Consolidated Statements of Comprehensive Income - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 467,711,000	$ 448,962,000	$ 444,715,000	$ 446,344,000	$ 441,920,000	$ 438,042,000	$ 460,147,000	$ 470,285,000	$ 1,807,732,000	$ 1,810,394,000	$ 1,779,149,000
Operating expenses
Direct cost of revenues	307,566,000	294,851,000	304,071,000	309,072,000	308,239,000	293,702,000	303,194,000	305,636,000	1,215,560,000	1,210,771,000	1,171,444,000
Selling, general and administrative expenses	111,176,000	103,909,000	107,342,000	107,295,000	116,478,000	106,220,000	108,245,000	103,609,000	429,722,000	434,552,000	432,668,000
Special charges recorded									40,885,000	10,445,000	0
Acquisition-related contingent consideration	867,000	252,000	777,000	395,000	623,000	201,000	206,000	1,134,000	2,291,000	2,164,000	(1,200,000)
Amortization of other intangible assets	2,766,000	2,882,000	2,422,000	2,493,000	2,265,000	2,845,000	2,590,000	2,606,000	10,563,000	10,306,000	11,726,000
Costs and Expenses	433,186,000	401,894,000	444,686,000	419,255,000	431,239,000	402,968,000	415,985,000	418,046,000	1,699,021,000	1,668,238,000	1,614,638,000
Operating income	34,525,000	47,068,000	29,000	27,089,000	10,681,000	35,074,000	44,162,000	52,239,000	108,711,000	142,156,000	164,511,000
Other income (expense)
Interest income and other	452,000	1,103,000	1,592,000	605,000	571,000	3,213,000	4,125,000	2,557,000	3,752,000	10,466,000	3,232,000
Interest expense	(6,547,000)	(6,760,000)	(6,250,000)	(5,801,000)	(5,983,000)	(6,304,000)	(6,303,000)	(6,229,000)	(25,358,000)	(24,819,000)	(42,768,000)
Loss on early extinguishment of debt									0	0	(19,589,000)
Other income (expense)									(21,606,000)	(14,353,000)	(59,125,000)
Income before income tax provision (benefit)	28,430,000	41,411,000	(4,629,000)	21,893,000	5,269,000	31,983,000	41,984,000	48,567,000	87,105,000	127,803,000	105,386,000
Income tax provision (benefit)	(38,458,000)	9,197,000	527,000	7,877,000	(1,832,000)	10,292,000	15,437,000	18,386,000	(20,857,000)	42,283,000	39,333,000
Net income	$ 66,888,000	$ 32,214,000	$ (5,156,000)	$ 14,016,000	$ 7,101,000	$ 21,691,000	$ 26,547,000	$ 30,181,000	$ 107,962,000	$ 85,520,000	$ 66,053,000
Earnings per common share — basic (in dollars per share)	$ 1.81	$ 0.86	$ (0.13)	$ 0.35	$ 0.17	$ 0.53	$ 0.65	$ 0.75	$ 2.79	$ 2.09	$ 1.62
Earnings per common share — diluted (in dollars per share)	$ 1.78	$ 0.85	$ (0.13)	$ 0.34	$ 0.17	$ 0.52	$ 0.64	$ 0.73	$ 2.75	$ 2.05	$ 1.58
Other comprehensive income (loss), net of tax
Foreign currency translation adjustments, net of tax expense of $0									$ 30,664,000	$ (41,884,000)	$ (28,727,000)
Other comprehensive income (loss), net of tax									30,664,000	(41,884,000)	(28,727,000)
Comprehensive income									$ 138,626,000	$ 43,636,000	$ 37,326,000