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	Jun. 30, 2021	Dec. 31, 2020
Fair value measurements
Trading account assets:	$ 14,349	$ 16,038
Securities available for sale:	21,125
U.S. Treasury and government agencies
Fair value measurements
Securities available for sale:	6,731
Other
Fair value measurements
Securities available for sale:	527
U.S. agencies
Fair value measurements
Securities available for sale:	6,724
Residential - non-agency
Fair value measurements
Securities available for sale:	801
Collateralized loan obligations
Fair value measurements
Securities available for sale:	1,154
Direct bank purchase bonds
Fair value measurements
Securities available for sale:	708
Fair Value, Measurements, Recurring
Fair value measurements
Netting Adjustment	$ (426)	$ (432)
Percentage of total	100.00%	100.00%
Netting Adjustment	1.00%	1.00%
Percentage of total Company assets	22.00%	21.00%
Netting Adjustment	0.00%	0.00%
Netting Adjustment	$ (536)	$ (675)
Percentage of total	100.00%	100.00%
Netting Adjustment	16.00%	20.00%
Percentage of total Company liabilities	3.00%	3.00%
Netting Adjustment	0.00%	0.00%
Fair Value, Measurements, Recurring | Trading derivatives
Fair value measurements
Netting Adjustment	$ (424)	$ (431)
Netting Adjustment	(535)	(674)
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets
Fair value measurements
Netting Adjustment	(2)	(1)
Fair Value, Measurements, Recurring | Interest rate derivative contracts
Fair value measurements
Netting Adjustment	(1)	(1)
Fair Value, Measurements, Recurring | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Netting Adjustment	(424)	(431)
Fair Value, Measurements, Recurring | Trading securities
Fair value measurements
Netting Adjustment	(535)	(674)
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting Adjustment	(2)	(1)
Fair Value, Measurements, Recurring | Total other liabilities
Fair value measurements
Netting Adjustment	(1)	(1)
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Securities available for sale:	0	0
Other assets:	25	20
Total assets	$ 675	$ 965
Percentage of total	2.00%	3.00%
Percentage of total Company assets	0.00%	1.00%
Trading account liabilities:	$ 34	$ 202
Other liabilities:	0	0
Total liabilities	$ 34	$ 202
Percentage of total	1.00%	6.00%
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:	641	925
Trading account liabilities:	0	144
Fair Value, Measurements, Recurring | Level 1 | Trading derivatives
Fair value measurements
Trading account assets:	9	20
Trading account liabilities:	34	58
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:	650	945
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:	25	20
Fair Value, Measurements, Recurring | Level 2
Fair value measurements
Securities available for sale:	19,881	16,785
Other assets:	3	2
Total assets	$ 34,006	$ 32,310
Percentage of total	95.00%	93.00%
Percentage of total Company assets	21.00%	19.00%
Trading account liabilities:	$ 3,909	$ 3,804
Other liabilities:	2	3
Total liabilities	$ 3,911	$ 3,807
Percentage of total	115.00%	114.00%
Percentage of total Company liabilities	3.00%	3.00%
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:	$ 2,368	$ 3,118
Securities available for sale:	6,731	4,473
Trading account liabilities:	2,497	2,359
Fair Value, Measurements, Recurring | Level 2 | Other
Fair value measurements
Securities available for sale:	2	11
Fair Value, Measurements, Recurring | Level 2 | Corporate debt
Fair value measurements
Trading account assets:	964	838
Fair Value, Measurements, Recurring | Level 2 | Mortgage-backed securities
Fair value measurements
Trading account assets:	8,637	8,936
Fair Value, Measurements, Recurring | Level 2 | Other debt
Fair value measurements
Trading account assets:	414	354
Trading account liabilities:	81	0
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:	1,739	2,277
Trading account liabilities:	781	962
Fair Value, Measurements, Recurring | Level 2 | Derivative Financial Instruments, Assets
Fair value measurements
Other assets:	3	2
Fair Value, Measurements, Recurring | Level 2 | Interest rate derivative contracts
Fair value measurements
Other liabilities:	2	3
Fair Value, Measurements, Recurring | Level 2 | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Trading account assets:	14,122	15,523
Fair Value, Measurements, Recurring | Level 2 | U.S. agencies
Fair value measurements
Securities available for sale:	6,724	5,922
Fair Value, Measurements, Recurring | Level 2 | Residential - non-agency
