Fair Value Measurement and Fair Value of Financial Instruments - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Fair value measurements
Trading account assets	$ 1,087	$ 1,114
Securities available for sale	14,336
Other liabilities	2,023	1,897
U.S. Treasury
Fair value measurements
Securities available for sale	594
Residential mortgage-backed securities, U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	7,201
Residential mortgage-backed securities, Privately issued
Fair value measurements
Securities available for sale	151
Collateralized loan obligations
Fair value measurements
Securities available for sale	3,233
Other
Fair value measurements
Securities available for sale	7
Other debt securities, Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,572
Other debt securities, Other
Fair value measurements
Securities available for sale	32
Level 3
Fair value measurements
Other assets	17	11
Fair Value, Measurements, Recurring [Member]
Fair value measurements
Trading account assets	1,087	1,114
Securities available for sale	14,344	13,724
Other assets	3	2
Total assets	$ 15,434	14,840
Percentage of Total	100.00%
Percentage of Total Company Assets	13.00%
Trading account liabilities	$ 796	894
FDIC clawback liability		105
Other liabilities	114	109
Total liabilities	$ 910	1,003
Percentage of Total	100.00%
Percentage of Total Company Liabilities	0.00%
Fair Value, Measurements, Recurring [Member] | U.S. Treasury
Fair value measurements
Trading account assets	$ 37	69
Securities available for sale	594
Fair Value, Measurements, Recurring [Member] | U.S. government-sponsored agencies
Fair value measurements
Trading account assets	106	125
Securities available for sale		0
Fair Value, Measurements, Recurring [Member] | State and municipal securities
Fair value measurements
Trading account assets	3	10
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair value measurements
Trading account assets	25
Fair Value, Measurements, Recurring [Member] | Interest rate derivative contracts
Fair value measurements
Trading account assets	839	761
Trading account liabilities	173	163
Fair Value, Measurements, Recurring [Member] | Commodity derivative contracts
Fair value measurements
Trading account assets	25	30
Trading account liabilities	308	315
Fair Value, Measurements, Recurring [Member] | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	47	43
Trading account liabilities	71	56
Fair Value, Measurements, Recurring [Member] | Equity Derivative contracts
Fair value measurements
Trading account assets	5	76
Trading account liabilities	221	299
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities, U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	7,201	7,560
Fair Value, Measurements, Recurring [Member] | Residential mortgage-backed securities, Privately issued
Fair value measurements
Securities available for sale	151	168
Fair Value, Measurements, Recurring [Member] | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	1,546	1,691
Fair Value, Measurements, Recurring [Member] | Collateralized loan obligations
Fair value measurements
Securities available for sale	3,233	2,494
Fair Value, Measurements, Recurring [Member] | Other
Fair value measurements
Securities available for sale	7	9
Fair Value, Measurements, Recurring [Member] | Other debt securities, Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,572	1,741
Fair Value, Measurements, Recurring [Member] | Other debt securities, Other
Fair value measurements
Securities available for sale	32	52
Fair Value, Measurements, Recurring [Member] | Equity securities
Fair value measurements
Securities available for sale	8	9
Fair Value, Measurements, Recurring [Member] | Interest rate hedging contracts
Fair value measurements
Other assets	2	0
Other liabilities	0	1
Fair Value, Measurements, Recurring [Member] | Other derivative contracts
Fair value measurements
Other assets	1	2
Other liabilities	2	3
Fair Value, Measurements, Recurring [Member] | Securities sold, not yet purchased
Fair value measurements
Trading account liabilities	23	61
Fair Value, Measurements, Recurring [Member] | Level 1
Fair value measurements
Trading account assets	1	1
Securities available for sale	8	9
Total assets	$ 9	$ 10
Percentage of Total	0.00%	0.00%
Percentage of Total Company Assets	0.00%	0.00%
Trading account liabilities	$ 2	$ 2
Total liabilities	$ 2	$ 2
Percentage of Total	0.00%	0.00%
Percentage of Total Company Liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring [Member] | Level 1 | Interest rate derivative contracts
Fair value measurements
Trading account assets		$ 0
Trading account liabilities	$ 1	1
Fair Value, Measurements, Recurring [Member] | Level 1 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	1	1
Trading account liabilities	1	1
Fair Value, Measurements, Recurring [Member] | Level 1 | Equity securities
Fair value measurements
Securities available for sale	8	9
Fair Value, Measurements, Recurring [Member] | Level 2
Fair value measurements
Trading account assets	1,692	1,653
Securities available for sale	12,733	11,925
Other assets	77	36
Total assets	$ 14,502	$ 13,614
Percentage of Total	94.00%	92.00%
Percentage of Total Company Assets	12.00%	12.00%
Trading account liabilities	$ 1,429	$ 1,371
Other liabilities	14	5
Total liabilities	$ 1,443	$ 1,376
Percentage of Total	159.00%	137.00%
Percentage of Total Company Liabilities	1.00%	1.00%
Fair Value, Measurements, Recurring [Member] | Level 2 | U.S. Treasury
Fair value measurements
Trading account assets	$ 37	$ 69
Securities available for sale	594
Fair Value, Measurements, Recurring [Member] | Level 2 | U.S. government-sponsored agencies
Fair value measurements
Trading account assets	106	125
Securities available for sale		0
