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Stock compensation plans - Additional Information (Detail) - 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 [Line Items]      
Maximum number of common stock authorized to be issued under plan 5,300,000.0    
Number of options granted (in shares) 260,171    
Unrecognized compensation expense $ 25,900    
Shares available for future grants 3,629,445    
Stock-based compensation $ 19,407 $ 16,871 $ 14,467
Tax benefit from compensation expense 12,800 $ 5,500 $ 4,400
Net excess tax benefit from compensation expense $ 6,600    
Stock option granted, weighted average grant date fair value (in dollars per share) $ 39.70 $ 27.42 $ 21.44
Stock option granted, weighted average grant date fair value $ 15,700 $ 11,300 $ 6,300
Stock option expenses including selling general and administrative expenses $ 8,200    
Non-vested restricted stock units issued (in shares) 82,865 93,367 105,239
Non vested restricted stock expense including selling general and administrative expense $ 11,200    
Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 9,400    
Period for recognition 1 year 9 months 18 days    
Common Stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of options granted (in shares) 260,171    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 82,865    
Restricted Stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 13,200    
Period for recognition 1 year 9 months 22 days    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 82,865    
Weighted average grant date fair value (in dollars per share) $ 187.85 $ 142.71 $ 118.00
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