Balance Sheets Details - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Related Disclosures [Abstract]
Accrued purchases	$ 941	$ 67
Accrued professional and outside service fees	2,393	746
Deferred rent	258	168
Lease incentive obligation	425	425
Other	345	705
Total	$ 4,362	$ 2,111