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Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Disclosure [Abstract]      
Provision for federal or state income taxes $ 0 $ 0 $ 0
Deferred income tax assets:      
Federal U.S. net operating loss carryforward 136,086,000 116,436,000  
State net operating loss carryforward 37,465,000 31,110,000  
Research and development credit, net 50,271,000 35,580,000  
Orphan drug credit, net 23,409,000 22,881,000  
Operating lease liabilities 8,048,000 8,413,000  
Deferred revenue 3,132,000 367,000  
Other 14,285,000 9,123,000  
Gross deferred income tax assets 272,696,000 223,910,000  
Valuation allowance (263,403,000) (214,893,000)  
Net deferred income tax assets 9,293,000 9,017,000  
Deferred income tax liabilities:      
Depreciation (2,123,000) (438,000)  
Operating lease ROU assets (5,316,000) (5,547,000)  
Prepaid expenditures (1,854,000) (3,032,000)  
Gross deferred income tax liabilities (9,293,000) (9,017,000)  
Net deferred income tax asset/(liability) 0 $ 0  
Operating Loss Carryforwards [Line Items]      
Remaining portion of net operating losses 634,500,000    
U.S. Federal and State      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards 648,000,000.0    
U.S. Federal Government      
Operating Loss Carryforwards [Line Items]      
US Federal tax credits carry forward 69,600,000    
Net operating loss for limited use 13,500,000    
U.S. Federal Government | Minimum      
Operating Loss Carryforwards [Line Items]      
Net operating losses limited for use utilized on an annual basis 200,000    
U.S. Federal Government | Maximum      
Operating Loss Carryforwards [Line Items]      
Net operating losses limited for use utilized on an annual basis 1,400,000    
Tax Year 2025 To 2037 | U.S. Federal and State      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards 237,700,000    
Indefinite | U.S. Federal and State      
Operating Loss Carryforwards [Line Items]      
Net operating loss carryforwards $ 410,300,000