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Regulatory Matters (Narrative) (Details) (USD $) | 12 Months Ended | 1 Months Ended | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |  |  | 1 Months Ended | 12 Months Ended |
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 Consumers Energy Company [Member] | Dec. 31, 2012 Consumers Energy Company [Member] | Dec. 31, 2011 Consumers Energy Company [Member] | Dec. 31, 2010 Consumers Energy Company [Member] | Jun. 30, 2012 Electric Rate Case [Member] Consumers Energy Company [Member] | Dec. 31, 2011 Power Supply Cost Recover (PSCR) [Member] Consumers Energy Company [Member] | Dec. 31, 2010 Power Supply Cost Recover (PSCR) [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Power Supply Cost Recover (PSCR) [Member] Consumers Energy Company [Member] | Mar. 31, 2012 Gas Cost Recover (GCR) [Member] Consumers Energy Company [Member] | Mar. 31, 2011 Gas Cost Recover (GCR) [Member] Consumers Energy Company [Member] | Mar. 31, 2010 Gas Cost Recover (GCR) [Member] Consumers Energy Company [Member] | Sep. 30, 2012 Gas Cost Recover (GCR) [Member] Consumers Energy Company [Member] | Mar. 31, 2012 Electric Revenue Decoupling Mechanism [Member] Consumers Energy Company [Member] | Dec. 31, 2011 Electric Revenue Decoupling Mechanism [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Gas Rate Case [Member] Consumers Energy Company [Member] | Jun. 30, 2012 Gas Rate Case [Member] Consumers Energy Company [Member] | Mar. 31, 2012 Gas Rate Case [Member] Consumers Energy Company [Member] | Sep. 30, 2011 Gas Rate Case [Member] Consumers Energy Company [Member] | Sep. 30, 2011 2010-2011 Gas Revenue Decoupling Mechanism [Member] Consumers Energy Company [Member] | Aug. 31, 2012 2011-2012 Gas Revenue Decoupling Mechanism [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Electric Energy Optimization Plan [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Gas Energy Optimization Plan [Member] Consumers Energy Company [Member] | Jun. 30, 2012 Cancelled Coal-Fueled Plant Costs [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Big Rock Decommissioning [Member] Consumers Energy Company [Member] | Dec. 31, 2011 Big Rock Decommissioning [Member] Consumers Energy Company [Member] | Sep. 30, 2011 Big Rock Decommissioning [Member] Consumers Energy Company [Member] | Dec. 31, 2012 DOE Settlement [Member] Consumers Energy Company [Member] | Jul. 31, 2011 DOE Settlement [Member] Consumers Energy Company [Member] | Dec. 31, 2012 DOE Settlement [Member] Consumers Energy Company [Member] | Dec. 31, 2011 DOE Settlement [Member] Consumers Energy Company [Member]
Requested recovery |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 16,000,000 | $ 17,000,000 |  |  |  |  |  |  |  |  |  |  |
Energy optimization incentive authorized for exceeding 2011 savings targets |  |  |  | 15,000,000 | 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percent of Savings Target Achieved |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 123.00% | 132.00% |  |  |  |  |  |  |  |  |
Energy Optimization Incentive Requested for Exceeding 2012 Savings Targets |  |  |  | 17,000,000 | 17,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Authorized Recovery |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14,000,000 |  |  |  |  |  |  |  |
Regulatory assets, noncurrent | 2,287,000,000 | 2,466,000,000 |  | 2,287,000,000 | 2,287,000,000 | 2,466,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 85,000,000 | [1] |  |  |  |  |
Regulatory asset |  |  |  | 2,322,000,000 | 2,322,000,000 | 2,467,000,000 |  |  |  |  |  |  |  |  |  |  | 59,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount paid to transfer ownership and responsibility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30,000,000 | 30,000,000 |  |  |  |  |  |
Nuclear fuel storage costs incurred as a result of the DOE's failure to accept spent nuclear fuel |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 55,000,000 |  |  |  |  |  |
Litigation settlement, gross |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 120,000,000 |  |
Proposed recovery of regulatory asset |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 85,000,000 |  |  |  |
Proposed refund of spent nuclear fuel disposal costs collected |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,000,000 |  |  |  |
Authorized Refund Of Spent Nuclear Fuel Disposal Costs Collected |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,000,000 |  |  |
Remaining Portion Of Litigation Settlement |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12,000,000 |
Trust assets assigned for future prescription drug costs |  |  |  | 792,000,000 | 792,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in regulatory liabilities |  |  |  | 151,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Write-off of regulatory asset |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 59,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual rate increase requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 49,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Rate of return on equity authorized |  |  |  |  |  |  |  | 10.30% |  |  |  |  |  |  |  |  |  |  | 10.30% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual rate increase self-implemented |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual rate increase authorized |  |  |  |  |  |  |  | 118,000,000 |  |  |  |  |  |  |  |  |  |  | 16,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual depreciation expense authorized |  |  |  |  |  |  |  | 20,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of Goods Sold, Electric |  |  |  |  |  |  |  |  | 1,800,000,000 | 1,700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of Goods Sold, Electric |  |  |  |  |  |  |  |  | 1,800,000,000 | 1,700,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cost of Natural Gas Purchases | 1,150,000,000 | 1,512,000,000 | 1,590,000,000 |  | 1,110,000,000 | 1,438,000,000 | 1,516,000,000 |  |  |  |  | 900,000,000 | 1,200,000,000 | 1,300,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Over/(Under) Recovery, Including Interest, Authorized by the MPSC |  |  |  |  |  |  |  |  |  |  | (11,000,000) | 1,000,000 |  |  | 6,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rate of return on equity requested |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.70% |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory liabilities, current | 25,000,000 | 125,000,000 |  | 25,000,000 | 25,000,000 | 125,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 23,000,000 |  | 23,000,000 | 120,000,000
Amount of rates to be refunded to customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | These regulatory assets represent expenditures for which the MPSC has provided, or Consumers expects, recovery without a return on investment.

