Condensed Consolidated Statements of Comprehensive Income (Unaudited) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenues	$ 448,962,000	$ 438,042,000	$ 1,340,021,000	$ 1,368,474,000
Operating expenses
Direct cost of revenues	294,851,000	293,702,000	907,994,000	902,532,000
Selling, general and administrative expenses	103,909,000	106,220,000	318,546,000	318,074,000
Special charges	0	0	30,074,000	6,811,000
Acquisition-related contingent consideration	252,000	201,000	1,424,000	1,541,000
Amortization of other intangible assets	2,882,000	2,845,000	7,797,000	8,041,000
Costs and Expenses, Total	401,894,000	402,968,000	1,265,835,000	1,236,999,000
Operating income	47,068,000	35,074,000	74,186,000	131,475,000
Other income (expense)
Interest income and other	1,103,000	3,213,000	3,300,000	9,895,000
Interest expense	(6,760,000)	(6,304,000)	(18,811,000)	(18,836,000)
Other (expense) income	(5,657,000)	(3,091,000)	(15,511,000)	(8,941,000)
Income before income tax provision	41,411,000	31,983,000	58,675,000	122,534,000
Income tax provision	9,197,000	10,292,000	17,601,000	44,115,000
Net income	$ 32,214,000	$ 21,691,000	$ 41,074,000	$ 78,419,000
Earnings per common share — basic	$ 0.86	$ 0.53	$ 1.05	$ 1.92
Earnings per common share — diluted	$ 0.85	$ 0.52	$ 1.03	$ 1.88
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0	$ 11,234,000	$ (4,478,000)	$ 28,778,000	$ (23,645,000)
Total other comprehensive income (loss), net of tax	11,234,000	(4,478,000)	28,778,000	(23,645,000)
Comprehensive income	$ 43,448,000	$ 17,213,000	$ 69,852,000	$ 54,774,000