Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income per consolidated statements of income	$ 9,541	$ 8,189	$ 18,097	$ 9,727
Net income allocated to participating securities	(41)	(49)	(91)	(61)
Net income allocated to common stock	9,500	8,140	18,006	9,666
Distributed earnings allocated to common stock	2,982	1,483	5,954	2,962
Undistributed earnings allocated to common stock	6,518	6,657	12,052	6,704
Net income allocated to common stock	$ 9,500	$ 8,140	$ 18,006	$ 9,666
Weighted average common shares outstanding, including shares considered participating securities	49,906	49,691	49,877	49,158
Less: Average participating securities	(212)	(299)	(250)	(309)
Weighted average shares	49,694	49,392	49,627	48,849
Basic earnings per common share	$ 0.19	$ 0.16	$ 0.36	$ 0.20
Weighted average common shares outstanding for basic earnings per common share	49,694	49,392	49,627	48,849
Add: Dilutive effects of assumed exercises of stock options and LTIP awards	250	403	262	398
Weighted average shares and dilutive potential common shares	49,944	49,795	49,889	49,247
Diluted earnings per common share	$ 0.19	$ 0.16	$ 0.36	$ 0.20