Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) 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	$ 2,989	$ 11,897	$ 13,589	$ 12,079	$ 12,533	$ 12,199	$ 9,405	$ 10,674	$ 40,554	$ 44,811	$ 43,818
Average number of common shares outstanding									22,912	22,814	22,750
Effect of dilutive stock options									338	273	248
Average number of common shares outstanding used to calculate diluted earnings per share									23,250	23,087	22,998