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Retirement Benefits (Schedule Of Major Components Of Actuarial Losses) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Pension Plan [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | $ 458 us-gaap_DefinedBenefitPlanActuarialGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember | $ (308)us-gaap_DefinedBenefitPlanActuarialGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Pension Plan [Member] | Discount Rate Changes [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 235 us-gaap_DefinedBenefitPlanActuarialGainLoss / cms_ActuarialAssumptionsAxis = cms_DiscountRateChangesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
Pension Plan [Member] | RP-2014 Mortality Table [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 150 us-gaap_DefinedBenefitPlanActuarialGainLoss / cms_ActuarialAssumptionsAxis = cms_RpCurrentYearMortalityTableMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember |
OPEB [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 230 us-gaap_DefinedBenefitPlanActuarialGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember | (440)us-gaap_DefinedBenefitPlanActuarialGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
OPEB [Member] | Consumers Energy Company [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 223 us-gaap_DefinedBenefitPlanActuarialGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember | (424)us-gaap_DefinedBenefitPlanActuarialGainLoss / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember
OPEB [Member] | Discount Rate Changes [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 155 us-gaap_DefinedBenefitPlanActuarialGainLoss / cms_ActuarialAssumptionsAxis = cms_DiscountRateChangesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
OPEB [Member] | Discount Rate Changes [Member] | Consumers Energy Company [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 150 us-gaap_DefinedBenefitPlanActuarialGainLoss / cms_ActuarialAssumptionsAxis = cms_DiscountRateChangesMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember |
OPEB [Member] | RP-2014 Mortality Table [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | 145 us-gaap_DefinedBenefitPlanActuarialGainLoss / cms_ActuarialAssumptionsAxis = cms_RpCurrentYearMortalityTableMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember |
OPEB [Member] | RP-2014 Mortality Table [Member] | Consumers Energy Company [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actuarial loss | $ 141 us-gaap_DefinedBenefitPlanActuarialGainLoss / cms_ActuarialAssumptionsAxis = cms_RpCurrentYearMortalityTableMember / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember |

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- Definition Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Gain or Loss -URI http://asc.fasb.org/extlink&oid=6514294 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Gain or Loss -URI http://asc.fasb.org/extlink&oid=6749293 + Details
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