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Fair Value Measurements - Rollforward of Fair Value of Level 3 Warrants and Transfers Between Levels (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Fair value measurements      
Asset transfers out of Level 1 into Level 2 $ 0 $ 0  
Asset transfers out of Level 2 into Level 1 0 0  
Asset transfers into (out of) Level 3 0 0  
Liability transfers out of Level 1 into Level 2 0 0  
Liability transfers out of Level 2 into Level 1 0 0  
Liability transfers into (out of) Level 3 0 0  
Common stock warrants | Liabilities      
Rollforward of the fair value of Level 3 warrants:      
Beginning balance 29 172 $ 406
Change in fair value (29) (143) (234)
Ending balance $ 0 $ 29 $ 172