Mortgage Banking Activities - Valuation Allowance for Mortgage Servicing Rights (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage Banking [Abstract]
Balance, beginning of period		$ (39)	$ (58)
Impairment charges	$ (15)	(727)	(299)
Recoveries	6	$ 766	318
Balance, end of period	$ (9)		$ (39)