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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, 2017
Dec. 31, 2016
Dec. 31, 2015
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 (in shares) 250,389    
Number of Non-Vested Shares, Granted (in shares) 82,865    
Number of Non-Vested Shares, Vested (in shares) (85,271)    
Number of Non-Vested Shares, Forfeited (in shares) (14,241)    
Number of Non-Vested Shares, Outstanding, end of the year (in shares) 233,742 250,389  
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 (in dollars per share) $ 119.44    
Weighted Average Grant Date Fair Value, Granted (in dollars per share) 187.85 $ 142.71 $ 118.00
Weighted Average Grant Date Fair Value, Vested (in dollars per share) 102.66    
Weighted Average Grant Date Fair Value, Forfeited (in dollars per share) 136.20    
Weighted Average Grant Date Fair Value, Outstanding, end of the year (in dollars per share) $ 148.79 $ 119.44  
Weighted Average Remaining Contractual Life In Years, Outstanding, end of the year 1 year 1 month 30 days    
Aggregate Intrinsic Value, Outstanding, end of the year $ 58,160