Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Commercial Mortgage Backed Securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 600	$ 565	$ 578	$ 539
Servicing retained from loan sales	31	30	88	107
Purchases	8	6	22	24
Amortization	(30)	(29)	(89)	(88)
Temporary (impairments) recoveries	0	(6)	10	(16)
Balance at end of period	609	566	609	566
Fair value at end of period	735	685	735	685
Agency residential mortgage-backed securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	77	46	58	46
Servicing retained from loan sales	9	12	32	25
Purchases	0	0	0	0
Amortization	(4)	(4)	(14)	(9)
Temporary (impairments) recoveries	3	(1)	9	(9)
Balance at end of period	85	53	85	53
Fair value at end of period	$ 89	$ 54	$ 89	$ 54