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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($)
$ in Millions
Sep. 30, 2025
Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months $ 2,144.2 $ 4,076.1
Unrealized losses, Less than 12 months (21.8) (122.9)
Estimated fair value, 12 months or longer 2,551.5 2,098.9
Unrealized losses, 12 months or longer (270.5) (361.7)
Estimated fair value, Total 4,695.7 6,175.0
Unrealized losses, Total (292.3) (484.6)
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 79.3 81.8
Unrealized losses, Less than 12 months (0.4) (1.8)
Estimated fair value, 12 months or longer 36.1 52.9
Unrealized losses, 12 months or longer (1.5) (3.4)
Estimated fair value, Total 115.4 134.7
Unrealized losses, Total (1.9) (5.2)
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 185.7 248.3
Unrealized losses, Less than 12 months (4.8) (4.4)
Estimated fair value, 12 months or longer 380.7 417.0
Unrealized losses, 12 months or longer (54.9) (69.3)
Estimated fair value, Total 566.4 665.3
Unrealized losses, Total (59.7) (73.7)
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 11.8 29.8
Unrealized losses, Less than 12 months 0.0 (0.2)
Estimated fair value, 12 months or longer 60.1 72.7
Unrealized losses, 12 months or longer (4.7) (6.9)
Estimated fair value, Total 71.9 102.5
Unrealized losses, Total (4.7) (7.1)
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 144.7 144.4
Unrealized losses, Less than 12 months (1.3) (5.3)
Estimated fair value, 12 months or longer 29.4 37.9
Unrealized losses, 12 months or longer (6.3) (8.7)
Estimated fair value, Total 174.1 182.3
Unrealized losses, Total (7.6) (14.0)
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 1,533.5 2,977.2
Unrealized losses, Less than 12 months (13.7) (98.4)
Estimated fair value, 12 months or longer 1,866.0 1,290.4
Unrealized losses, 12 months or longer (184.5) (245.4)
Estimated fair value, Total 3,399.5 4,267.6
Unrealized losses, Total (198.2) (343.8)
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 144.4 435.2
Unrealized losses, Less than 12 months (1.4) (9.6)
Estimated fair value, 12 months or longer 107.8 117.5
Unrealized losses, 12 months or longer (13.6) (18.8)
Estimated fair value, Total 252.2 552.7
Unrealized losses, Total (15.0) (28.4)
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 44.8 159.4
Unrealized losses, Less than 12 months (0.2) (3.2)
Estimated fair value, 12 months or longer 71.4 110.5
Unrealized losses, 12 months or longer (5.0) (9.2)
Estimated fair value, Total 116.2 269.9
Unrealized losses, Total $ (5.2) $ (12.4)