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Stock-Based Compensation - Summary of Restricted Performance Stock Unit Activity (Detail) - PSUs [Member] - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Number of Awards    
Number of Awards, Beginning balance (in shares) 239,674  
Granted (in shares) 289,698  
Vested (in shares) 0  
Canceled (in shares) (36,223)  
Number of Awards, Ending balance (in shares) 493,149 239,674
Weighted Average Grant Date Fair Value    
Weighted Average Grant Date Fair Value, Beginning balance (in dollars per share) $ 22.58  
Granted (in dollars per share) 16.84  
Vested (in dollars per share) 0  
Canceled (in dollars per share) 18.05  
Weighted Average Grant Date Fair Value, Ending balance (in dollars per share) $ 19.54 $ 22.58
Weighted Average Remaining Amortization Period (in Years) 1 year 10 months 24 days 1 year 8 months 12 days