AFS and HTM Debt Securities, Gross Unrealized Losses and Fair Value - AFS Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	$ (2,537)	$ (98)
Less than 12 months, Fair value, available-for-sale	53,540	27,690
12 months or more, Gross unrealized losses, available-for-sale	(684)	(289)
12 months or more, Fair value, available-for-sale	9,659	33,223
Total, Gross unrealized losses, available-for-sale	(3,221)	(387)
Total Fair value, available for sale	63,199	60,913
Securities of U.S. Treasury and federal agencies [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(4)	0
Less than 12 months, Fair value, available-for-sale	615	0
12 months or more, Gross unrealized losses, available-for-sale	0	(1)
12 months or more, Fair value, available-for-sale	0	2,423
Total, Gross unrealized losses, available-for-sale	(4)	(1)
Total Fair value, available for sale	615	2,423
Securities of U.S. states and political subdivisions [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(678)	(10)
Less than 12 months, Fair value, available-for-sale	22,740	2,776
12 months or more, Gross unrealized losses, available-for-sale	(70)	(26)
12 months or more, Fair value, available-for-sale	1,490	2,418
Total, Gross unrealized losses, available-for-sale	(748)	(36)
Total Fair value, available for sale	24,230	5,194
Mortgage-backed securities [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(275)	(54)
Less than 12 months, Fair value, available-for-sale	4,460	17,954
12 months or more, Gross unrealized losses, available-for-sale	(37)	(117)
12 months or more, Fair value, available-for-sale	980	10,885
Total, Gross unrealized losses, available-for-sale	(312)	(171)
Total Fair value, available for sale	5,440	28,839
Federal agencies [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(3)	(50)
Less than 12 months, Fair value, available-for-sale	594	16,807
12 months or more, Gross unrealized losses, available-for-sale	(12)	(114)
12 months or more, Fair value, available-for-sale	737	10,641
Total, Gross unrealized losses, available-for-sale	(15)	(164)
Total Fair value, available for sale	1,331	27,448
Residential [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(44)	(1)
Less than 12 months, Fair value, available-for-sale	591	149
12 months or more, Gross unrealized losses, available-for-sale	0	0
12 months or more, Fair value, available-for-sale	0	0
Total, Gross unrealized losses, available-for-sale	(44)	(1)
Total Fair value, available for sale	591	149
Commercial [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(228)	(3)
Less than 12 months, Fair value, available-for-sale	3,275	998
12 months or more, Gross unrealized losses, available-for-sale	(25)	(3)
12 months or more, Fair value, available-for-sale	243	244
Total, Gross unrealized losses, available-for-sale	(253)	(6)
Total Fair value, available for sale	3,518	1,242
Corporate debt securities [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(263)	(9)
Less than 12 months, Fair value, available-for-sale	3,337	303
12 months or more, Gross unrealized losses, available-for-sale	(12)	(23)
12 months or more, Fair value, available-for-sale	129	216
Total, Gross unrealized losses, available-for-sale	(275)	(32)
Total Fair value, available for sale	3,466	519
Collateralized loan and other debt obligations [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(1,226)	(13)
Less than 12 months, Fair value, available-for-sale	18,303	5,070
12 months or more, Gross unrealized losses, available-for-sale	(542)	(110)
12 months or more, Fair value, available-for-sale	6,442	16,789
Total, Gross unrealized losses, available-for-sale	(1,768)	(123)
Total Fair value, available for sale	24,745	21,859
Other [Member]
Available-for-sale debt securities:
Less than 12 months, Gross unrealized losses, available-for-sale	(91)	(12)
Less than 12 months, Fair value, available-for-sale	4,085	1,587
12 months or more, Gross unrealized losses, available-for-sale	(23)	(12)
12 months or more, Fair value, available-for-sale	618	492
Total, Gross unrealized losses, available-for-sale	(114)	(24)
Total Fair value, available for sale	$ 4,703	$ 2,079