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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 8,414.7 $ 9,169.6
Debt securities, Estimated fair value 7,227.2 8,169.6
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 6.4  
Amortized cost, Due after one through five years 124.7  
Amortized cost, Due after five through ten years 0.4  
Amortized cost, Due after ten years 4.6  
Debt Securities, Amortized cost 136.1 308.5
Estimated fair value, Due in one year or less 6.3  
Estimated fair value, Due after one through five years 117.7  
Estimated fair value, Due after five through ten years 0.3  
Estimated fair value, Due after ten years 3.8  
Debt securities, Estimated fair value 128.1 302.8
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 9.9  
Amortized cost, Due after one through five years 325.9  
Amortized cost, Due after five through ten years 527.4  
Amortized cost, Due after ten years 539.8  
Debt Securities, Amortized cost 1,403.0 1,670.6
Estimated fair value, Due in one year or less 9.8  
Estimated fair value, Due after one through five years 291.6  
Estimated fair value, Due after five through ten years 435.4  
Estimated fair value, Due after ten years 459.3  
Debt securities, Estimated fair value 1,196.1 1,478.7
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 41.6  
Amortized cost, Due after one through five years 105.5  
Amortized cost, Due after five through ten years 67.6  
Amortized cost, Due after ten years 9.3  
Debt Securities, Amortized cost 224.0 208.0
Estimated fair value, Due in one year or less 41.3  
Estimated fair value, Due after one through five years 102.8  
Estimated fair value, Due after five through ten years 55.4  
Estimated fair value, Due after ten years 7.4  
Debt securities, Estimated fair value 206.9 193.7
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 25.0  
Amortized cost, Due after one through five years 154.4  
Amortized cost, Due after five through ten years 6.7  
Amortized cost, Due after ten years 51.6  
Debt Securities, Amortized cost 237.7 247.9
Estimated fair value, Due in one year or less 24.9  
Estimated fair value, Due after one through five years 147.0  
Estimated fair value, Due after five through ten years 5.9  
Estimated fair value, Due after ten years 39.7  
Debt securities, Estimated fair value 217.5 228.6
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 10.6  
Amortized cost, Due after one through five years 690.9  
Amortized cost, Due after five through ten years 199.2  
Amortized cost, Due after ten years 64.3  
Debt Securities, Amortized cost 965.0 1,004.4
Estimated fair value, Due in one year or less 10.3  
Estimated fair value, Due after one through five years 634.5  
Estimated fair value, Due after five through ten years 185.4  
Estimated fair value, Due after ten years 49.9  
Debt securities, Estimated fair value 880.1 921.4
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 13.2  
Amortized cost, Due after one through five years 297.9  
Amortized cost, Due after five through ten years 115.8  
Amortized cost, Due after ten years 34.9  
Debt Securities, Amortized cost 461.8 476.8
Estimated fair value, Due in one year or less 12.9  
Estimated fair value, Due after one through five years 278.2  
Estimated fair value, Due after five through ten years 107.8  
Estimated fair value, Due after ten years 29.2  
Debt securities, Estimated fair value 428.1 $ 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 106.7  
Amortized cost, Due after one through five years 1,699.3  
Amortized cost, Due after five through ten years 917.1  
Amortized cost, Due after ten years 704.5  
Debt Securities, Amortized cost 3,427.6  
Estimated fair value, Due in one year or less 105.5  
Estimated fair value, Due after one through five years 1,571.8  
Estimated fair value, Due after five through ten years 790.2  
Estimated fair value, Due after ten years 589.3  
Debt securities, Estimated fair value 3,056.8  
Collateralized Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 4,987.1  
Estimated fair value $ 4,170.4