Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Revenues	$ 606,119	$ 512,098	$ 1,157,393	$ 1,009,872
Operating expenses
Direct cost of revenues	386,266	330,318	735,332	651,435
Selling, general and administrative expenses	129,906	117,897	243,091	230,025
Amortization of other intangible assets	1,852	2,052	3,713	4,322
Operating expenses	518,024	450,267	982,136	885,782
Operating income	88,095	61,831	175,257	124,090
Other income (expense)
Interest income and other	2,609	2,474	2,768	674
Interest expense	(4,793)	(6,583)	(9,539)	(12,827)
Other income (expense)	(2,184)	(4,109)	(6,771)	(12,153)
Income before income tax provision	85,911	57,722	168,486	111,937
Income tax provision	21,313	14,113	41,243	29,383
Net income	$ 64,598	$ 43,609	$ 127,243	$ 82,554
Earnings per common share — basic (in dollars per share)	$ 1.75	$ 1.18	$ 3.44	$ 2.24
Earnings per common share — diluted (in dollars per share)	$ 1.69	$ 1.14	$ 3.33	$ 2.18
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ (4,815)	$ (23,683)	$ 408	$ (13,237)
Total other comprehensive income (loss), net of tax	(4,815)	(23,683)	408	(13,237)
Comprehensive income	$ 59,783	$ 19,926	$ 127,651	$ 69,317