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UNAUDITED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - $ / shares
Sep. 30, 2018
Dec. 31, 2017
Stockholders' equity    
Common stock, par value (in dollars per share) $ 0.01 $ 0.01
Common stock, shares authorized (in shares) 350,000,000 350,000,000
Common stock, shares issued (in shares) 219,119,060 218,369,992
Treasury stock at cost (in shares) 79,127,753 75,454,111