Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 322	$ 323
Servicing retained from loan sales	80	55
Purchases	41	38
Amortization	(87)	(94)
Balance at end of period	356	322
Fair value at end of period	459	423
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	0
Servicing retained from loan sales	2
Purchases	28
Amortization	(2)
Balance at end of period	28	0
Fair value at end of period	$ 33	$ 28