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Stock-Based Compensation (Components of Stock Based Compensation) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Components of stock based-compensation [Abstract]      
Stock-based compensation expense $ 58 $ 61 $ 34
Tax benefit 23 24 13
Stock-based compensation cost capitalized in Property, plant, and equipment $ 9 $ 10 $ 5