Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) $ in Millions	Dec. 31, 2016 USD ($)
Electricity [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	$ 6
Other Commitment, Due in Second Year	7
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	77
Total	111
Natural Gas [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	20
Other Commitment, Due in Second Year	7
Other Commitment, Due in Third Year	6
Other Commitment, Due in Fourth Year	2
Other Commitment, Due in Fifth Year	0
Other Commitment, Due after Fifth Year	0
Total	35
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	26
Other Commitment, Due in Second Year	14
Other Commitment, Due in Third Year	13
Other Commitment, Due in Fourth Year	9
Other Commitment, Due in Fifth Year	7
Other Commitment, Due after Fifth Year	77
Total	$ 146