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Related Party Transactions - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Investor and Holder of August 2016 Series B Warrants    
Related Party Transaction [Line Items]    
Revenue from related party $ 0 $ 797
B . Riley Principal Investments    
Related Party Transaction [Line Items]    
Consulting fees $ 50 $ 50