Investment Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Accrued interest available-for-sale debt securities	$ 53	$ 26
Available-for-sale debt securities:
Cost	21,150
Estimated Fair Value	21,220
Equity securities
Cost	52	56
Gross Unrealized Gains	1	27
Gross Unrealized Losses	(3)	(2)
Estimated Fair Value	50	81
Total Cost	21,202	21,521
Total Gross Unrealized Gains	72	112
Total Gross Unrealized Losses	(4)	(2)
Total Estimated Fair Value	21,270	21,631
State and municipal obligations
Available-for-sale debt securities:
Cost	145	172
Gross Unrealized Gains	5	7
Gross Unrealized Losses	(1)	0
Estimated Fair Value	149	179
U.S. Government agency obligations
Available-for-sale debt securities:
Cost	7	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	7	7
U.S. Government treasury obligations
Available-for-sale debt securities:
Cost	20,356	20,655
Gross Unrealized Gains	64	76
Gross Unrealized Losses	0	0
Estimated Fair Value	20,420	20,731
Mortgage-backed securities
Available-for-sale debt securities:
Cost	25	28
Gross Unrealized Gains	2	2
Gross Unrealized Losses	0	0
Estimated Fair Value	27	30
Foreign government bonds and obligations
Available-for-sale debt securities:
Cost	575	581
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	575	581
Other Debt Obligations
Available-for-sale debt securities:
Cost	42	22
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 42	$ 22