Investment Securities - Schedule of Contractual Maturities of Debt Securities (Details) $ in Millions	Sep. 30, 2020 USD ($)
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	$ 8,589
Available for sale, under 1 year, fair value	8,612
Available for sale, 1 to 5 years, amortized cost	25,938
Available for sale, 1 to 5 years, fair value	26,326
Available for sale, 6 to 10 years, amortized cost	9,348
Available for sale, 6 to 10 years, fair value	9,590
Available for sale, over 10 years, amortized cost	16,839
Available for sale, over 10 years, fair value	17,252
Available for sale, amortized cost	60,714
Available for sale, fair value	61,780
Held to maturity, under 1 year, amortized cost	9,557
Held to maturity, under 1 year, fair value	9,610
Held to maturity, 1 to 5 years, amortized cost	3,964
Held to maturity, 1 to 5 years, fair value	4,043
Held to maturity, 6 to 10 years, amortized cost	5,290
Held to maturity, 6 to 10 years, fair value	5,453
Held to maturity, over 10 years, amortized cost	31,410
Held to maturity, over 10 years, fair value	32,245
Held to maturity, amortized cost	50,221
Held to maturity, fair value	51,351
US Treasury and federal agencies, direct obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	1,885
Available for sale, under 1 year, fair value	1,889
Available for sale, 1 to 5 years, amortized cost	3,196
Available for sale, 1 to 5 years, fair value	3,228
Available for sale, 6 to 10 years, amortized cost	2,048
Available for sale, 6 to 10 years, fair value	2,176
Available for sale, over 10 years, amortized cost	0
Available for sale, over 10 years, fair value	0
Available for sale, amortized cost	7,129
Available for sale, fair value	7,293
Held to maturity, under 1 year, amortized cost	3,644
Held to maturity, under 1 year, fair value	3,684
Held to maturity, 1 to 5 years, amortized cost	2,996
Held to maturity, 1 to 5 years, fair value	3,067
Held to maturity, 6 to 10 years, amortized cost	4
Held to maturity, 6 to 10 years, fair value	4
Held to maturity, over 10 years, amortized cost	24
Held to maturity, over 10 years, fair value	24
Held to maturity, amortized cost	6,668
Held to maturity, fair value	6,779
US Treasury and federal agencies, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	162
Available for sale, under 1 year, fair value	169
Available for sale, 1 to 5 years, amortized cost	700
Available for sale, 1 to 5 years, fair value	720
Available for sale, 6 to 10 years, amortized cost	2,951
Available for sale, 6 to 10 years, fair value	2,984
Available for sale, over 10 years, amortized cost	13,941
Available for sale, over 10 years, fair value	14,337
Available for sale, amortized cost	17,754
Available for sale, fair value	18,210
Held to maturity, under 1 year, amortized cost	64
Held to maturity, under 1 year, fair value	66
Held to maturity, 1 to 5 years, amortized cost	493
Held to maturity, 1 to 5 years, fair value	506
Held to maturity, 6 to 10 years, amortized cost	4,681
Held to maturity, 6 to 10 years, fair value	4,854
Held to maturity, over 10 years, amortized cost	27,858
Held to maturity, over 10 years, fair value	28,629
Held to maturity, amortized cost	33,096
Held to maturity, fair value	34,055
Total U.S. Treasury and federal agencies
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	2,047
Available for sale, under 1 year, fair value	2,058
Available for sale, 1 to 5 years, amortized cost	3,896
Available for sale, 1 to 5 years, fair value	3,948
Available for sale, 6 to 10 years, amortized cost	4,999
Available for sale, 6 to 10 years, fair value	5,160
Available for sale, over 10 years, amortized cost	13,941
Available for sale, over 10 years, fair value	14,337
Available for sale, amortized cost	24,883
Available for sale, fair value	25,503
Held to maturity, under 1 year, amortized cost	3,708
Held to maturity, under 1 year, fair value	3,750
Held to maturity, 1 to 5 years, amortized cost	3,489
Held to maturity, 1 to 5 years, fair value	3,573
Held to maturity, 6 to 10 years, amortized cost	4,685
Held to maturity, 6 to 10 years, fair value	4,858
Held to maturity, over 10 years, amortized cost	27,882
Held to maturity, over 10 years, fair value	28,653
Held to maturity, amortized cost	39,764
Held to maturity, fair value	40,834
Asset-backed securities, student loans
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	133
Available for sale, under 1 year, fair value	137
Available for sale, 1 to 5 years, amortized cost	109
Available for sale, 1 to 5 years, fair value	108
Available for sale, 6 to 10 years, amortized cost	1
Available for sale, 6 to 10 years, fair value	1
Available for sale, over 10 years, amortized cost	113
Available for sale, over 10 years, fair value	112
Available for sale, amortized cost	356
Available for sale, fair value	358
Held to maturity, under 1 year, amortized cost	362
Held to maturity, under 1 year, fair value	351
