Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies - Operating Leases (Details)

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Commitments and Contingencies - Operating Leases (Details)
 1 Months Ended 12 Months Ended
Jan. 08, 2019
ft²
Apr. 30, 2018
USD ($)
Mar. 31, 2015
USD ($)
ft²
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
2019       $ 1,325,000    
2020       1,130,000    
Operating Leases, Future Minimum Payments Due       $ 2,455,000    
Subsequent Event            
Area of office and laboratory space leased | ft² | ft² 52,859          
Operating ​Lease, ​March 2015            
Area of office and laboratory space leased | ft² | ft²     21,488      
Term of option to extend the lease       5 years    
Rent expense       $ 900,000 $ 900,000 $ 900,000
Letter of credit outstanding     $ 500,000      
Refund received from landlord of security deposit on lease   $ 200,000        
Letter of credit outstanding classified as restricted cash       $ 300,000    
Operating Lease, January 2019            
Term of option to extend the lease       10 years    
Letter of credit outstanding       $ 3,100,000    
Initial fixed rental rate per rentable square foot of the premises per annum       $ 67.50    
Percentage increase in fixed rental rate       3.00%    
Maximum allowance towards leased premises       $ 9,514,620    
Allowance per square foot of leased premises       $ 180    
Letter of credit expiration period       95 days    
Operating Lease, January 2019 | Subsequent Event            
Area of office and laboratory space leased | ft² | ft² 52,859          
Operating Lease, October 2023            
Expected letter of credit amount       $ 2,100,000