Investment Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	$ 46,554	$ 56,952
Available for sale, gross unrealized gains	156	437
Available for sale, gross unrealized losses	608	268
Investment securities available-for-sale	46,102	57,121
Held to maturity, amortized cost	40,567	40,458
Held to maturity, gross unrealized gains	171	186
Held to maturity, gross unrealized losses	1,147	389
Investment securities held-to-maturity	39,591	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	15,168	17,028
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	325	143
Investment securities held-to-maturity	14,843	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	15,841	10,975
Available for sale, gross unrealized gains	8	26
Available for sale, gross unrealized losses	376	129
Investment securities available-for-sale	15,473	10,872
Held to maturity, amortized cost	19,697	16,651
Held to maturity, gross unrealized gains	2	22
Held to maturity, gross unrealized losses	802	225
Investment securities held-to-maturity	18,897	16,448
Total U.S. Treasury and federal agencies
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	15,852	11,197
Available for sale, gross unrealized gains	8	28
Available for sale, gross unrealized losses	376	130
Investment securities available-for-sale	15,484	11,095
Held to maturity, amortized cost	34,865	33,679
Held to maturity, gross unrealized gains	2	22
Held to maturity, gross unrealized losses	1,127	368
Investment securities held-to-maturity	33,740	33,333
Asset-backed securities, student loans
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,422	3,325
Available for sale, gross unrealized gains	20	37
Available for sale, gross unrealized losses	1	4
Investment securities available-for-sale	1,441	3,358
Held to maturity, amortized cost	3,031	3,047
Held to maturity, gross unrealized gains	43	32
Held to maturity, gross unrealized losses	7	9
Investment securities held-to-maturity	3,067	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	24	25
Investment securities available-for-sale	616	1,542
Held to maturity, amortized cost	325	798
Held to maturity, gross unrealized gains	0	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	325	800
Asset-backed securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	609	1,440
Available for sale, gross unrealized gains	3	7
Available for sale, gross unrealized losses	1	0
Investment securities available-for-sale	611	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	2,670	6,330
Available for sale, gross unrealized gains	24	46
Available for sale, gross unrealized losses	26	29
Investment securities available-for-sale	2,668	6,347
Held to maturity, amortized cost	3,357	3,846
Held to maturity, gross unrealized gains	43	34
Held to maturity, gross unrealized losses	7	9
Investment securities held-to-maturity	3,393	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	2,348	6,664
Available for sale, gross unrealized gains	4	36
Available for sale, gross unrealized losses	4	5
Investment securities available-for-sale	2,348	6,695
Held to maturity, amortized cost	672	939
Held to maturity, gross unrealized gains	81	82
Held to maturity, gross unrealized losses	6	6
Investment securities held-to-maturity	747	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	1,523	2,942
Available for sale, gross unrealized gains	1	5
Available for sale, gross unrealized losses	3	0
Investment securities available-for-sale	1,521	2,947
Held to maturity, amortized cost	228	263
Held to maturity, gross unrealized gains	0	1
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	228	264
Non-U.S. debt securities, government securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	13,080	10,754
Available for sale, gross unrealized gains	8	16
Available for sale, gross unrealized losses	115	49
Investment securities available-for-sale	12,973	10,721
Held to maturity, amortized cost	357	474
Held to maturity, gross unrealized gains	1	2
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	358	476
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	4,747	6,076
Available for sale, gross unrealized gains	14	38
Available for sale, gross unrealized losses	17	6
Investment securities available-for-sale	4,744	6,108
Held to maturity, amortized cost	46	48
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	46	48
Total non-U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	21,698	26,436
Available for sale, gross unrealized gains	27	95
Available for sale, gross unrealized losses	139	60
Investment securities available-for-sale	21,586	26,471
Held to maturity, amortized cost	1,303	1,724
Held to maturity, gross unrealized gains	82	85
Held to maturity, gross unrealized losses	6	6
Investment securities held-to-maturity	1,379	1,803
State and political subdivisions
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	3,959	8,929
Available for sale, gross unrealized gains	93	245
Available for sale, gross unrealized losses	17	23
Investment securities available-for-sale	4,035	9,151
Collateralized mortgage obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	310	1,060
Available for sale, gross unrealized gains	0	3
Available for sale, gross unrealized losses	7	9
Investment securities available-for-sale	303	1,054
Held to maturity, amortized cost	1,042	1,209
Held to maturity, gross unrealized gains	44	45
Held to maturity, gross unrealized losses	7	6
Investment securities held-to-maturity	1,079	1,248
Other U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,065	2,563
Available for sale, gross unrealized gains	4	12
Available for sale, gross unrealized losses	43	15
Investment securities available-for-sale	2,026	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	$ 1,929	3,537
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	1,782	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	$ 1,131	$ 1,247