and ending liability for uncertain tax positions as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Liability at beginning of year, net $ 1.3 $ 3.0 $ 3.0 Additions for tax positions of current year — — — Subtractions as a result of a lapse of the applicable statute of limitations (0.4 ) (1.7 ) — Liability at end of year, net $ 0.9 $ 1.3 $ 3.0

Pension Plans (Tables)

Pension Plans (Tables)	12 Months Ended
	Dec. 31, 2020
Retirement Plans [Line Items]
Schedule of Weighted Average Assumptions	Weighted average assumptions used to determine pension benefit obligations are as follows: DECEMBER 31 2020 2019 2018 Discount rate - qualified plan 3.00 % 3.75 % 4.50 % Discount rate - non-qualified plan 2.88 % 4.00 % 4.50 % Cash balance interest crediting rate 3.00 % 3.50 % 3.50 % The Company utilizes a measurement date of January 1 st YEARS ENDED DECEMBER 31 2020 2019 2018 Qualified plan Discount rate 3.75 % 4.50 % 3.88 % Expected return on plan assets 4.75 % 5.50 % 4.75 % Cash balance interest crediting rate 3.50 % 3.50 % 3.50 % Non-qualified plan Discount rate 4.00 % 4.50 % 3.88 %
Summary of Target Allocations and Invested Asset Allocations	The following table provides its year-end 2020 target allocations and actual invested asset allocations at December 31, 2020 and 2019. DECEMBER 31 2020 TARGET LEVELS 2020 2019 Fixed income securities: Fixed maturities 88 % 87 % 88 % Money market funds 2 % 3 % 2 % Total fixed income securities 90 % 90 % 90 % Equity securities 10 % 10 % 10 % Total plan assets 100 % 100 % 100 %
Summary of Plan Assets Investment Measured at Fair Value	The following table presents, for each hierarchy level, the qualified defined benefit plan’s investment assets that are measured at fair value at December 31, 2020 and 2019. Refer to Note 5 – “Fair Value” for a description of the different levels in the Fair Value Hierarchy. DECEMBER 31 2020 2019 (in millions) Total Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Fixed income securities: Fixed maturities $ 29.5 $ 10.4 $ — $ 19.1 $ 33.6 $ 10.7 $ — $ 22.9 Money market mutual funds 11.3 11.3 — — 7.8 7.8 — — Total investments at fair value $ 40.8 $ 21.7 $ — $ 19.1 $ 41.4 $ 18.5 $ — $ 22.9
Summary of Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs	The table below provides a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). YEARS ENDED DECEMBER 31 2020 2019 (in millions) Balance at beginning of period $ 22.9 $ 25.8 Less: Assets transferred to Level 1 investments (4.3 ) (3.5 ) Actual return on plan assets related to assets still held 0.5 0.6 Balance at end of year $ 19.1 $ 22.9
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans	The following table reflects the benefit obligations, fair value of plan assets and funded status of the plans at December 31, 2020 and 2019. DECEMBER 31 Qualified Pension Plan Non-Qualified Pension Plan (in millions) 2020 2019 2020 2019 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 472.5 $ 469.5 $ 31.7 $ 33.8 Interest cost 17.1 19.9 1.2 1.4 Actuarial losses (gains) 25.9 20.4 3.0 (0.6 ) Benefits paid (32.2 ) (37.3 ) (2.9 ) (2.9 ) Benefit obligation, end of year (1) 483.3 472.5 33.0 31.7 Change in plan assets: Fair value of plan assets, beginning of period 487.6 450.7 — — Actual return on plan assets 55.3 64.2 — — Contributions — 10.0 2.9 2.9 Benefits paid (32.2 ) (37.3 ) (2.9 ) (2.9 ) Fair value of plan assets, end of year 510.7 487.6 — — Funded status of the plans $ 27.4 $ 15.1 $ (33.0 ) $ (31.7 )
Components of Net Periodic Pension Cost	The components of total net periodic pension cost are as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Interest cost $ 18.3 $ 21.3 $ 19.8 Expected return on plan assets (22.2 ) (23.4 ) (20.6 ) Recognized net actuarial loss 5.8 11.3 9.6 Net periodic pension cost $ 1.9 $ 9.2 $ 8.8
Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans	The following table reflects the total amounts recognized in accumulated other comprehensive income relating to the defined benefit pension plans as of December 31, 2020 and 2019. DECEMBER 31 2020 2019 (in millions) Net actuarial loss $ 67.4 $ 77.3
Schedule of Expected Benefit Payments	YEARS ENDED DECEMBER 31 2021 2022 2023 2024 2025 2026-2030 (in millions) Qualified pension plan $ 39.4 $ 38.1 $ 37.5 $ 35.8 $ 34.2 $ 152.4 Non-qualified pension plan $ 2.9 $ 2.9 $ 2.7 $ 2.6 $ 2.5 $ 11.1
Investments Net Asset Value [Member]
Retirement Plans [Line Items]
Schedule of Fair Values of Investments	The fair values of these investments are as follows: DECEMBER 31 2020 2019 Fixed maturities $ 414.4 $ 394.3 Equity securities 52.7 51.7 Total investments carried at NAV $ 467.1 $ 446.0

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Changes in Other Comprehensive Income (Loss)	The following table provides changes in other comprehensive income (loss). YEARS ENDED DECEMBER 31 2020 2019 2018 Tax Tax Tax Benefit Net of Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on investment securities: Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income $ 274.6 $ (57.7 ) $ 216.9 $ 326.0 $ (68.5 ) $ 257.5 $ (206.2 ) $ 43.3 $ (162.9 ) Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income 1.3 (0.2 ) 1.1 1.5 (0.4 ) 1.1 (3.5 ) 0.7 (2.8 ) Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income (13.5 ) (6.4 ) (19.9 ) (5.0 ) (13.7 ) (18.7 ) 2.5 (9.7 ) (7.2 ) Amount of credit losses recognized in Consolidated Statements of Income 1.2 (0.3 ) 0.9 2.1 (0.4 ) 1.7 3.5 (0.7 ) 2.8 Amount of additional impairment losses recognized in Consolidated Statements of Income 16.6 (3.5 ) 13.1 — — — — — — Unrealized losses (gains) realized with sale of Chaucer business — — — 0.1 — 0.1 24.2 (5.1 ) 19.1 Net unrealized gains (losses) 280.2 (68.1 ) 212.1 324.7 (83.0 ) 241.7 (179.5 ) 28.5 (151.0 ) Pension and postretirement benefits: Net gains (losses) arising in the period from net actuarial (gains) losses 3.9 (0.8 ) 3.1 20.5 (4.3 ) 16.2 (21.3 ) 4.3 (17.0 ) Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost 6.0 (1.3 ) 4.7 11.4 (2.4 ) 9.0 9.7 (2.0 ) 7.7 Pension obligations recognized with sale of Chaucer business — — — — — — 21.2 (4.8 ) 16.4 Total pension and postretirement benefits 9.9 (2.1 ) 7.8 31.9 (6.7 ) 25.2 9.6 (2.5 ) 7.1 Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period — — — — — — (2.2 ) 0.5 (1.7 ) Currency translation obligation recognized with sale of Chaucer business — — — 0.9 (0.2 ) 0.7 29.4 (6.2 ) 23.2 Total cumulative foreign currency translation adjustment — — — 0.9 (0.2 ) 0.7 27.2 (5.7 ) 21.5 Other comprehensive income (loss) $ 290.1 $ (70.2 ) $ 219.9 $ 357.5 $ (89.9 ) $ 267.6 $ (142.7 ) $ 20.3 $ (122.4 )
Reclassifications Out of Accumulated Other Comprehensive Income	Reclassifications out of accumulated other comprehensive income were as follows: YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Amount Reclassified from Details about Accumulated Other Accumulated Affected Line Item in the Statement Comprehensive Income Components Other Comprehensive Income Where Net Income is Presented Net unrealized gains (losses) on investment securities $ 13.4 $ 5.0 $ (3.2 ) Net realized gains (losses) from sales and other (17.6 ) (2.0 ) (2.6 ) Impairment losses on investments (4.2 ) 3.0 (5.8 ) Total before tax 10.2 14.1 10.3 Tax benefit 6.0 17.1 4.5 Continuing operations; net of tax — (0.1 ) (19.1 ) Gain on sale of Chaucer business — — (0.2 ) Discontinued operations - Chaucer business (0.1 ) (0.1 ) 0.1 Discontinued life businesses 5.9 16.9 (14.7 ) Net of tax Amortization of defined benefit pension and postretirement actuarial losses (6.0 ) (11.4 ) (9.5 ) Loss adjustment expenses and other operating expenses (1) 1.3 2.4 1.9 Tax benefit (4.7 ) (9.0 ) (7.6 ) Continuing operations; net of tax — — (16.4 ) Gain on sale of Chaucer business — — (0.1 ) Discontinued operations - Chaucer business (4.7 ) (9.0 ) (24.1 ) Net of tax Currency translation obligation recognized with sale of Chaucer business — (0.7 ) (23.2 ) Gain on sale of Chaucer business Total reclassifications for the period $ 1.2 $ 7.2 $ (62.0 ) Benefit (expense) 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 each of the years ended December 31, 2020, 2019 and 2018.

Stock-based Compensation Plans

Stock-based Compensation Plans (Tables)	12 Months Ended
	Dec. 31, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Stock Option Plan Activity	Information on the Company’s stock options is summarized below. YEARS ENDED DECEMBER 31 2020 2019 2018 (in whole shares and dollars) Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Outstanding, beginning of year 1,121,559 $ 87.88 1,099,076 $ 85.75 1,062,177 $ 75.53 Granted (1) 242,598 118.34 252,813 119.36 301,152 110.98 Exercised (54,721 ) 75.86 (191,601 ) 71.20 (210,190 ) 69.41 Forfeited or cancelled (27,158 ) 112.18 (38,729 ) 107.34 (54,063 ) 89.09 Outstanding, end of year 1,282,278 $ 93.64 1,121,559 $ 87.88 1,099,076 $ 85.75 Exercisable, end of year 852,804 $ 82.76 662,555 $ 75.63 515,286 $ 69.83 (1) In accordance with plan provisions, 2019 includes 67,605 options related to special dividends paid by the Company in January 2019 and December 2019, in order to retain the intrinsic value of outstanding awards. The remaining 185,208 option awards were granted at an exercise price of $119.36.
Schedule of Stock Options by Exercise Price Range	Additional information about employee options outstanding and exercisable at December 31, 2020 is included in the following table: Options Outstanding Options Currently Exercisable Range of Exercise Prices Number Weighted Average Remaining Contractual Lives Weighted Average Exercise Price Number Weighted Average Exercise Price $ 34.66 to $40.01 42,573 1.81 $ 38.33 42,573 $ 38.33 $ 54.61 66,107 3.14 54.61 66,107 54.61 $ 66.14 to $68.41 102,996 4.16 66.15 102,996 66.15 $ 70.51 to $77.91 154,014 5.35 75.45 154,014 75.45 $ 82.39 to $92.68 246,844 6.22 85.39 244,951 85.33 $ 104.11 to $105.53 266,274 7.16 104.12 181,613 104.12 $ 113.32 to $118.54 403,470 8.75 117.96 60,550 117.07
Schedule of Stock Option Valuation Assumptions	The following significant assumptions were used to determine the fair value for options granted in the years indicated. 2020 2019 2018 Dividend yield 2.193% to 2.805 % 2.011 % 1.740% to 1.954 % Expected volatility 17.671% to 24.495 % 18.495% to 19.100 % 17.943% to 18.500 % Weighted average expected volatility 18.10 % 18.86 % 18.20 % Risk-free interest rate 0.262 to 1.042% % 2.527% to 2.617 % 2.365% to 2.969 % Expected term, in years 2.5 to 6.5 2.5 to 6.5 2.5 to 6.0
Summary of Restricted Stock Activity	The following table summarizes information about employee restricted stock units: YEARS ENDED DECEMBER 31 2020 2019 2018 Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Shares Weighted Average Grant Date Fair Value Time-based restricted stock units: Outstanding, beginning of year 335,166 $ 109.55 332,481 $ 97.28 298,528 $ 83.45 Granted 143,128 117.72 149,698 117.60 152,529 111.66 Vested (94,993 ) 94.54 (109,493 ) 84.57 (72,710 ) 72.69 Forfeited (1) (32,821 ) 115.74 (37,520 ) 106.03 (45,866 ) 94.04 Outstanding, end of year 350,480 $ 116.37 335,166 $ 109.55 332,481 $ 97.28 Performance and market-based restricted stock units: Outstanding, beginning of year 86,252 $ 110.70 69,838 $ 95.58 102,586 $ 81.21 Granted 54,415 105.10 42,605 116.67 35,063 118.60 Vested (40,601 ) 80.92 (23,521 ) 79.57 (14,032 ) 70.24 Forfeited (1) (3,023 ) 124.78 (2,670 ) 135.92 (53,779 ) 89.79 Outstanding, end of year 97,043 $ 119.59 86,252 $ 110.70 69,838 $ 95.58 (1) As a result of the sale of Chaucer and included in forfeitures in 2018 are 19,655 shares of time-based restricted stock units, 43,449 shares of performance-based restricted stock units and 2,705 shares of market-based restricted stock units that were awarded to Chaucer employees.

Earnings Per Share and Shareh_2

Earnings Per Share and Shareholders' Equity Transactions (Tables)	12 Months Ended
	Dec. 31, 2020
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: DECEMBER 31 2020 2019 2018 (in millions, except per share data) Basic shares used in the calculation of earnings per share 37.7 40.0 42.4 Dilutive effect of securities: Employee stock options 0.2 0.3 0.3 Non-vested stock grants 0.2 0.3 0.3 Diluted shares used in the calculation of earnings per share 38.1 40.6 43.0 Per share effect of dilutive securities on income from continuing operations $ (0.11 ) $ (0.16 ) $ (0.07 ) Per share effect of dilutive securities on net income $ (0.10 ) $ (0.16 ) $ (0.12 )

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2020
Segment Reporting [Abstract]
Financial Information with Respect to Business Segments	Summarized below is financial information with respect to the Company’s business segments. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Operating revenues: Commercial Lines $ 2,870.8 $ 2,843.5 $ 2,739.5 Personal Lines 1,931.4 1,911.8 1,791.3 Other 20.1 26.0 14.2 Total 4,822.3 4,781.3 4,545.0 Net realized and unrealized investment gains (losses) 5.0 109.4 (50.7 ) Total revenues $ 4,827.3 $ 4,890.7 $ 4,494.3 Operating income (loss) before interest expense and income taxes: Commercial Lines: Underwriting income $ 106.3 $ 121.5 $ 85.0 Net investment income 175.3 180.1 182.2 Other expense (6.2 ) (1.5 ) (1.5 ) Commercial Lines operating income 275.4 300.1 265.7 Personal Lines: Underwriting income 132.9 59.1 66.6 Net investment income 76.7 80.1 73.7 Other income 2.9 5.7 5.9 Personal Lines operating income 212.5 144.9 146.2 Other: Underwriting loss (4.3 ) (1.3 ) (4.2 ) Net investment income 13.1 21.1 11.5 Other expense (12.0 ) (11.2 ) (12.7 ) Other operating income (loss) (3.2 ) 8.6 (5.4 ) Operating income before interest expense and income taxes 484.7 453.6 406.5 Interest on debt (37.1 ) (37.5 ) (45.1 ) Operating income before income taxes 447.6 416.1 361.4 Non-operating income (loss) items: Net realized and unrealized investment gains (losses) 5.0 109.4 (50.7 ) Net loss from repayment of debt (6.2 ) — (28.2 ) Other non-operating items (1.6 ) (3.4 ) — Income from continuing operations before income taxes $ 444.8 $ 522.1 $ 282.5
Identifiable Assets by Business Segment	The following table provides identifiable assets for the Company’s business segments and discontinued operations: DECEMBER 31 2020 2019 (in millions) Identifiable Assets Property and Casualty $ 13,333.5 $ 12,387.7 Assets of discontinued businesses 110.2 102.8 Total $ 13,443.7 $ 12,490.5 The Company reviews the assets of its Property and Casualty Insurance Companies collectively and does not allocate them among the Commercial Lines, Personal Lines and Other segments.

