Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 502	$ 412
Servicing retained from loan sales	108	117
Purchases	47	75
Amortization	(115)	(102)
Temporary impairments	(3)	0
Balance at end of period	539	502
Fair value at end of period	665	757
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	37	31
Servicing retained from loan sales	15	10
Purchases	0	0
Amortization	(6)	(4)
Balance at end of period	46	37
Fair value at end of period	$ 50	$ 52