The Progressive Corporation (PGR) 10-Q 31 Oct 23 2023 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2023 shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2023
Document Transition Report	false
Entity File Number	001-09518
Entity Incorporation, State or Country Code	OH
Entity Tax Identification Number	34-0963169
Entity Address, Address Line One	6300 Wilson Mills Road,
Entity Address, City or Town	Mayfield Village,
Entity Address, State or Province	OH
Entity Address, Postal Zip Code	44143
City Area Code	440
Local Phone Number	461-5000
Entity Information, Former Legal or Registered Name	Not Applicable
Title of 12(b) Security	Common Shares, $1.00 Par Value
Trading Symbol	PGR
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding	585,040,907
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	0000080661
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q3
Amendment Flag	false

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Revenues
Net premiums earned		$ 14,894.3	$ 12,398.9	$ 42,891.8	$ 36,349.7
Investment income		510.2	333.6	1,384.3	868.2
Net realized gains (losses) on securities:
Net realized gains (losses) on security sales		(66.2)	(62.1)	38.6	430
Net holding period gains (losses) on securities		(80.5)	(152.1)	17.9	(2,262.9)
Net impairment losses recognized in earnings		(2.3)	(2.2)	(6.8)	(6.5)
Total net realized gains (losses) on securities		(149)	(216.4)	49.7	(1,839.4)
Fees and other revenues		223.7	181.4	656.6	531.9
Service revenues		81.4	82.7	234.9	230.5
Total revenues		15,560.6	12,780.2	45,217.3	36,140.9
Expenses
Losses and loss adjustment expenses		11,387.9	10,018.7	34,182	28,298.2
Policy acquisition costs		1,173.2	970.9	3,442.6	2,867.9
Other underwriting expenses		1,420.7	1,496.4	4,710.3	4,433.9
Investment expenses		7.2	5.8	18.8	17.4
Service expenses		91.7	82.8	264.6	221.5
Interest expense		69.7	63.1	198.7	180.4
Goodwill impairment		0	0	0	224.8
Total expenses		14,150.4	12,637.7	42,817	36,244.1
Net Income (Loss)
Income (loss) before income taxes		1,410.2	142.5	2,400.3	(103.2)
Provision for income taxes		288.9	18.4	485.7	1.7
Net income (loss)		1,121.3	124.1	1,914.6	(104.9)
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on fixed-maturity securities		(388.6)	(920.3)	(241)	(3,170)
Net unrealized losses on forecasted transactions		0.2	0.1	0.4	0.3
Foreign currency translation adjustment		0	(0.7)	0.2	(0.9)
Other comprehensive income (loss)		(388.4)	(920.9)	(240.4)	(3,170.6)
Comprehensive income (loss)		732.9	(796.8)	1,674.2	(3,275.5)
Computation of Earnings Per Common Share
Net income (loss)		1,121.3	124.1	1,914.6	(104.9)
Less: Preferred share dividends	[1]	10.4	6.7	27.2	20.1
Net income (loss) available to common shareholders		$ 1,110.9	$ 117.4	$ 1,887.4	$ (125)
Average common shares outstanding - Basic (in shares)		584.8	584.5	584.9	584.4
Net effect of dilutive stock-based compensation (in shares)		2.7	2.6	2.6	2.7
Total average equivalent common shares - Diluted (in shares)		587.5	587.1	587.5	587.1
Basic: Earnings per common shares (in dollars per share)		$ 1.90	$ 0.20	$ 3.23	$ (0.21)
Diluted: Earnings per common share (in dollars per share)		$ 1.89	$ 0.20	$ 3.21	$ (0.21)
[1] Changed to a floating dividend rate in March 2023. See Note 1 – Basis of Reporting and Accounting for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Assets
Fixed maturities (amortized cost: $60,490.6, $48,205.7, and $50,264.0)		$ 56,591.9	$ 46,651.9	$ 44,173.1
Short-term investments (amortized cost: $1,795.2, $4,237.6, and $2,861.7)		1,795.2	2,861.7	4,237.6
Total available-for-sale securities		58,387.1	49,513.6	48,410.7
Nonredeemable preferred stocks (cost: $976.9, $1,417.6, and $1,364.2)		868.9	1,213.2	1,254.4
Common equities (cost: $662.6, $803.7, and $826.1)		2,614.5	2,821.5	2,665.3
Total equity securities		3,483.4	4,034.7	3,919.7
Total investments		61,870.5	53,548.3	52,330.4
Cash and cash equivalents		123.5	203.5	350.9
Restricted cash and cash equivalents		14.9	17.4	14.4
Total cash, cash equivalents, restricted cash, and restricted cash equivalents		138.4	220.9	365.3
Accrued investment income		378.1	282.5	217.4
Premiums receivable, net of allowance for credit losses of $361.2, $295.0, and $343.3		12,408	10,416.9	10,867.7
Reinsurance recoverables		5,331.2	5,832.1	6,306.8
Prepaid reinsurance premiums		235	295.5	367.5
Deferred acquisition costs		1,732.1	1,544.4	1,585.2
Property and equipment, net of accumulated depreciation of $1,596.9, $1,499.0, and $1,551.1		919.8	1,034	1,067.1
Goodwill		227.9	227.9	227.9
Intangible assets, net of accumulated amortization of $170.0, $153.1, and $158.6		74.9	86.3	91.8
Net federal deferred income taxes		1,357.1	1,131.5	1,269.5
Other assets		979.6	844.7	827.6
Total assets		85,652.6	75,465	75,524.2
Liabilities
Unearned premiums		20,761.7	17,293.6	17,796.9
Loss and loss adjustment expense reserves		33,577.3	30,359.3	30,631.8
Accounts payable, accrued expenses, and other liabilities		7,030.7	5,532.8	5,931.9
Debt	[1]	6,887.6	6,388.3	6,387.4
Total liabilities		68,257.3	59,574	60,748
Shareholders' Equity
Serial Preferred Shares, Series B, no par value (cumulative, liquidation preference $1,000 per share) (authorized, issued, and outstanding 0.5)		493.9	493.9	493.9
Common shares, $1.00 par value (authorized 900.0; issued 797.6, including treasury shares of 212.6, 212.5, and 212.7)		585	584.9	585.1
Paid-in capital		1,977.9	1,893	1,839.6
Retained earnings		17,380.9	15,721.2	14,987.5
Accumulated other comprehensive income (loss):
Net unrealized gains (losses) on fixed-maturity securities		(3,027.3)	(2,786.3)	(3,113.8)
Net unrealized losses on forecasted transactions		(14.1)	(14.5)	(14.6)
Foreign currency translation adjustment		(1)	(1.2)	(1.5)
Total accumulated other comprehensive income (loss)		(3,042.4)	(2,802)	(3,129.9)
Total shareholders’ equity		17,395.3	15,891	14,776.2
Total liabilities and shareholders’ equity		$ 85,652.6	$ 75,465	$ 75,524.2
[1] Consists of long-term debt. See Note 4 – Debt for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Common shares, par value (USD per share)	$ 1	$ 1	$ 1
Common shares, authorized (shares)	900,000,000	900,000,000	900,000,000
Common shares, issued (shares)	797,600,000	797,600,000	797,600,000
Treasury shares	212,600,000	212,700,000	212,500,000
Fixed maturities, amortized cost	$ 60,490.6	$ 50,264	$ 48,205.7
Short-term investments, amortized cost	1,795.2	2,861.7	4,237.6
Nonredeemable preferred stocks, cost	976.9	1,364.2	1,417.6
Common equities, cost	662.6	826.1	803.7
Premium receivable, allowance for credit loss	361.2	343.3	295
Property and equipment, accumulated depreciation	1,596.9	1,551.1	1,499
Intangible assets, accumulated amortization	$ 170	$ 158.6	$ 153.1
Serial Preferred Stock
Preferred stock, shares authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock | Serial Preferred Stock
Preferred stock, shares authorized (shares)	500,000	500,000	500,000
Preferred stock, no par value	$ 0	$ 0	$ 0
Preferred stock, liquidation preference per share (USD per share)	$ 1,000	$ 1,000	$ 1,000
Preferred stock, shares issued (shares)	500,000	500,000	500,000
Preferred stock, shares outstanding (shares)	500,000	500,000	500,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Serial Preferred Stock	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Series B Preferred Stock Serial Preferred Stock	Voting Preference Shares
Cash dividends declared on common stock (in dollars per share)	$ 0.30
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)								$ 26.875
Balance, Beginning of period at Dec. 31, 2021			$ 493.9	$ 584.4	$ 1,772.9	$ 15,339.7	$ 40.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ (104.9)					(104.9)
Treasury shares purchased				(0.7)	(2.2)	(75.7)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(175.2)
Cash dividends declared on Serial Preferred Shares, Series B ($20.67700, $26.875, $39.60163, and $26.875 per share)						(13.4)
Net restricted equity awards issued/vested				1.4	(1.4)
Amortization of equity-based compensation					69.7
Reinvested dividends on restricted stock units					0.6	(0.6)
Other, net						17.6
Other comprehensive income (loss)	(3,170.6)						(3,170.6)
Balance, End of period at Sep. 30, 2022	$ 14,776.2		493.9	585.1	1,839.6	14,987.5	(3,129.9)
Cash dividends declared on common stock (in dollars per share)	$ 0.10
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)								26.875
Balance, Beginning of period at Jun. 30, 2022			493.9	584.9	1,815.2	14,967.7	(2,209)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 124.1					124.1
Treasury shares purchased				(0.4)	(1.3)	(47.4)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(58.4)
Cash dividends declared on Serial Preferred Shares, Series B ($20.67700, $26.875, $39.60163, and $26.875 per share)						(13.4)
Net restricted equity awards issued/vested				0.6	(0.6)
Amortization of equity-based compensation					26
Reinvested dividends on restricted stock units					0.3	(0.3)
Other, net						15.2
Other comprehensive income (loss)	(920.9)						(920.9)
Balance, End of period at Sep. 30, 2022	$ 14,776.2		493.9	585.1	1,839.6	14,987.5	(3,129.9)
Preferred stock, no par value								$ 0
Common shares, par value (USD per share)	$ 1
Preferred stock, shares authorized (shares)		20,000,000						500,000	5,000,000
Preferred stock, shares issued (shares)								500,000	0
Preferred stock, no par value								$ 0
Common shares, par value (USD per share)	1
Preferred stock, shares authorized (shares)		20,000,000						500,000
Preferred stock, shares issued (shares)								500,000
Cash dividends declared on common stock (in dollars per share)	$ 0.30
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)								$ 39.60163
Balance, Beginning of period at Dec. 31, 2022	$ 15,891		493.9	584.9	1,893	15,721.2	(2,802)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	1,914.6					1,914.6
Treasury shares purchased				(0.6)	(2.1)	(77.1)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(175.4)
Cash dividends declared on Serial Preferred Shares, Series B ($20.67700, $26.875, $39.60163, and $26.875 per share)						(19.8)
Net restricted equity awards issued/vested				0.7	(0.7)
Amortization of equity-based compensation					86.7
Reinvested dividends on restricted stock units					1	(1)
Other, net						18.4
Other comprehensive income (loss)	(240.4)						(240.4)
Balance, End of period at Sep. 30, 2023	$ 17,395.3		493.9	585	1,977.9	17,380.9	(3,042.4)
Cash dividends declared on common stock (in dollars per share)	$ 0.10
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)								20.67700
Balance, Beginning of period at Jun. 30, 2023			493.9	585.3	1,935.7	16,350.4	(2,654)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 1,121.3					1,121.3
Treasury shares purchased				(0.3)	(1.1)	(38.3)
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($20.67700, $26.875, $39.60163, and $26.875 per share)						(10.3)
Net restricted equity awards issued/vested				0	0
Amortization of equity-based compensation					42.9
Reinvested dividends on restricted stock units					0.4	(0.4)
Other, net						16.7
Other comprehensive income (loss)	(388.4)						(388.4)
Balance, End of period at Sep. 30, 2023	$ 17,395.3		$ 493.9	$ 585	$ 1,977.9	$ 17,380.9	$ (3,042.4)
Preferred stock, no par value								$ 0
Common shares, par value (USD per share)	$ 1
Preferred stock, shares authorized (shares)		20,000,000						500,000	5,000,000
Preferred stock, shares issued (shares)								500,000	0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Cash Flows From Operating Activities
Net income (loss)	$ 1,914.6	$ (104.9)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation	212	227.5
Amortization of intangible assets	11.4	25.5
Net amortization (accretion) of fixed-income securities	(7.6)	1.6
Amortization of equity-based compensation	86.7	69.7
Net realized (gains) losses on securities	(49.7)	1,839.4
Net (gains) losses on disposition of property and equipment	23	5.3
Goodwill impairment	0	224.8
Changes in:
Premiums receivable	(1,991.1)	(1,468.2)
Reinsurance recoverables	500.9	(1,326.3)
Prepaid reinsurance premiums	60.5	90.1
Deferred acquisition costs	(187.7)	(229.6)
Income taxes	73.6	(591.6)
Unearned premiums	3,468.1	2,181.1
Loss and loss adjustment expense reserves	3,218	4,467.7
Accounts payable, accrued expenses, and other liabilities	1,089.1	546.5
Other, net	(215.8)	(39.4)
Net cash provided by operating activities	8,206	5,919.2
Purchases:
Fixed maturities	(18,956.2)	(18,672.7)
Equity securities	(23.2)	(126.5)
Sales:
Fixed maturities	4,780.3	9,737.6
Equity securities	745.5	1,467
Maturities, paydowns, calls, and other:
Fixed maturities	3,693.2	4,158.5
Equity securities	65.3	39.3
Net (purchases) sales of short-term investments	1,132	(3,266.6)
Net change in unsettled security transactions	201.7	(68.7)
Purchases of property and equipment	(169.9)	(245)
Sales of property and equipment	35	16
Net cash used in investing activities	(8,496.3)	(6,961.1)
Cash Flows From Financing Activities
Dividends paid to common shareholders	(175.5)	(175.5)
Dividends paid to preferred shareholders	(33.2)	(26.8)
Acquisition of treasury shares for restricted stock tax liabilities	(52.3)	(76.2)
Acquisition of treasury shares acquired in open market	(27.5)	(2.4)
Net proceeds from debt issuances	496.3	1,486
Net cash provided by financing activities	207.8	1,205.1
Increase (decrease) in cash, cash equivalents, restricted cash, and restricted cash equivalents	(82.5)	163.2
Cash, cash equivalents, restricted cash, and restricted cash equivalents – January 1	220.9	202.1
Cash, cash equivalents, restricted cash, and restricted cash equivalents – September 30	$ 138.4	$ 365.3

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Reporting and Accounting	BASIS OF REPORTING AND ACCOUNTING The accompanying consolidated financial statements include the accounts of The Progressive Corporation, our wholly owned insurance subsidiaries and non-insurance subsidiaries and affiliates in which we have a controlling financial interest (Progressive). The consolidated financial statements reflect all normal recurring adjustments that, in the opinion of management, were necessary for a fair statement of the results for the interim periods presented. The results of operations for the period ended September 30, 2023, are not necessarily indicative of the results expected for the full year. These consolidated financial statements and the notes thereto should be read in conjunction with Progressive’s audited financial statements and accompanying notes included in Exhibit 13 to our Annual Report on Form 10-K for the year ended December 31, 2022 (2022 Annual Report to Shareholders). Insurance Premiums and Receivables We perform analyses to evaluate our premiums receivable for expected credit losses. See our 2022 Annual Report to Shareholders for a discussion on our premiums receivable allowance for credit loss policy. The following table summarizes changes in our allowance for credit loss exposure on our premiums receivable: Three Months Ended September 30, Nine Months Ended September 30, (millions) 2023 2022 2023 2022 Allowance for credit losses, beginning of period $ 343.9 $ 265.8 $ 343.3 $ 280.4 Increase in allowance 1 157.5 130.9 399.7 320.6 Write-offs 2 (140.2) (101.7) (381.8) (306.0) Allowance for credit losses, end of period $ 361.2 $ 295.0 $ 361.2 $ 295.0 1 Represents the incremental increase in other underwriting expenses. 2 Represents the portion of the allowance that is reversed when premiums receivable are written off. Premiums receivable balances are written off once we have exhausted our collection efforts. Property and Equipment Included in other assets in our consolidated balance sheets are “held for sale” property. When property is transferred to held for sale, the property is written down to its fair value less estimated costs to sell the property. At September 30, 2023 and 2022, and December 31, 2022, we had held for sale property of $81.6 million, $47.4 million, and $48.7 million, respectively. Earnings Per Share Net income is reduced by preferred share dividends to determine net income available to common shareholders and is used in our calculation of the per common share amounts. Beginning March 15, 2023, the annual dividend rate for our Series B Preferred Shares switched to a floating rate. The floating nature of the dividend rate will impact the amount of the adjustment required to calculate net income available to common shareholders. See Note 9 – Dividends for further discussion. For periods when a net loss is reported, earnings per common share are calculated using basic average equivalent shares since diluted earnings per share would be antidilutive. New Accounting Standards During the first quarter 2023, the Financial Accounting Standards Board issued an Accounting Standards Update (ASU), which permits an election to amortize the cost of tax equity investments to the provision for income taxes if certain conditions are met. Currently, these investments are accounted for under the equity method of accounting. This ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2023 (2024 for calendar-year companies). If elected, this standard must be applied on either a modified retrospective or a retrospective basis. We intend to adopt the new standard under the modified retrospective basis, however, we do not expect this standard to have a material impact on our financial condition or results of operations.

