Supplemental Balance Sheet Information - Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Accrued expenses
Accrued payroll and compensation	$ 1,848	$ 2,479
Accrued research and development expenses	724	846
Accrued professional services	915	884
Other accrued expenses	573	699
Total accrued expenses	$ 4,060	$ 4,908