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Revenue (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Disaggregation of Revenue [Line Items] |  |  |
Unbilled receivables | $ 369 |  | $ 437
Consumers Energy Company |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Unbilled receivables | 369 |  | $ 437
Accounts Receivable |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | 6 | $ 5 |
Accounts Receivable | Consumers Energy Company |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | $ 6 | $ 5 |

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