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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Calculation of Basic Earnings Per Share:
Net Income	$ 6,833	$ 4,416	$ 32,023	$ 32,812	$ 44,675
Weighted shares outstanding (shares)	16,344,442	15,372,413	16,334,210	15,324,932
Basic Earnings Per Share (in dollars per share)	$ 0.42	$ 0.29	$ 1.96	$ 2.14
Reconciliation of Numerator:
Net Income	$ 6,833	$ 4,416	$ 32,023	$ 32,812	$ 44,675
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	16,344,442	15,372,413	16,334,210	15,324,932
Effect of dilutive securities:
Share-based compensation (shares)	45,193	40,370	44,423	41,023
Adjusted denominator-Diluted (shares)	16,389,635	15,412,783	16,378,633	15,365,955
Diluted Earnings Per Share (in dollars per share)	$ 0.42	$ 0.29	$ 1.96	$ 2.14