Contract Assets and Contract Liabilities - Summary of Advanced Bllings (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Contract With Customer Asset And Liability [Line Items]
Advanced billings	$ 147,935		$ 73,756
Pre-funded study costs	0		$ 57,406
ASC 606
Contract With Customer Asset And Liability [Line Items]
Advanced billings		$ 112,613
Pre-funded study costs		0
ASC 606 | Impact of Adoption of ASC 606 - Adjustments
Contract With Customer Asset And Liability [Line Items]
Advanced billings	(41,732)	38,857
Pre-funded study costs	$ 61,156	$ (57,406)