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2020
Reinsurance Disclosures [Abstract]
Schedule of Effects of Reinsurance	The following table provides the effects of reinsurance. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Premiums written: Direct $ 5,131.4 $ 5,090.8 $ 4,816.0 Assumed 23.1 27.7 27.6 Ceded (556.0 ) (536.8 ) (458.8 ) Net premiums written $ 4,598.5 $ 4,581.7 $ 4,384.8 Premiums earned: Direct $ 5,063.8 $ 4,953.3 $ 4,673.6 Assumed 24.6 26.3 26.6 Ceded (561.0 ) (505.1 ) (445.8 ) Net premiums earned $ 4,527.4 $ 4,474.5 $ 4,254.4 Percentage of assumed to net premiums earned 0.5 % 0.6 % 0.6 % Losses and LAE: Direct $ 3,200.5 $ 3,223.3 $ 2,986.2 Assumed 15.9 21.4 25.2 Ceded (371.2 ) (379.2 ) (286.8 ) Net losses and LAE $ 2,845.2 $ 2,865.5 $ 2,724.6

Liabilities for Outstanding C_2

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
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. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Gross reserve for losses and LAE, beginning of year $ 5,654.4 $ 5,304.1 $ 5,058.5 Reinsurance recoverable on unpaid losses 1,574.8 1,472.6 1,455.0 Net loss and LAE reserves, beginning of year 4,079.6 3,831.5 3,603.5 Net incurred losses and LAE in respect of losses occurring in: Current year 2,877.8 2,893.0 2,733.5 Prior years (32.6 ) (28.4 ) (8.9 ) Total incurred losses and LAE 2,845.2 2,864.6 2,724.6 Net payments of losses and LAE in respect of losses occurring in: Current year 1,347.7 1,315.4 1,232.3 Prior years 1,194.7 1,301.1 1,264.3 Total payments 2,542.4 2,616.5 2,496.6 Net reserve for losses and LAE, end of year 4,382.4 4,079.6 3,831.5 Reinsurance recoverable on unpaid losses 1,641.6 1,574.8 1,472.6 Gross reserve for losses and LAE, end of year $ 6,024.0 $ 5,654.4 $ 5,304.1
Schedule of (Favorable)/Unfavorable Loss and LAE Reserve Development	The following table provides a summary of (favorable)/unfavorable loss and LAE reserve development. YEARS ENDED DECEMBER 31 2020 2019 2018 (in millions) Commercial multiple peril $ (0.2 ) $ (19.5 ) $ (4.0 ) Workers’ compensation (36.0 ) (32.6 ) (31.0 ) Commercial automobile 15.4 5.8 23.2 Other commercial lines: Hanover Programs 10.6 19.8 17.0 General liability 0.3 (7.8 ) (27.9 ) Marine (17.6 ) (13.3 ) (10.7 ) Surety (3.5 ) (4.1 ) (9.0 ) Umbrella (1.5 ) 2.8 2.3 Other lines (5.3 ) (4.4 ) (0.8 ) Total other commercial lines (17.0 ) (7.0 ) (29.1 ) Total Commercial Lines (37.8 ) (53.3 ) (40.9 ) Personal automobile 5.2 21.6 14.3 Homeowners and other personal lines (4.2 ) 2.1 16.5 Total Personal Lines 1.0 23.7 30.8 Total Other Segment 4.2 1.2 1.2 Total loss and LAE reserve development, including catastrophes $ (32.6 ) $ (28.4 ) $ (8.9 )
Schedule of Carried Reserves	The table below summarizes the gross, ceded and net reserve for losses and LAE and reconciles to the incurred claims development in the following section. Accordingly, the commercial multiple peril, workers’ compensation, commercial automobile liability and general liability and umbrella - occurrence lines presentation includes Hanover Programs business. YEAR ENDED DECEMBER 31, 2020 2019 (in millions) Gross Ceded Net Gross Ceded Net Commercial multiple peril $ 1,429.4 $ (163.9 ) $ 1,265.5 $ 1,298.5 $ (126.5 ) $ 1,172.0 Workers’ compensation 795.4 (182.6 ) 612.8 795.3 (186.0 ) 609.3 Commercial automobile liability 504.1 (30.6 ) 473.5 467.7 (27.3 ) 440.4 General liability and umbrella - occurrence 612.0 (176.0 ) 436.0 575.9 (155.5 ) 420.4 General liability - claims made 323.7 (30.5 ) 293.2 268.1 (20.1 ) 248.0 Other lines 451.6 (85.1 ) 366.5 409.5 (89.5 ) 320.0 Total Commercial Lines and other 4,116.2 (668.7 ) 3,447.5 3,815.0 (604.9 ) 3,210.1 Personal automobile liability 1,649.9 (968.5 ) 681.4 1,626.4 (963.2 ) 663.2 Homeowners 210.6 (3.0 ) 207.6 172.4 (4.8 ) 167.6 Other personal lines 47.3 (1.4 ) 45.9 40.6 (1.9 ) 38.7 Total Personal Lines 1,907.8 (972.9 ) 934.9 1,839.4 (969.9 ) 869.5 Total loss and LAE reserves $ 6,024.0 $ (1,641.6 ) $ 4,382.4 $ 5,654.4 $ (1,574.8 ) $ 4,079.6
Schedule of Incurred Claims Development	Commercial multiple peril ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2013 $ 380.0 $ 362.5 $ 367.9 $ 391.8 $ 392.1 $ 393.0 $ 393.7 $ 393.6 $ 7.8 14,801 2014 443.9 439.6 464.8 459.4 449.1 444.4 441.0 13.1 15,881 2015 446.0 456.3 463.7 467.4 465.2 460.9 16.9 15,583 2016 447.1 449.6 448.7 447.4 449.6 25.8 15,750 2017 538.7 544.8 551.6 555.6 38.5 16,638 2018 578.2 560.9 556.1 77.8 17,123 2019 583.8 593.0 138.0 16,012 2020 628.9 245.5 13,136 Total $ 4,078.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2013 2014 2015 Year Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 2013 $ 137.6 $ 221.5 $ 262.3 $ 306.4 $ 334.6 $ 355.9 $ 366.3 $ 373.1 2014 171.7 267.8 316.0 363.4 395.2 407.7 411.7 2015 161.9 260.1 315.6 363.2 397.0 413.6 2016 140.3 237.9 290.2 342.2 369.6 2017 170.9 296.4 370.6 422.9 2018 178.8 306.2 363.7 2019 169.3 317.2 2020 217.4 Total 2,889.2 Total reserves for 2013 – 2020 accident years (incurred - paid) 1,189.5 Total reserves for 2012 and prior accident years 51.9 Unallocated loss adjustment expense 24.1 Net reserves at December 31, 2020 $ 1,265.5 Workers' compensation ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2011 $ 141.4 $ 144.2 $ 144.3 $ 145.5 $ 143.8 $ 146.7 $ 148.3 $ 147.9 $ 147.7 $ 146.9 $ 6.3 12,484 2012 176.3 171.1 165.2 157.2 162.9 163.7 163.7 161.9 161.4 8.4 13,083 2013 179.3 167.4 160.1 154.4 155.3 155.4 154.9 153.3 8.8 11,734 2014 182.1 172.9 154.7 152.1 150.3 148.2 145.8 11.8 10,943 2015 189.6 164.2 157.5 150.3 146.0 142.7 16.0 11,485 2016 189.6 180.5 164.6 158.2 152.8 16.1 15,894 2017 186.1 172.1 160.8 156.6 20.0 16,722 2018 187.2 182.1 172.1 24.2 17,454 2019 187.9 182.0 30.1 17,163 2020 182.5 76.9 12,549 Total $ 1,596.1 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2011 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 2011 $ 30.0 $ 71.3 $ 97.3 $ 114.0 $ 122.9 $ 128.0 $ 130.9 $ 132.8 $ 134.2 $ 135.3 2012 30.4 74.8 102.6 120.1 130.7 136.0 141.4 143.7 146.6 2013 30.9 74.6 101.2 114.0 122.8 127.4 131.4 133.8 2014 30.6 70.5 92.3 105.6 112.5 117.8 121.0 2015 28.0 65.7 87.2 99.4 105.8 109.6 2016 33.9 78.1 99.5 111.1 117.1 2017 32.8 73.0 94.1 106.6 2018 35.6 80.7 103.4 2019 33.3 83.1 2020 30.7 Total 1,087.2 Total reserves for 2011 – 2020 accident years (incurred - paid) 508.9 Total reserves for 2010 and prior accident years 84.6 Unallocated loss adjustment expense and other 19.3 Net reserves at December 31, 2020 $ 612.8 Commercial automobile liability ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2014 2015 Claim Year Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2014 168.5 $ 163.3 $ 177.3 $ 181.7 $ 185.1 $ 185.0 $ 183.5 $ 1.7 13,477 2015 163.4 168.3 166.9 167.8 167.6 170.3 1.2 12,829 2016 157.0 157.7 163.0 174.3 172.5 4.3 11,660 2017 159.5 170.1 182.1 188.5 7.6 11,504 2018 182.8 176.1 182.9 25.5 11,181 2019 180.5 192.0 57.0 10,210 2020 171.6 115.9 5,772 Total $ 1,261.3 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2014 2015 Year Unaudited Unaudited 2016 2017 2018 2019 2020 2014 33.1 $ 70.8 $ 102.7 $ 137.1 $ 168.2 $ 176.3 $ 178.2 2015 32.2 63.8 96.4 129.3 144.5 152.5 2016 27.8 60.7 98.3 134.0 147.2 2017 26.9 71.2 105.9 137.7 2018 29.2 59.7 92.5 2019 29.5 75.8 2020 23.4 Total 807.3 Total reserves for 2014 – 2020 accident years (incurred - paid) 454.0 Total reserves for 2013 and prior accident years 13.7 Unallocated loss adjustment expense 5.8 Net reserves at December 31, 2020 $ 473.5 General liability and umbrella - occurrence ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2011 $ 47.6 $ 43.8 $ 38.2 $ 42.2 $ 51.0 $ 55.6 $ 55.4 $ 57.5 $ 58.8 $ 59.3 $ 2.4 1,393 2012 77.2 59.3 62.2 64.3 71.1 74.2 69.1 71.2 72.8 2.5 1,774 2013 84.1 67.4 71.5 88.6 84.4 81.2 80.4 79.1 4.1 2,001 2014 100.9 82.3 98.6 101.4 98.3 99.8 98.4 6.4 2,108 2015 104.2 99.3 99.6 97.0 98.3 99.7 9.9 2,448 2016 95.6 100.7 101.5 100.8 98.7 15.7 1,913 2017 97.7 107.6 109.2 110.7 24.9 1,806 2018 99.8 106.4 104.0 35.2 1,823 2019 104.2 105.7 55.8 1,605 2020 99.8 81.0 1,213 Total $ 928.2 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2011 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2019 2020 2011 1.6 $ 6.7 $ 19.4 $ 31.1 $ 39.8 $ 45.8 $ 47.8 $ 50.6 $ 52.9 $ 54.8 2012 2.2 12.6 29.8 43.8 52.1 60.6 63.0 64.5 65.0 2013 2.4 11.0 26.8 43.1 56.1 63.5 67.6 70.1 2014 3.1 14.5 31.4 52.8 70.4 83.6 87.3 2015 3.3 15.2 30.8 48.5 65.7 77.3 2016 3.1 15.0 31.6 52.5 63.4 2017 4.4 17.0 34.5 49.8 2018 4.1 15.6 35.7 2019 7.5 16.7 2020 3.9 Total 524.0 Total reserves for 2011 – 2020 accident years (incurred - paid) 404.2 Total reserves for 2010 and prior accident years 18.1 Unallocated loss adjustment expense and other 13.7 Net reserves at December 31, 2020 $ 436.0 General liability - claims made ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2014 2015 Claim Year Unaudited Unaudited 2016 2017 2018 2019 2020 IBNR Count 2014 $ 61.7 $ 69.9 $ 83.9 $ 86.2 $ 82.5 $ 82.9 $ 82.4 $ 0.9 906 2015 93.2 98.8 98.4 90.2 90.3 91.0 1.9 1,002 2016 103.6 101.7 97.1 89.7 95.2 4.7 1,011 2017 103.3 104.6 98.1 96.0 8.8 1,122 2018 120.9 127.8 132.5 14.3 1,386 2019 126.8 127.1 31.7 1,554 2020 142.8 76.3 2,514 Total $ 767.0 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2014 2015 Year Unaudited Unaudited 2016 2017 2018 2019 2020 2014 $ 11.0 $ 38.8 $ 59.1 $ 71.5 $ 75.4 $ 79.1 $ 79.9 2015 10.0 43.6 64.2 76.0 81.8 83.7 2016 11.1 42.9 66.1 75.5 80.4 2017 12.3 42.3 67.2 76.5 2018 17.3 56.1 87.5 2019 17.8 56.7 2020 18.1 Total 482.8 Total reserves for 2014 – 2020 accident years (incurred - paid) 284.2 Total reserves for 2013 and prior accident years 3.8 Unallocated loss adjustment expense 5.1 Net reserves at December 31, 2020 $ 293.1 Personal automobile liability ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident Claim Year 2016 2017 2018 2019 2020 IBNR Count 2016 337.9 344.5 355.3 365.8 $ 367.7 $ 4.2 42,375 2017 363.6 362.3 383.4 392.2 9.7 42,993 2018 394.2 395.3 400.3 23.9 42,841 2019 431.2 431.8 67.0 41,703 2020 378.0 199.2 24,339 Total $ 1,970.0 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident Year 2016 2017 2018 2019 2020 2016 $ 112.8 $ 213.1 $ 288.0 $ 329.4 $ 346.3 2017 115.0 229.2 302.7 343.8 2018 121.7 237.2 307.4 2019 131.0 262.2 2020 95.9 Total 1,355.6 Total reserves for 2016 – 2020 accident years (incurred - paid) 614.4 Total reserves for 2015 and prior accident years 51.7 Unallocated loss adjustment expense 15.3 Net reserves at December 31, 2020 $ 681.4 Homeowners ($ in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2020 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident Claim Year 2017 2018 2019 2020 IBNR Count 2017 $ 291.4 $ 296.5 $ 297.3 $ 295.2 0.6 33,474 2018 312.9 316.2 317.3 6.1 32,725 2019 344.5 345.3 14.3 33,582 2020 418.8 77.2 31,392 Total $ 1,376.6 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident Year 2017 2018 2019 2020 2017 $ 204.5 $ 271.6 $ 283.5 $ 289.0 2018 213.7 290.1 300.8 2019 234.7 311.7 2020 281.5 Total 1,183.0 Total reserves for 2017 – 2020 accident years (incurred - paid) 193.6 Total reserves for 2016 and prior accident years 10.5 Unallocated loss adjustment expense 3.4 Net reserves at December 31, 2020 $ 207.5
Computation of Historical Claims on Paid and Incurred Claims Data, Net of Reinsurance	The following table is information about average historical claims duration as of December 31, 2020. The table is computed based on the paid and incurred claims data, net of reinsurance, for the accident years presented in the preceding claims development tables. Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance: Unaudited 1 2 3 4 5 6 7 8 9 10 Commercial multiple peril 33.3 % 22.4 % 11.4 % 10.7 % 7.0 % 4.0 % 1.8 % 1.7 % Workers' compensation 19.9 % 27.4 % 15.4 % 9.1 % 5.2 % 3.2 % 2.5 % 1.4 % 1.3 % 0.7 % Commercial automobile liability 16.0 % 20.4 % 18.9 % 18.9 % 11.2 % 4.5 % 1.0 % General liability and umbrella - occurrence 3.7 % 11.1 % 18.7 % 19.1 % 14.8 % 11.3 % 3.9 % 3.3 % 2.3 % 3.3 % General liability - claims made 12.7 % 32.5 % 24.3 % 11.9 % 5.4 % 3.3 % 0.9 % Personal automobile liability 29.2 % 28.9 % 18.9 % 10.9 % 4.6 % Homeowners 68.0 % 23.0 % 3.7 % 1.8 %

Statutory Financial Informati_2

Statutory Financial Information (Tables)	12 Months Ended
	Dec. 31, 2020
Statutory Accounting Practices [Abstract]
Statutory Accounting Practices Disclosure	The following table provides statutory net income for the years ended December 31 and statutory capital and surplus for the insurance subsidiaries as of December 31 for the periods indicated: (in millions) 2020 2019 2018 Statutory Net Income $ 360.9 $ 348.4 $ 296.1 Statutory Capital and Surplus 2,588.5 2,470.2 2,172.5

Quarterly Results of Operatio_2

Quarterly Results of Operations (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The quarterly results of operations for 2020 and 2019 are summarized below. THREE MONTHS ENDED (in millions, except per share data) 2020 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,056.2 $ 1,224.3 $ 1,249.8 $ 1,297.0 Income (loss) from continuing operations (38.7 ) 115.3 119.5 165.9 Net income (loss) (40.0 ) 115.2 118.9 164.6 Income (loss) from continuing operations per share: Basic (1.01 ) 3.03 3.17 4.52 Diluted (1.01 ) 3.01 3.14 4.47 Net income (loss) per share: Basic (1.04 ) 3.03 3.15 4.49 Diluted (1.04 ) 3.01 3.13 4.43 Dividends declared per share 0.65 0.65 0.65 0.70 THREE MONTHS ENDED (in millions, except per share data) 2019 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,219.5 $ 1,198.6 $ 1,214.7 $ 1,257.9 Income from continuing operations 122.6 85.0 111.2 110.2 Net income 122.4 74.0 118.9 109.8 Income from continuing operations per share: Basic 3.02 2.09 2.81 2.81 Diluted 2.98 2.06 2.77 2.77 Net income per share: Basic 3.01 1.82 3.00 2.80 Diluted 2.97 1.79 2.96 2.76 Dividends declared per share (1) 0.60 0.60 0.60 3.15 (1) On December 5, 2019 the Board of Directors declared a special cash dividend of $2.50 per share.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
Summary Of Significant Accounting Policies [Line Items]
Impairment recognized	$ 0	$ 0
Intangible assets	15,000,000	15,000,000	$ 15,000,000
Goodwill	178,800,000	178,800,000	$ 178,800,000
U.S. statutory income tax rate			21.00%
Retained earnings	2,668,000,000	2,410,900,000	$ 2,668,000,000
Total assets	13,443,700,000	12,490,500,000	13,443,700,000
Total liabilities	$ 10,241,500,000	$ 9,574,300,000	$ 10,241,500,000
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date					Jan. 1, 2018
Change in accounting principle, accounting standards update, adopted					true
Change in accounting principle, accounting standards update, immaterial effect					false
Accounting Standards Update 2018-02 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained earnings						$ 6,500,000
Accounting Standards Update 2018-14 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Dec. 31, 2020		Dec. 31, 2020
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, immaterial effect	true		true
Accounting Standards Update 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, immaterial effect	true		true
Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Jan. 1, 2020		Jan. 1, 2020
Change in accounting principle, accounting standards update, adopted	true		true
Change in accounting principle, accounting standards update, immaterial effect	true		true
Accounting Standards Update 2017-08 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2019
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, immaterial effect		false
Accounting Standards Update 2017-08 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained earnings				$ (1,500,000)
Accounting Standards Update 2016-02 and 2018-11 [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Total assets				35,000,000
Total liabilities				$ 35,000,000
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2019
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, immaterial effect		false
Accounting Standards Update 2018-11 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date		Jan. 1, 2019
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, immaterial effect		false
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			3 years
Minimum [Member] | Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			5 years
Minimum [Member] | Real Estate And Fleet Leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			4 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			30 years
Maximum [Member] | Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			7 years
Maximum [Member] | Real Estate And Fleet Leases [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, equipment and leasehold improvements, estimated useful lives			6 years

Discontinued Operations (Narrat

Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions	Apr. 10, 2019	Feb. 14, 2019	Dec. 28, 2018	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Income tax expense (benefit)													$ (0.5)		$ 0.2
Revenues				$ 1,297	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5		4,827.3	$ 4,890.7	4,494.3
Pre-tax operating losses													484.7	453.6	406.5
Assets held-for-sale				110.2				102.8					110.2	102.8
Liabilities held-for-sale				120.8				116.9					120.8	116.9
Accident and Health Insurance Business [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Income (loss) from discontinued businesses, net of taxes													(3.7)	(4.3)	0.1
Assets held-for-sale				90.3				82.4					90.3	82.4
Liabilities held-for-sale				$ 89.8				$ 84.4					89.8	84.4
Income tax benefit related to discontinued operations													1.7	0.9
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale			$ 174.4												174.4
Income tax expense (benefit)			42.5											(5.3)	42.5	[1]
Contingent proceeds			$ 31.7										22		31.7	[2],[3]
Increase (decrease) in estimate of pre-tax contingent proceeds													9.7
Recognized tax benefits related to prior years									$ 3.4
Revenues														6.7	912.4
Pre-tax operating losses														(0.5)
Income (loss) from discontinued businesses, net of taxes													0.4	1.6	20
Decrease in uncertain tax positions due to expiration of statute of limitations														2
Amount recognized as foreign currency transaction losses during the period															0.8
Income tax benefit related to discontinued operations															$ (4.7)
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													$ 1.2	$ 1.2
Chaucer Irish Entity [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale		$ 0.4
Income tax expense (benefit)		(0.5)
Proceeds from sale of businesses segment		$ 28
Australian Entities [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Pre-tax gain on sale	$ 1.2
Income tax expense (benefit)	0.1
Proceeds from sale of businesses segment												$ 13
Cash proceeds to be received	$ 13
[1]	The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that were no longer likely to be realized.
[2]	Contingent proceeds, as reflected in the sales and purchase agreement, could have been up to $45 million and was determined based upon 2018 catastrophe losses. In 2018, the Company’s best estimate of contingent consideration was $31.7 million.
[3]	Initial consideration for Chaucer as determined in the purchase and sales agreement was $779 million. This amount, along with $28 million in cash proceeds received from the sale of the Irish entity on February 14, 2019, $13 million from the sale of the Australian entities on April 10, 2019, estimated contingent consideration of $31.7 million, and an $85 million pre-signing dividend from Chaucer that was received in the second quarter of 2018, resulted in expected total proceeds from the entire transaction of $936.7 million. These amounts were partially offset by $17.0 million paid to China Re to adjust the purchase price for amounts received by the Company from Chaucer prior to December 28, 2018.

