Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended							9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 11,897	$ 13,589	$ 12,079	$ 12,533	$ 12,199	$ 9,405	$ 10,674	$ 37,565	$ 32,278
Average number of common shares outstanding	22,932				22,825			22,901	22,804
Effect of dilutive stock options and restricted stock units	312				274			338	273
Average number of common shares outstanding used to calculate diluted earnings per share	23,244				23,099			23,239	23,077
Options excluded from diluted earnings per share because the effect of these options was antidilutive					12				18