Retirement Benefits (Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates) (Details) (Other Postretirement Benefit Plans, Defined Benefit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	$ 19
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	245
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(16)
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	(213)
Consumers Energy Company [Member]
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components	18
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation	238
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components	(15)
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation	$ (207)