The Progressive Corporation (PGR) 10-Q 4 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 2021shares
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
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	585,153,282
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	0000080661
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
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, 2021	Mar. 31, 2020
Revenues
Net premiums earned	$ 10,420.2	$ 9,430.7
Investment income	220.2	241.2
Net realized gains (losses) on securities:
Net realized gains (losses) on security sales	143.8	315.2
Net holding period gains (losses) on securities	441.5	(868.8)
Total net realized gains (losses) on securities	585.3	(553.6)
Fees and other revenues	165.7	153.5
Service revenues	53.8	51.6
Total revenues	11,445.2	9,323.4
Expenses
Losses and loss adjustment expenses	7,110.5	6,155.2
Policy acquisition costs	874.4	782.8
Other underwriting expenses	1,481.1	1,409.9
Investment expenses	5.6	5.3
Service expenses	49.3	47.5
Interest expense	56.4	48
Total expenses	9,577.3	8,448.7
Net Income
Income before income taxes	1,867.9	874.7
Provision for income taxes	387.9	175.6
Net income	1,480	699.1
Net income attributable to noncontrolling interest (NCI)	0	(6.4)
Net income attributable to Progressive	1,480	692.7
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on fixed-maturity securities	(539.6)	62.8
Net unrealized losses on forecasted transactions	0.2	0.2
Other comprehensive income (loss)	(539.4)	63
Other comprehensive income attributable to NCI	0	(0.5)
Comprehensive income attributable to Progressive	940.6	755.2
Computation of Earnings Per Common Share
Net income attributable to Progressive	1,480	692.7
Less: Preferred share dividends	6.7	6.7
Net income available to common shareholders	$ 1,473.3	$ 686
Average common shares outstanding - Basic	584.9	584.7
Net effect of dilutive stock-based compensation	2	2.2
Total average equivalent common shares - Diluted	586.9	586.9
Basic: Earnings per common share	$ 2.52	$ 1.17
Diluted: Earnings per common share	$ 2.51	$ 1.17

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Assets
Fixed maturities (amortized cost: $38,564.0, $33,761.1, and $35,589.1)		$ 39,091.8	$ 36,810.9	$ 34,276.6
Short-term investments (amortized cost: $2,243.1, $2,524.2, and $5,218.5)		2,243.1	5,218.5	2,524.2
Total available-for-sale securities		41,334.9	42,029.4	36,800.8
Nonredeemable preferred stocks (cost: $1,453.9, $1,017.5, and $1,358.7)		1,507.2	1,447.9	933.4
Common equities (cost: $1,204.5, $1,113.2, and $1,187.3)		4,558.5	4,053	2,608.1
Total equity securities		6,065.7	5,500.9	3,541.5
Total investments		47,400.6	47,530.3	40,342.3
Cash and cash equivalents		122.9	76.5	369.5
Restricted cash		0.3	0	0.9
Cash and cash equivalents		123.2	76.5	370.4
Accrued investment income		166	176.4	171.6
Premiums receivable, net of allowance for credit losses of $265.3, $344.7, and $356.2		9,218.8	8,160.1	7,568.3
Reinsurance recoverables		4,143.2	4,019.4	3,639.6
Prepaid reinsurance premiums		667.6	368.1	438.2
Deferred acquisition costs		1,309.1	1,237.2	1,095.8
Property and equipment, net of accumulated depreciation of $1,318.5, $1,191.6, and $1,291.4		1,077.4	1,106	1,215.1
Goodwill		452.7	452.7	452.7
Intangible assets, net of accumulated amortization of $340.3, $328.5, and $326.1		157.2	171.4	213.8
Net federal deferred income taxes		0	0	20.6
Other assets		697.5	800.2	738
Total assets		65,413.3	64,098.3	56,266.4
Liabilities
Unearned premiums		15,045.9	13,437.5	12,641.1
Loss and loss adjustment expense reserves		21,063.7	20,265.8	18,306.5
Net federal deferred income taxes		241.5	310	0
Dividends Payable		58.5	2,694.5	58.5
Accounts payable, accrued expenses, and other liabilities		5,770.4	4,955.8	5,289.2
Debt	[1]	5,396.8	5,396.1	5,394
Total liabilities		47,576.8	47,059.7	41,689.3
Redeemable noncontrolling interest (NCI)		0	0	225.6
Shareholders’ Equity
Serial Preferred Shares, Series B, no par value (cumulative, liquidation preference $1,000 per share) (authorized, issued, and outstanding 0.5)		493.9	493.9	493.9
Common shares, $1.00 par value (authorized 900.0; issued 797.5, including treasury shares of 212.3, 212.2, and 212.3)		585.2	585.2	585.3
Paid-in capital		1,685.5	1,672.9	1,601.9
Retained earnings		14,679.6	13,354.9	11,266.2
Accumulated other comprehensive income (loss):
Net unrealized gains (losses) on fixed-maturity securities		407.7	947.3	423.6
Net unrealized losses on forecasted transactions		(15.4)	(15.6)	(16.2)
Accumulated other comprehensive income attributable to NCI		0	0	(3.2)
Total accumulated other comprehensive income (loss) attributable to Progressive		392.3	931.7	404.2
Total shareholders’ equity		17,836.5	17,038.6	14,351.5
Total liabilities, redeemable NCI, and shareholders’ equity		$ 65,413.3	$ 64,098.3	$ 56,266.4
[1]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Fixed maturities, amortized cost	$ 38,564	$ 35,589.1	$ 33,761.1
Short-term investments, amortized cost	2,243.1	5,218.5	2,524.2
Nonredeemable preferred stocks, cost	1,453.9	1,358.7	1,017.5
Common equities, cost	1,204.5	1,187.3	1,113.2
Allowance for credit loss, beginning of period	265.3	356.2	344.7
Property and equipment, accumulated depreciation	1,318.5	1,291.4	1,191.6
Intangible Assets, Accumulated Amortization	$ 340.3	$ 326.1	$ 328.5
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)	797,500,000	797,500,000	797,500,000
Common Shares, treasury shares (shares)	212,300,000	212,300,000	212,200,000
Serial Preferred Stock
Preferred Stock, Shares Authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock | Serial Preferred Stock
Preferred Stock, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)	500,000	500,000	500,000
Preferred Stock, Shares Issued (shares)	500,000	500,000	500,000
Preferred Stock, Shares Outstanding (shares)	500,000	500,000	500,000
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000	$ 1,000	$ 1,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Serial Preferred Stock	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss) Attributable to Progressive	Series B Preferred StockSerial Preferred Stock	Voting Preference Shares
Common Stock, Dividends, Per Share, Declared	$ 0.10
Preferred Stock, Dividends Per Share, Declared								$ 26.875
Balance, Beginning of period at Dec. 31, 2019			$ 493.9	$ 584.6	$ 1,573.4	$ 10,679.6	$ 341.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive	$ 692.7					692.7
Treasury shares purchased				(0.3)	(1)	(25.2)
Cash dividends declared on common shares ($0.10 and $0.10 per share)						(58.4)
Cash dividends declared on Serial Preferred Shares, Series B ($0 and $26.875 per share)						(13.4)
Reinvested dividends on restricted stock units					0.3	(0.3)
Net restricted equity awards issued/vested				1	(1)
Amortization of equity-based compensation					23.3
Adjustment to carrying amount of redeemable noncontrolling interest					6.9
Attributable to noncontrolling interest	(0.5)						(0.5)
Other comprehensive income (loss)	63						63
Other, net						(8.8)
Balance, End of period at Mar. 31, 2020	$ 14,351.5		493.9	585.3	1,601.9	11,266.2	404.2
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)		20,000,000						500,000	5,000,000
Preferred Stock, Shares Issued (shares)								500,000	0
Common Shares, par value (USD per share)	$ 1
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)		20,000,000						500,000
Preferred Stock, Shares Issued (shares)								500,000
Common Shares, par value (USD per share)	1
Common Stock, Dividends, Per Share, Declared	$ 0.10
Preferred Stock, Dividends Per Share, Declared								$ 0
Balance, Beginning of period at Dec. 31, 2020	$ 17,038.6		493.9	585.2	1,672.9	13,354.9	931.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive	1,480					1,480
Treasury shares purchased				(0.9)	(2.7)	(81.2)
Cash dividends declared on common shares ($0.10 and $0.10 per share)						(58.4)
Cash dividends declared on Serial Preferred Shares, Series B ($0 and $26.875 per share)						0
Reinvested dividends on restricted stock units					0.4	(0.4)
Net restricted equity awards issued/vested				0.9	(0.9)
Amortization of equity-based compensation					15.8
Adjustment to carrying amount of redeemable noncontrolling interest					0
Attributable to noncontrolling interest	0						0
Other comprehensive income (loss)	(539.4)						(539.4)
Other, net						(15.3)
Balance, End of period at Mar. 31, 2021	$ 17,836.5		$ 493.9	$ 585.2	$ 1,685.5	$ 14,679.6	$ 392.3
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)		20,000,000						500,000	5,000,000
Preferred Stock, Shares Issued (shares)								500,000	0
Common Shares, par value (USD per share)	$ 1

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Cash Flows [Abstract]
Net income	$ 1,480	$ 699.1
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	62.9	62.6
Amortization of intangible assets	14.2	14.5
Net amortization of fixed-income securities	31.7	18.9
Amortization of equity-based compensation	15.8	23.3
Net realized (gains) losses on securities	(585.3)	553.6
Net (gains) losses on disposition of property and equipment	(1.1)	0.1
Changes in:
Premiums receivable	(1,058.7)	(61)
Reinsurance recoverables	(123.8)	(260.7)
Prepaid reinsurance premiums	(299.5)	188.3
Deferred acquisition costs	(71.9)	(39.3)
Income taxes	283.8	128.7
Unearned premiums	1,608.4	252.3
Loss and loss adjustment expense reserves	797.9	201.1
Accounts payable, accrued expenses, and other liabilities	380.1	(189.9)
Other, net	80	34.2
Net cash provided by operating activities	2,614.5	1,625.8
Purchases:
Fixed maturities	(10,421.4)	(10,433)
Equity securities	(196.7)	(366.6)
Sales:
Fixed maturities	5,590.3	7,415.9
Equity securities	63	281
Maturities, paydowns, calls, and other:
Fixed maturities	1,948.6	2,161.6
Equity securities	39.7	79
Net (purchases) sales of short-term investments	2,976.8	(718.9)
Net unsettled security transactions	267.6	586.1
Purchases of property and equipment	(50.5)	(62.4)
Sales of property and equipment	7.5	3.5
Net cash provided by (used in) investing activities	224.9	(1,053.8)
Cash Flows From Financing Activities
Dividends paid to common shareholders	(2,694.5)	(1,375.4)
Dividends paid to preferred shareholders	(13.4)	(13.4)
Acquisition of treasury shares for restricted stock tax liabilities	(30)	(26.5)
Acquisition of treasury shares acquired in open market	(54.8)	0
Net proceeds from debt issuances	0	986.3
Net cash used in financing activities	(2,792.7)	(429)
Increase in cash, cash equivalents, and restricted cash	46.7	143
Cash, cash equivalents, and restricted cash – January 1	76.5	227.4
Cash, cash equivalents, and restricted cash – March 31	$ 123.2	$ 370.4

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation — The accompanying consolidated financial statements include the accounts of The Progressive Corporation, our wholly owned insurance and non-insurance subsidiaries and affiliates in which we have a controlling financial interest. 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, 2021, 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, 2020 (“2020 Annual Report to Shareholders”). We perform analyses to evaluate our premiums receivables for expected credit losses, based on historical and current collections experience using actuarial analysis, to determine estimated rates of default. These rates are applied to the stratified subsets of our consumer receivable balances, based on the age of the receivable, to establish an allowance for credit loss. Progressive’s premiums receivables are short-term in nature and, generally, premiums are collected prior to providing risk coverage, minimizing our exposure to credit risk. Premiums receivables balances are written off once we have exhausted our collection efforts. The following table summarizes changes in our allowance for credit loss exposure on our premiums receivables for the year-to-date periods ending: (millions) March 31, 2021 March 31, 2020 December 31, 2020 Allowance for credit loss, beginning of period $ 356.2 $ 283.2 $ 283.2 Increase in allowance 1 60.9 170.3 472.0 Write-offs 2 (151.8) (108.8) (399.0) Allowance for credit loss, end of period $ 265.3 $ 344.7 $ 356.2 1 Represents the incremental increase in other underwriting expenses. 2 Represents portion of allowance that is reversed when premiums receivables are written off. During the first quarter 2021, the increase in our allowance for credit losses was lower than prior periods reflecting greater collections received on outstanding premiums receivable balances, due in part to the government stimulus and tax refund checks distributed during the quarter. In contrast, during the first quarter 2020, the increase in the allowance in part reflects the greater potential for credit losses due to the financial hardships of policyholders as a result of the economic impacts related to the spread of the novel coronavirus, COVID-19. Other assets on the consolidated balance sheets include certain long-lived assets that are considered “ held for sale. ” The fair value of these held for sale assets, less the estimated costs to sell, was $55.1 million at March 31, 2021, $31.8 million at March 31, 2020, and $56.6 million at December 31, 2020.