Discontinued Operations (Compon

Discontinued Operations (Components of Estimated Gain From Disposal of Chaucer Business) (Details) - USD ($) $ in Millions		Dec. 28, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Initial consideration received from sale					$ 936.7
Income tax expense			$ (0.5)		0.2
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Initial consideration received from sale	[1]				779
Adjustment	[1]				(17)
Contingent proceeds		$ 31.7	$ 22		31.7	[1],[2]
Total cash proceeds expected from sale of Chaucer Holdings Limited	[1]				793.7
Carrying value of Chaucer business	[3]				530
Transaction and other sale related costs	[4]				30.6
Net realized losses on securities, pension and currency translation obligations related to Chaucer business	[5]				58.7
Total pre-tax reductions					619.3
Pre-tax gain on sale		174.4			174.4
Income tax expense		$ 42.5		$ (5.3)	42.5	[6]
Gain on sale				$ (1.2)	$ 131.9
[1]	Initial consideration for Chaucer as determined in the purchase and sales agreement was $779 million. This amount, along with $28 million in cash proceeds received from the sale of the Irish entity on February 14, 2019, $13 million from the sale of the Australian entities on April 10, 2019, estimated contingent consideration of $31.7 million, and an $85 million pre-signing dividend from Chaucer that was received in the second quarter of 2018, resulted in expected total proceeds from the entire transaction of $936.7 million. These amounts were partially offset by $17.0 million paid to China Re to adjust the purchase price for amounts received by the Company from Chaucer prior to December 28, 2018.
[2]	Contingent proceeds, as reflected in the sales and purchase agreement, could have been up to $45 million and was determined based upon 2018 catastrophe losses. In 2018, the Company’s best estimate of contingent consideration was $31.7 million.
[3]	The carrying value of the Chaucer business reflects its U.S. GAAP book value at December 28, 2018, excluding $7.9 million of U.S.-related deferred tax assets that are no longer likely to be realized and therefore are reflected in the income tax expense category.
[4]	Transaction and other sale related costs primarily include brokerage, legal, actuarial, tax and other professional fees, employee retention costs, costs for the purchase of aggregate excess of loss catastrophe coverage in consideration of the contingent proceeds provision, along with certain other miscellaneous charges related to the execution of the transaction.
[5]	As part of the transaction, investments held by Chaucer were transferred to China Re resulting in the recognition of net realized investment losses that were previously reflected in accumulated other comprehensive income.  Additionally, Chaucer’s deferred pension obligations and currency translation obligations previously recognized in accumulated other comprehensive income were recognized as losses associated with the transaction.
[6]	The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that were no longer likely to be realized.

Discontinued Operations (Comp_2

Discontinued Operations (Components of Estimated Gain From Disposal of Chaucer Business) (Parenthetical) (Details) - USD ($) $ in Millions		Feb. 14, 2019	Dec. 28, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration							$ 936.7
Maximum [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Contingent proceeds							45
Chaucer Irish Entity [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration							779
Proceeds from sale of businesses segment		$ 28
Australian Entities [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment					$ 13
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration	[1]						779
Contingent proceeds			$ 31.7			$ 22	31.7	[1],[2]
Proceeds from dividends received				$ 85
Adjustment	[1]						17
Deferred tax assets							7.9
Chaucer [Member] | China Re [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Adjustment							$ 17
[1]	Initial consideration for Chaucer as determined in the purchase and sales agreement was $779 million. This amount, along with $28 million in cash proceeds received from the sale of the Irish entity on February 14, 2019, $13 million from the sale of the Australian entities on April 10, 2019, estimated contingent consideration of $31.7 million, and an $85 million pre-signing dividend from Chaucer that was received in the second quarter of 2018, resulted in expected total proceeds from the entire transaction of $936.7 million. These amounts were partially offset by $17.0 million paid to China Re to adjust the purchase price for amounts received by the Company from Chaucer prior to December 28, 2018.
[2]	Contingent proceeds, as reflected in the sales and purchase agreement, could have been up to $45 million and was determined based upon 2018 catastrophe losses. In 2018, the Company’s best estimate of contingent consideration was $31.7 million.

Discontinued Operations (Summar

Discontinued Operations (Summary of Discontinued Chaucer Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Net premiums earned									$ 4,527.4	$ 4,474.5	$ 4,254.4
Net investment income									265.1	281.3	267.4
Total revenues	$ 1,297	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5	4,827.3	4,890.7	4,494.3
Losses and operating expenses
Losses and LAE									2,845.2	2,865.5	2,724.6
Amortization of deferred acquisition costs									951	926.7	891.8
Other expenses									543	538.9	522.1
Total losses and expenses									4,382.5	4,368.6	4,211.8
Other items:
Interest expense									(37.1)	(37.5)	(45.1)
Net realized and unrealized investment losses									5	109.4	(50.7)
Chaucer [Member]
Revenues
Net premiums earned											850
Net investment income											54.9
Other income											7.5
Total revenues										6.7	912.4
Losses and operating expenses
Losses and LAE											515.5
Amortization of deferred acquisition costs											252.5
Other expenses											115
Total losses and expenses											883
Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income)											29.4
Other items:
Interest expense											(3.8)
Net realized and unrealized investment losses											(1.3)
Other income											0.4
Income from Chaucer business before income taxes											24.7
Income tax expense											(4.7)
Income from Chaucer business, net of taxes									$ 0.4	$ 1.6	$ 20

Discontinued Operations (Schedu

Discontinued Operations (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct	$ 5,131.4	$ 5,090.8	$ 4,816
Property and casualty premiums written, Assumed	23.1	27.7	27.6
Property and casualty premiums written, Ceded	(556)	(536.8)	(458.8)
Property and casualty premiums written, Net	4,598.5	4,581.7	4,384.8
Property and casualty premiums earned, Direct	5,063.8	4,953.3	4,673.6
Property and casualty premiums earned, Assumed	24.6	26.3	26.6
Property and casualty premiums earned, Ceded	(561)	(505.1)	(445.8)
Premiums Earned, Net, Total	4,527.4	4,474.5	4,254.4
Property and casualty losses and LAE, Direct	3,200.5	3,223.3	2,986.2
Property and casualty losses and LAE, Assumed	15.9	21.4	25.2
Property and casualty losses and LAE, Ceded	$ (371.2)	$ (379.2)	(286.8)
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct			625.3
Property and casualty premiums written, Assumed			685.3
Property and casualty premiums written, Ceded			(459.9)
Property and casualty premiums written, Net			850.7
Property and casualty premiums earned, Direct			611.9
Property and casualty premiums earned, Assumed			667.2
Property and casualty premiums earned, Ceded			(429.1)
Premiums Earned, Net, Total			850
Property and casualty losses and LAE, Direct			362.2
Property and casualty losses and LAE, Assumed			568.7
Property and casualty losses and LAE, Ceded			(415.4)
Policyholder Benefits and Claims Incurred, Net, Total			$ 515.5

Discontinued Operations (Summ_2

Discontinued Operations (Summary of Cash Flows Associated with the Discontinued Chaucer Business) (Details) - Chaucer [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash provided by (used in) operating activities	$ 3.5	$ (22.8)
Net cash (used in) provided by investing activities	$ (6.8)	$ 131.1

Investments (Schedule of Availa

Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($)		Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 6,945,600,000	$ 6,452,200,000
Amortized Cost, Net of Allowance for Credit Losses		6,945,600,000
Fair Value		7,454,400,000	6,687,100,000
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		376,400,000	342,000,000
Amortized Cost, Net of Allowance for Credit Losses		376,400,000
Gross Unrealized Gains		17,600,000	9,100,000
Gross Unrealized Loss		2,000,000	1,300,000
Fair Value		392,000,000	349,800,000
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,200,000	15,700,000
Amortized Cost, Net of Allowance for Credit Losses		2,200,000
Gross Unrealized Gains		500,000	400,000
Fair Value		2,700,000	16,100,000
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,042,100,000	807,100,000
Amortized Cost, Net of Allowance for Credit Losses		1,042,100,000
Gross Unrealized Gains		61,700,000	27,600,000
Gross Unrealized Loss		100,000	1,200,000
Fair Value		1,103,700,000	833,500,000
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		3,770,000,000	3,653,500,000
Allowance for Credit Losses		(100,000)
Amortized Cost, Net of Allowance for Credit Losses		3,769,900,000
Gross Unrealized Gains		341,800,000	161,600,000
Gross Unrealized Loss		400,000	3,900,000
Fair Value		4,111,300,000	3,811,200,000
OTTI Unrealized Losses	[1]		3,000,000
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		978,200,000	905,400,000
Amortized Cost, Net of Allowance for Credit Losses		978,200,000
Gross Unrealized Gains		33,100,000	17,100,000
Gross Unrealized Loss		600,000	1,100,000
Fair Value		1,010,700,000	921,400,000
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		705,400,000	666,400,000
Amortized Cost, Net of Allowance for Credit Losses		705,400,000
Gross Unrealized Gains		54,800,000	25,600,000
Gross Unrealized Loss		200,000	100,000
Fair Value		760,000,000	691,900,000
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		71,400,000	62,100,000
Amortized Cost, Net of Allowance for Credit Losses		71,400,000
Gross Unrealized Gains		2,600,000	1,100,000
Fair Value		74,000,000	63,200,000
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		6,945,700,000	6,452,200,000
Allowance for Credit Losses		(100,000)	0
Amortized Cost, Net of Allowance for Credit Losses		6,945,600,000
Gross Unrealized Gains		512,100,000	242,500,000
Gross Unrealized Loss		3,300,000	7,600,000
Fair Value		$ 7,454,400,000	6,687,100,000
OTTI Unrealized Losses	[1]		$ 3,000,000
[1]	Other-than-temporary impairments (“OTTI”) unrealized losses represents OTTI recognized in accumulated other comprehensive income. This amount excludes net unrealized gains on impaired securities relating to changes in the value of such securities subsequent to the impairment measurement date through December 31, 2019 of $4.2 million

Investments (Schedule of Avai_2

Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Parenthetical) (Details) $ in Millions	Dec. 31, 2019USD ($)
Investments Debt And Equity Securities [Abstract]
Net unrealized gains on impaired securities	$ 4.2

Investments (Narrative) (Detail

Investments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Other investments	$ 793.2	$ 733.2
Concentration of investment in a single investee, maximum	10.00%	10.00%
Contractual investment commitments	$ 205.3
U.S. Government-Sponsored Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities	459.3	$ 432.3
Mortgage Participations [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities	459.3	432.3
U.S. Treasury and Government Agencies [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value	315.2	308.7
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	467.6	441.2
Due in year two, amortized cost	57.5
Due in year three, amortized cost	27.7
Due in year four, amortized cost	16.5
Due in year five, amortized cost	77.1
Due in year six and thereafter, amortized cost	288.8
Limited Partnerships [Member]
Schedule of Investments [Line Items]
Other investments	312.5	278.5
Federal Home Loan Bank of Boston [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value	$ 103.9	$ 94

Investments (Investments Classi

Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost, net of Allowance for Credit Losses	$ 334.1
Due after one year through five years, Amortized Cost, net of Allowance for Credit Losses	1,774.6
Due after five years through ten years, Amortized Cost, net of Allowance for Credit Losses	2,306.3
Due after ten years, Amortized Cost, net of Allowance for Credit Losses	775.6
Gross fixed maturities, Amortized Cost, net of Allowance for Credit Losses	5,190.6
Mortgage-backed and asset-backed securities, Amortized Cost, net of Allowance for Credit Losses	1,755
Amortized Cost, Net of Allowance for Credit Losses	6,945.6
Due in one year or less, Fair Value	339.1
Due after one year through five years, Fair Value	1,913.3
Due after five years through ten years, Fair Value	2,548.2
Due after ten years, Fair Value	809.1
Gross fixed maturities, Fair Value	5,609.7
Mortgage-backed and asset-backed securities, Fair Value	1,844.7
Fixed maturities, Fair Value	$ 7,454.4	$ 6,687.1

Investments (Unrealized Gains a

Investments (Unrealized Gains and Losses on Available-For-Sale and Other Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Provision for deferred income taxes	$ 68.1	$ 83	$ (28.5)
Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	216	(27.2)	205.4
Net appreciation (depreciation) on available-for-sale securities	280.2	324.7	(203.7)
Provision for deferred income taxes	(68.1)	(83)	33.6
Amount realized with sale of Chaucer business		0.1	19.1
Cumulative effect adjustment for ASU 2017-08, 2016-01 and 2018-02, net of tax		1.5	(81.6)
Total adjustment	212.1	243.2	(232.6)
Net appreciation (depreciation), end of year	428.1	216	(27.2)
Fixed Maturities [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	216	(27.2)	110.1
Net appreciation (depreciation) on available-for-sale securities	280.2	324.7	(203.7)
Provision for deferred income taxes	(68.1)	(83)	33.6
Amount realized with sale of Chaucer business			19.1
Cumulative effect adjustment for ASU 2017-08, 2016-01 and 2018-02, net of tax		1.5	13.7
Total adjustment	212.1	243.2	(137.3)
Net appreciation (depreciation), end of year	$ 428.1	$ 216	(27.2)
Equity Securities and Other [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year			95.3
Cumulative effect adjustment for ASU 2017-08, 2016-01 and 2018-02, net of tax			(95.3)
Total adjustment			$ (95.3)

Investments (Schedule of Unreal

Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 3.3	$ 6.1
12 months or less, Fair Value	345.6	368.9
Greater than 12 months, Gross Unrealized Losses		1.5
Greater than 12 months, Fair Value	0.7	65.6
Total, Gross Unrealized Losses	3.3	7.6
Total, Fair Value	346.3	434.5
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.3	3.9
12 months or less, Fair Value	341	341.8
Greater than 12 months, Gross Unrealized Losses		0.6
Greater than 12 months, Fair Value	0.7	56.6
Total, Gross Unrealized Losses	3.3	4.5
Total, Fair Value	341.7	398.4
U.S. Treasury and Government Agencies [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2	1.3
12 months or less, Fair Value	140.7	73
Greater than 12 months, Fair Value		9.3
Total, Gross Unrealized Losses	2	1.3
Total, Fair Value	140.7	82.3
Municipal [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	1.1
12 months or less, Fair Value	19.3	72.5
Greater than 12 months, Gross Unrealized Losses		0.1
Greater than 12 months, Fair Value	0.7	5.6
Total, Gross Unrealized Losses	0.1	1.2
Total, Fair Value	20	78.1
Corporate [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	0.7
12 months or less, Fair Value	53.4	86.5
Greater than 12 months, Gross Unrealized Losses		0.1
Greater than 12 months, Fair Value		4.7
Total, Gross Unrealized Losses	0.4	0.8
Total, Fair Value	53.4	91.2
Corporate [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		2.2
12 months or less, Fair Value	4.6	27.1
Greater than 12 months, Gross Unrealized Losses		0.9
Greater than 12 months, Fair Value		9
Total, Gross Unrealized Losses		3.1
Total, Fair Value	4.6	36.1
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.6	0.7
12 months or less, Fair Value	94.4	69.2
Greater than 12 months, Gross Unrealized Losses		0.4
Greater than 12 months, Fair Value		34.4
Total, Gross Unrealized Losses	0.6	1.1
Total, Fair Value	94.4	103.6
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.1
12 months or less, Fair Value	33.2	40.6
Greater than 12 months, Fair Value		0.9
Total, Gross Unrealized Losses	0.2	0.1
Total, Fair Value	$ 33.2	41.5
Asset-backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value		1.7
Total, Fair Value		$ 1.7

