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Fair Value of Financial Instruments Financial Assets and Liabilities Recognized at Fair Value (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Assets:    
Cash equivalents $ 0 $ 255
Debt securities:    
Domestic government 17 20
Corporate credit 13 13
Money market funds measured at NAV (2) 13 12
Non-qualified benefit plan trust: (2)    
Money market funds 2 1
Equity securities - domestic 8 7
Debt securities—domestic government 1 1
Assets from price risk management activities: (1) (3)    
Electricity 40 8
Natural gas 119 37
Total 13 12
Assets, Fair Value Disclosure 213 354
Liabilities from price risk management activities: (1) (3)    
Electricity 123 146
Natural gas 1 5
Total 124 151
Fair Value, Inputs, Level 1 [Member]    
Assets:    
Cash equivalents 0 255
Debt securities:    
Domestic government 9 9
Corporate credit 0 0
Non-qualified benefit plan trust: (2)    
Money market funds 2 1
Equity securities - domestic 8 7
Debt securities—domestic government 1 1
Assets from price risk management activities: (1) (3)    
Electricity 0 0
Natural gas 0 0
Total 20 273
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 8 11
Corporate credit 13 13
Non-qualified benefit plan trust: (2)    
Money market funds 0 0
Equity securities - domestic 0 0
Debt securities—domestic government 0 0
Assets from price risk management activities: (1) (3)    
Electricity 33 4
Natural gas 111 36
Total 165 64
Liabilities from price risk management activities: (1) (3)    
Electricity 50 5
Natural gas 1 4
Total 51 9
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
Equity securities - domestic 0 0
Debt securities—domestic government 0 0
Assets from price risk management activities: (1) (3)    
Electricity 7 4
Natural gas 8 1
Total 15 5
Liabilities from price risk management activities: (1) (3)    
Electricity 73 141
Natural gas 0 1
Total $ 73 $ 142