Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues	$ 77,941	$ 82,815	$ 78,661	$ 74,299	$ 82,672	$ 81,405	$ 77,281	$ 75,511	$ 313,716	$ 316,869	$ 302,195
Gross Profit	38,945	40,955	38,038	36,794	41,768	40,695	39,082	34,965	154,732	156,510	142,354
Net Income	$ 13,332	$ 11,034	$ 7,367	$ 8,685	$ 12,343	$ 12,188	$ 9,149	$ 7,601	$ 40,418	$ 41,281	$ 44,669
Basic	$ 0.89	$ 0.75	$ 0.50	$ 0.60	$ 0.86	$ 0.85	$ 0.65	$ 0.54	$ 2.75	$ 2.90	$ 3.23
Diluted	$ 0.86	$ 0.72	$ 0.48	$ 0.58	$ 0.82	$ 0.82	$ 0.63	$ 0.53	$ 2.65	$ 2.80	$ 3.15