Selected Balance Sheet Data - Property, Plant and Equipment (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 4,046	$ 3,880
Less accumulated depreciation	(1,456)	(1,396)
Property, plant and equipment, net	2,590	2,484
Electric plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	3,287	3,197
Natural gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	411	391
Common and other property
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	226	221
Construction work in progress
Public Utility, Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 122	$ 71