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Retirement Benefits (Narrative) (Details)	9 Months Ended	12 Months Ended
Retirement Benefits (Narrative) (Details)	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011
Expected long-term rate of return on pension plan assets	7.75%
Actual return on pension plan assets		18.60%	2.00%
Consumers Energy Company [Member]
Expected long-term rate of return on pension plan assets	7.75%
Actual return on pension plan assets		18.60%	2.00%

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

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