CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 3,399,781	$ 2,625,924	$ 2,440,413
Cost of services	2,376,130	1,775,851	1,645,230
Gross profit	1,023,651	850,073	795,183
Selling, general and administrative expenses	704,997	591,893	565,829
Amortization of intangible assets	58,506	33,444	39,628
Operating income	260,148	224,736	189,726
Interest expense	(55,973)	(27,643)	(32,327)
Income before income taxes	204,175	197,093	157,399
Provision for income taxes	46,191	39,219	60,203
Income from continuing operations	157,984	157,874	97,196
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(278)	(199)	5
Net income	$ 157,706	$ 157,675	$ 97,201
Earnings per share:
Basic	$ 3.02	$ 3.01	$ 1.83
Diluted	$ 2.98	$ 2.97	$ 1.81
Number of shares and share equivalents used to calculate earnings per share:
Basic (in shares)	52,333	52,503	53,192
Diluted (in shares)	53,061	53,205	53,747
Reconciliation of net income to comprehensive income:
Net income	$ 157,706	$ 157,675	$ 97,201
Foreign currency translation adjustment	(2,703)	6,384	(1,861)
Comprehensive income	$ 155,003	$ 164,059	$ 95,340