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Consolidated Balance Sheets (Parenthetical) - $ / shares
Dec. 31, 2018
Dec. 31, 2017
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) 420,288,886 418,323,134
Common stock, shares outstanding (in shares) 283,650,039 316,743,089
Treasury stock, shares (in shares) 136,638,847 101,580,045