Income Taxes - Significant Components of Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating losses	$ 67,653	$ 47,218
Tax credits	16,308	4,937
Accruals	1,285	1,322
Share-based compensation	7,444	8,966
Gross deferred tax assets	92,690	62,443
Valuation allowance	(83,972)	(56,382)
Net deferred tax assets	8,718	6,061
Deferred Tax Liabilities, Property, Plant and Equipment	(605)	(148)
Net deferred tax assets	$ 8,113	$ 5,913