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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]          
Federal statutory income tax rate 21.00% 21.00% 21.00% 21.00%  
Effective Income Tax Rate Reconciliation, Tax Credit, Percent (20.00%) (43.00%) (23.00%) (35.00%)  
State income tax, net of federal income tax benefit 3.00% (3.00%) 2.00% (6.00%)  
Income tax effect of foreign income 1.00% (1.00%) (0.00%) (2.00%)  
Effects of ratemaking (2.00%) (9.00%) (2.00%) (5.00%)  
Other, net (0.00%) (1.00%) (0.00%) (0.00%)  
Effective income tax rate 3.00% (36.00%) (2.00%) (27.00%)  
Production Tax Credit Carryforwards [Abstract]          
Years eligible for federal renewable energy production tax credit     10 years    
Related Party Tax Expense [Abstract]          
Long-term income tax receivable $ 530   $ 530   $ 530
Parent Company [Member]          
Related Party Tax Expense [Abstract]          
Related party transaction, cash paid for income taxes, net     $ 1,031 $ 534  
UNITED KINGDOM          
Production Tax Credit Carryforwards [Abstract]          
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount $ 35        
Corporate Income Tax Rate, Effective April 1, 2020 [Member] | UNITED KINGDOM          
Production Tax Credit Carryforwards [Abstract]          
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00%        
Corporate Income Tax Rate, Effective April 1, 2020 further reduction [Member] | UNITED KINGDOM          
Production Tax Credit Carryforwards [Abstract]          
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent 0.00%