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Commitments and Contingencies - Letters of Credit (Details) - Genesis [Member] - Standby Letters of Credit [Member]
$ in Millions
Sep. 30, 2015
USD ($)
Line of Credit Facility [Line Items]  
Credit facility, maximum borrowing capacity $ 83
Credit facility, amount outstanding 78
PPA Security [Member]  
Line of Credit Facility [Line Items]  
Credit facility, amount outstanding 25
Large Generator Interconnection Agreement [Member]  
Line of Credit Facility [Line Items]  
Credit facility, amount outstanding 8
Operations & Maintenance Reserve [Member]  
Line of Credit Facility [Line Items]  
Credit facility, amount outstanding 10
Debt service reserve [Member]  
Line of Credit Facility [Line Items]  
Credit facility, amount outstanding $ 35