Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues	$ 76,470	$ 76,458	$ 77,796	$ 71,471	$ 75,129	$ 75,482	$ 68,625	$ 63,723	$ 302,195	$ 282,959	$ 228,971
Gross Profit	35,953	35,574	36,201	34,626	37,016	33,777	31,890	26,287	142,354	128,970	106,276
Net income	$ 17,842	$ 9,261	$ 9,493	$ 8,073	$ 9,681	$ 8,817	$ 7,566	$ 3,859	$ 44,669	$ 29,923	$ 24,981
Basic	$ 1.29	$ 0.66	$ 0.69	$ 0.59	$ 0.71	$ 0.65	$ 0.56	$ 0.28	$ 3.23	$ 2.20	$ 1.70
Diluted	$ 1.26	$ 0.65	$ 0.67	$ 0.57	$ 0.70	$ 0.64	$ 0.55	$ 0.27	$ 3.15	$ 2.16	$ 1.67