Fair Value Measurement and Fair Value of Financial Instruments - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair value measurements
Trading account assets	$ 10,567	$ 8,942
Securities available for sale	17,553
Other liabilities	2,143	2,520
U.S. Treasury
Fair value measurements
Securities available for sale	3,252
Other
Fair value measurements
Securities available for sale	164
U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	9,208
Privately issued
Fair value measurements
Securities available for sale	694
Collateralized loan obligations
Fair value measurements
Securities available for sale	1,905
Other
Fair value measurements
Securities available for sale	5
Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,503
Fair Value, Measurements, Recurring
Fair value measurements
Trading account assets	10,567	8,942
Netting adjustment	(607)	(770)
Securities available for sale	17,563	14,141
Other assets	67	28
Total assets	$ 28,197	$ 23,111
Percentage of total	100.00%	100.00%
Percentage of total	(2.00%)	(3.00%)
Percentage of total Company assets	18.00%	15.00%
Percentage of total Company assets	(0.00%)	(1.00%)
Trading account liabilities	$ 3,600	$ 2,905
Netting adjustment	(620)	(1,047)
FDIC clawback liability		115
Other liabilities	119	176
Total liabilities	$ 3,719	$ 3,081
Percentage of total	100.00%	100.00%
Percentage of total	(17.00%)	(34.00%)
Percentage of total Company liabilities	3.00%	2.00%
Percentage of total Company liabilities	(0.00%)	(1.00%)
Fair Value, Measurements, Recurring | U.S. Treasury
Fair value measurements
Trading account assets	$ 1,926	$ 1,730
Securities available for sale	3,252	2,505
Trading account liabilities	2,709	1,973
Fair Value, Measurements, Recurring | U.S. government-sponsored agency securities
Fair value measurements
Trading account assets	118	73
Fair Value, Measurements, Recurring | State and municipal securities
Fair value measurements
Trading account assets	11	18
Fair Value, Measurements, Recurring | Commercial paper
Fair value measurements
Trading account assets	7	1
Fair Value, Measurements, Recurring | Other
Fair value measurements
Trading account assets	8	16
Securities available for sale	164	107
Trading account liabilities	7	11
Fair Value, Measurements, Recurring | Corporate bonds
Fair value measurements
Trading account assets	1,054	841
Fair Value, Measurements, Recurring | Asset-backed securities
Fair value measurements
Trading account assets	199	106
Fair Value, Measurements, Recurring | Mortgage-backed securities
Fair value measurements
Trading account assets	6,339	5,221
Fair Value, Measurements, Recurring | Equity securities
Fair value measurements
Trading account assets	193	85
Securities available for sale	10	5
Trading account liabilities	35	47
Fair Value, Measurements, Recurring | Interest rate derivative contracts
Fair value measurements
Trading account assets	525	731
Netting adjustment	(353)	(343)
Trading account liabilities	264	270
Netting adjustment	(382)	(718)
Fair Value, Measurements, Recurring | Commodity derivative contracts
Fair value measurements
Trading account assets	1	39
Netting adjustment	(49)	(106)
Trading account liabilities	13	44
Netting adjustment	(20)	(68)
Fair Value, Measurements, Recurring | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	182	79
Netting adjustment	(68)	(138)
Trading account liabilities	80	98
Netting adjustment	(66)	(33)
Fair Value, Measurements, Recurring | Equity derivative contracts
Fair value measurements
Trading account assets	4	2
Netting adjustment	(135)	(163)
Trading account liabilities	144	164
Netting adjustment	0
Fair Value, Measurements, Recurring | Trading Securities
Fair value measurements
Netting adjustment	(605)	(750)
Netting adjustment	(468)	(819)
Fair Value, Measurements, Recurring | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	9,208	6,695
Fair Value, Measurements, Recurring | Privately issued
Fair value measurements
Securities available for sale	694	327
Fair Value, Measurements, Recurring | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	822	664
Fair Value, Measurements, Recurring | Collateralized loan obligations
Fair value measurements
Securities available for sale	1,905	2,218
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting adjustment	(2)	(20)
Securities available for sale	5	7
Netting adjustment	(152)	(228)
Fair Value, Measurements, Recurring | Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,503	1,613
Fair Value, Measurements, Recurring | Mortgage servicing rights
Fair value measurements
Other assets	64	23
Fair Value, Measurements, Recurring | Interest rate hedging contracts
Fair value measurements
Netting adjustment	0	(20)
Other assets	2	2
Netting adjustment	(149)	(199)
Other liabilities		0
Fair Value, Measurements, Recurring | Other derivative contracts
Fair value measurements
Netting adjustment	(2)	0
Other assets	1	3
Netting adjustment	(3)	(29)
Other liabilities	6	61
Fair Value, Measurements, Recurring | Corporate bonds
Fair value measurements
Trading account liabilities	348	298
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Trading account assets	202	94
Securities available for sale	10	5
Total assets	$ 212	$ 99
Percentage of total	1.00%	0.00%
