Income Taxes - Summary of Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gross Deferred Tax Assets
Net Operating Loss Carryforwards	$ 172,745	$ 168,239
Foreign Net Operating Loss Carryforwards		4,485
Tax Credit Carryforwards	42,642	39,761
Deferred Research and Development Expenses	70,042	76,555
Stock-based Compensation	12,651	11,977
Fixed Assets	1,759	1,761
Accrued Expenses and Other	328	316
Deferred Tax Assets, Gross	300,167	303,094
Gross Deferred Tax Liabilities
IPR&D Intangibles	(12,748)	(12,748)
Total Deferred Tax Assets and Liabilities	287,419	290,346
Valuation Allowance	(290,426)	(293,353)
Net Deferred Tax Liability	$ 3,007	$ 3,007
Principal components of the deferred tax assets and liabilities
Increase in deferred tax and valuation allowance			$ 17,700
U.S. federal corporate income tax rate (as a percent)	21.00%	21.00%	35.00%
U.S. federal corporate income tax net operating losses (as a percent)			80.00%
Change in tax rate, deferred tax liability benefit		$ 6,900
Additional income tax benefit leading to tax liabilities		12,200
Revaluation of deferred tax asset		99,500
Income tax expense (benefit)		$ (765)	$ (24,282)