Fair Value Measurement and Fair Value of Financial Instruments - Reconciliation of Assets and Liabilities on a Recurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, end of period	$ 1
Securities Available for Sale
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	1,131	$ 1,331
Total gains (losses) (realized/unrealized) included in income before taxes, assets	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	13	31
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	382	57
Settlements, assets	(52)	(288)
Asset balance, end of period	1,474	1,131
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	0	0
Other Assets
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Asset balance, beginning of period	316	277
Total gains (losses) (realized/unrealized) included in income before taxes, assets	(166)	(88)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	16	396
Settlements, assets	0	0
Asset balance, end of period	166	316
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, assets	(166)	(103)
Trading Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(8)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	5
Liability balance, end of period		(8)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	2
Other Liabilities
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Liability balance, beginning of period	(124)	(118)
Total gains (losses) (realized/unrealized) included in income before taxes, assets, liability	(2)	(6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	122	0
Liability balance, end of period	(4)	(124)
Changes in unrealized gains (losses) included in income before taxes for assets still held at end of period, liabilities	(2)	(6)
Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	$ 432	$ 264
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Percentage of Assets Fair Value Disclosure	100.00%	100.00%
Percentage of Assets Fair Value Disclosure, Netting Adjustment	(1.00%)	(1.00%)
Percentage of Company Assets Fair Value Disclosure	21.00%	17.00%
Percentage of Company Assets Fair Value Disclosure, Netting Adjustment	0.00%	0.00%
Fair Value, Measurements, Recurring | Level 3
Reconciliation of assets measured using significant unobservable inputs
Other assets	$ 166	$ 316
Assets, Fair Value Disclosure	$ 1,641	$ 1,455
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Percentage of Assets Fair Value Disclosure	5.00%	5.00%
Percentage of Company Assets Fair Value Disclosure	1.00%	1.00%
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	$ 20	$ 10
Assets, Fair Value Disclosure	$ 965	$ 153
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Percentage of Assets Fair Value Disclosure	3.00%	1.00%
Percentage of Company Assets Fair Value Disclosure	1.00%	0.00%
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	$ 2	$ 10
Assets, Fair Value Disclosure	$ 32,310	$ 27,150
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Percentage of Assets Fair Value Disclosure	93.00%	95.00%
Percentage of Company Assets Fair Value Disclosure	19.00%	16.00%
Mortgage servicing rights [Domain] | Fair Value, Measurements, Recurring | Level 3
Reconciliation of assets measured using significant unobservable inputs
Other assets	$ 110	$ 270
Mortgage servicing rights [Domain] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
Mortgage servicing rights [Domain] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
loans held for sale [Domain] | Fair Value, Measurements, Recurring | Level 3
Reconciliation of assets measured using significant unobservable inputs
Other assets	43	42
loans held for sale [Domain] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
loans held for sale [Domain] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
Derivative Financial Instruments, Assets [Member] | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	1	8
Derivative Financial Instruments, Assets [Member] | Fair Value, Measurements, Recurring | Level 3
Reconciliation of assets measured using significant unobservable inputs
Other assets	13	4
Derivative Financial Instruments, Assets [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
Derivative Financial Instruments, Assets [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	2	10
Equity Securities, FV-NI [Member] | Fair Value, Measurements, Recurring | Level 3
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
Equity Securities, FV-NI [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	20	10
Equity Securities, FV-NI [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 [Member]
Reconciliation of assets measured using significant unobservable inputs
Other assets	0	0
Other Asset Liability Management Securities [Member] | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	1	8
Fair Value
Reconciliation of assets measured using significant unobservable inputs
Other assets	31	54
Fair Value | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Other assets	187	328
Assets, Fair Value Disclosure	34,484	28,494
Fair Value | Mortgage servicing rights [Domain] | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Other assets	110	270
Fair Value | loans held for sale [Domain] | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Other assets	43	42
Fair Value | Derivative Financial Instruments, Assets [Member] | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Other assets	14	6
Fair Value | Equity Securities, FV-NI [Member] | Fair Value, Measurements, Recurring
Reconciliation of assets measured using significant unobservable inputs
Other assets	$ 20	$ 10