Percentage of total Company assets	0.00%	0.00%
Trading account liabilities	$ 46	$ 49
Total liabilities	$ 46	$ 49
Percentage of total	1.00%	2.00%
Percentage of total Company liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Fair value measurements
Trading account assets	$ 193	$ 85
Securities available for sale	10	5
Trading account liabilities	35	47
Fair Value, Measurements, Recurring | Level 1 | Interest rate derivative contracts
Fair value measurements
Trading account assets	7	7
Trading account liabilities	3	1
Fair Value, Measurements, Recurring | Level 1 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	0	1
Trading account liabilities	1	1
Fair Value, Measurements, Recurring | Level 1 | Equity derivative contracts
Fair value measurements
Trading account assets	2
Trading account liabilities	7
Fair Value, Measurements, Recurring | Level 1 | Other derivative contracts
Fair value measurements
Other assets	0
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Trading account assets	10,831	9,430
Securities available for sale	15,954	12,498
Other assets	4	24
Total assets	$ 26,789	$ 21,952
Percentage of total	95.00%	95.00%
Percentage of total Company assets	17.00%	15.00%
Trading account liabilities	$ 3,884	$ 3,509
Other liabilities	152	283
Total liabilities	$ 4,036	$ 3,792
Percentage of total	109.00%	123.00%
Percentage of total Company liabilities	3.00%	3.00%
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury
Fair value measurements
Trading account assets	$ 1,926	$ 1,730
Securities available for sale	3,252	2,505
Trading account liabilities	2,709	1,973
Fair Value, Measurements, Recurring | Level 2 | U.S. government-sponsored agency securities
Fair value measurements
Trading account assets	118	73
Fair Value, Measurements, Recurring | Level 2 | State and municipal securities
Fair value measurements
Trading account assets	11	18
Fair Value, Measurements, Recurring | Level 2 | Commercial paper
Fair value measurements
Trading account assets	7	1
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Trading account assets	8	16
Securities available for sale	68	82
Trading account liabilities	7	11
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair value measurements
Trading account assets	1,054	841
Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Fair value measurements
Trading account assets	199	106
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed securities
Fair value measurements
Trading account assets	6,339	5,221
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivative contracts
Fair value measurements
Trading account assets	870	1,065
Trading account liabilities	643	987
Fair Value, Measurements, Recurring | Level 2 | Commodity derivative contracts
Fair value measurements
Trading account assets	50	144
Trading account liabilities	33	111
Fair Value, Measurements, Recurring | Level 2 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	249	215
Trading account liabilities	144	129
Fair Value, Measurements, Recurring | Level 2 | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	9,208	6,695
Fair Value, Measurements, Recurring | Level 2 | Privately issued
Fair value measurements
Securities available for sale	694	327
Fair Value, Measurements, Recurring | Level 2 | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	822	664
Fair Value, Measurements, Recurring | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale	1,905	2,218
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Securities available for sale	5	7
Fair Value, Measurements, Recurring | Level 2 | Interest rate hedging contracts
Fair value measurements
Other assets	2	22
Other liabilities	149	199
Fair Value, Measurements, Recurring | Level 2 | Other derivative contracts
Fair value measurements
Other assets	2	2
Other liabilities	3	84
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair value measurements
Trading account liabilities	348	298
Fair Value, Measurements, Recurring | Level 3
Fair value measurements
Trading account assets	139	168
Securities available for sale	1,599	1,638
Other assets	65	24
Total assets	$ 1,803	$ 1,830
Percentage of total	6.00%	8.00%
Percentage of total Company assets	1.00%	1.00%
Trading account liabilities	$ 138	$ 166
FDIC clawback liability	113	115
Other liabilities	119	121
Total liabilities	$ 257	$ 287
Percentage of total	7.00%	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	$ 96	$ 25
Fair Value, Measurements, Recurring | Level 3 | Interest rate derivative contracts
Fair value measurements
Trading account assets	1	2
Fair Value, Measurements, Recurring | Level 3 | Commodity derivative contracts
Fair value measurements
Trading account assets	0	1
Trading account liabilities	0	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	137	164
Trading account liabilities	137	164
Fair Value, Measurements, Recurring | Level 3 | Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,503	1,613
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights
Fair value measurements
Other assets	64	23
Fair Value, Measurements, Recurring | Level 3 | Other derivative contracts
Fair value measurements
Other assets	1	1
Other liabilities	6	$ 6
Fdic | Fair Value, Measurements, Recurring
Fair value measurements
Other liabilities	$ 113