The Hanover Insurance Group, Inc. (THG) 10-Q 31 Oct 24 2024 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2024	Oct. 29, 2024
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2024
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q3
Entity Registrant Name	HANOVER INSURANCE GROUP, INC.
Entity Central Index Key	0000944695
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		36,033,351
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity File Number	1-13754
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	04-3263626
Entity Address, Address Line One	440 Lincoln Street
Entity Address, City or Town	Worcester
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	01653
City Area Code	508
Local Phone Number	855-1000
Document Quarterly Report	true
Document Transition Report	false
Entity Interactive Data Current	Yes
Common Stock, $.01 Par Value [Member]
Document Information [Line Items]
Title of 12(b) Security	Common Stock, $.01 par value
Trading Symbol	THG
Security Exchange Name	NYSE
7 5/8% Senior Debentures Due 2025 [Member]
Document Information [Line Items]
Title of 12(b) Security	7 5/8% Senior Debentures due 2025
Trading Symbol	THG
Security Exchange Name	NYSE

Consolidated Statements of Inco

Consolidated Statements of Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues
Premiums	$ 1,479.2	$ 1,431.1	$ 4,401	$ 4,222.8
Net investment income	91.8	84.2	271.9	250.5
Net realized and unrealized investment gains (losses):
Net realized losses from sales and other	(23.5)	(0.9)	(55.2)	(1.9)
Net change in fair value of equity securities	11.7	(5.2)	19.3	(13.4)
Impairments on investments:
Credit-related impairments	(0.1)		(3.3)	(6.2)
Losses on intent to sell securities	(0.5)		(2.2)	(10.3)
Impairments on investments	(0.6)		(5.5)	(16.5)
Total net realized and unrealized investment losses	(12.4)	(6.1)	(41.4)	(31.8)
Fees and other income	6.7	7.4	21.6	23.2
Total revenues	1,565.3	1,516.6	4,653.1	4,464.7
Losses and expenses
Losses and loss adjustment expenses	954	1,061.5	2,896.8	3,218.8
Amortization of deferred acquisition costs	307.8	296.6	910.3	878.1
Interest expense	8.5	8.5	25.6	25.6
Other operating expenses	165.3	150.9	494.1	451.3
Total losses and expenses	1,435.6	1,517.5	4,326.8	4,573.8
Income (loss) from continuing operations before income taxes	129.7	(0.9)	326.3	(109.1)
Income tax expense (benefit):
Current	36.7	(1.1)	82.4	(13.7)
Deferred	(9.1)	(8)	(14.1)	(21.6)
Total income tax expense (benefit)	27.6	(9.1)	68.3	(35.3)
Income (loss) from continuing operations	102.1	8.2	258	(73.8)
Discontinued operations (net of taxes):
Net income (loss)	$ 102.1	$ 8.6	$ 258.1	$ (72.6)
Basic:
Income (loss) from continuing operations	$ 2.84	$ 0.23	$ 7.18	$ (2.07)
Net income (loss) per share	$ 2.84	$ 0.24	$ 7.18	$ (2.03)
Weighted average shares outstanding	36	35.8	35.9	35.7
Diluted:
Income (loss) from continuing operations	$ 2.80	$ 0.23	$ 7.09	$ (2.07)
Net income (loss) per share	$ 2.80	$ 0.24	$ 7.10	$ (2.03)
Weighted average shares outstanding	36.5	36.1	36.4	35.7
Life [Member]
Discontinued operations (net of taxes):
Income from discontinued life businesses			$ 0.1
Basic:
Income from discontinued life businesses	$ 0	$ 0	$ 0	$ 0
Diluted:
Income from discontinued life businesses			$ 0.01
Chaucer [Member]
Discontinued operations (net of taxes):
Income from discontinued life businesses		$ 0.4	$ 0	$ 1.2
Basic:
Income from discontinued life businesses		$ 0.01		$ 0.04
Diluted:
Income from discontinued life businesses		$ 0.01		$ 0.04

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 102.1	$ 8.6	$ 258.1	$ (72.6)
Changes in net unrealized gains (losses) on investment securities:
Having no credit losses recognized in the Consolidated Statements of Income (Loss)	242.4	(113.7)	212.3	(87)
Having credit losses recognized in the Consolidated Statements of Income (Loss)	0.6	(0.5)	2.6	0.9
Total available-for-sale securities	243	(114.2)	214.9	(86.1)
Pension and postretirement benefits:
Net change in net actuarial loss	1.4	1.5	4.1	4.7
Long-duration insurance contracts:
Net change in market risk	(3.1)	3.3	(1.3)	2.6
Total other comprehensive income (loss), net of tax	241.3	(109.4)	217.7	(78.8)
Comprehensive income (loss)	$ 343.4	$ (100.8)	$ 475.8	$ (151.4)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Assets
Fixed maturities, at fair value (amortized cost of $8,936.5 and $8,573.9)	$ 8,620	$ 7,985.3
Equity securities, at fair value	150.1	130.9
Other investments	760.6	796.9
Total investments	9,530.7	8,913.1
Cash and cash equivalents	427.1	316.1
Accrued investment income	64.2	58.2
Premiums and accounts receivable, net	1,881.5	1,705.6
Reinsurance recoverable on paid and unpaid losses and unearned premiums	1,960.9	2,056.1
Deferred acquisition costs	662.9	620.8
Deferred income tax asset	128.1	173.3
Goodwill	178.8	178.8
Other assets	443.7	504
Assets of discontinued businesses	89.1	86.6
Total assets	15,367	14,612.6
Liabilities
Loss and loss adjustment expense reserves	7,426.8	7,308.1
Unearned premiums	3,339.2	3,102.5
Expenses and taxes payable	775.1	775.9
Reinsurance premiums payable	50.9	64.3
Debt	783.9	783.2
Liabilities of discontinued businesses	113.4	113
Total liabilities	12,489.3	12,147
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	1,962.9	1,939.2
Accumulated other comprehensive loss	(299.5)	(517.2)
Retained earnings	3,074.5	2,909.4
Treasury stock at cost (24.5 million and 24.7 million shares)	(1,860.8)	(1,866.4)
Total shareholders’ equity	2,877.7	2,465.6
Total liabilities and shareholders’ equity	$ 15,367	$ 14,612.6

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 8,936.5	$ 8,573.9
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	24,500,000	24,700,000

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]	Long Duration Insurance Contracts [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net of Tax [Member]	Treasury Stock [Member]
Balance at beginning of period at Dec. 31, 2022		$ 0.6	$ 1,913.1	$ (641.4)	$ (64.1)	$ 4	$ 2,992.9		$ (1,871.4)
Net income (loss)	$ (72.6)						(72.6)
Employee and director stock-based awards and other			18.3
Net appreciation (depreciation) on available-for-sale securities	(86.1)			(86.1)
Net amount recognized as net periodic benefit cost					4.7
Net change in market risk						2.6
Dividends to shareholders							(88)
Net shares reissued at cost under employee stock-based compensation plans									3.7
Balance at end of period at Sep. 30, 2023	2,116.3		1,931.4	(727.5)	(59.4)	6.6	2,832.3	$ (780.3)	(1,867.7)
Balance at beginning of period at Jun. 30, 2023		0.6	1,923	(613.3)	(60.9)	3.3	2,853		(1,867.8)
Net income (loss)	8.6						8.6
Employee and director stock-based awards and other			8.4
Net appreciation (depreciation) on available-for-sale securities	(114.2)			(114.2)
Net amount recognized as net periodic benefit cost					1.5
Net change in market risk						3.3
Dividends to shareholders							(29.3)
Net shares reissued at cost under employee stock-based compensation plans									0.1
Balance at end of period at Sep. 30, 2023	2,116.3		1,931.4	(727.5)	(59.4)	6.6	2,832.3	(780.3)	(1,867.7)
Balance at beginning of period at Dec. 31, 2023	2,465.6	0.6	1,939.2	(462.8)	(54.8)	0.4	2,909.4		(1,866.4)
Balance at beginning of period at Dec. 31, 2023	(1,866.4)
Net income (loss)	258.1						258.1
Employee and director stock-based awards and other			23.7
Net appreciation (depreciation) on available-for-sale securities	214.9			214.9
Net amount recognized as net periodic benefit cost					4.1
Net change in market risk						(1.3)
Dividends to shareholders							(93)
Net shares reissued at cost under employee stock-based compensation plans									5.6
Balance at end of period at Sep. 30, 2024	2,877.7		1,962.9	(247.9)	(50.7)	(0.9)	3,074.5	(299.5)	(1,860.8)
Balance at end of period at Sep. 30, 2024	(1,860.8)
Balance at beginning of period at Jun. 30, 2024		$ 0.6	1,951.9	(490.9)	(52.1)	2.2	3,003.4		(1,862.9)
Net income (loss)	102.1						102.1
Employee and director stock-based awards and other			11
Net appreciation (depreciation) on available-for-sale securities	243			243
Net amount recognized as net periodic benefit cost					1.4
Net change in market risk						(3.1)
Dividends to shareholders							(31)
Net shares reissued at cost under employee stock-based compensation plans									2.1
Balance at end of period at Sep. 30, 2024	2,877.7		$ 1,962.9	$ (247.9)	$ (50.7)	$ (0.9)	$ 3,074.5	$ (299.5)	$ (1,860.8)
Balance at end of period at Sep. 30, 2024	$ (1,860.8)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Cash Flows From Operating Activities
Net income (loss)	$ 258.1	$ (72.6)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Net realized and unrealized investment losses	41.7	31.6
Net amortization and depreciation	2.1	5.5
Stock-based compensation expense	23.5	23.9
Amortization of defined benefit plan costs	5.1	5.9
Deferred income tax benefit	(14.3)	(21.5)
Change in deferred acquisition costs	(42.1)	(26.6)
Change in premiums receivable, net of reinsurance premiums payable	(189.2)	(176.6)
Change in loss, loss adjustment expense and unearned premium reserves	353.8	550.5
Change in reinsurance recoverable	95.2	(27.7)
Change in expenses and taxes payable	(13.7)	(9.6)
Other, net	72.7	(13.8)
Net cash provided by operating activities	592.9	269
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities	1,750.3	503.4
Proceeds from disposals of equity securities and other investments	61.3	121.5
Purchase of fixed maturities	(2,137.1)	(744)
Purchase of equity securities and other investments	(58.1)	(57.6)
Capital expenditures	(7.2)	(9.7)
Net cash used in investing activities	(390.8)	(186.4)
Cash Flows From Financing Activities
Proceeds from exercise of employee stock options	11.5	4.3
Dividends paid to shareholders	(91.7)	(86.9)
Other financing activities	(10.5)	(10.5)
Net cash used in financing activities	(90.7)	(93.1)
Net change in cash and cash equivalents	111.4	(10.5)
Net change in cash related to discontinued operations	(0.4)
Cash and cash equivalents, beginning of period	316.1	305
Cash and cash equivalents, end of period	$ 427.1	$ 294.5

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Pay vs Performance Disclosure
Net Income (Loss)	$ 102.1	$ 8.6	$ 258.1	$ (72.6)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Sep. 30, 2024
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false
Rule 10b5-1 Arrangement Modified	false
Non-Rule 10b5-1 Arrangement Modified	false

Basis of Presentation and Princ

Basis of Presentation and Principles of Consolidation	9 Months Ended
	Sep. 30, 2024
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	1. Basis of Presentation and Principles of Consolidation The accompanying unaudited consolidated financial statements of The Hanover Insurance Group, Inc. and its subsidiaries (“THG” or the “Company”) have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information and with the requirements of Form 10-Q. Certain financial information that is provided in annual financial statements, but is not required in interim reports, has been omitted. The interim consolidated financial statements of THG include the accounts of The Hanover Insurance Company and Citizens Insurance Company of America, THG’s principal property and casualty companies, and other insurance and non-insurance subsidiaries. These legal entities conduct their operations through several business segments discussed in Note 8 – “Segment Information.” All intercompany accounts and transactions have been eliminated. The preparation of financial statements in conformity with U.S. GAAP requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amount of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of the Company’s management, the accompanying interim consolidated financial statements reflect all adjustments, consisting of normal recurring items, necessary for a fair presentation of the financial position and results of operations. The results of operations for the nine months ended September 30, 2024 are not necessarily indicative of the results to be expected for the full year. These financial statements should be read in conjunction with the Company’s 2023 Annual Report on Form 10-K filed with the Securities and Exchange Commission (“SEC”) on February 22, 2024.

New Accounting Pronouncements

New Accounting Pronouncements	9 Months Ended
	Sep. 30, 2024
Accounting Standards Update and Change in Accounting Principle [Abstract]
New Accounting Pronouncements	2. New Accounting Pronouncements Recently Issued Standards In December 2023, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) Update No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures . This Update requires entities to disclose an annual tabular rate reconciliation, using both percentages and currency amounts, broken out into specific categories, to the extent those items exceed a specified threshold. In addition, all entities are required to disclose annual income taxes paid, net of refunds received, disaggregated by federal, state, and foreign jurisdictions, and for individual jurisdictions when the amount is at least five percent of total income tax payments, net of refunds received. This Update is effective for annual reporting periods beginning after December 15, 2024, with early adoption permitted. This guidance may be implemented either on a prospective or retrospective basis. The Company does not expect implementation of this guidance to have a material effect on its financial position or results of operations, as the Update is disclosure related. In November 2023, the FASB issued ASC Update No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures . This Update requires entities to disclose significant segment expenses and other segment items on an annual and interim basis, and to provide in interim periods all disclosures about a reportable segment’s profit or loss and assets that are currently required annually. Additionally, it requires entities to disclose the title and position of the Chief Operating Decision Maker (“CODM”), and an explanation of how the CODM uses the reported measures of segment profit or loss. The Update does not change how entities identify operating segments, aggregate them, or apply the quantitative thresholds to determine reportable segments. This Update is effective for annual reporting periods beginning after December 15, 2023, and interim reporting periods beginning after December 15, 2024, with early adoption permitted. The guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company does not expect implementation of this guidance to have a material effect on its financial position or results of operations, as the Update is disclosure related.

