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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost $ 9,317 $ 6,121
Debt Securities, Gross unrealized gains 127 238
Debt Securities, Gross unrealized losses (82) (4)
Debt securities, Estimated fair value 9,362 6,355
U.S. Treasury bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 123 80
Debt Securities, Gross unrealized gains 1 1
Debt Securities, Gross unrealized losses (1) 0
Debt securities, Estimated fair value 123 81
Municipal bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 1,607 1,168
Debt Securities, Gross unrealized gains 59 81
Debt Securities, Gross unrealized losses (17) (1)
Debt securities, Estimated fair value 1,649 1,248
Foreign government bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 228 194
Debt Securities, Gross unrealized gains 2 6
Debt Securities, Gross unrealized losses (3) (1)
Debt securities, Estimated fair value 227 199
Governmental agency bonds    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 175 254
Debt Securities, Gross unrealized gains 3 10
Debt Securities, Gross unrealized losses (1) 0
Debt securities, Estimated fair value 177 264
Governmental agency mortgage-backed securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 5,620 3,402
Debt Securities, Gross unrealized gains 34 74
Debt Securities, Gross unrealized losses (47) (2)
Debt securities, Estimated fair value 5,607 3,474
U.S. corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 1,071 638
Debt Securities, Gross unrealized gains 19 44
Debt Securities, Gross unrealized losses (9) 0
Debt securities, Estimated fair value 1,081 682
Foreign corporate debt securities    
Schedule Of Available For Sale Securities [Line Items]    
Debt Securities, Amortized cost 493 385
Debt Securities, Gross unrealized gains 9 22
Debt Securities, Gross unrealized losses (4) 0
Debt securities, Estimated fair value $ 498 $ 407