The Hanover Insurance Group, Inc. (THG) 10-K 22 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 15, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	THG
Entity Registrant Name	HANOVER INSURANCE GROUP, INC.
Entity Central Index Key	944,695
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 5,058,103,806
Entity Common Stock, Shares Outstanding		40,573,500
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Revenues
Premiums	$ 4,254.4		$ 3,980.4	$ 3,789.5
Net investment income	267.4		243.9	231.6
Net realized and unrealized investment gains (losses):
Net realized gains (losses) from sales and other	(2.7)		26.7	37.6
Net change in fair value of equity securities	(43.4)
Net other–than–temporary impairment losses on investments recognized in earnings	(4.6)		(5.6)	(27.4)
Total net realized and unrealized investment gains (losses)	(50.7)		21.1	10.2
Fees and other income	23.2		22.5	22.6
Total revenues	4,494.3		4,267.9	4,053.9
Losses and expenses
Losses and loss adjustment expenses	2,724.6		2,579.6	2,546
Amortization of deferred acquisition costs	891.8		840.7	803.6
Interest expense	45.1		45.2	51.4
Loss on repayment of debt	28.2			88.3
Other operating expenses	522.1		509.5	498.6
Total losses and expenses	4,211.8		3,975	3,987.9
Income from continuing operations before income taxes	282.5		292.9	66
Current	46.2		3.1	26.2
Deferred	(2.7)		73.7	(27.2)
Total income tax expense (benefit)	43.5		76.8	(1)
Income from continuing operations	239		216.1	67
Discontinued operations:
Net Income/gain (loss) from discontinued businesses, net of taxes	0.1		(16.8)	(1)
Net income	$ 391		$ 186.2	$ 155.1
Basic:
Income from continuing operations	$ 5.63		$ 5.08	$ 1.57
Net income per share	$ 9.21		$ 4.38	$ 3.63
Weighted average shares outstanding	42.4		42.5	42.8
Diluted:
Income from continuing operations	$ 5.56		$ 5.03	$ 1.55
Net income per share	$ 9.09		$ 4.33	$ 3.59
Weighted average shares outstanding	43		43	43.2
Chaucer [Member]
Revenues
Premiums	$ 850	[1]	$ 853	$ 838.6
Net investment income	54.9	[1]	52	45.7
Net realized and unrealized investment gains (losses):
Total revenues	912.4	[1]	911.7	891.4
Losses and expenses
Losses and loss adjustment expenses	515.5	[1]	549.5	419.3
Amortization of deferred acquisition costs	252.5	[1]	245.9	231.6
Interest expense	3.8	[1]	3.3	3.5
Other operating expenses	115	[1]	109.2	113.7
Total losses and expenses	883	[1]	904.6	764.6
Total income tax expense (benefit)	4.7	[1]	21.7	37.2
Discontinued operations:
Gain from sale of Chaucer business, net of taxes	131.9
Net Income/gain (loss) from discontinued businesses, net of taxes	$ 20	[1]	$ (13.1)	$ 89.1
Basic:
Gain from sale of business, net of taxes	$ 3.11
Net Income/gain (loss) from discontinued businesses, net of taxes	0.47		$ (0.30)	$ 2.08
Diluted:
Gain from sale of business, net of taxes	3.07
Net Income/gain (loss) from discontinued businesses, net of taxes	$ 0.46		$ (0.30)	$ 2.06
Life [Member]
Discontinued operations:
Net Income/gain (loss) from discontinued businesses, net of taxes	$ 0.1		$ (16.8)	$ (1)
Basic:
Net Income/gain (loss) from discontinued businesses, net of taxes			$ (0.40)	$ (0.02)
Diluted:
Net Income/gain (loss) from discontinued businesses, net of taxes			$ (0.40)	$ (0.02)
[1]	2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 391	$ 186.2	$ 155.1
Available-for-sale securities:
Net appreciation (depreciation) during the period	(170.1)	13.4	28.8
Change in other-than-temporary impairment losses recognized in other comprehensive income		6	7.3
Amount realized with sale of Chaucer business	19.1
Total available-for-sale securities	(151)	19.4	36.1
Pension and postretirement benefits:
Net actuarial losses and prior service costs arising in the period	(17)	13.6	(30.3)
Amortization recognized as net periodic benefit and postretirement cost	7.7	9.4	6.4
Amount realized with sale of Chaucer business	16.4
Total pension and postretirement benefits	7.1	23	(23.9)
Cumulative foreign currency translation adjustment:
Amount recognized as cumulative foreign currency translation during the period	(1.7)	2.4	(3.3)
Amount realized with sale of Chaucer business	23.2
Total cumulative foreign currency translation adjustment	21.5	2.4	(3.3)
Total other comprehensive income (loss), net of tax	(122.4)	44.8	8.9
Comprehensive income	$ 268.6	$ 231	$ 164

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets
Fixed maturities, at fair value (amortized cost of $6,245.9 and $5,657.1)	$ 6,161.5	$ 5,749.3
Equity securities, at fair value	464.4	576.2
Other investments	661.5	562.7
Total investments	7,287.4	6,888.2
Cash and cash equivalents	1,020.7	297.9
Accrued investment income	53.2	48.2
Premiums and accounts receivable, net	1,176.7	1,095.7
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,648.6	1,625.5
Deferred acquisition costs	450.8	430
Deferred income taxes	50.6	17
Goodwill	178.8	178.8
Other assets	371.6	333.7
Assets held-for-sale	57.4	4,466.6
Total assets	12,399.7	15,469.6
Liabilities
Loss and loss adjustment expense reserves	5,304.1	5,058.5
Unearned premiums	2,277.8	2,131.7
Expenses and taxes payable	909.8	620.7
Reinsurance premiums payable	37.3	51.5
Debt	777.9	786.9
Liabilities held-for-sale	22.2	3,707.2
Total liabilities	9,445	12,471.9
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	1,871.8	1,857
Accumulated other comprehensive income (loss)	(116.5)	107.6
Retained earnings	2,182.3	1,975
Treasury stock at cost (18.2 and 18.0 million shares)	(983.5)	(942.5)
Total shareholders’ equity	2,954.7	2,997.7
Total liabilities and shareholders’ equity	12,399.7	15,469.6
Discontinued Operations, Disposed of by Sale [Member]
Assets
Assets held-for-sale	103.9	88
Chaucer [Member] | Discontinued Operations, Disposed of by Sale [Member]
Assets
Assets held-for-sale	57.4	4,466.6
Liabilities
Liabilities held-for-sale	22.2	3,707.2
Life [Member] | Discontinued Operations, Disposed of by Sale [Member]
Assets
Assets held-for-sale	103.9	88
Liabilities
Liabilities held-for-sale	$ 115.9	$ 115.4

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 6,245.9	$ 5,657.1
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	18,200,000	18,000,000

Consolidated Statements of Shar

Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Cumulative Foreign Currency Translation Adjustment [Member]	Accumulated Other Comprehensive Income (Loss), net of tax [Member]	Retained Earnings [Member]	Treasury Stock
Balance at beginning year at Dec. 31, 2015		$ 0.6	$ 1,833.5	$ 149.9	$ (78.6)	$ (17.4)		$ 1,803.5	$ (847.1)
Balance at beginning of period, as adjusted at Dec. 31, 2015								1,803.5
Net income	$ 155.1							155.1
Shares purchased at cost									(105.6)
Employee and director stock-based awards and other			13.2
Net appreciation (depreciation) on available-for-sale securities	36.1			36.1
Net amount arising in the period	(30.3)				(30.3)
Net amount recognized as net periodic benefit cost	6.4				6.4
Amount recognized as cumulative foreign currency translation during the period	(3.3)					(3.3)
Dividends to shareholders								(80.4)
Net shares reissued at cost under employee stock-based compensation plans									24.5
Stock-based compensation								(2.6)
Balance at end of year at Dec. 31, 2016	2,857.5	0.6	1,846.7	186	(102.5)	(20.7)	$ 62.8	1,875.6	(928.2)
Balance at beginning of period, as adjusted at Dec. 31, 2016								1,875.6
Net income	186.2							186.2
Shares purchased at cost									(37.2)
Employee and director stock-based awards and other			10.3
Net appreciation (depreciation) on available-for-sale securities	19.4			19.4
Net amount arising in the period	13.6				13.6
Net amount recognized as net periodic benefit cost	9.4				9.4
Amount recognized as cumulative foreign currency translation during the period	2.4					2.4
Dividends to shareholders								(86.8)
Net shares reissued at cost under employee stock-based compensation plans									22.9
Balance at end of year at Dec. 31, 2017	2,997.7	$ 0.6	1,857	205.4	(79.5)	(18.3)	107.6	1,975	(942.5)
Cumulative effect of accounting change, net of taxes at Dec. 31, 2017								104.3
Balance at beginning of period, as adjusted at Dec. 31, 2017								2,079.3
Net income	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 period	21.5					(1.7)
Adoption of Accounting Standards Update | Accounting Standards Update 2018-02 [Member]					(16.2)	(3.9)
Adoption of Accounting Standards Update | Accounting Standards Update 2016-01 and 2018-02 [Member]				(81.6)
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		$ 1,871.8	$ (27.2)	$ (88.6)	$ (0.7)	$ (116.5)	$ 2,182.3	$ 983.5

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows From Operating Activities
Net income	$ 391	$ 186.2	$ 155.1
Adjustments to reconcile net income to net cash provided by operating activities:
Gain from sale of Chaucer business	(131.9)
Net loss from repayment of debt	1.9		88.3
Net realized investment losses (gains)	51.3	(23.7)	(8.6)
Net amortization and depreciation	24.6	30.1	30.7
Stock-based compensation expense	13.4	12.3	12
Amortization of defined benefit plan costs	9.7	14	9.9
Deferred income tax expense (benefit)	(23.4)	39.8	(13.2)
Change in deferred acquisition costs	(19.2)	(31.8)	(7.8)
Change in premiums receivable, net of reinsurance premiums payable	(73.9)	(29.6)	(28.2)
Change in loss, loss adjustment expense and unearned premium reserves	584.9	949.5	527.8
Change in reinsurance recoverable	(248.1)	(414)	(35.8)
Change in expenses and taxes payable	4.6	(0.5)	12.7
Other, net	(33.6)	(27.7)	0.5
Net cash provided by operating activities	551.3	704.6	743.4
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,094.2	1,225.3	1,602.3
Proceeds from disposals of equity securities and other investments	158.4	164.9	271.1
Purchase of fixed maturities	(1,425.6)	(1,645.8)	(2,020.9)
Purchase of equity securities and other investments	(178.3)	(222.6)	(351.2)
Capital expenditures	(13.1)	(18.6)	(15.7)
Net proceeds from sale of Chaucer business	635.7
Other investing activities		(9.5)	19
Net cash provided by (used in) investing activities	271.3	(506.3)	(495.4)
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	14.8	23.1	16.2
Proceeds from debt borrowings, net			370.7
Change in cash collateral related to securities lending program	(19)	(6.7)	(6.4)
Dividends paid to shareholders	(94.3)	(86.8)	(80.4)
Repayment of debt	(11.6)		(475.4)
Repurchases of common stock	(57.7)	(37.2)	(105.6)
Other financing activities	(3.2)	(4.1)	(19.9)
Net cash used in financing activities	(171)	(111.7)	(300.8)
Effect of exchange rate changes on cash	(4)	7.2	(3.4)
Net change in cash and cash equivalents	647.6	93.8	(56.2)
Net change in cash related to discontinued operations	75.2	56.8	14
Cash and cash equivalents, beginning of year	297.9	147.3	189.5
Cash and cash equivalents, end of year	1,020.7	297.9	147.3
Supplemental Cash Flow Information
Interest payments	47.8	47.5	53.1
Income tax net payments	$ 57	$ 51.1	$ 29.7

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
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 14 – “Segment Information”. The consolidated financial statements also include the Company’s discontinued operations, consisting primarily of Chaucer Holdings Limited (“Chaucer”), a specialist insurance underwriting group which operates through the Society and Corporation of Lloyd’s (“Lloyd’s”), and the Company’s accident and health and former life insurance businesses. 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”). The sale of the Chaucer-related Irish entity was completed on February 14, 2019 for proceeds of $28 million. The sale of the Australian entities for proceeds of $13 million is pending, subject only to local regulatory approval, and is expected to close in the first quarter of 2019. Accordingly, as of December 31, 2018 and for all prior periods presented in this Annual Report on Form 10-K, Chaucer’s accounts have been classified as discontinued operations in the Consolidated Statements of Income and as held-for-sale in the Consolidated Balance Sheets (See Note 2 – “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. Effective January 1, 2018, in accordance with the provisions of Accounting Standards Codification (“ ASC”) Update No. 2016-01, (Subtopic 825-10) Financial Instruments – Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU No. 2016-01”), 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. Prior to May 2018, certain reacquisition rights, which are no longer in effect, and which were previously retained by the third-party in the loan participations required that these investments be accounted for as secured borrowings under ASC 860, Transfers and Servicing Investments in limited partnerships include interests in private equity funds. Effective January 1, 2018, in accordance with the provisions of ASC Update No. 2016-01, when the Company’s interest in the partnership 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 that 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. When an other-than-temporary decline in value of a specific investment is deemed to have occurred, and a charge to earnings is required, the Company recognizes a realized investment loss. 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. Prior to the sale of Chaucer on December 28, 2018, the Company also held investments in overseas deposits; such deposits were maintained in overseas funds and were managed exclusively by Lloyd’s. These funds were required in order to protect policyholders in overseas markets and enabled the Company to operate in these markets. Net investment income and realized and unrealized investment gains (losses) related to overseas deposits are included in income (loss) from Chaucer business within discontinued operations. The Company reviews investments in an unrealized loss position to identify other-than-temporary declines in value. When an other-than-temporary decline in value of a debt security is deemed to have occurred, the Company must assess whether it intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis. If the debt security meets either of these two criteria, an other-than-temporary impairment (“OTTI”) 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 the Company does not intend to sell the debt security and it is not more likely than not the Company will be required to sell the security before recovery of its amortized cost basis, the credit loss portion of an OTTI is recorded through earnings while the portion attributable to all other factors is recorded separately as a component of other comprehensive income. The amount of the OTTI that relates to credit is 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. Once an OTTI has been recognized, the new amortized cost basis of the security is equal to the previous amortized cost less the amount of OTTI recognized in earnings. 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 and foreign currency 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 includes 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 and Capitalized Software 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 leases or the estimated useful life of the improvements. 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 2018 and 2017 with no impairments recognized. At December 31, 2018 and 2017, 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, 2018 includes senior debentures, subordinated debentures, and collateralized borrowings with the Federal Home Loan Bank (“FHLB”). 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 premium 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, 2018 and 2017. 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. 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 for income from the U.S. operations. In December 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted, which reduced the U.S. statutory rate from 35% to 21%, effective January 1, 2018. See Note 7 – “Income Taxes” for a discussion of this new law. 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, are 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 11 – “Stock-Based Compensation Plans”. N. Earnings Per Share Earnings per share (“EPS”) for the years ended December 31, 2018, 2017 and 2016 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 are 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 February 2018, the Financial Accounting Standards Board (“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. 2017-07, (Topic 715) Compensation – Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In January 2017, the FASB issued ASC Update No. 2017-01, (Topic 805) Business Combinations – Clarifying the Definition of a Business In November 2016, the FASB issued ASC Update No. 2016-18 (Topic 230) Statement of Cash Flows – Restricted Cash (a consensus of the FASB Emerging Issues Task Force) In October 2016, the FASB issued ASC Update No. 2016-16, (Topic 740) Income Taxes – Intra-Entity Transfers of Assets Other Than Inventory In August 2016, the FASB issued ASC Update No. 2016-15, (Topic 230) Classification of Certain Cash Receipts and Cash Payments In January 2016, the FASB issued ASC Update No. 2016-01, (Subtopic 825-10) Financial Instruments – Overall: Recognition and Measurement of Financial Assets and Financial Liabilities Technical Corrections and Improvements to Financial Instruments – Overall. In May 2014, the FASB issued ASC Update No. 2014-09, (Topic 606) Revenue from Contracts with Customers Revenue from Contracts with Customers Recently Issued Standards In August 2018, the FASB issued ASC Update No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASC Update No. 2018-14 (Topic 715-20) Compensation – Retirement Benefits – Defined Benefit Plans – General – Disclosure Framework – Changes to the Disclosure Requirements for the Defined Benefit Plans. In August 2018, the FASB issued ASC Update No. 2018-13 (Topic 820) Fair Value Measurement, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In March 2017, the FASB issued ASC Update No. 2017-08, (Subtopic 310-20) Receivables – Nonrefundable Fees and Other Costs: Premium Amortization on Purchased Callable Debt Securities In January 2017, the FASB issued ASC Update No. 2017-04, (Topic 350) Intangibles – Goodwill and Other: Simplifying the Test for Goodwill Impairment In June 2016, the FASB issued ASC Update No. 2016-13, (Topic 326) Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments In November 2018, the FASB issued ASC Update No. 2018-19, Codification Improvements to Topic 326, Financial Instruments–Credit Losses , which explicitly states that receivables arising from operating leases are not within the scope of Subtopic 326-20. ASC Update No. 2016-13 and related In February 2016, the FASB issued ASC Update No. 2016-02, (Topic 842) Leases Leases Targeted Improvements 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, 2018
Chaucer [Member]
Discontinued Operations	2. Discontinued Operations Gain on Sale of Chaucer Business On December 28, 2018, the Company completed the sale of Chaucer Holdings Limited to China Re. Total cash proceeds payable by China Re for this portion of the transaction is expected to total approximately $793.7 million (1) The following table summarizes the components of the estimated gain related to the sale of the Chaucer business as of December 28, 2018. This gain below excludes an expected gain on the Irish and Australian entities associated with the Chaucer business. The Company subsequently completed the sale of its Chaucer-related Irish entity on February 14, 2019. The sale of the Australian entities is pending, subject only to local regulatory approval, and is expected to close in the first quarter of 2019. The Company received $28 million of additional consideration for the Irish entity, and it expects to receive $13 million of additional consideration related to the Australian entities. 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 Holdings Limited as determined in the sales and purchase agreement was $779 million. This amount, along with $28 million in cash proceeds received for the sale of the Irish entity on February 14, 2019, $13 million in cash proceeds to be received upon the closing of the sale of the Australian entities, 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, results 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, may be up to $45 million and is determined based upon 2018 catastrophe losses. Based upon the Company’s best estimate of Chaucer’s 2018 catastrophe losses, expected contingent consideration is $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 are no longer likely to be realized . Income (loss) from Chaucer Business The following table summarizes the results of Chaucer’s operations: YEARS ENDED DECEMBER 31 2018 (1) 2017 2016 (in millions) Revenues Net premiums earned $ 850.0 $ 853.0 $ 838.6 Net investment income 54.9 52.0 45.7 Other income 7.5 6.7 7.1 912.4 911.7 891.4 Losses and operating expenses Losses and LAE 515.5 549.5 419.3 Amortization of deferred acquisition costs 252.5 245.9 231.6 Other expenses 115.0 109.2 113.7 883.0 904.6 764.6 Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income) 29.4 7.1 126.8 Other items: Interest expense (3.8 ) (3.3 ) (3.5 ) Net realized and unrealized investment gains (losses) (1.3 ) 2.6 (1.6 ) Other income 0.4 2.2 4.6 Income from Chaucer business before income taxes 24.7 8.6 126.3 Income tax expense (4.7 ) (21.7 ) (37.2 ) Income (loss) from Chaucer business, net of taxes $ 20.0 $ (13.1 ) $ 89.1 (1) 2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018. The Company recognized $0.8 million, $1.1 million and $22.3 million in foreign currency transaction losses in net income in 2018, 2017 and 2016, respectively. The following table details the carrying amounts of major assets and liabilities reflected in the Consolidated Balance Sheets under the caption “Assets held-for-sale” and “Liabilities held-for-sale”, respectively. DECEMBER 31 2018 2017 (in millions) Assets: Investments $ 24.5 $ 2,153.5 Cash and cash equivalents 3.3 78.5 Premiums and accounts receivable, net 2.8 471.9 Deferred acquisition costs 1.4 120.2 Reinsurance recoverable on paid and unpaid losses and unearned premiums 2.3 1,431.5 Goodwill 6.5 13.8 Other assets 16.6 197.2 Total assets $ 57.4 $ 4,466.6 Liabilities: Loss and loss adjustment expense reserves $ 2.0 $ 2,686.5 Unearned premiums 7.1 631.9 Reinsurance premium payable 1.5 293.3 Other liabilities 11.6 95.5 Total liabilities $ 22.2 $ 3,707.2 In accordance with Lloyd’s operating guidelines, the Company deposited funds at Lloyd’s to support Chaucer underwriting operations. These funds are available only to fund claim obligations. At December 31 2017, fixed maturities with a fair value of $528.1 million and cash of $7.1 million, respectively, were on deposit with Lloyd’s. The following table details the cash flows associated with the Chaucer business: DECEMBER 31 2018 2017 2016 (in millions) Net cash (used in) provided by operating activities (22.8 ) 28.8 95.9 Net cash provided by (used in) investing activities 131.1 (96.9 ) (104.7 ) Prior to the sale, the Company was subject to concentration of risk with respect to reinsurance ceded with respect to the Lloyd’s syndicates. The Lloyd’s Syndicates total reinsurance receivable balance for the Chaucer business was $443.0 million as of December 31, 2017. The following table provides the effects of reinsurance. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Premiums written: Direct $ 625.3 $ 597.6 $ 560.3 Assumed 685.3 646.9 546.3 Ceded (1) (459.9 ) (395.4 ) (290.5 ) Net premiums written $ 850.7 $ 849.1 $ 816.1 Premiums earned: Direct $ 611.9 $ 568.7 $ 584.0 Assumed 667.2 628.3 577.3 Ceded (1) (429.1 ) (344.0 ) (322.7 ) Net premiums earned $ 850.0 $ 853.0 $ 838.6 Losses and LAE: Direct $ 362.2 $ 365.2 $ 403.1 Assumed 568.7 570.7 258.8 Ceded (2) (415.4 ) (386.8 ) (243.2 ) Net losses and LAE $ 515.5 $ 549.1 $ 418.7 (1) The increase in ceded reinsurance premiums from 2016 through 2018 is primarily due to Chaucer’s planned increase in reinsurance purchases. (2) The increase in ceded losses and LAE from 2016 through 2018 is primarily due to higher catastrophe loss activity in certain Chaucer lines and due to the aforementioned increase in reinsurance purchases. For Chaucer’s 2016 U.S. casualty treaty lines, the Company entered into a whole account aggregate excess of loss contract. The contract covers the U.S. casualty treaty lines exposures, except specialty risks and workers’ compensation clash. This contract does not meet the risk transfer requirements of GAAP and was accounted for using the deposit accounting method. The impact of reinsurance contracts subject to deposit accounting were recognized through net investment income rather than losses. Net investment income for this contract of approximately $8 million was recognized for each of the years ended December 31, 2018 and December 31, 2017 and $3 million was recognized for the year ended December 31, 2016. Discontinued 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. Accordingly, the operating results of the discontinued segment have been reported in accordance with Accounting Principles Board Opinion No. 30, Reporting the Results of Operations—Reporting the Effects of Disposal of a Segment of a Business, and Extraordinary, Unusual and Infrequently Occurring Events and Transactions At December 31, 2018 and 2017, the portion of the discontinued accident and health business that was directly assumed had assets of $81.6 million and $76.9 million, respectively, consisting primarily of invested assets, and liabilities of $81.4 million and $83.1 million, respectively, consisting primarily of policy liabilities. At December 31, 2018 and 2017, 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 operations for the years ended December 31, 2018, 2017 and 2016 resulted in gains of $0.1 million, losses of $16.8 million and losses of $1.0 million, respectively, net of income tax benefits of $9.3 million and $2.8 million in 2017 and 2016, respectively. The increased losses in 2017 primarily relate to additional losses in the Company’s long-term care pool, which is included in the portion of the discontinued accident and health business that was directly assumed.

Investments

Investments	12 Months Ended
	Dec. 31, 2018
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, 2018 (in millions) Gross Gross OTTI Amortized Unrealized Unrealized Unrealized Cost Gains Losses Fair Value Losses Fixed maturities: U.S. Treasury and government agencies $ 414.7 $ 2.4 $ 7.2 $ 409.9 $ — Foreign government 7.3 0.1 — 7.4 — Municipal 879.0 16.6 9.8 885.8 — Corporate 3,476.6 26.1 92.0 3,410.7 6.6 Residential mortgage-backed 728.4 2.7 14.7 716.4 — Commercial mortgage-backed 648.4 1.7 9.8 640.3 — Asset-backed 91.5 0.2 0.7 91.0 — Fixed maturities, excluding held-for-sale (Chaucer) 6,245.9 49.8 134.2 6,161.5 6.6 Fixed maturities, held-for-sale 24.9 — 0.4 24.5 — Total fixed maturities $ 6,270.8 $ 49.8 $ 134.6 $ 6,186.0 $ 6.6 DECEMBER 31, 2017 (in millions) Gross Gross OTTI Amortized Unrealized Unrealized Unrealized Cost Gains Losses Fair Value Losses Fixed maturities: U.S. Treasury and government agencies $ 322.1 $ 3.4 $ 3.2 $ 322.3 $ — Foreign government 8.2 0.3 — 8.5 — Municipal 910.1 29.7 5.8 934.0 — Corporate 3,134.2 84.1 21.7 3,196.6 6.5 Residential mortgage-backed 632.3 5.8 4.9 633.2 — Commercial mortgage-backed 590.3 7.2 2.5 595.0 — Asset-backed 59.9 0.1 0.3 59.7 — Fixed maturities, excluding held-for-sale (Chaucer) 5,657.1 130.6 38.4 5,749.3 6.5 Fixed maturities, held-for-sale 2,031.7 14.6 15.9 2,030.4 0.4 Total fixed maturities $ 7,688.8 $ 145.2 $ 54.3 $ 7,779.7 $ 6.9 OTTI unrealized losses in the tables above represent 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 of $7.4 million and $11.0 million as of December 31, 2018 and 2017, respectively. The Company participates in a security lending program for the purpose of enhancing income. Securities on loan to various counterparties had a fair value of $4.8 million and $23.4 million at December 31, 2018 and 2017, respectively, and were fully collateralized by cash. The fair value of the loaned securities is monitored on a daily basis, and the collateral is maintained at a level of at least 102% of the fair value of the loaned securities. Securities lending collateral is recorded by the Company in cash and cash equivalents, with an offsetting liability included in expenses and taxes payable. At December 31, 2018 and 2017, fixed maturities with fair values of $299.9 million and $286.9 million, respectively, were on deposit with various state governmental authorities or trustees. The Company enters into various agreements that may require its fixed maturities to be held as collateral by others. At December 31, 2018 and 2017, fixed maturities with fair values of $234.9 million and $225.7 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. 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 2018 (in millions) Amortized Cost Fair Value Due in one year or less $ 207.4 $ 208.5 Due after one year through five years 1,830.9 1,830.0 Due after five years through ten years 2,465.1 2,398.7 Due after ten years 274.2 276.6 4,777.6 4,713.8 Mortgage-backed and asset-backed securities 1,468.3 1,447.7 Fixed maturities, excluding held-for-sale (Chaucer) 6,245.9 6,161.5 Fixed maturities, held-for-sale 24.9 24.5 Total fixed maturities $ 6,270.8 $ 6,186.0 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 2018 Maturities Other Total 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 ) 2017 Net appreciation, beginning of year $ 127.1 $ 58.9 $ 186.0 Net appreciation (depreciation) on available-for-sale securities (15.0 ) 55.9 40.9 Provision for deferred income taxes (11.2 ) (19.5 ) (30.7 ) Change in OTTI losses recognized in other comprehensive income 9.2 — 9.2 (17.0 ) 36.4 19.4 Net appreciation, end of year $ 110.1 $ 95.3 $ 205.4 2016 Net appreciation, beginning of year $ 116.5 $ 33.4 $ 149.9 Net appreciation on available-for-sale securities 36.9 39.2 76.1 Provision for deferred income taxes (37.5 ) (13.7 ) (51.2 ) Change in OTTI losses recognized in other comprehensive income 11.2 — 11.2 10.6 25.5 36.1 Net appreciation, end of year $ 127.1 $ 58.9 $ 186.0 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 (see “Equity Securities” below). 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, 2018 and 2017 including the length of time the securities have been in an unrealized loss position: DECEMBER 31, 2018 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 $ 1.1 $ 66.0 $ 6.1 $ 210.9 $ 7.2 $ 276.9 Foreign governments — 2.0 — 0.8 — 2.8 Municipal 0.8 110.0 9.0 248.0 9.8 358.0 Corporate 30.0 1,277.9 43.9 781.6 73.9 2,059.5 Residential mortgage-backed 2.6 201.2 12.1 323.7 14.7 524.9 Commercial mortgage-backed 3.4 293.0 6.4 175.5 9.8 468.5 Asset-backed 0.4 42.3 0.3 18.0 0.7 60.3 Total investment grade 38.3 1,992.4 77.8 1,758.5 116.1 3,750.9 Below investment grade: Municipal — — — 0.9 — 0.9 Corporate 8.1 185.6 10.0 54.0 18.1 239.6 Total below investment grade 8.1 185.6 10.0 54.9 18.1 240.5 Fixed maturities, excluding held-for-sale (Chaucer) 46.4 2,178.0 87.8 1,813.4 134.2 3,991.4 Fixed maturities, held-for-sale 0.1 4.0 0.3 18.0 0.4 22.0 Total fixed maturities $ 46.5 $ 2,182.0 $ 88.1 $ 1,831.4 $ 134.6 $ 4,013.4 DECEMBER 31, 2017 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 $ 1.1 $ 146.3 $ 2.1 $ 62.3 $ 3.2 $ 208.6 Foreign governments — 0.1 — 0.6 — 0.7 Municipal 1.7 183.8 4.1 101.2 5.8 285.0 Corporate 3.0 386.2 10.8 395.9 13.8 782.1 Residential mortgage-backed 1.4 196.3 3.5 159.0 4.9 355.3 Commercial mortgage-backed 0.9 139.2 1.6 61.9 2.5 201.1 Asset-backed 0.3 34.6 — 2.4 0.3 37.0 Total investment grade 8.4 1,086.5 22.1 783.3 30.5 1,869.8 Below investment grade: Corporate 1.5 48.7 6.4 39.0 7.9 87.7 Fixed maturities, excluding held-for-sale (Chaucer) 9.9 1,135.2 28.5 822.3 38.4 1,957.5 Fixed maturities, held-for-sale 5.7 803.3 10.2 389.6 15.9 1,192.9 Total fixed maturities $ 15.6 $ 1,938.5 $ 38.7 $ 1,211.9 $ 54.3 $ 3,150.4 The Company views gross unrealized losses on fixed maturities as being temporary since it is its assessment that these securities will recover in the near term, allowing the Company to realize the anticipated long-term economic value. The Company employs a systematic methodology to evaluate declines in fair value below amortized cost for fixed maturity securities. In determining OTTI, the Company evaluates several factors and circumstances, including the issuer’s overall financial condition; the issuer’s credit and financial strength ratings; the issuer’s financial performance, including earnings trends and asset quality; any specific events which may influence the operations of the issuer; the general outlook for market conditions in the industry or geographic region in which the issuer operates; and the length of time and the degree to which the fair value of an issuer’s securities remains below the Company’s amortized cost. The Company also considers any factors that might raise doubt about the issuer’s ability to make contractual payments as they come due and whether the Company expects to recover the entire amortized cost basis of the security. D. EQUITY SECURITIES Equity securities are carried at fair value. Effective January 1, 2018, increases or decreases in the fair value of equity securities are reported in net realized and unrealized investment gains (losses) in the Consolidated Statements of Income. Previously, equity securities were categorized as available-for-sale and unrealized gains and losses were reported in AOCI, a separate component of shareholders’ equity. On January 1, 2018, the Company recorded a cumulative effect adjustment which included the reclassification of net unrealized gains on equity securities of $142.5 million, pre-tax, from AOCI to retained earnings. As of December 31, 2017, the Company held equity securities with a fair value of $576.2 million and a cost of $433.7 million. E. OTHER INVESTMENTS The Company’s mortgage participations and other mortgage loans were $405.7 million and $365.8 million at December 31, 2018 and 2017, 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 2018 2017 (in millions) Property Type: Office $ 141.1 $ 134.6 Apartments 85.5 64.9 Retail 66.9 64.1 Hotel 62.7 58.1 Industrial 50.6 45.0 Valuation allowance (1.1 ) (0.9 ) Total $ 405.7 $ 365.8 DECEMBER 31 2018 2017 (in millions) Geographic Region: Pacific $ 92.5 $ 87.6 South Atlantic 82.1 76.9 West South Central 65.9 66.1 Mid-Atlantic 53.4 53.4 New England 35.3 28.8 East North Central 27.9 16.5 Mountain 12.5 5.0 Other 37.2 32.4 Valuation allowance (1.1 ) (0.9 ) Total $ 405.7 $ 365.8 At December 31, 2018, scheduled maturities of mortgage participations and other mortgage loans were as follows: due in 2020 - $10.0 million; in 2021 - $57.4 million; 2022 - $28.1 million; 2023 - $17.0 million and thereafter - $293.2 million. There were no scheduled loan maturities in 2019. 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 2018, 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 $235.7 million and $186.4 million at December 31, 2018 and December 31, 2017, respectively. F. OTHER At December 31, 2018 and 2017, the Company’s exposure to concentration of investments in a single investee that exceeded 10% of shareholders’ equity included securities of a U.S. government-sponsored agency, as well as mortgage participations with a highly rated single third party with a carry value of $396.7 million and $356.8 million, respectively. At December 31, 2018, there were contractual investment commitments of up to $162.9 million.

Investment Income and Gains and

Investment Income and Gains and Losses	12 Months Ended
	Dec. 31, 2018
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 from continuing operations were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Fixed maturities $ 217.7 $ 205.8 $ 202.1 Limited partnerships 24.1 15.3 8.0 Equity securities 17.0 18.0 18.6 Mortgage loans 14.0 11.4 9.1 Other investments 4.8 3.4 3.4 Gross investment income 277.6 253.9 241.2 Less investment expenses (10.2 ) (10.0 ) (9.6 ) Net investment income $ 267.4 $ 243.9 $ 231.6 Effective January 1, 2018, with the implementation of ASC No. 2016-01, the change in fair value of limited partnerships previously reported using the cost method are reported in net investment income, of which $5.3 million relates to holding gains on securities still owned at December 31, 2018. The carrying values of fixed maturity securities on non-accrual status at December 31, 2018 and 2017 were not material. The effects of non-accruals for the years ended December 31, 2018, 2017 and 2016, 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 from continuing operations, including OTTI, were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Equity securities $ (43.4 ) $ 19.8 $ 25.4 Fixed maturities (5.8 ) 3.6 (15.5 ) Other investments (1.5 ) (2.3 ) 0.3 Net realized and unrealized investment gains (losses) $ (50.7 ) $ 21.1 $ 10.2 Effective January 1, 2018, as a result of the implementation of ASC No. 2016-01, net realized and unrealized investment losses include changes in the fair value of equity securities. Previously, equity securities were categorized as available-for-sale and unrealized gains and losses were reported in AOCI, a separate component of shareholders’ equity. The following table provides pre-tax net realized and unrealized losses on equity securities from continuing operations: YEAR ENDED DECEMBER 31 2018 (in millions) Net losses recognized during the period $ (43.4 ) Less: net losses recognized on equity securities sold during the period (3.9 ) Net unrealized losses recognized during the period on equity securities still held $ (39.5 ) For the years ended December 31, 2017 and 2016, there were net unrealized gains on equity securities of $56.8 million and $37.8 million, respectively, recognized in AOCI. Other-than-temporary-impairments Included in the net realized and unrealized investment gains (losses) were OTTI of investment securities totaling $4.6 million, $5.6 million and $27.4 million in 2018, 2017 and 2016, respectively. For 2018, total OTTI from continuing operations was $5.4 million. Of this amount, $4.6 million was recognized in earnings and the remaining $0.8 million was recorded as unrealized losses in AOCI. The $4.6 million of OTTI recognized in earnings relates to $2.6 million of fixed maturity securities and $2.0 million of other invested assets. For 2017, total OTTI from continuing operations was $5.9 million. Of this amount, $5.6 million was recognized in earnings and the remaining $0.3 million was recorded as unrealized losses in AOCI. The $5.6 million of OTTI recognized in earnings relates to $2.0 million of other invested assets, $1.8 million of fixed maturity securities and $1.8 million of equities. For 2016, total OTTI from continuing operations was $20.6 million. Of this amount, $27.4 million was recognized in earnings including $6.8 million which was transferred from unrealized losses in AOCI. The $27.4 million of OTTI recognized in earnings was related primarily to $16.0 million of fixed maturity securities that the Company intended to sell, $8.4 million of credit impairments and $2.7 million of equities. The methodology and significant inputs used to measure the amount of credit losses on fixed maturities in 2018, 2017 and 2016 were as follows: Corporate bonds - the Company utilized a financial model that derives expected cash flows based on probability-of-default factors by credit rating and asset duration and loss-given-default factors based on security type. These factors are based on historical data provided by an independent third-party rating agency. In addition, other market data relevant to the realizability of contractual cash flows may be considered. The following table provides rollforwards of the cumulative amounts related to the Company’s credit loss portion of the OTTI losses on fixed maturity securities from continuing operations for which the non-credit portion of the loss is included in other comprehensive income. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Credit losses as of the beginning of the year $ 3.6 $ 9.6 $ 17.7 Credit losses on securities for which an OTTI was not previously recognized 1.0 0.4 6.1 Additional credit losses on securities for which an OTTI was previously recognized 0.1 0.1 2.3 Reductions for securities sold, matured or called (0.9 ) (6.1 ) (4.3 ) Reductions for securities reclassified as intend to sell — (0.4 ) (12.2 ) Credit losses as of the end of the year $ 3.8 $ 3.6 $ 9.6 The proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses on those sales, were as follows: YEARS ENDED DECEMBER 31 (in millions) Proceeds Gross Gross 2018 from Sales Gains Losses 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 2017 Fixed maturities, excluding held-for-sale (Chaucer) $ 306.8 $ 8.1 $ 7.8 Fixed maturities, held-for-sale 188.0 4.1 0.4 Total fixed maturities $ 494.8 $ 12.2 $ 8.2 2016 Fixed maturities, excluding held-for-sale (Chaucer) $ 348.2 $ 10.0 $ 5.1 Fixed maturities, held-for-sale 215.6 1.7 1.9 Total fixed maturities $ 563.8 $ 11.7 $ 7.0 Beginning in 2018, equity securities are no longer classified as available-for-sale. Proceeds from the sale of equity securities for the years ended December 31, 2017 and 2016 were $128.8 million and $245.0 million, respectively, resulting primarily in gross realized gains of $18.8 million and $31.1 million, respectively.

