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Stockholders' Equity - Additional Information (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Nov. 30, 2014
Class of Stock [Line Items]          
Common stock, shares authorized   90,000,000 90,000,000    
Common stock, par value (usd per share)   $ 1 $ 1    
Common stock, shares issued   66,400,000 66,100,000    
Common stock, shares outstanding   60,200,000 60,000,000    
Preference stock, shares authorized   800,000      
Preference stock, par value (usd per share)   $ 1      
Cash dividends declared per common share (usd per share)   $ 0.31 $ 0.28 $ 0.28  
Stock repurchase program, amount acquired   $ 1,200,000   $ 37,800,000  
Cash dividends paid to stockholders   $ 18,700,000 $ 16,800,000 $ 16,900,000  
Subsequent Event          
Class of Stock [Line Items]          
Cash dividends declared per common share (usd per share) $ 0.08        
November 2014 Repurchase Program [Member]          
Class of Stock [Line Items]          
Share repurchase program, authorized amount         $ 75,000,000
Shares acquired   62,500 0 2,961,007  
Stock repurchase program, amount acquired   $ 1,200,000   $ 37,800,000