Operations by Geographic Location - Total Revenue by Geographic Area (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Revenues by Geographic Location
Total net service revenue		$ 252,078	$ 248,997
Reimbursable out-of-pocket expenses		129,840	164,090
Total revenue		381,918	413,087
North America
Revenues by Geographic Location
Total net service revenue	[1]	184,287	185,714
United States
Revenues by Geographic Location
Total net service revenue		178,400	180,500
Europe, Middle East and Africa
Revenues by Geographic Location
Total net service revenue		57,262	56,898
Asia-Pacific
Revenues by Geographic Location
Total net service revenue		10,529	6,372
Latin America
Revenues by Geographic Location
Total net service revenue		$ 0	$ 13
Geographic Concentration Risk | Net Service Revenue | United States
Revenues by Geographic Location
Concentration risk percentage		70.80%	72.50%

[1] Net service revenue for the North America region includes revenue attributable to the United States of $178.4 million and $180.5 million, or 70.8% and 72.5% of net service revenue, for the three months ended March 31, 2017 and March 31, 2016, respectively. No other country represented more than 10% of net service revenue for any period.