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Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011
Financial Instruments [Line Items] |  |
Portion of long-term debt supported by third-party credit enhancements | $ 103 | $ 103
Consumers Energy Company [Member] |  |
Financial Instruments [Line Items] |  |
Portion of long-term debt supported by third-party credit enhancements | 103 | 103
Gain on donation of CMS Energy common stock | 5 | 4
Supplemental Executive Retirement Plan [Member] |  |
Financial Instruments [Line Items] |  |
Contributions made to the plan | 13 | 27
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member] |  |
Financial Instruments [Line Items] |  |
Contributions made to the plan | $ 9 | $ 20

X
- Definition This element represents the gross gains realized on donations of available-for-sale securities. + References No definition available. + Details
Name: | cms_AvailableForSaleSecuritiesGrossRealizedGainsOnDonations
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Contributions made to the plan Trust fund. + References No definition available. + Details
Name: | cms_ContributionsMadeToPlan
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Disclosure of the amount of the entity's issued liability supported by a third-party credit enhancement. + References No definition available. + Details
Name: | cms_DebtInstrumentSupportedByThirdPartyCreditEnhancement
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | cms_FinancialInstrumentsLineItems
Namespace Prefix: | cms_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration