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Summary of Significant Accounting Policies - Shares Excluded from the Computation of Diluted Earnings per Share (Details) - shares
6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Antidilutive securities excluded from computation of earnings per share (in shares) 5,830,302 3,232,551 6,968,141 14,049
Common Stock Warrants - Equity [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Antidilutive securities excluded from computation of earnings per share (in shares) 4,202,909   4,202,909 14,049
Common Stock Warrants - Liability [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Antidilutive securities excluded from computation of earnings per share (in shares) 125,029 2,116,296 189,029  
Assumed Conversion of Convertible Notes [Member]        
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]        
Antidilutive securities excluded from computation of earnings per share (in shares) 335,697 1,116,255 2,576,203