Premises, Equipment, and Leases - Lease Expense and Other Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease cost
Operating lease cost	$ 39	$ 25
Sublease income	(1)	0
Total lease cost	38	25
Other information
(Gain) loss on right-of-use asset impairment - operating leases	6	3
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	41	23
Right-of-use assets obtained in exchange for new lease obligations:
Operating leases	216	48
Finance leases	$ 2	$ 1