Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share [Abstract]
Net income per consolidated statements of income	$ 4,502	$ 7,556	$ 8,189	$ 1,538	$ 5,025	$ 5,153	$ 5,330	$ 3,320	$ 21,785	$ 18,828	$ 16,282
Net income allocated to participating securities									(136)	(121)	(81)
Net income allocated to common stock									21,649	18,707	16,201
Distributed earnings allocated to common stock									6,919	5,140	3,350
Undistributed earnings allocated to common stock									14,730	13,567	12,851
Net earnings allocated to common stock									$ 21,649	$ 18,707	$ 16,201
Weighted average common shares outstanding, including shares considered participating securities									49,459	46,967	48,320
Less: Average participating securities									(309)	(301)	(242)
Weighted average shares									49,150	46,666	48,078
Basic earnings per common share	$ 0.09	$ 0.15	$ 0.16	$ 0.03	$ 0.11	$ 0.11	$ 0.11	$ 0.07	$ 0.44	$ 0.40	$ 0.34
Weighted average common shares outstanding for basic earnings per common share									49,150	46,666	48,078
Add: Dilutive potential common shares									342	371	272
Weighted average shares and dilutive potential common shares									49,492	47,037	48,350
Diluted earnings per common share	$ 0.09	$ 0.15	$ 0.16	$ 0.03	$ 0.11	$ 0.11	$ 0.11	$ 0.07	$ 0.44	$ 0.40	$ 0.34