Income Taxes - Components of Company's Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carry forwards	$ 106,446	$ 60,771
Research credits	21,618	14,255
Stock based compensation	5,057	10,046
Deferred revenue	0	1,017
Development costs	3,938	4,143
Returns and allowances	1,976	1,636
Other, net	(5,066)	0
Total deferred tax assets before valuation allowance	133,969	91,868
Valuation allowance	(131,042)	(86,021)
Total deferred tax assets	2,927	5,847
Deferred tax liabilities:
Basis difference in debt	0	(28)
Depreciation and amortization differences	(4,396)	(6,836)
Other, net	0	(421)
Net deferred tax liabilities	$ (1,469)	$ (1,438)