Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Calculation of Basic Earnings Per Share:
Net Income	$ 5,119	$ 3,180	$ 32,521	$ 25,995	$ 36,092
Weighted shares outstanding - Basic (in shares)	15,258,819	14,574,678	15,035,569	14,539,841
Basic Earnings Per Share (in usd per share)	$ 0.34	$ 0.22	$ 2.16	$ 1.79
Reconciliation of Numerator:
Net Income	$ 5,119	$ 3,180	$ 32,521	$ 25,995	$ 36,092
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)	15,258,819	14,574,678	15,035,569	14,539,841
Effect of dilutive securities:
Share-based Compensation (in shares)	48,024	41,987	48,072	48,289
Adjusted denominator-Diluted (in shares)	15,306,843	14,616,665	15,083,641	14,588,130
Diluted Earnings Per Share (in usd per share)	$ 0.33	$ 0.22	$ 2.16	$ 1.78