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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 |

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