Fair Value Measurement and Fair Value of Financial Instruments - Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018
Fair value measurements
Trading account assets:		$ 11,741	$ 11,213
Securities available for sale:		17,251
U.S. Treasury and government agencies
Fair value measurements
Securities available for sale:		4,475
Other
Fair value measurements
Securities available for sale:		206
U.S. agencies
Fair value measurements
Securities available for sale:		6,674
Residential - non-agency
Fair value measurements
Securities available for sale:		852
Collateralized loan obligations
Fair value measurements
Securities available for sale:		1,524
Direct bank purchase bonds
Fair value measurements
Securities available for sale:		949
Level 3 | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		949
Fair Value, Measurements, Recurring
Fair value measurements
Netting Adjustment	[1]	$ (452)	$ (354)
Percentage of total		100.00%	100.00%
Netting Adjustment		1.00%	1.00%
Percentage of total Company assets		17.00%	17.00%
Netting Adjustment		(0.00%)	(0.00%)
Netting Adjustment	[1]	$ (384)	$ (322)
Percentage of total		100.00%	100.00%
Netting Adjustment		12.00%	8.00%
Percentage of total Company liabilities		2.00%	3.00%
Netting Adjustment		(0.00%)	(0.00%)
Fair Value, Measurements, Recurring | Trading derivatives
Fair value measurements
Netting Adjustment		$ (447)	$ (335)
Netting Adjustment		(380)	(322)
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Netting Adjustment		(5)	(19)
Fair Value, Measurements, Recurring | Interest rate derivative contracts
Fair value measurements
Netting Adjustment		(4)	0
Fair Value, Measurements, Recurring | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Netting Adjustment	[1]	(447)	(335)
Fair Value, Measurements, Recurring | Trading securities
Fair value measurements
Netting Adjustment	[1]	(380)	(322)
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting Adjustment	[2]	(5)	(19)
Fair Value, Measurements, Recurring | Total other liabilities
Fair value measurements
Netting Adjustment	[1]	(4)	0
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Securities available for sale:		0	0
Other assets:		10	9
Total assets		$ 121	$ 155
Percentage of total		0.00%	0.00%
Percentage of total Company assets		0.00%	0.00%
Trading account liabilities:		$ 119	$ 125
Other liabilities:		0	0
Total liabilities		$ 119	$ 125
Percentage of total		4.00%	3.00%
Percentage of total Company liabilities		0.00%	0.00%
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:		$ 0	$ 0
Securities available for sale:		0	0
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 1 | Other
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate debt
Fair value measurements
Trading account assets:		0	0
Fair Value, Measurements, Recurring | Level 1 | Mortgage-backed securities
Fair value measurements
Trading account assets:		0	0
Fair Value, Measurements, Recurring | Level 1 | Other debt
Fair value measurements
Trading account assets:		0	0
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Fair value measurements
Trading account assets:		97	127
Trading account liabilities:		99	70
Fair Value, Measurements, Recurring | Level 1 | Trading derivatives
Fair value measurements
Trading account assets:		14	19
Trading account liabilities:		20	55
Fair Value, Measurements, Recurring | Level 1 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Level 1 | Interest rate derivative contracts
Fair value measurements
Other liabilities:		0	0
Fair Value, Measurements, Recurring | Level 1 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		111	146
Fair Value, Measurements, Recurring | Level 1 | Fdic [Member]
Fair value measurements
Other liabilities:		0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. agencies
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 1 | Residential - non-agency
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 1 | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 1 | Collateralized loan obligations
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 1 | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 1 | Mortgage servicing rights
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Level 1 | loans held for sale [Domain]
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Level 1 | Corporate bonds
Fair value measurements
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 1 | Equity securities
Fair value measurements
Other assets:		10	9
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Securities available for sale:		16,110	14,983
Other assets:		6	32
Total assets		$ 28,181	$ 26,404
Percentage of total		96.00%	95.00%
Percentage of total Company assets		16.00%	16.00%
Trading account liabilities:		$ 3,437	$ 4,211
Other liabilities:		8	3
Total liabilities		$ 3,445	$ 4,214
Percentage of total		104.00%	102.00%
Percentage of total Company liabilities		2.00%	3.00%
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:		$ 3,617	$ 3,071
Securities available for sale:		4,475	3,429
Trading account liabilities:		2,173	2,753
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Securities available for sale:		32	29
Fair Value, Measurements, Recurring | Level 2 | Corporate debt
Fair value measurements
Trading account assets:		1,091	1,367
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed securities
Fair value measurements
Trading account assets:		5,012	5,785
Fair Value, Measurements, Recurring | Level 2 | Other debt
Fair value measurements
Trading account assets:		744	360
Trading account liabilities:		21	10
Fair Value, Measurements, Recurring | Level 2 | Equity securities
Fair value measurements
Trading account assets:		0	0
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 2 | Trading derivatives
Fair value measurements
Trading account assets:		1,601	806
Trading account liabilities:		678	731
Fair Value, Measurements, Recurring | Level 2 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		6	32
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivative contracts
Fair value measurements
Other liabilities:		8	3
Fair Value, Measurements, Recurring | Level 2 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		12,065	11,389
Fair Value, Measurements, Recurring | Level 2 | Fdic [Member]
Fair value measurements
Other liabilities:		0	0
Fair Value, Measurements, Recurring | Level 2 | U.S. agencies
Fair value measurements
Securities available for sale:		6,674	8,007
Fair Value, Measurements, Recurring | Level 2 | Residential - non-agency
Fair value measurements
Securities available for sale:		852	864
