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Summary of Significant Accounting Policies - Warrant liability (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Beginning balance $ 1,640 $ 1,697  
Change in fair value of warrants 5 (244) $ (54)
Conversion into common stock warrants $ (1,645)    
Ending balance   1,640 1,697
MidCap Credit Facility      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Change in fair value of warrants   $ 187  
Convertible promissory notes      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Change in fair value of warrants     1,408
Notes payable      
Fair Value Measurement Inputs and Valuation Techniques [Line Items]      
Change in fair value of warrants     $ 343