Premises and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Depreciation expense	$ 2,700	$ 2,300	$ 2,100
Rent expense	$ 790	$ 597	$ 338