Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	$ 4,643
Available for sale, 1 to 5 Years	14,067
Available for sale, 6 to 10 Years	13,581
Available for sale, over 10 Years	15,057
Available-for-sale, fair value	47,348
Held to maturity, under 1 Year	3,853
Held to maturity, 1 to 5 Years	14,629
Held to maturity, 6 to 10 Years	1,776
Held to maturity, over 10 Years	19,336
Held to maturity, amortized cost	39,594	$ 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	3,205
Held to maturity, 1 to 5 Years	12,725
Held to maturity, 6 to 10 Years	13
Held to maturity, over 10 Years	49
Held to maturity, amortized cost	15,992	17,028
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	105
Available for sale, 1 to 5 Years	621
Available for sale, 6 to 10 Years	2,708
Available for sale, over 10 Years	12,459
Available-for-sale, fair value	15,893
Held to maturity, under 1 Year	10
Held to maturity, 1 to 5 Years	185
Held to maturity, 6 to 10 Years	1,467
Held to maturity, over 10 Years	15,781
Held to maturity, amortized cost	17,443	16,651
Total U.S. Treasury and federal agencies
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	116
Available for sale, 1 to 5 Years	621
Available for sale, 6 to 10 Years	2,708
Available for sale, over 10 Years	12,459
Available-for-sale, fair value	15,904
Held to maturity, under 1 Year	3,215
Held to maturity, 1 to 5 Years	12,910
Held to maturity, 6 to 10 Years	1,480
Held to maturity, over 10 Years	15,830
Held to maturity, amortized cost	33,435	33,679
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	56
Available for sale, 1 to 5 Years	366
Available for sale, 6 to 10 Years	457
Available for sale, over 10 Years	688
Available-for-sale, fair value	1,567
Held to maturity, under 1 Year	32
Held to maturity, 1 to 5 Years	276
Held to maturity, 6 to 10 Years	226
Held to maturity, over 10 Years	2,358
Held to maturity, amortized cost	2,892	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	491
Available for sale, 6 to 10 Years	126
Available for sale, over 10 Years	0
Available-for-sale, fair value	617
Held to maturity, under 1 Year	173
Held to maturity, 1 to 5 Years	537
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	710	798
Asset-backed securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	100
Available for sale, 1 to 5 Years	589
Available for sale, 6 to 10 Years	142
Available for sale, over 10 Years	20
Available-for-sale, fair value	851
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	156
Available for sale, 1 to 5 Years	1,446
Available for sale, 6 to 10 Years	725
Available for sale, over 10 Years	708
Available-for-sale, fair value	3,035
Held to maturity, under 1 Year	205
Held to maturity, 1 to 5 Years	813
Held to maturity, 6 to 10 Years	226
Held to maturity, over 10 Years	2,359
Held to maturity, amortized cost	3,603	3,846
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	223
Available for sale, 1 to 5 Years	1,807
Available for sale, 6 to 10 Years	195
Available for sale, over 10 Years	390
Available-for-sale, fair value	2,615
Held to maturity, under 1 Year	94
Held to maturity, 1 to 5 Years	140
Held to maturity, 6 to 10 Years	21
Held to maturity, over 10 Years	472
Held to maturity, amortized cost	727	939
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	155
Available for sale, 1 to 5 Years	712
Available for sale, 6 to 10 Years	650
Available for sale, over 10 Years	140
Available-for-sale, fair value	1,657
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	231
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	231	263
Non-U.S. debt securities, government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	2,279
Available for sale, 1 to 5 Years	4,422
Available for sale, 6 to 10 Years	6,103
Available for sale, over 10 Years	268
Available-for-sale, fair value	13,072
Held to maturity, under 1 Year	287
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	404	474
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	1,232
Available for sale, 1 to 5 Years	2,478
Available for sale, 6 to 10 Years	705
Available for sale, over 10 Years	37
Available-for-sale, fair value	4,452
Held to maturity, under 1 Year	47
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	47	48
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	3,889
Available for sale, 1 to 5 Years	9,419
Available for sale, 6 to 10 Years	7,653
Available for sale, over 10 Years	835
Available-for-sale, fair value	21,796
Held to maturity, under 1 Year	428
Held to maturity, 1 to 5 Years	488
Held to maturity, 6 to 10 Years	21
Held to maturity, over 10 Years	472
Held to maturity, amortized cost	1,409	1,724
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	406
Available for sale, 1 to 5 Years	1,284
Available for sale, 6 to 10 Years	1,786
Available for sale, over 10 Years	752
Available-for-sale, fair value	4,228
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	0
Available for sale, 1 to 5 Years	16
Available for sale, 6 to 10 Years	0
Available for sale, over 10 Years	303
Available-for-sale, fair value	319
Held to maturity, under 1 Year	5
Held to maturity, 1 to 5 Years	418
Held to maturity, 6 to 10 Years	49
Held to maturity, over 10 Years	675
Held to maturity, amortized cost	1,147	$ 1,209
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	76
Available for sale, 1 to 5 Years	1,281
Available for sale, 6 to 10 Years	709
Available for sale, over 10 Years	0
Available-for-sale, fair value	$ 2,066