Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 435	$ 369	$ 412	$ 356
Servicing retained from loan sales	29	19	56	47
Purchases	12	7	33	14
Amortization	(25)	(22)	(50)	(44)
Balance at end of period	451	373	451	373
Fair value at end of period	646	495	646	495
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	32	29	31	28
Servicing retained from loan sales	3	1	5	3
Purchases	0	0	0	0
Amortization	(1)	(1)	(2)	(2)
Balance at end of period	34	29	34	29
Fair value at end of period	$ 43	$ 35	$ 43	$ 35