Investments

Investments	9 Months Ended
	Sep. 30, 2024
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: September 30, 2024 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 616.8 $ — $ 616.8 $ 4.0 $ 44.0 $ 576.8 Foreign governments 1.7 — 1.7 0.1 — 1.8 Municipals 1,135.0 — 1,135.0 4.3 90.0 1,049.3 Corporates 4,047.3 ( 0.6 ) 4,046.7 46.1 118.9 3,973.9 Residential mortgage-backed 1,813.2 — 1,813.2 20.7 105.1 1,728.8 Commercial mortgage-backed 621.9 — 621.9 0.7 36.4 586.2 Other asset-backed 701.2 — 701.2 4.2 2.2 703.2 Total fixed maturities $ 8,937.1 $ ( 0.6 ) $ 8,936.5 $ 80.1 $ 396.6 $ 8,620.0 December 31, 2023 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 512.9 $ — $ 512.9 $ 1.4 $ 51.7 $ 462.6 Foreign governments 2.2 — 2.2 — — 2.2 Municipals 1,309.3 — 1,309.3 5.5 129.6 1,185.2 Corporates 4,053.8 ( 1.9 ) 4,051.9 22.9 225.1 3,849.7 Residential mortgage-backed 1,435.6 — 1,435.6 5.5 130.2 1,310.9 Commercial mortgage-backed 890.3 — 890.3 0.1 72.2 818.2 Other asset-backed 371.7 — 371.7 0.7 15.9 356.5 Total fixed maturities $ 8,575.8 $ ( 1.9 ) $ 8,573.9 $ 36.1 $ 624.7 $ 7,985.3 The Company deposits funds with various state and governmental authorities. For a discussion of the Company’s deposits with state and governmental authorities, see also Note 2 – “Investments” in the Notes to Consolidated Financial Statements in the Company’s 2023 Annual Report on Form 10-K. The amortized cost and fair value by maturity periods for fixed maturities are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers. September 30, 2024 Amortized Cost, Net of Allowance for Fair (in millions) Credit Losses Value Due in one year or less $ 369.6 $ 367.8 Due after one year through five years 2,623.2 2,594.7 Due after five years through ten years 2,552.5 2,407.8 Due after ten years 254.9 231.5 5,800.2 5,601.8 Mortgage-backed and other asset-backed securities 3,136.3 3,018.2 Total fixed maturities $ 8,936.5 $ 8,620.0 B. Fixed maturity securities in an unrealized loss position The following tables provide information about the Company’s available-for-sale fixed maturity securities that were in an unrealized loss position at September 30, 2024 and December 31, 2023, including the length of time the securities have been in an unrealized loss position: September 30, 2024 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 0.3 $ 106.0 $ 43.7 $ 330.2 $ 44.0 $ 436.2 Municipals 0.2 39.8 89.8 848.0 90.0 887.8 Corporates 0.6 347.4 112.6 2,005.6 113.2 2,353.0 Residential mortgage-backed 0.1 78.1 105.0 808.1 105.1 886.2 Commercial mortgage-backed — — 36.4 545.4 36.4 545.4 Other asset-backed 0.2 60.8 2.0 126.1 2.2 186.9 Total investment grade 1.4 632.1 389.5 4,663.4 390.9 5,295.5 Below investment grade: Corporates 0.9 61.7 4.8 57.1 5.7 118.8 Commercial mortgage-backed — — — 0.9 — 0.9 Total below investment grade 0.9 61.7 4.8 58.0 5.7 119.7 Total fixed maturities $ 2.3 $ 693.8 $ 394.3 $ 4,721.4 $ 396.6 $ 5,415.2 December 31, 2023 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 0.6 $ 62.2 $ 51.1 $ 331.0 $ 51.7 $ 393.2 Foreign governments — — — 2.0 — 2.0 Municipals 2.5 72.5 127.1 935.2 129.6 1,007.7 Corporates 1.3 159.9 214.9 2,870.2 216.2 3,030.1 Residential mortgage-backed 1.2 139.3 129.0 865.4 130.2 1,004.7 Commercial mortgage-backed 0.2 14.0 72.0 770.0 72.2 784.0 Other asset-backed 0.1 28.1 15.8 260.2 15.9 288.3 Total investment grade 5.9 476.0 609.9 6,034.0 615.8 6,510.0 Below investment grade: Corporates 2.5 33.8 6.4 66.2 8.9 100.0 Commercial mortgage-backed — — — 0.9 — 0.9 Total below investment grade 2.5 33.8 6.4 67.1 8.9 100.9 Total fixed maturities $ 8.4 $ 509.8 $ 616.3 $ 6,101.1 $ 624.7 $ 6,610.9 The Company views gross unrealized losses on fixed maturities as non-credit related and through its assessment of unrealized losses has determined that these securities will recover, allowing the Company to realize the anticipated long-term economic value. The Company currently does not intend to sell, nor does it expect to be required to sell these securities before recovery of their amortized cost. The Company employs a systematic methodology to evaluate declines in fair value below amortized cost for fixed maturity securities. In determining impairments, the Company evaluates several factors and circumstances, including the issuer’s overall financial condition; the issuer’s credit and financial strength ratings; the issuer’s financial performance, including earnings trends and asset quality; any specific events which may influence the operations of the issuer; the general outlook for market conditions in the industry or geographic region in which the issuer operates; and the degree to which the fair value of an issuer’s securities is below the Company’s amortized cost. The Company also considers any factors that might raise doubt about the issuer’s ability to make contractual payments as they come due and whether the Company expects to recover the entire amortized cost basis of the security. C. Proceeds from sales The proceeds from sales of available-for-sale fixed maturities and gross realized gains and gross realized losses on those sales were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 (in millions) Proceeds from sales $ 479.5 $ 16.9 $ 1,145.9 $ 103.5 Gross gains 0.5 — 2.4 0.9 Gross losses 25.8 1.3 60.1 3.3 D. Impairments (Recoveries) For the three months ended September 30, 2024 , the Company recognized net impairment losses of $ 0.6 million, consisting of $ 0.4 million on mortgage loans and $ 0.2 million on fixed maturities. For the nine months ended September 30, 2024 , the Company recognized net impairment losses of $ 5.5 million, consisting of $ 4.0 million on mortgage loans and $ 1.5 million on fixed maturities. For the three months ended September 30, 2023 , impairments were not material. For the nine months ended September 30, 2023, the Company recognized net impairments of $ 16.5 million, consisting primarily of losses on intent to sell fixed maturities of $ 10.3 million and $ 5.4 million of estimated credit losses on mortgage loans. At September 30, 2024 and December 31, 2023 , the allowance for credit losses on mortgage loans was $ 7.5 million and $ 10.0 million respectively, and the allowance for credit losses on available-for-sale debt securities was $ 0.6 million and $ 1.9 million, respectively. The following table provides a rollforward of the allowance for credit losses on mortgage loans: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2024 2023 2024 2023 Allowance for credit losses as of the beginning of the period $ 8.6 $ 8.5 $ 10.0 $ 3.2 Reductions for writedowns — — ( 3.0 ) — Reductions for disposals ( 1.3 ) — ( 1.6 ) — Additional credit losses on investments for which an allowance 0.2 — 2.1 5.4 Recoveries — — — ( 0.1 ) Allowance for credit losses as of the end of the period $ 7.5 $ 8.5 $ 7.5 $ 8.5 The methodology and significant inputs used to measure the amount of credit losses were as follows: Mortgage loans – the Company estimated losses by applying expected loss rates, which are based on historical data. Embedded in expected loss rates are mortgage risk ratings and risk factors associated with property type such as office, retail, lodging, multi-family and industrial. Risk ratings, based on property characteristics and metrics including the geographic market, are predominantly driven by estimates of loan-to-value and debt service coverage ratios. Ratings may be adjusted to reflect current conditions and to incorporate reasonable and supportable forecasts, such as volatility of cash flows and valuation. Fixed maturities, Corporate bonds – the Company utilized a financial model that derives expected cash flows based on probability-of-default factors by credit rating and asset duration, and loss-given-default factors based on security type. These factors are based on historical data provided by an independent third-party rating agency. In addition, other qualitative market data relevant to the realizability of contractual cash flows may be considered, including current conditions and reasonable and supportable forecasts. E. Equity securities The following table provides pre-tax net unrealized gains (losses) on equity securities. There were no realized gains or losses from sales of equities for the three or nine months ended September 30, 2024. Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2024 2023 2024 2023 Net gains (losses) recognized during the period $ 11.7 $ ( 5.2 ) $ 19.3 $ ( 13.4 ) Less: net losses recognized on equity securities sold during — ( 11.9 ) — ( 11.9 ) Net unrealized gains (losses) recognized during the period $ 11.7 $ 6.7 $ 19.3 $ ( 1.5 )

Fair Value

Fair Value	9 Months Ended
	Sep. 30, 2024
Fair Value Disclosures [Abstract]
Fair Value	4. Fair Value Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability, i.e., exit price, in an orderly transaction between market participants. The Company emphasizes the use of observable market data whenever available in determining fair value. Fair values presented for certain financial instruments are estimates which, in many cases, may differ significantly from the amounts that could be realized upon immediate liquidation. A hierarchy of the three broad levels of fair value is as follows, with the highest priority given to Level 1 as these are the most observable, and the lowest priority given to Level 3: Level 1 – Unadjusted quoted prices in active markets for identical assets or liabilities. Level 2 – Quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data, including model-derived valuations. Level 3 – Unobservable inputs that are supported by little or no market activity. When more than one level of input is used to determine fair value, the financial instrument is classified as Level 2 or Level 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 third-party pricing services 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 services prepare 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 governments – estimates of appropriate market spread versus underlying related sovereign treasury curve(s) dependent on liquidity and direct or contingent support. • Municipals – overall credit quality, including assessments of the level and variability of: sources of payment such as income, sales or property taxes, levies or user fees; credit support such as insurance; state or local economic and political base; natural resource availability; and susceptibility to natural or man-made catastrophic events such as hurricanes, earthquakes or acts of terrorism. • Corporate fixed maturities – overall credit quality, including assessments of the level and variability of: economic sensitivity; liquidity; corporate financial policies; management quality; regulatory environment; competitive position; ownership; restrictive covenants; and security or collateral. • Residential mortgage-backed securities – estimates of prepayment speeds based upon: historical prepayment rate trends; underlying collateral interest rates; geographic concentration; vintage year; borrower credit quality characteristics; interest rate and yield curve forecasts; government or monetary authority support programs; tax policies; and delinquency/default trends; 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. • Other asset-backed securities – overall credit quality, including assessments of the underlying collateral type such as corporate loans, credit card receivables, automobile loan receivables and equipment lease receivables; geographic diversification; vintage year; historical collateral performance including delinquency, default and casualty trends; economic conditions influencing use rates and resale values; and contract structural support features. Generally, all prices provided by the pricing services, 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 services cannot provide fair values. The Company determines fair values for these securities using either matrix pricing, which utilizes the market approach, or broker quotes. The Company will use observable market data as inputs into the fair value techniques, as discussed in the determination of Level 2 fair values, to the extent it is available, but is also required to use a certain amount of unobservable judgment due to the illiquid nature of the securities involved. Unobservable judgment reflected in the Company’s matrix model accounts for estimates of additional spread required by market participants for factors such as issue size, credit stress, structural complexity, high bond coupon, or other unique features. These matrix-priced securities are reported as Level 2 or Level 3, depending on the significance of the impact of unobservable judgment on the security’s value. Additionally, the Company may obtain non-binding broker quotes, which are reported as Level 3. Equity Securities Level 1 consists of publicly traded securities, including exchange-traded funds, valued at quoted market prices. Level 2 includes securities that are valued using pricing for similar securities and pricing models that incorporate observable inputs. Level 3 consists of common or preferred stock of private companies for which observable inputs are not available. The Company utilizes a third-party pricing service for the valuation of the majority of its equity securities and receives one quote for each equity security. When quoted market prices in an active market are available, they are provided by the pricing service as the fair value and such values are classified as Level 1. The Company holds certain equity securities that have been issued by privately-held entities that do not have an active market and for which the pricing service cannot provide fair values. The Company estimates fair value for these securities based on prices from recent financing rounds, which may be adjusted for liquidity and other factors, or 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 limited partnerships not subject to the equity method of accounting and mortgage participations. The fair values of limited partnerships not subject to the equity method of accounting are based on the net asset value (“NAV”) provided by the general partner, adjusted for recent financial information, and are excluded from the fair value hierarchy. The estimated fair values of the financial instruments were as follows: September 30, 2024 December 31, 2023 Carrying Market Carrying Market (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 8,620.0 $ 8,620.0 $ 7,985.3 $ 7,985.3 Fair Value through Net Income: Equity securities 150.1 150.1 130.9 130.9 Other investments 101.2 101.2 118.4 118.4 Amortized Cost/Cost: Other investments 374.3 360.0 413.1 393.5 Cash and cash equivalents 427.1 427.1 316.1 316.1 Total financial instruments $ 9,672.7 $ 9,658.4 $ 8,963.8 $ 8,944.2 Financial Liabilities carried at: Amortized Cost: Debt $ 783.9 $ 752.5 $ 783.2 $ 727.5 The Company has processes designed to ensure that the values received from its third-party pricing services are accurately recorded, that the data inputs and valuation approaches and techniques utilized are appropriate and consistently applied, and that the assumptions are reasonable and consistent with the objective of determining fair value. The Company reviews the pricing services’ policies describing its methodology, processes, practices and inputs, including various financial models used to value securities. For assets carried at fair value, the Company performs a review of the fair value hierarchy classifications and of prices received from its pricing services on a quarterly basis. Also, the Company reviews the portfolio pricing, including a process for which securities with changes in prices that exceed a defined threshold are verified to independent sources, if available. If upon review, the Company is not satisfied with the validity of a given price, a pricing challenge would be submitted to the applicable pricing service along with supporting documentation for its review. The Company does not adjust quotes or prices obtained from the pricing services unless the pricing service agrees with the Company’s challenge. During the first nine months of 2024 and 2023, the Company did not adjust any prices received from its pricing services. Changes in the observability of valuation inputs may result in a reclassification of certain financial assets or liabilities within the fair value hierarchy. As previously discussed, the Company utilizes third party pricing services for the valuation of the majority of its fixed maturities and equity securities. The pricing services have indicated that they will only produce an estimate of fair value if there is objectively verifiable information to produce a valuation. If a pricing service discontinues pricing an investment, the Company will use observable market data to the extent it is available, but may also be required to make assumptions for market-based inputs that are unavailable due to market conditions. The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis. September 30, 2024 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 576.8 $ 436.2 $ 140.6 $ — Foreign governments 1.8 — 1.8 — Municipals 1,049.3 — 1,041.3 8.0 Corporates 3,973.9 — 3,973.9 — Residential mortgage-backed, U.S. agency backed 1,478.4 — 1,478.4 — Residential mortgage-backed, non-agency 250.4 — 250.4 — Commercial mortgage-backed 586.2 — 580.0 6.2 Other asset-backed 703.2 — 703.2 — Total fixed maturities 8,620.0 436.2 8,169.6 14.2 Equity securities 150.1 142.3 — 7.8 Other investments 3.8 — — 3.8 Total investment assets at fair value $ 8,773.9 $ 578.5 $ 8,169.6 $ 25.8 December 31, 2023 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 462.6 $ 324.3 $ 138.3 $ — Foreign governments 2.2 — 2.2 — Municipals 1,185.2 — 1,176.6 8.6 Corporates 3,849.7 — 3,849.6 0.1 Residential mortgage-backed, U.S. agency backed 1,168.7 — 1,168.7 — Residential mortgage-backed, non-agency 142.2 — 142.2 — Commercial mortgage-backed 818.2 — 811.2 7.0 Other asset-backed 356.5 — 356.5 — Total fixed maturities 7,985.3 324.3 7,645.3 15.7 Equity securities 130.9 123.3 — 7.6 Other investments 3.8 — — 3.8 Total investment assets at fair value $ 8,120.0 $ 447.6 $ 7,645.3 $ 27.1 Limited partnerships measured at fair value using the NAV based on an ownership interest in partners’ capital have not been included in the hierarchy tables. At September 30, 2024 and December 31, 2023 , the fair values of these investments were $ 97.4 million and $ 114.6 million, respectively, approximately 1 % of total investment assets. The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value: September 30, 2024 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 427.1 $ 427.1 $ — $ — Other investments 360.0 — 6.1 353.9 Total financial instruments $ 787.1 $ 427.1 $ 6.1 $ 353.9 Liabilities: Debt $ 752.5 $ — $ 752.5 $ — December 31, 2023 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 316.1 $ 316.1 $ — $ — Other investments 393.5 — 6.1 387.4 Total financial instruments $ 709.6 $ 316.1 $ 6.1 $ 387.4 Liabilities: Debt $ 727.5 $ — $ 727.5 $ — The following tables provide a reconciliation for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3). Fixed Maturities (in millions) Municipals Corporates Commercial Total Equities and Total Three Months Ended September 30, 2024 Balance July 1, 2024 $ 7.9 $ — $ 6.7 $ 14.6 $ 11.5 $ 26.1 Total gains (losses): Included in net realized and unrealized — — — — 0.1 0.1 Included in other comprehensive income (loss) - changes in 0.3 — ( 0.4 ) ( 0.1 ) — ( 0.1 ) Settlements ( 0.2 ) — ( 0.1 ) ( 0.3 ) — ( 0.3 ) Balance September 30, 2024 $ 8.0 $ — $ 6.2 $ 14.2 $ 11.6 $ 25.8 Changes in net unrealized gains (losses) for the period $ 0.3 $ — $ ( 0.4 ) $ ( 0.1 ) $ — $ ( 0.1 ) Three Months Ended September 30, 2023 Balance July 1, 2023 $ 8.8 $ 0.1 $ 7.2 $ 16.1 $ 11.2 $ 27.3 Total losses: Included in other comprehensive income (loss) - changes in ( 0.2 ) — ( 0.1 ) ( 0.3 ) — ( 0.3 ) Settlements ( 0.1 ) — ( 0.2 ) ( 0.3 ) — ( 0.3 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.2 ) $ — $ ( 0.1 ) $ ( 0.3 ) $ — $ ( 0.3 ) Fixed Maturities (in millions) Municipals Corporates Commercial Total Equities and Total Nine Months Ended September 30, 2024 Balance January 1, 2024 $ 8.6 $ 0.1 $ 7.0 $ 15.7 $ 11.4 $ 27.1 Transfers out of Level 3 — ( 0.1 ) — ( 0.1 ) — ( 0.1 ) Total gains (losses): Included in net realized and unrealized — — — — 0.2 0.2 Included in other comprehensive income (loss) - changes 0.2 — ( 0.4 ) ( 0.2 ) — ( 0.2 ) Settlements ( 0.8 ) — ( 0.4 ) ( 1.2 ) — ( 1.2 ) Balance September 30, 2024 $ 8.0 $ — $ 6.2 $ 14.2 $ 11.6 $ 25.8 Changes in net unrealized gains (losses) for the period $ 0.2 $ — $ ( 0.4 ) $ ( 0.2 ) $ — $ ( 0.2 ) Nine Months Ended September 30, 2023 Balance January 1, 2023 $ 9.3 $ 0.1 $ 7.5 $ 16.9 $ 11.1 $ 28.0 Total gains (losses): Included in net realized and unrealized — — — — 0.1 0.1 Included in other comprehensive income (loss) - changes ( 0.1 ) — ( 0.1 ) ( 0.2 ) — ( 0.2 ) Settlements ( 0.7 ) — ( 0.5 ) ( 1.2 ) — ( 1.2 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.1 ) $ — $ ( 0.1 ) $ ( 0.2 ) $ — $ ( 0.2 ) There were no transfers between Level 2 and Level 3 for the three months ended September 30, 2024 and 2023. During the nine months ended September 30, 2024, a fixed maturity security was transferred from Level 3 to Level 2 because it was valued by a pricing service with observable inputs rather than being valued by the Company’s internal matrix model. There were no transfers between Level 2 and Level 3 for the nine months ended September 30, 2023. The Company held no Level 3 liabilities for the nine months ended September 30, 2024 and 2023. The following table provides quantitative information about the significant unobservable inputs used by the Company in the fair value measurements of Level 3 assets. September 30, 2024 December 31, 2023 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipals Discounted Discount for: $ 8.0 6.1 - 6.8 % ( 6.7 %) $ 8.6 6.1 - 6.8 % ( 6.7 %) Corporates Discounted Discount for: — — 0.1 2.5 % ( 2.5 %) 0.3 % ( 0.3 %) Commercial Discounted Discount for: 6.2 3.0 - 5.0 % ( 4.7 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) 7.0 3.0 - 3.1 % ( 3.0 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) Equity securities Market Net tangible asset 1.4 N/A 1.2 N/A Internal price based on financing round Discount for: 6.4 27.0 % ( 27.0 %) 6.4 27.0 % ( 27.0 %) Other Discounted Discount rate 3.8 16.8 % ( 16.8 %) 3.8 16.8 % ( 16.8 %) The weighted average of the unobservable inputs was weighted by the relative fair value of the securities to which the inputs were applied. Each unobservable input is based on the Company’s subjective opinion and therefore inherently contains a degree of uncertainty. 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. Increases (decreases) in any of these inputs in isolation would result in a lower (higher) fair value measurement. Increases (decreases) in unobservable inputs used in the valuation of equity securities would result in a higher (lower) 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.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2024
Income Tax Disclosure [Abstract]
Income Taxes	5. Income Taxes Income tax expense (benefit) for the nine months ended September 30, 2024 and 2023 has been computed using estimated annual effective tax rates. These rates are revised, if necessary, at the end of each successive interim period to reflect current estimates of the annual effective tax rates. The tax provision was comprised of a U.S. federal income tax expense of $ 68.3 million and a benefit of $ 35.3 million for the nine months ended September 30, 2024 and 2023, respectively. The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction and various state jurisdictions, and have previously filed in foreign jurisdictions. The Company and its subsidiaries are subject to U.S. federal and state income tax examinations and foreign examinations for years after 2019.

