Fair Value Measurements - Financial Instruments Measured at Fair Value by Hierarchy Level (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Level 1
Assets:
Cash and cash equivalents	$ 7,776	$ 35,870
Total assets at fair value	$ 7,776	35,870
Level 3
Liabilities:
Total liabilities at fair value		29
Level 3 | Common stock warrants
Liabilities:
Common stock warrants		$ 29