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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Beginning balance 895,328 422,500 140,000
Number of Restricted Stock Units Outstanding, Granted 3,873,000 660,331 380,000
Number of Restricted Stock Units Outstanding, Vested (337,618) (187,503) (70,000)
Number of Restricted Stock Units Outstanding, Forfeited (36,054)   (27,500)
Ending balance 4,394,656 895,328 422,500
Weighted Average Grant Date Fair Value, Unvested $ 3.53 $ 3.64 $ 7.19
Weighted Average Grant Date Fair Value, Granted 0.82 3.74 2.94
Weighted Average Grant Date Fair Value, Vested 3.46 4.53 7.19
Weighted Average Grant Date Fair Value, Forfeited 3.60   2.94
Weighted Average Grant Date Fair Value, Unvested $ 1.15 $ 3.53 $ 3.64