License Agreements (Details) $ in Thousands, € in Millions	1 Months Ended					7 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($)	Apr. 30, 2017 EUR (€)	Sep. 30, 2015 USD ($)	Sep. 30, 2015 EUR (€)	Jun. 30, 2006 USD ($)	Dec. 31, 2006 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Research Agreements
Collaborative arrangement, milestone payments							$ 8,712		$ 0	$ 0
Eisai | License Agreement
Research Agreements
Period after the date of the first commercial sale of the product for license expiration							10 years	10 years
Initial license fee paid						$ 500				$ 400
Additional payments to be made upon achievement of certain clinical and regulatory milestones					$ 22,300
Duke University | License Agreement
Research Agreements
Nonrefundable, non-creditable payment							$ 1,300
Additional payments to be made upon achievement of certain milestones							$ 3,800
Collaborative arrangement, termination period							60 days	60 days
Clinical Trial Services Agreement | Ipsen
Research Agreements
Nonrefundable, non-creditable payment			$ 13,000
Additional payments to be made upon achievement of certain milestones			$ 28,700	€ 24.0
Collaborative arrangement, milestone payments	$ 8,700	€ 8.0					$ 8,700	€ 8.0
Royalty rate (as a percent)			5.00%	5.00%
Period after the date of the first commercial sale of the product for license expiration			10 years	10 years
Royalty expense							600
Clinical Trial Services Agreement | Ipsen | Sublicense Agreement
Research Agreements
Period after the date of the first commercial sale of the product for license expiration			10 years	10 years
Agreement expiration period, period after date of first commercial sale of product			10 years	10 years
Clinical Trial Services Agreement | Teijin Agreement
Research Agreements
Additional payments to be made upon achievement of certain milestones							40,000
License agreements, upfront payment received							10,000
Substantive milestones							$ 10,000