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Fair Value of Financial Instruments - Derivative Fair Value Measurement (Details)
$ in Millions
Dec. 31, 2018
USD ($)
$ / MWh
Dec. 31, 2017
USD ($)
$ / MWh
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Cash collateral posted in support of energy risk management activities $ 287 $ 171
Cash collateral received in support of energy risk management activities 33 37
Commodity contracts    
Assets    
Derivative assets 1,042 744
Liabilities    
Derivative liabilities 977 674
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Cash collateral posted in support of energy risk management activities 114 72
Cash collateral received in support of energy risk management activities 31 11
Commodity contracts | Fair Value, Measurements, Recurring    
Assets    
Derivative assets 1,042 744
Liabilities    
Derivative liabilities 977 674
Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Assets    
Derivative assets 109 44
Liabilities    
Derivative liabilities 89 59
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3    
Assets    
Derivative assets 89 33
Liabilities    
Derivative liabilities $ 75 $ 47
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Low    
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Forward Market Price (USD per MWh) | $ / MWh 1 10
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | High    
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Forward Market Price (USD per MWh) | $ / MWh 214 142
Commodity contracts | Fair Value, Measurements, Recurring | Power Contracts | Level 3 | Weighted Average    
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Forward Market Price (USD per MWh) | $ / MWh 31 24
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3    
Assets    
Derivative assets $ 20 $ 11
Liabilities    
Derivative liabilities $ 14 $ 12
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Low    
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Auction Prices (USD per MWh) | $ / MWh (90) (28)
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | High    
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Auction Prices (USD per MWh) | $ / MWh 34 46
Commodity contracts | Fair Value, Measurements, Recurring | Financial Transmission Rights | Level 3 | Weighted Average    
Fair Value Inputs [Abstract] (Deprecated 2018-01-31)    
Auction Prices (USD per MWh) | $ / MWh 0 0