Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Performance obligations remaining to be performed	$ 915,000
Accumulated deficit	(115,521)		$ (120,402)
Accounts receivable and unbilled, net	110,654		83,079
Deferred income taxes	9,970		6,343
Accrued expenses	63,576		23,673
Pre-funded study costs	0		57,406
Advanced billings	126,387		$ 73,756
ASC 606
Summary Of Significant Accounting Policies [Line Items]
Accounts receivable and unbilled, net		$ 92,832
Advanced billings		119,451
ASC 606 | Impact of Adoption of ASC 606 - Adjustments
Summary Of Significant Accounting Policies [Line Items]
Accumulated deficit	11,423	(9,700)
Accounts receivable and unbilled, net	(12,733)	9,753
Deferred income taxes	(3,347)	3,300
Accrued expenses	(45,145)	34,500
Pre-funded study costs	57,487	(57,406)
Advanced billings	$ (40,356)	$ 45,695