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Fair Value of Financial Instruments Fair Value of Financial Instruments (Derivative Fair Value Measurement - Details 4)
$ in Millions
Dec. 31, 2016
USD ($)
$ / T
$ / MWh
Dec. 31, 2015
USD ($)
$ / T
$ / MWh
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value Determined Using Valuation Techniques, Percentage 7.00%  
Derivative Liability, Fair Value Determined Using Valuation Techniques, Percentage 12.00%  
Fair Value Assets, Measured on Recurring Basis, Valuation Techniques, Impact of Credit Reserve to Fair Value $ 11 $ 5
Fair Value Assets Measured On Recurring Basis Valuation Techniques Impact Of Credit Reserve To Fair Value Included In Oci Derivative Contracts 2  
Fair Value Assets, Measured on Recurring Basis, Valuation Techniques, Impact of Credit Reserve to Fair Value Included in Operating Revenues and Cost of Operations, Derivative Contracts 3  
Derivative Asset, Fair Value, Gross Asset 1,251 2,220
Derivative Liability, Fair Value, Gross Liability 1,378 2,214
Derivative, Collateral, Right to Reclaim Cash 203 568
Cash collateral received in support of energy risk management activities 2 106
Commodity contracts    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value, Gross Asset 1,202 2,220
Derivative Liability, Fair Value, Gross Liability 1,290 2,086
Derivative, Collateral, Right to Reclaim Cash 14 271
Cash collateral received in support of energy risk management activities 1 113
Commodity contracts | Fair Value, Measurements, Recurring [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value, Gross Asset 1,202 2,220
Derivative Liability, Fair Value, Gross Liability 1,290 2,086
Commodity contracts | Fair Value, Measurements, Recurring [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value, Gross Asset 92 149
Derivative Liability, Fair Value, Gross Liability 161 182
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value, Gross Asset 40 86
Derivative Liability, Fair Value, Gross Liability $ 107 $ 100
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Minimum [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Forward Price | $ / MWh 11,000,000 10,000,000
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Maximum [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Forward Price | $ / MWh 104,000,000 92,000,000
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Weighted Average [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Forward Price | $ / MWh 31,000,000 27,000,000
Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value, Gross Asset $ 0 $ 0
Derivative Liability, Fair Value, Gross Liability $ 1 $ 12
Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Minimum [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Forward Price | $ / T 42,000,000 28,000,000
Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Maximum [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Forward Price | $ / T 51,000,000 45,000,000
Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Weighted Average [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Forward Price | $ / T 45,000,000 35,000,000
Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative Asset, Fair Value, Gross Asset $ 52 $ 63
Derivative Liability, Fair Value, Gross Liability $ 53 $ 70
Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Minimum [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Auction Price | $ / MWh (22,000,000) (98,000,000)
Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Maximum [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Auction Price | $ / MWh 17,000,000 87,000,000
Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Weighted Average [Member] | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative, Auction Price | $ / MWh 0 0