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Retirement Benefits (Schedule Of Pooled Funds Components) (Details) | Dec. 31, 2015 | Dec. 31, 2014
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Pension Plan [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
U.S. equity securities | 62.00% | 64.00%
Foreign equity securities | 18.00% | 16.00%
U.S. fixed-income securities | 11.00% | 9.00%
Foreign fixed-income securities | 6.00% | 6.00%
Alternative investments | 3.00% | 5.00%
Pension Plan [Member] | Consumers Energy Company [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
U.S. equity securities | 62.00% | 64.00%
Foreign equity securities | 18.00% | 16.00%
U.S. fixed-income securities | 11.00% | 9.00%
Foreign fixed-income securities | 6.00% | 6.00%
Alternative investments | 3.00% | 5.00%
OPEB [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
U.S. equity securities | 58.00% | 62.00%
Foreign equity securities | 13.00% | 12.00%
U.S. fixed-income securities | 22.00% | 18.00%
Foreign fixed-income securities | 5.00% | 5.00%
Alternative investments | 2.00% | 3.00%
OPEB [Member] | Consumers Energy Company [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
U.S. equity securities | 58.00% | 62.00%
Foreign equity securities | 13.00% | 12.00%
U.S. fixed-income securities | 22.00% | 18.00%
Foreign fixed-income securities | 5.00% | 5.00%
Alternative investments | 2.00% | 3.00%

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