Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Valuation allowance	$ 122,441	$ 95,817	$ 85,600
Effect Of Tax Cuts And Jobs Act Of 2017 [Abstract]
Corporate tax rate	21.00%	34.00%	34.00%
Deferred tax assets
Net operating loss carryforwards	$ 95,321	$ 77,356
Research and development tax credit carryforwards	15,632	12,675
Reserves and accruals	7,922	3,404
Stock based compensation	4,076	2,929
Total deferred tax assets before valuation allowance	122,951	96,364
Less: valuation allowance	(122,441)	(95,817)	$ (85,600)
Deferred tax assets, net	510	547
Deferred tax liabilities
Property and equipment	(510)	(547)
Net deferred tax assets