Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commercial Mortgage Backed Securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 578	$ 539
Servicing retained from loan sales	29	24
Purchases	7	11
Amortization	(29)	(29)
Temporary (impairments) recoveries	3	(2)
Balance at end of period	588	543
Fair value at end of period	682	655
Agency residential mortgage-backed securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	58	46
Servicing retained from loan sales	11	5
Purchases	0	0
Amortization	(5)	(2)
Temporary (impairments) recoveries	8	(9)
Balance at end of period	72	40
Fair value at end of period	$ 77	$ 41