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Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Current Assets, Commodity Contracts:    
Electricity $ 37 $ 4
Natural gas 88 29
Total current derivative assets 125 33
Noncurrent Assets, Commodity Contracts: [Abstract]    
Commodity Contract Asset, Noncurrent, Electricity 3 4
Commodity Contract Asset, Noncurrent, Natural Gas 31 8
Derivative Asset, Noncurrent 34 12
Total derivative assets 159 45
Current Liabilities, Commodity Contracts: [Abstract]    
Electricity 54 13
Natural gas 1 2
Total current derivative liabilities 55 15
Noncurrent Liabilities, Commodity Contracts: [Abstract]    
Electricity 69 133
Natural gas 0 3
Total noncurrent derivative liabilities 69 136
Total derivative liabilities $ 124 $ 151