Investment Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	$ 56,952	$ 62,056
Available for sale, gross unrealized gains	437	427
Available for sale, gross unrealized losses	268	485
Investment securities available-for-sale	57,121	61,998
Held to maturity, amortized cost	40,458	35,169
Held to maturity, gross unrealized gains	186	160
Held to maturity, gross unrealized losses	389	335
Investment securities held-to-maturity	40,255	34,994
US Treasury and federal agencies, direct obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	222	4,265
Available for sale, gross unrealized gains	2	7
Available for sale, gross unrealized losses	1	9
Investment securities available-for-sale	223	4,263
Held to maturity, amortized cost	17,028	17,527
Held to maturity, gross unrealized gains	0	17
Held to maturity, gross unrealized losses	143	58
Investment securities held-to-maturity	16,885	17,486
US Treasury and federal agencies, mortgage-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	10,975	13,340
Available for sale, gross unrealized gains	26	76
Available for sale, gross unrealized losses	129	159
Investment securities available-for-sale	10,872	13,257
Held to maturity, amortized cost	16,651	10,334
Held to maturity, gross unrealized gains	22	20
Held to maturity, gross unrealized losses	225	221
Investment securities held-to-maturity	16,448	10,133
Total U.S. Treasury and federal agencies
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	11,197	17,605
Available for sale, gross unrealized gains	28	83
Available for sale, gross unrealized losses	130	168
Investment securities available-for-sale	11,095	17,520
Held to maturity, amortized cost	33,679	27,861
Held to maturity, gross unrealized gains	22	37
Held to maturity, gross unrealized losses	368	279
Investment securities held-to-maturity	33,333	27,619
Asset-backed securities, student loans
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	3,325	5,659
Available for sale, gross unrealized gains	37	12
Available for sale, gross unrealized losses	4	75
Investment securities available-for-sale	3,358	5,596
Held to maturity, amortized cost	3,047	2,883
Held to maturity, gross unrealized gains	32	5
Held to maturity, gross unrealized losses	9	30
Investment securities held-to-maturity	3,070	2,858
Asset-backed securities, credit cards
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,565	1,377
Available for sale, gross unrealized gains	2	0
Available for sale, gross unrealized losses	25	26
Investment securities available-for-sale	1,542	1,351
Held to maturity, amortized cost	798	897
Held to maturity, gross unrealized gains	2	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	800	899
Asset-backed securities, sub-prime
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	0	289
Available for sale, gross unrealized gains	0	1
Available for sale, gross unrealized losses	0	18
Investment securities available-for-sale	0	272
Asset-backed securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,440	895
Available for sale, gross unrealized gains	7	10
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	1,447	905
Held to maturity, amortized cost	1	35
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	1	35
Total asset-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	6,330	8,220
Available for sale, gross unrealized gains	46	23
Available for sale, gross unrealized losses	29	119
Investment securities available-for-sale	6,347	8,124
Held to maturity, amortized cost	3,846	3,815
Held to maturity, gross unrealized gains	34	7
Held to maturity, gross unrealized losses	9	30
Investment securities held-to-maturity	3,871	3,792
Non-U.S. debt securities, mortgage-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	6,664	6,506
Available for sale, gross unrealized gains	36	35
Available for sale, gross unrealized losses	5	6
Investment securities available-for-sale	6,695	6,535
Held to maturity, amortized cost	939	1,150
Held to maturity, gross unrealized gains	82	70
Held to maturity, gross unrealized losses	6	15
Investment securities held-to-maturity	1,015	1,205
Non-U.S. debt securities, asset-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,942	2,513
Available for sale, gross unrealized gains	5	4
Available for sale, gross unrealized losses	0	1
Investment securities available-for-sale	2,947	2,516
Held to maturity, amortized cost	263	531
Held to maturity, gross unrealized gains	1	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	264	531
Non-U.S. debt securities, Government securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	10,754	5,834
Available for sale, gross unrealized gains	16	8
Available for sale, gross unrealized losses	49	6
Investment securities available-for-sale	10,721	5,836
Held to maturity, amortized cost	474	286
Held to maturity, gross unrealized gains	2	3
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	476	289
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	6,076	5,587
Available for sale, gross unrealized gains	38	31
Available for sale, gross unrealized losses	6	5
Investment securities available-for-sale	6,108	5,613
Held to maturity, amortized cost	48	113
Held to maturity, gross unrealized gains	0	1
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	48	114
Total non-U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	26,436	20,440
Available for sale, gross unrealized gains	95	78
Available for sale, gross unrealized losses	60	18
Investment securities available-for-sale	26,471	20,500
Held to maturity, amortized cost	1,724	2,080
Held to maturity, gross unrealized gains	85	74
Held to maturity, gross unrealized losses	6	15
Investment securities held-to-maturity	1,803	2,139
State and political subdivisions
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	8,929	10,233
Available for sale, gross unrealized gains	245	201
Available for sale, gross unrealized losses	23	112
Investment securities available-for-sale	9,151	10,322
Collateralized mortgage obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,060	2,610
Available for sale, gross unrealized gains	3	18
Available for sale, gross unrealized losses	9	35
Investment securities available-for-sale	1,054	2,593
Held to maturity, amortized cost	1,209	1,413
Held to maturity, gross unrealized gains	45	42
Held to maturity, gross unrealized losses	6	11
Investment securities held-to-maturity	1,248	1,444
Other U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,563	2,481
Available for sale, gross unrealized gains	12	18
Available for sale, gross unrealized losses	15	30
Investment securities available-for-sale	2,560	2,469
U.S. equity securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	40	39
Available for sale, gross unrealized gains	8	6
Available for sale, gross unrealized losses	2	3
Investment securities available-for-sale	46	42
Non-U.S. equity securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	0	3
Available for sale, gross unrealized gains	0	0
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	0	3
U.S. money-market mutual funds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	397	409
Available for sale, gross unrealized gains	0	0
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	397	409
Non-U.S. money-market mutual funds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	0	16
Available for sale, gross unrealized gains	0	0
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	$ 0	16
Federal family education loan program
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Federal government credit support guarantee, percentage minimum	97.00%
Collateralized loan obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	$ 1,447	905
Non-US debt securities, covered bonds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	3,537	3,769
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	$ 1,885	$ 988