Fair Value

Fair Value	12 Months Ended
	Dec. 31, 2018
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 are as follows, with the highest priority given to Level 1 as these are the most observable, and the lowest priority given to Level 3: Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities. Level 2 – Quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data, including model-derived valuations. Level 3 – Unobservable inputs that are supported by little or no market activity. When more than one level of input is used to determine fair value, the financial instrument is classified as Level 2 or 3 according to the lowest level input that has a significant impact on the fair value measurement. The following methods and assumptions were used to estimate the fair value of each class of financial instruments and have not changed since last year. FIXED MATURITIES Level 1 securities generally include U.S. Treasury issues and other securities that are highly liquid and for which quoted market prices are available. Level 2 securities are valued using pricing for similar securities and pricing models that incorporate observable inputs including, but not limited to yield curves and issuer spreads. Level 3 securities include issues for which little observable data can be obtained, primarily due to the illiquid nature of the securities, and for which significant inputs used to determine fair value are based on the Company’s own assumptions. The Company utilizes a third party pricing service for the valuation of the majority of its fixed maturity securities and receives one quote per security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. Since fixed maturities other than U.S. Treasury securities generally do not trade on a daily basis, the pricing service prepares estimates of fair value for those securities using pricing techniques based on a market approach. Inputs into the fair value pricing common to all asset classes include: benchmark U.S. Treasury security yield curves; reported trades of identical or similar fixed maturity securities; broker/dealer quotes of identical or similar fixed maturity securities and structural characteristics such as maturity date, coupon, mandatory principal payment dates, frequency of interest and principal payments, and optional redemption features. Inputs into the fair value applications that are unique by asset class include, but are not limited to: • U.S. government agencies – determination of direct versus indirect government support and whether any contingencies exist with respect to the timely payment of principal and interest. • Foreign government – estimates of appropriate market spread versus underlying related sovereign treasury curve(s) dependent on liquidity and direct or contingent support. • Municipals – overall credit quality, including assessments of the level and variability of: sources of payment such as income, sales or property taxes, levies or user fees; credit support such as insurance; state or local economic and political base; natural resource availability; and susceptibility to natural or man-made catastrophic events such as hurricanes, earthquakes or acts of terrorism. • Corporate fixed maturities – overall credit quality, including assessments of the level and variability of: economic sensitivity; liquidity; corporate financial policies; management quality; regulatory environment; competitive position; ownership; restrictive covenants; and security or collateral. • Residential mortgage-backed securities – estimates of prepayment speeds based upon: historical prepayment rate trends; underlying collateral interest rates; geographic concentration; vintage year; borrower credit quality characteristics; interest rate and yield curve forecasts; government or monetary authority support programs; tax policies; delinquency/default trends; and, in the case of non-agency collateralized mortgage obligations, severity of loss upon default and length of time to recover proceeds following default. • Commercial mortgage-backed securities – overall credit quality, including assessments of the value and supply/demand characteristics of: collateral type such as office, retail, residential, lodging, or other; geographic concentration by region, state, metropolitan statistical area and locale; vintage year; historical collateral performance including defeasance, delinquency, default and special servicer trends; and capital structure support features. • Asset-backed securities – overall credit quality, including assessments of the underlying collateral type such as credit card receivables, auto loan receivables and equipment lease receivables; geographic diversification; vintage year; historical collateral performance including delinquency, default and casualty trends; economic conditions influencing use rates and resale values; and contract structural support features. Generally, all prices provided by the pricing service, except actively traded securities with quoted market prices, are reported as Level 2. The Company holds privately placed fixed maturity securities and certain other fixed maturity securities that do not have an active market and for which the pricing service cannot provide fair values. The Company determines fair values for these securities using either matrix pricing utilizing the market approach or broker quotes. The Company will use observable market data as inputs into the fair value techniques, as discussed in the determination of Level 2 fair values, to the extent it is available, but is also required to use a certain amount of unobservable judgment due to the illiquid nature of the securities involved. Unobservable judgment reflected in the Company’s matrix model accounts for estimates of additional spread required by market participants for factors such as issue size, structural complexity, high bond coupon or other unique features. These matrix-priced securities are reported as Level 2 or Level 3, depending on the significance of the impact of unobservable judgment on the security’s value. Additionally, the Company may obtain non-binding broker quotes which are reported as Level 3. EQUITY SECURITIES Level 1 consists of publicly traded securities, including exchange traded funds, valued at quoted market prices. Level 2 includes securities that are valued using pricing for similar securities and pricing models that incorporate observable inputs. Level 3 consists of common or preferred stock of private companies for which observable inputs are not available. The Company utilizes a third-party pricing service for the valuation of the majority of its equity securities and receives one quote for each equity security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. The Company holds certain equity securities that have been issued by privately held entities that do not have an active market and for which the pricing service cannot provide fair values. Generally, the Company estimates fair value for these securities based on the issuer’s book value and market multiples and reports them as Level 3. Additionally, the Company may obtain non-binding broker quotes which are reported as Level 3. OTHER INVESTMENTS Other investments primarily include mortgage participations and limited partnerships not subject to the equity method of accounting. The fair values of limited partnerships not subject to the equity method of accounting are based on the net asset value provided by the general partner adjusted for recent financial information and are excluded from the fair value hierarchy. The estimated fair value of the financial instruments were as follows: December 31, 2018 December 31, 2017 Carrying Fair Carrying Fair (in millions) Value Value Value Value Financial Assets carried at (1) Fair Value through AOCI: Fixed maturities $ 6,161.5 $ 6,161.5 $ 5,749.3 $ 5,749.3 Equity securities — — 576.2 576.2 Other investments — — 3.6 3.6 Fair Value through Net Income: Equity securities 464.4 464.4 — — Other investments 175.0 175.0 — — Amortized Cost/Cost: Other investments 414.4 418.9 512.8 518.3 Cash and cash equivalents 1,020.7 1,020.7 297.9 297.9 Total financial instruments, excluding held-for-sale 8,236.0 8,240.5 7,139.8 7,145.3 Financial instruments, held-for-sale (Chaucer) 27.8 27.8 2,232.0 2,232.0 Total financial instruments $ 8,263.8 $ 8,268.3 $ 9,371.8 $ 9,377.3 Financial Liabilities carried at: Amortized Cost: Debt $ 777.9 $ 825.0 $ 786.9 $ 865.7 (1) Due to the adoption of ASU No. 2016-01, certain classifications are not comparable to the prior year. See Note 1 – “New Accounting Pronouncements”. The Company has processes designed to ensure that the values received from its third-party pricing service are accurately recorded, that the data inputs and valuation approaches and techniques utilized are appropriate and consistently applied, and that the assumptions are reasonable and consistent with the objective of determining fair value. The Company performs a review of the fair value hierarchy classifications and of prices received from its pricing service on a quarterly basis. The Company reviews the pricing services’ policies describing its methodology, processes, practices and inputs, including various financial models used to value securities. Also, the Company reviews the portfolio pricing, including a process for which securities with changes in prices that exceed a defined threshold are verified to independent sources, if available. If upon review, the Company is not satisfied with the validity of a given price, a pricing challenge would be submitted to the pricing service along with supporting documentation for its review. The Company does not adjust quotes or prices obtained from the pricing service unless the pricing service agrees with the Company’s challenge. During 2018 and 2017, 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. Reclassifications between levels of the fair value hierarchy are reported as of the beginning of the period in which the reclassification occurs. 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 assets that were measured at fair value on a recurring basis. DECEMBER 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 409.9 $ 154.9 $ 255.0 $ — Foreign government 7.4 — 7.4 — Municipal 885.8 — 864.7 21.1 Corporate 3,410.7 — 3,409.9 0.8 Residential mortgage-backed, U.S. agency backed 713.7 — 713.7 — Residential mortgage-backed, non-agency 2.7 — 2.7 — Commercial mortgage-backed 640.3 — 627.2 13.1 Asset backed 91.0 — 91.0 — Total fixed maturities 6,161.5 154.9 5,971.6 35.0 Equity securities 464.4 463.3 — 1.1 Other investments 3.5 — — 3.5 Total investment assets at fair value, excluding held-for-sale (Chaucer) 6,629.4 618.2 5,971.6 39.6 Investment assets, held-for-sale 24.5 5.7 18.8 — Total investment assets at fair value $ 6,653.9 $ 623.9 $ 5,990.4 $ 39.6 DECEMBER 31, 2017 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 322.3 $ 140.0 $ 182.3 $ — Foreign government 8.5 — 8.5 — Municipal 934.0 — 909.4 24.6 Corporate 3,196.6 — 3,195.7 0.9 Residential mortgage-backed, U.S. agency backed 605.0 — 605.0 — Residential mortgage-backed, non-agency 28.2 — 28.2 — Commercial mortgage-backed 595.0 — 580.8 14.2 Asset backed 59.7 — 59.7 — Total fixed maturities 5,749.3 140.0 5,569.6 39.7 Equity securities 567.8 566.7 — 1.1 Other investments 3.6 — — 3.6 Total investment assets at fair value, excluding held-for-sale (Chaucer) 6,320.7 706.7 5,569.6 44.4 Investment assets, held-for-sale 2,153.5 138.0 2,013.3 2.2 Total investment assets at fair value $ 8,474.2 $ 844.7 $ 7,582.9 $ 46.6 Limited partnerships measured at fair value using NAV based on an ownership interest in partners’ capital have not been included in the hierarchy tables. At December 31, 2018 and 2017, the fair values of these investments were $171.5 million and $149.4 million, respectively, approximately 2% of total investment assets, excluding assets-held-for-sale. 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, 2018 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 1,020.7 $ 1,020.7 $ — $ — Other investments 418.9 — 8.7 410.2 Total financial instruments, excluding held-for-sale 1,439.6 1,020.7 8.7 410.2 Financial instruments, held-for-sale (Chaucer) 3.3 3.3 — — Total financial instruments $ 1,442.9 $ 1,024.0 $ 8.7 $ 410.2 Liabilities: Debt $ 825.0 $ — $ 825.0 $ — DECEMBER 31, 2017 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 297.9 $ 297.9 $ — $ — Equity securities 8.4 — 8.4 — Other investments 368.9 — — 368.9 Total financial instruments, excluding held-for-sale 675.2 297.9 8.4 368.9 Financial instruments, held-for-sale (Chaucer) 78.5 78.5 — — Total financial instruments $ 753.7 $ 376.4 $ 8.4 $ 368.9 Liabilities: Debt $ 865.7 $ — $ 865.7 $ — 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, 2018 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year, excluding held-for-sale (Chaucer) $ 24.6 $ 0.9 $ 14.2 $ 39.7 $ 4.7 $ 44.4 Transfers out of Level 3 (0.5 ) — — (0.5 ) — (0.5 ) Total gains (losses): Included in total net realized and unrealized investment gains (losses) 0.1 — — 0.1 (0.1 ) — Included in other comprehensive income - net depreciation on available-for-sale securities (0.2 ) — (0.3 ) (0.5 ) — (0.5 ) Sales (2.9 ) (0.1 ) (0.8 ) (3.8 ) — (3.8 ) Balance at end of year (1) $ 21.1 $ 0.8 $ 13.1 $ 35.0 $ 4.6 $ 39.6 (1) There were no Level 3 held-for-sale securities (Chaucer) at December 31, 2018. YEAR ENDED DECEMBER 31, 2017 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year, excluding held-for-sale (Chaucer) $ 28.8 $ 4.0 $ 15.0 $ 47.8 $ 5.2 $ 53.0 Transfers out of Level 3 (1.9 ) — — (1.9 ) — (1.9 ) Total gains (losses): Included in total net realized and unrealized investment gains — 0.3 — 0.3 — 0.3 Included in other comprehensive income - net depreciation on available-for-sale securities (0.2 ) (0.2 ) — (0.4 ) (0.5 ) (0.9 ) Sales (2.1 ) (3.2 ) (0.8 ) (6.1 ) — (6.1 ) Balance at end of year, excluding held-for-sale 24.6 0.9 14.2 39.7 4.7 44.4 Balance at end of year, held-for-sale (Chaucer) 2.2 — — 2.2 — 2.2 Balance at end of year, including held-for-sale $ 26.8 $ 0.9 $ 14.2 $ 41.9 $ 4.7 $ 46.6 During the years ended December 31, 2018 and 2017, the Company transferred fixed maturities between Level 2 and Level 3 primarily as a result of assessing the significance of unobservable inputs on the fair value measurement. There were no transfers between Level 1 and Level 2 during 2018 or 2017. There were no Level 3 liabilities held by the Company for years ended December 31, 2018 and 2017. The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets, excluding amounts relating to the Chaucer business held-for-sale. 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, 2018 2017 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted Discount for: $ 21.1 $ 24.6 cash flow Small issue size 0.7 - 6.8% (3.4%) 0.7 - 6.8% (3.5%) Credit stress 1.3% (1.3%) 0.9 - 1.5% (1.2%) Above-market coupon 0.3 - 0.5% (0.5%) 0.3 - 0.5% (0.4%) Corporate Discounted Discount for: 0.8 0.9 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: 13.1 14.2 mortgage-backed cash flow Small issue size 1.9 - 3.1% (2.7%) 1.9 - 3.1% (2.6%) Above-market coupon 0.5% (0.5%) 0.5% (0.5%) Lease structure 0.3% (0.3%) 0.3% (0.3%) Equity securities Market comparables Net tangible asset market multiples 1.1 1.0X (1.0X) 1.1 1.0X (1.0X) Other Discounted cash flow Discount rate 3.5 18.0% (18.0%) 3.6 18.0% (18.0%) Significant increases (decreases) in any of the above inputs in isolation would result in a significantly lower (higher) fair value measurement. There were no interrelationships between these inputs which might magnify or mitigate the effect of changes in unobservable inputs on the fair value measurement.

Debt and Credit Arrangements

Debt and Credit Arrangements	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt and Credit Arrangements	6. DEBT AND CREDIT ARRANGEMENTS Debt consists of the following: DECEMBER 31 2018 2017 (in millions) Senior debentures maturing April 15, 2026 $ 375.0 $ 375.0 Senior debentures maturing October 15, 2025 62.6 62.6 Subordinated debentures maturing March 30, 2053 175.0 175.0 Subordinated debentures maturing February 3, 2027 50.1 59.7 FHLB borrowings (secured) 125.0 125.0 Total principal debt 787.7 797.3 Unamortized debt issuance costs (9.8 ) (10.4 ) Total $ 777.9 $ 786.9 The Company has outstanding 7.625% unsecured senior debentures with a par value of $62.6 million as of December 31, 2018 and 2017 and mature on October 15, 2025. Additionally, the Company has outstanding unsecured senior debentures that were issued on April 8, 2016 with a par value of $375.0 million, as of December 31, 2018 and 2017 and mature on April 15, 2026. Both 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 has outstanding $175.0 million aggregate principal amount of 6.35% subordinated unsecured debentures due March 30, 2053. These debentures pay interest quarterly. The Company may redeem these debentures in whole at any time, or in part from time to time, on or after March 30, 2018, at a redemption price equal to their principal amount plus accrued and unpaid interest. In addition, the Company’s subordinated debentures maturing February 3, 2027 have a par value of $50.1 million as of December 31, 2018 and $59.7 million as of December 31, 2017 and pay cumulative dividends semi-annually at 8.207%. During the third quarter of 2018, the Company repurchased subordinated debentures maturing February 3, 2027 with a net carrying value of $9.6 million at a cost of $11.5 million, resulting in a loss of $1.9 million. In 2009, Hanover Insurance received a $125.0 million advance through its membership in the FHLB. This collateralized advance bears interest at a fixed rate of 5.50% per annum over a twenty-year term. As collateral to FHLB, the Company pledged government agency securities with a fair value of $234.7 million and $223.6 million, for the aggregate borrowings of $125.0 million as of December 31, 2018 and December 31, 2017, respectively. The fair value of the collateral pledged must be maintained at certain specified levels of the borrowed amount, which can vary depending on the type of assets pledged. If the fair value of this collateral declines below these specified levels, the Company would be required to pledge additional collateral or repay outstanding borrowings. The Company is permitted to voluntarily repay the outstanding borrowings at any time, subject to a repayment fee. As a requirement of membership in the FHLB, the Company maintains a certain level of investment in FHLB stock. Total holdings of FHLB stock were $8.7 million and $8.5 million at December 31, 2018 and 2017, respectively. At December 31, 2018, the Company notified FHLB of its intent to repay its $125 million advance and recorded a pre-tax charge of $26.3 million related to the pre-payment provision. At December 31, 2018, the Company had a $200.0 million credit agreement which expires in May 2019. The Company had no borrowings under this agreement as of December 31, 2018 Interest expense was $ 45.1

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 (in millions) Income from continuing operations before income taxes $ 282.5 $ 292.9 $ 66.0 Income tax expense: Current $ 46.2 $ 3.1 $ 26.2 Deferred (2.7 ) 73.7 (27.2 ) Total income tax expense (benefit) $ 43.5 $ 76.8 $ (1.0 ) 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% in 2018 and 35% in both 2017 and 2016. The sources of the difference and the tax effects of each were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Expected income tax expense $ 59.3 $ 102.5 $ 23.1 Effect of the enactment of the Tax Cuts and Jobs Act (4.3 ) (3.9 ) — Tax difference related to investment disposals and maturities (9.2 ) (12.7 ) (20.7 ) Stock-based compensation windfall benefit (2.3 ) (5.3 ) — Nondeductible expenses 1.6 1.0 1.0 Dividend received deduction (1.2 ) (3.2 ) (3.3 ) Tax-exempt interest (0.4 ) (0.9 ) (1.1 ) Change in liability for uncertain tax positions — (0.5 ) — Other, net — (0.2 ) — Income tax expense $ 43.5 $ 76.8 $ (1.0 ) Effective tax rate 15.4 % 26.2 % -1.5 % The following are the components of the Company’s deferred tax assets and liabilities, (excluding those associated with its discontinued operations.) DECEMBER 31 2018 2017 (in millions) Deferred tax assets: Loss, LAE and unearned premium reserves, net $ 129.8 $ 115.7 Employee benefit plans 14.5 18.7 Tax credit carryforwards — 23.1 Investments, net 4.1 — Other 16.3 11.5 164.7 169.0 Less: Valuation allowance 0.2 0.2 164.5 168.8 Deferred tax liabilities: Deferred acquisition costs 95.0 90.2 Investments, net — 41.8 Software capitalization 18.3 18.0 Other 0.6 1.8 113.9 151.8 Net deferred tax asset $ 50.6 $ 17.0 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. 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, $12.7 million and $20.7 million, are recognized as part of income from continuing operations during 2018, 2017 and 2016, respectively. The remaining amount of $35.6 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 2018 2017 2016 (in millions) Liability at beginning of year, net $ 3.0 $ 2.7 $ 3.0 Additions for tax positions of current year — 0.9 0.4 Subtractions as a result of a lapse of the applicable statute of limitations — (0.6 ) (0.7 ) Liability at end of year, net $ 3.0 $ 3.0 $ 2.7 There are no tax positions at December 31, 2018, 2017 and 2016 for which the ultimate deductibility is highly certain, but for which there is 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, 2018, 2017 and 2016 the Company recognized a de minimis amount of net interest and has not recognized any penalties associated with unrecognized tax benefits. In 2019, the Company is expecting to release $1.7 million of liability due to the expiration of a statute of limitations. The Company or 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 2014. U.S. Tax Reform On December 22, 2017, the Tax Cuts and Jobs Act (“TCJA”, “U.S. Tax Reform”, or “the Act”) was enacted in the U.S. The Act substantially changed many aspects of the U.S. Tax code, including a reduction in the U.S. corporate income tax rate from 35% to 21%. While the new corporate rate is effective on January 1, 2018, the Company has recognized the impact of the rate change on its deferred tax balances as of the enactment date. The effect of this re-measurement of the Company’s deferred tax balances was a provision of $9.4 million for the year ended December 31, 2017. This provision was recorded as a component of income tax expense in continuing operations and as a component of the Chaucer business and reflected a tax benefit of $4.1 million and an expense of $13.5 million, respectively. This amount included the revaluation of deferred taxes initially recorded through other comprehensive income and recorded through discontinued operations, such as unrealized appreciation on investments, employee benefit plan-related items, foreign currency translation adjustments and reserve adjustments for discontinued business. Deferred taxes related to the revaluation of the Company’s pension plans at December 31, 2017, as well as changes in unrealized gains and losses occurring after the Act’s enactment date, were recorded at 21% in other comprehensive income. The Act also created a territorial tax system, which will generally allow companies to repatriate future non-U.S. sourced earnings without incurring additional U.S. taxes, by providing a 100% exemption on dividends received from certain non-U.S. subsidiaries. Although most of the Company’s non-U.S. income had been previously subject to U.S. taxes, a portion of its non-U.S. income had been indefinitely reinvested overseas and was not subject to U.S. tax until repatriated. These non-U.S. earnings were subject to a one-time mandatory toll charge totaling $12.7 million, which was recorded as a component of income tax expense in discontinued Chaucer business for the year ended December 31, 2017. In addition, the Act limited various existing deductions such as executive compensation and introduced new income taxes on certain low-taxed non-U.S. income. Under the Act, the exemption from the $1 million limitation on certain executive compensation has been eliminated. As a result, the Company recognized a provision of $0.2 million for the year ended December 31, 2017. The Act modified the provisions applicable to the determination of the tax basis of unpaid loss reserves. These modifications impact the payment pattern and applicable interest rate. The Act instructed the Treasury Department to provide discount factors and other guidance necessary to determine the appropriate transition adjustment. This information was released in November 2018 and the transition adjustment has been determined to total $126.2 million, resulting in a deferred tax liability of $26.5 million. This transitional amount will be taken into account ratably over eight years beginning in calendar year 2018. These provisions have no overall effect on the net deferred tax asset or income tax expense for the years ended December 31, 2017 and 2018. The cumulative effect of the enactment of TCJA was an expense of $22.3 million for the year ended December 31, 2017, comprising the aforementioned three components. The Company’s estimates were not based upon provisional amounts, as defined in the SEC’s Staff Accounting Bulletin No. 118.

Pension Plans

Pension Plans	12 Months Ended
	Dec. 31, 2018
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 is required to aggregate separately all overfunded plans from all underfunded plans. Prior to its sale of Chaucer on December 28, 2018, the Company’s pension obligation included defined benefit pension retirement benefits for Chaucer’s employees. U.S. 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 the employees’ 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, 2018, based on current estimates of plan liabilities and other assumptions, the projected benefit obligation of the qualified defined benefit pension plan exceeds plan assets by approximately $18.8 million. Assumptions U.S. 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 2018 2017 2016 U.S. Discount rate - qualified plan 4.50 % 3.88 % 4.25 % Discount rate - non-qualified plan 4.50 % 3.88 % 4.25 % Cash balance interest crediting rate 3.50 % 3.50 % 3.50 % The Company utilizes a measurement date of January 1 st YEARS ENDED DECEMBER 31 2018 2017 2016 U.S.Qualified plan Discount rate 3.88 % 4.25 % 4.88 % Expected return on plan assets 4.75 % 5.00 % 5.25 % Cash balance interest crediting rate 3.50 % 3.50 % 3.50 % U.S. Non-qualified plan Discount rate 3.88 % 4.25 % 4.75 % 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 U.S. defined benefit plans, the expected return on plan assets for 2018 of 4.75% reflects long-term expectations and remains generally consistent with 2017. The Company reviews and updates, at least annually, its expected return on plan assets based on changes in the actual assets held by the plans and market conditions. Plan Assets U.S. Qualified Defined Benefit Plan The Company utilizes a target allocation approach, which focuses on creating a mix of assets that will generate modest growth from equity securities while minimizing volatility in the Company’s earnings 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. The Company reviews and updates, at least annually, the target allocation and makes changes periodically. The following table provides 2018 target allocations and actual invested asset allocations for 2018 and 2017. DECEMBER 31 2018 TARGET LEVELS 2018 2017 Fixed income securities: Fixed maturities 83 % 85 % 83 % Money market funds 2 % 2 % 1 % Total fixed income securities 85 % 87 % 84 % Equity securities: Domestic 12 % 10 % 12 % International 3 % 3 % 4 % Total equity securities 15 % 13 % 16 % Total plan assets 100 % 100 % 100 % The following tables present, for each hierarchy level, the U.S. qualified defined benefit plan’s investment assets that are measured at fair value at December 31, 2018 and 2017. Refer to Note 5 – “Fair Value” for a description of the different levels in the Fair Value Hierarchy. DECEMBER 31 2018 2017 (in millions) Total Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Fixed income securities: Fixed maturities $ 35.1 $ 9.3 $ — $ 25.8 $ 38.8 $ 13.1 $ — $ 25.7 Money market mutual funds 9.1 9.1 — — 5.3 5.3 — — Mutual funds 36.3 36.3 — — 42.6 42.6 — — Total investments at fair value $ 80.5 $ 54.7 $ — $ 25.8 $ 86.7 $ 61.0 $ — $ 25.7 Fixed Income Securities and Mutual Funds Securities classified as Level 1 at December 31, 2018 and 2017 include actively traded mutual funds and publicly traded securities, which are valued at quoted market prices. Securities classified as Level 3 at December 31, 2018 and 2017 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 and international equity securities. 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 2018 2017 Fixed maturities $ 346.4 $ 343.5 Equity securities: Domestic 9.3 12.6 International 14.6 17.3 Total equity 23.9 29.9 Total investments carried at NAV $ 370.3 $ 373.4 The table below provides a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). YEAR ENDED DECEMBER 31 2018 (in millions) Balance at beginning of period $ 25.7 Plus: Assets transferred from investments measured at fair value using NAV 2.1 Less: Assets transferred to Level 1 investments (2.7 ) Actual return on plan assets related to assets still held 0.7 Balance at end of year $ 25.8 Obligations and Funded Status The Company recognizes the current net underfunded 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 loss or income. The components of accumulated other comprehensive loss or income are reflected as either a net actuarial gain or loss or a net prior service cost. The following table reflects the benefit obligations, fair value of plan assets and funded status of the U.S. plans at December 31, 2018 and 2017. DECEMBER 31 U.S. Qualified Pension Plan U.S. Non-Qualified Pension Plan (in millions) 2018 2017 2018 2017 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 501.2 $ 506.8 $ 36.6 $ 38.0 Interest cost 18.4 20.4 1.4 1.5 Actuarial losses (gains) (17.5 ) 7.3 (1.3 ) 0.1 Benefits paid (32.6 ) (33.3 ) (2.9 ) (3.0 ) Benefit obligation, end of year (1) 469.5 501.2 33.8 36.6 Change in plan assets: Fair value of plan assets, beginning of period 460.1 453.3 — — Actual return on plan assets (16.8 ) 40.1 — — Contributions 40.0 — 2.9 3.0 Benefits paid (32.6 ) (33.3 ) (2.9 ) (3.0 ) Fair value of plan assets, end of year 450.7 460.1 — — Funded status of the plans $ (18.8 ) $ (41.1 ) $ (33.8 ) $ (36.6 ) (1) The accumulated benefit obligation for these plans is equal to the projected benefit obligation. The following table reflects the benefit obligations and fair value for the Chaucer plan as of December 31, 2017 and related changes in those balances, as well as the changes to those balances during 2018 prior to the plan’s disposition in conjunction with the sale of Chaucer on December 28, 2018. DECEMBER 31 Chaucer Pension Plan (in millions) 2018 2017 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 146.1 $ 132.2 Plan amendment 0.3 — Interest cost 3.7 3.9 Actuarial losses (gains) (8.6 ) 3.2 Benefits paid (3.9 ) (5.8 ) Foreign currency translation (8.3 ) 12.6 Disposition of benefit obligations with the sale of Chaucer (129.3 ) — Benefit obligation, end of year (1) — 146.1 Change in plan assets: Fair value of plan assets, beginning of period 144.6 116.5 Actual return on plan assets (5.8 ) 14.4 Contributions — 7.7 Benefits paid (3.9 ) (5.8 ) Foreign currency translation (8.1 ) 11.8 Disposition of plan assets with the sale of Chaucer (126.8 ) — Fair value of plan assets, end of year — 144.6 Funded status of the plans $ — $ (1.5 ) (1) The accumulated benefit obligation for the plan is equal to the projected benefit obligation. Components of Net Periodic Pension Cost The components of total net periodic pension cost are as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Interest cost $ 19.8 $ 21.9 $ 24.9 Expected return on plan assets (20.6 ) (21.4 ) (23.5 ) Recognized net actuarial loss 9.6 13.5 11.2 Net periodic pension cost $ 8.8 $ 14.0 $ 12.6 Net periodic pension benefit for Chaucer’s pension plan of $2.3 million, $0.3 million and $0.9 million in 2018, 2017 and 2016, respectively, have been reported in discontinued operations and are not included in the amounts disclosed above. During 2016, the Chaucer plan was closed to future salary accruals, which eliminated any remaining benefits to be accumulated in future periods, and resulted in a $2.4 million curtailment gain. An equal and offsetting expense was included in recognized net actuarial losses. These items are reflected in discontinued operations and not included in the amounts disclosed in the table above. The following table reflects the total amounts recognized in accumulated other comprehensive income relating to the U.S. defined benefit pension plans as of December 31, 2018 and 2017. DECEMBER 31 2018 2017 (in millions) Net actuarial loss $ 109.7 $ 100.8 The unrecognized net actuarial gains (losses) which exceed 10% of the greater of the projected benefit obligation or the fair value of plan assets are amortized as a component of net periodic pension cost over the next five years. The total estimated amount of actuarial losses that will be amortized from accumulated other comprehensive income into net periodic pension cost in 2019 is $11.9 million. Contributions In accordance with ERISA guidelines, the Company is not required to fund its U.S. qualified benefit plan in 2019. The Company expects to contribute $3.1 million to its U.S. non-qualified pension plans to fund 2019 benefit payments. During 2018, the Company made a discretionary contribution of $40.0 million to its qualified benefit plan. At this time, no additional discretionary contributions are expected to be made into any of the U.S. plans during 2019, and the Company does not expect that any funds will be returned from the plans to the Company during 2019. Benefit Payments The Company estimates that benefit payments over the next 10 years will be as follows: YEARS ENDED DECEMBER 31 2019 2020 2021 2022 2023 2024-2028 (in millions) U.S. qualified pension plan $ 38.5 $ 37.4 $ 38.7 $ 37.3 $ 37.1 $ 163.7 U.S. non-qualified pension plan $ 3.1 $ 3.0 $ 3.0 $ 2.9 $ 2.7 $ 12.4 The benefit payments are based on the same assumptions used to measure the Company’s benefit obligations at the end of 2018. 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 plans 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 its U.S. employees, whereby the Company matches employee elective 401(k) contributions, up to a maximum of 6% of eligible compensation in 2018, 2017, and 2016. The Company’s expense for this matching provision was $21.7 million, $21.3 million and $21.4 million for 2018, 2017, and 2016, respectively. In addition to this matching provision, the Company can elect to make an annual contribution to employees’ accounts. Additional contributions amounted to $2.2 million and were contributed to the plan during 2017. Chaucer also provides a defined contribution plan for its employees which includes employer contributions. Prior to its sale, the Company’s expense included $4.5 million, $4.8 million and $3.9 million for 2018, 2017 and 2016, respectively, related to these benefits; such costs are reflected in discontinued operations.

Other Postretirement Benefit Pl

Other Postretirement Benefit Plans	12 Months Ended
	Dec. 31, 2018
Postemployment Benefits [Abstract]
Other Postretirement Benefit Plans	9. OTHER POSTRETIREMENT BENEFIT PLANS In addition to the Company’s pension plans, the Company has postretirement medical benefits that it provides to former agents of its discontinued life businesses and retirees and their dependents. The plans, which are funded by the Company through either a Health Reimbursement Arrangement (“HRA”) or directly, provide access to benefits including hospital and major medical, with certain limits, and have varying co-payments and deductibles, depending on the plan. Generally, employees who were actively employed on December 31, 1995 became eligible with at least 15 years The Company has recognized the funded status of its postretirement benefit plans in its Consolidated Balance Sheets. Since the plans are unfunded, the amount recognized in the Consolidated Balance Sheets is equal to the accumulated benefit obligation of the plans. The components of accumulated other comprehensive income or loss are reflected as either a net actuarial gain or loss or a net prior service cost. Obligation and Funded Status The following table reflects the funded status of these plans: DECEMBER 31 2018 2017 (in millions) Change in benefit obligation: Accumulated postretirement benefit obligation, beginning of year $ 10.1 $ 11.6 Interest cost 0.4 0.4 Net actuarial gain (0.9 ) (0.3 ) Benefits paid (1.2 ) (1.6 ) Accumulated postretirement benefit obligation, end of year 8.4 10.1 Fair value of plan assets, end of year — — Funded status of plans $ (8.4 ) $ (10.1 ) Benefit Payments The Company estimates that benefit payments over the next 10 years will be as follows: YEARS ENDING DECEMBER 31 (in millions) 2019 $ 1.1 2020 1.0 2021 0.9 2022 0.8 2023 0.8 2024 - 2028 3.1 The benefit payments are based on the same assumptions used to measure the Company’s benefit obligation at the end of 2018 and reflect benefits attributable to estimated future service. Components of Net Periodic Postretirement Expense (Benefit) The components of net periodic postretirement expense (benefit) were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Interest cost $ 0.4 $ 0.4 $ 0.5 Recognized net actuarial loss 0.2 0.2 0.2 Amortization of prior service cost (benefit) (0.3 ) (1.4 ) (1.4 ) Net periodic postretirement cost (benefit) $ 0.3 $ (0.8 ) $ (0.7 ) The following table reflects the balances in accumulated other comprehensive income relating to the Company’s postretirement benefit plans: DECEMBER 31 2018 2017 (in millions) Net actuarial loss $ 2.5 $ 3.6 Net prior service cost (benefit) (0.1 ) (0.4 ) $ 2.4 $ 3.2 The estimated amortization to be recognized in net periodic benefit cost in 2019 is de minimis. Assumptions Employers are required to measure the funded status of their plans as of the date of their year-end statement of financial position. As such, the Company has utilized a measurement date of December 31, 2018 and 2017, to determine its postretirement benefit obligations, consistent with the date of its Consolidated Balance Sheets. Weighted average discount rate assumptions used to determine postretirement benefit obligations and periodic postretirement costs are as follows: YEARS ENDED DECEMBER 31 2018 2017 Postretirement benefit obligations discount rate 4.38 % 3.75 % Postretirement benefit cost discount rate 3.75 % 4.13 % The following table provides the assumed health care cost trend rates used for 2018 and 2017. A one-percentage point change in these assumptions would be de minimis. DECEMBER 31 2018 2017 Health care cost trend rate assumed for next year 6.50 % 6.50 % Rate to which the cost trend is assumed to decline (ultimate trend rate) 4.50 % 4.50 % Year the rate reaches the ultimate trend rate 2024 2024

Other Comprehensive Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Other Comprehensive Income	10. OTHER COMPREHENSIVE INCOME The following table provides changes in other comprehensive income. YEARS ENDED DECEMBER 31 2018 2017 2016 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 Unrealized gains (losses) on available- for-sale securities: Unrealized gains (losses) arising during period $ (209.7 ) $ 44.0 $ (165.7 ) $ 77.0 $ (27.9 ) $ 49.1 $ 95.9 $ (33.4 ) $ 62.5 Amount of realized (gains) losses from sales and other 2.5 (9.7 ) (7.2 ) (30.8 ) (1.4 ) (32.2 ) (36.5 ) (8.0 ) (44.5 ) Portion of other-than-temporary impairment losses recognized in earnings 3.5 (0.7 ) 2.8 3.9 (1.4 ) 2.5 27.9 (9.8 ) 18.1 Unrealized losses (gains) realized with sale of Chaucer business 24.2 (5.1 ) 19.1 — — — — — — Net unrealized gains (losses) (179.5 ) 28.5 (151.0 ) 50.1 (30.7 ) 19.4 87.3 (51.2 ) 36.1 Pension and postretirement benefits: Net gains (losses) arising in the period from net actuarial (gains) losses and prior service costs (21.3 ) 4.3 (17.0 ) 17.0 (3.4 ) 13.6 (43.1 ) 12.8 (30.3 ) Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost 9.7 (2.0 ) 7.7 14.0 (4.6 ) 9.4 9.9 (3.5 ) 6.4 Pension obligations recognized with sale of Chaucer business 21.2 (4.8 ) 16.4 — — — — — — Total pension and postretirement benefits 9.6 (2.5 ) 7.1 31.0 (8.0 ) 23.0 (33.2 ) 9.3 (23.9 ) Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period (2.2 ) 0.5 (1.7 ) 3.7 (1.3 ) 2.4 (5.1 ) 1.8 (3.3 ) Currency translation obligation recognized with sale of Chaucer business 29.4 (6.2 ) 23.2 — — — — — — Total cumulative foreign currency translation adjustment 27.2 (5.7 ) 21.5 3.7 (1.3 ) 2.4 (5.1 ) 1.8 (3.3 ) Other comprehensive income (loss) $ (142.7 ) $ 20.3 $ (122.4 ) $ 84.8 $ (40.0 ) $ 44.8 $ 49.0 $ (40.1 ) $ 8.9 Reclassifications out of accumulated other comprehensive income were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (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 Unrealized gains (losses) on available-for- sale securities $ (3.2 ) $ 26.6 $ 36.2 Net realized gains (losses) from sales and other (2.6 ) (3.6 ) (27.4 ) Net other-than-temporary impairment losses on investments recognized in earnings (5.8 ) 23.0 8.8 Total before tax 10.3 4.2 17.7 Tax benefit 4.5 27.2 26.5 Continuing operations; net of tax (19.1 ) — — Gain on sale of Chaucer business (0.2 ) 2.5 (0.2 ) Discontinued operations - Chaucer business 0.1 — (0.1 ) Discontinued life business; net of tax (14.7 ) 29.7 26.2 Net of tax Amortization of defined benefit pension and postretirement plans (9.5 ) (12.4 ) (9.7 ) Loss adjustment expenses and other operating expenses (1) 1.9 4.3 3.4 Tax benefit (7.6 ) (8.1 ) (6.3 ) Continuing operations; net of tax (16.4 ) — — Gain on sale of Chaucer business (0.1 ) (1.3 ) (0.1 ) Discontinued operations - Chaucer business (24.1 ) (9.4 ) (6.4 ) Net of tax Currency translation obligation recognized with sale of Chaucer business (23.2 ) — — Gain on sale of Chaucer business Total reclassifications for the period $ (62.0 ) $ 20.3 $ 19.8 Benefit (detriment) to 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, 2018, 2017 and 2016.

