v3.26.3
Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 10,535.2 $ 8,656.7
Debt Securities, Gross unrealized gains 3.5 79.4
Debt Securities, Gross unrealized losses (776.1) (269.4)
Debt securities, Estimated fair value 9,762.6 8,466.7
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 257.0 244.2
Debt Securities, Gross unrealized gains 0.0 1.0
Debt Securities, Gross unrealized losses (9.5) (1.6)
Debt securities, Estimated fair value 247.5 243.6
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 1,146.3 1,003.5
Debt Securities, Gross unrealized gains 0.6 9.9
Debt Securities, Gross unrealized losses (117.7) (53.3)
Debt securities, Estimated fair value 1,029.2 960.1
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 241.8 239.4
Debt Securities, Gross unrealized gains 0.3 1.1
Debt Securities, Gross unrealized losses (5.8) (5.3)
Debt securities, Estimated fair value 236.3 235.2
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 474.3 268.3
Debt Securities, Gross unrealized gains 0.0 0.3
Debt Securities, Gross unrealized losses (30.3) (7.6)
Debt securities, Estimated fair value 444.0 261.0
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 6,623.2 5,401.9
Debt Securities, Gross unrealized gains 0.6 41.9
Debt Securities, Gross unrealized losses (529.6) (183.0)
Debt securities, Estimated fair value 6,094.2 5,260.8
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 1,299.1 1,032.8
Debt Securities, Gross unrealized gains 0.8 15.7
Debt Securities, Gross unrealized losses (65.6) (13.7)
Debt securities, Estimated fair value 1,234.3 1,034.8
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 493.5 466.6
Debt Securities, Gross unrealized gains 1.2 9.5
Debt Securities, Gross unrealized losses (17.6) (4.9)
Debt securities, Estimated fair value $ 477.1 $ 471.2