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Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Assets:    
Cash equivalents $ 60 $ 30
Debt securities:    
Domestic government 10 11
Corporate credit 6 6
Money market funds measured at NAV (2) 26 25
Non-qualified benefit plan trust: (2)    
Money market funds 1 1
Eqauity securities - domestic 6 7
Debt securities—domestic government 1 1
Assets from price risk management activities: (1) (3)    
Electricity 2 3
Natural gas 2 3
Total 26 25
Assets, Fair Value Disclosure 114 87
Liabilities from price risk management activities: (1) (3)    
Electricity 134 135
Natural gas 77 75
Total 211 210
Fair Value, Inputs, Level 1 [Member]    
Assets:    
Cash equivalents 60 30
Debt securities:    
Domestic government 3 4
Corporate credit 0 0
Non-qualified benefit plan trust: (2)    
Money market funds 1 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 71 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]    
Assets:    
Cash equivalents 0 0
Debt securities:    
Domestic government 7 7
Corporate credit 6 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 2 3
Natural gas 2 3
Total 17 19
Liabilities from price risk management activities: (1) (3)    
Electricity 11 5
Natural gas 66 66
Total 77 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 0 0
Natural gas 0 0
Total 0 0
Liabilities from price risk management activities: (1) (3)    
Electricity 123 130
Natural gas 11 9
Total $ 134 $ 139