Investments (Schedule of Other

Investments (Schedule of Other Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Other investments	$ 793.2	$ 733.2
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	467.6	441.2
Valuation allowance	(7.9)	(1.3)
Mortgage Loans on Real Estate [Member] | Pacific [Member]
Schedule of Investments [Line Items]
Other investments	117	102.3
Mortgage Loans on Real Estate [Member] | South Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	108.3	95.9
Mortgage Loans on Real Estate [Member] | West South Central [Member]
Schedule of Investments [Line Items]
Other investments	70.8	71.2
Mortgage Loans on Real Estate [Member] | Mid-Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	73.8	53.3
Mortgage Loans on Real Estate [Member] | New England [Member]
Schedule of Investments [Line Items]
Other investments	43.5	42.5
Mortgage Loans on Real Estate [Member] | East North Central [Member]
Schedule of Investments [Line Items]
Other investments	27.8	27.8
Mortgage Loans on Real Estate [Member] | Mountain [Member]
Schedule of Investments [Line Items]
Other investments	12.5	12.5
Mortgage Loans on Real Estate [Member] | Other Locations [Member]
Schedule of Investments [Line Items]
Other investments	21.8	37
Mortgage Loans on Real Estate [Member] | Office [Member]
Schedule of Investments [Line Items]
Other investments	160.4	138.2
Mortgage Loans on Real Estate [Member] | Apartments [Member]
Schedule of Investments [Line Items]
Other investments	128.3	125.1
Mortgage Loans on Real Estate [Member] | Retail [Member]
Schedule of Investments [Line Items]
Other investments	68.5	66.5
Mortgage Loans on Real Estate [Member] | Hotel [Member]
Schedule of Investments [Line Items]
Other investments	62.7	62.1
Mortgage Loans on Real Estate [Member] | Industrial [Member]
Schedule of Investments [Line Items]
Other investments	$ 55.6	$ 50.6

Investment Income and Gains a_3

Investment Income and Gains and Losses (Components of Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 274.7	$ 290	$ 277.6
Less: investment expenses	(9.6)	(8.7)	(10.2)
Net investment income	265.1	281.3	267.4
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	222.5	232.4	217.7
Limited Partnerships [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	16.7	19.7	24.1
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	17.5	16.3	14
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	14.8	16.3	17
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 3.2	$ 5.3	$ 4.8

Investment Income and Gains a_4

Investment Income and Gains and Losses (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 33,200,000	$ 103,700,000	$ (39,500,000)
Impairment losses on investment securities	26,300,000	2,000,000	4,600,000
Impairments on fixed maturities included intend to sell	16,500,000
Impairments on fixed maturities credit related losses	1,100,000
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Impairment losses on investment securities	17,600,000		2,600,000
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Impairment losses on investment securities	6,700,000
Allowance for credit losses	7,900,000	1,300,000
Other Invested Assets [Member]
Gain (Loss) on Investments [Line Items]
Impairment losses on investment securities			2,000,000
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Allowance for credit losses	100,000	0
Fair Value Measured Using NAV [Member]
Gain (Loss) on Investments [Line Items]
Net unrealized gains (losses) recognized during the period on equity securities still held	$ (6,900,000)	$ (4,600,000)	$ 5,300,000

Investment Income and Gains a_5

Investment Income and Gains and Losses (Schedule of Net Realized and Unrealized Gains (Losses) on Investments Including Impairments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ 5	$ 109.4	$ (50.7)
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	13.4	106.5	(43.4)
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(4.2)	3.1	(5.8)
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	2.5		(1.3)
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ (6.7)	$ (0.2)	$ (0.2)

Investment Income and Gains a_6

Investment Income and Gains and Losses (Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 13.4	$ 106.5	$ (43.4)
Less: net (losses) gains recognized on equity securities sold during the period	(19.8)	2.8	(3.9)
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 33.2	$ 103.7	$ (39.5)

Investment Income and Gains a_7

Investment Income and Gains and Losses (Proceeds from Sales of Available for Sale Fixed Maturities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 264.1
Gross Gains	9.9
Gross Losses	$ 2.9
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		$ 541.2	$ 296.5
Gross Gains		6.9	2.1
Gross Losses		5.7	8.4
Fixed Maturities Held-For-Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		0.3	276.8
Gross Gains			1.7
Gross Losses			1.7
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		541.5	573.3
Gross Gains		6.9	3.8
Gross Losses		$ 5.7	$ 10.1

Fair Value (Fair Values of Fina

Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 120.6	$ 215.7
Total financial instruments	8,820.6	8,108.9
Carrying Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through AOCI [Member] | Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	7,454.4	6,687.1
Carrying Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through Net Income [Member] | Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	176.8	187.1
Carrying Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through Net Income [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	598.5	575.7
Carrying Value [Member] | Cost Approach Valuation Technique [Member] | Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	120.6	215.7
Debt	780.8	653.4
Carrying Value [Member] | Cost Approach Valuation Technique [Member] | Amortized Cost/Cost [Member] | Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	470.3	443.3
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	120.6	215.7
Total financial instruments	8,855.1	8,129.3
Debt	887.7	722.1
Estimated Fair Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through AOCI [Member] | Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	7,454.4	6,687.1
Estimated Fair Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through Net Income [Member] | Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	176.8	187.1
Estimated Fair Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through Net Income [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	598.5	575.7
Estimated Fair Value [Member] | Cost Approach Valuation Technique [Member] | Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	120.6	215.7
Debt	887.7	722.1
Estimated Fair Value [Member] | Cost Approach Valuation Technique [Member] | Amortized Cost/Cost [Member] | Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	$ 504.8	$ 463.7

Fair Value (Fair Value, Investm

Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,454.4	$ 6,687.1
Equity securities	598.5	575.7
Other investments	793.2	733.2
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	392	349.8
Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.7	16.1
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,103.7	833.5
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,111.3	3,811.2
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,010.7	921.4
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	760	691.9
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,454.4	6,687.1
Equity securities	598.5	575.7
Other investments	4	3.5
Total investment assets at fair value	8,056.9	7,266.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	155.8	157.9
Equity securities	593	573.6
Total investment assets at fair value	748.8	731.5
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7,278.7	6,503.8
Total investment assets at fair value	7,278.7	6,503.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	19.9	25.4
Equity securities	5.5	2.1
Other investments	4	3.5
Total investment assets at fair value	29.4	31
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	392	349.8
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Government Agencies [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	155.8	157.9
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Government Agencies [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	236.2	191.9
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.7	16.1
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.7	16.1
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,103.7	833.5
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,096.7	821.4
Fair Value, Measurements, Recurring [Member] | Municipal [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7	12.1
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,111.3	3,811.2
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	4,110.8	3,810.6
Fair Value, Measurements, Recurring [Member] | Corporate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.5	0.6
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,010.7	921.4
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,010.7	921.4
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	760	691.9
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	747.6	679.2
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	12.4	12.7
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	74	63.2
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 74	$ 63.2

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2020
Fair Value Measurements [Line Items]
Investments measured at net asset value based on an ownership interest in partners'	2.00%	2.00%
Transfer between Level 2 and Level 3	$ 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	183,600,000	$ 172,800,000
Level 3 [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	3,500,000	4,000,000
Liabilities held	$ 0	$ 0

Fair Value (Estimated Fair Valu

Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	$ 4	$ 3.5
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	120.6	215.7
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	120.6	215.7
Other investments	504.8	463.7
Total financial instruments	625.4	679.4
Debt	887.7	722.1
Estimated Fair Value [Member] | Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	120.6	215.7
Total financial instruments	120.6	215.7
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	2.7	2.1
Total financial instruments	2.7	2.1
Debt	887.7	722.1
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	502.1	461.6
Total financial instruments	$ 502.1	$ 461.6

Fair Value (Fair Value on Recur

Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 31	$ 39.6
Transfers out of Level 3	(3.5)
Included in total net realized and unrealized investment gains	3.9	1.4
Included in other comprehensive income - net appreciation on available-for-sale securities	0.8
Purchases and Sales:
Purchases		1
Sales	(2.8)	(11)
Ending Balance	29.4	31
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	1
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.1	21.1
Transfers out of Level 3	(3.5)
Included in total net realized and unrealized investment gains		0.9
Included in other comprehensive income - net appreciation on available-for-sale securities	0.1
Purchases and Sales:
Sales	(1.7)	(9.9)
Ending Balance	7	12.1
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	0.3
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	0.6	0.8
Purchases and Sales:
Sales	(0.1)	(0.2)
Ending Balance	0.5	0.6
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	12.7	13.1
Included in total net realized and unrealized investment gains		0.5
Included in other comprehensive income - net appreciation on available-for-sale securities	0.7
Purchases and Sales:
Sales	(1)	(0.9)
Ending Balance	12.4	12.7
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	0.7
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	25.4	35
Transfers out of Level 3	(3.5)
Included in total net realized and unrealized investment gains		1.4
Included in other comprehensive income - net appreciation on available-for-sale securities	0.8
Purchases and Sales:
Sales	(2.8)	(11)
Ending Balance	19.9	25.4
Changes in unrealized gains for the period included in other comprehensive income for assets held at the end of the year	1
Equity Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	5.6	4.6
Included in total net realized and unrealized investment gains	3.9
Purchases and Sales:
Purchases		1
Ending Balance	$ 9.5	$ 5.6

Fair Value (Schedule of Additio

Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value [Line Items]
Fair Value	$ 7,454.4	$ 6,687.1
Equity securities, at fair value	598.5	575.7
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 4	$ 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
Range	0.162	0.180
Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Range	0.162	0.180
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 1,103.7	$ 833.5
Municipal [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 7	$ 12.1
Municipal [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Municipal [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size		0.50%
Municipal [Member] | Income Approach Valuation Technique [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] | Income Approach Valuation Technique [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] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Municipal [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	5.00%	4.30%
Municipal [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size		0.50%
Corporate [Member]
Fair Value [Line Items]
Fair Value	$ 4,111.3	$ 3,811.2
Corporate [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.5	$ 0.6
Corporate [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Measurement Input, Discount Rate
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Corporate [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount Rate
Fair Value [Line Items]
Small issue size	2.50%	2.50%
Above-market coupon	0.30%	0.30%
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	$ 760	$ 691.9
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 12.4	$ 12.7
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Measurement Input Discount Rate For Lease Structure
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [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] | Income Approach Valuation Technique [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] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	0.20%
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [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] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Commercial Mortgage-Backed [Member] | Income Approach Valuation Technique [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount Rate For Lease Structure
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Equity Securities [Member] | Market Comparables [Member] | Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.1
Fair value measurement market multiples	1	1
Equity Securities [Member] | Market Comparables [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1	1
Equity Securities [Member] | Internal Price [Member] | Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 4.4	$ 1
Fair value measurement market multiples	5.60	1.27
Equity Securities [Member] | Internal Price [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	5.60	1.27

Debt and Credit Arrangements (S

Debt and Credit Arrangements (Schedule of Debt) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Sep. 19, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Total principal debt	$ 786.9		$ 662.7
Unamortized debt issuance costs	(6.1)		(9.3)
Total	780.8		653.4
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Total principal debt	375		375
Senior debentures maturing September 1, 2030 [Member]
Debt Instrument [Line Items]
Total principal debt	300
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt	61.8		62.6
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt	$ 50.1		50.1
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Total principal debt			$ 175
Unamortized debt issuance costs		$ (6.1)

Debt and Credit Arrangements _2

Debt and Credit Arrangements (Schedule of Debt) (Parenthetical) (Details)	Sep. 19, 2020	Dec. 31, 2020	Dec. 31, 2019
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Maturity date		Apr. 15, 2026	Apr. 15, 2026
Senior debentures maturing September 1, 2030 [Member]
Debt Instrument [Line Items]
Maturity date		Sep. 1, 2030	Sep. 1, 2030
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Maturity date		Oct. 15, 2025	Oct. 15, 2025
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Maturity date		Feb. 3, 2027	Feb. 3, 2027
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Maturity date	Mar. 30, 2053	Mar. 30, 2053	Mar. 30, 2053

Debt and Credit Arrangements (N

Debt and Credit Arrangements (Narrative) (Details) - USD ($)	Sep. 19, 2020	Jan. 02, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Proceeds from debt borrowings, net			$ 296,400,000
Unamortized debt issuance costs			6,100,000	$ 9,300,000
Repayment of FHLB advance		$ 125,000,000
FHLB due date		2029
FHLB Interest rate		5.50%
Prepayment fees		$ 26,000,000
Federal Home Loan Bank Stock			2,600,000	2,100,000
Outstanding borrowings with FHLB			0	0
Line of Credit Facility, Maximum Borrowing Capacity			$ 200,000,000
Line Of Credit Facility Maturity Month And Year			2024-04
Outstanding borrowings under credit agreement			$ 0
Interest Expense Debt			37,100,000	37,500,000	$ 45,100,000
Collateralized Borrowing Program [Member]
Debt Instrument [Line Items]
Securities Held As Collateral At Fair Value			103,900,000	$ 94,000,000
Senior debentures maturing September 1, 2030 [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 300,000,000
Debt issuance date			Aug. 24, 2020
Debt instrument interest rate			2.50%
Maturity date			Sep. 1, 2030	Sep. 1, 2030
Proceeds from debt borrowings, net			$ 296,400,000
Long-term Debt, Gross			$ 300,000,000
Senior debentures unsecured mature on October 15, 2025 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate			7.625%
Maturity date			Oct. 15, 2025	Oct. 15, 2025
Long-term Debt, Gross			$ 61,800,000	$ 62,600,000
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Aggregate principal amount	$ 175,000,000
Debt instrument interest rate	6.35%
Maturity date	Mar. 30, 2053		Mar. 30, 2053	Mar. 30, 2053
Unamortized debt issuance costs	$ 6,100,000
Long-term Debt, Gross				$ 175,000,000
Senior debentures unsecured mature on April 15, 2026 [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 375,000,000	$ 375,000,000
Debt issuance date			Apr. 8, 2016
Maturity date			Apr. 15, 2026	Apr. 15, 2026
Long-term Debt, Gross			$ 375,000,000	$ 375,000,000
8.207% Subordinated Debentures [Member]
Debt Instrument [Line Items]
Aggregate principal amount			$ 50,100,000	$ 50,100,000
Maturity date			Feb. 3, 2027
Interest rate of Series B Subordinated Deferrable Interest Debentures			8.207%	8.207%

Income Taxes (Components of Inc

Income Taxes (Components of Income from Continuing Operations Before Income Taxes and Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income from continuing operations before income taxes	$ 444.8	$ 522.1	$ 282.5
Current	105.9	80.5	46.2
Deferred	(23.1)	12.6	(2.7)
Total income tax expense	$ 82.8	$ 93.1	$ 43.5

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
U.S. statutory federal income tax rate		21.00%
Valuation allowance		$ 0	$ 0
Income recognized in continuing operations related to non-segment income		9,200,000	14,800,000	$ 9,200,000
Liability for uncertain tax positions		0	0	0
Release the remaining of liability due to expiration of statute of limitations		400,000	1,700,000	$ 0
Release of accrued interest due to expiration of statute of limitations		100,000	300,000
Scenario Forecast [Member]
Income Taxes [Line Items]
Release the remaining of liability due to expiration of statute of limitations	$ 900,000
Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Income Taxes [Line Items]
Realized Gains In Accumulated Other Comprehensive Income To Be Released Into Income From Continuing Operations In Future		11,600,000
Chaucer [Member] | Federal [Member]
Income Taxes [Line Items]
Additional income tax incurred in sales of business as result from effect of tax regulations		$ 1,200,000	$ 1,200,000

Income Taxes (Schedule of Effec

Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Expected income tax expense	$ 93.4	$ 109.6	$ 59.3
Tax difference related to investment disposals and maturities	(9.2)	(14.8)	(9.2)
Stock-based compensation windfall benefit	(2.1)	(3)	(2.3)
Dividend received deduction	(1.1)	(1.3)	(1.2)
Tax-exempt interest	(0.2)	(0.3)	(0.4)
Nondeductible expenses	1.8	1.7	1.6
Effect of changes in the tax law and rates		1.2	(4.3)
Other, net	0.2
Total income tax expense	$ 82.8	$ 93.1	$ 43.5
Effective tax rate	18.60%	17.80%	15.40%

Income Taxes (Schedule of Defer

Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Loss, LAE and unearned premium reserves, net	$ 150.8	$ 139.9
Employee benefit plans	6	7.6
Other	16.9	8.3
Total deferred tax assets	173.7	155.8
Investments, net	146.7	87.9
Deferred acquisition costs	100.3	98.1
Software capitalization	24	21
Other		0.6
Total deferred tax liabilities	271	207.6
Net deferred tax liability	$ (97.3)	$ (51.8)

Income Taxes (Summary of Income

Income Taxes (Summary of Income Tax Uncertainties) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Liability at beginning of year, net	$ 1.3	$ 3	$ 3
Additions for tax positions of current year			0
Subtractions as a result of a lapse of the applicable statute of limitations	(0.4)	(1.7)	0
Liability at end of year, net	$ 0.9	$ 1.3	$ 3

Pension Plans (Narrative) (Deta

Pension Plans (Narrative) (Details) - USD ($)	Jan. 01, 2005	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]		us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Interest rate used calculating annual transition pension adjustment for specified completed period of service	5.00%
Defined benefit plan, maximum number of years of completed service used in calculating the annual transition pension adjustment	35 years
Interest rate used calculating annual transition pension adjustment, thereafter	3.00%
Projected benefit obligation, defined benefit pension plans exceeded plan assets		$ 27,400,000
Expected rate of return on plan assets		4.75%
Defined Benefit Plan, Sponsor Location [Extensible List]		country:US	country:US
Period of fair value of plan assets amortized as a component of net periodic pension cost		5 years
Estimated future contributions in next fiscal year		$ 0
Maximum percentage contributed by employer		6.00%	6.00%	6.00%
Defined contribution plan expense		$ 24,100,000	$ 22,200,000	$ 21,700,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of unrecognized net actuarial gains (losses) as projected benefit obligation, amortized as component of net periodic pension cost		10.00%
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]		us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Expected rate of return on plan assets		4.75%	5.50%	4.75%
Defined Benefit Plan, Sponsor Location [Extensible List]		country:US	country:US	country:US
Actuarial (losses) gains		$ (25,900,000)	$ (20,400,000)
Estimated future contributions in next fiscal year			$ 10,000,000
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]		us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]		country:US	country:US	country:US
Actuarial (losses) gains		$ (3,000,000)	$ 600,000
Estimated future contributions in next fiscal year		$ 2,900,000

