Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale securities, Amortized cost
Due in one year or less	$ 17,720
Due after one year through five years	109,597
Due after five years through 10 years	50,365
Due after 10 years	170,177
Amortized cost	347,859	$ 228,769
Available-for-sale securities, Fair value
Due in one year or less	17,729
Due after one year through five years	110,010
Due after five years through 10 years	50,994
Due after 10 years	174,688
Fair value	$ 353,421	230,394
Available-for-sale securities, Average yield
Due in one year or less	2.12%
Due after one year through five years	2.00%
Due after five years through 10 years	2.47%
Due after 10 years	3.55%
Average yield	2.83%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	51
Due after five years through 10 years	4,710
Due after 10 years	36,069
Amortized cost	40,830	31,434
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	51
Due after five years through 10 years	5,107
Due after 10 years	37,219
Fair value	$ 42,377	31,458
Total held-to-maturity 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	3.30%
Due after 10 years	3.24%
Average yield	3.24%
Supplemental information
US government agencies and US government sponsored enterprises residential MBS estimated duration	5 years
US government agencies and US government sponsored enterprises residential collateralized mortgage obligations estimated duration	3 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	$ 14
Due after one year through five years	59
Due after five years through 10 years	10,230
Due after 10 years	110,584
Amortized cost	120,887	84,190
Available-for-sale securities, Fair value
Due in one year or less	14
Due after one year through five years	60
Due after five years through 10 years	10,476
Due after 10 years	113,257
Fair value	$ 123,807	83,820
Available-for-sale securities, Average yield
Due in one year or less	3.59%
Due after one year through five years	2.73%
Due after five years through 10 years	2.92%
Due after 10 years	3.45%
Average yield	3.40%
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	4,655
Due after 10 years	31,321
Amortized cost	35,976	26,610
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	5,049
Due after 10 years	32,157
Fair value	$ 37,206	26,544
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	3.29%
Due after 10 years	3.12%
Average yield	3.14%
U.S. Treasury and government agencies
Available-for-sale securities, Amortized cost
Due in one year or less	$ 10,904
Due after one year through five years	95,139
Due after five years through 10 years	25,557
Due after 10 years	10,046
Amortized cost	141,646	55,771
Available-for-sale securities, Fair value
Due in one year or less	10,901
Due after one year through five years	95,231
Due after five years through 10 years	25,659
Due after 10 years	9,738
Fair value	$ 141,529	56,059
Available-for-sale securities, Average yield
Due in one year or less	1.99%
Due after one year through five years	1.96%
Due after five years through 10 years	2.02%
Due after 10 years	2.04%
Average yield	1.98%
Total held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	51
Due after five years through 10 years	0
Due after 10 years	0
Amortized cost	51	0
Total held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	51
Due after five years through 10 years	0
Due after 10 years	0
Fair value	$ 51	0
Total held-to-maturity 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	0.00%
Due after 10 years	0.00%
Average yield	1.47%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 170
Due after one year through five years	228
Due after five years through 10 years	1,089
Due after 10 years	27,384
Amortized cost	28,871	36,221
Available-for-sale securities, Fair value
Due in one year or less	172
Due after one year through five years	236
Due after five years through 10 years	1,135
Due after 10 years	29,521
Fair value	$ 31,064	37,723
Available-for-sale securities, Average yield
Due in one year or less	3.32%
Due after one year through five years	4.00%
Due after five years through 10 years	5.60%
Due after 10 years	4.94%
Average yield	4.95%
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	55
Due after 10 years	4,748
Amortized cost	4,803	4,824
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	58
Due after 10 years	5,062
Fair value	$ 5,120	4,914
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	3.89%
Due after 10 years	4.02%
Average yield	4.02%
Certificates of deposit
Available-for-sale securities, Amortized cost
Due in one year or less	$ 74
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Amortized cost	74	75
Available-for-sale securities, Fair value
Due in one year or less	74
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Fair value	$ 74	75
Available-for-sale securities, Average yield
Due in one year or less	0.49%
Due after one year through five years	0.00%
Due after five years through 10 years	0.00%
Due after 10 years	0.00%
Average yield	0.49%
Non-U.S. government debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 6,342
Due after one year through five years	11,580
Due after five years through 10 years	3,416
Due after 10 years	306
Amortized cost	21,644	23,771
Available-for-sale securities, Fair value
Due in one year or less	6,350
Due after one year through five years	11,870
Due after five years through 10 years	3,633
Due after 10 years	305
Fair value	$ 22,158	24,102
Available-for-sale securities, Average yield
Due in one year or less	2.23%
Due after one year through five years	1.99%
Due after five years through 10 years	1.34%
Due after 10 years	1.67%
Average yield	1.95%
Corporate debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 216
Due after one year through five years	718
Due after five years through 10 years	662
Due after 10 years	0
Amortized cost	1,596	1,904
Available-for-sale securities, Fair value
Due in one year or less	218
Due after one year through five years	737
Due after five years through 10 years	679
Due after 10 years	0
Fair value	$ 1,634	$ 1,918
Available-for-sale securities, Average yield
Due in one year or less	4.83%
Due after one year through five years	3.91%
Due after five years through 10 years	3.78%
Due after 10 years	0.00%
Average yield	3.98%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	1,873
Due after five years through 10 years	9,411
Due after 10 years	21,857
Amortized cost	33,141
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	1,876
Due after five years through 10 years	9,412
Due after 10 years	21,867
Fair value	$ 33,155
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	2.94%
Due after five years through 10 years	3.17%
Due after 10 years	3.03%
Average yield	3.06%