v3.26.1
Debt Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 7,208.2 $ 3,883.2
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 189.4  
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 727.0  
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 79.3  
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 354.4  
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 4,772.5  
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 826.9  
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position 258.7  
Investment Grade | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 6,811.5  
Percentage of debt securities in unrealized loss position 94.50%  
Investment Grade | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 298.6  
Percentage of debt securities in unrealized loss position 4.10%  
Investment Grade | U.S. Treasury bonds | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 189.4  
Percentage of debt securities in unrealized loss position 100.00%  
Investment Grade | U.S. Treasury bonds | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.0  
Percentage of debt securities in unrealized loss position 0.00%  
Investment Grade | Municipal bonds | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 712.1  
Percentage of debt securities in unrealized loss position 98.00%  
Investment Grade | Municipal bonds | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 14.6  
Percentage of debt securities in unrealized loss position 2.00%  
Investment Grade | Foreign government bonds | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 73.8  
Percentage of debt securities in unrealized loss position 93.10%  
Investment Grade | Foreign government bonds | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 5.0  
Percentage of debt securities in unrealized loss position 6.30%  
Investment Grade | Governmental agency bonds | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 354.4  
Percentage of debt securities in unrealized loss position 100.00%  
Investment Grade | Governmental agency bonds | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.0  
Percentage of debt securities in unrealized loss position 0.00%  
Investment Grade | Governmental agency mortgage-backed securities | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 4,772.5  
Percentage of debt securities in unrealized loss position 100.00%  
Investment Grade | Governmental agency mortgage-backed securities | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.0  
Percentage of debt securities in unrealized loss position 0.00%  
Investment Grade | U.S. corporate debt securities | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 549.3  
Percentage of debt securities in unrealized loss position 66.40%  
Investment Grade | U.S. corporate debt securities | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 197.6  
Percentage of debt securities in unrealized loss position 23.90%  
Investment Grade | Foreign corporate debt securities | A- or higher    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 160.0  
Percentage of debt securities in unrealized loss position 61.80%  
Investment Grade | Foreign corporate debt securities | BBB+ to BBB-    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 81.4  
Percentage of debt securities in unrealized loss position 31.50%  
Non-Investment Grade    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 98.1  
Percentage of debt securities in unrealized loss position 1.40%  
Non-Investment Grade | U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.0  
Percentage of debt securities in unrealized loss position 0.00%  
Non-Investment Grade | Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.3  
Percentage of debt securities in unrealized loss position 0.00%  
Non-Investment Grade | Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.5  
Percentage of debt securities in unrealized loss position 0.60%  
Non-Investment Grade | Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.0  
Percentage of debt securities in unrealized loss position 0.00%  
Non-Investment Grade | Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 0.0  
Percentage of debt securities in unrealized loss position 0.00%  
Non-Investment Grade | U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 80.0  
Percentage of debt securities in unrealized loss position 9.70%  
Non-Investment Grade | Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Estimated fair value of debt securities, Unrealized loss position $ 17.3  
Percentage of debt securities in unrealized loss position 6.70%