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Investments Accounted for by the Equity Method and Variable Interest Entities - Other Equity Investments (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2020
USD ($)
MW
Dec. 31, 2019
USD ($)
Schedule of Equity Method Investments [Line Items]    
Generation capacity (in MW) | MW 23,000  
Equity investments in affiliates $ 346 $ 388
Gladstone    
Schedule of Equity Method Investments [Line Items]    
Ownership percentage 37.50%  
Generation capacity (in MW) | MW 1,613  
Equity investments in affiliates $ 131  
Home Solar    
Schedule of Equity Method Investments [Line Items]    
Cash received $ 66