Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Available-for-sale securities, Amortized cost
Due in one year or less	$ 13,376
Due after one year through five years	97,339
Due after five years through 10 years	33,793
Due after 10 years	105,228
Amortized cost	249,736
Available-for-sale securities, Fair value
Due in one year or less	13,428
Due after one year through five years	97,529
Due after five years through 10 years	34,163
Due after 10 years	106,470
Fair value	$ 251,590	$ 388,178
Available-for-sale securities, Average yield
Due in one year or less	1.65%
Due after one year through five years	0.70%
Due after five years through 10 years	1.09%
Due after 10 years	2.48%
Average yield	1.56%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 14,411
Due after one year through five years	98,909
Due after five years through 10 years	96,509
Due after 10 years	133,783
Amortized cost	343,612
Total held-to-maturity securities, Fair value
Due in one year or less	14,418
Due after one year through five years	98,661
Due after five years through 10 years	96,242
Due after 10 years	135,072
Fair value	$ 344,393	205,472
Total held-to-maturity securities, Average yield
Due in one year or less	0.66%
Due after one year through five years	0.67%
Due after five years through 10 years	1.44%
Due after 10 years	2.71%
Average yield	1.68%
Supplemental information
US government agencies and US government sponsored enterprises residential MBS estimated duration	6 years
US government agencies and US government sponsored enterprises residential collateralized mortgage obligations estimated duration	4 years
U.S. nonagency residential collateralized mortgage obligations estimated duration	3 years
Minimum
Supplemental information
Due period of mortgage-backed securities and collateralized mortgage obligations	10 years
Mortgage-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	2,882
Due after five years through 10 years	5,326
Due after 10 years	73,925
Amortized cost	82,133
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	2,916
Due after five years through 10 years	5,650
Due after 10 years	73,988
Fair value	$ 82,554	126,390
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.47%
Due after five years through 10 years	1.79%
Due after 10 years	2.33%
Average yield	2.26%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	1,029
Due after five years through 10 years	11,737
Due after 10 years	99,091
Amortized cost	111,857
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	1,050
Due after five years through 10 years	12,197
Due after 10 years	100,069
Fair value	$ 113,316	117,830
Total held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.68%
Due after five years through 10 years	2.42%
Due after 10 years	2.90%
Average yield	2.84%
U.S. Treasury and government agencies
Available-for-sale securities, Amortized cost
Due in one year or less	$ 5,111
Due after one year through five years	88,001
Due after five years through 10 years	19,610
Due after 10 years	8,505
Amortized cost	121,227
Available-for-sale securities, Fair value
Due in one year or less	5,150
Due after one year through five years	88,082
Due after five years through 10 years	19,581
Due after 10 years	8,658
Fair value	$ 121,471	201,951
Available-for-sale securities, Average yield
Due in one year or less	1.28%
Due after one year through five years	0.53%
Due after five years through 10 years	0.68%
Due after 10 years	0.50%
Average yield	0.58%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 14,376
Due after one year through five years	97,814
Due after five years through 10 years	70,178
Due after 10 years	0
Amortized cost	182,368
Total held-to-maturity securities, Fair value
Due in one year or less	14,383
Due after one year through five years	97,545
Due after five years through 10 years	69,348
Due after 10 years	0
Fair value	$ 181,276	53,234
Total held-to-maturity securities, Average yield
Due in one year or less	0.65%
Due after one year through five years	0.66%
Due after five years through 10 years	1.29%
Due after 10 years	0.00%
Average yield	0.90%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 15
Due after one year through five years	167
Due after five years through 10 years	1,216
Due after 10 years	15,073
Amortized cost	16,471
Available-for-sale securities, Fair value
Due in one year or less	16
Due after one year through five years	172
Due after five years through 10 years	1,277
Due after 10 years	16,050
Fair value	$ 17,515	20,396
Available-for-sale securities, Average yield
Due in one year or less	3.91%
Due after one year through five years	4.37%
Due after five years through 10 years	4.88%
Due after 10 years	5.01%
Average yield	4.99%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 35
Due after one year through five years	66
Due after five years through 10 years	1,059
Due after 10 years	12,363
Amortized cost	13,523
Total held-to-maturity securities, Fair value
Due in one year or less	35
Due after one year through five years	66
Due after five years through 10 years	1,105
Due after 10 years	12,647
Fair value	$ 13,853	13,270
Total held-to-maturity securities, Average yield
Due in one year or less	3.78%
Due after one year through five years	2.67%
Due after five years through 10 years	3.67%
Due after 10 years	3.84%
Average yield	3.82%
Non-U.S. government debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 6,750
Due after one year through five years	4,980
Due after five years through 10 years	3,811
Due after 10 years	197
Amortized cost	15,738
Available-for-sale securities, Fair value
Due in one year or less	6,762
Due after one year through five years	5,050
Due after five years through 10 years	3,816
Due after 10 years	197
Fair value	$ 15,825	22,928
Available-for-sale securities, Average yield
Due in one year or less	2.02%
Due after one year through five years	2.54%
Due after five years through 10 years	0.84%
Due after 10 years	0.38%
Average yield	1.88%
Corporate debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	233
Due after five years through 10 years	30
Due after 10 years	0
Amortized cost	263
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	228
Due after five years through 10 years	32
Due after 10 years	0
Fair value	$ 260	$ 216
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	7.74%
Due after five years through 10 years	1.63%
Due after 10 years	0.00%
Average yield	7.01%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 1,500
Due after one year through five years	1,076
Due after five years through 10 years	3,800
Due after 10 years	7,528
Amortized cost	13,904
Available-for-sale securities, Fair value
Due in one year or less	1,500
Due after one year through five years	1,081
Due after five years through 10 years	3,807
Due after 10 years	7,577
Fair value	$ 13,965
Available-for-sale securities, Average yield
Due in one year or less	1.19%
Due after one year through five years	1.93%
Due after five years through 10 years	1.27%
Due after 10 years	1.26%
Average yield	1.31%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	0
Due after five years through 10 years	13,535
Due after 10 years	22,329
Amortized cost	35,864
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	0
Due after five years through 10 years	13,592
Due after 10 years	22,356
Fair value	$ 35,948
Total held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	0.00%
Due after five years through 10 years	1.19%
Due after 10 years	1.25%
Average yield	1.23%