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Retirement Benefits (Schedule Of Assumptions Used) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Actual rate of return on plan assets |  | (2.00%) | 7.40% | 12.50%
Pension Plan [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | [1],[2] | 4.52% | 4.10% | 4.90%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | [2] | 3.00% | 3.00% | 3.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | [1],[2],[3] | 4.10% | 4.90% | 4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | [2],[4] | 7.50% | 7.50% | 7.75%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | [2] | 3.00% | 3.00% | 3.00%
Actual rate of return on plan assets |  | (2.00%) | 7.40% | 12.50%
DB SERP [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | [1],[2] | 4.43% | 4.10% | 4.90%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | [2] | 5.50% | 5.50% | 5.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | [1],[2],[3] | 4.10% | 4.90% | 4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | [2] | 5.50% | 5.50% | 5.50%
OPEB [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | [1],[2] | 4.70% | 4.30% | 5.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate | [1],[2],[3] | 4.30% | 5.10% | 4.40%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | [2],[4] | 7.25% | 7.25% | 7.25%
[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' DB Pension Plan and OPEB Plan and the yields on high-quality corporate bonds rated Aa or better.
[2] | The mortality assumption for 2015 and 2014 for benefit obligations was based on the RP-2014 mortality table, with projection scales MP-2015 for 2015 and MP-2014 for 2014. The mortality assumption for 2013 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. The mortality assumption for net periodic benefit cost for 2015 was based on the RP-2014 mortality table with projection scale MP-2014, and for 2014 and 2013 was based on the RP-2000 mortality table.
[3] | In January 2016, CMS Energy and Consumers changed the method in which they determine the discount rate used to calculate the service cost and interest expense components of net periodic benefit costs for the DB Pension and OPEB Plans. Historically, the discount rate used for this purpose represented a single weighted-average rate derived from the yield curve used to determine the benefit obligation. CMS Energy and Consumers have elected to use instead a full-yield-curve approach in the estimation of service cost and interest expense; this approach is more accurate in that it applies individual spot rates along the yield curve to future projected benefit payments based on the time of payment. CMS Energy and Consumers expect that this change will result in a decrease in the service cost and interest expense components of net periodic benefit costs for the DB Pension and OPEB Plans, with an offsetting impact to the actuarial gain or loss recorded in, and later amortized from, the associated regulatory asset and AOCI. This change represents a change in accounting estimate and will not impact years prior to 2016.
[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 DB Pension Plan assets was 7.5 percent in 2015. The actual return (loss) on DB Pension Plan assets was (2.0) percent in 2015, 7.4 percent in 2014, and 12.5 percent in 2013.

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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=58740216&loc=d3e1928-114920 + Details
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