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Fair Value of Financial Instruments Fair Value of Financial Instruments (Details 4 - Derivative FV Measurements)
$ / T in Millions, $ / MWh in Millions, $ in Millions
Mar. 31, 2016
USD ($)
$ / T
$ / MWh
Dec. 31, 2015
USD ($)
$ / T
$ / MWh
Mar. 31, 2015
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Total derivative assets valued with prices provied by models and other valuation techniques (as a percent) 6.00%    
Total derivative liabilities valued with prices provied by models and other valuation techniques (as a percent) 7.00%    
Derivative Asset, Fair Value, Gross Asset $ 2,578 $ 2,220  
Derivative Liability, Fair Value, Gross Liability 2,574 2,214  
Valuation Allowances and Reserves, Balance 5   $ 5
Increase in fair value as a result of the credit reserve, portion recorded in OCI 3   3
Increase in fair value as a result of the credit reserve, portion recorded in operating revenue and cost of operations 2   $ 2
Commodity contracts      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset, Fair Value, Gross Asset 2,578 2,220  
Derivative Liability, Fair Value, Gross Liability 2,389 2,086  
Commodity contracts | Fair Value, Measurements, Recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset, Fair Value, Gross Asset 2,578 2,220  
Derivative Liability, Fair Value, Gross Liability 2,389 2,086  
Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset, Fair Value, Gross Asset 164 149  
Derivative Liability, Fair Value, Gross Liability 181 182  
Power Contracts [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset, Fair Value, Gross Asset 120 86  
Derivative Liability, Fair Value, Gross Liability 115 100  
Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset, Fair Value, Gross Asset 0 0  
Derivative Liability, Fair Value, Gross Liability 15 12  
Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative Asset, Fair Value, Gross Asset 44 63  
Derivative Liability, Fair Value, Gross Liability $ 51 $ 70  
Minimum [Member] | Power Contracts [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Forward Price | $ / MWh 9 10  
Minimum [Member] | Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Forward Price | $ / T 28 28  
Minimum [Member] | Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Auction Price | $ / MWh (62) (98)  
Maximum [Member] | Power Contracts [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Forward Price | $ / MWh 95 92  
Maximum [Member] | Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Forward Price | $ / T 41 45  
Maximum [Member] | Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Auction Price | $ / MWh 51 87  
Weighted Average [Member] | Power Contracts [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Forward Price | $ / MWh 25 27  
Weighted Average [Member] | Coal Contract [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Forward Price | $ / T 33 35  
Weighted Average [Member] | Financial Transmission Rights [Member] | Commodity contracts | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Derivative, Auction Price | $ / MWh 0 0