Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues	$ 82,672	$ 81,405	$ 77,281	$ 75,511	$ 76,470	$ 76,458	$ 77,796	$ 71,471	$ 316,869	$ 302,195	$ 282,959
Gross Profit	41,768	40,695	39,082	34,965	35,953	35,574	36,201	34,626	156,510	142,354	128,970
Net Income	$ 12,343	$ 12,188	$ 9,149	$ 7,601	$ 17,842	$ 9,261	$ 9,493	$ 8,073	$ 41,281	$ 44,669	$ 29,923
Basic	$ 0.86	$ 0.85	$ 0.65	$ 0.54	$ 1.29	$ 0.66	$ 0.69	$ 0.59	$ 2.90	$ 3.23	$ 2.20
Diluted	$ 0.82	$ 0.82	$ 0.63	$ 0.53	$ 1.26	$ 0.65	$ 0.67	$ 0.57	$ 2.80	$ 3.15	$ 2.16