Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) $ in Millions	Dec. 31, 2019 USD ($)
Electricity [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	$ (5)
Other Commitment, Due in Second Year	(1)
Other Commitment, Due in Third Year	(7)
Other Commitment, Due in Fourth Year	7
Other Commitment, Due in Fifth Year	7
Other Commitment, Due after Fifth Year	76
Total	(103)
Natural Gas [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	(7)
Other Commitment, Due in Second Year	(2)
Other Commitment, Due in Third Year	(1)
Other Commitment, Due in Fourth Year	0
Other Commitment, Due in Fifth Year	0
Other Commitment, Due after Fifth Year	0
Total	(10)
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	(2)
Other Commitment, Due in Second Year	(1)
Other Commitment, Due in Third Year	(6)
Other Commitment, Due in Fourth Year	7
Other Commitment, Due in Fifth Year	7
Other Commitment, Due after Fifth Year	76
Total	$ (93)