Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended						6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income	$ 15,029	$ 13,910	$ 2,989	$ 11,897	$ 13,589	$ 12,079	$ 28,939	$ 25,668
Average number of common shares outstanding	22,983				22,900		22,970	22,885
Effect of dilutive stock options and restricted stock	293				340		310	351
Average number of common shares outstanding used to calculate diluted earnings per share	23,276				23,240		23,280	23,236
Options excluded from diluted earnings per share because the effect of these options was antidilutive	0				0		0	0