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Revenue (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |
Uncollectable expense | $ 6 | $ 7 | $ 12 | $ 14 |
Consumers Energy Company |  |  |  |  |
Revenue, Initial Application Period Cumulative Effect Transition [Line Items] |  |  |  |  |
Uncollectable expense | 6 | $ 7 | 12 | $ 14 |
Unbilled receivables | $ 257 |  | $ 257 |  | $ 409

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