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, 2020	Dec. 31, 2019
Fair value measurements
Trading account assets:	$ 12,503	$ 10,377
Securities available for sale:	16,670
U.S. Treasury and government agencies
Fair value measurements
Securities available for sale:	3,282
Other
Fair value measurements
Securities available for sale:	538
U.S. agencies
Fair value measurements
Securities available for sale:	5,461
Residential - non-agency
Fair value measurements
Securities available for sale:	655
Collateralized loan obligations
Fair value measurements
Securities available for sale:	1,371
Direct bank purchase bonds
Fair value measurements
Securities available for sale:	937
Fair Value, Measurements, Recurring
Fair value measurements
Netting Adjustment	$ (526)	$ (264)
Percentage of total	100.00%	100.00%
Netting Adjustment	2.00%	1.00%
Percentage of total Company assets	18.00%	17.00%
Netting Adjustment	(0.00%)	(0.00%)
Netting Adjustment	$ (649)	$ (306)
Percentage of total	100.00%	100.00%
Netting Adjustment	23.00%	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	$ (525)	$ (256)
Netting Adjustment	(648)	(306)
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets
Fair value measurements
Netting Adjustment	(1)	(8)
Fair Value, Measurements, Recurring | Interest rate derivative contracts
Fair value measurements
Netting Adjustment	(1)	0
Fair Value, Measurements, Recurring | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Netting Adjustment	(525)	(256)
Fair Value, Measurements, Recurring | Trading securities
Fair value measurements
Netting Adjustment	(648)	(306)
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting Adjustment	(1)	(8)
Fair Value, Measurements, Recurring | Total other liabilities
Fair value measurements
Netting Adjustment	(1)	0
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Securities available for sale:	0	0
Other assets:	20	10
Total assets	$ 191	$ 153
Percentage of total	1.00%	1.00%
Percentage of total Company assets	0.00%	0.00%
Trading account liabilities:	$ 108	$ 116
Other liabilities:	0	0
Total liabilities	$ 108	$ 116
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:	160	127
Trading account liabilities:	96	105
Fair Value, Measurements, Recurring | Level 1 | Trading derivatives
Fair value measurements
Trading account assets:	11	16
Trading account liabilities:	12	11
Fair Value, Measurements, Recurring | Level 1 | Derivative Financial Instruments, Assets
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
Fair value measurements
Trading account assets:	171	143
Fair Value, Measurements, Recurring | Level 1 | Fdic
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:	20	10
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Securities available for sale:	15,189	16,658
Other assets:	2	10
Total assets	$ 28,047	$ 27,150
Percentage of total	97.00%	95.00%
Percentage of total Company assets	17.00%	16.00%
Trading account liabilities:	$ 3,345	$ 3,448
Other liabilities:	3	2
Total liabilities	$ 3,348	$ 3,450
Percentage of total	119.00%	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:	$ 2,357	$ 2,393
Securities available for sale:	3,282	5,438
Trading account liabilities:	1,998	2,299
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Securities available for sale:	11	28
Fair Value, Measurements, Recurring | Level 2 | Corporate debt
Fair value measurements
Trading account assets:	721	1,212
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed securities
Fair value measurements
Trading account assets:	6,949	5,376
Fair Value, Measurements, Recurring | Level 2 | Other debt
Fair value measurements
Trading account assets:	312	306
Trading account liabilities:	71	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:	2,517	1,195
Trading account liabilities:	907	499
Fair Value, Measurements, Recurring | Level 2 | Derivative Financial Instruments, Assets
Fair value measurements
Other assets:	2	10
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivative contracts
Fair value measurements
Other liabilities:	3	2
Fair Value, Measurements, Recurring | Level 2 | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Trading account assets:	12,856	10,482
Fair Value, Measurements, Recurring | Level 2 | Fdic
Fair value measurements
Other liabilities:	0	0
Fair Value, Measurements, Recurring | Level 2 | U.S. agencies
Fair value measurements
Securities available for sale:	5,461	5,468
Fair Value, Measurements, Recurring | Level 2 | Residential - non-agency
