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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, 2021
Dec. 31, 2020
Dec. 31, 2019
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) 150,812    
Number of Non-Vested Shares, Granted (in shares) 59,210    
Number of Non-Vested Shares, Vested (in shares) (50,098)    
Number of Non-Vested Shares, Forfeited (in shares) (24,546)    
Number of Non-Vested Shares, Outstanding, end of the year (in shares) 135,378 150,812  
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) $ 293.10    
Weighted Average Grant Date Fair Value, Granted (in dollars per share) 398.59 $ 344.70 $ 286.51
Weighted Average Grant Date Fair Value, Vested (in dollars per share) 260.32    
Weighted Average Grant Date Fair Value, Forfeited (in dollars per share) 334.38    
Weighted Average Grant Date Fair Value, Outstanding, end of the year (in dollars per share) $ 343.89 $ 293.10  
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 $ 44,469