Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) $ in Millions	Dec. 31, 2018 USD ($)
Electricity [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	$ 4
Other Commitment, Due in Second Year	6
Other Commitment, Due in Third Year	6
Other Commitment, Due in Fourth Year	6
Other Commitment, Due in Fifth Year	6
Other Commitment, Due after Fifth Year	55
Total	83
Natural Gas [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	28
Other Commitment, Due in Second Year	14
Other Commitment, Due in Third Year	6
Other Commitment, Due in Fourth Year	1
Other Commitment, Due in Fifth Year	0
Other Commitment, Due after Fifth Year	0
Total	49
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	32
Other Commitment, Due in Second Year	20
Other Commitment, Due in Third Year	12
Other Commitment, Due in Fourth Year	7
Other Commitment, Due in Fifth Year	6
Other Commitment, Due after Fifth Year	55
Total	$ 132