Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues	$ 607,852	$ 606,119	$ 1,212,445	$ 1,157,393
Operating expenses
Direct cost of revenues	413,011	386,266	815,258	735,332
Selling, general and administrative expenses	126,928	129,906	253,887	243,091
Amortization of other intangible assets	2,314	1,852	4,645	3,713
Operating expenses	542,253	518,024	1,073,790	982,136
Operating income	65,599	88,095	138,655	175,257
Other income (expense)
Interest income and other	2,202	2,609	7,219	2,768
Interest expense	(5,157)	(4,793)	(10,018)	(9,539)
Other income (expense)	(2,955)	(2,184)	(2,799)	(6,771)
Income before income tax provision	62,644	85,911	135,856	168,486
Income tax provision	14,470	21,313	30,935	41,243
Net income	$ 48,174	$ 64,598	$ 104,921	$ 127,243
Earnings per common share — basic (in dollars per share)	$ 1.33	$ 1.75	$ 2.89	$ 3.44
Earnings per common share — diluted (in dollars per share)	$ 1.27	$ 1.69	$ 2.76	$ 3.33
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ 9,568	$ (4,815)	$ (21,534)	$ 408
Total other comprehensive income (loss), net of tax	9,568	(4,815)	(21,534)	408
Comprehensive income	$ 57,742	$ 59,783	$ 83,387	$ 127,651