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Investments - Components of Net Realized Gains (Losses) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities $ 45.1 $ 0.9 $ 51.5 $ 5.0
Gross realized gains on equity securities 14.1 0.3 25.3 354.5
Subtotal gross realized gains on security sales 59.2 1.2 76.8 359.5
Gross realized loss on available-for-sale securities (8.0) (59.5) (387.9) (203.1)
Gross realized loss on equity securities (19.5) (16.3) (30.1) (139.4)
Subtotal gross realized losses on security sales (27.5) (75.8) (418.0) (342.5)
Net realized gains (losses) on available-for-sale securities sales 37.1 (58.6) (336.4) (198.1)
Net realized gains (losses) on equity securities sales (5.4) (16.0) (4.8) 215.1
Subtotal net realized gains (losses) on security sales 31.7 (74.6) (341.2) 17.0
Gain 36.3 8.4 36.3 21.6
Impairment 0.0 (2.3) 0.0 (6.8)
Subtotal net realized gains (losses) on other assets 36.3 6.1 36.3 14.8
Hybrid securities 19.7 (0.3) 30.2 18.4
Equity securities 199.7 (80.2) 591.4 (0.5)
Subtotal net holding period gains (losses) 219.4 (80.5) 621.6 17.9
Total net realized gains (losses) on securities 287.4 (149.0) 316.7 49.7
U.S. government obligations        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 43.1 0.2 44.4 4.2
Gross realized loss on available-for-sale securities (1.4) (14.6) (328.6) (27.2)
Net realized gains (losses) on available-for-sale securities sales 41.7 (14.4) (284.2) (23.0)
State and local government obligations        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.0 0.0 0.3 0.0
Gross realized loss on available-for-sale securities (1.3) 0.0 (1.4) 0.0
Net realized gains (losses) on available-for-sale securities sales (1.3) 0.0 (1.1) 0.0
Corporate and other debt securities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 2.0 0.7 5.6 0.8
Gross realized loss on available-for-sale securities (5.3) (19.1) (42.8) (69.3)
Net realized gains (losses) on available-for-sale securities sales (3.3) (18.4) (37.2) (68.5)
Residential mortgage-backed securities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.0 0.0 1.1 0.0
Net realized gains (losses) on available-for-sale securities sales 0.0 0.0 1.1 0.0
Commercial mortgage-backed securities        
Gain (Loss) on Securities [Line Items]        
Gross realized loss on available-for-sale securities 0.0 (20.6) (15.0) (100.6)
Net realized gains (losses) on available-for-sale securities sales 0.0 (20.6) (15.0) (100.6)
Other asset-backed securities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.0 0.0 0.1 0.0
Gross realized loss on available-for-sale securities 0.0 (5.2) (0.1) (5.6)
Net realized gains (losses) on available-for-sale securities sales 0.0 (5.2) 0.0 (5.6)
Short-term investments        
Gain (Loss) on Securities [Line Items]        
Gross realized loss on available-for-sale securities 0.0 0.0 0.0 (0.4)
Net realized gains (losses) on available-for-sale securities sales 0.0 0.0 0.0 (0.4)
Nonredeemable preferred stocks        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on equity securities 0.0 0.0 0.0 0.3
Gross realized loss on equity securities (7.8) (7.4) (18.3) (117.6)
Net realized gains (losses) on equity securities sales (7.8) (7.4) (18.3) (117.3)
Common equities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on equity securities 14.1 0.3 25.3 354.2
Gross realized loss on equity securities (11.7) (8.9) (11.8) (21.8)
Net realized gains (losses) on equity securities sales $ 2.4 $ (8.6) $ 13.5 $ 332.4