v3.26.3
Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months $ 7,650.8 $ 1,657.6
Unrealized losses, Less than 12 months (425.5) (14.0)
Estimated fair value, 12 months or longer 1,770.8 2,225.6
Unrealized losses, 12 months or longer (350.6) (255.4)
Estimated fair value, Total 9,421.6 3,883.2
Unrealized losses, Total (776.1) (269.4)
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 230.1 61.7
Unrealized losses, Less than 12 months (8.5) (0.3)
Estimated fair value, 12 months or longer 17.2 36.3
Unrealized losses, 12 months or longer (1.0) (1.3)
Estimated fair value, Total 247.3 98.0
Unrealized losses, Total (9.5) (1.6)
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 619.2 118.6
Unrealized losses, Less than 12 months (36.6) (1.1)
Estimated fair value, 12 months or longer 385.9 451.3
Unrealized losses, 12 months or longer (81.1) (52.2)
Estimated fair value, Total 1,005.1 569.9
Unrealized losses, Total (117.7) (53.3)
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 148.2 76.2
Unrealized losses, Less than 12 months (1.9) (0.3)
Estimated fair value, 12 months or longer 34.9 59.8
Unrealized losses, 12 months or longer (3.9) (5.0)
Estimated fair value, Total 183.1 136.0
Unrealized losses, Total (5.8) (5.3)
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 419.2 210.6
Unrealized losses, Less than 12 months (23.8) (1.5)
Estimated fair value, 12 months or longer 24.8 28.9
Unrealized losses, 12 months or longer (6.5) (6.1)
Estimated fair value, Total 444.0 239.5
Unrealized losses, Total (30.3) (7.6)
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 4,850.6 988.8
Unrealized losses, Less than 12 months (294.9) (9.8)
Estimated fair value, 12 months or longer 1,196.9 1,479.1
Unrealized losses, 12 months or longer (234.7) (173.2)
Estimated fair value, Total 6,047.5 2,467.9
Unrealized losses, Total (529.6) (183.0)
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 1,021.8 169.4
Unrealized losses, Less than 12 months (48.2) (0.8)
Estimated fair value, 12 months or longer 72.1 101.2
Unrealized losses, 12 months or longer (17.4) (12.9)
Estimated fair value, Total 1,093.9 270.6
Unrealized losses, Total (65.6) (13.7)
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 361.7 32.3
Unrealized losses, Less than 12 months (11.6) (0.2)
Estimated fair value, 12 months or longer 39.0 69.0
Unrealized losses, 12 months or longer (6.0) (4.7)
Estimated fair value, Total 400.7 101.3
Unrealized losses, Total $ (17.6) $ (4.9)