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Stockholders Equity - Additional Information (Detail)
$ / shares in Units, $ in Millions
 12 Months Ended
Oct. 05, 2016
Sep. 07, 2016
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Stockholders Equity [Line Items]          
Preferred stock, shares authorized     3,000,000    
Preferred stock, par value | $ / shares     $ 1    
Common Stock, shares authorized     240,000,000    
Common stock, par value | $ / shares     $ 1    
Dividends paid | $ / shares     $ 0.56 $ 0.48 $ 0.38
Stock split ratio 2 2      
Stock repurchase program number of shares authorized to repurchase       3,000,000 4,000,000
Shares repurchased     2,533,350 3,273,109 3,816,474
Shares repurchased, value | $     $ 139.1 $ 135.2 $ 128.1
Remaining authorization stock repurchase     2,373,053    
Common Class A          
Stockholders Equity [Line Items]          
Common Stock, shares authorized     27,000,000    
Common stock, par value | $ / shares     $ 5    
Shares converted into Common Stock     73,792 60,045 67,544
Treasury stock, number of shares held     130,380 130,380  
Common Stock          
Stockholders Equity [Line Items]          
Treasury stock, number of shares held     18,914,082 17,135,628