Mortgage Banking Activity - Residential Mortgage Loans (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Loans Receivable Held-for-sale, Net, Reconciliation to Cash Flow [Roll Forward]
Balance at beginning of period	$ 1,022
Balance at end of period	1,052	$ 1,022
Mortgage loans
Loans Receivable Held-for-sale, Net, Reconciliation to Cash Flow [Roll Forward]
Balance at beginning of period	409	4
Acquired	0	320
Originations and purchases	2,154	2,499
Sales, net of gains	(2,302)	(2,405)
Mortgage loans transferred from (to) held for investment	30	(9)
Balance at end of period	$ 291	$ 409