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Share-Based Compensation Expense (Details) - USD ($)
shares in Thousands, $ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-Based Compensation Expense [Abstract]      
Granted (in shares) 483 500 503
Compensation cost for share-based awards [1] $ 58 $ 45 $ 57
Tax benefit recognized in income 15 12 $ 22
Unrecognized compensation cost related to nonvested share-based compensation awards $ 40 $ 38  
Weighted-average period for recognition of unrecognized compensation cost related to nonvested share-based compensation awards (in years) 1 year 7 months 6 days    
Award Vesting Period (in years) 3 years    
Service-based awards [Member]      
Share-Based Compensation Expense [Abstract]      
Granted (in shares) 300 300 300
[1]
Compensation costs for share-based payment are included in O&M expense.