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Future year net unrealized gain/loss recorded at balance sheet date expected to become realized (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Electricity [Member]  
Commodity contracts:  
Other Commitment, Due in Next Twelve Months $ 10
Other Commitment, Due in Second Year 8
Other Commitment, Due in Third Year 8
Other Commitment, Due in Fourth Year 8
Other Commitment, Due in Fifth Year 7
Other Commitment, Due after Fifth Year 91
Total 132
Natural Gas [Member]  
Commodity contracts:  
Other Commitment, Due in Next Twelve Months 43
Other Commitment, Due in Second Year 20
Other Commitment, Due in Third Year 7
Other Commitment, Due in Fourth Year 2
Other Commitment, Due in Fifth Year 0
Other Commitment, Due after Fifth Year 0
Total 72
Unrealized Gain Loss On Derivatives [Member]  
Commodity contracts:  
Other Commitment, Due in Next Twelve Months 53
Other Commitment, Due in Second Year 28
Other Commitment, Due in Third Year 15
Other Commitment, Due in Fourth Year 10
Other Commitment, Due in Fifth Year 7
Other Commitment, Due after Fifth Year 91
Total $ 204