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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2018
Jan. 01, 2018
Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Accumulated deficit $ (556,232,609) $ (556,232,609) $ 200,000 $ (537,375,776)
Revenue, remaining performance obligation 11,500,000 11,500,000    
Difference between Revenue Guidance in Effect before and after Topic 606        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Deferred revenue     1,800,000  
Deferred costs     2,100,000  
Accounts receivable     $ 100,000  
Revenue 0 0    
Secured Debt | Loan Agreement        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Restricted cash and cash equivalents 5,000,000 5,000,000    
Accounting Standards Update 2016-18        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Restricted cash $ 5,100,000 $ 5,100,000    
Accrued expenses and other current liabilities        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Deferred revenue       $ 1,300,000