Mortgage Banking Activities - Principal Balances of Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 1,200,000	$ 1,200,000
FHLMC [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	958,111	956,278
FNMA [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 206,472	$ 208,114