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Commitments and Contingencies Commitments and Contingencies - Table (Details)
$ in Millions
Sep. 30, 2018
USD ($)
megawatts
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Obligations      
Megawatts provided under transmission obligations | megawatts 1,450    
Accrual for Environmental Loss Contingencies, Gross $ 12 $ 7  
Estimated future decommissioning cost [1] 4,779 4,304 $ 4,052
2018 51    
2019 51    
2020 51    
2021 51    
2022 51    
Thereafter 468    
Total 723    
2018 69    
2019 64    
2020 62    
2021 47    
2022 7    
Thereafter 1    
Total 250    
CT and QTE outstanding leaseback obligation 301 $ 338  
2018 49    
2019 50    
2020 207    
2021 25    
2022 0    
Thereafter 0    
Total $ 331    
[1] (1) The current portions of the ARO liability in the amounts of $115 million and $128 million as of September 30, 2018 and 2017, respectively, are included in Accounts payable and accrued liabilities.