Investments

Investments	9 Months Ended
	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Investments	INVESTMENTS The following tables present the composition of our investment portfolio by major security type. Our securities are reported in our consolidated balance sheets at fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income (loss), net of deferred income taxes, in our consolidated balance sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value for the hybrid and equity securities. The changes in the net holding period gains (losses) between periods are recorded as a component of net realized gains (losses) on securities in our consolidated statements of comprehensive income. ($ in millions) Cost Gross Gross Net Fair % of September 30, 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 36,392.2 $ 0 $ (2,098.4) $ 0 $ 34,293.8 55.4 % State and local government obligations 2,257.6 0 (203.3) 0 2,054.3 3.3 Foreign government obligations 16.9 0 (1.5) 0 15.4 0.1 Corporate debt securities 10,880.8 1.3 (591.8) (36.1) 10,254.2 16.6 Residential mortgage-backed securities 530.4 0.2 (13.0) (1.8) 515.8 0.8 Commercial mortgage-backed securities 4,699.5 2.1 (723.9) 0 3,977.7 6.4 Other asset-backed securities 5,524.2 0.1 (210.1) (0.6) 5,313.6 8.6 Redeemable preferred stocks 189.0 0 (4.3) (17.6) 167.1 0.3 Total fixed maturities 60,490.6 3.7 (3,846.3) (56.1) 56,591.9 91.5 Short-term investments 1,795.2 0 0 0 1,795.2 2.9 Total available-for-sale securities 62,285.8 3.7 (3,846.3) (56.1) 58,387.1 94.4 Equity securities: Nonredeemable preferred stocks 976.9 0 0 (108.0) 868.9 1.4 Common equities 662.6 0 0 1,951.9 2,614.5 4.2 Total equity securities 1,639.5 0 0 1,843.9 3,483.4 5.6 Total portfolio 1 $ 63,925.3 $ 3.7 $ (3,846.3) $ 1,787.8 $ 61,870.5 100.0 % ($ in millions) Cost Gross Gross Net Fair % of September 30, 2022 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 24,260.9 $ 0 $ (1,855.4) $ 0 $ 22,405.5 42.8 % State and local government obligations 2,140.9 0 (215.4) 0 1,925.5 3.7 Foreign government obligations 16.7 0 (1.4) 0 15.3 0.1 Corporate debt securities 10,119.8 0 (832.5) (58.9) 9,228.4 17.6 Residential mortgage-backed securities 754.2 0.4 (16.5) (15.1) 723.0 1.4 Commercial mortgage-backed securities 5,833.9 1.4 (747.4) 0 5,087.9 9.7 Other asset-backed securities 4,876.7 0 (268.9) (1.9) 4,605.9 8.8 Redeemable preferred stocks 202.6 0 (5.6) (15.4) 181.6 0.3 Total fixed maturities 48,205.7 1.8 (3,943.1) (91.3) 44,173.1 84.4 Short-term investments 4,237.6 0 0 0 4,237.6 8.1 Total available-for-sale securities 52,443.3 1.8 (3,943.1) (91.3) 48,410.7 92.5 Equity securities: Nonredeemable preferred stocks 1,417.6 0 0 (163.2) 1,254.4 2.4 Common equities 803.7 0 0 1,861.6 2,665.3 5.1 Total equity securities 2,221.3 0 0 1,698.4 3,919.7 7.5 Total portfolio 1 $ 54,664.6 $ 1.8 $ (3,943.1) $ 1,607.1 $ 52,330.4 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2022 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 26,770.7 $ 1.4 $ (1,604.7) $ 0 $ 25,167.4 47.0 % State and local government obligations 2,180.0 0 (202.9) 0 1,977.1 3.7 Foreign government obligations 16.8 0 (1.3) 0 15.5 0.1 Corporate debt securities 10,125.8 9.8 (676.1) (46.8) 9,412.7 17.6 Residential mortgage-backed securities 696.1 0.3 (17.5) (12.1) 666.8 1.2 Commercial mortgage-backed securities 5,446.0 1.5 (784.0) 0 4,663.5 8.7 Other asset-backed securities 4,826.0 0.9 (260.5) (1.8) 4,564.6 8.5 Redeemable preferred stocks 202.6 0 (4.5) (13.8) 184.3 0.3 Total fixed maturities 50,264.0 13.9 (3,551.5) (74.5) 46,651.9 87.1 Short-term investments 2,861.7 0 0 0 2,861.7 5.4 Total available-for-sale securities 53,125.7 13.9 (3,551.5) (74.5) 49,513.6 92.5 Equity securities: Nonredeemable preferred stocks 1,364.2 0 0 (151.0) 1,213.2 2.3 Common equities 826.1 0 0 1,995.4 2,821.5 5.2 Total equity securities 2,190.3 0 0 1,844.4 4,034.7 7.5 Total portfolio 1 $ 55,316.0 $ 13.9 $ (3,551.5) $ 1,769.9 $ 53,548.3 100.0 % 1 At September 30, 2023 and 2022, we had $167.3 million and $74.7 million, respectively, of net unsettled security purchase transactions included in other liabilities, compared to $34.4 million included in other assets at December 31, 2022. The total fair value of the portfolio at September 30, 2023 and 2022, and December 31, 2022, included $4.1 billion, $4.2 billion, and $4.4 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions. At September 30, 2023, bonds and certificates of deposit in the principal amount of $612.4 million were on deposit to meet state insurance regulatory requirements. We did not hold any securities of any one issuer, excluding U.S. government obligations, with an aggregate cost or fair value exceeding 10% of total shareholders’ equity at September 30, 2023 or 2022, or December 31, 2022. At September 30, 2023, we did not hold any debt securities that were non-income producing during the preceding 12 months. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature or are redeemable within one year. We did not have any open repurchase or reverse repurchase transactions positions at September 30, 2023 and 2022, or December 31, 2022, and did not enter into any such transactions during the first nine months of 2023. During 2022, however, we did invest in repurchase transactions that had an immaterial impact on our results of operations and cash flows. To the extent we enter into repurchase or reverse repurchase transactions, consistent with past practice, we would elect not to offset these transactions and would report them on a gross basis in our consolidated balance sheets, despite the option to elect to offset these transactions as long as they were with the same counterparty and subject to an enforceable master netting arrangement. Hybrid Securities Certain securities in our fixed-maturity portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. These securities are reported at fair value: September 30, (millions) 2023 2022 Dec. 31, 2022 Fixed Maturities: Corporate debt securities $ 511.1 $ 500.4 $ 535.4 Residential mortgage-backed securities 410.8 550.8 509.6 Other asset-backed securities 19.4 51.4 42.0 Redeemable preferred stocks 136.4 133.0 134.7 Total hybrid securities $ 1,077.7 $ 1,235.6 $ 1,221.7 Since the embedded derivatives (e.g., change-in-control put option, debt-to-equity conversion, or any other feature unrelated to the credit quality or risk of default of the issuer that could impact the amount or timing of our expected future cash flows) do not have observable intrinsic values, we use the fair value option to record the changes in fair value of these securities through income as a component of net realized gains or losses. Fixed Maturities The composition of fixed maturities by maturity at September 30, 2023, was: (millions) Cost Fair Value Less than one year $ 9,639.9 $ 9,387.8 One to five years 37,572.4 35,419.0 Five to ten years 13,197.7 11,710.3 Ten years or greater 80.6 74.8 Total $ 60,490.6 $ 56,591.9 Asset-backed securities are classified in the maturity distribution table based upon their projected cash flows. All other securities that do not have a single maturity date are reported based upon expected average maturity. Contractual maturities may differ from expected maturities because the issuers of the securities may have the right to call or prepay obligations. Gross Unrealized Losses The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2023 U.S. government obligations 173 $ 34,293.8 $ (2,098.4) 37 $ 16,686.2 $ (557.6) 136 $ 17,607.6 $ (1,540.8) State and local government obligations 356 2,033.9 (203.3) 46 359.6 (7.6) 310 1,674.3 (195.7) Foreign government obligations 1 15.4 (1.5) 0 0 0 1 15.4 (1.5) Corporate debt securities 467 9,490.0 (591.8) 164 3,380.4 (94.1) 303 6,109.6 (497.7) Residential mortgage-backed securities 40 99.9 (13.0) 2 0.4 0 38 99.5 (13.0) Commercial mortgage-backed securities 196 3,960.8 (723.9) 0 0 0 196 3,960.8 (723.9) Other asset-backed securities 295 5,001.8 (210.1) 104 2,056.9 (8.8) 191 2,944.9 (201.3) Redeemable preferred stocks 3 30.6 (4.3) 0 0 0 3 30.6 (4.3) Total fixed maturities 1,531 $ 54,926.2 $ (3,846.3) 353 $ 22,483.5 $ (668.1) 1,178 $ 32,442.7 $ (3,178.2) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2022 U.S. government obligations 157 $ 22,385.4 $ (1,855.4) 103 $ 16,085.8 $ (1,095.6) 54 $ 6,299.6 $ (759.8) State and local government obligations 353 1,911.0 (215.4) 292 1,363.1 (129.8) 61 547.9 (85.6) Foreign government obligations 1 15.3 (1.4) 1 15.3 (1.4) 0 0 0 Corporate debt securities 452 8,885.6 (832.5) 391 7,875.5 (699.0) 61 1,010.1 (133.5) Residential mortgage-backed securities 44 165.1 (16.5) 34 126.0 (11.8) 10 39.1 (4.7) Commercial mortgage-backed securities 234 5,074.3 (747.4) 182 3,856.0 (425.4) 52 1,218.3 (322.0) Other asset-backed securities 277 4,544.8 (268.9) 217 3,597.1 (183.4) 60 947.7 (85.5) Redeemable preferred stocks 4 48.6 (5.6) 3 37.7 (4.0) 1 10.9 (1.6) Total fixed maturities 1,522 $ 43,030.1 $ (3,943.1) 1,223 $ 32,956.5 $ (2,550.4) 299 $ 10,073.6 $ (1,392.7) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized December 31, 2022 U.S. government obligations 160 $ 24,802.5 $ (1,604.7) 90 $ 17,327.2 $ (699.2) 70 $ 7,475.3 $ (905.5) State and local government obligations 348 1,948.8 (202.9) 239 1,124.2 (76.8) 109 824.6 (126.1) Foreign government obligations 1 15.5 (1.3) 0 0 0 1 15.5 (1.3) Corporate debt securities 422 8,449.6 (676.1) 285 5,717.6 (426.1) 137 2,732.0 (250.0) Residential mortgage-backed securities 45 151.0 (17.5) 27 65.1 (6.8) 18 85.9 (10.7) Commercial mortgage-backed securities 226 4,651.1 (784.0) 99 1,702.0 (192.1) 127 2,949.1 (591.9) Other asset-backed securities 262 4,247.8 (260.5) 130 2,144.8 (100.9) 132 2,103.0 (159.6) Redeemable preferred stocks 4 49.6 (4.5) 3 38.5 (3.1) 1 11.1 (1.4) Total fixed maturities 1,468 $ 44,315.9 $ (3,551.5) 873 $ 28,119.4 $ (1,505.0) 595 $ 16,196.5 $ (2,046.5) During the third quarter 2023, the credit ratings of our U.S. government obligations went down due to two major credit rating agencies rating U.S. government debt at AA+ instead of AAA. Additionally, we had 14 securities in other fixed-maturity sectors that had their credit ratings downgraded, with a combined fair value of $144.2 million and an unrealized loss of $7.1 million as of September 30, 2023. A review of the securities in an unrealized loss position indicated that the issuers were current with respect to their interest obligations and that there was no evidence of deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. Allowance For Credit and Uncollectible Losses We are required to measure the amount of potential credit losses for all fixed-maturity securities in an unrealized loss position. We did not record any allowances for credit losses or any write-offs for amounts deemed to be uncollectible during the first nine months of 2023 or 2022, and did not have a material credit loss allowance balance as of September 30, 2023 and 2022, or December 31, 2022. We considered several factors and inputs related to the individual securities as part of our analysis. The methodology and significant inputs used to measure the amount of credit losses in our portfolio included: • current performance indicators on the business model or underlying assets (e.g., delinquency rates, foreclosure rates, and default rates); • credit support (via current levels of subordination); • historical credit ratings; and • updated cash flow expectations based upon these performance indicators. In order to determine the amount of credit loss, if any, we initially reviewed securities in a loss position to determine whether it was likely that we would be required, or intended, to sell any of the securities prior to the recovery of their respective cost bases (which could be maturity). If we were likely to, or intended to, sell prior to a potential recovery, we would write off the unrealized loss. For those securities that we determined we were not likely to, or did not intend to, sell prior to a potential recovery, we performed additional analysis to determine if the loss was credit related. For securities subject to credit related loss, we calculated the net present value (NPV) of the cash flows expected (i.e., expected recovery value) using the current book yield for each security. The NPV was then compared to the security’s current amortized value to determine if a credit loss existed. In the event that the NPV was below the amortized value, and the amount was determined to be material individually, or in the aggregate, a credit loss would be deemed to exist, and either an allowance for credit losses would be created, or if an allowance currently existed, either a recovery of the previous allowance, or an incremental loss, would be recorded to net realized gains (losses) on securities. As of September 30, 2023 and 2022, and December 31, 2022, we believe none of the unrealized losses were related to material credit losses on any individual security, or in the aggregate. We continue to expect all the securities in our portfolio to pay their principal and interest obligations. In addition, we reviewed our accrued investment income outstanding on those securities in an unrealized loss position at September 30, 2023 and 2022, and December 31, 2022, to determine if the accrued interest amounts were determined to be uncollectible. Based on our analysis, we believe the issuers have sufficient liquidity and capital reserves to meet their current interest, and future principal, obligations and, therefore, did not write off any accrued income as uncollectible at September 30, 2023 and 2022, or December 31, 2022. Realized Gains (Losses) The components of net realized gains (losses) for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2023 2022 2023 2022 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0.2 $ 0 $ 4.2 $ 4.6 Corporate debt securities 0.7 0.3 0.8 6.8 Residential mortgage-backed securities 0 0 0 0.7 Other asset-backed securities 0 0 0 0.1 Total available-for-sale securities 0.9 0.3 5.0 12.2 Equity securities: Nonredeemable preferred stocks 0 0.1 0.3 17.6 Common equities 0.3 1.4 354.2 832.5 Total equity securities 0.3 1.5 354.5 850.1 Subtotal gross realized gains on security sales 1.2 1.8 359.5 862.3 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (14.6) 0 (27.2) (233.5) State and local government obligations 0 0 0 (1.0) Corporate debt securities (19.1) (14.0) (69.3) (51.8) Commercial mortgage-backed securities (20.6) (44.8) (100.6) (58.6) Other asset-backed securities (5.2) 0 (5.6) (2.1) Short-term investments 0 0 (0.4) (0.3) Total available-for-sale securities (59.5) (58.8) (203.1) (347.3) Equity securities: Nonredeemable preferred stocks (7.4) (5.1) (117.6) (7.0) Common equities (8.9) 0 (21.8) (78.0) Total equity securities (16.3) (5.1) (139.4) (85.0) Subtotal gross realized losses on security sales (75.8) (63.9) (342.5) (432.3) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (14.4) 0 (23.0) (228.9) State and local government obligations 0 0 0 (1.0) Corporate debt securities (18.4) (13.7) (68.5) (45.0) Residential mortgage-backed securities 0 0 0 0.7 Commercial mortgage-backed securities (20.6) (44.8) (100.6) (58.6) Other asset-backed securities (5.2) 0 (5.6) (2.0) Short-term investments 0 0 (0.4) (0.3) Total available-for-sale securities (58.6) (58.5) (198.1) (335.1) Equity securities: Nonredeemable preferred stocks (7.4) (5.0) (117.3) 10.6 Common equities (8.6) 1.4 332.4 754.5 Total equity securities (16.0) (3.6) 215.1 765.1 Subtotal net realized gains (losses) on security sales (74.6) (62.1) 17.0 430.0 Other assets Gain 8.4 0 21.6 0 Impairment (2.3) (2.2) (6.8) (6.5) Subtotal net realized gains (losses) on other assets 6.1 (2.2) 14.8 (6.5) Net holding period gains (losses) Hybrid securities (0.3) (11.4) 18.4 (98.8) Equity securities (80.2) (140.7) (0.5) (2,164.1) Subtotal net holding period gains (losses) (80.5) (152.1) 17.9 (2,262.9) Total net realized gains (losses) on securities $ (149.0) $ (216.4) $ 49.7 $ (1,839.4) Realized gains (losses) on securities sold are computed using the first-in-first-out method. For the first nine months of both 2023 and 2022, the gross gains in common equities reflected sales of securities, as part of our plan to incrementally reduce risk in the portfolio in response to our view of the potential of a more difficult economic environment in both years. The gross loss from the fixed-maturity sales reflected the continued rise in interest rates throughout 2022, which resulted in valuation declines for most of our available-for-sale securities. In addition, during 2023, we selectively sold securities, which were primarily corporate debt securities and commercial mortgage-backed securities. The gross loss incurred in our nonredeemable preferred stocks was primarily related to the sale of certain holdings in U.S. bank preferred stocks. The other asset gain for 2023, related to proceeds received as the result of litigation in conjunction with three renewable energy investments we made from 2016 through 2018 (the original investments were previously written down in full). The other asset impairment loss was recorded as a result of our investment in a federal new markets tax credit fund, which was entered into during the second quarter 2021, and is reported in other assets in our consolidated balance sheets. The following table reflects our holding period realized gains (losses) recognized on equity securities held at the respective periods ended September 30: Three Months Nine Months (millions) 2023 2022 2023 2022 Total net gains (losses) recognized during the period on equity securities $ (96.2) $ (144.3) $ 214.6 $ (1,399.0) Less: Net gains (losses) recognized on equity securities sold during the period (16.0) (3.6) 215.1 765.1 Net holding period gains (losses) recognized during the period on equity securities held at period end $ (80.2) $ (140.7) $ (0.5) $ (2,164.1) Net Investment Income The components of net investment income for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2023 2022 2023 2022 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 237.0 $ 86.1 $ 591.7 $ 205.4 State and local government obligations 12.3 9.8 34.5 29.3 Foreign government obligations 0.1 0.1 0.2 0.2 Corporate debt securities 94.8 71.8 261.9 215.4 Residential mortgage-backed securities 7.6 8.8 22.6 23.2 Commercial mortgage-backed securities 48.7 48.3 147.9 138.1 Other asset-backed securities 62.9 52.1 173.0 116.5 Redeemable preferred stocks 2.6 3.0 8.2 8.5 Total fixed maturities 466.0 280.0 1,240.0 736.6 Short-term investments 21.6 24.8 71.1 29.6 Total available-for-sale securities 487.6 304.8 1,311.1 766.2 Equity securities: Nonredeemable preferred stocks 11.9 17.3 40.3 53.6 Common equities 10.7 11.5 32.9 48.4 Total equity securities 22.6 28.8 73.2 102.0 Investment income 510.2 333.6 1,384.3 868.2 Investment expenses (7.2) (5.8) (18.8) (17.4) Net investment income $ 503.0 $ 327.8 $ 1,365.5 $ 850.8 On a year-over-year basis, investment income (interest and dividends) increased 53% and 59% for the three and nine months ended September 30, 2023, respectively, and the recurring investment book yield increased 32% and 39% for the three and nine months ended September 30, 2023, respectively, compared to the same periods last year. The increases are primarily due to an increase in interest rates on floating-rate securities in our portfolio, an increase in average assets resulting from premium growth, and investing new cash and cash from maturities in higher interest rate securities given the rising interest rate environment.