Stock-based Compensation Plans

Stock-based Compensation Plans	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation Plans	11. 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, 2018, there were 4,343,301 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”) and the Chaucer Share Incentive Plan (the “SIP Plan”), authorizing the issuance of 2,500,000 and 750,000 shares, respectively, under such plans. As of December 31, 2018, 2,371,060 shares were available for grant under the ESPP Plan. As a result of the sale of the Chaucer business in 2018, new grants of stock to the SIP Plan were terminated and the SIP Plan was transferred as part of the sale on December 28, 2018. Total shares awarded under this SIP Plan since 2014 were 74,189. Compensation cost, excluding the discontinued operations related to the Chaucer business, for the years ended December 31, 2018, 2017, and 2016 totaled $14.9 million, $10.5 million and $10.5 million, respectively. Related tax benefits were $3.1 million, $3.7 million and $3.7 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 Committee of the Board of Directors (the “Committee”) at the time of grant. Options granted in 2018, 2017 and 2016 generally vest over 3 years with 33 1/3% vesting in each year. Options must be exercised not later than ten years from the date of grant. Information on the Company’s stock options is summarized below. YEARS ENDED DECEMBER 31 2018 2017 2016 (in whole shares and dollars) Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Outstanding, beginning of year 1,062,177 $ 75.53 1,396,152 $ 68.63 1,619,948 $ 56.57 Granted 301,152 110.98 460,610 90.85 587,340 82.17 Exercised (210,190 ) 69.41 (464,726 ) 63.12 (589,666 ) 48.99 Forfeited or cancelled (54,063 ) 89.09 (329,859 ) 85.22 (221,470 ) 68.61 Outstanding, end of year 1,099,076 $ 85.75 1,062,177 $ 75.53 1,396,152 $ 68.63 Exercisable, end of year 515,286 $ 69.83 423,883 $ 62.41 441,256 $ 53.59 Cash received for options exercised for the years ended December 31, 2018, 2017 and 2016 was $14.3 million, $20.5 million and $15.8 million, respectively. The intrinsic value of options exercised for the years ended December 31, 2018, 2017 and 2016 was $9.6 million, $15.6 million and $21.1 million, respectively. The excess tax expense realized from options exercised for the years ended December 31, 2018, 2017 and 2016 was $1.6 million, $4.0 million, and $5.6 million, respectively. The aggregate intrinsic value at December 31, 2018 for shares outstanding and shares exercisable was $34.1 million and $24.2 million, respectively. At December 31, 2018, the weighted average remaining contractual life for shares outstanding and shares exercisable was 7.5 years and 6.2 years, respectively. Additional information about employee options outstanding and exercisable at December 31, 2018 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 $ 36.81 to $46.47 64,367 3.44 $ 41.61 64,367 $ 41.61 $ 57.99 93,778 5.14 57.99 93,778 57.99 $ 70.24 to $72.66 165,924 6.16 70.27 165,924 70.27 $ 74.88 to $82.74 190,120 7.30 80.75 112,847 80.49 $ 87.50 to $91.19 285,280 8.20 90.66 78,370 90.89 $ 110.57 to $124.14 299,607 9.18 110.98 — — 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, 2018, 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, 2018, 2017 and 2016 was $16.30, $13.05 and $10.83, respectively. The following significant assumptions were used to determine the fair value for options granted in the years indicated. 2018 2017 2016 Dividend yield 1.740% to 1.954 % 2.193% to 2.286 % 2.04% to 2.46 % Expected volatility 17.943% to 18.500 % 18.341% to 21.694 % 18.08% to 21.31 % Weighted average expected volatility 18.20 % 19.52 % 19.57 % Risk-free interest rate 2.365% to 2.969 % 1.324% to 2.201 % 0.77% to 1.97 % Expected term, in years 2.5 to 6.0 2.5 to 6.0 2.5 to 5.5 The expected dividend yield is based on the Company’s dividend payout rate(s), in the year noted. 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, 2018 and 2017 was $9.3 million and $8.3 million, respectively. As of December 31, 2018, the Company had unrecognized compensation expense of $3.7 million related to unvested stock options that is expected to be recognized over a weighted average period of 1.5 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 2018, 2017 and 2016. These units generally vest after 3 years of continued employment and after the achievement of certain stock performance targets. The Company also granted time-based restricted stock units to eligible employees in 2018, 2017 and 2016 that generally vest after 3 years of continued employment. The following table summarizes information about employee restricted stock units: YEARS ENDED DECEMBER 31 2018 2017 2016 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 298,528 $ 83.45 269,063 $ 73.91 301,897 $ 54.54 Granted 152,529 111.66 130,075 90.40 143,107 83.43 Vested (72,710 ) 72.69 (70,590 ) 59.29 (139,183 ) 42.97 Forfeited (1) (45,866 ) 94.04 (30,020 ) 84.88 (36,758 ) 69.07 Outstanding, end of year 332,481 $ 97.28 298,528 $ 83.45 269,063 $ 73.91 Performance and market-based restricted stock units: Outstanding, beginning of year 102,586 $ 81.21 115,057 $ 78.82 196,142 $ 47.89 Granted 35,063 118.60 60,101 79.48 126,796 73.42 Vested (14,032 ) 70.24 (17,642 ) 56.45 (144,141 ) 40.95 Forfeited (1) (53,779 ) 89.79 (54,930 ) 82.27 (63,740 ) 58.54 Outstanding, end of year (2) 69,838 $ 95.58 102,586 $ 81.21 115,057 $ 78.82 (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. (2) At December 31, 2018, only market-based awards were outstanding . In 2018, 2017 and 2016, the Company granted market-based awards totaling 31,688, 56,571, and 87,213, respectively, to certain members of senior management, which are included in the table above as performance and market-based restricted stock activity. The vesting of these stock units is based on the relative total shareholder return (“TSR”) of the Company. This metric is generally based on relative TSR for a three year period as compared to a group of Property and Casualty peer companies. The fair value of market-based awards was estimated at the date of grant using a valuation model. These units have the potential to range from 0% to 150% of the shares disclosed. In 2018, market-based restricted stock units of 3,115 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 2017, market-based restricted stock units of 5,881 were included as granted due to completion levels in excess of 100% for units granted in 2014. The weighted average grant date fair value of these awards was $56.45. In 2016, performance and market-based stock units of 2,268 and 30,134 were included as granted due to completion levels in excess of 100% for units granted in 2014 and 2013, respectively. The weighted average grant date fair value of these awards was $57.00 and $41.77, respectively. The intrinsic value of restricted stock and restricted stock units that vested during the years ended December 31, 2018, 2017 and 2016 was $2.9 million, $2.3 million and $5.4 million, respectively. The intrinsic value for performance and market-based restricted stock units that vested in 2018, 2017 and 2016 was $0.6 million, $0.6 million and $5.8 million, respectively. There were 10,330 shares, 53,840 shares and 63,740 shares of market-based awards that forfeited in 2018, 2017 and 2016, respectively. Also, during 2018 and 2017, there were 43,449 shares and 1,090 shares, respectively, of performance-based awards that forfeited. At December 31, 2018, the aggregate intrinsic value of restricted stock units was $39.1 million and the weighted average remaining contractual life was 1.3 years. The aggregate intrinsic value of market-based restricted stock units was $8.2 million and the weighted average remaining contractual life was 1.4 years. As of December 31, 2018, there was $19.7 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, 2018
Earnings Per Share [Abstract]
Earnings Per Share and Shareholders' Equity Transactions	12. EARNINGS PER SHARE AND SHAREHOLDERS’ EQUITY TRANSACTIONS The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: DECEMBER 31 2018 2017 2016 (in millions, except per share data) Basic shares used in the calculation of earnings per share 42.4 42.5 42.8 Dilutive effect of securities: Employee stock options 0.3 0.3 0.2 Non-vested stock grants 0.3 0.2 0.2 Diluted shares used in the calculation of earnings per share 43.0 43.0 43.2 Per share effect of dilutive securities on income from continuing operations $ (0.07 ) $ (0.05 ) $ (0.04 ) Per share effect of dilutive securities on net income $ (0.12 ) $ (0.05 ) $ (0.04 ) Diluted earnings per share during 2018, 2017 and 2016 excludes 0.3 million, 0.4 million and 0.6 million, respectively, of common shares issuable under the Company’s stock compensation plans, because their effect would be antidilutive. Prior to December 30, 2018, the Board of Directors had authorized a stock repurchase program, which, including subsequent amendments to increase the number of shares subject to repurchase, provided for aggregate repurchases of up to $900 million. This program was terminated on December 30, 2018 in conjunction with the establishment of the new program discussed below. Prior to termination, the Company purchased 0.5 57.7 On December 30, 2018, the Board of Directors of the Company declared a special dividend of $4.75 per share to be paid on January 25, 2019. The Company held liabilities of $193.4 million at December 31, 2018 for payment of this obligation.

Dividend Restrictions

Dividend Restrictions	12 Months Ended
	Dec. 31, 2018
Disclosure Of Restrictions On Dividends Loans And Advances Disclosure [Abstract]
Dividend Restrictions	13. DIVIDEND RESTRICTIONS U.S. 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 $140.0 million, $296.8 million and $218.8 million in 2018, 2017 and 2016, respectively. In 2017, the $296.8 million dividend included $80.0 million of extra ordinary dividends. At January 1, 2019, the maximum dividend payable without prior approval is $76.7 million. In May 2019, the maximum dividend declared payable without prior approval will increase by $140.0 million to a total amount of $216.7 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 dividends to its parent, Hanover Insurance, totaling $87.9 million, $99.9 million and $70.0 million in 2018, 2017 and 2016, respectively. At January 1, 2019, the maximum dividend payable without prior approval is $19.2 million. In December 2019, the maximum dividend declared payable without prior approval will increase by $87.9 million to a total amount of $107.1 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. CHAUCER Dividend payments previously paid by Chaucer to THG were regulated by U.K. law, whereby advance notice was provided to the U.K.’s Prudential Regulatory Authority. Dividend payments of $85.0 million and $79.5 million were made in 2018 and 2016, respectively.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment Information	14. SEGMENT INFORMATION The Company’s primary business operations include insurance products and services provided through three operating segments: Commercial Lines, Personal Lines and Other. Commercial Lines includes commercial multiple peril, commercial automobile, workers’ compensation, and other commercial coverages, such as inland marine, specialty program business, management and professional liability, surety and specialty property. Personal Lines includes personal automobile, homeowners and other personal coverages. Included in the Other segment are Opus Investment Management, Inc., which markets investment management services to institutions, pension funds and other organizations; earnings on holding company assets; holding company and other expenses, including certain costs associated with retirement benefits due to the Company’s former life insurance employees and agents; and, a run-off voluntary pools business. On December 28, 2018, the Company completed the sale of Chaucer, the major portion of our Lloyd’s international specialty business, to China Re. The Chaucer-related Irish entity was subsequently sold on February 14, 2019. The sale of the Australian entities is pending, subject only to local regulatory approval, and is expected to close in the first quarter of 2019. Accordingly, as of December 31, 2018, and for all prior periods, Chaucer’s results of operations have been classified as Discontinued Operations in the Consolidated Statements of Income and assets and liabilities related to the Chaucer business have been classified as held-for-sale in the Consolidated Balance Sheets (see Note 2 – “Discontinued Operations”). Certain ongoing expenses have been reclassified from Chaucer to the other three operating segments. 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 2018, this consisted of interest on the Company’s senior debentures, subordinated debentures, collateralized borrowings with the FHLB, and a letter of credit facility through the completion of the sale of Chaucer on December 28, 2018. Management evaluates the results of the aforementioned segments based on operating income before taxes, excluding interest expense on debt. Operating income before taxes excludes certain items which are included in net income, such as net realized and unrealized investment gains (losses). Such gains and losses are excluded since they are determined by interest rates, financial markets and the timing of sales. Also, operating income before taxes excludes net gains and losses on disposals of businesses, gains and losses related to the repayment of debt, discontinued operations, costs to acquire businesses, restructuring costs, the cumulative effect of accounting changes and certain other items. Although the items excluded from operating income before taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income before taxes enhances an investor’s understanding of the Company’s results of operations by highlighting net income attributable to the core operations of the business. However, operating income before taxes should not be construed as a substitute for income before income taxes and operating income should not be construed as a substitute for net income. Summarized below is financial information with respect to the Company’s business segments. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Operating revenues: Commercial Lines $ 2,739.5 $ 2,573.8 $ 2,485.0 Personal Lines 1,791.3 1,662.3 1,552.4 Other 14.2 10.7 6.3 Total 4,545.0 4,246.8 4,043.7 Net realized and unrealized investment gains (losses) (50.7 ) 21.1 10.2 Total revenues $ 4,494.3 $ 4,267.9 $ 4,053.9 Operating income (loss) before interest expense and income taxes: Commercial Lines: Underwriting income (loss) $ 85.0 $ 12.2 $ (123.2 ) Net investment income 182.2 165.8 158.5 Other expense (1.5 ) (0.6 ) (0.6 ) Commercial Lines operating income 265.7 177.4 34.7 Personal Lines: Underwriting income 66.6 83.7 103.1 Net investment income 73.7 70.1 69.5 Other income 5.9 4.9 5.1 Personal Lines operating income 146.2 158.7 177.7 Other: Underwriting loss (4.2 ) (4.2 ) (9.9 ) Net investment income 11.5 8.0 3.6 Other expense (12.7 ) (12.6 ) (13.2 ) Other operating loss (5.4 ) (8.8 ) (19.5 ) Operating income before interest expense and income taxes 406.5 327.3 192.9 Interest on debt (45.1 ) (45.2 ) (51.4 ) Operating income before income taxes 361.4 282.1 141.5 Non-operating income (loss) items: Net realized and unrealized investment gains (losses) (50.7 ) 21.1 10.2 Net loss from repayment of debt (28.2 ) — (88.3 ) Other non-operating items — (10.3 ) 2.6 Income from continuing operations before income taxes $ 282.5 $ 292.9 $ 66.0 The following table provides identifiable assets for the Company’s business segments and discontinued operations: DECEMBER 31 2018 2017 (in millions) Identifiable Assets U.S. Companies $ 12,238.4 $ 10,915.0 Assets held-for-sale (1) 57.4 4,466.6 Assets of discontinued life business 103.9 88.0 Total $ 12,399.7 $ 15,469.6 (1) See also Note 2 – “Discontinued Operations” for a discussion on the sale of the Chaucer business. The Company reviews the assets of its U.S. Companies collectively and does not allocate them among the Commercial Lines, Personal Lines and Other segments.

Lease Commitments

Lease Commitments	12 Months Ended
	Dec. 31, 2018
Leases [Abstract]
Lease Commitments	15. LEASE COMMITMENTS Rental expenses for operating leases from continuing operations amounted to $16.7 million, $14.8 million and $15.6 million in 2018, 2017 and 2016, respectively. These expenses relate primarily to building leases of the Company. At December 31, 2018, future minimum rental payments under non-cancelable operating leases were $41.1 million, payable as follows: 2019 - $13.5 million; 2020 - $11.0 million; 2021 - $8.3 million and 2022 - $4.5 million and 2023 and thereafter - $3.8 million. It is expected that in the normal course of business, leases that expire will generally be renewed or replaced by leases on similar property and equipment. Rental expenses for operating leases related to the former Chaucer business amounted to $3.8 million, $3.3 million and $2.9 million in 2018, 2017 and 2016, respectively. These expenses relate primarily to building leases of Chaucer and are included in discontinued operations.

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2018
Reinsurance Disclosures [Abstract]
Reinsurance	16. 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 THG’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. Insurers are allowed to pass along this cost to Michigan automobile policyholders. The Company ceded to the MCCA premiums earned and losses and LAE incurred of $70.9 million and $108.8 million in 2018, $62.8 million and $81.4 million in 2017, and $58.6 million and $75.0 million in 2016. The MCCA represented 59.3% of the total reinsurance receivable balance at December 31, 2018. Reinsurance recoverables related to MCCA were $977.1 million and $930.6 million at December 31, 2018 and 2017, 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, 2018, 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 2018 2017 2016 (in millions) Premiums written: Direct $ 4,816.0 $ 4,540.0 $ 4,265.7 Assumed 27.6 23.6 25.1 Ceded (458.8 ) (454.5 ) (408.1 ) Net premiums written $ 4,384.8 $ 4,109.1 $ 3,882.7 Premiums earned: Direct $ 4,673.6 $ 4,399.9 $ 4,162.6 Assumed 26.6 23.6 24.4 Ceded (445.8 ) (443.1 ) (397.5 ) Net premiums earned $ 4,254.4 $ 3,980.4 $ 3,789.5 Percentage of assumed to net premiums earned 0.6 % 0.6 % 0.6 % Losses and LAE: Direct $ 2,986.2 $ 2,934.6 $ 2,785.1 Assumed 25.2 20.5 30.6 Ceded (1) (286.8 ) (375.5 ) (269.7 ) Net losses and LAE $ 2,724.6 $ 2,579.6 $ 2,546.0 (1) The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines.

Liabilities For Outstanding Cla

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses	12 Months Ended
	Dec. 31, 2018
Insurance [Abstract]
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses	17. 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 which 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 review 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 loss adjustment expenses. 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 loss adjustment expenses 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 loss expense 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 or segment of business, effects of the economy and political outlook, perceived anomalies in the historical results, evolving trends or other factors. 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 “prior years’ loss reserves”. Such development can be either favorable or unfavorable to the Company’s financial results and may vary by line of business. In this section, all amounts presented include catastrophe losses and LAE, unless otherwise indicated. The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Gross loss and LAE reserves, beginning of year $ 5,058.5 $ 4,660.0 $ 4,201.9 Reinsurance recoverable on unpaid losses 1,455.0 1,349.2 1,295.3 Net loss and LAE reserves, beginning of year 3,603.5 3,310.8 2,906.6 Net incurred losses and LAE in respect of losses occurring in: Current year 2,733.5 2,579.8 2,307.8 Prior years (8.9 ) (0.2 ) 238.2 Total incurred losses and LAE 2,724.6 2,579.6 2,546.0 Net payments of losses and LAE in respect of losses occurring in: Current year 1,232.3 1,203.8 1,063.0 Prior years 1,264.3 1,083.1 1,078.8 Total payments 2,496.6 2,286.9 2,141.8 Net reserve for losses and LAE, end of year 3,831.5 3,603.5 3,310.8 Reinsurance recoverable on unpaid losses 1,472.6 1,455.0 1,349.2 Gross reserve for losses and LAE, end of year $ 5,304.1 $ 5,058.5 $ 4,660.0 The following table provides a summary of (favorable)/unfavorable loss and LAE reserve development. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Commercial multiple peril $ (4.0 ) $ 2.9 $ 68.8 Workers’ compensation (31.0 ) (9.1 ) (46.7 ) Commercial automobile 23.2 2.3 27.5 Other commercial lines: AIX program business 17.0 (1.9 ) 75.1 General liability (21.1 ) (1.9 ) 56.0 Surety (9.0 ) 0.1 35.3 Umbrella 2.3 0.9 (8.9 ) Other lines (18.3 ) (4.1 ) 12.2 Total other commercial lines (29.1 ) (6.9 ) 169.7 Total Commercial Lines (40.9 ) (10.8 ) 219.3 Personal automobile 14.3 3.4 4.8 Homeowners and other personal lines 16.5 6.0 5.8 Total Personal Lines 30.8 9.4 10.6 Total Other Segment 1.2 1.2 8.3 Total loss and LAE reserve development, including catastrophes $ (8.9 ) $ (0.2 ) $ 238.2 Within other commercial lines, general liability is comprised of both monoline general liability and certain management and professional liability coverages and other lines is primarily comprised of lawyers professional liability, fidelity, marine, miscellaneous property, and healthcare lines. Loss and LAE reserve development in the Other segment is related to run-off voluntary assumed 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 $8.9 million in 2018, decreased by $0.2 million in 2017 and increased by $238.2 million in 2016. 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, $29.1 million in other commercial lines, primarily in our professional and management liability and monoline general liability lines, related to the 2014 through 2016 accident years, and our surety line in accident years 2015 and 2017, partially offset by higher than expected losses in AIX programs and business classes which have since been terminated. Also partially offsetting the favorable Commercial Lines development was higher than expected losses of $23.2 million in the commercial automobile line, 2017 In 2017, net favorable loss and LAE development was $0.2 million, primarily as a result of net favorable development of $10.8 million for Commercial Lines, partially offset by unfavorable development of $9.4 million for Personal Lines. Commercial Lines favorable development was primarily due to lower than expected losses within the workers’ compensation line 2016 In 2016, net unfavorable loss and LAE development was $238.2 million, primarily as a result of net unfavorable development of $219.3 million for Commercial Lines. The net unfavorable Commercial Lines development primarily resulted from higher than expected losses in other commercial lines of $169.7 million, which includes the AIX program business. This was primarily driven by AIX programs and business classes, general liability coverages in accident years 2012 through 2015, and surety bonds in accident years 2012 through 2015. The Company also experienced higher than expected losses within the commercial multiple peril lines of $68.8 million for accident years 2012 through 2015 and commercial automobile of $27.5 million in accident years 2012 through 2014, both primarily within liability coverages. Partially offsetting the unfavorable development was lower than expected losses of $46.7 million within the workers’ compensation line, primarily related to accident years 2013 through 2015, and, to a lesser extent, the commercial umbrella line primarily related to the 2015 accident year. As a result of the 2016 fourth quarter reserve review, carried reserves for prior accident years, excluding catastrophes, were increased by $174.1 million in the fourth quarter of 2016, of which $161.5 million related to Commercial Lines. The majority of this adjustment was attributed to long-tailed commercial liability coverages, including AIX program business and was largely driven by worsening trends in the number and nature of high severity losses and higher than anticipated legal defense costs. The Company reacted to this adverse emergence by updating its assumptions about loss severity, loss development patterns, expected loss ratios and loss adjustment expenses for the most recent accident years, placing greater weight on the adverse severity trends experienced in the most recent calendar years. The adverse prior year development for the Other segment was due to run-off voluntary assumed reinsurance pools business. The reserve increase was primarily based on an updated third-party actuarial study received in the fourth quarter for a pool that primarily consists of asbestos and environmental exposures. 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 lines and general liability and umbrella - occurrence presentation includes AIX program business. YEARS ENDED DECEMBER 31, 2018 2017 (in millions) Gross Ceded Net Gross Ceded Net Commercial multiple peril $ 1,206.5 $ (110.8 ) $ 1,095.7 $ 1,104.9 $ (108.9 ) $ 996.0 Workers’ compensation 775.9 (183.0 ) 592.9 757.8 (180.8 ) 577.0 Commercial automobile liability 433.5 (25.5 ) 408.0 428.1 (27.6 ) 400.5 General liability and umbrella - occurrence 534.1 (133.5 ) 400.6 508.5 (126.2 ) 382.3 General liability - claims made 245.9 (16.2 ) 229.7 231.6 (17.9 ) 213.7 Other lines 400.6 (78.4 ) 322.2 408.3 (108.8 ) 299.5 Total Commercial Lines and other 3,596.5 (547.4 ) 3,049.1 3,439.2 (570.2 ) 2,869.0 Personal automobile liability 1,514.0 (917.3 ) 596.7 1,453.8 (878.8 ) 575.0 Homeowners 158.2 (6.0 ) 152.2 133.5 (4.4 ) 129.1 Other personal lines 35.4 (1.9 ) 33.5 32.0 (1.6 ) 30.4 Total Personal Lines 1,707.6 (925.2 ) 782.4 1,619.3 (884.8 ) 734.5 Total loss and LAE reserves $ 5,304.1 $ (1,472.6 ) $ 3,831.5 $ 5,058.5 $ (1,455.0 ) $ 3,603.5 General liability and umbrella - occurrence is primarily comprised of our commercial monoline general liability and umbrella coverages. General liability - claims made is primarily comprised of our commercial professional and management liability lines. Within total Commercial Lines and other, other lines is primarily comprised of marine, surety, miscellaneous commercial property, product liability, voluntary pools, healthcare and fidelity lines. Included in the above table, primarily in other lines, are $57.6 million and $59.4 million of gross asbestos and environmental reserves as of December 31, 2018 and 2017, 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 and general liability and umbrella - occurrence lines presentation includes AIX program 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 5% 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, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2012 $ 480.0 $ 462.8 $ 456.2 $ 459.7 $ 475.6 $ 474.9 $ 476.6 $ 8.8 16,983 2013 380.0 362.5 367.9 391.8 392.1 393.0 10.2 14,830 2014 443.9 439.6 464.8 459.4 449.1 22.6 15,922 2015 446.0 456.3 463.7 467.4 33.0 15,731 2016 447.1 449.6 448.7 55.5 15,981 2017 538.7 544.8 122.2 16,674 2018 578.2 228.3 16,716 Total $ 3,357.8 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2012 $ 175.8 $ 307.5 $ 350.8 $ 389.3 $ 424.2 $ 442.8 $ 454.9 2013 137.6 221.5 262.3 306.4 334.6 355.9 2014 171.7 267.8 316.0 363.4 395.2 2015 161.9 260.1 315.6 363.2 2016 140.3 237.9 290.2 2017 170.9 296.4 2018 178.8 Total $ 2,334.6 Total reserves for 2012 – 2018 accident years (incurred - paid) 1,023.2 Total reserves for 2011 and prior accident years 53.2 Unallocated loss adjustment expense 19.3 Net reserves at December 31, 2018 $ 1,095.7 Workers' compensation (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2009 2010 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2009 $ 93.8 $ 87.6 $ 82.5 $ 80.3 $ 79.4 $ 79.2 $ 77.5 $ 77.5 $ 77.9 $ 78.7 $ 1.9 8,032 2010 115.5 116.9 115.8 115.9 116.3 114.9 114.7 115.9 116.5 3.3 10,174 2011 141.4 144.2 144.3 145.5 143.8 146.7 148.3 147.9 7.1 12,465 2012 176.3 171.1 165.2 157.2 162.9 163.7 163.7 10.2 13,042 2013 179.3 167.4 160.1 154.4 155.3 155.4 12.3 11,683 2014 182.1 172.9 154.7 152.1 150.3 15.2 10,880 2015 189.6 164.2 157.5 150.3 22.5 11,417 2016 189.6 180.5 164.6 24.5 15,790 2017 186.1 172.1 35.4 16,405 2018 187.2 69.4 15,711 Total $ 1,486.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2009 2010 2011 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2009 $ 19.2 $ 41.2 $ 54.6 $ 61.6 $ 65.4 $ 68.6 $ 69.8 $ 70.8 $ 72.4 $ 73.1 2010 22.1 57.2 76.9 89.5 96.5 101.0 104.2 106.3 107.2 2011 30.0 71.3 97.3 114.0 122.9 128.0 130.9 132.8 2012 30.4 74.8 102.6 120.1 130.7 136.0 141.4 2013 30.9 74.6 101.2 114.0 122.8 127.4 2014 30.6 70.5 92.3 105.6 112.5 2015 28.0 65.7 87.2 99.4 2016 33.9 78.1 99.5 2017 32.8 73.0 2018 35.6 Total $ 1,001.9 Total reserves for 2009 – 2018 accident years (incurred - paid) 484.8 Total reserves for 2008 and prior accident years 91.1 Unallocated loss adjustment expense and other 17.0 Net reserves at December 31, 2018 $ 592.9 Commercial automobile liability (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2013 $ 177.7 $ 179.4 $ 205.3 $ 227.5 $ 225.6 $ 231.0 $ 1.4 15,274 2014 168.5 163.3 177.3 181.7 185.1 6.4 13,477 2015 163.4 168.3 166.9 167.8 10.0 12,838 2016 157.0 157.7 163.0 16.4 11,629 2017 159.5 170.1 43.4 11,343 2018 182.8 110.7 10,338 Total $ 1,099.8 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2013 2014 2015 Year Unaudited Unaudited Unaudited 2016 2017 2018 2013 $ 35.9 $ 89.6 $ 137.6 $ 176.3 $ 200.6 $ 214.9 2014 33.1 70.8 102.7 137.1 168.2 2015 32.2 63.8 96.4 129.3 2016 27.8 60.7 98.3 2017 26.9 71.2 2018 29.2 Total $ 711.1 Total reserves for 2013 – 2018 accident years (incurred - paid) 388.7 Total reserves for 2012 and prior accident years 14.4 Unallocated loss adjustment expense 4.9 Net reserves at December 31, 2018 $ 408.0 General liability and umbrella - occurrence (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2011 $ 47.6 $ 43.8 $ 38.2 $ 42.2 $ 51.0 $ 55.6 $ 55.4 $ 57.5 $ 3.3 1,398 2012 77.2 59.3 62.2 64.3 71.1 74.2 69.1 3.7 1,779 2013 84.1 67.4 71.5 88.6 84.4 81.2 8.2 2,031 2014 100.9 82.3 98.6 101.4 98.3 10.6 2,133 2015 104.2 99.3 99.6 97.0 21.3 2,550 2016 95.6 100.7 101.5 33.9 2,015 2017 97.7 107.6 54.2 1,844 2018 99.8 74.4 1,917 Total $ 712.0 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 2011 $ 1.6 $ 6.7 $ 19.4 $ 31.1 $ 39.8 $ 45.8 $ 47.8 $ 50.6 2012 2.2 12.6 29.8 43.8 52.1 60.6 63.0 2013 2.4 11.0 26.8 43.1 56.1 63.5 2014 3.1 14.5 31.4 52.8 70.4 2015 3.3 15.2 30.8 48.5 2016 3.1 15.0 31.6 2017 4.4 17.0 2018 4.1 Total $ 348.7 Total reserves for 2011 – 2018 accident years (incurred - paid) 363.3 Total reserves for 2010 and prior accident years 25.8 Unallocated loss adjustment expense and other 11.5 Net reserves at December 31, 2018 $ 400.6 General liability - claims made (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2013 $ 54.9 $ 55.4 $ 60.6 $ 71.4 $ 69.2 $ 68.4 $ 0.9 645 2014 61.7 69.9 83.9 86.2 82.5 2.9 907 2015 93.2 98.8 98.4 90.2 4.9 1,035 2016 103.6 101.7 97.1 18.4 1,082 2017 103.3 104.6 35.5 1,310 2018 120.9 59.5 2,586 Total $ 563.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2013 2014 2015 Year Unaudited Unaudited Unaudited 2016 2017 2018 2013 $ 9.3 $ 36.2 $ 53.7 $ 61.1 $ 64.5 $ 65.7 2014 11.0 38.8 59.1 71.5 75.4 2015 10.0 43.6 64.2 76.0 2016 11.1 42.9 66.1 2017 12.3 42.3 2018 17.3 Total $ 342.8 Total reserves for 2013– 2018 accident years (incurred - paid) 220.9 Total reserves for 2012 and prior accident years 4.0 Unallocated loss adjustment expense 4.8 Net reserves at December 31, 2018 $ 229.7 Personal automobile liability (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2014 2015 Claim Year Unaudited Unaudited 2016 2017 2018 IBNR Count 2014 $ 323.1 $ 304.1 $ 308.8 $ 310.2 $ 312.5 $ 4.7 43,356 2015 327.4 334.1 328.6 332.4 5.6 42,358 2016 337.9 344.5 355.3 11.5 42,214 2017 363.6 362.3 35.0 42,200 2018 394.2 151.7 39,164 Total $ 1,756.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2014 2015 Year Unaudited Unaudited 2016 2017 2018 2014 $ 107.2 $ 188.6 $ 241.1 $ 279.4 $ 298.4 2015 112.9 205.4 261.5 301.9 2016 112.8 213.1 288.0 2017 115.0 229.2 2018 121.7 Total $ 1,239.2 Total reserves for 2014 – 2018 accident years (incurred - paid) 517.5 Total reserves for 2013 and prior accident years 65.4 Unallocated loss adjustment expense 13.8 Net reserves at December 31, 2018 $ 596.7 Homeowners (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2015 Claim Year Unaudited 2016 2017 2018 IBNR Count 2015 $ 267.1 $ 272.0 $ 272.0 $ 274.2 1.7 30,436 2016 222.1 226.5 231.4 3.2 25,456 2017 291.4 296.5 3.4 33,394 2018 312.9 46.0 30,487 Total $ 1,115.0 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2015 Year Unaudited 2016 2017 2018 2015 $ 199.9 $ 253.8 $ 260.6 $ 267.5 2016 154.3 207.9 218.2 2017 204.5 271.6 2018 213.7 Total $ 971.0 Total reserves for 2015 – 2018 accident years (incurred - paid) 144.0 Total reserves for 2014 and prior accident years 4.8 Unallocated loss adjustment expense 3.4 Net reserves at December 31, 2018 $ 152.2 The following table is information about average historical claims duration as of December 31, 2018. 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 34.0 % 22.7 % 10.7 % 10.0 % 7.2 % 4.7 % 2.5 % Workers' compensation 20.0 % 27.0 % 15.9 % 9.6 % 5.6 % 3.5 % 2.4 % 1.4 % 1.4 % 0.8 % Commercial automobile liability 16.9 % 21.7 % 20.1 % 18.3 % 13.7 % 6.2 % General liability and umbrella - occurrence 3.4 % 11.7 % 19.4 % 20.1 % 15.3 % 10.6 % 3.5 % 4.9 % General liability - claims made 12.6 % 34.4 % 24.3 % 12.9 % 4.8 % 1.7 % Personal automobile liability 32.5 % 28.4 % 18.3 % 12.2 % 6.1 % Homeowners 69.2 % 21.8 % 3.4 % 2.5 %

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	18. COMMITMENTS AND CONTINGENCIES Legal Proceedings Durand Litigation On March 12, 2007, a putative class action suit captioned Jennifer A. Durand v. The Hanover Insurance Group, Inc., and The Allmerica Financial Cash Balance Pension Plan The plaintiff filed an Amended Complaint adding two new named plaintiffs and additional claims on December 11, 2009. Two of the three new claims set forth in the Amended Complaint were dismissed by the District Court, which action was upheld in November 2015 by the U.S. Court of Appeals, Sixth Circuit. The District Court, however, did allow to stand the new claim in the Amended Complaint for breach of fiduciary duty and failure to meet notice requirements arising under the Employee Retirement Income Security Act of 1974 (“ERISA”) from the various interest crediting and lump sum distribution matters of which plaintiffs complain, but only as to plaintiffs’ “whipsaw” claim that remained in the case. On December 17, 2013, the Court entered an order certifying a class to bring “whipsaw” and related breach of fiduciary duty claims consisting of all persons who received a lump sum distribution between March 1, 1997 and December 31, 2003. On February 16, 2018, the Court entered an order limiting the claims of those participants who received lump sum distributions prior to March 13, 2002 to alleged violations of ERISA’s disclosure requirements and breaches of fiduciary duty. Subsequently, the parties entered into settlement discussions and have reached a settlement, which the Court preliminarily approved on December 18, 2018. The Court will consider whether to give final approval to the settlement at a hearing on March 22, 2019. Provided the Court approves the settlement, the Company does not believe the resulting settlement amount will be material to the Company’s financial position or have a material effect on its results of operations. Other Matters The Company has been named a defendant in various other legal proceedings arising in the normal course of business. In addition, the Company is involved, from time to time, in examinations, investigations and proceedings by governmental and self-regulatory agencies. The potential outcome of any such action or regulatory proceedings in which the Company has been named a defendant or the subject of an inquiry or investigation, and its ultimate liability, if any, from such action or regulatory proceedings, is difficult to predict at this time. The ultimate resolutions of such proceedings are not expected to have a material effect on its financial position, although they could have a material effect on the results of operations for a particular quarter or annual period. Residual Markets The Company is required to participate in residual markets in various states, which generally pertain to high risk insureds, disrupted markets or lines of business or geographic areas where rates are regarded as excessive. The results of the residual markets are not subject to the predictability associated with the Company’s own managed business, and are significant to both the personal and commercial automobile lines of business.

Statutory Financial Information

Statutory Financial Information	12 Months Ended
	Dec. 31, 2018
Statutory Accounting Practices [Abstract]
Statutory Financial Information	19. 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 U.S. insurance subsidiaries did not have any permitted practices as of or for the years ended December 31, 2018, 2017, and 2016. 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 as of December 31 for the periods indicated: (in millions) 2018 2017 2016 Statutory Net Income U.S. Insurance Subsidiaries $ 296.1 $ 254.5 $ 158.0 Statutory Capital and Surplus U.S. Insurance Subsidiaries 2,172.5 2,077.1 2,173.4 The minimum statutory capital and surplus necessary to satisfy the Company’s regulatory requirements was $518.4 million, $500.7 million and $460.1 million, which equals the Authorized Control Level at December 31, 2018, 2017, and 2016, respectively. Beginning in 2017, the NAIC modified the RBC formula to require the inclusion of catastrophe risk. Accordingly, the Authorized Control Level for 2016 has been restated to reflect this change.

Quarterly Results of Operations

Quarterly Results of Operations	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results of Operations	20. QUARTERLY RESULTS OF OPERATIONS (UNAUDITED) The quarterly results of operations for 2018 and 2017 are summarized below. THREE MONTHS ENDED (in millions, except per share data) 2018 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,091.5 $ 1,133.4 $ 1,166.6 $ 1,102.8 Income from continuing operations $ 50.5 $ 82.4 $ 104.0 $ 2.1 Net income $ 67.7 $ 99.3 $ 100.4 $ 123.6 Income from continuing operations per share: Basic $ 1.19 $ 1.94 $ 2.45 $ 0.05 Diluted $ 1.17 $ 1.91 $ 2.41 $ 0.05 Net income per share: Basic $ 1.59 $ 2.34 $ 2.36 $ 2.92 Diluted $ 1.57 $ 2.31 $ 2.33 $ 2.88 Dividends declared per share (1) $ 0.54 $ 0.54 $ 0.54 $ 5.35 THREE MONTHS ENDED (in millions, except per share data) 2017 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,036.2 $ 1,053.3 $ 1,085.9 $ 1,092.5 Income from continuing operations $ 28.4 $ 56.3 $ 59.5 $ 71.9 Net income $ 45.2 $ 78.4 $ 11.1 $ 51.5 Income from continuing operations per share: Basic $ 0.67 $ 1.32 $ 1.40 $ 1.69 Diluted $ 0.66 $ 1.32 $ 1.38 $ 1.67 Net income per share: Basic $ 1.06 $ 1.84 $ 0.26 $ 1.21 Diluted $ 1.05 $ 1.83 $ 0.26 $ 1.20 Dividends declared per share $ 0.50 $ 0.50 $ 0.50 $ 0.54 (1) As a result of the sale of Chaucer, (see Note 2 – “Discontinued Operations”), on December 30, 2018 the Board of Directors declared a special dividend of $4.75 per share (see Note 12 – “Earnings Per Share and Shareholders’ Equity Transactions”). 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. In accordance with ASU No. 2016-01, changes in fair value of equity securities are now reflected in income from continuing operations; whereas previously these changes were captured in AOCI. The fourth quarter of 2018 reflects $50.1 million of losses related to such changes in fair value. Additionally, during the fourth quarter of 2018, the Company recognized $26.3 million of prepayment fees related to its FHLB advance (see Note 6 – “Debt and Credit Arrangements”).

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	On January 2, 2019, pursuant to the terms of an ASR agreement, the Company paid $250.0 million and received an initial delivery of approximately 1.8 million shares of our common stock, which is approximately 80% of the total number of shares expected to be repurchased under this agreement. On February 14, 2019, the Company completed the sale of its Chaucer-related Irish entity for proceeds of $28 million.

SCHEDULE I SUMMARY OF INVESTMEN

SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	12 Months Ended
	Dec. 31, 2018
Summary Of Investments Other Than Investments In Related Parties [Abstract]
SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES	THE HANOVER INSURANCE GROUP, INC. DECEMBER 31, 2018 (in millions) Type of investment Cost (1) Fair Value Amount at which shown in the balance sheet Fixed maturities: Bonds: United States Government and government agencies and authorities $ 1,140.2 $ 1,123.6 $ 1,123.6 States, municipalities and political subdivisions 879.0 885.8 885.8 Foreign governments 7.3 7.4 7.4 Public utilities 410.3 403.1 403.1 All other corporate bonds 3,809.1 3,741.6 3,741.6 Total fixed maturities 6,245.9 6,161.5 6,161.5 Equity securities: Common stocks: Public utilities 63.9 90.7 90.7 Banks, trusts and insurance companies 28.5 27.4 27.4 Industrial, miscellaneous and all other 290.6 346.3 346.3 Total equity securities 383.0 464.4 464.4 Mortgage loans on real estate 405.7 410.2 405.7 Real estate 7.9 7.9 7.9 Other long-term investments 230.3 247.9 247.9 Total investments, excluding held-for-sale 7,272.8 7,291.9 7,287.4 Total investments, held-for-sale (Chaucer) 24.9 24.5 24.5 Total investments $ 7,297.7 $ 7,316.4 $ 7,311.9 (1) For equity securities and other long-term investments, represents original cost; for fixed maturities, represents original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts. The condensed financial information should be read in conjunction with the consolidated financial statements and notes thereto.

