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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Sep. 30, 2014
Facilities
Hours
Megawatts
Sep. 30, 2013
Sep. 30, 2012
Aug. 09, 2013
Obligations        
Membership interests of VIE subject to mandatory redemption $ 50 $ 50   $ 40
Megawatts provided under power purchase obligations 1,621      
Remaining terms of the agreements, high end of range 18 years      
Power purchased under agreement 293 322 447  
Number of U.S. Army Corps of Engineers hydroelectric facilities contracted with to obtain power 8      
Agreement termination notice period 3 years      
Minimum required hours of power 1,500      
Megawatt allocation 405      
Maximum term length for the purchase and transportation of coal 6 years      
Maximum term for purchase of enriched uranium and fabrication of nuclear fuel assemblies 16 years      
Purchase Agreements Required by Federal Law
        
Obligations        
Megawatts provided under power purchase obligations 890      
Number of suppliers 6      
Purchase Obligations - Coal
        
Obligations        
Long-term purchase commitment 1,600      
Purchase Obligations - Nuclear Fuel
        
Obligations        
Long-term purchase commitment 3,700      
Purchase Obligations - Other
        
Obligations        
Long-term purchase commitment $ 2,752