Fair Value

Fair Value	9 Months Ended
	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Fair Value	FAIR VALUE We have categorized our financial instruments, based on the degree of subjectivity inherent in the method by which they are valued, into a fair value hierarchy of three levels, as follows: • Level 1 : Inputs are unadjusted, quoted prices in active markets for identical instruments at the measurement date (e.g., U.S. government obligations, which are continually priced on a daily basis, active exchange-traded equity securities, and certain short-term securities). • Level 2 : Inputs (other than quoted prices included within Level 1) that are observable for the instrument either directly or indirectly (e.g., certain corporate and municipal bonds and certain preferred stocks). This includes: (i) quoted prices for similar instruments in active markets, (ii) quoted prices for identical or similar instruments in markets that are not active, (iii) inputs other than quoted prices that are observable for the instruments, and (iv) inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 : Inputs that are unobservable. Unobservable inputs reflect our subjective evaluation about the assumptions market participants would use in pricing the financial instrument (e.g., certain structured securities and privately held investments). Determining the fair value of the investment portfolio is the responsibility of management. As part of that responsibility, we evaluate whether a market is distressed or inactive in determining the fair value for our portfolio. We review certain market level inputs to evaluate whether sufficient activity, volume, and new issuances exist to create an active market. Based on this evaluation, we concluded that there was sufficient activity related to the sectors and securities for which we obtained valuations. The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2023 Fixed maturities: U.S. government obligations $ 34,293.8 $ 0 $ 0 $ 34,293.8 $ 36,392.2 State and local government obligations 0 2,054.3 0 2,054.3 2,257.6 Foreign government obligations 0 15.4 0 15.4 16.9 Corporate debt securities 0 10,254.2 0 10,254.2 10,880.8 Subtotal 34,293.8 12,323.9 0 46,617.7 49,547.5 Asset-backed securities: Residential mortgage-backed 0 515.8 0 515.8 530.4 Commercial mortgage-backed 0 3,977.7 0 3,977.7 4,699.5 Other asset-backed 0 5,313.6 0 5,313.6 5,524.2 Subtotal asset-backed securities 0 9,807.1 0 9,807.1 10,754.1 Redeemable preferred stocks: Financials 0 21.3 0 21.3 24.5 Utilities 0 9.3 0 9.3 10.4 Industrials 0 136.5 0 136.5 154.1 Subtotal redeemable preferred stocks 0 167.1 0 167.1 189.0 Total fixed maturities 34,293.8 22,298.1 0 56,591.9 60,490.6 Short-term investments 1,763.2 32.0 0 1,795.2 1,795.2 Total available-for-sale securities 36,057.0 22,330.1 0 58,387.1 62,285.8 Equity securities: Nonredeemable preferred stocks: Financials 0 770.7 47.9 818.6 921.9 Utilities 0 34.5 0 34.5 40.0 Industrials 0 0 15.8 15.8 15.0 Subtotal nonredeemable preferred stocks 0 805.2 63.7 868.9 976.9 Common equities: Common stocks 2,573.6 0 18.3 2,591.9 640.0 Other risk investments 0 0 22.6 22.6 22.6 Subtotal common equities 2,573.6 0 40.9 2,614.5 662.6 Total equity securities 2,573.6 805.2 104.6 3,483.4 1,639.5 Total portfolio $ 38,630.6 $ 23,135.3 $ 104.6 $ 61,870.5 $ 63,925.3 Debt $ 0 $ 5,932.1 $ 0 $ 5,932.1 $ 6,887.6 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2022 Fixed maturities: U.S. government obligations $ 22,405.5 $ 0 $ 0 $ 22,405.5 $ 24,260.9 State and local government obligations 0 1,925.5 0 1,925.5 2,140.9 Foreign government obligations 0 15.3 0 15.3 16.7 Corporate debt securities 0 9,228.4 0 9,228.4 10,119.8 Subtotal 22,405.5 11,169.2 0 33,574.7 36,538.3 Asset-backed securities: Residential mortgage-backed 0 723.0 0 723.0 754.2 Commercial mortgage-backed 0 5,087.9 0 5,087.9 5,833.9 Other asset-backed 0 4,605.9 0 4,605.9 4,876.7 Subtotal asset-backed securities 0 10,416.8 0 10,416.8 11,464.8 Redeemable preferred stocks: Financials 0 39.8 0 39.8 43.6 Utilities 0 8.8 0 8.8 10.5 Industrials 8.9 124.1 0 133.0 148.5 Subtotal redeemable preferred stocks 8.9 172.7 0 181.6 202.6 Total fixed maturities 22,414.4 21,758.7 0 44,173.1 48,205.7 Short-term investments 4,219.6 18.0 0 4,237.6 4,237.6 Total available-for-sale securities 26,634.0 21,776.7 0 48,410.7 52,443.3 Equity securities: Nonredeemable preferred stocks: Financials 60.9 1,015.6 64.9 1,141.4 1,297.6 Utilities 0 71.3 0 71.3 79.9 Industrials 0 24.7 17.0 41.7 40.1 Subtotal nonredeemable preferred stocks 60.9 1,111.6 81.9 1,254.4 1,417.6 Common equities: Common stocks 2,597.3 49.3 0 2,646.6 785.0 Other risk investments 0 0 18.7 18.7 18.7 Subtotal common equities 2,597.3 49.3 18.7 2,665.3 803.7 Total equity securities 2,658.2 1,160.9 100.6 3,919.7 2,221.3 Total portfolio $ 29,292.2 $ 22,937.6 $ 100.6 $ 52,330.4 $ 54,664.6 Debt $ 0 $ 5,634.7 $ 0 $ 5,634.7 $ 6,387.4 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2022 Fixed maturities: U.S. government obligations $ 25,167.4 $ 0 $ 0 $ 25,167.4 $ 26,770.7 State and local government obligations 0 1,977.1 0 1,977.1 2,180.0 Foreign government obligations 0 15.5 0 15.5 16.8 Corporate debt securities 0 9,412.7 0 9,412.7 10,125.8 Subtotal 25,167.4 11,405.3 0 36,572.7 39,093.3 Asset-backed securities: Residential mortgage-backed 0 666.8 0 666.8 696.1 Commercial mortgage-backed 0 4,663.5 0 4,663.5 5,446.0 Other asset-backed 0 4,564.6 0 4,564.6 4,826.0 Subtotal asset-backed securities 0 9,894.9 0 9,894.9 10,968.1 Redeemable preferred stocks: Financials 0 40.5 0 40.5 43.6 Utilities 0 9.1 0 9.1 10.5 Industrials 9.2 125.5 0 134.7 148.5 Subtotal redeemable preferred stocks 9.2 175.1 0 184.3 202.6 Total fixed maturities 25,176.6 21,475.3 0 46,651.9 50,264.0 Short-term investments 2,800.7 61.0 0 2,861.7 2,861.7 Total available-for-sale securities 27,977.3 21,536.3 0 49,513.6 53,125.7 Equity securities: Nonredeemable preferred stocks: Financials 39.0 994.4 67.4 1,100.8 1,244.2 Utilities 0 71.2 0 71.2 79.9 Industrials 0 24.8 16.4 41.2 40.1 Subtotal nonredeemable preferred stocks 39.0 1,090.4 83.8 1,213.2 1,364.2 Common equities: Common stocks 2,783.4 0 18.3 2,801.7 806.3 Other risk investments 0 0 19.8 19.8 19.8 Subtotal common equities 2,783.4 0 38.1 2,821.5 826.1 Total equity securities 2,822.4 1,090.4 121.9 4,034.7 2,190.3 Total portfolio $ 30,799.7 $ 22,626.7 $ 121.9 $ 53,548.3 $ 55,316.0 Debt $ 0 $ 5,717.9 $ 0 $ 5,717.9 $ 6,388.3 Our portfolio valuations, excluding short-term investments, classified as either Level 1 or Level 2 in the above tables are priced exclusively by external sources, including pricing vendors, dealers/market makers, and exchange-quoted prices. Our short-term investments classified as Level 1 are highly liquid, actively marketed, and have a very short duration, primarily 90 days or less to redemption. These securities are held at their original cost, adjusted for any accretion of discount, since that value very closely approximates what an active market participant would be willing to pay for such securities. The remainder of our short-term investments are classified as Level 2 and are not priced externally since these securities continually trade at par value. These securities are classified as Level 2 since they are primarily longer-dated securities issued by municipalities that contain either liquidity facilities or mandatory put features within one year. At September 30, 2023, vendor-quoted prices represented 93% of our Level 1 classifications (excluding short-term investments), compared to 90% at both September 30, 2022 and December 31, 2022. The securities quoted by vendors in Level 1 primarily represent our holdings in U.S. Treasury Notes, which are frequently traded, and the quotes are considered similar to exchange-traded quotes. The balance of our Level 1 pricing comes from quotes obtained directly from trades made on active exchanges. All Level 1 preferred stocks with active exchange quotes were sold as of September 30, 2023. At September 30, 2023, vendor-quoted prices comprised 99% of our Level 2 classifications (excluding short-term investments and common stock), while dealer-quoted prices represented the remaining 1%, compared to 98% and 2%, respectively, at both September 30, 2022 and December 31, 2022. In our process for selecting a source (e.g., dealer or pricing service) to provide pricing for securities in our portfolio, we reviewed documentation from the sources that detailed the pricing techniques and methodologies used by these sources and determined if their policies adequately considered market activity, either based on specific transactions for the particular security type or based on modeling of securities with similar credit quality, duration, yield, and structure that were recently transacted. Once a source is chosen, we continue to monitor any changes or modifications to their processes by reviewing their documentation on internal controls for pricing and market reviews. We review quality control measures of our sources as they become available to determine if any significant changes have occurred from period to period that might indicate issues or concerns regarding their evaluation or market coverage. As part of our pricing procedures, we obtain quotes from more than one source to help us fully evaluate the market price of securities. However, our internal pricing policy is to use a consistent source for individual securities in order to maintain the integrity of our valuation process. Quotes obtained from the sources are not considered binding offers to transact. Under our policy, when a review of the valuation received from our selected source appears to be outside of what is considered market level activity (which is defined as trading at spreads or yields significantly different than those of comparable securities or outside the general sector level movement without a reasonable explanation), we may use an alternate source’s price. To the extent we determine that it may be prudent to substitute one source’s price for another, we will contact the initial source to obtain an understanding of the factors that may be contributing to the significant price variance. To allow us to determine if our initial source is providing a price that is outside of a reasonable range, we review our portfolio pricing on a weekly basis. When necessary, we challenge prices from our sources when a price provided does not match our expectations based on our evaluation of market trends and activity. Initially, we perform a review of our portfolio by sector to identify securities whose prices appear outside of a reasonable range. We then perform a more detailed review of fair values for securities disclosed as Level 2. We review dealer bids and quotes for these and/or similar securities to determine the market level context for our valuations. We then evaluate inputs relevant for each class of securities disclosed in the preceding hierarchy tables. For structured debt securities, including commercial, residential, and other asset-backed securities, we evaluate available market-related data for these and similar securities related to collateral, delinquencies, and defaults for historical trends and reasonably estimable projections, as well as historical prepayment rates and current prepayment assumptions and cash flow estimates. We further stratify each class of our structured debt securities into more finite sectors (e.g., planned amortization class, first pay, second pay, senior, subordinated, etc.) and use duration, credit quality, and coupon to determine if the fair value is appropriate. For corporate debt and preferred stock (redeemable and nonredeemable) portfolios, as well as the notes issued by The Progressive Corporation (see Note 4 – Debt ), we review securities by duration, credit quality, and coupon, as well as changes in interest rate and credit spread movements within that stratification. The review also includes recent trades, including: volume traded at various levels that establish a market; issuer specific fundamentals; and industry-specific economic news as it comes to light. For municipal securities (e.g., general obligations, revenue, and housing), we stratify the portfolio to evaluate securities by type, duration, credit quality, and coupon to review price changes relative to credit spread and interest rate changes. Additionally, we look to economic data as it relates to geographic location as an indication of price-to-call or maturity predictors. For municipal housing securities, we look to changes in cash flow projections, both historical and reasonably estimable projections, to understand yield changes and their effect on valuation. For short-term securities, we look at acquisition price relative to the coupon or yield. Since our short-term securities are typically 90 days or less to maturity, with the majority listed in Level 2 being 30 days or less to redemption, we believe that acquisition price is the best estimate of fair value. We also review data assumptions as supplied by our sources to determine if that data is relevant to current market conditions. In addition, we independently review each sector for transaction volumes, new issuances, and changes in spreads, as well as the overall movement of interest rates along the yield curve to determine if sufficient activity and liquidity exists to provide a credible source for our market valuations. During each valuation period, we create internal estimations of portfolio valuation (performance returns), based on current market-related activity (i.e., interest rate and credit spread movements and other credit-related factors) within each major sector of our portfolio. We compare our internally generated portfolio results with those generated based on quotes we receive externally and research material valuation differences. We compare our results to index returns for each major sector adjusting for duration and credit quality differences to better understand our portfolio’s results. Additionally, we review on a monthly basis our external sales transactions and compare the actual final market sales prices to previous market valuation prices. This review provides us further validation that our pricing sources are providing market level prices, since we are able to explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a particular security’s price at sale. This analysis provides us with additional comfort regarding the source’s process, the quality of its review, and its willingness to improve its analysis based on feedback from clients. We believe this effort helps ensure that we are reporting the most representative fair values for our securities. After all the valuations are received and our review of Level 2 securities is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected securities to Level 3. Except as described below, our Level 3 securities are priced externally; however, due to several factors (e.g., nature of the securities, level of activity, and lack of similar securities trading to obtain observable market level inputs), these valuations are more subjective in nature. To the extent we receive prices from external sources (e.g., broker, valuation firm) for the Level 3 securities, we review those prices for reasonableness using internally developed assumptions and then compare our derived prices to the prices received from the external sources. Based on our review, all prices received from external sources remained unadjusted. If we do not receive prices from an external source, we perform an internal fair value comparison, which includes a review and analysis of market-comparable securities, to determine if fair value changes are needed. Based on this analysis, certain private equity investments included in the Level 3 category remain valued at cost or were priced using a recent transaction as the basis for fair value. At least annually, these private equity investments are priced by an external source. Our Level 3 other risk investments include securities accounted for under the equity method of accounting and, therefore, are not subject to fair value reporting. Since these securities represent less than 0.1% of our total portfolio, we will continue to include them in our Level 3 disclosures and report the activity from these investments as “other” changes in the summary of changes in fair value table and categorize these securities as “pricing exemption securities” in the quantitative information table. One of our Level 3 financial nonredeemable preferred stocks was exchanged into common equity during third quarter 2023 and the security was sold prior to September 30, 2023. The Level 2 common stock held at September 30, 2022, was transferred to Level 3 at December 31, 2022. At September 30, 2023 and 2022, and December 31, 2022, we did not have any securities in our fixed-maturity portfolio listed as Level 3. During the first nine months of 2023 and 2022, there were no material assets or liabilities measured at fair value on a nonrecurring basis, other than goodwill. See Note 12 – Goodwill and Intangible Assets for further discussion. Due to the relative size of the Level 3 securities’ fair values compared to the total portfolio’s fair value, any changes in pricing methodology would not have a significant change in valuation that would materially impact net or comprehensive income. The following tables provide a summary of changes in fair value associated with Level 3 assets for the three and nine months ended September 30, 2023 and 2022: Level 3 Fair Value (millions) Fair Value at June 30, 2023 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2023 Equity securities: Nonredeemable preferred stocks: Financials $ 56.0 $ (8.1) $ 0 $ 0 $ 0 $ 0 $ 0 $ 47.9 Industrials 15.8 0 0 0 0 0 0 15.8 Common equities: Common stocks 18.3 8.1 0 (10.0) 8.9 (7.0) 0 18.3 Other risk investments 21.9 0.7 0 0 0 0 0 22.6 Total Level 3 securities $ 112.0 $ 0.7 $ 0 $ (10.0) $ 8.9 $ (7.0) $ 0 $ 104.6 Level 3 Fair Value (millions) Fair Value at June 30, 2022 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2022 Equity securities: Nonredeemable preferred stocks: Financials $ 64.9 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 64.9 Industrials 17.0 0 0 0 0 0 0 17.0 Common equities: Common stocks 0 0 0 0 0 0 0 0 Other risk investments 18.5 0.2 0 0 0 0 0 18.7 Total Level 3 securities $ 100.4 $ 0.2 $ 0 $ 0 $ 0 $ 0 $ 0 $ 100.6 Level 3 Fair Value (millions) Fair Value at December 31, 2022 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2023 Equity securities: Nonredeemable preferred stocks: Financials $ 67.4 $ (8.1) $ 0 $ 0 $ 0 $ (11.4) $ 0 $ 47.9 Industrials 16.4 0 0 0 0 (0.6) 0 15.8 Common equities: Common stocks 18.3 8.1 0 (10.0) 8.9 (7.0) 0 18.3 Other risk investments 19.8 2.8 0 0 0 0 0 22.6 Total Level 3 securities $ 121.9 $ 2.8 $ 0 $ (10.0) $ 8.9 $ (19.0) $ 0 $ 104.6 Level 3 Fair Value (millions) Fair Value at December 31, 2021 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2022 Equity securities: Nonredeemable preferred stocks: Financials $ 76.4 $ 0 $ 0 $ (15.0) $ (17.2) $ 20.7 $ 0 $ 64.9 Industrials 34.4 (0.5) 0 0 0 (16.9) 0 17.0 Common equities: Other risk investments 16.9 1.8 0 0 0 0 0 18.7 Total Level 3 securities $ 127.7 $ 1.3 $ 0 $ (15.0) $ (17.2) $ 3.8 $ 0 $ 100.6 The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at September 30, 2023 and 2022, and December 31, 2022: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2023 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Equity securities: Nonredeemable preferred stocks $ 63.7 Market comparables Weighted average market capitalization price change % (10.8)% to 4.5% 1.7 % Common stocks 18.3 Market comparables Weighted average market capitalization price change % (37.4)% to 130.2% (7.9) % Subtotal Level 3 securities 82.0 Pricing exemption securities 22.6 Total Level 3 securities $ 104.6 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2022 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Equity securities: Nonredeemable preferred stocks $ 81.9 Market comparables Weighted average market capitalization price change % (20.7)% to (1.1)% (6.6) % Subtotal Level 3 securities 81.9 Pricing exemption securities 18.7 Total Level 3 securities $ 100.6 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2022 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Equity securities: Nonredeemable preferred stocks $ 83.8 Market comparables Weighted average market capitalization price change % (0.6)% to 19.9% 10.5% Common stocks 18.3 Market comparables Weighted average market capitalization price change % (42.5)% to 59.1% 0.3% Subtotal Level 3 securities 102.1 Pricing exemption securities 19.8 Total Level 3 securities $ 121.9

Debt

Debt	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Debt	DEBT Debt at each of the balance sheet periods consisted of: September 30, 2023 September 30, 2022 December 31, 2022 (millions) Carrying Fair Carrying Fair Carrying Fair 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) $ 498.5 $ 455.8 $ 498.1 $ 459.7 $ 498.2 $ 457.7 2.50% Senior Notes due 2027 (issued: $500.0, March 2022) 497.9 454.0 497.4 451.6 497.5 460.3 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.7 320.8 297.4 322.1 297.5 326.8 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 546.8 518.9 546.3 519.5 546.4 527.8 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 497.2 435.1 496.8 439.3 496.9 448.6 3.00% Senior Notes due 2032 (issued: $500.0, March 2022) 496.2 420.5 495.8 418.8 495.9 438.1 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.6 414.3 396.4 426.0 396.4 435.4 4.95% Senior Notes due 2033 (issued: $500.0, May 2023) 496.4 474.7 0 0 0 0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 347.0 280.9 346.9 298.5 346.9 298.4 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.8 288.2 395.7 305.7 395.7 310.2 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 842.2 666.3 842.0 710.3 842.1 716.2 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.6 473.3 590.3 505.9 590.4 507.0 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 491.1 373.5 490.9 396.6 490.9 404.9 3.70% Senior Notes due 2052 (issued: $500.0, March 2022) 493.6 355.8 493.4 380.7 493.5 386.5 Total $ 6,887.6 $ 5,932.1 $ 6,387.4 $ 5,634.7 $ 6,388.3 $ 5,717.9 The Progressive Corporation issued $500.0 million of 4.95% Senior Notes due 2033 in May 2023 in an underwritten public offering. The net proceeds from the issuance, after deducting underwriters’ discounts, commissions, and other issuance costs, was approximately $496.3 million. Consistent with the other senior notes issued by Progressive, interest on this note is payable semiannually, principal is due at maturity, and the note is redeemable, in whole or in part, at any time, subject to a treasury “make whole” provision. There was no short-term debt outstanding at September 30, 2023 and 2022, and December 31, 2022. During the second quarter 2023, The Progressive Corporation amended its line of credit with PNC Bank, National Association (PNC), to increase the maximum principal amount to $300 million from the previous amount of $250 million and extend the expiration date to April 30, 2024. Subject to the terms and conditions of the line of credit documents, advances under the line of credit (if any) will bear interest at a variable rate equal to the 1-month term Secured Overnight Financing Rate (SOFR) plus 1.10%. Each advance must be repaid on the 30th day after the date of the advance or, if earlier, April 30, 2024, the expiration date of the line of credit. Prepayments are permitted without penalty. The line of credit is uncommitted and, as such, all advances are subject to PNC’s discretion. We had no borrowings under the line of credit during the periods presented.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2023
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXESDeferred tax assets and liabilities are recorded based on the difference between the financial statement and tax bases of assets and liabilities at the enacted tax rates. We review our deferred tax assets regularly for recoverability. At September 30, 2023 and 2022, and December 31, 2022, we determined that we did not need a valuation allowance on our gross deferred tax assets. Although realization of the deferred tax assets is not assured, management believes that it is more likely than not the deferred tax assets will be realized based on our expectation that we will be able to fully utilize the deductions that are ultimately recognized for tax purposes. At September 30, 2023 and 2022, and December 31, 2022, the net deferred tax asset includes a gross deferred tax asset of $807.0 million, $827.7 million, and $742.9 million, respectively, related to unrealized losses on fixed-maturity securities. We believe this deferred tax asset will be realized based on the existence of prior year capital gains, current temporary differences related to unrealized gains in our equity portfolio, and other tax planning strategies. At September 30, 2023 and 2022, and December 31, 2022, we had no reserves for uncertain tax positions. The effective tax rate for the three and nine months ended September 30, 2023, was 20.5% and 20.2%, respectively, compared to 12.9% and (1.6)% for the same periods last year. The lower effective tax rates for the three and nine months ended September 30, 2022, were due in part to the low amount of income (loss) before taxes during those periods, resulting in permanent tax differences having a greater impact on the effective rate. The low effective tax rate for the nine months ended September 30, 2022, was also due to a goodwill impairment charge (see Note 12 – Goodwill and Intangible Assets for further discussion).

