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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 7,895.2 $ 9,169.6
Debt securities, Estimated fair value 7,157.5 8,169.6
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 39.1  
Amortized cost, Due after one through five years 147.0  
Amortized cost, Due after five through ten years 0.4  
Amortized cost, Due after ten years 16.8  
Debt Securities, Amortized cost 203.3 308.5
Estimated fair value, Due in one year or less 39.1  
Estimated fair value, Due after one through five years 143.3  
Estimated fair value, Due after five through ten years 0.4  
Estimated fair value, Due after ten years 16.5  
Debt securities, Estimated fair value 199.3 302.8
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 9.8  
Amortized cost, Due after one through five years 361.8  
Amortized cost, Due after five through ten years 509.9  
Amortized cost, Due after ten years 492.2  
Debt Securities, Amortized cost 1,373.7 1,670.6
Estimated fair value, Due in one year or less 9.7  
Estimated fair value, Due after one through five years 333.5  
Estimated fair value, Due after five through ten years 444.2  
Estimated fair value, Due after ten years 458.4  
Debt securities, Estimated fair value 1,245.8 1,478.7
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 51.5  
Amortized cost, Due after one through five years 97.8  
Amortized cost, Due after five through ten years 70.5  
Amortized cost, Due after ten years 8.6  
Debt Securities, Amortized cost 228.4 208.0
Estimated fair value, Due in one year or less 51.3  
Estimated fair value, Due after one through five years 98.1  
Estimated fair value, Due after five through ten years 62.4  
Estimated fair value, Due after ten years 7.5  
Debt securities, Estimated fair value 219.3 193.7
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 0.0  
Amortized cost, Due after one through five years 154.4  
Amortized cost, Due after five through ten years 6.6  
Amortized cost, Due after ten years 46.7  
Debt Securities, Amortized cost 207.7 247.9
Estimated fair value, Due in one year or less 0.0  
Estimated fair value, Due after one through five years 150.8  
Estimated fair value, Due after five through ten years 6.1  
Estimated fair value, Due after ten years 38.5  
Debt securities, Estimated fair value 195.4 228.6
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 10.5  
Amortized cost, Due after one through five years 696.3  
Amortized cost, Due after five through ten years 224.7  
Amortized cost, Due after ten years 75.5  
Debt Securities, Amortized cost 1,007.0 1,004.4
Estimated fair value, Due in one year or less 10.4  
Estimated fair value, Due after one through five years 659.5  
Estimated fair value, Due after five through ten years 220.6  
Estimated fair value, Due after ten years 67.9  
Debt securities, Estimated fair value 958.4 921.4
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 21.7  
Amortized cost, Due after one through five years 302.3  
Amortized cost, Due after five through ten years 117.8  
Amortized cost, Due after ten years 37.1  
Debt Securities, Amortized cost 478.9 476.8
Estimated fair value, Due in one year or less 21.3  
Estimated fair value, Due after one through five years 291.7  
Estimated fair value, Due after five through ten years 116.4  
Estimated fair value, Due after ten years 34.2  
Debt securities, Estimated fair value 463.6 $ 442.0
Aggregate Debt Securities Excluding Mortgage Backed And Asset Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 132.6  
Amortized cost, Due after one through five years 1,759.6  
Amortized cost, Due after five through ten years 929.9  
Amortized cost, Due after ten years 676.9  
Debt Securities, Amortized cost 3,499.0  
Estimated fair value, Due in one year or less 131.8  
Estimated fair value, Due after one through five years 1,676.9  
Estimated fair value, Due after five through ten years 850.1  
Estimated fair value, Due after ten years 623.0  
Debt securities, Estimated fair value 3,281.8  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 4,396.2  
Estimated fair value $ 3,875.7