Investment Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	$ 44,439	$ 56,952
Available for sale, gross unrealized gains	274	437
Available for sale, gross unrealized losses	409	268
Investment securities available-for-sale	44,304	57,121
Held to maturity, amortized cost	41,158	40,458
Held to maturity, gross unrealized gains	176	186
Held to maturity, gross unrealized losses	851	389
Investment securities held-to-maturity	40,483	40,255
US Treasury and federal agencies, direct obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	88	222
Available for sale, gross unrealized gains	1	2
Available for sale, gross unrealized losses	0	1
Investment securities available-for-sale	89	223
Held to maturity, amortized cost	16,903	17,028
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	259	143
Investment securities held-to-maturity	16,644	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	10,521	10,975
Available for sale, gross unrealized gains	13	26
Available for sale, gross unrealized losses	244	129
Investment securities available-for-sale	10,290	10,872
Held to maturity, amortized cost	17,879	16,651
Held to maturity, gross unrealized gains	1	22
Held to maturity, gross unrealized losses	571	225
Investment securities held-to-maturity	17,309	16,448
Total U.S. Treasury and federal agencies
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	10,609	11,197
Available for sale, gross unrealized gains	14	28
Available for sale, gross unrealized losses	244	130
Investment securities available-for-sale	10,379	11,095
Held to maturity, amortized cost	34,782	33,679
Held to maturity, gross unrealized gains	1	22
Held to maturity, gross unrealized losses	830	368
Investment securities held-to-maturity	33,953	33,333
Asset-backed securities, student loans
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,719	3,325
Available for sale, gross unrealized gains	26	37
Available for sale, gross unrealized losses	2	4
Investment securities available-for-sale	1,743	3,358
Held to maturity, amortized cost	2,973	3,047
Held to maturity, gross unrealized gains	38	32
Held to maturity, gross unrealized losses	9	9
Investment securities held-to-maturity	3,002	3,070
Asset-backed securities, credit cards
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,461	1,565
Available for sale, gross unrealized gains	1	2
Available for sale, gross unrealized losses	31	25
Investment securities available-for-sale	1,431	1,542
Held to maturity, amortized cost	709	798
Held to maturity, gross unrealized gains	1	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	710	800
Asset-backed securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	821	1,440
Available for sale, gross unrealized gains	5	7
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	826	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	4,001	6,330
Available for sale, gross unrealized gains	32	46
Available for sale, gross unrealized losses	33	29
Investment securities available-for-sale	4,000	6,347
Held to maturity, amortized cost	3,683	3,846
Held to maturity, gross unrealized gains	39	34
Held to maturity, gross unrealized losses	9	9
Investment securities held-to-maturity	3,713	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	2,940	6,664
Available for sale, gross unrealized gains	13	36
Available for sale, gross unrealized losses	1	5
Investment securities available-for-sale	2,952	6,695
Held to maturity, amortized cost	791	939
Held to maturity, gross unrealized gains	87	82
Held to maturity, gross unrealized losses	5	6
Investment securities held-to-maturity	873	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	1,630	2,942
Available for sale, gross unrealized gains	3	5
Available for sale, gross unrealized losses	0	0
Investment securities available-for-sale	1,633	2,947
Held to maturity, amortized cost	259	263
Held to maturity, gross unrealized gains	1	1
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	260	264
Government securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	10,876	10,754
Available for sale, gross unrealized gains	31	16
Available for sale, gross unrealized losses	32	49
Investment securities available-for-sale	10,875	10,721
Held to maturity, amortized cost	411	474
Held to maturity, gross unrealized gains	2	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	413	476
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	4,525	6,076
Available for sale, gross unrealized gains	21	38
Available for sale, gross unrealized losses	15	6
Investment securities available-for-sale	4,531	6,108
Held to maturity, amortized cost	49	48
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	49	48
Total non-U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	19,971	26,436
Available for sale, gross unrealized gains	68	95
Available for sale, gross unrealized losses	48	60
Investment securities available-for-sale	19,991	26,471
Held to maturity, amortized cost	1,510	1,724
Held to maturity, gross unrealized gains	90	85
Held to maturity, gross unrealized losses	5	6
Investment securities held-to-maturity	1,595	1,803
State and political subdivisions
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	7,197	8,929
Available for sale, gross unrealized gains	154	245
Available for sale, gross unrealized losses	44	23
Investment securities available-for-sale	7,307	9,151
Collateralized mortgage obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	352	1,060
Available for sale, gross unrealized gains	0	3
Available for sale, gross unrealized losses	5	9
Investment securities available-for-sale	347	1,054
Held to maturity, amortized cost	1,183	1,209
Held to maturity, gross unrealized gains	46	45
Held to maturity, gross unrealized losses	7	6
Investment securities held-to-maturity	1,222	1,248
Other U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,309	2,563
Available for sale, gross unrealized gains	6	12
Available for sale, gross unrealized losses	35	15
Investment securities available-for-sale	2,280	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	$ 2,193	3,537
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	1,738	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	$ 1,213	$ 1,247