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Summary of Significant Accounting Policies - Narrative (Details)
 2 Months Ended 12 Months Ended
Mar. 01, 2019
USD ($)
Mar. 12, 2019
USD ($)
Dec. 31, 2018
USD ($)
segment
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jan. 01, 2018
USD ($)
Jan. 01, 2017
USD ($)
Property, Plant and Equipment [Line Items]              
Net loss from operations     $ (96,967,830) $ (88,205,772) $ (73,740,339)    
Working capital     52,500,000        
Accumulated deficit     (620,426,436) (523,356,317)   $ (523,124,951)  
Cash, cash equivalents, and short-term investments     81,200,000        
Long-term investments     9,400,000        
Proceeds from issuance of common stock     $ 29,227,776 94,347,922 $ 6,295,761    
Number of operating segments | segment     1        
Allowance for doubtful accounts     $ 0 0      
Intangible assets, net     4,760,145 6,009,729      
Deferred tax assets, valuation allowance     115,007,000 94,039,000      
Cumulative effect of accounting change           0 $ 0
Grant proceeds received     $ 9,500,000        
Minimum              
Property, Plant and Equipment [Line Items]              
Property, plant, and equipment, useful life     3 years        
Intangible asset, useful life     2 years        
Maximum              
Property, Plant and Equipment [Line Items]              
Property, plant, and equipment, useful life     5 years        
Intangible asset, useful life     18 years        
Patents | Minimum              
Property, Plant and Equipment [Line Items]              
Intangible asset, useful life     8 years        
Patents | Maximum              
Property, Plant and Equipment [Line Items]              
Intangible asset, useful life     17 years        
Accounting Standards Update 2016-01              
Property, Plant and Equipment [Line Items]              
Accumulated deficit           231,366  
Retained earnings              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change           231,366 (312,310)
Retained earnings | Accounting Standards Update 2016-09              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change             (312,000)
Retained earnings | Accounting Standards Update 2016-01              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change           (231,000)  
Additional paid-in capital              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change             312,310
Additional paid-in capital | Accounting Standards Update 2016-09              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change             $ 312,000
Accumulated other comprehensive income (loss)              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change           (231,366)  
Accumulated other comprehensive income (loss) | Accounting Standards Update 2016-01              
Property, Plant and Equipment [Line Items]              
Cumulative effect of accounting change           $ (231,000)  
Common stock | Sales Agreement              
Property, Plant and Equipment [Line Items]              
Stock sales agreement, remaining authorized sales amount     $ 71,800,000        
Proceeds from issuance of common stock     29,200,000        
Common stock | Underwritten Public Offering And Sales Agreement              
Property, Plant and Equipment [Line Items]              
Proceeds from issuance of common stock       $ 94,300,000      
Plumbline Life Sciences              
Property, Plant and Equipment [Line Items]              
Equity securities, FV-NI, gain (loss)     $ 699,000        
Convertible Debt | 6.50% Convertible Senior Notes Due 2024              
Property, Plant and Equipment [Line Items]              
Stated interest rate     6.50%        
Convertible Debt | Subsequent Event | 6.50% Convertible Senior Notes Due 2024              
Property, Plant and Equipment [Line Items]              
Proceeds from issuance of debt $ 75,800,000 $ 75,800,000          
Debt instrument, face amount $ 78,500,000 $ 78,500,000          
VGX Animal Health, Inc              
Property, Plant and Equipment [Line Items]              
Non-controlling interest     15.00%