Investments

Investments	3 Months Ended
	Mar. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Investments	Investments — The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. 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, 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 of the securities. The changes in the net holding period gains (losses) between periods for the hybrid securities and equity securities 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, 2021 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 16,002.7 $ 181.7 $ (111.0) $ 0 $ 16,073.4 33.9 % State and local government obligations 2,574.8 65.3 (16.0) 0 2,624.1 5.6 Corporate debt securities 10,095.2 368.1 (24.0) (3.1) 10,436.2 22.0 Residential mortgage-backed securities 548.3 5.7 (0.8) 0 553.2 1.2 Commercial mortgage-backed securities 5,868.3 68.8 (45.1) 0 5,892.0 12.4 Other asset-backed securities 3,293.5 30.6 (0.7) 0 3,323.4 7.0 Redeemable preferred stocks 181.2 2.9 (2.0) 7.4 189.5 0.4 Total fixed maturities 38,564.0 723.1 (199.6) 4.3 39,091.8 82.5 Short-term investments 2,243.1 0 0 0 2,243.1 4.7 Total available-for-sale securities 40,807.1 723.1 (199.6) 4.3 41,334.9 87.2 Equity securities: Nonredeemable preferred stocks 1,453.9 0 0 53.3 1,507.2 3.2 Common equities 1,204.5 0 0 3,354.0 4,558.5 9.6 Total equity securities 2,658.4 0 0 3,407.3 6,065.7 12.8 Total portfolio 1 $ 43,465.5 $ 723.1 $ (199.6) $ 3,411.6 $ 47,400.6 100.0 % ($ in millions) Cost Gross Gross Net Fair % of March 31, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 10,841.2 $ 654.5 $ 0 $ 0 $ 11,495.7 28.5 % State and local government obligations 2,243.9 64.8 (4.5) 0 2,304.2 5.7 Corporate debt securities 9,452.9 160.8 (120.1) (1.7) 9,491.9 23.5 Residential mortgage-backed securities 574.3 1.5 (19.2) 0 556.6 1.4 Commercial mortgage-backed securities 5,822.4 30.2 (188.3) 0 5,664.3 14.0 Other asset-backed securities 4,674.9 4.3 (40.0) (0.1) 4,639.1 11.5 Redeemable preferred stocks 151.5 0 (5.1) (21.6) 124.8 0.3 Total fixed maturities 33,761.1 916.1 (377.2) (23.4) 34,276.6 84.9 Short-term investments 2,524.2 0 0 0 2,524.2 6.3 Total available-for-sale securities 36,285.3 916.1 (377.2) (23.4) 36,800.8 91.2 Equity securities: Nonredeemable preferred stocks 1,017.5 0 0 (84.1) 933.4 2.3 Common equities 1,113.2 0 0 1,494.9 2,608.1 6.5 Total equity securities 2,130.7 0 0 1,410.8 3,541.5 8.8 Total portfolio 1 $ 38,416.0 $ 916.1 $ (377.2) $ 1,387.4 $ 40,342.3 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 12,437.9 $ 305.8 $ (3.7) $ 0 $ 12,740.0 26.8 % State and local government obligations 3,099.4 123.1 (0.7) 0 3,221.8 6.8 Corporate debt securities 9,579.7 601.7 (0.1) 3.9 10,185.2 21.4 Residential mortgage-backed securities 503.3 7.1 (0.9) 0 509.5 1.1 Commercial mortgage-backed securities 6,042.6 142.5 (10.0) 0 6,175.1 13.0 Other asset-backed securities 3,745.0 40.1 (0.5) 0 3,784.6 7.9 Redeemable preferred stocks 181.2 3.6 (1.4) 11.3 194.7 0.4 Total fixed maturities 35,589.1 1,223.9 (17.3) 15.2 36,810.9 77.4 Short-term investments 5,218.5 0 0 0 5,218.5 11.0 Total available-for-sale securities 40,807.6 1,223.9 (17.3) 15.2 42,029.4 88.4 Equity securities: Nonredeemable preferred stocks 1,358.7 0 0 89.2 1,447.9 3.1 Common equities 1,187.3 0 0 2,865.7 4,053.0 8.5 Total equity securities 2,546.0 0 0 2,954.9 5,500.9 11.6 Total portfolio 1 $ 43,353.6 $ 1,223.9 $ (17.3) $ 2,970.1 $ 47,530.3 100.0 % 1 Our portfolio reflects the effect of net unsettled security transactions; at March 31, 2021, we had $363.1 million in other liabilities, compared to $598.0 million and $95.5 million at March 31, 2020 and December 31, 2020, respectively. The total fair value of the portfolio at March 31, 2021 and 2020, and December 31, 2020, included $3.7 billion, $2.6 billion, and $6.2 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions. During the first quarter 2021, we used a portion of these investments to pay our fourth quarter and annual 2020 common share dividends. At March 31, 2021, bonds and certificates of deposit in the principal amount of $296.1 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, 2021 and 2020, or December 31, 2020. At March 31, 2021, we did not hold any debt securities that were non-income producing during the preceding 12 months. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature or are redeemable within one year. We invested in repurchase and reverse repurchase transactions during 2021 and 2020, but did not have any open positions at March 31, 2021 and 2020, or December 31, 2020. To the extent we enter into repurchase or reverse repurchase transactions, consistent with past practice, we would elect not to offset these transactions and would report them on a gross basis on our consolidated balance sheets, despite the option to elect to offset these transactions as long as they were with the same counterparty and subject to an enforceable master netting arrangement. Hybrid Securities Certain securities in our fixed-maturity portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. These securities are reported at fair value: March 31, (millions) 2021 2020 December 31, 2020 Fixed Maturities: State and local government obligations $ 0 $ 3.4 $ 0 Corporate debt securities 295.0 116.5 188.4 Residential mortgage-backed securities 35.0 0 0 Other asset-backed securities 31.9 2.2 34.8 Redeemable preferred stocks 127.5 68.4 131.4 Total hybrid securities $ 489.4 $ 190.5 $ 354.6 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 have elected to record the changes in fair value of these securities through income as a component of net realized gains or losses. Fixed Maturities The composition of fixed maturities by maturity at March 31, 2021, was: (millions) Cost Fair Value Less than one year $ 4,918.5 $ 4,940.6 One to five years 24,370.8 24,814.6 Five to ten years 9,214.5 9,277.3 Ten years or greater 60.2 59.3 Total $ 38,564.0 $ 39,091.8 Asset-backed securities are classified in the maturity distribution table based upon their projected cash flows. All other securities, which 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, for which no credit related impairment has been recognized, 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, 2021 U.S. government obligations 36 $ 8,612.2 $ (111.0) 35 $ 8,598.6 $ (109.7) 1 $ 13.6 $ (1.3) State and local government obligations 92 816.0 (16.0) 92 816.0 (16.0) 0 0 0 Corporate debt securities 79 1,751.4 (24.0) 76 1,698.4 (23.9) 3 53.0 (0.1) Residential mortgage-backed securities 21 123.4 (0.8) 6 94.4 (0.2) 15 29.0 (0.6) Commercial mortgage-backed securities 95 2,542.2 (45.1) 77 2,023.9 (41.7) 18 518.3 (3.4) Other asset-backed securities 43 505.8 (0.7) 36 477.6 (0.5) 7 28.2 (0.2) Redeemable preferred stocks 1 10.5 (2.0) 0 0 0 1 10.5 (2.0) Total fixed maturities 367 $ 14,361.5 $ (199.6) 322 $ 13,708.9 $ (192.0) 45 $ 652.6 $ (7.6) 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, 2020 U.S. government obligations 1 $ 100.3 $ 0 1 $ 100.3 $ 0 0 $ 0 $ 0 State and local government obligations 89 528.4 (4.5) 78 507.7 (4.4) 11 20.7 (0.1) Corporate debt securities 233 4,205.6 (120.1) 233 4,205.6 (120.1) 0 0 0 Residential mortgage-backed securities 70 532.5 (19.2) 46 425.0 (11.9) 24 107.5 (7.3) Commercial mortgage-backed securities 204 4,441.5 (188.3) 187 4,109.2 (173.8) 17 332.3 (14.5) Other asset-backed securities 216 3,558.1 (40.0) 201 3,514.2 (39.0) 15 43.9 (1.0) Redeemable preferred stocks 4 56.3 (5.1) 3 45.2 (3.8) 1 11.1 (1.3) Total fixed maturities 817 $ 13,422.7 $ (377.2) 749 $ 12,907.2 $ (353.0) 68 $ 515.5 $ (24.2) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized December 31, 2020 U.S. government obligations 9 $ 1,511.0 $ (3.7) 9 $ 1,511.0 $ (3.7) 0 $ 0 $ 0 State and local government obligations 30 208.7 (0.7) 30 208.7 (0.7) 0 0 0 Corporate debt securities 7 129.4 (0.1) 7 129.4 (0.1) 0 0 0 Residential mortgage-backed securities 21 44.4 (0.9) 0 0 0 21 44.4 (0.9) Commercial mortgage-backed securities 43 893.3 (10.0) 9 93.6 (0.3) 34 799.7 (9.7) Other asset-backed securities 22 183.7 (0.5) 9 74.4 (0.1) 13 109.3 (0.4) Redeemable preferred stocks 1 11.0 (1.4) 0 0 0 1 11.0 (1.4) Total fixed maturities 133 $ 2,981.5 $ (17.3) 64 $ 2,017.1 $ (4.9) 69 $ 964.4 $ (12.4) As of March 31, 2021, we had three securities in the table above, all in our corporate portfolio, that had their credit ratings downgraded during the quarter, with a combined fair value of $31.9 million and an unrealized loss of $0.7 million. A review of the securities in an unrealized loss position indicated that the issuers were current with respect to their interest obligations and that there was no evidence of deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. Allowance For Credit and Uncollectible Losses We are required to measure the amount of potential credit losses for all fixed-maturity securities in an unrealized loss position. We did not record any allowances for credit losses or any write-offs for amounts deemed to be uncollectible for the first three months of 2021 and did not have a credit loss allowance balance as of March 31, 2021 and 2020, or December 31, 2020. We considered several factors and inputs related to the individual securities as part of this analysis, including: • 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 whether we 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 calculated the net present value (NPV) of the cash flows expected (i.e., expected recovery value) using the current book yield for each security. The NPV was then compared to the security’s current amortized value to determine if a credit loss existed. In the event that the NPV was below the amortized value, and the amount was determined to be material individually, or in 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, 2021, we believe none of the unrealized losses relate to material credit losses on any specific securities, or in the aggregate, based on our review. We continue to expect all the securities in our portfolio to pay their principal and interest obligations. In addition, we reviewed our accrued investment income outstanding on those securities in an unrealized loss position at March 31, 2021, to determine if the accrued interest amounts were determined to be uncollectible. Based on our analysis, we believe the issuers have sufficient liquidity and capital reserves to meet their current interest, and future principal, obligations and, therefore, did not write off any accrued income as uncollectible at March 31, 2021. Realized Gains (Losses) The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2021 2020 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 66.6 $ 251.1 State and local government obligations 30.2 2.0 Corporate and other debt securities 20.7 32.4 Commercial mortgage-backed securities 29.8 6.1 Other asset-backed securities 0.7 0 Total available-for-sale securities 148.0 291.6 Equity securities: Nonredeemable preferred stocks 17.3 19.6 Common equities 1.1 66.0 Total equity securities 18.4 85.6 Subtotal gross realized gains on security sales 166.4 377.2 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (19.6) (2.7) State and local government obligations (0.2) 0 Corporate and other debt securities (2.1) (1.0) Commercial mortgage-backed securities (0.6) 0 Other asset-backed securities (0.1) 0 Total available-for-sale securities (22.6) (3.7) Equity securities: Nonredeemable preferred stocks 0 (4.4) Common equities 0 (53.9) Total equity securities 0 (58.3) Subtotal gross realized losses on security sales (22.6) (62.0) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations 47.0 248.4 State and local government obligations 30.0 2.0 Corporate and other debt securities 18.6 31.4 Commercial mortgage-backed securities 29.2 6.1 Other asset-backed securities 0.6 0 Total available-for-sale securities 125.4 287.9 Equity securities: Nonredeemable preferred stocks 17.3 15.2 Common equities 1.1 12.1 Total equity securities 18.4 27.3 Subtotal net realized gains (losses) on security sales 143.8 315.2 Net holding period gains (losses) Hybrid securities (10.9) (31.2) Equity securities 452.4 (837.6) Subtotal net holding period gains (losses) 441.5 (868.8) Total net realized gains (losses) on securities $ 585.3 $ (553.6) Realized gains (losses) on securities sold are computed using the first-in-first-out method. The following table reflects our holding period realized gains (losses) on equity securities recognized for the three months ended March 31, 2021 and 2020, for equity securities held at the respective quarter end: Three Months (millions) 2021 2020 Total net gains (losses) recognized during the period on equity securities $ 470.8 $ (810.3) Less: Net gains (losses) recognized on equity securities sold during the period 18.4 27.3 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 452.4 $ (837.6) During the first quarter 2021, as expectations of U.S. economic growth increased, the value of our equity securities also rose. In contrast, during the first quarter 2020, we recognized significant declines in the equity security valuations due to investors reducing exposure to risk in response to the economic impact of COVID-19 restrictions. Net Investment Income The components of net investment income for the three months ended March 31, were: Three Months (millions) 2021 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 30.4 $ 62.6 State and local government obligations 13.4 9.6 Corporate debt securities 85.2 58.0 Residential mortgage-backed securities 3.4 4.2 Commercial mortgage-backed securities 35.8 34.3 Other asset-backed securities 15.9 29.2 Redeemable preferred stocks 2.5 7.9 Total fixed maturities 186.6 205.8 Short-term investments 1.5 7.0 Total available-for-sale securities 188.1 212.8 Equity securities: Nonredeemable preferred stocks 17.9 13.8 Common equities 14.2 14.6 Total equity securities 32.1 28.4 Investment income 220.2 241.2 Investment expenses (5.6) (5.3) Net investment income $ 214.6 $ 235.9

