v3.19.3.a.u2
Summary of Significant Accounting Policies - Liquidity Matters (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Oct. 17, 2019
Apr. 26, 2019
Oct. 31, 2019
Apr. 30, 2019
Jul. 31, 2018
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Apr. 23, 2019
Aug. 31, 2017
Aug. 14, 2017
Aug. 08, 2017
Dec. 31, 2016
Policies                                          
Net (loss) income           $ (35,183) $ (23,137) $ (32,416) $ (34,091) $ (31,841) $ (29,616) $ (33,824) $ (32,873) $ (124,827) $ (128,154) $ (137,628)          
Accumulated deficit           699,171       574,529       699,171 574,529            
Cash, cash equivalents and restricted cash           61,981       51,004       61,981 51,004 13,021         $ 16,690
Marketable securities           379,065       107,461       379,065 107,461            
Short-term Credit Line, outstanding balance           50,123       50,153       50,123 50,153            
Long-term Term Loan, carrying amount           $ 73,656       $ 73,357       $ 73,656 $ 73,357            
Common stock, shares issued 5,714,286 789,473 5,714,286 5,263,158 4,500,000 78,004,669       62,083,000       78,004,669 62,083,000   5,263,158        
Stock issued (in dollars per share) $ 35   $ 35 $ 19 $ 20                       $ 19        
Payment of offering expenses $ 400 $ 600 $ 400 $ 600 $ 500                                
Proceeds from issuance of common stock $ 216,200 $ 108,100 $ 216,200 108,100 $ 97,300                                
2017 Term Loan                                          
Policies                                          
Long-term Term Loan, carrying amount           $ 73,656       $ 73,357       $ 73,656 $ 73,357            
Remaining borrowing capacity       $ 50,000         $ 50,000 $ 25,000         25,000     $ 50,000   $ 25,000  
Common stock, shares issued       25,000                           300,000 300,000    
Stock issued (in dollars per share)                                       $ 8.16  
Option for additional shares                                          
Policies                                          
Common stock, shares issued 857,142 789,473 857,142   675,000                                
Accumulated Deficit                                          
Policies                                          
Net (loss) income                           (124,827) $ (128,154) $ (137,628)          
Cumulative-effect adjustment upon adoption of ASU 2018-07                           $ (185)