Pension Plans (Schedule of Weig

Pension Plans (Schedule of Weighted Average Assumptions Used to Determine Pension Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Cash balance interest crediting rate	3.00%	3.50%	3.50%
Expected return on plan assets	4.75%
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US
Discount rate	3.00%	3.75%	4.50%
Discount rate	3.75%	4.50%	3.88%
Expected return on plan assets	4.75%	5.50%	4.75%
Cash balance interest crediting rate	3.50%	3.50%	3.50%
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US
Discount rate	2.88%	4.00%	4.50%
Discount rate	4.00%	4.50%	3.88%

Pension Plans (Summary of Targe

Pension Plans (Summary of Target Allocations and Invested Asset Allocations) (Details)	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Target allocations, fixed and equity securities	100.00%
Actual invested asset allocations, fixed and equity securities	100.00%	100.00%
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations, fixed and equity securities	88.00%
Actual invested asset allocations, fixed and equity securities	87.00%	88.00%
Money Market Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations, fixed and equity securities	2.00%
Actual invested asset allocations, fixed and equity securities	3.00%	2.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations, fixed and equity securities	90.00%
Actual invested asset allocations, fixed and equity securities	90.00%	90.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocations, fixed and equity securities	10.00%
Actual invested asset allocations, fixed and equity securities	10.00%	10.00%

Pension Plans (Summary of Plan

Pension Plans (Summary of Plan Assets Investment Measured At Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 40.8	$ 41.4
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	19.1	22.9	$ 25.8
Fixed Maturities [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	29.5	33.6
Money Market Funds [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	11.3	7.8
Level 1 [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	21.7	18.5
Level 1 [Member] | Fixed Maturities [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	10.4	10.7
Level 1 [Member] | Money Market Funds [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	11.3	7.8
Level 3 [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	19.1	22.9
Level 3 [Member] | Fixed Maturities [Member] | Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 19.1	$ 22.9

Pension Plans (Schedule of Fair

Pension Plans (Schedule of Fair Values of Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 7,454.4	$ 6,687.1
Equity securities	598.5	575.7
Investments Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	414.4	394.3
Equity securities	52.7	51.7
Total investment assets at fair value	$ 467.1	$ 446

Pension Plans (Summary for Asse

Pension Plans (Summary for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	$ 22.9	$ 25.8
Less: Assets transferred to Level 1 investments	(4.3)	(3.5)
Actual return on plan assets related to assets still held	0.5	0.6
Balance at end of year	$ 19.1	$ 22.9

Pension Plans (Fair Value of Pl

Pension Plans (Fair Value of Plan Assets and Funded Status of Plans) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Interest cost	$ 18.3	$ 21.3	$ 19.8
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US
Benefit obligation, beginning of period	$ 472.5	$ 469.5
Interest cost	17.1	19.9
Actuarial losses (gains)	25.9	20.4
Benefits paid	(32.2)	(37.3)
Benefit obligation, end of year	483.3	472.5	$ 469.5
Balance at beginning of period	487.6	450.7
Actual return on plan assets	55.3	64.2
Contributions		10
Benefits paid	(32.2)	(37.3)
Balance at end of year	510.7	487.6	$ 450.7
Funded status of the plans	$ 27.4	$ 15.1
Nonqualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US
Benefit obligation, beginning of period	$ 31.7	$ 33.8
Interest cost	1.2	1.4
Actuarial losses (gains)	3	(0.6)
Benefits paid	(2.9)	(2.9)
Benefit obligation, end of year	33	31.7	$ 33.8
Contributions	2.9	2.9
Benefits paid	(2.9)	(2.9)
Funded status of the plans	$ (33)	$ (31.7)

Pension Plans (Components of Ne

Pension Plans (Components of Net Periodic Pension Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Compensation And Retirement Disclosure [Abstract]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Interest cost	$ 18.3	$ 21.3	$ 19.8
Expected return on plan assets	(22.2)	(23.4)	(20.6)
Recognized net actuarial loss	5.8	11.3	9.6
Net periodic pension cost	$ 1.9	$ 9.2	$ 8.8

Pension Plans (Summary of Amoun

Pension Plans (Summary of Amounts Recognized in Accumulated Other Comprehensive Income Loss) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Compensation And Retirement Disclosure [Abstract]
Net actuarial loss	$ 67.4	$ 77.3

Pension Plans (Summary of Estim

Pension Plans (Summary of Estimated Benefit Payments) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US
Qualified Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US
2021	$ 39.4
2022	38.1
2023	37.5
2024	35.8
2025	34.2
2026-2030	$ 152.4
Nonqualified Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Defined Benefit Plan, Sponsor Location [Extensible List]	country:US	country:US	country:US
2021	$ 2.9
2022	2.9
2023	2.7
2024	2.6
2025	2.5
2026-2030	$ 11.1

Other Comprehensive Income (Cha

Other Comprehensive Income (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Pre-Tax	$ 274.6	$ 326	$ (206.2)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Pre-Tax	1.3	1.5	(3.5)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Pre-Tax	(13.5)	(5)	2.5
Amount of credit losses recognized in Consolidated Statements of Income, Pre-Tax	1.2	2.1	3.5
Amount of additional impairment losses recognized in Consolidated Statements of Income, Pre-tax	16.6
Unrealized losses (gains) realized with sale of Chaucer business, Pre-Tax		0.1	24.2
Net unrealized gains (losses), Pre-Tax	280.2	324.7	(179.5)
Net gains (losses) arising in the period from net actuarial (gains) losses, Pre-Tax	3.9	20.5	(21.3)
Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost, Pre-Tax	6	11.4	9.7
Pension obligations recognized with sale of Chaucer business, Pre-Tax			21.2
Total pension and postretirement benefits, Pre-Tax	9.9	31.9	9.6
Foreign currency translation recognized during the period, Pre-Tax			(2.2)
Currency translation obligation recognized with sale of Chaucer business, Pre-Tax		0.9	29.4
Total cumulative foreign currency translation adjustment, Pre-Tax		0.9	27.2
Other comprehensive income (loss), Pre-Tax	290.1	357.5	(142.7)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	(57.7)	(68.5)	43.3
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	(0.2)	(0.4)	0.7
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(6.4)	(13.7)	(9.7)
Amount of credit losses recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(0.3)	(0.4)	(0.7)
Amount of additional impairment losses recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(3.5)
Unrealized losses (gains) realized with sale of Chaucer business, Tax Benefit (Expense)			(5.1)
Net unrealized gains (losses), Tax Benefit (Expense)	(68.1)	(83)	28.5
Net gains (losses) arising in the period from net actuarial (gains) losses, Tax Benefit (Expense)	(0.8)	(4.3)	4.3
Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost, Tax Benefit (Expense)	(1.3)	(2.4)	(2)
Pension obligations recognized with sale of Chaucer business, Tax Benefit (Expense)			(4.8)
Total pension and postretirement benefits, Tax Benefit (Expense)	(2.1)	(6.7)	(2.5)
Foreign currency translation recognized during the period, Tax Benefit (Expense)			0.5
Currency translation obligation recognized with sale of Chaucer business, Tax Benefit (Expense)		(0.2)	(6.2)
Total cumulative foreign currency translation adjustment, Tax Benefit (Expense)		(0.2)	(5.7)
Other comprehensive income (loss), Tax Benefit (Expense)	(70.2)	(89.9)	20.3
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Net of Tax	216.9	257.5	(162.9)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Net of Tax	1.1	1.1	(2.8)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Net of Tax	(19.9)	(18.7)	(7.2)
Amount of credit losses recognized in Consolidated Statements of Income, Net of Tax	0.9	1.7	2.8
Amount of additional impairment losses recognized in Consolidated Statements of Income, Net of Tax	13.1
Unrealized losses (gains) realized with sale of Chaucer business, Net of Tax		0.1	19.1
Total available-for-sale securities	212.1	241.7	(151)
Net actuarial gains (losses) and prior service costs arising in the period	3.1	16.2	(17)
Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost, Net of Tax	4.7	9	7.7
Pension obligations recognized with sale of Chaucer business, Net of Tax			16.4
Total pension and postretirement benefits	7.8	25.2	7.1
Foreign currency translation recognized during the period, Net of Tax			(1.7)
Currency translation obligation recognized with sale of Chaucer business, Net of Tax		0.7	23.2
Total cumulative foreign currency translation adjustment		0.7	21.5
Other comprehensive income, net of tax	$ 219.9	$ 267.6	$ (122.4)

Other Comprehensive Income (Rec

Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other									$ 17.9	$ 4.9	$ (2.7)
Income from continuing operations before income taxes									444.8	522.1	282.5
Loss adjustment expenses and other operating expenses									(2,845.2)	(2,865.5)	(2,724.6)
Tax benefit									(82.8)	(93.1)	(43.5)
Income from continuing operations	$ 165.9	$ 119.5	$ 115.3	$ (38.7)	$ 110.2	$ 111.2	$ 85	$ 122.6	362	429	239
Gain on sale of Chaucer business										(1.2)	131.9
Net of tax	$ 164.6	$ 118.9	$ 115.2	$ (40)	$ 109.8	$ 118.9	$ 74	$ 122.4	358.7	425.1	391
Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses											(515.5)
Income (loss) from discontinued businesses, net of taxes									0.4	1.6	20
Gain from sale of Chaucer business										(1.2)	131.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									1.2	7.2	(62)
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									13.4	5	(3.2)
Impairment losses on investments									(17.6)	(2)	(2.6)
Income from continuing operations before income taxes									(4.2)	3	(5.8)
Tax benefit									10.2	14.1	10.3
Income from continuing operations									6	17.1	4.5
Gain from sale of Chaucer business									(0.1)	(0.1)	0.1
Net of tax									5.9	16.9	(14.7)
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]
Gain on sale of Chaucer business										(0.1)	(19.1)
Income (loss) from discontinued businesses, net of taxes											(0.2)
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									(6)	(11.4)	(9.5)
Tax benefit									1.3	2.4	1.9
Income from continuing operations									(4.7)	(9)	(7.6)
Gain on sale of Chaucer business											(16.4)
Income (loss) from discontinued businesses, net of taxes											(0.1)
Net of tax									$ (4.7)	(9)	(24.1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Currency Translation Obligation Recognized With Sale [Member] | Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of Chaucer business										$ (0.7)	$ (23.2)

Other Comprehensive Income (R_2

Other Comprehensive Income (Reclassifications out of Accumulated Other Comprehensive Income) (Parenthetical) (Details) - Defined Benefit Pension and Postretirement Plans [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses	40.00%	40.00%	40.00%
Percentage of other operating expenses	60.00%	60.00%	60.00%

Stock-based Compensation Plan_2

Stock-based Compensation Plans (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		May 20, 2014	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Arrangement by Share Based Payment Award, Reduction in Shares Available for Grant Per Share Issued		3.8
Stock-based compensation expense			$ 20.1	$ 17.4	$ 14.9
Tax benefit from compensation expense			$ 4.2	3.7	3.1
Vesting period			3 years
Award expiration Period			10 years
Cash received for options exercised			$ 6.3	14.4	14.8
Share-based Compensation Arrangement by Share-Based Payment Award, Options, Tax Benefit Realized from Options Exercised			2.1	$ 1.7	$ 1.6
Aggregate intrinsic value, outstanding			29.9
Aggregate intrinsic value, exercisable			$ 29.1
Weighted average remaining contractual life, outstanding (in years)			6 years 7 months 6 days
Weighted average remaining contractual life, exercisable (in years)			5 years 8 months 12 days
Weighted average grant date fair value of options granted			$ 14.45	$ 18.12	$ 16.30
Fair value of vested shares			$ 4.6	$ 9.5
Vesting Each Year [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting rate			33.33%	33.33%	33.33%
ESPP Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized under plan		2,500,000
Number of shares available for grant			2,332,094
Option or SAR [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share Based Compensation Arrangement by Share Based Payment Award, Reduction in Shares Available for Grant Per Share Issued		1
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense			$ 2.6
Unrecognized compensation expense, weighted average period of recognition			1 year 4 months 24 days
Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			3 years	3 years	3 years
Granted, Shares			21,379	24,410	31,866
Forfeited, Shares			1,949	1,674	10,330
Market-Based Restricted Stock Units [Member] | 2017 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares			13,532	5,820
Increase as grant in period, percentage			100.00%		100.00%
Granted, Weighted Average Grant Date Fair Value			$ 80.92	$ 75.95
Market-Based Restricted Stock Units [Member] | 2016 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares				5,820
Increase as grant in period, percentage			100.00%
Granted, Weighted Average Grant Date Fair Value				$ 75.95
Market-Based Restricted Stock Units [Member] | 2015 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares						3,115
Increase as grant in period, percentage				100.00%
Granted, Weighted Average Grant Date Fair Value					$ 70.24
Time-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period			3 years	3 years	3 years
Granted, Shares			143,128	149,698	152,529
Granted, Weighted Average Grant Date Fair Value			$ 117.72	$ 117.60	$ 111.66
Forfeited, Shares	[1]		32,821	37,520	45,866
Performance and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in intrinsic value of awards vested			$ 2	$ 1.1	$ 0.6
Aggregate intrinsic value			$ 11.6
Weighted average remaining contractual life			1 year 3 months 18 days
Performance 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 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%
Performance-based restricted stock units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares			19,504	18,195
Forfeited, Shares			1,074	996	43,449
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%
Restricted Stock and Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase in intrinsic value of awards vested			$ 4.5	$ 3.7	$ 2.9
Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Aggregate intrinsic value			$ 40.8
Weighted average remaining contractual life			1 year 3 months 18 days
Unvested Restricted Stock Units and Performance and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense			$ 25.1
Unrecognized compensation expense, weighted average period of recognition			1 year 9 months 18 days
2014 Stock Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized under plan		6,100,000
Number of shares available for grant			2,790,071
Cash received for options exercised			$ 3.9	12.3	14.3
Intrinsic value of options exercised			$ 2.7	$ 10.1	$ 9.6
[1]	As a result of the sale of Chaucer and included in forfeitures in 2018 are 19,655 shares of time-based restricted stock units, 43,449 shares of performance-based restricted stock units and 2,705 shares of market-based restricted stock units that were awarded to Chaucer employees.

Stock-based Compensation Plan_3

Stock-based Compensation Plans (Summary of Stock Option Plan Activity) (Details) - $ / shares		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares
Outstanding, beginning of period, Shares		1,121,559	1,099,076	1,062,177
Granted, Shares	[1]	242,598	252,813	301,152
Exercised, Shares		(54,721)	(191,601)	(210,190)
Forfeited or cancelled, Shares		(27,158)	(38,729)	(54,063)
Outstanding, end of period, Shares		1,282,278	1,121,559	1,099,076
Exercisable, end of period, Shares		852,804	662,555	515,286
Weighted Average Exercise Price
Outstanding, beginning of period, Weighted Average Exercise Price		$ 87.88	$ 85.75	$ 75.53
Granted, Weighted Average Exercise Price	[1]	118.34	119.36	110.98
Exercised, Weighted Average Exercise Price		75.86	71.20	69.41
Forfeited or cancelled, Weighted Average Exercise Price		112.18	107.34	89.09
Outstanding, end of period, Weighted Average Exercise Price		93.64	87.88	85.75
Exercisable, end of period, Weighted Average Exercise Price		$ 82.76	$ 75.63	$ 69.83
[1]	In accordance with plan provisions, 2019 includes 67,605 options related to special dividends paid by the Company in January 2019 and December 2019, in order to retain the intrinsic value of outstanding awards. The remaining 185,208 option awards were granted at an exercise price of $119.36.

Stock-based Compensation Plan_4

Stock-based Compensation Plans (Summary of Stock Option Plan Activity) (Parenthetical) (Details) - $ / shares		1 Months Ended	12 Months Ended
		Jan. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options related to special dividends paid		67,605		67,605
Remaining option awards granted			185,208
Granted, Weighted Average Exercise Price	[1]		$ 118.34	$ 119.36	$ 110.98
[1]	In accordance with plan provisions, 2019 includes 67,605 options related to special dividends paid by the Company in January 2019 and December 2019, in order to retain the intrinsic value of outstanding awards. The remaining 185,208 option awards were granted at an exercise price of $119.36.

