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Stock-Based Compensation (Performance Share Awards Compensation Expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Compensation Expense Recorded [Line Items]      
Compensation expense $ 58 $ 61 $ 34
Performance Share Awards      
Compensation Expense Recorded [Line Items]      
Compensation expense 47 50 24
Cash settlements 15 7 13
Stock settlements $ 66 $ 38 $ 71