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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details)
$ in Millions
Dec. 31, 2020
USD ($)
$ / MWh
Dec. 31, 2019
USD ($)
$ / MWh
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Cash collateral posted in support of energy risk management activities $ 50 $ 190
Cash collateral received in support of energy risk management activities 19 32
Commodity contracts    
Assets    
Derivative assets 821 1,170
Liabilities    
Commodity contracts 884 1,103
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Cash collateral posted in support of energy risk management activities 0 73
Cash collateral received in support of energy risk management activities 5 7
Commodity contracts | Level 3    
Assets    
Derivative assets 139 193
Liabilities    
Commodity contracts 155 155
Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Assets    
Derivative assets 139 193
Liabilities    
Commodity contracts 155 155
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3    
Assets    
Derivative assets 111 151
Liabilities    
Commodity contracts $ 143 $ 139
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Low    
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Forward Market Price (USD per MWh) | $ / MWh 10 8
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | High    
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Forward Market Price (USD per MWh) | $ / MWh 105 218
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Forward Market Price (USD per MWh) | $ / MWh 21 24
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3    
Assets    
Derivative assets $ 28 $ 42
Liabilities    
Commodity contracts $ 12 $ 16
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Low    
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Auction Prices (USD per MWh) | $ / MWh (28) (105)
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | High    
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Auction Prices (USD per MWh) | $ / MWh 43 213
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Weighted Average    
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Auction Prices (USD per MWh) | $ / MWh 0 0