Pension Plans

Pension Plans	9 Months Ended
	Sep. 30, 2024
Retirement Benefits [Abstract]
Pension Plans	6. Pension Plans The components of net periodic pension cost for the defined benefit pension plans included in the Company’s results of operations are as follows: Three Months Ended September 30, 2024 2023 (in millions) Pension Plans Interest cost $ 5.1 $ 5.5 Expected return on plan assets ( 5.1 ) ( 5.5 ) Recognized net actuarial loss 1.7 1.9 Net periodic pension cost $ 1.7 $ 1.9 Nine Months Ended September 30, 2024 2023 (in millions) Pension Plans Interest cost $ 15.3 $ 16.4 Expected return on plan assets ( 15.2 ) ( 16.5 ) Recognized net actuarial loss 5.0 5.8 Net periodic pension cost $ 5.1 $ 5.7

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2024
Equity [Abstract]
Other Comprehensive Income (Loss)	7. Other Comprehensive Income (Loss) The following tables provide changes in other comprehensive income (loss). Three Months Ended September 30, 2024 2023 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) Net unrealized gains (losses) arising during period $ 282.3 $ ( 59.3 ) $ 223.0 $ ( 144.3 ) $ 30.3 $ ( 114.0 ) Net unrealized gains (losses) arising during period 1.0 ( 0.1 ) 0.9 ( 0.7 ) 0.2 ( 0.5 ) Amount of losses realized from sales and 24.8 ( 5.8 ) 19.0 0.9 ( 0.6 ) 0.3 Amount of credit-related recoveries on prior ( 0.3 ) — ( 0.3 ) — — — Amount of additional impairment losses 0.5 ( 0.1 ) 0.4 — — — Net unrealized gains (losses) 308.3 ( 65.3 ) 243.0 ( 144.1 ) 29.9 ( 114.2 ) Pension and postretirement benefits: Amortization of net actuarial losses 1.7 ( 0.3 ) 1.4 1.9 ( 0.4 ) 1.5 Long-duration insurance contracts: Net change in market risk ( 4.0 ) 0.9 ( 3.1 ) 4.2 ( 0.9 ) 3.3 Other comprehensive income (loss) $ 306.0 $ ( 64.7 ) $ 241.3 $ ( 138.0 ) $ 28.6 $ ( 109.4 ) Nine Months Ended September 30, 2024 2023 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on Net unrealized gains (losses) arising during period $ 211.2 $ ( 44.4 ) $ 166.8 $ ( 120.8 ) $ 25.4 $ ( 95.4 ) Net unrealized gains arising during period for 4.0 ( 0.8 ) 3.2 0.3 — 0.3 Amount of losses realized from sales and 56.9 ( 13.1 ) 43.8 1.9 ( 1.6 ) 0.3 Amount of credit-related impairments (recoveries) ( 0.7 ) 0.1 ( 0.6 ) 0.8 ( 0.2 ) 0.6 Amount of additional impairment losses 2.2 ( 0.5 ) 1.7 10.3 ( 2.2 ) 8.1 Net unrealized gains (losses) 273.6 ( 58.7 ) 214.9 ( 107.5 ) 21.4 ( 86.1 ) Pension and postretirement benefits: Amortization of net actuarial losses 5.1 ( 1.0 ) 4.1 5.9 ( 1.2 ) 4.7 Long-duration insurance contracts: Net change in market risk ( 1.7 ) 0.4 ( 1.3 ) 3.3 ( 0.7 ) 2.6 Other comprehensive income (loss) $ 277.0 $ ( 59.3 ) $ 217.7 $ ( 98.3 ) $ 19.5 $ ( 78.8 ) Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2024 2023 2024 2023 Amount Reclassified from Details about Accumulated Other Accumulated Other Affected Line Item in the Statement Comprehensive Loss Components Comprehensive Loss Where Net Income (Loss) is Presented Net unrealized losses on investment $ ( 24.7 ) $ ( 0.9 ) $ ( 56.6 ) $ ( 1.9 ) Net realized losses from sales ( 0.2 ) — ( 1.5 ) ( 11.1 ) Impairments on investments ( 24.9 ) ( 0.9 ) ( 58.1 ) ( 13.0 ) Total before tax 5.9 0.6 13.5 4.0 Income tax (expense) benefit ( 19.0 ) ( 0.3 ) ( 44.6 ) ( 9.0 ) Continuing operations; net of tax ( 0.1 ) — ( 0.3 ) — Income from discontinued life ( 19.1 ) ( 0.3 ) ( 44.9 ) ( 9.0 ) Net of tax Amortization of defined benefit pension ( 1.7 ) ( 1.9 ) ( 5.1 ) ( 5.9 ) Loss adjustment expenses and other (1) 0.3 0.4 1.0 1.2 Income tax (expense) benefit ( 1.4 ) ( 1.5 ) ( 4.1 ) ( 4.7 ) Continuing operations; net of tax Net change in market risk 0.3 — 0.8 — Income from discontinued life ( 0.1 ) — ( 0.2 ) — Income tax (expense) benefit 0.2 — 0.6 — Discontinued operations; net of tax Total reclassifications for the period $ ( 20.3 ) $ ( 1.8 ) $ ( 48.4 ) $ ( 13.7 ) Benefit (expense) reflected in income (1) The amount reclassified from accumulated other comprehensive loss for the pension and postretirement benefits was allocated approximately 40 % to loss adjustment expenses and 60 % to other operating expenses for the three and nine months ended September 30, 2024 and 2023 .

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2024
Segment Reporting [Abstract]
Segment Information	8. Segment Information The Company’s primary business operations include insurance products and services provided through four reporting segments: Core Commercial, Specialty, Personal Lines and Other. Core Commercial includes commercial multiple peril, commercial automobile, workers’ compensation, and other commercial coverages provided to small and mid-sized businesses. Specialty includes four divisions of business: Professional and Executive Lines, Specialty Property and Casualty (“Specialty P&C”), Marine, and Surety and Other. Specialty P&C includes coverages such as program business, which provides commercial insurance to markets with specialized coverage or risk management needs related to groups of similar businesses, specialty industrial and commercial property, excess and surplus lines and specialty general liability coverage. Personal Lines includes personal automobile, homeowners and other personal coverages. The Other segment includes 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; run-off voluntary assumed property and casualty pools and run-off direct asbestos and environmental, and product liability businesses. Also included in the Other segment during the first half of 2024 and prior were the operations of Opus Investment Management, Inc. (“Opus”), which provided investment management services to THG, as well as institutions, pension funds, and other organizations. During the second and third quarters of 2024, the Company exited all of Opus’ business operations serving unaffiliated entities. Investment management services provided by Opus to THG related to its investment-grade fixed maturities portfolio were also transferred to an external manager during the second quarter of 2024. The separate financial information is presented consistent with the way results are regularly evaluated by the CODM in deciding how to allocate resources and in assessing performance. The Company reports interest expense related to debt separately from the earnings of its reporting segments. This consists primarily of interest on the Company’s senior and subordinated debentures. Management evaluates the results of its segments based on operating income (loss) before interest expense and income taxes. Operating income (loss) before interest expense and income taxes excludes certain items which are included in net income (loss), such as net realized and unrealized investment gains and losses. Such gains and losses are excluded since they are determined by interest rates, financial markets and the timing of sales. Also, operating income (loss) before interest expense and income taxes excludes net gains and losses on disposals of businesses, gains and losses related to the repayment of debt, discontinued operations, costs to acquire businesses, restructuring costs, the cumulative effect of accounting changes and certain other items. Although the items excluded from operating income (loss) before interest expense and income taxes may be important components in understanding and assessing the Company’s overall financial performance, management believes that the presentation of operating income (loss) before interest expense and income taxes enhances an investor’s understanding of the Company’s results of operations by highlighting net income (loss) attributable to the core operations of the business. However, operating income (loss) before income taxes should not be construed as a substitute for income (loss) before income taxes or income (loss) from continuing operations and operating income (loss) should not be construed as a substitute for net income (loss). Summarized below is financial information with respect to the Company’s reporting segments. Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2024 2023 2024 2023 Operating revenues: Core Commercial $ 576.6 $ 557.1 $ 1,727.6 $ 1,658.2 Specialty 353.7 340.9 1,049.0 1,011.6 Personal Lines 644.9 620.5 1,908.5 1,815.1 Other 2.5 4.2 9.4 11.6 Total 1,577.7 1,522.7 4,694.5 4,496.5 Net realized and unrealized investment losses ( 12.4 ) ( 6.1 ) ( 41.4 ) ( 31.8 ) Total revenues $ 1,565.3 $ 1,516.6 $ 4,653.1 $ 4,464.7 Operating income (loss) before interest expense Core Commercial: Underwriting income $ 14.6 $ 5.3 $ 88.3 $ 1.0 Net investment income 42.1 38.4 124.3 114.2 Other expense ( 0.8 ) ( 0.6 ) ( 2.0 ) ( 0.8 ) Core Commercial operating income 55.9 43.1 210.6 114.4 Specialty: Underwriting income 52.6 52.7 113.5 119.5 Net investment income 20.9 18.0 61.7 53.6 Other expense ( 0.5 ) ( 0.4 ) ( 0.8 ) ( 0.1 ) Specialty operating income 73.0 70.3 174.4 173.0 Personal Lines: Underwriting loss ( 6.7 ) ( 127.3 ) ( 74.4 ) ( 422.4 ) Net investment income 26.3 24.3 77.9 73.3 Other income 2.1 2.6 6.7 8.0 Personal Lines operating income (loss) 21.7 ( 100.4 ) 10.2 ( 341.1 ) Other: Underwriting income — — — — Net investment income 2.5 3.5 8.0 9.4 Other expense ( 2.5 ) ( 2.8 ) ( 7.5 ) ( 8.2 ) Other operating income — 0.7 0.5 1.2 Operating income (loss) before interest expense 150.6 13.7 395.7 ( 52.5 ) Interest on debt ( 8.5 ) ( 8.5 ) ( 25.6 ) ( 25.6 ) Operating income (loss) before income taxes 142.1 5.2 370.1 ( 78.1 ) Non-operating items: Net realized and unrealized investment losses ( 12.4 ) ( 6.1 ) ( 41.4 ) ( 31.8 ) Other non-operating — — ( 2.4 ) 0.8 Income (loss) from continuing operations before income taxes $ 129.7 $ ( 0.9 ) $ 326.3 $ ( 109.1 ) The following table provides identifiable assets for the Company’s segments and discontinued operations: (in millions) September 30, 2024 December 31, 2023 Property and Casualty $ 15,277.9 $ 14,526.0 Assets of discontinued businesses 89.1 86.6 Total $ 15,367.0 $ 14,612.6 The Company reviews the assets of its insurance subsidiaries collectively and does not allocate them among the Core Commercial, Specialty, Personal Lines and Other segments.

