CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenues	$ 2,440,413	$ 2,065,008	$ 1,724,741
Costs of services	1,645,230	1,386,263	1,167,306
Gross profit	795,183	678,745	557,435
Selling, general and administrative expenses	565,829	492,170	397,523
Amortization of intangible assets	39,628	34,467	22,130
Operating income	189,726	152,108	137,782
Interest expense	(32,327)	(26,444)	(12,730)
Write-off of loan costs	0	(3,751)	0
Income before income taxes	157,399	121,913	125,052
Provision for income taxes	60,203	50,491	51,557
Income from continuing operations	97,196	71,422	73,495
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	5	525	3,689
Net income	$ 97,201	$ 97,650	$ 77,184
Basic earnings per common share:
Continuing operations	$ 1.83	$ 1.37	$ 1.38
Discontinued operations	0.00	0.50	0.06
Net income	1.83	1.87	1.44
Diluted earnings per common share:
Continuing operations	1.81	1.35	1.35
Discontinued operations	0.00	0.49	0.07
Net income	$ 1.81	$ 1.84	$ 1.42
Number of shares and share equivalents used to calculate earnings per share:
Basic (in shares)	53,192	52,259	53,437
Diluted (in shares)	53,747	53,005	54,294
Reconciliation of net income to comprehensive income:
Net income	$ 97,201	$ 97,650	$ 77,184
Changes in fair value of derivative, net of tax	0	122	86
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax	(1,861)	(3,714)	(4,772)
Comprehensive income	$ 95,340	$ 94,058	$ 72,498