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Financial Instruments (Schedule Of Investment Securities) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Investment Securities [Line Items] |  |
Held to maturity securities - Cost | $ 11 us-gaap_HeldToMaturitySecurities | $ 10 us-gaap_HeldToMaturitySecurities
Held to maturity securities - Fair Value | 11 us-gaap_HeldToMaturitySecuritiesFairValue | 10 us-gaap_HeldToMaturitySecuritiesFairValue
CMS Energy Common Stock [Member] | Consumers Energy Company [Member] |  |
Investment Securities [Line Items] |  |
Held to maturity securities - Cost | 5 us-gaap_HeldToMaturitySecurities / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 5 us-gaap_HeldToMaturitySecurities / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Held to maturity securities - Unrealized Gains | 33 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 24 us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Held to maturity securities - Fair Value | 38 us-gaap_HeldToMaturitySecuritiesFairValue / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 29 us-gaap_HeldToMaturitySecuritiesFairValue / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
DB SERP [Member] | Mutual Fund [Member] |  |
Investment Securities [Line Items] |  |
Available for sale - Cost | 129 us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember | 136 us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember
Available for sale - Unrealized Losses | 2 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember |
Available for sale - Fair Value | 127 us-gaap_AvailableForSaleSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember | 136 us-gaap_AvailableForSaleSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember
DB SERP [Member] | Mutual Fund [Member] | Consumers Energy Company [Member] |  |
Investment Securities [Line Items] |  |
Available for sale - Cost | 92 us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember | 95 us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember
Available for sale - Unrealized Losses | 2 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember |
Available for sale - Fair Value | $ 90 us-gaap_AvailableForSaleSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember | $ 95 us-gaap_AvailableForSaleSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_SupplementalEmployeeRetirementPlanDefinedBenefitMember / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = cms_MutualFundMember

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- Definition This item represents the cost of equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments for other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References No definition available. + Details
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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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