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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Calculation of Basic Earnings Per Share:
Net Income	$ 6,294	$ 5,134	$ 27,403	$ 22,815	$ 36,092
Weighted shares outstanding - Basic (in shares)	15,235,860	14,556,242	14,922,094	14,522,133
Basic Earnings Per Share (in usd per share)	$ 0.41	$ 0.35	$ 1.84	$ 1.57
Reconciliation of Numerator:
Net Income	$ 6,294	$ 5,134	$ 27,403	$ 22,815	$ 36,092
Reconciliation of Denominator:
Weighted shares outstanding - Basic (in shares)	15,235,860	14,556,242	14,922,094	14,522,133
Effect of dilutive securities:
Share-based Compensation (in shares)	44,797	50,537	48,096	51,510
Adjusted denominator-Diluted (in shares)	15,280,657	14,606,779	14,970,190	14,573,643
Diluted Earnings Per Share (in usd per share)	$ 0.41	$ 0.35	$ 1.83	$ 1.57