Fair Value Measurement and Fair Value of Financial Instruments - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair value measurements
Trading account assets	$ 8,942	$ 3,734
Securities available for sale	14,136
Other liabilities	2,520	2,251
U.S. Treasury
Fair value measurements
Securities available for sale	2,505
Other
Fair value measurements
Securities available for sale	107
U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	6,695
Privately issued
Fair value measurements
Securities available for sale	327
Collateralized loan obligations
Fair value measurements
Securities available for sale	2,218
Other
Fair value measurements
Securities available for sale	7
Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,613
Level 1
Fair value measurements
Other assets		0
Level 2
Fair value measurements
Other assets		0
Level 3
Fair value measurements
Other assets		17
Fair Value, Measurements, Recurring
Fair value measurements
Trading account assets	8,942	3,734
Netting adjustment	(770)	(909)
Securities available for sale	14,141	14,359
Other assets	28	14
Total assets	$ 23,111	$ 18,107
Percentage of total	100.00%	100.00%
Percentage of total	(3.00%)	(5.00%)
Percentage of total Company assets	15.00%	12.00%
Percentage of total Company assets	(1.00%)	(0.00%)
Trading account liabilities	$ 2,905	$ 3,712
Netting adjustment	(1,047)	(872)
FDIC clawback liability		112
Other liabilities	176	116
Total liabilities	$ 3,081	$ 3,828
Percentage of total	100.00%	100.00%
Percentage of total	(34.00%)	(23.00%)
Percentage of total Company liabilities	2.00%	2.00%
Percentage of total Company liabilities	(1.00%)	(1.00%)
Fair Value, Measurements, Recurring | U.S. Treasury
Fair value measurements
Trading account assets	$ 1,730	$ 1,429
Securities available for sale	2,505	594
Trading account liabilities	1,973	2,469
Fair Value, Measurements, Recurring | U.S. government-sponsored agency securities
Fair value measurements
Trading account assets	73	363
Fair Value, Measurements, Recurring | State and municipal securities
Fair value measurements
Trading account assets	18	5
Fair Value, Measurements, Recurring | Commercial Paper
Fair value measurements
Trading account assets	1	25
Fair Value, Measurements, Recurring | Other
Fair value measurements
Trading account assets	16	5
Securities available for sale	107	32
Trading account liabilities	11	16
Fair Value, Measurements, Recurring | Corporate bonds
Fair value measurements
Trading account assets	841	828
Fair Value, Measurements, Recurring | Asset-backed securities
Fair value measurements
Trading account assets	106	133
Fair Value, Measurements, Recurring | Mortgage-backed securities
Fair value measurements
Trading account assets	5,221
Fair Value, Measurements, Recurring | Equity securities
Fair value measurements
Trading account assets	85	30
Securities available for sale	5	8
Trading account liabilities	47	42
Fair Value, Measurements, Recurring | Interest rate derivative contracts
Fair value measurements
Trading account assets	731	839
Netting adjustment	(343)	(163)
Trading account liabilities	270	173
Netting adjustment	(718)	(775)
Fair Value, Measurements, Recurring | Commodity derivative contracts
Fair value measurements
Trading account assets	39	25
Netting adjustment	(106)	(384)
Trading account liabilities	44	308
Netting adjustment	(68)	(61)
Fair Value, Measurements, Recurring | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	79	47
Netting adjustment	(138)	(70)
Trading account liabilities	98	71
Netting adjustment	(33)	(22)
Fair Value, Measurements, Recurring | Equity derivative contracts
Fair value measurements
Trading account assets	2	5
Netting adjustment	(163)	(217)
Trading account liabilities	164	221
Netting adjustment	0
Fair Value, Measurements, Recurring | Trading Securities
Fair value measurements
Netting adjustment	(750)	(834)
Netting adjustment	(819)	(858)
Fair Value, Measurements, Recurring | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	6,695	7,201
Fair Value, Measurements, Recurring | Privately issued
Fair value measurements
Securities available for sale	327	151
Fair Value, Measurements, Recurring | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	664	1,546
Fair Value, Measurements, Recurring | Collateralized loan obligations
Fair value measurements
Securities available for sale	2,218	3,233
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting adjustment		(75)
Securities available for sale	7	22
Netting adjustment	(228)
Fair Value, Measurements, Recurring | Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,613	1,572
Fair Value, Measurements, Recurring | Mortgage servicing rights
Fair value measurements
Other assets	23
Fair Value, Measurements, Recurring | Interest rate hedging contracts
Fair value measurements
Netting adjustment	(20)	(71)
Other assets	2	4
Netting adjustment	(199)	(14)
Other liabilities		2
Fair Value, Measurements, Recurring | Other derivative contracts
Fair value measurements
Netting adjustment		(4)
Other assets	3	10
Netting adjustment	(29)
Other liabilities	61	2
Fair Value, Measurements, Recurring | Corporate bonds
Fair value measurements
Trading account liabilities	298	412
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Trading account assets	94	31
Securities available for sale	5	8
Total assets	$ 99	$ 39
Percentage of total	0.00%	0.00%
Percentage of total Company assets	0.00%	0.00%
Trading account liabilities	$ 49	$ 44
Total liabilities	$ 49	$ 44
Percentage of total	2.00%	1.00%
