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Stock compensation plans - summary of non vested restricted stock unit activity (Detail) - Restricted Stock Units - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Number of Non-Vested Shares, Outstanding, beginning of the year 313,703    
Number of Non-Vested Shares, Granted 105,239    
Number of Non-Vested Shares, Vested (106,667)    
Number of Non-Vested Shares, Forfeited (30,867)    
Number of Non-Vested Shares, Outstanding, end of the year 281,408 313,703  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]      
Weighted Average Grant Date Fair Value, Outstanding, beginning of the year $ 76.80    
Weighted Average Grant Date Fair Value, Granted 118.00 $ 97.87 $ 75.60
Weighted Average Grant Date Fair Value, Vested 61.79    
Weighted Average Grant Date Fair Value, Forfeited 88.73    
Weighted Average Grant Date Fair Value, Outstanding, end of the year $ 96.59 $ 76.80  
Weighted Average Remaining Contractual Life In Years, Outstanding, end of the year 1 year 2 months 12 days    
Aggregate Intrinsic Value, Outstanding, end of the year $ 36,818