The Progressive Corporation (PGR) 10-Q 6 May 24 2024 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 2024 shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2024
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	585,698,387
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	0000080661
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Amendment Flag	false

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Revenues
Net premiums earned		$ 16,148.6	$ 13,533.1
Investment income		617.6	419.6
Net realized gains (losses) on securities:
Net realized gains (losses) on security sales		(146.5)	(30.3)
Net holding period gains (losses) on securities		302.1	104.4
Net impairment losses recognized in earnings		0	(2.3)
Total net realized gains (losses) on securities		155.6	71.8
Fees and other revenues		236.5	206.2
Service revenues		84.2	72.5
Total revenues		17,242.5	14,303.2
Expenses
Losses and loss adjustment expenses		10,971.6	10,624
Policy acquisition costs		1,232.2	1,115.8
Other underwriting expenses		1,931.4	1,857.9
Investment expenses		5.7	5.5
Service expenses		92.1	82.3
Interest expense		69.6	63.3
Total expenses		14,302.6	13,748.8
Net Income
Income before income taxes		2,939.9	554.4
Provision for income taxes		608.5	106.5
Net income		2,331.4	447.9
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on fixed-maturity securities		(207.8)	603.2
Net unrealized losses on forecasted transactions		0.1	0.1
Foreign currency translation adjustment		(0.2)	0
Other comprehensive income (loss)		(207.9)	603.3
Comprehensive income (loss)		2,123.5	1,051.2
Computation of Earnings Per Common Share
Net income		2,331.4	447.9
Less: Preferred share dividends and other	[1]	17	7.3
Net income available to common shareholders		$ 2,314.4	$ 440.6
Average common shares outstanding - Basic (in shares)		585.4	584.9
Net effect of dilutive stock-based compensation (in shares)		1.9	2.1
Total average equivalent common shares - Diluted (in shares)		587.3	587
Basic: Earnings per common shares (in dollars per share)		$ 3.95	$ 0.75
Diluted: Earnings per common share (in dollars per share)		$ 3.94	$ 0.75
[1] All of our outstanding Serial Preferred Shares, Series B, were redeemed in February 2024. See Note 9 – Dividends for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Assets
Fixed maturities (amortized cost: $65,949.3, $53,123.9, and $62,441.9)		$ 63,629.7	$ 60,378.2	$ 50,289.2
Short-term investments (amortized cost: $1,326.7, $2,524.1, and $1,789.9)		1,326.7	1,789.9	2,524.1
Total available-for-sale securities		64,956.4	62,168.1	52,813.3
Nonredeemable preferred stocks (cost: $931.1, $1,197.7, and $977.1)		886.7	902.1	1,078.8
Common equities (cost: $708.2, $740.5, and $706.0)		3,194.9	2,928.4	2,794.3
Total equity securities		4,081.6	3,830.5	3,873.1
Total investments		69,038	65,998.6	56,686.4
Cash and cash equivalents		154.5	84.9	273.7
Restricted cash and cash equivalents		13.4	14.7	14.9
Total cash, cash equivalents, restricted cash, and restricted cash equivalents		167.9	99.6	288.6
Accrued investment income		464.2	438	299.5
Premiums receivable, net of allowance for credit losses of $327.7, $340.9, and $369.1		14,192.5	11,958.2	12,411.4
Reinsurance recoverables		5,003.4	5,093.9	5,616.2
Prepaid reinsurance premiums		209.8	249.8	269.6
Deferred acquisition costs		1,818.2	1,687.4	1,626.8
Property and equipment, net of accumulated depreciation of $1,580.2, $1,576.2, and $1,655.1		756.3	880.8	949
Net federal deferred income taxes		1,031.8	936	1,057
Other assets		1,445.8	1,348.5	1,202.6
Total assets		94,127.9	88,690.8	80,407.1
Liabilities
Unearned premiums		22,907.3	20,133.7	19,844.3
Loss and loss adjustment expense reserves		34,831	34,389.2	31,026.4
Accounts payable, accrued expenses, and other liabilities		7,688.6	7,002.2	6,278.7
Debt	[1]	6,889.7	6,888.6	6,389.3
Total liabilities		72,316.6	68,413.7	63,538.7
Shareholders' Equity
Serial Preferred Shares, Series B, no par value (cumulative, liquidation preference $1,000 per share) (authorized, issued, and outstanding of $0, $0.5, and $0.5)		0	493.9	493.9
Common shares, $1.00 par value (authorized 900.0; issued 797.6, including treasury shares of 211.9, 212.2, and 212.3)		585.7	585.3	585.4
Paid-in capital		2,028.7	2,013.1	1,907.7
Retained earnings		21,020.5	18,800.5	16,080.1
Accumulated other comprehensive income (loss):
Net unrealized gains (losses) on fixed-maturity securities		(1,808.6)	(1,600.8)	(2,183.1)
Net unrealized losses on forecasted transactions		(13.9)	(14)	(14.4)
Foreign currency translation adjustment		(1.1)	(0.9)	(1.2)
Total accumulated other comprehensive income (loss)		(1,823.6)	(1,615.7)	(2,198.7)
Total shareholders’ equity		21,811.3	20,277.1	16,868.4
Total liabilities and shareholders’ equity		$ 94,127.9	$ 88,690.8	$ 80,407.1
[1] Consists of long-term debt. See Note 4 – Debt for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
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	211,900,000	212,300,000	212,200,000
Fixed maturities, amortized cost	$ 65,949.3	$ 62,441.9	$ 53,123.9
Short-term investments, amortized cost	1,326.7	1,789.9	2,524.1
Nonredeemable preferred stocks, cost	931.1	977.1	1,197.7
Common equities, cost	708.2	706	740.5
Premium receivable, allowance for credit loss	327.7	369.1	340.9
Property and equipment, accumulated depreciation	$ 1,580.2	$ 1,655.1	$ 1,576.2
Serial Preferred Stock
Preferred stock, shares authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock
Preferred stock, liquidation preference per share (USD per share)	$ 1,000
Series B Preferred Stock | Serial Preferred Stock
Preferred stock, shares authorized (shares)	0	500,000	500,000
Preferred stock, no par value		$ 0	$ 0
Preferred stock, liquidation preference per share (USD per share)		$ 1,000	$ 1,000
Preferred stock, shares issued (shares)	0	500,000	500,000
Preferred stock, shares outstanding (shares)	0	500,000	500,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity - 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)		$ 0.10
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)									$ 0
Balance, Beginning of period at Dec. 31, 2022				$ 493.9	$ 584.9	$ 1,893	$ 15,721.2	$ (2,802)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of Serial Preferred Shares, Series B	[1]			0
Net income		$ 447.9					447.9
Treasury shares purchased					(0.2)	(0.8)	(31.7)
Cash dividends declared on common shares ($0.10 and $0.10 per share)	[1]						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($15.688377 and $0 per share)	[1]						0
Net restricted equity awards issued/vested					0.7	(0.7)
Amortization of equity-based compensation						15.9
Reinvested dividends on restricted stock units						0.3	(0.3)
Other, net							1.5
Other comprehensive income (loss)		603.3						603.3
Balance, End of period at Mar. 31, 2023		$ 16,868.4		493.9	585.4	1,907.7	16,080.1	(2,198.7)
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)		$ 0.10
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)									$ 15.688377
Balance, Beginning of period at Dec. 31, 2023		$ 20,277.1		493.9	585.3	2,013.1	18,800.5	(1,615.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of Serial Preferred Shares, Series B	[1]			(493.9)
Net income		2,331.4					2,331.4
Treasury shares purchased					(0.2)	(0.8)	(36.1)
Cash dividends declared on common shares ($0.10 and $0.10 per share)	[1]						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($15.688377 and $0 per share)	[1]						(7.8)
Net restricted equity awards issued/vested					0.6	(0.6)
Amortization of equity-based compensation						16.8
Reinvested dividends on restricted stock units						0.2	(0.2)
Other, net							(8.8)
Other comprehensive income (loss)		(207.9)						(207.9)
Balance, End of period at Mar. 31, 2024		$ 21,811.3		$ 0	$ 585.7	$ 2,028.7	$ 21,020.5	$ (1,823.6)
Common shares, par value (USD per share)		$ 1
Preferred stock, shares authorized (shares)			20,000,000						0	5,000,000
Preferred stock, shares issued (shares)									0	0
[1] See Note 9 – Dividends for further discussion.

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Cash Flows From Operating Activities
Net income	$ 2,331.4	$ 447.9
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	69.7	68
Net amortization (accretion) of fixed-income securities	(7.5)	(5)
Amortization of equity-based compensation	16.8	15.9
Net realized (gains) losses on securities	(155.6)	(71.8)
Net (gains) losses on disposition of property and equipment	(1.7)	16.5
Changes in:
Premiums receivable	(2,234.3)	(1,994.5)
Reinsurance recoverables	90.5	215.9
Prepaid reinsurance premiums	40	25.9
Deferred acquisition costs	(130.8)	(82.4)
Income taxes	609.5	106.6
Unearned premiums	2,773.6	2,550.7
Loss and loss adjustment expense reserves	441.8	667.1
Accounts payable, accrued expenses, and other liabilities	457.8	565.5
Other, net	(65.8)	(76.8)
Net cash provided by operating activities	4,235.4	2,449.5
Purchases:
Fixed maturities	(13,288.2)	(6,119.5)
Equity securities	(32.4)	(17.8)
Sales:
Fixed maturities	7,765.3	2,202.6
Equity securities	58.2	277.3
Maturities, paydowns, calls, and other:
Fixed maturities	1,855.2	976
Equity securities	23.3	25.1
Net (purchases) sales of short-term investments	479.2	360.8
Net change in unsettled security transactions	61.8	57.2
Purchases of property and equipment	(50.2)	(43.2)
Sales of property and equipment	3.5	4.3
Net cash used in investing activities	(3,124.3)	(2,277.2)
Cash Flows From Financing Activities
Redemption of Serial Preferred Shares, Series B	(500)	0
Dividends paid to common shareholders	(497.9)	(58.5)
Dividends paid to preferred shareholders	(7.8)	(13.4)
Acquisition of treasury shares for restricted stock tax liabilities	(37.1)	(32.7)
Net cash used in financing activities	(1,042.8)	(104.6)
Increase in cash, cash equivalents, restricted cash, and restricted cash equivalents	68.3	67.7
Cash, cash equivalents, restricted cash, and restricted cash equivalents – January 1	99.6	220.9
Cash, cash equivalents, restricted cash, and restricted cash equivalents – March 31	$ 167.9	$ 288.6

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2024
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 and 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 March 31, 2024, 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, 2023 (2023 Annual Report to Shareholders). Premiums Receivable We perform analyses to evaluate our premiums receivable for expected credit losses. See our 2023 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 March 31, (millions) 2024 2023 Allowance for credit losses, beginning of period $ 369.1 $ 343.3 Increase in allowance 1 106.8 116.9 Write-offs 2 (148.2) (119.3) Allowance for credit losses, end of period $ 327.7 $ 340.9 1 Represents the incremental increase in other underwriting expenses. 2 Represents the portion of allowance that is reversed when the premiums receivable is 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” properties of $169.8 million, $67.8 million, and $77.2 million at March 31, 2024 and 2023, and December 31, 2023, respectively. When properties are determined to be “held for sale,” the property is written down to its fair value less estimated costs to sell, as applicable. The increase in the March 31, 2024, balance from December 31, 2023, was primarily driven by a decision in the first quarter to sell certain regional properties to optimize our real estate portfolio by consolidating employees into already existing alternative properties. Earnings Per Share We redeemed all of our outstanding Serial Preferred Shares, Series B, in February 2024. See Note 9 – Dividends for further discussion. To determine net income available to common shareholders, which is used in the calculation of the per common share amounts, we reduced net income by preferred share dividends, and, for 2024, • underwriting discounts and commissions on the preferred share issuance, • initial issuance costs related to the preferred shares, and • excise taxes related to the preferred share redemption. New Accounting Standards On January 1, 2024, we started to amortize the remaining original cost of tax equity investments to the provision for income taxes, since certain conditions were met, on the modified retrospective basis, pursuant to an Accounting Standards Update (ASU) issued by the Financial Accounting Standards Board. Previously, these investments were accounted for under the equity method of accounting. The adoption of the ASU had no cumulative effect on retained earnings and did not have a material impact on our financial condition or results of operations. The amount of income tax credits and investment amortization recognized for the first quarter 2024, and the carrying amount of the tax credit investments at March 31, 2024, were not material to our financial condition or results of operations and, therefore, no additional disclosure is provided. Reclassification

