Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	$ 7,471
Available for sale, 1 to 5 Years	20,298
Available for sale, 6 to 10 Years	16,230
Available for sale, over 10 Years	11,799
Available-for-sale, fair value	55,798
Held to maturity, under 1 Year	2,722
Held to maturity, 1 to 5 Years	17,329
Held to maturity, 6 to 10 Years	2,161
Held to maturity, over 10 Years	14,638
Held to maturity, amortized cost	36,850	$ 35,169
US Treasury and federal agencies, direct obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	232
Available for sale, 1 to 5 Years	7
Available for sale, 6 to 10 Years	55
Available for sale, over 10 Years	326
Available-for-sale, fair value	620
Held to maturity, under 1 Year	1,827
Held to maturity, 1 to 5 Years	15,552
Held to maturity, 6 to 10 Years	14
Held to maturity, over 10 Years	63
Held to maturity, amortized cost	17,456	17,527
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	270
Available for sale, 1 to 5 Years	1,311
Available for sale, 6 to 10 Years	3,424
Available for sale, over 10 Years	5,995
Available-for-sale, fair value	11,000
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	172
Held to maturity, 6 to 10 Years	1,427
Held to maturity, over 10 Years	10,776
Held to maturity, amortized cost	12,375	10,334
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	408
Available for sale, 1 to 5 Years	1,634
Available for sale, 6 to 10 Years	1,159
Available for sale, over 10 Years	1,625
Available-for-sale, fair value	4,826
Held to maturity, under 1 Year	87
Held to maturity, 1 to 5 Years	240
Held to maturity, 6 to 10 Years	298
Held to maturity, over 10 Years	2,491
Held to maturity, amortized cost	3,116	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	1,296
Available for sale, 6 to 10 Years	252
Available for sale, over 10 Years	0
Available-for-sale, fair value	1,548
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	120
Available for sale, 6 to 10 Years	1,101
Available for sale, over 10 Years	0
Available-for-sale, fair value	1,221
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	408
Available for sale, 1 to 5 Years	3,050
Available for sale, 6 to 10 Years	2,512
Available for sale, over 10 Years	1,625
Available-for-sale, fair value	7,595
Held to maturity, under 1 Year	265
Held to maturity, 1 to 5 Years	860
Held to maturity, 6 to 10 Years	298
Held to maturity, over 10 Years	2,492
Held to maturity, amortized cost	3,915	3,815
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	837
Available for sale, 1 to 5 Years	4,128
Available for sale, 6 to 10 Years	1,046
Available for sale, over 10 Years	1,063
Available-for-sale, fair value	7,074
Held to maturity, under 1 Year	173
Held to maturity, 1 to 5 Years	221
Held to maturity, 6 to 10 Years	49
Held to maturity, over 10 Years	557
Held to maturity, amortized cost	1,000	1,150
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	395
Available for sale, 1 to 5 Years	2,182
Available for sale, 6 to 10 Years	262
Available for sale, over 10 Years	0
Available-for-sale, fair value	2,839
Held to maturity, under 1 Year	84
Held to maturity, 1 to 5 Years	241
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	325	531
Government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	3,000
Available for sale, 1 to 5 Years	2,288
Available for sale, 6 to 10 Years	1,370
Available for sale, over 10 Years	0
Available-for-sale, fair value	6,658
Held to maturity, under 1 Year	364
Held to maturity, 1 to 5 Years	119
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	483	286
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	1,485
Available for sale, 1 to 5 Years	3,607
Available for sale, 6 to 10 Years	726
Available for sale, over 10 Years	0
Available-for-sale, fair value	5,818
Held to maturity, under 1 Year	0
Held to maturity, 1 to 5 Years	47
Held to maturity, 6 to 10 Years	0
Held to maturity, over 10 Years	0
Held to maturity, amortized cost	47	113
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	5,717
Available for sale, 1 to 5 Years	12,205
Available for sale, 6 to 10 Years	3,404
Available for sale, over 10 Years	1,063
Available-for-sale, fair value	22,389
Held to maturity, under 1 Year	621
Held to maturity, 1 to 5 Years	628
Held to maturity, 6 to 10 Years	49
Held to maturity, over 10 Years	557
Held to maturity, amortized cost	1,855	2,080
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	433
Available for sale, 1 to 5 Years	2,525
Available for sale, 6 to 10 Years	5,020
Available for sale, over 10 Years	1,760
Available-for-sale, fair value	9,738
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	7
Available for sale, 1 to 5 Years	148
Available for sale, 6 to 10 Years	343
Available for sale, over 10 Years	1,030
Available-for-sale, fair value	1,528
Held to maturity, under 1 Year	9
Held to maturity, 1 to 5 Years	117
Held to maturity, 6 to 10 Years	373
Held to maturity, over 10 Years	750
Held to maturity, amortized cost	1,249	$ 1,413
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 Year	404
Available for sale, 1 to 5 Years	1,052
Available for sale, 6 to 10 Years	1,472
Available for sale, over 10 Years	0
Available-for-sale, fair value	$ 2,928