Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Revenues	$ 686,277	$ 604,593
Operating expenses
Direct cost of revenues	468,424	402,247
Selling, general and administrative expenses	126,546	126,959
Amortization of intangible assets	2,801	2,331
Costs and Expenses	597,771	531,537
Operating income	88,506	73,056
Other income (expense)
Interest income and other	1,034	5,017
Interest expense	(4,797)	(4,861)
Other income (expense)	(3,763)	156
Income before income tax provision	84,743	73,212
Income tax provision	20,247	16,465
Net income	$ 64,496	$ 56,747
Earnings per common share — basic (in dollars per share)	$ 1.93	$ 1.56
Earnings per common share — diluted (in dollars per share)	$ 1.84	$ 1.49
Other comprehensive loss, net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ (5,242)	$ (31,102)
Total other comprehensive loss, net of tax	(5,242)	(31,102)
Comprehensive income	$ 59,254	$ 25,645