Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale debt securities
Available-for-sale debt securities, Cost	$ 21,465
Available-for-sale debt securities, Estimated Fair Value	21,550	$ 8,328
Equity securities
Equity securities, Cost	56	55
Equity securities, Unrealized Gains	27	25
Equity securities, Unrealized Losses	(2)	(2)
Equity securities	81	78
Total Cost	21,521	8,339
Total Unrealized Gains	112	71
Total Unrealized Losses	(2)	(4)
Total Estimated Fair Value	21,631	8,406
State and municipal obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	172	236
Available-for-sale debt securities, Unrealized Gains	7	8
Available-for-sale debt securities, Unrealized Losses	0	(1)
Available-for-sale debt securities, Estimated Fair Value	179	243
US Government agencies obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	7	9
Available-for-sale debt securities, Unrealized Gains	0	0
Available-for-sale debt securities, Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	7	9
US government treasury obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	20,655	7,395
Available-for-sale debt securities, Unrealized Gains	76	35
Available-for-sale debt securities, Unrealized Losses	0	(1)
Available-for-sale debt securities, Estimated Fair Value	20,731	7,429
Corporate debt securities [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	22	27
Available-for-sale debt securities, Unrealized Gains	0	0
Available-for-sale debt securities, Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	22	27
Mortgage-backed securities [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	28	39
Available-for-sale debt securities, Unrealized Gains	2	2
Available-for-sale debt securities, Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	30	41
Foreign government bonds and obligations [Member]
Available-for-sale debt securities
Available-for-sale debt securities, Cost	581	578
Available-for-sale debt securities, Unrealized Gains	0	1
Available-for-sale debt securities, Unrealized Losses	0	0
Available-for-sale debt securities, Estimated Fair Value	$ 581	$ 579

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