Investment Securities - Amortized Cost, Fair Value, by Contract Maturity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale securities, Amortized Cost:
Due in one year or less	$ 42,502
Due after one year through five years	121,698
Due after five years through 10 years	67,347
Due after 10 years	150,182
Amortized cost	381,729	$ 345,306
Available-for-sale securities, Fair value
Due in one year or less	42,562
Due after one year through five years	122,936
Due after five years through 10 years	68,718
Due after 10 years	153,962
Fair value	$ 388,178	350,699
Available-for-sale securities, Average yield
Due in one year or less	0.59%
Due after one year through five years	0.65%
Due after five years through 10 years	1.00%
Due after 10 years	2.64%
Average yield	1.49%
Held-to-maturity securities, Amortized cost
Due in one year or less	$ 501
Due after one year through five years	42,700
Due after five years through 10 years	34,263
Due after 10 years	124,435
Amortized cost	201,899
Held-to-maturity securities, Fair value
Due in one year or less	501
Due after one year through five years	42,738
Due after five years through 10 years	35,152
Due after 10 years	127,081
Fair value	$ 205,472	48,941
Held-to-maturity securities, Average yield
Due in one year or less	1.86%
Due after one year through five years	0.60%
Due after five years through 10 years	1.65%
Due after 10 years	2.88%
Average yield	2.19%
Supplemental information
US government agencies and US government sponsored enterprises residential mortgage-backed securities estimated duration	5 years
US government agencies and US government sponsored enterprises residential collateralized mortgage obligations estimated duration	4 years
Non-agency 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	741
Due after five years through 10 years	7,797
Due after 10 years	115,257
Amortized cost	123,795	125,648
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	756
Due after five years through 10 years	8,139
Due after 10 years	117,495
Fair value	$ 126,390	128,295
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.66%
Due after five years through 10 years	1.67%
Due after 10 years	2.57%
Average yield	2.51%
Held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	158
Due after five years through 10 years	11,908
Due after 10 years	102,791
Amortized cost	114,857
Held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	160
Due after five years through 10 years	12,707
Due after 10 years	104,963
Fair value	$ 117,830	37,626
Held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.56%
Due after five years through 10 years	2.42%
Due after 10 years	2.94%
Average yield	2.88%
U.S. Treasury and government agencies
Available-for-sale securities, Amortized Cost:
Due in one year or less	$ 33,633
Due after one year through five years	110,033
Due after five years through 10 years	46,827
Due after 10 years	9,417
Amortized cost	199,910	139,162
Available-for-sale securities, Fair value
Due in one year or less	33,678
Due after one year through five years	111,014
Due after five years through 10 years	47,675
Due after 10 years	9,584
Fair value	$ 201,951	139,436
Available-for-sale securities, Average yield
Due in one year or less	0.42%
Due after one year through five years	0.53%
Due after five years through 10 years	0.79%
Due after 10 years	0.48%
Average yield	0.57%
Held-to-maturity securities, Amortized cost
Due in one year or less	$ 501
Due after one year through five years	42,477
Due after five years through 10 years	10,206
Due after 10 years	0
Amortized cost	53,184
Held-to-maturity securities, Fair value
Due in one year or less	501
Due after one year through five years	42,511
Due after five years through 10 years	10,222
Due after 10 years	0
Fair value	$ 53,234	50
Held-to-maturity securities, Average yield
Due in one year or less	1.86%
Due after one year through five years	0.60%
Due after five years through 10 years	0.94%
Due after 10 years	0.00%
Average yield	0.67%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized Cost:
Due in one year or less	$ 33
Due after one year through five years	203
Due after five years through 10 years	1,047
Due after 10 years	17,710
Amortized cost	18,993	27,693
Available-for-sale securities, Fair value
Due in one year or less	33
Due after one year through five years	211
Due after five years through 10 years	1,111
Due after 10 years	19,041
Fair value	$ 20,396	29,810
Available-for-sale securities, Average yield
Due in one year or less	4.11%
Due after one year through five years	4.59%
Due after five years through 10 years	4.84%
Due after 10 years	4.80%
Average yield	4.80%
Held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	65
Due after five years through 10 years	532
Due after 10 years	12,211
Amortized cost	12,808
Held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	67
Due after five years through 10 years	565
Due after 10 years	12,638
Fair value	$ 13,270	5,096
Held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	3.09%
Due after five years through 10 years	3.57%
Due after 10 years	3.62%
Average yield	3.62%
Non-U.S. government debt securities
Available-for-sale securities, Amortized Cost:
Due in one year or less	$ 8,282
Due after one year through five years	8,011
Due after five years through 10 years	5,615
Due after 10 years	679
Amortized cost	22,587	21,427
Available-for-sale securities, Fair value
Due in one year or less	8,297
Due after one year through five years	8,225
Due after five years through 10 years	5,726
Due after 10 years	680
Fair value	$ 22,928	21,787
Available-for-sale securities, Average yield
Due in one year or less	1.25%
Due after one year through five years	1.70%
Due after five years through 10 years	0.68%
Due after 10 years	0.17%
Average yield	1.24%
Corporate debt securities
Available-for-sale securities, Amortized Cost:
Due in one year or less	$ 0
Due after one year through five years	141
Due after five years through 10 years	74
Due after 10 years	0
Amortized cost	215	823
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	139
Due after five years through 10 years	77
Due after 10 years	0
Fair value	$ 216	$ 845
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.21%
Due after five years through 10 years	1.92%
Due after 10 years	0.00%
Average yield	1.45%
Asset-backed securities
Available-for-sale securities, Amortized Cost:
Due in one year or less	$ 554
Due after one year through five years	2,569
Due after five years through 10 years	5,987
Due after 10 years	7,119
Amortized cost	16,229
Available-for-sale securities, Fair value
Due in one year or less	554
Due after one year through five years	2,591
Due after five years through 10 years	5,990
Due after 10 years	7,162
Fair value	$ 16,297
Available-for-sale securities, Average yield
Due in one year or less	1.31%
Due after one year through five years	2.00%
Due after five years through 10 years	1.33%
Due after 10 years	1.48%
Average yield	1.50%
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	11,617
Due after 10 years	9,433
Amortized cost	21,050
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	11,658
Due after 10 years	9,480
Fair value	$ 21,138
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	1.40%
Due after 10 years	1.33%
Average yield	1.37%