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
	Sep. 30, 2023
Loss and Loss Adjustment Expenses Reserves [Abstract]
Loss and Loss Adjustment Expense Reserves	LOSS AND LOSS ADJUSTMENT EXPENSE RESERVES Activity in the loss and loss adjustment expense reserves is summarized as follows: September 30, (millions) 2023 2022 Balance at January 1 $ 30,359.3 $ 26,164.1 Less reinsurance recoverables on unpaid losses 5,559.2 4,733.6 Net balance at January 1 24,800.1 21,430.5 Incurred related to: Current year 33,035.0 28,245.4 Prior years 1,147.0 52.8 Total incurred 34,182.0 28,298.2 Paid related to: Current year 18,581.3 15,712.4 Prior years 11,764.7 9,418.5 Total paid 30,346.0 25,130.9 Net balance at September 30 28,636.1 24,597.8 Plus reinsurance recoverables on unpaid losses 4,941.2 6,034.0 Balance at September 30 $ 33,577.3 $ 30,631.8 We experienced unfavorable reserve development of $1,147.0 million and $52.8 million during the first nine months of 2023 and 2022, respectively, which is reflected as “incurred related to prior years ” in the table above. Year-to-date September 30, 2023 • The unfavorable prior year reserve development included approximately $929 million attributable to accident year 2022, $108 million to accident year 2021, and the remainder to accident years 2020 and prior. • Our perso nal auto products incurred about $866 million of unfavorable loss and loss adjustment expense (LAE) reserve development, with the Agency and Direct auto businesses each contributing about half. Over half of the unfavorable development was attributable to higher than anticipated severity in auto property and physical damage coverages, while the remaining unfavorable development was primarily due to increased loss costs in Florida injury and medical coverages and, to a lesser extent, higher than anticipated late reported injury claims; partially offset by lower than expected loss adjustment expenses. • Our Commercial Lines business experienced about $277 million of unfavorable development, primarily driven by higher than anticipated severity and frequency of late reported injury claims, with about half of the unfavorable development attributable to our transportation network company (TNC) business. Year-to-date September 30, 2022 • Approximately $45 million of the unfavorable prior year reserve development was attributable to accident year 2021 and $29 million to 2019 and prior accident years, partially offset by $21 million of favorable development attributable to accident year 2020. • Our personal auto products incurred about $28 million of favorable loss and LAE reserve development, with about $25 million attributable to the Direct business. The favorable development was primarily attributable to more subrogation and salvage recoveries and lower LAE than originally anticipated, partially offset by higher than anticipated severity and frequency of auto property damage payments on previously closed claims and late reported injury claims. • Our Commercial Lines business experienced about $77 million of unfavorable development, primarily due to higher than anticipated severity of injury case reserves and higher than anticipated severity and frequency of late reported claims in our TNC business.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2023
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	SUPPLEMENTAL CASH FLOW INFORMATION Cash and cash equivalents include bank demand deposits and daily overnight reverse repurchase commitments of funds held in bank demand deposit accounts by certain subsidiaries. The amount of reverse repurchase commitments, which are not considered part of the investment portfolio, held by these subsidiaries at September 30, 2023 and 2022, and December 31, 2022, were $79.7 million, $123.0 million, and $125.9 million, respectively. Restricted cash and cash equivalents include collateral held against unpaid deductibles and cash that is restricted to pay flood claims under the National Flood Insurance Program’s “Write Your Own” program, for which certain subsidiaries are participants. Non-cash activity included the following in the respective periods: Nine Months Ended September 30, (millions) 2023 2022 Common share dividends 1 $ 58.5 $ 58.5 Operating lease liabilities 2 45.6 29.7 1 Declared but unpaid. See Note 9 – Dividends for further discussion. 2 From obtaining right-of-use assets. In the respective periods, we paid the following: Nine Months Ended September 30, (millions) 2023 2022 Income taxes $ 413.0 $ 595.9 Interest 213.5 191.3 Operating lease liabilities 57.2 62.5

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2023
Segment Reporting [Abstract]
Segment Information	SEGMENT INFORMATION Our Personal Lines segment writes insurance for personal autos and recreational vehicles (our special lines products). Our Commercial Lines segment writes auto-related liability and physical damage insurance, business-related general liability and property insurance predominately for small businesses, and workers’ compensation insurance primarily for the transportation industry. Our Property segment writes residential property insurance for homeowners, other property owners, and renters. Our service businesses provide insurance-related services, including serving as an agent for homeowners, general liability, and workers’ compensation insurance, among other products, through programs in our direct Personal Lines and Commercial Lines businesses. As previously disclosed in our 2022 Annual Report to Shareholders, during 2022, our contract to act as a servicing agent for processing Commercial Automobile Insurance Procedures/Plans (CAIP) business expired during the third quarter 2022 and we did not renew the contract. This non-renewal did not materially affect our financial condition, results of operations, or cash flows. All segment revenues are generated from external customers; all intercompany transactions are eliminated in consolidation. Following are the operating results for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 (millions) Revenues Pretax Revenues Pretax Revenues Pretax Revenues Pretax Personal Lines Agency $ 5,414.0 $ 335.7 $ 4,441.9 $ (41.3) $ 15,481.4 $ 427.1 $ 13,131.7 $ 507.6 Direct 6,361.8 706.2 5,077.4 40.6 18,259.9 810.6 14,776.9 389.4 Total Personal Lines 1 11,775.8 1,041.9 9,519.3 (0.7) 33,741.3 1,237.7 27,908.6 897.0 Commercial Lines 2,486.6 23.0 2,317.9 238.1 7,296.8 147.4 6,749.5 683.5 Property 2 631.8 74.6 561.0 (141.0) 1,852.8 (164.9) 1,689.6 (289.6) Other indemnity 3 0.1 (3.3) 0.7 (2.1) 0.9 (6.7) 2.0 (9.3) Total underwriting operations 14,894.3 1,136.2 12,398.9 94.3 42,891.8 1,213.5 36,349.7 1,281.6 Fees and other revenues 4 223.7 NA 181.4 NA 656.6 NA 531.9 NA Service businesses 81.4 (10.3) 82.7 (0.1) 234.9 (29.7) 230.5 9.0 Investments 5 361.2 354.0 117.2 111.4 1,434.0 1,415.2 (971.2) (988.6) Interest expense NA (69.7) NA (63.1) NA (198.7) NA (180.4) Property - Goodwill impairment 2 NA 0 NA 0 NA 0 NA (224.8) Consolidated total $ 15,560.6 $ 1,410.2 $ 12,780.2 $ 142.5 $ 45,217.3 $ 2,400.3 $ 36,140.9 $ (103.2) NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and nine months ended September 30, 2023 and 2022; insurance for our special lines products (e.g., motorcycles, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2023, pretax profit (loss) included $2.9 million and $10.7 million, respectively, of amortization expense associated with intangible assets attributable to our Property segment, and $5.0 million and $24.1 million for the same periods in 2022. For the nine months ended September 30, 2022, the total pretax loss, including goodwill impairment, for the Property segment was $514.4 million. See Note 12 – Goodwill and Intangible Assets for further discussion. 3 Includes other underwriting business and run-off operations. 4 Pretax profit (loss) for fees and other revenues is allocated to operating segments based on revenue. 5 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit (loss) is net of investment expenses. Our management uses underwriting margin and combined ratio as primary measures of underwriting profitability. The underwriting margin is the pretax underwriting profit (loss) expressed as a percentage of net premiums earned (i.e., revenues from underwriting operations). Pretax underwriting profit (loss) is calculated as net premiums earned plus fees and other revenues, less: (i) losses and loss adjustment expenses; (ii) policy acquisition costs; and (iii) other underwriting expenses. Combined ratio is the complement of the underwriting margin. Following are the underwriting margins and combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Under-writing Combined Under-writing Combined Under-writing Combined Under-writing Combined Personal Lines Agency 6.2 % 93.8 (0.9) % 100.9 2.8 % 97.2 3.9 % 96.1 Direct 11.1 88.9 0.8 99.2 4.4 95.6 2.6 97.4 Total Personal Lines 8.8 91.2 0 100.0 3.7 96.3 3.2 96.8 Commercial Lines 0.9 99.1 10.3 89.7 2.0 98.0 10.1 89.9 Property 1 11.8 88.2 (25.1) 125.1 (8.9) 108.9 (17.1) 117.1 Total underwriting operations 7.6 92.4 0.8 99.2 2.8 97.2 3.5 96.5 1 Included in the three and nine months ended September 30, 2023, is 0.5 points and 0.6 points, respectively, of amortization expense associated with intangible assets and 0.9 points and 1.4 points, respectively, for the three and nine months ended September 30, 2022.

Dividends

Dividends	9 Months Ended
	Sep. 30, 2023
Dividends [Abstract]
Dividends	DIVIDENDS Following is a summary of our common and preferred share dividends that were declared and/or paid during the nine months ended September 30, 2023 and 2022: (millions, except per share amounts) Amount Declared Payable Per Share Accrued/Paid 1 Common – Quarterly Dividends: August 2023 October 2023 $ 0.10 $ 58.5 May 2023 July 2023 0.10 58.5 March 2023 April 2023 0.10 58.5 December 2022 January 2023 0.10 58.5 August 2022 October 2022 0.10 58.5 May 2022 July 2022 0.10 58.5 March 2022 April 2022 0.10 58.5 December 2021 January 2022 0.10 58.5 Preferred Dividends: August 2023 September 2023 20.67700 10.3 May 2023 June 2023 18.92463 9.5 December 2022 March 2023 26.875 13.4 August 2022 September 2022 26.875 13.4 December 2021 March 2022 26.875 13.4 1 The accrual is based on an estimate of shares outstanding as of the record date and recorded as a component of accounts payable, accrued expenses, and other liabilities on our consolidated balance sheets until paid. Beginning March 15, 2023, the annual dividend rate for our Series B Preferred Shares switched to a floating rate equal to the three-month London Interbank Offered Rate (LIBOR) plus a spread of 2.539% applied to the stated amount per share. We, as calculation agent under our Series B Preferred Shares, determined that the reference rate for the Series B Preferred Shares, for any determination date after June 30, 2023, shall be the sum of (i) 3-Month CME Term SOFR plus (ii) a tenor spread adjustment of 0.26161%, and (iii) a spread of 2.539%. The new reference rate became effective for the dividend period determination date commencing September 15, 2023. The reference rate was determined in accordance with the successor base rate provisions of the Series B Preferred Shares and the Adjustable Interest Rate (LIBOR) Act and the regulation issued by the Board of Governors of the Federal Reserve System on December 16, 2022, implementing the LIBOR Act. During the floating rate period, dividends on the Series B Preferred Shares will be payable quarterly, if and when declared by the Board of Directors. See Note 14 – Dividends in our 2022 Annual Report to Shareholders for a discussion of our quarterly and annual common share dividends and our preferred share dividend policies.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2023
Equity [Abstract]
Other Comprehensive Income (Loss)	OTHER COMPREHENSIVE INCOME (LOSS) The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized losses on forecasted transactions Foreign Balance at June 30, 2023 $ (3,369.6) $ 715.6 $ (2,654.0) $ (2,638.7) $ (14.3) $ (1.0) Other comprehensive income (loss) before reclassifications: Investment securities (547.4) 114.9 (432.5) (432.5) 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications (547.4) 114.9 (432.5) (432.5) 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (55.6) 11.7 (43.9) (43.9) 0 0 Interest expense (0.2) 0 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income (55.8) 11.7 (44.1) (43.9) (0.2) 0 Total other comprehensive income (loss) (491.6) 103.2 (388.4) (388.6) 0.2 0 Balance at September 30, 2023 $ (3,861.2) $ 818.8 $ (3,042.4) $ (3,027.3) $ (14.1) $ (1.0) Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized losses on forecasted transactions Foreign Balance at June 30, 2022 $ (2,795.5) $ 586.5 $ (2,209.0) $ (2,193.5) $ (14.7) $ (0.8) Other comprehensive income (loss) before reclassifications: Investment securities (1,223.3) 256.9 (966.4) (966.4) 0 0 Foreign currency translation adjustment (0.9) 0.2 (0.7) 0 0 (0.7) Total other comprehensive income (loss) before reclassifications (1,224.2) 257.1 (967.1) (966.4) 0 (0.7) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (58.4) 12.3 (46.1) (46.1) 0 0 Interest expense (0.2) 0.1 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income (58.6) 12.4 (46.2) (46.1) (0.1) 0 Total other comprehensive income (loss) (1,165.6) 244.7 (920.9) (920.3) 0.1 (0.7) Balance at September 30, 2022 $ (3,961.1) $ 831.2 $ (3,129.9) $ (3,113.8) $ (14.6) $ (1.5) Components of Changes in (millions) Pretax total Total tax After tax total Total net Net unrealized losses on forecasted transactions Foreign Balance at December 31, 2022 $ (3,556.9) $ 754.9 $ (2,802.0) $ (2,786.3) $ (14.5) $ (1.2) Other comprehensive income (loss) before reclassifications: Investment securities (493.3) 103.5 (389.8) (389.8) 0 0 Foreign currency translation adjustment 0.3 (0.1) 0.2 0 0 0.2 Total other comprehensive income (loss) before reclassifications (493.0) 103.4 (389.6) (389.8) 0 0.2 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (188.3) 39.5 (148.8) (148.8) 0 0 Interest expense (0.4) 0 (0.4) 0 (0.4) 0 Total reclassification adjustment for amounts realized in net income (188.7) 39.5 (149.2) (148.8) (0.4) 0 Total other comprehensive income (loss) (304.3) 63.9 (240.4) (241.0) 0.4 0.2 Balance at September 30, 2023 $ (3,861.2) $ 818.8 $ (3,042.4) $ (3,027.3) $ (14.1) $ (1.0) Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized losses on forecasted transactions Foreign Balance at December 31, 2021 $ 52.3 $ (11.6) $ 40.7 $ 56.2 $ (14.9) $ (0.6) Other comprehensive income (loss) before reclassifications: Investment securities (4,321.7) 907.6 (3,414.1) (3,414.1) 0 0 Foreign currency translation adjustment (1.1) 0.2 (0.9) 0 0 (0.9) Total other comprehensive income (loss) before reclassifications (4,322.8) 907.8 (3,415.0) (3,414.1) 0 (0.9) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (309.0) 64.9 (244.1) (244.1) 0 0 Interest expense (0.4) 0.1 (0.3) 0 (0.3) 0 Total reclassification adjustment for amounts realized in net income (309.4) 65.0 (244.4) (244.1) (0.3) 0 Total other comprehensive income (loss) (4,013.4) 842.8 (3,170.6) (3,170.0) 0.3 (0.9) Balance at September 30, 2022 $ (3,961.1) $ 831.2 $ (3,129.9) $ (3,113.8) $ (14.6) $ (1.5) In an effort to manage interest rate risk, we entered into forecasted transactions on certain of Progressive’s debt issuances. We expect to reclassify $0.6 million (pretax) into interest expense during the next 12 months, related to net unrealized losses on these forecasted transactions (see Note 4 – Debt in our 2022 Annual Report to Shareholders for further discussion).

Litigation

Litigation	9 Months Ended
	Sep. 30, 2023
Commitments and Contingencies Disclosure [Abstract]
Litigation	LITIGATION The Progressive Corporation and/or its insurance subsidiaries are named as defendants in various lawsuits arising out of claims made under insurance policies written by our insurance subsidiaries in the ordinary course of business. We consider all legal actions relating to such claims in establishing our loss and loss adjustment expense reserves. In addition, The Progressive Corporation and/or its subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the subsidiaries. These cases include and/or typically have included those alleging damages as a result of, among other things: our subsidiaries’ methods used for evaluating and paying medical or injury claims or benefits, including, but not limited to, certain bodily injury, personal injury protection (PIP), uninsured motorist/underinsured motorist (UM/UIM), and medical payment claims and for reimbursing medical costs incurred by Medicare/Medicaid beneficiaries; other claims handling procedures, including, but not limited to, challenges relating to our network of repair facilities, our methods used for estimating physical damage to vehicles for repair purposes and for evaluating the actual cash value of total loss vehicles, the application of a negotiation adjustment in calculating total loss valuations, our payment of fees and taxes, our subrogation practices, our salvage practices, and our handling of diminution of value claims; homeowner claims handling practices and procedures; interpretations of the provisions of our insurance policies; our insurance product design; certain of our premium actions, including those in response to the COVID-19 pandemic; rating practices; certain marketing, sales, services, implementation and renewal practices and procedures, including with respect to accessibility and privacy; our usage-based insurance program; certain relationships with independent insurance agents; patent matters; alleged violation of the Telephone Consumer Protection Act; vendor data security; commercial disputes, including breach of contract; a small business grant program; and certain employment practices, including claims relating to pay practices and fair employment practices, among other matters. As of September 30, 2023, lawsuits have been certified or conditionally certified as class/collective actions in cases alleging: we improperly reduce or deny UIM benefits in New Mexico; we improperly value total loss claims in Florida, Georgia, New York, Pennsylvania, and Washington; we improperly fail to pay fees and taxes associated with total losses in Florida, Michigan, and New York; we improperly adjust medical bills in Washington; we improperly calculate basic economic loss as it relates to wage loss coverage in New York; we improperly fail to timely process and pay PIP claims in Texas; we improperly reduce or deny first-party medical benefits in Arkansas; and that certain of our compensation practices and overtime payment practices are improper, including our classification of certain employees as exempt from overtime pay requirements. Other insurance companies face many of these same issues. The nature and volume of litigation pending against The Progressive Corporation and/or its insurance subsidiaries is similar to that which was disclosed in Note 12 – Litigation in our 2022 Annual Report to Shareholders. We plan to contest the pending lawsuits vigorously, but may pursue settlement negotiations in some cases, as we deem appropriate. Although outcomes of pending cases are uncertain until final disposition, we establish accruals for these lawsuits when it is probable that a loss has been or will be incurred and we can reasonably estimate potential loss exposure, which may include a range of loss. As to lawsuits for which the loss is considered neither probable nor estimable, or is considered probable but not estimable, we do not establish an accrual. Nevertheless, we continue to evaluate pending litigation to determine if any losses not deemed probable and estimable become so, at which point we would establish an accrual at our best estimate of the loss or range of loss. With respect to our pending lawsuits that are not related to claims under insurance policies, the accruals that we have established, if any, were not material at September 30, 2023 and 2022, or December 31, 2022, and there were no material settlements during 2022 or the first nine months of 2023. For most of these lawsuits, we do not consider any losses to be both probable and estimable, and we are unable to estimate a meaningful range of loss, if any, at this time, due to the factors discussed in Note 12 – Litigation in our 2022 Annual Report to Shareholders. In the event that any one or more of these lawsuits results in a substantial judgment against us, or settlement by us, or if our accruals (if any) prove to be inadequate by a significant amount, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. For a further discussion on our pending litigation and related reserving policies, see Note 12 – Litigation in our 2022 Annual Report to Shareholders.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended
	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	GOODWILL AND INTANGIBLE ASSETS Goodwill The majority of the goodwill recorded as of September 30, 2023 and 2022, and December 31, 2022, related to the April 1, 2015, acquisition of a controlling interest in ARX Holding Corp. (ARX). During the nine months ended September 30, 2023, there were no changes to the carrying amount of goodwill. In the second quarter 2022, we performed an interim impairment test of our goodwill allocated to the ARX reporting unit and recorded an impairment loss of $224.8 million . The impairment loss was fully allocated to our Property operating segment. There were no previously recorded goodwill impairment losses on any of the outstanding goodwill. See Note 15 – Goodwill and Intangible Assets in our 2022 Annual Report to Shareholders for a discussion of our goodwill impairment evaluation. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets: (millions) September 30, 2023 September 30, 2022 December 31, 2022 Intangible assets subject to amortization $ 62.5 $ 79.4 $ 73.9 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 74.9 $ 91.8 $ 86.3 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: (millions) September 30, 2023 September 30, 2022 December 31, 2022 Category Gross Accumulated Net Gross Accumulated Net Gross Accumulated Net Agency relationships $ 159.2 $ 96.7 $ 62.5 $ 159.2 $ 85.3 $ 73.9 $ 159.2 $ 88.1 $ 71.1 Software rights 69.1 69.1 0 69.1 64.8 4.3 69.1 67.0 2.1 Trade name 3.6 3.6 0 3.6 2.4 1.2 3.6 2.9 0.7 Total $ 231.9 $ 169.4 $ 62.5 $ 231.9 $ 152.5 $ 79.4 $ 231.9 $ 158.0 $ 73.9 Amortization expense was $2.9 million and $11.4 million for the three and nine months ended September 30, 2023, respectively, compared to $5.4 million and $25.5 million during the same periods last year.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pay vs Performance Disclosure
Net income (loss)	$ 1,121.3	$ 124.1	$ 1,914.6	$ (104.9)

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Sep. 30, 2023
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

Basis of Presentation (Policies

Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Earnings Per Share	Earnings Per ShareNet income is reduced by preferred share dividends to determine net income available to common shareholders and is used in our calculation of the per common share amounts.
New Accounting Standards	New Accounting Standards During the first quarter 2023, the Financial Accounting Standards Board issued an Accounting Standards Update (ASU), which permits an election to amortize the cost of tax equity investments to the provision for income taxes if certain conditions are met. Currently, these investments are accounted for under the equity method of accounting. This ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2023 (2024 for calendar-year companies). If elected, this standard must be applied on either a modified retrospective or a retrospective basis. We intend to adopt the new standard under the modified retrospective basis, however, we do not expect this standard to have a material impact on our financial condition or results of operations.