SCHEDULE II CONDENSED FINANCIAL

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY	12 Months Ended
	Dec. 31, 2018
Condensed Financial Information Of Parent Company Only Disclosure [Abstract]
SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY STATEMENTS OF INCOME	THE HANOVER INSURANCE GROUP, INC. PARENT COMPANY ONLY STATEMENTS OF INCOME YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Revenues Net investment income $ 11.8 $ 7.6 $ 4.9 Net realized losses from sales and other (0.5 ) (0.2 ) — Interest income from loan to subsidiary 0.3 0.2 22.4 Other income 0.5 0.5 0.5 Total revenues 12.1 8.1 27.8 Expenses Interest expense 37.8 38.1 46.1 Employee benefit related expenses 5.1 7.4 7.0 Interest expense on loan from subsidiary 6.8 7.0 6.3 Loss from repayment of debt 1.9 — 88.3 Other operating expenses 12.1 10.5 8.6 Total expenses 63.7 63.0 156.3 Net loss before income taxes and equity in income of subsidiaries (51.6 ) (54.9 ) (128.5 ) Income tax benefit 21.4 44.4 75.6 Equity in income of subsidiaries 288.4 196.1 207.0 Income from continuing operations 258.2 185.6 154.1 Gain from sale of Chaucer business (net of income tax expense of $42.5 in 2018) 131.9 — — Income from discontinued life business (net of income tax (expense) benefit of $(0.2), $(0.1) and $1.7 in 2018, 2017 and 2016) 0.9 0.6 1.0 Net income 391.0 186.2 155.1 Other comprehensive (loss) income, net of tax (122.4 ) 44.8 8.9 Comprehensive income $ 268.6 $ 231.0 $ 164.0 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 2018 2017 (in millions, except per share data) Assets Fixed maturities - at fair value (amortized cost of $380.5 and $281.9) $ 374.3 $ 280.9 Equity securities - at fair value 1.1 1.1 Cash and cash equivalents (1) 827.4 32.7 Investments in subsidiaries 2,800.1 3,455.4 Net receivable from subsidiaries 25.0 31.3 Other assets (2) 34.6 2.6 Total assets $ 4,062.5 $ 3,804.0 Liabilities Expenses and state taxes payable (3) $ 275.2 $ 7.9 Current income tax payable 46.9 3.2 Interest payable 7.8 8.1 Debt 777.9 787.1 Total liabilities 1,107.8 806.3 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 1,871.8 1,857.0 Accumulated other comprehensive income (loss) (116.5 ) 107.6 Retained earnings 2,182.3 1,975.0 Treasury stock at cost (18.2 and 18.0 million shares) (983.5 ) (942.5 ) Total shareholders' equity 2,954.7 2,997.7 Total liabilities and shareholders' equity $ 4,062.5 $ 3,804.0 (1) Included in 2018 were net cash proceeds of $762.0 million as a result of the sale of the Chaucer business to China Re on December 28, 2018. (2) In 2018, other assets include a $31.7 million receivable for estimated contingent consideration related to the sale of the Chaucer business. (3) Included in 2018 was a payable related to a special dividend of $193.4 million declared on December 30, 2018. 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 2018 2017 2016 (in millions) Cash flows from operating activities Net income $ 391.0 $ 186.2 $ 155.1 Adjustments to reconcile net income to net cash provided by operating activities: Gain from sale of Chaucer business (131.9 ) — — Net realized investment losses 0.6 0.2 — Loss from retirement of debt 1.9 — 88.3 Equity in net income of subsidiaries (288.4 ) (196.1 ) (207.0 ) Dividends received from subsidiaries 86.4 55.1 228.1 Deferred income tax expense (benefit) (5.3 ) 18.9 (39.0 ) Change in expenses and taxes payable 59.7 (27.4 ) 6.6 Change in net receivable from subsidiaries 8.9 9.0 50.5 Other, net (0.4 ) (0.1 ) 7.3 Net cash provided by operating activities 122.5 45.8 289.9 Cash flows from investing activities Proceeds from disposals and maturities of fixed maturities 82.8 94.1 82.4 Purchase of fixed maturities (4.0 ) — — Net cash received from sale of Chaucer business 762.0 — — Net cash used for business acquisitions — (12.3 ) (2.2 ) Net cash provided by investing activities 840.8 81.8 80.2 Cash flows from financing activities Proceeds from debt borrowings — — 370.7 Proceeds from long-term intercompany borrowings — — 125.0 Net cash related to short-term intercompany borrowings (19.8 ) — (102.7 ) Repurchases of debt (11.6 ) — (571.9 ) Repurchase of common stock (57.7 ) (37.2 ) (105.6 ) Dividends paid to shareholders (94.3 ) (86.8 ) (80.4 ) Proceeds from exercise of employee stock options 14.8 23.1 16.2 Other financing activities — (4.1 ) (19.9 ) Net cash used in financing activities (168.6 ) (105.0 ) (368.6 ) Net change in cash and cash equivalents 794.7 22.6 1.5 Cash and cash equivalents, beginning of year 32.7 10.1 8.6 Cash and cash equivalents, end of year $ 827.4 $ 32.7 $ 10.1 Included in other operating cash flows was the cash portion of dividends received from unconsolidated subsidiaries. Additionally, investment assets of $179.8 million, $261.6 million and $78.5 million were transferred to the parent company in 2018, 2017 and 2016, 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, 2018
Supplementary Insurance Information [Abstract]
SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION	THE HANOVER INSURANCE GROUP, INC. 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 DECEMBER 31, 2017 (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 $ 286.4 $ 3,399.3 $ 1,299.5 $ 8.2 $ 2,399.6 $ 165.8 $ 1,531.4 $ 557.9 $ 312.8 $ 2,462.0 Personal Lines 143.6 1,619.3 833.7 — 1,580.8 70.1 1,046.9 282.8 178.5 1,647.1 Other — 38.7 — — — 8.0 1.3 — 26.0 — Interest on Debt — — — — — — — — 45.2 — Eliminations — (7.0 ) (1.5 ) — — — — — (7.8 ) — Total (4) $ 430.0 $ 5,050.3 $ 2,131.7 $ 8.2 $ 3,980.4 $ 243.9 $ 2,579.6 $ 840.7 $ 554.7 $ 4,109.1 DECEMBER 31, 2016 (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 $ 269.6 $ 3,069.5 $ 1,231.5 $ 6.4 $ 2,318.0 $ 158.5 $ 1,601.5 $ 537.4 $ 308.9 $ 2,361.5 Personal Lines 133.2 1,554.0 763.6 — 1,471.5 69.5 936.1 266.2 172.4 1,521.2 Other — 40.3 — — — 3.6 8.4 — 113.2 — Interest on Debt — — — — — — — — 51.4 — Eliminations — (10.2 ) (3.1 ) — — — — — (7.6 ) — Total (4) $ 402.8 $ 4,653.6 $ 1,992.0 $ 6.4 $ 3,789.5 $ 231.6 $ 2,546.0 $ 803.6 $ 638.3 $ 3,882.7 (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 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 companies, expenses are generally allocated based upon either net premiums written or net premiums earned. (3) In 2018 and 2016, the Other segment includes $28.2 million and $88.3 million of losses on the repayment of debt, respectively. (4) Information for all periods excludes results and balances related to the 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, 2018
Supplemental Schedule Of Reinsurance Premiums For Insurance Companies Abstract
SCHEDULE IV REINSURANCE	THE HANOVER INSURANCE GROUP, INC. Incorporated herein by reference to Note 16 —“Reinsurance” in the Notes to Consolidated Financial Statements.

SCHEDULE V VALUATION AND QUALIF

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	12 Months Ended
	Dec. 31, 2018
Valuation And Qualifying Accounts [Abstract]
SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS	THE HANOVER INSURANCE GROUP, INC. 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 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 2017 Allowance for doubtful accounts $ 3.7 $ 9.4 $ — $ (8.8 ) $ 4.3 Allowance for uncollectible reinsurance recoverables (2) 13.1 0.3 0.1 (3.8 ) 9.7 $ 16.8 $ 9.7 $ 0.1 $ (12.6 ) $ 14.0 2016 Allowance for doubtful accounts $ 3.2 $ 8.7 $ — $ (8.2 ) $ 3.7 Allowance for uncollectible reinsurance recoverables (2)(3) 9.7 3.4 0.1 (0.1 ) 13.1 $ 12.9 $ 12.1 $ 0.1 $ (8.3 ) $ 16.8 (1) Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its Chaucer business; and accordingly, the $6.4 million balance associated with this business was reduced as part of the sale. (2) The balance at beginning of period and the balance at end of period include $6.3 million and $6.4 million, respectively, for the year-ended 2017 and $6.2 million and $6.3 million, respectively, for the year-ended 2016 related to the former Chaucer business. (3) Activity primarily relates to the impairment in 2016 and subsequent write-off in 2017 of a receivable that is associated with a single reinsurance counterparty that was placed into conservation by the state of California in July 2016. This counterparty is not involved in the Company’s ongoing reinsurance program and the entire receivable related to this counterparty resulted from a one-time block reinsurance transaction that occurred in 2012 as part of the Company’s exposure management actions.

SCHEDULE VI SUPPLEMENTAL INFORM

SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	12 Months Ended
	Dec. 31, 2018
Supplemental Information For Property Casualty Insurance Underwriters [Abstract]
SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS	THE HANOVER INSURANCE GROUP, INC. 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) 2018 $ 450.8 $ 5,304.1 $ — $ 2,277.8 $ 4,254.4 $ 267.4 2017 $ 430.0 $ 5,058.5 $ — $ 2,131.7 $ 3,980.4 $ 243.9 2016 $ 402.8 $ 4,660.0 $ — $ 1,992.0 $ 3,789.5 $ 231.6 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 2018 $ 2,733.5 $ (8.9 ) $ 891.8 $ 2,496.6 $ 4,384.8 2017 $ 2,575.1 $ (0.2 ) $ 840.7 $ 2,282.2 $ 4,109.1 2016 $ 2,307.8 $ 238.2 $ 803.6 $ 2,141.8 $ 3,882.7 (1) Reserves for unpaid claims and claim adjustment expenses are shown gross of $1,472.6 million, $1,455.0 million and $1,349.2 million of reinsurance recoverable on unpaid losses in 2018, 2017 and 2016, respectively. Unearned premiums are shown gross of prepaid premiums of $117.6 million, $101.8 million and $90.9 million in 2018, 2017 and 2016, 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, 2018
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 14 – “Segment Information”. The consolidated financial statements also include the Company’s discontinued operations, consisting primarily of Chaucer Holdings Limited (“Chaucer”), a specialist insurance underwriting group which operates through the Society and Corporation of Lloyd’s (“Lloyd’s”), and the Company’s accident and health and former life insurance businesses. 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”). The sale of the Chaucer-related Irish entity was completed on February 14, 2019 for proceeds of $28 million. The sale of the Australian entities for proceeds of $13 million is pending, subject only to local regulatory approval, and is expected to close in the first quarter of 2019. Accordingly, as of December 31, 2018 and for all prior periods presented in this Annual Report on Form 10-K, Chaucer’s accounts have been classified as discontinued operations in the Consolidated Statements of Income and as held-for-sale in the Consolidated Balance Sheets (See Note 2 – “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. Effective January 1, 2018, in accordance with the provisions of Accounting Standards Codification (“ ASC”) Update No. 2016-01, (Subtopic 825-10) Financial Instruments – Overall: Recognition and Measurement of Financial Assets and Financial Liabilities (“ASU No. 2016-01”), 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. Prior to May 2018, certain reacquisition rights, which are no longer in effect, and which were previously retained by the third-party in the loan participations required that these investments be accounted for as secured borrowings under ASC 860, Transfers and Servicing Investments in limited partnerships include interests in private equity funds. Effective January 1, 2018, in accordance with the provisions of ASC Update No. 2016-01, when the Company’s interest in the partnership 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 that 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. When an other-than-temporary decline in value of a specific investment is deemed to have occurred, and a charge to earnings is required, the Company recognizes a realized investment loss. 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. Prior to the sale of Chaucer on December 28, 2018, the Company also held investments in overseas deposits; such deposits were maintained in overseas funds and were managed exclusively by Lloyd’s. These funds were required in order to protect policyholders in overseas markets and enabled the Company to operate in these markets. Net investment income and realized and unrealized investment gains (losses) related to overseas deposits are included in income (loss) from Chaucer business within discontinued operations. The Company reviews investments in an unrealized loss position to identify other-than-temporary declines in value. When an other-than-temporary decline in value of a debt security is deemed to have occurred, the Company must assess whether it intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis. If the debt security meets either of these two criteria, an other-than-temporary impairment (“OTTI”) 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 the Company does not intend to sell the debt security and it is not more likely than not the Company will be required to sell the security before recovery of its amortized cost basis, the credit loss portion of an OTTI is recorded through earnings while the portion attributable to all other factors is recorded separately as a component of other comprehensive income. The amount of the OTTI that relates to credit is 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. Once an OTTI has been recognized, the new amortized cost basis of the security is equal to the previous amortized cost less the amount of OTTI recognized in earnings.
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 and foreign currency 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 includes 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 AND CAPITALIZED SOFTWARE	G. Property, Equipment and Capitalized Software 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 leases or the estimated useful life of the improvements. 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 2018 and 2017 with no impairments recognized. At December 31, 2018 and 2017, 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, 2018 includes senior debentures, subordinated debentures, and collateralized borrowings with the Federal Home Loan Bank (“FHLB”). 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 premium 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, 2018 and 2017. 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. 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 for income from the U.S. operations. In December 2017, the Tax Cuts and Jobs Act (“TCJA”) was enacted, which reduced the U.S. statutory rate from 35% to 21%, effective January 1, 2018. See Note 7 – “Income Taxes” for a discussion of this new law. 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, are 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 11 – “Stock-Based Compensation Plans”.
EARNINGS PER SHARE	N. Earnings Per Share Earnings per share (“EPS”) for the years ended December 31, 2018, 2017 and 2016 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 are 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 February 2018, the Financial Accounting Standards Board (“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. 2017-07, (Topic 715) Compensation – Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In January 2017, the FASB issued ASC Update No. 2017-01, (Topic 805) Business Combinations – Clarifying the Definition of a Business In November 2016, the FASB issued ASC Update No. 2016-18 (Topic 230) Statement of Cash Flows – Restricted Cash (a consensus of the FASB Emerging Issues Task Force) In October 2016, the FASB issued ASC Update No. 2016-16, (Topic 740) Income Taxes – Intra-Entity Transfers of Assets Other Than Inventory In August 2016, the FASB issued ASC Update No. 2016-15, (Topic 230) Classification of Certain Cash Receipts and Cash Payments In January 2016, the FASB issued ASC Update No. 2016-01, (Subtopic 825-10) Financial Instruments – Overall: Recognition and Measurement of Financial Assets and Financial Liabilities Technical Corrections and Improvements to Financial Instruments – Overall. In May 2014, the FASB issued ASC Update No. 2014-09, (Topic 606) Revenue from Contracts with Customers Revenue from Contracts with Customers Recently Issued Standards In August 2018, the FASB issued ASC Update No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract In August 2018, the FASB issued ASC Update No. 2018-14 (Topic 715-20) Compensation – Retirement Benefits – Defined Benefit Plans – General – Disclosure Framework – Changes to the Disclosure Requirements for the Defined Benefit Plans. In August 2018, the FASB issued ASC Update No. 2018-13 (Topic 820) Fair Value Measurement, Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement In March 2017, the FASB issued ASC Update No. 2017-08, (Subtopic 310-20) Receivables – Nonrefundable Fees and Other Costs: Premium Amortization on Purchased Callable Debt Securities In January 2017, the FASB issued ASC Update No. 2017-04, (Topic 350) Intangibles – Goodwill and Other: Simplifying the Test for Goodwill Impairment In June 2016, the FASB issued ASC Update No. 2016-13, (Topic 326) Financial Instruments – Credit Losses: Measurement of Credit Losses on Financial Instruments In November 2018, the FASB issued ASC Update No. 2018-19, Codification Improvements to Topic 326, Financial Instruments–Credit Losses , which explicitly states that receivables arising from operating leases are not within the scope of Subtopic 326-20. ASC Update No. 2016-13 and related In February 2016, the FASB issued ASC Update No. 2016-02, (Topic 842) Leases Leases Targeted Improvements
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, 2018
Components of Estimated Gain From Disposal of Chaucer Business	The following table summarizes the components of the estimated gain related to the sale of the Chaucer business as of December 28, 2018. This gain below excludes an expected gain on the Irish and Australian entities associated with the Chaucer business. The Company subsequently completed the sale of its Chaucer-related Irish entity on February 14, 2019. The sale of the Australian entities is pending, subject only to local regulatory approval, and is expected to close in the first quarter of 2019. The Company received $28 million of additional consideration for the Irish entity, and it expects to receive $13 million of additional consideration related to the Australian entities. 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 Holdings Limited as determined in the sales and purchase agreement was $779 million. This amount, along with $28 million in cash proceeds received for the sale of the Irish entity on February 14, 2019, $13 million in cash proceeds to be received upon the closing of the sale of the Australian entities, 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, results 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, may be up to $45 million and is determined based upon 2018 catastrophe losses. Based upon the Company’s best estimate of Chaucer’s 2018 catastrophe losses, expected contingent consideration is $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 are no longer likely to be realized .
Summary of Carrying Amounts of Major Assets and Liabilities Reflected in Consolidated Balance Sheets	The following table details the carrying amounts of major assets and liabilities reflected in the Consolidated Balance Sheets under the caption “Assets held-for-sale” and “Liabilities held-for-sale”, respectively. DECEMBER 31 2018 2017 (in millions) Assets: Investments $ 24.5 $ 2,153.5 Cash and cash equivalents 3.3 78.5 Premiums and accounts receivable, net 2.8 471.9 Deferred acquisition costs 1.4 120.2 Reinsurance recoverable on paid and unpaid losses and unearned premiums 2.3 1,431.5 Goodwill 6.5 13.8 Other assets 16.6 197.2 Total assets $ 57.4 $ 4,466.6 Liabilities: Loss and loss adjustment expense reserves $ 2.0 $ 2,686.5 Unearned premiums 7.1 631.9 Reinsurance premium payable 1.5 293.3 Other liabilities 11.6 95.5 Total liabilities $ 22.2 $ 3,707.2
Schedule of Effects of Reinsurance	The following table provides the effects of reinsurance. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Premiums written: Direct $ 625.3 $ 597.6 $ 560.3 Assumed 685.3 646.9 546.3 Ceded (1) (459.9 ) (395.4 ) (290.5 ) Net premiums written $ 850.7 $ 849.1 $ 816.1 Premiums earned: Direct $ 611.9 $ 568.7 $ 584.0 Assumed 667.2 628.3 577.3 Ceded (1) (429.1 ) (344.0 ) (322.7 ) Net premiums earned $ 850.0 $ 853.0 $ 838.6 Losses and LAE: Direct $ 362.2 $ 365.2 $ 403.1 Assumed 568.7 570.7 258.8 Ceded (2) (415.4 ) (386.8 ) (243.2 ) Net losses and LAE $ 515.5 $ 549.1 $ 418.7 YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Premiums written: Direct $ 4,816.0 $ 4,540.0 $ 4,265.7 Assumed 27.6 23.6 25.1 Ceded (458.8 ) (454.5 ) (408.1 ) Net premiums written $ 4,384.8 $ 4,109.1 $ 3,882.7 Premiums earned: Direct $ 4,673.6 $ 4,399.9 $ 4,162.6 Assumed 26.6 23.6 24.4 Ceded (445.8 ) (443.1 ) (397.5 ) Net premiums earned $ 4,254.4 $ 3,980.4 $ 3,789.5 Percentage of assumed to net premiums earned 0.6 % 0.6 % 0.6 % Losses and LAE: Direct $ 2,986.2 $ 2,934.6 $ 2,785.1 Assumed 25.2 20.5 30.6 Ceded (1) (286.8 ) (375.5 ) (269.7 ) Net losses and LAE $ 2,724.6 $ 2,579.6 $ 2,546.0 (1) The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines.
Chaucer [Member]
Summary of Discontinued Chaucer Business	Income (loss) from Chaucer Business The following table summarizes the results of Chaucer’s operations: YEARS ENDED DECEMBER 31 2018 (1) 2017 2016 (in millions) Revenues Net premiums earned $ 850.0 $ 853.0 $ 838.6 Net investment income 54.9 52.0 45.7 Other income 7.5 6.7 7.1 912.4 911.7 891.4 Losses and operating expenses Losses and LAE 515.5 549.5 419.3 Amortization of deferred acquisition costs 252.5 245.9 231.6 Other expenses 115.0 109.2 113.7 883.0 904.6 764.6 Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income) 29.4 7.1 126.8 Other items: Interest expense (3.8 ) (3.3 ) (3.5 ) Net realized and unrealized investment gains (losses) (1.3 ) 2.6 (1.6 ) Other income 0.4 2.2 4.6 Income from Chaucer business before income taxes 24.7 8.6 126.3 Income tax expense (4.7 ) (21.7 ) (37.2 ) Income (loss) from Chaucer business, net of taxes $ 20.0 $ (13.1 ) $ 89.1 (1) 2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.
Summary of Cash Flows Associated with the Discontinued Chaucer Business	The following table details the cash flows associated with the Chaucer business: DECEMBER 31 2018 2017 2016 (in millions) Net cash (used in) provided by operating activities (22.8 ) 28.8 95.9 Net cash provided by (used in) investing activities 131.1 (96.9 ) (104.7 )

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Schedule of Available-for-sale Securities Reconciliation	The amortized cost and fair value of available-for-sale fixed maturities were as follows: DECEMBER 31, 2018 (in millions) Gross Gross OTTI Amortized Unrealized Unrealized Unrealized Cost Gains Losses Fair Value Losses Fixed maturities: U.S. Treasury and government agencies $ 414.7 $ 2.4 $ 7.2 $ 409.9 $ — Foreign government 7.3 0.1 — 7.4 — Municipal 879.0 16.6 9.8 885.8 — Corporate 3,476.6 26.1 92.0 3,410.7 6.6 Residential mortgage-backed 728.4 2.7 14.7 716.4 — Commercial mortgage-backed 648.4 1.7 9.8 640.3 — Asset-backed 91.5 0.2 0.7 91.0 — Fixed maturities, excluding held-for-sale (Chaucer) 6,245.9 49.8 134.2 6,161.5 6.6 Fixed maturities, held-for-sale 24.9 — 0.4 24.5 — Total fixed maturities $ 6,270.8 $ 49.8 $ 134.6 $ 6,186.0 $ 6.6 DECEMBER 31, 2017 (in millions) Gross Gross OTTI Amortized Unrealized Unrealized Unrealized Cost Gains Losses Fair Value Losses Fixed maturities: U.S. Treasury and government agencies $ 322.1 $ 3.4 $ 3.2 $ 322.3 $ — Foreign government 8.2 0.3 — 8.5 — Municipal 910.1 29.7 5.8 934.0 — Corporate 3,134.2 84.1 21.7 3,196.6 6.5 Residential mortgage-backed 632.3 5.8 4.9 633.2 — Commercial mortgage-backed 590.3 7.2 2.5 595.0 — Asset-backed 59.9 0.1 0.3 59.7 — Fixed maturities, excluding held-for-sale (Chaucer) 5,657.1 130.6 38.4 5,749.3 6.5 Fixed maturities, held-for-sale 2,031.7 14.6 15.9 2,030.4 0.4 Total fixed maturities $ 7,688.8 $ 145.2 $ 54.3 $ 7,779.7 $ 6.9
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 2018 (in millions) Amortized Cost Fair Value Due in one year or less $ 207.4 $ 208.5 Due after one year through five years 1,830.9 1,830.0 Due after five years through ten years 2,465.1 2,398.7 Due after ten years 274.2 276.6 4,777.6 4,713.8 Mortgage-backed and asset-backed securities 1,468.3 1,447.7 Fixed maturities, excluding held-for-sale (Chaucer) 6,245.9 6,161.5 Fixed maturities, held-for-sale 24.9 24.5 Total fixed maturities $ 6,270.8 $ 6,186.0
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 2018 Maturities Other Total 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 ) 2017 Net appreciation, beginning of year $ 127.1 $ 58.9 $ 186.0 Net appreciation (depreciation) on available-for-sale securities (15.0 ) 55.9 40.9 Provision for deferred income taxes (11.2 ) (19.5 ) (30.7 ) Change in OTTI losses recognized in other comprehensive income 9.2 — 9.2 (17.0 ) 36.4 19.4 Net appreciation, end of year $ 110.1 $ 95.3 $ 205.4 2016 Net appreciation, beginning of year $ 116.5 $ 33.4 $ 149.9 Net appreciation on available-for-sale securities 36.9 39.2 76.1 Provision for deferred income taxes (37.5 ) (13.7 ) (51.2 ) Change in OTTI losses recognized in other comprehensive income 11.2 — 11.2 10.6 25.5 36.1 Net appreciation, end of year $ 127.1 $ 58.9 $ 186.0
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, 2018 and 2017 including the length of time the securities have been in an unrealized loss position: DECEMBER 31, 2018 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 $ 1.1 $ 66.0 $ 6.1 $ 210.9 $ 7.2 $ 276.9 Foreign governments — 2.0 — 0.8 — 2.8 Municipal 0.8 110.0 9.0 248.0 9.8 358.0 Corporate 30.0 1,277.9 43.9 781.6 73.9 2,059.5 Residential mortgage-backed 2.6 201.2 12.1 323.7 14.7 524.9 Commercial mortgage-backed 3.4 293.0 6.4 175.5 9.8 468.5 Asset-backed 0.4 42.3 0.3 18.0 0.7 60.3 Total investment grade 38.3 1,992.4 77.8 1,758.5 116.1 3,750.9 Below investment grade: Municipal — — — 0.9 — 0.9 Corporate 8.1 185.6 10.0 54.0 18.1 239.6 Total below investment grade 8.1 185.6 10.0 54.9 18.1 240.5 Fixed maturities, excluding held-for-sale (Chaucer) 46.4 2,178.0 87.8 1,813.4 134.2 3,991.4 Fixed maturities, held-for-sale 0.1 4.0 0.3 18.0 0.4 22.0 Total fixed maturities $ 46.5 $ 2,182.0 $ 88.1 $ 1,831.4 $ 134.6 $ 4,013.4 DECEMBER 31, 2017 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 $ 1.1 $ 146.3 $ 2.1 $ 62.3 $ 3.2 $ 208.6 Foreign governments — 0.1 — 0.6 — 0.7 Municipal 1.7 183.8 4.1 101.2 5.8 285.0 Corporate 3.0 386.2 10.8 395.9 13.8 782.1 Residential mortgage-backed 1.4 196.3 3.5 159.0 4.9 355.3 Commercial mortgage-backed 0.9 139.2 1.6 61.9 2.5 201.1 Asset-backed 0.3 34.6 — 2.4 0.3 37.0 Total investment grade 8.4 1,086.5 22.1 783.3 30.5 1,869.8 Below investment grade: Corporate 1.5 48.7 6.4 39.0 7.9 87.7 Fixed maturities, excluding held-for-sale (Chaucer) 9.9 1,135.2 28.5 822.3 38.4 1,957.5 Fixed maturities, held-for-sale 5.7 803.3 10.2 389.6 15.9 1,192.9 Total fixed maturities $ 15.6 $ 1,938.5 $ 38.7 $ 1,211.9 $ 54.3 $ 3,150.4
Schedule of Other Investments	Mortgage participations and other mortgage loans were comprised of the following property types and geographic locations DECEMBER 31 2018 2017 (in millions) Property Type: Office $ 141.1 $ 134.6 Apartments 85.5 64.9 Retail 66.9 64.1 Hotel 62.7 58.1 Industrial 50.6 45.0 Valuation allowance (1.1 ) (0.9 ) Total $ 405.7 $ 365.8 DECEMBER 31 2018 2017 (in millions) Geographic Region: Pacific $ 92.5 $ 87.6 South Atlantic 82.1 76.9 West South Central 65.9 66.1 Mid-Atlantic 53.4 53.4 New England 35.3 28.8 East North Central 27.9 16.5 Mountain 12.5 5.0 Other 37.2 32.4 Valuation allowance (1.1 ) (0.9 ) Total $ 405.7 $ 365.8

Investment Income and Gains a_2

Investment Income and Gains and Losses (Tables)	12 Months Ended
	Dec. 31, 2018
Investments Debt And Equity Securities [Abstract]
Components of Net Investment Income from Continuing Operations	The components of net investment income from continuing operations were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Fixed maturities $ 217.7 $ 205.8 $ 202.1 Limited partnerships 24.1 15.3 8.0 Equity securities 17.0 18.0 18.6 Mortgage loans 14.0 11.4 9.1 Other investments 4.8 3.4 3.4 Gross investment income 277.6 253.9 241.2 Less investment expenses (10.2 ) (10.0 ) (9.6 ) Net investment income $ 267.4 $ 243.9 $ 231.6
Schedule of Net Realized and Unrealized Gains (Losses) on Investments from Continuing Operations Including OTTI	Net realized and unrealized gains (losses) on investments from continuing operations, including OTTI, were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Equity securities $ (43.4 ) $ 19.8 $ 25.4 Fixed maturities (5.8 ) 3.6 (15.5 ) Other investments (1.5 ) (2.3 ) 0.3 Net realized and unrealized investment gains (losses) $ (50.7 ) $ 21.1 $ 10.2
Schedule of Pre-tax Net Realized and Unrealized Losses on Equity Securities	The following table provides pre-tax net realized and unrealized losses on equity securities from continuing operations: YEAR ENDED DECEMBER 31 2018 (in millions) Net losses recognized during the period $ (43.4 ) Less: net losses recognized on equity securities sold during the period (3.9 ) Net unrealized losses recognized during the period on equity securities still held $ (39.5 )
Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses	The following table provides rollforwards of the cumulative amounts related to the Company’s credit loss portion of the OTTI losses on fixed maturity securities from continuing operations for which the non-credit portion of the loss is included in other comprehensive income. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Credit losses as of the beginning of the year $ 3.6 $ 9.6 $ 17.7 Credit losses on securities for which an OTTI was not previously recognized 1.0 0.4 6.1 Additional credit losses on securities for which an OTTI was previously recognized 0.1 0.1 2.3 Reductions for securities sold, matured or called (0.9 ) (6.1 ) (4.3 ) Reductions for securities reclassified as intend to sell — (0.4 ) (12.2 ) Credit losses as of the end of the year $ 3.8 $ 3.6 $ 9.6
Schedule of Realized Gain (Loss)	The proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses on those sales, were as follows: YEARS ENDED DECEMBER 31 (in millions) Proceeds Gross Gross 2018 from Sales Gains Losses 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 2017 Fixed maturities, excluding held-for-sale (Chaucer) $ 306.8 $ 8.1 $ 7.8 Fixed maturities, held-for-sale 188.0 4.1 0.4 Total fixed maturities $ 494.8 $ 12.2 $ 8.2 2016 Fixed maturities, excluding held-for-sale (Chaucer) $ 348.2 $ 10.0 $ 5.1 Fixed maturities, held-for-sale 215.6 1.7 1.9 Total fixed maturities $ 563.8 $ 11.7 $ 7.0

Fair Value (Tables)

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	The estimated fair value of the financial instruments were as follows: December 31, 2018 December 31, 2017 Carrying Fair Carrying Fair (in millions) Value Value Value Value Financial Assets carried at (1) Fair Value through AOCI: Fixed maturities $ 6,161.5 $ 6,161.5 $ 5,749.3 $ 5,749.3 Equity securities — — 576.2 576.2 Other investments — — 3.6 3.6 Fair Value through Net Income: Equity securities 464.4 464.4 — — Other investments 175.0 175.0 — — Amortized Cost/Cost: Other investments 414.4 418.9 512.8 518.3 Cash and cash equivalents 1,020.7 1,020.7 297.9 297.9 Total financial instruments, excluding held-for-sale 8,236.0 8,240.5 7,139.8 7,145.3 Financial instruments, held-for-sale (Chaucer) 27.8 27.8 2,232.0 2,232.0 Total financial instruments $ 8,263.8 $ 8,268.3 $ 9,371.8 $ 9,377.3 Financial Liabilities carried at: Amortized Cost: Debt $ 777.9 $ 825.0 $ 786.9 $ 865.7
Fair Value, Assets Measured on Recurring Basis	The following tables provide, for each hierarchy level, the Company’s assets that were measured at fair value on a recurring basis. DECEMBER 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 409.9 $ 154.9 $ 255.0 $ — Foreign government 7.4 — 7.4 — Municipal 885.8 — 864.7 21.1 Corporate 3,410.7 — 3,409.9 0.8 Residential mortgage-backed, U.S. agency backed 713.7 — 713.7 — Residential mortgage-backed, non-agency 2.7 — 2.7 — Commercial mortgage-backed 640.3 — 627.2 13.1 Asset backed 91.0 — 91.0 — Total fixed maturities 6,161.5 154.9 5,971.6 35.0 Equity securities 464.4 463.3 — 1.1 Other investments 3.5 — — 3.5 Total investment assets at fair value, excluding held-for-sale (Chaucer) 6,629.4 618.2 5,971.6 39.6 Investment assets, held-for-sale 24.5 5.7 18.8 — Total investment assets at fair value $ 6,653.9 $ 623.9 $ 5,990.4 $ 39.6 DECEMBER 31, 2017 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 322.3 $ 140.0 $ 182.3 $ — Foreign government 8.5 — 8.5 — Municipal 934.0 — 909.4 24.6 Corporate 3,196.6 — 3,195.7 0.9 Residential mortgage-backed, U.S. agency backed 605.0 — 605.0 — Residential mortgage-backed, non-agency 28.2 — 28.2 — Commercial mortgage-backed 595.0 — 580.8 14.2 Asset backed 59.7 — 59.7 — Total fixed maturities 5,749.3 140.0 5,569.6 39.7 Equity securities 567.8 566.7 — 1.1 Other investments 3.6 — — 3.6 Total investment assets at fair value, excluding held-for-sale (Chaucer) 6,320.7 706.7 5,569.6 44.4 Investment assets, held-for-sale 2,153.5 138.0 2,013.3 2.2 Total investment assets at fair value $ 8,474.2 $ 844.7 $ 7,582.9 $ 46.6
Estimated Fair Values of Financial Instruments Not Carried at Fair Value	The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value. DECEMBER 31, 2018 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 1,020.7 $ 1,020.7 $ — $ — Other investments 418.9 — 8.7 410.2 Total financial instruments, excluding held-for-sale 1,439.6 1,020.7 8.7 410.2 Financial instruments, held-for-sale (Chaucer) 3.3 3.3 — — Total financial instruments $ 1,442.9 $ 1,024.0 $ 8.7 $ 410.2 Liabilities: Debt $ 825.0 $ — $ 825.0 $ — DECEMBER 31, 2017 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 297.9 $ 297.9 $ — $ — Equity securities 8.4 — 8.4 — Other investments 368.9 — — 368.9 Total financial instruments, excluding held-for-sale 675.2 297.9 8.4 368.9 Financial instruments, held-for-sale (Chaucer) 78.5 78.5 — — Total financial instruments $ 753.7 $ 376.4 $ 8.4 $ 368.9 Liabilities: Debt $ 865.7 $ — $ 865.7 $ —
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, 2018 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year, excluding held-for-sale (Chaucer) $ 24.6 $ 0.9 $ 14.2 $ 39.7 $ 4.7 $ 44.4 Transfers out of Level 3 (0.5 ) — — (0.5 ) — (0.5 ) Total gains (losses): Included in total net realized and unrealized investment gains (losses) 0.1 — — 0.1 (0.1 ) — Included in other comprehensive income - net depreciation on available-for-sale securities (0.2 ) — (0.3 ) (0.5 ) — (0.5 ) Sales (2.9 ) (0.1 ) (0.8 ) (3.8 ) — (3.8 ) Balance at end of year (1) $ 21.1 $ 0.8 $ 13.1 $ 35.0 $ 4.6 $ 39.6 (1) There were no Level 3 held-for-sale securities (Chaucer) at December 31, 2018. YEAR ENDED DECEMBER 31, 2017 Fixed Maturities (in millions) Municipal Corporate Commercial mortgage- backed Total Equity and Other Total Assets Balance at beginning of year, excluding held-for-sale (Chaucer) $ 28.8 $ 4.0 $ 15.0 $ 47.8 $ 5.2 $ 53.0 Transfers out of Level 3 (1.9 ) — — (1.9 ) — (1.9 ) Total gains (losses): Included in total net realized and unrealized investment gains — 0.3 — 0.3 — 0.3 Included in other comprehensive income - net depreciation on available-for-sale securities (0.2 ) (0.2 ) — (0.4 ) (0.5 ) (0.9 ) Sales (2.1 ) (3.2 ) (0.8 ) (6.1 ) — (6.1 ) Balance at end of year, excluding held-for-sale 24.6 0.9 14.2 39.7 4.7 44.4 Balance at end of year, held-for-sale (Chaucer) 2.2 — — 2.2 — 2.2 Balance at end of year, including held-for-sale $ 26.8 $ 0.9 $ 14.2 $ 41.9 $ 4.7 $ 46.6
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, excluding amounts relating to the Chaucer business held-for-sale. 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, 2018 2017 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipal Discounted Discount for: $ 21.1 $ 24.6 cash flow Small issue size 0.7 - 6.8% (3.4%) 0.7 - 6.8% (3.5%) Credit stress 1.3% (1.3%) 0.9 - 1.5% (1.2%) Above-market coupon 0.3 - 0.5% (0.5%) 0.3 - 0.5% (0.4%) Corporate Discounted Discount for: 0.8 0.9 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: 13.1 14.2 mortgage-backed cash flow Small issue size 1.9 - 3.1% (2.7%) 1.9 - 3.1% (2.6%) Above-market coupon 0.5% (0.5%) 0.5% (0.5%) Lease structure 0.3% (0.3%) 0.3% (0.3%) Equity securities Market comparables Net tangible asset market multiples 1.1 1.0X (1.0X) 1.1 1.0X (1.0X) Other Discounted cash flow Discount rate 3.5 18.0% (18.0%) 3.6 18.0% (18.0%)

Debt and Credit Arrangements (T

Debt and Credit Arrangements (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Long-Term Debt	Debt consists of the following: DECEMBER 31 2018 2017 (in millions) Senior debentures maturing April 15, 2026 $ 375.0 $ 375.0 Senior debentures maturing October 15, 2025 62.6 62.6 Subordinated debentures maturing March 30, 2053 175.0 175.0 Subordinated debentures maturing February 3, 2027 50.1 59.7 FHLB borrowings (secured) 125.0 125.0 Total principal debt 787.7 797.3 Unamortized debt issuance costs (9.8 ) (10.4 ) Total $ 777.9 $ 786.9

