Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended						6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Line Items]
Net income	$ 9,405	$ 10,674	$ 11,422	$ 12,694	$ 11,366	$ 8,336	$ 20,079	$ 19,702
Average number of common shares outstanding	22,803				22,745		22,793	22,736
Effect of dilutive stock options	267				235		269	229
Average number of common shares outstanding used to calculate diluted earnings per share	23,070				22,980		23,062	22,965
Options excluded from diluted earnings per share because the effect of these options was antidilutive	21				23		22	23
Restricted Stock Units (RSUs) [Member]
Earnings Per Share [Line Items]
Options excluded from diluted earnings per share because the effect of these options was antidilutive	19						10