Stock-based Compensation Plan_5

Stock-based Compensation Plans (Schedule of Stock Options by Exercise Price Range) (Details)	12 Months Ended
	Dec. 31, 2020$ / sharesshares
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price	$ 54.61
Options Outstanding, Number | shares	66,107
Options Outstanding, Weighted Average Remaining Contractual Lives	3 years 1 month 20 days
Options Outstanding, Weighted Average Exercise Price	$ 54.61
Options Currently Exercisable, Number | shares	66,107
Options Currently Exercisable, Weighted Average Exercise Price	$ 54.61
Exercise Price Range One [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Range of Exercise Prices, Lower Range Limit	34.66
Range of Exercise Prices, Upper Range Limit	$ 40.01
Options Outstanding, Number | shares	42,573
Options Outstanding, Weighted Average Remaining Contractual Lives	1 year 9 months 21 days
Options Outstanding, Weighted Average Exercise Price	$ 38.33
Options Currently Exercisable, Number | shares	42,573
Options Currently Exercisable, Weighted Average Exercise Price	$ 38.33
Exercise Price Range Two [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Range of Exercise Prices, Lower Range Limit	66.14
Range of Exercise Prices, Upper Range Limit	$ 68.41
Options Outstanding, Number | shares	102,996
Options Outstanding, Weighted Average Remaining Contractual Lives	4 years 1 month 28 days
Options Outstanding, Weighted Average Exercise Price	$ 66.15
Options Currently Exercisable, Number | shares	102,996
Options Currently Exercisable, Weighted Average Exercise Price	$ 66.15
Exercise Price Range Three [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Range of Exercise Prices, Lower Range Limit	70.51
Range of Exercise Prices, Upper Range Limit	$ 77.91
Options Outstanding, Number | shares	154,014
Options Outstanding, Weighted Average Remaining Contractual Lives	5 years 4 months 6 days
Options Outstanding, Weighted Average Exercise Price	$ 75.45
Options Currently Exercisable, Number | shares	154,014
Options Currently Exercisable, Weighted Average Exercise Price	$ 75.45
Exercise Price Range Four [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Range of Exercise Prices, Lower Range Limit	82.39
Range of Exercise Prices, Upper Range Limit	$ 92.68
Options Outstanding, Number | shares	246,844
Options Outstanding, Weighted Average Remaining Contractual Lives	6 years 2 months 19 days
Options Outstanding, Weighted Average Exercise Price	$ 85.39
Options Currently Exercisable, Number | shares	244,951
Options Currently Exercisable, Weighted Average Exercise Price	$ 85.33
Exercise Price Range Five [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Range of Exercise Prices, Lower Range Limit	104.11
Range of Exercise Prices, Upper Range Limit	$ 105.53
Options Outstanding, Number | shares	266,274
Options Outstanding, Weighted Average Remaining Contractual Lives	7 years 1 month 28 days
Options Outstanding, Weighted Average Exercise Price	$ 104.12
Options Currently Exercisable, Number | shares	181,613
Options Currently Exercisable, Weighted Average Exercise Price	$ 104.12
Exercise Price Range Six [Member]
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Range of Exercise Prices, Lower Range Limit	113.32
Range of Exercise Prices, Upper Range Limit	$ 118.54
Options Outstanding, Number | shares	403,470
Options Outstanding, Weighted Average Remaining Contractual Lives	8 years 9 months
Options Outstanding, Weighted Average Exercise Price	$ 117.96
Options Currently Exercisable, Number | shares	60,550
Options Currently Exercisable, Weighted Average Exercise Price	$ 117.07

Stock-based Compensation Plan_6

Stock-based Compensation Plans (Schedule of Stock Option Valuation Assumptions) (Details) - Employee Stock Option [Member]	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield		2.011%
Expected volatility, minimum	17.671%	18.495%	17.943%
Expected volatility, maximum	24.495%	19.10%	18.50%
Weighted average expected volatility	18.10%	18.86%	18.20%
Risk-free interest rate, minimum	0.262%	2.527%	2.365%
Risk-free interest rate, maximum	1.042%	2.617%	2.969%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield	2.193%		1.74%
Expected term, in years	2 years 6 months	2 years 6 months	2 years 6 months
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield	2.805%		1.954%
Expected term, in years	6 years 6 months	6 years 6 months	6 years

Stock-based Compensation Plan_7

Stock-based Compensation Plans (Summary of Restricted Stock Activity) (Details) - $ / shares		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Time-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		335,166	332,481	298,528
Granted, Shares		143,128	149,698	152,529
Vested, Shares		(94,993)	(109,493)	(72,710)
Forfeited, Shares	[1]	(32,821)	(37,520)	(45,866)
Outstanding, end of period, Shares		350,480	335,166	332,481
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 109.55	$ 97.28	$ 83.45
Granted, Weighted Average Grant Date Fair Value		117.72	117.60	111.66
Vested, Weighted Average Grant Date Fair Value		94.54	84.57	72.69
Forfeited, Weighted Average Grant Date Fair Value		115.74	106.03	94.04
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 116.37	$ 109.55	$ 97.28
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		86,252	69,838	102,586
Granted, Shares		54,415	42,605	35,063
Vested, Shares		(40,601)	(23,521)	(14,032)
Forfeited, Shares	[1]	(3,023)	(2,670)	(53,779)
Outstanding, end of period, Shares		97,043	86,252	69,838
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 110.70	$ 95.58	$ 81.21
Granted, Weighted Average Grant Date Fair Value		105.10	116.67	118.60
Vested, Weighted Average Grant Date Fair Value		80.92	79.57	70.24
Forfeited, Weighted Average Grant Date Fair Value		124.78	135.92	89.79
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 119.59	$ 110.70	$ 95.58
[1]	As a result of the sale of Chaucer and included in forfeitures in 2018 are 19,655 shares of time-based restricted stock units, 43,449 shares of performance-based restricted stock units and 2,705 shares of market-based restricted stock units that were awarded to Chaucer employees.

Stock-based Compensation Plan_8

Stock-based Compensation Plans (Summary of Restricted Stock Activity) (Parenthetical) (Details)	12 Months Ended
	Dec. 31, 2018shares
Time-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted Stock Award, Forfeitures	19,655
Performance-based restricted stock units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted Stock Award, Forfeitures	43,449
Market-based restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted Stock Award, Forfeitures	2,705

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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic shares used in the calculation of earnings per share	37.7	40	42.4
Diluted shares used in the calculation of earnings per share	38.1	40.6	43
Per share effect of dilutive securities on income from continuing operations	$ (0.11)	$ (0.16)	$ (0.07)
Per share effect of dilutive securities on net income	$ (0.10)	$ (0.16)	$ (0.12)
Employee Stock Option [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.2	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	0.2	0.3	0.3

Earnings Per Share and Shareh_4

Earnings Per Share and Shareholders' Equity Transactions (Narrative) (Details) - USD ($)	Jan. 29, 2021	Oct. 29, 2020	Feb. 26, 2020	Dec. 09, 2019	Jun. 28, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020
Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Stock repurchase program, available authorized repurchase amount						$ 124,000,000			$ 124,000,000
Repurchased common shares									5,200,000
Accelerated Share Repurchase Agreement [Member] | Maximum [Member]
Class Of Stock [Line Items]
Repurchases common stock, authorized						$ 900,000,000			$ 900,000,000
December 30, 2018 and June 28, 2019 Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Repurchased common shares					3,200,000
Repurchased common shares, amount					$ 400,000,000
December 2019 Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Repurchased common shares				900,000
Repurchased common shares, amount				$ 150,000,000
Common stock shares received as final settlement of shares repurchased			200,000
Shares repurchased under repurchase agreement, percentage				80.00%
October 2020 ASR [Member]
Class Of Stock [Line Items]
Amount paid by company		$ 100,000,000
Initial delivery of shares of common stock received		800,000
Shares repurchased under repurchase agreement, percentage		80.00%
October 2020 ASR [Member] | Subsequent Event [Member]
Class Of Stock [Line Items]
Common stock shares received as final settlement of shares repurchased	45,000
Stock Compensation Plans [Member]
Class Of Stock [Line Items]
Antidilutive securities excluded from calculation of earnings per share						400,000	200,000	300,000

Dividend Restrictions (Details)

Dividend Restrictions (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2021	May 31, 2021	Jan. 01, 2021
NEW HAMPSHIRE [Member]
Statutory Accounting Practices [Line Items]
Percentage of policyholder's surplus distributes as dividends and other distributions		10.00%
Dividends declared	$ 245,000,000	$ 245,000,000	$ 140,000,000	$ 140,000,000
Dividends paid to parent		$ 245,000,000	140,000,000	140,000,000
NEW HAMPSHIRE [Member] | Scenario Forecast [Member]
Statutory Accounting Practices [Line Items]
Statutory amount available for dividend payments						$ 258,300,000
NEW HAMPSHIRE [Member] | Maximum [Member] | Scenario Forecast [Member]
Statutory Accounting Practices [Line Items]
Maximum dividends payable without prior approval of state regulators						$ 245,000,000
NEW HAMPSHIRE [Member] | Maximum [Member] | Subsequent Event [Member]
Statutory Accounting Practices [Line Items]
Maximum dividends payable without prior approval of state regulators							$ 13,300,000
MICHIGAN [Member]
Statutory Accounting Practices [Line Items]
Percentage of policyholder's surplus distributes as dividends and other distributions		10.00%
Dividends declared			106,000,000	87,900,000
Dividends paid to parent			$ 106,000,000	$ 87,900,000
Extraordinary dividends declared	82,000,000	$ 82,000,000
Extraordinary dividends paid to parent	$ 82,000,000
MICHIGAN [Member] | Scenario Forecast [Member]
Statutory Accounting Practices [Line Items]
Statutory amount available for dividend payments					$ 108,700,000
MICHIGAN [Member] | Maximum [Member] | Scenario Forecast [Member]
Statutory Accounting Practices [Line Items]
Maximum dividends payable without prior approval of state regulators					$ 82,000,000
MICHIGAN [Member] | Maximum [Member] | Subsequent Event [Member]
Statutory Accounting Practices [Line Items]
Maximum dividends payable without prior approval of state regulators							$ 26,700,000

Segment Information (Narrative)

Segment Information (Narrative) (Details)	12 Months Ended
	Dec. 31, 2020segment
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								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating revenues									$ 4,822.3	$ 4,781.3	$ 4,545
Net realized and unrealized investment gains (losses)									5	109.4	(50.7)
Total revenues	$ 1,297	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5	4,827.3	4,890.7	4,494.3
Net investment income									265.1	281.3	267.4
Operating income (loss) before interest expense and income taxes									484.7	453.6	406.5
Interest on debt									(37.1)	(37.5)	(45.1)
Operating income before income taxes									447.6	416.1	361.4
Net loss from repayment of debt									(6.2)		(28.2)
Other non-operating items									(1.6)	(3.4)
Income from continuing operations before income taxes									444.8	522.1	282.5
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,870.8	2,843.5	2,739.5
Underwriting income (loss)									106.3	121.5	85
Net investment income									175.3	180.1	182.2
Other income (expense)									(6.2)	(1.5)	(1.5)
Operating income (loss) before interest expense and income taxes									275.4	300.1	265.7
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,931.4	1,911.8	1,791.3
Underwriting income (loss)									132.9	59.1	66.6
Net investment income									76.7	80.1	73.7
Other income (expense)									2.9	5.7	5.9
Operating income (loss) before interest expense and income taxes									212.5	144.9	146.2
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues									20.1	26	14.2
Underwriting income (loss)									(4.3)	(1.3)	(4.2)
Net investment income									13.1	21.1	11.5
Other income (expense)									(12)	(11.2)	(12.7)
Operating income (loss) before interest expense and income taxes									$ (3.2)	$ 8.6	$ (5.4)

Segment Information (Identifiab

Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total assets	$ 13,443.7	$ 12,490.5
Identifiable assets	110.2	102.8
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	110.2	102.8
Property and Casualty [Member]
Segment Reporting Information [Line Items]
Total assets	$ 13,333.5	$ 12,387.7

Reinsurance (Narrative) (Detail

Reinsurance (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded	$ 561	$ 505.1	$ 445.8
Property and casualty losses and LAE, Ceded	371.2	379.2	286.8
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,874.3	1,814
MCCA [Member]
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded	84.6	84.3	70.9
Property and casualty losses and LAE, Ceded	99.2	108.5	$ 108.8
Reinsurance recoverable on paid and unpaid losses and unearned premiums	$ 1,024.7	$ 1,023.7
MCCA [Member] | Reinsurance Receivable [Member] | Reinsurer Concentration Risk [Member]
Effects of Reinsurance [Line Items]
Percentage of reinsurance receivable represented by segment	54.70%

Reinsurance (Schedule of Effect

Reinsurance (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance Disclosures [Abstract]
Property and casualty premiums written, Direct	$ 5,131.4	$ 5,090.8	$ 4,816
Property and casualty premiums written, Assumed	23.1	27.7	27.6
Property and casualty premiums written, Ceded	(556)	(536.8)	(458.8)
Property and casualty premiums written, Net	4,598.5	4,581.7	4,384.8
Property and casualty premiums earned, Direct	5,063.8	4,953.3	4,673.6
Property and casualty premiums earned, Assumed	24.6	26.3	26.6
Property and casualty premiums earned, Ceded	(561)	(505.1)	(445.8)
Premiums Earned, Net, Total	$ 4,527.4	$ 4,474.5	$ 4,254.4
Percentage of assumed to net premiums earned	0.50%	0.60%	0.60%
Property and casualty losses and LAE, Direct	$ 3,200.5	$ 3,223.3	$ 2,986.2
Property and casualty losses and LAE, Assumed	15.9	21.4	25.2
Property and casualty losses and LAE, Ceded	(371.2)	(379.2)	(286.8)
Policyholder Benefits and Claims Incurred, Net, Total	$ 2,845.2	$ 2,865.5	$ 2,724.6

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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Gross reserve for losses and LAE, beginning of year	$ 5,654.4	$ 5,304.1	$ 5,058.5
Reinsurance recoverable on unpaid losses, beginning of year	1,574.8	1,472.6	1,455
Net loss and LAE reserves, beginning of year	4,079.6	3,831.5	3,603.5
Net incurred losses and LAE in respect of losses occurring in current year	2,877.8	2,893	2,733.5
Net incurred losses and LAE in respect of losses occurring in prior years	(32.6)	(28.4)	(8.9)
Total incurred losses and LAE	2,845.2	2,864.6	2,724.6
Net payments of losses and LAE in respect of losses occurring in current year	1,347.7	1,315.4	1,232.3
Net payments of losses and LAE in respect of losses occurring in prior years	1,194.7	1,301.1	1,264.3
Total payments	2,542.4	2,616.5	2,496.6
Net reserve for losses and LAE, end of year	4,382.4	4,079.6	3,831.5
Reinsurance recoverable on unpaid losses, end of year	1,641.6	1,574.8	1,472.6
Gross reserve for losses and LAE, end of year	$ 6,024	$ 5,654.4	$ 5,304.1

Liabilities for Outstanding C_4

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of (Favorable)/Unfavorable Loss and LAE Reserve Development) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (32.6)	$ (28.4)	$ (8.9)
Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(37.8)	(53.3)	(40.9)
Commercial Lines [Member] | Commercial Multiple Peril Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(0.2)	(19.5)	(4)
Commercial Lines [Member] | Workers' Compensation Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(36)	(32.6)	(31)
Commercial Lines [Member] | Commercial Automobile Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	15.4	5.8	23.2
Commercial Lines [Member] | Hanover Programs Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	10.6	19.8	17
Commercial Lines [Member] | Marine Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(17.6)	(13.3)	(10.7)
Commercial Lines [Member] | Surety Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(3.5)	(4.1)	(9)
Commercial Lines [Member] | Umbrella Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(1.5)	2.8	2.3
Commercial Lines [Member] | Other Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(5.3)	(4.4)	(0.8)
Commercial Lines [Member] | Other Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(17)	(7)	(29.1)
Commercial Lines [Member] | General Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	0.3	(7.8)	(27.9)
Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	1	23.7	30.8
Personal Lines [Member] | Personal Automobile Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	5.2	21.6	14.3
Personal Lines [Member] | Homeowners and Other Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	(4.2)	2.1	16.5
Other Segment [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ 4.2	$ 1.2	$ 1.2

Liabilities for Outstanding C_5

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (32.6)	$ (28.4)	$ (8.9)
Loss and loss adjustment expense reserves	$ 6,024	5,654.4	5,304.1	$ 5,058.5
U.S. Companies [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross incurred claims, percentage	4.00%
Asbestos Issue [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 57.7	55.5
Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(37.8)	(53.3)	(40.9)
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	1	23.7	30.8
Loss and loss adjustment expense reserves	1,907.8	1,839.4
Other Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	4.2	1.2	1.2
Workers' Compensation Line [Member] | Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(36)	(32.6)	(31)
Personal Automobile Lines [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	5.2	21.6	14.3
Loss and loss adjustment expense reserves	1,649.9	1,626.4
Commercial Automobile Line [Member] | Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	15.4	5.8	23.2
Other Commercial Lines [Member] | Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(17)	(7)	(29.1)
Years 2016 to 2019 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (36)
Years 2015 to 2018 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(32.6)
Years 2016 to 2017 [Member] | Personal Automobile Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		$ (21.6)
Years 2014, 2016 and 2017 [Member] | Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			(23.2)
Years 2014, 2016 and 2017 [Member] | Other Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			(29.1)
Years 2015 to 2017 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			(31)
Years 2015 and 2016 [Member] | Personal Automobile Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development			$ (14.3)

Liabilities for Outstanding C_6

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	$ 6,024	$ 5,654.4	$ 5,304.1	$ 5,058.5
Ceded	(1,641.6)	(1,574.8)	(1,472.6)	(1,455)
Net	4,382.4	4,079.6	$ 3,831.5	$ 3,603.5
Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	4,116.2	3,815
Ceded	(668.7)	(604.9)
Net	3,447.5	3,210.1
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,907.8	1,839.4
Ceded	(972.9)	(969.9)
Net	934.9	869.5
Commercial Multiple Peril Line [Member] | Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,429.4	1,298.5
Ceded	(163.9)	(126.5)
Net	1,265.5	1,172
Workers' Compensation Line [Member] | Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	795.4	795.3
Ceded	(182.6)	(186)
Net	612.8	609.3
Commercial Automobile Line [Member] | Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	504.1	467.7
Ceded	(30.6)	(27.3)
Net	473.5	440.4
General Liability and Umbrella - Occurrence [Member] | Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	612	575.9
Ceded	(176)	(155.5)
Net	436	420.4
General Liability - Claims Made [Member] | Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	323.7	268.1
Ceded	(30.5)	(20.1)
Net	293.2	248
Other Commercial Lines [Member] | Commercial and Other Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	451.6	409.5
Ceded	(85.1)	(89.5)
Net	366.5	320
Personal Automobile Lines [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	1,649.9	1,626.4
Ceded	(968.5)	(963.2)
Net	681.4	663.2
Homeowners [Member] | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	210.6	172.4
Ceded	(3)	(4.8)
Net	207.6	167.6
Other Personal Lines | Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross	47.3	40.6
Ceded	(1.4)	(1.9)
Net	$ 45.9	$ 38.7

