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Stock compensation plans - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
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 353,688    
Unrecognized compensation expense $ 20,000    
Shares available for future grants 4,446,967    
Stock-based compensation $ 14,467 $ 12,227 $ 11,871
Tax benefit from compensation expense $ 4,400 $ 3,300 $ 3,800
Stock option granted, weighted average grant date fair value $ 21.44 $ 18.01 $ 14.30
Stock option granted, weighted average grant date fair value $ 6,300 $ 15,400 $ 4,100
Stock option expenses including selling general and administrative expenses $ 5,700    
Non-vested restricted stock units issued 105,239 116,258 148,191
Non vested restricted stock expense including selling general and administrative expense $ 8,800    
Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 6,600    
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 353,688    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards 105,239    
Restricted Stock      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Unrecognized compensation expense $ 11,200    
Period for recognition 1 year 8 months 12 days    
Restricted Stock Units      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Grant of restricted stock awards 105,239    
Weighted average grant date fair value $ 118.00 $ 97.87 $ 75.60
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