Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect Of Tax Cuts And Jobs Act Of 2017 [Abstract]
Corporate tax rate		34.00%	34.00%	34.00%
Decrease related to deferred tax assets		$ 42,100
Increase related to deferred tax liabilities		200
Valuation allowance reduction		42,200
Deferred tax assets
Net operating loss carryforwards		77,356	$ 68,504
Research and development tax credit carryforwards		12,675	8,634
Reserves and accruals		3,404	5,719
Stock based compensation		2,929	3,220
Total deferred tax assets before valuation allowance		96,364	86,077
Less: valuation allowance		(95,817)	(85,606)
Deferred tax assets, net		547	471
Deferred tax liabilities
Property and equipment		(547)	(471)
Net deferred tax assets
Forecast
Effect Of Tax Cuts And Jobs Act Of 2017 [Abstract]
Corporate tax rate	21.00%