Contract Assets and Contract Liabilities - Summary of Advanced Bllings (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Contract With Customer Asset And Liability [Line Items]
Advanced billings	$ 126,387		$ 73,756
Pre-funded study costs (includes $1.0 million with related parties at December 31, 2017)	0		$ 57,406
ASC 606
Contract With Customer Asset And Liability [Line Items]
Advanced billings		$ 119,451
ASC 606 | Impact of Adoption of ASC 606 - Adjustments
Contract With Customer Asset And Liability [Line Items]
Advanced billings	(40,356)	45,695
Pre-funded study costs (includes $1.0 million with related parties at December 31, 2017)	$ 57,487	$ (57,406)