Mortgage Servicing Assets - Changes in Carrying Amount of Mortgage Servicing Assets (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Commercial Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	$ 451	$ 373	$ 412	$ 356
Servicing retained from loan sales	25	24	81	71
Purchases	17	10	50	24
Amortization	(25)	(22)	(75)	(66)
Balance at end of period	468	385	468	385
Fair value at end of period	672	508	672	508
Residential Mortgage Backed Securities [Member]
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	34	29	31	28
Servicing retained from loan sales	2	2	7	5
Purchases	0	0	0	0
Amortization	(1)	(1)	(3)	(3)
Balance at end of period	35	30	35	30
Fair value at end of period	$ 49	$ 34	$ 49	$ 34