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Income Taxes - Narrative (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Operating Loss Carryforwards [Line Items]              
Provision for income taxes $ 0 $ 0 $ 0 $ 2,169,811 $ 2,169,811 $ 0 $ 0
Tax benefits expired 11,200,000       11,200,000    
Decrease in deferred tax assets         50,000,000    
Decrease in valuation allowance         50,200,000    
Provisional income tax benefit         200,000    
Unrecognized tax benefits that would impact effective tax rate 8,300,000       8,300,000 $ 7,100,000 $ 5,700,000
Federal              
Operating Loss Carryforwards [Line Items]              
Net operating loss carryforwards that will expire due to IRC Section 382 limitations 383,300,000       383,300,000    
Net operating loss carryforwards 86,500,000       86,500,000    
State              
Operating Loss Carryforwards [Line Items]              
Net operating loss carryforwards 144,200,000.0       144,200,000.0    
California              
Operating Loss Carryforwards [Line Items]              
Net operating loss carryforwards that will expire due to IRC Section 382 limitations 68,600,000       68,600,000    
Pennsylvania              
Operating Loss Carryforwards [Line Items]              
Net operating loss carryforwards that will expire due to IRC Section 382 limitations 75,600,000       75,600,000    
Research Tax Credit Carryforward | Federal              
Operating Loss Carryforwards [Line Items]              
Tax credit carryforward amount 13,800,000       13,800,000    
Research Tax Credit Carryforward | State              
Operating Loss Carryforwards [Line Items]              
Tax credit carryforward amount $ 2,700,000       $ 2,700,000