Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	$ 5,187
Available for sale, 1 to 5 Years	21,238
Available for sale, 6 to 10 Years	17,388
Available for sale, over 10 Years	12,865
Available-for-sale, fair value	56,678
Held to maturity, under 1 Year	2,720
Held to maturity, 1 to 5 Years	16,762
Held to maturity, 6 to 10 Years	2,272
Held to maturity, over 10 Years	18,704
Held to maturity, amortized cost	40,458	$ 35,169
US Treasury and federal agencies, direct obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	0
Available for sale, 1 to 5 Years	12
Available for sale, 6 to 10 Years	6
Available for sale, over 10 Years	205
Available-for-sale, fair value	223
Held to maturity, under 1 Year	1,988
Held to maturity, 1 to 5 Years	14,968
Held to maturity, 6 to 10 Years	14
Held to maturity, over 10 Years	58
Held to maturity, amortized cost	17,028	17,527
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	96
Available for sale, 1 to 5 Years	762
Available for sale, 6 to 10 Years	3,123
Available for sale, over 10 Years	6,891
Available-for-sale, fair value	10,872
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	162
Held to maturity, 6 to 10 Years	1,605
Held to maturity, over 10 Years	14,884
Held to maturity, amortized cost	16,651	10,334
Total U.S. Treasury and federal agencies
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	96
Available for sale, 1 to 5 Years	774
Available for sale, 6 to 10 Years	3,129
Available for sale, over 10 Years	7,096
Available-for-sale, fair value	11,095
Held to maturity, under 1 Year	1,988
Held to maturity, 1 to 5 Years	15,130
Held to maturity, 6 to 10 Years	1,619
Held to maturity, over 10 Years	14,942
Held to maturity, amortized cost	33,679	27,861
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	289
Available for sale, 1 to 5 Years	1,044
Available for sale, 6 to 10 Years	685
Available for sale, over 10 Years	1,340
Available-for-sale, fair value	3,358
Held to maturity, under 1 Year	35
Held to maturity, 1 to 5 Years	245
Held to maturity, 6 to 10 Years	265
Held to maturity, over 10 Years	2,502
Held to maturity, amortized cost	3,047	2,883
Asset-backed securities, credit cards
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	0
Available for sale, 1 to 5 Years	1,290
Available for sale, 6 to 10 Years	252
Available for sale, over 10 Years	0
Available-for-sale, fair value	1,542
Held to maturity, under 1 Year	178
Held to maturity, 1 to 5 Years	620
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	798	897
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	350
Available for sale, 6 to 10 Years	956
Available for sale, over 10 Years	141
Available-for-sale, fair value	1,447
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	35
Total asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	289
Available for sale, 1 to 5 Years	2,684
Available for sale, 6 to 10 Years	1,893
Available for sale, over 10 Years	1,481
Available-for-sale, fair value	6,347
Held to maturity, under 1 Year	213
Held to maturity, 1 to 5 Years	865
Held to maturity, 6 to 10 Years	265
Held to maturity, over 10 Years	2,503
Held to maturity, amortized cost	3,846	3,815
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	551
Available for sale, 1 to 5 Years	4,502
Available for sale, 6 to 10 Years	602
Available for sale, over 10 Years	1,040
Available-for-sale, fair value	6,695
Held to maturity, under 1 Year	132
Held to maturity, 1 to 5 Years	217
Held to maturity, 6 to 10 Years	45
Held to maturity, over 10 Years	545
Held to maturity, amortized cost	939	1,150
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	205
Available for sale, 1 to 5 Years	2,185
Available for sale, 6 to 10 Years	557
Available for sale, over 10 Years	0
Available-for-sale, fair value	2,947
Held to maturity, under 1 Year	26
Held to maturity, 1 to 5 Years	237
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	263	531
Government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	2,195
Available for sale, 1 to 5 Years	3,201
Available for sale, 6 to 10 Years	4,448
Available for sale, over 10 Years	877
Available-for-sale, fair value	10,721
Held to maturity, under 1 Year	353
Held to maturity, 1 to 5 Years	121
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	474	286
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	1,078
Available for sale, 1 to 5 Years	4,235
Available for sale, 6 to 10 Years	758
Available for sale, over 10 Years	37
Available-for-sale, fair value	6,108
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	48
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	48	113
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	4,029
Available for sale, 1 to 5 Years	14,123
Available for sale, 6 to 10 Years	6,365
Available for sale, over 10 Years	1,954
Available-for-sale, fair value	26,471
Held to maturity, under 1 Year	511
Held to maturity, 1 to 5 Years	623
Held to maturity, 6 to 10 Years	45
Held to maturity, over 10 Years	545
Held to maturity, amortized cost	1,724	2,080
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	474
Available for sale, 1 to 5 Years	2,415
Available for sale, 6 to 10 Years	4,724
Available for sale, over 10 Years	1,538
Available-for-sale, fair value	9,151
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	3
Available for sale, 1 to 5 Years	145
Available for sale, 6 to 10 Years	170
Available for sale, over 10 Years	736
Available-for-sale, fair value	1,054
Held to maturity, under 1 Year	8
Held to maturity, 1 to 5 Years	144
Held to maturity, 6 to 10 Years	343
Held to maturity, over 10 Years	714
Held to maturity, amortized cost	1,209	$ 1,413
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	296
Available for sale, 1 to 5 Years	1,097
Available for sale, 6 to 10 Years	1,107
Available for sale, over 10 Years	60
Available-for-sale, fair value	$ 2,560