Unaudited Quarterly Financial Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035		$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Gross profit	271,896	270,172	263,846	217,737	220,677		218,315	212,937	198,144	1,023,651	850,073	795,183
Income from continuing operations	45,926	49,217	33,601	29,240	67,377	[1]	34,879	33,236	22,382	157,984	157,874	97,196
Income from discontinued operations, net of income taxes										(278)	(199)	5
Net income	$ 45,881	$ 49,172	$ 33,561	$ 29,092	$ 67,331		$ 34,856	$ 33,097	$ 22,391	$ 157,706	$ 157,675	$ 97,201
Earnings per share:
Basic	$ 0.88	$ 0.94	$ 0.64	$ 0.56	$ 1.29		$ 0.66	$ 0.63	$ 0.43	$ 3.02	$ 3.01	$ 1.83
Diluted	$ 0.86	$ 0.93	$ 0.63	$ 0.55	$ 1.28		$ 0.66	$ 0.62	$ 0.42	$ 2.98	$ 2.97	$ 1.81
Discontinued Operations, Disposed of by Sale [Member]
Income from discontinued operations, net of income taxes	$ (45)	$ (45)	$ (40)	$ (148)	$ (46)		$ (23)	$ (139)	$ 9	$ (278)	$ (199)

[1] On December 22, 2017, the U.S. government enacted comprehensive tax legislation, the TCJA. In the fourth quarter of 2017, the Company recorded an estimated net tax benefit of $31.4 million for the impact of the TCJA, which is included in the provision for income taxes in the consolidated statement of operations and comprehensive income, see Note 11 – Income Taxes.