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Stock compensation plans - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
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) 338,902    
Unrecognized compensation expense $ 22,600    
Shares available for future grants 3,999,156    
Stock-based compensation $ 16,871 $ 14,467 $ 12,227
Tax benefit from compensation expense $ 5,500 $ 4,400 $ 3,300
Stock option granted, weighted average grant date fair value (in dollars per share) $ 27.42 $ 21.44 $ 18.01
Stock option granted, weighted average grant date fair value $ 11,300 $ 6,300 $ 15,400
Stock option expenses including selling general and administrative expenses $ 6,900    
Non-vested restricted stock units issued (in shares) 93,367 105,239 116,258
Non vested restricted stock expense including selling general and administrative expense $ 10,000    
Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 7,800    
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) 338,902    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 93,367    
Restricted Stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 11,300    
Period for recognition 1 year 9 months 18 days    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards (in shares) 93,367    
Weighted average grant date fair value (in dollars per share) $ 142.71 $ 118.00 $ 97.87
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