Segment Information (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment	2
Revenue:
Segment revenue	$ 252,078	$ 248,997
Reimbursable out-of-pocket expenses not allocated to segments	129,840	164,090
Total revenue	381,918	413,087
Direct costs:
Segment direct costs	154,835	152,058
Reimbursable out-of-pocket expenses not allocated to segments	129,840	164,090
Direct costs and reimbursable out-of-pocket expenses	284,675	316,148
Segment contribution margin:
Segment contribution margin	97,243	96,939
Less expenses not allocated to segments:
Selling, general, and administrative	44,934	43,479
Restructuring, CEO transition and other costs	1,927	6,038
Transaction expenses	2	561
Depreciation and amortization	15,628	14,353
Income from operations	34,752	32,508
Clinical Development Services
Revenue:
Segment revenue	249,727	245,973
Direct costs:
Segment direct costs	152,620	149,314
Segment contribution margin:
Segment contribution margin	97,107	96,659
Phase I Services
Revenue:
Segment revenue	2,351	3,024
Direct costs:
Segment direct costs	2,215	2,744
Segment contribution margin:
Segment contribution margin	$ 136	$ 280