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Fair Value of FInancial Instruments Schedule of Fair Value (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 18 $ 65
Domestic government, Debt securities 14 14
Corporate debt securities held in decommissioning trust assets 8 11
Fair Value - Money market funds 1  
Domestic Equity Securities 5 5
International Equity Securities 0 1
Debt securities - domestic government 1  
Electricity 7 5
Natural gas 3 2
Financial Instruments, Owned, at Fair Value 57 103
Liabilities from price risk management activities: [Abstract]    
Electricity 133 112
Natural gas 158 116
Liabilities, Fair Value Disclosure 291 228
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Domestic government, Debt securities 6 7
Corporate debt securities held in decommissioning trust assets 0 0
Fair Value - Money market funds 0  
Domestic Equity Securities 3 4
International Equity Securities 0 1
Debt securities - domestic government 1  
Electricity 0 0
Natural gas 0 0
Financial Instruments, Owned, at Fair Value 10 12
Liabilities from price risk management activities: [Abstract]    
Electricity 0 0
Natural gas 0 0
Liabilities, Fair Value Disclosure 0 0
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 18 65
Domestic government, Debt securities 8 7
Corporate debt securities held in decommissioning trust assets 8 11
Fair Value - Money market funds 1  
Domestic Equity Securities 2 1
International Equity Securities 0 0
Debt securities - domestic government 0  
Electricity 7 4
Natural gas 3 2
Financial Instruments, Owned, at Fair Value 47 90
Liabilities from price risk management activities: [Abstract]    
Electricity 28 32
Natural gas 144 95
Liabilities, Fair Value Disclosure 172 127
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Domestic government, Debt securities 0 0
Corporate debt securities held in decommissioning trust assets 0 0
Fair Value - Money market funds 0  
Domestic Equity Securities 0 0
International Equity Securities 0 0
Debt securities - domestic government 0  
Electricity 0 1
Natural gas 0 0
Financial Instruments, Owned, at Fair Value 0 1
Liabilities from price risk management activities: [Abstract]    
Electricity 105 80
Natural gas 14 21
Liabilities, Fair Value Disclosure 119 101
Non Qualified Benefit Plans [Member]    
Liabilities from price risk management activities: [Abstract]    
Insurance contracts, at cash surrender value $ 26 $ 26