Research and Development Collaborative Arrangements - Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenues
Accumulated deficit	$ (594,929)		$ (594,929)		$ (333,446)	$ (330,517)
Collaboration
Revenues
Revenue	21,378	$ 0	30,672	$ 0
Collaboration, reimbursement of research and development costs
Revenues
Revenue	18,175		25,730
Collaboration, research and development service
Revenues
Revenue	3,203		4,942
Collaboration, research and development service, upfront consideration
Revenues
Revenue	1,703		3,442
Collaboration, research and development service, milestone payment
Revenues
Revenue	$ 1,500		$ 1,500
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenues
Accumulated deficit					$ 16,307