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Restricted Stock Units - Summary of Restricted Stock Unit Activity (Detail) - Nonvested Restricted Stock Units [Member] - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Restricted Stock Units Outstanding, Beginning balance 4,394,656 895,328 422,500
Number of Restricted Stock Units Outstanding, Granted 2,215,151 3,873,000 660,331
Number of Restricted Stock Units Outstanding, Vested (1,605,997) (337,618) (187,503)
Number of Restricted Stock Units Outstanding, Forfeited (36,666) (36,054)  
Number of Restricted Stock Units Outstanding, Ending balance 4,967,144 4,394,656 895,328
Weighted Average Grant Date Fair Value, Unvested beginning balance $ 1.15 $ 3.53 $ 3.64
Weighted Average Grant Date Fair Value, Granted 2.20 0.82 3.74
Weighted Average Grant Date Fair Value, Vested 1.34 3.46 4.53
Weighted Average Grant Date Fair Value, Forfeited 1.36 3.60  
Weighted Average Grant Date Fair Value, Unvested ending balance $ 1.56 $ 1.15 $ 3.53