Segment & Geographical Information - Revenues and Property and Equipment, Net Based on Geographical Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Revenues, net
Revenues and reimbursements	$ 234,903	$ 231,124	$ 210,112	$ 206,973	$ 197,875	$ 192,345	$ 189,057	$ 183,033	$ 883,112	$ 762,310	[1]	$ 685,988	[1]
Property, Plant and Equipment, net
Property and equipment, net	73,510				66,757				73,510	66,757
United States
Revenues, net
Revenues and reimbursements									732,589	626,336		554,945
Property, Plant and Equipment, net
Property and equipment, net	28,254				16,371				28,254	16,371
United Kingdom
Revenues, net
Revenues and reimbursements									114,515	108,640		109,905
Rest of World
Revenues, net
Revenues and reimbursements									36,008	27,334		21,138
Property, Plant and Equipment, net
Property and equipment, net	3,119				3,026				3,119	3,026
Total Non-United States
Revenues, net
Revenues and reimbursements									150,523	135,974		$ 131,043
India
Property, Plant and Equipment, net
Property and equipment, net	36,152				39,143				36,152	39,143
Philippines
Property, Plant and Equipment, net
Property and equipment, net	$ 5,985				$ 8,217				$ 5,985	$ 8,217

[1] Adjusted pursuant to adoption of ASU No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Post-retirement Benefit Cost. Refer to Note 21 and Note 27 to the consolidated financial statements.