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Fair Value Measurements (Assets and Liabilities Measured At Fair Value on a Recurring Basis using Significant Level 3 Inputs) (Details) (USD $) In Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010
Balance at beginning of period |  |  | $ (5) |  | $ (3) | $ (8) |
Total gains (losses) included in earnings | (1) |  | 2 |  | (1) | 4 |
Total gains (losses) offset through regulatory accounting | (1) |  | 3 |  | 2 | 4 |
Settlements | 1 |  | (3) |  | 1 | (3) |
Balance at end of period | (1) |  | (3) |  | (1) | (3) |
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at end of period | (1) | [1] | 2 | [1] |  | 4 | [1]
Consumers Energy Company [Member] |  |  |  |  |  |  |
Balance at beginning of period | 3 |  |  |  | 1 |  |
Total gains (losses) offset through regulatory accounting | (1) |  | 3 |  | 2 | 4 |
Settlements |  |  | (2) |  | (1) | (3) |
Balance at end of period | $ 2 |  | $ 1 |  | $ 2 | $ 1 |
[1] | CMS Energy records realized and unrealized gains and losses for Level 3 recurring fair values in earnings as a component of operating revenue or maintenance and other operating expenses on its consolidated statements of income.

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- Definition This item represents the amount of total change in unrealized (holding) gains or losses for the period which are included in the statement of income (or changes in net assets) in earnings. Such unrealized (holding) gains or losses relate to those assets or liablities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + Details
Name: | cms_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLossIncludedInEarnings
Namespace Prefix: | cms
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition This element represents total gains or losses for the period (realized), arising from assets and liablities measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset and liability values. + Details
Name: | cms_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesGainLossIncludedInEarnings
Namespace Prefix: | cms
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This element represents total gains or losses for the period, arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (Level 3), which were offset through regulatory accounting. + Details
Name: | cms_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesGainLossOffsetThroughRegulatoryAccounting
Namespace Prefix: | cms
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets and Liabilities Settlements + Details
Name: | cms_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesSettlements
Namespace Prefix: | cms
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This element represents assets and liabilities measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances. + Details
Name: | cms_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetsAndLiabilitiesValue
Namespace Prefix: | cms
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant