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Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months $ 1,433.5 $ 3,865.7
Unrealized losses, Less than 12 months (64.3) (268.4)
Estimated fair value, 12 months or longer 5,595.6 3,780.1
Unrealized losses, 12 months or longer (1,125.6) (741.1)
Estimated fair value, Total 7,029.1 7,645.8
Unrealized losses, Total (1,189.9) (1,009.5)
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 72.5 108.0
Unrealized losses, Less than 12 months (2.0) (1.4)
Estimated fair value, 12 months or longer 55.5 49.5
Unrealized losses, 12 months or longer (6.0) (5.7)
Estimated fair value, Total 128.0 157.5
Unrealized losses, Total (8.0) (7.1)
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 237.3 813.4
Unrealized losses, Less than 12 months (11.8) (55.8)
Estimated fair value, 12 months or longer 926.5 540.0
Unrealized losses, 12 months or longer (196.0) (139.3)
Estimated fair value, Total 1,163.8 1,353.4
Unrealized losses, Total (207.8) (195.1)
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 103.7 142.1
Unrealized losses, Less than 12 months (1.3) (5.9)
Estimated fair value, 12 months or longer 97.3 45.7
Unrealized losses, 12 months or longer (15.8) (8.5)
Estimated fair value, Total 201.0 187.8
Unrealized losses, Total (17.1) (14.4)
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 74.6 172.7
Unrealized losses, Less than 12 months (1.7) (8.2)
Estimated fair value, 12 months or longer 142.9 55.8
Unrealized losses, 12 months or longer (18.5) (11.1)
Estimated fair value, Total 217.5 228.5
Unrealized losses, Total (20.2) (19.3)
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 641.3 1,859.6
Unrealized losses, Less than 12 months (37.5) (141.4)
Estimated fair value, 12 months or longer 3,516.5 2,626.8
Unrealized losses, 12 months or longer (779.3) (511.3)
Estimated fair value, Total 4,157.8 4,486.4
Unrealized losses, Total (816.8) (652.7)
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 196.7 528.3
Unrealized losses, Less than 12 months (6.5) (38.2)
Estimated fair value, 12 months or longer 593.0 325.2
Unrealized losses, 12 months or longer (79.0) (46.3)
Estimated fair value, Total 789.7 853.5
Unrealized losses, Total (85.5) (84.5)
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value, Less than 12 months 107.4 241.6
Unrealized losses, Less than 12 months (3.5) (17.5)
Estimated fair value, 12 months or longer 263.9 137.1
Unrealized losses, 12 months or longer (31.0) (18.9)
Estimated fair value, Total 371.3 378.7
Unrealized losses, Total $ (34.5) $ (36.4)