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Consolidated Balance Sheets (Unaudited) (Parenthetical) - $ / shares
Jun. 30, 2018
Dec. 31, 2017
Statement Of Financial Position [Abstract]    
Common stock, par value $ 0.01 $ 0.01
Common stock, shares authorized 200,000,000 200,000,000
Common stock, shares issued 79,652,000 79,652,000
Common stock, shares outstanding 47,733,000 48,485,000
Treasury stock, shares 31,919,000 31,167,000