Other Assets and Liabilities - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Prepaid Expenses and Other Current Assets:
Valued-added tax recoverable	$ 1,100	$ 1,378
Prepaid and other expenses	2,607	2,145
Guarantee from Armistice	543	534
Income tax receivable	996	921
Research and development tax credit receivable	270	283
Short-term deposit	0	3,350
Other	1,040	225
Total	$ 6,556	$ 8,836