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Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 9,762.6 $ 8,466.7
Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 9,079.4  
Percentage of debt securities by credit rating agencies 93.00%  
Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 510.2  
Percentage of debt securities by credit rating agencies 5.20%  
Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 173.0  
Percentage of debt securities by credit rating agencies 1.80%  
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 247.5 243.6
U.S. Treasury bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 247.5  
Percentage of debt securities by credit rating agencies 100.00%  
U.S. Treasury bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.0  
Percentage of debt securities by credit rating agencies 0.00%  
U.S. Treasury bonds | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.0  
Percentage of debt securities by credit rating agencies 0.00%  
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 1,029.2 960.1
Municipal bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 1,014.1  
Percentage of debt securities by credit rating agencies 98.60%  
Municipal bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 14.8  
Percentage of debt securities by credit rating agencies 1.40%  
Municipal bonds | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.3  
Percentage of debt securities by credit rating agencies 0.00%  
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 236.3 235.2
Foreign government bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 228.1  
Percentage of debt securities by credit rating agencies 96.50%  
Foreign government bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 7.5  
Percentage of debt securities by credit rating agencies 3.20%  
Foreign government bonds | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.7  
Percentage of debt securities by credit rating agencies 0.30%  
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 444.0 261.0
Governmental agency bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 444.0  
Percentage of debt securities by credit rating agencies 100.00%  
Governmental agency bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.0  
Percentage of debt securities by credit rating agencies 0.00%  
Governmental agency bonds | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.0  
Percentage of debt securities by credit rating agencies 0.00%  
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 6,094.2 5,260.8
Governmental agency mortgage-backed securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 6,094.2  
Percentage of debt securities by credit rating agencies 100.00%  
Governmental agency mortgage-backed securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.0  
Percentage of debt securities by credit rating agencies 0.00%  
Governmental agency mortgage-backed securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.0  
Percentage of debt securities by credit rating agencies 0.00%  
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 1,234.3 1,034.8
U.S. corporate debt securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 786.8  
Percentage of debt securities by credit rating agencies 63.70%  
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 302.4  
Percentage of debt securities by credit rating agencies 24.50%  
U.S. corporate debt securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 145.1  
Percentage of debt securities by credit rating agencies 11.80%  
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 477.1 $ 471.2
Foreign corporate debt securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 264.7  
Percentage of debt securities by credit rating agencies 55.50%  
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 185.5  
Percentage of debt securities by credit rating agencies 38.90%  
Foreign corporate debt securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 26.9  
Percentage of debt securities by credit rating agencies 5.60%