Investments

Investments	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 39,823.8 $ 66.2 $ (1,327.3) $ 0 $ 38,562.7 55.8 % State and local government obligations 2,200.7 1.5 (142.4) 0 2,059.8 3.0 Foreign government obligations 16.9 0 (1.0) 0 15.9 0.1 Corporate debt securities 12,821.9 45.9 (328.1) (18.4) 12,521.3 18.1 Residential mortgage-backed securities 390.1 0.2 (9.2) 2.0 383.1 0.6 Commercial mortgage-backed securities 4,368.6 2.5 (519.3) 0 3,851.8 5.6 Other asset-backed securities 6,146.1 9.2 (93.9) (0.2) 6,061.2 8.8 Redeemable preferred stocks 181.2 0 (2.0) (5.3) 173.9 0.2 Total fixed maturities 65,949.3 125.5 (2,423.2) (21.9) 63,629.7 92.2 Short-term investments 1,326.7 0 0 0 1,326.7 1.9 Total available-for-sale securities 67,276.0 125.5 (2,423.2) (21.9) 64,956.4 94.1 Equity securities: Nonredeemable preferred stocks 931.1 0 0 (44.4) 886.7 1.3 Common equities 708.2 0 0 2,486.7 3,194.9 4.6 Total equity securities 1,639.3 0 0 2,442.3 4,081.6 5.9 Total portfolio 1 $ 68,915.3 $ 125.5 $ (2,423.2) $ 2,420.4 $ 69,038.0 100.0 % ($ in millions) Cost Gross Gross Net Fair % of March 31, 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 28,490.9 $ 68.8 $ (1,209.6) $ 0 $ 27,350.1 48.3 % State and local government obligations 2,222.4 0.9 (161.7) 0 2,061.6 3.6 Foreign government obligations 16.9 0 (1.1) 0 15.8 0.1 Corporate debt securities 11,195.8 45.6 (525.4) (34.7) 10,681.3 18.8 Residential mortgage-backed securities 655.0 0.2 (16.3) (8.9) 630.0 1.1 Commercial mortgage-backed securities 5,252.6 1.6 (751.2) 0 4,503.0 7.9 Other asset-backed securities 5,087.8 1.8 (222.5) (1.3) 4,865.8 8.6 Redeemable preferred stocks 202.5 0 (5.2) (15.7) 181.6 0.3 Total fixed maturities 53,123.9 118.9 (2,893.0) (60.6) 50,289.2 88.7 Short-term investments 2,524.1 0 0 0 2,524.1 4.5 Total available-for-sale securities 55,648.0 118.9 (2,893.0) (60.6) 52,813.3 93.2 Equity securities: Nonredeemable preferred stocks 1,197.7 0 0 (118.9) 1,078.8 1.9 Common equities 740.5 0 0 2,053.8 2,794.3 4.9 Total equity securities 1,938.2 0 0 1,934.9 3,873.1 6.8 Total portfolio 1 $ 57,586.2 $ 118.9 $ (2,893.0) $ 1,874.3 $ 56,686.4 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 37,823.2 $ 204.1 $ (1,157.9) $ 0 $ 36,869.4 55.9 % State and local government obligations 2,338.4 2.8 (138.4) 0 2,202.8 3.3 Foreign government obligations 17.3 0 (1.0) 0 16.3 0.1 Corporate debt securities 11,446.0 87.2 (332.3) (17.2) 11,183.7 16.9 Residential mortgage-backed securities 426.9 0.2 (10.0) 0.1 417.2 0.6 Commercial mortgage-backed securities 4,535.2 2.2 (597.7) 0 3,939.7 6.0 Other asset-backed securities 5,667.2 15.7 (107.1) (0.4) 5,575.4 8.4 Redeemable preferred stocks 187.7 0 (2.4) (11.6) 173.7 0.3 Total fixed maturities 62,441.9 312.2 (2,346.8) (29.1) 60,378.2 91.5 Short-term investments 1,789.9 0 0 0 1,789.9 2.7 Total available-for-sale securities 64,231.8 312.2 (2,346.8) (29.1) 62,168.1 94.2 Equity securities: Nonredeemable preferred stocks 977.1 0 0 (75.0) 902.1 1.4 Common equities 706.0 0 0 2,222.4 2,928.4 4.4 Total equity securities 1,683.1 0 0 2,147.4 3,830.5 5.8 Total portfolio 1 $ 65,914.9 $ 312.2 $ (2,346.8) $ 2,118.3 $ 65,998.6 100.0 % 1 At March 31, 2024 and 2023, we had $16.2 million and $22.8 million, respectively, of net unsettled security purchase transactions included in other liabilities, compared to $45.6 million included in other assets at December 31, 2023. The total fair value of the portfolio at March 31, 2024 and 2023, and December 31, 2023, included $3.2 billion, $4.1 billion, and $4.2 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions. At March 31, 2024, bonds and certificates of deposit in the principal amount of $672.4 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 March 31, 2024 or 2023, or December 31, 2023. At March 31, 2024, 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. 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: March 31, (millions) 2024 2023 Dec. 31, 2023 Fixed Maturities: Corporate debt securities $ 507.1 $ 548.6 $ 531.3 Residential mortgage-backed securities 300.0 482.1 323.9 Other asset-backed securities 9.5 33.8 13.9 Redeemable preferred stocks 141.0 132.8 141.2 Total hybrid securities $ 957.6 $ 1,197.3 $ 1,010.3 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 (losses). Fixed Maturities The composition of fixed maturities by maturity at March 31, 2024, was: (millions) Cost Fair Value Less than one year $ 7,227.5 $ 7,084.8 One to five years 45,277.4 43,850.9 Five to ten years 13,394.2 12,645.2 Ten years or greater 50.2 48.8 Total $ 65,949.3 $ 63,629.7 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 March 31, 2024 U.S. government obligations 139 $ 32,968.1 $ (1,327.3) 37 $ 21,634.6 $ (295.1) 102 $ 11,333.5 $ (1,032.2) State and local government obligations 330 1,922.7 (142.4) 41 250.7 (1.8) 289 1,672.0 (140.6) Foreign government obligations 1 15.9 (1.0) 0 0 0 1 15.9 (1.0) Corporate debt securities 363 8,094.3 (328.1) 106 2,369.6 (20.6) 257 5,724.7 (307.5) Residential mortgage-backed securities 35 78.0 (9.2) 1 0 0 34 78.0 (9.2) Commercial mortgage-backed securities 181 3,741.7 (519.3) 4 190.5 (1.8) 177 3,551.2 (517.5) Other asset-backed securities 193 3,259.5 (93.9) 59 1,303.5 (2.3) 134 1,956.0 (91.6) Redeemable preferred stocks 3 32.9 (2.0) 0 0 0 3 32.9 (2.0) Total fixed maturities 1,245 $ 50,113.1 $ (2,423.2) 248 $ 25,748.9 $ (321.6) 997 $ 24,364.2 $ (2,101.6) 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 March 31, 2023 U.S. government obligations 148 $ 19,446.8 $ (1,209.6) 40 $ 5,938.5 $ (127.5) 108 $ 13,508.3 $ (1,082.1) State and local government obligations 343 1,943.6 (161.7) 72 350.7 (7.4) 271 1,592.9 (154.3) Foreign government obligations 1 15.8 (1.1) 0 0 0 1 15.8 (1.1) Corporate debt securities 390 8,129.9 (525.4) 93 2,239.3 (75.5) 297 5,890.6 (449.9) Residential mortgage-backed securities 43 141.7 (16.3) 13 10.0 (0.4) 30 131.7 (15.9) Commercial mortgage-backed securities 220 4,484.5 (751.2) 11 53.4 (1.5) 209 4,431.1 (749.7) Other asset-backed securities 267 4,391.9 (222.5) 61 884.8 (6.2) 206 3,507.1 (216.3) Redeemable preferred stocks 4 48.8 (5.2) 1 10.6 (1.4) 3 38.2 (3.8) Total fixed maturities 1,416 $ 38,603.0 $ (2,893.0) 291 $ 9,487.3 $ (219.9) 1,125 $ 29,115.7 $ (2,673.1) 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, 2023 U.S. government obligations 147 $ 28,225.0 $ (1,157.9) 25 $ 11,890.0 $ (100.0) 122 $ 16,335.0 $ (1,057.9) State and local government obligations 324 1,846.2 (138.4) 31 169.9 (0.9) 293 1,676.3 (137.5) Foreign government obligations 1 16.3 (1.0) 0 0 0 1 16.3 (1.0) Corporate debt securities 313 6,642.4 (332.3) 26 617.2 (14.7) 287 6,025.2 (317.6) Residential mortgage-backed securities 39 88.4 (10.0) 2 0.4 0 37 88.0 (10.0) Commercial mortgage-backed securities 189 3,912.2 (597.7) 1 30.7 (2.5) 188 3,881.5 (595.2) Other asset-backed securities 207 3,299.1 (107.1) 41 639.4 (1.2) 166 2,659.7 (105.9) Redeemable preferred stocks 3 32.5 (2.4) 0 0 0 3 32.5 (2.4) Total fixed maturities 1,223 $ 44,062.1 $ (2,346.8) 126 $ 13,347.6 $ (119.3) 1,097 $ 30,714.5 $ (2,227.5) 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. We had seven securities that had their credit ratings downgraded during the first quarter 2024, with a combined fair value of $58.2 million and an unrealized loss of $8.2 million as of March 31, 2024. 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 three months of 2024 or 2023, and did not have a material credit loss allowance balance as of March 31, 2024 and 2023, or December 31, 2023. 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 on any specific security, 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 March 31, 2024 and 2023, and December 31, 2023, we believe that none of the unrealized losses on our fixed-maturity securities were related to material credit losses on any specific securities, or in the aggregate. We continue to expect all the securities in our fixed-maturity 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 March 31, 2024 and 2023, and December 31, 2023, to determine if the accrued interest amounts were 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 March 31, 2024 and 2023, or December 31, 2023. Realized Gains (Losses) The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2024 2023 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0 $ 3.8 State and local government obligations 0.3 0 Corporate debt securities 2.9 0.1 Total available-for-sale securities 3.2 3.9 Equity securities: Nonredeemable preferred stocks 0 0.1 Common equities 11.2 132.0 Total equity securities 11.2 132.1 Subtotal gross realized gains on security sales 14.4 136.0 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (135.3) (11.5) State and local government obligations (0.1) 0 Corporate debt securities (14.8) (20.4) Commercial mortgage-backed securities (4.3) (34.5) Other asset-backed securities (0.1) (0.2) Redeemable preferred stocks (0.3) 0 Short-term investments 0 (0.1) Total available-for-sale securities (154.9) (66.7) Equity securities: Nonredeemable preferred stocks (5.9) (101.0) Common equities (0.1) (11.8) Total equity securities (6.0) (112.8) Subtotal gross realized losses on security sales (160.9) (179.5) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (135.3) (7.7) State and local government obligations 0.2 0 Corporate debt securities (11.9) (20.3) Commercial mortgage-backed securities (4.3) (34.5) Other asset-backed securities (0.1) (0.2) Redeemable preferred stocks (0.3) 0 Short-term investments 0 (0.1) Total available-for-sale securities (151.7) (62.8) Equity securities: Nonredeemable preferred stocks (5.9) (100.9) Common equities 11.1 120.2 Total equity securities 5.2 19.3 Subtotal net realized gains (losses) on security sales (146.5) (43.5) Other assets Gain 0 13.2 Impairment 0 (2.3) Subtotal net realized gains (losses) on other assets 0 10.9 Net holding period gains (losses) Hybrid securities 7.2 13.9 Equity securities 294.9 90.5 Subtotal net holding period gains (losses) 302.1 104.4 Total net realized gains (losses) on securities $ 155.6 $ 71.8 Realized gains (losses) on securities sold are computed using the first-in-first-out method. During the first three months of 2024, the majority of our gross realized losses on security sales were U.S. Treasury Notes that were sold for duration management. We also sold corporate debt securities with less attractive risk/reward profiles and select commercial mortgage-backed securities. During the first quarter 2023, 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. The gross loss incurred in our nonredeemable preferred stocks, during the first quarter 2023, was primarily related to the sale of certain holdings in U.S. bank preferred stocks. In addition, during the first quarter last year, we selectively sold securities, which were primarily corporate debt securities and commercial mortgage-backed securities. The other asset gain for the first three months of 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 following table reflects our holding period realized gains (losses) recognized on equity securities held at the respective periods ended March 31: Three Months (millions) 2024 2023 Total net gains (losses) recognized during the period on equity securities $ 300.1 $ 109.8 Less: Net gains (losses) recognized on equity securities sold during the period 5.2 19.3 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 294.9 $ 90.5 Net Investment Income The components of net investment income for the three months ended March 31, were: Three Months (millions) 2024 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 306.9 $ 163.2 State and local government obligations 12.4 10.6 Foreign government obligations 0.1 0.1 Corporate debt securities 121.5 83.6 Residential mortgage-backed securities 5.4 9.2 Commercial mortgage-backed securities 46.4 50.0 Other asset-backed securities 78.2 48.5 Redeemable preferred stocks 2.9 2.9 Total fixed maturities 573.8 368.1 Short-term investments 18.9 24.7 Total available-for-sale securities 592.7 392.8 Equity securities: Nonredeemable preferred stocks 11.0 15.1 Common equities 13.9 11.7 Total equity securities 24.9 26.8 Investment income 617.6 419.6 Investment expenses (5.7) (5.5) Net investment income $ 611.9 $ 414.1 On a year-over-year basis, investment income (interest and dividends) increased 47% and recurring investment book yield increased 23% for the three months ended March 31, 2024, compared to the same period last year. The increases primarily reflected investing new cash from operations and proceeds from maturing bonds in higher coupon rate securities.

