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Investments - Components of Net Realized Gains (Losses) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities $ 3.2 $ 0.2 $ 6.4 $ 4.1
Gross realized gains on equity securities 0.0 222.1 11.2 354.2
Subtotal gross realized gains on security sales 3.2 222.3 17.6 358.3
Gross realized loss on available-for-sale securities (225.0) (76.9) (379.9) (143.6)
Gross realized loss on equity securities (4.6) (10.3) (10.6) (123.1)
Subtotal gross realized losses on security sales (229.6) (87.2) (390.5) (266.7)
Net realized gains (losses) on available-for-sale securities sales (221.8) (76.7) (373.5) (139.5)
Net realized gains (losses) on equity securities sales (4.6) 211.8 0.6 231.1
Subtotal net realized gains (losses) on security sales (226.4) 135.1 (372.9) 91.6
Gain 0.0 0.0 0.0 13.2
Impairment 0.0 (2.2) 0.0 (4.5)
Subtotal net realized gains (losses) on other assets 0.0 (2.2) 0.0 8.7
Hybrid securities 3.3 4.8 10.5 18.7
Equity securities 96.8 (10.8) 391.7 79.7
Subtotal net holding period gains (losses) 100.1 (6.0) 402.2 98.4
Total net realized gains (losses) on securities (126.3) 126.9 29.3 198.7
U.S. government obligations        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 1.3 0.2 1.3 4.0
Gross realized loss on available-for-sale securities (191.9) (1.1) (327.2) (12.6)
Net realized gains (losses) on available-for-sale securities sales (190.6) (0.9) (325.9) (8.6)
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 0.0 0.0 (0.1) 0.0
Net realized gains (losses) on available-for-sale securities sales 0.0 0.0 0.2 0.0
Corporate debt securities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.7 0.0 3.6 0.1
Gross realized loss on available-for-sale securities (21.6) (29.8) (36.4) (50.2)
Net realized gains (losses) on available-for-sale securities sales (20.9) (29.8) (32.8) (50.1)
Residential mortgage-backed securities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 1.1 0.0 1.1 0.0
Net realized gains (losses) on available-for-sale securities sales 1.1 0.0 1.1 0.0
Commercial mortgage-backed securities        
Gain (Loss) on Securities [Line Items]        
Gross realized loss on available-for-sale securities (10.7) (45.5) (15.0) (80.0)
Net realized gains (losses) on available-for-sale securities sales (10.7) (45.5) (15.0) (80.0)
Other asset-backed securities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on available-for-sale securities 0.1 0.0 0.1 0.0
Gross realized loss on available-for-sale securities 0.0 (0.2) (0.1) (0.4)
Net realized gains (losses) on available-for-sale securities sales 0.1 (0.2) 0.0 (0.4)
Redeemable preferred stocks        
Gain (Loss) on Securities [Line Items]        
Gross realized loss on available-for-sale securities     (1.1) 0.0
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment (0.8) 0.0    
Net realized gains (losses) on available-for-sale securities sales (0.8) 0.0 (1.1) 0.0
Short-term investments        
Gain (Loss) on Securities [Line Items]        
Gross realized loss on available-for-sale securities 0.0 (0.3) 0.0 (0.4)
Net realized gains (losses) on available-for-sale securities sales 0.0 (0.3) 0.0 (0.4)
Nonredeemable preferred stocks        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on equity securities 0.0 0.2 0.0 0.3
Gross realized loss on equity securities (4.6) (9.2) (10.5) (110.2)
Net realized gains (losses) on equity securities sales (4.6) (9.0) (10.5) (109.9)
Common equities        
Gain (Loss) on Securities [Line Items]        
Gross realized gains on equity securities 0.0 221.9 11.2 353.9
Gross realized loss on equity securities 0.0 (1.1) (0.1) (12.9)
Net realized gains (losses) on equity securities sales $ 0.0 $ 220.8 $ 11.1 $ 341.0