v3.26.3
Investments - Components of Net Realized Gains (Losses) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities $ 16.0 $ 7.0 $ 203.0 $ 88.0
Gross realized gains on equity securities 29.0 12.0 75.0 53.0
Subtotal gross realized gains on security sales 45.0 19.0 278.0 141.0
Gross realized loss on available-for-sale securities (69.0) (4.0) (169.0) (97.0)
Gross realized loss on equity securities (3.0) (3.0) (24.0) (12.0)
Subtotal gross realized losses on security sales (72.0) (7.0) (193.0) (109.0)
Net realized gains (losses) on available-for-sale securities sales (52.0) 3.0 35.0 (9.0)
Net realized gains (losses) on equity securities sales 25.0 9.0 50.0 41.0
Subtotal net realized gains (losses) on security sales (27.0) 12.0 85.0 32.0
Gain 18.0 0.0 18.0 0.0
Hybrid securities (218.0) 3.0 (228.0) 17.0
Equity securities 57.0 280.0 439.0 421.0
Subtotal net holding period gains (losses) (161.0) 283.0 211.0 438.0
Total net realized gains (losses) on securities (170.0) 295.0 314.0 470.0
US Treasury Future        
Gain (Loss) on Securities [Line Items]        
Gain (Loss) on Sale of Derivatives 18.0   69.0  
Unrealized Gain (Loss) on Derivatives 199.0   198.0  
U.S. government        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 9.0 1.0 159.0 78.0
Gross realized loss on available-for-sale securities (48.0) 0.0 (133.0) (78.0)
Net realized gains (losses) on available-for-sale securities sales (38.0) 1.0 27.0 0.0
State and local government        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.0 0.0 1.0 0.0
Gross realized loss on available-for-sale securities (4.0) 0.0 (8.0) (2.0)
Net realized gains (losses) on available-for-sale securities sales (4.0) 0.0 (7.0) (2.0)
Corporate and other debt        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 7.0 6.0 42.0 9.0
Gross realized loss on available-for-sale securities (15.0) (4.0) (26.0) (7.0)
Net realized gains (losses) on available-for-sale securities sales (8.0) 2.0 16.0 2.0
Residential mortgage-backed        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.0 0.0 1.0 1.0
Gross realized loss on available-for-sale securities (1.0) 0.0 (1.0) 0.0
Net realized gains (losses) on available-for-sale securities sales (1.0) 0.0 0.0 1.0
Commercial mortgage-backed        
Gain (Loss) on Securities [Line Items]        
Gross realized loss on available-for-sale securities (1.0) 0.0 (1.0) (10.0)
Net realized gains (losses) on available-for-sale securities sales (1.0) 0.0 (1.0) (10.0)
Nonredeemable preferred stocks        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on equity securities 2.0 1.0 12.0 3.0
Gross realized loss on equity securities 0.0 (1.0) (8.0) (6.0)
Net realized gains (losses) on equity securities sales 2.0 0.0 4.0 (3.0)
Common equities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on equity securities 27.0 11.0 63.0 50.0
Gross realized loss on equity securities (3.0) (2.0) (16.0) (6.0)
Net realized gains (losses) on equity securities sales $ 23.0 $ 9.0 $ 46.0 $ 44.0