Fair Value, Measurements, Recurring [Member] | Level 2 | State and municipal securities
Fair value measurements
Trading account assets	3	10
Fair Value, Measurements, Recurring [Member] | Level 2 | Commercial Paper [Member]
Fair value measurements
Trading account assets	25
Fair Value, Measurements, Recurring [Member] | Level 2 | Interest rate derivative contracts
Fair value measurements
Trading account assets	998	928
Trading account liabilities	947	829
Fair Value, Measurements, Recurring [Member] | Level 2 | Commodity derivative contracts
Fair value measurements
Trading account assets	408	417
Trading account liabilities	368	403
Fair Value, Measurements, Recurring [Member] | Level 2 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	115	104
Trading account liabilities	91	78
Fair Value, Measurements, Recurring [Member] | Level 2 | Residential mortgage-backed securities, U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	7,201	7,560
Fair Value, Measurements, Recurring [Member] | Level 2 | Residential mortgage-backed securities, Privately issued
Fair value measurements
Securities available for sale	151	168
Fair Value, Measurements, Recurring [Member] | Level 2 | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	1,546	1,691
Fair Value, Measurements, Recurring [Member] | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale	3,233	2,494
Fair Value, Measurements, Recurring [Member] | Level 2 | Other
Fair value measurements
Securities available for sale	7	9
Fair Value, Measurements, Recurring [Member] | Level 2 | Other debt securities, Other
Fair value measurements
Securities available for sale	1	3
Fair Value, Measurements, Recurring [Member] | Level 2 | Interest rate hedging contracts
Fair value measurements
Other assets	73	36
Other liabilities	14	4
Fair Value, Measurements, Recurring [Member] | Level 2 | Other derivative contracts
Fair value measurements
Other assets	4	0
Other liabilities	0	1
Fair Value, Measurements, Recurring [Member] | Level 2 | Securities sold, not yet purchased
Fair value measurements
Trading account liabilities	23	61
Fair Value, Measurements, Recurring [Member] | Level 3
Fair value measurements
Trading account assets	228	311
Securities available for sale	1,603	1,790
Other assets	1	2
Total assets	$ 1,832	$ 2,103
Percentage of Total	12.00%	14.00%
Percentage of Total Company Assets	2.00%	2.00%
Trading account liabilities	$ 223	$ 305
FDIC clawback liability		105
Other liabilities	114	107
Total liabilities	$ 337	$ 412
Percentage of Total	37.00%	41.00%
Percentage of Total Company Liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring [Member] | Level 3 | Interest rate derivative contracts
Fair value measurements
Trading account assets	$ 4	$ 5
Fair Value, Measurements, Recurring [Member] | Level 3 | Commodity derivative contracts
Fair value measurements
Trading account assets	1	5
Trading account liabilities	1	5
Fair Value, Measurements, Recurring [Member] | Level 3 | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	1	1
Trading account liabilities	1	1
Fair Value, Measurements, Recurring [Member] | Level 3 | Equity Derivative contracts
Fair value measurements
Trading account assets	222	300
Trading account liabilities	221	299
Fair Value, Measurements, Recurring [Member] | Level 3 | Other debt securities, Direct bank purchase bonds
Fair value measurements
Securities available for sale	1,572	1,741
Fair Value, Measurements, Recurring [Member] | Level 3 | Other debt securities, Other
Fair value measurements
Securities available for sale	31	49
Fair Value, Measurements, Recurring [Member] | Level 3 | Other derivative contracts
Fair value measurements
Other assets	1	2
Other liabilities	2	2
Fair Value, Measurements, Recurring [Member] | Netting Adjustment
Fair value measurements
Trading account assets	(834)	(851)
Other assets	(75)	(36)
Total assets	$ (909)	$ (887)
Percentage of Total	(6.00%)	(6.00%)
Percentage of Total Company Assets	(1.00%)	(1.00%)
Trading account liabilities	$ (858)	$ (784)
Other liabilities	(14)	(3)
Total liabilities	$ (872)	$ (787)
Percentage of Total	(96.00%)	(78.00%)
Percentage of Total Company Liabilities	(1.00%)	(1.00%)
Fair Value, Measurements, Recurring [Member] | Netting Adjustment | Interest rate derivative contracts
Fair value measurements
Trading account assets	$ (163)	$ (172)
Trading account liabilities	(775)	(667)
Fair Value, Measurements, Recurring [Member] | Netting Adjustment | Commodity derivative contracts
Fair value measurements
Trading account assets	(384)	(392)
Trading account liabilities	(61)	(93)
Fair Value, Measurements, Recurring [Member] | Netting Adjustment | Foreign exchange derivative contracts
Fair value measurements
Trading account assets	(70)	(63)
Trading account liabilities	(22)	(24)
Fair Value, Measurements, Recurring [Member] | Netting Adjustment | Equity Derivative contracts
Fair value measurements
Trading account assets	(217)	(224)
Fair Value, Measurements, Recurring [Member] | Netting Adjustment | Interest rate hedging contracts
Fair value measurements
Other assets	(71)	(36)
Trading account liabilities		(3)
Other liabilities	(14)
Fair Value, Measurements, Recurring [Member] | Netting Adjustment | Other derivative contracts
Fair value measurements
Other assets	(4)
Fdic | Fair Value, Measurements, Recurring [Member]
Fair value measurements
Other liabilities	112
Fdic | Fair Value, Measurements, Recurring [Member] | Level 3
Fair value measurements
FDIC clawback liability	112
Estimate of Fair Value Measurement
Fair value measurements
Other assets	$ 17	$ 11
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring [Member]
Fair value measurements
Percentage of Total		100.00%
Percentage of Total Company Assets		13.00%
Percentage of Total		100.00%
Percentage of Total Company Liabilities		0.00%