Fair Value

Fair Value	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Fixed maturities: U.S. government obligations $ 38,562.7 $ 0 $ 0 $ 38,562.7 $ 39,823.8 State and local government obligations 0 2,059.8 0 2,059.8 2,200.7 Foreign government obligations 0 15.9 0 15.9 16.9 Corporate debt securities 0 12,518.3 3.0 12,521.3 12,821.9 Subtotal 38,562.7 14,594.0 3.0 53,159.7 54,863.3 Asset-backed securities: Residential mortgage-backed 0 383.1 0 383.1 390.1 Commercial mortgage-backed 0 3,851.8 0 3,851.8 4,368.6 Other asset-backed 0 6,061.2 0 6,061.2 6,146.1 Subtotal asset-backed securities 0 10,296.1 0 10,296.1 10,904.8 Redeemable preferred stocks: Financials 0 23.2 0 23.2 24.5 Utilities 0 9.6 0 9.6 10.4 Industrials 0 141.1 0 141.1 146.3 Subtotal redeemable preferred stocks 0 173.9 0 173.9 181.2 Total fixed maturities 38,562.7 25,064.0 3.0 63,629.7 65,949.3 Short-term investments 1,326.7 0 0 1,326.7 1,326.7 Total available-for-sale securities 39,889.4 25,064.0 3.0 64,956.4 67,276.0 Equity securities: Nonredeemable preferred stocks: Financials 0 785.3 49.2 834.5 876.1 Utilities 0 37.4 0 37.4 40.0 Industrials 0 0 14.8 14.8 15.0 Subtotal nonredeemable preferred stocks 0 822.7 64.0 886.7 931.1 Common equities: Common stocks 3,148.1 0 22.5 3,170.6 683.9 Other risk investments 0 0 24.3 24.3 24.3 Subtotal common equities 3,148.1 0 46.8 3,194.9 708.2 Total equity securities 3,148.1 822.7 110.8 4,081.6 1,639.3 Total portfolio $ 43,037.5 $ 25,886.7 $ 113.8 $ 69,038.0 $ 68,915.3 Debt $ 0 $ 6,297.8 $ 0 $ 6,297.8 $ 6,889.7 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2023 Fixed maturities: U.S. government obligations $ 27,350.1 $ 0 $ 0 $ 27,350.1 $ 28,490.9 State and local government obligations 0 2,061.6 0 2,061.6 2,222.4 Foreign government obligations 0 15.8 0 15.8 16.9 Corporate debt securities 0 10,681.3 0 10,681.3 11,195.8 Subtotal 27,350.1 12,758.7 0 40,108.8 41,926.0 Asset-backed securities: Residential mortgage-backed 0 630.0 0 630.0 655.0 Commercial mortgage-backed 0 4,503.0 0 4,503.0 5,252.6 Other asset-backed 0 4,865.8 0 4,865.8 5,087.8 Subtotal asset-backed securities 0 9,998.8 0 9,998.8 10,995.4 Redeemable preferred stocks: Financials 0 40.0 0 40.0 43.5 Utilities 0 8.8 0 8.8 10.5 Industrials 9.6 123.2 0 132.8 148.5 Subtotal redeemable preferred stocks 9.6 172.0 0 181.6 202.5 Total fixed maturities 27,359.7 22,929.5 0 50,289.2 53,123.9 Short-term investments 2,524.1 0 0 2,524.1 2,524.1 Total available-for-sale securities 29,883.8 22,929.5 0 52,813.3 55,648.0 Equity securities: Nonredeemable preferred stocks: Financials 40.0 884.5 67.4 991.9 1,102.7 Utilities 0 70.5 0 70.5 80.0 Industrials 0 0 16.4 16.4 15.0 Subtotal nonredeemable preferred stocks 40.0 955.0 83.8 1,078.8 1,197.7 Common equities: Common stocks 2,755.7 0 18.3 2,774.0 720.2 Other risk investments 0 0 20.3 20.3 20.3 Subtotal common equities 2,755.7 0 38.6 2,794.3 740.5 Total equity securities 2,795.7 955.0 122.4 3,873.1 1,938.2 Total portfolio $ 32,679.5 $ 23,884.5 $ 122.4 $ 56,686.4 $ 57,586.2 Debt $ 0 $ 5,881.0 $ 0 $ 5,881.0 $ 6,389.3 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2023 Fixed maturities: U.S. government obligations $ 36,869.4 $ 0 $ 0 $ 36,869.4 $ 37,823.2 State and local government obligations 0 2,202.8 0 2,202.8 2,338.4 Foreign government obligations 0 16.3 0 16.3 17.3 Corporate debt securities 0 11,180.7 3.0 11,183.7 11,446.0 Subtotal 36,869.4 13,399.8 3.0 50,272.2 51,624.9 Asset-backed securities: Residential mortgage-backed 0 417.2 0 417.2 426.9 Commercial mortgage-backed 0 3,939.7 0 3,939.7 4,535.2 Other asset-backed 0 5,575.4 0 5,575.4 5,667.2 Subtotal asset-backed securities 0 9,932.3 0 9,932.3 10,629.3 Redeemable preferred stocks: Financials 0 23.1 0 23.1 24.5 Utilities 0 9.4 0 9.4 10.4 Industrials 0 141.2 0 141.2 152.8 Subtotal redeemable preferred stocks 0 173.7 0 173.7 187.7 Total fixed maturities 36,869.4 23,505.8 3.0 60,378.2 62,441.9 Short-term investments 1,757.0 32.9 0 1,789.9 1,789.9 Total available-for-sale securities 38,626.4 23,538.7 3.0 62,168.1 64,231.8 Equity securities: Nonredeemable preferred stocks: Financials 0 802.7 49.2 851.9 922.1 Utilities 0 35.4 0 35.4 40.0 Industrials 0 0 14.8 14.8 15.0 Subtotal nonredeemable preferred stocks 0 838.1 64.0 902.1 977.1 Common equities: Common stocks 2,885.3 0 22.5 2,907.8 685.4 Other risk investments 0 0 20.6 20.6 20.6 Subtotal common equities 2,885.3 0 43.1 2,928.4 706.0 Total equity securities 2,885.3 838.1 107.1 3,830.5 1,683.1 Total portfolio $ 41,511.7 $ 24,376.8 $ 110.1 $ 65,998.6 $ 65,914.9 Debt $ 0 $ 6,431.3 $ 0 $ 6,431.3 $ 6,888.6 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 March 31, 2024, vendor-quoted prices represented 92% of our Level 1 classifications (excluding short-term investments), compared to 91% at March 31, 2023 and 93% at December 31, 2023. 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 during 2023. At both March 31, 2024 and December 31, 2023, vendor-quoted prices comprised 100% of our Level 2 classifications (excluding short-term investments), compared to 98% at March 31, 2023; the remaining 2% at March 31, 2023 were dealer-quoted prices. 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. At both March 31, 2024 and December 31, 2023, we held one privately held fixed-maturity security that is classified as a Level 3 investment. At March 31, 2023, we did not have any securities in our fixed-maturity portfolio listed as Level 3. During the first three months of 2024 and for the full year of 2023, there were no material assets or liabilities measured at fair value on a nonrecurring basis. 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 months ended March 31, 2024 and 2023: Level 3 Fair Value (millions) Fair Value at December 31, 2023 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2024 Fixed maturities: Corporate debt securities $ 3.0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 3.0 Equity securities: Nonredeemable preferred stocks: Financials 49.2 0 0 0 0 0 0 49.2 Industrials 14.8 0 0 0 0 0 0 14.8 Common equities: Common stocks 22.5 0 0 0 0 0 0 22.5 Other risk investments 20.6 3.7 0 0 0 0 0 24.3 Total Level 3 securities $ 110.1 $ 3.7 $ 0 $ 0 $ 0 $ 0 $ 0 $ 113.8 Level 3 Fair Value (millions) Fair Value at December 31, 2022 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2023 Equity securities: Nonredeemable preferred stocks: Financials $ 67.4 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 67.4 Industrials 16.4 0 0 0 0 0 0 16.4 Common equities: Common stocks 18.3 0 0 0 0 0 0 18.3 Other risk investments 19.8 0.5 0 0 0 0 0 20.3 Total Level 3 securities $ 121.9 $ 0.5 $ 0 $ 0 $ 0 $ 0 $ 0 $ 122.4 The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2024 and 2023, and December 31, 2023: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2024 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Fixed maturities: Corporate debt securities $ 3.0 Market comparables Weighted average market capitalization price change % (2.3)% to 1.2% (0.1) % Equity securities: Nonredeemable preferred stocks 64.0 Market comparables Weighted average market capitalization price change % 3.4% to 28.4% 22.6 % Common stocks 22.5 Market comparables Weighted average market capitalization price change % (37.9)% to 39.6% 3.4 % Subtotal Level 3 securities 89.5 Pricing exemption securities 24.3 Total Level 3 securities $ 113.8 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2023 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 % 2.7% to 23.3% 9.2 % Common stocks 18.3 Market comparables Weighted average market capitalization price change % (51.3)% to 48.3% 15.6 % Subtotal Level 3 securities 102.1 Pricing exemption securities 20.3 Total Level 3 securities $ 122.4 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2023 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Fixed maturities: Corporate debt securities $ 3.0 Market comparables Weighted average market capitalization price change % 0.3% to 7.7% 2.6 % Equity securities: Nonredeemable preferred stocks 64.0 Market comparables Weighted average market capitalization price change % 17.2% to 39.7% 21.7% Common stocks 22.5 Market comparables Weighted average market capitalization price change % (45.8)% to 95.6% 39.7% Subtotal Level 3 securities 89.5 Pricing exemption securities 20.6 Total Level 3 securities $ 110.1

Debt

Debt	3 Months Ended
	Mar. 31, 2024
Debt Disclosure [Abstract]
Debt	DEBT Debt at each of the balance sheet periods consisted of: March 31, 2024 March 31, 2023 December 31, 2023 (millions) Carrying Fair Carrying Fair Carrying Fair 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) $ 498.7 $ 468.6 $ 498.3 $ 464.2 $ 498.6 $ 468.9 2.50% Senior Notes due 2027 (issued: $500.0, March 2022) 498.2 468.5 497.7 463.8 498.1 469.1 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.9 323.5 297.6 329.6 297.8 328.7 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 547.1 532.5 546.5 538.5 546.9 542.6 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 497.4 456.1 497.0 460.0 497.3 462.2 3.00% Senior Notes due 2032 (issued: $500.0, March 2022) 496.4 435.7 496.0 446.9 496.3 446.0 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.8 434.2 396.5 448.1 396.7 445.6 4.95% Senior Notes due 2033 (issued: $500.0, May 2023) 496.5 499.2 0 0 496.4 513.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 347.0 312.8 346.9 312.3 347.0 314.2 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.9 327.9 395.8 324.0 395.9 325.1 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 842.3 719.2 842.1 743.8 842.3 756.2 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.7 519.8 590.4 530.0 590.6 534.1 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 491.2 409.8 491.0 420.3 491.1 422.3 3.70% Senior Notes due 2052 (issued: $500.0, March 2022) 493.6 390.0 493.5 399.5 493.6 403.3 Total $ 6,889.7 $ 6,297.8 $ 6,389.3 $ 5,881.0 $ 6,888.6 $ 6,431.3 There was no short-term debt outstanding as of the end of all the periods presented. The Progressive Corporation has a line of credit with PNC Bank, National Association, in the maximum principal amount of $300 million. See the 2023 Annual Report to Shareholders for a discussion of the terms of this line of credit. We had no borrowings under the line of credit that was available during the periods presented.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2024
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES The effective tax rate for the three months ended March 31, 2024, was 20.7% , compared to 19.2% for the same period last year, with the change primarily due to our permanent tax differences having a lower impact on our effective rate due to increased profitability in the current year. Deferred 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 March 31, 2024 and 2023, and December 31, 2023, 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 March 31, 2024 and 2023, and December 31, 2023, the net deferred tax asset includes a gross deferred tax asset of $482.5 million , $582.6 million, and $427.3 million, respectively, related to unrealized losses on fixed-maturity securities. We believe this deferred tax asset will be realized based on the existence of current temporary differences related to unrealized gains in our equity portfolio, prior year capital gains, and other tax planning strategies. We had net current income taxes payable of $961.8 million , $203.3 million, and $311.8 million at March 31, 2024 and 2023, and December 31, 2023, respectively, included in accounts payable, accrued expenses, and other liabilities on our consolidated balance sheets. The increase in income taxes payable was primarily driven by the increase in profitability during the first quarter 2024. At March 31, 2024 and 2023, and December 31, 2023, we had no reserves for uncertain tax positions.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 2024
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: March 31, (millions) 2024 2023 Balance at January 1 $ 34,389.2 $ 30,359.3 Less reinsurance recoverables on unpaid losses 4,789.0 5,559.2 Net balance at January 1 29,600.2 24,800.1 Incurred related to: Current year 10,983.0 10,002.8 Prior years (11.4) 621.2 Total incurred 10,971.6 10,624.0 Paid related to: Current year 4,421.7 3,962.6 Prior years 5,971.0 5,736.2 Total paid 10,392.7 9,698.8 Net balance at March 31 30,179.1 25,725.3 Plus reinsurance recoverables on unpaid losses 4,651.9 5,301.1 Balance at March 31 $ 34,831.0 $ 31,026.4 We experienced favorable reserve development of $11.4 million during the first three months of 2024, compared to unfavorable development of $621.2 million for the same period last year, which is reflected as “incurred related to prior years ” in the table above. First Quarter 2024 • The favorable prior year reserve development included approximately $50 million of favorable development attributable to accident year 2023 and $15 million to accident year 2022; partially offset by unfavorable development attributable to accident years 2021 and prior. • Our perso nal auto products incurred about $100 million of favorable loss and loss adjustment expense (LAE) reserve development, in part due to lower than anticipated frequency in Florida following tort reform that passed in the first quarter 2023, with about 60% attributable to the Agency auto business and the balance in the Direct auto business. • Our Commercial Lines business experienced about $70 million of unfavorable development, primarily driven by higher than anticipated severity in our commercial auto business for California and New York. First Quarter 2023 • The unfavorable prior year reserve development included approximately $498 million attributable to accident year 2022, $30 million to accident year 2021, and the remainder to accident years 2020 and prior. • Our personal auto products incurred about $428 million of unfavorable loss and LAE reserve development, with the Agency and Direct auto businesses each contributing about half. The unfavorable development was primarily attributable to higher than anticipated severity in auto property and physical damage coverages, higher than anticipated late reported injury claims, and, to a lessor extent, increased loss costs in Florida injury and medical coverages in small part due to the impact of the legislation enacted in March 2023 in Florida. • Our Commercial Lines business experienced about $144 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 injury claims. • Our Property business experienced $42 million of unfavorable development, primarily from higher than anticipated claims expenses and higher than anticipated severity in our homeowner liability peril and umbrella products.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2024
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 overnight reverse repurchase commitments, which are not considered part of the investment portfolio, held by these subsidiaries at March 31, 2024 and 2023, and December 31, 2023, were $107.7 million, $177.5 million, and $68.2 million, respectively. Restricted cash and restricted 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: Three Months Ended March 31, (millions) 2024 2023 Common share dividends 1 $ 58.6 $ 58.5 Operating lease liabilities 2 28.0 9.3 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: Three Months Ended March 31, (millions) 2024 2023 Income taxes $ 0 $ 0.1 Interest 87.9 87.9 Operating lease liabilities 21.6 21.5

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2024
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, and umbrella insurance. 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. 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 March 31, 2024 2023 (millions) Revenues Pretax Revenues Pretax Personal Lines Agency $ 5,857.7 $ 950.7 $ 4,860.2 $ 162.6 Direct 7,020.5 1,043.6 5,717.4 (22.1) Total Personal Lines 1 12,878.2 1,994.3 10,577.6 140.5 Commercial Lines 2,557.4 209.0 2,356.1 37.2 Property 712.8 46.9 598.7 (32.7) Other indemnity 2 0.2 (0.3) 0.7 (3.4) Total underwriting operations 16,148.6 2,249.9 13,533.1 141.6 Fees and other revenues 3 236.5 NA 206.2 NA Service businesses 84.2 (7.9) 72.5 (9.8) Investments 4 773.2 767.5 491.4 485.9 Interest expense NA (69.6) NA (63.3) Consolidated total $ 17,242.5 $ 2,939.9 $ 14,303.2 $ 554.4 NA = Not applicable 1 Personal auto insurance accounted for 95% and 94% of the total Personal Lines segment net premiums earned during the three months ended March 31, 2024 and 2023, respectively; 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 Includes other underwriting business and run-off operations. 3 Pretax profit (loss) for fees and other revenues is allocated to operating segments based on revenue. 4 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 March 31, 2024 2023 Under-writing Combined Under-writing Combined Personal Lines Agency 16.2 % 83.8 3.3 % 96.7 Direct 14.9 85.1 (0.4) 100.4 Total Personal Lines 15.5 84.5 1.3 98.7 Commercial Lines 8.2 91.8 1.6 98.4 Property 6.6 93.4 (5.5) 105.5 Total underwriting operations 13.9 86.1 1.0 99.0