Fair Value

Fair Value	3 Months Ended
	Mar. 31, 2021
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 the 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, 2021 Fixed maturities: U.S. government obligations $ 16,073.4 $ 0 $ 0 $ 16,073.4 $ 16,002.7 State and local government obligations 0 2,624.1 0 2,624.1 2,574.8 Corporate debt securities 0 10,436.2 0 10,436.2 10,095.2 Subtotal 16,073.4 13,060.3 0 29,133.7 28,672.7 Asset-backed securities: Residential mortgage-backed 0 553.2 0 553.2 548.3 Commercial mortgage-backed 0 5,892.0 0 5,892.0 5,868.3 Other asset-backed 0 3,323.4 0 3,323.4 3,293.5 Subtotal asset-backed securities 0 9,768.6 0 9,768.6 9,710.1 Redeemable preferred stocks: Financials 0 50.6 0 50.6 51.0 Utilities 0 11.4 0 11.4 10.0 Industrials 10.6 116.9 0 127.5 120.2 Subtotal redeemable preferred stocks 10.6 178.9 0 189.5 181.2 Total fixed maturities 16,084.0 23,007.8 0 39,091.8 38,564.0 Short-term investments 2,227.8 15.3 0 2,243.1 2,243.1 Total available-for-sale securities 18,311.8 23,023.1 0 41,334.9 40,807.1 Equity securities: Nonredeemable preferred stocks: Financials 113.8 1,274.7 35.0 1,423.5 1,373.9 Utilities 0 41.9 0 41.9 39.9 Industrials 0 25.2 16.6 41.8 40.1 Subtotal nonredeemable preferred stocks 113.8 1,341.8 51.6 1,507.2 1,453.9 Common equities: Common stocks 4,555.3 0 0 4,555.3 1,201.3 Other risk investments 0 0 3.2 3.2 3.2 Subtotal common equities 4,555.3 0 3.2 4,558.5 1,204.5 Total equity securities 4,669.1 1,341.8 54.8 6,065.7 2,658.4 Total portfolio $ 22,980.9 $ 24,364.9 $ 54.8 $ 47,400.6 $ 43,465.5 Debt $ 0 $ 6,195.2 $ 0 $ 6,195.2 $ 5,396.8 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2020 Fixed maturities: U.S. government obligations $ 11,495.7 $ 0 $ 0 $ 11,495.7 $ 10,841.2 State and local government obligations 0 2,304.2 0 2,304.2 2,243.9 Corporate debt securities 0 9,491.9 0 9,491.9 9,452.9 Subtotal 11,495.7 11,796.1 0 23,291.8 22,538.0 Asset-backed securities: Residential mortgage-backed 0 556.6 0 556.6 574.3 Commercial mortgage-backed 0 5,664.3 0 5,664.3 5,822.4 Other asset-backed 0 4,639.1 0 4,639.1 4,674.9 Subtotal asset-backed securities 0 10,860.0 0 10,860.0 11,071.6 Redeemable preferred stocks: Financials 0 47.2 0 47.2 51.4 Utilities 0 9.2 0 9.2 10.0 Industrials 7.3 61.1 0 68.4 90.1 Subtotal redeemable preferred stocks 7.3 117.5 0 124.8 151.5 Total fixed maturities 11,503.0 22,773.6 0 34,276.6 33,761.1 Short-term investments 2,438.4 85.8 0 2,524.2 2,524.2 Total available-for-sale securities 13,941.4 22,859.4 0 36,800.8 36,285.3 Equity securities: Nonredeemable preferred stocks: Financials 105.5 752.3 13.1 870.9 937.4 Utilities 0 32.0 0 32.0 40.0 Industrials 0 15.3 15.2 30.5 40.1 Subtotal nonredeemable preferred stocks 105.5 799.6 28.3 933.4 1,017.5 Common equities: Common stocks 2,607.8 0 0 2,607.8 1,112.9 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,607.8 0 0.3 2,608.1 1,113.2 Total equity securities 2,713.3 799.6 28.6 3,541.5 2,130.7 Total portfolio $ 16,654.7 $ 23,659.0 $ 28.6 $ 40,342.3 $ 38,416.0 Debt $ 0 $ 6,108.6 $ 0 $ 6,108.6 $ 5,394.0 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2020 Fixed maturities: U.S. government obligations $ 12,740.0 $ 0 $ 0 $ 12,740.0 $ 12,437.9 State and local government obligations 0 3,221.8 0 3,221.8 3,099.4 Corporate debt securities 0 10,185.2 0 10,185.2 9,579.7 Subtotal 12,740.0 13,407.0 0 26,147.0 25,117.0 Asset-backed securities: Residential mortgage-backed 0 509.5 0 509.5 503.3 Commercial mortgage-backed 0 6,175.1 0 6,175.1 6,042.6 Other asset-backed 0 3,784.6 0 3,784.6 3,745.0 Subtotal asset-backed securities 0 10,469.2 0 10,469.2 10,290.9 Redeemable preferred stocks: Financials 0 51.6 0 51.6 51.1 Utilities 0 11.7 0 11.7 10.0 Industrials 10.8 120.6 0 131.4 120.1 Subtotal redeemable preferred stocks 10.8 183.9 0 194.7 181.2 Total fixed maturities 12,750.8 24,060.1 0 36,810.9 35,589.1 Short-term investments 4,704.9 513.6 0 5,218.5 5,218.5 Total available-for-sale securities 17,455.7 24,573.7 0 42,029.4 40,807.6 Equity securities: Nonredeemable preferred stocks: Financials 117.7 1,212.3 35.0 1,365.0 1,278.6 Utilities 0 41.9 0 41.9 40.0 Industrials 0 24.3 16.7 41.0 40.1 Subtotal nonredeemable preferred stocks 117.7 1,278.5 51.7 1,447.9 1,358.7 Common equities: Common stocks 4,049.9 0 0 4,049.9 1,184.2 Other risk investments 0 0 3.1 3.1 3.1 Subtotal common equities 4,049.9 0 3.1 4,053.0 1,187.3 Total equity securities 4,167.6 1,278.5 54.8 5,500.9 2,546.0 Total portfolio $ 21,623.3 $ 25,852.2 $ 54.8 $ 47,530.3 $ 43,353.6 Debt $ 0 $ 6,793.5 $ 0 $ 6,793.5 $ 5,396.1 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, 2021, vendor-quoted prices represented 78% of our Level 1 classifications (excluding short-term investments), compared to 82% and 76% at March 31, 2020 and December 31, 2020, respectively. 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. At March 31, 2021 and December 31, 2020, vendor-quoted prices comprised 99% of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented the remaining 1%, compared to 98% and 2%, respectively, at March 31, 2020. 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 at least 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 our structured debt securities, including commercial, residential, and 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 our 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, coupon, and credit quality, 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 our municipal securities (e.g., general obligations, revenue, and housing), we stratify the portfolio to evaluate securities by type, coupon, credit quality, and duration 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. Lastly, for our 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 explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a 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. Except as described below, our Level 3 securities are also 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. Certain private equity investments included in the Level 3 category are valued using external pricing supplemented by internal review and analysis. After all the valuations are received and our review is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected security valuations to Level 3. At March 31, 2021 and 2020, and December 31, 2020, we did not have any securities in our fixed-maturity portfolio listed as Level 3. At March 31, 2021 and December 31, 2020, we held four private nonredeemable preferred securities that were priced internally or by a pricing firm, and we held three private nonredeemable preferred securities at March 31, 2020. At March 31, 2021 and December 31, 2020, we held two Level 3 other risk investments that were priced using the cost method, compared to one Level 3 other risk investment at March 31, 2020. To the extent we receive prices from external sources for the Level 3 securities, we would review those prices for reasonableness using internally developed assumptions and then compare our derived prices to the prices we received. During 2021 and 2020, there were no material assets or liabilities measured at fair value on a nonrecurring basis. Based on our review, all prices received from external sources for 2021 remained unadjusted. 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, 2021 and 2020: Level 3 Fair Value (millions) Fair Value at December 31, 2020 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2021 Equity securities: Nonredeemable preferred stocks: Financials $ 35.0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 35.0 Industrials 16.7 0 0 0 0 (0.1) 0 16.6 Common equities: Other risk investments 3.1 0 0.1 0 0 0 0 3.2 Total Level 3 securities $ 54.8 $ 0 $ 0.1 $ 0 $ 0 $ (0.1) $ 0 $ 54.8 Level 3 Fair Value (millions) Fair Value at December 31, 2019 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2020 Equity securities: Nonredeemable preferred stocks: Financials $ 27.1 $ 0 $ 0 $ 0 $ 0 $ (14.0) $ 0 $ 13.1 Industrials 16.0 0 0 0 0 $ (0.8) 0 15.2 Common equities: Other risk investments 0.3 0 0 0 0 0 0 0.3 Total Level 3 securities $ 43.4 $ 0 $ 0 $ 0 $ 0 $ (14.8) $ 0 $ 28.6 The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2021 and 2020, and December 31, 2020: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2021 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials 1 $ 25.0 Internal price Unadjusted purchase price per share 3.7 Financials 2 10.0 Internal price Unadjusted purchase price per share 16.9 Industrials 6.9 Pricing firm Market approach (guideline public company method) 5.4 Industrials 9.7 Pricing firm Market approach (guideline public company method) 4.8 Subtotal Level 3 securities 51.6 Pricing exemption securities 3.2 Total Level 3 securities $ 54.8 1 This security was purchased in May 2020. 2 This security was purchased in November 2020. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials $ 13.1 Internal price Market approach (guideline public company method) 4.4 Industrials 1 7.1 Internal price Market approach (guideline public company method) 5.6 Industrials 1 8.1 Internal price Market approach (guideline public company method) 4.0 Subtotal Level 3 securities 28.3 Pricing exemption securities 0.3 Total Level 3 securities $ 28.6 1 Modified prior year disclosure to present securities individually to conform to the current year presentation. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials 1 $ 25.0 Internal price Unadjusted purchase price per share 3.7 Financials 2 10.0 Internal price Unadjusted purchase price per share 16.9 Industrials 6.9 Pricing firm Market approach (guideline public company method) 5.4 Industrials 9.8 Pricing firm Market approach (guideline public company method) 4.8 Subtotal Level 3 securities 51.7 Pricing exemption securities 3.1 Total Level 3 securities $ 54.8 1 This security was purchased in May 2020. 2 This security was purchased in November 2020.

Debt

Debt	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: March 31, 2021 March 31, 2020 December 31, 2020 (millions) Carrying Fair Carrying Fair Carrying Fair 3.75% Senior Notes due 2021 (issued: $500.0, August 2011) $ 499.9 $ 506.7 $ 499.5 $ 510.7 $ 499.8 $ 510.9 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) 497.4 526.8 497.0 488.2 497.3 541.1 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.0 386.1 296.7 395.3 296.9 409.4 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 545.6 623.7 545.1 605.7 545.5 660.4 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 496.2 536.7 495.8 543.2 496.1 575.5 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.0 531.4 395.8 539.6 396.0 582.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 346.8 413.9 346.7 391.7 346.7 459.7 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.6 430.2 395.4 406.2 395.5 481.0 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 841.7 977.1 841.6 991.9 841.7 1,113.1 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.0 698.7 589.9 675.8 590.0 806.7 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 490.6 563.9 490.5 560.3 490.6 653.7 Total $ 5,396.8 $ 6,195.2 $ 5,394.0 $ 6,108.6 $ 5,396.1 $ 6,793.5 At March 31, 2021 and December 31, 2020, short-term debt consisted of the $500 million 3.75% Senior Notes that mature in August 2021; there was no short-term debt outstanding at March 31, 2020. The Progressive Corporation has a line of credit with PNC Bank, National Association (PNC), in the maximum principal amount of $250 million. See the 2020 Annual Report to Shareholders for terms of this line of credit. We had no borrowings under the line of credit during the periods presented.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes — 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, 2021 and 2020, and December 31, 2020, 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. For the three months ended March 31, 2021, there have been no material changes in our reserve for uncertain tax positions. The effective tax rate for the three months ended March 31, 2021, was 20.8%, compared to 20.1% for the same period last year.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 2021
Loss and Loss Adjustment Expenses Reserves [Abstract]
Loss and Loss Adjustment Expenses Reserves	Loss and Loss Adjustment Expense Reserves — Activity in the loss and loss adjustment expense reserves is summarized as follows: March 31, (millions) 2021 2020 Balance at January 1 $ 20,265.8 $ 18,105.4 Less reinsurance recoverables on unpaid losses 3,798.2 3,212.2 Net balance at January 1 16,467.6 14,893.2 Incurred related to: Current year 6,986.1 6,011.1 Prior years 124.4 144.1 Total incurred 7,110.5 6,155.2 Paid related to: Current year 3,029.3 2,916.4 Prior years 3,453.7 3,269.2 Total paid 6,483.0 6,185.6 Net balance at March 31 17,095.1 14,862.8 Plus reinsurance recoverables on unpaid losses 3,968.6 3,443.7 Balance at March 31 $ 21,063.7 $ 18,306.5 We experienced unfavorable reserve development of $124.4 million and $144.1 million during the first three months of 2021 and 2020, respectively, which is reflected as “Incurred related to prior years ” in the table above. First Quarter 2021 • Approximately 34% of the unfavorable prior year reserve development was attributable to accident year 2020, 34% to accident year 2019, and the remainder to accident years 2018 and prior. • Our personal auto products incurred about $92 million of unfavorable loss and loss adjustment expense (LAE) reserve development, with about 60% attributable to the Agency business. The unfavorable development was primarily attributable to a higher than anticipated frequency of reopened personal injury protection (PIP) claims, primarily in Florida, and higher than anticipated late reported losses occurring toward the end of 2020 but not reported until 2021. • Our Commercial Lines business experienced about $29 million of unfavorable development, primarily due to the emergence of large injury claims at rates higher than originally anticipated. • Our special lines business experienced about $5 million of favorable development and our Property business experienced about $8 million of unfavorable development during the first quarter. First Quarter 2020 • Approximately 52% of the unfavorable prior year reserve development was attributable to accident year 2019, 36% to accident year 2018, and the remainder to accident years 2017 and prior. • Our personal auto products incurred about $104 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 revised estimates of our per claim settlement costs and late reported losses occurring toward the end of 2019 but not reported until 2020. • Our Commercial Lines business experienced about $54 million of unfavorable development, primarily due to increased injury severity and the emergence of large injury claims at rates higher than originally anticipated. • Our special lines business experienced about $14 million of favorable development and our Property business had minimal development during the first quarter.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 2021
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, and are not considered part of the investment portfolio. The amount of reverse repurchase commitments held by these subsidiaries at March 31, 2021 and 2020, and December 31, 2020, were $154.0 million, $80.6 million, and $93.5 million, respectively. Restricted cash on our consolidated balance sheets represents cash that is restricted to pay flood claims under the National Flood Insurance Program’s “Write Your Own” program, for which certain subsidiaries are administrators. During the first quarter 2021 and 2020, non-cash activity included declared but unpaid common share dividends of $58.5 million (see Note 9 – Dividends for further discussion) for both periods, and operating lease liabilities arising from obtaining right-of-use assets of $6.2 million and $9.7 million, respectively. We paid the following in the respective periods: Three Months Ended March 31, (millions) 2021 2020 Income taxes $ 0.8 $ 0 Interest 74.3 56.4 Operating lease liabilities 24.5 22.7

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2021
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 primary liability and physical damage insurance, and business-related general liability and property insurance, predominately for small businesses. Our Property segment writes residential property insurance for homeowners, other property owners, and renters. Our service businesses provide insurance-related services, including processing Commercial Automobile Insurance Procedures/Plans (CAIP) business and 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, 2021 2020 (millions) Revenues Pretax Revenues Pretax Personal Lines Agency $ 4,098.2 $ 547.5 $ 3,828.7 $ 601.6 Direct 4,431.7 414.6 3,992.4 473.0 Total Personal Lines 1 8,529.9 962.1 7,821.1 1,074.6 Commercial Lines 1,417.8 228.5 1,189.0 112.5 Property 2 472.5 (70.7) 420.6 49.2 Total underwriting operations 10,420.2 1,119.9 9,430.7 1,236.3 Fees and other revenues 3 165.7 NA 153.5 NA Service businesses 53.8 4.5 51.6 4.1 Investments 4 805.5 799.9 (312.4) (317.7) Interest expense NA (56.4) NA (48.0) Consolidated total $ 11,445.2 $ 1,867.9 $ 9,323.4 $ 874.7 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three months ended March 31, 2021 and 2020; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three months ended March 31, 2021 and 2020, pretax profit (loss) includes $14.2 million and $14.5 million, respectively, of amortization expense predominately associated with intangible assets. See Note 12 – Goodwill and Intangible Assets for further discussion. 3 Pretax profit (loss) for fees and other revenues is allocated to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense. 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, 2021 2020 Underwriting Combined Underwriting Combined Personal Lines Agency 13.4 % 86.6 15.7 % 84.3 Direct 9.4 90.6 11.9 88.1 Total Personal Lines 11.3 88.7 13.7 86.3 Commercial Lines 16.1 83.9 9.5 90.5 Property 1 (15.0) 115.0 11.7 88.3 Total underwriting operations 10.7 89.3 13.1 86.9 1 Included in the three months ended March 31, 2021 and 2020, is 3.0 points and 3.4 points, respectively, of amortization expense predominately associated with intangible assets.

