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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2015
USD ($)
Megawatts
Hours
Suppliers
Facilities
Sep. 30, 2014
USD ($)
Sep. 30, 2013
USD ($)
Obligations      
Megawatts provided under power purchase obligations | Megawatts 1,255    
Remaining terms of the agreements, high end of range 17 years    
Power purchased under agreement $ 230 $ 237 $ 267
Number of U.S. Army Corps of Engineers hydroelectric facilities contracted with to obtain power | Facilities 8    
Agreement termination notice period 3 years    
Minimum required hours of power | Hours 1,500    
Megawatt allocation | Megawatts 405    
Maximum term length for the purchase and transportation of coal 5 years    
Maximum term for purchase of enriched uranium and fabrication of nuclear fuel assemblies 15 years    
Purchase Agreements Required by Federal Law      
Obligations      
Megawatts provided under power purchase obligations | Megawatts 882    
Number of suppliers | Suppliers 24    
Purchase Obligations - Coal      
Obligations      
Long-term purchase commitment $ 1,500    
Purchase Obligations - Nuclear Fuel      
Obligations      
Long-term purchase commitment 3,400    
Purchase Obligations - Other      
Obligations      
Long-term purchase commitment $ 2,845