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New Accounting Standards (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Dec. 31, 2016
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Accumulated Deficit |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | $ 0 | $ 8 | $ 0 | $ 0
Retirement benefits liability |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | 0 | (11) | 0 | 0
Retirement benefits liability | Accounting Standards Update 2018-02 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | (11) |  |
Consumers Energy Company | Accounting Standards Update 2016-01 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | 19 |  |
Consumers Energy Company | Accumulated Deficit |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | 0 | 19 | 0 | 0
Consumers Energy Company | Accumulated Deficit | Accounting Standards Update 2016-01 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | 19 |  |
Consumers Energy Company | Accumulated Other Comprehensive Loss |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle | $ (19) |  |  |  |
Consumers Energy Company | Retirement benefits liability |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | $ 0 | (5) | $ 0 | $ 0
Consumers Energy Company | Retirement benefits liability | Accounting Standards Update 2018-02 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  |  | $ (5) |  |

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