X
- Definition The amount that Consumers paid to transfer ownership and responsibility for the Big Rock ISFSI + References No definition available. + Details
Name: | cms_AmountPaidToTransferOwnershipAndResponsibility
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The annual rate increase authorized by the MPSC. + References No definition available. + Details
Name: | cms_AnnualRateIncreaseAuthorized
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The annual rate increase requested by Consumers. + References No definition available. + Details
Name: | cms_AnnualRateIncreaseRequested
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The annual rate increase self-implemented by Consumers. + References No definition available. + Details
Name: | cms_AnnualRateIncreaseSelfImplemented
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount the MPSC authorized collection from customers. + References No definition available. + Details
Name: | cms_AuthorizedRecovery
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of spent nuclear fuel disposal costs incurred and collected from customers through rates that the MPSC has authorized for refund + References No definition available. + Details
Name: | cms_AuthorizedRefundOfSpentNuclearFuelDisposalCostsCollected
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Energy optimization incentive authorized by the MPSC for achieving electric and gas savings targets. + References No definition available. + Details
Name: | cms_EnergyOptimizationIncentiveAuthorized
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Increase in annual depreciation expense authorized by the MPSC. + References No definition available. + Details
Name: | cms_IncreaseInAnnualDepreciationExpenseAuthorized
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Net over-/(under-) recovery of expense, including interest, authorized by the MPSC. + References No definition available. + Details
Name: | cms_NetOverUnderRecoveryOfExpenseIncludingInterestAuthorized
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Nuclear fuel storage costs incurred by Consumers as a result of the DOE's failure to accept spent nuclear fuel. + References No definition available. + Details
Name: | cms_NuclearFuelStorageCostsIncurred
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Percent of energy optimization savings targets achieved during the year. + References No definition available. + Details
Name: | cms_PercentOfSavingsTargetAchieved
Namespace Prefix: | cms_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The portion of the settlement proceeds Consumers has requested to use as recovery for its Big Rock Nuclear Decommissioning And Related Costs regulatory asset. + References No definition available. + Details
Name: | cms_ProposedRecoveryOfRegulatoryAsset
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The amount of spent nuclear fuel disposal costs incurred and collected from customers through rates that Consumers has proposed to refund. + References No definition available. + Details
Name: | cms_ProposedRefundOfSpentNuclearFuelDisposalCostsCollected
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The rate of return on equity authorized by the MPSC. + References No definition available. + Details
Name: | cms_RateOfReturnOnEquityAuthorized
Namespace Prefix: | cms_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The rate of return on equity requested. + References No definition available. + Details
Name: | cms_RateOfReturnOnEquityRequested
Namespace Prefix: | cms_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The remaining portion of the settlement proceeds received from the DOE. + References No definition available. + Details
Name: | cms_RemainingPortionOfLitigationProceeds
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Energy optimization incentive requested for achieving electric and gas savings targets. + References No definition available. + Details
Name: | cms_RequestedEnergyOptimizationIncentive
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount for which Consumers requested recovery. + References No definition available. + Details
Name: | cms_RequestedRecovery
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Trust assets assigned for future prescription drug costs + References No definition available. + Details
Name: | cms_TrustAssetsAssignedForFuturePrescriptionDrugCosts
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Write-Off of the value of Consumers' electric revenue decoupling mechanism regulatory asset. + References No definition available. + Details
Name: | cms_WriteOffOfRegulatoryAsset
Namespace Prefix: | cms_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total costs related to electricity produced and sold during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 2 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.2(b)) -URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688 + Details
Name: | us-gaap_CostOfGoodsSoldElectric
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Cost of natural gas purchased and associated transportation and storage costs during the period. + References No definition available. + Details
Name: | us-gaap_CostOfNaturalGasPurchases
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Current regulatory liabilities generally represent obligations to make refunds to customers for various reasons including overpayment. + References No definition available. + Details
Name: | us-gaap_CustomerRefundLiabilityCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The increase (decrease) during the reporting period in regulated liabilities. Regulated liabilities are created when regulatory agency permits an entity to defer revenues to the balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 28 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585 + Details
Name: | us-gaap_IncreaseDecreaseInRegulatoryLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This element represents the gross amount awarded, to be received by, or to be remitted to the entity in settlement of litigation occurring during the period. + References No definition available. + Details
Name: | us-gaap_LitigationSettlementGross
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9, 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
Name: | us-gaap_RegulatoryAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 9, 10 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
Name: | us-gaap_RegulatoryAssetsNoncurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 71 -Paragraph 11 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
Name: | us-gaap_RegulatoryLiabilityCurrent
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant