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Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Assets:    
Cash equivalents $ 170 $ 30
Debt securities:    
Domestic government 24 11
Corporate credit 12 6
Money market funds measured at NAV (2) 6 25
Non-qualified benefit plan trust: (2)    
Money market funds 3 1
Eqauity securities - domestic 6 7
Debt securities—domestic government 1 1
Assets from price risk management activities: (1) (3)    
Electricity 5 3
Natural gas 1 3
Total 6 25
Assets, Fair Value Disclosure 234 87
Liabilities from price risk management activities: (1) (3)    
Electricity 116 135
Natural gas 50 75
Total 166 210
Fair Value, Inputs, Level 1 [Member]    
Assets:    
Cash equivalents 170 30
Debt securities:    
Domestic government 8 4
Corporate credit 0 0
Non-qualified benefit plan trust: (2)    
Money market funds 3 1
Eqauity securities - domestic 6 7
Debt securities—domestic government 1 1
Assets from price risk management activities: (1) (3)    
Electricity 0 0
Natural gas 0 0
Total 188 43
Liabilities from price risk management activities: (1) (3)    
Electricity 0 0
Natural gas 0 0
Total 0 0
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset 6  
Assets:    
Cash equivalents 0 0
Debt securities:    
Domestic government 16 7
Corporate credit 12 6
Non-qualified benefit plan trust: (2)    
Money market funds 0 0
Eqauity securities - domestic 0 0
Debt securities—domestic government 0 0
Assets from price risk management activities: (1) (3)    
Electricity 4 3
Natural gas 1 3
Total 39 19
Liabilities from price risk management activities: (1) (3)    
Electricity 4 5
Natural gas 36 66
Total 40 71
Fair Value, Inputs, Level 3 [Member]    
Assets:    
Cash equivalents 0 0
Debt securities:    
Domestic government 0 0
Corporate credit 0 0
Non-qualified benefit plan trust: (2)    
Money market funds 0 0
Eqauity securities - domestic 0 0
Debt securities—domestic government 0 0
Assets from price risk management activities: (1) (3)    
Electricity 1 0
Natural gas 0 0
Total 1 0
Liabilities from price risk management activities: (1) (3)    
Electricity 112 130
Natural gas 14 9
Total $ 126 $ 139