Investment Securities - Schedule of Gross Pre-Tax Unrealized Losses on Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	$ 9,143	$ 17,740
Available for sale, gross unrealized losses less than 12 months	18	110
Available for sale, fair value 12 months or longer	2,914	6,886
Available for sale, gross unrealized losses 12 months or longer	17	208
Available for sale, fair value total	12,057	24,626
Available for sale, gross unrealized losses total	35	318
Held to maturity, fair value less than 12 months	5,644	9,461
Held-to-maturity, gross unrealized losses, less than 12 months	25	155
Held to maturity, fair value 12 months or longer	8,079	23,757
Held to maturity, gross unrealized losses 12 months or longer	44	583
Held to maturity, fair value total	13,723	33,218
Held to maturity, gross unrealized losses total	69	738
US Treasury and federal agencies, direct obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	622
Available for sale, gross unrealized losses less than 12 months	7
Available for sale, fair value 12 months or longer	0
Available for sale, gross unrealized losses 12 months or longer	0
Available for sale, fair value total	622
Available for sale, gross unrealized losses total	7
Held to maturity, fair value less than 12 months	253	2,192
Held-to-maturity, gross unrealized losses, less than 12 months	0	45
Held to maturity, fair value 12 months or longer	5,645	12,403
Held to maturity, gross unrealized losses 12 months or longer	11	154
Held to maturity, fair value total	5,898	14,595
Held to maturity, gross unrealized losses total	11	199
US Treasury and federal agencies, mortgage-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	2,465	5,058
Available for sale, gross unrealized losses less than 12 months	4	21
Available for sale, fair value 12 months or longer	1,727	5,089
Available for sale, gross unrealized losses 12 months or longer	14	160
Available for sale, fair value total	4,192	10,147
Available for sale, gross unrealized losses total	18	181
Held to maturity, fair value less than 12 months	3,780	6,502
Held-to-maturity, gross unrealized losses, less than 12 months	14	103
Held to maturity, fair value 12 months or longer	1,731	10,648
Held to maturity, gross unrealized losses 12 months or longer	15	415
Held to maturity, fair value total	5,511	17,150
Held to maturity, gross unrealized losses total	29	518
Total U.S. Treasury and federal agencies
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	3,087	5,058
Available for sale, gross unrealized losses less than 12 months	11	21
Available for sale, fair value 12 months or longer	1,727	5,089
Available for sale, gross unrealized losses 12 months or longer	14	160
Available for sale, fair value total	4,814	10,147
Available for sale, gross unrealized losses total	25	181
Held to maturity, fair value less than 12 months	4,033	8,694
Held-to-maturity, gross unrealized losses, less than 12 months	14	148
Held to maturity, fair value 12 months or longer	7,376	23,051
Held to maturity, gross unrealized losses 12 months or longer	26	569
Held to maturity, fair value total	11,409	31,745
Held to maturity, gross unrealized losses total	40	717
Asset-backed securities, student loans
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	65	106
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	154	218
Available for sale, gross unrealized losses 12 months or longer	0	1
Available for sale, fair value total	219	324
Available for sale, gross unrealized losses total	0	1
Held to maturity, fair value less than 12 months	1,504	481
Held-to-maturity, gross unrealized losses, less than 12 months	10	4
Held to maturity, fair value 12 months or longer	530	536
Held to maturity, gross unrealized losses 12 months or longer	11	6
Held to maturity, fair value total	2,034	1,017
Held to maturity, gross unrealized losses total	21	10
Asset-backed securities, credit cards
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	90	90
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	0	493
Available for sale, gross unrealized losses 12 months or longer	0	26
Available for sale, fair value total	90	583
Available for sale, gross unrealized losses total	0	26
Asset-backed securities, collateralized loan obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	643	548
Available for sale, gross unrealized losses less than 12 months	1	2
Available for sale, fair value 12 months or longer	276	0
Available for sale, gross unrealized losses 12 months or longer	1	0
Available for sale, fair value total	919	548
Available for sale, gross unrealized losses total	2	2
Asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	798	744
Available for sale, gross unrealized losses less than 12 months	1	2
