Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenues	$ 2,027,877	$ 1,807,732	$ 1,810,394
Operating expenses
Direct cost of revenues	1,328,074	1,215,560	1,210,771
Selling, general and administrative expenses	465,636	432,013	436,716
Special charges	0	40,885	10,445
Amortization of other intangible assets	8,162	10,563	10,306
Costs and Expenses	1,801,872	1,699,021	1,668,238
Operating income	226,005	108,711	142,156
Other income (expense)
Interest income and other	4,977	3,752	10,466
Interest expense	(27,149)	(25,358)	(24,819)
Gain on sale of business	(13,031)	0	0
Loss on early extinguishment of debt	(9,072)	0	0
Other income (expense)	(18,213)	(21,606)	(14,353)
Income before income tax provision	207,792	87,105	127,803
Income tax provision	57,181	(20,857)	42,283
Net income	$ 150,611	$ 107,962	$ 85,520
Earnings per common share — basic (in dollars per share)	$ 4.06	$ 2.79	$ 2.09
Earnings per common share — diluted (in dollars per share)	$ 3.93	$ 2.75	$ 2.05
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $373, $— and $—	$ (27,602)	$ 30,664	$ (41,884)
Other comprehensive income (loss), net of tax	(27,602)	30,664	(41,884)
Comprehensive income	$ 123,009	$ 138,626	$ 43,636