Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Commodity contracts:
Other Commitment, Due in Next Twelve Months	$ 14
Other Commitment, Due in Second Year	11
Other Commitment, Due in Third Year	11
Other Commitment, Due in Fourth Year	7
Total	43
Electricity [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	50
Other Commitment, Due in Second Year	19
Other Commitment, Due in Third Year	6
Other Commitment, Due in Fourth Year	5
Other Commitment, Due in Fifth Year	5
Other Commitment, Due after Fifth Year	22
Total	107
Natural Gas [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	49
Other Commitment, Due in Second Year	44
Other Commitment, Due in Third Year	18
Other Commitment, Due in Fourth Year	3
Other Commitment, Due in Fifth Year	0
Other Commitment, Due after Fifth Year	0
Total	114
Unrealized Gain Loss On Derivatives [Member]
Commodity contracts:
Other Commitment, Due in Next Twelve Months	99
Other Commitment, Due in Second Year	63
Other Commitment, Due in Third Year	24
Other Commitment, Due in Fourth Year	8
Other Commitment, Due in Fifth Year	5
Other Commitment, Due after Fifth Year	22
Total	$ 221