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 (in millions) Income from continuing operations before income taxes $ 282.5 $ 292.9 $ 66.0 Income tax expense: Current $ 46.2 $ 3.1 $ 26.2 Deferred (2.7 ) 73.7 (27.2 ) Total income tax expense (benefit) $ 43.5 $ 76.8 $ (1.0 )
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% in 2018 and 35% in both 2017 and 2016. The sources of the difference and the tax effects of each were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Expected income tax expense $ 59.3 $ 102.5 $ 23.1 Effect of the enactment of the Tax Cuts and Jobs Act (4.3 ) (3.9 ) — Tax difference related to investment disposals and maturities (9.2 ) (12.7 ) (20.7 ) Stock-based compensation windfall benefit (2.3 ) (5.3 ) — Nondeductible expenses 1.6 1.0 1.0 Dividend received deduction (1.2 ) (3.2 ) (3.3 ) Tax-exempt interest (0.4 ) (0.9 ) (1.1 ) Change in liability for uncertain tax positions — (0.5 ) — Other, net — (0.2 ) — Income tax expense $ 43.5 $ 76.8 $ (1.0 ) Effective tax rate 15.4 % 26.2 % -1.5 %
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 2018 2017 (in millions) Deferred tax assets: Loss, LAE and unearned premium reserves, net $ 129.8 $ 115.7 Employee benefit plans 14.5 18.7 Tax credit carryforwards — 23.1 Investments, net 4.1 — Other 16.3 11.5 164.7 169.0 Less: Valuation allowance 0.2 0.2 164.5 168.8 Deferred tax liabilities: Deferred acquisition costs 95.0 90.2 Investments, net — 41.8 Software capitalization 18.3 18.0 Other 0.6 1.8 113.9 151.8 Net deferred tax asset $ 50.6 $ 17.0
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 2018 2017 2016 (in millions) Liability at beginning of year, net $ 3.0 $ 2.7 $ 3.0 Additions for tax positions of current year — 0.9 0.4 Subtractions as a result of a lapse of the applicable statute of limitations — (0.6 ) (0.7 ) Liability at end of year, net $ 3.0 $ 3.0 $ 2.7

Pension Plans (Tables)

Pension Plans (Tables)	12 Months Ended
	Dec. 31, 2018
Retirement Plans [Line Items]
Schedule of Weighted Average Assumptions	Weighted average assumptions used to determine pension benefit obligations are as follows: DECEMBER 31 2018 2017 2016 U.S. Discount rate - qualified plan 4.50 % 3.88 % 4.25 % Discount rate - non-qualified plan 4.50 % 3.88 % 4.25 % Cash balance interest crediting rate 3.50 % 3.50 % 3.50 % The Company utilizes a measurement date of January 1 st YEARS ENDED DECEMBER 31 2018 2017 2016 U.S.Qualified plan Discount rate 3.88 % 4.25 % 4.88 % Expected return on plan assets 4.75 % 5.00 % 5.25 % Cash balance interest crediting rate 3.50 % 3.50 % 3.50 % U.S. Non-qualified plan Discount rate 3.88 % 4.25 % 4.75 %
Summary of Plan Assets Investment Measured at Fair Value	The following tables present, for each hierarchy level, the U.S. qualified defined benefit plan’s investment assets that are measured at fair value at December 31, 2018 and 2017. Refer to Note 5 – “Fair Value” for a description of the different levels in the Fair Value Hierarchy. DECEMBER 31 2018 2017 (in millions) Total Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Fixed income securities: Fixed maturities $ 35.1 $ 9.3 $ — $ 25.8 $ 38.8 $ 13.1 $ — $ 25.7 Money market mutual funds 9.1 9.1 — — 5.3 5.3 — — Mutual funds 36.3 36.3 — — 42.6 42.6 — — Total investments at fair value $ 80.5 $ 54.7 $ — $ 25.8 $ 86.7 $ 61.0 $ — $ 25.7
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 U.S. plans at December 31, 2018 and 2017. DECEMBER 31 U.S. Qualified Pension Plan U.S. Non-Qualified Pension Plan (in millions) 2018 2017 2018 2017 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 501.2 $ 506.8 $ 36.6 $ 38.0 Interest cost 18.4 20.4 1.4 1.5 Actuarial losses (gains) (17.5 ) 7.3 (1.3 ) 0.1 Benefits paid (32.6 ) (33.3 ) (2.9 ) (3.0 ) Benefit obligation, end of year (1) 469.5 501.2 33.8 36.6 Change in plan assets: Fair value of plan assets, beginning of period 460.1 453.3 — — Actual return on plan assets (16.8 ) 40.1 — — Contributions 40.0 — 2.9 3.0 Benefits paid (32.6 ) (33.3 ) (2.9 ) (3.0 ) Fair value of plan assets, end of year 450.7 460.1 — — Funded status of the plans $ (18.8 ) $ (41.1 ) $ (33.8 ) $ (36.6 ) (1) The accumulated benefit obligation for these plans is equal to the projected benefit obligation.
Components of Net Periodic Pension Cost	The components of total net periodic pension cost are as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Interest cost $ 19.8 $ 21.9 $ 24.9 Expected return on plan assets (20.6 ) (21.4 ) (23.5 ) Recognized net actuarial loss 9.6 13.5 11.2 Net periodic pension cost $ 8.8 $ 14.0 $ 12.6
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 U.S. defined benefit pension plans as of December 31, 2018 and 2017. DECEMBER 31 2018 2017 (in millions) Net actuarial loss $ 109.7 $ 100.8
Schedule of Expected Benefit Payments	The Company estimates that benefit payments over the next 10 years will be as follows: YEARS ENDED DECEMBER 31 2019 2020 2021 2022 2023 2024-2028 (in millions) U.S. qualified pension plan $ 38.5 $ 37.4 $ 38.7 $ 37.3 $ 37.1 $ 163.7 U.S. non-qualified pension plan $ 3.1 $ 3.0 $ 3.0 $ 2.9 $ 2.7 $ 12.4
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 2018 2017 Fixed maturities $ 346.4 $ 343.5 Equity securities: Domestic 9.3 12.6 International 14.6 17.3 Total equity 23.9 29.9 Total investments carried at NAV $ 370.3 $ 373.4
US [Member]
Retirement Plans [Line Items]
Summary of Target Allocations and Invested Asset Allocations	The following table provides 2018 target allocations and actual invested asset allocations for 2018 and 2017. DECEMBER 31 2018 TARGET LEVELS 2018 2017 Fixed income securities: Fixed maturities 83 % 85 % 83 % Money market funds 2 % 2 % 1 % Total fixed income securities 85 % 87 % 84 % Equity securities: Domestic 12 % 10 % 12 % International 3 % 3 % 4 % Total equity securities 15 % 13 % 16 % Total plan assets 100 % 100 % 100 %
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). YEAR ENDED DECEMBER 31 2018 (in millions) Balance at beginning of period $ 25.7 Plus: Assets transferred from investments measured at fair value using NAV 2.1 Less: Assets transferred to Level 1 investments (2.7 ) Actual return on plan assets related to assets still held 0.7 Balance at end of year $ 25.8
Chaucer Pension Plan [Member]
Retirement Plans [Line Items]
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans	The following table reflects the benefit obligations and fair value for the Chaucer plan as of December 31, 2017 and related changes in those balances, as well as the changes to those balances during 2018 prior to the plan’s disposition in conjunction with the sale of Chaucer on December 28, 2018. DECEMBER 31 Chaucer Pension Plan (in millions) 2018 2017 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 146.1 $ 132.2 Plan amendment 0.3 — Interest cost 3.7 3.9 Actuarial losses (gains) (8.6 ) 3.2 Benefits paid (3.9 ) (5.8 ) Foreign currency translation (8.3 ) 12.6 Disposition of benefit obligations with the sale of Chaucer (129.3 ) — Benefit obligation, end of year (1) — 146.1 Change in plan assets: Fair value of plan assets, beginning of period 144.6 116.5 Actual return on plan assets (5.8 ) 14.4 Contributions — 7.7 Benefits paid (3.9 ) (5.8 ) Foreign currency translation (8.1 ) 11.8 Disposition of plan assets with the sale of Chaucer (126.8 ) — Fair value of plan assets, end of year — 144.6 Funded status of the plans $ — $ (1.5 ) (1) The accumulated benefit obligation for the plan is equal to the projected benefit obligation.

Other Postretirement Benefit _2

Other Postretirement Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2018
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 U.S. plans at December 31, 2018 and 2017. DECEMBER 31 U.S. Qualified Pension Plan U.S. Non-Qualified Pension Plan (in millions) 2018 2017 2018 2017 Change in benefit obligation: Benefit obligation, beginning of period (1) $ 501.2 $ 506.8 $ 36.6 $ 38.0 Interest cost 18.4 20.4 1.4 1.5 Actuarial losses (gains) (17.5 ) 7.3 (1.3 ) 0.1 Benefits paid (32.6 ) (33.3 ) (2.9 ) (3.0 ) Benefit obligation, end of year (1) 469.5 501.2 33.8 36.6 Change in plan assets: Fair value of plan assets, beginning of period 460.1 453.3 — — Actual return on plan assets (16.8 ) 40.1 — — Contributions 40.0 — 2.9 3.0 Benefits paid (32.6 ) (33.3 ) (2.9 ) (3.0 ) Fair value of plan assets, end of year 450.7 460.1 — — Funded status of the plans $ (18.8 ) $ (41.1 ) $ (33.8 ) $ (36.6 ) (1) The accumulated benefit obligation for these plans is equal to the projected benefit obligation.
Schedule of Expected Benefit Payments	The Company estimates that benefit payments over the next 10 years will be as follows: YEARS ENDED DECEMBER 31 2019 2020 2021 2022 2023 2024-2028 (in millions) U.S. qualified pension plan $ 38.5 $ 37.4 $ 38.7 $ 37.3 $ 37.1 $ 163.7 U.S. non-qualified pension plan $ 3.1 $ 3.0 $ 3.0 $ 2.9 $ 2.7 $ 12.4
Components of Net Periodic Pension Cost	The components of total net periodic pension cost are as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Interest cost $ 19.8 $ 21.9 $ 24.9 Expected return on plan assets (20.6 ) (21.4 ) (23.5 ) Recognized net actuarial loss 9.6 13.5 11.2 Net periodic pension cost $ 8.8 $ 14.0 $ 12.6
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 U.S. defined benefit pension plans as of December 31, 2018 and 2017. DECEMBER 31 2018 2017 (in millions) Net actuarial loss $ 109.7 $ 100.8
Postretirement Plans [Member]
Schedule of Benefit Obligations, Plan Assets and Funded Status of Plans	DECEMBER 31 2018 2017 (in millions) Change in benefit obligation: Accumulated postretirement benefit obligation, beginning of year $ 10.1 $ 11.6 Interest cost 0.4 0.4 Net actuarial gain (0.9 ) (0.3 ) Benefits paid (1.2 ) (1.6 ) Accumulated postretirement benefit obligation, end of year 8.4 10.1 Fair value of plan assets, end of year — — Funded status of plans $ (8.4 ) $ (10.1 )
Schedule of Expected Benefit Payments	YEARS ENDING DECEMBER 31 (in millions) 2019 $ 1.1 2020 1.0 2021 0.9 2022 0.8 2023 0.8 2024 - 2028 3.1
Components of Net Periodic Pension Cost	YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Interest cost $ 0.4 $ 0.4 $ 0.5 Recognized net actuarial loss 0.2 0.2 0.2 Amortization of prior service cost (benefit) (0.3 ) (1.4 ) (1.4 ) Net periodic postretirement cost (benefit) $ 0.3 $ (0.8 ) $ (0.7 )
Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans	DECEMBER 31 2018 2017 (in millions) Net actuarial loss $ 2.5 $ 3.6 Net prior service cost (benefit) (0.1 ) (0.4 ) $ 2.4 $ 3.2
Summary of Weighted-Average Assumptions Used to Determine Pension Benefit Obligations	YEARS ENDED DECEMBER 31 2018 2017 Postretirement benefit obligations discount rate 4.38 % 3.75 % Postretirement benefit cost discount rate 3.75 % 4.13 %
Assumed Health Care Cost Trend Rates	DECEMBER 31 2018 2017 Health care cost trend rate assumed for next year 6.50 % 6.50 % Rate to which the cost trend is assumed to decline (ultimate trend rate) 4.50 % 4.50 % Year the rate reaches the ultimate trend rate 2024 2024

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Changes in Other Comprehensive Income	The following table provides changes in other comprehensive income. YEARS ENDED DECEMBER 31 2018 2017 2016 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 Unrealized gains (losses) on available- for-sale securities: Unrealized gains (losses) arising during period $ (209.7 ) $ 44.0 $ (165.7 ) $ 77.0 $ (27.9 ) $ 49.1 $ 95.9 $ (33.4 ) $ 62.5 Amount of realized (gains) losses from sales and other 2.5 (9.7 ) (7.2 ) (30.8 ) (1.4 ) (32.2 ) (36.5 ) (8.0 ) (44.5 ) Portion of other-than-temporary impairment losses recognized in earnings 3.5 (0.7 ) 2.8 3.9 (1.4 ) 2.5 27.9 (9.8 ) 18.1 Unrealized losses (gains) realized with sale of Chaucer business 24.2 (5.1 ) 19.1 — — — — — — Net unrealized gains (losses) (179.5 ) 28.5 (151.0 ) 50.1 (30.7 ) 19.4 87.3 (51.2 ) 36.1 Pension and postretirement benefits: Net gains (losses) arising in the period from net actuarial (gains) losses and prior service costs (21.3 ) 4.3 (17.0 ) 17.0 (3.4 ) 13.6 (43.1 ) 12.8 (30.3 ) Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost 9.7 (2.0 ) 7.7 14.0 (4.6 ) 9.4 9.9 (3.5 ) 6.4 Pension obligations recognized with sale of Chaucer business 21.2 (4.8 ) 16.4 — — — — — — Total pension and postretirement benefits 9.6 (2.5 ) 7.1 31.0 (8.0 ) 23.0 (33.2 ) 9.3 (23.9 ) Cumulative foreign currency translation adjustment: Foreign currency translation recognized during the period (2.2 ) 0.5 (1.7 ) 3.7 (1.3 ) 2.4 (5.1 ) 1.8 (3.3 ) Currency translation obligation recognized with sale of Chaucer business 29.4 (6.2 ) 23.2 — — — — — — Total cumulative foreign currency translation adjustment 27.2 (5.7 ) 21.5 3.7 (1.3 ) 2.4 (5.1 ) 1.8 (3.3 ) Other comprehensive income (loss) $ (142.7 ) $ 20.3 $ (122.4 ) $ 84.8 $ (40.0 ) $ 44.8 $ 49.0 $ (40.1 ) $ 8.9
Reclassifications Out of Accumulated Other Comprehensive Income	Reclassifications out of accumulated other comprehensive income were as follows: YEARS ENDED DECEMBER 31 2018 2017 2016 (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 Unrealized gains (losses) on available-for- sale securities $ (3.2 ) $ 26.6 $ 36.2 Net realized gains (losses) from sales and other (2.6 ) (3.6 ) (27.4 ) Net other-than-temporary impairment losses on investments recognized in earnings (5.8 ) 23.0 8.8 Total before tax 10.3 4.2 17.7 Tax benefit 4.5 27.2 26.5 Continuing operations; net of tax (19.1 ) — — Gain on sale of Chaucer business (0.2 ) 2.5 (0.2 ) Discontinued operations - Chaucer business 0.1 — (0.1 ) Discontinued life business; net of tax (14.7 ) 29.7 26.2 Net of tax Amortization of defined benefit pension and postretirement plans (9.5 ) (12.4 ) (9.7 ) Loss adjustment expenses and other operating expenses (1) 1.9 4.3 3.4 Tax benefit (7.6 ) (8.1 ) (6.3 ) Continuing operations; net of tax (16.4 ) — — Gain on sale of Chaucer business (0.1 ) (1.3 ) (0.1 ) Discontinued operations - Chaucer business (24.1 ) (9.4 ) (6.4 ) Net of tax Currency translation obligation recognized with sale of Chaucer business (23.2 ) — — Gain on sale of Chaucer business Total reclassifications for the period $ (62.0 ) $ 20.3 $ 19.8 Benefit (detriment) to 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, 2018, 2017 and 2016.

Stock-based Compensation Plans

Stock-based Compensation Plans (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 (in whole shares and dollars) Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Shares Weighted Average Exercise Price Outstanding, beginning of year 1,062,177 $ 75.53 1,396,152 $ 68.63 1,619,948 $ 56.57 Granted 301,152 110.98 460,610 90.85 587,340 82.17 Exercised (210,190 ) 69.41 (464,726 ) 63.12 (589,666 ) 48.99 Forfeited or cancelled (54,063 ) 89.09 (329,859 ) 85.22 (221,470 ) 68.61 Outstanding, end of year 1,099,076 $ 85.75 1,062,177 $ 75.53 1,396,152 $ 68.63 Exercisable, end of year 515,286 $ 69.83 423,883 $ 62.41 441,256 $ 53.59
Schedule of Stock Options by Exercise Price Range	Additional information about employee options outstanding and exercisable at December 31, 2018 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 $ 36.81 to $46.47 64,367 3.44 $ 41.61 64,367 $ 41.61 $ 57.99 93,778 5.14 57.99 93,778 57.99 $ 70.24 to $72.66 165,924 6.16 70.27 165,924 70.27 $ 74.88 to $82.74 190,120 7.30 80.75 112,847 80.49 $ 87.50 to $91.19 285,280 8.20 90.66 78,370 90.89 $ 110.57 to $124.14 299,607 9.18 110.98 — —
Schedule of Stock Option Valuation Assumptions	The following significant assumptions were used to determine the fair value for options granted in the years indicated. 2018 2017 2016 Dividend yield 1.740% to 1.954 % 2.193% to 2.286 % 2.04% to 2.46 % Expected volatility 17.943% to 18.500 % 18.341% to 21.694 % 18.08% to 21.31 % Weighted average expected volatility 18.20 % 19.52 % 19.57 % Risk-free interest rate 2.365% to 2.969 % 1.324% to 2.201 % 0.77% to 1.97 % Expected term, in years 2.5 to 6.0 2.5 to 6.0 2.5 to 5.5
Summary of Restricted Stock Activity	The following table summarizes information about employee restricted stock units: YEARS ENDED DECEMBER 31 2018 2017 2016 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 298,528 $ 83.45 269,063 $ 73.91 301,897 $ 54.54 Granted 152,529 111.66 130,075 90.40 143,107 83.43 Vested (72,710 ) 72.69 (70,590 ) 59.29 (139,183 ) 42.97 Forfeited (1) (45,866 ) 94.04 (30,020 ) 84.88 (36,758 ) 69.07 Outstanding, end of year 332,481 $ 97.28 298,528 $ 83.45 269,063 $ 73.91 Performance and market-based restricted stock units: Outstanding, beginning of year 102,586 $ 81.21 115,057 $ 78.82 196,142 $ 47.89 Granted 35,063 118.60 60,101 79.48 126,796 73.42 Vested (14,032 ) 70.24 (17,642 ) 56.45 (144,141 ) 40.95 Forfeited (1) (53,779 ) 89.79 (54,930 ) 82.27 (63,740 ) 58.54 Outstanding, end of year (2) 69,838 $ 95.58 102,586 $ 81.21 115,057 $ 78.82 (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. (2) At December 31, 2018, only market-based awards were outstanding .

Earnings Per Share and Shareh_2

Earnings Per Share and Shareholders' Equity Transactions (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 (in millions, except per share data) Basic shares used in the calculation of earnings per share 42.4 42.5 42.8 Dilutive effect of securities: Employee stock options 0.3 0.3 0.2 Non-vested stock grants 0.3 0.2 0.2 Diluted shares used in the calculation of earnings per share 43.0 43.0 43.2 Per share effect of dilutive securities on income from continuing operations $ (0.07 ) $ (0.05 ) $ (0.04 ) Per share effect of dilutive securities on net income $ (0.12 ) $ (0.05 ) $ (0.04 )

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 (in millions) Operating revenues: Commercial Lines $ 2,739.5 $ 2,573.8 $ 2,485.0 Personal Lines 1,791.3 1,662.3 1,552.4 Other 14.2 10.7 6.3 Total 4,545.0 4,246.8 4,043.7 Net realized and unrealized investment gains (losses) (50.7 ) 21.1 10.2 Total revenues $ 4,494.3 $ 4,267.9 $ 4,053.9 Operating income (loss) before interest expense and income taxes: Commercial Lines: Underwriting income (loss) $ 85.0 $ 12.2 $ (123.2 ) Net investment income 182.2 165.8 158.5 Other expense (1.5 ) (0.6 ) (0.6 ) Commercial Lines operating income 265.7 177.4 34.7 Personal Lines: Underwriting income 66.6 83.7 103.1 Net investment income 73.7 70.1 69.5 Other income 5.9 4.9 5.1 Personal Lines operating income 146.2 158.7 177.7 Other: Underwriting loss (4.2 ) (4.2 ) (9.9 ) Net investment income 11.5 8.0 3.6 Other expense (12.7 ) (12.6 ) (13.2 ) Other operating loss (5.4 ) (8.8 ) (19.5 ) Operating income before interest expense and income taxes 406.5 327.3 192.9 Interest on debt (45.1 ) (45.2 ) (51.4 ) Operating income before income taxes 361.4 282.1 141.5 Non-operating income (loss) items: Net realized and unrealized investment gains (losses) (50.7 ) 21.1 10.2 Net loss from repayment of debt (28.2 ) — (88.3 ) Other non-operating items — (10.3 ) 2.6 Income from continuing operations before income taxes $ 282.5 $ 292.9 $ 66.0
Identifiable Assets by Business Segment	The following table provides identifiable assets for the Company’s business segments and discontinued operations: DECEMBER 31 2018 2017 (in millions) Identifiable Assets U.S. Companies $ 12,238.4 $ 10,915.0 Assets held-for-sale (1) 57.4 4,466.6 Assets of discontinued life business 103.9 88.0 Total $ 12,399.7 $ 15,469.6

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2018
Reinsurance Disclosures [Abstract]
Schedule of Effects of Reinsurance	The following table provides the effects of reinsurance. YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Premiums written: Direct $ 625.3 $ 597.6 $ 560.3 Assumed 685.3 646.9 546.3 Ceded (1) (459.9 ) (395.4 ) (290.5 ) Net premiums written $ 850.7 $ 849.1 $ 816.1 Premiums earned: Direct $ 611.9 $ 568.7 $ 584.0 Assumed 667.2 628.3 577.3 Ceded (1) (429.1 ) (344.0 ) (322.7 ) Net premiums earned $ 850.0 $ 853.0 $ 838.6 Losses and LAE: Direct $ 362.2 $ 365.2 $ 403.1 Assumed 568.7 570.7 258.8 Ceded (2) (415.4 ) (386.8 ) (243.2 ) Net losses and LAE $ 515.5 $ 549.1 $ 418.7 YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Premiums written: Direct $ 4,816.0 $ 4,540.0 $ 4,265.7 Assumed 27.6 23.6 25.1 Ceded (458.8 ) (454.5 ) (408.1 ) Net premiums written $ 4,384.8 $ 4,109.1 $ 3,882.7 Premiums earned: Direct $ 4,673.6 $ 4,399.9 $ 4,162.6 Assumed 26.6 23.6 24.4 Ceded (445.8 ) (443.1 ) (397.5 ) Net premiums earned $ 4,254.4 $ 3,980.4 $ 3,789.5 Percentage of assumed to net premiums earned 0.6 % 0.6 % 0.6 % Losses and LAE: Direct $ 2,986.2 $ 2,934.6 $ 2,785.1 Assumed 25.2 20.5 30.6 Ceded (1) (286.8 ) (375.5 ) (269.7 ) Net losses and LAE $ 2,724.6 $ 2,579.6 $ 2,546.0 (1) The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines.

Liabilities For Outstanding C_2

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
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 2018 2017 2016 (in millions) Gross loss and LAE reserves, beginning of year $ 5,058.5 $ 4,660.0 $ 4,201.9 Reinsurance recoverable on unpaid losses 1,455.0 1,349.2 1,295.3 Net loss and LAE reserves, beginning of year 3,603.5 3,310.8 2,906.6 Net incurred losses and LAE in respect of losses occurring in: Current year 2,733.5 2,579.8 2,307.8 Prior years (8.9 ) (0.2 ) 238.2 Total incurred losses and LAE 2,724.6 2,579.6 2,546.0 Net payments of losses and LAE in respect of losses occurring in: Current year 1,232.3 1,203.8 1,063.0 Prior years 1,264.3 1,083.1 1,078.8 Total payments 2,496.6 2,286.9 2,141.8 Net reserve for losses and LAE, end of year 3,831.5 3,603.5 3,310.8 Reinsurance recoverable on unpaid losses 1,472.6 1,455.0 1,349.2 Gross reserve for losses and LAE, end of year $ 5,304.1 $ 5,058.5 $ 4,660.0
Schedule of (Favorable)/Unfavorable Loss and LAE Reserve Development	YEARS ENDED DECEMBER 31 2018 2017 2016 (in millions) Commercial multiple peril $ (4.0 ) $ 2.9 $ 68.8 Workers’ compensation (31.0 ) (9.1 ) (46.7 ) Commercial automobile 23.2 2.3 27.5 Other commercial lines: AIX program business 17.0 (1.9 ) 75.1 General liability (21.1 ) (1.9 ) 56.0 Surety (9.0 ) 0.1 35.3 Umbrella 2.3 0.9 (8.9 ) Other lines (18.3 ) (4.1 ) 12.2 Total other commercial lines (29.1 ) (6.9 ) 169.7 Total Commercial Lines (40.9 ) (10.8 ) 219.3 Personal automobile 14.3 3.4 4.8 Homeowners and other personal lines 16.5 6.0 5.8 Total Personal Lines 30.8 9.4 10.6 Total Other Segment 1.2 1.2 8.3 Total loss and LAE reserve development, including catastrophes $ (8.9 ) $ (0.2 ) $ 238.2
Schedule of Carried Reserves	YEARS ENDED DECEMBER 31, 2018 2017 (in millions) Gross Ceded Net Gross Ceded Net Commercial multiple peril $ 1,206.5 $ (110.8 ) $ 1,095.7 $ 1,104.9 $ (108.9 ) $ 996.0 Workers’ compensation 775.9 (183.0 ) 592.9 757.8 (180.8 ) 577.0 Commercial automobile liability 433.5 (25.5 ) 408.0 428.1 (27.6 ) 400.5 General liability and umbrella - occurrence 534.1 (133.5 ) 400.6 508.5 (126.2 ) 382.3 General liability - claims made 245.9 (16.2 ) 229.7 231.6 (17.9 ) 213.7 Other lines 400.6 (78.4 ) 322.2 408.3 (108.8 ) 299.5 Total Commercial Lines and other 3,596.5 (547.4 ) 3,049.1 3,439.2 (570.2 ) 2,869.0 Personal automobile liability 1,514.0 (917.3 ) 596.7 1,453.8 (878.8 ) 575.0 Homeowners 158.2 (6.0 ) 152.2 133.5 (4.4 ) 129.1 Other personal lines 35.4 (1.9 ) 33.5 32.0 (1.6 ) 30.4 Total Personal Lines 1,707.6 (925.2 ) 782.4 1,619.3 (884.8 ) 734.5 Total loss and LAE reserves $ 5,304.1 $ (1,472.6 ) $ 3,831.5 $ 5,058.5 $ (1,455.0 ) $ 3,603.5
Schedule of Incurred Claims Development	Commercial multiple peril (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2012 $ 480.0 $ 462.8 $ 456.2 $ 459.7 $ 475.6 $ 474.9 $ 476.6 $ 8.8 16,983 2013 380.0 362.5 367.9 391.8 392.1 393.0 10.2 14,830 2014 443.9 439.6 464.8 459.4 449.1 22.6 15,922 2015 446.0 456.3 463.7 467.4 33.0 15,731 2016 447.1 449.6 448.7 55.5 15,981 2017 538.7 544.8 122.2 16,674 2018 578.2 228.3 16,716 Total $ 3,357.8 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2012 $ 175.8 $ 307.5 $ 350.8 $ 389.3 $ 424.2 $ 442.8 $ 454.9 2013 137.6 221.5 262.3 306.4 334.6 355.9 2014 171.7 267.8 316.0 363.4 395.2 2015 161.9 260.1 315.6 363.2 2016 140.3 237.9 290.2 2017 170.9 296.4 2018 178.8 Total $ 2,334.6 Total reserves for 2012 – 2018 accident years (incurred - paid) 1,023.2 Total reserves for 2011 and prior accident years 53.2 Unallocated loss adjustment expense 19.3 Net reserves at December 31, 2018 $ 1,095.7 Workers' compensation (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2009 2010 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2009 $ 93.8 $ 87.6 $ 82.5 $ 80.3 $ 79.4 $ 79.2 $ 77.5 $ 77.5 $ 77.9 $ 78.7 $ 1.9 8,032 2010 115.5 116.9 115.8 115.9 116.3 114.9 114.7 115.9 116.5 3.3 10,174 2011 141.4 144.2 144.3 145.5 143.8 146.7 148.3 147.9 7.1 12,465 2012 176.3 171.1 165.2 157.2 162.9 163.7 163.7 10.2 13,042 2013 179.3 167.4 160.1 154.4 155.3 155.4 12.3 11,683 2014 182.1 172.9 154.7 152.1 150.3 15.2 10,880 2015 189.6 164.2 157.5 150.3 22.5 11,417 2016 189.6 180.5 164.6 24.5 15,790 2017 186.1 172.1 35.4 16,405 2018 187.2 69.4 15,711 Total $ 1,486.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2009 2010 2011 2012 2013 2014 2015 Year Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 2009 $ 19.2 $ 41.2 $ 54.6 $ 61.6 $ 65.4 $ 68.6 $ 69.8 $ 70.8 $ 72.4 $ 73.1 2010 22.1 57.2 76.9 89.5 96.5 101.0 104.2 106.3 107.2 2011 30.0 71.3 97.3 114.0 122.9 128.0 130.9 132.8 2012 30.4 74.8 102.6 120.1 130.7 136.0 141.4 2013 30.9 74.6 101.2 114.0 122.8 127.4 2014 30.6 70.5 92.3 105.6 112.5 2015 28.0 65.7 87.2 99.4 2016 33.9 78.1 99.5 2017 32.8 73.0 2018 35.6 Total $ 1,001.9 Total reserves for 2009 – 2018 accident years (incurred - paid) 484.8 Total reserves for 2008 and prior accident years 91.1 Unallocated loss adjustment expense and other 17.0 Net reserves at December 31, 2018 $ 592.9 Commercial automobile liability (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2013 $ 177.7 $ 179.4 $ 205.3 $ 227.5 $ 225.6 $ 231.0 $ 1.4 15,274 2014 168.5 163.3 177.3 181.7 185.1 6.4 13,477 2015 163.4 168.3 166.9 167.8 10.0 12,838 2016 157.0 157.7 163.0 16.4 11,629 2017 159.5 170.1 43.4 11,343 2018 182.8 110.7 10,338 Total $ 1,099.8 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2013 2014 2015 Year Unaudited Unaudited Unaudited 2016 2017 2018 2013 $ 35.9 $ 89.6 $ 137.6 $ 176.3 $ 200.6 $ 214.9 2014 33.1 70.8 102.7 137.1 168.2 2015 32.2 63.8 96.4 129.3 2016 27.8 60.7 98.3 2017 26.9 71.2 2018 29.2 Total $ 711.1 Total reserves for 2013 – 2018 accident years (incurred - paid) 388.7 Total reserves for 2012 and prior accident years 14.4 Unallocated loss adjustment expense 4.9 Net reserves at December 31, 2018 $ 408.0 General liability and umbrella - occurrence (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2011 2012 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2011 $ 47.6 $ 43.8 $ 38.2 $ 42.2 $ 51.0 $ 55.6 $ 55.4 $ 57.5 $ 3.3 1,398 2012 77.2 59.3 62.2 64.3 71.1 74.2 69.1 3.7 1,779 2013 84.1 67.4 71.5 88.6 84.4 81.2 8.2 2,031 2014 100.9 82.3 98.6 101.4 98.3 10.6 2,133 2015 104.2 99.3 99.6 97.0 21.3 2,550 2016 95.6 100.7 101.5 33.9 2,015 2017 97.7 107.6 54.2 1,844 2018 99.8 74.4 1,917 Total $ 712.0 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 2011 $ 1.6 $ 6.7 $ 19.4 $ 31.1 $ 39.8 $ 45.8 $ 47.8 $ 50.6 2012 2.2 12.6 29.8 43.8 52.1 60.6 63.0 2013 2.4 11.0 26.8 43.1 56.1 63.5 2014 3.1 14.5 31.4 52.8 70.4 2015 3.3 15.2 30.8 48.5 2016 3.1 15.0 31.6 2017 4.4 17.0 2018 4.1 Total $ 348.7 Total reserves for 2011 – 2018 accident years (incurred - paid) 363.3 Total reserves for 2010 and prior accident years 25.8 Unallocated loss adjustment expense and other 11.5 Net reserves at December 31, 2018 $ 400.6 General liability - claims made (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2013 2014 2015 Claim Year Unaudited Unaudited Unaudited 2016 2017 2018 IBNR Count 2013 $ 54.9 $ 55.4 $ 60.6 $ 71.4 $ 69.2 $ 68.4 $ 0.9 645 2014 61.7 69.9 83.9 86.2 82.5 2.9 907 2015 93.2 98.8 98.4 90.2 4.9 1,035 2016 103.6 101.7 97.1 18.4 1,082 2017 103.3 104.6 35.5 1,310 2018 120.9 59.5 2,586 Total $ 563.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2013 2014 2015 Year Unaudited Unaudited Unaudited 2016 2017 2018 2013 $ 9.3 $ 36.2 $ 53.7 $ 61.1 $ 64.5 $ 65.7 2014 11.0 38.8 59.1 71.5 75.4 2015 10.0 43.6 64.2 76.0 2016 11.1 42.9 66.1 2017 12.3 42.3 2018 17.3 Total $ 342.8 Total reserves for 2013– 2018 accident years (incurred - paid) 220.9 Total reserves for 2012 and prior accident years 4.0 Unallocated loss adjustment expense 4.8 Net reserves at December 31, 2018 $ 229.7 Personal automobile liability (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2014 2015 Claim Year Unaudited Unaudited 2016 2017 2018 IBNR Count 2014 $ 323.1 $ 304.1 $ 308.8 $ 310.2 $ 312.5 $ 4.7 43,356 2015 327.4 334.1 328.6 332.4 5.6 42,358 2016 337.9 344.5 355.3 11.5 42,214 2017 363.6 362.3 35.0 42,200 2018 394.2 151.7 39,164 Total $ 1,756.7 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2014 2015 Year Unaudited Unaudited 2016 2017 2018 2014 $ 107.2 $ 188.6 $ 241.1 $ 279.4 $ 298.4 2015 112.9 205.4 261.5 301.9 2016 112.8 213.1 288.0 2017 115.0 229.2 2018 121.7 Total $ 1,239.2 Total reserves for 2014 – 2018 accident years (incurred - paid) 517.5 Total reserves for 2013 and prior accident years 65.4 Unallocated loss adjustment expense 13.8 Net reserves at December 31, 2018 $ 596.7 Homeowners (in millions) As of Incurred Losses and ALAE, Net of Reinsurance December 31, 2018 YEARS ENDED DECEMBER 31, Cumulative Incurred Accident 2015 Claim Year Unaudited 2016 2017 2018 IBNR Count 2015 $ 267.1 $ 272.0 $ 272.0 $ 274.2 1.7 30,436 2016 222.1 226.5 231.4 3.2 25,456 2017 291.4 296.5 3.4 33,394 2018 312.9 46.0 30,487 Total $ 1,115.0 Cumulative Paid Losses and ALAE, Net of Reinsurance YEARS ENDED DECEMBER 31, Accident 2015 Year Unaudited 2016 2017 2018 2015 $ 199.9 $ 253.8 $ 260.6 $ 267.5 2016 154.3 207.9 218.2 2017 204.5 271.6 2018 213.7 Total $ 971.0 Total reserves for 2015 – 2018 accident years (incurred - paid) 144.0 Total reserves for 2014 and prior accident years 4.8 Unallocated loss adjustment expense 3.4 Net reserves at December 31, 2018 $ 152.2
Computation of Historical Claims on Paid and Incurred Claims Data, Net of Reinsurance	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 34.0 % 22.7 % 10.7 % 10.0 % 7.2 % 4.7 % 2.5 % Workers' compensation 20.0 % 27.0 % 15.9 % 9.6 % 5.6 % 3.5 % 2.4 % 1.4 % 1.4 % 0.8 % Commercial automobile liability 16.9 % 21.7 % 20.1 % 18.3 % 13.7 % 6.2 % General liability and umbrella - occurrence 3.4 % 11.7 % 19.4 % 20.1 % 15.3 % 10.6 % 3.5 % 4.9 % General liability - claims made 12.6 % 34.4 % 24.3 % 12.9 % 4.8 % 1.7 % Personal automobile liability 32.5 % 28.4 % 18.3 % 12.2 % 6.1 % Homeowners 69.2 % 21.8 % 3.4 % 2.5 %

Statutory Financial Informati_2

Statutory Financial Information (Tables)	12 Months Ended
	Dec. 31, 2018
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 as of December 31 for the periods indicated: (in millions) 2018 2017 2016 Statutory Net Income U.S. Insurance Subsidiaries $ 296.1 $ 254.5 $ 158.0 Statutory Capital and Surplus U.S. Insurance Subsidiaries 2,172.5 2,077.1 2,173.4

Quarterly Results of Operatio_2

Quarterly Results of Operations (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The quarterly results of operations for 2018 and 2017 are summarized below. THREE MONTHS ENDED (in millions, except per share data) 2018 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,091.5 $ 1,133.4 $ 1,166.6 $ 1,102.8 Income from continuing operations $ 50.5 $ 82.4 $ 104.0 $ 2.1 Net income $ 67.7 $ 99.3 $ 100.4 $ 123.6 Income from continuing operations per share: Basic $ 1.19 $ 1.94 $ 2.45 $ 0.05 Diluted $ 1.17 $ 1.91 $ 2.41 $ 0.05 Net income per share: Basic $ 1.59 $ 2.34 $ 2.36 $ 2.92 Diluted $ 1.57 $ 2.31 $ 2.33 $ 2.88 Dividends declared per share (1) $ 0.54 $ 0.54 $ 0.54 $ 5.35 THREE MONTHS ENDED (in millions, except per share data) 2017 March 31 June 30 Sept. 30 Dec. 31 Total revenues $ 1,036.2 $ 1,053.3 $ 1,085.9 $ 1,092.5 Income from continuing operations $ 28.4 $ 56.3 $ 59.5 $ 71.9 Net income $ 45.2 $ 78.4 $ 11.1 $ 51.5 Income from continuing operations per share: Basic $ 0.67 $ 1.32 $ 1.40 $ 1.69 Diluted $ 0.66 $ 1.32 $ 1.38 $ 1.67 Net income per share: Basic $ 1.06 $ 1.84 $ 0.26 $ 1.21 Diluted $ 1.05 $ 1.83 $ 0.26 $ 1.20 Dividends declared per share $ 0.50 $ 0.50 $ 0.50 $ 0.54 (1) As a result of the sale of Chaucer, (see Note 2 – “Discontinued Operations”), on December 30, 2018 the Board of Directors declared a special dividend of $4.75 per share (see Note 12 – “Earnings Per Share and Shareholders’ Equity Transactions”).

