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Retirement Benefits (Schedule of Assumptions Used) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Pension And DB SERP [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | 4.10% |  | 4.90% |  | 5.40% |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Mortality table | 2000 |  | 2000 |  | 2000 |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | 4.90% |  | 5.40% |  | 5.85% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | 7.75% |  | 8.00% |  | 8.00% |
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Mortality Table | 2000 |  | 2000 |  | 2000 |
Pension And DB SERP [Member] | Consumers Energy Company [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | 4.10% | [1] | 4.90% | [1] | 5.40% | [1]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Mortality table | 2000 | [2] | 2000 | [2] | 2000 | [2]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | 4.90% | [1] | 5.40% | [1] | 5.85% | [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | 7.75% | [3] | 8.00% | [3] | 8.00% | [3]
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Mortality Table | 2000 | [2] | 2000 | [2] | 2000 | [2]
Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | 4.40% |  | 5.10% |  | 5.60% |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Mortality table | 2000 |  | 2000 |  | 2000 |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | 5.10% |  | 5.60% |  | 6.00% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | 7.25% |  | 7.50% |  | 7.50% |
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Mortality Table | 2000 |  | 2000 |  | 2000 |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | 4.40% | [1] | 5.10% | [1] | 5.60% | [1]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Mortality table | 2000 | [2] | 2000 | [2] | 2000 | [2]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | 5.10% | [1] | 5.60% | [1] | 6.00% | [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | 7.25% | [3] | 7.50% | [3] | 7.50% | [3]
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Mortality Table | 2000 | [2] | 2000 | [2] | 2000 | [2]
Pension Plans, Defined Benefit [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | 3.00% |  | 3.50% |  | 4.00% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | 3.50% |  | 4.00% |  | 4.00% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected long-term rate of return on plan assets | 7.75% |  |  |  |  |
Actual rate of return on plan assets | 14.10% |  | 4.00% |  | 13.00% |
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | 3.00% |  | 3.50% |  | 4.00% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | 3.50% |  | 4.00% |  | 4.00% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected long-term rate of return on plan assets | 7.75% |  |  |  |  |
Actual rate of return on plan assets | 14.10% |  | 4.00% |  | 13.00% |
Supplemental Executive Retirement Plan [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | 5.50% |  | 5.50% |  | 5.50% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | 5.50% |  | 5.50% |  | 5.50% |
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member] |  |  |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | 5.50% |  | 5.50% |  | 5.50% |
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | 5.50% |  | 5.50% |  | 5.50% |
[1] | The discount rate reflects the rate at which benefits could be effectively settled and is equal to the equivalent single rate resulting from a yield curve analysis. This analysis incorporated the projected benefit payments specific to CMS Energy’s and Consumers’ Pension Plan and OPEB plan and the yields on high quality corporate bonds rated Aa or better.
[2] | The mortality assumption was based on the RP-2000 mortality tables with projection of future mortality improvements using Scale AA, which aligned with the IRS prescriptions for cash funding valuations under the Pension Protection Act of 2006.
[3] | CMS Energy and Consumers determined the long-term rate of return using historical market returns, the present and expected future economic environment, the capital market principles of risk and return, and the expert opinions of individuals and firms with financial market knowledge. CMS Energy and Consumers considered the asset allocation of the portfolio in forecasting the future expected total return of the portfolio. The goal was to determine a long-term rate of return that could be incorporated into the planning of future cash flow requirements in conjunction with the change in the liability. Annually, CMS Energy and Consumers review for reasonableness and appropriateness the forecasted returns for various classes of assets used to construct an expected return model. CMS Energy’s and Consumers’ expected long-term rate of return on Pension Plan assets was 7.75 percent in 2012. The actual return on Pension Plan assets was 14.1 percent in 2012, four percent in 2011, and 13 percent in 2010.

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- Definition The mortality assumption was based on the RP-2000 mortality tables with projection of future mortality improvements using Scale AA, which aligned with the IRS prescriptions for cash funding valuations under the Pension Protection Act of 2006. + References No definition available. + Details
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- Definition The mortality assumption was based on the RP-2000 mortality tables with projection of future mortality improvements using Scale AA, which aligned with the IRS prescriptions for cash funding valuations under the Pension Protection Act of 2006. + References No definition available. + Details
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- Definition The interest rate used to adjust for the time value of money for the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph j -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 (k)(1) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph j -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
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- Definition An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(3) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 87 -Paragraph 264 -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 j -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 Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Long-Term Rate of Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512013 + Details
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- Definition Expected rate of compensation increases (for pay-related plans). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(2) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph j -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
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