Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) $ in Millions	Dec. 31, 2015 USD ($)
Commodity contracts:
Other Commitment, Due in Next Twelve Months	$ 15
Other Commitment, Due in Second Year	11
Other Commitment, Due in Third Year	7
Total	33
Electricity [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	29
Other Commitment, Due in Second Year	8
Other Commitment, Due in Third Year	7
Other Commitment, Due in Fourth Year	7
Other Commitment, Due in Fifth Year	6
Other Commitment, Due after Fifth Year	69
Total	126
Natural Gas [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	91
Other Commitment, Due in Second Year	50
Other Commitment, Due in Third Year	12
Other Commitment, Due in Fourth Year	2
Other Commitment, Due in Fifth Year	0
Other Commitment, Due after Fifth Year	0
Total	155
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	120
Other Commitment, Due in Second Year	58
Other Commitment, Due in Third Year	19
Other Commitment, Due in Fourth Year	9
Other Commitment, Due in Fifth Year	6
Other Commitment, Due after Fifth Year	69
Total	$ 281