Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 539	$ 502
Servicing retained from loan sales	24	18
Purchases	11	6
Amortization	(29)	(29)
Temporary impairments	(2)	0
Balance at end of period	543	497
Fair value at end of period	655	727
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	46	37
Servicing retained from loan sales	5	2
Purchases	0	0
Amortization	(2)	(1)
Temporary impairments	(9)	0
Balance at end of period	40	38
Fair value at end of period	$ 41	$ 50