Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	$ 4,680
Available for sale, 1 to 5 Years	14,558
Available for sale, 6 to 10 Years	12,381
Available for sale, over 10 Years	14,483
Available-for-sale, fair value	46,102
Held to maturity, under 1 Year	3,452
Held to maturity, 1 to 5 Years	13,781
Held to maturity, 6 to 10 Years	1,619
Held to maturity, over 10 Years	21,715
Held to maturity, amortized cost	40,567	$ 40,458
US Treasury and federal agencies, direct obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	11
Available for sale, 1 to 5 Years	0
Available for sale, 6 to 10 Years	0
Available for sale, over 10 Years	0
Available-for-sale, fair value	11
Held to maturity, under 1 Year	2,897
Held to maturity, 1 to 5 Years	12,213
Held to maturity, 6 to 10 Years	12
Held to maturity, over 10 Years	46
Held to maturity, amortized cost	15,168	17,028
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	129
Available for sale, 1 to 5 Years	830
Available for sale, 6 to 10 Years	2,547
Available for sale, over 10 Years	11,967
Available-for-sale, fair value	15,473
Held to maturity, under 1 Year	8
Held to maturity, 1 to 5 Years	173
Held to maturity, 6 to 10 Years	1,403
Held to maturity, over 10 Years	18,113
Held to maturity, amortized cost	19,697	16,651
Total U.S. Treasury and federal agencies
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	140
Available for sale, 1 to 5 Years	830
Available for sale, 6 to 10 Years	2,547
Available for sale, over 10 Years	11,967
Available-for-sale, fair value	15,484
Held to maturity, under 1 Year	2,905
Held to maturity, 1 to 5 Years	12,386
Held to maturity, 6 to 10 Years	1,415
Held to maturity, over 10 Years	18,159
Held to maturity, amortized cost	34,865	33,679
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	71
Available for sale, 1 to 5 Years	311
Available for sale, 6 to 10 Years	391
Available for sale, over 10 Years	668
Available-for-sale, fair value	1,441
Held to maturity, under 1 Year	32
Held to maturity, 1 to 5 Years	300
Held to maturity, 6 to 10 Years	185
Held to maturity, over 10 Years	2,514
Held to maturity, amortized cost	3,031	3,047
Asset-backed securities, credit cards
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	198
Available for sale, 1 to 5 Years	292
Available for sale, 6 to 10 Years	126
Available for sale, over 10 Years	0
Available-for-sale, fair value	616
Held to maturity, under 1 Year	58
Held to maturity, 1 to 5 Years	267
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	325	798
Asset-backed securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	0
Available for sale, 1 to 5 Years	449
Available for sale, 6 to 10 Years	141
Available for sale, over 10 Years	21
Available-for-sale, fair value	611
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	0
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	1
Held to maturity, amortized cost	1	1
Total asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	269
Available for sale, 1 to 5 Years	1,052
Available for sale, 6 to 10 Years	658
Available for sale, over 10 Years	689
Available-for-sale, fair value	2,668
Held to maturity, under 1 Year	90
Held to maturity, 1 to 5 Years	567
Held to maturity, 6 to 10 Years	185
Held to maturity, over 10 Years	2,515
Held to maturity, amortized cost	3,357	3,846
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	175
Available for sale, 1 to 5 Years	1,638
Available for sale, 6 to 10 Years	158
Available for sale, over 10 Years	377
Available-for-sale, fair value	2,348
Held to maturity, under 1 Year	71
Held to maturity, 1 to 5 Years	143
Held to maturity, 6 to 10 Years	8
Held to maturity, over 10 Years	450
Held to maturity, amortized cost	672	939
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	103
Available for sale, 1 to 5 Years	637
Available for sale, 6 to 10 Years	607
Available for sale, over 10 Years	174
Available-for-sale, fair value	1,521
Held to maturity, under 1 Year	98
Held to maturity, 1 to 5 Years	130
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	228	263
Non-U.S. debt securities, government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	2,100
Available for sale, 1 to 5 Years	5,184
Available for sale, 6 to 10 Years	5,430
Available for sale, over 10 Years	259
Available-for-sale, fair value	12,973
Held to maturity, under 1 Year	240
Held to maturity, 1 to 5 Years	117
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	357	474
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	1,346
Available for sale, 1 to 5 Years	2,647
Available for sale, 6 to 10 Years	731
Available for sale, over 10 Years	20
Available-for-sale, fair value	4,744
Held to maturity, under 1 Year	46
Held to maturity, 1 to 5 Years	0
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	46	48
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	3,724
Available for sale, 1 to 5 Years	10,106
Available for sale, 6 to 10 Years	6,926
Available for sale, over 10 Years	830
Available-for-sale, fair value	21,586
Held to maturity, under 1 Year	455
Held to maturity, 1 to 5 Years	390
Held to maturity, 6 to 10 Years	8
Held to maturity, over 10 Years	450
Held to maturity, amortized cost	1,303	1,724
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	379
Available for sale, 1 to 5 Years	1,329
Available for sale, 6 to 10 Years	1,625
Available for sale, over 10 Years	702
Available-for-sale, fair value	4,035
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	8
Available for sale, 1 to 5 Years	0
Available for sale, 6 to 10 Years	0
Available for sale, over 10 Years	295
Available-for-sale, fair value	303
Held to maturity, under 1 Year	2
Held to maturity, 1 to 5 Years	438
Held to maturity, 6 to 10 Years	11
Held to maturity, over 10 Years	591
Held to maturity, amortized cost	1,042	$ 1,209
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	160
Available for sale, 1 to 5 Years	1,241
Available for sale, 6 to 10 Years	625
Available for sale, over 10 Years	0
Available-for-sale, fair value	$ 2,026