v3.26.1
Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months $ 5,285.7 $ 1,657.6
Unrealized losses, Less than 12 months (78.5) (14.0)
Estimated fair value, 12 months or longer 1,922.5 2,225.6
Unrealized losses, 12 months or longer (247.7) (255.4)
Estimated fair value, Total 7,208.2 3,883.2
Unrealized losses, Total (326.2) (269.4)
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 158.9 61.7
Unrealized losses, Less than 12 months (2.2) (0.3)
Estimated fair value, 12 months or longer 30.5 36.3
Unrealized losses, 12 months or longer (1.0) (1.3)
Estimated fair value, Total 189.4 98.0
Unrealized losses, Total (3.2) (1.6)
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 296.7 118.6
Unrealized losses, Less than 12 months (3.9) (1.1)
Estimated fair value, 12 months or longer 430.3 451.3
Unrealized losses, 12 months or longer (51.0) (52.2)
Estimated fair value, Total 727.0 569.9
Unrealized losses, Total (54.9) (53.3)
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 40.7 76.2
Unrealized losses, Less than 12 months (0.1) (0.3)
Estimated fair value, 12 months or longer 38.6 59.8
Unrealized losses, 12 months or longer (3.3) (5.0)
Estimated fair value, Total 79.3 136.0
Unrealized losses, Total (3.4) (5.3)
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 327.1 210.6
Unrealized losses, Less than 12 months (4.0) (1.5)
Estimated fair value, 12 months or longer 27.3 28.9
Unrealized losses, 12 months or longer (5.4) (6.1)
Estimated fair value, Total 354.4 239.5
Unrealized losses, Total (9.4) (7.6)
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 3,497.8 988.8
Unrealized losses, Less than 12 months (56.9) (9.8)
Estimated fair value, 12 months or longer 1,274.7 1,479.1
Unrealized losses, 12 months or longer (170.4) (173.2)
Estimated fair value, Total 4,772.5 2,467.9
Unrealized losses, Total (227.3) (183.0)
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 747.5 169.4
Unrealized losses, Less than 12 months (9.2) (0.8)
Estimated fair value, 12 months or longer 79.4 101.2
Unrealized losses, 12 months or longer (12.2) (12.9)
Estimated fair value, Total 826.9 270.6
Unrealized losses, Total (21.4) (13.7)
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 217.0 32.3
Unrealized losses, Less than 12 months (2.2) (0.2)
Estimated fair value, 12 months or longer 41.7 69.0
Unrealized losses, 12 months or longer (4.4) (4.7)
Estimated fair value, Total 258.7 101.3
Unrealized losses, Total $ (6.6) $ (4.9)