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Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]        
Beginning balance $ 761 $ 47 $ 67 $ 43
Acquired balance from Direct Energy 0 0 112 0
Provision for credit losses 64 26 715 74
Write-offs (41) (19) (124) (71)
Recoveries collected 8 3 22 11
Ending balance $ 792 $ 57 792 $ 57
Increase in provision for credit losses due finance hedge risk     403  
Increase in provision due to counterparty credit Risk     152  
Increase in provision for credit Loss due to shortfall payments     $ 83