Available for sale, fair value 12 months or longer	430	711
Available for sale, gross unrealized losses 12 months or longer	1	27
Available for sale, fair value total	1,228	1,455
Available for sale, gross unrealized losses total	2	29
Held to maturity, fair value less than 12 months	1,504	481
Held-to-maturity, gross unrealized losses, less than 12 months	10	4
Held to maturity, fair value 12 months or longer	530	536
Held to maturity, gross unrealized losses 12 months or longer	11	6
Held to maturity, fair value total	2,034	1,017
Held to maturity, gross unrealized losses total	21	10
Non-U.S. debt securities, mortgage-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	245	1,407
Available for sale, gross unrealized losses less than 12 months	1	4
Available for sale, fair value 12 months or longer	288	118
Available for sale, gross unrealized losses 12 months or longer	1	1
Available for sale, fair value total	533	1,525
Available for sale, gross unrealized losses total	2	5
Held to maturity, fair value less than 12 months	13	184
Held-to-maturity, gross unrealized losses, less than 12 months	1	2
Held to maturity, fair value 12 months or longer	145	119
Held to maturity, gross unrealized losses 12 months or longer	6	7
Held to maturity, fair value total	158	303
Held to maturity, gross unrealized losses total	7	9
Non-U.S. debt securities, asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	279	1,479
Available for sale, gross unrealized losses less than 12 months	1	6
Available for sale, fair value 12 months or longer	217	0
Available for sale, gross unrealized losses 12 months or longer	0	0
Available for sale, fair value total	496	1,479
Available for sale, gross unrealized losses total	1	6
Non-U.S. debt securities, government securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	3,075	5,478
Available for sale, gross unrealized losses less than 12 months	1	45
Available for sale, fair value 12 months or longer	0	0
Available for sale, gross unrealized losses 12 months or longer	0	0
Available for sale, fair value total	3,075	5,478
Available for sale, gross unrealized losses total	1	45
Non-U.S. debt securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,289	2,167
Available for sale, gross unrealized losses less than 12 months	1	12
Available for sale, fair value 12 months or longer	190	226
Available for sale, gross unrealized losses 12 months or longer	0	4
Available for sale, fair value total	1,479	2,393
Available for sale, gross unrealized losses total	1	16
Total non-U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	4,888	10,531
Available for sale, gross unrealized losses less than 12 months	4	67
Available for sale, fair value 12 months or longer	695	344
Available for sale, gross unrealized losses 12 months or longer	1	5
Available for sale, fair value total	5,583	10,875
Available for sale, gross unrealized losses total	5	72
Held to maturity, fair value less than 12 months	13	184
Held-to-maturity, gross unrealized losses, less than 12 months	1	2
Held to maturity, fair value 12 months or longer	145	119
Held to maturity, gross unrealized losses 12 months or longer	6	7
Held to maturity, fair value total	158	303
Held to maturity, gross unrealized losses total	7	9
State and political subdivisions
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	199	365
Available for sale, gross unrealized losses less than 12 months	2	3
Available for sale, fair value 12 months or longer	0	244
Available for sale, gross unrealized losses 12 months or longer	0	4
Available for sale, fair value total	199	609
Available for sale, gross unrealized losses total	2	7
Collateralized mortgage obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	181
Available for sale, gross unrealized losses less than 12 months	0	3
Available for sale, fair value 12 months or longer	6	14
Available for sale, gross unrealized losses 12 months or longer	0	0
Available for sale, fair value total	6	195
Available for sale, gross unrealized losses total	0	3
Held to maturity, fair value less than 12 months	94	102
Held-to-maturity, gross unrealized losses, less than 12 months	0	1
Held to maturity, fair value 12 months or longer	28	51
Held to maturity, gross unrealized losses 12 months or longer	1	1
Held to maturity, fair value total	122	153
Held to maturity, gross unrealized losses total	1	2
Other U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	171	861
Available for sale, gross unrealized losses less than 12 months	0	14
Available for sale, fair value 12 months or longer	56	484
Available for sale, gross unrealized losses 12 months or longer	1	12
Available for sale, fair value total	227	1,345
Available for sale, gross unrealized losses total	$ 1	$ 26