Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 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	18	27
Purchases	6	21
Amortization	(29)	(25)
Balance at end of period	497	435
Fair value at end of period	727	537
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	2	2
Purchases	0	0
Amortization	(1)	(1)
Balance at end of period	38	32
Fair value at end of period	$ 50	$ 42