Reconciliation of Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earning Per Share Basic And Diluted [Abstract]
Net income	$ 17,743	$ 18,201	$ 19,184	$ 25,929	$ 19,275	$ 31,225	$ 16,861	$ 27,968	$ 81,057	$ 95,329	$ 63,466
Basic weighted average shares									14,392,522	14,259,768	14,133,925
Effect of common stock equivalents:
Assumed exercise of stock options and vesting of stock grants									433,223	386,601	211,628
Diluted weighted average shares									14,825,745	14,646,369	14,345,553
Net income per common share:
Basic	$ 1.23	$ 1.26	$ 1.33	$ 1.81	$ 1.35	$ 2.19	$ 1.18	$ 1.96	$ 5.63	$ 6.69	$ 4.49
Diluted	$ 1.19	$ 1.23	$ 1.30	$ 1.77	$ 1.31	$ 2.14	$ 1.17	$ 1.94	$ 5.47	$ 6.51	$ 4.42