Dividends

Dividends	3 Months Ended
	Mar. 31, 2021
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, 2021 and 2020: (millions, except per share amounts) Amount Declared Payable Per Share Accrued 1 Common - Quarterly Dividends: March 2021 April 2021 $ 0.10 $ 58.5 December 2020 January 2021 0.10 58.6 February 2020 April 2020 0.10 58.5 December 2019 January 2020 0.10 58.5 Common - Annual Variable Dividends: December 2020 January 2021 4.50 2,635.9 December 2019 January 2020 2.25 1,316.9 Preferred Dividends: December 2020 March 2021 26.875 13.4 February 2020 March 2020 26.875 13.4 1 The accrual is based on an estimate of shares outstanding as of the record date. The dividends accrued at March 31, 2020, were reclassified from accounts payable, accrued expenses, and other liabilities into dividends payable on common shares on the consolidated balance sheets, to conform to the current year presentation. See Note 14 – Dividends in our 2020 Annual Report to Shareholders for a discussion of our common and preferred share dividend policies.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 2021
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 gains (losses) on forecasted transactions (Income) Balance at December 31, 2020 $ 1,187.4 $ (255.7) $ 931.7 $ 947.3 $ (15.6) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities (551.4) 115.8 (435.6) (435.6) 0 0 Loss attributable to noncontrolling interest (NCI) 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications (551.4) 115.8 (435.6) (435.6) 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 131.7 (27.7) 104.0 104.0 0 0 Interest expense (0.3) 0.1 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income 131.4 (27.6) 103.8 104.0 (0.2) 0 Total other comprehensive income (loss) (682.8) 143.4 (539.4) (539.6) 0.2 0 Balance at March 31, 2021 $ 504.6 $ (112.3) $ 392.3 $ 407.7 $ (15.4) $ 0 Components of Changes in (millions) Pretax total Total tax After tax total Total net Net (Income) Balance at December 31, 2019 $ 435.7 $ (94.0) $ 341.7 $ 360.8 $ (16.4) $ (2.7) Other comprehensive income (loss) before reclassifications: Investment securities 318.2 (66.8) 251.4 251.4 0 0 Loss attributable to noncontrolling interest (NCI) (0.6) 0.1 (0.5) 0 0 (0.5) Total other comprehensive income (loss) before reclassifications 317.6 (66.7) 250.9 251.4 0 (0.5) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 238.7 (50.1) 188.6 188.6 0 0 Interest expense (0.3) 0.1 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income 238.4 (50.0) 188.4 188.6 (0.2) 0 Total other comprehensive income (loss) 79.2 (16.7) 62.5 62.8 0.2 (0.5) Balance at March 31, 2020 $ 514.9 $ (110.7) $ 404.2 $ 423.6 $ (16.2) $ (3.2) In an effort to manage interest rate risk, we may enter into forecasted transactions on Progressive’s debt issuances. We expect to reclassify $0.7 million (pretax) into interest expense during the next 12 months, related to net unrealized losses on forecasted transactions (see Note 4 – Debt in our 2020 Annual Report to Shareholders for further discussion).

Litigation

Litigation	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Litigation	Litigation — The Progressive Corporation and/or its insurance subsidiaries are named as defendants in various lawsuits arising out of claims made under insurance policies written by our insurance subsidiaries in the ordinary course of business. We consider all legal actions relating to such claims in establishing our loss and loss adjustment expense reserves. In addition, The Progressive Corporation and/or its subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the subsidiaries. Other insurance companies face many of these same issues. These cases include those alleging damages as a result of, among other things, our subsidiaries’ practices in evaluating or paying medical or injury claims or benefits, including, but not limited to, personal injury protection, medical payments, uninsured motorist/underinsured motorist, and bodily injury benefits, and for reimbursing medical costs incurred by Medicare/Medicaid beneficiaries; our practices in evaluating or paying physical damage claims, including, but not limited to, our payment of total loss claims and labor rates paid to auto body repair shops; our insurance product design; employment matters; and cases challenging other aspects of our claims, marketing, pricing or sales practices or other business operations. Other insurance companies face many of these same issues. The nature and volume of litigation to which The Progressive Corporation is subject is similar to that which was disclosed in Note 12 – Litigation in our 2020 Annual Report to Shareholders. We plan to contest the pending lawsuits vigorously, but may pursue settlement negotiations in some cases, as we deem appropriate. The outcomes of pending cases are uncertain at this time. 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 or estimable, or is considered probable but not estimable, we do not establish an accrual. Nevertheless, we continue to evaluate this pending litigation to determine if any losses not deemed probable and estimable become so, at which point we would establish an accrual at our best estimate of the loss or range of loss. With respect to our pending lawsuits that are not related to claims under insurance policies, the accruals that we have established, if any, were not material at March 31, 2021 or 2020, and there were no material settlements during 2020 or the first three months of 2021. 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 2020 Annual Report to Shareholders. In the event that any one or more of these lawsuits results in a substantial judgment against us, or settlement by us, or if our accruals (if any) prove to be inadequate by a significant amount, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. For a further discussion on our pending litigation and related reserving policies, see Note 12 – Litigation in our 2020 Annual Report to Shareholders.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill During the three months ended March 31, 2021, there were no changes to the carrying amount of goodwill. No accumulated goodwill impairment losses exist. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets: (millions) March 31, 2021 March 31, 2020 December 31, 2020 Intangible assets subject to amortization $ 144.8 $ 201.4 $ 159.0 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 157.2 $ 213.8 $ 171.4 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: March 31, 2021 March 31, 2020 December 31, 2020 (millions) Gross Accumulated Net Gross Accumulated Net Gross Accumulated Net Category: Policies in force $ 256.2 $ 219.6 $ 36.6 $ 256.2 $ 183.0 $ 73.2 $ 256.2 $ 210.4 $ 45.8 Agency relationships 159.2 68.3 90.9 159.2 56.9 102.3 159.2 65.4 93.8 Software rights 69.1 51.8 17.3 79.1 53.2 25.9 69.1 49.7 19.4 Trade name 0 0 0 34.8 34.8 0 0 0 0 Total $ 484.5 $ 339.7 $ 144.8 $ 529.3 $ 327.9 $ 201.4 $ 484.5 $ 325.5 $ 159.0

New Accounting Standards

New Accounting Standards	3 Months Ended
	Mar. 31, 2021
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards

Basis of Presentation (Policies

Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Financing Receivable, Allowance for Credit Losses, Policy for Uncollectible Amounts	We perform analyses to evaluate our premiums receivables for expected credit losses, based on historical and current collections experience using actuarial analysis, to determine estimated rates of default. These rates are applied to the stratified subsets of our consumer receivable balances, based on the age of the receivable, to establish an allowance for credit loss. Progressive’s premiums receivables are short-term in nature and, generally, premiums are collected prior to providing risk coverage, minimizing our exposure to credit risk. Premiums receivables balances are written off once we have exhausted our collection efforts.

Basis of Presentation (Tables)

