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Stock Option Plan - Summary of Option Activity (Detail) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Shares      
Outstanding at January 1 (in shares) 748,208 753,625 570,840
Granted (in shares) 225,000 200,000 237,500
Exercised (in shares) (112,934) (167,917) (49,715)
Expired/Forfeited (in shares) 0 (37,500) (5,000)
Outstanding December 31 (in shares) 860,274 748,208 753,625
Options exercisable (in shares) 435,899 380,708 413,000
Weighted Average Exercise Price      
Outstanding at January 1 (in usd per share) $ 44.79 $ 42.55 $ 41.04
Granted (in usd per share) 51.07 47.03 44.42
Exercised (in usd per share) 43.67 37.71 33.15
Expired/Forfeited (in usd per share) 0.00 43.56 52.16
Outstanding December 31 (in usd per share) $ 46.58 44.79 42.55
Exercisable at December 31 (in usd per share) $ 44.85 $ 42.42
Intrinsic value of options exercised $ 1,500,000 $ 2,000,000 $ 600,000
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Intrinsic value of options outstanding $ 4,200,000    
Options exercisable (in shares) 435,899 380,708 413,000
Intrinsic value of options exercisable $ 2,800,000    
Closing price (in usd per share) $ 51.27    
Weighted average remaining life of options exercisable 5 years 8 months 3 days    
Weighted average remaining life of options outstanding 7 years 22 days    
Options Issued in 2007      
Shares      
Options exercisable (in shares) 70,903    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Intrinsic value of options outstanding $ 0    
Options exercisable (in shares) 70,903