Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Revenues	$ 512,098,000	$ 444,715,000	$ 1,009,872,000	$ 891,059,000
Operating expenses
Direct cost of revenues	330,318,000	304,071,000	651,435,000	613,143,000
Selling, general and administrative expenses	117,897,000	108,119,000	230,025,000	215,809,000
Special charges	0	30,074,000	0	30,074,000
Amortization of other intangible assets	2,052,000	2,422,000	4,322,000	4,915,000
Operating expenses	450,267,000	444,686,000	885,782,000	863,941,000
Operating income	61,831,000	29,000	124,090,000	27,118,000
Other income (expense)
Interest income and other	2,474,000	1,592,000	674,000	2,197,000
Interest expense	(6,583,000)	(6,250,000)	(12,827,000)	(12,051,000)
Other income (expense)	(4,109,000)	(4,658,000)	(12,153,000)	(9,854,000)
Income (loss) before income tax provision	57,722,000	(4,629,000)	111,937,000	17,264,000
Income tax provision	14,113,000	527,000	29,383,000	8,404,000
Net income (loss)	$ 43,609,000	$ (5,156,000)	$ 82,554,000	$ 8,860,000
Earnings (loss) per common share — basic (in dollars per share)	$ 1.18	$ (0.13)	$ 2.24	$ 0.22
Earnings (loss) per common share — diluted (in dollars per share)	$ 1.14	$ (0.13)	$ 2.18	$ 0.22
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ (23,683,000)	$ 10,174,000	$ (13,237,000)	$ 17,544,000
Total other comprehensive income (loss), net of tax	(23,683,000)	10,174,000	(13,237,000)	17,544,000
Comprehensive income	$ 19,926,000	$ 5,018,000	$ 69,317,000	$ 26,404,000