Basis of Presentation (Tables)

Basis of Presentation (Tables)	9 Months Ended
	Sep. 30, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Premium Receivable, Allowance for Credit Loss	The following table summarizes changes in our allowance for credit loss exposure on our premiums receivable: Three Months Ended September 30, Nine Months Ended September 30, (millions) 2023 2022 2023 2022 Allowance for credit losses, beginning of period $ 343.9 $ 265.8 $ 343.3 $ 280.4 Increase in allowance 1 157.5 130.9 399.7 320.6 Write-offs 2 (140.2) (101.7) (381.8) (306.0) Allowance for credit losses, end of period $ 361.2 $ 295.0 $ 361.2 $ 295.0 1 Represents the incremental increase in other underwriting expenses. 2 Represents the portion of the allowance that is reversed when premiums receivable are written off. Premiums receivable balances are written off once we have exhausted our collection efforts.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2023
Investments, Debt and Equity Securities [Abstract]
Schedule of Debt and Equity Securities Reconciliation	The following tables present the composition of our investment portfolio by major security type. Our securities are reported in our consolidated balance sheets at fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income (loss), net of deferred income taxes, in our consolidated balance sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value for the hybrid and equity securities. The changes in the net holding period gains (losses) between periods are recorded as a component of net realized gains (losses) on securities in our consolidated statements of comprehensive income. ($ in millions) Cost Gross Gross Net Fair % of September 30, 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 36,392.2 $ 0 $ (2,098.4) $ 0 $ 34,293.8 55.4 % State and local government obligations 2,257.6 0 (203.3) 0 2,054.3 3.3 Foreign government obligations 16.9 0 (1.5) 0 15.4 0.1 Corporate debt securities 10,880.8 1.3 (591.8) (36.1) 10,254.2 16.6 Residential mortgage-backed securities 530.4 0.2 (13.0) (1.8) 515.8 0.8 Commercial mortgage-backed securities 4,699.5 2.1 (723.9) 0 3,977.7 6.4 Other asset-backed securities 5,524.2 0.1 (210.1) (0.6) 5,313.6 8.6 Redeemable preferred stocks 189.0 0 (4.3) (17.6) 167.1 0.3 Total fixed maturities 60,490.6 3.7 (3,846.3) (56.1) 56,591.9 91.5 Short-term investments 1,795.2 0 0 0 1,795.2 2.9 Total available-for-sale securities 62,285.8 3.7 (3,846.3) (56.1) 58,387.1 94.4 Equity securities: Nonredeemable preferred stocks 976.9 0 0 (108.0) 868.9 1.4 Common equities 662.6 0 0 1,951.9 2,614.5 4.2 Total equity securities 1,639.5 0 0 1,843.9 3,483.4 5.6 Total portfolio 1 $ 63,925.3 $ 3.7 $ (3,846.3) $ 1,787.8 $ 61,870.5 100.0 % ($ in millions) Cost Gross Gross Net Fair % of September 30, 2022 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 24,260.9 $ 0 $ (1,855.4) $ 0 $ 22,405.5 42.8 % State and local government obligations 2,140.9 0 (215.4) 0 1,925.5 3.7 Foreign government obligations 16.7 0 (1.4) 0 15.3 0.1 Corporate debt securities 10,119.8 0 (832.5) (58.9) 9,228.4 17.6 Residential mortgage-backed securities 754.2 0.4 (16.5) (15.1) 723.0 1.4 Commercial mortgage-backed securities 5,833.9 1.4 (747.4) 0 5,087.9 9.7 Other asset-backed securities 4,876.7 0 (268.9) (1.9) 4,605.9 8.8 Redeemable preferred stocks 202.6 0 (5.6) (15.4) 181.6 0.3 Total fixed maturities 48,205.7 1.8 (3,943.1) (91.3) 44,173.1 84.4 Short-term investments 4,237.6 0 0 0 4,237.6 8.1 Total available-for-sale securities 52,443.3 1.8 (3,943.1) (91.3) 48,410.7 92.5 Equity securities: Nonredeemable preferred stocks 1,417.6 0 0 (163.2) 1,254.4 2.4 Common equities 803.7 0 0 1,861.6 2,665.3 5.1 Total equity securities 2,221.3 0 0 1,698.4 3,919.7 7.5 Total portfolio 1 $ 54,664.6 $ 1.8 $ (3,943.1) $ 1,607.1 $ 52,330.4 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2022 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 26,770.7 $ 1.4 $ (1,604.7) $ 0 $ 25,167.4 47.0 % State and local government obligations 2,180.0 0 (202.9) 0 1,977.1 3.7 Foreign government obligations 16.8 0 (1.3) 0 15.5 0.1 Corporate debt securities 10,125.8 9.8 (676.1) (46.8) 9,412.7 17.6 Residential mortgage-backed securities 696.1 0.3 (17.5) (12.1) 666.8 1.2 Commercial mortgage-backed securities 5,446.0 1.5 (784.0) 0 4,663.5 8.7 Other asset-backed securities 4,826.0 0.9 (260.5) (1.8) 4,564.6 8.5 Redeemable preferred stocks 202.6 0 (4.5) (13.8) 184.3 0.3 Total fixed maturities 50,264.0 13.9 (3,551.5) (74.5) 46,651.9 87.1 Short-term investments 2,861.7 0 0 0 2,861.7 5.4 Total available-for-sale securities 53,125.7 13.9 (3,551.5) (74.5) 49,513.6 92.5 Equity securities: Nonredeemable preferred stocks 1,364.2 0 0 (151.0) 1,213.2 2.3 Common equities 826.1 0 0 1,995.4 2,821.5 5.2 Total equity securities 2,190.3 0 0 1,844.4 4,034.7 7.5 Total portfolio 1 $ 55,316.0 $ 13.9 $ (3,551.5) $ 1,769.9 $ 53,548.3 100.0 % 1 At September 30, 2023 and 2022, we had $167.3 million and $74.7 million, respectively, of net unsettled security purchase transactions included in other liabilities, compared to $34.4 million included in other assets at December 31, 2022. The total fair value of the portfolio at September 30, 2023 and 2022, and December 31, 2022, included $4.1 billion, $4.2 billion, and $4.4 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions.
Hybrid Instruments Table	Certain securities in our fixed-maturity portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. These securities are reported at fair value: September 30, (millions) 2023 2022 Dec. 31, 2022 Fixed Maturities: Corporate debt securities $ 511.1 $ 500.4 $ 535.4 Residential mortgage-backed securities 410.8 550.8 509.6 Other asset-backed securities 19.4 51.4 42.0 Redeemable preferred stocks 136.4 133.0 134.7 Total hybrid securities $ 1,077.7 $ 1,235.6 $ 1,221.7
Investments Classified by Contractual Maturity Date	The composition of fixed maturities by maturity at September 30, 2023, was: (millions) Cost Fair Value Less than one year $ 9,639.9 $ 9,387.8 One to five years 37,572.4 35,419.0 Five to ten years 13,197.7 11,710.3 Ten years or greater 80.6 74.8 Total $ 60,490.6 $ 56,591.9
Schedule of Unrealized Loss on Investments	The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2023 U.S. government obligations 173 $ 34,293.8 $ (2,098.4) 37 $ 16,686.2 $ (557.6) 136 $ 17,607.6 $ (1,540.8) State and local government obligations 356 2,033.9 (203.3) 46 359.6 (7.6) 310 1,674.3 (195.7) Foreign government obligations 1 15.4 (1.5) 0 0 0 1 15.4 (1.5) Corporate debt securities 467 9,490.0 (591.8) 164 3,380.4 (94.1) 303 6,109.6 (497.7) Residential mortgage-backed securities 40 99.9 (13.0) 2 0.4 0 38 99.5 (13.0) Commercial mortgage-backed securities 196 3,960.8 (723.9) 0 0 0 196 3,960.8 (723.9) Other asset-backed securities 295 5,001.8 (210.1) 104 2,056.9 (8.8) 191 2,944.9 (201.3) Redeemable preferred stocks 3 30.6 (4.3) 0 0 0 3 30.6 (4.3) Total fixed maturities 1,531 $ 54,926.2 $ (3,846.3) 353 $ 22,483.5 $ (668.1) 1,178 $ 32,442.7 $ (3,178.2) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized September 30, 2022 U.S. government obligations 157 $ 22,385.4 $ (1,855.4) 103 $ 16,085.8 $ (1,095.6) 54 $ 6,299.6 $ (759.8) State and local government obligations 353 1,911.0 (215.4) 292 1,363.1 (129.8) 61 547.9 (85.6) Foreign government obligations 1 15.3 (1.4) 1 15.3 (1.4) 0 0 0 Corporate debt securities 452 8,885.6 (832.5) 391 7,875.5 (699.0) 61 1,010.1 (133.5) Residential mortgage-backed securities 44 165.1 (16.5) 34 126.0 (11.8) 10 39.1 (4.7) Commercial mortgage-backed securities 234 5,074.3 (747.4) 182 3,856.0 (425.4) 52 1,218.3 (322.0) Other asset-backed securities 277 4,544.8 (268.9) 217 3,597.1 (183.4) 60 947.7 (85.5) Redeemable preferred stocks 4 48.6 (5.6) 3 37.7 (4.0) 1 10.9 (1.6) Total fixed maturities 1,522 $ 43,030.1 $ (3,943.1) 1,223 $ 32,956.5 $ (2,550.4) 299 $ 10,073.6 $ (1,392.7) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized December 31, 2022 U.S. government obligations 160 $ 24,802.5 $ (1,604.7) 90 $ 17,327.2 $ (699.2) 70 $ 7,475.3 $ (905.5) State and local government obligations 348 1,948.8 (202.9) 239 1,124.2 (76.8) 109 824.6 (126.1) Foreign government obligations 1 15.5 (1.3) 0 0 0 1 15.5 (1.3) Corporate debt securities 422 8,449.6 (676.1) 285 5,717.6 (426.1) 137 2,732.0 (250.0) Residential mortgage-backed securities 45 151.0 (17.5) 27 65.1 (6.8) 18 85.9 (10.7) Commercial mortgage-backed securities 226 4,651.1 (784.0) 99 1,702.0 (192.1) 127 2,949.1 (591.9) Other asset-backed securities 262 4,247.8 (260.5) 130 2,144.8 (100.9) 132 2,103.0 (159.6) Redeemable preferred stocks 4 49.6 (4.5) 3 38.5 (3.1) 1 11.1 (1.4) Total fixed maturities 1,468 $ 44,315.9 $ (3,551.5) 873 $ 28,119.4 $ (1,505.0) 595 $ 16,196.5 $ (2,046.5)
Realized Gain (Loss) on Investments	The components of net realized gains (losses) for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2023 2022 2023 2022 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0.2 $ 0 $ 4.2 $ 4.6 Corporate debt securities 0.7 0.3 0.8 6.8 Residential mortgage-backed securities 0 0 0 0.7 Other asset-backed securities 0 0 0 0.1 Total available-for-sale securities 0.9 0.3 5.0 12.2 Equity securities: Nonredeemable preferred stocks 0 0.1 0.3 17.6 Common equities 0.3 1.4 354.2 832.5 Total equity securities 0.3 1.5 354.5 850.1 Subtotal gross realized gains on security sales 1.2 1.8 359.5 862.3 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (14.6) 0 (27.2) (233.5) State and local government obligations 0 0 0 (1.0) Corporate debt securities (19.1) (14.0) (69.3) (51.8) Commercial mortgage-backed securities (20.6) (44.8) (100.6) (58.6) Other asset-backed securities (5.2) 0 (5.6) (2.1) Short-term investments 0 0 (0.4) (0.3) Total available-for-sale securities (59.5) (58.8) (203.1) (347.3) Equity securities: Nonredeemable preferred stocks (7.4) (5.1) (117.6) (7.0) Common equities (8.9) 0 (21.8) (78.0) Total equity securities (16.3) (5.1) (139.4) (85.0) Subtotal gross realized losses on security sales (75.8) (63.9) (342.5) (432.3) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (14.4) 0 (23.0) (228.9) State and local government obligations 0 0 0 (1.0) Corporate debt securities (18.4) (13.7) (68.5) (45.0) Residential mortgage-backed securities 0 0 0 0.7 Commercial mortgage-backed securities (20.6) (44.8) (100.6) (58.6) Other asset-backed securities (5.2) 0 (5.6) (2.0) Short-term investments 0 0 (0.4) (0.3) Total available-for-sale securities (58.6) (58.5) (198.1) (335.1) Equity securities: Nonredeemable preferred stocks (7.4) (5.0) (117.3) 10.6 Common equities (8.6) 1.4 332.4 754.5 Total equity securities (16.0) (3.6) 215.1 765.1 Subtotal net realized gains (losses) on security sales (74.6) (62.1) 17.0 430.0 Other assets Gain 8.4 0 21.6 0 Impairment (2.3) (2.2) (6.8) (6.5) Subtotal net realized gains (losses) on other assets 6.1 (2.2) 14.8 (6.5) Net holding period gains (losses) Hybrid securities (0.3) (11.4) 18.4 (98.8) Equity securities (80.2) (140.7) (0.5) (2,164.1) Subtotal net holding period gains (losses) (80.5) (152.1) 17.9 (2,262.9) Total net realized gains (losses) on securities $ (149.0) $ (216.4) $ 49.7 $ (1,839.4)
Components of Equity Realized Gains (Losses)	The following table reflects our holding period realized gains (losses) recognized on equity securities held at the respective periods ended September 30: Three Months Nine Months (millions) 2023 2022 2023 2022 Total net gains (losses) recognized during the period on equity securities $ (96.2) $ (144.3) $ 214.6 $ (1,399.0) Less: Net gains (losses) recognized on equity securities sold during the period (16.0) (3.6) 215.1 765.1 Net holding period gains (losses) recognized during the period on equity securities held at period end $ (80.2) $ (140.7) $ (0.5) $ (2,164.1)
Components of Net Investment Income	The components of net investment income for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2023 2022 2023 2022 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 237.0 $ 86.1 $ 591.7 $ 205.4 State and local government obligations 12.3 9.8 34.5 29.3 Foreign government obligations 0.1 0.1 0.2 0.2 Corporate debt securities 94.8 71.8 261.9 215.4 Residential mortgage-backed securities 7.6 8.8 22.6 23.2 Commercial mortgage-backed securities 48.7 48.3 147.9 138.1 Other asset-backed securities 62.9 52.1 173.0 116.5 Redeemable preferred stocks 2.6 3.0 8.2 8.5 Total fixed maturities 466.0 280.0 1,240.0 736.6 Short-term investments 21.6 24.8 71.1 29.6 Total available-for-sale securities 487.6 304.8 1,311.1 766.2 Equity securities: Nonredeemable preferred stocks 11.9 17.3 40.3 53.6 Common equities 10.7 11.5 32.9 48.4 Total equity securities 22.6 28.8 73.2 102.0 Investment income 510.2 333.6 1,384.3 868.2 Investment expenses (7.2) (5.8) (18.8) (17.4) Net investment income $ 503.0 $ 327.8 $ 1,365.5 $ 850.8

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
	Sep. 30, 2023
Fair Value Disclosures [Abstract]
Composition of Investment Portfolio by Major Security Type	The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2023 Fixed maturities: U.S. government obligations $ 34,293.8 $ 0 $ 0 $ 34,293.8 $ 36,392.2 State and local government obligations 0 2,054.3 0 2,054.3 2,257.6 Foreign government obligations 0 15.4 0 15.4 16.9 Corporate debt securities 0 10,254.2 0 10,254.2 10,880.8 Subtotal 34,293.8 12,323.9 0 46,617.7 49,547.5 Asset-backed securities: Residential mortgage-backed 0 515.8 0 515.8 530.4 Commercial mortgage-backed 0 3,977.7 0 3,977.7 4,699.5 Other asset-backed 0 5,313.6 0 5,313.6 5,524.2 Subtotal asset-backed securities 0 9,807.1 0 9,807.1 10,754.1 Redeemable preferred stocks: Financials 0 21.3 0 21.3 24.5 Utilities 0 9.3 0 9.3 10.4 Industrials 0 136.5 0 136.5 154.1 Subtotal redeemable preferred stocks 0 167.1 0 167.1 189.0 Total fixed maturities 34,293.8 22,298.1 0 56,591.9 60,490.6 Short-term investments 1,763.2 32.0 0 1,795.2 1,795.2 Total available-for-sale securities 36,057.0 22,330.1 0 58,387.1 62,285.8 Equity securities: Nonredeemable preferred stocks: Financials 0 770.7 47.9 818.6 921.9 Utilities 0 34.5 0 34.5 40.0 Industrials 0 0 15.8 15.8 15.0 Subtotal nonredeemable preferred stocks 0 805.2 63.7 868.9 976.9 Common equities: Common stocks 2,573.6 0 18.3 2,591.9 640.0 Other risk investments 0 0 22.6 22.6 22.6 Subtotal common equities 2,573.6 0 40.9 2,614.5 662.6 Total equity securities 2,573.6 805.2 104.6 3,483.4 1,639.5 Total portfolio $ 38,630.6 $ 23,135.3 $ 104.6 $ 61,870.5 $ 63,925.3 Debt $ 0 $ 5,932.1 $ 0 $ 5,932.1 $ 6,887.6 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2022 Fixed maturities: U.S. government obligations $ 22,405.5 $ 0 $ 0 $ 22,405.5 $ 24,260.9 State and local government obligations 0 1,925.5 0 1,925.5 2,140.9 Foreign government obligations 0 15.3 0 15.3 16.7 Corporate debt securities 0 9,228.4 0 9,228.4 10,119.8 Subtotal 22,405.5 11,169.2 0 33,574.7 36,538.3 Asset-backed securities: Residential mortgage-backed 0 723.0 0 723.0 754.2 Commercial mortgage-backed 0 5,087.9 0 5,087.9 5,833.9 Other asset-backed 0 4,605.9 0 4,605.9 4,876.7 Subtotal asset-backed securities 0 10,416.8 0 10,416.8 11,464.8 Redeemable preferred stocks: Financials 0 39.8 0 39.8 43.6 Utilities 0 8.8 0 8.8 10.5 Industrials 8.9 124.1 0 133.0 148.5 Subtotal redeemable preferred stocks 8.9 172.7 0 181.6 202.6 Total fixed maturities 22,414.4 21,758.7 0 44,173.1 48,205.7 Short-term investments 4,219.6 18.0 0 4,237.6 4,237.6 Total available-for-sale securities 26,634.0 21,776.7 0 48,410.7 52,443.3 Equity securities: Nonredeemable preferred stocks: Financials 60.9 1,015.6 64.9 1,141.4 1,297.6 Utilities 0 71.3 0 71.3 79.9 Industrials 0 24.7 17.0 41.7 40.1 Subtotal nonredeemable preferred stocks 60.9 1,111.6 81.9 1,254.4 1,417.6 Common equities: Common stocks 2,597.3 49.3 0 2,646.6 785.0 Other risk investments 0 0 18.7 18.7 18.7 Subtotal common equities 2,597.3 49.3 18.7 2,665.3 803.7 Total equity securities 2,658.2 1,160.9 100.6 3,919.7 2,221.3 Total portfolio $ 29,292.2 $ 22,937.6 $ 100.6 $ 52,330.4 $ 54,664.6 Debt $ 0 $ 5,634.7 $ 0 $ 5,634.7 $ 6,387.4 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2022 Fixed maturities: U.S. government obligations $ 25,167.4 $ 0 $ 0 $ 25,167.4 $ 26,770.7 State and local government obligations 0 1,977.1 0 1,977.1 2,180.0 Foreign government obligations 0 15.5 0 15.5 16.8 Corporate debt securities 0 9,412.7 0 9,412.7 10,125.8 Subtotal 25,167.4 11,405.3 0 36,572.7 39,093.3 Asset-backed securities: Residential mortgage-backed 0 666.8 0 666.8 696.1 Commercial mortgage-backed 0 4,663.5 0 4,663.5 5,446.0 Other asset-backed 0 4,564.6 0 4,564.6 4,826.0 Subtotal asset-backed securities 0 9,894.9 0 9,894.9 10,968.1 Redeemable preferred stocks: Financials 0 40.5 0 40.5 43.6 Utilities 0 9.1 0 9.1 10.5 Industrials 9.2 125.5 0 134.7 148.5 Subtotal redeemable preferred stocks 9.2 175.1 0 184.3 202.6 Total fixed maturities 25,176.6 21,475.3 0 46,651.9 50,264.0 Short-term investments 2,800.7 61.0 0 2,861.7 2,861.7 Total available-for-sale securities 27,977.3 21,536.3 0 49,513.6 53,125.7 Equity securities: Nonredeemable preferred stocks: Financials 39.0 994.4 67.4 1,100.8 1,244.2 Utilities 0 71.2 0 71.2 79.9 Industrials 0 24.8 16.4 41.2 40.1 Subtotal nonredeemable preferred stocks 39.0 1,090.4 83.8 1,213.2 1,364.2 Common equities: Common stocks 2,783.4 0 18.3 2,801.7 806.3 Other risk investments 0 0 19.8 19.8 19.8 Subtotal common equities 2,783.4 0 38.1 2,821.5 826.1 Total equity securities 2,822.4 1,090.4 121.9 4,034.7 2,190.3 Total portfolio $ 30,799.7 $ 22,626.7 $ 121.9 $ 53,548.3 $ 55,316.0 Debt $ 0 $ 5,717.9 $ 0 $ 5,717.9 $ 6,388.3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation	The following tables provide a summary of changes in fair value associated with Level 3 assets for the three and nine months ended September 30, 2023 and 2022: Level 3 Fair Value (millions) Fair Value at June 30, 2023 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2023 Equity securities: Nonredeemable preferred stocks: Financials $ 56.0 $ (8.1) $ 0 $ 0 $ 0 $ 0 $ 0 $ 47.9 Industrials 15.8 0 0 0 0 0 0 15.8 Common equities: Common stocks 18.3 8.1 0 (10.0) 8.9 (7.0) 0 18.3 Other risk investments 21.9 0.7 0 0 0 0 0 22.6 Total Level 3 securities $ 112.0 $ 0.7 $ 0 $ (10.0) $ 8.9 $ (7.0) $ 0 $ 104.6 Level 3 Fair Value (millions) Fair Value at June 30, 2022 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2022 Equity securities: Nonredeemable preferred stocks: Financials $ 64.9 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 64.9 Industrials 17.0 0 0 0 0 0 0 17.0 Common equities: Common stocks 0 0 0 0 0 0 0 0 Other risk investments 18.5 0.2 0 0 0 0 0 18.7 Total Level 3 securities $ 100.4 $ 0.2 $ 0 $ 0 $ 0 $ 0 $ 0 $ 100.6 Level 3 Fair Value (millions) Fair Value at December 31, 2022 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2023 Equity securities: Nonredeemable preferred stocks: Financials $ 67.4 $ (8.1) $ 0 $ 0 $ 0 $ (11.4) $ 0 $ 47.9 Industrials 16.4 0 0 0 0 (0.6) 0 15.8 Common equities: Common stocks 18.3 8.1 0 (10.0) 8.9 (7.0) 0 18.3 Other risk investments 19.8 2.8 0 0 0 0 0 22.6 Total Level 3 securities $ 121.9 $ 2.8 $ 0 $ (10.0) $ 8.9 $ (19.0) $ 0 $ 104.6 Level 3 Fair Value (millions) Fair Value at December 31, 2021 Calls/ Purchases Sales Net Realized Change in Net Fair Value at September 30, 2022 Equity securities: Nonredeemable preferred stocks: Financials $ 76.4 $ 0 $ 0 $ (15.0) $ (17.2) $ 20.7 $ 0 $ 64.9 Industrials 34.4 (0.5) 0 0 0 (16.9) 0 17.0 Common equities: Other risk investments 16.9 1.8 0 0 0 0 0 18.7 Total Level 3 securities $ 127.7 $ 1.3 $ 0 $ (15.0) $ (17.2) $ 3.8 $ 0 $ 100.6
Fair Value Measurement Inputs and Valuation Techniques	The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at September 30, 2023 and 2022, and December 31, 2022: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2023 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Equity securities: Nonredeemable preferred stocks $ 63.7 Market comparables Weighted average market capitalization price change % (10.8)% to 4.5% 1.7 % Common stocks 18.3 Market comparables Weighted average market capitalization price change % (37.4)% to 130.2% (7.9) % Subtotal Level 3 securities 82.0 Pricing exemption securities 22.6 Total Level 3 securities $ 104.6 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2022 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Equity securities: Nonredeemable preferred stocks $ 81.9 Market comparables Weighted average market capitalization price change % (20.7)% to (1.1)% (6.6) % Subtotal Level 3 securities 81.9 Pricing exemption securities 18.7 Total Level 3 securities $ 100.6 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2022 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Equity securities: Nonredeemable preferred stocks $ 83.8 Market comparables Weighted average market capitalization price change % (0.6)% to 19.9% 10.5% Common stocks 18.3 Market comparables Weighted average market capitalization price change % (42.5)% to 59.1% 0.3% Subtotal Level 3 securities 102.1 Pricing exemption securities 19.8 Total Level 3 securities $ 121.9