Dividends

Dividends	3 Months Ended
	Mar. 31, 2024
Dividends [Abstract]
Dividends	DIVIDENDS Following is a summary of our common and preferred share dividends that were declared and/or paid during the three months ended March 31, 2024 and 2023: (millions, except per share amounts) Amount Declared Payable Per Share Accrued/Paid 1 Common – Annual-Variable Dividends: December 2023 January 2024 $ 0.75 $ 439.3 Common – Quarterly Dividends: March 2024 April 2024 0.10 58.6 December 2023 January 2024 0.10 58.6 March 2023 April 2023 0.10 58.5 December 2022 January 2023 0.10 58.5 Preferred Dividends: January 2024 2 February 2024 15.688377 7.8 December 2022 March 2023 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. 2 During the first quarter 2024, we redeemed all of the outstanding Serial Preferred Shares, Series B, at the stated amount of $1,000 per share, for an aggregate payout of $507.8 million, including accrued and unpaid dividends of $7.8 million to, but excluding, February 22, 2024, which was the redemption date. See Note 14 – Dividends in our 2023 Annual Report to Shareholders for a discussion of the dividend policies related to our common shares and our preferred shares, prior to redemption.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2024
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 December 31, 2023 $ (2,052.9) $ 437.2 $ (1,615.7) $ (1,600.8) $ (14.0) $ (0.9) Other comprehensive income (loss) before reclassifications: Investment securities (413.6) 86.9 (326.7) (326.7) 0 0 Foreign currency translation adjustment (0.3) 0.1 (0.2) 0 0 (0.2) Total other comprehensive income (loss) before reclassifications (413.9) 87.0 (326.9) (326.7) 0 (0.2) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (150.5) 31.6 (118.9) (118.9) 0 0 Interest expense (0.1) 0 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income (150.6) 31.6 (119.0) (118.9) (0.1) 0 Total other comprehensive income (loss) (263.3) 55.4 (207.9) (207.8) 0.1 (0.2) Balance at March 31, 2024 $ (2,316.2) $ 492.6 $ (1,823.6) $ (1,808.6) $ (13.9) $ (1.1) 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 705.6 (148.2) 557.4 557.4 0 0 Total other comprehensive income (loss) before reclassifications 705.6 (148.2) 557.4 557.4 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (57.9) 12.1 (45.8) (45.8) 0 0 Interest expense (0.1) 0 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income (58.0) 12.1 (45.9) (45.8) (0.1) 0 Total other comprehensive income (loss) 763.6 (160.3) 603.3 603.2 0.1 0 Balance at March 31, 2023 $ (2,793.3) $ 594.6 $ (2,198.7) $ (2,183.1) $ (14.4) $ (1.2) 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 2023 Annual Report to Shareholders for further discussion).

Litigation

Litigation	3 Months Ended
	Mar. 31, 2024
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 insurance subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the subsidiaries. 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 2023 Annual Report to Shareholders. As of March 31, 2024, lawsuits have been certified or conditionally certified as class/collective actions in cases alleging: we improperly value total loss claims in Colorado, Georgia, Indiana, New York, and Pennsylvania; we improperly fail to pay fees and taxes associated with total losses in 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 personal injury protection 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. 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 either our best estimate of the loss or the lower end of the range of loss. Lawsuits arising from insurance policies and operations, including but not limited to allegations involving claims adjustment and vehicle valuation, may be filed contemporaneously in multiple states. As of March 31, 2024, we are named as defendants in class action lawsuits pending in multiple states alleging that we improperly value total loss vehicle physical damage claims through the application of a negotiation adjustment in calculating such valuations, which includes five states in which classes have been certified, as noted above, and lawsuits styled as putative class actions pending in additional states. These lawsuits, which were filed at different times by different plaintiffs, feature certain similar claims and also include different allegations and are subject to various state laws. While we believe we have meritorious defenses and we are vigorously contesting these lawsuits, an unfavorable result in, or a settlement of, a significant number of these lawsuits could, in aggregation, have a material adverse effect on our financial condition, cash flows, and/or results of operations. Based on information known at March 31, 2024, we determined that losses from these lawsuits are reasonably possible but neither probable nor reasonably estimable. 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 March 31, 2024 and 2023, or December 31, 2023, and there were no material settlements during 2023 or the first three months of 2024. 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 2023 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, 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 2023 Annual Report to Shareholders.

Pay vs Performance Disclosure

Pay vs Performance Disclosure - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pay vs Performance Disclosure
Net income	$ 2,331.4	$ 447.9

Insider Trading Arrangements

Insider Trading Arrangements	3 Months Ended
	Mar. 31, 2024 shares
Trading Arrangements, by Individual
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false
Lori Niederst [Member]
Trading Arrangements, by Individual
Name	Lori Niederst
Title	Customer Relationship Management President
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	2/28/2024
Termination Date	12/31/2024
Arrangement Duration	307 days
Aggregate Available	0
Susan Patricia Griffith [Member]
Trading Arrangements, by Individual
Name	Susan Patricia Griffith
Title	President and Chief Executive Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	2/28/2024
Termination Date	2/28/2025
Arrangement Duration	366 days
Aggregate Available	13,359
Andrew J. Quigg [Member]
Trading Arrangements, by Individual
Name	Andrew J. Quigg
Title	Chief Strategy Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	1/25/2024
Termination Date	2/3/2025
Arrangement Duration	341 days
Aggregate Available	0
Steven A. Broz [Member]
Trading Arrangements, by Individual
Name	Steven A. Broz
Title	Chief Information Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	1/26/2024
Termination Date	11/1/2024
Arrangement Duration	280 days
Aggregate Available	16,511
John P. Sauerland [Member]
Trading Arrangements, by Individual
Name	John P. Sauerland
Title	Vice President and Chief Financial Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	2/28/2024
Termination Date	2/28/2025
Arrangement Duration	366 days
Aggregate Available	20,000
Jonathan S. Bauer [Member]
Trading Arrangements, by Individual
Name	Jonathan S. Bauer
Title	Chief Investment Officer
Rule 10b5-1 Arrangement Adopted	true
Adoption Date	3/18/2024
Termination Date	9/3/2024
Arrangement Duration	169 days
Aggregate Available	2,047
Officer Trading Arrangement [Member]
Trading Arrangements, by Individual
Material Terms of Trading Arrangement	Below are the details of each executive's Rule 10b5-1 trading arrangement: Executive Officer Title Date Entered Date Expires* Additional Shares Jonathan S. Bauer Chief Investment Officer 3/18/2024 9/3/2024 2,047 Steven A. Broz Chief Information Officer 1/26/2024 11/1/2024 16,511 Susan Patricia Griffith President and Chief Executive Officer 2/28/2024 2/28/2025 13,359 Lori Niederst Customer Relationship Management President 2/28/2024 12/31/2024 0 Andrew J. Quigg Chief Strategy Officer 1/25/2024 2/3/2025 0 John P. Sauerland Vice President and Chief Financial Officer 2/28/2024 2/28/2025 20,000 * subject to the plan’s earlier expiration or completion in accordance with its terms.

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Earnings Per Share	To determine net income available to common shareholders, which is used in the calculation of the per common share amounts, we reduced net income by preferred share dividends, and, for 2024, • underwriting discounts and commissions on the preferred share issuance, • initial issuance costs related to the preferred shares, and • excise taxes related to the preferred share redemption.
New Accounting Standards	New Accounting Standards On January 1, 2024, we started to amortize the remaining original cost of tax equity investments to the provision for income taxes, since certain conditions were met, on the modified retrospective basis, pursuant to an Accounting Standards Update (ASU) issued by the Financial Accounting Standards Board. Previously, these investments were accounted for under the equity method of accounting. The adoption of the ASU had no cumulative effect on retained earnings and did not have a material impact on our financial condition or results of operations. The amount of income tax credits and investment amortization recognized for the first quarter 2024, and the carrying amount of the tax credit investments at March 31, 2024, were not material to our financial condition or results of operations and, therefore, no additional disclosure is provided.

Basis of Presentation (Tables)

