Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 412	$ 356
Servicing retained from loan sales	117	110
Purchases	75	36
Amortization	(102)	(90)
Balance at end of period	502	412
Fair value at end of period	757	537
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	31	28
Servicing retained from loan sales	10	7
Purchases	0	0
Amortization	(4)	(4)
Balance at end of period	37	31
Fair value at end of period	$ 52	$ 37