Basis of Presentation (Tables)	3 Months Ended
	Mar. 31, 2021
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 receivables for the year-to-date periods ending: (millions) March 31, 2021 March 31, 2020 December 31, 2020 Allowance for credit loss, beginning of period $ 356.2 $ 283.2 $ 283.2 Increase in allowance 1 60.9 170.3 472.0 Write-offs 2 (151.8) (108.8) (399.0) Allowance for credit loss, end of period $ 265.3 $ 344.7 $ 356.2 1 Represents the incremental increase in other underwriting expenses. 2 Represents portion of allowance that is reversed when premiums receivables are written off.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2021
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, consistent with our classification of how we manage, monitor, and measure the portfolio. 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, 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 of the securities. The changes in the net holding period gains (losses) between periods for the hybrid securities and equity securities 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, 2021 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 16,002.7 $ 181.7 $ (111.0) $ 0 $ 16,073.4 33.9 % State and local government obligations 2,574.8 65.3 (16.0) 0 2,624.1 5.6 Corporate debt securities 10,095.2 368.1 (24.0) (3.1) 10,436.2 22.0 Residential mortgage-backed securities 548.3 5.7 (0.8) 0 553.2 1.2 Commercial mortgage-backed securities 5,868.3 68.8 (45.1) 0 5,892.0 12.4 Other asset-backed securities 3,293.5 30.6 (0.7) 0 3,323.4 7.0 Redeemable preferred stocks 181.2 2.9 (2.0) 7.4 189.5 0.4 Total fixed maturities 38,564.0 723.1 (199.6) 4.3 39,091.8 82.5 Short-term investments 2,243.1 0 0 0 2,243.1 4.7 Total available-for-sale securities 40,807.1 723.1 (199.6) 4.3 41,334.9 87.2 Equity securities: Nonredeemable preferred stocks 1,453.9 0 0 53.3 1,507.2 3.2 Common equities 1,204.5 0 0 3,354.0 4,558.5 9.6 Total equity securities 2,658.4 0 0 3,407.3 6,065.7 12.8 Total portfolio 1 $ 43,465.5 $ 723.1 $ (199.6) $ 3,411.6 $ 47,400.6 100.0 % ($ in millions) Cost Gross Gross Net Fair % of March 31, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 10,841.2 $ 654.5 $ 0 $ 0 $ 11,495.7 28.5 % State and local government obligations 2,243.9 64.8 (4.5) 0 2,304.2 5.7 Corporate debt securities 9,452.9 160.8 (120.1) (1.7) 9,491.9 23.5 Residential mortgage-backed securities 574.3 1.5 (19.2) 0 556.6 1.4 Commercial mortgage-backed securities 5,822.4 30.2 (188.3) 0 5,664.3 14.0 Other asset-backed securities 4,674.9 4.3 (40.0) (0.1) 4,639.1 11.5 Redeemable preferred stocks 151.5 0 (5.1) (21.6) 124.8 0.3 Total fixed maturities 33,761.1 916.1 (377.2) (23.4) 34,276.6 84.9 Short-term investments 2,524.2 0 0 0 2,524.2 6.3 Total available-for-sale securities 36,285.3 916.1 (377.2) (23.4) 36,800.8 91.2 Equity securities: Nonredeemable preferred stocks 1,017.5 0 0 (84.1) 933.4 2.3 Common equities 1,113.2 0 0 1,494.9 2,608.1 6.5 Total equity securities 2,130.7 0 0 1,410.8 3,541.5 8.8 Total portfolio 1 $ 38,416.0 $ 916.1 $ (377.2) $ 1,387.4 $ 40,342.3 100.0 % ($ in millions) Cost Gross Gross Net Fair % of December 31, 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 12,437.9 $ 305.8 $ (3.7) $ 0 $ 12,740.0 26.8 % State and local government obligations 3,099.4 123.1 (0.7) 0 3,221.8 6.8 Corporate debt securities 9,579.7 601.7 (0.1) 3.9 10,185.2 21.4 Residential mortgage-backed securities 503.3 7.1 (0.9) 0 509.5 1.1 Commercial mortgage-backed securities 6,042.6 142.5 (10.0) 0 6,175.1 13.0 Other asset-backed securities 3,745.0 40.1 (0.5) 0 3,784.6 7.9 Redeemable preferred stocks 181.2 3.6 (1.4) 11.3 194.7 0.4 Total fixed maturities 35,589.1 1,223.9 (17.3) 15.2 36,810.9 77.4 Short-term investments 5,218.5 0 0 0 5,218.5 11.0 Total available-for-sale securities 40,807.6 1,223.9 (17.3) 15.2 42,029.4 88.4 Equity securities: Nonredeemable preferred stocks 1,358.7 0 0 89.2 1,447.9 3.1 Common equities 1,187.3 0 0 2,865.7 4,053.0 8.5 Total equity securities 2,546.0 0 0 2,954.9 5,500.9 11.6 Total portfolio 1 $ 43,353.6 $ 1,223.9 $ (17.3) $ 2,970.1 $ 47,530.3 100.0 % 1 Our portfolio reflects the effect of net unsettled security transactions; at March 31, 2021, we had $363.1 million in other liabilities, compared to $598.0 million and $95.5 million at March 31, 2020 and December 31, 2020, respectively. The total fair value of the portfolio at March 31, 2021 and 2020, and December 31, 2020, included $3.7 billion, $2.6 billion, and $6.2 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of unsettled security transactions. During the first quarter 2021, we used a portion of these investments to pay our fourth quarter and annual 2020 common share dividends.
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) 2021 2020 December 31, 2020 Fixed Maturities: State and local government obligations $ 0 $ 3.4 $ 0 Corporate debt securities 295.0 116.5 188.4 Residential mortgage-backed securities 35.0 0 0 Other asset-backed securities 31.9 2.2 34.8 Redeemable preferred stocks 127.5 68.4 131.4 Total hybrid securities $ 489.4 $ 190.5 $ 354.6
Investments Classified by Contractual Maturity Date	The composition of fixed maturities by maturity at March 31, 2021, was: (millions) Cost Fair Value Less than one year $ 4,918.5 $ 4,940.6 One to five years 24,370.8 24,814.6 Five to ten years 9,214.5 9,277.3 Ten years or greater 60.2 59.3 Total $ 38,564.0 $ 39,091.8
Schedule of Unrealized Loss on Investments	The following tables show the composition of gross unrealized losses, for which no credit related impairment has been recognized, 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, 2021 U.S. government obligations 36 $ 8,612.2 $ (111.0) 35 $ 8,598.6 $ (109.7) 1 $ 13.6 $ (1.3) State and local government obligations 92 816.0 (16.0) 92 816.0 (16.0) 0 0 0 Corporate debt securities 79 1,751.4 (24.0) 76 1,698.4 (23.9) 3 53.0 (0.1) Residential mortgage-backed securities 21 123.4 (0.8) 6 94.4 (0.2) 15 29.0 (0.6) Commercial mortgage-backed securities 95 2,542.2 (45.1) 77 2,023.9 (41.7) 18 518.3 (3.4) Other asset-backed securities 43 505.8 (0.7) 36 477.6 (0.5) 7 28.2 (0.2) Redeemable preferred stocks 1 10.5 (2.0) 0 0 0 1 10.5 (2.0) Total fixed maturities 367 $ 14,361.5 $ (199.6) 322 $ 13,708.9 $ (192.0) 45 $ 652.6 $ (7.6) 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, 2020 U.S. government obligations 1 $ 100.3 $ 0 1 $ 100.3 $ 0 0 $ 0 $ 0 State and local government obligations 89 528.4 (4.5) 78 507.7 (4.4) 11 20.7 (0.1) Corporate debt securities 233 4,205.6 (120.1) 233 4,205.6 (120.1) 0 0 0 Residential mortgage-backed securities 70 532.5 (19.2) 46 425.0 (11.9) 24 107.5 (7.3) Commercial mortgage-backed securities 204 4,441.5 (188.3) 187 4,109.2 (173.8) 17 332.3 (14.5) Other asset-backed securities 216 3,558.1 (40.0) 201 3,514.2 (39.0) 15 43.9 (1.0) Redeemable preferred stocks 4 56.3 (5.1) 3 45.2 (3.8) 1 11.1 (1.3) Total fixed maturities 817 $ 13,422.7 $ (377.2) 749 $ 12,907.2 $ (353.0) 68 $ 515.5 $ (24.2) Total No. of Sec. Total Gross Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized No. of Sec. Fair Unrealized December 31, 2020 U.S. government obligations 9 $ 1,511.0 $ (3.7) 9 $ 1,511.0 $ (3.7) 0 $ 0 $ 0 State and local government obligations 30 208.7 (0.7) 30 208.7 (0.7) 0 0 0 Corporate debt securities 7 129.4 (0.1) 7 129.4 (0.1) 0 0 0 Residential mortgage-backed securities 21 44.4 (0.9) 0 0 0 21 44.4 (0.9) Commercial mortgage-backed securities 43 893.3 (10.0) 9 93.6 (0.3) 34 799.7 (9.7) Other asset-backed securities 22 183.7 (0.5) 9 74.4 (0.1) 13 109.3 (0.4) Redeemable preferred stocks 1 11.0 (1.4) 0 0 0 1 11.0 (1.4) Total fixed maturities 133 $ 2,981.5 $ (17.3) 64 $ 2,017.1 $ (4.9) 69 $ 964.4 $ (12.4)
Realized Gain (Loss) on Investments	The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2021 2020 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 66.6 $ 251.1 State and local government obligations 30.2 2.0 Corporate and other debt securities 20.7 32.4 Commercial mortgage-backed securities 29.8 6.1 Other asset-backed securities 0.7 0 Total available-for-sale securities 148.0 291.6 Equity securities: Nonredeemable preferred stocks 17.3 19.6 Common equities 1.1 66.0 Total equity securities 18.4 85.6 Subtotal gross realized gains on security sales 166.4 377.2 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (19.6) (2.7) State and local government obligations (0.2) 0 Corporate and other debt securities (2.1) (1.0) Commercial mortgage-backed securities (0.6) 0 Other asset-backed securities (0.1) 0 Total available-for-sale securities (22.6) (3.7) Equity securities: Nonredeemable preferred stocks 0 (4.4) Common equities 0 (53.9) Total equity securities 0 (58.3) Subtotal gross realized losses on security sales (22.6) (62.0) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations 47.0 248.4 State and local government obligations 30.0 2.0 Corporate and other debt securities 18.6 31.4 Commercial mortgage-backed securities 29.2 6.1 Other asset-backed securities 0.6 0 Total available-for-sale securities 125.4 287.9 Equity securities: Nonredeemable preferred stocks 17.3 15.2 Common equities 1.1 12.1 Total equity securities 18.4 27.3 Subtotal net realized gains (losses) on security sales 143.8 315.2 Net holding period gains (losses) Hybrid securities (10.9) (31.2) Equity securities 452.4 (837.6) Subtotal net holding period gains (losses) 441.5 (868.8) Total net realized gains (losses) on securities $ 585.3 $ (553.6)
Components of Equity Realized Gains (Losses)	The following table reflects our holding period realized gains (losses) on equity securities recognized for the three months ended March 31, 2021 and 2020, for equity securities held at the respective quarter end: Three Months (millions) 2021 2020 Total net gains (losses) recognized during the period on equity securities $ 470.8 $ (810.3) Less: Net gains (losses) recognized on equity securities sold during the period 18.4 27.3 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 452.4 $ (837.6)
Components of Net Investment Income	The components of net investment income for the three months ended March 31, were: Three Months (millions) 2021 2020 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 30.4 $ 62.6 State and local government obligations 13.4 9.6 Corporate debt securities 85.2 58.0 Residential mortgage-backed securities 3.4 4.2 Commercial mortgage-backed securities 35.8 34.3 Other asset-backed securities 15.9 29.2 Redeemable preferred stocks 2.5 7.9 Total fixed maturities 186.6 205.8 Short-term investments 1.5 7.0 Total available-for-sale securities 188.1 212.8 Equity securities: Nonredeemable preferred stocks 17.9 13.8 Common equities 14.2 14.6 Total equity securities 32.1 28.4 Investment income 220.2 241.2 Investment expenses (5.6) (5.3) Net investment income $ 214.6 $ 235.9

Fair Value (Tables)

Fair Value (Tables)	3 Months Ended
	Mar. 31, 2021
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, 2021 Fixed maturities: U.S. government obligations $ 16,073.4 $ 0 $ 0 $ 16,073.4 $ 16,002.7 State and local government obligations 0 2,624.1 0 2,624.1 2,574.8 Corporate debt securities 0 10,436.2 0 10,436.2 10,095.2 Subtotal 16,073.4 13,060.3 0 29,133.7 28,672.7 Asset-backed securities: Residential mortgage-backed 0 553.2 0 553.2 548.3 Commercial mortgage-backed 0 5,892.0 0 5,892.0 5,868.3 Other asset-backed 0 3,323.4 0 3,323.4 3,293.5 Subtotal asset-backed securities 0 9,768.6 0 9,768.6 9,710.1 Redeemable preferred stocks: Financials 0 50.6 0 50.6 51.0 Utilities 0 11.4 0 11.4 10.0 Industrials 10.6 116.9 0 127.5 120.2 Subtotal redeemable preferred stocks 10.6 178.9 0 189.5 181.2 Total fixed maturities 16,084.0 23,007.8 0 39,091.8 38,564.0 Short-term investments 2,227.8 15.3 0 2,243.1 2,243.1 Total available-for-sale securities 18,311.8 23,023.1 0 41,334.9 40,807.1 Equity securities: Nonredeemable preferred stocks: Financials 113.8 1,274.7 35.0 1,423.5 1,373.9 Utilities 0 41.9 0 41.9 39.9 Industrials 0 25.2 16.6 41.8 40.1 Subtotal nonredeemable preferred stocks 113.8 1,341.8 51.6 1,507.2 1,453.9 Common equities: Common stocks 4,555.3 0 0 4,555.3 1,201.3 Other risk investments 0 0 3.2 3.2 3.2 Subtotal common equities 4,555.3 0 3.2 4,558.5 1,204.5 Total equity securities 4,669.1 1,341.8 54.8 6,065.7 2,658.4 Total portfolio $ 22,980.9 $ 24,364.9 $ 54.8 $ 47,400.6 $ 43,465.5 Debt $ 0 $ 6,195.2 $ 0 $ 6,195.2 $ 5,396.8 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2020 Fixed maturities: U.S. government obligations $ 11,495.7 $ 0 $ 0 $ 11,495.7 $ 10,841.2 State and local government obligations 0 2,304.2 0 2,304.2 2,243.9 Corporate debt securities 0 9,491.9 0 9,491.9 9,452.9 Subtotal 11,495.7 11,796.1 0 23,291.8 22,538.0 Asset-backed securities: Residential mortgage-backed 0 556.6 0 556.6 574.3 Commercial mortgage-backed 0 5,664.3 0 5,664.3 5,822.4 Other asset-backed 0 4,639.1 0 4,639.1 4,674.9 Subtotal asset-backed securities 0 10,860.0 0 10,860.0 11,071.6 Redeemable preferred stocks: Financials 0 47.2 0 47.2 51.4 Utilities 0 9.2 0 9.2 10.0 Industrials 7.3 61.1 0 68.4 90.1 Subtotal redeemable preferred stocks 7.3 117.5 0 124.8 151.5 Total fixed maturities 11,503.0 22,773.6 0 34,276.6 33,761.1 Short-term investments 2,438.4 85.8 0 2,524.2 2,524.2 Total available-for-sale securities 13,941.4 22,859.4 0 36,800.8 36,285.3 Equity securities: Nonredeemable preferred stocks: Financials 105.5 752.3 13.1 870.9 937.4 Utilities 0 32.0 0 32.0 40.0 Industrials 0 15.3 15.2 30.5 40.1 Subtotal nonredeemable preferred stocks 105.5 799.6 28.3 933.4 1,017.5 Common equities: Common stocks 2,607.8 0 0 2,607.8 1,112.9 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,607.8 0 0.3 2,608.1 1,113.2 Total equity securities 2,713.3 799.6 28.6 3,541.5 2,130.7 Total portfolio $ 16,654.7 $ 23,659.0 $ 28.6 $ 40,342.3 $ 38,416.0 Debt $ 0 $ 6,108.6 $ 0 $ 6,108.6 $ 5,394.0 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2020 Fixed maturities: U.S. government obligations $ 12,740.0 $ 0 $ 0 $ 12,740.0 $ 12,437.9 State and local government obligations 0 3,221.8 0 3,221.8 3,099.4 Corporate debt securities 0 10,185.2 0 10,185.2 9,579.7 Subtotal 12,740.0 13,407.0 0 26,147.0 25,117.0 Asset-backed securities: Residential mortgage-backed 0 509.5 0 509.5 503.3 Commercial mortgage-backed 0 6,175.1 0 6,175.1 6,042.6 Other asset-backed 0 3,784.6 0 3,784.6 3,745.0 Subtotal asset-backed securities 0 10,469.2 0 10,469.2 10,290.9 Redeemable preferred stocks: Financials 0 51.6 0 51.6 51.1 Utilities 0 11.7 0 11.7 10.0 Industrials 10.8 120.6 0 131.4 120.1 Subtotal redeemable preferred stocks 10.8 183.9 0 194.7 181.2 Total fixed maturities 12,750.8 24,060.1 0 36,810.9 35,589.1 Short-term investments 4,704.9 513.6 0 5,218.5 5,218.5 Total available-for-sale securities 17,455.7 24,573.7 0 42,029.4 40,807.6 Equity securities: Nonredeemable preferred stocks: Financials 117.7 1,212.3 35.0 1,365.0 1,278.6 Utilities 0 41.9 0 41.9 40.0 Industrials 0 24.3 16.7 41.0 40.1 Subtotal nonredeemable preferred stocks 117.7 1,278.5 51.7 1,447.9 1,358.7 Common equities: Common stocks 4,049.9 0 0 4,049.9 1,184.2 Other risk investments 0 0 3.1 3.1 3.1 Subtotal common equities 4,049.9 0 3.1 4,053.0 1,187.3 Total equity securities 4,167.6 1,278.5 54.8 5,500.9 2,546.0 Total portfolio $ 21,623.3 $ 25,852.2 $ 54.8 $ 47,530.3 $ 43,353.6 Debt $ 0 $ 6,793.5 $ 0 $ 6,793.5 $ 5,396.1
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, 2021 and 2020: Level 3 Fair Value (millions) Fair Value at December 31, 2020 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2021 Equity securities: Nonredeemable preferred stocks: Financials $ 35.0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 35.0 Industrials 16.7 0 0 0 0 (0.1) 0 16.6 Common equities: Other risk investments 3.1 0 0.1 0 0 0 0 3.2 Total Level 3 securities $ 54.8 $ 0 $ 0.1 $ 0 $ 0 $ (0.1) $ 0 $ 54.8 Level 3 Fair Value (millions) Fair Value at December 31, 2019 Calls/ Purchases Sales Net Realized Change in Net Fair Value at March 31, 2020 Equity securities: Nonredeemable preferred stocks: Financials $ 27.1 $ 0 $ 0 $ 0 $ 0 $ (14.0) $ 0 $ 13.1 Industrials 16.0 0 0 0 0 $ (0.8) 0 15.2 Common equities: Other risk investments 0.3 0 0 0 0 0 0 0.3 Total Level 3 securities $ 43.4 $ 0 $ 0 $ 0 $ 0 $ (14.8) $ 0 $ 28.6
Fair Value Measurement Inputs and Valuation Techniques	The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2021 and 2020, and December 31, 2020: Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2021 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials 1 $ 25.0 Internal price Unadjusted purchase price per share 3.7 Financials 2 10.0 Internal price Unadjusted purchase price per share 16.9 Industrials 6.9 Pricing firm Market approach (guideline public company method) 5.4 Industrials 9.7 Pricing firm Market approach (guideline public company method) 4.8 Subtotal Level 3 securities 51.6 Pricing exemption securities 3.2 Total Level 3 securities $ 54.8 1 This security was purchased in May 2020. 2 This security was purchased in November 2020. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials $ 13.1 Internal price Market approach (guideline public company method) 4.4 Industrials 1 7.1 Internal price Market approach (guideline public company method) 5.6 Industrials 1 8.1 Internal price Market approach (guideline public company method) 4.0 Subtotal Level 3 securities 28.3 Pricing exemption securities 0.3 Total Level 3 securities $ 28.6 1 Modified prior year disclosure to present securities individually to conform to the current year presentation. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at December 31, 2020 Valuation Technique Unobservable Input Unobservable Input Assumption Equity securities: Nonredeemable preferred stocks: Financials 1 $ 25.0 Internal price Unadjusted purchase price per share 3.7 Financials 2 10.0 Internal price Unadjusted purchase price per share 16.9 Industrials 6.9 Pricing firm Market approach (guideline public company method) 5.4 Industrials 9.8 Pricing firm Market approach (guideline public company method) 4.8 Subtotal Level 3 securities 51.7 Pricing exemption securities 3.1 Total Level 3 securities $ 54.8 1 This security was purchased in May 2020. 2 This security was purchased in November 2020.

