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Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 7,015.3 $ 7,157.5
Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 6,269.1  
Percentage of debt securities by credit rating agencies 89.30%  
Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 545.7  
Percentage of debt securities by credit rating agencies 7.80%  
Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 200.5  
Percentage of debt securities by credit rating agencies 2.90%  
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 190.4 199.3
U.S. Treasury bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 190.4  
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 $ 731.4 1,245.8
Municipal bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 708.9  
Percentage of debt securities by credit rating agencies 97.00%  
Municipal bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 20.8  
Percentage of debt securities by credit rating agencies 2.80%  
Municipal bonds | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 1.7  
Percentage of debt securities by credit rating agencies 0.20%  
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 226.6 219.3
Foreign government bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 221.0  
Percentage of debt securities by credit rating agencies 97.50%  
Foreign government bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 4.8  
Percentage of debt securities by credit rating agencies 2.10%  
Foreign government bonds | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 0.8  
Percentage of debt securities by credit rating agencies 0.40%  
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 222.0 195.4
Governmental agency bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 222.0  
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 $ 4,285.9 3,875.7
Governmental agency mortgage-backed securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 4,285.9  
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 $ 864.5 958.4
U.S. corporate debt securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 384.4  
Percentage of debt securities by credit rating agencies 44.40%  
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 325.7  
Percentage of debt securities by credit rating agencies 37.70%  
U.S. corporate debt securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 154.4  
Percentage of debt securities by credit rating agencies 17.90%  
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 494.5 $ 463.6
Foreign corporate debt securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 256.5  
Percentage of debt securities by credit rating agencies 51.90%  
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 194.4  
Percentage of debt securities by credit rating agencies 39.30%  
Foreign corporate debt securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 43.6  
Percentage of debt securities by credit rating agencies 8.80%