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Revenue (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | $ 90 | $ 67 | $ 54
Regulatory assets | 2,653 | 2,489 |
Unbilled receivables | 437 | 426 |
Regulatory liabilities | 151 | 87 |
Consumers Energy Company |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense |  |  | 29
Regulatory assets | 2,653 | 2,489 |
Unbilled receivables | 437 | 426 |
Regulatory liabilities | 151 | 87 |
Consumers Energy Company | Reserve for customer refunds |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Regulatory liabilities | 28 | 2 |
Consumers Energy Company | COVID-19 costs accounting deferral |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Regulatory assets | 4 | 0 |
Accounts Receivable |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | 33 | 29 | 29
Accounts Receivable | Consumers Energy Company |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | $ 33 | $ 29 | $ 29

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