Unaudited Quarterly Financial Results (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	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 620,884	$ 629,401	$ 608,088	$ 582,040	$ 577,517	$ 572,123	$ 485,323	$ 430,045	$ 2,440,413	$ 2,065,008	$ 1,724,741
Gross profit	198,195	207,120	202,086	187,782	192,932	191,404	158,534	135,875	795,183	678,745	557,435
Income from continuing operations	24,060	29,775	26,013	17,348	19,173	24,891	14,335	13,023	97,196	71,422	73,495
Net income	$ 24,028	$ 29,768	$ 26,004	$ 17,401	$ 19,338	$ 24,925	$ 14,252	$ 39,135	$ 97,201	$ 97,650	$ 77,184
Earnings Per Share, Basic [Abstract]
Continuing operations	$ 0.45	$ 0.56	$ 0.49	$ 0.33	$ 0.36	$ 0.47	$ 0.28	$ 0.25	$ 1.83	$ 1.37	$ 1.38
Discontinued operations	0.00	0.00	0.00	0.00	0.01	0.00	(0.01)	0.51	0.00	0.50	0.06
Net income	0.45	0.56	0.49	0.33	0.37	0.47	0.27	0.76	1.83	1.87	1.44
Earnings Per Share, Diluted [Abstract]
Continuing operations	0.45	0.55	0.48	0.32	0.36	0.47	0.27	0.25	1.81	1.35	1.35
Discontinued operations	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.50	0.00	0.49	0.07
Net income	$ 0.45	$ 0.55	$ 0.48	$ 0.32	$ 0.36	$ 0.47	$ 0.27	$ 0.75	$ 1.81	$ 1.84	$ 1.42
Segment Reporting Information
Income from discontinued operations, net of income taxes									$ 5	$ 525	$ 3,689
Discontinued Operations, Disposed of by Sale [Member]
Segment Reporting Information
Income from discontinued operations, net of income taxes	$ (32)	$ (7)	$ (9)	$ 53	$ 165	$ 34	$ (83)	$ 26,112	$ 5	$ 26,228