Mortgage Banking Activity - Residential Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Loans Receivable Held-for-sale, Net, Reconciliation to Cash Flow [Roll Forward]
Balance at beginning of period	$ 1,022
Acquired	0	$ 320
Originations and purchases	446	2,499
Sales, net of gains	(421)	(2,405)
Mortgage loans transferred from (to) held for investment	3	(9)
Balance at end of period	811
Mortgage loans
Loans Receivable Held-for-sale, Net, Reconciliation to Cash Flow [Roll Forward]
Balance at beginning of period	409	4
Balance at end of period	$ 437	$ 409