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Equity - Schedule of Common Stock Purchase Warrant Activity (Detail) - Warrant - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Number of Warrants, Outstanding, Beginning balance 3,864,551 748,060
Number of Warrants, Issued 0 3,166,491
Number of Warrants, Cancelled or expired   (15,385)
Number of Warrants, Exercised   (34,615)
Number of Warrants, Outstanding, Ending balance 3,864,551 3,864,551
Weighted Avg. Exercise Price, Outstanding, Beginning balance $ 4.60 $ 5.17
Weighted Avg. Exercise Price, Issued   4.47
Weighted Avg. Exercise Price, Outstanding, Ending balance $ 4.60 $ 4.60