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Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($)
$ in Millions
Dec. 31, 2022
Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]    
Debt securities, amortized cost $ 9,170 $ 9,317
Debt securities, estimated fair value 8,170 9,362
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 143  
Amortized cost, Due after one through five years 141  
Amortized cost, Due after five through ten years 6  
Amortized cost, Due after ten years 19  
Debt securities, amortized cost 309 123
Estimated fair value, Due in one year or less 143  
Estimated fair value, Due after one through five years 137  
Estimated fair value, Due after five through ten years 5  
Estimated fair value, Due after ten years 18  
Debt securities, estimated fair value 303 123
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 21  
Amortized cost, Due after one through five years 288  
Amortized cost, Due after five through ten years 758  
Amortized cost, Due after ten years 604  
Debt securities, amortized cost 1,671 1,607
Estimated fair value, Due in one year or less 21  
Estimated fair value, Due after one through five years 268  
Estimated fair value, Due after five through ten years 669  
Estimated fair value, Due after ten years 521  
Debt securities, estimated fair value 1,479 1,649
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 36  
Amortized cost, Due after one through five years 95  
Amortized cost, Due after five through ten years 71  
Amortized cost, Due after ten years 6  
Debt securities, amortized cost 208 228
Estimated fair value, Due in one year or less 36  
Estimated fair value, Due after one through five years 93  
Estimated fair value, Due after five through ten years 60  
Estimated fair value, Due after ten years 5  
Debt securities, estimated fair value 194 227
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 26  
Amortized cost, Due after one through five years 157  
Amortized cost, Due after five through ten years 7  
Amortized cost, Due after ten years 58  
Debt securities, amortized cost 248 175
Estimated fair value, Due in one year or less 25  
Estimated fair value, Due after one through five years 151  
Estimated fair value, Due after five through ten years 6  
Estimated fair value, Due after ten years 47  
Debt securities, estimated fair value 229 177
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 55  
Amortized cost, Due after one through five years 687  
Amortized cost, Due after five through ten years 199  
Amortized cost, Due after ten years 63  
Debt securities, amortized cost 1,004 1,071
Estimated fair value, Due in one year or less 54  
Estimated fair value, Due after one through five years 629  
Estimated fair value, Due after five through ten years 186  
Estimated fair value, Due after ten years 52  
Debt securities, estimated fair value 921 1,081
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 20  
Amortized cost, Due after one through five years 300  
Amortized cost, Due after five through ten years 116  
Amortized cost, Due after ten years 41  
Debt securities, amortized cost 477 493
Estimated fair value, Due in one year or less 20  
Estimated fair value, Due after one through five years 280  
Estimated fair value, Due after five through ten years 107  
Estimated fair value, Due after ten years 36  
Debt securities, estimated fair value 443 $ 498
Debt Securities Excluding Mortgage Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost, Due in one year or less 301  
Amortized cost, Due after one through five years 1,668  
Amortized cost, Due after five through ten years 1,157  
Amortized cost, Due after ten years 791  
Debt securities, amortized cost 3,917  
Estimated fair value, Due in one year or less 299  
Estimated fair value, Due after one through five years 1,558  
Estimated fair value, Due after five through ten years 1,033  
Estimated fair value, Due after ten years 679  
Debt securities, estimated fair value 3,569  
Total Mortgage-Backed Securities    
Schedule Of Available For Sale Securities [Line Items]    
Amortized cost 5,253  
Estimated fair value $ 4,601