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Selected Balance Sheet Data Selected Balance Sheet Data, Inventories (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Public Utilities, Inventory [Line Items]    
Total inventories $ 587 $ 535
Materials and supplies    
Public Utilities, Inventory [Line Items]    
Inventories 283 275
Fuel    
Public Utilities, Inventory [Line Items]    
Inventories 155 176
Natural gas    
Public Utilities, Inventory [Line Items]    
Inventories $ 149 $ 84