Accrued Expenses - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued expenses
Accrued license obligations	$ 2,523	$ 806
Compensation expense	2,380	2,325
Research and development fees	1,945	791
Professional fees	943	516
Audit and tax fees	378	424
Other current liabilities	945	1,308
Total	$ 9,114	$ 6,170