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Retirement Benefits (Schedule of SERP trust assets, ABO and contributions) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010
Supplemental Executive Retirement Plan [Member] |  |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation | $ 117 |  | $ 107 |
Contributions made to the plan | 27 |  | 17 |
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation | 76 |  | 66 |
Contributions made to the plan | 20 |  | 11 |
Supplemental Employee Retirement Plan Defined Contribution [Member] |  |  |  |
Minimum years of participation before vesting | 5 |  |  |
Supplemental Employee Retirement Plan Defined Contribution [Member] | Consumers Energy Company [Member] |  |  |  |
Minimum years of participation before vesting | 5 |  |  |
Supplemental Employee Retirement Plan Defined Contribution [Member] | Minimum [Member] |  |  |  |
Plan Contribution Percentage | 5.00% |  |  |
Supplemental Employee Retirement Plan Defined Contribution [Member] | Minimum [Member] | Consumers Energy Company [Member] |  |  |  |
Plan Contribution Percentage | 5.00% |  |  |
Supplemental Employee Retirement Plan Defined Contribution [Member] | Maximum [Member] |  |  |  |
Plan Contribution Percentage | 15.00% |  |  |
Supplemental Employee Retirement Plan Defined Contribution [Member] | Maximum [Member] | Consumers Energy Company [Member] |  |  |  |
Plan Contribution Percentage | 15.00% |  |  |
DB SERP Trust Assets [Member] | Supplemental Executive Retirement Plan [Member] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | 114 | [1] | 91 | [1]
DB SERP Trust Assets [Member] | Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Fair Value of Plan Assets | $ 75 | [1] | $ 57 | [1]
[1] | Trust assets are included in other non-current assets on CMS Energy's and Consumers' consolidated balance sheets.

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- Definition Contributions made to the plan Trust fund. + References No definition available. + Details
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- Definition Minimum years of participation required before becoming vested. + References No definition available. + Details
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- Definition Percentage of base pay contributed by the Company to the plan for employees. + References No definition available. + Details
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- Definition For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph e -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 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Accumulated Benefit Obligation -URI http://asc.fasb.org/extlink&oid=6503844 + Details
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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=6867990&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph d(iv)(b)(i) -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph b -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 87 -Paragraph 49 -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920 + Details
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