Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 356	$ 322
Servicing retained from loan sales	110	80
Purchases	36	41
Amortization	(90)	(87)
Balance at end of period	412	356
Fair value at end of period	537	459
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	28	0
Servicing retained from loan sales	7	2
Purchases	0	28
Amortization	(4)	(2)
Balance at end of period	31	28
Fair value at end of period	$ 37	$ 33