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Supplemental Information (Allowance for Doubtful Accounts) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2022 | Jun. 30, 2021 | Jun. 30, 2022 | Jun. 30, 2021 | Dec. 31, 2021
Allowance for Doubtful Accounts Receivable [Roll Forward] |  |  |  |  |
Beginning of period | $ 28 | $ 47 | $ 29 | $ 50 |
Bad debt expense | 7 | (3) | 11 | 1 |
Net write-offs | (5) | (2) | (10) | (9) |
End of period | 30 | 42 | 30 | 42 |
Payables for purchased receivables | 32 |  | 32 |  | $ 27
Union Electric Company |  |  |  |  |
Allowance for Doubtful Accounts Receivable [Roll Forward] |  |  |  |  |
Beginning of period | 11 | 15 | 13 | 16 |
Bad debt expense | 2 | 2 | 3 | 3 |
Net write-offs | (1) | (1) | (4) | (3) |
End of period | 12 | 16 | 12 | 16 |
Ameren Illinois Company |  |  |  |  |
Allowance for Doubtful Accounts Receivable [Roll Forward] |  |  |  |  |
Beginning of period | 17 | 32 | 16 | 34 |
Bad debt expense | 5 | (5) | 8 | (2) |
Net write-offs | (4) | (1) | (6) | (6) |
End of period | 18 | $ 26 | 18 | $ 26 |
Payables for purchased receivables | $ 32 |  | $ 32 |  | $ 27

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- Definition Amount of expense (reversal of expense) for expected credit loss on accounts receivable. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(5)) -URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227 + Details
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