Stock-based Compensation

Stock-based Compensation	9 Months Ended
	Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]
Stock-based Compensation	9. Stock-based Compensation As of September 30, 2024 , there were 2,356,776 and 1,229,903 shares available for grant under The Hanover Insurance Group 2022 Long-Term Incentive Plan and 2023 Employee Stock Purchase plan, respectively. Compensation cost for the Company’s stock-based awards and the related tax benefits were as follows: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2024 2023 2024 2023 Stock-based compensation expense $ 8.7 $ 8.6 $ 23.5 $ 23.9 Tax benefit ( 1.8 ) ( 1.8 ) ( 4.9 ) ( 5.0 ) Stock-based compensation expense, net of taxes $ 6.9 $ 6.8 $ 18.6 $ 18.9 Stock Options Information on the Company’s stock option activity for the nine months ended September 30, 2024 and 2023 is summarized below. Nine Months Ended September 30, 2024 2023 (in whole shares and dollars) Shares Weighted Average Shares Weighted Average Outstanding, beginning of period 1,137,042 $ 111.57 1,080,852 $ 107.07 Granted 150,731 134.26 135,019 140.01 Exercised ( 131,891 ) 88.87 ( 40,544 ) 92.39 Forfeited or cancelled ( 2,642 ) 134.99 ( 22,184 ) 125.15 Outstanding, end of period 1,153,240 117.08 1,153,143 111.09 Restricted Stock Units The Company currently issues time-based, market-based and performance-based restricted stock units to eligible employees, all of which generally vest after three years of continued employment. The following table summarizes activity information about employee restricted stock units: Nine Months Ended September 30, 2024 2023 (in whole shares and dollars) Shares Weighted Shares Weighted Time-based restricted stock units: Outstanding, beginning of period 376,626 $ 132.04 378,256 $ 124.88 Granted 158,460 133.30 149,509 138.35 Vested ( 123,973 ) 116.25 ( 132,086 ) 118.21 Forfeited ( 21,705 ) 135.85 ( 19,132 ) 131.14 Outstanding, end of period 389,408 137.36 376,547 132.25 Performance-based and market-based restricted stock units: Outstanding, beginning of period 120,504 $ 132.76 119,163 $ 122.81 Granted 53,336 132.27 50,115 137.90 Vested ( 41,290 ) 112.36 ( 44,781 ) 114.14 Forfeited ( 6,339 ) 108.06 ( 3,993 ) 109.19 Outstanding, end of period 126,211 140.47 120,504 132.76 In the first nine months of 2024 and 2023 , the Company granted market-based awards totaling 25,414 shares and 21,789 shares, respectively, to certain members of senior management, which are included in the table above as performance and market-based restricted stock activity. The vesting of these stock units is based on the relative total shareholder return (“TSR”) of the Company. This metric is generally based on relative TSR for a three-year period as compared to a pre-selected group of property and casualty companies. The fair value of market-based awards was estimated at the date of grant using a valuation model. These units have the potential to range from 0 % to 150 % of the shares disclosed. Included in the amount forfeited above in 2024 and 2023 are 6,339 shares and 2,836 shares, respectively, related to market-based awards that achieved a payout below 100 %. These awards were forfeited in the first quarter of 2024 and 2023, respectively. The Company also granted performance-based restricted stock units in 2024 and 2023 , totaling 27,922 shares and 28,326 shares, respectively, which are based upon the Company’s achievement of return on equity objectives. These units have the potential to range from 0 % to 150 % of the shares disclosed. Increases above the 100% target level are reflected as granted in the period after which performance-based stock unit goals are achieved. Decreases below the 100% target level are reflected as forfeited. Included in the amounts granted above in 2024 and 2023 are 2,615 shares and 5,961 shares, respectively, related to performance-based awards that achieved a payout in excess of 100 %. These awards vested in the first quarter of 2024 and 2023 , respectively.

Earnings Per Share and Sharehol

Earnings Per Share and Shareholders' Equity Transactions	9 Months Ended
	Sep. 30, 2024
Earnings Per Share [Abstract]
Earnings Per Share and Shareholders' Equity Transactions	10. Earnings Per Share and Shareholders’ Equity Transactions The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: Three Months Ended Nine Months Ended September 30, September 30, (in millions, except per share data) 2024 2023 2024 2023 Basic shares used in the calculation of earnings per share 36.0 35.8 35.9 35.7 Dilutive effect of securities: Employee stock options 0.2 0.1 0.2 — Non-vested stock grants 0.3 0.2 0.3 — Diluted shares used in the calculation of earnings per share 36.5 36.1 36.4 35.7 Per share effect of dilutive securities on income (loss) from $ ( 0.04 ) $ — $ ( 0.09 ) $ — Per share effect of dilutive securities on net income (loss) $ ( 0.04 ) $ — $ ( 0.08 ) $ — Diluted earnings per share for both the three and nine months ended September 30, 2024 excludes 0.4 million shares of common stock issuable under the Company’s stock compensation plans, because their effect would be antidilutive. Diluted earnings per share for the three and nine months ended September 30, 2023 excludes 0.7 million and 0.6 million, respectively, shares of common stock issuable under the Company’s stock compensation plans, because their effect would be antidilutive. T he Board of Directors authorized a stock repurchase program which provides for aggregate repurchases of the Company’s common stock of up to $ 1.3 billion. Under this program, the Company had approximately $ 330 million available at September 30, 2024 . Under the repurchase authorization, the Company may repurchase, from time to time, common stock in amounts, at prices and at such times as the Company deems appropriate, subject to market conditions and other considerations. Repurchases may be executed using open market purchases, privately negotiated transactions, accelerated repurchase programs or other transactions. The Company is not required to purchase any specific number of shares or to make purchases by any certain date under this program.

Liabilities for Outstanding Cla

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	9 Months Ended
	Sep. 30, 2024
Insurance [Abstract]
Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses	11. Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses Reserve Rollforward and Prior Year Development The Company regularly updates its reserve estimates as new information becomes available and further events occur which may impact the resolution of unsettled claims. Reserve adjustments are reflected in results of operations as adjustments to losses and loss adjustment expenses (“LAE”). Often these adjustments are recognized in periods subsequent to the period in which the underlying policy was written and loss event occurred. These types of subsequent adjustments are described as loss and LAE “development.” Such development can be either favorable or unfavorable to the Company’s financial results and may vary by line of business. In this section, all amounts presented include catastrophe losses and LAE, unless otherwise indicated. The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. Nine Months Ended September 30, (in millions) 2024 2023 Gross reserve for losses and LAE, beginning of period $ 7,308.1 $ 7,012.6 Reinsurance recoverable on unpaid losses 1,795.0 1,748.6 Net reserve for losses and LAE, beginning of period 5,513.1 5,264.0 Net incurred losses and LAE in respect of losses occurring in: Current year 2,975.6 3,225.6 Prior years ( 78.8 ) ( 6.8 ) Total incurred losses and LAE 2,896.8 3,218.8 Net payments of losses and LAE in respect of losses occurring in: Current year 1,225.3 1,375.7 Prior years 1,517.5 1,523.8 Total payments 2,742.8 2,899.5 Net reserve for losses and LAE, end of period 5,667.1 5,583.3 Reinsurance recoverable on unpaid losses 1,759.7 1,746.5 Gross reserve for losses and LAE, end of period $ 7,426.8 $ 7,329.8 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 $ 78.8 million and $ 6.8 million in 2024 and 2023, respectively. 2024 For the nine months ended September 30, 2024 , net favorable loss and LAE development was $ 78.8 million, primarily as a result of net favorable development of $ 39.7 million in Core Commercial, $ 34.8 million in Specialty and $ 4.3 million in Personal Lines. The favorable development in Core Commercial was primarily due to favorable catastrophe development of $ 24.8 million, primarily due to lower than expected losses related to events from accident years 2020 through 2023, including several convective storms across multiple states, Winter Storm Elliot, Hurricane Ian, and Hurricane Ida. Additionally, and to a lesser extent, Core Commercial favorable development resulted from lower than expected non-catastrophe losses across each of the main product lines: commercial multiple peril, commercial automobile, workers’ compensation and other commercial lines. Within non-catastrophe losses, l ower than expected property losses of $ 25.6 million were partially offset by higher than expected losses of $ 10.7 million across several of the liability lines of business. The favorable development in Specialty was primarily due to lower than expected non-catastrophe losses in the Professional and Executive Lines division’s liability claims-made coverage of $ 14.1 million and, to a lesser extent, in the surety line. In addition, there was lower than expected catastrophe losses in our Marine and Specialty P&C divisions. The net favorable development in Personal Lines was primarily due to lower than expected non-catastrophe losses of $ 13.4 million in the personal automobile line within physical damage coverage. This favorable development was partially offset by higher than expected losses of $ 10.3 million in other personal lines within the standalone umbrella coverage. 2023 For the nine months ended September 30, 2023 , net favorable loss and LAE development was $ 6.8 million, primarily as a result of favorable development of $ 43.1 million in Specialty, partially offset by net unfavorable development of $ 35.1 million in Personal Lines and, to a lesser extent, net unfavorable development of $ 1.2 million in Core Commercial. The favorable development in Specialty was primarily due to lower than expected losses and LAE of $ 28.6 million in the Professional and Executive Lines division, primarily in accident years 2019 through 2022 and, to a lesser extent, lower than expected losses and LAE of $ 9.2 million in the surety line. The unfavorable development in Personal Lines was primarily due to higher than expected losses of $ 16.4 million within the personal automobile line, higher than expected losses of $ 11.4 million in the homeowners line, primarily in accident year 2022 due to Winter Storm Elliott and, to a lesser extent, higher than expected losses in the standalone personal umbrella coverage. The higher than expected losses in the personal automobile line were primarily within bodily injury in accident years 2018, 2019 and 2022, and within property damage coverages in accident years 2021 and 2022, partially offset by lower than expected losses within personal injury protection coverages in accident years 2021 through 2022. The net unfavorable development in Core Commercial was primarily due to higher than expected losses of $ 13.5 million within the commercial automobile line driven by higher bodily injury and personal injury protection losses in accident years 2014 through 2019 and, to a lesser extent, higher than expected losses in miscellaneous property and commercial umbrella coverages. The unfavorable development in Core Commercial was partially offset by lower than expected losses of $ 17.7 million within the workers’ compensation line, primarily in accident years 2013 through 2021, and within the commercial multiple peril line, primarily in accident year 2022, due to Winter Storm Elliott.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2024
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Legal Proceedings The Company has been named a defendant in various legal proceedings arising in the normal course of business. In addition, the Company is involved, from time to time, in examinations, investigations and proceedings by governmental and self-regulatory agencies. The potential outcome of any such action or regulatory proceedings in which the Company has been named a defendant or the subject of an inquiry, examination 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 the Company’s financial position, although they could have a material effect on the results of operations for a particular quarterly or annual period. Residual Markets The Company is required to participate in residual markets in various states, which generally pertain to high risk insureds, disrupted markets or lines of business or geographic areas where rates are regarded as excessive. The results of the residual markets are not subject to the predictability associated with the Company’s own managed business, and are significant to both the personal and commercial automobile lines of business.

Subsequent Events

Subsequent Events	9 Months Ended
	Sep. 30, 2024
Subsequent Events [Abstract]
Subsequent Events	13. Subsequent Events There were no subsequent events requiring adjustment to the financial statements and no additional disclosure required in the notes to the consolidated financial statements.

