Investment Securities - Schedule of Gross Pre-Tax Unrealized Losses on Investment Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	$ 13,816	$ 16,928
Available for sale, gross unrealized losses less than 12 months	130	308
Available for sale, fair value 12 months or longer	4,684	7,800
Available for sale, gross unrealized losses 12 months or longer	73	177
Available for sale, fair value total	18,500	24,728
Available for sale, gross unrealized losses total	203	485
Held to maturity, fair value less than 12 months	17,388	17,284
Held-to-maturity, gross, less than 12 months	198	293
Held to maturity, fair value 12 months or longer	1,394	1,899
Held to maturity, gross unrealized losses 12 months or longer	33	42
Held to maturity, fair value total	18,782	19,183
Held to maturity, gross unrealized losses total	231	335
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	0	651
Available for sale, gross unrealized losses less than 12 months	0	8
Available for sale, fair value 12 months or longer	153	180
Available for sale, gross unrealized losses 12 months or longer	1	1
Available for sale, fair value total	153	831
Available for sale, gross unrealized losses total	1	9
Held to maturity, fair value less than 12 months	9,660	8,891
Held-to-maturity, gross, less than 12 months	36	57
Held to maturity, fair value 12 months or longer	77	86
Held to maturity, gross unrealized losses 12 months or longer	1	1
Held to maturity, fair value total	9,737	8,977
Held to maturity, gross unrealized losses total	37	58
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	4,382	7,072
Available for sale, gross unrealized losses less than 12 months	51	131
Available for sale, fair value 12 months or longer	1,880	1,114
Available for sale, gross unrealized losses 12 months or longer	44	28
Available for sale, fair value total	6,262	8,186
Available for sale, gross unrealized losses total	95	159
Held to maturity, fair value less than 12 months	6,939	6,838
Held-to-maturity, gross, less than 12 months	152	221
Held to maturity, fair value 12 months or longer	493	0
Held to maturity, gross unrealized losses 12 months or longer	17	0
Held to maturity, fair value total	7,432	6,838
Held to maturity, gross unrealized losses total	169	221
Asset-backed securities, student loans
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	54
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	1,191	3,745
Available for sale, gross unrealized losses 12 months or longer	8	75
Available for sale, fair value total	1,191	3,799
Available for sale, gross unrealized losses total	8	75
Held to maturity, fair value less than 12 months	544	705
Held-to-maturity, gross, less than 12 months	8	9
Held to maturity, fair value 12 months or longer	389	1,235
Held to maturity, gross unrealized losses 12 months or longer	5	21
Held to maturity, fair value total	933	1,940
Held to maturity, gross unrealized losses total	13	30
Asset-backed securities, credit cards
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	499	795
Available for sale, gross unrealized losses less than 12 months	22	1
Available for sale, fair value 12 months or longer	0	494
Available for sale, gross unrealized losses 12 months or longer	0	25
Available for sale, fair value total	499	1,289
Available for sale, gross unrealized losses total	22	26
Held to maturity, fair value less than 12 months		33
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		0
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		33
Held to maturity, gross unrealized losses total		0
Asset-backed securities, sub-prime
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		1
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		252
Available for sale, gross unrealized losses 12 months or longer		18
Available for sale, fair value total		253
Available for sale, gross unrealized losses total		18
Asset-backed securities, other
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months		75
Available for sale, gross unrealized losses less than 12 months		0
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		75
Available for sale, gross unrealized losses total		0
Held to maturity, fair value less than 12 months		18
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		9
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		27
Held to maturity, gross unrealized losses total		0
Asset-backed securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	499	925
Available for sale, gross unrealized losses less than 12 months	22	1
Available for sale, fair value 12 months or longer	1,191	4,491
Available for sale, gross unrealized losses 12 months or longer	8	118
Available for sale, fair value total	1,690	5,416
Available for sale, gross unrealized losses total	30	119
Held to maturity, fair value less than 12 months	544	756
Held-to-maturity, gross, less than 12 months	8	9
Held to maturity, fair value 12 months or longer	389	1,244
Held to maturity, gross unrealized losses 12 months or longer	5	21
