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Investments Components of Net Realized Gains (Losses) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain $ 14.0 $ 20.4 $ 305.9
Equity Securities, FV-NI, Realized Gain 381.3 863.7 397.6
Debt and Equity Securities, Realized Gains 395.3 884.1 703.5
Debt Securities, Available-for-Sale, Realized Loss (263.1) (599.8) (67.4)
Equity Securities, FV-NI, Realized Loss (140.1) (87.8) (21.8)
Debt and Equity Securities, Realized Losses (403.2) (687.6) (89.2)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (249.1) (579.4) 238.5
Equity Securities, FV-NI, Realized Gain (Loss) 241.2 775.9 375.8
Net realized gains (losses) on security sales (7.9) 196.5 614.3
Realized Gain (Loss) on Security Sales, Other Assets 21.6 0.0 0.0
Net impairment losses recognized in earnings (9.0) (8.6) (5.0)
Realized Gains (Losses), Other Assets 12.6 (8.6) (5.0)
Unrealized Gain (Loss) on Hybrid Instrument, Net 45.4 (82.0) (7.7)
Equity Securities, FV-NI, Unrealized Gain (Loss) 303.0 (2,018.1) 907.6
Net holding period gains (losses) on securities 348.4 (2,100.1) 899.9
Total net realized gains (losses) on securities 353.1 (1,912.2) 1,509.2
U.S. government obligations      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 12.1 10.8 105.8
Debt Securities, Available-for-Sale, Realized Loss (64.6) (433.3) (52.0)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (52.5) (422.5) 53.8
State and local government obligations      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 0.0 51.2
Debt Securities, Available-for-Sale, Realized Loss 0.0 (1.0) (3.4)
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.0 (1.0) 47.8
Foreign government obligations      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Loss 0.0 0.0 (0.1)
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.0 0.0 (0.1)
Corporate debt securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 1.0 8.7 104.0
Debt Securities, Available-for-Sale, Realized Loss (85.7) (88.3) (8.3)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (84.7) (79.6) 95.7
Residential mortgage-backed securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.9 0.7 0.3
Debt Securities, Available-for-Sale, Realized Loss 0.0 0.0 (1.1)
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.9 0.7 (0.8)
Commercial mortgage-backed securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 0.0 41.9
Debt Securities, Available-for-Sale, Realized Loss (106.8) (72.4) (1.9)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (106.8) (72.4) 40.0
Other asset-backed securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 0.2 1.2
Debt Securities, Available-for-Sale, Realized Loss (5.6) (4.4) (0.6)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (5.6) (4.2) 0.6
Redeemable preferred stocks      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 0.0 1.5
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.0 0.0 1.5
Short-term investments      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Loss (0.4) (0.4) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) (0.4) (0.4) 0.0
Nonredeemable preferred stocks      
Gain (Loss) on Securities [Line Items]      
Equity Securities, FV-NI, Realized Gain 0.3 17.6 28.0
Equity Securities, FV-NI, Realized Loss (118.3) (9.8) (1.7)
Equity Securities, FV-NI, Realized Gain (Loss) (118.0) 7.8 26.3
Common equities      
Gain (Loss) on Securities [Line Items]      
Equity Securities, FV-NI, Realized Gain 381.0 846.1 369.6
Equity Securities, FV-NI, Realized Loss (21.8) (78.0) (20.1)
Equity Securities, FV-NI, Realized Gain (Loss) $ 359.2 $ 768.1 $ 349.5