Securities - Amortized Cost, Fair Value, by Contract Maturity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 10,274
Due after one year through five years	23,583
Due after five years through 10 years	84,124
Due after 10 years	117,535
Amortized cost	235,516	$ 235,391
Available-for-sale debt securities, Fair value
Due in one year or less	10,303
Due after one year through five years	23,999
Due after five years through 10 years	84,175
Due after 10 years	119,488
Fair value	$ 237,965	239,667
Available-for-sale debt securities, Average yield
Due in one year or less	2.73%
Due after one year through five years	1.63%
Due after five years through 10 years	1.74%
Due after 10 years	3.92%
Average yield	2.86%
Available-for-sale equity 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	0
Due after 10 years	914
Amortized cost	914
Available-for-sale equity 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	0
Due after 10 years	926
Fair value	$ 926
Available-for-sale equity securities, Average yield
Due in 1 year or less, average yield	0.00%
Due after 1 year through 5 years, average yield	0.00%
Due after 5 years through 10 years, average yield	0.00%
Due after 10 years, average yield	0.58%
Available-for-sale securities, equity maturities, average yield, total	0.58%
Available-for-sale securities, Amortized cost
Due in 1 year or less, amortized cost	$ 10,274
Due after 1 year through 5 years, amortized cost	23,583
Due after 5 years through 10 years, amortized cost	84,124
Due after 10 years, amortized cost	118,449
Amortized cost	236,430	237,458
Available-for-sale securities, Fair value
Due in 1 year or less, fair value	10,303
Due after 1 year through 5 years, fair value	23,999
Due after 5 years through 10 years, fair value	84,175
Due after 10 years, fair value	120,414
Fair value	$ 238,891	241,754
Available-for-sale securities, Average yield
Due in one year or less	2.73%
Due after one year through five years	1.63%
Due after five years through 10 years	1.74%
Due after 10 years	3.89%
Average yield	2.85%
Held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	29
Due after five years through 10 years	1,439
Due after 10 years	48,700
Amortized cost	50,168	49,073
Held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	29
Due after five years through 10 years	1,467
Due after 10 years	49,393
Fair value	$ 50,889	50,587
Held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	6.61%
Due after five years through 10 years	5.11%
Due after 10 years	3.94%
Average yield	3.97%
Supplemental information
US government agencies and US government sponsored enterprises residential mortgage-backed securities estimated duration	7 years
US government agencies and US government sponsored enterprises residential collateralized mortgage obligations estimated duration	3 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 debt securities, Amortized Cost:
Due in one year or less	$ 2,012
Due after one year through five years	2,393
Due after five years through 10 years	7,574
Due after 10 years	74,610
Amortized cost	86,589	103,980
Available-for-sale debt securities, Fair value
Due in one year or less	2,022
Due after one year through five years	2,449
Due after five years through 10 years	7,756
Due after 10 years	75,325
Fair value	$ 87,552	105,582
Available-for-sale debt securities, Average yield
Due in one year or less	2.04%
Due after one year through five years	2.36%
Due after five years through 10 years	3.03%
Due after 10 years	3.26%
Average yield	3.19%
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	0
Due after 10 years	35,693
Amortized cost	35,693	36,271
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	0
Due after 10 years	36,165
Fair value	$ 36,165	37,081
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	0.00%
Due after 10 years	3.30%
Average yield	3.30%
U.S. Treasury and government agencies
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 132
Due after one year through five years	4,573
Due after five years through 10 years	38,976
Due after 10 years	1,141
Amortized cost	44,822	11,202
Available-for-sale debt securities, Fair value
Due in one year or less	132
Due after one year through five years	4,561
Due after five years through 10 years	38,317
Due after 10 years	1,091
Fair value	$ 44,101	11,036
Available-for-sale debt securities, Average yield
Due in one year or less	0.42%
Due after one year through five years	0.86%
Due after five years through 10 years	1.27%
Due after 10 years	1.13%
Average yield	1.22%
Obligations of U.S. states and municipalities
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 134
Due after one year through five years	752
Due after five years through 10 years	1,096
Due after 10 years	28,302
Amortized cost	30,284	31,328
Available-for-sale debt securities, Fair value
Due in one year or less	135
Due after one year through five years	767
Due after five years through 10 years	1,148
Due after 10 years	29,542
Fair value	$ 31,592	33,550
Available-for-sale debt securities, Average yield
Due in one year or less	5.85%
Due after one year through five years	3.58%
Due after five years through 10 years	6.29%
Due after 10 years	6.63%
Average yield	6.54%
Held-to-maturity securities, Amortized cost
Due in one year or less	$ 0
Due after one year through five years	29
Due after five years through 10 years	1,439
Due after 10 years	13,007
Amortized cost	14,475	12,802
Held-to-maturity securities, Fair value
Due in one year or less	0
Due after one year through five years	29
Due after five years through 10 years	1,467
Due after 10 years	13,228
Fair value	$ 14,724	13,506
Held-to-maturity securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	6.61%
Due after five years through 10 years	5.11%
Due after 10 years	5.68%
Average yield	5.63%
Certificates of deposit
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 106
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Amortized cost	106	282
Available-for-sale debt securities, Fair value
Due in one year or less	106
Due after one year through five years	0
Due after five years through 10 years	0
Due after 10 years	0
Fair value	$ 106	283
Available-for-sale debt securities, Average yield
Due in one year or less	1.78%
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	1.78%
Non-U.S. government debt securities
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 5,831
Due after one year through five years	14,109
Due after five years through 10 years	13,503
Due after 10 years	1,054
Amortized cost	34,497	35,864
Available-for-sale debt securities, Fair value
Due in one year or less	5,838
Due after one year through five years	14,444
Due after five years through 10 years	13,944
Due after 10 years	1,062
Fair value	$ 35,288	36,676
Available-for-sale debt securities, Average yield
Due in one year or less	2.92%
Due after one year through five years	1.55%
Due after five years through 10 years	0.93%
Due after 10 years	0.58%
Average yield	1.51%
Corporate debt securities
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 2,059
Due after one year through five years	1,312
Due after five years through 10 years	1,424
Due after 10 years	121
Amortized cost	4,916	12,464
Available-for-sale debt securities, Fair value
Due in one year or less	2,070
Due after one year through five years	1,332
Due after five years through 10 years	1,433
Due after 10 years	123
Fair value	$ 4,958	$ 12,436
Available-for-sale debt securities, Average yield
Due in one year or less	2.88%
Due after one year through five years	3.11%
Due after five years through 10 years	3.24%
Due after 10 years	3.52%
Average yield	3.06%
Asset-backed securities
Available-for-sale debt securities, Amortized Cost:
Due in one year or less	$ 0
Due after one year through five years	444
Due after five years through 10 years	21,551
Due after 10 years	12,307
Amortized cost	34,302
Available-for-sale debt securities, Fair value
Due in one year or less	0
Due after one year through five years	446
Due after five years through 10 years	21,577
Due after 10 years	12,345
Fair value	$ 34,368
Available-for-sale debt securities, Average yield
Due in one year or less	0.00%
Due after one year through five years	0.49%
Due after five years through 10 years	2.33%
Due after 10 years	2.21%
Average yield	2.26%