New Accounting Pronouncements (

New Accounting Pronouncements (Policies)	9 Months Ended
	Sep. 30, 2024
Accounting Standards Update and Change in Accounting Principle [Abstract]
New Accounting Pronouncements	New Accounting Pronouncements Recently Issued Standards In December 2023, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Codification (“ASC”) Update No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures . This Update requires entities to disclose an annual tabular rate reconciliation, using both percentages and currency amounts, broken out into specific categories, to the extent those items exceed a specified threshold. In addition, all entities are required to disclose annual income taxes paid, net of refunds received, disaggregated by federal, state, and foreign jurisdictions, and for individual jurisdictions when the amount is at least five percent of total income tax payments, net of refunds received. This Update is effective for annual reporting periods beginning after December 15, 2024, with early adoption permitted. This guidance may be implemented either on a prospective or retrospective basis. The Company does not expect implementation of this guidance to have a material effect on its financial position or results of operations, as the Update is disclosure related. In November 2023, the FASB issued ASC Update No. 2023-07, Segment Reporting (Topic 280): Improvements to Reportable Segment Disclosures . This Update requires entities to disclose significant segment expenses and other segment items on an annual and interim basis, and to provide in interim periods all disclosures about a reportable segment’s profit or loss and assets that are currently required annually. Additionally, it requires entities to disclose the title and position of the Chief Operating Decision Maker (“CODM”), and an explanation of how the CODM uses the reported measures of segment profit or loss. The Update does not change how entities identify operating segments, aggregate them, or apply the quantitative thresholds to determine reportable segments. This Update is effective for annual reporting periods beginning after December 15, 2023, and interim reporting periods beginning after December 15, 2024, with early adoption permitted. The guidance should be applied retrospectively to all prior periods presented in the financial statements. The Company does not expect implementation of this guidance to have a material effect on its financial position or results of operations, as the Update is disclosure related.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract]
Schedule of Available-for-sale Fixed Maturities Reconciliation	The amortized cost and fair value of available-for-sale fixed maturities were as follows: September 30, 2024 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 616.8 $ — $ 616.8 $ 4.0 $ 44.0 $ 576.8 Foreign governments 1.7 — 1.7 0.1 — 1.8 Municipals 1,135.0 — 1,135.0 4.3 90.0 1,049.3 Corporates 4,047.3 ( 0.6 ) 4,046.7 46.1 118.9 3,973.9 Residential mortgage-backed 1,813.2 — 1,813.2 20.7 105.1 1,728.8 Commercial mortgage-backed 621.9 — 621.9 0.7 36.4 586.2 Other asset-backed 701.2 — 701.2 4.2 2.2 703.2 Total fixed maturities $ 8,937.1 $ ( 0.6 ) $ 8,936.5 $ 80.1 $ 396.6 $ 8,620.0 December 31, 2023 Amortized Cost, Allowance Net of Allowance Gross Gross Amortized for Credit for Credit Unrealized Unrealized (in millions) Cost Losses Losses Gains Losses Fair Value U.S. Treasury and government agencies $ 512.9 $ — $ 512.9 $ 1.4 $ 51.7 $ 462.6 Foreign governments 2.2 — 2.2 — — 2.2 Municipals 1,309.3 — 1,309.3 5.5 129.6 1,185.2 Corporates 4,053.8 ( 1.9 ) 4,051.9 22.9 225.1 3,849.7 Residential mortgage-backed 1,435.6 — 1,435.6 5.5 130.2 1,310.9 Commercial mortgage-backed 890.3 — 890.3 0.1 72.2 818.2 Other asset-backed 371.7 — 371.7 0.7 15.9 356.5 Total fixed maturities $ 8,575.8 $ ( 1.9 ) $ 8,573.9 $ 36.1 $ 624.7 $ 7,985.3
Investments Classified by Contractual Maturity Date	The amortized cost and fair value by maturity periods for fixed maturities are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties, or the Company may have the right to put or sell the obligations back to the issuers. September 30, 2024 Amortized Cost, Net of Allowance for Fair (in millions) Credit Losses Value Due in one year or less $ 369.6 $ 367.8 Due after one year through five years 2,623.2 2,594.7 Due after five years through ten years 2,552.5 2,407.8 Due after ten years 254.9 231.5 5,800.2 5,601.8 Mortgage-backed and other asset-backed securities 3,136.3 3,018.2 Total fixed maturities $ 8,936.5 $ 8,620.0
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 September 30, 2024 and December 31, 2023, including the length of time the securities have been in an unrealized loss position: September 30, 2024 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 0.3 $ 106.0 $ 43.7 $ 330.2 $ 44.0 $ 436.2 Municipals 0.2 39.8 89.8 848.0 90.0 887.8 Corporates 0.6 347.4 112.6 2,005.6 113.2 2,353.0 Residential mortgage-backed 0.1 78.1 105.0 808.1 105.1 886.2 Commercial mortgage-backed — — 36.4 545.4 36.4 545.4 Other asset-backed 0.2 60.8 2.0 126.1 2.2 186.9 Total investment grade 1.4 632.1 389.5 4,663.4 390.9 5,295.5 Below investment grade: Corporates 0.9 61.7 4.8 57.1 5.7 118.8 Commercial mortgage-backed — — — 0.9 — 0.9 Total below investment grade 0.9 61.7 4.8 58.0 5.7 119.7 Total fixed maturities $ 2.3 $ 693.8 $ 394.3 $ 4,721.4 $ 396.6 $ 5,415.2 December 31, 2023 12 months or less Greater than 12 months Total Gross Gross Gross Unrealized Fair Unrealized Fair Unrealized Fair (in millions) Losses Value Losses Value Losses Value Investment grade: U.S. Treasury and government agencies $ 0.6 $ 62.2 $ 51.1 $ 331.0 $ 51.7 $ 393.2 Foreign governments — — — 2.0 — 2.0 Municipals 2.5 72.5 127.1 935.2 129.6 1,007.7 Corporates 1.3 159.9 214.9 2,870.2 216.2 3,030.1 Residential mortgage-backed 1.2 139.3 129.0 865.4 130.2 1,004.7 Commercial mortgage-backed 0.2 14.0 72.0 770.0 72.2 784.0 Other asset-backed 0.1 28.1 15.8 260.2 15.9 288.3 Total investment grade 5.9 476.0 609.9 6,034.0 615.8 6,510.0 Below investment grade: Corporates 2.5 33.8 6.4 66.2 8.9 100.0 Commercial mortgage-backed — — — 0.9 — 0.9 Total below investment grade 2.5 33.8 6.4 67.1 8.9 100.9 Total fixed maturities $ 8.4 $ 509.8 $ 616.3 $ 6,101.1 $ 624.7 $ 6,610.9
Schedule of Realized Gain (Loss)	The proceeds from sales of available-for-sale fixed maturities and gross realized gains and gross realized losses on those sales were as follows: Three Months Ended September 30, Nine Months Ended September 30, 2024 2023 2024 2023 (in millions) Proceeds from sales $ 479.5 $ 16.9 $ 1,145.9 $ 103.5 Gross gains 0.5 — 2.4 0.9 Gross losses 25.8 1.3 60.1 3.3
Schedule of Allowance for Credit Losses	The following table provides a rollforward of the allowance for credit losses on mortgage loans: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2024 2023 2024 2023 Allowance for credit losses as of the beginning of the period $ 8.6 $ 8.5 $ 10.0 $ 3.2 Reductions for writedowns — — ( 3.0 ) — Reductions for disposals ( 1.3 ) — ( 1.6 ) — Additional credit losses on investments for which an allowance 0.2 — 2.1 5.4 Recoveries — — — ( 0.1 ) Allowance for credit losses as of the end of the period $ 7.5 $ 8.5 $ 7.5 $ 8.5
Schedule of Pre-tax Net Realized and Unrealized Gains (Losses) on Equity Securities	The following table provides pre-tax net unrealized gains (losses) on equity securities. There were no realized gains or losses from sales of equities for the three or nine months ended September 30, 2024. Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2024 2023 2024 2023 Net gains (losses) recognized during the period $ 11.7 $ ( 5.2 ) $ 19.3 $ ( 13.4 ) Less: net losses recognized on equity securities sold during — ( 11.9 ) — ( 11.9 ) Net unrealized gains (losses) recognized during the period $ 11.7 $ 6.7 $ 19.3 $ ( 1.5 )

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
	Sep. 30, 2024
Fair Value Disclosures [Abstract]
Fair Values of Financial Instruments	The estimated fair values of the financial instruments were as follows: September 30, 2024 December 31, 2023 Carrying Market Carrying Market (in millions) Value Value Value Value Financial Assets carried at: Fair Value through AOCI: Fixed maturities $ 8,620.0 $ 8,620.0 $ 7,985.3 $ 7,985.3 Fair Value through Net Income: Equity securities 150.1 150.1 130.9 130.9 Other investments 101.2 101.2 118.4 118.4 Amortized Cost/Cost: Other investments 374.3 360.0 413.1 393.5 Cash and cash equivalents 427.1 427.1 316.1 316.1 Total financial instruments $ 9,672.7 $ 9,658.4 $ 8,963.8 $ 8,944.2 Financial Liabilities carried at: Amortized Cost: Debt $ 783.9 $ 752.5 $ 783.2 $ 727.5
Fair Value, Investment Assets Measured on Recurring Basis	The following tables provide, for each hierarchy level, the Company’s investment assets that were measured at fair value on a recurring basis. September 30, 2024 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 576.8 $ 436.2 $ 140.6 $ — Foreign governments 1.8 — 1.8 — Municipals 1,049.3 — 1,041.3 8.0 Corporates 3,973.9 — 3,973.9 — Residential mortgage-backed, U.S. agency backed 1,478.4 — 1,478.4 — Residential mortgage-backed, non-agency 250.4 — 250.4 — Commercial mortgage-backed 586.2 — 580.0 6.2 Other asset-backed 703.2 — 703.2 — Total fixed maturities 8,620.0 436.2 8,169.6 14.2 Equity securities 150.1 142.3 — 7.8 Other investments 3.8 — — 3.8 Total investment assets at fair value $ 8,773.9 $ 578.5 $ 8,169.6 $ 25.8 December 31, 2023 (in millions) Total Level 1 Level 2 Level 3 Fixed maturities: U.S. Treasury and government agencies $ 462.6 $ 324.3 $ 138.3 $ — Foreign governments 2.2 — 2.2 — Municipals 1,185.2 — 1,176.6 8.6 Corporates 3,849.7 — 3,849.6 0.1 Residential mortgage-backed, U.S. agency backed 1,168.7 — 1,168.7 — Residential mortgage-backed, non-agency 142.2 — 142.2 — Commercial mortgage-backed 818.2 — 811.2 7.0 Other asset-backed 356.5 — 356.5 — Total fixed maturities 7,985.3 324.3 7,645.3 15.7 Equity securities 130.9 123.3 — 7.6 Other investments 3.8 — — 3.8 Total investment assets at fair value $ 8,120.0 $ 447.6 $ 7,645.3 $ 27.1
Estimated Fair Values of Financial Instruments Not Carried at Fair Value	The following tables provide, for each hierarchy level, the Company’s estimated fair values of financial instruments that were not carried at fair value: September 30, 2024 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 427.1 $ 427.1 $ — $ — Other investments 360.0 — 6.1 353.9 Total financial instruments $ 787.1 $ 427.1 $ 6.1 $ 353.9 Liabilities: Debt $ 752.5 $ — $ 752.5 $ — December 31, 2023 (in millions) Total Level 1 Level 2 Level 3 Assets: Cash and cash equivalents $ 316.1 $ 316.1 $ — $ — Other investments 393.5 — 6.1 387.4 Total financial instruments $ 709.6 $ 316.1 $ 6.1 $ 387.4 Liabilities: Debt $ 727.5 $ — $ 727.5 $ —
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). Fixed Maturities (in millions) Municipals Corporates Commercial Total Equities and Total Three Months Ended September 30, 2024 Balance July 1, 2024 $ 7.9 $ — $ 6.7 $ 14.6 $ 11.5 $ 26.1 Total gains (losses): Included in net realized and unrealized — — — — 0.1 0.1 Included in other comprehensive income (loss) - changes in 0.3 — ( 0.4 ) ( 0.1 ) — ( 0.1 ) Settlements ( 0.2 ) — ( 0.1 ) ( 0.3 ) — ( 0.3 ) Balance September 30, 2024 $ 8.0 $ — $ 6.2 $ 14.2 $ 11.6 $ 25.8 Changes in net unrealized gains (losses) for the period $ 0.3 $ — $ ( 0.4 ) $ ( 0.1 ) $ — $ ( 0.1 ) Three Months Ended September 30, 2023 Balance July 1, 2023 $ 8.8 $ 0.1 $ 7.2 $ 16.1 $ 11.2 $ 27.3 Total losses: Included in other comprehensive income (loss) - changes in ( 0.2 ) — ( 0.1 ) ( 0.3 ) — ( 0.3 ) Settlements ( 0.1 ) — ( 0.2 ) ( 0.3 ) — ( 0.3 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.2 ) $ — $ ( 0.1 ) $ ( 0.3 ) $ — $ ( 0.3 ) Fixed Maturities (in millions) Municipals Corporates Commercial Total Equities and Total Nine Months Ended September 30, 2024 Balance January 1, 2024 $ 8.6 $ 0.1 $ 7.0 $ 15.7 $ 11.4 $ 27.1 Transfers out of Level 3 — ( 0.1 ) — ( 0.1 ) — ( 0.1 ) Total gains (losses): Included in net realized and unrealized — — — — 0.2 0.2 Included in other comprehensive income (loss) - changes 0.2 — ( 0.4 ) ( 0.2 ) — ( 0.2 ) Settlements ( 0.8 ) — ( 0.4 ) ( 1.2 ) — ( 1.2 ) Balance September 30, 2024 $ 8.0 $ — $ 6.2 $ 14.2 $ 11.6 $ 25.8 Changes in net unrealized gains (losses) for the period $ 0.2 $ — $ ( 0.4 ) $ ( 0.2 ) $ — $ ( 0.2 ) Nine Months Ended September 30, 2023 Balance January 1, 2023 $ 9.3 $ 0.1 $ 7.5 $ 16.9 $ 11.1 $ 28.0 Total gains (losses): Included in net realized and unrealized — — — — 0.1 0.1 Included in other comprehensive income (loss) - changes ( 0.1 ) — ( 0.1 ) ( 0.2 ) — ( 0.2 ) Settlements ( 0.7 ) — ( 0.5 ) ( 1.2 ) — ( 1.2 ) Balance September 30, 2023 $ 8.5 $ 0.1 $ 6.9 $ 15.5 $ 11.2 $ 26.7 Changes in net unrealized gains (losses) for the period $ ( 0.1 ) $ — $ ( 0.1 ) $ ( 0.2 ) $ — $ ( 0.2 )
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. September 30, 2024 December 31, 2023 Valuation Significant Fair Range Fair Range (in millions) Technique Unobservable Inputs Value (Wtd Average) Value (Wtd Average) Fixed maturities: Municipals Discounted Discount for: $ 8.0 6.1 - 6.8 % ( 6.7 %) $ 8.6 6.1 - 6.8 % ( 6.7 %) Corporates Discounted Discount for: — — 0.1 2.5 % ( 2.5 %) 0.3 % ( 0.3 %) Commercial Discounted Discount for: 6.2 3.0 - 5.0 % ( 4.7 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) 7.0 3.0 - 3.1 % ( 3.0 %) 0.5 % ( 0.5 %) 0.3 % ( 0.3 %) Equity securities Market Net tangible asset 1.4 N/A 1.2 N/A Internal price based on financing round Discount for: 6.4 27.0 % ( 27.0 %) 6.4 27.0 % ( 27.0 %) Other Discounted Discount rate 3.8 16.8 % ( 16.8 %) 3.8 16.8 % ( 16.8 %)

Pension Plans (Tables)

Pension Plans (Tables)	9 Months Ended
	Sep. 30, 2024
Retirement Benefits [Abstract]
Components of Net Periodic Pension Cost	The components of net periodic pension cost for the defined benefit pension plans included in the Company’s results of operations are as follows: Three Months Ended September 30, 2024 2023 (in millions) Pension Plans Interest cost $ 5.1 $ 5.5 Expected return on plan assets ( 5.1 ) ( 5.5 ) Recognized net actuarial loss 1.7 1.9 Net periodic pension cost $ 1.7 $ 1.9 Nine Months Ended September 30, 2024 2023 (in millions) Pension Plans Interest cost $ 15.3 $ 16.4 Expected return on plan assets ( 15.2 ) ( 16.5 ) Recognized net actuarial loss 5.0 5.8 Net periodic pension cost $ 5.1 $ 5.7

