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Summary of Significant Accounting Policies - Activity in the Allowance for Credit Losses (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Accounts Receivable, Allowance for Credit Loss [Roll Forward]    
Beginning balance $ 67 $ 43
Acquired balance from Direct Energy 112 0
Provision for credit losses 611 24
Write-offs (48) (32)
Recoveries collected 7 4
Ending balance 749 $ 39
Increase in provision for credit losses due finance hedge risk 393  
Increase in provision due to counterparty credit Risk 109  
Increase in provision for credit Loss due to shortfall payments $ 83