Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) $ in Millions	Dec. 31, 2020 USD ($)
Electricity [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	$ (9)
Other Commitment, Due in Second Year	(4)
Other Commitment, Due in Third Year	(8)
Other Commitment, Due in Fourth Year	8
Other Commitment, Due in Fifth Year	9
Other Commitment, Due after Fifth Year	100
Total	(138)
Natural Gas [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	(27)
Other Commitment, Due in Second Year	(5)
Other Commitment, Due in Third Year	0
Other Commitment, Due in Fourth Year	0
Other Commitment, Due in Fifth Year	0
Other Commitment, Due after Fifth Year	0
Total	(32)
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	(18)
Other Commitment, Due in Second Year	(1)
Other Commitment, Due in Third Year	(8)
Other Commitment, Due in Fourth Year	8
Other Commitment, Due in Fifth Year	9
Other Commitment, Due after Fifth Year	100
Total	$ (106)