Income Taxes - Deferred Tax (Details) - USD ($) $ in Thousands		12 Months Ended
	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017
Gross Deferred Tax Assets
Net Operating Loss Carryforwards		$ 168,239	$ 146,228
Foreign Net Operating Loss Carryforwards		4,485	3,572
Tax Credit Carryforwards		39,761	36,458
Deferred Research and Development Expenses		76,555	79,272
Stock-based Compensation		11,977	10,718
Fixed Assets		1,761	1,305
Deferred Revenue		13	686
Accrued Expenses and Other		183	316
Deferred Tax Assets, Gross		302,974	278,555
Gross Deferred Tax Liabilities
Other Acquired Intangibles		120	(1,792)
IPR&D Intangibles		(12,748)	(15,992)
Total Deferred Tax Assets and Liabilities		290,346	260,771
Valuation Allowance		(293,353)	(264,543)
Net Deferred Tax Liability		(3,007)	(3,772)
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%		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)