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		Mar. 31, 2020	Dec. 31, 2019
Fair value measurements
Trading account assets:		$ 11,799	$ 10,377
Securities available for sale:		15,140
U.S. Treasury and government agencies
Fair value measurements
Securities available for sale:		3,551
Other
Fair value measurements
Securities available for sale:		179
U.S. agencies
Fair value measurements
Securities available for sale:		4,778
Residential - non-agency
Fair value measurements
Securities available for sale:		701
Collateralized loan obligations
Fair value measurements
Securities available for sale:		1,328
Direct bank purchase bonds
Fair value measurements
Securities available for sale:		940
Level 3 | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		940
Fair Value, Measurements, Recurring
Fair value measurements
Netting Adjustment	[1]	$ (705)	$ (264)
Percentage of total		100.00%	100.00%
Netting Adjustment		2.00%	1.00%
Percentage of total Company assets		16.00%	17.00%
Netting Adjustment		(0.00%)	(0.00%)
Netting Adjustment	[1]	$ (663)	$ (306)
Percentage of total		100.00%	100.00%
Netting Adjustment		26.00%	9.00%
Percentage of total Company liabilities		2.00%	2.00%
Netting Adjustment		(0.00%)	(0.00%)
Fair Value, Measurements, Recurring | Trading derivatives
Fair value measurements
Netting Adjustment		$ (676)	$ (256)
Netting Adjustment		(643)	(306)
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Netting Adjustment		(29)	(8)
Fair Value, Measurements, Recurring | Interest rate derivative contracts
Fair value measurements
Netting Adjustment		(20)	0
Fair Value, Measurements, Recurring | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Netting Adjustment	[1]	(676)	(256)
Fair Value, Measurements, Recurring | Trading securities
Fair value measurements
Netting Adjustment	[1]	(643)	(306)
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting Adjustment	[2]	(29)	(8)
Fair Value, Measurements, Recurring | Total other liabilities
Fair value measurements
Netting Adjustment	[1]	(20)	0
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Securities available for sale:		0	0
Other assets:		10	10
Total assets		$ 78	$ 153
Percentage of total		0.00%	1.00%
Percentage of total Company assets		0.00%	0.00%
Trading account liabilities:		$ 72	$ 116
Other liabilities:		0	0
Total liabilities		$ 72	$ 116
Percentage of total		3.00%	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:		40	127
Trading account liabilities:		37	105
Fair Value, Measurements, Recurring | Level 1 | Trading derivatives
Fair value measurements
Trading account assets:		28	16
Trading account liabilities:		35	11
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:		68	143
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	10
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Securities available for sale:		14,017	16,658
Other assets:		39	10
Total assets		$ 26,460	$ 27,150
Percentage of total		97.00%	95.00%
Percentage of total Company assets		15.00%	16.00%
Trading account liabilities:		$ 3,024	$ 3,448
Other liabilities:		34	2
Total liabilities		$ 3,058	$ 3,450
Percentage of total		118.00%	102.00%
Percentage of total Company liabilities		2.00%	2.00%
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:		$ 1,706	$ 2,393
Securities available for sale:		3,551	5,438
Trading account liabilities:		1,284	2,299
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Securities available for sale:		12	28
Fair Value, Measurements, Recurring | Level 2 | Corporate debt
Fair value measurements
Trading account assets:		1,110	1,212
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed securities
Fair value measurements
Trading account assets:		6,611	5,376
Fair Value, Measurements, Recurring | Level 2 | Other debt
Fair value measurements
Trading account assets:		361	306
Trading account liabilities:		5	4
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:		2,616	1,195
Trading account liabilities:		1,110	499
Fair Value, Measurements, Recurring | Level 2 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		39	10
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivative contracts
Fair value measurements
Other liabilities:		34	2
Fair Value, Measurements, Recurring | Level 2 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		12,404	10,482
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:		4,778	5,468
Fair Value, Measurements, Recurring | Level 2 | Residential - non-agency
Fair value measurements
Securities available for sale:		701	762
