Unaudited Quarterly Financial Results (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]
Revenues	$ 679,035		$ 667,048	$ 653,313	$ 626,528	$ 620,884		$ 629,401	$ 608,088	$ 582,040	$ 2,625,924	$ 2,440,413	$ 2,065,008
Gross profit	220,677		218,315	212,937	198,144	198,195	[1]	207,120	202,086	187,782	850,073	795,183	678,745
Income from continuing operations	67,377	[2]	34,879	33,236	22,382	24,060	[1]	29,775	26,013	17,348	157,874	97,196	71,422
Net income	$ 67,331		$ 34,856	$ 33,097	$ 22,391	$ 24,028	[1]	$ 29,768	$ 26,004	$ 17,401	$ 157,675	$ 97,201	$ 97,650
Earnings Per Share, Basic [Abstract]
Continuing operations	$ 1.29		$ 0.66	$ 0.63	$ 0.43	$ 0.45	[1]	$ 0.56	$ 0.49	$ 0.33	$ 3.01	$ 1.83	$ 1.37
Discontinued operations											0.00	0.00	0.50
Net income											3.01	1.83	1.87
Earnings Per Share, Diluted [Abstract]
Continuing operations	$ 1.28		$ 0.66	$ 0.62	$ 0.42	$ 0.45	[1]	$ 0.55	$ 0.48	$ 0.32	2.97	1.81	1.35
Discontinued operations											0.00	0.00	0.49
Net income											$ 2.97	$ 1.81	$ 1.84
Segment Reporting Information
Tax Adjustments, Settlements, and Unusual Provisions											$ (31,420)	$ 0	$ 0
Income from discontinued operations, net of income taxes											(199)	5	$ 525
Discontinued Operations, Disposed of by Sale [Member]
Segment Reporting Information
Income from discontinued operations, net of income taxes	$ (46)		$ (23)	$ (139)	$ 9	$ (32)		$ (7)	$ (9)	$ 53	$ (199)	$ 5

[1] The fourth quarter of 2016 included out-of-period adjustments of $5.6 million for costs of services that were understated in prior quarters. As a result, costs of services for the fourth quarter of 2016 were overstated by $5.6 million and gross profit, net income and diluted earnings per common share were understated by $5.6 million, $3.4 million and $0.06, respectively.

[2] 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 9. Income Taxes”.