Mortgage Banking Activities - Principal Balance of Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 1,300,000	$ 1,200,000
FHLMC [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	1,003,441	956,278
FNMA [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	242,444	$ 208,114
Private Investor [Member]
Mortgage loan portfolios serviced for:
Mortgage loans serviced for others	$ 23,404