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Share-based Compensation and Employee Benefits (Schedule of One-Percentage Point Change in Assumed Health Care Cost Trends) (Details) - OPEB [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
One-percentage point increase:    
Aggregate of service cost and interest cost $ 1 $ 1
Accumulated postretirement benefit obligation 16 22
One-percentage point decrease:    
Aggregate of service cost and interest cost (1) (1)
Accumulated postretirement benefit obligation $ (14) $ (19)