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Commitments and Contingencies - Commitments (Details) - Fuel, capacity and transmission contract commitments, Not commercially operable [Member]
$ in Millions
Sep. 30, 2015
USD ($)
ContractualObligationFiscalYearMaturityScheduleTable [Line Items]  
Purchase Obligation, Due in Second Year $ 70
Purchase Obligation, Due in Third Year 112
Purchase Obligation, Due in Fourth Year 127
Purchase Obligation, Due in Fifth Year 127
Purchase Obligation, Due after Fifth Year $ 1,601