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Compensation - Activity Associated with Restricted Stock Units (Detail) - Restricted Stock Units (RSUs) [Member] - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Units, Balance outstanding at beginning of period 928,713    
Number of Units, Granted 138,784 49,000  
Number of Units, Delivered units (273,163)    
Number of Units, Forfeited units (27,404)    
Number of Units, Balance outstanding at end of period 766,930   928,713
Weighted-Average Grant Date Fair Value, Balance outstanding at beginning of period $ 10.83    
Weighted-Average Grant Date Fair Value, Granted 5.86    
Weighted-Average Grant Date Fair Value, Delivered units 10.15    
Weighted-Average Grant Date Fair Value, Forfeited units 10.17    
Weighted-Average Grant Date Fair Value, Balance outstanding at end of period $ 10.20   $ 10.83
Weighted-Average Remaining Contractual Term (Years) 2 years 1 month 2 days   1 year 9 months