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2023
Debt Disclosure [Abstract]
Debt Table	Debt at each of the balance sheet periods consisted of: September 30, 2023 September 30, 2022 December 31, 2022 (millions) Carrying Fair Carrying Fair Carrying Fair 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) $ 498.5 $ 455.8 $ 498.1 $ 459.7 $ 498.2 $ 457.7 2.50% Senior Notes due 2027 (issued: $500.0, March 2022) 497.9 454.0 497.4 451.6 497.5 460.3 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.7 320.8 297.4 322.1 297.5 326.8 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 546.8 518.9 546.3 519.5 546.4 527.8 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 497.2 435.1 496.8 439.3 496.9 448.6 3.00% Senior Notes due 2032 (issued: $500.0, March 2022) 496.2 420.5 495.8 418.8 495.9 438.1 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.6 414.3 396.4 426.0 396.4 435.4 4.95% Senior Notes due 2033 (issued: $500.0, May 2023) 496.4 474.7 0 0 0 0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 347.0 280.9 346.9 298.5 346.9 298.4 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.8 288.2 395.7 305.7 395.7 310.2 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 842.2 666.3 842.0 710.3 842.1 716.2 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.6 473.3 590.3 505.9 590.4 507.0 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 491.1 373.5 490.9 396.6 490.9 404.9 3.70% Senior Notes due 2052 (issued: $500.0, March 2022) 493.6 355.8 493.4 380.7 493.5 386.5 Total $ 6,887.6 $ 5,932.1 $ 6,387.4 $ 5,634.7 $ 6,388.3 $ 5,717.9

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
	Sep. 30, 2023
Loss and Loss Adjustment Expenses Reserves [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense	Activity in the loss and loss adjustment expense reserves is summarized as follows: September 30, (millions) 2023 2022 Balance at January 1 $ 30,359.3 $ 26,164.1 Less reinsurance recoverables on unpaid losses 5,559.2 4,733.6 Net balance at January 1 24,800.1 21,430.5 Incurred related to: Current year 33,035.0 28,245.4 Prior years 1,147.0 52.8 Total incurred 34,182.0 28,298.2 Paid related to: Current year 18,581.3 15,712.4 Prior years 11,764.7 9,418.5 Total paid 30,346.0 25,130.9 Net balance at September 30 28,636.1 24,597.8 Plus reinsurance recoverables on unpaid losses 4,941.2 6,034.0 Balance at September 30 $ 33,577.3 $ 30,631.8

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2023
Supplemental Cash Flow Elements [Abstract]
Schedule of Supplemental Cash Flow Information, Non Cash Activity	Non-cash activity included the following in the respective periods: Nine Months Ended September 30, (millions) 2023 2022 Common share dividends 1 $ 58.5 $ 58.5 Operating lease liabilities 2 45.6 29.7 1 Declared but unpaid. See Note 9 – Dividends for further discussion. 2 From obtaining right-of-use assets.
Supplemental Cash Flow Information Table	In the respective periods, we paid the following: Nine Months Ended September 30, (millions) 2023 2022 Income taxes $ 413.0 $ 595.9 Interest 213.5 191.3 Operating lease liabilities 57.2 62.5

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2023
Segment Reporting [Abstract]
Reconciliation of Revenue and operating Income from Segments to Consolidated	Following are the operating results for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 (millions) Revenues Pretax Revenues Pretax Revenues Pretax Revenues Pretax Personal Lines Agency $ 5,414.0 $ 335.7 $ 4,441.9 $ (41.3) $ 15,481.4 $ 427.1 $ 13,131.7 $ 507.6 Direct 6,361.8 706.2 5,077.4 40.6 18,259.9 810.6 14,776.9 389.4 Total Personal Lines 1 11,775.8 1,041.9 9,519.3 (0.7) 33,741.3 1,237.7 27,908.6 897.0 Commercial Lines 2,486.6 23.0 2,317.9 238.1 7,296.8 147.4 6,749.5 683.5 Property 2 631.8 74.6 561.0 (141.0) 1,852.8 (164.9) 1,689.6 (289.6) Other indemnity 3 0.1 (3.3) 0.7 (2.1) 0.9 (6.7) 2.0 (9.3) Total underwriting operations 14,894.3 1,136.2 12,398.9 94.3 42,891.8 1,213.5 36,349.7 1,281.6 Fees and other revenues 4 223.7 NA 181.4 NA 656.6 NA 531.9 NA Service businesses 81.4 (10.3) 82.7 (0.1) 234.9 (29.7) 230.5 9.0 Investments 5 361.2 354.0 117.2 111.4 1,434.0 1,415.2 (971.2) (988.6) Interest expense NA (69.7) NA (63.1) NA (198.7) NA (180.4) Property - Goodwill impairment 2 NA 0 NA 0 NA 0 NA (224.8) Consolidated total $ 15,560.6 $ 1,410.2 $ 12,780.2 $ 142.5 $ 45,217.3 $ 2,400.3 $ 36,140.9 $ (103.2) NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and nine months ended September 30, 2023 and 2022; insurance for our special lines products (e.g., motorcycles, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2023, pretax profit (loss) included $2.9 million and $10.7 million, respectively, of amortization expense associated with intangible assets attributable to our Property segment, and $5.0 million and $24.1 million for the same periods in 2022. For the nine months ended September 30, 2022, the total pretax loss, including goodwill impairment, for the Property segment was $514.4 million. See Note 12 – Goodwill and Intangible Assets for further discussion. 3 Includes other underwriting business and run-off operations. 4 Pretax profit (loss) for fees and other revenues is allocated to operating segments based on revenue. 5 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit (loss) is net of investment expenses.
Underwriting Margins and Combined Ratios for our Underwriting Operations	Following are the underwriting margins and combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2023 2022 2023 2022 Under-writing Combined Under-writing Combined Under-writing Combined Under-writing Combined Personal Lines Agency 6.2 % 93.8 (0.9) % 100.9 2.8 % 97.2 3.9 % 96.1 Direct 11.1 88.9 0.8 99.2 4.4 95.6 2.6 97.4 Total Personal Lines 8.8 91.2 0 100.0 3.7 96.3 3.2 96.8 Commercial Lines 0.9 99.1 10.3 89.7 2.0 98.0 10.1 89.9 Property 1 11.8 88.2 (25.1) 125.1 (8.9) 108.9 (17.1) 117.1 Total underwriting operations 7.6 92.4 0.8 99.2 2.8 97.2 3.5 96.5 1 Included in the three and nine months ended September 30, 2023, is 0.5 points and 0.6 points, respectively, of amortization expense associated with intangible assets and 0.9 points and 1.4 points, respectively, for the three and nine months ended September 30, 2022.

Dividends (Tables)

Dividends (Tables)	9 Months Ended
	Sep. 30, 2023
Dividends [Abstract]
Dividends	Following is a summary of our common and preferred share dividends that were declared and/or paid during the nine months ended September 30, 2023 and 2022: (millions, except per share amounts) Amount Declared Payable Per Share Accrued/Paid 1 Common – Quarterly Dividends: August 2023 October 2023 $ 0.10 $ 58.5 May 2023 July 2023 0.10 58.5 March 2023 April 2023 0.10 58.5 December 2022 January 2023 0.10 58.5 August 2022 October 2022 0.10 58.5 May 2022 July 2022 0.10 58.5 March 2022 April 2022 0.10 58.5 December 2021 January 2022 0.10 58.5 Preferred Dividends: August 2023 September 2023 20.67700 10.3 May 2023 June 2023 18.92463 9.5 December 2022 March 2023 26.875 13.4 August 2022 September 2022 26.875 13.4 December 2021 March 2022 26.875 13.4 1 The accrual is based on an estimate of shares outstanding as of the record date and recorded as a component of accounts payable, accrued expenses, and other liabilities on our consolidated balance sheets until paid.

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2023
Equity [Abstract]
Components of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized losses on forecasted transactions Foreign Balance at June 30, 2023 $ (3,369.6) $ 715.6 $ (2,654.0) $ (2,638.7) $ (14.3) $ (1.0) Other comprehensive income (loss) before reclassifications: Investment securities (547.4) 114.9 (432.5) (432.5) 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications (547.4) 114.9 (432.5) (432.5) 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (55.6) 11.7 (43.9) (43.9) 0 0 Interest expense (0.2) 0 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income (55.8) 11.7 (44.1) (43.9) (0.2) 0 Total other comprehensive income (loss) (491.6) 103.2 (388.4) (388.6) 0.2 0 Balance at September 30, 2023 $ (3,861.2) $ 818.8 $ (3,042.4) $ (3,027.3) $ (14.1) $ (1.0) Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized losses on forecasted transactions Foreign Balance at June 30, 2022 $ (2,795.5) $ 586.5 $ (2,209.0) $ (2,193.5) $ (14.7) $ (0.8) Other comprehensive income (loss) before reclassifications: Investment securities (1,223.3) 256.9 (966.4) (966.4) 0 0 Foreign currency translation adjustment (0.9) 0.2 (0.7) 0 0 (0.7) Total other comprehensive income (loss) before reclassifications (1,224.2) 257.1 (967.1) (966.4) 0 (0.7) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (58.4) 12.3 (46.1) (46.1) 0 0 Interest expense (0.2) 0.1 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income (58.6) 12.4 (46.2) (46.1) (0.1) 0 Total other comprehensive income (loss) (1,165.6) 244.7 (920.9) (920.3) 0.1 (0.7) Balance at September 30, 2022 $ (3,961.1) $ 831.2 $ (3,129.9) $ (3,113.8) $ (14.6) $ (1.5) Components of Changes in (millions) Pretax total Total tax After tax total Total net Net unrealized losses on forecasted transactions Foreign Balance at December 31, 2022 $ (3,556.9) $ 754.9 $ (2,802.0) $ (2,786.3) $ (14.5) $ (1.2) Other comprehensive income (loss) before reclassifications: Investment securities (493.3) 103.5 (389.8) (389.8) 0 0 Foreign currency translation adjustment 0.3 (0.1) 0.2 0 0 0.2 Total other comprehensive income (loss) before reclassifications (493.0) 103.4 (389.6) (389.8) 0 0.2 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (188.3) 39.5 (148.8) (148.8) 0 0 Interest expense (0.4) 0 (0.4) 0 (0.4) 0 Total reclassification adjustment for amounts realized in net income (188.7) 39.5 (149.2) (148.8) (0.4) 0 Total other comprehensive income (loss) (304.3) 63.9 (240.4) (241.0) 0.4 0.2 Balance at September 30, 2023 $ (3,861.2) $ 818.8 $ (3,042.4) $ (3,027.3) $ (14.1) $ (1.0) Components of Changes in (millions) Pretax total Total tax After tax total Total net unrealized gains (losses) on securities Net unrealized losses on forecasted transactions Foreign Balance at December 31, 2021 $ 52.3 $ (11.6) $ 40.7 $ 56.2 $ (14.9) $ (0.6) Other comprehensive income (loss) before reclassifications: Investment securities (4,321.7) 907.6 (3,414.1) (3,414.1) 0 0 Foreign currency translation adjustment (1.1) 0.2 (0.9) 0 0 (0.9) Total other comprehensive income (loss) before reclassifications (4,322.8) 907.8 (3,415.0) (3,414.1) 0 (0.9) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (309.0) 64.9 (244.1) (244.1) 0 0 Interest expense (0.4) 0.1 (0.3) 0 (0.3) 0 Total reclassification adjustment for amounts realized in net income (309.4) 65.0 (244.4) (244.1) (0.3) 0 Total other comprehensive income (loss) (4,013.4) 842.8 (3,170.6) (3,170.0) 0.3 (0.9) Balance at September 30, 2022 $ (3,961.1) $ 831.2 $ (3,129.9) $ (3,113.8) $ (14.6) $ (1.5)

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2023
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Indefinite-Lived Intangible Assets	The following table is a summary of the net carrying amount of other intangible assets: (millions) September 30, 2023 September 30, 2022 December 31, 2022 Intangible assets subject to amortization $ 62.5 $ 79.4 $ 73.9 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 74.9 $ 91.8 $ 86.3
Intangible Assets Subject to Amortization	The following table is a summary of the net carrying amount of other intangible assets: (millions) September 30, 2023 September 30, 2022 December 31, 2022 Intangible assets subject to amortization $ 62.5 $ 79.4 $ 73.9 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 74.9 $ 91.8 $ 86.3 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: (millions) September 30, 2023 September 30, 2022 December 31, 2022 Category Gross Accumulated Net Gross Accumulated Net Gross Accumulated Net Agency relationships $ 159.2 $ 96.7 $ 62.5 $ 159.2 $ 85.3 $ 73.9 $ 159.2 $ 88.1 $ 71.1 Software rights 69.1 69.1 0 69.1 64.8 4.3 69.1 67.0 2.1 Trade name 3.6 3.6 0 3.6 2.4 1.2 3.6 2.9 0.7 Total $ 231.9 $ 169.4 $ 62.5 $ 231.9 $ 152.5 $ 79.4 $ 231.9 $ 158.0 $ 73.9

Basis of Presentation - Reporti

Basis of Presentation - Reporting and Accounting Policies - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning of period	$ 343.9	$ 265.8	$ 343.3	$ 280.4
Increase in allowance	157.5	130.9	399.7	320.6
Write-offs	(140.2)	(101.7)	(381.8)	(306)
Allowance for credit losses, end of period	$ 361.2	$ 295	$ 361.2	$ 295

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Assets held-for-sale	$ 81.6	$ 48.7	$ 47.4

Investments - Investment Portfo

Investments - Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 62,285.8	$ 53,125.7	$ 52,443.3
Gross Unrealized Gains	3.7	13.9	1.8
Gross Unrealized Losses	(3,846.3)	(3,551.5)	(3,943.1)
Net Holding Period Gains (Losses)	(56.1)	(74.5)	(91.3)
Total available-for-sale securities	$ 58,387.1	$ 49,513.6	$ 48,410.7
% of Total Fair Value	94.40%	92.50%	92.50%
Equity securities, at Fair Value
Cost	$ 1,639.5	$ 2,190.3	$ 2,221.3
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	1,843.9	1,844.4	1,698.4
Fair Value	$ 3,483.4	$ 4,034.7	$ 3,919.7
% of Total Fair Value	5.60%	7.50%	7.50%
Debt and Equity Securities, at Fair Value
Cost	$ 63,925.3	$ 55,316	$ 54,664.6
Gross Unrealized Gains	3.7	13.9	1.8
Gross Unrealized Losses	(3,846.3)	(3,551.5)	(3,943.1)
Net Holding Period Gains (Losses)	1,787.8	1,769.9	1,607.1
Fair Value	$ 61,870.5	$ 53,548.3	$ 52,330.4
% of Total Fair Value	100%	100%	100%
Unsettled security transactions, payable	$ 167.3		$ 74.7
Other receivable from broker-dealer and clearing organization		$ 34.4
Total fair value of non-insurance subsidiary portfolio	4,100	4,400	4,200
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	976.9	1,364.2	1,417.6
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	(108)	(151)	(163.2)
Fair Value	$ 868.9	$ 1,213.2	$ 1,254.4
% of Total Fair Value	1.40%	2.30%	2.40%
Common equities
Equity securities, at Fair Value
Cost	$ 662.6	$ 826.1	$ 803.7
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	1,951.9	1,995.4	1,861.6
Fair Value	$ 2,614.5	$ 2,821.5	$ 2,665.3
% of Total Fair Value	4.20%	5.20%	5.10%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 60,490.6	$ 50,264	$ 48,205.7
Gross Unrealized Gains	3.7	13.9	1.8
Gross Unrealized Losses	(3,846.3)	(3,551.5)	(3,943.1)
Net Holding Period Gains (Losses)	(56.1)	(74.5)	(91.3)
Total available-for-sale securities	$ 56,591.9	$ 46,651.9	$ 44,173.1
% of Total Fair Value	91.50%	87.10%	84.40%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 36,392.2	$ 26,770.7	$ 24,260.9
Gross Unrealized Gains	0	1.4	0
Gross Unrealized Losses	(2,098.4)	(1,604.7)	(1,855.4)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 34,293.8	$ 25,167.4	$ 22,405.5
% of Total Fair Value	55.40%	47%	42.80%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,257.6	$ 2,180	$ 2,140.9
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(203.3)	(202.9)	(215.4)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 2,054.3	$ 1,977.1	$ 1,925.5
% of Total Fair Value	3.30%	3.70%	3.70%
Total fixed maturities | Foreign government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 16.9	$ 16.8	$ 16.7
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(1.5)	(1.3)	(1.4)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 15.4	$ 15.5	$ 15.3
% of Total Fair Value	0.10%	0.10%	0.10%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 10,880.8	$ 10,125.8	$ 10,119.8
Gross Unrealized Gains	1.3	9.8	0
Gross Unrealized Losses	(591.8)	(676.1)	(832.5)
Net Holding Period Gains (Losses)	(36.1)	(46.8)	(58.9)
Total available-for-sale securities	$ 10,254.2	$ 9,412.7	$ 9,228.4
% of Total Fair Value	16.60%	17.60%	17.60%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 530.4	$ 696.1	$ 754.2
Gross Unrealized Gains	0.2	0.3	0.4
Gross Unrealized Losses	(13)	(17.5)	(16.5)
Net Holding Period Gains (Losses)	(1.8)	(12.1)	(15.1)
Total available-for-sale securities	$ 515.8	$ 666.8	$ 723
% of Total Fair Value	0.80%	1.20%	1.40%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,699.5	$ 5,446	$ 5,833.9
Gross Unrealized Gains	2.1	1.5	1.4
Gross Unrealized Losses	(723.9)	(784)	(747.4)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 3,977.7	$ 4,663.5	$ 5,087.9
% of Total Fair Value	6.40%	8.70%	9.70%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 5,524.2	$ 4,826	$ 4,876.7
Gross Unrealized Gains	0.1	0.9	0
Gross Unrealized Losses	(210.1)	(260.5)	(268.9)
Net Holding Period Gains (Losses)	(0.6)	(1.8)	(1.9)
Total available-for-sale securities	$ 5,313.6	$ 4,564.6	$ 4,605.9
% of Total Fair Value	8.60%	8.50%	8.80%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 189	$ 202.6	$ 202.6
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(4.3)	(4.5)	(5.6)
Net Holding Period Gains (Losses)	(17.6)	(13.8)	(15.4)
Total available-for-sale securities	$ 167.1	$ 184.3	$ 181.6
% of Total Fair Value	0.30%	0.30%	0.30%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,795.2	$ 2,861.7	$ 4,237.6
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 1,795.2	$ 2,861.7	$ 4,237.6
% of Total Fair Value	2.90%	5.40%	8.10%

