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Consolidated Balance Sheets (Parenthetical) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
Class of Stock [Line Items]    
Preferred stock, par value (dollars per share) $ 0.10 $ 0.10
Preferred stock, authorized (shares) 5,000,000 5,000,000
Preferred stock, issued (shares) 0 0
Preferred stock, outstanding (shares) 0 0
Common stock, par value (dollars per share) $ 0.20 $ 0.20
Common stock, authorized (shares) 500,000,000 500,000,000
Common stock, issued (shares) 121,514,852 121,193,599
Common stock, outstanding (shares) 121,514,852 121,193,599
Treasury stock, at cost (shares) 113,603,066 113,924,319
Common Stock [Member]    
Class of Stock [Line Items]    
Common stock, outstanding (shares)   121,193,599