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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 31, 2018 | Feb. 28, 2018 | Jan. 31, 2018 | Oct. 31, 2017 | Sep. 30, 2017 | Mar. 31, 2017 | Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory liability current | $ 134 |  |  |  |  |  | $ 134 |  | $ 80
Revenue |  |  |  |  |  |  | 1,953 | $ 1,829 |
Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory liability current | 134 |  |  |  |  |  | 134 |  | 80
Revenue |  |  |  |  |  |  | 1,855 | $ 1,737 |
Revenue Subject to Refund - Tax Reform Rate Change [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory liability current | 51 |  |  |  |  |  | 51 |  |
Electric Rate Case [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Annual rate increase requested |  |  |  |  |  | $ 173 |  |  |
Annual rate increase requested, amended |  |  |  |  | $ 148 |  |  |  |
Rate of return on equity requested |  |  |  |  |  | 10.50% |  |  |
Annual rate increase self-implemented |  |  |  | $ 130 |  |  |  |  |
Annual rate increase authorized | $ 66 |  |  |  |  |  |  |  |
Rate of return on equity authorized | 10.00% |  |  |  |  |  |  |  |
Electric Rate Case [Member] | Revenue Subject to Refund - Other [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Regulatory liability current | $ 36 |  |  |  |  |  | 36 |  |
Gas Rate Case [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Recommended disallowance of cost recovery |  | $ (115) |  |  |  |  |  |  |
Electric Rate Case Tax Reform Rate Change [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Estimated change in rate |  |  | $ (116) |  |  |  |  |  |
Gas Rate Case Tax Reform Rate Change [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Annual rate increase requested | (48) |  |  |  |  |  |  |  |
Estimated change in rate |  |  | $ (49) |  |  |  |  |  |
Energy Waste Reduction Plan [Member] | Consumers Energy Company [Member] |  |  |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |  |  |
Requested recovery/collection | $ 31 |  |  |  |  |  | $ 31 |  |
Revenue |  |  |  |  |  |  |  |  | $ 31

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