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2024
Equity [Abstract]
Changes in Other Comprehensive Income (Loss)	The following tables provide changes in other comprehensive income (loss). Three Months Ended September 30, 2024 2023 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) Net unrealized gains (losses) arising during period $ 282.3 $ ( 59.3 ) $ 223.0 $ ( 144.3 ) $ 30.3 $ ( 114.0 ) Net unrealized gains (losses) arising during period 1.0 ( 0.1 ) 0.9 ( 0.7 ) 0.2 ( 0.5 ) Amount of losses realized from sales and 24.8 ( 5.8 ) 19.0 0.9 ( 0.6 ) 0.3 Amount of credit-related recoveries on prior ( 0.3 ) — ( 0.3 ) — — — Amount of additional impairment losses 0.5 ( 0.1 ) 0.4 — — — Net unrealized gains (losses) 308.3 ( 65.3 ) 243.0 ( 144.1 ) 29.9 ( 114.2 ) Pension and postretirement benefits: Amortization of net actuarial losses 1.7 ( 0.3 ) 1.4 1.9 ( 0.4 ) 1.5 Long-duration insurance contracts: Net change in market risk ( 4.0 ) 0.9 ( 3.1 ) 4.2 ( 0.9 ) 3.3 Other comprehensive income (loss) $ 306.0 $ ( 64.7 ) $ 241.3 $ ( 138.0 ) $ 28.6 $ ( 109.4 ) Nine Months Ended September 30, 2024 2023 Tax Tax Benefit Net of Benefit Net of (in millions) Pre-Tax (Expense) Tax Pre-Tax (Expense) Tax Changes in net unrealized gains (losses) on Net unrealized gains (losses) arising during period $ 211.2 $ ( 44.4 ) $ 166.8 $ ( 120.8 ) $ 25.4 $ ( 95.4 ) Net unrealized gains arising during period for 4.0 ( 0.8 ) 3.2 0.3 — 0.3 Amount of losses realized from sales and 56.9 ( 13.1 ) 43.8 1.9 ( 1.6 ) 0.3 Amount of credit-related impairments (recoveries) ( 0.7 ) 0.1 ( 0.6 ) 0.8 ( 0.2 ) 0.6 Amount of additional impairment losses 2.2 ( 0.5 ) 1.7 10.3 ( 2.2 ) 8.1 Net unrealized gains (losses) 273.6 ( 58.7 ) 214.9 ( 107.5 ) 21.4 ( 86.1 ) Pension and postretirement benefits: Amortization of net actuarial losses 5.1 ( 1.0 ) 4.1 5.9 ( 1.2 ) 4.7 Long-duration insurance contracts: Net change in market risk ( 1.7 ) 0.4 ( 1.3 ) 3.3 ( 0.7 ) 2.6 Other comprehensive income (loss) $ 277.0 $ ( 59.3 ) $ 217.7 $ ( 98.3 ) $ 19.5 $ ( 78.8 )
Reclassifications Out of Accumulated Other Comprehensive Loss	Reclassifications out of accumulated other comprehensive loss were as follows: Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2024 2023 2024 2023 Amount Reclassified from Details about Accumulated Other Accumulated Other Affected Line Item in the Statement Comprehensive Loss Components Comprehensive Loss Where Net Income (Loss) is Presented Net unrealized losses on investment $ ( 24.7 ) $ ( 0.9 ) $ ( 56.6 ) $ ( 1.9 ) Net realized losses from sales ( 0.2 ) — ( 1.5 ) ( 11.1 ) Impairments on investments ( 24.9 ) ( 0.9 ) ( 58.1 ) ( 13.0 ) Total before tax 5.9 0.6 13.5 4.0 Income tax (expense) benefit ( 19.0 ) ( 0.3 ) ( 44.6 ) ( 9.0 ) Continuing operations; net of tax ( 0.1 ) — ( 0.3 ) — Income from discontinued life ( 19.1 ) ( 0.3 ) ( 44.9 ) ( 9.0 ) Net of tax Amortization of defined benefit pension ( 1.7 ) ( 1.9 ) ( 5.1 ) ( 5.9 ) Loss adjustment expenses and other (1) 0.3 0.4 1.0 1.2 Income tax (expense) benefit ( 1.4 ) ( 1.5 ) ( 4.1 ) ( 4.7 ) Continuing operations; net of tax Net change in market risk 0.3 — 0.8 — Income from discontinued life ( 0.1 ) — ( 0.2 ) — Income tax (expense) benefit 0.2 — 0.6 — Discontinued operations; net of tax Total reclassifications for the period $ ( 20.3 ) $ ( 1.8 ) $ ( 48.4 ) $ ( 13.7 ) Benefit (expense) reflected in income (1) The amount reclassified from accumulated other comprehensive loss for the pension and postretirement benefits was allocated approximately 40 % to loss adjustment expenses and 60 % to other operating expenses for the three and nine months ended September 30, 2024 and 2023 .

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2024
Segment Reporting [Abstract]
Financial Information with Respect to Reporting Segments	Summarized below is financial information with respect to the Company’s reporting segments. Three Months Ended Nine Months Ended September 30, September 30, (in millions) 2024 2023 2024 2023 Operating revenues: Core Commercial $ 576.6 $ 557.1 $ 1,727.6 $ 1,658.2 Specialty 353.7 340.9 1,049.0 1,011.6 Personal Lines 644.9 620.5 1,908.5 1,815.1 Other 2.5 4.2 9.4 11.6 Total 1,577.7 1,522.7 4,694.5 4,496.5 Net realized and unrealized investment losses ( 12.4 ) ( 6.1 ) ( 41.4 ) ( 31.8 ) Total revenues $ 1,565.3 $ 1,516.6 $ 4,653.1 $ 4,464.7 Operating income (loss) before interest expense Core Commercial: Underwriting income $ 14.6 $ 5.3 $ 88.3 $ 1.0 Net investment income 42.1 38.4 124.3 114.2 Other expense ( 0.8 ) ( 0.6 ) ( 2.0 ) ( 0.8 ) Core Commercial operating income 55.9 43.1 210.6 114.4 Specialty: Underwriting income 52.6 52.7 113.5 119.5 Net investment income 20.9 18.0 61.7 53.6 Other expense ( 0.5 ) ( 0.4 ) ( 0.8 ) ( 0.1 ) Specialty operating income 73.0 70.3 174.4 173.0 Personal Lines: Underwriting loss ( 6.7 ) ( 127.3 ) ( 74.4 ) ( 422.4 ) Net investment income 26.3 24.3 77.9 73.3 Other income 2.1 2.6 6.7 8.0 Personal Lines operating income (loss) 21.7 ( 100.4 ) 10.2 ( 341.1 ) Other: Underwriting income — — — — Net investment income 2.5 3.5 8.0 9.4 Other expense ( 2.5 ) ( 2.8 ) ( 7.5 ) ( 8.2 ) Other operating income — 0.7 0.5 1.2 Operating income (loss) before interest expense 150.6 13.7 395.7 ( 52.5 ) Interest on debt ( 8.5 ) ( 8.5 ) ( 25.6 ) ( 25.6 ) Operating income (loss) before income taxes 142.1 5.2 370.1 ( 78.1 ) Non-operating items: Net realized and unrealized investment losses ( 12.4 ) ( 6.1 ) ( 41.4 ) ( 31.8 ) Other non-operating — — ( 2.4 ) 0.8 Income (loss) from continuing operations before income taxes $ 129.7 $ ( 0.9 ) $ 326.3 $ ( 109.1 )
Identifiable Assets by Business Segment	The following table provides identifiable assets for the Company’s segments and discontinued operations: (in millions) September 30, 2024 December 31, 2023 Property and Casualty $ 15,277.9 $ 14,526.0 Assets of discontinued businesses 89.1 86.6 Total $ 15,367.0 $ 14,612.6

Stock-based Compensation (Table

Stock-based Compensation (Tables)	9 Months Ended
	Sep. 30, 2024
Share-Based Payment Arrangement [Abstract]
Compensation Cost and Related Tax Benefits	Compensation cost for the Company’s stock-based awards and the related tax benefits were as follows: Three Months Ended September 30, Nine Months Ended September 30, (in millions) 2024 2023 2024 2023 Stock-based compensation expense $ 8.7 $ 8.6 $ 23.5 $ 23.9 Tax benefit ( 1.8 ) ( 1.8 ) ( 4.9 ) ( 5.0 ) Stock-based compensation expense, net of taxes $ 6.9 $ 6.8 $ 18.6 $ 18.9
Summary of Stock Option Activity	Information on the Company’s stock option activity for the nine months ended September 30, 2024 and 2023 is summarized below. Nine Months Ended September 30, 2024 2023 (in whole shares and dollars) Shares Weighted Average Shares Weighted Average Outstanding, beginning of period 1,137,042 $ 111.57 1,080,852 $ 107.07 Granted 150,731 134.26 135,019 140.01 Exercised ( 131,891 ) 88.87 ( 40,544 ) 92.39 Forfeited or cancelled ( 2,642 ) 134.99 ( 22,184 ) 125.15 Outstanding, end of period 1,153,240 117.08 1,153,143 111.09
Summary of Activity Information about Employee Restricted Stock Units	The following table summarizes activity information about employee restricted stock units: Nine Months Ended September 30, 2024 2023 (in whole shares and dollars) Shares Weighted Shares Weighted Time-based restricted stock units: Outstanding, beginning of period 376,626 $ 132.04 378,256 $ 124.88 Granted 158,460 133.30 149,509 138.35 Vested ( 123,973 ) 116.25 ( 132,086 ) 118.21 Forfeited ( 21,705 ) 135.85 ( 19,132 ) 131.14 Outstanding, end of period 389,408 137.36 376,547 132.25 Performance-based and market-based restricted stock units: Outstanding, beginning of period 120,504 $ 132.76 119,163 $ 122.81 Granted 53,336 132.27 50,115 137.90 Vested ( 41,290 ) 112.36 ( 44,781 ) 114.14 Forfeited ( 6,339 ) 108.06 ( 3,993 ) 109.19 Outstanding, end of period 126,211 140.47 120,504 132.76

Earnings Per Share and Shareh_2

Earnings Per Share and Shareholders' Equity Transactions (Tables)	9 Months Ended
	Sep. 30, 2024
Earnings Per Share [Abstract]
Information Regarding Basic and Diluted Earnings Per Share	The following table provides weighted average share information used in the calculation of the Company’s basic and diluted earnings per share: Three Months Ended Nine Months Ended September 30, September 30, (in millions, except per share data) 2024 2023 2024 2023 Basic shares used in the calculation of earnings per share 36.0 35.8 35.9 35.7 Dilutive effect of securities: Employee stock options 0.2 0.1 0.2 — Non-vested stock grants 0.3 0.2 0.3 — Diluted shares used in the calculation of earnings per share 36.5 36.1 36.4 35.7 Per share effect of dilutive securities on income (loss) from $ ( 0.04 ) $ — $ ( 0.09 ) $ — Per share effect of dilutive securities on net income (loss) $ ( 0.04 ) $ — $ ( 0.08 ) $ —

Liabilities for Outstanding C_2

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Tables)	9 Months Ended
	Sep. 30, 2024
Insurance [Abstract]
Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses	The table below provides a reconciliation of the gross beginning and ending reserve for unpaid losses and loss adjustment expenses. Nine Months Ended September 30, (in millions) 2024 2023 Gross reserve for losses and LAE, beginning of period $ 7,308.1 $ 7,012.6 Reinsurance recoverable on unpaid losses 1,795.0 1,748.6 Net reserve for losses and LAE, beginning of period 5,513.1 5,264.0 Net incurred losses and LAE in respect of losses occurring in: Current year 2,975.6 3,225.6 Prior years ( 78.8 ) ( 6.8 ) Total incurred losses and LAE 2,896.8 3,218.8 Net payments of losses and LAE in respect of losses occurring in: Current year 1,225.3 1,375.7 Prior years 1,517.5 1,523.8 Total payments 2,742.8 2,899.5 Net reserve for losses and LAE, end of period 5,667.1 5,583.3 Reinsurance recoverable on unpaid losses 1,759.7 1,746.5 Gross reserve for losses and LAE, end of period $ 7,426.8 $ 7,329.8

Investments (Schedule of Availa

Investments (Schedule of Available-for-sale Fixed Maturities Reconciliation) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Schedule of Available-for-sale Securities [Line Items]
Allowance for Credit Losses	$ (7.5)	$ (8.6)	$ (10)	$ (8.5)	$ (8.5)	$ (3.2)
Amortized Cost, Net of Allowance for Credit Losses	8,936.5		8,573.9
Fair Value	8,620		7,985.3
U.S. Treasury and Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	616.8		512.9
Amortized Cost, Net of Allowance for Credit Losses	616.8		512.9
Gross Unrealized Gains	4		1.4
Gross Unrealized Loss	44		51.7
Fair Value	576.8		462.6
Foreign Governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1.7		2.2
Amortized Cost, Net of Allowance for Credit Losses	1.7		2.2
Gross Unrealized Gains	0.1
Fair Value	1.8		2.2
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,135		1,309.3
Amortized Cost, Net of Allowance for Credit Losses	1,135		1,309.3
Gross Unrealized Gains	4.3		5.5
Gross Unrealized Loss	90		129.6
Fair Value	1,049.3		1,185.2
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	4,047.3		4,053.8
Allowance for Credit Losses	(0.6)		(1.9)
Amortized Cost, Net of Allowance for Credit Losses	4,046.7		4,051.9
Gross Unrealized Gains	46.1		22.9
Gross Unrealized Loss	118.9		225.1
Fair Value	3,973.9		3,849.7
Residential Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	1,813.2		1,435.6
Amortized Cost, Net of Allowance for Credit Losses	1,813.2		1,435.6
Gross Unrealized Gains	20.7		5.5
Gross Unrealized Loss	105.1		130.2
Fair Value	1,728.8		1,310.9
Commercial Mortgage-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	621.9		890.3
Amortized Cost, Net of Allowance for Credit Losses	621.9		890.3
Gross Unrealized Gains	0.7		0.1
Gross Unrealized Loss	36.4		72.2
Fair Value	586.2		818.2
Other Asset-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	701.2		371.7
Amortized Cost, Net of Allowance for Credit Losses	701.2		371.7
Gross Unrealized Gains	4.2		0.7
Gross Unrealized Loss	2.2		15.9
Fair Value	703.2		356.5
Fixed Maturities Including Held For Sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, amortized cost	8,937.1		8,575.8
Allowance for Credit Losses	(0.6)		(1.9)
Amortized Cost, Net of Allowance for Credit Losses	8,936.5		8,573.9
Gross Unrealized Gains	80.1		36.1
Gross Unrealized Loss	396.6		624.7
Fair Value	$ 8,620		$ 7,985.3