Held to maturity, 1 to 5 years, amortized cost	180
Held to maturity, 1 to 5 years, fair value	175
Held to maturity, 6 to 10 years, amortized cost	598
Held to maturity, 6 to 10 years, fair value	588
Held to maturity, over 10 years, amortized cost	3,183
Held to maturity, over 10 years, fair value	3,170
Held to maturity, amortized cost	4,323
Held to maturity, fair value	4,284
Asset-backed securities, credit cards
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	0
Available for sale, under 1 year, fair value	0
Available for sale, 1 to 5 years, amortized cost	0
Available for sale, 1 to 5 years, fair value	0
Available for sale, 6 to 10 years, amortized cost	90
Available for sale, 6 to 10 years, fair value	90
Available for sale, over 10 years, amortized cost	0
Available for sale, over 10 years, fair value	0
Available for sale, amortized cost	90
Available for sale, fair value	90
Asset-backed securities, collateralized loan obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	81
Available for sale, under 1 year, fair value	80
Available for sale, 1 to 5 years, amortized cost	1,005
Available for sale, 1 to 5 years, fair value	998
Available for sale, 6 to 10 years, amortized cost	773
Available for sale, 6 to 10 years, fair value	767
Available for sale, over 10 years, amortized cost	694
Available for sale, over 10 years, fair value	693
Available for sale, amortized cost	2,553
Available for sale, fair value	2,538
Total asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	214
Available for sale, under 1 year, fair value	217
Available for sale, 1 to 5 years, amortized cost	1,114
Available for sale, 1 to 5 years, fair value	1,106
Available for sale, 6 to 10 years, amortized cost	864
Available for sale, 6 to 10 years, fair value	858
Available for sale, over 10 years, amortized cost	807
Available for sale, over 10 years, fair value	805
Available for sale, amortized cost	2,999
Available for sale, fair value	2,986
Held to maturity, under 1 year, amortized cost	362
Held to maturity, under 1 year, fair value	351
Held to maturity, 1 to 5 years, amortized cost	180
Held to maturity, 1 to 5 years, fair value	175
Held to maturity, 6 to 10 years, amortized cost	598
Held to maturity, 6 to 10 years, fair value	588
Held to maturity, over 10 years, amortized cost	3,183
Held to maturity, over 10 years, fair value	3,170
Held to maturity, amortized cost	4,323
Held to maturity, fair value	4,284
Non-U.S. debt securities, mortgage-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	166
Available for sale, under 1 year, fair value	165
Available for sale, 1 to 5 years, amortized cost	599
Available for sale, 1 to 5 years, fair value	599
Available for sale, 6 to 10 years, amortized cost	126
Available for sale, 6 to 10 years, fair value	126
Available for sale, over 10 years, amortized cost	974
Available for sale, over 10 years, fair value	973
Available for sale, amortized cost	1,865
Available for sale, fair value	1,863
Held to maturity, under 1 year, amortized cost	100
Held to maturity, under 1 year, fair value	96
Held to maturity, 1 to 5 years, amortized cost	26
Held to maturity, 1 to 5 years, fair value	26
Held to maturity, 6 to 10 years, amortized cost	3
Held to maturity, 6 to 10 years, fair value	3
Held to maturity, over 10 years, amortized cost	192
Held to maturity, over 10 years, fair value	253
Held to maturity, amortized cost	321
Held to maturity, fair value	378
Non-U.S. debt securities, asset-backed securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	327
Available for sale, under 1 year, fair value	326
Available for sale, 1 to 5 years, amortized cost	1,059
Available for sale, 1 to 5 years, fair value	1,055
Available for sale, 6 to 10 years, amortized cost	271
Available for sale, 6 to 10 years, fair value	271
Available for sale, over 10 years, amortized cost	380
Available for sale, over 10 years, fair value	379
Available for sale, amortized cost	2,037
Available for sale, fair value	2,031
Non-U.S. debt securities, government securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	3,757
Available for sale, under 1 year, fair value	3,759
Available for sale, 1 to 5 years, amortized cost	7,751
Available for sale, 1 to 5 years, fair value	7,913
Available for sale, 6 to 10 years, amortized cost	880
Available for sale, 6 to 10 years, fair value	894
Available for sale, over 10 years, amortized cost	282
Available for sale, over 10 years, fair value	284
Available for sale, amortized cost	12,670
Available for sale, fair value	12,850
Held to maturity, under 1 year, amortized cost	419
Held to maturity, under 1 year, fair value	419
Held to maturity, 1 to 5 years, amortized cost	0
Held to maturity, 1 to 5 years, fair value	0
Held to maturity, 6 to 10 years, amortized cost	0
Held to maturity, 6 to 10 years, fair value	0
Held to maturity, over 10 years, amortized cost	0
