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Summary of Significant Accounting Policies - Narrative (Details)
 1 Months Ended 2 Months Ended 3 Months Ended 12 Months Ended
Jul. 31, 2019
Mar. 11, 2020
USD ($)
shares
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
segment
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2019
KRW (₩)
Dec. 26, 2019
Aug. 01, 2019
Mar. 01, 2019
USD ($)
Jan. 01, 2019
USD ($)
Property, Plant and Equipment [Line Items]                            
Net loss from operations     $ (37,662,038) $ (23,090,693) $ (29,387,226) $ (29,219,262) $ (119,359,219) $ (96,967,830) $ (88,205,772)          
Working capital     62,200,000       62,200,000              
Accumulated deficit     (739,785,655)       (739,785,655) (620,426,436)            
Proceeds from issuance of common stock             9,089,010 29,227,776 $ 94,347,922          
Cash, cash equivalents, and short-term investments     89,500,000       89,500,000              
Investments in affiliated entities     6,315,356       $ 6,315,356 9,405,913            
Expected number of positions to be eliminated, percent 28.00%                          
Restructuring charges       $ 2,200,000                    
Number of operating segments | segment             1              
Allowance for doubtful accounts     0       $ 0 0            
Finite lived intangible assets, net     3,693,851       3,693,851 4,760,145            
Deferred tax assets, valuation allowance     137,159,000       137,159,000 115,007,000            
Operating right-of-use assets     13,783,009       13,783,009             $ 14,634,769
Total operating lease liabilities     22,484,764       $ 22,484,764              
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              
Accounting Standards Update 2016-02                            
Property, Plant and Equipment [Line Items]                            
Operating right-of-use assets                           14,600,000
Total operating lease liabilities                           24,200,000
Deferred rent credit                           $ (9,600,000)
Sales Agreement                            
Property, Plant and Equipment [Line Items]                            
Proceeds from issuance of common stock             $ 9,100,000              
6.50% Convertible Senior Notes Due 2024                            
Property, Plant and Equipment [Line Items]                            
Debt instrument, face amount     78,500,000       78,500,000           $ 78,500,000.0  
Stated interest rate           6.50%             6.50%  
August 2019 Convertible Bonds                            
Property, Plant and Equipment [Line Items]                            
Debt instrument, face amount     15,578,211       15,578,211              
December 2019 Convertible Bonds                            
Property, Plant and Equipment [Line Items]                            
Debt instrument, face amount     4,067,644       4,067,644              
Convertible Debt | 6.50% Convertible Senior Notes Due 2024                            
Property, Plant and Equipment [Line Items]                            
Proceeds from issuance of debt             75,700,000              
Debt instrument, face amount     $ 78,500,000.0       $ 78,500,000.0           $ 78,500,000.0  
Stated interest rate     6.50%       6.50%     6.50%        
Convertible Debt | August 2019 Convertible Bonds                            
Property, Plant and Equipment [Line Items]                            
Proceeds from issuance of debt             $ 14,500,000              
Debt instrument, face amount     $ 15,000,000.0       $ 15,000,000.0     ₩ 18,000,000,000        
Stated interest rate     1.00%       1.00%     1.00%   1.00%    
Convertible Debt | December 2019 Convertible Bonds                            
Property, Plant and Equipment [Line Items]                            
Proceeds from issuance of debt             $ 4,000,000              
Debt instrument, face amount     $ 4,100,000.0       $ 4,100,000.0     ₩ 4,700,000,000.0        
Stated interest rate     1.00%       1.00%     1.00% 1.00%      
Geneos Therapeutics, Inc.                            
Property, Plant and Equipment [Line Items]                            
Non-controlling interest     39.00%       39.00%     39.00%        
VGX Animal Health, Inc                            
Property, Plant and Equipment [Line Items]                            
Non-controlling interest     15.00%       15.00%     15.00%        
Fair Value, Measurements, Recurring                            
Property, Plant and Equipment [Line Items]                            
Investments in affiliated entities     $ 6,315,356       $ 6,315,356 9,405,913            
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1                            
Property, Plant and Equipment [Line Items]                            
Investments in affiliated entities     6,315,356       $ 6,315,356 9,405,913            
Subsequent Event | Common stock | Sales Agreement                            
Property, Plant and Equipment [Line Items]                            
Proceeds from issuance of common stock   $ 208,200,000                        
Number of shares issued in transaction (in shares) | shares   43,148,952                        
Patents                            
Property, Plant and Equipment [Line Items]                            
Intangible asset, useful life             17 years              
Finite lived intangible assets, net     $ 0       $ 0 $ 60,449