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Debt Table	Debt at each of the balance sheet periods consisted of: March 31, 2021 March 31, 2020 December 31, 2020 (millions) Carrying Fair Carrying Fair Carrying Fair 3.75% Senior Notes due 2021 (issued: $500.0, August 2011) $ 499.9 $ 506.7 $ 499.5 $ 510.7 $ 499.8 $ 510.9 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) 497.4 526.8 497.0 488.2 497.3 541.1 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 297.0 386.1 296.7 395.3 296.9 409.4 4.00% Senior Notes due 2029 (issued: $550.0, October 2018) 545.6 623.7 545.1 605.7 545.5 660.4 3.20% Senior Notes due 2030 (issued: $500.0, March 2020) 496.2 536.7 495.8 543.2 496.1 575.5 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 396.0 531.4 395.8 539.6 396.0 582.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 346.8 413.9 346.7 391.7 346.7 459.7 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.6 430.2 395.4 406.2 395.5 481.0 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 841.7 977.1 841.6 991.9 841.7 1,113.1 4.20% Senior Notes due 2048 (issued: $600.0, March 2018) 590.0 698.7 589.9 675.8 590.0 806.7 3.95% Senior Notes due 2050 (issued: $500.0, March 2020) 490.6 563.9 490.5 560.3 490.6 653.7 Total $ 5,396.8 $ 6,195.2 $ 5,394.0 $ 6,108.6 $ 5,396.1 $ 6,793.5

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 2021
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) 2021 2020 Balance at January 1 $ 20,265.8 $ 18,105.4 Less reinsurance recoverables on unpaid losses 3,798.2 3,212.2 Net balance at January 1 16,467.6 14,893.2 Incurred related to: Current year 6,986.1 6,011.1 Prior years 124.4 144.1 Total incurred 7,110.5 6,155.2 Paid related to: Current year 3,029.3 2,916.4 Prior years 3,453.7 3,269.2 Total paid 6,483.0 6,185.6 Net balance at March 31 17,095.1 14,862.8 Plus reinsurance recoverables on unpaid losses 3,968.6 3,443.7 Balance at March 31 $ 21,063.7 $ 18,306.5

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2021
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Three Months Ended March 31, (millions) 2021 2020 Income taxes $ 0.8 $ 0 Interest 74.3 56.4 Operating lease liabilities 24.5 22.7

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2021
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, 2021 2020 (millions) Revenues Pretax Revenues Pretax Personal Lines Agency $ 4,098.2 $ 547.5 $ 3,828.7 $ 601.6 Direct 4,431.7 414.6 3,992.4 473.0 Total Personal Lines 1 8,529.9 962.1 7,821.1 1,074.6 Commercial Lines 1,417.8 228.5 1,189.0 112.5 Property 2 472.5 (70.7) 420.6 49.2 Total underwriting operations 10,420.2 1,119.9 9,430.7 1,236.3 Fees and other revenues 3 165.7 NA 153.5 NA Service businesses 53.8 4.5 51.6 4.1 Investments 4 805.5 799.9 (312.4) (317.7) Interest expense NA (56.4) NA (48.0) Consolidated total $ 11,445.2 $ 1,867.9 $ 9,323.4 $ 874.7 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three months ended March 31, 2021 and 2020; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three months ended March 31, 2021 and 2020, pretax profit (loss) includes $14.2 million and $14.5 million, respectively, of amortization expense predominately associated with intangible assets. See Note 12 – Goodwill and Intangible Assets for further discussion. 3 Pretax profit (loss) for fees and other revenues is allocated to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense.
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, 2021 2020 Underwriting Combined Underwriting Combined Personal Lines Agency 13.4 % 86.6 15.7 % 84.3 Direct 9.4 90.6 11.9 88.1 Total Personal Lines 11.3 88.7 13.7 86.3 Commercial Lines 16.1 83.9 9.5 90.5 Property 1 (15.0) 115.0 11.7 88.3 Total underwriting operations 10.7 89.3 13.1 86.9 1 Included in the three months ended March 31, 2021 and 2020, is 3.0 points and 3.4 points, respectively, of amortization expense predominately associated with intangible assets.

Dividends (Tables)

Dividends (Tables)	3 Months Ended
	Mar. 31, 2021
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, 2021 and 2020: (millions, except per share amounts) Amount Declared Payable Per Share Accrued 1 Common - Quarterly Dividends: March 2021 April 2021 $ 0.10 $ 58.5 December 2020 January 2021 0.10 58.6 February 2020 April 2020 0.10 58.5 December 2019 January 2020 0.10 58.5 Common - Annual Variable Dividends: December 2020 January 2021 4.50 2,635.9 December 2019 January 2020 2.25 1,316.9 Preferred Dividends: December 2020 March 2021 26.875 13.4 February 2020 March 2020 26.875 13.4 1 The accrual is based on an estimate of shares outstanding as of the record date. The dividends accrued at March 31, 2020, were reclassified from accounts payable, accrued expenses, and other liabilities into dividends payable on common shares on the consolidated balance sheets, to conform to the current year presentation.

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 2021
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 gains (losses) on forecasted transactions (Income) Balance at December 31, 2020 $ 1,187.4 $ (255.7) $ 931.7 $ 947.3 $ (15.6) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities (551.4) 115.8 (435.6) (435.6) 0 0 Loss attributable to noncontrolling interest (NCI) 0 0 0 0 0 0 Total other comprehensive income (loss) before reclassifications (551.4) 115.8 (435.6) (435.6) 0 0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 131.7 (27.7) 104.0 104.0 0 0 Interest expense (0.3) 0.1 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income 131.4 (27.6) 103.8 104.0 (0.2) 0 Total other comprehensive income (loss) (682.8) 143.4 (539.4) (539.6) 0.2 0 Balance at March 31, 2021 $ 504.6 $ (112.3) $ 392.3 $ 407.7 $ (15.4) $ 0 Components of Changes in (millions) Pretax total Total tax After tax total Total net Net (Income) Balance at December 31, 2019 $ 435.7 $ (94.0) $ 341.7 $ 360.8 $ (16.4) $ (2.7) Other comprehensive income (loss) before reclassifications: Investment securities 318.2 (66.8) 251.4 251.4 0 0 Loss attributable to noncontrolling interest (NCI) (0.6) 0.1 (0.5) 0 0 (0.5) Total other comprehensive income (loss) before reclassifications 317.6 (66.7) 250.9 251.4 0 (0.5) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net realized gains (losses) on securities 238.7 (50.1) 188.6 188.6 0 0 Interest expense (0.3) 0.1 (0.2) 0 (0.2) 0 Total reclassification adjustment for amounts realized in net income 238.4 (50.0) 188.4 188.6 (0.2) 0 Total other comprehensive income (loss) 79.2 (16.7) 62.5 62.8 0.2 (0.5) Balance at March 31, 2020 $ 514.9 $ (110.7) $ 404.2 $ 423.6 $ (16.2) $ (3.2)

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Indefinite-Lived Intangible Assets	The following table is a summary of the net carrying amount of other intangible assets: (millions) March 31, 2021 March 31, 2020 December 31, 2020 Intangible assets subject to amortization $ 144.8 $ 201.4 $ 159.0 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 157.2 $ 213.8 $ 171.4
Intangible Assets Subject to Amortization	The following table is a summary of the net carrying amount of other intangible assets: (millions) March 31, 2021 March 31, 2020 December 31, 2020 Intangible assets subject to amortization $ 144.8 $ 201.4 $ 159.0 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 157.2 $ 213.8 $ 171.4 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: March 31, 2021 March 31, 2020 December 31, 2020 (millions) Gross Accumulated Net Gross Accumulated Net Gross Accumulated Net Category: Policies in force $ 256.2 $ 219.6 $ 36.6 $ 256.2 $ 183.0 $ 73.2 $ 256.2 $ 210.4 $ 45.8 Agency relationships 159.2 68.3 90.9 159.2 56.9 102.3 159.2 65.4 93.8 Software rights 69.1 51.8 17.3 79.1 53.2 25.9 69.1 49.7 19.4 Trade name 0 0 0 34.8 34.8 0 0 0 0 Total $ 484.5 $ 339.7 $ 144.8 $ 529.3 $ 327.9 $ 201.4 $ 484.5 $ 325.5 $ 159.0

Basis of Presentation - Reporti

Basis of Presentation - Reporting and Accounting Policies - Allowance for Credit Loss (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Allowance for Credit Loss [Abstract]
Premium Receivable, Allowance for Credit Loss, Beginning Balance	$ 356.2	$ 283.2	$ 283.2
Increase in allowance	60.9	170.3	472
Write-offs	(151.8)	(108.8)	(399)
Allowance for credit loss, end of period	$ 265.3	$ 344.7	$ 356.2

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Assets held-for-sale	$ 55.1	$ 56.6	$ 31.8

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 40,807.1	$ 40,807.6	$ 36,285.3
Gross Unrealized Gains	723.1	1,223.9	916.1
Gross Unrealized Losses	(199.6)	(17.3)	(377.2)
Net Holding Period Gains (Losses)	4.3	15.2	(23.4)
Total available-for-sale securities	$ 41,334.9	$ 42,029.4	$ 36,800.8
% of Total Fair Value	87.20%	88.40%	91.20%
Equity securities, at Fair Value
Cost	$ 2,658.4	$ 2,546	$ 2,130.7
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	3,407.3	2,954.9	1,410.8
Fair Value	$ 6,065.7	$ 5,500.9	$ 3,541.5
% of Total Fair Value	12.80%	11.60%	8.80%
Debt and Equity Securities, at Fair Value
Cost	$ 43,465.5	$ 43,353.6	$ 38,416
Gross Unrealized Gains	723.1	1,223.9	916.1
Gross Unrealized Losses	(199.6)	(17.3)	(377.2)
Net Holding Period Gains (Losses)	3,411.6	2,970.1	1,387.4
Fair Value	$ 47,400.6	$ 47,530.3	$ 40,342.3
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 363.1	$ 95.5	$ 598
Total fair value of non-insurance subsidiary portfolio	3,700	6,200	2,600
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	1,453.9	1,358.7	1,017.5
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	53.3	89.2	(84.1)
Fair Value	$ 1,507.2	$ 1,447.9	$ 933.4
% of Total Fair Value	3.20%	3.10%	2.30%
Common equities
Equity securities, at Fair Value
Cost	$ 1,204.5	$ 1,187.3	$ 1,113.2
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Holding Period Gains (Losses)	3,354	2,865.7	1,494.9
Fair Value	$ 4,558.5	$ 4,053	$ 2,608.1
% of Total Fair Value	9.60%	8.50%	6.50%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 38,564	$ 35,589.1	$ 33,761.1
Gross Unrealized Gains	723.1	1,223.9	916.1
Gross Unrealized Losses	(199.6)	(17.3)	(377.2)
Net Holding Period Gains (Losses)	4.3	15.2	(23.4)
Total available-for-sale securities	$ 39,091.8	$ 36,810.9	$ 34,276.6
% of Total Fair Value	82.50%	77.40%	84.90%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 16,002.7	$ 12,437.9	$ 10,841.2
Gross Unrealized Gains	181.7	305.8	654.5
Gross Unrealized Losses	(111)	(3.7)	0
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 16,073.4	$ 12,740	$ 11,495.7
% of Total Fair Value	33.90%	26.80%	28.50%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,574.8	$ 3,099.4	$ 2,243.9
Gross Unrealized Gains	65.3	123.1	64.8
Gross Unrealized Losses	(16)	(0.7)	(4.5)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 2,624.1	$ 3,221.8	$ 2,304.2
% of Total Fair Value	5.60%	6.80%	5.70%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 10,095.2	$ 9,579.7	$ 9,452.9
Gross Unrealized Gains	368.1	601.7	160.8
Gross Unrealized Losses	(24)	(0.1)	(120.1)
Net Holding Period Gains (Losses)	(3.1)	3.9	(1.7)
Total available-for-sale securities	$ 10,436.2	$ 10,185.2	$ 9,491.9
% of Total Fair Value	22.00%	21.40%	23.50%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 548.3	$ 503.3	$ 574.3
Gross Unrealized Gains	5.7	7.1	1.5
Gross Unrealized Losses	(0.8)	(0.9)	(19.2)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 553.2	$ 509.5	$ 556.6
% of Total Fair Value	1.20%	1.10%	1.40%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 5,868.3	$ 6,042.6	$ 5,822.4
Gross Unrealized Gains	68.8	142.5	30.2
Gross Unrealized Losses	(45.1)	(10)	(188.3)
Net Holding Period Gains (Losses)	0	0	0
Total available-for-sale securities	$ 5,892	$ 6,175.1	$ 5,664.3
% of Total Fair Value	12.40%	13.00%	14.00%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 3,293.5	$ 3,745	$ 4,674.9
Gross Unrealized Gains	30.6	40.1	4.3
Gross Unrealized Losses	(0.7)	(0.5)	(40)
Net Holding Period Gains (Losses)	0	0	(0.1)
Total available-for-sale securities	$ 3,323.4	$ 3,784.6	$ 4,639.1
% of Total Fair Value	7.00%	7.90%	11.50%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 181.2	$ 181.2	$ 151.5
Gross Unrealized Gains	2.9	3.6	0
Gross Unrealized Losses	(2)	(1.4)	(5.1)
Net Holding Period Gains (Losses)	7.4	11.3	(21.6)
Total available-for-sale securities	$ 189.5	$ 194.7	$ 124.8
% of Total Fair Value	0.40%	0.40%	0.30%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,243.1	$ 5,218.5	$ 2,524.2
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	$ 2,243.1	$ 5,218.5	$ 2,524.2
% of Total Fair Value	4.70%	11.00%	6.30%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Hybrid Instruments [Line Items]
Hybrid securities	$ 489.4	$ 354.6	$ 190.5
State and local government obligations
Hybrid Instruments [Line Items]
Hybrid securities	0	0	3.4
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	295	188.4	116.5
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	35	0	0
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	31.9	34.8	2.2
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 127.5	$ 131.4	$ 68.4