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Narrative) (Details) - USD ($)	Feb. 14, 2019	Jan. 01, 2018	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Impairment recognized				$ 0	$ 0
Intangible assets				15,000,000	15,000,000
Goodwill				$ 178,800,000	$ 178,800,000
U.S. statutory income tax rate				21.00%	35.00%	35.00%
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect on retained earnings, net of tax		$ 6,500,000
Accounting Standards Update 2016-01 And 2018-03 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment benefit to retained earnings				$ 97,800,000
Cumulative effect on retained earnings, net of tax				95,200,000
Gains on partnership investments at NAV				2,600,000
Accounting Standards Update 2016-02 And 2018-11 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect to increase assets and liabilities				$ 35,000,000
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
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
Chaucer Irish Entity[Member] | Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
Proceeds from sale of businesses segment			$ 13,000,000
Chaucer Irish Entity[Member] | Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Proceeds from sale of businesses segment	$ 28,000,000

Discontinued Operations (Narrat

Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions		Feb. 14, 2019	Mar. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total cash proceeds expected from sale of Chaucer Holdings Limited				$ 793.7
Pre-tax gain on sale				$ 174.4
Excess loss percentage of net earned premium				10.00%
Restricted assets, fixed maturities						$ 528.1
Restricted assets, cash and cash equivalents						7.1
Reinsurance recoverable on paid and unpaid losses and unearned premiums				$ 1,648.6		1,625.5
Assets held-for-sale				57.4		4,466.6
Liabilities held-for-sale				22.2		3,707.2
Net Income/gain (loss) from discontinued businesses, net of taxes				0.1		(16.8)	$ (1)
Income tax benefit related to discontinued operations						9.3	2.8
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net investment income for contract				8		8	3
Lloyd's Syndicates [Member] | Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Reinsurance recoverable on paid and unpaid losses and unearned premiums						443
Discontinued Accident and Health Insurance Business [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Assets held-for-sale				81.6		76.9
Liabilities held-for-sale				81.4		83.1
Chaucer Irish Entity[Member] | Scenario, Forecast [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment			$ 13
Chaucer Irish Entity[Member] | Subsequent Event [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment		$ 28
Australian Entities [Member] | Scenario, Forecast [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Cash proceeds to be received			$ 13
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total cash proceeds expected from sale of Chaucer Holdings Limited	[1]			793.7
Pre-tax gain on sale				174.4
Amount recognized as foreign currency transaction losses during the period				0.8		1.1	22.3
Net Income/gain (loss) from discontinued businesses, net of taxes				$ 20	[2]	$ (13.1)	$ 89.1
[1]	Initial consideration for Chaucer Holdings Limited as determined in the sales and purchase agreement was $779 million. This amount, along with $28 million in cash proceeds received for the sale of the Irish entity on February 14, 2019, $13 million in cash proceeds to be received upon the closing of the sale of the Australian entities, 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, results 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]	2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.

Discontinued Operations (Compon

Discontinued Operations (Components of Estimated Gain From Disposal of Chaucer Business) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Initial consideration received from sale		$ 936.7
Total cash proceeds expected from sale of Chaucer Holdings Limited		793.7
Pre-tax gain on sale		174.4
Income tax expense		0.2	$ 0.1	$ (1.7)
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	[1],[2]	31.7
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
Income tax expense	[6]	42.5
Gain on sale		$ 131.9
[1]	Initial consideration for Chaucer Holdings Limited as determined in the sales and purchase agreement was $779 million. This amount, along with $28 million in cash proceeds received for the sale of the Irish entity on February 14, 2019, $13 million in cash proceeds to be received upon the closing of the sale of the Australian entities, 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, results 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, may be up to $45 million and is determined based upon 2018 catastrophe losses. Based upon the Company’s best estimate of Chaucer’s 2018 catastrophe losses, expected contingent consideration is $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 are 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	Mar. 31, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 28, 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
Chaucer Irish Entity[Member] | Scenario, Forecast [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment			$ 13
Chaucer Irish Entity[Member] | Subsequent Event [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Proceeds from sale of businesses segment		$ 28
Australian Entities [Member] | Scenario, Forecast [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Cash proceeds to be received			$ 13
Chaucer [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total consideration	[1]				779
Contingent proceeds	[1],[2]				31.7
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 Holdings Limited as determined in the sales and purchase agreement was $779 million. This amount, along with $28 million in cash proceeds received for the sale of the Irish entity on February 14, 2019, $13 million in cash proceeds to be received upon the closing of the sale of the Australian entities, 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, results 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, may be up to $45 million and is determined based upon 2018 catastrophe losses. Based upon the Company’s best estimate of Chaucer’s 2018 catastrophe losses, expected contingent consideration is $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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Revenues
Premiums									$ 4,254.4		$ 3,980.4	$ 3,789.5
Net investment income									267.4		243.9	231.6
Total revenues	$ 1,102.8	$ 1,166.6	$ 1,133.4	$ 1,091.5	$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	4,494.3		4,267.9	4,053.9
Losses and operating expenses
Losses and LAE									2,724.6		2,579.6	2,546
Amortization of deferred acquisition costs									891.8		840.7	803.6
Other operating expenses									522.1		509.5	498.6
Total losses and expenses									4,211.8		3,975	3,987.9
Other items:
Interest expense									(45.1)		(45.2)	(51.4)
Net realized and unrealized investment gains (losses)									(50.7)		21.1	10.2
Income tax expense									(43.5)		(76.8)	1
Income (loss) from Chaucer business, net of taxes									0.1		(16.8)	(1)
Chaucer [Member]
Revenues
Premiums									850	[1]	853	838.6
Net investment income									54.9	[1]	52	45.7
Other income									7.5	[1]	6.7	7.1
Total revenues									912.4	[1]	911.7	891.4
Losses and operating expenses
Losses and LAE									515.5	[1]	549.5	419.3
Amortization of deferred acquisition costs									252.5	[1]	245.9	231.6
Other operating expenses									115	[1]	109.2	113.7
Total losses and expenses									883	[1]	904.6	764.6
Income from Chaucer business before income taxes and other items (previously presented as Chaucer's operating income)									29.4	[1]	7.1	126.8
Other items:
Interest expense									(3.8)	[1]	(3.3)	(3.5)
Net realized and unrealized investment gains (losses)									(1.3)	[1]	2.6	(1.6)
Other income									0.4	[1]	2.2	4.6
Income from Chaucer business before income taxes									24.7	[1]	8.6	126.3
Income tax expense									(4.7)	[1]	(21.7)	(37.2)
Income (loss) from Chaucer business, net of taxes									$ 20	[1]	$ (13.1)	$ 89.1
[1]	2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.

Discontinued Chaucer Business (

Discontinued Chaucer Business (Summary of Carrying Amounts of Major Assets and Liabilities Reflected in Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets
Investments	$ 24.5	$ 2,153.5
Cash and cash equivalents	3.3	78.5
Premiums and accounts receivable, net	2.8	471.9
Deferred acquisition costs	1.4	120.2
Reinsurance recoverable on paid and unpaid losses and unearned premiums	2.3	1,431.5
Goodwill	6.5	13.8
Other assets	16.6	197.2
Total assets	57.4	4,466.6
Liabilities
Loss and loss adjustment expense reserves	2	2,686.5
Unearned premiums	7.1	631.9
Reinsurance premium payable	1.5	293.3
Other liabilities	11.6	95.5
Total liabilities	$ 22.2	$ 3,707.2

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash (used in) provided by operating activities	$ (22.8)	$ 28.8	$ 95.9
Net cash provided by (used in) investing activities	$ 131.1	$ (96.9)	$ (104.7)

Discontinued Operations (Schedu

Discontinued Operations (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Property and casualty premiums written, Direct		$ 4,816		$ 4,540	$ 4,265.7
Property and casualty premiums written, Assumed		27.6		23.6	25.1
Property and casualty premiums written, Ceded		(458.8)		(454.5)	(408.1)
Property and casualty premiums written, Net		4,384.8		4,109.1	3,882.7
Property and casualty premiums earned, Direct		4,673.6		4,399.9	4,162.6
Property and casualty premiums earned, Assumed		26.6		23.6	24.4
Property and casualty premiums earned, Ceded		(445.8)		(443.1)	(397.5)
Premiums Earned, Net, Total		4,254.4		3,980.4	3,789.5
Property and casualty losses and LAE, Direct		2,986.2		2,934.6	2,785.1
Property and casualty losses and LAE, Assumed		25.2		20.5	30.6
Property and casualty losses and LAE, Ceded	[1]	(286.8)		(375.5)	(269.7)
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		597.6	560.3
Property and casualty premiums written, Assumed		685.3		646.9	546.3
Property and casualty premiums written, Ceded	[2]	(459.9)		(395.4)	(290.5)
Property and casualty premiums written, Net		850.7		849.1	816.1
Property and casualty premiums earned, Direct		611.9		568.7	584
Property and casualty premiums earned, Assumed		667.2		628.3	577.3
Property and casualty premiums earned, Ceded	[2]	(429.1)		(344)	(322.7)
Premiums Earned, Net, Total		850	[3]	853	838.6
Property and casualty losses and LAE, Direct		362.2		365.2	403.1
Property and casualty losses and LAE, Assumed		568.7		570.7	258.8
Property and casualty losses and LAE, Ceded	[4]	(415.4)		(386.8)	(243.2)
Policyholder Benefits and Claims Incurred, Net, Total		$ 515.5		$ 549.1	$ 418.7
[1]	The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines
[2]	The increase in ceded reinsurance premiums from 2016 through 2018 is primarily due to Chaucer’s planned increase in reinsurance purchases.
[3]	2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.
[4]	The increase in ceded losses and LAE from 2016 through 2018 is primarily due to higher catastrophe loss activity in certain Chaucer lines and due to the aforementioned increase in reinsurance purchases.

Investments (Schedule of Availa

Investments (Schedule of Available-for-sale Securities Reconciliation) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	$ 6,245.9	$ 5,657.1
Fair Value	6,161.5	5,749.3
Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	879	910.1
Gross Unrealized Gains	16.6	29.7
Gross Unrealized Loss	9.8	5.8
Fair Value	885.8	934
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	6,245.9	5,657.1
Gross Unrealized Gains	49.8	130.6
Gross Unrealized Loss	134.2	38.4
Fair Value	6,161.5	5,749.3
OTTI Unrealized Losses	6.6	6.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	6,270.8	7,688.8
Gross Unrealized Gains	49.8	145.2
Gross Unrealized Loss	134.6	54.3
Fair Value	6,186	7,779.7
OTTI Unrealized Losses	6.6	6.9
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	24.9	2,031.7
Gross Unrealized Gains		14.6
Gross Unrealized Loss	0.4	15.9
Fair Value	24.5	2,030.4
OTTI Unrealized Losses		0.4
US Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	414.7	322.1
Gross Unrealized Gains	2.4	3.4
Gross Unrealized Loss	7.2	3.2
Fair Value	409.9	322.3
Foreign Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	7.3	8.2
Gross Unrealized Gains	0.1	0.3
Fair Value	7.4	8.5
Corporate [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	3,476.6	3,134.2
Gross Unrealized Gains	26.1	84.1
Gross Unrealized Loss	92	21.7
Fair Value	3,410.7	3,196.6
OTTI Unrealized Losses	6.6	6.5
Residential Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	728.4	632.3
Gross Unrealized Gains	2.7	5.8
Gross Unrealized Loss	14.7	4.9
Fair Value	716.4	633.2
Commercial Mortgage Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	648.4	590.3
Gross Unrealized Gains	1.7	7.2
Gross Unrealized Loss	9.8	2.5
Fair Value	640.3	595
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost	91.5	59.9
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Loss	0.7	0.3
Fair Value	$ 91	$ 59.7

Investments (Narrative) (Detail

Investments (Narrative) (Details) - USD ($)	Jan. 02, 2018	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Net unrealized gains on impaired securities		$ 7,400,000	$ 11,000,000
Cash collateral maintained		102.00%
Equity securities, at fair value		$ 464,400,000	576,200,000
Equity securities, cost			433,700,000
Other investments		661,500,000	$ 562,700,000
Due in one year or less, Amortized Cost		$ 207,400,000
Concentration of investment in a single investee, maximum		10.00%	10.00%
Contractual investment commitments		$ 162,900,000
U.S. Government-Sponsored Agencies and Authorities [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities		396,700,000	$ 396,700,000
Mortgage Participations [Member]
Schedule of Investments [Line Items]
Marketable securities, fixed maturities		356,800,000	356,800,000
Federal Home Loan Bank of Boston [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value		234,900,000	225,700,000
Security Lending Program [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value		4,800,000	23,400,000
US Treasury and Government Agencies [Member]
Schedule of Investments [Line Items]
Securities on loan, fair value		299,900,000	286,900,000
Equity Securities [Member] | Retained Earnings [Member]
Schedule of Investments [Line Items]
Cumulative effect on retained earnings, before tax	$ 142,500,000
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments		405,700,000	365,800,000
Due in one year or less, Amortized Cost		0
Due in year two, Amortized Cost		10,000,000
Due in year three, Amortized Cost		57,400,000
Due in year four, Amortized Cost		28,100,000
Due in year five, Amortized Cost		17,000,000
Due in year six and thereafter, Amortized Cost			293,200,000
Limited Partnership [Member]
Schedule of Investments [Line Items]
Other investments		$ 235,700,000	$ 186,400,000

Investments (Investments Classi

Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost	$ 207.4
Due after one year through five years, Amortized Cost	1,830.9
Due after five years through ten years, Amortized Cost	2,465.1
Due after ten years, Amortized Cost	274.2
Gross fixed maturities, Amortized Cost	4,777.6
Mortgage-backed and asset-backed securities, Amortized Cost	1,468.3
Amortized Cost or Cost	6,245.9	$ 5,657.1
Fixed maturities, held-for-sale	24.9
Total fixed maturities, Amortized Cost	6,270.8
Due in one year or less, Fair Value	208.5
Due after one year through five years, Fair Value	1,830
Due after five years through ten years, Fair Value	2,398.7
Due after ten years, Fair Value	276.6
Gross fixed maturities, Fair Value	4,713.8
Mortgage-backed and asset-backed securities, Fair Value	1,447.7
Fixed maturities, excluding held-for-sale (Chaucer)	6,161.5	$ 5,749.3
Fixed maturities, held-for-sale	24.5
Fixed maturities, Fair Value	$ 6,186

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Provision for deferred income taxes	$ (28.5)	$ 30.7	$ 51.2
Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	205.4	186	149.9
Net appreciation (depreciation) on available-for-sale securities	(203.7)	40.9	76.1
Provision for deferred income taxes	33.6	(30.7)	(51.2)
Change in OTTI losses recognized in other comprehensive income		9.2	11.2
Amount realized with sale of Chaucer business	19.1
Cumulative effect adjustment for ASUs 2016-01 and 2018-02, net of tax	(81.6)
Total adjustment	(232.6)	19.4	36.1
Net appreciation (depreciation), end of year	(27.2)	205.4	186
Fixed Maturities [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Net appreciation (depreciation), beginning of year	110.1	127.1	116.5
Net appreciation (depreciation) on available-for-sale securities	(203.7)	(15)	36.9
Provision for deferred income taxes	33.6	(11.2)	(37.5)
Change in OTTI losses recognized in other comprehensive income		9.2	11.2
Amount realized with sale of Chaucer business	19.1
Cumulative effect adjustment for ASUs 2016-01 and 2018-02, net of tax	13.7
Total adjustment	(137.3)	(17)	10.6
Net appreciation (depreciation), end of year	(27.2)	110.1	127.1
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	58.9	33.4
Net appreciation (depreciation) on available-for-sale securities		55.9	39.2
Provision for deferred income taxes		(19.5)	(13.7)
Cumulative effect adjustment for ASUs 2016-01 and 2018-02, net of tax	(95.3)
Total adjustment	$ (95.3)	36.4	25.5
Net appreciation (depreciation), end of year		$ 95.3	$ 58.9

Investments (Schedule of Unreal

Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 38.3	$ 8.4
12 months or less, Fair Value	1,992.4	1,086.5
Greater than 12 months, Gross Unrealized Losses	77.8	22.1
Greater than 12 months, Fair Value	1,758.5	783.3
Total, Gross Unrealized Losses	116.1	30.5
Total, Fair Value	3,750.9	1,869.8
Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.1
12 months or less, Fair Value	185.6
Greater than 12 months, Gross Unrealized Losses	10
Greater than 12 months, Fair Value	54.9
Total, Gross Unrealized Losses	18.1
Total, Fair Value	240.5
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	46.4	9.9
12 months or less, Fair Value	2,178	1,135.2
Greater than 12 months, Gross Unrealized Losses	87.8	28.5
Greater than 12 months, Fair Value	1,813.4	822.3
Total, Gross Unrealized Losses	134.2	38.4
Total, Fair Value	3,991.4	1,957.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	46.5	15.6
12 months or less, Fair Value	2,182	1,938.5
Greater than 12 months, Gross Unrealized Losses	88.1	38.7
Greater than 12 months, Fair Value	1,831.4	1,211.9
Total, Gross Unrealized Losses	134.6	54.3
Total, Fair Value	4,013.4	3,150.4
Fixed Maturities Held-For-Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	5.7
12 months or less, Fair Value	4	803.3
Greater than 12 months, Gross Unrealized Losses	0.3	10.2
Greater than 12 months, Fair Value	18	389.6
Total, Gross Unrealized Losses	0.4	15.9
Total, Fair Value	22	1,192.9
US Treasury and Government Agencies [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.1	1.1
12 months or less, Fair Value	66	146.3
Greater than 12 months, Gross Unrealized Losses	6.1	2.1
Greater than 12 months, Fair Value	210.9	62.3
Total, Gross Unrealized Losses	7.2	3.2
Total, Fair Value	276.9	208.6
Foreign Government [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Fair Value	2	0.1
Greater than 12 months, Fair Value	0.8	0.6
Total, Fair Value	2.8	0.7
Municipal [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.8	1.7
12 months or less, Fair Value	110	183.8
Greater than 12 months, Gross Unrealized Losses	9	4.1
Greater than 12 months, Fair Value	248	101.2
Total, Gross Unrealized Losses	9.8	5.8
Total, Fair Value	358	285
Municipal [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value	0.9
Total, Fair Value	0.9
Corporate [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	30	3
12 months or less, Fair Value	1,277.9	386.2
Greater than 12 months, Gross Unrealized Losses	43.9	10.8
Greater than 12 months, Fair Value	781.6	395.9
Total, Gross Unrealized Losses	73.9	13.8
Total, Fair Value	2,059.5	782.1
Corporate [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	8.1	1.5
12 months or less, Fair Value	185.6	48.7
Greater than 12 months, Gross Unrealized Losses	10	6.4
Greater than 12 months, Fair Value	54	39
Total, Gross Unrealized Losses	18.1	7.9
Total, Fair Value	239.6	87.7
Residential Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.6	1.4
12 months or less, Fair Value	201.2	196.3
Greater than 12 months, Gross Unrealized Losses	12.1	3.5
Greater than 12 months, Fair Value	323.7	159
Total, Gross Unrealized Losses	14.7	4.9
Total, Fair Value	524.9	355.3
Commercial Mortgage Backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	3.4	0.9
12 months or less, Fair Value	293	139.2
Greater than 12 months, Gross Unrealized Losses	6.4	1.6
Greater than 12 months, Fair Value	175.5	61.9
Total, Gross Unrealized Losses	9.8	2.5
Total, Fair Value	468.5	201.1
Asset-backed [Member] | Investment grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	0.3
12 months or less, Fair Value	42.3	34.6
Greater than 12 months, Gross Unrealized Losses	0.3
Greater than 12 months, Fair Value	18	2.4
Total, Gross Unrealized Losses	0.7	0.3
Total, Fair Value	$ 60.3	$ 37

Investments (Schedule of Other

Investments (Schedule of Other Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Other investments	$ 661.5	$ 562.7
Mortgage Loans on Real Estate [Member]
Schedule of Investments [Line Items]
Other investments	405.7	365.8
Valuation allowance	(1.1)	(0.9)
Mortgage Loans on Real Estate [Member] | South Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	82.1	76.9
Mortgage Loans on Real Estate [Member] | Pacific [Member]
Schedule of Investments [Line Items]
Other investments	92.5	87.6
Mortgage Loans on Real Estate [Member] | Mid-Atlantic [Member]
Schedule of Investments [Line Items]
Other investments	53.4	53.4
Mortgage Loans on Real Estate [Member] | West South Central [Member]
Schedule of Investments [Line Items]
Other investments	65.9	66.1
Mortgage Loans on Real Estate [Member] | New England [Member]
Schedule of Investments [Line Items]
Other investments	35.3	28.8
Mortgage Loans on Real Estate [Member] | East North Central [Member]
Schedule of Investments [Line Items]
Other investments	27.9	16.5
Mortgage Loans on Real Estate [Member] | Mountain [Member]
Schedule of Investments [Line Items]
Other investments	12.5	5
Mortgage Loans on Real Estate [Member] | Other Locations [Member]
Schedule of Investments [Line Items]
Other investments	37.2	32.4
Mortgage Loans on Real Estate [Member] | Office [Member]
Schedule of Investments [Line Items]
Other investments	141.1	134.6
Mortgage Loans on Real Estate [Member] | Retail [Member]
Schedule of Investments [Line Items]
Other investments	66.9	64.1
Mortgage Loans on Real Estate [Member] | Hotel [Member]
Schedule of Investments [Line Items]
Other investments	62.7	58.1
Mortgage Loans on Real Estate [Member] | Apartments [Member]
Schedule of Investments [Line Items]
Other investments	85.5	64.9
Mortgage Loans on Real Estate [Member] | Industrial [Member]
Schedule of Investments [Line Items]
Other investments	$ 50.6	$ 45

Investment Income and Gains a_3

Investment Income and Gains and Losses (Components of Net Investment Income from Continuing Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 277.6	$ 253.9	$ 241.2
Less investment expenses	(10.2)	(10)	(9.6)
Net investment income	267.4	243.9	231.6
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	217.7	205.8	202.1
Limited Partnerships [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	24.1	15.3	8
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	17	18	18.6
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	14	11.4	9.1
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 4.8	$ 3.4	$ 3.4

Investment Income and Gains a_4

Investment Income and Gains and Losses (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held	$ (39.5)
Net other-than-temporary impairment losses on securities recognized in earnings	4.6	$ 5.6	$ 27.4
Other-than-temporary impairments from continuing operations	5.4	5.9	20.6
Other-than-temporary impairments recorded as unrealized losses in AOCI	0.8	0.3	6.8
Proceeds from sale of equity securities	128.8	245
Gross realized gains on sale of equity securities	18.8	31.1
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings	2.6	1.8	16
Other Invested Assets [Member]
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings	2	2
Equity Securities And Other [Member]
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings		1.8	2.7
Credit Impairments
Gain (Loss) on Investments [Line Items]
Net other-than-temporary impairment losses on securities recognized in earnings			8.4
Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Gain (Loss) on Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held		$ 56.8	$ 37.8
ASC No. 2016-01 [Member]
Gain (Loss) on Investments [Line Items]
Net unrealized gains recognized during the period on equity securities still held	$ 5.3

Investment Income and Gains a_5

Investment Income and Gains and Losses (Schedule of Net Realized and Unrealized Gains (Losses) on Investments from Continuing Operations Including OTTI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ (50.7)	$ 21.1	$ 10.2
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(43.4)	19.8	25.4
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	(5.8)	3.6	(15.5)
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Net realized and unrealized investment gains (losses)	$ (1.5)	$ (2.3)	$ 0.3

Investment Income and Gains a_6

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

Investment Income and Gains a_7

Investment Income and Gains and Losses (Rollforward of Cumulative Amounts Related to Credit Loss Portion of OTTI Losses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Credit losses at beginning of period	$ 3.6	$ 9.6	$ 17.7
Credit losses on securities for which an OTTI was not previously recognized	1	0.4	6.1
Additional credit losses on securities for which an OTTI was previously recognized	0.1	0.1	2.3
Reductions for securities sold, matured or called	(0.9)	(6.1)	(4.3)
Reductions for securities reclassified as intend to sell		(0.4)	(12.2)
Credit losses at end of period	$ 3.8	$ 3.6	$ 9.6

Investment Income and Gains a_8

Investment Income and Gains and Losses (Proceeds from Sale of Available for Sale Securities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 296.5	$ 306.8	$ 348.2
Gross Gains	2.1	8.1	10
Gross Losses	8.4	7.8	5.1
Fixed Maturities Held-For-Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	276.8	188	215.6
Gross Gains	1.7	4.1	1.7
Gross Losses	1.7	0.4	1.9
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	573.3	494.8	563.8
Gross Gains	3.8	12.2	11.7
Gross Losses	$ 10.1	$ 8.2	$ 7

Fair Value (Fair Value of Finan

Fair Value (Fair Value of Financial Instruments, Including Chaucer Business Held-for-Sale) (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents		$ 1,020.7	$ 297.9
Total financial instruments, excluding held-for-sale		8,236	7,139.8
Total financial instruments		8,263.8	9,371.8
Carrying Value [Member] | Chaucer [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)		27.8	2,232
Carrying Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through AOCI [Member] | Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	[1]		3.6
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	[1]	6,161.5	5,749.3
Carrying Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through AOCI [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	[1]		576.2
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	[1]	175
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	[1]	464.4
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	[1]	1,020.7	297.9
Debt		777.9	786.9
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	[1]	414.4	512.8
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents		1,020.7	297.9
Total financial instruments, excluding held-for-sale		8,240.5	7,145.3
Total financial instruments		8,268.3	9,377.3
Debt		825	865.7
Estimated Fair Value [Member] | Chaucer [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)		27.8	2,232
Estimated Fair Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through AOCI [Member] | Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	[1]		3.6
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	[1]	6,161.5	5,749.3
Estimated Fair Value [Member] | Income Approach Valuation Technique [Member] | Fair Value Through AOCI [Member] | Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	[1]		576.2
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	[1]	175
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	[1]	464.4
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	[1]	1,020.7	297.9
Debt		825	865.7
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	[1]	$ 418.9	$ 518.3
[1]	Due to the adoption of ASU No. 2016-01, certain classifications are not comparable to the prior year. See Note 1 – “New Accounting Pronouncements”.

Fair Value (Fair Value, Assets

Fair Value (Fair Value, Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,161.5	$ 5,749.3
Equity securities	464.4	576.2
Other investments	661.5	562.7
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	1.1	1.1
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	885.8	934
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,161.5	5,749.3
Equity securities	464.4	567.8
Other investments	3.5	3.6
Investment assets, held-for-sale	24.5	2,153.5
Total investment assets at fair value	6,653.9	8,474.2
Fair Value, Measurements, Recurring [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)	6,629.4	6,320.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	154.9	140
Equity securities	463.3	566.7
Investment assets, held-for-sale	5.7	138
Total investment assets at fair value	623.9	844.7
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)	618.2	706.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	5,971.6	5,569.6
Investment assets, held-for-sale	18.8	2,013.3
Total investment assets at fair value	5,990.4	7,582.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)	5,971.6	5,569.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	35	39.7
Equity securities	1.1	1.1
Other investments	3.5	3.6
Investment assets, held-for-sale		2.2
Total investment assets at fair value	39.6	46.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value, excluding held-for-sale (Chaucer)	39.6	44.4
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	409.9	322.3
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	154.9	140
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	255	182.3
Fair Value, Measurements, Recurring [Member] | Foreign Government [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	7.4	8.5
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	7.4	8.5
Fair Value, Measurements, Recurring [Member] | Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	885.8	934
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	864.7	909.4
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	21.1	24.6
Fair Value, Measurements, Recurring [Member] | Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,410.7	3,196.6
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	3,409.9	3,195.7
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	0.8	0.9
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities U S Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	713.7	605
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed Securities U S Agency Backed [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	713.7	605
Fair Value, Measurements, Recurring [Member] | Residential Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	2.7	28.2
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	2.7	28.2
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	640.3	595
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	627.2	580.8
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	13.1	14.2
Fair Value, Measurements, Recurring [Member] | Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	91	59.7
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	$ 91	$ 59.7

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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 1 and Level 2	$ 0	$ 0
Fair Value Measured Using NAV [Member] | Limited Partnerships [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	171,500,000	149,400,000
Level 3 [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	3,500,000	3,600,000
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, 2018	Dec. 31, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Equity securities	$ 464.4	$ 576.2
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Equity securities	1.1	1.1
Other investments	3.5	3.6
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	1,020.7	297.9
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	1,020.7	297.9
Equity securities		8.4
Total financial instruments, excluding held-for-sale	1,439.6	675.2
Other investments	418.9	368.9
Total financial instruments	1,442.9	753.7
Debt	825	865.7
Estimated Fair Value [Member] | Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	1,020.7	297.9
Total financial instruments, excluding held-for-sale	1,020.7	297.9
Total financial instruments	1,024	376.4
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Equity securities		8.4
Total financial instruments, excluding held-for-sale	8.7	8.4
Other investments	8.7
Total financial instruments	8.7	8.4
Debt	825	865.7
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Total financial instruments, excluding held-for-sale	410.2	368.9
Other investments	410.2	368.9
Total financial instruments	410.2	368.9
Estimated Fair Value [Member] | Chaucer [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	3.3	78.5
Estimated Fair Value [Member] | Chaucer [Member] | Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	$ 3.3	$ 78.5

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, 2018		Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	$ 46.6		$ 53
Transfers out of Level 3	(0.5)		(1.9)
Included in total net realized and unrealized investment gains (losses)			0.3
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.5)		(0.9)
Sales	(3.8)		(6.1)
Balance at end of year	39.6	[1]	46.6
Balance, Excluding held-for-sale			44.4
Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	44.4
Balance at end of year			44.4
Balance, Held-for-sale (Chaucer)			2.2
Municipal [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	26.8		28.8
Transfers out of Level 3	(0.5)		(1.9)
Included in total net realized and unrealized investment gains (losses)	0.1
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.2)		(0.2)
Sales	(2.9)		(2.1)
Balance at end of year	21.1	[1]	26.8
Balance, Excluding held-for-sale			24.6
Municipal [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	24.6
Balance at end of year			24.6
Balance, Held-for-sale (Chaucer)			2.2
Corporate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	0.9		4
Included in total net realized and unrealized investment gains (losses)			0.3
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities			(0.2)
Sales	(0.1)		(3.2)
Balance at end of year	0.8	[1]	0.9
Balance, Excluding held-for-sale			0.9
Corporate [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	0.9
Balance at end of year			0.9
Commercial Mortgage Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	14.2		15
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.3)
Sales	(0.8)		(0.8)
Balance at end of year	13.1	[1]	14.2
Balance, Excluding held-for-sale			14.2
Commercial Mortgage Backed [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	14.2
Balance at end of year			14.2
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	41.9		47.8
Transfers out of Level 3	(0.5)		(1.9)
Included in total net realized and unrealized investment gains (losses)	0.1		0.3
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities	(0.5)		(0.4)
Sales	(3.8)		(6.1)
Balance at end of year	35	[1]	41.9
Balance, Excluding held-for-sale			39.7
Fixed Maturities [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	39.7
Balance at end of year			39.7
Balance, Held-for-sale (Chaucer)			2.2
Equity Securities and Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	4.7		5.2
Included in total net realized and unrealized investment gains (losses)	(0.1)
Included in other comprehensive income - net appreciation (depreciation) on available- for-sale securities			(0.5)
Balance at end of year	4.6	[1]	4.7
Balance, Excluding held-for-sale			4.7
Equity Securities and Other Investments [Member] | Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	$ 4.7
Balance at end of year			$ 4.7
[1]	There were no Level 3 held-for-sale securities (Chaucer) at December 31, 2018.

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, 2018	Dec. 31, 2017
Fair Value [Line Items]
Fair Value	$ 6,161.5	$ 5,749.3
Equity securities, at fair value	464.4	576.2
Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.1	$ 1.1
Equity Securities, FV-NI, Valuation Technique [Extensible List]	us-gaap:MarketApproachValuationTechniqueMember	us-gaap:MarketApproachValuationTechniqueMember
Equity Securities, FV-NI, Measurement Input [Extensible List]	thg:MeasurementInputNetTangibleAssetMarketMultipleMember	thg:MeasurementInputNetTangibleAssetMarketMultipleMember
Fair value measurement market multiples	1	1
Investments measured at fair value using net asset value	$ 3.5	$ 3.6
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	0.180	0.180
Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	1	1
Range	0.180	0.180
Municipal [Member]
Fair Value [Line Items]
Fair Value	$ 885.8	$ 934
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 21.1	$ 24.6
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	1.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size		0.90%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	0.70%	0.70%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.30%	0.30%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size		1.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	6.80%	6.80%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount For Credit Stress
Fair Value [Line Items]
Small issue size	1.30%	1.20%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	3.40%	3.50%
Municipal [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input Discount Rate For Above Market Coupon
Fair Value [Line Items]
Small issue size	0.50%	0.40%
Corporate [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 0.8	$ 0.9
Corporate [Member] | Discounted cash flow [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] | Discounted cash flow [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] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 13.1	$ 14.2
Commercial Mortgage Backed [Member] | Discounted cash flow [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] | Discounted cash flow [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] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	1.90%	1.90%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	3.10%	3.10%
Commercial Mortgage Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member] | Measurement Input, Discount for Small Issue Size [Member]
Fair Value [Line Items]
Small issue size	2.70%	2.60%
Commercial Mortgage Backed [Member] | Discounted cash flow [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] | Discounted cash flow [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%

Debt and Credit Arrangements (S

Debt and Credit Arrangements (Schedule of Long Term Debt) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total principal debt		$ 787.7	$ 797.3
Unamortized debt discount		(9.8)	(10.4)
Total		777.9	786.9
Senior debentures maturing April 15, 2026 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 375	$ 375
Maturity date		Apr. 15, 2026	Apr. 15, 2026
Senior debentures maturing October 15, 2025 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 62.6	$ 62.6
Maturity date		Oct. 15, 2025	Oct. 15, 2025
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 175	$ 175
Maturity date		Mar. 30, 2053	Mar. 30, 2053
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Total principal debt		$ 50.1	$ 59.7
Maturity date	Feb. 3, 2027	Feb. 3, 2027	Feb. 3, 2027
FHLB borrowings (secured) [Member]
Debt Instrument [Line Items]
Total principal debt		$ 125	$ 125

Debt and Credit Arrangements (N

Debt and Credit Arrangements (Narrative) (Details) - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2009
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 787,700,000		$ 787,700,000	$ 797,300,000
Gain (loss) on the repurchase of debt			(1,900,000)		$ (88,300,000)
Federal Home Loan Bank Stock	8,700,000		8,700,000	8,500,000
Advance repayment fee			125,000,000
Advance repayment charges before tax	26,300,000		26,300,000
Line of Credit Facility, Maximum Borrowing Capacity	200,000,000		$ 200,000,000
Line Of Credit Facility Maturity Month And Year			2019-05
Outstanding borrowings under credit agreement	0		$ 0
Interest Expense Debt			45,100,000	45,200,000	51,400,000
Collateralized Borrowing Program [Member]
Debt Instrument [Line Items]
Advance received	125,000,000		125,000,000	125,000,000
Securities Held As Collateral At Fair Value	234,700,000		234,700,000	223,600,000
Federal Home Loan Bank of Boston [Member]
Debt Instrument [Line Items]
Debt instrument interest rate						5.50%
Advance received						$ 125,000,000
Length of debt instrument						20 years
Securities Held As Collateral At Fair Value	234,900,000		$ 234,900,000	225,700,000
Senior debentures unsecured mature on October 15, 2025 [Member]
Debt Instrument [Line Items]
Maturity date			Oct. 15, 2025
Long-term Debt, Gross	$ 62,600,000		$ 62,600,000	62,600,000
Debt instrument interest rate	7.625%		7.625%
Senior debentures unsecured mature on April 15, 2026 [Member]
Debt Instrument [Line Items]
Maturity date			Apr. 15, 2026
Aggregate principal amount	$ 375,000,000		$ 375,000,000	$ 375,000,000
Debt issuance date			Apr. 8, 2016
Subordinated debentures maturing March 30, 2053 [Member]
Debt Instrument [Line Items]
Maturity date			Mar. 30, 2053	Mar. 30, 2053
Long-term Debt, Gross	$ 175,000,000		$ 175,000,000	$ 175,000,000
Debt instrument interest rate	6.35%		6.35%
Aggregate principal amount	$ 175,000,000		$ 175,000,000
Debt Instrument Redemption Date			Mar. 30, 2018
8.207% Subordinated Debentures [Member]
Debt Instrument [Line Items]
Maturity date			Feb. 3, 2027
Aggregate principal amount	50,100,000		$ 50,100,000	$ 59,700,000
Interest rate of Series B Subordinated Deferrable Interest Debentures			8.207%	8.207%
Subordinated debentures maturing February 3, 2027 [Member]
Debt Instrument [Line Items]
Maturity date		Feb. 3, 2027	Feb. 3, 2027	Feb. 3, 2027
Long-term Debt, Gross	$ 50,100,000		$ 50,100,000	$ 59,700,000
Repurchase of subordinated debentures					9,600,000
Repurchase of subordinated debentures at cost					$ 11,500,000
Gain (loss) on the repurchase of debt		$ (1,900,000)

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income from continuing operations before income taxes	$ 282.5	$ 292.9	$ 66
Current	46.2	3.1	26.2
Deferred	(2.7)	73.7	(27.2)
Total income tax expense (benefit)	$ 43.5	$ 76.8	$ (1)

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
U.S. statutory federal income tax rate		21.00%	35.00%	35.00%
Income recognized in continuing operations related to non-segment income		$ 9,200,000	$ 12,700,000	$ 20,700,000
Liability for uncertain tax positions		0	0	0
Release of liability due to expiration of statute of limitations		0	600,000	$ 700,000
Effect of re-measurement of company's deferred tax balance			9,400,000
Component of income tax expense in continuing operations and income tax (benefit) expense			$ (4,100,000)
Exemption on dividends received, percentage			100.00%
Non- US earnings of income tax expense in continuing operations			$ 12,700,000
Elimination of executive compensation			1,000,000
Recognized provision upon elimination of executive compensation			200,000
Deferred tax liability			26,500,000
Transition adjustment			126,200,000
Effect on net deferred tax asset or income tax expense		0	0
Cumulative effect of TCJA enactment, tax expense			22,300,000
Chaucer [Member]
Income Taxes [Line Items]
Component of income tax expense in continuing operations and income tax (benefit) expense			$ 13,500,000
Scenario, Forecast [Member]
Income Taxes [Line Items]
Release of liability due to expiration of statute of limitations	$ 1,700,000
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		$ 35,600,000