Investments - Hybrid Securities

Investments - Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Hybrid Instruments [Line Items]
Hybrid securities	$ 1,077.7	$ 1,221.7	$ 1,235.6
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	511.1	535.4	500.4
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	410.8	509.6	550.8
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	19.4	42	51.4
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 136.4	$ 134.7	$ 133

Investments - Composition of Fi

Investments - Composition of Fixed Maturities by Maturity (Detail) - Total fixed maturities $ in Millions	Sep. 30, 2023 USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Less than one year	$ 9,639.9
One to five years	37,572.4
Five to ten years	13,197.7
Ten years or greater	80.6
Cost	60,490.6
Fair Value
Less than one year	9,387.8
One to five years	35,419
Five to ten years	11,710.3
Ten years or greater	74.8
Fair Value	$ 56,591.9

Investments - Gross Unrealized

Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($) security	Sep. 30, 2022 USD ($) security
Number of Securities
Total | security	1,531	1,468	1,522
Less than 12 Months | security	353	873	1,223
12 Months or Greater | security	1,178	595	299
Fair Value
Total	$ 54,926.2	$ 44,315.9	$ 43,030.1
Less than 12 Months	22,483.5	28,119.4	32,956.5
12 Months or Greater	32,442.7	16,196.5	10,073.6
Unrealized Losses
Total	(3,846.3)	(3,551.5)	(3,943.1)
Less than 12 Months	(668.1)	(1,505)	(2,550.4)
12 Months or Greater	$ (3,178.2)	$ (2,046.5)	$ (1,392.7)
U.S. government obligations
Number of Securities
Total | security	173	160	157
Less than 12 Months | security	37	90	103
12 Months or Greater | security	136	70	54
Fair Value
Total	$ 34,293.8	$ 24,802.5	$ 22,385.4
Less than 12 Months	16,686.2	17,327.2	16,085.8
12 Months or Greater	17,607.6	7,475.3	6,299.6
Unrealized Losses
Total	(2,098.4)	(1,604.7)	(1,855.4)
Less than 12 Months	(557.6)	(699.2)	(1,095.6)
12 Months or Greater	$ (1,540.8)	$ (905.5)	$ (759.8)
State and local government obligations
Number of Securities
Total | security	356	348	353
Less than 12 Months | security	46	239	292
12 Months or Greater | security	310	109	61
Fair Value
Total	$ 2,033.9	$ 1,948.8	$ 1,911
Less than 12 Months	359.6	1,124.2	1,363.1
12 Months or Greater	1,674.3	824.6	547.9
Unrealized Losses
Total	(203.3)	(202.9)	(215.4)
Less than 12 Months	(7.6)	(76.8)	(129.8)
12 Months or Greater	$ (195.7)	$ (126.1)	$ (85.6)
Foreign government obligations
Number of Securities
Total | security	1	1	1
Less than 12 Months | security	0	0	1
12 Months or Greater | security	1	1	0
Fair Value
Total	$ 15.4	$ 15.5	$ 15.3
Less than 12 Months	0	0	15.3
12 Months or Greater	15.4	15.5	0
Unrealized Losses
Total	(1.5)	(1.3)	(1.4)
Less than 12 Months	0	0	(1.4)
12 Months or Greater	$ (1.5)	$ (1.3)	$ 0
Corporate debt securities
Number of Securities
Total | security	467	422	452
Less than 12 Months | security	164	285	391
12 Months or Greater | security	303	137	61
Fair Value
Total	$ 9,490	$ 8,449.6	$ 8,885.6
Less than 12 Months	3,380.4	5,717.6	7,875.5
12 Months or Greater	6,109.6	2,732	1,010.1
Unrealized Losses
Total	(591.8)	(676.1)	(832.5)
Less than 12 Months	(94.1)	(426.1)	(699)
12 Months or Greater	$ (497.7)	$ (250)	$ (133.5)
Residential mortgage-backed securities
Number of Securities
Total | security	40	45	44
Less than 12 Months | security	2	27	34
12 Months or Greater | security	38	18	10
Fair Value
Total	$ 99.9	$ 151	$ 165.1
Less than 12 Months	0.4	65.1	126
12 Months or Greater	99.5	85.9	39.1
Unrealized Losses
Total	(13)	(17.5)	(16.5)
Less than 12 Months	0	(6.8)	(11.8)
12 Months or Greater	$ (13)	$ (10.7)	$ (4.7)
Commercial mortgage-backed securities
Number of Securities
Total | security	196	226	234
Less than 12 Months | security	0	99	182
12 Months or Greater | security	196	127	52
Fair Value
Total	$ 3,960.8	$ 4,651.1	$ 5,074.3
Less than 12 Months	0	1,702	3,856
12 Months or Greater	3,960.8	2,949.1	1,218.3
Unrealized Losses
Total	(723.9)	(784)	(747.4)
Less than 12 Months	0	(192.1)	(425.4)
12 Months or Greater	$ (723.9)	$ (591.9)	$ (322)
Other asset-backed securities
Number of Securities
Total | security	295	262	277
Less than 12 Months | security	104	130	217
12 Months or Greater | security	191	132	60
Fair Value
Total	$ 5,001.8	$ 4,247.8	$ 4,544.8
Less than 12 Months	2,056.9	2,144.8	3,597.1
12 Months or Greater	2,944.9	2,103	947.7
Unrealized Losses
Total	(210.1)	(260.5)	(268.9)
Less than 12 Months	(8.8)	(100.9)	(183.4)
12 Months or Greater	$ (201.3)	$ (159.6)	$ (85.5)
Redeemable preferred stocks
Number of Securities
Total | security	3	4	4
Less than 12 Months | security	0	3	3
12 Months or Greater | security	3	1	1
Fair Value
Total	$ 30.6	$ 49.6	$ 48.6
Less than 12 Months	0	38.5	37.7
12 Months or Greater	30.6	11.1	10.9
Unrealized Losses
Total	(4.3)	(4.5)	(5.6)
Less than 12 Months	0	(3.1)	(4)
12 Months or Greater	$ (4.3)	$ (1.4)	$ (1.6)

Investments - Components of Net

Investments - Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	$ 0.9	$ 0.3	$ 5	$ 12.2
Gross realized gains on equity securities	0.3	1.5	354.5	850.1
Subtotal gross realized gains on security sales	1.2	1.8	359.5	862.3
Gross realized loss on available-for-sale securities	(59.5)	(58.8)	(203.1)	(347.3)
Gross realized loss on equity securities	(16.3)	(5.1)	(139.4)	(85)
Subtotal gross realized losses on security sales	(75.8)	(63.9)	(342.5)	(432.3)
Net realized gains (losses) on available-for-sale securities sales	(58.6)	(58.5)	(198.1)	(335.1)
Net realized gains (losses) on equity securities sales	(16)	(3.6)	215.1	765.1
Subtotal net realized gains (losses) on security sales	(74.6)	(62.1)	17	430
Gain	8.4	0	21.6	0
Impairment	(2.3)	(2.2)	(6.8)	(6.5)
Subtotal net realized gains (losses) on other assets	6.1	(2.2)	14.8	(6.5)
Hybrid securities	(0.3)	(11.4)	18.4	(98.8)
Equity securities	(80.2)	(140.7)	(0.5)	(2,164.1)
Subtotal net holding period gains (losses)	(80.5)	(152.1)	17.9	(2,262.9)
Total net realized gains (losses) on securities	(149)	(216.4)	49.7	(1,839.4)
U.S. government obligations
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	0.2	0	4.2	4.6
Gross realized loss on available-for-sale securities	(14.6)	0	(27.2)	(233.5)
Net realized gains (losses) on available-for-sale securities sales	(14.4)	0	(23)	(228.9)
State and local government obligations
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	0	0	0	(1)
Net realized gains (losses) on available-for-sale securities sales	0	0	0	(1)
Corporate debt securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	0.7	0.3	0.8	6.8
Gross realized loss on available-for-sale securities	(19.1)	(14)	(69.3)	(51.8)
Net realized gains (losses) on available-for-sale securities sales	(18.4)	(13.7)	(68.5)	(45)
Residential mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	0	0	0	0.7
Net realized gains (losses) on available-for-sale securities sales	0	0	0	0.7
Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	(20.6)	(44.8)	(100.6)	(58.6)
Net realized gains (losses) on available-for-sale securities sales	(20.6)	(44.8)	(100.6)	(58.6)
Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	0	0	0	0.1
Gross realized loss on available-for-sale securities	(5.2)	0	(5.6)	(2.1)
Net realized gains (losses) on available-for-sale securities sales	(5.2)	0	(5.6)	(2)
Short-term investments
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	0	0	(0.4)	(0.3)
Net realized gains (losses) on available-for-sale securities sales	0	0	(0.4)	(0.3)
Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Gross realized gains on equity securities	0	0.1	0.3	17.6
Gross realized loss on equity securities	(7.4)	(5.1)	(117.6)	(7)
Net realized gains (losses) on equity securities sales	(7.4)	(5)	(117.3)	10.6
Common equities
Gain (Loss) on Securities [Line Items]
Gross realized gains on equity securities	0.3	1.4	354.2	832.5
Gross realized loss on equity securities	(8.9)	0	(21.8)	(78)
Net realized gains (losses) on equity securities sales	$ (8.6)	$ 1.4	$ 332.4	$ 754.5

Investments - Components of Equ

Investments - Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]
Total net gains (losses) recognized during the period on equity securities	$ (96.2)	$ (144.3)	$ 214.6	$ (1,399)
Less: Net gains (losses) recognized on equity securities sold during the period	(16)	(3.6)	215.1	765.1
Net holding period gains (losses) recognized during the period on equity securities held at period end	$ (80.2)	$ (140.7)	$ (0.5)	$ (2,164.1)

Investments - Components of N_2

Investments - Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net Investment Income [Line Items]
Gross investment income	$ 510.2	$ 333.6	$ 1,384.3	$ 868.2
Investment expenses	(7.2)	(5.8)	(18.8)	(17.4)
Net Investment Income	503	327.8	1,365.5	850.8
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	487.6	304.8	1,311.1	766.2
Total equity securities
Net Investment Income [Line Items]
Gross investment income	22.6	28.8	73.2	102
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	11.9	17.3	40.3	53.6
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	10.7	11.5	32.9	48.4
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	466	280	1,240	736.6
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	237	86.1	591.7	205.4
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	12.3	9.8	34.5	29.3
Total fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Gross investment income	0.1	0.1	0.2	0.2
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	94.8	71.8	261.9	215.4
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	7.6	8.8	22.6	23.2
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	48.7	48.3	147.9	138.1
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	62.9	52.1	173	116.5
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.6	3	8.2	8.5
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 21.6	$ 24.8	$ 71.1	$ 29.6

Investments - Additional Inform

Investments - Additional Information (Detail)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2023 USD ($) security	Sep. 30, 2023 USD ($) security	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Schedule of Debt and Equity Securities [Line Items]
Principal amount of bonds and certificates of deposit	$ 612,400,000	$ 612,400,000
Securities exceeding 10% of Shareholders' Equity	0	0	$ 0	$ 0
Non-income producing securities	0	0
Open reverse repurchase commitments	0	0	0	0
Securities sold under agreements to repurchase	$ 0	$ 0	0	0
Debt securities, available-for-sale, unrealized loss position, number of positions downgraded | security	14	14
Debt securities, available-for-sale, unrealized loss position, fair value, downgraded in period	$ 144,200,000	$ 144,200,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss, downgraded in period	7,100,000	7,100,000
Debt securities, available-for-sale, allowance for credit loss, not previously recorded		0	0
Debt securities, available-for-sale, allowance for credit loss	$ 0	0	0	0
Debt securities, available-for-sale, allowance for credit loss, write-off		$ 0	$ 0	$ 0
Recovery of impairment, other assets, number of positions | security		3
Percent change in investment income	59%	53%
Percent change in book yield	32%	39%

Fair Value - Composition of Inv

Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 61,870,500,000	$ 53,548,300,000	$ 52,330,400,000
Fair value	5,932,100,000	5,717,900,000	5,634,700,000
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	58,387,100,000	49,513,600,000	48,410,700,000
Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	56,591,900,000	46,651,900,000	44,173,100,000
Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	46,617,700,000	36,572,700,000	33,574,700,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34,293,800,000	25,167,400,000	22,405,500,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,054,300,000	1,977,100,000	1,925,500,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	15,400,000	15,500,000	15,300,000
Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	10,254,200,000	9,412,700,000	9,228,400,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9,807,100,000	9,894,900,000	10,416,800,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	515,800,000	666,800,000	723,000,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3,977,700,000	4,663,500,000	5,087,900,000
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,313,600,000	4,564,600,000	4,605,900,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	167,100,000	184,300,000	181,600,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21,300,000	40,500,000	39,800,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9,300,000	9,100,000	8,800,000
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	136,500,000	134,700,000	133,000,000
Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,795,200,000	2,861,700,000	4,237,600,000
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3,483,400,000	4,034,700,000	3,919,700,000
Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	868,900,000	1,213,200,000	1,254,400,000
Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	818,600,000	1,100,800,000	1,141,400,000
Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34,500,000	71,200,000	71,300,000
Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	15,800,000	41,200,000	41,700,000
Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,614,500,000	2,821,500,000	2,665,300,000
Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,591,900,000	2,801,700,000	2,646,600,000
Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22,600,000	19,800,000	18,700,000
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	38,630,600,000	30,799,700,000	29,292,200,000
Fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	36,057,000,000	27,977,300,000	26,634,000,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34,293,800,000	25,176,600,000	22,414,400,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34,293,800,000	25,167,400,000	22,405,500,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34,293,800,000	25,167,400,000	22,405,500,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	9,200,000	8,900,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	9,200,000	8,900,000
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,763,200,000	2,800,700,000	4,219,600,000
Fair Value, Inputs, Level 1 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,573,600,000	2,822,400,000	2,658,200,000
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	39,000,000	60,900,000
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	39,000,000	60,900,000
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 1 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,573,600,000	2,783,400,000	2,597,300,000
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,573,600,000	2,783,400,000	2,597,300,000
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	23,135,300,000	22,626,700,000	22,937,600,000
Fair value	5,932,100,000	5,717,900,000	5,634,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22,330,100,000	21,536,300,000	21,776,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22,298,100,000	21,475,300,000	21,758,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	12,323,900,000	11,405,300,000	11,169,200,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,054,300,000	1,977,100,000	1,925,500,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	15,400,000	15,500,000	15,300,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	10,254,200,000	9,412,700,000	9,228,400,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9,807,100,000	9,894,900,000	10,416,800,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	515,800,000	666,800,000	723,000,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	3,977,700,000	4,663,500,000	5,087,900,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,313,600,000	4,564,600,000	4,605,900,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	167,100,000	175,100,000	172,700,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21,300,000	40,500,000	39,800,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	9,300,000	9,100,000	8,800,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	136,500,000	125,500,000	124,100,000
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	32,000,000	61,000,000	18,000,000
Fair Value, Inputs, Level 2 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	805,200,000	1,090,400,000	1,160,900,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	805,200,000	1,090,400,000	1,111,600,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	770,700,000	994,400,000	1,015,600,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	34,500,000	71,200,000	71,300,000
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	24,800,000	24,700,000
Fair Value, Inputs, Level 2 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	49,300,000
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	49,300,000
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	104,600,000	121,900,000	100,600,000
Fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	104,600,000	121,900,000	100,600,000
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	63,700,000	83,800,000	81,900,000
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	47,900,000	67,400,000	64,900,000
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	0	0	0
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	15,800,000	16,400,000	17,000,000
Fair Value, Inputs, Level 3 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,900,000	38,100,000	18,700,000
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	18,300,000	18,300,000	0
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	22,600,000	19,800,000	18,700,000
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	63,925,300,000	55,316,000,000	54,664,600,000
Long-term debt	6,887,600,000	6,388,300,000	6,387,400,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	62,285,800,000	53,125,700,000	52,443,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	60,490,600,000	50,264,000,000	48,205,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	49,547,500,000	39,093,300,000	36,538,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	36,392,200,000	26,770,700,000	24,260,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	2,257,600,000	2,180,000,000	2,140,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	16,900,000	16,800,000	16,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	10,880,800,000	10,125,800,000	10,119,800,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	10,754,100,000	10,968,100,000	11,464,800,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	530,400,000	696,100,000	754,200,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	4,699,500,000	5,446,000,000	5,833,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	5,524,200,000	4,826,000,000	4,876,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	189,000,000	202,600,000	202,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	24,500,000	43,600,000	43,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	10,400,000	10,500,000	10,500,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	154,100,000	148,500,000	148,500,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,795,200,000	2,861,700,000	4,237,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	1,639,500,000	2,190,300,000	2,221,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	976,900,000	1,364,200,000	1,417,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	921,900,000	1,244,200,000	1,297,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	40,000,000	79,900,000	79,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	15,000,000	40,100,000	40,100,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	662,600,000	826,100,000	803,700,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	640,000,000	806,300,000	785,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 22,600,000	$ 19,800,000	$ 18,700,000

Fair Value - Summary of Changes

Fair Value - Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 112	$ 100.4	$ 121.9	$ 127.7
Calls/ Maturities/ Paydowns	0.7	0.2	2.8	1.3
Purchases	0	0	0	0
Sales	(10)	0	(10)	(15)
Net Realized (Gain)/Loss on Sales	8.9	0	8.9	(17.2)
Change in Valuation	(7)	0	(19)	3.8
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	104.6	100.6	104.6	100.6
Total equity securities | Financials | Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	56	64.9	67.4	76.4
Calls/ Maturities/ Paydowns	(8.1)	0	(8.1)	0
Purchases	0	0	0	0
Sales	0	0	0	(15)
Net Realized (Gain)/Loss on Sales	0	0	0	(17.2)
Change in Valuation	0	0	(11.4)	20.7
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	47.9	64.9	47.9	64.9
Total equity securities | Industrials | Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	15.8	17	16.4	34.4
Calls/ Maturities/ Paydowns	0	0	0	(0.5)
Purchases	0	0	0	0
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	0	0
Change in Valuation	0	0	(0.6)	(16.9)
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	15.8	17	15.8	17
Total equity securities | Common Stocks, excluding Other Risk | Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	18.3	0	18.3
Calls/ Maturities/ Paydowns	8.1	0	8.1
Purchases	0	0	0
Sales	(10)	0	(10)
Net Realized (Gain)/Loss on Sales	8.9	0	8.9
Change in Valuation	(7)	0	(7)
Net Transfers In (Out)	0	0	0
Fair value, ending balance	18.3	0	18.3	0
Total equity securities | Other risk investments | Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	21.9	18.5	19.8	16.9
Calls/ Maturities/ Paydowns	0.7	0.2	2.8	1.8
Purchases	0	0	0	0
Sales	0	0	0	0
Net Realized (Gain)/Loss on Sales	0	0	0	0
Change in Valuation	0	0	0	0
Net Transfers In (Out)	0	0	0	0
Fair value, ending balance	$ 22.6	$ 18.7	$ 22.6	$ 18.7

Fair Value - Summary of Quantit

Fair Value - Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	$ 61,870.5	$ 52,330.4	$ 53,548.3
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	104.6	100.6	121.9
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	82	81.9	102.1
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	22.6	18.7	19.8
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	$ 63.7	$ 81.9	$ 83.8
Fair Value Inputs, Market Cap Price Change Percentage, Range, Minimum	(10.80%)	(20.70%)	(0.60%)
Fair Value Inputs, Market Cap Price Change Percentage, Range, Maximum	4.50%	(1.10%)	19.90%
Fair Value Inputs, Market Cap Price Change Percentage, Weighted Average	1.70%	(6.60%)	10.50%
Fair Value, Inputs, Level 3 | Common equities | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	$ 18.3		$ 18.3
Fair Value Inputs, Market Cap Price Change Percentage, Range, Minimum	(37.40%)		(42.50%)
Fair Value Inputs, Market Cap Price Change Percentage, Range, Maximum	130.20%		59.10%
Fair Value Inputs, Market Cap Price Change Percentage, Weighted Average	(7.90%)		0.30%

