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Fair Value of Financial Assets and Liabilities - Consideration of Credit Risk and Concentrations (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2021
USD ($)
Counterparty
Jun. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
Counterparty
Jun. 30, 2020
USD ($)
Consideration of Credit Risk and Concentrations [Abstract]        
Derivative instruments designated as fair value hedges $ 0 $ 0 $ 0 $ 0
Other Derivative Instruments        
Consideration of Credit Risk and Concentrations [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities (2) (3) (2) (3)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 (6) 3
Pre-tax gains (losses) recognized during the period in income 10 (1) 8 (8)
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Designated as Hedging Instrument | Cash Flow Hedges        
Consideration of Credit Risk and Concentrations [Abstract]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net     1 1
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     0 0
Natural Gas Commodity | Other Derivative Instruments        
Consideration of Credit Risk and Concentrations [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities (2) (3) (2) (3)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net     0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) [1]     (6) 3
Pre-tax gains (losses) recognized during the period in income [1]     (6) (3)
Pre-tax fair value gains (losses) recognized during the period in accumulated other comprehensive loss 0 0 0 0
Commodity Trading | Other Derivative Instruments        
Consideration of Credit Risk and Concentrations [Abstract]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 0 0 0 0
Pre-tax (gains) losses reclassified into income during the period from regulatory assets and (liabilities) 0 0 0 0
Pre-tax gains (losses) recognized during the period in income [2] 10 (1) 14 (5)
Natural Gas | Other Derivative Instruments        
Consideration of Credit Risk and Concentrations [Abstract]        
Pre-tax fair value gains (losses) recognized during the period in regulatory (assets) and liabilities $ 0 $ 0 0 0
Interest Rate | Designated as Hedging Instrument | Cash Flow Hedges        
Consideration of Credit Risk and Concentrations [Abstract]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net [3]     1 1
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     $ 0 $ 0
Credit Concentration Risk        
Consideration of Credit Risk and Concentrations [Abstract]        
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 10   10  
Credit Concentration Risk | Municipal or Cooperative Entities or Other Utilities [Member]        
Consideration of Credit Risk and Concentrations [Abstract]        
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 7   7  
Credit Concentration Risk | External Credit Rating, Investment Grade [Member]        
Consideration of Credit Risk and Concentrations [Abstract]        
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 5   5  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties $ 80   $ 80  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 49.00%   49.00%  
Credit Concentration Risk | No Investment Grade Ratings from External Credit Rating Agencies [Member]        
Consideration of Credit Risk and Concentrations [Abstract]        
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 4   4  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties $ 16   $ 16  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 10.00%   10.00%  
Credit Concentration Risk | External Credit Rating, Non Investment Grade [Member]        
Consideration of Credit Risk and Concentrations [Abstract]        
Number of most significant counterparties for wholesale, trading and non-trading commodity activities with credit exposure | Counterparty 1   1  
Wholesale, trading and non-trading commodity credit exposure for the most significant counterparties $ 46   $ 46  
Percentage of wholesale, trading and non-trading commodity credit exposure for the most significant counterparties (in hundredths) 28.00%   28.00%  
[1] Amounts for both the three and six months ended June 30, 2021 and 2020, included no settlement gains or losses on derivatives entered to mitigate natural gas price risk for electric generation recorded to electric fuel and purchased power, subject to cost-recovery mechanisms and reclassified to a regulatory asset, as appropriate. The derivative settlement losses for both the three and six months ended June 30, 2021 and 2020 relate to natural gas operations and were recorded to cost of natural gas sold and transported. These gains and losses are subject to cost-recovery mechanisms and reclassified out of income to a regulatory asset or liability, as appropriate.
[2] 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.
[3] Amounts are recorded to interest charges.