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Stock-Based Compensation (Performance Share Awards Compensation Expense) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Compensation Expense Recorded [Line Items]      
Stock-based compensation expense $ 61 $ 34 $ 103
Performance Shares      
Compensation Expense Recorded [Line Items]      
Stock-based compensation expense 50 24 93
Cash settlements 7 13 11
Stock settlements $ 38 $ 71 $ 61