Liabilities for Outstanding C_7

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Incurred Claims Development) (Details) $ in Millions	Dec. 31, 2020USD ($)claim	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)
Commercial Multiple Peril Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	$ 4,078.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	2,889.2
Unallocated loss adjustment expense	24.1
Net reserves	1,265.5
Commercial Multiple Peril Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	393.6	$ 393.7	$ 393	$ 392.1	$ 391.8	$ 367.9	$ 362.5	$ 380
IBNR	$ 7.8
Cumulative Incurred Claim Count | claim	14,801
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 373.1	366.3	355.9	334.6	306.4	262.3	221.5	137.6
Commercial Multiple Peril Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	441	444.4	449.1	459.4	464.8	439.6	443.9
IBNR	$ 13.1
Cumulative Incurred Claim Count | claim	15,881
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 411.7	407.7	395.2	363.4	316	267.8	171.7
Commercial Multiple Peril Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	460.9	465.2	467.4	463.7	456.3	446
IBNR	$ 16.9
Cumulative Incurred Claim Count | claim	15,583
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 413.6	397	363.2	315.6	260.1	161.9
Commercial Multiple Peril Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	449.6	447.4	448.7	449.6	447.1
IBNR	$ 25.8
Cumulative Incurred Claim Count | claim	15,750
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 369.6	342.2	290.2	237.9	140.3
Commercial Multiple Peril Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	555.6	551.6	544.8	538.7
IBNR	$ 38.5
Cumulative Incurred Claim Count | claim	16,638
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 422.9	370.6	296.4	170.9
Commercial Multiple Peril Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	556.1	560.9	578.2
IBNR	$ 77.8
Cumulative Incurred Claim Count | claim	17,123
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 363.7	306.2	178.8
Commercial Multiple Peril Line [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	593	583.8
IBNR	$ 138
Cumulative Incurred Claim Count | claim	16,012
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 317.2	169.3
Commercial Multiple Peril Line [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	628.9
IBNR	$ 245.5
Cumulative Incurred Claim Count | claim	13,136
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 217.4
Commercial Multiple Peril Line [Member] | Years 2013 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	1,189.5
Commercial Multiple Peril Line [Member] | Years 2012 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	51.9
Workers' Compensation Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,596.1
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,087.2
Unallocated loss adjustment expense	19.3
Net reserves	612.8
Workers' Compensation Line [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	146.9	147.7	147.9	148.3	146.7	143.8	145.5	144.3	$ 144.2	$ 141.4
IBNR	$ 6.3
Cumulative Incurred Claim Count | claim	12,484
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 135.3	134.2	132.8	130.9	128	122.9	114	97.3	71.3	30
Workers' Compensation Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	161.4	161.9	163.7	163.7	162.9	157.2	165.2	171.1	176.3
IBNR	$ 8.4
Cumulative Incurred Claim Count | claim	13,083
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 146.6	143.7	141.4	136	130.7	120.1	102.6	74.8	30.4
Workers' Compensation Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	153.3	154.9	155.4	155.3	154.4	160.1	167.4	179.3
IBNR	$ 8.8
Cumulative Incurred Claim Count | claim	11,734
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 133.8	131.4	127.4	122.8	114	101.2	74.6	30.9
Workers' Compensation Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	145.8	148.2	150.3	152.1	154.7	172.9	182.1
IBNR	$ 11.8
Cumulative Incurred Claim Count | claim	10,943
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 121	117.8	112.5	105.6	92.3	70.5	30.6
Workers' Compensation Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	142.7	146	150.3	157.5	164.2	189.6
IBNR	$ 16
Cumulative Incurred Claim Count | claim	11,485
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 109.6	105.8	99.4	87.2	65.7	28
Workers' Compensation Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	152.8	158.2	164.6	180.5	189.6
IBNR	$ 16.1
Cumulative Incurred Claim Count | claim	15,894
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 117.1	111.1	99.5	78.1	33.9
Workers' Compensation Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	156.6	160.8	172.1	186.1
IBNR	$ 20
Cumulative Incurred Claim Count | claim	16,722
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 106.6	94.1	73	32.8
Workers' Compensation Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	172.1	182.1	187.2
IBNR	$ 24.2
Cumulative Incurred Claim Count | claim	17,454
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 103.4	80.7	35.6
Workers' Compensation Line [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182	187.9
IBNR	$ 30.1
Cumulative Incurred Claim Count | claim	17,163
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 83.1	33.3
Workers' Compensation Line [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.5
IBNR	$ 76.9
Cumulative Incurred Claim Count | claim	12,549
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 30.7
Workers' Compensation Line [Member] | Years 2011 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	508.9
Workers' Compensation Line [Member] | Years 2010 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	84.6
Commercial Automobile Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,261.3
Cumulative Paid Losses and ALAE, Net of Reinsurance	807.3
Unallocated loss adjustment expense	5.8
Net reserves	473.5
Commercial Automobile Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	183.5	185	185.1	181.7	177.3	163.3	168.5
IBNR	$ 1.7
Cumulative Incurred Claim Count | claim	13,477
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 178.2	176.3	168.2	137.1	102.7	70.8	33.1
Commercial Automobile Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	170.3	167.6	167.8	166.9	168.3	163.4
IBNR	$ 1.2
Cumulative Incurred Claim Count | claim	12,829
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 152.5	144.5	129.3	96.4	63.8	32.2
Commercial Automobile Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	172.5	174.3	163	157.7	157
IBNR	$ 4.3
Cumulative Incurred Claim Count | claim	11,660
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 147.2	134	98.3	60.7	27.8
Commercial Automobile Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	188.5	182.1	170.1	159.5
IBNR	$ 7.6
Cumulative Incurred Claim Count | claim	11,504
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 137.7	105.9	71.2	26.9
Commercial Automobile Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.9	176.1	182.8
IBNR	$ 25.5
Cumulative Incurred Claim Count | claim	11,181
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 92.5	59.7	29.2
Commercial Automobile Line [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	192	180.5
IBNR	$ 57
Cumulative Incurred Claim Count | claim	10,210
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 75.8	29.5
Commercial Automobile Line [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	171.6
IBNR	$ 115.9
Cumulative Incurred Claim Count | claim	5,772
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 23.4
Commercial Automobile Line [Member] | Years 2014 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	454
Commercial Automobile Line [Member] | Years 2013 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	13.7
General Liability and Umbrella - Occurrence [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	928.2
Cumulative Paid Losses and ALAE, Net of Reinsurance	524
Unallocated loss adjustment expense	13.7
Net reserves	436
General Liability and Umbrella - Occurrence [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	59.3	58.8	57.5	55.4	55.6	51	42.2	38.2	43.8	47.6
IBNR	$ 2.4
Cumulative Incurred Claim Count | claim	1,393
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 54.8	52.9	50.6	47.8	45.8	39.8	31.1	19.4	6.7	$ 1.6
General Liability and Umbrella - Occurrence [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	72.8	71.2	69.1	74.2	71.1	64.3	62.2	59.3	77.2
IBNR	$ 2.5
Cumulative Incurred Claim Count | claim	1,774
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 65	64.5	63	60.6	52.1	43.8	29.8	12.6	$ 2.2
General Liability and Umbrella - Occurrence [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	79.1	80.4	81.2	84.4	88.6	71.5	67.4	84.1
IBNR	$ 4.1
Cumulative Incurred Claim Count | claim	2,001
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 70.1	67.6	63.5	56.1	43.1	26.8	11	$ 2.4
General Liability and Umbrella - Occurrence [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	98.4	99.8	98.3	101.4	98.6	82.3	100.9
IBNR	$ 6.4
Cumulative Incurred Claim Count | claim	2,108
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 87.3	83.6	70.4	52.8	31.4	14.5	3.1
General Liability and Umbrella - Occurrence [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	99.7	98.3	97	99.6	99.3	104.2
IBNR	$ 9.9
Cumulative Incurred Claim Count | claim	2,448
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 77.3	65.7	48.5	30.8	15.2	3.3
General Liability and Umbrella - Occurrence [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	98.7	100.8	101.5	100.7	95.6
IBNR	$ 15.7
Cumulative Incurred Claim Count | claim	1,913
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 63.4	52.5	31.6	15	3.1
General Liability and Umbrella - Occurrence [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	110.7	109.2	107.6	97.7
IBNR	$ 24.9
Cumulative Incurred Claim Count | claim	1,806
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 49.8	34.5	17	4.4
General Liability and Umbrella - Occurrence [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	104	106.4	99.8
IBNR	$ 35.2
Cumulative Incurred Claim Count | claim	1,823
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 35.7	15.6	4.1
General Liability and Umbrella - Occurrence [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	105.7	104.2
IBNR	$ 55.8
Cumulative Incurred Claim Count | claim	1,605
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 16.7	7.5
General Liability and Umbrella - Occurrence [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	99.8
IBNR	$ 81
Cumulative Incurred Claim Count | claim	1,213
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 3.9
General Liability and Umbrella - Occurrence [Member] | Years 2011 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	404.2
General Liability and Umbrella - Occurrence [Member] | Years 2010 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	18.1
General Liability - Claims Made [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	767
Cumulative Paid Losses and ALAE, Net of Reinsurance	482.8
Unallocated loss adjustment expense	5.1
Net reserves	293.1
General Liability - Claims Made [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	82.4	82.9	82.5	86.2	83.9	69.9	61.7
IBNR	$ 0.9
Cumulative Incurred Claim Count | claim	906
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 79.9	79.1	75.4	71.5	59.1	38.8	$ 11
General Liability - Claims Made [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	91	90.3	90.2	98.4	98.8	93.2
IBNR	$ 1.9
Cumulative Incurred Claim Count | claim	1,002
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 83.7	81.8	76	64.2	43.6	$ 10
General Liability - Claims Made [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	95.2	89.7	97.1	101.7	103.6
IBNR	$ 4.7
Cumulative Incurred Claim Count | claim	1,011
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 80.4	75.5	66.1	42.9	11.1
General Liability - Claims Made [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	96	98.1	104.6	103.3
IBNR	$ 8.8
Cumulative Incurred Claim Count | claim	1,122
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 76.5	67.2	42.3	12.3
General Liability - Claims Made [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	132.5	127.8	120.9
IBNR	$ 14.3
Cumulative Incurred Claim Count | claim	1,386
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 87.5	56.1	17.3
General Liability - Claims Made [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	127.1	126.8
IBNR	$ 31.7
Cumulative Incurred Claim Count | claim	1,554
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 56.7	17.8
General Liability - Claims Made [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	142.8
IBNR	$ 76.3
Cumulative Incurred Claim Count | claim	2,514
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 18.1
General Liability - Claims Made [Member] | Years 2014 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	284.2
General Liability - Claims Made [Member] | Years 2013 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	3.8
Personal Automobile Lines [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,970
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,355.6
Unallocated loss adjustment expense	15.3
Net reserves	681.4
Personal Automobile Lines [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	367.7	365.8	355.3	344.5	337.9
IBNR	$ 4.2
Cumulative Incurred Claim Count | claim	42,375
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 346.3	329.4	288	213.1	$ 112.8
Personal Automobile Lines [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	392.2	383.4	362.3	363.6
IBNR	$ 9.7
Cumulative Incurred Claim Count | claim	42,993
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 343.8	302.7	229.2	115
Personal Automobile Lines [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	400.3	395.3	394.2
IBNR	$ 23.9
Cumulative Incurred Claim Count | claim	42,841
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 307.4	237.2	121.7
Personal Automobile Lines [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	431.8	431.2
IBNR	$ 67
Cumulative Incurred Claim Count | claim	41,703
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 262.2	131
Personal Automobile Lines [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	378
IBNR	$ 199.2
Cumulative Incurred Claim Count | claim	24,339
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 95.9
Personal Automobile Lines [Member] | Years 2016 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	614.4
Personal Automobile Lines [Member] | Years 2015 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	51.7
Homeowners [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,376.6
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,183
Unallocated loss adjustment expense	3.4
Net reserves	207.5
Homeowners [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	295.2	297.3	296.5	291.4
IBNR	$ 0.6
Cumulative Incurred Claim Count | claim	33,474
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 289	283.5	271.6	$ 204.5
Homeowners [Member] | Year 2018 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	317.3	316.2	312.9
IBNR	$ 6.1
Cumulative Incurred Claim Count | claim	32,725
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 300.8	290.1	$ 213.7
Homeowners [Member] | Year 2019 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	345.3	344.5
IBNR	$ 14.3
Cumulative Incurred Claim Count | claim	33,582
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 311.7	$ 234.7
Homeowners [Member] | Year 2020 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	418.8
IBNR	$ 77.2
Cumulative Incurred Claim Count | claim	31,392
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 281.5
Homeowners [Member] | Years 2017 to 2020 [Member]
Claims Development [Line Items]
Total reserves for accident years	193.6
Homeowners [Member] | Years 2016 and Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	$ 10.5

Liabilities for Outstanding C_8

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Computation of Historical Claims on Paid and Incurred Claims Data, Net of Reinsurance) (Details)	Dec. 31, 2020
Commercial Multiple Peril Line [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	33.30%
2	22.40%
3	11.40%
4	10.70%
5	7.00%
6	4.00%
7	1.80%
8	1.70%
Workers' Compensation Line [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	19.90%
2	27.40%
3	15.40%
4	9.10%
5	5.20%
6	3.20%
7	2.50%
8	1.40%
9	1.30%
10	0.70%
Commercial Automobile Line [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	16.00%
2	20.40%
3	18.90%
4	18.90%
5	11.20%
6	4.50%
7	1.00%
General Liability and Umbrella - Occurrence [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	3.70%
2	11.10%
3	18.70%
4	19.10%
5	14.80%
6	11.30%
7	3.90%
8	3.30%
9	2.30%
10	3.30%
General Liability - Claims Made [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	12.70%
2	32.50%
3	24.30%
4	11.90%
5	5.40%
6	3.30%
7	0.90%
Personal Automobile Lines [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	29.20%
2	28.90%
3	18.90%
4	10.90%
5	4.60%
Homeowners [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	68.00%
2	23.00%
3	3.70%
4	1.80%

Statutory Financial Informati_3

Statutory Financial Information (Statutory Accounting Practices Disclosure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statutory Accounting Practices [Abstract]
Statutory Net Income	$ 360.9	$ 348.4	$ 296.1
Statutory Capital and Surplus	$ 2,588.5	$ 2,470.2	$ 2,172.5

Statutory Financial Informati_4

Statutory Financial Information (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statutory Accounting Practices [Abstract]
Minimum statutory capital and surplus required	$ 552.6	$ 531.8	$ 518.4

Quarterly Results of Operatio_3

Quarterly Results of Operations (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 05, 2019	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Total revenues		$ 1,297	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9		$ 1,214.7		$ 1,198.6		$ 1,219.5		$ 4,827.3	$ 4,890.7	$ 4,494.3
Income (loss) from continuing operations		165.9	119.5	115.3	(38.7)	110.2		111.2		85		122.6		362	429	239
Net income		$ 164.6	$ 118.9	$ 115.2	$ (40)	$ 109.8		$ 118.9		$ 74		$ 122.4		$ 358.7	$ 425.1	$ 391
Income from continuing operations per share, Basic		$ 4.52	$ 3.17	$ 3.03	$ (1.01)	$ 2.81		$ 2.81		$ 2.09		$ 3.02		$ 9.61	$ 10.72	$ 5.63
Income from continuing operations per share, Diluted		4.47	3.14	3.01	(1.01)	2.77		2.77		2.06		2.98		9.50	10.56	5.56
Net income per share, Basic		4.49	3.15	3.03	(1.04)	2.80		3		1.82		3.01		9.52	10.62	9.21
Net income per share, Diluted		4.43	3.13	3.01	(1.04)	2.76		2.96		1.79		2.97		$ 9.42	$ 10.46	$ 9.09
Dividends declared per share	$ 2.50	$ 0.70	$ 0.65	$ 0.65	$ 0.65	$ 3.15	[1]	$ 0.60	[1]	$ 0.60	[1]	$ 0.60	[1]
[1]	On December 5, 2019 the Board of Directors declared a special cash dividend of $2.50 per share.

Quarterly Results of Operatio_4

Quarterly Results of Operations (Schedule of Quarterly Financial Information) (Parenthetical) (Details) - $ / shares

Dec. 05, 2019

Dec. 31, 2020

Sep. 30, 2020

Jun. 30, 2020

Mar. 31, 2020

Dec. 31, 2019

Sep. 30, 2019

Jun. 30, 2019

Mar. 31, 2019

Quarterly Financial Information Disclosure [Abstract]

Special dividend declared per share

$ 2.50

$ 0.70

$ 0.65

$ 0.65

$ 0.65

$ 3.15

[1]

$ 0.60

[1]

$ 0.60

[1]

$ 0.60

[1]

[1]

On December 5, 2019 the Board of Directors declared a special cash dividend of $2.50 per share.

