Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes
Valuation allowance increase	$ 34,500	$ 24,500	$ 900
Deferred tax assets
Net operating loss carryforwards	68,504	41,451
Research and development tax credit carryforwards	8,634	4,794
Reserves and accruals	5,719	3,108
Stock based compensation	3,220	2,206
Total deferred tax assets before valuation allowance	86,077	51,559
Less: valuation allowance	(85,606)	(51,153)
Deferred tax assets, net	471	406
Deferred tax liabilities
Property and equipment	$ (471)	$ (406)