Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Corporate tax rate change impact on deferred income taxes									$ 14,039	$ 0	$ 0
Selected Quarterly Financial Information [Abstract]
Revenue	$ 509,076	$ 494,406	$ 489,803	$ 495,169	$ 487,858	$ 472,636	$ 473,729	$ 468,002	1,988,454	1,902,225	1,463,065
Gross profit	162,092	159,539	161,012	161,776	158,606	154,467	154,753	151,898	644,419	619,724	469,363
Net income	$ 41,167	$ 28,128	$ 31,255	$ 32,008	$ 26,351	$ 27,296	$ 26,322	$ 25,869	$ 132,558	$ 105,838	$ 81,891
Basic (in dollars per share)	$ 0.86	$ 0.59	$ 0.65	$ 0.67	$ 0.55	$ 0.57	$ 0.55	$ 0.54	$ 2.77	$ 2.21	$ 1.72
Diluted (in dollars per share)	$ 0.84	$ 0.57	$ 0.63	$ 0.65	$ 0.54	$ 0.55	$ 0.53	$ 0.53	$ 2.68	$ 2.15	$ 1.68