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Share-Based Compensation (Summary of Unrecognized Compensation Cost And Weighted Average Period For Recognition) (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
Restricted Stock Awards And Units [Member]  
Unrecognized Compensation And Weighted Average Recognition [Line Items]  
Unrecognized compensation cost $ 161
Weighted average period for recognition (years) 2 years 8 months 12 days
Performance-Based Restricted Stock Awards [Member]  
Unrecognized Compensation And Weighted Average Recognition [Line Items]  
Unrecognized compensation cost $ 2
Weighted average period for recognition (years) 1 year 4 months 24 days
Performance Share Units [Member]  
Unrecognized Compensation And Weighted Average Recognition [Line Items]  
Unrecognized compensation cost $ 39
Weighted average period for recognition (years) 2 years