Basis of Presentation (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, (millions) 2024 2023 Allowance for credit losses, beginning of period $ 369.1 $ 343.3 Increase in allowance 1 106.8 116.9 Write-offs 2 (148.2) (119.3) Allowance for credit losses, end of period $ 327.7 $ 340.9 1 Represents the incremental increase in other underwriting expenses. 2 Represents the portion of allowance that is reversed when the premiums receivable is written off. Premiums receivable balances are written off once we have exhausted our collection efforts.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 39,823.8 $ 66.2 $ (1,327.3) $ 0 $ 38,562.7 55.8 % State and local government obligations 2,200.7 1.5 (142.4) 0 2,059.8 3.0 Foreign government obligations 16.9 0 (1.0) 0 15.9 0.1 Corporate debt securities 12,821.9 45.9 (328.1) (18.4) 12,521.3 18.1 Residential mortgage-backed securities 390.1 0.2 (9.2) 2.0 383.1 0.6 Commercial mortgage-backed securities 4,368.6 2.5 (519.3) 0 3,851.8 5.6 Other asset-backed securities 6,146.1 9.2 (93.9) (0.2) 6,061.2 8.8 Redeemable preferred stocks 181.2 0 (2.0) (5.3) 173.9 0.2 Total fixed maturities 65,949.3 125.5 (2,423.2) (21.9) 63,629.7 92.2 Short-term investments 1,326.7 0 0 0 1,326.7 1.9 Total available-for-sale securities 67,276.0 125.5 (2,423.2) (21.9) 64,956.4 94.1 Equity securities: Nonredeemable preferred stocks 931.1 0 0 (44.4) 886.7 1.3 Common equities 708.2 0 0 2,486.7 3,194.9 4.6 Total equity securities 1,639.3 0 0 2,442.3 4,081.6 5.9 Total portfolio 1 $ 68,915.3 $ 125.5 $ (2,423.2) $ 2,420.4 $ 69,038.0 100.0 % ($ in millions) Cost Gross Gross Net Fair % of March 31, 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 28,490.9 $ 68.8 $ (1,209.6) $ 0 $ 27,350.1 48.3 % State and local government obligations 2,222.4 0.9 (161.7) 0 2,061.6 3.6 Foreign government obligations 16.9 0 (1.1) 0 15.8 0.1 Corporate debt securities 11,195.8 45.6 (525.4) (34.7) 10,681.3 18.8 Residential mortgage-backed securities 655.0 0.2 (16.3) (8.9) 630.0 1.1 Commercial mortgage-backed securities 5,252.6 1.6 (751.2) 0 4,503.0 7.9 Other asset-backed securities 5,087.8 1.8 (222.5) (1.3) 4,865.8 8.6 Redeemable preferred stocks 202.5 0 (5.2) (15.7) 181.6 0.3 Total fixed maturities 53,123.9 118.9 (2,893.0) (60.6) 50,289.2 88.7 Short-term investments 2,524.1 0 0 0 2,524.1 4.5 Total available-for-sale securities 55,648.0 118.9 (2,893.0) (60.6) 52,813.3 93.2 Equity securities: Nonredeemable preferred stocks 1,197.7 0 0 (118.9) 1,078.8 1.9 Common equities 740.5 0 0 2,053.8 2,794.3 4.9 Total equity securities 1,938.2 0 0 1,934.9 3,873.1 6.8 Total portfolio 1 $ 57,586.2 $ 118.9 $ (2,893.0) $ 1,874.3 $ 56,686.4 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 37,823.2 $ 204.1 $ (1,157.9) $ 0 $ 36,869.4 55.9 % State and local government obligations 2,338.4 2.8 (138.4) 0 2,202.8 3.3 Foreign government obligations 17.3 0 (1.0) 0 16.3 0.1 Corporate debt securities 11,446.0 87.2 (332.3) (17.2) 11,183.7 16.9 Residential mortgage-backed securities 426.9 0.2 (10.0) 0.1 417.2 0.6 Commercial mortgage-backed securities 4,535.2 2.2 (597.7) 0 3,939.7 6.0 Other asset-backed securities 5,667.2 15.7 (107.1) (0.4) 5,575.4 8.4 Redeemable preferred stocks 187.7 0 (2.4) (11.6) 173.7 0.3 Total fixed maturities 62,441.9 312.2 (2,346.8) (29.1) 60,378.2 91.5 Short-term investments 1,789.9 0 0 0 1,789.9 2.7 Total available-for-sale securities 64,231.8 312.2 (2,346.8) (29.1) 62,168.1 94.2 Equity securities: Nonredeemable preferred stocks 977.1 0 0 (75.0) 902.1 1.4 Common equities 706.0 0 0 2,222.4 2,928.4 4.4 Total equity securities 1,683.1 0 0 2,147.4 3,830.5 5.8 Total portfolio 1 $ 65,914.9 $ 312.2 $ (2,346.8) $ 2,118.3 $ 65,998.6 100.0 % 1 At March 31, 2024 and 2023, we had $16.2 million and $22.8 million, respectively, of net unsettled security purchase transactions included in other liabilities, compared to $45.6 million included in other assets at December 31, 2023. The total fair value of the portfolio at March 31, 2024 and 2023, and December 31, 2023, included $3.2 billion, $4.1 billion, and $4.2 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: March 31, (millions) 2024 2023 Dec. 31, 2023 Fixed Maturities: Corporate debt securities $ 507.1 $ 548.6 $ 531.3 Residential mortgage-backed securities 300.0 482.1 323.9 Other asset-backed securities 9.5 33.8 13.9 Redeemable preferred stocks 141.0 132.8 141.2 Total hybrid securities $ 957.6 $ 1,197.3 $ 1,010.3
Investments Classified by Contractual Maturity Date	The composition of fixed maturities by maturity at March 31, 2024, was: (millions) Cost Fair Value Less than one year $ 7,227.5 $ 7,084.8 One to five years 45,277.4 43,850.9 Five to ten years 13,394.2 12,645.2 Ten years or greater 50.2 48.8 Total $ 65,949.3 $ 63,629.7
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 March 31, 2024 U.S. government obligations 139 $ 32,968.1 $ (1,327.3) 37 $ 21,634.6 $ (295.1) 102 $ 11,333.5 $ (1,032.2) State and local government obligations 330 1,922.7 (142.4) 41 250.7 (1.8) 289 1,672.0 (140.6) Foreign government obligations 1 15.9 (1.0) 0 0 0 1 15.9 (1.0) Corporate debt securities 363 8,094.3 (328.1) 106 2,369.6 (20.6) 257 5,724.7 (307.5) Residential mortgage-backed securities 35 78.0 (9.2) 1 0 0 34 78.0 (9.2) Commercial mortgage-backed securities 181 3,741.7 (519.3) 4 190.5 (1.8) 177 3,551.2 (517.5) Other asset-backed securities 193 3,259.5 (93.9) 59 1,303.5 (2.3) 134 1,956.0 (91.6) Redeemable preferred stocks 3 32.9 (2.0) 0 0 0 3 32.9 (2.0) Total fixed maturities 1,245 $ 50,113.1 $ (2,423.2) 248 $ 25,748.9 $ (321.6) 997 $ 24,364.2 $ (2,101.6) 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 March 31, 2023 U.S. government obligations 148 $ 19,446.8 $ (1,209.6) 40 $ 5,938.5 $ (127.5) 108 $ 13,508.3 $ (1,082.1) State and local government obligations 343 1,943.6 (161.7) 72 350.7 (7.4) 271 1,592.9 (154.3) Foreign government obligations 1 15.8 (1.1) 0 0 0 1 15.8 (1.1) Corporate debt securities 390 8,129.9 (525.4) 93 2,239.3 (75.5) 297 5,890.6 (449.9) Residential mortgage-backed securities 43 141.7 (16.3) 13 10.0 (0.4) 30 131.7 (15.9) Commercial mortgage-backed securities 220 4,484.5 (751.2) 11 53.4 (1.5) 209 4,431.1 (749.7) Other asset-backed securities 267 4,391.9 (222.5) 61 884.8 (6.2) 206 3,507.1 (216.3) Redeemable preferred stocks 4 48.8 (5.2) 1 10.6 (1.4) 3 38.2 (3.8) Total fixed maturities 1,416 $ 38,603.0 $ (2,893.0) 291 $ 9,487.3 $ (219.9) 1,125 $ 29,115.7 $ (2,673.1) 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, 2023 U.S. government obligations 147 $ 28,225.0 $ (1,157.9) 25 $ 11,890.0 $ (100.0) 122 $ 16,335.0 $ (1,057.9) State and local government obligations 324 1,846.2 (138.4) 31 169.9 (0.9) 293 1,676.3 (137.5) Foreign government obligations 1 16.3 (1.0) 0 0 0 1 16.3 (1.0) Corporate debt securities 313 6,642.4 (332.3) 26 617.2 (14.7) 287 6,025.2 (317.6) Residential mortgage-backed securities 39 88.4 (10.0) 2 0.4 0 37 88.0 (10.0) Commercial mortgage-backed securities 189 3,912.2 (597.7) 1 30.7 (2.5) 188 3,881.5 (595.2) Other asset-backed securities 207 3,299.1 (107.1) 41 639.4 (1.2) 166 2,659.7 (105.9) Redeemable preferred stocks 3 32.5 (2.4) 0 0 0 3 32.5 (2.4) Total fixed maturities 1,223 $ 44,062.1 $ (2,346.8) 126 $ 13,347.6 $ (119.3) 1,097 $ 30,714.5 $ (2,227.5)
Realized Gain (Loss) on Investments	The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2024 2023 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0 $ 3.8 State and local government obligations 0.3 0 Corporate debt securities 2.9 0.1 Total available-for-sale securities 3.2 3.9 Equity securities: Nonredeemable preferred stocks 0 0.1 Common equities 11.2 132.0 Total equity securities 11.2 132.1 Subtotal gross realized gains on security sales 14.4 136.0 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (135.3) (11.5) State and local government obligations (0.1) 0 Corporate debt securities (14.8) (20.4) Commercial mortgage-backed securities (4.3) (34.5) Other asset-backed securities (0.1) (0.2) Redeemable preferred stocks (0.3) 0 Short-term investments 0 (0.1) Total available-for-sale securities (154.9) (66.7) Equity securities: Nonredeemable preferred stocks (5.9) (101.0) Common equities (0.1) (11.8) Total equity securities (6.0) (112.8) Subtotal gross realized losses on security sales (160.9) (179.5) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (135.3) (7.7) State and local government obligations 0.2 0 Corporate debt securities (11.9) (20.3) Commercial mortgage-backed securities (4.3) (34.5) Other asset-backed securities (0.1) (0.2) Redeemable preferred stocks (0.3) 0 Short-term investments 0 (0.1) Total available-for-sale securities (151.7) (62.8) Equity securities: Nonredeemable preferred stocks (5.9) (100.9) Common equities 11.1 120.2 Total equity securities 5.2 19.3 Subtotal net realized gains (losses) on security sales (146.5) (43.5) Other assets Gain 0 13.2 Impairment 0 (2.3) Subtotal net realized gains (losses) on other assets 0 10.9 Net holding period gains (losses) Hybrid securities 7.2 13.9 Equity securities 294.9 90.5 Subtotal net holding period gains (losses) 302.1 104.4 Total net realized gains (losses) on securities $ 155.6 $ 71.8
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 March 31: Three Months (millions) 2024 2023 Total net gains (losses) recognized during the period on equity securities $ 300.1 $ 109.8 Less: Net gains (losses) recognized on equity securities sold during the period 5.2 19.3 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 294.9 $ 90.5
Components of Net Investment Income	The components of net investment income for the three months ended March 31, were: Three Months (millions) 2024 2023 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 306.9 $ 163.2 State and local government obligations 12.4 10.6 Foreign government obligations 0.1 0.1 Corporate debt securities 121.5 83.6 Residential mortgage-backed securities 5.4 9.2 Commercial mortgage-backed securities 46.4 50.0 Other asset-backed securities 78.2 48.5 Redeemable preferred stocks 2.9 2.9 Total fixed maturities 573.8 368.1 Short-term investments 18.9 24.7 Total available-for-sale securities 592.7 392.8 Equity securities: Nonredeemable preferred stocks 11.0 15.1 Common equities 13.9 11.7 Total equity securities 24.9 26.8 Investment income 617.6 419.6 Investment expenses (5.7) (5.5) Net investment income $ 611.9 $ 414.1

Fair Value (Tables)

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 Fixed maturities: U.S. government obligations $ 38,562.7 $ 0 $ 0 $ 38,562.7 $ 39,823.8 State and local government obligations 0 2,059.8 0 2,059.8 2,200.7 Foreign government obligations 0 15.9 0 15.9 16.9 Corporate debt securities 0 12,518.3 3.0 12,521.3 12,821.9 Subtotal 38,562.7 14,594.0 3.0 53,159.7 54,863.3 Asset-backed securities: Residential mortgage-backed 0 383.1 0 383.1 390.1 Commercial mortgage-backed 0 3,851.8 0 3,851.8 4,368.6 Other asset-backed 0 6,061.2 0 6,061.2 6,146.1 Subtotal asset-backed securities 0 10,296.1 0 10,296.1 10,904.8 Redeemable preferred stocks: Financials 0 23.2 0 23.2 24.5 Utilities 0 9.6 0 9.6 10.4 Industrials 0 141.1 0 141.1 146.3 Subtotal redeemable preferred stocks 0 173.9 0 173.9 181.2 Total fixed maturities 38,562.7 25,064.0 3.0 63,629.7 65,949.3 Short-term investments 1,326.7 0 0 1,326.7 1,326.7 Total available-for-sale securities 39,889.4 25,064.0 3.0 64,956.4 67,276.0 Equity securities: Nonredeemable preferred stocks: Financials 0 785.3 49.2 834.5 876.1 Utilities 0 37.4 0 37.4 40.0 Industrials 0 0 14.8 14.8 15.0 Subtotal nonredeemable preferred stocks 0 822.7 64.0 886.7 931.1 Common equities: Common stocks 3,148.1 0 22.5 3,170.6 683.9 Other risk investments 0 0 24.3 24.3 24.3 Subtotal common equities 3,148.1 0 46.8 3,194.9 708.2 Total equity securities 3,148.1 822.7 110.8 4,081.6 1,639.3 Total portfolio $ 43,037.5 $ 25,886.7 $ 113.8 $ 69,038.0 $ 68,915.3 Debt $ 0 $ 6,297.8 $ 0 $ 6,297.8 $ 6,889.7 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2023 Fixed maturities: U.S. government obligations $ 27,350.1 $ 0 $ 0 $ 27,350.1 $ 28,490.9 State and local government obligations 0 2,061.6 0 2,061.6 2,222.4 Foreign government obligations 0 15.8 0 15.8 16.9 Corporate debt securities 0 10,681.3 0 10,681.3 11,195.8 Subtotal 27,350.1 12,758.7 0 40,108.8 41,926.0 Asset-backed securities: Residential mortgage-backed 0 630.0 0 630.0 655.0 Commercial mortgage-backed 0 4,503.0 0 4,503.0 5,252.6 Other asset-backed 0 4,865.8 0 4,865.8 5,087.8 Subtotal asset-backed securities 0 9,998.8 0 9,998.8 10,995.4 Redeemable preferred stocks: Financials 0 40.0 0 40.0 43.5 Utilities 0 8.8 0 8.8 10.5 Industrials 9.6 123.2 0 132.8 148.5 Subtotal redeemable preferred stocks 9.6 172.0 0 181.6 202.5 Total fixed maturities 27,359.7 22,929.5 0 50,289.2 53,123.9 Short-term investments 2,524.1 0 0 2,524.1 2,524.1 Total available-for-sale securities 29,883.8 22,929.5 0 52,813.3 55,648.0 Equity securities: Nonredeemable preferred stocks: Financials 40.0 884.5 67.4 991.9 1,102.7 Utilities 0 70.5 0 70.5 80.0 Industrials 0 0 16.4 16.4 15.0 Subtotal nonredeemable preferred stocks 40.0 955.0 83.8 1,078.8 1,197.7 Common equities: Common stocks 2,755.7 0 18.3 2,774.0 720.2 Other risk investments 0 0 20.3 20.3 20.3 Subtotal common equities 2,755.7 0 38.6 2,794.3 740.5 Total equity securities 2,795.7 955.0 122.4 3,873.1 1,938.2 Total portfolio $ 32,679.5 $ 23,884.5 $ 122.4 $ 56,686.4 $ 57,586.2 Debt $ 0 $ 5,881.0 $ 0 $ 5,881.0 $ 6,389.3 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2023 Fixed maturities: U.S. government obligations $ 36,869.4 $ 0 $ 0 $ 36,869.4 $ 37,823.2 State and local government obligations 0 2,202.8 0 2,202.8 2,338.4 Foreign government obligations 0 16.3 0 16.3 17.3 Corporate debt securities 0 11,180.7 3.0 11,183.7 11,446.0 Subtotal 36,869.4 13,399.8 3.0 50,272.2 51,624.9 Asset-backed securities: Residential mortgage-backed 0 417.2 0 417.2 426.9 Commercial mortgage-backed 0 3,939.7 0 3,939.7 4,535.2 Other asset-backed 0 5,575.4 0 5,575.4 5,667.2 Subtotal asset-backed securities 0 9,932.3 0 9,932.3 10,629.3 Redeemable preferred stocks: Financials 0 23.1 0 23.1 24.5 Utilities 0 9.4 0 9.4 10.4 Industrials 0 141.2 0 141.2 152.8 Subtotal redeemable preferred stocks 0 173.7 0 173.7 187.7 Total fixed maturities 36,869.4 23,505.8 3.0 60,378.2 62,441.9 Short-term investments 1,757.0 32.9 0 1,789.9 1,789.9 Total available-for-sale securities 38,626.4 23,538.7 3.0 62,168.1 64,231.8 Equity securities: Nonredeemable preferred stocks: Financials 0 802.7 49.2 851.9 922.1 Utilities 0 35.4 0 35.4 40.0 Industrials 0 0 14.8 14.8 15.0 Subtotal nonredeemable preferred stocks 0 838.1 64.0 902.1 977.1 Common equities: Common stocks 2,885.3 0 22.5 2,907.8 685.4 Other risk investments 0 0 20.6 20.6 20.6 Subtotal common equities 2,885.3 0 43.1 2,928.4 706.0 Total equity securities 2,885.3 838.1 107.1 3,830.5 1,683.1 Total portfolio $ 41,511.7 $ 24,376.8 $ 110.1 $ 65,998.6 $ 65,914.9 Debt $ 0 $ 6,431.3 $ 0 $ 6,431.3 $ 6,888.6
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 months ended March 31, 2024 and 2023: Level 3 Fair Value (millions) Fair Value at December 31, 2023 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2024 Fixed maturities: Corporate debt securities $ 3.0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 3.0 Equity securities: Nonredeemable preferred stocks: Financials 49.2 0 0 0 0 0 0 49.2 Industrials 14.8 0 0 0 0 0 0 14.8 Common equities: Common stocks 22.5 0 0 0 0 0 0 22.5 Other risk investments 20.6 3.7 0 0 0 0 0 24.3 Total Level 3 securities $ 110.1 $ 3.7 $ 0 $ 0 $ 0 $ 0 $ 0 $ 113.8 Level 3 Fair Value (millions) Fair Value at December 31, 2022 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2023 Equity securities: Nonredeemable preferred stocks: Financials $ 67.4 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 67.4 Industrials 16.4 0 0 0 0 0 0 16.4 Common equities: Common stocks 18.3 0 0 0 0 0 0 18.3 Other risk investments 19.8 0.5 0 0 0 0 0 20.3 Total Level 3 securities $ 121.9 $ 0.5 $ 0 $ 0 $ 0 $ 0 $ 0 $ 122.4
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 March 31, 2024 and 2023, and December 31, 2023: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2024 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Fixed maturities: Corporate debt securities $ 3.0 Market comparables Weighted average market capitalization price change % (2.3)% to 1.2% (0.1) % Equity securities: Nonredeemable preferred stocks 64.0 Market comparables Weighted average market capitalization price change % 3.4% to 28.4% 22.6 % Common stocks 22.5 Market comparables Weighted average market capitalization price change % (37.9)% to 39.6% 3.4 % Subtotal Level 3 securities 89.5 Pricing exemption securities 24.3 Total Level 3 securities $ 113.8 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2023 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 % 2.7% to 23.3% 9.2 % Common stocks 18.3 Market comparables Weighted average market capitalization price change % (51.3)% to 48.3% 15.6 % Subtotal Level 3 securities 102.1 Pricing exemption securities 20.3 Total Level 3 securities $ 122.4 Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2023 Valuation Technique Unobservable Input Range of Input Values Increase (Decrease) Weighted Average Increase (Decrease) Fixed maturities: Corporate debt securities $ 3.0 Market comparables Weighted average market capitalization price change % 0.3% to 7.7% 2.6 % Equity securities: Nonredeemable preferred stocks 64.0 Market comparables Weighted average market capitalization price change % 17.2% to 39.7% 21.7% Common stocks 22.5 Market comparables Weighted average market capitalization price change % (45.8)% to 95.6% 39.7% Subtotal Level 3 securities 89.5 Pricing exemption securities 20.6 Total Level 3 securities $ 110.1

