Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Revenues	$ 2,461,275	$ 2,352,717	$ 2,027,877
Operating expenses
Direct cost of revenues	1,672,711	1,534,896	1,328,074
Selling, general and administrative expenses	488,411	504,074	465,636
Special charges	7,103	0	0
Amortization of intangible assets	10,387	8,152	8,162
Costs and Expenses	2,178,612	2,047,122	1,801,872
Operating income	282,663	305,595	226,005
Other income (expense)
Interest income and other	(412)	2,061	4,977
Interest expense	(19,805)	(19,206)	(27,149)
Gain on sale of business	0	0	13,031
Loss on early extinguishment of debt	0	0	(9,072)
Other income (expense)	(20,217)	(17,145)	(18,213)
Income before income tax provision	262,446	288,450	207,792
Income tax provision	51,764	71,724	57,181
Net income	$ 210,682	$ 216,726	$ 150,611
Earnings per common share — basic (in dollars per share)	$ 5.92	$ 5.89	$ 4.06
Earnings per common share — diluted (in dollars per share)	$ 5.67	$ 5.69	$ 3.93
Other comprehensive income (loss), net of tax
Cumulative translation adjustment	$ 34,412	$ 6,970	$ (27,602)
Total other comprehensive income (loss), net of tax	34,412	6,970	(27,602)
Comprehensive income	$ 245,094	$ 223,696	$ 123,009