Quarterly Results Of Operations (Schedule Of Quarterly Results Of Operations) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2010
Quarterly Results Of Operations [Abstract]
Revenue	$ 89,462	$ 83,297	$ 72,624	$ 63,296	$ 77,980	$ 76,336	$ 69,413	$ 66,811	$ 308,679	$ 290,540	$ 260,342
Gross profit	70,502	66,058	57,047	49,400	61,593	60,244	55,106	52,236	243,007	229,179	204,278
Net income	$ 12,947	$ 10,031	$ 7,051	$ 2,399	$ 10,771	$ 11,101	$ 8,248	$ 7,145	$ 32,428	$ 37,265	$ 27,917
Diluted net income per share	$ 0.30	$ 0.24	$ 0.17	$ 0.06	$ 0.25	$ 0.26	$ 0.19	$ 0.17	$ 0.76	$ 0.87	$ 0.67