Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total interest and dividend income	$ 36,344	$ 34,040	$ 32,099	$ 29,387	$ 28,698	$ 28,069	$ 27,003	$ 25,104	$ 131,870	$ 108,874	$ 94,315
Net interest income	28,116	26,878	25,826	24,100	23,801	23,517	22,989	21,202	104,920	91,509	81,287
Provision for loan losses	1,441	1,095	1,905	1,631	3,105	1,400	1,134	1,016	6,072	6,655	4,149
Non-interest income	4,433	5,933	5,606	4,751	5,541	5,032	5,089	5,773	20,723	21,435	17,691
Non-interest expense	20,006	20,794	19,543	18,999	17,588	17,618	17,797	17,034	79,342	70,037	67,118
Net income	$ 8,945	$ 9,236	$ 8,441	$ 7,097	$ 3,451	$ 7,246	$ 6,683	$ 6,480	$ 33,719	$ 23,860	$ 20,540
Basic earnings per common share (in dollars per share)	$ 0.59	$ 0.60	$ 0.55	$ 0.46	$ 0.23	$ 0.47	$ 0.44	$ 0.43	$ 2.21	$ 1.57	$ 1.42
Diluted earnings per common share (in dollars per share)	$ 0.59	$ 0.60	$ 0.55	$ 0.46	$ 0.23	$ 0.47	$ 0.44	$ 0.43	$ 2.21	$ 1.57	$ 1.42