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Commitments and Contingencies Commitments and Contingencies - Purchase Obligations (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2016
USD ($)
Megawatts
Suppliers
Sep. 30, 2015
USD ($)
Sep. 30, 2014
USD ($)
Obligations      
Megawatts provided under power purchase obligations | Megawatts 2,227    
Remaining terms of the agreements, high end of range 16 years    
Megawatts provided under transmission obligations | Megawatts 1,450    
Power purchased under agreement $ 261 $ 262 $ 264
Maximum term length for the purchase and transportation of coal 4 years    
Maximum term length for the purchase, storage, and transportation of natural gas 20 years    
Maximum term for purchase of enriched uranium and fabrication of nuclear fuel assemblies 14 years    
Fuel purchased under agreement $ 1,500 973 718
Other purchased under agreement 174 93 $ 91
CT and QTE outstanding leaseback obligation $ 467 $ 616  
Purchase Agreements Required by Federal Law      
Obligations      
Megawatts provided under power purchase obligations | Megawatts 259    
Number of suppliers | Suppliers 25    
Purchase Obligations - Coal      
Obligations      
Long-term purchase commitment $ 900    
Purchase Obligations - Natural Gas      
Obligations      
Long-term purchase commitment 1,300    
Purchase Obligations - Fuel      
Obligations      
Long-term purchase commitment [1] 4,455    
Purchase Obligations - Nuclear Fuel      
Obligations      
Long-term purchase commitment 2,300    
Purchase Obligations - Other      
Obligations      
Long-term purchase commitment [2] $ 896    
[1] (4) Includes commitments to purchase nuclear fuel, coal, and natural gas, as well as related transportation and storage services.
[2] (5) Primarily includes long-term service contracts, contracts that contain minimum purchase levels for the purchase of limestone along with related storage and transportation, and contractual obligations related to load control programs.