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Revenue (Narrative) (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) product | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | $ 54 | $ 49 | $ 50
Unbilled receivables | $ 0 | 481 |
EnerBank |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Threshold period past due when company ceases recognizing interest income | 120 days |  |
EnerBank | Consumer Loan |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Number of products | product | 4 |  |
EnerBank | Same As Cash Program |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Description of timing of revenue recognition | EnerBank recognizes the fee on a straight-line basis over the same-as-cash period, which typically ranges from three to 24 months. |  |
EnerBank | Zero Interest and Reduced Interest Programs |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Description of timing of revenue recognition | EnerBank recognizes the fee using the interest method over the term of the loan, which ranges from one to 12 years. |  |
Consumers Energy Company |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense |  | 29 | $ 31
Unbilled receivables | $ 409 | $ 481 |
Consumers Energy Company | Alternative Revenue Programs |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Financial incentive as percentage of energy waste reduction program spending | 20.00% |  |
Accounts Receivable |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | $ 29 |  |
Accounts Receivable | Consumers Energy Company |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Bad debt expense | $ 29 |  |

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