Reconciliation of Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended																12 Months Ended
		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Earning Per Share Basic And Diluted [Abstract]
Net income		$ 21,591		$ 22,013		$ 16,190		$ 22,932		$ 17,743		$ 18,201		$ 19,184		$ 25,929		$ 82,726		$ 81,057		$ 95,329
Basic weighted average shares	[1]																	29,073,803		28,785,044		28,519,536
Effect of common stock equivalents:
Assumed exercise of stock options and vesting of stock grants																		957,806		866,446		773,202
Diluted weighted average shares																		30,031,609		29,651,490		29,292,738
Net income per common share:
Basic		$ 0.74	[1]	$ 0.76	[1]	$ 0.56	[1]	$ 0.79	[1]	$ 0.62	[2]	$ 0.63	[2]	$ 0.67	[2]	$ 0.90	[2]	$ 2.85	[1]	$ 2.82	[1]	$ 3.34	[1]
Diluted		$ 0.71	[1]	$ 0.73	[1]	$ 0.54	[1]	$ 0.77	[1]	$ 0.59	[2]	$ 0.61	[2]	$ 0.65	[2]	$ 0.89	[2]	$ 2.75	[1]	$ 2.73	[1]	$ 3.25	[1]

[1] We completed a two-for-one stock split on January 20, 2017. All share and per share data has been retroactively restated on a post-split basis.

[2] We completed a two-for-one stock split on January 20, 2017. All per share data has been retroactively restated on a post-split basis.