Calculation of Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Calculation of Basic Earnings Per Share:
Net Income	$ 8,029	$ 6,294	$ 28,396	$ 27,403	$ 41,140
Weighted shares outstanding (shares)	15,315,020	15,235,860	15,300,931	14,922,094
Basic Earnings Per Share (in dollars per share)	$ 0.52	$ 0.41	$ 1.86	$ 1.84
Reconciliation of Numerator:
Net Income	$ 8,029	$ 6,294	$ 28,396	$ 27,403	$ 41,140
Reconciliation of Denominator:
Weighted shares outstanding - Basic (shares)	15,315,020	15,235,860	15,300,931	14,922,094
Effect of dilutive securities:
Share-based compensation (shares)	37,682	44,797	41,356	48,096
Adjusted denominator-Diluted (shares)	15,352,702	15,280,657	15,342,287	14,970,190
Diluted Earnings Per Share (in dollars per share)	$ 0.52	$ 0.41	$ 1.85	$ 1.83