Fair Value Measurement and Fair Value of Financial Instruments - Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value measurements
Trading account assets	$ 10,377	$ 11,213
Securities available for sale	17,789
Other liabilities	2,837	2,048
US Treasury and Government [Member]
Fair value measurements
Securities available for sale	5,438
Other
Fair value measurements
Securities available for sale	205
U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	5,468
Privately issued
Fair value measurements
Securities available for sale	762
Collateralized loan obligations
Fair value measurements
Securities available for sale	1,491
Direct bank purchase bonds
Fair value measurements
Securities available for sale	936
Fair Value, Measurements, Recurring
Fair value measurements
Netting adjustment	$ (264)	$ (354)
Percentage of total	100.00%	100.00%
Percentage of total	1.00%	1.00%
Percentage of total Company assets	17.00%	17.00%
Percentage of total Company assets	(0.00%)	(0.00%)
Netting adjustment	$ (306)	$ (322)
Percentage of total	100.00%	100.00%
Percentage of total	9.00%	8.00%
Percentage of total Company liabilities	2.00%	3.00%
Percentage of total Company liabilities	(0.00%)	(0.00%)
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Liabilities [Member]
Fair value measurements
Netting adjustment	$ 0	$ 0
Fair Value, Measurements, Recurring | Trading Derivatives [Member]
Fair value measurements
Netting adjustment	(256)	(335)
Netting adjustment	(306)	(322)
Fair Value, Measurements, Recurring | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Netting adjustment	(256)	(335)
Fair Value, Measurements, Recurring | Other
Fair value measurements
Netting adjustment	(8)	(19)
Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Netting adjustment	(8)	(19)
Fair Value, Measurements, Recurring | Trading Liabilities
Fair value measurements
Netting adjustment	(306)	(322)
Fair Value, Measurements, Recurring | Total Other Liabilities [Member]
Fair value measurements
Netting adjustment	0	0
Fair Value, Measurements, Recurring | Level 1
Fair value measurements
Securities available for sale	0	0
Other assets	10	9
Total assets	$ 153	$ 155
Percentage of total	1.00%	0.00%
Percentage of total Company assets	0.00%	0.00%
Trading account liabilities	$ 116	$ 125
Other liabilities	0	0
Total liabilities	$ 116	$ 125
Percentage of total	3.00%	3.00%
Percentage of total Company liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring | Level 1 | Derivative Financial Instruments, Liabilities [Member]
Fair value measurements
Other liabilities	$ 0	$ 0
Fair Value, Measurements, Recurring | Level 1 | US Treasury and Government [Member]
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 bonds
Fair value measurements
Trading account assets	0	0
Fair Value, Measurements, Recurring | Level 1 | Other contracts
Fair value measurements
Trading account assets	0	0
Trading account liabilities	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 | Equity securities
Fair value measurements
Trading account assets	127	127
Trading account liabilities	105	70
Fair Value, Measurements, Recurring | Level 1 | Trading Derivatives [Member]
Fair value measurements
Trading account assets	16	19
Trading account liabilities	11	55
Fair Value, Measurements, Recurring | Level 1 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets	143	146
Fair Value, Measurements, Recurring | Level 1 | Fdic
Fair value measurements
Other liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	0	0
Fair Value, Measurements, Recurring | Level 1 | Privately issued
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(2)
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 | Equity Securities, FV-NI [Member]
Fair value measurements
Other assets	10	9
Fair Value, Measurements, Recurring | Level 1 | Corporate bonds
Fair value measurements
Trading account liabilities	0	0
Fair Value, Measurements, Recurring | Level 1 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Fair value measurements
Securities available for sale	16,658	14,983
Other assets	10	32
Total assets	$ 27,150	$ 26,404
Percentage of total	95.00%	95.00%
Percentage of total Company assets	16.00%	16.00%
Trading account liabilities	$ 3,448	$ 4,211
Other liabilities	2	3
Total liabilities	$ 3,450	$ 4,214
Percentage of total	102.00%	102.00%
Percentage of total Company liabilities	2.00%	3.00%
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair value measurements
Other liabilities	$ 2	$ 3
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | US Treasury and Government [Member]
Fair value measurements
Trading account assets	2,393	3,071
Securities available for sale	5,438	3,429
Trading account liabilities	2,299	2,753
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Other
Fair value measurements
Securities available for sale	28	29
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Corporate bonds
Fair value measurements
Trading account assets	1,212	1,367
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Other contracts
Fair value measurements
Trading account assets	306	360
Trading account liabilities	4	10
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Mortgage-backed securities
Fair value measurements
Trading account assets	5,376	5,785
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Equity securities
Fair value measurements
Trading account assets	0	0
Trading account liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Trading Derivatives [Member]
Fair value measurements
Trading account assets	1,195	806
Trading account liabilities	499	731
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets	10,482	11,389
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Fdic
Fair value measurements
Other liabilities	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	5,468	8,007
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Privately issued
Fair value measurements
Securities available for sale	762	864
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	3,471	1,180
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Collateralized loan obligations
Fair value measurements
Securities available for sale	1,491	1,474
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Direct bank purchase bonds