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) - Total fixed maturities $ in Millions	Mar. 31, 2021USD ($)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Amortized Cost [Abstract]
Less than one year	$ 4,918.5
One to five years	24,370.8
Five to ten years	9,214.5
Ten years or greater	60.2
Cost	38,564
Fair Value
Less than one year	4,940.6
One to five years	24,814.6
Five to ten years	9,277.3
Ten years or greater	59.3
Fair Value	$ 39,091.8

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Mar. 31, 2021USD ($)security	Dec. 31, 2020USD ($)security	Mar. 31, 2020USD ($)security
Number of Securities
Total | security	367	133	817
Less than 12 Months | security	322	64	749
12 Months or Greater | security	45	69	68
Fair Value
Total	$ 14,361.5	$ 2,981.5	$ 13,422.7
Less than 12 Months	13,708.9	2,017.1	12,907.2
12 Months or Greater	652.6	964.4	515.5
Unrealized Losses
Total	(199.6)	(17.3)	(377.2)
Less than 12 Months	(192)	(4.9)	(353)
12 Months or Greater	$ (7.6)	$ (12.4)	$ (24.2)
U.S. government obligations
Number of Securities
Total | security	36	9	1
Less than 12 Months | security	35	9	1
12 Months or Greater | security	1	0	0
Fair Value
Total	$ 8,612.2	$ 1,511	$ 100.3
Less than 12 Months	8,598.6	1,511	100.3
12 Months or Greater	13.6	0	0
Unrealized Losses
Total	(111)	(3.7)	0
Less than 12 Months	(109.7)	(3.7)	0
12 Months or Greater	$ (1.3)	$ 0	$ 0
State and local government obligations
Number of Securities
Total | security	92	30	89
Less than 12 Months | security	92	30	78
12 Months or Greater | security	0	0	11
Fair Value
Total	$ 816	$ 208.7	$ 528.4
Less than 12 Months	816	208.7	507.7
12 Months or Greater	0	0	20.7
Unrealized Losses
Total	(16)	(0.7)	(4.5)
Less than 12 Months	(16)	(0.7)	(4.4)
12 Months or Greater	$ 0	$ 0	$ (0.1)
Corporate debt securities
Number of Securities
Total | security	79	7	233
Less than 12 Months | security	76	7	233
12 Months or Greater | security	3	0	0
Fair Value
Total	$ 1,751.4	$ 129.4	$ 4,205.6
Less than 12 Months	1,698.4	129.4	4,205.6
12 Months or Greater	53	0	0
Unrealized Losses
Total	(24)	(0.1)	(120.1)
Less than 12 Months	(23.9)	(0.1)	(120.1)
12 Months or Greater	$ (0.1)	$ 0	$ 0
Residential mortgage-backed securities
Number of Securities
Total | security	21	21	70
Less than 12 Months | security	6	0	46
12 Months or Greater | security	15	21	24
Fair Value
Total	$ 123.4	$ 44.4	$ 532.5
Less than 12 Months	94.4	0	425
12 Months or Greater	29	44.4	107.5
Unrealized Losses
Total	(0.8)	(0.9)	(19.2)
Less than 12 Months	(0.2)	0	(11.9)
12 Months or Greater	$ (0.6)	$ (0.9)	$ (7.3)
Commercial mortgage-backed securities
Number of Securities
Total | security	95	43	204
Less than 12 Months | security	77	9	187
12 Months or Greater | security	18	34	17
Fair Value
Total	$ 2,542.2	$ 893.3	$ 4,441.5
Less than 12 Months	2,023.9	93.6	4,109.2
12 Months or Greater	518.3	799.7	332.3
Unrealized Losses
Total	(45.1)	(10)	(188.3)
Less than 12 Months	(41.7)	(0.3)	(173.8)
12 Months or Greater	$ (3.4)	$ (9.7)	$ (14.5)
Other asset-backed securities
Number of Securities
Total | security	43	22	216
Less than 12 Months | security	36	9	201
12 Months or Greater | security	7	13	15
Fair Value
Total	$ 505.8	$ 183.7	$ 3,558.1
Less than 12 Months	477.6	74.4	3,514.2
12 Months or Greater	28.2	109.3	43.9
Unrealized Losses
Total	(0.7)	(0.5)	(40)
Less than 12 Months	(0.5)	(0.1)	(39)
12 Months or Greater	$ (0.2)	$ (0.4)	$ (1)
Redeemable preferred stocks
Number of Securities
Total | security	1	1	4
Less than 12 Months | security	0	0	3
12 Months or Greater | security	1	1	1
Fair Value
Total	$ 10.5	$ 11	$ 56.3
Less than 12 Months	0	0	45.2
12 Months or Greater	10.5	11	11.1
Unrealized Losses
Total	(2)	(1.4)	(5.1)
Less than 12 Months	0	0	(3.8)
12 Months or Greater	$ (2)	$ (1.4)	$ (1.3)

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	$ 148	$ 291.6
Equity Securities, FV-NI, Gross Realized Gains on Sales	18.4	85.6
Debt and Equity Securities, Gross Realized Gains on Sales	166.4	377.2
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(22.6)	(3.7)
Equity Securities, FV-NI, Gross Realized Losses on Sales	0	(58.3)
Debt and Equity Securities, Gross Realized Losses on Sales	(22.6)	(62)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	125.4	287.9
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	18.4	27.3
Gross Realized Gain (Loss), excluding Holding Period and OTTI	143.8	315.2
Unrealized Gain (Loss) on Hybrid Instrument, Net	(10.9)	(31.2)
Net holding period gains (losses), equity securities	452.4	(837.6)
Net Holding Period Realized Gain (Loss)	441.5	(868.8)
Total net realized gains (losses) on securities	585.3	(553.6)
U.S. government obligations
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	66.6	251.1
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(19.6)	(2.7)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	47	248.4
State and local government obligations
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	30.2	2
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.2)	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	30	2
Corporate debt securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	20.7	32.4
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(2.1)	(1)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	18.6	31.4
Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	29.8	6.1
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.6)	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	29.2	6.1
Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0.7	0
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.1)	0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0.6	0
Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Equity Securities, FV-NI, Gross Realized Gains on Sales	17.3	19.6
Equity Securities, FV-NI, Gross Realized Losses on Sales	0	(4.4)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	17.3	15.2
Common equities
Gain (Loss) on Securities [Line Items]
Equity Securities, FV-NI, Gross Realized Gains on Sales	1.1	66
Equity Securities, FV-NI, Gross Realized Losses on Sales	0	(53.9)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	$ 1.1	$ 12.1

Investments Components of Equit

Investments Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Realized Investment Gains (Losses)	$ 470.8	$ (810.3)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	18.4	27.3
Equity Securities, FV-NI, Unrealized Gain (Loss)	$ 452.4	$ (837.6)

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Net Investment Income [Line Items]
Gross investment income	$ 220.2	$ 241.2
Investment expenses	(5.6)	(5.3)
Net Investment Income	214.6	235.9
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	188.1	212.8
Total equity securities
Net Investment Income [Line Items]
Gross investment income	32.1	28.4
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	17.9	13.8
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	14.2	14.6
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	186.6	205.8
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	30.4	62.6
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	13.4	9.6
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	85.2	58
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	3.4	4.2
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	35.8	34.3
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	15.9	29.2
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	2.5	7.9
Short-term investments
Net Investment Income [Line Items]
Gross investment income	$ 1.5	$ 7

Investments - Additional Inform

Investments - Additional Information (Detail)	1 Months Ended	3 Months Ended
	Mar. 31, 2020USD ($)	Mar. 31, 2021USD ($)security	Dec. 31, 2020USD ($)
Schedule of Debt and Equity Securities [Line Items]
Principal amount of bonds and certificates of deposit		$ 296,100,000
Securities Exceeding 10% of Shareholders' Equity	$ 0	0	$ 0
Non-Income Producing Securities		0
Open reverse repurchase commitments	0	0	0
Securities Sold under Agreements to Repurchase	0	$ 0	0
Debt Securities, Available-for-sale, Unrealized Loss Position, Positions Downgraded | security		3
Combined fair value of securities		$ 31,900,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss, Downgrades in Period		700,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not Previously Recorded		0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0	$ 0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Writeoff		$ 0
Percent Change in Investment Income		9.00%
Percent Change in Book Yield		22.00%
Proceeds from Debt, Net of Issuance Costs	$ 1,000,000,000

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 47,400.6	$ 47,530.3	$ 40,342.3
Fair Value	6,195.2	6,793.5	6,108.6
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	41,334.9	42,029.4	36,800.8
Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	39,091.8	36,810.9	34,276.6
Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	29,133.7	26,147	23,291.8
Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,073.4	12,740	11,495.7
Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,624.1	3,221.8	2,304.2
Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,436.2	10,185.2	9,491.9
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,768.6	10,469.2	10,860
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	553.2	509.5	556.6
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,892	6,175.1	5,664.3
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,323.4	3,784.6	4,639.1
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	189.5	194.7	124.8
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	50.6	51.6	47.2
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	11.4	11.7	9.2
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	127.5	131.4	68.4
Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,243.1	5,218.5	2,524.2
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,065.7	5,500.9	3,541.5
Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,507.2	1,447.9	933.4
Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,423.5	1,365	870.9
Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	41.9	41.9	32
Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	41.8	41	30.5
Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,558.5	4,053	2,608.1
Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,555.3	4,049.9	2,607.8
Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3.2	3.1	0.3
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	22,980.9	21,623.3	16,654.7
Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,311.8	17,455.7	13,941.4
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,084	12,750.8	11,503
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,073.4	12,740	11,495.7
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,073.4	12,740	11,495.7
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10.6	10.8	7.3
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10.6	10.8	7.3
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,227.8	4,704.9	2,438.4
Fair Value, Inputs, Level 1 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,669.1	4,167.6	2,713.3
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	113.8	117.7	105.5
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	113.8	117.7	105.5
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,555.3	4,049.9	2,607.8
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,555.3	4,049.9	2,607.8
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	24,364.9	25,852.2	23,659
Fair Value	6,195.2	6,793.5	6,108.6
Fair Value, Inputs, Level 2 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	23,023.1	24,573.7	22,859.4
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	23,007.8	24,060.1	22,773.6
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	13,060.3	13,407	11,796.1
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,624.1	3,221.8	2,304.2
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,436.2	10,185.2	9,491.9
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,768.6	10,469.2	10,860
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	553.2	509.5	556.6
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,892	6,175.1	5,664.3
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,323.4	3,784.6	4,639.1
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	178.9	183.9	117.5
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	50.6	51.6	47.2
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	11.4	11.7	9.2
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	116.9	120.6	61.1
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	15.3	513.6	85.8
Fair Value, Inputs, Level 2 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,341.8	1,278.5	799.6
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,341.8	1,278.5	799.6
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,274.7	1,212.3	752.3
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	41.9	41.9	32
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	25.2	24.3	15.3
Fair Value, Inputs, Level 2 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	54.8	54.8	28.6
Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	54.8	54.8	28.6
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	51.6	51.7	28.3
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	35	35	13.1
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16.6	16.7	15.2
Fair Value, Inputs, Level 3 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3.2	3.1	0.3
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3.2	3.1	0.3
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	43,465.5	43,353.6	38,416
Long-term Debt	5,396.8	5,396.1	5,394
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	40,807.1	40,807.6	36,285.3
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	38,564	35,589.1	33,761.1
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	28,672.7	25,117	22,538
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,002.7	12,437.9	10,841.2
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,574.8	3,099.4	2,243.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,095.2	9,579.7	9,452.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,710.1	10,290.9	11,071.6
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	548.3	503.3	574.3
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,868.3	6,042.6	5,822.4
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,293.5	3,745	4,674.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	181.2	181.2	151.5
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	51	51.1	51.4
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10	10	10
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	120.2	120.1	90.1
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,243.1	5,218.5	2,524.2
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,658.4	2,546	2,130.7
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,453.9	1,358.7	1,017.5
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,373.9	1,278.6	937.4
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	39.9	40	40
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	40.1	40.1	40.1
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,204.5	1,187.3	1,113.2
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,201.3	1,184.2	1,112.9
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 3.2	$ 3.1	$ 0.3

Fair Value Summary of Changes i

Fair Value Summary of Changes in Fair Value Associated with Level 3 Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	$ 54.8	$ 43.4
Calls/ Maturities/ Paydowns	0	0
Purchases	0.1	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	(0.1)	(14.8)
Net Transfers In (Out)	0	0
Fair value, ending balance	54.8	28.6
Total equity securities | Financials | Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	35	27.1
Calls/ Maturities/ Paydowns	0	0
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	(14)
Net Transfers In (Out)	0	0
Fair value, ending balance	35	13.1
Total equity securities | Industrials | Nonredeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	16.7	16
Calls/ Maturities/ Paydowns	0	0
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	(0.1)	(0.8)
Net Transfers In (Out)	0	0
Fair value, ending balance	16.6	15.2
Total equity securities | Other risk investments | Common equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value, beginning balance	3.1	0.3
Calls/ Maturities/ Paydowns	0	0
Purchases	0.1	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	$ 3.2	$ 0.3

