Parent Company Financial statements - Condensed Statements of Income and Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income
Interest income	$ 24,849	$ 24,048	$ 23,553	$ 21,060	$ 18,495	$ 17,815	$ 17,399	$ 17,020	$ 93,510	$ 70,729	$ 65,635
Non-interest income	6,460	6,305	7,090	5,384	5,635	6,003	5,780	4,658	25,239	22,076	19,717
Expenses
Interest expenses	3,969	3,545	3,059	2,584	2,161	2,055	2,065	2,147	13,157	8,428	9,113
Non-interest expenses	17,329	15,464	15,176	20,290	13,717	12,978	12,860	12,464	68,259	52,019	49,929
Income tax benefit	4,294	3,067	3,377	557	1,734	2,288	2,529	1,592	11,295	8,143	7,893
Net income	$ 4,502	$ 7,556	$ 8,189	$ 1,538	$ 5,025	$ 5,153	$ 5,330	$ 3,320	21,785	18,828	16,282
Comprehensive income									24,492	17,008	17,060
Parent Company [Member]
Expenses
Interest expenses									1
Non-interest expenses									1,046	1,658	1,017
Total expenses									1,047	1,658	1,017
Loss before income taxes									(1,047)	(1,658)	(1,017)
Income tax benefit									(353)	(481)	(356)
Income before equity in undistributed net earnings of subsidiaries									(694)	(1,177)	(661)
Earnings distributed by subsidiaries									15,000	23,500	15,000
Undistributed earnings (loss) of subsidiaries									7,479	(3,495)	1,943
Net income									21,785	18,828	16,282
Comprehensive income									$ 24,492	$ 17,008	$ 17,060