Fair Value of Assets and Liabilities - Summary Of Changes In Level 3 Assets And Liabilities Measured At Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Acquisition	$ 0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	(23)	$ (32)	$ (6)
Total net gains (losses) included in net income	(1)	(4)	(5)
Purchases	0	0	(28)
Sales	0	0	0
Settlements	10	13	7
Net transfers into (out of) Level 3	0	0	0
Ending balance	(14)	(23)	(32)
Net unrealized gains (losses) included in net income	(1)	(4)	(5)
Trading securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	2	2
Total net gains (losses) included in net income	(1)	0	1
Purchases	0	0	0
Sales	0	0	0
Settlements	0	(1)	(1)
Net transfers into (out of) Level 3	0	0	0
Ending balance	0	1	2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Acquisition	0
Net unrealized gains (losses) included in net income	0	0	0
Interest-only strips- AFS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19	10	1
Total net gains (losses) included in net income	(6)	(5)	0
Purchases	6	0	0
Sales	(11)	(47)	(17)
Settlements	0	0	0
Net transfers into (out of) Level 3	24	61	26
Ending balance	32	19	10
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Acquisition	0
Net unrealized gains (losses) included in net income	(4)	(2)	(1)
Loans held for sale
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	14	16	19
Total net gains (losses) included in net income	1	2	1
Purchases	0	0	0
Sales	0	0	0
Settlements	(3)	(4)	(4)
Net transfers into (out of) Level 3	0	0	0
Ending balance	12	14	16
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Acquisition	0
Net unrealized gains (losses) included in net income	1	2	$ 1
Loans held for investment
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0
Total net gains (losses) included in net income	0
Purchases	0
Sales	(4)
Settlements	(3)
Net transfers into (out of) Level 3	9
Ending balance	16	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Acquisition	14
Net unrealized gains (losses) included in net income	$ 0