Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Commercial Mortgage Backed Securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 588	$ 543	$ 578	$ 539
Servicing retained from loan sales	28	53	57	77
Purchases	7	7	14	18
Amortization	(30)	(30)	(59)	(59)
Temporary (impairments) recoveries	7	(8)	10	(10)
Balance at end of period	600	565	600	565
Fair value at end of period	704	663	704	663
Agency residential mortgage-backed securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	72	40	58	46
Servicing retained from loan sales	12	8	23	13
Purchases	0	0	0	0
Amortization	(5)	(3)	(10)	(5)
Temporary (impairments) recoveries	(2)	1	6	(8)
Balance at end of period	77	46	77	46
Fair value at end of period	$ 81	$ 46	$ 81	$ 46