Quarterly Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Quarterly Financial Information [Line Items]
Revenue	$ 98,077	$ 91,567	$ 84,545	$ 76,361	$ 89,462	$ 83,297	$ 72,624	$ 63,296	$ 350,550	$ 308,679	$ 290,540
Gross profit	77,037	72,403	67,604	59,498	70,502	66,058	57,047	49,400	276,542	243,007	229,179
Net income	$ 14,553	$ 11,138	$ 9,910	$ 5,008	$ 12,947	$ 10,031	$ 7,051	$ 2,399	$ 40,609	$ 32,428	$ 37,265
Diluted net income per share	$ 0.34	$ 0.26	$ 0.23	$ 0.12	$ 0.30	$ 0.24	$ 0.17	$ 0.06	$ 0.96	$ 0.76	$ 0.87