Retirement Benefits (Schedule of SERP trust assets, ABO and contributions) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 117		$ 107
Contributions made to the plan	27		17
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Accumulated Benefit Obligation	76		66
Contributions made to the plan	20		11
Supplemental Employee Retirement Plan Defined Contribution [Member]
Minimum years of participation before vesting	5
Supplemental Employee Retirement Plan Defined Contribution [Member] | Consumers Energy Company [Member]
Minimum years of participation before vesting	5
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%
Supplemental Employee Retirement Plan Defined Contribution [Member] | Maximum [Member] | Consumers Energy Company [Member]
Plan Contribution Percentage	15.00%
DB SERP Trust Assets [Member] | Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Fair Value of Plan Assets	114	[1]	91	[1]
DB SERP Trust Assets [Member] | Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Fair Value of Plan Assets	$ 75	[1]	$ 57	[1]

[1] Trust assets are included in other non-current assets on CMS Energy's and Consumers' consolidated balance sheets.