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Revenue (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Disaggregation of Revenue [Line Items] |  |  |  |  |
Unbilled receivables | $ 340 |  | $ 340 |  | $ 437
Consumers Energy Company |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Unbilled receivables | 340 |  | 340 |  | $ 437
Accounts Receivable |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Bad debt expense | 6 | $ 5 | 17 | $ 18 |
Accounts Receivable | Consumers Energy Company |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Bad debt expense | $ 6 | $ 5 | $ 17 | $ 18 |

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