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Stock Option Plan - Summary of Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Shares      
Outstanding at January 1 (in shares) 913,320 833,630 860,274
Granted (in shares) 245,000 232,500 226,500
Exercised (in shares) (39,151) (149,060) (246,894)
Expired/Forfeited (in shares) (5,000) (3,750) (6,250)
Outstanding December 31 (in shares) 1,114,169 913,320 833,630
Options exercisable (in shares) 600,919 430,945 375,255
Weighted Average Exercise Price      
Outstanding at January 1 (in usd per share) $ 52.80 $ 49.92 $ 46.58
Granted (in usd per share) 49.46 59.41 57.74
Exercised (in usd per share) 42.98 46.97 45.59
Expired/Forfeited (in usd per share) 54.78 53.73 45.31
Outstanding December 31 (in usd per share) 52.40 52.80 49.92
Exercisable at December 31 (in usd per share) $ 50.93 $ 48.94 $ 46.68
Intrinsic value of options exercised $ 0.5 $ 2.2 $ 3.4
Intrinsic value of options outstanding 0.5    
Intrinsic value of options exercisable $ 0.5    
Closing price (in usd per share) $ 47.22    
Weighted average remaining life of options exercisable 6 years 2 months 16 days    
Weighted average remaining life of options outstanding 7 years 2 months 19 days