Investments (Investments Classi

Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost, Net of Allowance for Credit Losses	$ 369.6
Due after one year through five years, Amortized Cost, Net of Allowance for Credit Losses	2,623.2
Due after five years through ten years, Amortized Cost, Net of Allowance for Credit Losses	2,552.5
Due after ten years, Amortized Cost, Net of Allowance for Credit Losses	254.9
Gross fixed maturities, Amortized Cost, Net of Allowance for Credit Losses	5,800.2
Mortgage-backed and other asset-backed securities, Amortized Cost, Net of Allowance for Credit Losses	3,136.3
Amortized Cost, Net of Allowance for Credit Losses	8,936.5
Due in one year or less, Fair Value	367.8
Due after one year through five years, Fair Value	2,594.7
Due after five years through ten years, Fair Value	2,407.8
Due after ten years, Fair Value	231.5
Gross fixed maturities, Fair Value	5,601.8
Mortgage-backed and other asset-backed securities, Fair Value	3,018.2
Fixed maturities, Fair Value	$ 8,620	$ 7,985.3

Investments (Schedule of Unreal

Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 2.3	$ 8.4
12 months or less, Fair Value	693.8	509.8
Greater than 12 months, Gross Unrealized Losses	394.3	616.3
Greater than 12 months, Fair Value	4,721.4	6,101.1
Total, Gross Unrealized Losses	396.6	624.7
Total, Fair Value	5,415.2	6,610.9
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	1.4	5.9
12 months or less, Fair Value	632.1	476
Greater than 12 months, Gross Unrealized Losses	389.5	609.9
Greater than 12 months, Fair Value	4,663.4	6,034
Total, Gross Unrealized Losses	390.9	615.8
Total, Fair Value	5,295.5	6,510
Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	2.5
12 months or less, Fair Value	61.7	33.8
Greater than 12 months, Gross Unrealized Losses	4.8	6.4
Greater than 12 months, Fair Value	58	67.1
Total, Gross Unrealized Losses	5.7	8.9
Total, Fair Value	119.7	100.9
U.S. Treasury and Government Agencies [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3	0.6
12 months or less, Fair Value	106	62.2
Greater than 12 months, Gross Unrealized Losses	43.7	51.1
Greater than 12 months, Fair Value	330.2	331
Total, Gross Unrealized Losses	44	51.7
Total, Fair Value	436.2	393.2
Foreign Governments [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value		2
Total, Fair Value		2
Municipals [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	2.5
12 months or less, Fair Value	39.8	72.5
Greater than 12 months, Gross Unrealized Losses	89.8	127.1
Greater than 12 months, Fair Value	848	935.2
Total, Gross Unrealized Losses	90	129.6
Total, Fair Value	887.8	1,007.7
Corporates [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.6	1.3
12 months or less, Fair Value	347.4	159.9
Greater than 12 months, Gross Unrealized Losses	112.6	214.9
Greater than 12 months, Fair Value	2,005.6	2,870.2
Total, Gross Unrealized Losses	113.2	216.2
Total, Fair Value	2,353	3,030.1
Corporates [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.9	2.5
12 months or less, Fair Value	61.7	33.8
Greater than 12 months, Gross Unrealized Losses	4.8	6.4
Greater than 12 months, Fair Value	57.1	66.2
Total, Gross Unrealized Losses	5.7	8.9
Total, Fair Value	118.8	100
Residential Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	1.2
12 months or less, Fair Value	78.1	139.3
Greater than 12 months, Gross Unrealized Losses	105	129
Greater than 12 months, Fair Value	808.1	865.4
Total, Gross Unrealized Losses	105.1	130.2
Total, Fair Value	886.2	1,004.7
Commercial Mortgage-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.2
12 months or less, Fair Value		14
Greater than 12 months, Gross Unrealized Losses	36.4	72
Greater than 12 months, Fair Value	545.4	770
Total, Gross Unrealized Losses	36.4	72.2
Total, Fair Value	545.4	784
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Greater than 12 months, Fair Value	0.9	0.9
Total, Fair Value	0.9	0.9
Other Asset-Backed [Member] | Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.2	0.1
12 months or less, Fair Value	60.8	28.1
Greater than 12 months, Gross Unrealized Losses	2	15.8
Greater than 12 months, Fair Value	126.1	260.2
Total, Gross Unrealized Losses	2.2	15.9
Total, Fair Value	$ 186.9	$ 288.3

Investment Income and Gains and

Investment Income and Gains and Losses (Proceeds from Sales of Available for Sale Fixed Maturities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 479.5	$ 16.9	$ 1,145.9	$ 103.5
Gross gains	0.5		2.4	0.9
Gross losses	$ 25.8	$ 1.3	$ 60.1	$ 3.3

Investments (Narrative) (Detail

Investments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Schedule of Investments [Line Items]
Impairments losses on investments	$ 0.6	$ 5.5	$ 16.5
Allowance for credit losses	7.5	7.5	8.5	$ 8.6	$ 10	$ 8.5	$ 3.2
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Impairments losses on investments	0.2	1.5	10.3
Fixed Maturities Including Held For Sale [Member]
Schedule of Investments [Line Items]
Allowance for credit losses	0.6	0.6			1.9
Mortgage Loans [Member]
Schedule of Investments [Line Items]
Impairments on fixed maturities credit related losses	0.4	4	$ 5.4
Allowance for credit losses	$ 7.5	$ 7.5			$ 10

Investments - (Schedule of Allo

Investments - (Schedule of Allowance for Credit Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Allowance for credit losses as of the beginning of the period	$ 8.6	$ 8.5	$ 10	$ 3.2
Reductions for writedowns			(3)
Reductions for disposals	(1.3)		(1.6)
Additional credit losses on investments for which an allowance was previously recognized	0.2		2.1	5.4
Recoveries				(0.1)
Allowance for credit losses as of the end of the period	$ 7.5	$ 8.5	$ 7.5	$ 8.5

Investments (Schedule of Pre-ta

Investments (Schedule of Pre-tax Net Realized and Unrealized Losses on Equity Securities) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Net gains (losses) recognized during the period	$ 11.7	$ (5.2)	$ 19.3	$ (13.4)
Less: net losses recognized on equity securities sold during the period		(11.9)		(11.9)
Net unrealized gains (losses) recognized during the period on equity securities still held	$ 11.7	$ 6.7	$ 19.3	$ (1.5)

Fair Value (Fair Values of Fina

Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 427.1	$ 316.1
Total financial instruments	9,672.7	8,963.8
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	8,620	7,985.3
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	101.2	118.4
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	150.1	130.9
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	427.1	316.1
Debt	783.9	783.2
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	374.3	413.1
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	427.1	316.1
Total financial instruments	9,658.4	8,944.2
Debt	752.5	727.5
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	8,620	7,985.3
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	101.2	118.4
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	150.1	130.9
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	427.1	316.1
Debt	752.5	727.5
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	$ 360	$ 393.5

Fair Value (Fair Value, Investm

Fair Value (Fair Value, Investment Assets Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 8,620	$ 7,985.3
Equity securities	150.1	130.9
Other investments	760.6	796.9
U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	576.8	462.6
Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,049.3	1,185.2
Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,973.9	3,849.7
Residential Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,728.8	1,310.9
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	586.2	818.2
Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	703.2	356.5
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	8,620	7,985.3
Equity securities	150.1	130.9
Other investments	3.8	3.8
Total investment assets at fair value	8,773.9	8,120
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	576.8	462.6
Fair Value, Measurements, Recurring [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1.8	2.2
Fair Value, Measurements, Recurring [Member] | Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,049.3	1,185.2
Fair Value, Measurements, Recurring [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,973.9	3,849.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed, U.S. Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,478.4	1,168.7
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed, Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	250.4	142.2
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	586.2	818.2
Fair Value, Measurements, Recurring [Member] | Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	703.2	356.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	436.2	324.3
Equity securities	142.3	123.3
Total investment assets at fair value	578.5	447.6
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	436.2	324.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	8,169.6	7,645.3
Total investment assets at fair value	8,169.6	7,645.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury and Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	140.6	138.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1.8	2.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,041.3	1,176.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,973.9	3,849.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed, U.S. Agency Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,478.4	1,168.7
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed, Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	250.4	142.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	580	811.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Other Asset-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	703.2	356.5
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	14.2	15.7
Equity securities	7.8	7.6
Other investments	3.8	3.8
Total investment assets at fair value	25.8	27.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	8	8.6
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities		0.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6.2	$ 7

Fair Value (Narrative) (Details

Fair Value (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2023
Fair Value Measurements [Line Items]
Investments measured at net asset value based on an ownership interest in partners'	1%			1%
Transfer between Level 2 and Level 3	$ 0	$ 0	$ 0
Fair Value Measured Using NAV [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	$ 97,400,000			$ 114,600,000
Investment, Type [Extensible Enumeration]	srt:PartnershipInterestMember			srt:PartnershipInterestMember
Level 3 [Member]
Fair Value Measurements [Line Items]
Investments measured at fair value using net asset value	$ 3,800,000			$ 3,800,000
Liabilities held	$ 0	$ 0	$ 0

Fair Value (Estimated Fair Valu

Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	$ 3.8	$ 3.8
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	427.1	316.1
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	427.1	316.1
Other investments	360	393.5
Total financial instruments	787.1	709.6
Debt	752.5	727.5
Estimated Fair Value [Member] | Level 1 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	427.1	316.1
Total financial instruments	427.1	316.1
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	6.1	6.1
Total financial instruments	6.1	6.1
Debt	752.5	727.5
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Other investments	353.9	387.4
Total financial instruments	$ 353.9	$ 387.4

Fair Value (Fair Value on Recur

Fair Value (Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 26.1	$ 27.3	$ 27.1	$ 28
Transfers out of Level 3			(0.1)
Included in net realized and unrealized investment gains (losses)	$ 0.1		$ 0.2	$ 0.1
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Gain Loss On Investments From Continuing Operations		Gain Loss On Investments From Continuing Operations	Gain Loss On Investments From Continuing Operations
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	$ (0.1)	$ (0.3)	$ (0.2)	$ (0.2)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax	OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax	OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax	OCI, Debt Securities, Available-for-Sale, Gain (Loss), after Adjustment and Tax
Settlements	$ (0.3)	$ (0.3)	$ (1.2)	$ (1.2)
Ending Balance	25.8	26.7	25.8	26.7
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	$ (0.1)	$ (0.3)	$ (0.2)	$ (0.2)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Gain Loss On Investments From Continuing Operations	Gain Loss On Investments From Continuing Operations	Gain Loss On Investments From Continuing Operations	Gain Loss On Investments From Continuing Operations
Municipals [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	$ 7.9	$ 8.8	$ 8.6	$ 9.3
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	0.3	(0.2)	0.2	(0.1)
Settlements	(0.2)	(0.1)	(0.8)	(0.7)
Ending Balance	8	8.5	8	8.5
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	0.3	(0.2)	0.2	(0.1)
Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance		0.1	0.1	0.1
Transfers out of Level 3			(0.1)
Ending Balance		0.1		0.1
Commercial Mortgage-Backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	6.7	7.2	7	7.5
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	(0.4)	(0.1)	(0.4)	(0.1)
Settlements	(0.1)	(0.2)	(0.4)	(0.5)
Ending Balance	6.2	6.9	6.2	6.9
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	(0.4)	(0.1)	(0.4)	(0.1)
Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	14.6	16.1	15.7	16.9
Transfers out of Level 3			(0.1)
Included in other comprehensive income (loss) - changes in net unrealized gains (losses) on investment securities	(0.1)	(0.3)	(0.2)	(0.2)
Settlements	(0.3)	(0.3)	(1.2)	(1.2)
Ending Balance	14.2	15.5	14.2	15.5
Changes in net unrealized gains (losses) for the period included in other comprehensive income (loss) for assets held at the end of the period	(0.1)	(0.3)	(0.2)	(0.2)
Equities Securities and Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	11.5	11.2	11.4	11.1
Included in net realized and unrealized investment gains (losses)	0.1		0.2	0.1
Ending Balance	$ 11.6	$ 11.2	$ 11.6	$ 11.2

Fair Value (Schedule of Additio

Fair Value (Schedule of Additional Information About Significant Unobservable Inputs Used in Fair Valuations of Level 3) (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Fair Value [Line Items]
Fair Value	$ 8,620	$ 7,985.3
Equity securities, at fair value	150.1	130.9
Level 3 [Member]
Fair Value [Line Items]
Fair Value	$ 3.8	$ 3.8
Alternative Investment, Valuation Technique [Extensible Enumeration]	Discounted cash flow [Member]	Discounted cash flow [Member]
Alternative Investment, Measurement Input [Extensible Enumeration]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Range	0.168	0.168
Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Range	0.168	0.168
Municipals [Member]
Fair Value [Line Items]
Fair Value	$ 1,049.3	$ 1,185.2
Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	8	8.6
Corporates [Member]
Fair Value [Line Items]
Fair Value	3,973.9	3,849.7
Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value		0.1
Commercial Mortgage-Backed [Member]
Fair Value [Line Items]
Fair Value	586.2	818.2
Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Fair Value	6.2	7
Equity Securities [Member] | Internal Model Pricing Approach Valuation Technique | Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 6.4	$ 6.4
Fair value measurement market multiples	27	27
Equity Securities [Member] | Internal Model Pricing Approach Valuation Technique | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Fair value measurement market multiples	27	27
Net Tangible Asset [Member] | Equity Securities [Member] | Market comparables [Member] | Level 3 [Member]
Fair Value [Line Items]
Equity securities, at fair value	$ 1.4	$ 1.2
Measurement Input, Discount for Small Issue Size [Member] | Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.10%	6.10%
Measurement Input, Discount for Small Issue Size [Member] | Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.80%	6.80%
Measurement Input, Discount for Small Issue Size [Member] | Municipals [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	6.70%	6.70%
Measurement Input, Discount for Small Issue Size [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		2.50%
Measurement Input, Discount for Small Issue Size [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		2.50%
Measurement Input, Discount for Small Issue Size [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Minimum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	3%	3%
Measurement Input, Discount for Small Issue Size [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Maximum [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	5%	3.10%
Measurement Input, Discount for Small Issue Size [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	4.70%	3%
Measurement Input, Discount For Above-market coupon [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		0.30%
Measurement Input, Discount For Above-market coupon [Member] | Corporates [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs		0.30%
Measurement Input, Discount For Above-market coupon [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Above-market coupon [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.50%	0.50%
Measurement Input, Discount For Lease Structure [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%
Measurement Input, Discount For Lease Structure [Member] | Commercial Mortgage-Backed [Member] | Discounted cash flow [Member] | Level 3 [Member] | Weighted Average [Member]
Fair Value [Line Items]
Significant Unobservable Inputs	0.30%	0.30%

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Federal income tax expense (benefit)	$ 68.3	$ 35.3

