Investment Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	$ 54,059	$ 45,359
Available for sale, gross unrealized gains	733	107
Available for sale, gross unrealized losses	35	318
Investment securities available-for-sale	54,757	45,148
Held to maturity, amortized cost	39,119	41,914
Held to maturity, gross unrealized gains	485	175
Held to maturity, gross unrealized losses	69	738
Investment securities held-to-maturity	39,535	41,351
US Treasury and federal agencies, direct obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,035	1,035
Available for sale, gross unrealized gains	11	4
Available for sale, gross unrealized losses	7	0
Investment securities available-for-sale	2,039	1,039
Held to maturity, amortized cost	11,781	14,794
Held to maturity, gross unrealized gains	20	0
Held to maturity, gross unrealized losses	11	199
Investment securities held-to-maturity	11,790	14,595
US Treasury and federal agencies, mortgage-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	22,076	16,112
Available for sale, gross unrealized gains	321	37
Available for sale, gross unrealized losses	18	181
Investment securities available-for-sale	22,379	15,968
Held to maturity, amortized cost	21,966	21,647
Held to maturity, gross unrealized gains	324	24
Held to maturity, gross unrealized losses	29	518
Investment securities held-to-maturity	22,261	21,153
Total U.S. Treasury and federal agencies
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	24,111	17,147
Available for sale, gross unrealized gains	332	41
Available for sale, gross unrealized losses	25	181
Investment securities available-for-sale	24,418	17,007
Held to maturity, amortized cost	33,747	36,441
Held to maturity, gross unrealized gains	344	24
Held to maturity, gross unrealized losses	40	717
Investment securities held-to-maturity	34,051	35,748
Asset-backed securities, student loans
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	469	538
Available for sale, gross unrealized gains	2	4
Available for sale, gross unrealized losses	0	1
Investment securities available-for-sale	471	541
Held to maturity, amortized cost	3,881	3,191
Held to maturity, gross unrealized gains	22	35
Held to maturity, gross unrealized losses	21	10
Investment securities held-to-maturity	3,882	3,216
Asset-backed securities, credit cards
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	90	609
Available for sale, gross unrealized gains	0	0
Available for sale, gross unrealized losses	0	26
Investment securities available-for-sale	90	583
Held to maturity, amortized cost	0	193
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	0	193
Asset-backed securities, collateralized loan obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,448	594
Available for sale, gross unrealized gains	0	1
Available for sale, gross unrealized losses	2	2
Investment securities available-for-sale	1,446	593
Held to maturity, amortized cost	0	1
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	0	1
Total asset-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,007	1,741
Available for sale, gross unrealized gains	2	5
Available for sale, gross unrealized losses	2	29
Investment securities available-for-sale	2,007	1,717
Held to maturity, amortized cost	3,881	3,385
Held to maturity, gross unrealized gains	22	35
Held to maturity, gross unrealized losses	21	10
Investment securities held-to-maturity	3,882	3,410
Non-U.S. debt securities, mortgage-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,916	1,687
Available for sale, gross unrealized gains	3	0
Available for sale, gross unrealized losses	2	5
Investment securities available-for-sale	1,917	1,682
Held to maturity, amortized cost	471	638
Held to maturity, gross unrealized gains	80	77
Held to maturity, gross unrealized losses	7	9
Investment securities held-to-maturity	544	706
Non-U.S. debt securities, asset-backed securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,829	1,580
Available for sale, gross unrealized gains	3	0
Available for sale, gross unrealized losses	1	6
Investment securities available-for-sale	1,831	1,574
Held to maturity, amortized cost	0	223
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	0	223
Non-U.S. debt securities, government securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	12,996	12,816
Available for sale, gross unrealized gains	194	22
Available for sale, gross unrealized losses	1	45
Investment securities available-for-sale	13,189	12,793
Held to maturity, amortized cost	294	358
Held to maturity, gross unrealized gains	1	1
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	295	359
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	6,891	6,600
Available for sale, gross unrealized gains	108	18
Available for sale, gross unrealized losses	1	16
Investment securities available-for-sale	6,998	6,602
Held to maturity, amortized cost	0	46
Held to maturity, gross unrealized gains	0	0
Held to maturity, gross unrealized losses	0	0
Investment securities held-to-maturity	0	46
Total non-U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	23,632	22,683
Available for sale, gross unrealized gains	308	40
Available for sale, gross unrealized losses	5	72
Investment securities available-for-sale	23,935	22,651
Held to maturity, amortized cost	765	1,265
Held to maturity, gross unrealized gains	81	78
Held to maturity, gross unrealized losses	7	9
Investment securities held-to-maturity	839	1,334
State and political subdivisions
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	1,765	1,905
Available for sale, gross unrealized gains	56	20
Available for sale, gross unrealized losses	2	7
Investment securities available-for-sale	1,819	1,918
Collateralized mortgage obligations
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	112	200
Available for sale, gross unrealized gains	1	0
Available for sale, gross unrealized losses	0	3
Investment securities available-for-sale	113	197
Held to maturity, amortized cost	726	823
Held to maturity, gross unrealized gains	38	38
Held to maturity, gross unrealized losses	1	2
Investment securities held-to-maturity	763	859
Other U.S. debt securities
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Available for sale, amortized cost	2,432	1,683
Available for sale, gross unrealized gains	34	1
Available for sale, gross unrealized losses	1	26
Investment securities available-for-sale	$ 2,465	1,658
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, supranational bonds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	$ 4,290	3,200
Non-US debt securities, covered bonds
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	730	1,300
Non-U.S. debt securities, other
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	1,520	1,330
State and political subdivisions, securities in trusts
Available-For-Sale and Held-To-Maturity-Securities [Line Items]
Investment securities available-for-sale	$ 990	$ 1,050