Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended							12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 154,174	$ 156,017	$ 160,258	$ 163,524	$ 162,437	$ 171,549	$ 175,814	$ 633,973	$ 681,100	$ 654,988
Gross Profit	90,935	91,684	97,375	99,424	96,685	96,807	100,286	379,418	399,006	375,115
Net income	$ 21,347	$ 24,273	$ 26,829	$ 25,543	$ 24,257	$ 26,897	$ 28,412	$ 97,992	$ 104,923	$ 36,453
Basic (USD per share)	$ 1.18	$ 1.33	$ 1.46	$ 1.38	$ 1.29	$ 1.41	$ 1.49	$ 5.36	$ 5.55	$ 1.92
Diluted (USD per share)	$ 1.18	$ 1.33	$ 1.45	$ 1.37	$ 1.28	$ 1.40	$ 1.47	$ 5.33	$ 5.52	$ 1.90