Balance Sheets Details - Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Balance Sheets Details [Abstract]
Accrued purchases	$ 688	$ 941
Accrued professional and outside service fees	2,229	2,393
Accrued expenses - cost of sales over time recognition	1,761	0
Deferred rent	318	258
Lease incentive obligation	425	425
Other	512	345
Total	$ 5,933	$ 4,362