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Retirement Benefits (Schedule of Assumptions Used) (Details)
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Pension and SERP [Member]
      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.90% [1] 5.40% [1] 5.85% [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 5.40% [1] 5.85% [1] 6.50% [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets 8.00% [3] 8.00% [3] 8.25% [3]
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Mortality Table 2000 [2] 2000 [2] 2000 [2]
Pension and SERP [Member] | Consumers Energy Company [Member]
      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 4.90% [1] 5.40% [1] 5.85% [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 5.40% [1] 5.85% [1] 6.50% [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets 8.00% [4] 8.00% [4] 8.25% [4]
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 5.10% [1] 5.60% [1] 6.00% [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 5.60% [1] 6.00% [1] 6.50% [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets 7.50% [3] 7.50% [3] 7.75% [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] | Consumers Energy Company [Member]
      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 5.10% [1] 5.60% [1] 6.00% [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 5.60% [1] 6.00% [1] 6.50% [1]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets 7.50% [4] 7.50% [4] 7.75% [4]
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 3.50% 4.00% 4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase 4.00% 4.00% 4.00%
Actual rate of return on plan assets 4.00% 13.00% 21.00%
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase 3.50% 4.00% 4.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase 4.00% 4.00% 4.00%
Actual rate of return on plan assets 4.00% 13.00% 21.00%
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 the 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 eight percent in 2011. The actual return on Pension Plan assets was four percent in 2011, 13 percent in 2010, and 21 percent in 2009.
[4] 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 eight percent in 2011. The actual return on Pension Plan assets was four percent in 2011, 13 percent in 2010, and 21 percent in 2009.