Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 6,572	$ 5,739
Unrealized losses, Less than 12 months	(409)	(80)
Estimated fair value, 12 months or longer	638	33
Unrealized losses, 12 months or longer	(71)	(2)
Estimated fair value, Total	7,210	5,772
Unrealized losses, Total	(480)	(82)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	76	76
Unrealized losses, Less than 12 months	(5)	(1)
Estimated fair value, 12 months or longer	11	0
Unrealized losses, 12 months or longer	(1)	0
Estimated fair value, Total	87	76
Unrealized losses, Total	(6)	(1)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	961	684
Unrealized losses, Less than 12 months	(78)	(17)
Estimated fair value, 12 months or longer	63	0
Unrealized losses, 12 months or longer	(9)	0
Estimated fair value, Total	1,024	684
Unrealized losses, Total	(87)	(17)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	174	103
Unrealized losses, Less than 12 months	(7)	(1)
Estimated fair value, 12 months or longer	39	33
Unrealized losses, 12 months or longer	(5)	(2)
Estimated fair value, Total	213	136
Unrealized losses, Total	(12)	(3)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	97	73
Unrealized losses, Less than 12 months	(5)	(1)
Estimated fair value, 12 months or longer	4	0
Unrealized losses, 12 months or longer	(1)	0
Estimated fair value, Total	101	73
Unrealized losses, Total	(6)	(1)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	4,071	4,036
Unrealized losses, Less than 12 months	(248)	(47)
Estimated fair value, 12 months or longer	469	0
Unrealized losses, 12 months or longer	(50)	0
Estimated fair value, Total	4,540	4,036
Unrealized losses, Total	(298)	(47)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	818	533
Unrealized losses, Less than 12 months	(46)	(9)
Estimated fair value, 12 months or longer	33	0
Unrealized losses, 12 months or longer	(3)	0
Estimated fair value, Total	851	533
Unrealized losses, Total	(49)	(9)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	375	234
Unrealized losses, Less than 12 months	(20)	(4)
Estimated fair value, 12 months or longer	19	0
Unrealized losses, 12 months or longer	(2)	0
Estimated fair value, Total	394	234
Unrealized losses, Total	$ (22)	$ (4)