Fair value measurements
Securities available for sale	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Mortgage servicing rights(2)
Fair value measurements
Other assets	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | loans held for sale [Domain]
Fair value measurements
Other assets	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Equity Securities, FV-NI [Member]
Fair value measurements
Other assets	0	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Corporate bonds
Fair value measurements
Trading account liabilities	646	717
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets	10	32
Fair Value, Measurements, Recurring | Level 3
Fair value measurements
Securities available for sale	1,131	1,331
Other assets	316	277
Total assets	$ 1,455	$ 1,621
Percentage of total	5.00%	6.00%
Percentage of total Company assets	1.00%	1.00%
Trading account liabilities	$ 8	$ 13
Other liabilities	124	118
Total liabilities	$ 132	$ 131
Percentage of total	4.00%	3.00%
Percentage of total Company liabilities	0.00%	0.00%
Fair Value, Measurements, Recurring | Level 3 | Derivative Financial Instruments, Liabilities [Member]
Fair value measurements
Other liabilities	$ 3	$ 2
Fair Value, Measurements, Recurring | Level 3 | US Treasury and Government [Member]
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	177	141
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds
Fair value measurements
Trading account assets	0	0
Fair Value, Measurements, Recurring | Level 3 | Other contracts
Fair value measurements
Trading account assets	0	0
Trading account liabilities	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 | Equity securities
Fair value measurements
Trading account assets	0	0
Trading account liabilities	0	0
Fair Value, Measurements, Recurring | Level 3 | Trading Derivatives [Member]
Fair value measurements
Trading account assets	8	13
Trading account liabilities	8	13
Fair Value, Measurements, Recurring | Level 3 | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets	8	13
Fair Value, Measurements, Recurring | Level 3 | Fdic
Fair value measurements
Other liabilities	121	116
Fair Value, Measurements, Recurring | Level 3 | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	0	0
Fair Value, Measurements, Recurring | Level 3 | Privately issued
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	936	1,190
Fair Value, Measurements, Recurring | Level 3 | Mortgage servicing rights(2)
Fair value measurements
Other assets	270	159
Fair Value, Measurements, Recurring | Level 3 | loans held for sale [Domain]
Fair value measurements
Other assets	42	117
Fair Value, Measurements, Recurring | Level 3 | Equity Securities, FV-NI [Member]
Fair value measurements
Other assets	0	0
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds
Fair value measurements
Trading account liabilities	0	0
Fair Value, Measurements, Recurring | Level 3 | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets	4	1
Fair Value
Fair value measurements
Other assets	54	48
Fair Value | Fair Value, Measurements, Recurring
Fair value measurements
Securities available for sale	17,789	16,314
Other assets	328	299
Total assets	28,494	27,826
Trading account liabilities	3,266	4,027
Other liabilities	126	121
Total liabilities	3,392	4,148
Fair Value | Fair Value, Measurements, Recurring | Derivative Financial Instruments, Liabilities [Member]
Fair value measurements
Other liabilities	5	5
Fair Value | Fair Value, Measurements, Recurring | US Treasury and Government [Member]
Fair value measurements
Trading account assets	2,393	3,071
Securities available for sale	5,438	3,429
Trading account liabilities	2,299	2,753
Fair Value | Fair Value, Measurements, Recurring | Other
Fair value measurements
Securities available for sale	205	170
Fair Value | Fair Value, Measurements, Recurring | Corporate bonds
Fair value measurements
Trading account assets	1,212	1,367
Fair Value | Fair Value, Measurements, Recurring | Other contracts
Fair value measurements
Trading account assets	306	360
Trading account liabilities	4	10
Fair Value | Fair Value, Measurements, Recurring | Mortgage-backed securities
Fair value measurements
Trading account assets	5,376	5,785
Fair Value | Fair Value, Measurements, Recurring | Equity securities
Fair value measurements
Trading account assets	127	127
Trading account liabilities	105	70
Fair Value | Fair Value, Measurements, Recurring | Trading Derivatives [Member]
Fair value measurements
Trading account assets	963	503
Trading account liabilities	212	477
Fair Value | Fair Value, Measurements, Recurring | Trading Assets, Excluding Debt and Equity Securities [Member]
Fair value measurements
Trading account assets	10,377	11,213
Fair Value | Fair Value, Measurements, Recurring | Fdic
Fair value measurements
Other liabilities	121	116
Fair Value | Fair Value, Measurements, Recurring | U.S. government agency and government-sponsored agencies
Fair value measurements
Securities available for sale	5,468	8,007
Fair Value | Fair Value, Measurements, Recurring | Privately issued
Fair value measurements
Securities available for sale	762	864
Fair Value | Fair Value, Measurements, Recurring | Privately issued - commercial mortgage-backed securities
Fair value measurements
Securities available for sale	3,489	1,180
Fair Value | Fair Value, Measurements, Recurring | Collateralized loan obligations
Fair value measurements
Securities available for sale	1,491	1,474
Fair Value | Fair Value, Measurements, Recurring | Direct bank purchase bonds
Fair value measurements
Securities available for sale	936	1,190
Fair Value | Fair Value, Measurements, Recurring | Mortgage servicing rights(2)
Fair value measurements
Other assets	270	159
Fair Value | Fair Value, Measurements, Recurring | loans held for sale [Domain]
Fair value measurements
Other assets	42	117
Fair Value | Fair Value, Measurements, Recurring | Equity Securities, FV-NI [Member]
Fair value measurements
Other assets	10	9
Fair Value | Fair Value, Measurements, Recurring | Corporate bonds
Fair value measurements
Trading account liabilities	646	717
Fair Value | Fair Value, Measurements, Recurring | Derivative Financial Instruments, Assets [Member]
Fair value measurements
Other assets	$ 6	$ 14