Fair Value Summary of Quantitat

Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 47,400.6	$ 40,342.3	$ 47,530.3
Fair Value, Inputs, Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	54.8	28.6	54.8
Fair Value, Inputs, Level 3 | Internal Pricing | Nonredeemable preferred stocks | Financials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 25	$ 13.1	$ 25
Fair Value Inputs, Original Issue Price	$ 3.7		$ 3.7
Fair Value Inputs, Guideline Public Company Method		$ 4.4
Fair Value, Inputs, Level 3 | Internal Pricing | Nonredeemable preferred stocks | Industrials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value		$ 7.1
Fair Value Inputs, Guideline Public Company Method		$ 5.6
Fair Value, Inputs, Level 3 | Unadjusted Purchase Price | Nonredeemable preferred stocks | Financials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 10		$ 10
Fair Value Inputs, Original Issue Price	$ 16.9		$ 16.9
Fair Value, Inputs, Level 3 | Third Party Pricing Firm | Nonredeemable preferred stocks | Industrials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 6.9		$ 6.9
Fair Value Inputs, Guideline Public Company Method	$ 5.4		$ 5.4
Fair Value, Inputs, Level 3 | Guideline Public Company Method [Member] | Nonredeemable preferred stocks | Industrials
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 9.7	$ 8.1	$ 9.8
Fair Value Inputs, Guideline Public Company Method	$ 4.8	$ 4	$ 4.8
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 51.6	$ 28.3	$ 51.7
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Investments, Fair Value	$ 3.2	$ 0.3	$ 3.1

Fair Value - Additional Informa

Fair Value - Additional Information (Detail)	Mar. 31, 2021USD ($)security	Dec. 31, 2020USD ($)security	Mar. 31, 2020USD ($)security
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 47,400,600,000	$ 47,530,300,000	$ 40,342,300,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	78.00%	76.00%	82.00%
Investments, Fair Value	$ 22,980,900,000	$ 21,623,300,000	$ 16,654,700,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	99.00%	99.00%	98.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	1.00%	1.00%	2.00%
Investments, Fair Value	$ 24,364,900,000	$ 25,852,200,000	$ 23,659,000,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	54,800,000	54,800,000	28,600,000
Total fixed maturities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 0	$ 0	$ 0
Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities | security	4	4	3
Common equities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities | security	2	2	1

Debt (Detail)

Debt (Detail) - USD ($)		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Debt Instrument [Line Items]
Fair Value		$ 6,195,200,000	$ 6,793,500,000	$ 6,108,600,000
Debt, Long-term and Short-term, Combined Amount	[1]	5,396,800,000	5,396,100,000	5,394,000,000
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Long-term Debt		499,900,000	499,800,000	499,500,000
Fair Value		$ 506,700,000	510,900,000	510,700,000
Debt, stated interest rate		3.75%
Debt, face amount		$ 500,000,000	500,000,000	500,000,000
2.45% Senior Notes due 2027 (issued: $500.0, August 2016)
Debt Instrument [Line Items]
Long-term Debt		497,400,000	497,300,000	497,000,000
Fair Value		$ 526,800,000	541,100,000	488,200,000
Debt, stated interest rate		2.45%
Debt, face amount		$ 500,000,000	500,000,000	500,000,000
6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999)
Debt Instrument [Line Items]
Long-term Debt		297,000,000	296,900,000	296,700,000
Fair Value		$ 386,100,000	409,400,000	395,300,000
Debt, stated interest rate		6.625%
Debt, face amount		$ 300,000,000	300,000,000	300,000,000
4.00% Senior Notes due 2029 (issued: $550.0, October 2018)
Debt Instrument [Line Items]
Long-term Debt		545,600,000	545,500,000	545,100,000
Fair Value		$ 623,700,000	660,400,000	605,700,000
Debt, stated interest rate		4.00%
Debt, face amount		$ 550,000,000	550,000,000	550,000,000
Senior Notes Three Point Two Zero Percent Due Twenty Thirty [Member]
Debt Instrument [Line Items]
Long-term Debt		496,200,000	496,100,000	495,800,000
Fair Value		$ 536,700,000	575,500,000	543,200,000
Debt, stated interest rate		3.20%
Debt, face amount		$ 500,000,000	500,000,000	500,000,000
6.25% Senior Notes due 2032 (issued: $400.0, November 2002)
Debt Instrument [Line Items]
Long-term Debt		396,000,000	396,000,000	395,800,000
Fair Value		$ 531,400,000	582,000,000	539,600,000
Debt, stated interest rate		6.25%
Debt, face amount		$ 400,000,000	400,000,000	400,000,000
4.35% Senior Notes due 2044 (issued: $350.0, April 2014)
Debt Instrument [Line Items]
Long-term Debt		346,800,000	346,700,000	346,700,000
Fair Value		$ 413,900,000	459,700,000	391,700,000
Debt, stated interest rate		4.35%
Debt, face amount		$ 350,000,000	350,000,000	350,000,000
3.70% Senior Notes due 2045 (issued: $400.0, January 2015)
Debt Instrument [Line Items]
Long-term Debt		395,600,000	395,500,000	395,400,000
Fair Value		$ 430,200,000	481,000,000	406,200,000
Debt, stated interest rate		3.70%
Debt, face amount		$ 400,000,000	400,000,000	400,000,000
4.125% Senior Notes due 2047 (issued: $850.0, April 2017)
Debt Instrument [Line Items]
Long-term Debt		841,700,000	841,700,000	841,600,000
Fair Value		$ 977,100,000	1,113,100,000	991,900,000
Debt, stated interest rate		4.125%
Debt, face amount		$ 850,000,000	850,000,000	850,000,000
4.20% Senior Notes due 2048 (issued: $600.0, March 2018)
Debt Instrument [Line Items]
Long-term Debt		590,000,000	590,000,000	589,900,000
Fair Value		$ 698,700,000	806,700,000	675,800,000
Debt, stated interest rate		4.20%
Debt, face amount		$ 600,000,000	600,000,000	600,000,000
Senior Notes Three Point Nine Five Percent Due Twenty Fifty [Member]
Debt Instrument [Line Items]
Long-term Debt		490,600,000	490,600,000	490,500,000
Fair Value		$ 563,900,000	653,700,000	560,300,000
Debt, stated interest rate		3.95%
Debt, face amount		$ 500,000,000	$ 500,000,000	$ 500,000,000
[1]	Consists of both short-term and 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, 2021	Mar. 31, 2020	Dec. 31, 2020
Debt Instrument [Line Items]
Long-Term Debt, Maturity, Remainder of Fiscal Year		$ 0
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Debt, stated interest rate	3.75%
Debt, face amount	$ 500,000,000	500,000,000	$ 500,000,000
Long-Term Debt, Maturity, Remainder of Fiscal Year	500,000,000		500,000,000
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	250,000,000
Borrowings outstanding	$ 0	$ 0	$ 0

Income Taxes Additional Informa

Income Taxes Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Income Taxes [Line Items]
Valuation allowance	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent	20.80%	20.10%

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 21,063.7	$ 18,306.5	$ 20,265.8	$ 18,105.4
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	3,968.6	3,443.7	3,798.2	3,212.2
Liability for Unpaid Claims and Claims Adjustment Expense, Net	17,095.1	14,862.8	$ 16,467.6	$ 14,893.2
Current Year Claims and Claims Adjustment Expense	6,986.1	6,011.1
Prior Year Claims and Claims Adjustment Expense	124.4	144.1
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	7,110.5	6,155.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	3,029.3	2,916.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	3,453.7	3,269.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	$ 6,483	$ 6,185.6
Short-Duration Insurance Contract, Accident Year 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense	34.00%
Short-Duration Insurance Contract, Accident Year 2019
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense	34.00%	52.00%
Short-duration Insurance Contracts, Accident Year 2018
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense		36.00%
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 92	$ 104
Personal Lines | Special Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(5)	(14)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	29	$ 54
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 8
Agency | Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Percent of Prior Year Claims and Claims Adjustment Expense	60.00%

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Open reverse repurchase commitments	$ 0	$ 0	$ 0
Dividends Payable	58,500,000	58,500,000	2,694,500,000
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	6,200,000	9,700,000
Operating Lease, Payments	24,500,000	22,700,000
Income Taxes Paid	800,000	0
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	74,300,000	56,400,000
Cash and Cash Equivalents | ARX Holding Corp.
Open reverse repurchase commitments	154,000,000	80,600,000	93,500,000
Common equities | Quarterly dividend [Member]
Dividends Payable	$ 58,500,000	$ 58,500,000	$ 58,600,000	$ 58,500,000

Segment Information Operating R

Segment Information Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues	$ 11,445.2	$ 9,323.4
Pretax Profit (Loss)	1,867.9	874.7
Amortization of intangible assets	14.2	14.5
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	4,098.2	3,828.7
Pretax Profit (Loss)	547.5	601.6
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	4,431.7	3,992.4
Pretax Profit (Loss)	414.6	473
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	10,420.2	9,430.7
Pretax Profit (Loss)	1,119.9	1,236.3
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	8,529.9	7,821.1
Pretax Profit (Loss)	$ 962.1	$ 1,074.6
Personal auto insurance percentage of the total personal lines segment net premiums earned	94.00%	94.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 1,417.8	$ 1,189
Pretax Profit (Loss)	228.5	112.5
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	472.5	420.6
Pretax Profit (Loss)	(70.7)	49.2
Amortization of intangible assets	14.2	14.5
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	165.7	153.5
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	53.8	51.6
Pretax Profit (Loss)	4.5	4.1
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	805.5	(312.4)
Pretax Profit (Loss)	799.9	(317.7)
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (56.4)	$ (48)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	13.40%	15.70%
Combined Ratio	86.60%	84.30%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	9.40%	11.90%
Combined Ratio	90.60%	88.10%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	10.70%	13.10%
Combined Ratio	89.30%	86.90%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	11.30%	13.70%
Combined Ratio	88.70%	86.30%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	16.10%	9.50%
Combined Ratio	83.90%	90.50%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(15.00%)	11.70%
Combined Ratio	115.00%	88.30%
Combined ratio amortization points	0.030	0.034

Dividends (Details)

Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended					3 Months Ended
	Mar. 31, 2021	Jan. 31, 2021	Apr. 30, 2020	Mar. 31, 2020	Jan. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Feb. 29, 2020	Dec. 31, 2019
Common Stock, Dividends, Per Share, Declared						$ 0.10	$ 0.10
Dividends Payable	$ 58.5			$ 58.5		$ 58.5	$ 58.5	$ 2,694.5
Quarterly dividend [Member]
Common Stock, Dividends, Per Share, Declared	$ 0.10
Common Stock, Dividends, Per Share, Cash Paid		$ 0.10	$ 0.10		$ 0.10
Annual variable dividend
Common Stock, Dividends, Per Share, Cash Paid		$ 4.50			$ 2.25
Series B Preferred Stock
Preferred Stock, Dividends, Per Share, Cash Paid	$ 26.875			$ 26.875
Dividends Payable								13.4	$ 13.4
Common equities | Quarterly dividend [Member]
Dividends Payable	$ 58.5			$ 58.5		$ 58.5	$ 58.5	58.6		$ 58.5
Common equities | Annual variable dividend
Dividends Payable								$ 2,635.9		$ 1,316.9

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, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 1,187.4	$ 435.7
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment and Tax	(551.4)	318.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	0	(0.6)
OCI, before Reclassifications, before Tax, Attributable to Parent	(551.4)	317.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	131.7	238.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.3)	(0.3)
Total reclassification adjustment for amounts realized in net income, pre tax	131.4	238.4
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(682.8)	79.2
AOCI before Tax, Attributable to Parent, ending balance	504.6	514.9
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(255.7)	(94)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	115.8	(66.8)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	0	0.1
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax	115.8	(66.7)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(27.7)	(50.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.1	0.1
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(27.6)	(50)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	143.4	(16.7)
AOCI Tax, Attributable to Parent, ending balance	(112.3)	(110.7)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	931.7	341.7
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(435.6)	251.4
Other comprehensive income attributable to NCI	0	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(435.6)	250.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	104	188.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	(0.2)
Total reclassification adjustment for amounts realized in net income, after tax	103.8	188.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(539.4)	62.5
After tax total accumulated other comprehensive income, ending balance	392.3	404.2
Total net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	947.3	360.8
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(435.6)	251.4
Other comprehensive income attributable to NCI	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(435.6)	251.4
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	104	188.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0
Total reclassification adjustment for amounts realized in net income, after tax	104	188.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(539.6)	62.8
After tax total accumulated other comprehensive income, ending balance	407.7	423.6
Net unrealized gains (losses) on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(15.6)	(16.4)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0	0
Other comprehensive income attributable to NCI	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	(0.2)
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	(0.2)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.2	0.2
After tax total accumulated other comprehensive income, ending balance	(15.4)	(16.2)
(Income) loss attributable to NCI
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	0	(2.7)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	0	0
Other comprehensive income attributable to NCI	0	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	(0.5)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0
Total reclassification adjustment for amounts realized in net income, after tax	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0	(0.5)
After tax total accumulated other comprehensive income, ending balance	$ 0	$ (3.2)

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, 2021USD ($)
Statement of Comprehensive Income [Abstract]
Net unrealized losses on forecasted transactions, expected to reclassify into income within the next 12 months	$ 0.7

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill, Acquired During Period	$ 0
Goodwill, Impaired, Accumulated Impairment Loss	0
Amortization of intangible assets	$ 14,200,000	$ 14,500,000

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Schedule of Other Intangible Assets [Line Items]
Indefinite lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	157.2	171.4	213.8
Accumulated amortization	340.3	326.1	328.5
Licensing Agreements
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	0.6	0.6	0.6
Finite-Lived Intangible Assets
Schedule of Other Intangible Assets [Line Items]
Intangible assets subject to amortization	144.8	159	201.4
Accumulated amortization	$ 339.7	$ 325.5	$ 327.9

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 340.3	$ 326.1	$ 328.5
Policies in force
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	219.6	210.4	183
Net Carrying Amount	36.6	45.8	73.2
Agency relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	68.3	65.4	56.9
Net Carrying Amount	90.9	93.8	102.3
Software rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	69.1	69.1	79.1
Accumulated Amortization	51.8	49.7	53.2
Net Carrying Amount	17.3	19.4	25.9
Trade name
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	0	0	34.8
Accumulated Amortization	0	0	34.8
Net Carrying Amount	0	0	0
Finite-Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	484.5	484.5	529.3
Accumulated Amortization	339.7	325.5	327.9
Net Carrying Amount	$ 144.8	$ 159	$ 201.4