Prior-Period Financial Statementss - (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenue
Collaborations, licenses and milestones	$ 12,500			$ 16,000
ASU 2014-09 | As Reported Before ASC Topic 606
Revenue
Collaborations, licenses and milestones	13,340			17,681
ASU 2014-09 | Effect of Adoption of ASC Topic 606
Revenue
Collaborations, licenses and milestones	$ (840)			$ (1,681)
ASU 2014-09 | Collaboration
Revenue
Collaborations, licenses and milestones					$ 20,500	$ 6,250
ASU 2014-09 | License
Revenue
Period revenue recognized			5 years
Collaborations, licenses and milestones		$ 11,300