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Commitments and Contingencies - Purchase Obligations (Details)
$ in Millions
 12 Months Ended
Sep. 30, 2020
USD ($)
Megawatts
megawatts
Sep. 30, 2019
USD ($)
Sep. 30, 2018
USD ($)
Obligations      
Megawatts provided under power purchase obligations 2,384    
Remaining terms of the agreements, high end of range 12 years    
Megawatts provided under transmission obligations | megawatts 1,450    
Power purchased under agreement | $ $ 202 $ 195 $ 188
Purchase Agreements Required by Federal Law      
Obligations      
Megawatts provided under power purchase obligations 268    
Number of generation sources under PPAs 111