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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Calculation of Basic Earnings Per Share:
Net Income	$ 4,416	$ 5,119	$ 32,812	$ 32,521	$ 41,140
Weighted shares outstanding (shares)	15,372,413	15,258,819	15,324,932	15,035,569
Basic Earnings Per Share (in dollars per share)	$ 0.29	$ 0.34	$ 2.14	$ 2.16
Reconciliation of Numerator:
Net Income	$ 4,416	$ 5,119	$ 32,812	$ 32,521	$ 41,140
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	15,372,413	15,258,819	15,324,932	15,035,569
Effect of dilutive securities:
Share-based compensation (shares)	40,370	48,024	41,023	48,072
Adjusted denominator-Diluted (shares)	15,412,783	15,306,843	15,365,955	15,083,641
Diluted Earnings Per Share (in dollars per share)	$ 0.29	$ 0.33	$ 2.14	$ 2.16