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Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 1 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Feb. 28, 2015 | Oct. 31, 2014
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Postretirement benefits contributions | $ 31 us-gaap_PensionAndOtherPostretirementBenefitContributions | $ 2 us-gaap_PensionAndOtherPostretirementBenefitContributions |  |
Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Postretirement benefits contributions | 30 us-gaap_PensionAndOtherPostretirementBenefitContributions / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember | 1 us-gaap_PensionAndOtherPostretirementBenefitContributions / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember |  |
Consumers Energy Company [Member] | OPEB [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Postretirement benefits contributions |  |  | $ 29 us-gaap_PensionAndOtherPostretirementBenefitContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember | $ 25 us-gaap_PensionAndOtherPostretirementBenefitContributions / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember

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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=31042434&loc=d3e3536-108585 + Details
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