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Commodity Derivatives (Details)
MWh in Millions, MMBTU in Millions, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
USD ($)
MWh
MMBTU
Sep. 30, 2020
USD ($)
Sep. 30, 2021
USD ($)
MWh
MMBTU
Sep. 30, 2020
USD ($)
Dec. 31, 2020
MWh
MMBTU
Other Derivative Instruments          
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]          
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities $ 62 $ (1) $ 75 $ (4)  
Cash Flow Hedge Commodity [Member]          
Derivative [Line Items]          
Commodity contracts designated as cash flow hedges $ 0   $ 0    
Electric Commodity [Member]          
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]          
Derivative, Nonmonetary Notional Amount | MWh [1],[2] 101   101   87
Natural Gas Commodity          
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]          
Derivative, Nonmonetary Notional Amount | MMBTU [1],[2] 184   184   175
Natural Gas Commodity | Other Derivative Instruments          
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]          
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities $ 57 2 $ 57 (1)  
Natural Gas Commodity for Electric Generation | Other Derivative Instruments          
Gross Notional Amounts of Commodity Forwards, Options and FTRs [Abstract]          
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities $ 0 $ 0 $ 0 $ 0  
[1] Not reflective of net positions in the underlying commodities.
[2] Notional amounts for options included on a gross basis but weighted for the probability of exercise.