Investment Securities - Amortized Cost and Estimated Fair Value by Contractual Maturity (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale securities, Amortized cost
Due in one year or less	$ 11,686
Due after one year through five years	58,388
Due after five years through 10 years	41,585
Due after 10 years	159,767
Amortized cost	271,426	$ 228,769
Available-for-sale securities, Fair value
Due in one year or less	11,696
Due after one year through five years	58,793
Due after five years through 10 years	42,086
Due after 10 years	163,782
Fair value	$ 276,357	230,394
Available-for-sale securities, Average yield
Due in one year or less	2.46%
Due after one year through five years	2.46%
Due after five years through 10 years	2.76%
Due after 10 years	3.68%
Average yield	3.23%
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	4,325
Due after 10 years	26,582
Amortized cost	30,907	31,434
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	4,610
Due after 10 years	27,462
Fair value	$ 32,072	31,458
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	3.38%
Due after 10 years	3.45%
Average yield	3.44%
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	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	$ 287
Due after one year through five years	89
Due after five years through 10 years	9,642
Due after 10 years	103,021
Amortized cost	113,039	84,190
Available-for-sale securities, Fair value
Due in one year or less	287
Due after one year through five years	90
Due after five years through 10 years	9,827
Due after 10 years	105,141
Fair value	$ 115,345	83,820
Available-for-sale securities, Average yield
Due in one year or less	2.27%
Due after one year through five years	2.55%
Due after five years through 10 years	3.18%
Due after 10 years	3.50%
Average yield	3.47%
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	4,293
Due after 10 years	21,804
Amortized cost	26,097	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	4,576
Due after 10 years	22,424
Fair value	$ 27,000	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	3.37%
Due after 10 years	3.33%
Average yield	3.34%
U.S. Treasury and government agencies
Available-for-sale securities, Amortized cost
Due in one year or less	$ 4,550
Due after one year through five years	42,673
Due after five years through 10 years	18,309
Due after 10 years	8,387
Amortized cost	73,919	55,771
Available-for-sale securities, Fair value
Due in one year or less	4,551
Due after one year through five years	42,760
Due after five years through 10 years	18,385
Due after 10 years	8,294
Fair value	$ 73,990	56,059
Available-for-sale securities, Average yield
Due in one year or less	2.55%
Due after one year through five years	2.53%
Due after five years through 10 years	2.34%
Due after 10 years	2.55%
Average yield	2.49%
Obligations of U.S. states and municipalities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 158
Due after one year through five years	410
Due after five years through 10 years	1,317
Due after 10 years	27,984
Amortized cost	29,869	36,221
Available-for-sale securities, Fair value
Due in one year or less	159
Due after one year through five years	417
Due after five years through 10 years	1,365
Due after 10 years	29,960
Fair value	$ 31,901	37,723
Available-for-sale securities, Average yield
Due in one year or less	2.83%
Due after one year through five years	4.27%
Due after five years through 10 years	5.69%
Due after 10 years	4.99%
Average yield	5.00%
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	32
Due after 10 years	4,778
Amortized cost	4,810	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	34
Due after 10 years	5,038
Fair value	$ 5,072	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	3.82%
Due after 10 years	4.02%
Average yield	4.01%
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	$ 6,497
Due after one year through five years	11,961
Due after five years through 10 years	3,559
Due after 10 years	0
Amortized cost	22,017	23,771
Available-for-sale securities, Fair value
Due in one year or less	6,504
Due after one year through five years	12,246
Due after five years through 10 years	3,742
Due after 10 years	0
Fair value	$ 22,492	24,102
Available-for-sale securities, Average yield
Due in one year or less	2.38%
Due after one year through five years	1.94%
Due after five years through 10 years	1.20%
Due after 10 years	0.00%
Average yield	1.95%
Corporate debt securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 120
Due after one year through five years	910
Due after five years through 10 years	561
Due after 10 years	145
Amortized cost	1,736	1,904
Available-for-sale securities, Fair value
Due in one year or less	121
Due after one year through five years	932
Due after five years through 10 years	572
Due after 10 years	153
Fair value	$ 1,778	$ 1,918
Available-for-sale securities, Average yield
Due in one year or less	4.89%
Due after one year through five years	4.34%
Due after five years through 10 years	4.12%
Due after 10 years	4.58%
Average yield	4.33%
Asset-backed securities
Available-for-sale securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	2,345
Due after five years through 10 years	8,197
Due after 10 years	20,230
Amortized cost	30,772
Available-for-sale securities, Fair value
Due in one year or less	0
Due after one year through five years	2,348
Due after five years through 10 years	8,195
Due after 10 years	20,234
Fair value	$ 30,777
Available-for-sale securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	2.91%
Due after five years through 10 years	3.32%
Due after 10 years	3.27%
Average yield	3.25%