Held to maturity, over 10 years, fair value	0
Held to maturity, amortized cost	419
Held to maturity, fair value	419
Non-U.S. debt securities, other
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	1,371
Available for sale, under 1 year, fair value	1,376
Available for sale, 1 to 5 years, amortized cost	8,083
Available for sale, 1 to 5 years, fair value	8,196
Available for sale, 6 to 10 years, amortized cost	1,586
Available for sale, 6 to 10 years, fair value	1,611
Available for sale, over 10 years, amortized cost	122
Available for sale, over 10 years, fair value	126
Available for sale, amortized cost	11,162
Available for sale, fair value	11,309
Total non-U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	5,621
Available for sale, under 1 year, fair value	5,626
Available for sale, 1 to 5 years, amortized cost	17,492
Available for sale, 1 to 5 years, fair value	17,763
Available for sale, 6 to 10 years, amortized cost	2,863
Available for sale, 6 to 10 years, fair value	2,902
Available for sale, over 10 years, amortized cost	1,758
Available for sale, over 10 years, fair value	1,762
Available for sale, amortized cost	27,734
Available for sale, fair value	28,053
Held to maturity, under 1 year, amortized cost	519
Held to maturity, under 1 year, fair value	515
Held to maturity, 1 to 5 years, amortized cost	26
Held to maturity, 1 to 5 years, fair value	26
Held to maturity, 6 to 10 years, amortized cost	3
Held to maturity, 6 to 10 years, fair value	3
Held to maturity, over 10 years, amortized cost	192
Held to maturity, over 10 years, fair value	253
Held to maturity, amortized cost	740
Held to maturity, fair value	797
State and political subdivisions
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	218
Available for sale, under 1 year, fair value	218
Available for sale, 1 to 5 years, amortized cost	641
Available for sale, 1 to 5 years, fair value	660
Available for sale, 6 to 10 years, amortized cost	511
Available for sale, 6 to 10 years, fair value	552
Available for sale, over 10 years, amortized cost	250
Available for sale, over 10 years, fair value	264
Available for sale, amortized cost	1,620
Available for sale, fair value	1,694
Collateralized mortgage obligations
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	0
Available for sale, under 1 year, fair value	0
Available for sale, 1 to 5 years, amortized cost	0
Available for sale, 1 to 5 years, fair value	0
Available for sale, 6 to 10 years, amortized cost	0
Available for sale, 6 to 10 years, fair value	0
Available for sale, over 10 years, amortized cost	83
Available for sale, over 10 years, fair value	84
Available for sale, amortized cost	83
Available for sale, fair value	84
Held to maturity, under 1 year, amortized cost	143
Held to maturity, under 1 year, fair value	153
Held to maturity, 1 to 5 years, amortized cost	269
Held to maturity, 1 to 5 years, fair value	269
Held to maturity, 6 to 10 years, amortized cost	4
Held to maturity, 6 to 10 years, fair value	4
Held to maturity, over 10 years, amortized cost	153
Held to maturity, over 10 years, fair value	169
Held to maturity, amortized cost	569
Held to maturity, fair value	595
Total
Contractual Maturities Of Debt Investment Securities [Line Items]
Held to maturity, under 1 year, amortized cost	4,732
Held to maturity, under 1 year, fair value	4,769
Held to maturity, 1 to 5 years, amortized cost	3,964
Held to maturity, 1 to 5 years, fair value	4,043
Held to maturity, 6 to 10 years, amortized cost	5,290
Held to maturity, 6 to 10 years, fair value	5,453
Held to maturity, over 10 years, amortized cost	31,410
Held to maturity, over 10 years, fair value	32,245
Held to maturity, amortized cost	45,396
Held to maturity, fair value	46,510
Other U.S. debt securities
Contractual Maturities Of Debt Investment Securities [Line Items]
Available for sale, under 1 year, amortized cost	489
Available for sale, under 1 year, fair value	493
Available for sale, 1 to 5 years, amortized cost	2,795
Available for sale, 1 to 5 years, fair value	2,849
Available for sale, 6 to 10 years, amortized cost	111
Available for sale, 6 to 10 years, fair value	118
Available for sale, over 10 years, amortized cost	0
Available for sale, over 10 years, fair value	0
Available for sale, amortized cost	3,395
Available for sale, fair value	3,460
Money Market Liquidity Facility
Contractual Maturities Of Debt Investment Securities [Line Items]
Held to maturity, under 1 year, amortized cost	4,825
Held to maturity, under 1 year, fair value	4,841
Held to maturity, 1 to 5 years, amortized cost	0
Held to maturity, 1 to 5 years, fair value	0
Held to maturity, 6 to 10 years, amortized cost	0
Held to maturity, 6 to 10 years, fair value	0
Held to maturity, over 10 years, amortized cost	0
Held to maturity, over 10 years, fair value	0
Held to maturity, amortized cost	4,825
Held to maturity, fair value	$ 4,841