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Consolidated Balance Sheets (Parenthetical) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
Statement of Financial Position [Abstract]    
Common stock, par value (in usd per share) $ 0.01 $ 0.01
Common stock, shares authorized (in shares) 500,000,000 500,000,000
Common stock, shares issued (in shares) 418,323,134 417,583,825
Common stock, shares outstanding (in shares) 316,743,089 315,443,011
Treasury stock, shares (in shares) 101,580,045 102,140,814