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Financial Instruments (Summary of the Sales Activity for Investment Securities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Proceeds from Sales of Available-for-Sale Securities | $ 8 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds | $ 3 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds | $ 3 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Consumers Energy Company [Member] |  |  |
Proceeds from Sales of Available-for-Sale Securities | $ 6 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember | $ 2 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember | $ 2 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds / dei_LegalEntityAxis = cms_ConsumersEnergyCompanyMember

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- Definition The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Data Type: | na
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