Income Taxes (Schedule of Effec

Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Expected income tax expense	$ 59.3	$ 102.5	$ 23.1
Effect of the enactment of the Tax Cuts and Jobs Act	(4.3)	(3.9)
Tax difference related to investment disposals and maturities	(9.2)	(12.7)	(20.7)
Stock-based compensation windfall benefit	(2.3)	(5.3)
Nondeductible expenses	1.6	1	1
Dividend received deduction	(1.2)	(3.2)	(3.3)
Tax-exempt interest	(0.4)	(0.9)	(1.1)
Change in liability for uncertain tax positions		(0.5)
Other, net		(0.2)
Total income tax expense (benefit)	$ 43.5	$ 76.8	$ (1)
Effective tax rate	15.40%	26.20%	(1.50%)

Income Taxes (Schedule of Defer

Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Loss, LAE and unearned premium reserves, net	$ 129.8	$ 115.7
Employee benefit plans	14.5	18.7
Tax credit carryforwards		23.1
Investments, net	4.1
Other	16.3	11.5
Deferred tax assets, gross	164.7	169
Less: Valuation allowance	0.2	0.2
Deferred tax assets, total	164.5	168.8
Deferred acquisition costs	95	90.2
Investments, net		41.8
Software capitalization	18.3	18
Other	0.6	1.8
Deferred tax liabilities, total	113.9	151.8
Net deferred tax asset	$ 50.6	$ 17

Income Taxes (Summary of Income

Income Taxes (Summary of Income Tax Uncertainties) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Liability at beginning of year, net	$ 3	$ 2.7	$ 3
Additions for tax positions of current year	0	0.9	0.4
Subtractions as a result of a lapse of the applicable statute of limitations	0	(0.6)	(0.7)
Liability at end of year, net	$ 3	$ 3	$ 2.7

Pension Plan (Narrative) (Detai

Pension Plan (Narrative) (Details) - USD ($)	Jan. 01, 2005	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Plans [Line Items]
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 exceeds plan assets		$ 18,800,000
Net periodic pension benefit cost		$ 8,800,000	$ 14,000,000	$ 12,600,000
Curtailment gain				$ 2,400,000
Period of fair value of plan assets amortized as a component of net periodic pension cost		5 years
Actuarial losses, amortized amount		$ 11,900,000
Maximum percentage contributed by employer		6.00%	6.00%	6.00%
Defined contribution plan expense		$ 21,700,000	$ 21,300,000	$ 21,400,000
Additional cost contribution			2,200,000
Additional cost contribution		$ 4,500,000	4,800,000	3,900,000
Minimum [Member]
Retirement Plans [Line Items]
Percentage of unrecognized net actuarial gains (losses) as projected benefit obligation, amortized as component of net periodic pension cost		10.00%
Chaucer Pension Plan [Member]
Retirement Plans [Line Items]
Net periodic pension benefit cost		$ 2,300,000	$ 300,000	$ 900,000
Estimated future contributions in next fiscal year		$ 0
Pension Plans [Member] | US [Member]
Retirement Plans [Line Items]
Expected rate of return on plan assets		4.75%
Estimated future contributions in next fiscal year		$ 0
Pension Plans [Member] | US [Member] | Nonqualified Plan [Member]
Retirement Plans [Line Items]
Estimated future contributions in next fiscal year		$ 3,100,000

Pension Plans (Schedule of Weig

Pension Plans (Schedule of Weighted Average Assumptions Used to Determine Pension Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Cash balance interest crediting rate	3.50%	3.50%	3.50%
Pension Plans [Member] | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	4.75%
Pension Plans [Member] | Qualified Plan [Member] | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.50%	3.88%	4.25%
Discount rate	3.88%	4.25%	4.88%
Expected return on plan assets	4.75%	5.00%	5.25%
Cash balance interest crediting rate	3.50%	3.50%	3.50%
Pension Plans [Member] | Nonqualified Plan | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.50%	3.88%	4.25%
Discount rate	3.88%	4.25%	4.75%

Pension Plans (Summary of Targe

Pension Plans (Summary of Target Allocations and Invested Asset Allocations) (Details) - Pension Plans [Member] - US [Member]	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	100.00%
Total Fixed Income and Equity Securities	100.00%	100.00%
Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	2.00%
Total Fixed Income and Equity Securities	2.00%	1.00%
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	85.00%
Total Fixed Income and Equity Securities	87.00%	84.00%
Domestic
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	12.00%
Total Fixed Income and Equity Securities	10.00%	12.00%
International
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	3.00%
Total Fixed Income and Equity Securities	3.00%	4.00%
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	15.00%
Total Fixed Income and Equity Securities	13.00%	16.00%
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Fixed Income and Equity Securities, TARGET LEVELS	83.00%
Total Fixed Income and Equity Securities	85.00%	83.00%

Pension Plans (Summary of Plan

Pension Plans (Summary of Plan Assets Investment Measured At Fair Value) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Chaucer Pension Plan [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value		$ 144.6	$ 116.5
Pension Plans [Member] | Estimated Fair Value [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 80.5	86.7
Pension Plans [Member] | Estimated Fair Value [Member] | Fixed Maturities | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	35.1	38.8
Pension Plans [Member] | Estimated Fair Value [Member] | Money Market Mutual Funds | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	9.1	5.3
Pension Plans [Member] | Estimated Fair Value [Member] | Mutual Fund | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	36.3	42.6
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	54.7	61
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Fixed Maturities | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	9.3	13.1
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Money Market Mutual Funds | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	9.1	5.3
Level 1 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Mutual Fund | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	36.3	42.6
Level 3 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	25.8	25.7
Level 3 [Member] | Pension Plans [Member] | Estimated Fair Value [Member] | Fixed Maturities | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Invested assets, fair value	$ 25.8	$ 25.7

Pension Plans (Schedule of Fair

Pension Plans (Schedule of Fair Values of Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,161.5	$ 5,749.3
Equity securities	464.4	576.2
Investments Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	346.4	343.5
Equity securities	23.9	29.9
Total investment assets at fair value	370.3	373.4
Investments Net Asset Value [Member] | Domestic
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	9.3	12.6
Investments Net Asset Value [Member] | International
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	$ 14.6	$ 17.3

Pension Plans (Summary for Asse

Pension Plans (Summary for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	$ 46.6
Balance at end of year	39.6	[1]
Pension Plans [Member] | US [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of year, excluding held-for-sale (Chaucer)	25.7
Plus: Assets transferred from investments measured at fair value using NAV	2.1
Less: Assets transferred to Level 1 investments	(2.7)
Actual return on plan assets related to assets still held	0.7
Balance at end of year	$ 25.8
[1]	There were no Level 3 held-for-sale securities (Chaucer) at December 31, 2018.

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 19.8	$ 21.9	$ 24.9
Chaucer Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, beginning of period	146.1	132.2
Plan amendment	0.3
Interest cost	3.7	3.9
Actuarial losses (gains)	(8.6)	3.2
Benefits paid	(3.9)	(5.8)
Foreign currency translation	(8.3)	12.6
Disposition of benefit obligations with the sale of Chaucer	(129.3)
Benefit obligation, end of year		146.1	132.2
Fair value of plan assets, beginning of period	144.6	116.5
Actual return on plan assets	(5.8)	14.4
Contributions		7.7
Benefits paid	(3.9)	(5.8)
Foreign currency translation	(8.1)	11.8
Disposition of plan assets with the sale of Chaucer	(126.8)
Fair value of plan assets, end of year		144.6	116.5
Funded status of the plans		(1.5)
Pension Plans [Member] | Qualified Plan [Member] | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, beginning of period	501.2	506.8
Interest cost	18.4	20.4
Actuarial losses (gains)	(17.5)	7.3
Benefits paid	(32.6)	(33.3)
Benefit obligation, end of year	469.5	501.2	506.8
Fair value of plan assets, beginning of period	460.1	453.3
Actual return on plan assets	(16.8)	40.1
Contributions	40
Benefits paid	(32.6)	(33.3)
Fair value of plan assets, end of year	450.7	460.1	453.3
Funded status of the plans	(18.8)	(41.1)
Pension Plans [Member] | Nonqualified Plan | US [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation, beginning of period	36.6	38
Interest cost	1.4	1.5
Actuarial losses (gains)	(1.3)	0.1
Benefits paid	(2.9)	(3)
Benefit obligation, end of year	33.8	36.6	$ 38
Contributions	2.9	3
Benefits paid	(2.9)	(3)
Funded status of the plans	$ (33.8)	$ (36.6)

Pension Plans (Components of Ne

Pension Plans (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Interest cost	$ 19.8	$ 21.9	$ 24.9
Expected return on plan assets	(20.6)	(21.4)	(23.5)
Recognized net actuarial loss	9.6	13.5	11.2
Net periodic pension cost (benefit)	$ 8.8	$ 14	$ 12.6

Pension Plans (Summary of Amoun

Pension Plans (Summary of Amounts Recognized in Accumulated Other Comprehensive Income Loss) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Net actuarial loss	$ 109.7	$ 100.8

Pension Plans (Summary of Estim

Pension Plans (Summary of Estimated Benefit Payments) (Details) - Pension Plans [Member] - US [Member] $ in Millions	Dec. 31, 2018USD ($)
Qualified Plan [Member]
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2,019	$ 38.5
2,020	37.4
2,021	38.7
2,022	37.3
2,023	37.1
2024-2028	163.7
Nonqualified Plan
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
2,019	3.1
2,020	3
2,021	3
2,022	2.9
2,023	2.7
2024-2028	$ 12.4

Other Postretirement Benefit _3

Other Postretirement Benefit Plans (Narrative) (Details) - Postretirement Plans [Member]	12 Months Ended
	Dec. 31, 2018
1995 Postretirement Health Coverage [Member]
Retirement Plans [Line Items]
Eligibility age	40 years
2018 Postretirement Health Coverage [Member]
Retirement Plans [Line Items]
Eligibility age	65 years
Minimum [Member] | 1995 Postretirement Health Coverage [Member]
Retirement Plans [Line Items]
Service period required to avail postretirement benefits (in years)	15 years

Other Postretirement Benefit _4

Other Postretirement Benefit Plans (Fair Value of Plan Assets and Funded Status of Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 19.8	$ 21.9	$ 24.9
Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated postretirement benefit obligation, beginning of year	10.1	11.6
Interest cost	0.4	0.4	0.5
Net actuarial gain	(0.9)	(0.3)
Benefits paid	(1.2)	(1.6)
Accumulated postretirement benefit obligation, end of year	8.4	10.1	$ 11.6
Fair value of plan assets, end of year	0	0
Funded status of plans	$ (8.4)	$ (10.1)

Other Postretirement Benefit _5

Other Postretirement Benefit Plans (Schedule of Expected Benefit Payments) (Details) - Postretirement Plans [Member] $ in Millions	Dec. 31, 2018USD ($)
Defined Benefit Plan Disclosure [Line Items]
2,019	$ 1.1
2,020	1
2,021	0.9
2,022	0.8
2,023	0.8
2024-2028	$ 3.1

Other Postretirement Benefit _6

Other Postretirement Benefit Plans (Components of Net Periodic Pension Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 19.8	$ 21.9	$ 24.9
Recognized net actuarial loss	9.6	13.5	11.2
Net periodic pension cost (benefit)	8.8	14	12.6
Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.4	0.4	0.5
Recognized net actuarial loss	0.2	0.2	0.2
Amortization of prior service cost (benefit)	(0.3)	(1.4)	(1.4)
Net periodic pension cost (benefit)	$ 0.3	$ (0.8)	$ (0.7)

Other Postretirement Benefit _7

Other Postretirement Benefit Plans (Accumulated Other Comprehensive (Income) Loss Related to Postretirement Benefit Plans) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	$ 109.7	$ 100.8
Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss	2.5	3.6
Net prior service cost (benefit)	(0.1)	(0.4)
Amounts recognized in accumulated other comprehensive income	$ 2.4	$ 3.2

Other Postretirement Benefit _8

Other Postretirement Benefit Plans (Summary of Weighted-Average Assumptions Used to Determine Pension Benefit Obligations) (Details) - Postretirement Plans [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefit obligations discount rate	4.38%	3.75%
Postretirement benefit cost discount rate	3.75%	4.13%

Other Postretirement Benefit _9

Other Postretirement Benefit Plans (Assumed Health Care Cost Trend Rates) (Details) - Postretirement Plans [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate assumed for next year	6.50%	6.50%
Rate to which the cost trend is assumed to decline (ultimate trend rate)	4.50%	4.50%
Year the rate reaches the ultimate trend rate	2,024	2,024

Other Comprehensive Income (Cha

Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Unrealized gains (losses) arising during period, Pre-Tax	$ (209.7)	$ 77	$ 95.9
Amount of realized (gains) losses from sales and other, Pre-tax	2.5	(30.8)	(36.5)
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	3.5	3.9	27.9
Unrealized losses (gains) realized with sale of Chaucer business, Pre-tax	24.2
Net unrealized gains (losses), Pre-tax	(179.5)	50.1	87.3
Net gains (losses) arising in the period from net actuarial (gains) losses and prior service costs, Pre-tax	(21.3)	17	(43.1)
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Pre-tax	9.7	14	9.9
Pension obligations recognized with sale of Chaucer business, Pre-tax	21.2
Total pension and postretirement benefits, Pre-tax	9.6	31	(33.2)
Foreign currency translation recognized during the period, Pre-tax	(2.2)	3.7	(5.1)
Currency translation obligation recognized with sale of Chaucer business, Pre-tax	29.4
Total cumulative foreign currency translation adjustment, Pre-tax	27.2	3.7	(5.1)
Other comprehensive income (loss), Pre-tax	(142.7)	84.8	49
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	44	(27.9)	(33.4)
Amount of realized (gains) losses from sales and other, Tax Benefit (Expense)	(9.7)	(1.4)	(8)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	(0.7)	(1.4)	(9.8)
Unrealized losses (gains) realized with sale of Chaucer business, Tax Benefit (Expense)	(5.1)
Net unrealized gains, Tax Benefit (Expense)	28.5	(30.7)	(51.2)
Net gains (losses) arising in the period from net actuarial (gains) losses and prior service costs, Tax Benefit (Expense)	4.3	(3.4)	12.8
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Tax Benefit (Expense)	(2)	(4.6)	(3.5)
Pension obligations recognized with sale of Chaucer business, Tax Benefit (Expense)	(4.8)
Total pension and postretirement benefits, Tax Benefit (Expense)	(2.5)	(8)	9.3
Foreign currency translation recognized during the period, Tax Benefit (Expense)	0.5	(1.3)	1.8
Currency translation obligation recognized with sale of Chaucer business, Tax Benefit (Expense)	(6.2)
Total cumulative foreign currency translation adjustment, Tax Benefit (Expense)	(5.7)	(1.3)	1.8
Other comprehensive income (loss), Tax Benefit (Expense)	20.3	(40)	(40.1)
Unrealized gains (losses) arising during period, Net of Tax	(165.7)	49.1	62.5
Amount of realized (gains) losses from sales and other, Net of Tax	(7.2)	(32.2)	(44.5)
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	2.8	2.5	18.1
Unrealized losses (gains) realized with sale of Chaucer business, Net of Tax	19.1
Total available-for-sale securities	(151)	19.4	36.1
Net actuarial losses and prior service costs arising in the period	(17)	13.6	(30.3)
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Net of Tax	7.7	9.4	6.4
Pension obligations recognized with sale of Chaucer business, Net of Tax	16.4
Total pension and postretirement benefits	7.1	23	(23.9)
Amount recognized as cumulative foreign currency translation during the period	(1.7)	2.4	(3.3)
Currency translation obligation recognized with sale of Chaucer business, Net of Tax	23.2
Total cumulative foreign currency translation adjustment	21.5	2.4	(3.3)
Total other comprehensive income (loss), net of tax	$ (122.4)	$ 44.8	$ 8.9

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other									$ (2.7)		$ 26.7	$ 37.6
Net other–than–temporary impairment losses on investments recognized in earnings									(4.6)		(5.6)	(27.4)
Income from continuing operations before income taxes									282.5		292.9	66
Loss adjustment expenses and other operating expenses									(4,211.8)		(3,975)	(3,987.9)
Income tax expense									(43.5)		(76.8)	1
Continuing operations; net of tax	$ 2.1	$ 104	$ 82.4	$ 50.5	$ 71.9	$ 59.5	$ 56.3	$ 28.4	239		216.1	67
Gain on sale of Chaucer business									131.9
Net Income/gain (loss) from discontinued businesses, net of taxes									0.1		(16.8)	(1)
Net of tax	$ 123.6	$ 100.4	$ 99.3	$ 67.7	$ 51.5	$ 11.1	$ 78.4	$ 45.2	391		186.2	155.1
Chaucer [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss adjustment expenses and other operating expenses									(883)	[1]	(904.6)	(764.6)
Income tax expense									(4.7)	[1]	(21.7)	(37.2)
Net Income/gain (loss) from discontinued businesses, net of taxes									20	[1]	(13.1)	89.1
Gain from sale of Chaucer business, net of taxes									131.9
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									(62)		20.3	19.8
Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized gains (losses) from sales and other									(3.2)		26.6	36.2
Net other–than–temporary impairment losses on investments recognized in earnings									(2.6)		(3.6)	(27.4)
Income from continuing operations before income taxes									(5.8)		23	8.8
Income tax expense									10.3		4.2	17.7
Continuing operations; net of tax									4.5		27.2	26.5
Gain from sale of Chaucer business, net of taxes									0.1			(0.1)
Net of tax									(14.7)		29.7	26.2
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									(19.1)
Net Income/gain (loss) from discontinued businesses, net of taxes									(0.2)		2.5	(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									(9.5)		(12.4)	(9.7)
Income tax expense									1.9		4.3	3.4
Continuing operations; net of tax									(7.6)		(8.1)	(6.3)
Gain on sale of Chaucer business									(16.4)
Net Income/gain (loss) from discontinued businesses, net of taxes									(0.1)		(1.3)	(0.1)
Net of tax									(24.1)		$ (9.4)	$ (6.4)
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									$ (23.2)
[1]	2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss Adjustment Expense	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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 28, 2018
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
Shares awarded		301,152	460,610	587,340
Stock-based compensation expense		$ 14.9	$ 10.5	$ 10.5
Tax benefit from compensation expense		$ 3.1	3.7	3.7
Vesting period		3 years
Award expiration Period		10 years
Cash received for options exercised		$ 14.8	23.1	16.2
Intrinsic value of options exercised		9.6	15.6	21.1
Share-based Compensation Arrangement by Share-Based Payment Award, Options, Tax Expense Realized from Options Exercised		1.6	$ 4	$ 5.6
Aggregate intrinsic value, outstanding		34.1
Aggregate intrinsic value, exercisable		$ 24.2
Weighted average remaining contractual life, outstanding (in years)		7 years 6 months
Weighted average remaining contractual life, exercisable (in years)		6 years 2 months 12 days
Weighted average grant date fair value of options granted		$ 16.30	$ 13.05	$ 10.83
Fair value of vested shares		$ 9.3	$ 8.3
Vesting Each Year [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting rate		33.33%	33.33%	33.33%
Chaucer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 1.2	$ 1
Option or SAR
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		$ 3.7
Unrecognized compensation expense, weighted average period of recognition		1 year 6 months
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
Forfeited, Shares		10,330	53,840	63,740
Aggregate intrinsic value		$ 8.2
Weighted average remaining contractual life		1 year 4 months 24 days
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
Market-Based Restricted Stock Units [Member] | 2014 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares			5,881
Granted, Weighted Average Grant Date Fair Value			$ 56.45
Market-Based Restricted Stock Units [Member] | 2014 and 2013 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Increase as grant in period, percentage			100.00%
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
Market-based awards
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period		3 years
Granted, Shares		31,688	56,571	87,213
Performance And Market-Based Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense		$ 19.7
Unrecognized compensation expense, weighted average period of recognition		1 year 9 months 18 days
Intrinsic value of awards vested		$ 0.6	$ 0.6	$ 5.8
Performance And Market-Based Restricted Stock Units | 2014 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares				2,268
Granted, Weighted Average Grant Date Fair Value				$ 57
Performance And Market-Based Restricted Stock Units | 2013 Grants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares				30,134
Granted, Weighted Average Grant Date Fair Value				$ 41.77
Performance And Market-Based Restricted Stock Units | Minimum [Member] | Senior management
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range		0.00%
Performance And Market-Based Restricted Stock Units | Maximum [Member] | Senior management
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]
Intrinsic value of awards vested		$ 2.9	$ 2.3	$ 5.4
Aggregate intrinsic value		$ 39.1
Weighted average remaining contractual life		1 year 3 months 18 days
Performance-based restricted stock units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Forfeited, Shares		43,449	1,090
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		4,343,301
ESPP Plan [Member]
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		2,371,060
SIP Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares authorized under plan	750,000
Shares awarded					74,189
Two Thousand Fourteen Stock Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Cash received for options exercised		$ 14.3	$ 20.5	$ 15.8

Stock-based Compensation Plan_3

Stock-based Compensation Plans (Summary of Stock Option Plan Activity) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Outstanding, beginning of year, Shares	1,062,177	1,396,152	1,619,948
Granted, Shares	301,152	460,610	587,340
Exercised, Shares	(210,190)	(464,726)	(589,666)
Forfeited or cancelled, Shares	(54,063)	(329,859)	(221,470)
Outstanding, end of year, Shares	1,099,076	1,062,177	1,396,152
Additional Disclosures
Exercisable, end of year, Shares	515,286	423,883	441,256
Exercisable, end of year, Weighted Average Exercise Price	$ 69.83	$ 62.41	$ 53.59
Weighted Average Exercise Price
Outstanding, beginning of year, Weighted Average Exercise Price	75.53	68.63	56.57
Granted, Weighted Average Exercise Price	110.98	90.85	82.17
Exercised, Weighted Average Exercise Price	69.41	63.12	48.99
Forfeited or cancelled, Weighted Average Exercise Price	89.09	85.22	68.61
Outstanding, end of year, Weighted Average Exercise Price	$ 85.75	$ 75.53	$ 68.63

Stock-based Compensation Plan_4

Stock-based Compensation Plans (Schedule of Stock Options by Exercise Price Range) (Details)	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Exercise Price	$ 57.99
Options Outstanding, Number | shares	93,778
Options Outstanding, Weighted Average Remaining Contractual Lives	5 years 1 month 20 days
Options Outstanding, Weighted Average Exercise Price	$ 57.99
Options Currently Exercisable, Number | shares	93,778
Options Currently Exercisable, Weighted Average Exercise Price	$ 57.99
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	36.81
Range of Exercise Prices, Upper Range Limit	$ 46.47
Options Outstanding, Number | shares	64,367
Options Outstanding, Weighted Average Remaining Contractual Lives	3 years 5 months 8 days
Options Outstanding, Weighted Average Exercise Price	$ 41.61
Options Currently Exercisable, Number | shares	64,367
Options Currently Exercisable, Weighted Average Exercise Price	$ 41.61
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	70.24
Range of Exercise Prices, Upper Range Limit	$ 72.66
Options Outstanding, Number | shares	165,924
Options Outstanding, Weighted Average Remaining Contractual Lives	6 years 1 month 28 days
Options Outstanding, Weighted Average Exercise Price	$ 70.27
Options Currently Exercisable, Number | shares	165,924
Options Currently Exercisable, Weighted Average Exercise Price	$ 70.27
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	74.88
Range of Exercise Prices, Upper Range Limit	$ 82.74
Options Outstanding, Number | shares	190,120
Options Outstanding, Weighted Average Remaining Contractual Lives	7 years 3 months 18 days
Options Outstanding, Weighted Average Exercise Price	$ 80.75
Options Currently Exercisable, Number | shares	112,847
Options Currently Exercisable, Weighted Average Exercise Price	$ 80.49
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	87.50
Range of Exercise Prices, Upper Range Limit	$ 91.19
Options Outstanding, Number | shares	285,280
Options Outstanding, Weighted Average Remaining Contractual Lives	8 years 2 months 12 days
Options Outstanding, Weighted Average Exercise Price	$ 90.66
Options Currently Exercisable, Number | shares	78,370
Options Currently Exercisable, Weighted Average Exercise Price	$ 90.89
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	110.57
Range of Exercise Prices, Upper Range Limit	$ 124.14
Options Outstanding, Number | shares	299,607
Options Outstanding, Weighted Average Remaining Contractual Lives	9 years 2 months 4 days
Options Outstanding, Weighted Average Exercise Price	$ 110.98

Stock-based Compensation Plan_5

Stock-based Compensation Plans (Schedule of Stock Option Valuation Assumptions) (Details) - Employee Stock Option [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility,minimum	17.943%	18.341%	18.08%
Expected volatility, maximum	18.50%	21.694%	21.31%
Weighted average expected volatility	18.20%	19.52%	19.57%
Risk-free interest rate,minimum	2.365%	1.324%	0.77%
Risk-free interest rate, maximum	2.969%	2.201%	1.97%
Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Dividend yield	1.74%	2.193%	2.04%
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	1.954%	2.286%	2.46%
Expected term, in years	6 years	6 years	5 years 6 months

Stock-based Compensation Plan_6

Stock-based Compensation Plans (Summary of Restricted Stock Activity) (Details) - $ / shares		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Restricted Stock Units (RSUs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		298,528		269,063		301,897
Granted, Shares		152,529		130,075		143,107
Vested, Shares		(72,710)		(70,590)		(139,183)
Forfeited, Shares	[1]	(45,866)		(30,020)		(36,758)
Outstanding, end of period, Shares		332,481		298,528		269,063
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 83.45		$ 73.91		$ 54.54
Granted, Weighted Average Grant Date Fair Value		111.66		90.40		83.43
Vested, Weighted Average Grant Date Fair Value		72.69		59.29		42.97
Forfeited, Weighted Average Grant Date Fair Value		94.04		84.88		69.07
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 97.28		$ 83.45		$ 73.91
Performance Shares [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares		102,586	[2]	115,057	[2]	196,142
Granted, Shares		35,063		60,101		126,796
Vested, Shares		(14,032)		(17,642)		(144,141)
Forfeited, Shares	[1]	(53,779)		(54,930)		(63,740)
Outstanding, end of period, Shares	[2]	69,838		102,586		115,057
Outstanding, beginning of period, Weighted Average Grant Date Fair Value		$ 81.21		$ 78.82		$ 47.89
Granted, Weighted Average Grant Date Fair Value		118.60		79.48		73.42
Vested, Weighted Average Grant Date Fair Value		70.24		56.45		40.95
Forfeited, Weighted Average Grant Date Fair Value		89.79		82.27		58.54
Outstanding, end of period, Weighted Average Grant Date Fair Value		$ 95.58		$ 81.21		$ 78.82
[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.
[2]	At December 31, 2018, only market-based awards were outstanding.

Stock-based Compensation Plan_7

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
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic shares used in the calculation of earnings per share	42.4	42.5	42.8
Diluted shares used in the calculation of earnings per share	43	43	43.2
Per share effect of dilutive securities on income from continuing operations	$ (0.07)	$ (0.05)	$ (0.04)
Per share effect of dilutive securities on net income	$ (0.12)	$ (0.05)	$ (0.04)
Employee Stock Option [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.3	0.3	0.2
Non-Vested Stock Grants [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.3	0.2	0.2

Earnings Per Share and Shareh_4

Earnings Per Share and Shareholders' Equity Transactions (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions

Jan. 02, 2019

Dec. 30, 2018

Dec. 31, 2018

Sep. 30, 2018

[1]

Jun. 30, 2018

[1]

Mar. 31, 2018

[1]

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2018

Dec. 29, 2018

Dec. 31, 2017

Dec. 31, 2016

Class Of Stock [Line Items]

Antidilutive securities excluded from calculation of earnings per share

0.3

0.4

0.6

Repurchases common stock, shares

0.5

Repurchases common stock, value

$ 57,700,000

Repurchases of common stock under program, shares

14.9

Repurchases of common stock under program, value

$ 811,200,000

Special dividend declared per share

$ 4.75

$ 5.35

[1]

$ 0.54

$ 0.54

$ 0.54

$ 0.54

$ 0.50

$ 0.50

$ 0.50

Special dividend, declared date

Dec. 30, 2018

Special dividend, paid date

Jan. 25, 2019

Liabilities held for dividends payable

$ 193,400,000

$ 193,400,000

Subsequent Event [Member]

Class Of Stock [Line Items]

Initial delivery of shares of common stock received

1.8

Amount paid by company

$ 250,000,000

Accelerated Share Repurchase Agreement [Member]

Class Of Stock [Line Items]

Repurchases common stock, available for repurchases

$ 600,000,000

$ 600,000,000

Accelerated Share Repurchase Agreement [Member] | Subsequent Event [Member]

Class Of Stock [Line Items]

Shares repurchased under repurchase agreement, percentage

80.00%

Maximum [Member]

Class Of Stock [Line Items]

Repurchases common stock, authorized

$ 900,000,000

Maximum [Member] | Accelerated Share Repurchase Agreement [Member]

Class Of Stock [Line Items]

Repurchases common stock, authorized

$ 600,000,000

[1]

As a result of the sale of Chaucer, (see Note 2 – “Discontinued Operations”), on December 30, 2018 the Board of Directors declared a special dividend of $4.75 per share (see Note 12 – “Earnings Per Share and Shareholders’ Equity Transactions”).

Dividend Restrictions (Details)

Dividend Restrictions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2019	May 31, 2019	Jan. 01, 2019
U.K. [Member]
Statutory Accounting Practices [Line Items]
Dividends declared	$ 85		$ 79.5
NEW HAMPSHIRE
Statutory Accounting Practices [Line Items]
Percentage of policyholder's surplus distributes as dividends and other distributions	10.00%
Dividends declared	$ 140	$ 296.8	218.8
Dividends paid to parent	$ 140	296.8	218.8
Payment of extraordinary dividends		80
NEW HAMPSHIRE | Scenario, Forecast [Member]
Statutory Accounting Practices [Line Items]
Statutory amount available for dividend payments					$ 216.7
NEW HAMPSHIRE | Maximum [Member] | Scenario, Forecast [Member]
Statutory Accounting Practices [Line Items]
Maximum dividends payable without prior approval of state regulators					$ 140	$ 76.7
MICHIGAN
Statutory Accounting Practices [Line Items]
Percentage of policyholder's surplus distributes as dividends and other distributions	10.00%
Dividends declared	$ 87.9	$ 99.9	$ 70
MICHIGAN | Scenario, Forecast [Member]
Statutory Accounting Practices [Line Items]
Statutory amount available for dividend payments				$ 107.1
MICHIGAN | Maximum [Member] | Scenario, Forecast [Member]
Statutory Accounting Practices [Line Items]
Maximum dividends payable without prior approval of state regulators				$ 19.2		$ 87.9

Segment Information (Narrative)

Segment Information (Narrative) (Details)	12 Months Ended
	Dec. 31, 2018segment
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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating revenues									$ 4,545	$ 4,246.8	$ 4,043.7
Net realized and unrealized investment gains (losses)									(50.7)	21.1	10.2
Total revenues	$ 1,102.8	$ 1,166.6	$ 1,133.4	$ 1,091.5	$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	4,494.3	4,267.9	4,053.9
Net investment income									267.4	243.9	231.6
Operating income (loss) before interest expense and income taxes									406.5	327.3	192.9
Interest on debt									(45.1)	(45.2)	(51.4)
Operating income before income taxes									361.4	282.1	141.5
Net loss from repayment of debt									(28.2)		(88.3)
Other non-operating items										(10.3)	2.6
Income from continuing operations before income taxes									282.5	292.9	66
Commercial Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									2,739.5	2,573.8	2,485
Underwriting income (loss)									85	12.2	(123.2)
Net investment income									182.2	165.8	158.5
Other income (expense)									(1.5)	(0.6)	(0.6)
Operating income (loss) before interest expense and income taxes									265.7	177.4	34.7
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues									1,791.3	1,662.3	1,552.4
Underwriting income (loss)									66.6	83.7	103.1
Net investment income									73.7	70.1	69.5
Other income (expense)									5.9	4.9	5.1
Operating income (loss) before interest expense and income taxes									146.2	158.7	177.7
Other (including eliminations) [Member]
Segment Reporting Information [Line Items]
Operating revenues									14.2	10.7	6.3
Underwriting income (loss)									(4.2)	(4.2)	(9.9)
Net investment income									11.5	8	3.6
Other expense									(12.7)	(12.6)	(13.2)
Operating income (loss) before interest expense and income taxes									$ (5.4)	$ (8.8)	$ (19.5)

Segment Information (Identifiab

Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable assets	$ 12,399.7	$ 15,469.6
Identifiable assets	57.4	4,466.6
Assets Held-for-Sale [Member]
Segment Reporting Information [Line Items]
Identifiable assets	57.4	4,466.6
Assets of Discontinued Life Business [Member]
Segment Reporting Information [Line Items]
Identifiable assets	103.9	88
U.S. Companies [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 12,238.4	$ 10,915

Lease Commitments (Narrative) (

Lease Commitments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Leased Assets [Line Items]
Rental expenses for operating leases	$ 16.7	$ 14.8	$ 15.6
Future minimum rental payments	41.1
Future minimum rental payments, 2019	13.5
Future minimum rental payments, 2020	11
Future minimum rental payments, 2021	8.3
Future minimum rental payments, 2022	4.5
Future minimum rental payments, 2023 and thereafter	3.8
Chaucer [Member]
Operating Leased Assets [Line Items]
Rental expenses for operating leases	$ 3.8	$ 3.3	$ 2.9

Reinsurance (Narrative) (Detail

Reinsurance (Narrative) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded		$ 445.8	$ 443.1	$ 397.5
Property and casualty losses and LAE, Ceded	[1]	286.8	375.5	269.7
Reinsurance recoverable on paid and unpaid losses and unearned premiums		1,648.6	1,625.5
MCCA [Member]
Effects of Reinsurance [Line Items]
Property and casualty premiums earned, Ceded		70.9	62.8	58.6
Property and casualty losses and LAE, Ceded		108.8	81.4	$ 75
Reinsurance recoverable on paid and unpaid losses and unearned premiums		$ 977.1	$ 930.6
MCCA [Member] | Reinsurance Receivable [Member] | Reinsurer Concentration Risk [Member]
Effects of Reinsurance [Line Items]
Percentage of reinsurance receivable represented by segment		59.30%
[1]	The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines

Reinsurance (Schedule of Effect

Reinsurance (Schedule of Effects of Reinsurance) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Disclosures [Abstract]
Property and casualty premiums written, Direct		$ 4,816	$ 4,540	$ 4,265.7
Property and casualty premiums written, Assumed		27.6	23.6	25.1
Property and casualty premiums written, Ceded		(458.8)	(454.5)	(408.1)
Property and casualty premiums written, Net		4,384.8	4,109.1	3,882.7
Property and casualty premiums earned, Direct		4,673.6	4,399.9	4,162.6
Property and casualty premiums earned, Assumed		26.6	23.6	24.4
Property and casualty premiums earned, Ceded		(445.8)	(443.1)	(397.5)
Premiums Earned, Net, Total		$ 4,254.4	$ 3,980.4	$ 3,789.5
Percentage of assumed to net premiums earned		0.60%	0.60%	0.60%
Property and casualty losses and LAE, Direct		$ 2,986.2	$ 2,934.6	$ 2,785.1
Property and casualty losses and LAE, Assumed		25.2	20.5	30.6
Property and casualty losses and LAE, Ceded	[1]	(286.8)	(375.5)	(269.7)
Policyholder Benefits and Claims Incurred, Net, Total		$ 2,724.6	$ 2,579.6	$ 2,546
[1]	The higher level of ceded losses and LAE in 2017 as compared to 2016 and 2018 is primarily due to higher non-catastrophe large loss activity occurring in certain domestic lines

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	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Gross loss and LAE reserves, beginning of year		$ 5,058.5	$ 4,660	$ 4,201.9
Reinsurance recoverable on unpaid losses, beginning of year		1,455	1,349.2	1,295.3
Net loss and LAE reserves, beginning of year		3,603.5	3,310.8	2,906.6
Net incurred losses and LAE in respect of losses occurring in current year		2,733.5	2,579.8	2,307.8
Net incurred losses and LAE in respect of losses occurring in prior years	$ (174.1)	(8.9)	(0.2)	238.2
Total incurred losses and LAE		2,724.6	2,579.6	2,546
Net payments of losses and LAE in respect of losses occurring in Current year		1,232.3	1,203.8	1,063
Net payments of losses and LAE in respect of losses occurring in Prior years		1,264.3	1,083.1	1,078.8
Total payments		2,496.6	2,286.9	2,141.8
Net reserve for losses and LAE, end of year	3,310.8	3,831.5	3,603.5	3,310.8
Reinsurance recoverable on unpaid losses, end of year	1,349.2	1,472.6	1,455	1,349.2
Gross reserve for losses and LAE, end of year	$ 4,660	$ 5,304.1	$ 5,058.5	$ 4,660

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	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (174.1)	$ (8.9)	$ (0.2)	$ 238.2
Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(40.9)	(10.8)	219.3
Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		30.8	9.4	10.6
General Liability [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(21.1)	(1.9)	56
Commercial Multiple Peril Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(4)	2.9	68.8
Workers' Compensation Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(31)	(9.1)	(46.7)
Commercial Automobile Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		23.2	2.3	27.5
AIX Program Business [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		17	(1.9)	75.1
Surety Product Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(9)	0.1	35.3
Umbrella Line [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		2.3	0.9	(8.9)
Other Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(18.3)	(4.1)	12.2
Other Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		(29.1)	(6.9)	169.7
Other Commercial Lines [Member] | Commercial Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes	$ (161.5)
Personal Automobile Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		14.3	3.4	4.8
Homeowners And Other Personal Lines [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		16.5	6	5.8
Other Segment [Member]
Shortduration Insurance Contracts Reconciliation Of Claims Development To Liability [Line Items]
Loss and LAE reserve development, including catastrophes		$ 1.2	$ 1.2	$ 8.3

Liabilities For Outstanding C_5

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (174.1)	$ (8.9)	$ (0.2)	$ 238.2
Loss and loss adjustment expense reserves	4,660	5,304.1	$ 5,058.5	4,660	$ 4,201.9
U.S. Companies [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Gross incurred claims, percentage			5.00%
Asbestos Issue [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves		57.6	$ 59.4
Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(40.9)	(10.8)	219.3
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(30.8)	(9.4)
Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		23.2	2.3	27.5
Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(31)	(9.1)	(46.7)
Other Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(29.1)	(6.9)	169.7
Other Commercial Lines [Member] | Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (161.5)
Personal Automobile Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		14.3	3.4	4.8
Other Segment [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		1.2	1.2	8.3
Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(4)	$ 2.9	68.8
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 2015 And 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 2017 [Member] | Homeowners [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(13.8)
Short Duration Insurance Contracts Accident Years2014 Through2016 | Other Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(29.1)
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)
Years 2012 To 2015 [Member] | Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development				68.8
Years 2012 To 2014 [Member] | Commercial Automobile Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development				27.5
Years 2013 To 2015 [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development				$ 46.7