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2024
Debt Disclosure [Abstract]
Debt Table	Debt at each of the balance sheet periods consisted of: March 31, 2024 March 31, 2023 December 31, 2023 (millions) Carrying Fair Carrying Fair Carrying Fair 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) $ 498.7 $ 468.6 $ 498.3 $ 464.2 $ 498.6 $ 468.9 2.50% Senior Notes due 2027 (issued: $500.0, March 2022) 498.2 468.5 497.7 463.8 498.1 469.1 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.9 323.5 297.6 329.6 297.8 328.7 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 547.1 532.5 546.5 538.5 546.9 542.6 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 497.4 456.1 497.0 460.0 497.3 462.2 3.00% Senior Notes due 2032 (issued: $500.0, March 2022) 496.4 435.7 496.0 446.9 496.3 446.0 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.8 434.2 396.5 448.1 396.7 445.6 4.95% Senior Notes due 2033 (issued: $500.0, May 2023) 496.5 499.2 0 0 496.4 513.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 347.0 312.8 346.9 312.3 347.0 314.2 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.9 327.9 395.8 324.0 395.9 325.1 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 842.3 719.2 842.1 743.8 842.3 756.2 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.7 519.8 590.4 530.0 590.6 534.1 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 491.2 409.8 491.0 420.3 491.1 422.3 3.70% Senior Notes due 2052 (issued: $500.0, March 2022) 493.6 390.0 493.5 399.5 493.6 403.3 Total $ 6,889.7 $ 6,297.8 $ 6,389.3 $ 5,881.0 $ 6,888.6 $ 6,431.3

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 2024
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: March 31, (millions) 2024 2023 Balance at January 1 $ 34,389.2 $ 30,359.3 Less reinsurance recoverables on unpaid losses 4,789.0 5,559.2 Net balance at January 1 29,600.2 24,800.1 Incurred related to: Current year 10,983.0 10,002.8 Prior years (11.4) 621.2 Total incurred 10,971.6 10,624.0 Paid related to: Current year 4,421.7 3,962.6 Prior years 5,971.0 5,736.2 Total paid 10,392.7 9,698.8 Net balance at March 31 30,179.1 25,725.3 Plus reinsurance recoverables on unpaid losses 4,651.9 5,301.1 Balance at March 31 $ 34,831.0 $ 31,026.4

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2024
Supplemental Cash Flow Elements [Abstract]
Schedule of Supplemental Cash Flow Information, Non Cash Activity	Non-cash activity included the following in the respective periods: Three Months Ended March 31, (millions) 2024 2023 Common share dividends 1 $ 58.6 $ 58.5 Operating lease liabilities 2 28.0 9.3 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: Three Months Ended March 31, (millions) 2024 2023 Income taxes $ 0 $ 0.1 Interest 87.9 87.9 Operating lease liabilities 21.6 21.5

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2024
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 March 31, 2024 2023 (millions) Revenues Pretax Revenues Pretax Personal Lines Agency $ 5,857.7 $ 950.7 $ 4,860.2 $ 162.6 Direct 7,020.5 1,043.6 5,717.4 (22.1) Total Personal Lines 1 12,878.2 1,994.3 10,577.6 140.5 Commercial Lines 2,557.4 209.0 2,356.1 37.2 Property 712.8 46.9 598.7 (32.7) Other indemnity 2 0.2 (0.3) 0.7 (3.4) Total underwriting operations 16,148.6 2,249.9 13,533.1 141.6 Fees and other revenues 3 236.5 NA 206.2 NA Service businesses 84.2 (7.9) 72.5 (9.8) Investments 4 773.2 767.5 491.4 485.9 Interest expense NA (69.6) NA (63.3) Consolidated total $ 17,242.5 $ 2,939.9 $ 14,303.2 $ 554.4 NA = Not applicable 1 Personal auto insurance accounted for 95% and 94% of the total Personal Lines segment net premiums earned during the three months ended March 31, 2024 and 2023, respectively; 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 Includes other underwriting business and run-off operations. 3 Pretax profit (loss) for fees and other revenues is allocated to operating segments based on revenue. 4 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 March 31, 2024 2023 Under-writing Combined Under-writing Combined Personal Lines Agency 16.2 % 83.8 3.3 % 96.7 Direct 14.9 85.1 (0.4) 100.4 Total Personal Lines 15.5 84.5 1.3 98.7 Commercial Lines 8.2 91.8 1.6 98.4 Property 6.6 93.4 (5.5) 105.5 Total underwriting operations 13.9 86.1 1.0 99.0

Dividends (Tables)

Dividends (Tables)	3 Months Ended
	Mar. 31, 2024
Dividends [Abstract]
Dividends	Following is a summary of our common and preferred share dividends that were declared and/or paid during the three months ended March 31, 2024 and 2023: (millions, except per share amounts) Amount Declared Payable Per Share Accrued/Paid 1 Common – Annual-Variable Dividends: December 2023 January 2024 $ 0.75 $ 439.3 Common – Quarterly Dividends: March 2024 April 2024 0.10 58.6 December 2023 January 2024 0.10 58.6 March 2023 April 2023 0.10 58.5 December 2022 January 2023 0.10 58.5 Preferred Dividends: January 2024 2 February 2024 15.688377 7.8 December 2022 March 2023 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)	3 Months Ended
	Mar. 31, 2024
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 December 31, 2023 $ (2,052.9) $ 437.2 $ (1,615.7) $ (1,600.8) $ (14.0) $ (0.9) Other comprehensive income (loss) before reclassifications: Investment securities (413.6) 86.9 (326.7) (326.7) 0 0 Foreign currency translation adjustment (0.3) 0.1 (0.2) 0 0 (0.2) Total other comprehensive income (loss) before reclassifications (413.9) 87.0 (326.9) (326.7) 0 (0.2) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (150.5) 31.6 (118.9) (118.9) 0 0 Interest expense (0.1) 0 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income (150.6) 31.6 (119.0) (118.9) (0.1) 0 Total other comprehensive income (loss) (263.3) 55.4 (207.9) (207.8) 0.1 (0.2) Balance at March 31, 2024 $ (2,316.2) $ 492.6 $ (1,823.6) $ (1,808.6) $ (13.9) $ (1.1) 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 705.6 (148.2) 557.4 557.4 0 0 Total other comprehensive income (loss) before reclassifications 705.6 (148.2) 557.4 557.4 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities (57.9) 12.1 (45.8) (45.8) 0 0 Interest expense (0.1) 0 (0.1) 0 (0.1) 0 Total reclassification adjustment for amounts realized in net income (58.0) 12.1 (45.9) (45.8) (0.1) 0 Total other comprehensive income (loss) 763.6 (160.3) 603.3 603.2 0.1 0 Balance at March 31, 2023 $ (2,793.3) $ 594.6 $ (2,198.7) $ (2,183.1) $ (14.4) $ (1.2)

Basis of Presentation - Reporti

Basis of Presentation - Reporting and Accounting Policies - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Premium Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning of period	$ 369.1	$ 343.3
Increase in allowance	106.8	116.9
Write-offs	(148.2)	(119.3)
Allowance for credit losses, end of period	$ 327.7	$ 340.9

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Assets held-for-sale	$ 169.8	$ 77.2	$ 67.8

Investments - Investment Portfo

Investments - Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 67,276	$ 64,231.8	$ 55,648
Gross Unrealized Gains	125.5	312.2	118.9
Gross Unrealized Losses	(2,423.2)	(2,346.8)	(2,893)
Net Holding Period Gains (Losses)	(21.9)	(29.1)	(60.6)
Total available-for-sale securities	$ 64,956.4	$ 62,168.1	$ 52,813.3
% of Total Fair Value	94.10%	94.20%	93.20%
Equity securities, at Fair Value
Cost	$ 1,639.3	$ 1,683.1	$ 1,938.2
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	2,442.3	2,147.4	1,934.9
Fair Value	$ 4,081.6	$ 3,830.5	$ 3,873.1
% of Total Fair Value	5.90%	5.80%	6.80%
Debt and Equity Securities, at Fair Value
Cost	$ 68,915.3	$ 65,914.9	$ 57,586.2
Gross Unrealized Gains	125.5	312.2	118.9
Gross Unrealized Losses	(2,423.2)	(2,346.8)	(2,893)
Net Holding Period Gains (Losses)	2,420.4	2,118.3	1,874.3
Fair Value	$ 69,038	$ 65,998.6	$ 56,686.4
% of Total Fair Value	100%	100%	100%
Unsettled security transactions, payable	$ 16.2		$ 22.8
Other receivable from broker-dealer and clearing organization		$ 45.6
Total fair value of non-insurance subsidiary portfolio	3,200	4,200	4,100
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	931.1	977.1	1,197.7
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	(44.4)	(75)	(118.9)
Fair Value	$ 886.7	$ 902.1	$ 1,078.8
% of Total Fair Value	1.30%	1.40%	1.90%
Common equities
Equity securities, at Fair Value
Cost	$ 708.2	$ 706	$ 740.5
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	2,486.7	2,222.4	2,053.8
Fair Value	$ 3,194.9	$ 2,928.4	$ 2,794.3
% of Total Fair Value	4.60%	4.40%	4.90%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 65,949.3	$ 62,441.9	$ 53,123.9
Gross Unrealized Gains	125.5	312.2	118.9
Gross Unrealized Losses	(2,423.2)	(2,346.8)	(2,893)
Net Holding Period Gains (Losses)	(21.9)	(29.1)	(60.6)
Total available-for-sale securities	$ 63,629.7	$ 60,378.2	$ 50,289.2
% of Total Fair Value	92.20%	91.50%	88.70%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 39,823.8	$ 37,823.2	$ 28,490.9
Gross Unrealized Gains	66.2	204.1	68.8
Gross Unrealized Losses	(1,327.3)	(1,157.9)	(1,209.6)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 38,562.7	$ 36,869.4	$ 27,350.1
% of Total Fair Value	55.80%	55.90%	48.30%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,200.7	$ 2,338.4	$ 2,222.4
Gross Unrealized Gains	1.5	2.8	0.9
Gross Unrealized Losses	(142.4)	(138.4)	(161.7)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 2,059.8	$ 2,202.8	$ 2,061.6
% of Total Fair Value	3%	3.30%	3.60%
Total fixed maturities | Foreign government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 16.9	$ 17.3	$ 16.9
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(1)	(1)	(1.1)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 15.9	$ 16.3	$ 15.8
% 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	$ 12,821.9	$ 11,446	$ 11,195.8
Gross Unrealized Gains	45.9	87.2	45.6
Gross Unrealized Losses	(328.1)	(332.3)	(525.4)
Net Holding Period Gains (Losses)	(18.4)	(17.2)	(34.7)
Total available-for-sale securities	$ 12,521.3	$ 11,183.7	$ 10,681.3
% of Total Fair Value	18.10%	16.90%	18.80%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 390.1	$ 426.9	$ 655
Gross Unrealized Gains	0.2	0.2	0.2
Gross Unrealized Losses	(9.2)	(10)	(16.3)
Net Holding Period Gains (Losses)	2	0.1	(8.9)
Total available-for-sale securities	$ 383.1	$ 417.2	$ 630
% of Total Fair Value	0.60%	0.60%	1.10%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,368.6	$ 4,535.2	$ 5,252.6
Gross Unrealized Gains	2.5	2.2	1.6
Gross Unrealized Losses	(519.3)	(597.7)	(751.2)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 3,851.8	$ 3,939.7	$ 4,503
% of Total Fair Value	5.60%	6%	7.90%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 6,146.1	$ 5,667.2	$ 5,087.8
Gross Unrealized Gains	9.2	15.7	1.8
Gross Unrealized Losses	(93.9)	(107.1)	(222.5)
Net Holding Period Gains (Losses)	(0.2)	(0.4)	(1.3)
Total available-for-sale securities	$ 6,061.2	$ 5,575.4	$ 4,865.8
% of Total Fair Value	8.80%	8.40%	8.60%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 181.2	$ 187.7	$ 202.5
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	(2)	(2.4)	(5.2)
Net Holding Period Gains (Losses)	(5.3)	(11.6)	(15.7)
Total available-for-sale securities	$ 173.9	$ 173.7	$ 181.6
% of Total Fair Value	0.20%	0.30%	0.30%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,326.7	$ 1,789.9	$ 2,524.1
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	$ 1,326.7	$ 1,789.9	$ 2,524.1
% of Total Fair Value	1.90%	2.70%	4.50%

Investments - Hybrid Securities

Investments - Hybrid Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Hybrid Instruments [Line Items]
Hybrid securities	$ 957.6	$ 1,010.3	$ 1,197.3
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	507.1	531.3	548.6
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	300	323.9	482.1
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	9.5	13.9	33.8
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 141	$ 141.2	$ 132.8

