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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 7,774.6 $ 7,730.9
Debt securities, Estimated fair value 7,434.0 7,265.9
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 38.7  
Amortized cost, Due after one through five years 106.6  
Amortized cost, Due after five through ten years 4.7  
Amortized cost, Due after ten years 25.5  
Debt Securities, Amortized cost 175.5 180.7
Estimated fair value, Due in one year or less 38.6  
Estimated fair value, Due after one through five years 105.3  
Estimated fair value, Due after five through ten years 4.8  
Estimated fair value, Due after ten years 24.2  
Debt securities, Estimated fair value 172.9 175.6
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 13.0  
Amortized cost, Due after one through five years 85.7  
Amortized cost, Due after five through ten years 286.1  
Amortized cost, Due after ten years 479.4  
Debt Securities, Amortized cost 864.2 844.9
Estimated fair value, Due in one year or less 13.1  
Estimated fair value, Due after one through five years 83.2  
Estimated fair value, Due after five through ten years 256.2  
Estimated fair value, Due after ten years 444.0  
Debt securities, Estimated fair value 796.5 776.5
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 8.2  
Amortized cost, Due after one through five years 125.7  
Amortized cost, Due after five through ten years 78.2  
Amortized cost, Due after ten years 3.7  
Debt Securities, Amortized cost 215.8 217.1
Estimated fair value, Due in one year or less 8.2  
Estimated fair value, Due after one through five years 127.7  
Estimated fair value, Due after five through ten years 74.1  
Estimated fair value, Due after ten years 2.7  
Debt securities, Estimated fair value 212.7 211.6
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 8.6  
Amortized cost, Due after one through five years 1.4  
Amortized cost, Due after five through ten years 148.8  
Amortized cost, Due after ten years 43.4  
Debt Securities, Amortized cost 202.2 203.8
Estimated fair value, Due in one year or less 8.6  
Estimated fair value, Due after one through five years 1.4  
Estimated fair value, Due after five through ten years 147.0  
Estimated fair value, Due after ten years 35.5  
Debt securities, Estimated fair value 192.5 189.8
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 14.1  
Amortized cost, Due after one through five years 555.9  
Amortized cost, Due after five through ten years 319.5  
Amortized cost, Due after ten years 92.4  
Debt Securities, Amortized cost 981.9 948.4
Estimated fair value, Due in one year or less 14.1  
Estimated fair value, Due after one through five years 554.0  
Estimated fair value, Due after five through ten years 316.9  
Estimated fair value, Due after ten years 81.5  
Debt securities, Estimated fair value 966.5 925.6
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 24.1  
Amortized cost, Due after one through five years 308.5  
Amortized cost, Due after five through ten years 122.9  
Amortized cost, Due after ten years 31.3  
Debt Securities, Amortized cost 486.8 491.6
Estimated fair value, Due in one year or less 24.1  
Estimated fair value, Due after one through five years 309.6  
Estimated fair value, Due after five through ten years 122.0  
Estimated fair value, Due after ten years 28.4  
Debt securities, Estimated fair value 484.1 $ 484.5
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 1,183.8  
Amortized cost, Due after five through ten years 960.2  
Amortized cost, Due after ten years 675.7  
Debt Securities, Amortized cost 2,926.4  
Estimated fair value, Due in one year or less 106.7  
Estimated fair value, Due after one through five years 1,181.2  
Estimated fair value, Due after five through ten years 921.0  
Estimated fair value, Due after ten years 616.3  
Debt securities, Estimated fair value 2,825.2  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 4,848.2  
Estimated fair value $ 4,608.8