Fair value measurements
Securities available for sale:	801	539
Fair Value, Measurements, Recurring | Level 2 | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:	4,469	4,473
Fair Value, Measurements, Recurring | Level 2 | Collateralized loan obligations
Fair value measurements
Securities available for sale:	1,154	1,367
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:	550	483
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:	1,244	1,474
Other assets:	116	166
Total assets	$ 1,361	$ 1,641
Percentage of total	4.00%	5.00%
Percentage of total Company assets	1.00%	1.00%
Trading account liabilities:	$ 1	$ 1
Other liabilities:	7	4
Total liabilities	$ 8	$ 5
Percentage of total	0.00%	0.00%
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:	525	528
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	1
Trading account liabilities:	1	1
Fair Value, Measurements, Recurring | Level 3 | Derivative Financial Instruments, Assets
Fair value measurements
Other assets:	7	13
Fair Value, Measurements, Recurring | Level 3 | Interest rate derivative contracts
Fair value measurements
Other liabilities:	7	4
Fair Value, Measurements, Recurring | Level 3 | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Trading account assets:	1	1
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:	11	16
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:	708	930
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights
Fair value measurements
Other assets:	109	110
Fair Value, Measurements, Recurring | Level 3 | loans held for sale [Domain]
Fair value measurements
Other assets:	0	43
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:	21,125	18,259
Other assets:	142	187
Total assets	35,616	34,484
Trading account liabilities:	3,409	3,333
Other liabilities:	8	6
Total liabilities	3,417	3,339
Fair Value, Measurements, Recurring | Fair Value | U.S. Treasury and government agencies
Fair value measurements
Trading account assets:	2,368	3,118
Securities available for sale:	6,731	4,473
Trading account liabilities:	2,497	2,359
Fair Value, Measurements, Recurring | Fair Value | Other
Fair value measurements
Securities available for sale:	527	539
Fair Value, Measurements, Recurring | Fair Value | Corporate debt
Fair value measurements
Trading account assets:	964	838
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:	8,637	8,936
Fair Value, Measurements, Recurring | Fair Value | Other debt
Fair value measurements
Trading account assets:	414	354
Trading account liabilities:	81	0
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Trading account assets:	641	925
Trading account liabilities:	0	144
Fair Value, Measurements, Recurring | Fair Value | Trading derivatives
Fair value measurements
Trading account assets:	1,325	1,867
Trading account liabilities:	281	347
Fair Value, Measurements, Recurring | Fair Value | Derivative Financial Instruments, Assets
Fair value measurements
Other assets:	8	14
Fair Value, Measurements, Recurring | Fair Value | Interest rate derivative contracts
Fair value measurements
Other liabilities:	8	6
Fair Value, Measurements, Recurring | Fair Value | Trading Assets, Excluding Debt and Equity Securities
Fair value measurements
Trading account assets:	14,349	16,038
Fair Value, Measurements, Recurring | Fair Value | U.S. agencies
Fair value measurements
Securities available for sale:	6,724	5,922
Fair Value, Measurements, Recurring | Fair Value | Residential - non-agency
Fair value measurements
Securities available for sale:	801	539
Fair Value, Measurements, Recurring | Fair Value | Privately issued commercial mortgage-backed securities
Fair value measurements
Securities available for sale:	4,480	4,489
Fair Value, Measurements, Recurring | Fair Value | Collateralized loan obligations
Fair value measurements
Securities available for sale:	1,154	1,367
Fair Value, Measurements, Recurring | Fair Value | Direct bank purchase bonds
Fair value measurements
Securities available for sale:	708	930
Fair Value, Measurements, Recurring | Fair Value | Mortgage servicing rights
Fair value measurements
Other assets:	109	110
Fair Value, Measurements, Recurring | Fair Value | loans held for sale [Domain]
Fair value measurements
Other assets:	0	43
Fair Value, Measurements, Recurring | Fair Value | Corporate bonds
Fair value measurements
Trading account liabilities:	550	483
Fair Value, Measurements, Recurring | Fair Value | Equity securities
Fair value measurements
Other assets:	$ 25	$ 20