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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Mar. 31, 2023
Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 8,478.2 $ 9,169.6
Debt securities, Estimated fair value 7,624.7 8,169.6
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 144.4  
Amortized cost, Due after one through five years 70.8  
Amortized cost, Due after five through ten years 4.5  
Amortized cost, Due after ten years 6.4  
Debt Securities, Amortized cost 226.1 308.5
Estimated fair value, Due in one year or less 144.3  
Estimated fair value, Due after one through five years 66.6  
Estimated fair value, Due after five through ten years 4.0  
Estimated fair value, Due after ten years 6.3  
Debt securities, Estimated fair value 221.2 302.8
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 10.7  
Amortized cost, Due after one through five years 257.0  
Amortized cost, Due after five through ten years 564.4  
Amortized cost, Due after ten years 565.0  
Debt Securities, Amortized cost 1,397.1 1,670.6
Estimated fair value, Due in one year or less 10.7  
Estimated fair value, Due after one through five years 237.2  
Estimated fair value, Due after five through ten years 494.2  
Estimated fair value, Due after ten years 507.0  
Debt securities, Estimated fair value 1,249.1 1,478.7
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 30.8  
Amortized cost, Due after one through five years 97.8  
Amortized cost, Due after five through ten years 68.7  
Amortized cost, Due after ten years 9.3  
Debt Securities, Amortized cost 206.6 208.0
Estimated fair value, Due in one year or less 30.6  
Estimated fair value, Due after one through five years 96.7  
Estimated fair value, Due after five through ten years 60.6  
Estimated fair value, Due after ten years 8.2  
Debt securities, Estimated fair value 196.1 193.7
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 25.8  
Amortized cost, Due after one through five years 157.6  
Amortized cost, Due after five through ten years 6.7  
Amortized cost, Due after ten years 56.7  
Debt Securities, Amortized cost 246.8 247.9
Estimated fair value, Due in one year or less 25.4  
Estimated fair value, Due after one through five years 152.1  
Estimated fair value, Due after five through ten years 6.0  
Estimated fair value, Due after ten years 47.0  
Debt securities, Estimated fair value 230.5 228.6
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 9.3  
Amortized cost, Due after one through five years 646.1  
Amortized cost, Due after five through ten years 191.0  
Amortized cost, Due after ten years 59.9  
Debt Securities, Amortized cost 906.3 1,004.4
Estimated fair value, Due in one year or less 9.2  
Estimated fair value, Due after one through five years 596.6  
Estimated fair value, Due after five through ten years 182.1  
Estimated fair value, Due after ten years 50.4  
Debt securities, Estimated fair value 838.3 921.4
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 18.5  
Amortized cost, Due after one through five years 313.6  
Amortized cost, Due after five through ten years 101.3  
Amortized cost, Due after ten years 38.9  
Debt Securities, Amortized cost 472.3 476.8
Estimated fair value, Due in one year or less 18.2  
Estimated fair value, Due after one through five years 295.4  
Estimated fair value, Due after five through ten years 96.5  
Estimated fair value, Due after ten years 34.5  
Debt securities, Estimated fair value 444.6 $ 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 239.5  
Amortized cost, Due after one through five years 1,542.9  
Amortized cost, Due after five through ten years 936.6  
Amortized cost, Due after ten years 736.2  
Debt Securities, Amortized cost 3,455.2  
Estimated fair value, Due in one year or less 238.4  
Estimated fair value, Due after one through five years 1,444.6  
Estimated fair value, Due after five through ten years 843.4  
Estimated fair value, Due after ten years 653.4  
Debt securities, Estimated fair value 3,179.8  
Collateralized Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 5,023.0  
Estimated fair value $ 4,444.9