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Fair Value of Financial Assets and Liabilities - Consideration of Credit Risk and Concentrations (Details)
$ in Millions
3 Months Ended
Mar. 31, 2021
USD ($)
Counterparty
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Fair Value Hedges, Net $ 0 $ 0  
Derivative instruments in a gross liability position 4   $ 4
Other Derivative Instruments      
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0    
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 1    
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0  
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0  
Pre-tax gains (losses) recognized during the period in income 28 2  
Commodity Trading Contract | Other Derivative Instruments      
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0  
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0  
Pre-tax gains (losses) recognized during the period in income [1] 31 4  
Natural Gas Commodity | Other Derivative Instruments      
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0  
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities 1    
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0  
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) [2] 1 1  
Pre-tax gains (losses) recognized during the period in income [2] (3) (2)  
Electric Commodity | Other Derivative Instruments      
Impact of Derivative Activity on Accumulated Other Comprehensive Loss, Regulatory Assets and Liabilities, and Income [Abstract]      
Pre-tax (gains) losses reclassified into income during the period from accumulated other comprehensive loss 0 0  
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) [3] 1 (1)  
Pre-tax gains (losses) recognized during the period in income $ 0 $ 0  
Credit Concentration Risk      
Derivative [Line Items]      
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 10    
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member]      
Derivative [Line Items]      
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 8    
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]      
Derivative [Line Items]      
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 6    
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties $ 26    
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties 52.00%    
Credit Concentration Risk | Internal Investment Grade [Member]      
Derivative [Line Items]      
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 3    
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties $ 10    
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties 20.00%    
Credit Concentration Risk | External Credit Rating, Non Investment Grade [Member]      
Derivative [Line Items]      
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 1    
Wholesale, trading and non trading commodity credit exposure for the most significant counterparties $ 8    
Percentage of wholesale, trading and non trading commodity credit exposure for the most significant counterparties 16.00%    
[1] Amounts are recorded to electric operating revenues. Portions of these gains and losses are subject to sharing with electric customers through margin-sharing mechanisms and deducted from gross revenue, as appropriate.
[2] Amounts are recorded to cost of natural gas sold and transported. These derivative settlement gains and losses are shared with natural gas customers through purchased natural gas cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.
[3] Amounts are recorded to electric fuel and purchased power. These derivative settlement gains and losses are shared with electric customers through fuel and purchased energy cost-recovery mechanisms, and reclassified out of income as regulatory assets or liabilities, as appropriate.