Fair Value, Measurements, Recurring | Level 2 | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:		3,647	3,471
Fair Value, Measurements, Recurring | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale:		1,328	1,491
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:		625	646
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:		1,123	1,131
Other assets:		221	316
Total assets		$ 1,347	$ 1,455
Percentage of total		5.00%	5.00%
Percentage of total Company assets		1.00%	1.00%
Trading account liabilities:		$ 3	$ 8
Other liabilities:		124	124
Total liabilities		$ 127	$ 132
Percentage of total		5.00%	4.00%
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:		167	177
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:		3	8
Trading account liabilities:		3	8
Fair Value, Measurements, Recurring | Level 3 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		11	4
Fair Value, Measurements, Recurring | Level 3 | Interest rate derivative contracts
Fair value measurements
Other liabilities:		2	3
Fair Value, Measurements, Recurring | Level 3 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		3	8
Fair Value, Measurements, Recurring | Level 3 | Fdic [Member]
Fair value measurements
Other liabilities:		122	121
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:		16	18
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:		940	936
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights
Fair value measurements
Other assets:		164	270
Fair Value, Measurements, Recurring | Level 3 | loans held for sale [Domain]
Fair value measurements
Other assets:		46	42
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:		15,140	17,789
Other assets:		241	328
Total assets		27,180	28,494
Trading account liabilities:		2,456	3,266
Other liabilities:		138	126
Total liabilities		2,594	3,392
Fair Value, Measurements, Recurring | Fair Value | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:		1,706	2,393
Securities available for sale:		3,551	5,438
Trading account liabilities:		1,284	2,299
Fair Value, Measurements, Recurring | Fair Value | Other
Fair value measurements
Securities available for sale:		179	205
Fair Value, Measurements, Recurring | Fair Value | Corporate debt
Fair value measurements
Trading account assets:		1,110	1,212
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:		6,611	5,376
Fair Value, Measurements, Recurring | Fair Value | Other debt
Fair value measurements
Trading account assets:		361	306
Trading account liabilities:		5	4
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Trading account assets:		40	127
Trading account liabilities:		37	105
Fair Value, Measurements, Recurring | Fair Value | Trading derivatives
Fair value measurements
Trading account assets:		1,971	963
Trading account liabilities:		505	212
Fair Value, Measurements, Recurring | Fair Value | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets:		21	6
Fair Value, Measurements, Recurring | Fair Value | Interest rate derivative contracts
Fair value measurements
Other liabilities:		16	5
Fair Value, Measurements, Recurring | Fair Value | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets:		11,799	10,377
Fair Value, Measurements, Recurring | Fair Value | Fdic [Member]
Fair value measurements
Other liabilities:		122	121
Fair Value, Measurements, Recurring | Fair Value | U.S. agencies
Fair value measurements
Securities available for sale:		4,778	5,468
Fair Value, Measurements, Recurring | Fair Value | Residential - non-agency
Fair value measurements
Securities available for sale:		701	762
Fair Value, Measurements, Recurring | Fair Value | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:		3,663	3,489
Fair Value, Measurements, Recurring | Fair Value | Collateralized loan obligations
Fair value measurements
Securities available for sale:		1,328	1,491
Fair Value, Measurements, Recurring | Fair Value | Direct bank purchase bonds
Fair value measurements
Securities available for sale:		940	936
Fair Value, Measurements, Recurring | Fair Value | Mortgage servicing rights
Fair value measurements
Other assets:		164	270
Fair Value, Measurements, Recurring | Fair Value | loans held for sale [Domain]
Fair value measurements
Other assets:		46	42
Fair Value, Measurements, Recurring | Fair Value | Corporate bonds
Fair value measurements
Trading account liabilities:		625	646
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Other assets:		$ 10	$ 10

[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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