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Stock Plans - Share Based Compensation Activity (Detail) - Restricted Stock Units [Member] - Omnibus Incentive 2016 Plan [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]      
Nonvested shares balance, Shares 501,992    
Granted, Shares 159,992    
Vested, Shares (237,262)    
Canceled, Shares (10,958)    
Nonvested shares balance, Shares 413,764 501,992  
Nonvested shares balance, Weighted Average Grant-Date Fair-Value $ 46.52    
Granted, Weighted Average Grant-Date Fair-Value 66.54 $ 47.71 $ 57.21
Vested, Weighted Average Grant-Date Fair-Value 42.98    
Canceled, Weighted Average Grant-Date Fair-Value 57.15    
Nonvested shares balance, Weighted Average Grant-Date Fair-Value $ 54.57 $ 46.52