Collaboration Agreements (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2014	Jun. 30, 2018	Jun. 30, 2017
Collaboration agreement
Revenue from royalty agreement	$ 1,500	$ 1,200		$ 2,780	$ 2,328
Noncash interest expense	$ 1,204	$ 160		$ 1,905	$ 1,119
License and collaboration agreements | United Therapeutics
Collaboration agreement
Revenue from royalty agreement			$ 30,000