Liabilities For Outstanding C_6

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	$ 5,304.1	$ 5,058.5	$ 4,660	$ 4,201.9
Ceded	(1,472.6)	(1,455)
Net	3,831.5	3,603.5	$ 3,310.8	$ 2,906.6
Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	3,596.5	3,439.2
Ceded	(547.4)	(570.2)
Net	3,049.1	2,869
Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	1,707.6	1,619.3
Ceded	(925.2)	(884.8)
Net	782.4	734.5
Commercial Multiple Peril Line [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	1,206.5	1,104.9
Ceded	(110.8)	(108.9)
Net	1,095.7	996
Workers' Compensation Line [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	775.9	757.8
Ceded	(183)	(180.8)
Net	592.9	577
Commercial Automobile Line [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	433.5	428.1
Ceded	(25.5)	(27.6)
Net	408	400.5
General Liability And Umbrella - Occurrence [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	534.1	508.5
Ceded	(133.5)	(126.2)
Net	400.6	382.3
General Liability - Claims Made [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	245.9	231.6
Ceded	(16.2)	(17.9)
Net	229.7	213.7
Other Commercial Lines [Member] | Commercial And Other Line [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	400.6	408.3
Ceded	(78.4)	(108.8)
Net	322.2	299.5
Personal Automobile Lines [Member] | Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	1,514	1,453.8
Ceded	(917.3)	(878.8)
Net	596.7	575
Homeowners [Member] | Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	158.2	133.5
Ceded	(6)	(4.4)
Net	152.2	129.1
Other Personal Lines | Personal Lines [Member]
Liabilities For Outstanding Claims Losses And Loss Adjustment Expenses [Line Items]
Gross	35.4	32
Ceded	(1.9)	(1.6)
Net	$ 33.5	$ 30.4

Liabilities For Outstanding C_7

Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses (Schedule of Incurred Claims Development) (Details) $ in Millions	Dec. 31, 2018USD ($)claim	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)	Dec. 31, 2011USD ($)	Dec. 31, 2010USD ($)	Dec. 31, 2009USD ($)
Commercial Multiple Peril Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	$ 3,357.8
Cumulative Paid Losses and ALAE, Net of Reinsurance	2,334.6
Unallocated loss adjustment expense	19.3
Net reserves	1,095.7
Commercial Multiple Peril Line [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	476.6	$ 474.9	$ 475.6	$ 459.7	$ 456.2	$ 462.8	$ 480
IBNR	$ 8.8
Cumulative Incurred Claim Count | claim	16,983
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 454.9	442.8	424.2	389.3	350.8	307.5	175.8
Commercial Multiple Peril Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	393	392.1	391.8	367.9	362.5	380
IBNR	$ 10.2
Cumulative Incurred Claim Count | claim	14,830
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 355.9	334.6	306.4	262.3	221.5	137.6
Commercial Multiple Peril Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	449.1	459.4	464.8	439.6	443.9
IBNR	$ 22.6
Cumulative Incurred Claim Count | claim	15,922
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 395.2	363.4	316	267.8	171.7
Commercial Multiple Peril Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	467.4	463.7	456.3	446
IBNR	$ 33
Cumulative Incurred Claim Count | claim	15,731
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 363.2	315.6	260.1	161.9
Commercial Multiple Peril Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	448.7	449.6	447.1
IBNR	$ 55.5
Cumulative Incurred Claim Count | claim	15,981
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 290.2	237.9	140.3
Commercial Multiple Peril Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	544.8	538.7
IBNR	$ 122.2
Cumulative Incurred Claim Count | claim	16,674
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 296.4	170.9
Commercial Multiple Peril Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	578.2
IBNR	$ 228.3
Cumulative Incurred Claim Count | claim	16,716
Commercial Multiple Peril Line [Member] | Years 2010 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	$ 53.2
Commercial Multiple Peril Line [Member] | Years 2011 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	1,023.2
Commercial Multiple Peril Line [Member] | Year 2018 [Member]
Claims Development [Line Items]
Cumulative Paid Losses and ALAE, Net of Reinsurance	178.8
Workers' Compensation Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,486.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,001.9
Unallocated loss adjustment expense	17
Net reserves	592.9
Workers' Compensation Line [Member] | Year 2008 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	78.7	77.9	77.5	77.5	79.2	79.4	80.3	$ 82.5	$ 87.6	$ 93.8
IBNR	$ 1.9
Cumulative Incurred Claim Count | claim	8,032
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 73.1	72.4	70.8	69.8	68.6	65.4	61.6	54.6	41.2	$ 19.2
Workers' Compensation Line [Member] | Year 2009 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	116.5	115.9	114.7	114.9	116.3	115.9	115.8	116.9	115.5
IBNR	$ 3.3
Cumulative Incurred Claim Count | claim	10,174
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 107.2	106.3	104.2	101	96.5	89.5	76.9	57.2	$ 22.1
Workers' Compensation Line [Member] | Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	147.9	148.3	146.7	143.8	145.5	144.3	144.2	141.4
IBNR	$ 7.1
Cumulative Incurred Claim Count | claim	12,465
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 132.8	130.9	128	122.9	114	97.3	71.3	30
Workers' Compensation Line [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	163.7	163.7	162.9	157.2	165.2	171.1	176.3
IBNR	$ 10.2
Cumulative Incurred Claim Count | claim	13,042
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 141.4	136	130.7	120.1	102.6	74.8	30.4
Workers' Compensation Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	155.4	155.3	154.4	160.1	167.4	179.3
IBNR	$ 12.3
Cumulative Incurred Claim Count | claim	11,683
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 127.4	122.8	114	101.2	74.6	30.9
Workers' Compensation Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	150.3	152.1	154.7	172.9	182.1
IBNR	$ 15.2
Cumulative Incurred Claim Count | claim	10,880
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 112.5	105.6	92.3	70.5	30.6
Workers' Compensation Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	150.3	157.5	164.2	189.6
IBNR	$ 22.5
Cumulative Incurred Claim Count | claim	11,417
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 99.4	87.2	65.7	28
Workers' Compensation Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	164.6	180.5	189.6
IBNR	$ 24.5
Cumulative Incurred Claim Count | claim	15,790
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 99.5	78.1	33.9
Workers' Compensation Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	172.1	186.1
IBNR	$ 35.4
Cumulative Incurred Claim Count | claim	16,405
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 73	32.8
Workers' Compensation Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	187.2
IBNR	$ 69.4
Cumulative Incurred Claim Count | claim	15,711
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 35.6
Workers' Compensation Line [Member] | Years 2008 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	484.8
Workers' Compensation Line [Member] | Years 2007 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	91.1
Commercial Automobile Line [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,099.8
Cumulative Paid Losses and ALAE, Net of Reinsurance	711.1
Unallocated loss adjustment expense	4.9
Net reserves	408
Commercial Automobile Line [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	231	225.6	227.5	205.3	179.4		177.7
IBNR	$ 1.4
Cumulative Incurred Claim Count | claim	15,274
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 214.9	200.6	176.3	137.6	89.6		35.9
Commercial Automobile Line [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	185.1	181.7	177.3	163.3	168.5
IBNR	$ 6.4
Cumulative Incurred Claim Count | claim	13,477
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 168.2	137.1	102.7	70.8	33.1
Commercial Automobile Line [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	167.8	166.9	168.3	163.4
IBNR	$ 10
Cumulative Incurred Claim Count | claim	12,838
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 129.3	96.4	63.8	32.2
Commercial Automobile Line [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	163	157.7	157
IBNR	$ 16.4
Cumulative Incurred Claim Count | claim	11,629
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 98.3	60.7	27.8
Commercial Automobile Line [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	170.1	159.5
IBNR	$ 43.4
Cumulative Incurred Claim Count | claim	11,343
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 71.2	26.9
Commercial Automobile Line [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	182.8
IBNR	$ 110.7
Cumulative Incurred Claim Count | claim	10,338
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 29.2
Commercial Automobile Line [Member] | Years 2012 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	388.7
Commercial Automobile Line [Member] | Years 2011 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	14.4
General Liability And Umbrella - Occurrence [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	712
Cumulative Paid Losses and ALAE, Net of Reinsurance	348.7
Unallocated loss adjustment expense	11.5
Net reserves	400.6
General Liability And Umbrella - Occurrence [Member] | Year 2010 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	57.5	55.4	55.6	51	42.2	38.2	43.8	47.6
IBNR	$ 3.3
Cumulative Incurred Claim Count | claim	1,398
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 50.6	47.8	45.8	39.8	31.1	19.4	6.7	$ 1.6
General Liability And Umbrella - Occurrence [Member] | Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	69.1	74.2	71.1	64.3	62.2	59.3	77.2
IBNR	$ 3.7
Cumulative Incurred Claim Count | claim	1,779
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 63	60.6	52.1	43.8	29.8	12.6	$ 2.2
General Liability And Umbrella - Occurrence [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	81.2	84.4	88.6	71.5	67.4	84.1
IBNR	$ 8.2
Cumulative Incurred Claim Count | claim	2,031
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 63.5	56.1	43.1	26.8	11	2.4
General Liability And Umbrella - Occurrence [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	98.3	101.4	98.6	82.3	100.9
IBNR	$ 10.6
Cumulative Incurred Claim Count | claim	2,133
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 70.4	52.8	31.4	14.5	3.1
General Liability And Umbrella - Occurrence [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	97	99.6	99.3	104.2
IBNR	$ 21.3
Cumulative Incurred Claim Count | claim	2,550
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 48.5	30.8	15.2	3.3
General Liability And Umbrella - Occurrence [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	101.5	100.7	95.6
IBNR	$ 33.9
Cumulative Incurred Claim Count | claim	2,015
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 31.6	15	3.1
General Liability And Umbrella - Occurrence [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	107.6	97.7
IBNR	$ 54.2
Cumulative Incurred Claim Count | claim	1,844
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 17	4.4
General Liability And Umbrella - Occurrence [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	99.8
IBNR	$ 74.4
Cumulative Incurred Claim Count | claim	1,917
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 4.1
General Liability And Umbrella - Occurrence [Member] | Years 2010 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	363.3
General Liability And Umbrella - Occurrence [Member] | Years 2009 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	25.8
General Liability - Claims Made [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	563.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	342.8
Unallocated loss adjustment expense	4.8
Net reserves	229.7
General Liability - Claims Made [Member] | Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	68.4	69.2	71.4	60.6	55.4	54.9
IBNR	$ 0.9
Cumulative Incurred Claim Count | claim	645
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 65.7	64.5	61.1	53.7	36.2	$ 9.3
General Liability - Claims Made [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	82.5	86.2	83.9	69.9	61.7
IBNR	$ 2.9
Cumulative Incurred Claim Count | claim	907
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 75.4	71.5	59.1	38.8	11
General Liability - Claims Made [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	90.2	98.4	98.8	93.2
IBNR	$ 4.9
Cumulative Incurred Claim Count | claim	1,035
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 76	64.2	43.6	10
General Liability - Claims Made [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	97.1	101.7	103.6
IBNR	$ 18.4
Cumulative Incurred Claim Count | claim	1,082
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 66.1	42.9	11.1
General Liability - Claims Made [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	104.6	103.3
IBNR	$ 35.5
Cumulative Incurred Claim Count | claim	1,310
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 42.3	12.3
General Liability - Claims Made [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	120.9
IBNR	$ 59.5
Cumulative Incurred Claim Count | claim	2,586
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 17.3
General Liability - Claims Made [Member] | Years 2012 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	220.9
General Liability - Claims Made [Member] | Years 2011 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	4
Personal Automobile Lines [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,756.7
Cumulative Paid Losses and ALAE, Net of Reinsurance	1,239.2
Unallocated loss adjustment expense	13.8
Net reserves	596.7
Personal Automobile Lines [Member] | Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	312.5	310.2	308.8	304.1	323.1
IBNR	$ 4.7
Cumulative Incurred Claim Count | claim	43,356
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 298.4	279.4	241.1	188.6	$ 107.2
Personal Automobile Lines [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	332.4	328.6	334.1	327.4
IBNR	$ 5.6
Cumulative Incurred Claim Count | claim	42,358
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 301.9	261.5	205.4	112.9
Personal Automobile Lines [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	355.3	344.5	337.9
IBNR	$ 11.5
Cumulative Incurred Claim Count | claim	42,214
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 288	213.1	112.8
Personal Automobile Lines [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	362.3	363.6
IBNR	$ 35
Cumulative Incurred Claim Count | claim	42,200
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 229.2	115
Personal Automobile Lines [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	394.2
IBNR	$ 151.7
Cumulative Incurred Claim Count | claim	39,164
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 121.7
Personal Automobile Lines [Member] | Years 2013 To 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	517.5
Personal Automobile Lines [Member] | Years 2012 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	65.4
Homeowners [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	1,115
Cumulative Paid Losses and ALAE, Net of Reinsurance	971
Unallocated loss adjustment expense	3.4
Net reserves	152.2
Homeowners [Member] | Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	274.2	272		267.1
IBNR	$ 1.7
Cumulative Incurred Claim Count | claim	30,436
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 267.5	260.6	253.8	$ 199.9
Homeowners [Member] | Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	231.4	226.5	222.1
IBNR	$ 3.2
Cumulative Incurred Claim Count | claim	25,456
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 218.2	207.9	$ 154.3
Homeowners [Member] | Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	296.5	291.4
IBNR	$ 3.4
Cumulative Incurred Claim Count | claim	33,394
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 271.6	$ 204.5
Homeowners [Member] | Year 2017 [Member]
Claims Development [Line Items]
Incurred Losses and ALAE, Net of Reinsurance	312.9
IBNR	$ 46
Cumulative Incurred Claim Count | claim	30,487
Cumulative Paid Losses and ALAE, Net of Reinsurance	$ 213.7
Homeowners [Member] | Years 2015 And 2017 [Member]
Claims Development [Line Items]
Total reserves for accident years	144
Homeowners [Member] | Years 2014 And Prior [Member]
Claims Development [Line Items]
Total reserves for accident years	$ 4.8

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, 2018
Commercial Multiple Peril Line [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	34.00%
2	22.70%
3	10.70%
4	10.00%
5	7.20%
6	4.70%
7	2.50%
Workers' Compensation Line [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	20.00%
2	27.00%
3	15.90%
4	9.60%
5	5.60%
6	3.50%
7	2.40%
8	1.40%
9	1.40%
10	0.80%
Commercial Automobile Line [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	16.90%
2	21.70%
3	20.10%
4	18.30%
5	13.70%
6	6.20%
General Liability And Umbrella - Occurrence [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	3.40%
2	11.70%
3	19.40%
4	20.10%
5	15.30%
6	10.60%
7	3.50%
8	4.90%
General Liability - Claims Made [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	12.60%
2	34.40%
3	24.30%
4	12.90%
5	4.80%
6	1.70%
Personal Automobile Lines [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	32.50%
2	28.40%
3	18.30%
4	12.20%
5	6.10%
Homeowners [Member]
Shortduration Insurance Contracts Historical Claims Duration [Line Items]
1	69.20%
2	21.80%
3	3.40%
4	2.50%

Commitments and Contingencies (

Commitments and Contingencies (Narrative) (Details)	Dec. 11, 2009claimplaintiff	Nov. 30, 2015claim	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Participant's retirement age			65 years
Number of plaintiffs | plaintiff	2
New claims	2	3
Claims Dismissed		2

Statutory Financial Informati_3

Statutory Financial Information (Statutory Accounting Practices Disclosure) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Abstract]
Statutory Net Income	$ 296.1	$ 254.5	$ 158
Statutory Capital and Surplus	$ 2,172.5	$ 2,077.1	$ 2,173.4

Statutory Financial Informati_4

Statutory Financial Information (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Abstract]
Minimum statutory capital and surplus required	$ 518.4	$ 500.7	$ 460.1

Quarterly Results of Operatio_3

Quarterly Results of Operations (Schedule of Quarterly Financial Information) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 30, 2018	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues		$ 1,102.8		$ 1,166.6		$ 1,133.4		$ 1,091.5		$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	$ 4,494.3	$ 4,267.9	$ 4,053.9
Income from continuing operations		2.1		104		82.4		50.5		71.9	59.5	56.3	28.4	239	216.1	67
Net income		$ 123.6		$ 100.4		$ 99.3		$ 67.7		$ 51.5	$ 11.1	$ 78.4	$ 45.2	$ 391	$ 186.2	$ 155.1
Income from continuing operations per share, Basic		$ 0.05		$ 2.45		$ 1.94		$ 1.19		$ 1.69	$ 1.40	$ 1.32	$ 0.67	$ 5.63	$ 5.08	$ 1.57
Income from continuing operations per share, Diluted		0.05		2.41		1.91		1.17		1.67	1.38	1.32	0.66	5.56	5.03	1.55
Net income per share, Basic		2.92		2.36		2.34		1.59		1.21	0.26	1.84	1.06	9.21	4.38	3.63
Net income per share, Diluted		2.88		2.33		2.31		1.57		1.20	0.26	1.83	1.05	$ 9.09	$ 4.33	$ 3.59
Dividends declared per share	$ 4.75	$ 5.35	[1]	$ 0.54	[1]	$ 0.54	[1]	$ 0.54	[1]	$ 0.54	$ 0.50	$ 0.50	$ 0.50
[1]	As a result of the sale of Chaucer, (see Note 2 – “Discontinued Operations”), on December 30, 2018 the Board of Directors declared a special dividend of $4.75 per share (see Note 12 – “Earnings Per Share and Shareholders’ Equity Transactions”).

Quarterly Results of Operatio_4

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

Dec. 30, 2018

Dec. 31, 2018

Sep. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Special dividend declared per share

$ 4.75

$ 5.35

[1]

$ 0.54

[1]

$ 0.54

[1]

$ 0.54

[1]

$ 0.54

$ 0.50

$ 0.50

$ 0.50

[1]

As a result of the sale of Chaucer, (see Note 2 – “Discontinued Operations”), on December 30, 2018 the Board of Directors declared a special dividend of $4.75 per share (see Note 12 – “Earnings Per Share and Shareholders’ Equity Transactions”).

Quarterly Results of Operatio_5

Quarterly Results of Operations (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Losses related to changes in fair value of equity securities	$ 50.1
Advance repayment charges before tax	$ 26.3	$ 26.3

Subsequent Events (Narrative) (

Subsequent Events (Narrative) (Details) - Subsequent Event [Member] - USD ($) shares in Millions, $ in Millions	Feb. 14, 2019	Jan. 02, 2019
Subsequent Event [Line Items]
Amount paid by company		$ 250
Initial delivery of shares of common stock received		1.8
Chaucer Irish Entity[Member]
Subsequent Event [Line Items]
Proceeds from sale of businesses segment	$ 28
Accelerated Share Repurchase Agreement [Member]
Subsequent Event [Line Items]
Shares repurchased under repurchase agreement, percentage		80.00%

SCHEDULE I SUMMARY OF INVESTM_2

SCHEDULE I SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Millions	Dec. 31, 2018USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 7,297.7	[1]
Fair Value	7,316.4
Amount at which shown in the balance sheet	7,311.9
US Treasury and Government Agencies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,140.2	[1]
Fair Value	1,123.6
Amount at which shown in the balance sheet	1,123.6
States, municipalities and political subdivisions [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	879	[1]
Fair Value	885.8
Amount at which shown in the balance sheet	885.8
Foreign governments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7.3	[1]
Fair Value	7.4
Amount at which shown in the balance sheet	7.4
Public Utilities Bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	410.3	[1]
Fair Value	403.1
Amount at which shown in the balance sheet	403.1
All other corporate bonds [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	3,809.1	[1]
Fair Value	3,741.6
Amount at which shown in the balance sheet	3,741.6
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,245.9	[1]
Fair Value	6,161.5
Amount at which shown in the balance sheet	6,161.5
Public utilities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	63.9	[1]
Fair Value	90.7
Amount at which shown in the balance sheet	90.7
Banks, trust and insurance companies [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	28.5	[1]
Fair Value	27.4
Amount at which shown in the balance sheet	27.4
Industrial, miscellaneous and all other [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	290.6	[1]
Fair Value	346.3
Amount at which shown in the balance sheet	346.3
Equity Securities Investment Summary [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	383	[1]
Fair Value	464.4
Amount at which shown in the balance sheet	464.4
Mortgage Loans on Real Estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	405.7	[1]
Fair Value	410.2
Amount at which shown in the balance sheet	405.7
Real estate [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7.9	[1]
Fair Value	7.9
Amount at which shown in the balance sheet	7.9
Other long-term investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	230.3	[1]
Fair Value	247.9	[1]
Amount at which shown in the balance sheet	247.9	[1]
Total investments, excluding held-for-sale [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	7,272.8	[1]
Fair Value	7,291.9
Amount at which shown in the balance sheet	7,287.4
Assets Held-for-Sale [Member] | Chaucer [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	24.9	[1]
Fair Value	24.5
Amount at which shown in the balance sheet	$ 24.5
[1]	For equity securities and other long-term investments, represents original cost; for fixed maturities, represents original cost reduced by repayments and adjusted for amortization of premiums and accretion of discounts.

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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements Captions [Line Items]
Net investment income									$ 267.4		$ 243.9	$ 231.6
Net realized gains (losses) from sales and other									(2.7)		26.7	37.6
Total revenues	$ 1,102.8	$ 1,166.6	$ 1,133.4	$ 1,091.5	$ 1,092.5	$ 1,085.9	$ 1,053.3	$ 1,036.2	4,494.3		4,267.9	4,053.9
Interest expense									45.1		45.2	51.4
Loss from repayment of debt									1.9			88.3
Other operating expenses									522.1		509.5	498.6
Total losses and expenses									4,211.8		3,975	3,987.9
Income tax benefit									(43.5)		(76.8)	1
Income from continuing operations	2.1	104	82.4	50.5	71.9	59.5	56.3	28.4	239		216.1	67
Gain on sale of Chaucer business									131.9
Income from discontinued life business (net of income tax (expense) benefit									0.1		(16.8)	(1)
Net income	$ 123.6	$ 100.4	$ 99.3	$ 67.7	$ 51.5	$ 11.1	$ 78.4	$ 45.2	391		186.2	155.1
Income from discontinued operations, income tax (expense) benefit											(9.3)	(2.8)
Other comprehensive (loss) income, net of tax									(122.4)		44.8	8.9
Comprehensive income									268.6		231	164
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income									54.9	[1]	52	45.7
Total revenues									912.4	[1]	911.7	891.4
Interest expense									3.8	[1]	3.3	3.5
Other operating expenses									115	[1]	109.2	113.7
Total losses and expenses									883	[1]	904.6	764.6
Income tax benefit									(4.7)	[1]	(21.7)	(37.2)
Income from discontinued life business (net of income tax (expense) benefit									20	[1]	(13.1)	89.1
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net investment income									11.8		7.6	4.9
Net realized gains (losses) from sales and other									(0.5)		(0.2)
Interest income from loan to subsidiary									0.3		0.2	22.4
Other income									0.5		0.5	0.5
Total revenues									12.1		8.1	27.8
Interest expense									37.8		38.1	46.1
Employee benefit related expenses									5.1		7.4	7
Interest expense on loan from subsidiary									6.8		7	6.3
Loss from repayment of debt									1.9			88.3
Other operating expenses									12.1		10.5	8.6
Total losses and expenses									63.7		63	156.3
Net loss before income taxes and equity in income of subsidiaries									(51.6)		(54.9)	(128.5)
Income tax benefit									21.4		44.4	75.6
Equity in income of subsidiaries									288.4		196.1	207
Income from continuing operations									258.2		185.6	154.1
Gain on sale of Chaucer business									131.9
Income from discontinued life business (net of income tax (expense) benefit									0.9		0.6	1
Net income									391		186.2	155.1
Income from discontinued operations, income tax (expense) benefit											(0.1)	1.7
Other comprehensive (loss) income, net of tax									(122.4)		44.8	8.9
Comprehensive income									268.6		$ 231	$ 164
The Hanover Insurance Group [Member] | Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Gain on sale of Chaucer business									$ 131.9
[1]	2018 reflects the results of operations for the period in which THG owned Chaucer, which concluded with the sale of the Chaucer business on December 28, 2018.

SCHEDULE II CONDENSED FINANCI_3

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF INCOME) (Parenthetical) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements Captions [Line Items]
Income from discontinued operation, income tax (expense) benefit		$ (0.2)	$ (0.1)	$ 1.7
Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Income from discontinued operation, income tax (expense) benefit	[1]	$ (42.5)
[1]	The income tax expense represents the current tax obligation on the sale and the derecognition of deferred tax assets that are 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 ($) $ / shares in Units, $ in Millions		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Assets
Fair Value		$ 6,161.5		$ 5,749.3
Equity securities - at fair value		464.4		576.2
Cash and cash equivalents		1,020.7		297.9
Other assets		371.6		333.7
Total assets		12,399.7		15,469.6
Liabilities
Debt		777.9		786.9
Total liabilities		9,445		12,471.9
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		1,871.8		1,857
Accumulated other comprehensive income (loss)		(116.5)		107.6
Retained earnings		2,182.3		1,975
Treasury stock at cost (18.2 and 18.0 million shares)		(983.5)		(942.5)
Total shareholders’ equity		2,954.7		2,997.7		$ 2,857.5
Total liabilities and shareholders’ equity		12,399.7		15,469.6
Fixed maturities, amortized cost		$ 6,245.9		$ 5,657.1
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		18,200,000		18,000,000
The Hanover Insurance Group [Member]
Assets
Fair Value		$ 374.3		$ 280.9
Equity securities - at fair value		1.1		1.1
Cash and cash equivalents		827.4	[1]	32.7	[1]	$ 10.1	$ 8.6
Investments in subsidiaries		2,800.1		3,455.4
Net receivable from subsidiaries		25		31.3
Other assets	[2]	34.6		2.6
Total assets		4,062.5		3,804
Liabilities
Expenses and state taxes payable	[3]	275.2		7.9
Current income tax payable		46.9		3.2
Interest payable		7.8		8.1
Debt		777.9		787.1
Total liabilities		1,107.8		806.3
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		1,871.8		1,857
Accumulated other comprehensive income (loss)		(116.5)		107.6
Retained earnings		2,182.3		1,975
Treasury stock at cost (18.2 and 18.0 million shares)		(983.5)		(942.5)
Total shareholders’ equity		2,954.7		2,997.7
Total liabilities and shareholders’ equity		4,062.5		3,804
Fixed maturities, amortized cost		$ 380.5		$ 281.9
Preferred stock, par value		$ 0.01		$ 0.01
Preferred stock, shares authorized		20,000,000		20,000,000
Preferred stock, issued		0		0
Common stock, par value		$ 0.01		$ 0.01
Common stock, shares authorized		300,000,000		300,000,000
Common stock, shares issued		60,500,000		60,500,000
Treasury stock, shares		18,200,000		18,000,000
[1]	Included in 2018 were net cash proceeds of $762.0 million as a result of the sale of the Chaucer business to China Re on December 28, 2018.
[2]	In 2018, other assets include a $31.7 million receivable for estimated contingent consideration related to the sale of the Chaucer business.
[3]	Included in 2018 was a payable related to a special dividend of $193.4 million declared on December 30, 2018.

SCHEDULE II CONDENSED FINANCI_5

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Parenthetical) (Details) - USD ($) $ in Millions	Dec. 28, 2018	Dec. 31, 2018	Dec. 30, 2018
Condensed Financial Statements Captions [Line Items]
Net proceeds from sale of Chaucer business		$ 635.7
The Hanover Insurance Group [Member]
Condensed Financial Statements Captions [Line Items]
Net proceeds from sale of Chaucer business		762
Dividends declared			$ 193.4
The Hanover Insurance Group [Member] | Chaucer [Member]
Condensed Financial Statements Captions [Line Items]
Net proceeds from sale of Chaucer business	$ 762
Receivable		$ 31.7

SCHEDULE II CONDENSED FINANCI_6

SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Cash Flows From Operating Activities
Net income	$ 123.6		$ 100.4	$ 99.3	$ 67.7		$ 51.5		$ 11.1	$ 78.4	$ 45.2	$ 391		$ 186.2		$ 155.1
Adjustments to reconcile net income to net cash provided by operating activities:
Gain from sale of Chaucer business												(131.9)
Net loss from repayment of debt												1.9				88.3
Deferred income tax expense (benefit)												(23.4)		39.8		(13.2)
Change in expenses and taxes payable												4.6		(0.5)		12.7
Net cash provided by operating activities												551.3		704.6		743.4
Cash Flows From Investing Activities
Purchase of fixed maturities												(1,425.6)		(1,645.8)		(2,020.9)
Net cash received from sale of Chaucer business												635.7
Net cash provided by (used in) investing activities												271.3		(506.3)		(495.4)
Cash Flows From Financing Activities
Proceeds from debt borrowings, net																370.7
Repayment of debt												(11.6)				(475.4)
Repurchases of common stock												(57.7)		(37.2)		(105.6)
Dividends paid to shareholders												(94.3)		(86.8)		(80.4)
Proceeds from exercise of employee stock options												14.8		23.1		16.2
Other financing activities												(3.2)		(4.1)		(19.9)
Net cash used in financing activities												(171)		(111.7)		(300.8)
Net change in cash and cash equivalents												647.6		93.8		(56.2)
Cash and cash equivalents, beginning of year					297.9							297.9
Cash and cash equivalents, end of year	1,020.7						297.9					1,020.7		297.9
The Hanover Insurance Group [Member]
Cash Flows From Operating Activities
Net income												391		186.2		155.1
Adjustments to reconcile net income to net cash provided by operating activities:
Gain from sale of Chaucer business												(131.9)
Net realized investment losses												0.6		0.2
Net loss from repayment of debt												1.9				88.3
Equity in net income of subsidiaries												(288.4)		(196.1)		(207)
Dividends received from subsidiaries												86.4		55.1		228.1
Deferred income tax expense (benefit)												(5.3)		18.9		(39)
Change in expenses and taxes payable												59.7		(27.4)		6.6
Change in net receivable from subsidiaries												8.9		9		50.5
Other, net												(0.4)		(0.1)		7.3
Net cash provided by operating activities												122.5		45.8		289.9
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities												82.8		94.1		82.4
Purchase of fixed maturities												(4)
Net cash received from sale of Chaucer business												762
Net cash used for business acquisitions														(12.3)		(2.2)
Net cash provided by (used in) investing activities												840.8		81.8		80.2
Cash Flows From Financing Activities
Proceeds from debt borrowings, net																370.7
Proceeds from long-term intercompany borrowings																125
Net cash related to short-term intercompany borrowings												(19.8)				(102.7)
Repayment of debt												(11.6)				(571.9)
Repurchases of common stock												(57.7)		(37.2)		(105.6)
Dividends paid to shareholders												(94.3)		(86.8)		(80.4)
Proceeds from exercise of employee stock options												14.8		23.1		16.2
Other financing activities														(4.1)		(19.9)
Net cash used in financing activities												(168.6)		(105)		(368.6)
Net change in cash and cash equivalents												794.7		22.6		1.5
Cash and cash equivalents, beginning of year					$ 32.7	[1]					$ 10.1	32.7	[1]	10.1		8.6
Cash and cash equivalents, end of year	$ 827.4	[1]					$ 32.7	[1]				$ 827.4	[1]	$ 32.7	[1]	$ 10.1
[1]	Included in 2018 were net cash proceeds of $762.0 million as a result of the sale of the Chaucer business to China Re on December 28, 2018.

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, 2018	Dec. 31, 2017	Dec. 31, 2016
The Hanover Insurance Group [Member]
Supplementary Insurance Information By Segment [Line Items]
Investment assets transferred to the parent company to settle dividend balances	$ 179.8	$ 261.6	$ 78.5

SCHEDULE III SUPPLEMENTARY IN_2

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs	[1]	$ 450.8		$ 430	$ 402.8
Future policy benefits, losses, claims and loss expenses	[1]	5,296.1		5,050.3	4,653.6
Unearned premiums	[1]	2,277.8		2,131.7	1,992
Other policy claims and benefits payable	[1]	8		8.2	6.4
Premium revenue	[1]	4,254.4		3,980.4	3,789.5
Net investment income	[1],[2]	267.4		243.9	231.6
Benefits, claims, losses and settlement expenses	[1]	2,724.6		2,579.6	2,546
Amortization of deferred acquisition costs	[1]	891.8		840.7	803.6
Other operating expenses	[1],[3]	595.4	[4]	554.7	638.3	[4]
Premiums written	[1]	4,384.8		4,109.1	3,882.7
Eliminations [Member]
Supplementary Insurance Information By Segment [Line Items]
Future policy benefits, losses, claims and loss expenses				(7)	(10.2)
Unearned premiums				(1.5)	(3.1)
Other operating expenses	[3]	(7.8)	[4]	(7.8)	(7.6)	[4]
Commercial Lines [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		296.9		286.4	269.6
Future policy benefits, losses, claims and loss expenses		3,550.6		3,399.3	3,069.5
Unearned premiums		1,369.5		1,299.5	1,231.5
Other policy claims and benefits payable		8		8.2	6.4
Premium revenue		2,548.4		2,399.6	2,318
Net investment income	[2]	182.2		165.8	158.5
Benefits, claims, losses and settlement expenses		1,568.3		1,531.4	1,601.5
Amortization of deferred acquisition costs		587.5		557.9	537.4
Other operating expenses	[3]	318	[4]	312.8	308.9	[4]
Premiums written		2,610.7		2,462	2,361.5
Personal Lines [Member]
Supplementary Insurance Information By Segment [Line Items]
Deferred acquisition costs		153.9		143.6	133.2
Future policy benefits, losses, claims and loss expenses		1,707.6		1,619.3	1,554
Unearned premiums		908.3		833.7	763.6
Premium revenue		1,706		1,580.8	1,471.5
Net investment income	[2]	73.7		70.1	69.5
Benefits, claims, losses and settlement expenses		1,155		1,046.9	936.1
Amortization of deferred acquisition costs		304.3		282.8	266.2
Other operating expenses	[3]	185.8	[4]	178.5	172.4	[4]
Premiums written		1,774.1		1,647.1	1,521.2
Interest On Debt [Member]
Supplementary Insurance Information By Segment [Line Items]
Other operating expenses	[3]	45.1	[4]	45.2	51.4	[4]
Other [Member]
Supplementary Insurance Information By Segment [Line Items]
Future policy benefits, losses, claims and loss expenses		37.9		38.7	40.3
Net investment income	[2]	11.5		8	3.6
Benefits, claims, losses and settlement expenses		1.3		1.3	8.4
Other operating expenses	[3]	$ 54.3	[4]	$ 26	$ 113.2	[4]
[1]	Information for all periods excludes results and balances related to the 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 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 companies, expenses are generally allocated based upon either net premiums written or net premiums earned.
[4]	In 2018 and 2016, the Other segment includes $28.2 million and $88.3 million of losses on the repayment of debt, respectively.

SCHEDULE III SUPPLEMENTARY IN_3

SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Parenthetical) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016
Supplementary Insurance Information [Abstract]
Loss on repayment of debt	$ 28.2	$ 88.3

SCHEDULE V VALUATION AND QUAL_2

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		$ 14	$ 16.8		$ 12.9
Charged to costs and expenses		10.1	9.7		12.1
Charged to other accounts	[1]	(6.4)	0.1		0.1
Deductions		(9.6)	(12.6)		(8.3)
Balance at end of period		8.1	14		16.8
Allowance for doubtful accounts [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period		4.3	3.7		3.2
Charged to costs and expenses		9.5	9.4		8.7
Deductions		(9.6)	(8.8)		(8.2)
Balance at end of period		4.2	4.3		3.7
Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at beginning of period	[2]	9.7	13.1	[3]	9.7	[3]
Charged to costs and expenses		0.6	0.3	[2]	3.4	[2],[3]
Charged to other accounts	[1]	(6.4)	0.1	[2]	0.1	[2],[3]
Deductions	[2]		(3.8)		(0.1)	[3]
Balance at end of period		$ 3.9	$ 9.7	[2]	$ 13.1	[2],[3]
[1]	Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its Chaucer business; and accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.
[2]	The balance at beginning of period and the balance at end of period include $6.3 million and $6.4 million, respectively, for the year-ended 2017 and $6.2 million and $6.3 million, respectively, for the year-ended 2016 related to the former Chaucer business.
[3]	Activity primarily relates to the impairment in 2016 and subsequent write-off in 2017 of a receivable that is associated with a single reinsurance counterparty that was placed into conservation by the state of California in July 2016. This counterparty is not involved in the Company’s ongoing reinsurance program and the entire receivable related to this counterparty resulted from a one-time block reinsurance transaction that occurred in 2012 as part of the Company’s exposure management actions.

SCHEDULE V VALUATION AND QUAL_3

SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Parenthetical) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	[1]	$ (6.4)	$ 0.1		$ 0.1
Valuation allowance, balance		8.1	14		16.8		$ 12.9
Allowance for uncollectible reinsurance recoverables [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Valuation allowance, balance reduced as part of sale of business	[1]	(6.4)	0.1	[2]	0.1	[2],[3]
Valuation allowance, balance		3.9	9.7	[2]	13.1	[2],[3]	9.7	[2],[3]
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
Valuation allowance, balance			$ 6.4		$ 6.3		$ 6.2
[1]	Amounts charged to other accounts include foreign exchange gains and losses. Additionally, in 2018 the Company sold its Chaucer business; and accordingly, the $6.4 million balance associated with this business was reduced as part of the sale.
[2]	The balance at beginning of period and the balance at end of period include $6.3 million and $6.4 million, respectively, for the year-ended 2017 and $6.2 million and $6.3 million, respectively, for the year-ended 2016 related to the former Chaucer business.
[3]	Activity primarily relates to the impairment in 2016 and subsequent write-off in 2017 of a receivable that is associated with a single reinsurance counterparty that was placed into conservation by the state of California in July 2016. This counterparty is not involved in the Company’s ongoing reinsurance program and the entire receivable related to this counterparty resulted from a one-time block reinsurance transaction that occurred in 2012 as part of the Company’s exposure management actions.

SCHEDULE VI SUPPLEMENTAL INFO_2

SCHEDULE VI SUPPLEMENTAL INFORMATION CONCERNING PROPERTY AND CASUALTY INSURANCE OPERATIONS (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net incurred losses and LAE in respect of losses occurring in current year			$ 2,733.5	$ 2,579.8	$ 2,307.8
Net incurred losses and LAE in respect of losses occurring in prior years		$ 174.1	8.9	0.2	(238.2)
Reinsurance Recoverable On Unpaid Losses Property Casualty Liability		1,349.2	1,472.6	1,455	1,349.2	$ 1,295.3
Prepaid premiums, gross		90.9	117.6	101.8	90.9
Consolidated Property and Casualty Insurance Entity [Member]
Deferred acquisition costs	[1]	402.8	450.8	430	402.8
Reserves for unpaid claims and claim adjustment expenses	[1],[2]	4,660	5,304.1	5,058.5	4,660
Unearned premiums	[1],[2]	$ 1,992	2,277.8	2,131.7	1,992
Earned premiums	[1]		4,254.4	3,980.4	3,789.5
Net investment income	[1]		267.4	243.9	231.6
Net incurred losses and LAE in respect of losses occurring in current year	[1],[2]		2,733.5	2,575.1	2,307.8
Net incurred losses and LAE in respect of losses occurring in prior years	[1],[3]		(8.9)	(0.2)	238.2
Amortization of deferred acquisition costs	[1],[2]		891.8	840.7	803.6
Paid claims and claim adjustment expenses	[1]		2,496.6	2,282.2	2,141.8
Premiums written	[1]		$ 4,384.8	$ 4,109.1	$ 3,882.7
[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 $1,472.6 million, $1,455.0 million and $1,349.2 million of reinsurance recoverable on unpaid losses in 2018, 2017 and 2016, respectively. Unearned premiums are shown gross of prepaid premiums of $117.6 million, $101.8 million and $90.9 million in 2018, 2017 and 2016, respectively. Reserves for unpaid claims and claims adjustment expense also include policyholder dividends.
[3]	The Company does not use discounting techniques.