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Fair Value Measurements - Rollforward of Fair Value of Level 3 Warrants and Transfers Between Levels (Details) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
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 $ 0 29,000 $ 172,000
Change in fair value   (29,000) (143,000)
Ending balance   $ 0 $ 29,000