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CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
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) 218,369,992 217,707,600
Treasury stock at cost (in shares) 75,454,111 71,269,474