Investments - Composition of Fi

Investments - Composition of Fixed Maturities by Maturity (Detail) - Total fixed maturities $ in Millions	Mar. 31, 2024 USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Less than one year	$ 7,227.5
One to five years	45,277.4
Five to ten years	13,394.2
Ten years or greater	50.2
Cost	65,949.3
Fair Value
Less than one year	7,084.8
One to five years	43,850.9
Five to ten years	12,645.2
Ten years or greater	48.8
Fair Value	$ 63,629.7

Investments - Gross Unrealized

Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions	Mar. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security	Mar. 31, 2023 USD ($) security
Number of Securities
Total | security	1,245	1,223	1,416
Less than 12 Months | security	248	126	291
12 Months or Greater | security	997	1,097	1,125
Fair Value
Total	$ 50,113.1	$ 44,062.1	$ 38,603
Less than 12 Months	25,748.9	13,347.6	9,487.3
12 Months or Greater	24,364.2	30,714.5	29,115.7
Unrealized Losses
Total	(2,423.2)	(2,346.8)	(2,893)
Less than 12 Months	(321.6)	(119.3)	(219.9)
12 Months or Greater	$ (2,101.6)	$ (2,227.5)	$ (2,673.1)
U.S. government obligations
Number of Securities
Total | security	139	147	148
Less than 12 Months | security	37	25	40
12 Months or Greater | security	102	122	108
Fair Value
Total	$ 32,968.1	$ 28,225	$ 19,446.8
Less than 12 Months	21,634.6	11,890	5,938.5
12 Months or Greater	11,333.5	16,335	13,508.3
Unrealized Losses
Total	(1,327.3)	(1,157.9)	(1,209.6)
Less than 12 Months	(295.1)	(100)	(127.5)
12 Months or Greater	$ (1,032.2)	$ (1,057.9)	$ (1,082.1)
State and local government obligations
Number of Securities
Total | security	330	324	343
Less than 12 Months | security	41	31	72
12 Months or Greater | security	289	293	271
Fair Value
Total	$ 1,922.7	$ 1,846.2	$ 1,943.6
Less than 12 Months	250.7	169.9	350.7
12 Months or Greater	1,672	1,676.3	1,592.9
Unrealized Losses
Total	(142.4)	(138.4)	(161.7)
Less than 12 Months	(1.8)	(0.9)	(7.4)
12 Months or Greater	$ (140.6)	$ (137.5)	$ (154.3)
Foreign government obligations
Number of Securities
Total | security	1	1	1
Less than 12 Months | security	0	0	0
12 Months or Greater | security	1	1	1
Fair Value
Total	$ 15.9	$ 16.3	$ 15.8
Less than 12 Months	0	0	0
12 Months or Greater	15.9	16.3	15.8
Unrealized Losses
Total	(1)	(1)	(1.1)
Less than 12 Months	0	0	0
12 Months or Greater	$ (1)	$ (1)	$ (1.1)
Corporate debt securities
Number of Securities
Total | security	363	313	390
Less than 12 Months | security	106	26	93
12 Months or Greater | security	257	287	297
Fair Value
Total	$ 8,094.3	$ 6,642.4	$ 8,129.9
Less than 12 Months	2,369.6	617.2	2,239.3
12 Months or Greater	5,724.7	6,025.2	5,890.6
Unrealized Losses
Total	(328.1)	(332.3)	(525.4)
Less than 12 Months	(20.6)	(14.7)	(75.5)
12 Months or Greater	$ (307.5)	$ (317.6)	$ (449.9)
Residential mortgage-backed securities
Number of Securities
Total | security	35	39	43
Less than 12 Months | security	1	2	13
12 Months or Greater | security	34	37	30
Fair Value
Total	$ 78	$ 88.4	$ 141.7
Less than 12 Months	0	0.4	10
12 Months or Greater	78	88	131.7
Unrealized Losses
Total	(9.2)	(10)	(16.3)
Less than 12 Months	0	0	(0.4)
12 Months or Greater	$ (9.2)	$ (10)	$ (15.9)
Commercial mortgage-backed securities
Number of Securities
Total | security	181	189	220
Less than 12 Months | security	4	1	11
12 Months or Greater | security	177	188	209
Fair Value
Total	$ 3,741.7	$ 3,912.2	$ 4,484.5
Less than 12 Months	190.5	30.7	53.4
12 Months or Greater	3,551.2	3,881.5	4,431.1
Unrealized Losses
Total	(519.3)	(597.7)	(751.2)
Less than 12 Months	(1.8)	(2.5)	(1.5)
12 Months or Greater	$ (517.5)	$ (595.2)	$ (749.7)
Other asset-backed securities
Number of Securities
Total | security	193	207	267
Less than 12 Months | security	59	41	61
12 Months or Greater | security	134	166	206
Fair Value
Total	$ 3,259.5	$ 3,299.1	$ 4,391.9
Less than 12 Months	1,303.5	639.4	884.8
12 Months or Greater	1,956	2,659.7	3,507.1
Unrealized Losses
Total	(93.9)	(107.1)	(222.5)
Less than 12 Months	(2.3)	(1.2)	(6.2)
12 Months or Greater	$ (91.6)	$ (105.9)	$ (216.3)
Redeemable preferred stocks
Number of Securities
Total | security	3	3	4
Less than 12 Months | security	0	0	1
12 Months or Greater | security	3	3	3
Fair Value
Total	$ 32.9	$ 32.5	$ 48.8
Less than 12 Months	0	0	10.6
12 Months or Greater	32.9	32.5	38.2
Unrealized Losses
Total	(2)	(2.4)	(5.2)
Less than 12 Months	0	0	(1.4)
12 Months or Greater	$ (2)	$ (2.4)	$ (3.8)

Investments - Components of Net

Investments - Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	$ 3.2	$ 3.9
Gross realized gains on equity securities	11.2	132.1
Subtotal gross realized gains on security sales	14.4	136
Gross realized loss on available-for-sale securities	(154.9)	(66.7)
Gross realized loss on equity securities	(6)	(112.8)
Subtotal gross realized losses on security sales	(160.9)	(179.5)
Net realized gains (losses) on available-for-sale securities sales	(151.7)	(62.8)
Net realized gains (losses) on equity securities sales	5.2	19.3
Subtotal net realized gains (losses) on security sales	(146.5)	(43.5)
Gain	0	13.2
Impairment	0	(2.3)
Subtotal net realized gains (losses) on other assets	0	10.9
Hybrid securities	7.2	13.9
Equity securities	294.9	90.5
Subtotal net holding period gains (losses)	302.1	104.4
Total net realized gains (losses) on securities	155.6	71.8
U.S. government obligations
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	0	3.8
Gross realized loss on available-for-sale securities	(135.3)	(11.5)
Net realized gains (losses) on available-for-sale securities sales	(135.3)	(7.7)
State and local government obligations
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	0.3	0
Gross realized loss on available-for-sale securities	(0.1)	0
Net realized gains (losses) on available-for-sale securities sales	0.2	0
Corporate debt securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on available-for-sale securities	2.9	0.1
Gross realized loss on available-for-sale securities	(14.8)	(20.4)
Net realized gains (losses) on available-for-sale securities sales	(11.9)	(20.3)
Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	(4.3)	(34.5)
Net realized gains (losses) on available-for-sale securities sales	(4.3)	(34.5)
Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	(0.1)	(0.2)
Net realized gains (losses) on available-for-sale securities sales	(0.1)	(0.2)
Redeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	(0.3)	0
Net realized gains (losses) on available-for-sale securities sales	(0.3)	0
Short-term investments
Gain (Loss) on Securities [Line Items]
Gross realized loss on available-for-sale securities	0	(0.1)
Net realized gains (losses) on available-for-sale securities sales	0	(0.1)
Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Gross realized gains on equity securities	0	0.1
Gross realized loss on equity securities	(5.9)	(101)
Net realized gains (losses) on equity securities sales	(5.9)	(100.9)
Common equities
Gain (Loss) on Securities [Line Items]
Gross realized gains on equity securities	11.2	132
Gross realized loss on equity securities	(0.1)	(11.8)
Net realized gains (losses) on equity securities sales	$ 11.1	$ 120.2

Investments - Components of Equ

Investments - Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Investments, Debt and Equity Securities [Abstract]
Total net gains (losses) recognized during the period on equity securities	$ 300.1	$ 109.8
Less: Net gains (losses) recognized on equity securities sold during the period	5.2	19.3
Net holding period gains (losses) recognized during the period on equity securities held at period end	$ 294.9	$ 90.5

Investments - Components of N_2

Investments - Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Net Investment Income [Line Items]
Gross investment income	$ 617.6	$ 419.6
Investment expenses	(5.7)	(5.5)
Net Investment Income	611.9	414.1
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	592.7	392.8
Total equity securities
Net Investment Income [Line Items]
Gross investment income	24.9	26.8
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	11	15.1
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	13.9	11.7
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	573.8	368.1
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	306.9	163.2
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	12.4	10.6
Total fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Gross investment income	0.1	0.1
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	121.5	83.6
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	5.4	9.2
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	46.4	50
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	78.2	48.5
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.9	2.9
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 18.9	$ 24.7

Investments - Additional Inform

Investments - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2024 USD ($) security	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Investments, Debt and Equity Securities [Abstract]
Principal amount of bonds and certificates of deposit	$ 672,400,000
Securities exceeding 10% of Shareholders' Equity	0	$ 0	$ 0
Non-income producing securities	$ 0
Debt securities, available-for-sale, unrealized loss position, number of positions downgraded | security	7
Debt securities, available-for-sale, unrealized loss position, fair value, downgraded in period	$ 58,200,000
Debt securities, available-for-sale, unrealized loss position, accumulated loss, downgraded in period	8,200,000
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
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	47%
Percent change in book yield	23%

Fair Value - Composition of Inv

Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 64,956.4	$ 62,168.1	$ 52,813.3
Equity securities	4,081.6	3,830.5	3,873.1
Investments, fair value	69,038	65,998.6	56,686.4
Fair value	6,297.8	6,431.3	5,881
Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	886.7	902.1	1,078.8
Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	834.5	851.9	991.9
Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	37.4	35.4	70.5
Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	14.8	14.8	16.4
Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,194.9	2,928.4	2,794.3
Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,170.6	2,907.8	2,774
Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	24.3	20.6	20.3
Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	63,629.7	60,378.2	50,289.2
Total fixed maturities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	38,562.7	36,869.4	27,350.1
Total fixed maturities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,059.8	2,202.8	2,061.6
Total fixed maturities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	15.9	16.3	15.8
Total fixed maturities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,521.3	11,183.7	10,681.3
Total fixed maturities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	383.1	417.2	630
Total fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3,851.8	3,939.7	4,503
Total fixed maturities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,061.2	5,575.4	4,865.8
Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	53,159.7	50,272.2	40,108.8
Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	38,562.7	36,869.4	27,350.1
Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,059.8	2,202.8	2,061.6
Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	15.9	16.3	15.8
Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,521.3	11,183.7	10,681.3
Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,296.1	9,932.3	9,998.8
Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	383.1	417.2	630
Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3,851.8	3,939.7	4,503
Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,061.2	5,575.4	4,865.8
Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	173.9	173.7	181.6
Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	23.2	23.1	40
Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	9.6	9.4	8.8
Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	141.1	141.2	132.8
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,326.7	1,789.9	2,524.1
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	39,889.4	38,626.4	29,883.8
Equity securities	3,148.1	2,885.3	2,795.7
Investments, fair value	43,037.5	41,511.7	32,679.5
Fair value	0	0	0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	40
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	40
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 1 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,148.1	2,885.3	2,755.7
Fair Value, Inputs, Level 1 | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,148.1	2,885.3	2,755.7
Fair Value, Inputs, Level 1 | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	38,562.7	36,869.4	27,359.7
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	38,562.7	36,869.4	27,350.1
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	38,562.7	36,869.4	27,350.1
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	9.6
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 1 | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	9.6
Fair Value, Inputs, Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,326.7	1,757	2,524.1
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	25,064	23,538.7	22,929.5
Equity securities	822.7	838.1	955
Investments, fair value	25,886.7	24,376.8	23,884.5
Fair value	6,297.8	6,431.3	5,881
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	822.7	838.1	955
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	785.3	802.7	884.5
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	37.4	35.4	70.5
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 2 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 2 | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 2 | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 2 | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	25,064	23,505.8	22,929.5
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	14,594	13,399.8	12,758.7
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,059.8	2,202.8	2,061.6
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	15.9	16.3	15.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,518.3	11,180.7	10,681.3
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,296.1	9,932.3	9,998.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	383.1	417.2	630
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3,851.8	3,939.7	4,503
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,061.2	5,575.4	4,865.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	173.9	173.7	172
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	23.2	23.1	40
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	9.6	9.4	8.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	141.1	141.2	123.2
Fair Value, Inputs, Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	32.9	0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3	3	0
Equity securities	110.8	107.1	122.4
Investments, fair value	113.8	110.1	122.4
Fair value	0	0	0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	64	64	83.8
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	49.2	49.2	67.4
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0	0	0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	14.8	14.8	16.4
Fair Value, Inputs, Level 3 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	46.8	43.1	38.6
Fair Value, Inputs, Level 3 | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	22.5	22.5	18.3
Fair Value, Inputs, Level 3 | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	24.3	20.6	20.3
Fair Value, Inputs, Level 3 | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3	3	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3	3	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3	3	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Fair Value, Inputs, Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0	0	0
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	67,276	64,231.8	55,648
Equity securities	1,639.3	1,683.1	1,938.2
Investments, fair value	68,915.3	65,914.9	57,586.2
Long-term debt	6,889.7	6,888.6	6,389.3
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	931.1	977.1	1,197.7
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	876.1	922.1	1,102.7
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	40	40	80
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	15	15	15
Carrying (Reported) Amount, Fair Value Disclosure | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	708.2	706	740.5
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	683.9	685.4	720.2
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	24.3	20.6	20.3
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	65,949.3	62,441.9	53,123.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	54,863.3	51,624.9	41,926
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	39,823.8	37,823.2	28,490.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,200.7	2,338.4	2,222.4
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	16.9	17.3	16.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,821.9	11,446	11,195.8
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10,904.8	10,629.3	10,995.4
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	390.1	426.9	655
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	4,368.6	4,535.2	5,252.6
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,146.1	5,667.2	5,087.8
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	181.2	187.7	202.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	24.5	24.5	43.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	10.4	10.4	10.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	146.3	152.8	148.5
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 1,326.7	$ 1,789.9	$ 2,524.1

