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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details)
$ in Millions
Dec. 31, 2019
USD ($)
$ / MWh
Dec. 31, 2018
USD ($)
$ / MWh
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Cash collateral posted in support of energy risk management activities $ 190 $ 287
Cash collateral received in support of energy risk management activities 32 33
Commodity contracts    
Assets    
Derivative assets 1,170 1,042
Liabilities    
Commodity contracts 1,103 977
Fair Value Measurement Inputs and Valuation Techniques [Abstract    
Cash collateral posted in support of energy risk management activities 73 114
Cash collateral received in support of energy risk management activities 7 31
Commodity contracts | Level 3    
Assets    
Derivative assets 193 109
Liabilities    
Commodity contracts 155 89
Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Assets    
Derivative assets 193 109
Liabilities    
Commodity contracts 155 89
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3    
Assets    
Derivative assets 151 89
Liabilities    
Commodity contracts $ 139 $ 75
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 8 1
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 218 214
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 24 31
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3    
Assets    
Derivative assets $ 42 $ 20
Liabilities    
Commodity contracts $ 16 $ 14
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 (105) (90)
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 213 34
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