Investment Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	$ 47,560	$ 56,952
Available for sale, gross unrealized gains	228	437
Available for sale, gross unrealized losses	440	268
Investment securities available-for-sale	47,348	57,121
Held to maturity, amortized cost	39,594	40,458
Held to maturity, gross unrealized gains	175	186
Held to maturity, gross unrealized losses	964	389
Investment securities held-to-maturity	38,805	40,255
US Treasury and federal agencies, direct obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	11	222
Available for sale, gross unrealized gains	0	2
Available for sale, gross unrealized losses	0	1
Investment securities available-for-sale	11	223
Held to maturity, amortized cost	15,992	17,028
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	292	143
Investment securities held-to-maturity	15,700	16,885
US Treasury and federal agencies, mortgage-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	16,158	10,975
Available for sale, gross unrealized gains	22	26
Available for sale, gross unrealized losses	287	129
Investment securities available-for-sale	15,893	10,872
Held to maturity, amortized cost	17,443	16,651
Held to maturity, gross unrealized gains	1	22
Held to maturity, gross unrealized losses	652	225
Investment securities held-to-maturity	16,792	16,448
Total U.S. Treasury and federal agencies
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	16,169	11,197
Available for sale, gross unrealized gains	22	28
Available for sale, gross unrealized losses	287	130
Investment securities available-for-sale	15,904	11,095
Held to maturity, amortized cost	33,435	33,679
Held to maturity, gross unrealized gains	1	22
Held to maturity, gross unrealized losses	944	368
Investment securities held-to-maturity	32,492	33,333
Asset-backed securities, student loans
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,546	3,325
Available for sale, gross unrealized gains	22	37
Available for sale, gross unrealized losses	1	4
Investment securities available-for-sale	1,567	3,358
Held to maturity, amortized cost	2,892	3,047
Held to maturity, gross unrealized gains	44	32
Held to maturity, gross unrealized losses	8	9
Investment securities held-to-maturity	2,928	3,070
Asset-backed securities, credit cards
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	639	1,565
Available for sale, gross unrealized gains	1	2
Available for sale, gross unrealized losses	23	25
Investment securities available-for-sale	617	1,542
Held to maturity, amortized cost	710	798
Held to maturity, gross unrealized gains	1	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	711	800
Asset-backed securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	848	1,440
Available for sale, gross unrealized gains	4	7
Available for sale, gross unrealized losses	1	0
Investment securities available-for-sale	851	1,447
Held to maturity, amortized cost	1	1
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	1	1
Total asset-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	3,033	6,330
Available for sale, gross unrealized gains	27	46
Available for sale, gross unrealized losses	25	29
Investment securities available-for-sale	3,035	6,347
Held to maturity, amortized cost	3,603	3,846
Held to maturity, gross unrealized gains	45	34
Held to maturity, gross unrealized losses	8	9
Investment securities held-to-maturity	3,640	3,871
Non-U.S. debt securities, mortgage-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,609	6,664
Available for sale, gross unrealized gains	8	36
Available for sale, gross unrealized losses	2	5
Investment securities available-for-sale	2,615	6,695
Held to maturity, amortized cost	727	939
Held to maturity, gross unrealized gains	82	82
Held to maturity, gross unrealized losses	5	6
Investment securities held-to-maturity	804	1,015
Non-U.S. debt securities, asset-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,655	2,942
Available for sale, gross unrealized gains	2	5
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	1,657	2,947
Held to maturity, amortized cost	231	263
Held to maturity, gross unrealized gains	0	1
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	231	264
Non-U.S. debt securities, government securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	13,089	10,754
Available for sale, gross unrealized gains	31	16
Available for sale, gross unrealized losses	48	49
Investment securities available-for-sale	13,072	10,721
Held to maturity, amortized cost	404	474
Held to maturity, gross unrealized gains	2	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	406	476
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	4,449	6,076
Available for sale, gross unrealized gains	19	38
Available for sale, gross unrealized losses	16	6
Investment securities available-for-sale	4,452	6,108
Held to maturity, amortized cost	47	48
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	47	48
Total non-U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	21,802	26,436
Available for sale, gross unrealized gains	60	95
Available for sale, gross unrealized losses	66	60
Investment securities available-for-sale	21,796	26,471
Held to maturity, amortized cost	1,409	1,724
Held to maturity, gross unrealized gains	84	85
Held to maturity, gross unrealized losses	5	6
Investment securities held-to-maturity	1,488	1,803
State and political subdivisions
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	4,127	8,929
Available for sale, gross unrealized gains	114	245
Available for sale, gross unrealized losses	13	23
Investment securities available-for-sale	4,228	9,151
Collateralized mortgage obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	325	1,060
Available for sale, gross unrealized gains	0	3
Available for sale, gross unrealized losses	6	9
Investment securities available-for-sale	319	1,054
Held to maturity, amortized cost	1,147	1,209
Held to maturity, gross unrealized gains	45	45
Held to maturity, gross unrealized losses	7	6
Investment securities held-to-maturity	1,185	1,248
Other U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,104	2,563
Available for sale, gross unrealized gains	5	12
Available for sale, gross unrealized losses	43	15
Investment securities available-for-sale	2,066	2,560
U.S. equity securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	0	40
Available for sale, gross unrealized gains	0	8
Available for sale, gross unrealized losses	0	2
Investment securities available-for-sale	0	46
U.S. money-market mutual funds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	0	397
Available for sale, gross unrealized gains	0	0
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	$ 0	397
Federal family education loan program
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Federal government credit support guarantee, percentage minimum	97.00%
Non-US debt securities, covered bonds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	$ 1,959	3,537
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	1,735	1,885
US States and Political Subdivisions Debt Securities - Trusts
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	$ 1,207	$ 1,247