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Stock compensation plans - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Maximum number of common stock authorized to be issued under plan 5,300,000    
Number of options granted (in shares) 162,087    
Unrecognized compensation expense $ 34.0    
Shares available for future grants 3,357,048    
Stock option granted, weighted average grant date fair value (in dollars per share) $ 68.22 $ 58.16 $ 39.70
Stock option granted, weighted average grant date fair value $ 64.3 $ 69.4 $ 15.7
Stock option expenses including selling general and administrative expenses $ 9.6    
Non-vested restricted stock units issued (in shares) 69,799 62,221 82,865
Non vested restricted stock expense including selling general and administrative expense $ 14.1    
Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 10.6    
Period for recognition 1 year 4 months 24 days    
Common Stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of options granted (in shares) 162,087    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 69,799    
Restricted Stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 18.6    
Period for recognition 1 year 2 months 12 days    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 69,799    
Weighted average grant date fair value (in dollars per share) $ 286.51 $ 250.66 $ 187.85
Performance Shares Units (PSUs)      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Weighted average grant date fair value (in dollars per share) $ 305.58    
2014 Plan | Stock Options      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of award shares 1    
2014 Plan | Stock Compensation Plan      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of award shares 1.8    
Prior Plans Before December 31, 2013 | Stock Options      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of award shares 1    
Prior Plans Before December 31, 2013 | Stock Compensation Plan      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of award shares 1.8    
Prior Plans After January 1, 2014 | Stock Options      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of award shares 1    
Prior Plans After January 1, 2014 | Stock Compensation Plan      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of award shares 1.8    
Minimum | Performance Shares Units (PSUs)      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 18,663    
Maximum | Performance Shares Units (PSUs)      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 46,660