Supplemental Balance Sheet Information - Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accrued expenses
Accrued payroll and compensation	$ 2,479	$ 2,418
Accrued research and development expenses	846	458
Accrued professional services	884	542
Other accrued expenses	699	744
Total accrued expenses	$ 4,908	$ 4,162