Collaborative Arrangements and Acquisitions - Janssen Pharmaceuticals, Inc. (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative revenues	$ 1,714		$ 232,545
Janssen Pharmaceuticals, Inc.
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaborative arrangement, up-front payment				$ 35,000
Collaborative revenues		$ 25,000
Time period of notice required to terminate				6 months