Percentage of total Company liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Fair value measurements
Trading account assets	$ 85	$ 30
Securities available for sale	5	8
Trading account liabilities	47	42
Fair Value, Measurements, Recurring | Level 1 | Interest rate derivative contracts
Fair value measurements
Trading account assets	7	0
Trading account liabilities	1	1
Fair Value, Measurements, Recurring | Level 1 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	1	1
Trading account liabilities	1	1
Fair Value, Measurements, Recurring | Level 1 | Equity derivative contracts
Fair value measurements
Trading account assets	1
Fair Value, Measurements, Recurring | Level 1 | Other derivative contracts
Fair value measurements
Other assets
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Trading account assets	9,430	4,309
Securities available for sale	12,498	12,748
Other assets	24	88
Total assets	$ 21,952	$ 17,145
Percentage of total	95.00%	95.00%
Percentage of total Company assets	15.00%	11.00%
Trading account liabilities	$ 3,509	$ 4,303
Other liabilities	283	16
Total liabilities	$ 3,792	$ 4,319
Percentage of total	123.00%	113.00%
Percentage of total Company liabilities	3.00%	3.00%
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury
Fair value measurements
Trading account assets	$ 1,730	$ 1,429
Securities available for sale	2,505	594
Trading account liabilities	1,973	2,469
Fair Value, Measurements, Recurring | Level 2 | U.S. government-sponsored agency securities
Fair value measurements
Trading account assets	73	363
Fair Value, Measurements, Recurring | Level 2 | State and municipal securities
Fair value measurements
Trading account assets	18	5
Fair Value, Measurements, Recurring | Level 2 | Commercial Paper
Fair value measurements
Trading account assets	1	25
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Trading account assets	16	5
Securities available for sale	82	1
Trading account liabilities	11	16
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair value measurements
Trading account assets	841	828
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Fair value measurements
Trading account assets	106	133
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed securities
Fair value measurements
Trading account assets	5,221
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivative contracts
Fair value measurements
Trading account assets	1,065	998
Trading account liabilities	987	947
Fair Value, Measurements, Recurring | Level 2 | Commodity derivative contracts
Fair value measurements
Trading account assets	144	408
Trading account liabilities	111	368
Fair Value, Measurements, Recurring | Level 2 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	215	115
Trading account liabilities	129	91
Fair Value, Measurements, Recurring | Level 2 | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	6,695	7,201
Fair Value, Measurements, Recurring | Level 2 | Privately issued
Fair value measurements
Securities available for sale	327	151
Fair Value, Measurements, Recurring | Level 2 | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	664	1,546
Fair Value, Measurements, Recurring | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale	2,218	3,233
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Securities available for sale	7	22
Fair Value, Measurements, Recurring | Level 2 | Interest rate hedging contracts
Fair value measurements
Other assets	22	75
Other liabilities	199	16
Fair Value, Measurements, Recurring | Level 2 | Other derivative contracts
Fair value measurements
Other assets	2	13
Other liabilities	84	0
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair value measurements
Trading account liabilities	298	412
Fair Value, Measurements, Recurring | Level 3
Fair value measurements
Trading account assets	168	228
Securities available for sale	1,638	1,603
Other assets	24	1
Total assets	$ 1,830	$ 1,832
Percentage of total	8.00%	10.00%
Percentage of total Company assets	1.00%	1.00%
Trading account liabilities	$ 166	$ 223
FDIC clawback liability		112
Other liabilities	121	114
Total liabilities	$ 287	$ 337
Percentage of total	9.00%	9.00%
Percentage of total Company liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring | Level 3 | Other
Fair value measurements
Securities available for sale	$ 25	$ 31
Fair Value, Measurements, Recurring | Level 3 | Interest rate derivative contracts
Fair value measurements
Trading account assets	2	4
Fair Value, Measurements, Recurring | Level 3 | Commodity derivative contracts
Fair value measurements
Trading account assets	1	1
Trading account liabilities	1	1
Fair Value, Measurements, Recurring | Level 3 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	1	1
Trading account liabilities	1	1
Fair Value, Measurements, Recurring | Level 3 | Equity derivative contracts
Fair value measurements
Trading account assets	164	222
Trading account liabilities	164	221
Fair Value, Measurements, Recurring | Level 3 | Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,613	1,572
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights
Fair value measurements
Other assets	23
Fair Value, Measurements, Recurring | Level 3 | Other derivative contracts
Fair value measurements
Other assets	1	1
Other liabilities	6	$ 2
Fdic | Fair Value, Measurements, Recurring
Fair value measurements
Other liabilities	115
Fdic | Fair Value, Measurements, Recurring | Level 3
Fair value measurements
FDIC clawback liability	$ 115