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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Sep. 30, 2013
Hours
Facilities
Megawatts
Sep. 30, 2012
Sep. 30, 2011
Aug. 09, 2013
Obligations        
Membership interests of VIE subject to mandatory redemption $ 40 $ 0   $ 40
Megawatts provided under power purchase obligations 1,621      
Remaining terms of the agreements, high end of range 19 years      
Power purchased under agreement 322 447 713  
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      
Number of hydroelectic facilities with reduced production 2      
Maximum term length for the purchase and transportation of coal 10 years      
Long-term purchase commitment 149 148    
Maximum term for purchase of enriched uranium and fabrication of nuclear fuel assemblies 17 years      
Purchase Agreements Required by Federal Law
        
Obligations        
Megawatts provided under power purchase obligations 913      
Number of suppliers 6      
Purchase Obligations - Coal [Member]
        
Obligations        
Long-term purchase commitment 2,300      
Purchase Obligations - Nuclear Fuel [Member]
        
Obligations        
Long-term purchase commitment 4,000      
Purchase Obligations - Other
        
Obligations        
Long-term purchase commitment $ 2,554