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Retirement Benefits (Schedule Of Assumptions Used) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
DB Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Actual rate of return on plan assets |  | 8.00% | (2.00%) | 7.40%
DB 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],[3] | 4.30% | 4.52% | 4.10%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | [3] | 3.60% | 3.00% | 3.00%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | [3],[4] | 7.25% | 7.50% | 7.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate | [1],[2],[3] | 4.79% | 4.10% | 4.90%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate | [3] | 3.66% | 4.10% | 4.90%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | [3] | 3.00% | 3.00% | 3.00%
Actual rate of return on plan assets |  | 8.00% | (2.00%) | 7.40%
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],[3] | 4.16% | 4.43% | 4.10%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase | [3] | 5.50% | 5.50% | 5.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate | [1],[2],[3] | 4.87% | 4.10% | 4.90%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate | [3] | 3.64% | 4.10% | 4.90%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase | [3] | 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],[3] | 4.49% | 4.70% | 4.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets | [3],[4] | 7.25% | 7.25% | 7.25%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate | [1],[2],[3] | 4.75% | 4.30% | 5.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate | [3] | 3.89% | 4.30% | 5.10%
Change in Assumptions for Pension Plans [Member] | DB Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Reduction in service cost and interest cost |  | $ 23 |  |
Change in Assumptions for Pension Plans [Member] | DB Pension Plan [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Reduction in service cost and interest cost |  | 22 |  |
Change in Assumptions for Pension Plans [Member] | OPEB [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Reduction in service cost and interest cost |  | 12 |  |
Change in Assumptions for Pension Plans [Member] | OPEB [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Reduction in service cost and interest cost |  | $ 11 |  |
[1] | In January 2016, CMS Energy and Consumers changed the method they use to determine the discount rate used to calculate the service cost and interest cost 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 cost; 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. This change represented a change in accounting estimate and did not impact years prior to 2016. As a result of changing to the full-yield-curve approach to determine the discount rate, for the year ended December 31, 2016, the service cost and interest cost components of net periodic benefit costs were reduced by $23 million for the DB Pension Plan and $12 million for the OPEB Plan for CMS Energy and by $22 million for the DB Pension Plan and $11 million for the OPEB Plan for Consumers.
[2] | 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.
[3] | The mortality assumption for benefit obligations was based on the RP-2014 mortality table, with projection scales MP-2016 for 2016, MP-2015 for 2015, and MP-2014 for 2014. The mortality assumption for net periodic benefit cost for 2016 and 2015 was based on the RP-2014 mortality table, with projection scales MP-2015 for 2016 and MP-2014 for 2015, and for 2014 was based on the RP-2000 mortality table.
[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 7.25 percent in 2016. The actual return (loss) on DB Pension Plan assets was 8.0 percent in 2016, (2.0) percent in 2015, and 7.4 percent in 2014.

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