Debt Securities (Investments in Debt Securities by Contractual Maturity) (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 9,388	$ 9,317
Debt securities, Estimated fair value	8,937	9,362
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	2
Amortized cost, Due after one through five years	58
Amortized cost, Due after five through ten years	25
Amortized cost, Due after ten years	9
Debt Securities, Amortized cost	94	123
Estimated fair value, Due in one year or less	2
Estimated fair value, Due after one through five years	55
Estimated fair value, Due after five through ten years	23
Estimated fair value, Due after ten years	8
Debt securities, Estimated fair value	88	123
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	23
Amortized cost, Due after one through five years	114
Amortized cost, Due after five through ten years	817
Amortized cost, Due after ten years	642
Debt Securities, Amortized cost	1,596	1,607
Estimated fair value, Due in one year or less	23
Estimated fair value, Due after one through five years	113
Estimated fair value, Due after five through ten years	777
Estimated fair value, Due after ten years	608
Debt securities, Estimated fair value	1,521	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	111
Amortized cost, Due after five through ten years	76
Amortized cost, Due after ten years	12
Debt Securities, Amortized cost	235	228
Estimated fair value, Due in one year or less	36
Estimated fair value, Due after one through five years	108
Estimated fair value, Due after five through ten years	68
Estimated fair value, Due after ten years	11
Debt securities, Estimated fair value	223	227
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	10
Amortized cost, Due after one through five years	75
Amortized cost, Due after five through ten years	7
Amortized cost, Due after ten years	61
Debt Securities, Amortized cost	153	175
Estimated fair value, Due in one year or less	11
Estimated fair value, Due after one through five years	72
Estimated fair value, Due after five through ten years	7
Estimated fair value, Due after ten years	57
Debt securities, Estimated fair value	147	177
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	46
Amortized cost, Due after one through five years	628
Amortized cost, Due after five through ten years	372
Amortized cost, Due after ten years	95
Debt Securities, Amortized cost	1,141	1,071
Estimated fair value, Due in one year or less	46
Estimated fair value, Due after one through five years	609
Estimated fair value, Due after five through ten years	352
Estimated fair value, Due after ten years	89
Debt securities, Estimated fair value	1,096	1,081
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	24
Amortized cost, Due after one through five years	286
Amortized cost, Due after five through ten years	135
Amortized cost, Due after ten years	56
Debt Securities, Amortized cost	501	493
Estimated fair value, Due in one year or less	24
Estimated fair value, Due after one through five years	277
Estimated fair value, Due after five through ten years	127
Estimated fair value, Due after ten years	53
Debt securities, Estimated fair value	481	$ 498
Aggregate Debt Securities Excluding Mortgage Backed And Asset Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost, Due in one year or less	141
Amortized cost, Due after one through five years	1,272
Amortized cost, Due after five through ten years	1,432
Amortized cost, Due after ten years	875
Debt Securities, Amortized cost	3,720
Estimated fair value, Due in one year or less	142
Estimated fair value, Due after one through five years	1,234
Estimated fair value, Due after five through ten years	1,354
Estimated fair value, Due after ten years	826
Debt securities, Estimated fair value	3,556
Collateralized Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized cost	5,668
Estimated fair value	$ 5,381