Held to maturity, fair value total	933	2,000
Held to maturity, gross unrealized losses total	13	30
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	780	442
Available for sale, gross unrealized losses less than 12 months	2	1
Available for sale, fair value 12 months or longer	569	893
Available for sale, gross unrealized losses 12 months or longer	1	5
Available for sale, fair value total	1,349	1,335
Available for sale, gross unrealized losses total	3	6
Held to maturity, fair value less than 12 months	0	54
Held-to-maturity, gross, less than 12 months	0	2
Held to maturity, fair value 12 months or longer	259	330
Held to maturity, gross unrealized losses 12 months or longer	7	13
Held to maturity, fair value total	259	384
Held to maturity, gross unrealized losses total	7	15
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		253
Available for sale, gross unrealized losses less than 12 months		0
Available for sale, fair value 12 months or longer		276
Available for sale, gross unrealized losses 12 months or longer		1
Available for sale, fair value total		529
Available for sale, gross unrealized losses total		1
Held to maturity, fair value less than 12 months		28
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		35
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		63
Held to maturity, gross unrealized losses total		0
Government securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	4,965	1,314
Available for sale, gross unrealized losses less than 12 months	16	6
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	4,965	1,314
Available for sale, gross unrealized losses total	16	6
Held to maturity, fair value less than 12 months		180
Held-to-maturity, gross, less than 12 months		0
Held to maturity, fair value 12 months or longer		0
Held to maturity, gross unrealized losses 12 months or longer		0
Held to maturity, fair value total		180
Held to maturity, gross unrealized losses total		0
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	447	670
Available for sale, gross unrealized losses less than 12 months	3	4
Available for sale, fair value 12 months or longer	229	218
Available for sale, gross unrealized losses 12 months or longer	1	1
Available for sale, fair value total	676	888
Available for sale, gross unrealized losses total	4	5
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	6,192	2,679
Available for sale, gross unrealized losses less than 12 months	21	11
Available for sale, fair value 12 months or longer	798	1,387
Available for sale, gross unrealized losses 12 months or longer	2	7
Available for sale, fair value total	6,990	4,066
Available for sale, gross unrealized losses total	23	18
Held to maturity, fair value less than 12 months	0	262
Held-to-maturity, gross, less than 12 months	0	2
Held to maturity, fair value 12 months or longer	259	365
Held to maturity, gross unrealized losses 12 months or longer	7	13
Held to maturity, fair value total	259	627
Held to maturity, gross unrealized losses total	7	15
State and political subdivisions
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,052	3,390
Available for sale, gross unrealized losses less than 12 months	15	102
Available for sale, fair value 12 months or longer	525	304
Available for sale, gross unrealized losses 12 months or longer	13	10
Available for sale, fair value total	1,577	3,694
Available for sale, gross unrealized losses total	28	112
Collateralized mortgage obligations
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	550	1,259
Available for sale, gross unrealized losses less than 12 months	7	31
Available for sale, fair value 12 months or longer	56	162
Available for sale, gross unrealized losses 12 months or longer	2	4
Available for sale, fair value total	606	1,421
Available for sale, gross unrealized losses total	9	35
Held to maturity, fair value less than 12 months	245	537
Held-to-maturity, gross, less than 12 months	2	4
Held to maturity, fair value 12 months or longer	176	204
Held to maturity, gross unrealized losses 12 months or longer	3	7
Held to maturity, fair value total	421	741
Held to maturity, gross unrealized losses total	5	11
Other U.S. debt securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	1,141	944
Available for sale, gross unrealized losses less than 12 months	14	24
Available for sale, fair value 12 months or longer	75	157
Available for sale, gross unrealized losses 12 months or longer	1	6
Available for sale, fair value total	1,216	1,101
Available for sale, gross unrealized losses total	15	30
U.S. equity securities
Gross Pre-Tax Unrealized Losses On Investment Securities [Line Items]
Available for sale, fair value less than 12 months	0	8
Available for sale, gross unrealized losses less than 12 months	0	0
Available for sale, fair value 12 months or longer	6	5
Available for sale, gross unrealized losses 12 months or longer	2	3
Available for sale, fair value total	6	13
Available for sale, gross unrealized losses total	$ 2	$ 3