Quarterly Results of Operatio_5

Quarterly Results of Operations (Narrative) (Details) - October 2020 Accelerated Share Repurchase Agreement [Member] - USD ($) $ in Millions	Jan. 29, 2021	Oct. 29, 2020	Dec. 31, 2020
Class Of Stock [Line Items]
Amount paid by company			$ 100
Initial delivery of shares of common stock received		800,000
Shares repurchased under repurchase agreement, percentage		80.00%
Subsequent Event [Member]
Class Of Stock [Line Items]
Common stock shares received as final settlement of shares repurchased	45,000

SCHEDULE I SUMMARY OF INVESTM_2

SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2020USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 8,131.9	[1]
Fair Value	8,880.6
Amount at which shown in the balance sheet	8,846.1	[2]
U.S. Treasury and Government Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,353.1	[1]
Fair Value	1,401.1
Amount at which shown in the balance sheet	1,401.1	[2]
States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,042.1	[1]
Fair Value	1,103.7
Amount at which shown in the balance sheet	1,103.7	[2]
Foreign governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2.2	[1]
Fair Value	2.7
Amount at which shown in the balance sheet	2.7	[2]
Public Utilities Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	392.3	[1]
Fair Value	430.1
Amount at which shown in the balance sheet	430.1	[2]
All other corporate bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,156	[1]
Fair Value	4,516.8
Amount at which shown in the balance sheet	4,516.8	[2]
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,945.7	[1]
Fair Value	7,454.4
Amount at which shown in the balance sheet	7,454.4	[2]
Public utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	56.8	[1]
Fair Value	89.6
Amount at which shown in the balance sheet	89.6	[2]
Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	44.1	[1]
Fair Value	47.6
Amount at which shown in the balance sheet	47.6	[2]
Industrial, miscellaneous and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	302	[1]
Fair Value	456.9
Amount at which shown in the balance sheet	456.9	[2]
Nonredeemable preferred stock [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1	[1]
Fair Value	4.4
Amount at which shown in the balance sheet	4.4	[2]
Equity Securities Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	403.9	[1]
Fair Value	598.5
Amount at which shown in the balance sheet	598.5	[2]
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	475.5	[1]
Fair Value	502.1
Amount at which shown in the balance sheet	467.6	[2]
Real estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6.4	[1]
Fair Value	6.4
Amount at which shown in the balance sheet	6.4	[2]
Other long-term investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	300.4	[1]
Fair Value	319.2	[1]
Amount at which shown in the balance sheet	$ 319.2	[1],[2]
[1]	Original cost of equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of        premiums and accretion of discounts.
[2]	Mortgage loans on real estate are shown on the balance sheet net of the allowance for credit losses.

SCHEDULE II CONDENSED FINANCI_2

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF INCOME) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements Captions [Line Items]
Net investment income									$ 265.1	$ 281.3	$ 267.4
Net realized gains (losses) from sales and other									17.9	4.9	(2.7)
Total revenues	$ 1,297	$ 1,249.8	$ 1,224.3	$ 1,056.2	$ 1,257.9	$ 1,214.7	$ 1,198.6	$ 1,219.5	4,827.3	4,890.7	4,494.3
Interest expense									37.1	37.5	45.1
Loss on repayment of debt									6.2		28.2
Other operating expenses									543	538.9	522.1
Total losses and expenses									4,382.5	4,368.6	4,211.8
Income tax benefit									(82.8)	(93.1)	(43.5)
Income from continuing operations	165.9	119.5	115.3	(38.7)	110.2	111.2	85	122.6	362	429	239
Gain on sale of Chaucer business										(1.2)	131.9
Net income	$ 164.6	$ 118.9	$ 115.2	$ (40)	$ 109.8	$ 118.9	$ 74	$ 122.4	358.7	425.1	391
Other comprehensive income (loss), net of tax									219.9	267.6	(122.4)
Comprehensive income									578.6	692.7	268.6
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income											54.9
Total revenues										6.7	912.4
Interest expense											3.8
Other operating expenses											115
Total losses and expenses											883
Income (loss) from discontinued businesses, net of taxes									0.4	1.6	20
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income									10.7	19.8	11.8
Net realized gains (losses) from sales and other									0.4	5.3	(0.5)
Other income										0.4	0.8
Total revenues									11.1	25.5	12.1
Interest expense									37	37.4	37.8
Employee benefit related expenses									2.3	5	5.1
Interest expense on loan from subsidiary									7	6.9	6.8
Loss on repayment of debt									6.2		1.9
Other operating expenses									9.8	7.1	12.1
Total losses and expenses									62.3	56.4	63.7
Net loss before income taxes and equity in income of subsidiaries									(51.2)	(30.9)	(51.6)
Income tax benefit									22.1	25.6	21.4
Equity in income of subsidiaries									387.2	432.4	288.4
Income from continuing operations									358.1	427.1	258.2
Gain on sale of Chaucer business										(2.1)	131.9
Income (loss) from discontinued businesses, net of taxes									0.6	0.1	0.9
Net income									358.7	425.1	391
Other comprehensive income (loss), net of tax									219.9	267.6	(122.4)
Comprehensive income									$ 578.6	692.7	268.6
The Hanover Insurance Group [Member] | Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Gain on sale of Chaucer business										$ (2.1)	$ 131.9

SCHEDULE II CONDENSED FINANCI_3

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF INCOME) (Parenthetical) (Details) - USD ($) $ in Millions	Dec. 28, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements Captions [Line Items]
Income from discontinued operation, income tax expense		$ 0.5		$ (0.2)
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Income from discontinued operation, income tax expense	$ (42.5)		$ 5.3	$ (42.5)	[1]
[1]	The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that were no longer likely to be realized.

SCHEDULE II CONDENSED FINANCI_4

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets
Fair Value		$ 7,454.4	$ 6,687.1
Equity securities - at fair value		598.5	575.7
Cash and cash equivalents		120.6	215.7
Other assets		445.7	402.4
Total assets		13,443.7	12,490.5
Liabilities
Debt		780.8	653.4
Total liabilities		10,241.5	9,574.3
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		0.6	0.6
Additional paid-in capital		1,857.4	1,837.3
Accumulated other comprehensive income		372.5	152.6
Retained earnings		2,668	2,410.9
Treasury stock at cost (24.1 and 22.1 million shares)		(1,696.3)	(1,485.2)
Total shareholders’ equity		3,202.2	2,916.2	$ 2,954.7
Total liabilities and shareholders’ equity		13,443.7	12,490.5
The Hanover Insurance Group [Member]
Assets
Fair Value		381.3	328.5
Equity securities - at fair value		1.1	1.1
Cash and cash equivalents		15.6	12.2
Investments in subsidiaries		3,718.3	3,352.2
Net receivable from subsidiaries		25.2	25.1
Other assets		2.9	3.2
Total assets		4,144.4	3,722.3
Liabilities
Expenses and state taxes payable		23.2	16.4
Current income tax payable		2.7	3.5
Interest payable		10.5	7.8
Debt	[1]	905.8	778.4
Total liabilities		942.2	806.1
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		0.6	0.6
Additional paid-in capital		1,857.4	1,837.3
Accumulated other comprehensive income		372.5	152.6
Retained earnings		2,668	2,410.9
Treasury stock at cost (24.1 and 22.1 million shares)		(1,696.3)	(1,485.2)
Total shareholders’ equity		3,202.2	2,916.2
Total liabilities and shareholders’ equity		$ 4,144.4	$ 3,722.3
[1]	In 2020, the Company issued $300.0 million aggregate principal amount of 2.50% unsecured senior debentures. With the proceeds, THG retired its outstanding $175.0 million principal amount of 6.35% subordinated unsecured debentures. (See Note 6 – “Debt and Credit Arrangements” in Notes to Consolidated Financial Statements).

SCHEDULE II CONDENSED FINANCI_5

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Parenthetical) (Details) - USD ($)	Dec. 31, 2020	Sep. 19, 2020	Dec. 31, 2019
Condensed Financial Statements Captions [Line Items]
Fixed maturities, amortized cost	$ 6,945,600,000		$ 6,452,200,000
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	24,100,000		22,100,000
Senior debentures maturing September 1, 2030 [Member]
Condensed Financial Statements Captions [Line Items]
Aggregate principal amount	$ 300,000,000
Debt instrument interest rate	2.50%
Subordinated debentures maturing March 30, 2053 [Member]
Condensed Financial Statements Captions [Line Items]
Aggregate principal amount		$ 175,000,000
Debt instrument interest rate		6.35%
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Fixed maturities, amortized cost	$ 369,500,000		$ 321,200,000
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	24,100,000		22,100,000
The Hanover Insurance Group [Member] | Senior debentures maturing September 1, 2030 [Member]
Condensed Financial Statements Captions [Line Items]
Aggregate principal amount	$ 300,000,000
Debt instrument interest rate	2.50%
The Hanover Insurance Group [Member] | Subordinated debentures maturing March 30, 2053 [Member]
Condensed Financial Statements Captions [Line Items]
Aggregate principal amount	$ 175,000,000
Debt instrument interest rate	6.35%

SCHEDULE II CONDENSED FINANCI_6

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 358.7	$ 425.1	$ 391
Adjustments to reconcile net income to net cash provided by operating activities:
(Gain) loss from sale of Chaucer business		1.2	(131.9)
Loss on repayment of debt	6.2		28.2
Deferred income tax benefit	(23)	12.5	(23.4)
Change in expenses and taxes payable	67.5	11	4.6
Other, net	(24.7)	(14.7)	(31.7)
Net cash provided by operating activities	707.6	602.9	551.3
Cash flows from investing activities
Proceeds from disposals of equity securities and other investments	172.8	250.3	158.4
Purchase of fixed maturities	(1,829.1)	(1,588.3)	(1,425.6)
Purchase of equity securities and other investments	(255.1)	(332.2)	(178.3)
Net cash received from sale of Chaucer business		34.7	635.7
Net cash provided by (used in) investing activities	(608.8)	(311.9)	271.3
Cash flows from financing activities
Proceeds from exercise of employee stock options	6.3	14.4	14.8
Proceeds from debt borrowings, net	296.4
Dividends paid to shareholders	(99.5)	(386.2)	(94.3)
Repayment of debt	(175.8)	(151.1)	(11.6)
Repurchases of common stock	(212.8)	(563.6)	(57.7)
Net cash used in financing activities	(193.9)	(1,099.3)	(171)
Cash and cash equivalents, beginning of year	215.7	1,020.7	297.9
Cash and cash equivalents, end of year	120.6	215.7	1,020.7
The Hanover Insurance Group [Member]
Cash flows from operating activities
Net income	358.7	425.1	391
Adjustments to reconcile net income to net cash provided by operating activities:
(Gain) loss from sale of Chaucer business		2.1	(131.9)
Net realized investment (gains) losses	(0.4)	(5.3)	0.6
Loss on repayment of debt	6.2		1.9
Equity in net income of subsidiaries	(387.2)	(432.3)	(288.4)
Dividends received from subsidiaries	60.3	69.9	86.4
Deferred income tax benefit	(8.5)	(13.9)	(5.3)
Change in expenses and taxes payable	5.6	(107.8)	59.7
Change in net receivable from subsidiaries	15.5	11.6	8.9
Other, net	4.4	3.9	(0.4)
Net cash provided by operating activities	54.6	(46.7)	122.5
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities	166.6	447.9	82.8
Proceeds from disposals of equity securities and other investments		121.9
Purchase of fixed maturities	(32.4)	(316.1)	(4)
Purchase of equity securities and other investments		(121.9)
Net cash received from sale of Chaucer business		35.1	762
Net cash provided by (used in) investing activities	134.2	166.9	840.8
Cash flows from financing activities
Proceeds from exercise of employee stock options	6.3	14.4	14.8
Proceeds from debt borrowings, net	296.4
Dividends paid to shareholders	(99.5)	(386.2)	(94.3)
Repayment of debt	(175.8)		(11.6)
Repurchases of common stock	(212.8)	(563.6)	(57.7)
Net cash related to short-term intercompany borrowings			(19.8)
Net cash used in financing activities	(185.4)	(935.4)	(168.6)
Net change in cash and cash equivalents	3.4	(815.2)	794.7
Cash and cash equivalents, beginning of year	12.2	827.4	32.7
Cash and cash equivalents, end of year	$ 15.6	$ 12.2	$ 827.4

SCHEDULE II CONDENSED FINANCI_7

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
The Hanover Insurance Group [Member]
Supplementary Insurance Information By Segment [Line Items]
Investment assets transferred to the parent company to settle dividend balances	$ 185.6	$ 70.1	$ 179.8

SCHEDULE III SUPPLEMENTARY IN_2

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	[1]	$ 477.5	$ 467.4	$ 450.8
Future policy benefits, losses, claims and loss expenses	[1]	6,015.4	5,646	5,296.1
Unearned premiums	[1]	2,482.7	2,416.7	2,277.8
Other policy claims and benefits payable	[1]	8.6	8.4	8
Premium revenue	[1]	4,527.4	4,474.5	4,254.4
Net investment income	[1],[2]	265.1	281.3	267.4
Benefits, claims, losses and settlement expenses	[1]	2,845.2	2,865.5	2,724.6
Amortization of deferred acquisition costs	[1]	951	926.7	891.8
Other operating expenses	[1],[3]	586.3	576.4	595.4	[4]
Premiums written	[1]	4,598.5	4,581.7	4,384.8
Interest On Debt [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[3]	37.1	37.5	45.1	[4]
Eliminations [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[3]	(6.2)	(6.1)	(7.8)	[4]
Commercial Lines [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		312.1	307.4	296.9
Future policy benefits, losses, claims and loss expenses		4,067.4	3,768.9	3,550.6
Unearned premiums		1,513.9	1,446.7	1,369.5
Other policy claims and benefits payable		8.6	8.4	8
Premium revenue		2,683.3	2,654.2	2,548.4
Net investment income	[2]	175.3	180.1	182.2
Benefits, claims, losses and settlement expenses		1,647.6	1,610	1,568.3
Amortization of deferred acquisition costs		618.4	604.4	587.5
Other operating expenses	[3]	330.3	330.3	318	[4]
Premiums written		2,733.1	2,707.2	2,610.7
Personal Lines [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		165.4	160	153.9
Future policy benefits, losses, claims and loss expenses		1,907.6	1,839.4	1,707.6
Unearned premiums		968.8	970	908.3
Premium revenue		1,844.1	1,820.3	1,706
Net investment income	[2]	76.7	80.1	73.7
Benefits, claims, losses and settlement expenses		1,193.3	1,254.2	1,155
Amortization of deferred acquisition costs		332.6	322.3	304.3
Other operating expenses	[3]	193.7	191.6	185.8	[4]
Premiums written		1,865.4	1,874.5	1,774.1
Other Segment [Member] | Operating Segments [Member]
Supplementary Insurance Information By Segment [Line Items]
Future policy benefits, losses, claims and loss expenses		40.4	37.7	37.9
Net investment income	[2]	13.1	21.1	11.5
Benefits, claims, losses and settlement expenses		4.3	1.3	1.3
Other operating expenses	[3]	$ 31.4	$ 23.1	$ 54.3	[4]
[1]	Information for all periods excludes results and balances related to the former Chaucer business (See Note 2 – “Discontinued Operations” in the Notes to Consolidated Financial Statements).
[2]	The Company manages investment assets for its Commercial Lines, Personal Lines and Other segments on a combined basis, based on the requirements of its combined property and casualty insurance companies. Net investment income is allocated to these segments based on actuarial information related to the underlying businesses.
[3]	For other operating expenses that are not directly attributable to a single segment for combined property and casualty insurance companies, expenses are generally allocated based upon either net premiums written or net premiums earned.
[4]	In 2020 and 2018, the Other segment includes $6.2 million and $28.2 million, respectively, of losses from the repayment of debt.

SCHEDULE III SUPPLEMENTARY IN_3

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Parenthetical) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2018
Supplementary Insurance Information [Abstract]
Loss on repayment of debt	$ 6.2	$ 28.2

SCHEDULE V VALUATION AND QUAL_2

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		$ 7.4	$ 8.1	$ 14
Charged to costs and expenses		19.6	9.5	10.1
Charged to other accounts	[1]			(6.4)
Deductions		(11.1)	(10.2)	(9.6)
Balance at end of period		15.9	7.4	8.1
Allowance for doubtful accounts [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		3.5	4.2	4.3
Charged to costs and expenses		16.9	9.5	9.5
Deductions		(11.1)	(10.2)	(9.6)
Balance at end of period		9.3	3.5	4.2
Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		3.9	3.9	9.7
Charged to costs and expenses		2.7		0.6
Charged to other accounts	[1]			(6.4)
Balance at end of period		$ 6.6	$ 3.9	$ 3.9
[1]	Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its former Chaucer business; accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.

SCHEDULE V VALUATION AND QUAL_3

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Parenthetical) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	$ (6.4)	[1]
Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	(6.4)	[1]
Chaucer [Member] | Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	$ (6.4)
[1]	Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its former Chaucer business; accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.

SCHEDULE VI SUPPLEMENTAL INFO_2

SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net incurred losses and LAE in respect of losses occurring in current year		$ 2,877.8	$ 2,893	$ 2,733.5
Net incurred losses and LAE in respect of losses occurring in prior years		32.6	28.4	8.9
Reinsurance Recoverable On Unpaid Losses Property Casualty Liability		1,641.6	1,574.8	1,472.6	$ 1,455
Prepaid premiums, gross		144.2	149.3	117.6
Consolidated Property and Casualty Insurance Entity [Member]
Deferred acquisition costs	[1]	477.5	467.4	450.8
Reserves for unpaid claims and claim adjustment expenses	[1],[2]	6,024	5,654.4	5,304.1
Unearned premiums	[1],[2]	2,482.7	2,416.7	2,277.8
Earned premiums	[1]	4,527.4	4,474.5	4,254.4
Net investment income	[1]	265.1	281.3	267.4
Net incurred losses and LAE in respect of losses occurring in current year	[1],[2]	2,877.8	2,893.9	2,733.5
Net incurred losses and LAE in respect of losses occurring in prior years	[1],[3]	(32.6)	(28.4)	(8.9)
Amortization of deferred acquisition costs	[1],[2]	951	926.7	891.8
Paid claims and claim adjustment expenses	[1]	2,542.4	2,616.5	2,496.6
Premiums written	[1]	$ 4,598.5	$ 4,581.7	$ 4,384.8
[1]	Information for all periods excludes results and balances related to the discontinued Chaucer business. See Note 2 – “Discontinued Operations” in the Notes to Consolidated Financial Statements for further information on that business.
[2]	Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,641.6 million, $1,574.8 million and $1,472.6 million of reinsurance recoverable on unpaid losses in 2020, 2019 and 2018, respectively. Unearned premiums are shown gross of prepaid premiums of $144.2 million, $149.3 million and $117.6 million in 2020, 2019 and 2018, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.
[3]	The Company does not use discounting techniques.