Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Real estate — commercial mortgage
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 543	$ 497	$ 539	$ 502
Servicing retained from loan sales	53	23	77	41
Purchases	7	17	18	23
Amortization	(30)	(28)	(59)	(57)
Temporary impairments	(8)	0	(10)	0
Balance at end of period	565	509	565	509
Fair value at end of period	663	723	663	723
Agency residential mortgage-backed securities
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	40	38	46	37
Servicing retained from loan sales	8	2	13	4
Purchases	0	0	0	0
Amortization	(3)	(1)	(5)	(2)
Temporary impairments	1	0	(8)	0
Balance at end of period	46	39	46	39
Fair value at end of period	$ 46	$ 45	$ 46	$ 45