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UNAUDITED CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical) - $ / shares
Mar. 31, 2019
Dec. 31, 2018
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) 219,523,859 219,140,523
Treasury stock at cost (in shares) 80,996,065 79,413,989