Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Revenues	$ 702,228	$ 622,249	$ 2,099,991	$ 1,834,694
Operating expenses
Direct cost of revenues	472,235	417,179	1,431,381	1,232,437
Selling, general and administrative expenses	138,600	122,102	399,076	375,989
Special charges	0	7,103	0	7,103
Amortization of intangible assets	2,860	2,795	8,515	7,440
Costs and Expenses	613,695	549,179	1,838,972	1,622,969
Operating income	88,533	73,070	261,019	211,725
Other income (expense)
Interest income and other	5,175	(3,340)	5,297	3,879
Interest expense	(5,073)	(5,151)	(15,164)	(15,169)
Other income (expense)	102	(8,491)	(9,867)	(11,290)
Income before income tax provision	88,635	64,579	251,152	200,435
Income tax provision	19,155	14,407	54,394	45,342
Net income	$ 69,480	$ 50,172	$ 196,758	$ 155,093
Earnings per common share — basic (in dollars per share)	$ 2.07	$ 1.41	$ 5.88	$ 4.30
Earnings per common share — diluted (in dollars per share)	$ 1.96	$ 1.35	$ 5.58	$ 4.11
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ (18,607)	$ 21,330	$ (18,042)	$ (204)
Total other comprehensive income (loss), net of tax	(18,607)	21,330	(18,042)	(204)
Comprehensive income	$ 50,873	$ 71,502	$ 178,716	$ 154,889