Mortgage Banking Activities - Valuation Allowance for Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Mortgage Banking [Abstract]
Balance, beginning of period	$ (3)	$ (474)		$ (39)
Impairment charges	(2)	(292)	$ (5)	(727)
Balance, end of period	$ (5)	$ (766)	$ (5)	$ (766)