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Share-based Compensation - Summary of Restricted Stock Unit Activity (Detail) - Restricted Stock Units - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]    
Non-vested — beginning of period 1,055,977  
Granted 966,747  
Vested (1,110,138)  
Forfeited (458,204)  
Non-vested — end of period 454,382 1,055,977
Weighted-Average Grant-Date Fair Value    
Non-vested — beginning of period $ 2.06  
Granted 2.01 $ 1.83
Vested 1.96  
Forfeited 2.07  
Non-vested — end of period $ 2.17 $ 2.06