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Investments Components of Net Realized Gains (Losses) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain $ 20.4 $ 305.9 $ 900.7
Equity Securities, FV-NI, Realized Gain 863.7 397.6 113.0
Debt and Equity Securities, Realized Gains 884.1 703.5 1,013.7
Debt Securities, Available-for-Sale, Realized Loss (599.8) (67.4) (29.6)
Equity Securities, FV-NI, Realized Loss (87.8) (21.8) (69.4)
Debt and Equity Securities, Realized Losses (687.6) (89.2) (99.0)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (579.4) 238.5 871.1
Equity Securities, FV-NI, Realized Gain (Loss) 775.9 375.8 43.6
Net realized gains (losses) on security sales 196.5 614.3 914.7
Unrealized Gain (Loss) on Hybrid Instrument, Net (82.0) (7.7) 7.4
Equity Securities, FV-NI, Unrealized Gain (Loss) (2,018.1) 907.6 706.5
Derivative, Gain (Loss) on Derivative, Net 0.0 0.0 1.4
Net holding period gains (losses) on securities (2,100.1) 899.9 715.3
Net impairment losses recognized in earnings (8.6) (5.0) 0.0
Total net realized gains (losses) on securities (1,912.2) 1,509.2 1,630.0
U.S. government obligations      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 10.8 105.8 612.5
Debt Securities, Available-for-Sale, Realized Loss (433.3) (52.0) (9.6)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (422.5) 53.8 602.9
State and local government obligations      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 51.2 102.4
Debt Securities, Available-for-Sale, Realized Loss (1.0) (3.4) (0.7)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (1.0) 47.8 101.7
Foreign government obligations      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Loss 0.0 (0.1) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.0 (0.1) 0.0
Corporate debt securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 8.7 104.0 161.9
Debt Securities, Available-for-Sale, Realized Loss (88.3) (8.3) (6.5)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (79.6) 95.7 155.4
Residential mortgage-backed securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.7 0.3 0.0
Debt Securities, Available-for-Sale, Realized Loss 0.0 (1.1) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.7 (0.8) 0.0
Commercial mortgage-backed securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 41.9 23.7
Debt Securities, Available-for-Sale, Realized Loss (72.4) (1.9) (12.8)
Debt Securities, Available-for-Sale, Realized Gain (Loss) (72.4) 40.0 10.9
Other asset-backed securities      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.2 1.2 0.2
Debt Securities, Available-for-Sale, Realized Loss (4.4) (0.6) 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) (4.2) 0.6 0.2
Redeemable preferred stocks      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Gain 0.0 1.5 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) 0.0 1.5 0.0
Short-term investments      
Gain (Loss) on Securities [Line Items]      
Debt Securities, Available-for-Sale, Realized Loss (0.4) 0.0 0.0
Debt Securities, Available-for-Sale, Realized Gain (Loss) (0.4) 0.0 0.0
Nonredeemable preferred stocks      
Gain (Loss) on Securities [Line Items]      
Equity Securities, FV-NI, Realized Gain 17.6 28.0 24.4
Equity Securities, FV-NI, Realized Loss (9.8) (1.7) (8.7)
Equity Securities, FV-NI, Realized Gain (Loss) 7.8 26.3 15.7
Common equities      
Gain (Loss) on Securities [Line Items]      
Equity Securities, FV-NI, Realized Gain 846.1 369.6 88.6
Equity Securities, FV-NI, Realized Loss (78.0) (20.1) (60.7)
Equity Securities, FV-NI, Realized Gain (Loss) 768.1 349.5 27.9
Other Assets      
Gain (Loss) on Securities [Line Items]      
Net impairment losses recognized in earnings $ (8.6) $ (5.0) $ 0.0