Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	$ 4,814
Available for sale, 1 to 5 Years	15,022
Available for sale, 6 to 10 Years	13,951
Available for sale, over 10 Years	10,517
Available-for-sale, fair value	44,304
Held to maturity, under 1 Year	3,504
Held to maturity, 1 to 5 Years	15,644
Held to maturity, 6 to 10 Years	2,117
Held to maturity, over 10 Years	19,893
Held to maturity, amortized cost	41,158	$ 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	78
Available-for-sale, fair value	89
Held to maturity, under 1 Year	2,939
Held to maturity, 1 to 5 Years	13,898
Held to maturity, 6 to 10 Years	13
Held to maturity, over 10 Years	53
Held to maturity, amortized cost	16,903	17,028
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	112
Available for sale, 1 to 5 Years	647
Available for sale, 6 to 10 Years	2,899
Available for sale, over 10 Years	6,632
Available-for-sale, fair value	10,290
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	219
Held to maturity, 6 to 10 Years	1,470
Held to maturity, over 10 Years	16,190
Held to maturity, amortized cost	17,879	16,651
Total U.S. Treasury and federal agencies
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	123
Available for sale, 1 to 5 Years	647
Available for sale, 6 to 10 Years	2,899
Available for sale, over 10 Years	6,710
Available-for-sale, fair value	10,379
Held to maturity, under 1 Year	2,939
Held to maturity, 1 to 5 Years	14,117
Held to maturity, 6 to 10 Years	1,483
Held to maturity, over 10 Years	16,243
Held to maturity, amortized cost	34,782	33,679
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	216
Available for sale, 1 to 5 Years	395
Available for sale, 6 to 10 Years	374
Available for sale, over 10 Years	758
Available-for-sale, fair value	1,743
Held to maturity, under 1 Year	25
Held to maturity, 1 to 5 Years	262
Held to maturity, 6 to 10 Years	249
Held to maturity, over 10 Years	2,437
Held to maturity, amortized cost	2,973	3,047
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	1,280
Available for sale, 6 to 10 Years	151
Available for sale, over 10 Years	0
Available-for-sale, fair value	1,431
Held to maturity, under 1 Year	173
Held to maturity, 1 to 5 Years	536
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	709	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	406
Available for sale, 6 to 10 Years	400
Available for sale, over 10 Years	20
Available-for-sale, fair value	826
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	216
Available for sale, 1 to 5 Years	2,081
Available for sale, 6 to 10 Years	925
Available for sale, over 10 Years	778
Available-for-sale, fair value	4,000
Held to maturity, under 1 Year	198
Held to maturity, 1 to 5 Years	798
Held to maturity, 6 to 10 Years	249
Held to maturity, over 10 Years	2,438
Held to maturity, amortized cost	3,683	3,846
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	302
Available for sale, 1 to 5 Years	2,054
Available for sale, 6 to 10 Years	194
Available for sale, over 10 Years	402
Available-for-sale, fair value	2,952
Held to maturity, under 1 Year	63
Held to maturity, 1 to 5 Years	172
Held to maturity, 6 to 10 Years	42
Held to maturity, over 10 Years	514
Held to maturity, amortized cost	791	939
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	172
Available for sale, 1 to 5 Years	1,103
Available for sale, 6 to 10 Years	233
Available for sale, over 10 Years	125
Available-for-sale, fair value	1,633
Held to maturity, under 1 Year	13
Held to maturity, 1 to 5 Years	246
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	259	263
Government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	2,296
Available for sale, 1 to 5 Years	3,115
Available for sale, 6 to 10 Years	4,556
Available for sale, over 10 Years	908
Available-for-sale, fair value	10,875
Held to maturity, under 1 Year	287
Held to maturity, 1 to 5 Years	124
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	411	474
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	992
Available for sale, 1 to 5 Years	2,783
Available for sale, 6 to 10 Years	718
Available for sale, over 10 Years	38
Available-for-sale, fair value	4,531
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	49
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	49	48
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	3,762
Available for sale, 1 to 5 Years	9,055
Available for sale, 6 to 10 Years	5,701
Available for sale, over 10 Years	1,473
Available-for-sale, fair value	19,991
Held to maturity, under 1 Year	363
Held to maturity, 1 to 5 Years	591
Held to maturity, 6 to 10 Years	42
Held to maturity, over 10 Years	514
Held to maturity, amortized cost	1,510	1,724
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	443
Available for sale, 1 to 5 Years	2,063
Available for sale, 6 to 10 Years	3,569
Available for sale, over 10 Years	1,232
Available-for-sale, fair value	7,307
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	2
Available for sale, 1 to 5 Years	22
Available for sale, 6 to 10 Years	0
Available for sale, over 10 Years	323
Available-for-sale, fair value	347
Held to maturity, under 1 Year	4
Held to maturity, 1 to 5 Years	138
Held to maturity, 6 to 10 Years	343
Held to maturity, over 10 Years	698
Held to maturity, amortized cost	1,183	$ 1,209
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	268
Available for sale, 1 to 5 Years	1,154
Available for sale, 6 to 10 Years	857
Available for sale, over 10 Years	1
Available-for-sale, fair value	$ 2,280