Condensed Consolidated Statements of Comprehensive Income - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Revenues	$ 622,249,000	$ 593,106,000	$ 1,834,694,000	$ 1,750,499,000
Operating expenses
Direct cost of revenues	417,179,000	380,892,000	1,232,437,000	1,116,224,000
Selling, general and administrative expenses	122,102,000	127,951,000	375,989,000	371,042,000
Special charges	7,103,000	0	7,103,000	0
Amortization of other intangible assets	2,795,000	2,125,000	7,440,000	5,838,000
Operating expenses	549,179,000	510,968,000	1,622,969,000	1,493,104,000
Operating income	73,070,000	82,138,000	211,725,000	257,395,000
Other income (expense)
Interest income and other	(3,340,000)	2,973,000	3,879,000	5,741,000
Interest expense	(5,151,000)	(4,832,000)	(15,169,000)	(14,371,000)
Other income (expense)	(8,491,000)	(1,859,000)	(11,290,000)	(8,630,000)
Income before income tax provision	64,579,000	80,279,000	200,435,000	248,765,000
Income tax provision	14,407,000	19,857,000	45,342,000	61,100,000
Net income	$ 50,172,000	$ 60,422,000	$ 155,093,000	$ 187,665,000
Earnings per common share — basic (in dollars per share)	$ 1.41	$ 1.65	$ 4.30	$ 5.09
Earnings per common share — diluted (in dollars per share)	$ 1.35	$ 1.59	$ 4.11	$ 4.92
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ 21,330,000	$ (16,633,000)	$ (204,000)	$ (16,225,000)
Total other comprehensive income (loss), net of tax	21,330,000	(16,633,000)	(204,000)	(16,225,000)
Comprehensive income	$ 71,502,000	$ 43,789,000	$ 154,889,000	$ 171,440,000