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Risk Management Fair values of price risk management assets and liabilities (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Current Assets, Commodity Contracts:    
Electricity $ 34 $ 4
Natural gas 160 29
Total current derivative assets 194 33
Noncurrent Assets, Commodity Contracts: [Abstract]    
Commodity Contract Asset, Noncurrent, Electricity 9 4
Commodity Contract Asset, Noncurrent, Natural Gas 69 8
Derivative Asset, Noncurrent 78 12
Total derivative assets 272 45
Current Liabilities, Commodity Contracts: [Abstract]    
Electricity 37 13
Natural gas 2 2
Total current derivative liabilities 39 15
Noncurrent Liabilities, Commodity Contracts: [Abstract]    
Electricity 89 133
Natural gas 0 3
Total noncurrent derivative liabilities 89 136
Total derivative liabilities $ 128 $ 151