Quarterly Financial Information - Summary of Selected Financial Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 127,440	$ 128,664	$ 124,371	$ 112,626	$ 110,735	$ 107,021	$ 103,088	$ 95,799	$ 493,101	$ 416,643	$ 363,304
Gross profit	60,491	61,792	58,558	52,530	52,767	49,549	47,042	43,845	233,371	193,203	167,702
Income before income taxes	24,760	26,698	23,406	19,603	19,062	17,572	15,053	14,132	94,467	65,819	53,868
Net income	$ 15,317	$ 17,000	$ 14,740	$ 11,883	$ 10,512	$ 11,049	$ 9,047	$ 8,493	$ 58,940	$ 39,101	$ 32,994
Earnings per diluted share	$ 0.43	$ 0.48	$ 0.42	$ 0.33	$ 0.30	$ 0.32	$ 0.26	$ 0.25	$ 1.66	$ 1.13	$ 1.00
Shares used in computing diluted earnings per share	35,661	35,284	35,161	35,500	35,348	34,764	34,290	33,948	35,401	34,590	32,916