Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Revenues	$ 2,776,222	$ 2,461,275	$ 2,352,717
Operating expenses
Direct cost of revenues	1,915,507	1,672,711	1,534,896
Selling, general and administrative expenses	537,844	488,411	504,074
Special charges	0	7,103	0
Amortization of intangible assets	10,823	10,387	8,152
Costs and Expenses	2,464,174	2,178,612	2,047,122
Operating income	312,048	282,663	305,595
Other income (expense)
Interest income and other	6,193	(412)	2,061
Interest expense	(20,294)	(19,805)	(19,206)
Other income (expense)	(14,101)	(20,217)	(17,145)
Income before income tax provision	297,947	262,446	288,450
Income tax provision	62,981	51,764	71,724
Net income	$ 234,966	$ 210,682	$ 216,726
Earnings per common share — basic (in dollars per share)	$ 7.02	$ 5.92	$ 5.89
Earnings per common share — diluted (in dollars per share)	$ 6.65	$ 5.67	$ 5.69
Other comprehensive income (loss), net of tax
Cumulative translation adjustment	$ (22,405)	$ 34,412	$ 6,970
Total other comprehensive income (loss), net of tax	(22,405)	34,412	6,970
Comprehensive income	$ 212,561	$ 245,094	$ 223,696