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
	Mar. 31, 2024	Mar. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 110.1	$ 121.9
Calls/ Maturities/ Paydowns	3.7	0.5
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Fair value, ending balance	113.8	122.4
Corporate debt securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	3
Calls/ Maturities/ Paydowns	0
Purchases	0
Sales	0
Net Realized (Gain)/Loss on Sales	0
Change in Valuation	0
Net Transfers In (Out)	0
Fair value, ending balance	3
Nonredeemable preferred stocks | Financials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	49.2	67.4
Calls/ Maturities/ Paydowns	0	0
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Fair value, ending balance	49.2	67.4
Nonredeemable preferred stocks | Industrials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	14.8	16.4
Calls/ Maturities/ Paydowns	0	0
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Fair value, ending balance	14.8	16.4
Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	22.5	18.3
Calls/ Maturities/ Paydowns	0	0
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Fair value, ending balance	22.5	18.3
Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	20.6	19.8
Calls/ Maturities/ Paydowns	3.7	0.5
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Fair value, ending balance	$ 24.3	$ 20.3

Fair Value - Summary of Quantit

Fair Value - Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	$ 69,038	$ 56,686.4	$ 65,998.6
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	113.8	122.4	110.1
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	89.5	102.1	89.5
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	24.3	20.3	20.6
Fair Value, Inputs, Level 3 | Corporate debt securities | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	$ 3		$ 3
Fair value inputs, market cap price change percentage, range, minimum	(2.30%)		0.30%
Fair value inputs, market cap price change percentage, range, maximum	1.20%		7.70%
Fair value inputs, market cap price change percentage, weighted average	(0.10%)		2.60%
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	$ 64	$ 83.8	$ 64
Fair value inputs, market cap price change percentage, range, minimum	3.40%	2.70%	17.20%
Fair value inputs, market cap price change percentage, range, maximum	28.40%	23.30%	39.70%
Fair value inputs, market cap price change percentage, weighted average	22.60%	9.20%	21.70%
Fair Value, Inputs, Level 3 | Common equities | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, fair value	$ 22.5	$ 18.3	$ 22.5
Fair value inputs, market cap price change percentage, range, minimum	(37.90%)	(51.30%)	(45.80%)
Fair value inputs, market cap price change percentage, range, maximum	39.60%	48.30%	95.60%
Fair value inputs, market cap price change percentage, weighted average	3.40%	15.60%	39.70%

Fair Value - Additional Informa

Fair Value - Additional Information (Detail) - USD ($)	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor quoted prices, percent of FV hierarchy level	92%	93%	91%
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor quoted prices, percent of FV hierarchy level	100%	100%	98%
Dealer quoted prices, percent of FV hierarchy level			2%
Fair Value, Inputs, Level 3 | Total fixed maturities
Fair Value Measurements [Line Items]
Number of securities	1	1	0

Debt - Summary of Debt (Detail)

Debt - Summary of Debt (Detail) - USD ($)		Mar. 31, 2024	Dec. 31, 2023	May 31, 2023	Mar. 31, 2023	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]	$ 6,889,700,000	$ 6,888,600,000		$ 6,389,300,000
Fair value		6,297,800,000	6,431,300,000		5,881,000,000
2.45% Senior Notes due 2027 (issued: $500.0, August 2016)
Debt Instrument [Line Items]
Long-term debt		498,700,000	498,600,000		498,300,000
Fair value		468,600,000	468,900,000		464,200,000
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		498,200,000	498,100,000		497,700,000
Fair value		468,500,000	469,100,000		463,800,000
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		297,900,000	297,800,000		297,600,000
Fair value		323,500,000	328,700,000		329,600,000
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		547,100,000	546,900,000		546,500,000
Fair value		532,500,000	542,600,000		538,500,000
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		497,400,000	497,300,000		497,000,000
Fair value		456,100,000	462,200,000		460,000,000
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		496,400,000	496,300,000		496,000,000
Fair value		435,700,000	446,000,000		446,900,000
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		396,800,000	396,700,000		396,500,000
Fair value		434,200,000	445,600,000		448,100,000
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		496,500,000	496,400,000		0
Fair value		499,200,000	513,000,000		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	347,000,000		346,900,000
Fair value		312,800,000	314,200,000		312,300,000
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		395,900,000	395,900,000		395,800,000
Fair value		327,900,000	325,100,000		324,000,000
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		842,300,000	842,300,000		842,100,000
Fair value		719,200,000	756,200,000		743,800,000
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		590,700,000	590,600,000		590,400,000
Fair value		519,800,000	534,100,000		530,000,000
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		491,200,000	491,100,000		491,000,000
Fair value		409,800,000	422,300,000		420,300,000
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	493,600,000		493,500,000
Fair value		$ 390,000,000	$ 403,300,000		$ 399,500,000
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 ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Debt Instrument [Line Items]
Long-term debt, current maturities	$ 0	$ 0	$ 0
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	300,000,000
Borrowings outstanding	$ 0	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Income Tax Disclosure [Abstract]
Provision for income taxes	20.70%	19.20%
Valuation allowance	$ 0	$ 0	$ 0
Gross deferred tax asset related to unrealized losses on fixed-maturity securities	482,500,000	582,600,000	427,300,000
Accrued income taxes	$ 961,800,000	$ 203,300,000	$ 311,800,000

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves Activity in Loss and Loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at January 1	$ 34,389.2	$ 30,359.3
Less reinsurance recoverables on unpaid losses	4,651.9	5,301.1	$ 4,789	$ 5,559.2
Net balance at January 1	29,600.2	24,800.1
Incurred related to:
Current year	10,983	10,002.8
Prior years	(11.4)	621.2
Total incurred	10,971.6	10,624
Paid related to:
Current year	4,421.7	3,962.6
Prior years	5,971	5,736.2
Total paid	10,392.7	9,698.8
Net balance at March 31	30,179.1	25,725.3
Plus reinsurance recoverables on unpaid losses	4,651.9	5,301.1
Balance at March 31	$ 34,831	$ 31,026.4

Loss and Loss Adjustment Expe_4

Loss and Loss Adjustment Expense Reserves (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (11.4)	$ 621.2
Personal Lines | Personal auto business
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (100)	428
Personal Lines | Personal auto business | Agency
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of prior year claims and claims adjustment expense	0.60
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ 70	144
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		42
Short-Duration Insurance Contract, Accident Year 2023
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	(50)
Short-Duration Insurance Contract, Accident Year 2022
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years	$ (15)	498
Short-Duration Insurance Contract, Accident Year 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior years		$ 30

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Operating lease liabilities	$ 28	$ 9.3
Income taxes	0	0.1
Interest	87.9	87.9
Operating lease liabilities	21.6	21.5
Cash and Cash Equivalents
Open reverse repurchase commitments	107.7	177.5	$ 68.2
Common equities | Quarterly dividend
Common share dividends	$ 58.6	$ 58.5

Segment Information - Operating

Segment Information - Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 17,242.5	$ 14,303.2
Pretax Profit (Loss)	2,939.9	554.4
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	5,857.7	4,860.2
Pretax Profit (Loss)	950.7	162.6
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,020.5	5,717.4
Pretax Profit (Loss)	1,043.6	(22.1)
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	16,148.6	13,533.1
Pretax Profit (Loss)	2,249.9	141.6
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	12,878.2	10,577.6
Pretax Profit (Loss)	$ 1,994.3	$ 140.5
Personal auto insurance percentage of the total personal lines segment net premiums earned	95%	94%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 2,557.4	$ 2,356.1
Pretax Profit (Loss)	209	37.2
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	712.8	598.7
Pretax Profit (Loss)	46.9	(32.7)
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0.2	0.7
Pretax Profit (Loss)	(0.3)	(3.4)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Interest expense	(69.6)	(63.3)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	236.5	206.2
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	84.2	72.5
Pretax Profit (Loss)	(7.9)	(9.8)
Segment Reconciling Items | Investments
Segment Reporting Information [Line Items]
Revenues	773.2	491.4
Pretax Profit (Loss)	$ 767.5	$ 485.9

Segment Information - Underwrit

Segment Information - Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Personal Lines | Agency
Segment Reporting Information [Line Items]
Under-writing Margin	16.20%	3.30%
Combined Ratio	83.80%	96.70%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Under-writing Margin	14.90%	(0.40%)
Combined Ratio	85.10%	100.40%
Operating Segments
Segment Reporting Information [Line Items]
Under-writing Margin	13.90%	1%
Combined Ratio	86.10%	99%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Under-writing Margin	15.50%	1.30%
Combined Ratio	84.50%	98.70%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Under-writing Margin	8.20%	1.60%
Combined Ratio	91.80%	98.40%
Operating Segments | Property
Segment Reporting Information [Line Items]
Under-writing Margin	6.60%	(5.50%)
Combined Ratio	93.40%	105.50%

Dividends (Details)

Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended					3 Months Ended
	Feb. 28, 2024	Jan. 31, 2024	Apr. 30, 2023	Mar. 31, 2023	Jan. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Cash dividends declared on common stock (in dollars per share)						$ 0.10	$ 0.10
Dividends paid to common shareholders						$ 497.9	$ 58.5
Dividends paid to preferred shareholders						$ 7.8	13.4
Series B Preferred Stock
Cash dividends paid on preferred stock (in dollars per share)	$ 15.688377			$ 26.875
Dividends paid to preferred shareholders	$ 7.8			$ 13.4
Preferred stock, liquidation preference per share (USD per share)						$ 1,000
Aggregate redemption amount						$ 507.8
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
Dividends paid to common shareholders		$ 58.6	$ 58.5		$ 58.5
Annual variable dividend
Cash dividends paid on common stock (in dollars per share)		$ 0.75
Dividends paid to common shareholders		$ 439.3
Common equities | Quarterly dividend
Common share dividends				$ 58.5		$ 58.6	$ 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
	Mar. 31, 2024	Mar. 31, 2023
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pretax total accumulated other comprehensive income (loss), beginning balance	$ (2,052.9)	$ (3,556.9)
Pretax total accumulated other comprehensive income (loss), Investment securities	(413.6)	705.6
Pretax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	(0.3)
Pretax total accumulated other comprehensive income (loss), before reclassifications	(413.9)	705.6
Pretax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	(150.5)	(57.9)
Pretax total accumulated other comprehensive income (loss), Interest expense	(0.1)	(0.1)
Total reclassification adjustment for amounts realized in net income, pre tax	(150.6)	(58)
Total other comprehensive income (loss), pre tax	(263.3)	763.6
Pretax total accumulated other comprehensive income (loss), ending balance	(2,316.2)	(2,793.3)
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Total tax (provision) benefit, beginning balance	437.2	754.9
Total tax (provision) benefit, Investment securities	86.9	(148.2)
Total tax (provision) benefit, Foreign currency translation adjustment	0.1
Total tax (provision) benefit, before reclassifications	87	(148.2)
Total tax (provision) benefit, Net realized gains (losses) on securities	31.6	12.1
Total tax (provision) benefit, Interest expense	0	0
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	31.6	12.1
Total other comprehensive income (loss), tax (provision) benefit	55.4	(160.3)
Total tax (provision) benefit, ending balance	492.6	594.6
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(1,615.7)	(2,802)
After tax total accumulated other comprehensive income (loss), Investment securities	(326.7)	557.4
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	(0.2)	0
After tax total accumulated other comprehensive income (loss), before reclassifications	(326.9)	557.4
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	(118.9)	(45.8)
After tax total accumulated other comprehensive income (loss), Interest expense	(0.1)	(0.1)
Total reclassification adjustment for amounts realized in net income, after tax	(119)	(45.9)
Total other comprehensive income (loss), after tax	(207.9)	603.3
After tax total accumulated other comprehensive income (loss), ending balance	(1,823.6)	(2,198.7)
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	(1,600.8)	(2,786.3)
After tax total accumulated other comprehensive income (loss), Investment securities	(326.7)	557.4
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0
After tax total accumulated other comprehensive income (loss), before reclassifications	(326.7)	557.4
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	(118.9)	(45.8)
After tax total accumulated other comprehensive income (loss), Interest expense	0	0
Total reclassification adjustment for amounts realized in net income, after tax	(118.9)	(45.8)
Total other comprehensive income (loss), after tax	(207.8)	603.2
After tax total accumulated other comprehensive income (loss), ending balance	(1,808.6)	(2,183.1)
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)	(14.5)
After tax total accumulated other comprehensive income (loss), Investment securities	0	0
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	0
After tax total accumulated other comprehensive income (loss), before reclassifications	0	0
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	0	0
After tax total accumulated other comprehensive income (loss), Interest expense	(0.1)	(0.1)
Total reclassification adjustment for amounts realized in net income, after tax	(0.1)	(0.1)
Total other comprehensive income (loss), after tax	0.1	0.1
After tax total accumulated other comprehensive income (loss), ending balance	(13.9)	(14.4)
Foreign currency translation adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income (loss), beginning balance	(0.9)	(1.2)
After tax total accumulated other comprehensive income (loss), Investment securities	0	0
After tax total accumulated other comprehensive income (loss), Foreign currency translation adjustment	(0.2)
After tax total accumulated other comprehensive income (loss), before reclassifications	(0.2)	0
After tax total accumulated other comprehensive income (loss), Net realized gains (losses) on securities	0	0
After tax total accumulated other comprehensive income (loss), Interest expense	0	0
Total reclassification adjustment for amounts realized in net income, after tax	0	0
Total other comprehensive income (loss), after tax	(0.2)	0
After tax total accumulated other comprehensive income (loss), ending balance	$ (1.1)	$ (1.2)

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	Mar. 31, 2024 USD ($)
Equity [Abstract]
Net unrealized losses on forecasted transactions, expected to reclassify into income within the next 12 months	$ 0.6

Litigation - Additional Informa

Litigation - Additional Information (Details)	Mar. 31, 2024 segment
Commitments and Contingencies Disclosure [Abstract]
Number of states, classes certified	5