Pension Plans (Components of Ne

Pension Plans (Components of Net Periodic Pension Cost) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Retirement Benefits [Abstract]
Defined Benefit Plan, Type [Extensible List]	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember	us-gaap:PensionPlansDefinedBenefitMember
Interest cost	$ 5.1	$ 5.5	$ 15.3	$ 16.4
Expected return on plan assets	(5.1)	(5.5)	(15.2)	(16.5)
Recognized net actuarial loss	1.7	1.9	5	5.8
Net periodic pension cost	$ 1.7	$ 1.9	$ 5.1	$ 5.7

Other Comprehensive Income (L_3

Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income (Loss), Pre-Tax	$ 282.3	$ (144.3)	$ 211.2	$ (120.8)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income (Loss), Pre-Tax	1	(0.7)	4	0.3
Amount of losses realized from sales and other recognized in Consolidated Statements of Income (Loss), Pre-Tax	24.8	0.9	56.9	1.9
Amount of credit-related impairments (recoveries) recognized in the Consolidated Statement of Income (Loss), Pre-Tax	(0.3)	0	(0.7)	0.8
Amount of additional impairment losses recognized in the Consolidated Statements of Income (Loss), Pre-Tax	0.5	0	2.2	10.3
Net unrealized gains (losses), Pre-Tax	308.3	(144.1)	273.6	(107.5)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	1.7	1.9	5.1	5.9
Net change in market risk, Pre-tax	(4)	4.2	(1.7)	3.3
Other comprehensive income (loss), Pre-Tax	306	(138)	277	(98.3)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income (Loss), Tax Benefit (Expense)	(59.3)	30.3	(44.4)	25.4
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income (Loss), Tax Benefit (Expense)	(0.1)	0.2	(0.8)	0
Amount of losses realized from sales and other recognized in Consolidated Statements of Income (Loss), Tax Benefit (Expense)	(5.8)	(0.6)	(13.1)	(1.6)
Amount of credit-related impairments (recoveries) recognized in the Consolidated Statement of Income (Loss), Tax Benefit (Expense)	0	0	0.1	(0.2)
Amount of additional impairment losses recognized in the Consolidated Statements of Income (Loss), Tax Benefit (Expense)	(0.1)	0	(0.5)	(2.2)
Net unrealized gains (losses), Tax Benefit (Expense)	(65.3)	29.9	(58.7)	21.4
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.3)	(0.4)	(1)	(1.2)
Net change in market risk, Tax Benefit (Expense)	0.9	(0.9)	0.4	(0.7)
Other comprehensive income (loss), Tax Benefit (Expense)	(64.7)	28.6	(59.3)	19.5
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income (Loss), Net of Tax	223	(114)	166.8	(95.4)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income (Loss), Net of Tax	0.9	(0.5)	3.2	0.3
Amount of losses realized from sales and other recognized in Consolidated Statements of Income (Loss), Net of Tax	19	0.3	43.8	0.3
Amount of credit-related impairments (recoveries) recognized in the Consolidated Statement of Income (Loss), Net of Tax	(0.3)	0	(0.6)	0.6
Amount of additional impairment losses recognized in the Consolidated Statements of Income (Loss), Net of Tax	0.4	0	1.7	8.1
Total available-for-sale securities	243	(114.2)	214.9	(86.1)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	1.4	1.5	4.1	4.7
Net change in market risk, Net of Tax	(3.1)	3.3	(1.3)	2.6
Other comprehensive income (loss), Net of tax	$ 241.3	$ (109.4)	$ 217.7	$ (78.8)

Other Comprehensive Income (L_4

Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses from sales and other	$ (23.5)	$ (0.9)	$ (55.2)	$ (1.9)
Income (loss) from continuing operations before income taxes	129.7	(0.9)	326.3	(109.1)
Loss adjustment expenses and other operating expenses	(954)	(1,061.5)	(2,896.8)	(3,218.8)
Income tax (expense) benefit	(27.6)	9.1	(68.3)	35.3
Income (loss) from continuing operations	102.1	8.2	258	(73.8)
Net income (loss)	102.1	8.6	258.1	(72.6)
Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes			0.1
Reclassification out of Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income (loss)	(20.3)	(1.8)	(48.4)	(13.7)
Reclassification out of Accumulated Other Comprehensive Loss [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	(1.7)	(1.9)	(5.1)	(5.9)
Income tax (expense) benefit	0.3	0.4	1	1.2
Income (loss) from continuing operations	(1.4)	(1.5)	(4.1)	(4.7)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized losses from sales and other	(24.7)	(0.9)	(56.6)	(1.9)
Impairments on investments	(0.2)	0	(1.5)	(11.1)
Income (loss) from continuing operations before income taxes	(24.9)	(0.9)	(58.1)	(13)
Income tax (expense) benefit	5.9	0.6	13.5	4
Income (loss) from continuing operations	(19)	(0.3)	(44.6)	(9)
Net income (loss)	(19.1)	(0.3)	(44.9)	(9)
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net Unrealized Appreciation (Depreciation) on Investments [Member] | Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes	(0.1)	0	(0.3)	0
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net change in market risk
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	(0.1)	0	(0.2)	0
Income (loss) from discontinued businesses, net of taxes	0.2	0	0.6	0
Reclassification out of Accumulated Other Comprehensive Loss [Member] | Net change in market risk | Life [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income (loss) from discontinued businesses, net of taxes	$ 0.3	$ 0	$ 0.8	$ 0

Other Comprehensive Income (L_5

Other Comprehensive Income (Loss) (Reclassifications out of Accumulated Other Comprehensive Loss) (Parenthetical) (Details) - Defined Benefit Pension and Postretirement Plans [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Condensed Statement Of Income Captions [Line Items]
Loss adjustment expenses	40%	40%	40%	40%
Percentage of other operating expenses	60%	60%	60%	60%

Segment Information (Narrative)

Segment Information (Narrative) (Details)	9 Months Ended
	Sep. 30, 2024 Segment
Segment Reporting [Abstract]
Operating segments	4

Segment Information (Financial

Segment Information (Financial Information with Respect to Business Segments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Operating revenues	$ 1,577.7	$ 1,522.7	$ 4,694.5	$ 4,496.5
Net realized and unrealized investment losses	(12.4)	(6.1)	(41.4)	(31.8)
Total revenues	1,565.3	1,516.6	4,653.1	4,464.7
Net investment income	91.8	84.2	271.9	250.5
Operating income (loss) before interest expense and income taxes	150.6	13.7	395.7	(52.5)
Interest on debt	(8.5)	(8.5)	(25.6)	(25.6)
Operating income (loss) before income taxes	142.1	5.2	370.1	(78.1)
Other non-operating	0		(2.4)	0.8
Income (loss) from continuing operations before income taxes	129.7	(0.9)	326.3	(109.1)
Core Commercial [Member]
Segment Reporting Information [Line Items]
Operating revenues	576.6	557.1	1,727.6	1,658.2
Underwriting (loss) income	14.6	5.3	88.3	1
Net investment income	42.1	38.4	124.3	114.2
Other income (expense)	(0.8)	(0.6)	(2)	(0.8)
Operating income (loss) before interest expense and income taxes	55.9	43.1	210.6	114.4
Specialty [Member]
Segment Reporting Information [Line Items]
Operating revenues	353.7	340.9	1,049	1,011.6
Underwriting (loss) income	52.6	52.7	113.5	119.5
Net investment income	20.9	18	61.7	53.6
Other income (expense)	(0.5)	(0.4)	(0.8)	(0.1)
Operating income (loss) before interest expense and income taxes	73	70.3	174.4	173
Personal Lines [Member]
Segment Reporting Information [Line Items]
Operating revenues	644.9	620.5	1,908.5	1,815.1
Underwriting (loss) income	(6.7)	(127.3)	(74.4)	(422.4)
Net investment income	26.3	24.3	77.9	73.3
Other income (expense)	2.1	2.6	6.7	8
Operating income (loss) before interest expense and income taxes	21.7	(100.4)	10.2	(341.1)
Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	2.5	4.2	9.4	11.6
Net investment income	2.5	3.5	8	9.4
Other income (expense)	$ (2.5)	(2.8)	(7.5)	(8.2)
Operating income (loss) before interest expense and income taxes		$ 0.7	$ 0.5	$ 1.2

Segment Information (Identifiab

Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Identifiable assets	$ 15,367	$ 14,612.6
Identifiable assets	89.1	86.6
Property and Casualty [Member]
Segment Reporting Information [Line Items]
Identifiable assets	15,277.9	14,526
Assets of Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 89.1	$ 86.6

Stock-based Compensation (Narra

Stock-based Compensation (Narrative) (Details) - shares	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Time-based, Market-based and Performance-based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Vesting period	3 years
Granted, Shares	25,414	21,789
Forfeited, Shares	6,339	2,836
Decrease as grant in period, percentage	100%
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	53,336	50,115
Forfeited, Shares	6,339	3,993
Performance-Based and Market-Based Restricted Stock Units [Member] | Minimum [Member] | Senior Management [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	0%
Performance-Based and Market-Based Restricted Stock Units [Member] | Maximum [Member] | Senior Management [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	150%
Performance-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	27,922	28,326
Performance-Based Restricted Stock Units [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	0%
Performance-Based Restricted Stock Units [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Performance metric, potential range	150%
Performance-Based Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Granted, Shares	2,615	5,961
Increase as grant in period, percentage	100%
Hanover Insurance Group 2022 Long-Term Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	2,356,776
Hanover Insurance Group 2023 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of shares available for grant	1,229,903

Stock-based Compensation (Compe

Stock-based Compensation (Compensation Cost and Related Tax Benefits) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Share-Based Payment Arrangement [Abstract]
Stock-based compensation expense	$ 8.7	$ 8.6	$ 23.5	$ 23.9
Tax benefit	(1.8)	(1.8)	(4.9)	(5)
Stock-based compensation expense, net of taxes	$ 6.9	$ 6.8	$ 18.6	$ 18.9

Stock-based Compensation (Summa

Stock-based Compensation (Summary of Stock Option Activity) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Shares
Outstanding, beginning of period, Shares	1,137,042	1,080,852
Granted, Shares	150,731	135,019
Exercised, Shares	(131,891)	(40,544)
Forfeited or cancelled, Shares	(2,642)	(22,184)
Outstanding, end of period, Shares	1,153,240	1,153,143
Weighted Average Exercise Price
Outstanding, beginning of period, Weighted Average Exercise Price	$ 111.57	$ 107.07
Granted, Weighted Average Exercise Price	134.26	140.01
Exercised, Weighted Average Exercise Price	88.87	92.39
Forfeited or cancelled, Weighted Average Exercise Price	134.99	125.15
Outstanding, end of period, Weighted Average Exercise Price	$ 117.08	$ 111.09

Stock-based Compensation (Sum_2

Stock-based Compensation (Summary of Activity Information about Employee Restricted Stock Units) (Details) - $ / shares	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Time-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	376,626	378,256
Granted, Shares	158,460	149,509
Vested, Shares	(123,973)	(132,086)
Forfeited, Shares	(21,705)	(19,132)
Outstanding, end of period, Shares	389,408	376,547
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 132.04	$ 124.88
Granted, Weighted Average Grant Date Fair Value	133.3	138.35
Vested, Weighted Average Grant Date Fair Value	116.25	118.21
Forfeited, Weighted Average Grant Date Fair Value	135.85	131.14
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 137.36	$ 132.25
Performance-Based and Market-Based Restricted Stock Units [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Outstanding, beginning of period, Shares	120,504	119,163
Granted, Shares	53,336	50,115
Vested, Shares	(41,290)	(44,781)
Forfeited, Shares	(6,339)	(3,993)
Outstanding, end of period, Shares	126,211	120,504
Outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 132.76	$ 122.81
Granted, Weighted Average Grant Date Fair Value	132.27	137.9
Vested, Weighted Average Grant Date Fair Value	112.36	114.14
Forfeited, Weighted Average Grant Date Fair Value	108.06	109.19
Outstanding, end of period, Weighted Average Grant Date Fair Value	$ 140.47	$ 132.76

Earnings Per Share and Shareh_3

Earnings Per Share and Shareholders' Equity Transactions (Information Regarding Basic and Diluted Earnings Per Share) (Details) - $ / shares shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Basic shares used in the calculation of earnings per share	36	35.8	35.9	35.7
Diluted shares used in the calculation of earnings per share	36.5	36.1	36.4	35.7
Per share effect of dilutive securities on income (loss) from continuing operations	$ (0.04)		$ (0.09)
Per share effect of dilutive securities on net income (loss)	$ (0.04)		$ (0.08)
Employee Stock Options [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of securities	0.2	0.1	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	0.3

Earnings Per Share and Shareh_4

Earnings Per Share and Shareholders' Equity Transactions (Narrative) (Details) - USD ($) shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Stock repurchase program, available authorized repurchase amount	$ 330,000,000		$ 330,000,000
Maximum [Member] | Accelerated Share Repurchase Agreement [Member]
Class Of Stock [Line Items]
Repurchases common stock, authorized	$ 1,300,000,000		$ 1,300,000,000
Stock Compensation Plans [Member]
Class Of Stock [Line Items]
Antidilutive securities excluded from calculation of earnings per share	0.4	0.7	0.4	0.6

Liabilities for Outstanding C_3

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Liability for Unpaid Losses and Loss Adjustment Expenses) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Insurance [Abstract]
Gross reserve for losses and LAE, beginning of period	$ 7,308.1	$ 7,012.6
Reinsurance recoverable on unpaid losses, beginning of period	1,795	1,748.6
Net reserve for losses and LAE, beginning of period	5,513.1	5,264
Net incurred losses and LAE in respect of losses occurring in current year	2,975.6	3,225.6
Net incurred losses and LAE in respect of losses occurring in prior years	(78.8)	(6.8)
Total incurred losses and LAE	2,896.8	3,218.8
Net payments of losses and LAE in respect of losses occurring in current year	1,225.3	1,375.7
Net payments of losses and LAE in respect of losses occurring in prior years	1,517.5	1,523.8
Total payments	2,742.8	2,899.5
Net reserve for losses and LAE, end of period	5,667.1	5,583.3
Reinsurance recoverable on unpaid losses, end of period	1,759.7	1,746.5
Gross reserve for losses and LAE, end of period	$ 7,426.8	$ 7,329.8

Liabilities for Outstanding C_4

Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (78.8)	$ (6.8)
Personal Automobile Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		16.4
Other Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(10.3)
Surety Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	14.1	9.2
Specialty [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(34.8)	43.1
Specialty [Member] | Professional and Executive Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(28.6)
Specialty [Member] | Favorable Catastrophe Development [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	24.8
Core Commercial [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(39.7)	1.2
Core Commercial [Member] | Commercial Multiple Peril Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		(13.5)
Core Commercial [Member] | Workers' Compensation Line [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development		17.7
Core Commercial [Member] | Certain Liability Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(10.7)
Core Commercial [Member] | Commercial Multiple Peril, Commercial Automobile, Workers' Compensation And Other Commercial Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(25.6)
Personal Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	(4.3)	35.1
Personal Lines [Member] | Personal Automobile Lines [Member]
Liability For Claims And Claims Adjustment Expense [Line Items]
(Favorable) unfavorable loss and LAE development	$ (13.4)	$ 11.4