Fair Value - Additional Informa

Fair Value - Additional Information (Detail)	Sep. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)
Fair Value Measurements [Line Items]
Investments, fair value	$ 61,870,500,000	$ 53,548,300,000	$ 52,330,400,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor quoted prices, percent of FV hierarchy level	93%		90%
Investments, fair value	$ 38,630,600,000	$ 30,799,700,000	$ 29,292,200,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor quoted prices, percent of FV hierarchy level	99%	98%	98%
Dealer quoted prices, percent of FV hierarchy level	1%	2%	2%
Investments, fair value	$ 23,135,300,000	$ 22,626,700,000	$ 22,937,600,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, fair value	$ 104,600,000	121,900,000	100,600,000
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Security type reclassification, level 3 securities | security	1
Available-for-sale Securities
Fair Value Measurements [Line Items]
Investments, fair value	$ 58,387,100,000	49,513,600,000	48,410,700,000
Available-for-sale Securities | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	56,591,900,000	46,651,900,000	44,173,100,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, fair value	36,057,000,000	27,977,300,000	26,634,000,000
Available-for-sale Securities | Fair Value, Inputs, Level 1 | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	34,293,800,000	25,176,600,000	22,414,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, fair value	22,330,100,000	21,536,300,000	21,776,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 2 | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	22,298,100,000	21,475,300,000	21,758,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, fair value	0	0	0
Available-for-sale Securities | Fair Value, Inputs, Level 3 | Total fixed maturities
Fair Value Measurements [Line Items]
Investments, fair value	$ 0	$ 0	$ 0

Debt - Summary of Debt (Detail)

Debt - Summary of Debt (Detail) - USD ($)		Sep. 30, 2023	May 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Mar. 31, 2022	Mar. 31, 2020	Oct. 31, 2018	Mar. 31, 2018	Apr. 30, 2017	Aug. 31, 2016	Jan. 31, 2015	Apr. 30, 2014	Nov. 30, 2002	Mar. 31, 1999
Debt Instrument [Line Items]
Debt	[1]	$ 6,887,600,000		$ 6,388,300,000	$ 6,387,400,000
Fair value		5,932,100,000		5,717,900,000	5,634,700,000
2.45% Senior Notes due 2027 (issued: $500.0, August 2016)
Debt Instrument [Line Items]
Long-term debt		498,500,000		498,200,000	498,100,000
Fair value		455,800,000		457,700,000	459,700,000
Debt, stated interest rate											2.45%
Debt, face amount											$ 500,000,000
2.50% Senior Notes due 2027 (issued: $500.0, March 2022)
Debt Instrument [Line Items]
Long-term debt		497,900,000		497,500,000	497,400,000
Fair value		454,000,000		460,300,000	451,600,000
Debt, stated interest rate						2.50%
Debt, face amount						$ 500,000,000
6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999)
Debt Instrument [Line Items]
Long-term debt		297,700,000		297,500,000	297,400,000
Fair value		320,800,000		326,800,000	322,100,000
Debt, stated interest rate															6.625%
Debt, face amount															$ 300,000,000
4.00% Senior Notes due 2029 (issued: $550.0, October 2018)
Debt Instrument [Line Items]
Long-term debt		546,800,000		546,400,000	546,300,000
Fair value		518,900,000		527,800,000	519,500,000
Debt, stated interest rate								4%
Debt, face amount								$ 550,000,000
3.20% Senior Notes due 2030 (issued: $500.0, March 2020)
Debt Instrument [Line Items]
Long-term debt		497,200,000		496,900,000	496,800,000
Fair value		435,100,000		448,600,000	439,300,000
Debt, stated interest rate							3.20%
Debt, face amount							$ 500,000,000
3.00% Senior Notes due 2032 (issued: $500.0, March 2022)
Debt Instrument [Line Items]
Long-term debt		496,200,000		495,900,000	495,800,000
Fair value		420,500,000		438,100,000	418,800,000
Debt, stated interest rate						3%
Debt, face amount						$ 500,000,000
6.25% Senior Notes due 2032 (issued: $400.0, November 2002)
Debt Instrument [Line Items]
Long-term debt		396,600,000		396,400,000	396,400,000
Fair value		414,300,000		435,400,000	426,000,000
Debt, stated interest rate														6.25%
Debt, face amount														$ 400,000,000
4.95% Senior Notes due 2033 (issued: $500.0, May 2023)
Debt Instrument [Line Items]
Long-term debt		496,400,000		0	0
Fair value		474,700,000		0	0
Debt, stated interest rate			4.95%
Debt, face amount			$ 500,000,000
4.35% Senior Notes due 2044 (issued: $350.0, April 2014)
Debt Instrument [Line Items]
Long-term debt		347,000,000		346,900,000	346,900,000
Fair value		280,900,000		298,400,000	298,500,000
Debt, stated interest rate													4.35%
Debt, face amount													$ 350,000,000
3.70% Senior Notes due 2045 (issued: $400.0, January 2015)
Debt Instrument [Line Items]
Long-term debt		395,800,000		395,700,000	395,700,000
Fair value		288,200,000		310,200,000	305,700,000
Debt, stated interest rate												3.70%
Debt, face amount												$ 400,000,000
4.125% Senior Notes due 2047 (issued: $850.0, April 2017)
Debt Instrument [Line Items]
Long-term debt		842,200,000		842,100,000	842,000,000
Fair value		666,300,000		716,200,000	710,300,000
Debt, stated interest rate										4.125%
Debt, face amount										$ 850,000,000
4.20% Senior Notes due 2048 (issued: $600.0, March 2018)
Debt Instrument [Line Items]
Long-term debt		590,600,000		590,400,000	590,300,000
Fair value		473,300,000		507,000,000	505,900,000
Debt, stated interest rate									4.20%
Debt, face amount									$ 600,000,000
3.95% Senior Notes due 2050 (issued: $500.0, March 2020)
Debt Instrument [Line Items]
Long-term debt		491,100,000		490,900,000	490,900,000
Fair value		373,500,000		404,900,000	396,600,000
Debt, stated interest rate							3.95%
Debt, face amount							$ 500,000,000
3.70% Senior Notes due 2052 (issued: $500.0, March 2022)
Debt Instrument [Line Items]
Long-term debt		493,600,000		493,500,000	493,400,000
Fair value		$ 355,800,000		$ 386,500,000	$ 380,700,000
Debt, stated interest rate						3.70%
Debt, face amount						$ 500,000,000
[1] Consists of long-term debt. See Note 4 – Debt for further discussion.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	1 Months Ended	6 Months Ended	9 Months Ended		12 Months Ended
	Aug. 31, 2023	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	May 31, 2023	Mar. 31, 2023
Debt Instrument [Line Items]
Proceeds from debt, net of issuance costs	$ 496,300,000
Long-term debt, current maturities		$ 0	$ 0	$ 0	$ 0
4.95% Senior Notes due 2033 (issued: $500.0, May 2023)
Debt Instrument [Line Items]
Debt, face amount						$ 500,000,000
Debt, stated interest rate						4.95%
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 300,000,000	300,000,000				$ 250,000,000
Discretionary line of credit repayment period		30 days
Borrowings outstanding			$ 0	$ 0	$ 0
Discretionary Line of Credit | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.10%

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Valuation allowance	$ 0	$ 0	$ 0	$ 0	$ 0
Gross deferred tax asset related to unrealized losses on fixed-maturity securities	$ 807,000,000	$ 827,700,000	$ 807,000,000	$ 827,700,000	$ 742,900,000
Provision for income taxes	20.50%	12.90%	20.20%	(1.60%)

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves Activity in Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at January 1	$ 30,359.3	$ 26,164.1
Less reinsurance recoverables on unpaid losses	4,941.2	6,034	$ 5,559.2	$ 4,733.6
Net balance at January 1	24,800.1	21,430.5
Incurred related to:
Current year	33,035	28,245.4
Prior years	1,147	52.8
Total incurred	34,182	28,298.2
Paid related to:
Current year	18,581.3	15,712.4
Prior years	11,764.7	9,418.5
Total paid	30,346	25,130.9
Net balance at September 30	28,636.1	24,597.8
Plus reinsurance recoverables on unpaid losses	4,941.2	6,034
Balance at September 30	$ 33,577.3	$ 30,631.8

Loss and Loss Adjustment Expe_4

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 1,147	$ 52.8
Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	866	(28)
Personal Lines | Personal auto business | Direct
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		(25)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	277	77
Short-Duration Insurance Contract, Accident Year 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	929
Short-Duration Insurance Contract, Accident Year 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 108	45
Short Duration Insurance Contract Accident Years 2019 and Prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		29
Short-Duration Insurance Contract, Accident Year 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ (21)

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Open reverse repurchase commitments	$ 0	$ 0	$ 0
Operating lease liabilities	45,600,000	29,700,000
Income taxes	413,000,000	595,900,000
Interest	213,500,000	191,300,000
Operating lease liabilities	57,200,000	62,500,000
Cash and Cash Equivalents
Open reverse repurchase commitments	79,700,000	123,000,000	$ 125,900,000
Common equities | Quarterly dividend
Common share dividends	$ 58,500,000	$ 58,500,000

Segment Information - Operating

Segment Information - Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 15,560.6	$ 12,780.2	$ 45,217.3	$ 36,140.9
Pretax Profit (Loss)	1,410.2	142.5	2,400.3	(103.2)
Amortization of intangible assets	2.9	5.4	11.4	25.5
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	5,414	4,441.9	15,481.4	13,131.7
Pretax Profit (Loss)	335.7	(41.3)	427.1	507.6
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,361.8	5,077.4	18,259.9	14,776.9
Pretax Profit (Loss)	706.2	40.6	810.6	389.4
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	14,894.3	12,398.9	42,891.8	36,349.7
Pretax Profit (Loss)	1,136.2	94.3	1,213.5	1,281.6
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	11,775.8	9,519.3	33,741.3	27,908.6
Pretax Profit (Loss)	$ 1,041.9	$ (0.7)	$ 1,237.7	$ 897
Personal auto insurance percentage of the total personal lines segment net premiums earned	94%	94%	94%	94%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 2,486.6	$ 2,317.9	$ 7,296.8	$ 6,749.5
Pretax Profit (Loss)	23	238.1	147.4	683.5
Operating Segments | Property
Segment Reporting Information [Line Items]
Pretax Profit (Loss)				514.4
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	631.8	561	1,852.8	1,689.6
Pretax Profit (Loss)	74.6	(141)	(164.9)	(289.6)
Amortization of intangible assets	2.9	5	10.7	24.1
Operating Segments | Property | Property - Goodwill impairment
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	0	0	0	(224.8)
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.1	0.7	0.9	2
Pretax Profit (Loss)	(3.3)	(2.1)	(6.7)	(9.3)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	223.7	181.4	656.6	531.9
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	81.4	82.7	234.9	230.5
Pretax Profit (Loss)	(10.3)	(0.1)	(29.7)	9
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	361.2	117.2	1,434	(971.2)
Pretax Profit (Loss)	354	111.4	1,415.2	(988.6)
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (69.7)	$ (63.1)	$ (198.7)	$ (180.4)

Segment Information - Underwrit

Segment Information - Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	6.20%	(0.90%)	2.80%	3.90%
Combined Ratio	93.80%	100.90%	97.20%	96.10%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	11.10%	0.80%	4.40%	2.60%
Combined Ratio	88.90%	99.20%	95.60%	97.40%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	7.60%	0.80%	2.80%	3.50%
Combined Ratio	92.40%	99.20%	97.20%	96.50%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	8.80%	0%	3.70%	3.20%
Combined Ratio	91.20%	100%	96.30%	96.80%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	0.90%	10.30%	2%	10.10%
Combined Ratio	99.10%	89.70%	98%	89.90%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	11.80%	(25.10%)	(8.90%)	(17.10%)
Combined Ratio	88.20%	125.10%	108.90%	117.10%
Combined ratio amortization points	0.005	0.009	0.006	0.014

Dividends (Details)

Dividends (Details) - USD ($) $ / shares in Units, $ in Millions				1 Months Ended												3 Months Ended		9 Months Ended
	Sep. 15, 2023	Jul. 01, 2023	Mar. 15, 2023	Sep. 30, 2023	Aug. 31, 2023	Jun. 30, 2023	Apr. 30, 2023	Mar. 31, 2023	Jan. 31, 2023	Oct. 31, 2022	Sep. 30, 2022	Jul. 31, 2022	Apr. 30, 2022	Mar. 31, 2022	Jan. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Cash dividends declared on common stock (in dollars per share)																$ 0.10	$ 0.10	$ 0.30	$ 0.30
Dividends paid to common shareholders																		$ 175.5	$ 175.5
Dividends paid to preferred shareholders																		33.2	26.8
Series B Preferred Stock
Cash dividends paid on preferred stock (in dollars per share)				$ 20.67700		$ 18.92463		$ 26.875			$ 26.875			$ 26.875
Dividends paid to preferred shareholders				$ 10.3		$ 9.5		$ 13.4			$ 13.4			$ 13.4
Series B Preferred Stock | London Interbank Offered Rate (LIBOR)
Preferred stock, basis spread on variable rate			2.539%
Series B Preferred Stock | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Preferred stock, basis spread on variable rate	2.539%
Preferred stock, basis spread on variable rate, tenor spread adjustment	0.26161%
Quarterly dividend
Cash dividends declared on common stock (in dollars per share)					$ 0.10
Cash dividends paid on common stock (in dollars per share)		$ 0.10					$ 0.10		$ 0.10	$ 0.10		$ 0.10	$ 0.10		$ 0.10
Dividends paid to common shareholders		$ 58.5					$ 58.5		$ 58.5	$ 58.5		$ 58.5	$ 58.5		$ 58.5
Common equities | Quarterly dividend
Common share dividends				$ 58.5							$ 58.5					$ 58.5	$ 58.5	$ 58.5	$ 58.5

Other Comprehensive Income (L_3

Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pretax total accumulated other comprehensive income (loss), beginning balance	$ (3,369.6)	$ (2,795.5)	$ (3,556.9)	$ 52.3
Pretax total accumulated other comprehensive income (loss), Investment securities	(547.4)	(1,223.3)	(493.3)	(4,321.7)
Pretax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0	(0.9)	0.3	(1.1)
Pretax total accumulated other comprehensive income (loss), before reclassifications	(547.4)	(1,224.2)	(493)	(4,322.8)
Pretax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	(55.6)	(58.4)	(188.3)	(309)
Pretax total accumulated other comprehensive income (loss), Interest expense	(0.2)	(0.2)	(0.4)	(0.4)
Total reclassification adjustment for amounts realized in net income, pre tax	(55.8)	(58.6)	(188.7)	(309.4)
Total other comprehensive income (loss), pre tax	(491.6)	(1,165.6)	(304.3)	(4,013.4)
Pretax total accumulated other comprehensive income (loss), ending balance	(3,861.2)	(3,961.1)	(3,861.2)	(3,961.1)
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Total tax (provision) benefit, beginning balance	715.6	586.5	754.9	(11.6)
Total tax (provision) benefit, Investment securities	114.9	256.9	103.5	907.6
Total tax (provision) benefit, Foreign currency translation adjustment	0	0.2	(0.1)	0.2
Total tax (provision) benefit, before reclassifications	114.9	257.1	103.4	907.8
Total tax (provision) benefit, Net realized gains (losses) on securities	11.7	12.3	39.5	64.9
Total tax (provision) benefit, Interest expense	0	0.1	0	0.1
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	11.7	12.4	39.5	65
Total other comprehensive income (loss), tax (provision) benefit	103.2	244.7	63.9	842.8
Total tax (provision) benefit, ending balance	818.8	831.2	818.8	831.2
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(2,654)	(2,209)	(2,802)	40.7
After tax total accumulated other comprehensive income (loss), Investment securities	(432.5)	(966.4)	(389.8)	(3,414.1)
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0	(0.7)	0.2	(0.9)
After tax total accumulated other comprehensive income (loss), before reclassifications	(432.5)	(967.1)	(389.6)	(3,415)
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	(43.9)	(46.1)	(148.8)	(244.1)
After tax total accumulated other comprehensive income (loss), Interest expense	(0.2)	(0.1)	(0.4)	(0.3)
Total reclassification adjustment for amounts realized in net income, after tax	(44.1)	(46.2)	(149.2)	(244.4)
Total other comprehensive income (loss), after tax	(388.4)	(920.9)	(240.4)	(3,170.6)
After tax total accumulated other comprehensive income (loss), ending balance	(3,042.4)	(3,129.9)	(3,042.4)	(3,129.9)
Total net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(2,638.7)	(2,193.5)	(2,786.3)	56.2
After tax total accumulated other comprehensive income (loss), Investment securities	(432.5)	(966.4)	(389.8)	(3,414.1)
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0	0	0	0
After tax total accumulated other comprehensive income (loss), before reclassifications	(432.5)	(966.4)	(389.8)	(3,414.1)
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	(43.9)	(46.1)	(148.8)	(244.1)
After tax total accumulated other comprehensive income (loss), Interest expense	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	(43.9)	(46.1)	(148.8)	(244.1)
Total other comprehensive income (loss), after tax	(388.6)	(920.3)	(241)	(3,170)
After tax total accumulated other comprehensive income (loss), ending balance	(3,027.3)	(3,113.8)	(3,027.3)	(3,113.8)
Net unrealized losses on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(14.3)	(14.7)	(14.5)	(14.9)
After tax total accumulated other comprehensive income (loss), Investment securities	0	0	0	0
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0	0	0	0
After tax total accumulated other comprehensive income (loss), before reclassifications	0	0	0	0
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	0	0	0	0
After tax total accumulated other comprehensive income (loss), Interest expense	(0.2)	(0.1)	(0.4)	(0.3)
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	(0.1)	(0.4)	(0.3)
Total other comprehensive income (loss), after tax	0.2	0.1	0.4	0.3
After tax total accumulated other comprehensive income (loss), ending balance	(14.1)	(14.6)	(14.1)	(14.6)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(1)	(0.8)	(1.2)	(0.6)
After tax total accumulated other comprehensive income (loss), Investment securities	0	0	0	0
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0	(0.7)	0.2	(0.9)
After tax total accumulated other comprehensive income (loss), before reclassifications	0	(0.7)	0.2	(0.9)
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	0	0	0	0
After tax total accumulated other comprehensive income (loss), Interest expense	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Total other comprehensive income (loss), after tax	0	(0.7)	0.2	(0.9)
After tax total accumulated other comprehensive income (loss), ending balance	$ (1)	$ (1.5)	$ (1)	$ (1.5)

Other Comprehensive Income (L_4

Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement, Additional Information (Detail) $ in Millions	Sep. 30, 2023 USD ($)
Equity [Abstract]
Net unrealized losses on forecasted transactions, expected to reclassify into income within the next 12 months	$ 0.6

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Schedule of Other Intangible Assets [Line Items]
Indefinite-lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	74.9	86.3	91.8
Accumulated amortization	170	158.6	153.1
Licensing Agreements
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	0.6	0.6	0.6
Finite-Lived Intangible Assets
Schedule of Other Intangible Assets [Line Items]
Intangible assets subject to amortization	62.5	73.9	79.4
Accumulated amortization	$ 169.4	$ 158	$ 152.5

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 170	$ 158.6	$ 153.1
Agency relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	96.7	88.1	85.3
Net Carrying Amount	62.5	71.1	73.9
Software rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	69.1	69.1	69.1
Accumulated Amortization	69.1	67	64.8
Net Carrying Amount	0	2.1	4.3
Trade name
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	3.6	3.6	3.6
Accumulated Amortization	3.6	2.9	2.4
Net Carrying Amount	0	0.7	1.2
Finite-Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	231.9	231.9	231.9
Accumulated Amortization	169.4	158	152.5
Net Carrying Amount	$ 62.5	$ 73.9	$ 79.4

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Goodwill (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Mar. 31, 2022
Finite-Lived Intangible Assets [Line Items]
Changes to the carrying amount of goodwill				$ 0
Goodwill impairment	$ 0	$ 0		0	$ 224,800,000
Previously recorded goodwill impairment losses						$ 0
Amortization of intangible assets	$ 2,900,000	$ 5,400,000		$ 11,400,000	$ 25,500,000
Property
Finite-Lived Intangible Assets [Line Items]
Goodwill impairment			$ 224,800,000