Fair Value, Measurements, Recurring | Level 2 | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:		2,553	1,180
Fair Value, Measurements, Recurring | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale:		1,524	1,474
Fair Value, Measurements, Recurring | Level 2 | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 2 | Mortgage servicing rights
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Level 2 | loans held for sale [Domain]
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds
Fair value measurements
Trading account liabilities:		565	717
Fair Value, Measurements, Recurring | Level 2 | Equity securities
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Level 3
Fair value measurements
Securities available for sale:		1,141	1,331
Other assets:		308	277
Total assets		$ 1,461	$ 1,621
Percentage of total		5.00%	6.00%
Percentage of total Company assets		1.00%	1.00%
Trading account liabilities:		$ 12	$ 13
Other liabilities:		125	118
Total liabilities		$ 137	$ 131
Percentage of total		4.00%	3.00%
Percentage of total Company liabilities		0.00%	0.00%
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:		$ 0	$ 0
Securities available for sale:		0	0
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 3 | Other
Fair value measurements
Securities available for sale:		174	141
Fair Value, Measurements, Recurring | Level 3 | Corporate debt
Fair value measurements
Trading account assets:		0	0
Fair Value, Measurements, Recurring | Level 3 | Mortgage-backed securities
Fair value measurements
Trading account assets:		0	0
Fair Value, Measurements, Recurring | Level 3 | Other debt
Fair value measurements
Trading account assets:		0	0
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 3 | Equity securities
Fair value measurements
Trading account assets:		0	0
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 3 | Trading derivatives
Fair value measurements
Trading account assets:		12	13
Trading account liabilities:		12	13
Fair Value, Measurements, Recurring | Level 3 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		4	1
Fair Value, Measurements, Recurring | Level 3 | Interest rate derivative contracts
Fair value measurements
Other liabilities:		5	2
Fair Value, Measurements, Recurring | Level 3 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		12	13
Fair Value, Measurements, Recurring | Level 3 | Fdic [Member]
Fair value measurements
Other liabilities:		120	116
Fair Value, Measurements, Recurring | Level 3 | U.S. agencies
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 3 | Residential - non-agency
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 3 | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:		18	0
Fair Value, Measurements, Recurring | Level 3 | Collateralized loan obligations
Fair value measurements
Securities available for sale:		0	0
Fair Value, Measurements, Recurring | Level 3 | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		949	1,190
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights
Fair value measurements
Other assets:		261	159
Fair Value, Measurements, Recurring | Level 3 | loans held for sale [Domain]
Fair value measurements
Other assets:		43	117
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds
Fair value measurements
Trading account liabilities:		0	0
Fair Value, Measurements, Recurring | Level 3 | Equity securities
Fair value measurements
Other assets:		0	0
Fair Value, Measurements, Recurring | Fair Value
Fair value measurements
Securities available for sale:		17,251	16,314
Other assets:		319	299
Total assets		29,311	27,826
Trading account liabilities:		3,188	4,027
Other liabilities:		129	121
Total liabilities		3,317	4,148
Fair Value, Measurements, Recurring | Fair Value | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:		3,617	3,071
Securities available for sale:		4,475	3,429
Trading account liabilities:		2,173	2,753
Fair Value, Measurements, Recurring | Fair Value | Other
Fair value measurements
Securities available for sale:		206	170
Fair Value, Measurements, Recurring | Fair Value | Corporate debt
Fair value measurements
Trading account assets:		1,091	1,367
Fair Value, Measurements, Recurring | Fair Value | Asset-backed securities
Fair value measurements
Trading account assets:
Fair Value, Measurements, Recurring | Fair Value | Mortgage-backed securities
Fair value measurements
Trading account assets:		5,012	5,785
Fair Value, Measurements, Recurring | Fair Value | Other debt
Fair value measurements
Trading account assets:		744	360
Trading account liabilities:		21	10
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Trading account assets:		97	127
Trading account liabilities:		99	70
Fair Value, Measurements, Recurring | Fair Value | Trading derivatives
Fair value measurements
Trading account assets:		1,180	503
Trading account liabilities:		330	477
Fair Value, Measurements, Recurring | Fair Value | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		5	14
Fair Value, Measurements, Recurring | Fair Value | Interest rate derivative contracts
Fair value measurements
Other liabilities:		9	5
Fair Value, Measurements, Recurring | Fair Value | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		11,741	11,213
Fair Value, Measurements, Recurring | Fair Value | Fdic [Member]
Fair value measurements
Other liabilities:		120	116
Fair Value, Measurements, Recurring | Fair Value | U.S. agencies
Fair value measurements
Securities available for sale:		6,674	8,007
Fair Value, Measurements, Recurring | Fair Value | Residential - non-agency
Fair value measurements
Securities available for sale:		852	864
Fair Value, Measurements, Recurring | Fair Value | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:		2,571	1,180
Fair Value, Measurements, Recurring | Fair Value | Collateralized loan obligations
Fair value measurements
Securities available for sale:		1,524	1,474
Fair Value, Measurements, Recurring | Fair Value | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		949	1,190
Fair Value, Measurements, Recurring | Fair Value | Mortgage servicing rights
Fair value measurements
Other assets:		261	159
Fair Value, Measurements, Recurring | Fair Value | loans held for sale [Domain]
Fair value measurements
Other assets:		43	117
Fair Value, Measurements, Recurring | Fair Value | Corporate bonds
Fair value measurements
Trading account liabilities:		565	717
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Other assets:		$ 10	$ 9

[1] Amounts represent the impact of legally enforceable master netting agreements between the same counterparties that allow the Company to net settle all contracts.

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