Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income per consolidated statements of income	$ 9,529	$ 7,556	$ 27,626	$ 17,283
Net income allocated to participating securities	(49)	(42)	(140)	(108)
Net income allocated to common stock	9,480	7,514	27,486	17,175
Distributed earnings allocated to common stock	3,478	1,979	9,432	4,941
Undistributed earnings allocated to common stock	6,002	5,535	18,054	12,234
Net income allocated to common stock	$ 9,480	$ 7,514	$ 27,486	$ 17,175
Weighted average common shares outstanding, including shares considered participating securities	49,909	49,736	49,892	49,353
Less: Average participating securities	(226)	(276)	(252)	(308)
Weighted average shares	49,683	49,460	49,640	49,045
Basic earnings per common share	$ 0.19	$ 0.15	$ 0.55	$ 0.35
Weighted average common shares outstanding for basic earnings per common share	49,683	49,460	49,640	49,045
Add: Dilutive effects of assumed exercises of stock options and LTIP awards	264	391	262	396
Weighted average shares and dilutive potential common shares	49,947	49,851	49,902	49,441
Diluted earnings per common share	$ 0.19	$ 0.15	$ 0.55	$ 0.35