Retirement Benefits (Schedule Of Assumptions Used) (Details)		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual rate of return on plan assets		18.00%		8.00%	(2.00%)
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	[1],[2]	7.25%		7.25%	7.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate	[2],[3],[4]	4.53%		4.79%	4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate	[2],[3],[4]	3.56%		3.66%	4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	[2]	3.60%		3.00%	3.00%
Actual rate of return on plan assets		18.00%		8.00%	(2.00%)
DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	[2],[4]	3.65%		4.16%	4.43%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[2],[4]	5.50%		5.50%	5.50%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate	[2],[3],[4]	4.51%		4.87%	4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate	[2],[3],[4]	3.51%		3.64%	4.10%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase	[2]	5.50%		5.50%	5.50%
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	[2],[4]	3.74%		4.49%	4.70%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	[1],[2]	7.25%		7.25%	7.25%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Service Cost Discount Rate	[2],[3],[4]	4.89%		4.75%	4.30%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Interest Cost Discount Rate	[2],[4],[5]	3.79%		3.89%	4.30%
Pension Plan A [Member] | DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	[2],[4],[5]	3.78%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	[2],[5]	3.50%
Pension Plan B [Member] | DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate				4.30%	4.52%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				3.60%	3.00%
Pension Plan B [Member] | DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate		3.64%	[2],[4],[5]	4.30%	4.52%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase				3.60%	3.00%

[1] 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 the assets of the DB Pension Plans was 7.25 percent in 2017. The actual return (loss) on the assets of the DB Pension Plans was 18.0 percent in 2017, 8.0 percent in 2016, and (2.0) percent in 2015.

[2] The mortality assumption for benefit obligations was based on the RP-2014 mortality table, with projection scales MP-2017 for 2017, MP-2016 for 2016, and MP-2015 for 2015. The mortality assumption for net periodic benefit cost for 2017, 2016, and 2015 was based on the RP-2014 mortality table, with projection scales MP-2016 for 2017, MP-2015 for 2016, and MP-2014 for 2015.

[3] 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.

[4] 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 Plans and OPEB Plan and the yields on high-quality corporate bonds rated Aa or better.

[5] Effective December 31, 2017, CMS Energy's and Consumers' existing defined benefit pension plan was amended to include only retired or inactive employees; this amended plan is referred to as DB Pension Plan B. Active employees were moved to a newly created pension plan, referred to as DB Pension Plan A. The discount rate used to measure the existing plan was 4.30 percent at December 31, 2016 and 4.52 percent at December 31, 2015. The weighted-average rate of compensation increase used to measure the existing plan was 3.60 percent at December 31, 2016 and 3.00 percent at December 31, 2015.