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Fair Value Measurements - Additional Information (Details) - shares
Jun. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Class Of Warrant Or Right [Line Items]        
Number of warrants outstanding (in shares) 57,722,416 65,685,269 14,049 78
Febuary 2018 Warrants [Member]        
Class Of Warrant Or Right [Line Items]        
Number of warrants outstanding (in shares) 125,000