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Retirement Benefits (Schedule Of Retirement Benefit Plan Assets (Liabilities)) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |
Non-current liabilities | $ 152 | $ 674
Consumers Energy Company |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Non-current liabilities | 112 | 622
DB Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Non-current assets | 136 | 104
Non-current liabilities | 0 | 531
DB Pension Plans | Consumers Energy Company |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Non-current assets | 138 | 109
Non-current liabilities | 0 | 517
OPEB Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Non-current assets | 440 | 344
OPEB Plan | Consumers Energy Company |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Non-current assets | 377 | 290
DB SERP |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Current liabilities | 10 | 10
Non-current liabilities | 150 | 140
DB SERP | Consumers Energy Company |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Current liabilities | 7 | 7
Non-current liabilities | $ 110 | $ 102

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