Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements (Details) - USD ($)
Jan. 01, 2018
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Jan. 01, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Recently Adopted Accounting Pronouncements                    
Additional paid-in capital   $ 575,365,000       $ 543,040,000       $ 17,277,000
Accumulated deficit   (268,268,000)     $ (149,282,000) (150,082,000)       (60,979,000)
Change in fair value of the unvested awards $ 100,000                  
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment                    
Operating lease, right-of-use-asset   47,911,000     1,751,000          
Property, plant and equipment, net   52,478,000     17,654,000 62,796,000        
Deferred rent           143,000        
Accrued expenses and other current liabilities         7,667,000 12,118,000        
Lease liability, net of current portion           41,441,000        
Operating lease liabilities, current portion   6,998,000     616,000 4,502,000        
Operating lease liabilities, net of current portion   41,439,000     1,226,000 41,441,000        
Stockholders Equity   $ 307,343,000 $ 343,644,000 $ 371,636,000   $ 393,008,000 $ 411,007,000 $ (70,709,000) $ (55,526,000) $ (43,687,000)
Non-employees                    
Assumptions used to determine the fair value of outstanding awards granted                    
Weighted average grant-date fair value (per share) $ 5.88                  
Risk-free interest rate 2.30%                  
Expected volatility 74.00%                  
Expected term (in years) 6 years 1 month 6 days                  
Common stock value $ 6.28                  
Restricted Common Stock | Non-employees                    
Assumptions used to determine the fair value of outstanding awards granted                    
Number of outstanding options 4,767,014                  
Stock options | Non-employees                    
Assumptions used to determine the fair value of outstanding awards granted                    
Number of outstanding options 330,917                  
ASU 2018-07                    
Recently Adopted Accounting Pronouncements                    
Additional paid-in capital $ 17,185,000                  
Accumulated deficit (60,887,000)                  
ASU 2016-02                    
Recently Adopted Accounting Pronouncements                    
Change in fair value of the unvested awards       $ 800,000            
ASU 2016-02 | Adjustments                    
Recently Adopted Accounting Pronouncements                    
Accumulated deficit         800,000          
Financial impact on the Company’s condensed consolidated balance sheet upon the adoption of ASUs and the cumulative effect adjustment                    
Operating lease, right-of-use-asset         1,751,000          
Property, plant and equipment, net         (45,142,000)          
Deferred rent         (143,000)          
Accrued expenses and other current liabilities         (4,451,000)          
Lease liability, net of current portion         (41,441,000)          
Operating lease liabilities, current portion         616,000          
Operating lease liabilities, net of current portion         $ 1,226,000          
Adjustments for New Accounting Principle, Early Adoption | ASU 2018-07                    
Recently Adopted Accounting Pronouncements                    
Additional paid-in capital (92,000)                  
Accumulated deficit $ 92,000