Retirement Benefits (Schedule of SERP Trust Assets, ABO and contributions) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 130	$ 117
Contributions made to the plan	13	27
Supplemental Executive Retirement Plan [Member] \| Consumers Energy Company [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	86	76
Contributions made to the plan	9	20
Supplemental Employee Retirement Plan Defined Contribution [Member]
Defined Benefit Plan, Fair Value of Plan Assets	1	1
Minimum years of participation before vesting	5 years
Supplemental Employee Retirement Plan Defined Contribution [Member] \| Consumers Energy Company [Member]
Defined Benefit Plan, Fair Value of Plan Assets	1	1
Minimum years of participation before vesting	5 years
Supplemental Employee Retirement Plan Defined Contribution [Member] \| Minimum [Member]
Plan Contribution Percentage	5.00%
Supplemental Employee Retirement Plan Defined Contribution [Member] \| Minimum [Member] \| Consumers Energy Company [Member]
Plan Contribution Percentage	5.00%
Supplemental Employee Retirement Plan Defined Contribution [Member] \| Maximum [Member]
Plan Contribution Percentage	15.00%
Defined Contribution Plan, Cost Recognized, less than	1	1	1
Supplemental Employee Retirement Plan Defined Contribution [Member] \| Maximum [Member] \| Consumers Energy Company [Member]
Plan Contribution Percentage	15.00%
Defined Contribution Plan, Cost Recognized, less than	1	1	1
DB SERP Trust Assets [Member] \| Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Fair Value of Plan Assets	128	114
DB SERP Trust Assets [Member] \| Supplemental Executive Retirement Plan [Member] \| Consumers Energy Company [Member]
Defined Benefit Plan, Fair Value of Plan Assets	$ 87	$ 75

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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