Fair value measurements
Securities available for sale:	655	762
Fair Value, Measurements, Recurring | Level 2 | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:	4,409	3,471
Fair Value, Measurements, Recurring | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale:	1,371	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:	369	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:	1,481	1,131
Other assets:	180	316
Total assets	$ 1,662	$ 1,455
Percentage of total	4.00%	5.00%
Percentage of total Company assets	1.00%	1.00%
Trading account liabilities:	$ 1	$ 8
Other liabilities:	5	124
Total liabilities	$ 6	$ 132
Percentage of total	0.00%	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:	527	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:	1	8
Trading account liabilities:	1	8
Fair Value, Measurements, Recurring | Level 3 | Derivative Financial Instruments, Assets
Fair value measurements
Other assets:	14	4
Fair Value, Measurements, Recurring | Level 3 | Interest rate derivative contracts
Fair value measurements
Other liabilities:	5	3
Fair Value, Measurements, Recurring | Level 3 | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Trading account assets:	1	8
Fair Value, Measurements, Recurring | Level 3 | Fdic
Fair value measurements
Other liabilities:	0	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:	17	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:	937	936
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights
Fair value measurements
Other assets:	122	270
Fair Value, Measurements, Recurring | Level 3 | loans held for sale [Domain]
Fair value measurements
Other assets:	44	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:	16,670	17,789
Other assets:	201	328
Total assets	29,374	28,494
Trading account liabilities:	2,806	3,266
Other liabilities:	7	126
Total liabilities	2,813	3,392
Fair Value, Measurements, Recurring | Fair Value | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:	2,357	2,393
Securities available for sale:	3,282	5,438
Trading account liabilities:	1,998	2,299
Fair Value, Measurements, Recurring | Fair Value | Other
Fair value measurements
Securities available for sale:	538	205
Fair Value, Measurements, Recurring | Fair Value | Corporate debt
Fair value measurements
Trading account assets:	721	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:	6,949	5,376
Fair Value, Measurements, Recurring | Fair Value | Other debt
Fair value measurements
Trading account assets:	312	306
Trading account liabilities:	71	4
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Trading account assets:	160	127
Trading account liabilities:	96	105
Fair Value, Measurements, Recurring | Fair Value | Trading derivatives
Fair value measurements
Trading account assets:	2,004	963
Trading account liabilities:	272	212
Fair Value, Measurements, Recurring | Fair Value | Derivative Financial Instruments, Assets
Fair value measurements
Other assets:	15	6
Fair Value, Measurements, Recurring | Fair Value | Interest rate derivative contracts
Fair value measurements
Other liabilities:	7	5
Fair Value, Measurements, Recurring | Fair Value | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Trading account assets:	12,503	10,377
Fair Value, Measurements, Recurring | Fair Value | Fdic
Fair value measurements
Other liabilities:	0	121
Fair Value, Measurements, Recurring | Fair Value | U.S. agencies
Fair value measurements
Securities available for sale:	5,461	5,468
Fair Value, Measurements, Recurring | Fair Value | Residential - non-agency
Fair value measurements
Securities available for sale:	655	762
Fair Value, Measurements, Recurring | Fair Value | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:	4,426	3,489
Fair Value, Measurements, Recurring | Fair Value | Collateralized loan obligations
Fair value measurements
Securities available for sale:	1,371	1,491
Fair Value, Measurements, Recurring | Fair Value | Direct bank purchase bonds
Fair value measurements
Securities available for sale:	937	936
Fair Value, Measurements, Recurring | Fair Value | Mortgage servicing rights
Fair value measurements
Other assets:	122	270
Fair Value, Measurements, Recurring | Fair Value | loans held for sale [Domain]
Fair value measurements
Other assets:	44	42
Fair Value, Measurements, Recurring | Fair Value | Corporate bonds
Fair value measurements
Trading account liabilities:	369	646
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Other assets:	$ 20	$ 10