v3.26.1
Debt Securities (Composition of Debt Securities Portfolio by Credit Rating Agencies) (Detail) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 9,774.7 $ 8,466.7
Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 9,029.9  
Percentage of debt securities by credit rating agencies 92.30%  
Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 572.7  
Percentage of debt securities by credit rating agencies 5.90%  
Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 172.1  
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 $ 218.7 243.6
U.S. Treasury bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 218.7  
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 $ 1,053.3 960.1
Municipal bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 1,032.5  
Percentage of debt securities by credit rating agencies 98.10%  
Municipal bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 20.5  
Percentage of debt securities by credit rating agencies 1.90%  
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 $ 233.3 235.2
Foreign government bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 224.4  
Percentage of debt securities by credit rating agencies 96.20%  
Foreign government bonds | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 8.1  
Percentage of debt securities by credit rating agencies 3.50%  
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 0.30%  
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 374.6 261.0
Governmental agency bonds | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 374.6  
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 $ 6,130.6 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 $ 6,130.6  
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 $ 1,269.9 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 $ 776.1  
Percentage of debt securities by credit rating agencies 61.10%  
U.S. corporate debt securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 351.4  
Percentage of debt securities by credit rating agencies 27.70%  
U.S. corporate debt securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 142.4  
Percentage of debt securities by credit rating agencies 11.20%  
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 494.3 $ 471.2
Foreign corporate debt securities | Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 273.0  
Percentage of debt securities by credit rating agencies 55.20%  
Foreign corporate debt securities | Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 192.7  
Percentage of debt securities by credit rating agencies 39.00%  
Foreign corporate debt securities | Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, Estimated fair value $ 28.6  
Percentage of debt securities by credit rating agencies 5.80%