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Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 6 Months Ended
--- | --- | ---
Feb. 28, 2015 | Oct. 31, 2014 | Jun. 30, 2015 | Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Postretirement benefits contributions |  |  | $ 33 | $ 4
Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Postretirement benefits contributions |  |  | $ 31 | $ 2
Consumers Energy Company [Member] | OPEB [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Postretirement benefits contributions | $ 29 | $ 25 |  |

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- Definition The amount of cash or cash equivalents contributed during the reporting period by the entity to fund its pension plans and its non-pension postretirement benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3536-108585 + Details
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