Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Available-for-sale securities, Amortized cost
Due in one year or less	$ 9,108
Due after one year through five years	69,512
Due after five years through 10 years	42,476
Due after 10 years	109,017
Amortized cost	230,113
Available-for-sale securities, Fair value
Due in one year or less	9,138
Due after one year through five years	69,916
Due after five years through 10 years	42,514
Due after 10 years	110,593
Fair value	$ 232,161	$ 388,178
Available-for-sale securities, Average yield
Due in one year or less	1.63%
Due after one year through five years	0.67%
Due after five years through 10 years	1.01%
Due after 10 years	2.44%
Average yield	1.61%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 6,521
Due after one year through five years	105,372
Due after five years through 10 years	98,360
Due after 10 years	131,307
Amortized cost	341,560
Total held-to-maturity securities, Fair value
Due in one year or less	6,525
Due after one year through five years	105,185
Due after five years through 10 years	98,520
Due after 10 years	132,623
Fair value	$ 342,853	205,472
Total held-to-maturity securities, Average yield
Due in one year or less	0.26%
Due after one year through five years	0.63%
Due after five years through 10 years	1.45%
Due after 10 years	2.80%
Average yield	1.69%
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	$ 1
Due after one year through five years	2,285
Due after five years through 10 years	5,712
Due after 10 years	73,391
Amortized cost	81,389
Available-for-sale securities, Fair value
Due in one year or less	1
Due after one year through five years	2,315
Due after five years through 10 years	6,050
Due after 10 years	73,757
Fair value	$ 82,123	126,390
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.43%
Due after five years through 10 years	1.77%
Due after 10 years	2.36%
Average yield	2.29%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	422
Due after five years through 10 years	12,046
Due after 10 years	103,470
Amortized cost	115,938
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	423
Due after five years through 10 years	12,595
Due after 10 years	104,360
Fair value	$ 117,378	117,830
Total held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.13%
Due after five years through 10 years	2.39%
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	$ 2,168
Due after one year through five years	60,911
Due after five years through 10 years	28,155
Due after 10 years	8,671
Amortized cost	99,905
Available-for-sale securities, Fair value
Due in one year or less	2,187
Due after one year through five years	61,190
Due after five years through 10 years	27,776
Due after 10 years	8,694
Fair value	$ 99,847	201,951
Available-for-sale securities, Average yield
Due in one year or less	1.47%
Due after one year through five years	0.49%
Due after five years through 10 years	0.67%
Due after 10 years	0.46%
Average yield	0.56%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 6,486
Due after one year through five years	104,920
Due after five years through 10 years	71,123
Due after 10 years	0
Amortized cost	182,529
Total held-to-maturity securities, Fair value
Due in one year or less	6,489
Due after one year through five years	104,732
Due after five years through 10 years	70,634
Due after 10 years	0
Fair value	$ 181,855	53,234
Total held-to-maturity securities, Average yield
Due in one year or less	0.24%
Due after one year through five years	0.62%
Due after five years through 10 years	1.30%
Due after 10 years	0.00%
Average yield	0.87%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 35
Due after one year through five years	167
Due after five years through 10 years	1,217
Due after 10 years	15,830
Amortized cost	17,249
Available-for-sale securities, Fair value
Due in one year or less	35
Due after one year through five years	173
Due after five years through 10 years	1,283
Due after 10 years	16,964
Fair value	$ 18,455	20,396
Available-for-sale securities, Average yield
Due in one year or less	3.72%
Due after one year through five years	4.40%
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	30
Due after five years through 10 years	766
Due after 10 years	12,756
Amortized cost	13,587
Total held-to-maturity securities, Fair value
Due in one year or less	36
Due after one year through five years	30
Due after five years through 10 years	808
Due after 10 years	13,134
Fair value	$ 14,008	13,270
Total held-to-maturity securities, Average yield
Due in one year or less	4.28%
Due after one year through five years	2.65%
Due after five years through 10 years	3.81%
Due after 10 years	3.78%
Average yield	3.78%
Non-U.S. government debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 5,894
Due after one year through five years	4,589
Due after five years through 10 years	3,506
Due after 10 years	3,660
Amortized cost	17,649
Available-for-sale securities, Fair value
Due in one year or less	5,905
Due after one year through five years	4,668
Due after five years through 10 years	3,509
Due after 10 years	3,658
Fair value	$ 17,740	22,928
Available-for-sale securities, Average yield
Due in one year or less	1.75%
Due after one year through five years	2.15%
Due after five years through 10 years	0.81%
Due after 10 years	0.19%
Average yield	1.34%
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	73
Due after 10 years	0
Amortized cost	214
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	140
Due after five years through 10 years	76
Due after 10 years	0
Fair value	$ 216	$ 216
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	1.16%
Due after five years through 10 years	2.29%
Due after 10 years	0.00%
Average yield	1.54%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 1,010
Due after one year through five years	1,419
Due after five years through 10 years	3,813
Due after 10 years	7,465
Amortized cost	13,707
Available-for-sale securities, Fair value
Due in one year or less	1,010
Due after one year through five years	1,430
Due after five years through 10 years	3,820
Due after 10 years	7,520
Fair value	$ 13,780
Available-for-sale securities, Average yield
Due in one year or less	1.20%
Due after one year through five years	1.94%
Due after five years through 10 years	1.33%
Due after 10 years	1.24%
Average yield	1.34%
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	14,425
Due after 10 years	15,081
Amortized cost	29,506
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	14,483
Due after 10 years	15,129
Fair value	$ 29,612
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	1.27%
Due after 10 years	1.30%
Average yield	1.29%