Retirement Benefits (Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	$ 7		$ 7
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	1,289		1,135
Consumers Energy Company [Member]
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	4		4
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	1,225		1,076
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	1,896	[1]	1,717
Defined Benefit Plan, Service Cost	48		44
Defined Benefit Plan, Interest Cost	100		98
Defined Benefit Plan, Actuarial Net (Gains) Losses	107		150
Defined Benefit Plan, Benefits Paid	(79)	[2]	(113)	[2]
Defined Benefit Plan, Benefit Obligation, Ending Balance	2,072	[1]	1,896	[1]
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	1,401		1,007
Defined Benefit Plan, Actual Return on Plan Assets	54		132
Defined Benefit Plan, Contributions by Employer	250		375
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	1,626		1,401
Defined Benefit Plan, Funded Status of Plan	(446)	[3],[4]	(495)	[3],[4]
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Funded Status of Plan	414		463
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	118	[1]	106
Defined Benefit Plan, Service Cost	1		1
Defined Benefit Plan, Interest Cost	6		6
Defined Benefit Plan, Actuarial Net (Gains) Losses	8		11
Defined Benefit Plan, Benefits Paid	(6)	[2]	(6)	[2]
Defined Benefit Plan, Benefit Obligation, Ending Balance	127	[1]	118	[1]
Defined Benefit Plan, Contributions by Employer	6		6
Defined Benefit Plan, Funded Status of Plan	127	[3]	118	[3]
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	77	[1]	67
Defined Benefit Plan, Service Cost	1		1
Defined Benefit Plan, Interest Cost	4		4
Defined Benefit Plan, Actuarial Net (Gains) Losses	6		8
Defined Benefit Plan, Benefits Paid	(3)	[2]	(3)	[2]
Defined Benefit Plan, Benefit Obligation, Ending Balance	85	[1]	77	[1]
Defined Benefit Plan, Contributions by Employer	3		3
Defined Benefit Plan, Funded Status of Plan	(85)	[5]	(77)	[5]
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	1,410	[1]	1,423
Defined Benefit Plan, Service Cost	27		26		24
Defined Benefit Plan, Interest Cost	77		80		80
Defined Benefit Plan, Plan Amendments			(101)	[6]
Defined Benefit Plan, Actuarial Net (Gains) Losses	180		36
Defined Benefit Plan, Benefits Paid	(53)	[2]	(54)	[2]
Defined Benefit Plan, Benefit Obligation, Ending Balance	1,641	[1]	1,410	[1]	1,423
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	887		782
Defined Benefit Plan, Actual Return on Plan Assets	23		88
Defined Benefit Plan, Contributions by Employer	67		71
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	924		887		782
Defined Benefit Plan, Funded Status of Plan	717	[3]	523	[3]
Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	26		28		19
Capitalization Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	9		10		6
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	5		5		4
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	1,358	[1]	1,373
Defined Benefit Plan, Service Cost	26		25		24
Defined Benefit Plan, Interest Cost	74		77		77
Defined Benefit Plan, Plan Amendments			(100)	[6]
Defined Benefit Plan, Actuarial Net (Gains) Losses	178		34
Defined Benefit Plan, Benefits Paid	(51)	[2]	(51)	[2]
Defined Benefit Plan, Benefit Obligation, Ending Balance	1,585	[1]	1,358	[1]	1,373
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	825		725
Defined Benefit Plan, Actual Return on Plan Assets	21		81
Defined Benefit Plan, Contributions by Employer	66		70
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	861		825		725
Defined Benefit Plan, Funded Status of Plan	(724)	[5]	(533)	[5]
Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	25		26		18
Capitalization Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	9		10		6
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	$ 5		$ 5		$ 4

[1] The Medicare Prescription Drug, Improvement, and Modernization Act of 2003 established a prescription drug benefit under Medicare (Medicare Part D) and a federal subsidy, which is tax-exempt, to sponsors of retiree health care benefit plans that provide a benefit that is actuarially equivalent to Medicare Part D. In 2010, the Health Care Acts repealed these tax-exempt deductions for years beginning after December 31, 2012. The Medicare Part D annualized reduction in net OPEB cost for CMS Energy was $26 million for 2011, $28 million for 2010, and $19 million for 2009. Consumers' Medicare Part D annualized reduction in net OPEB costs was $25 million for 2011, $26 million for 2010, and $18 million for 2009. The reduction for CMS Energy and Consumers included $9 million for 2011, $10 million for 2010, and $6 million for 2009 in capitalized OPEB costs.

[2] CMS Energy received payments of $5 million in each of 2011 and 2010 and $4 million in 2009 for the Medicare Part D subsidies. Consumers received payments of $5 million in each of 2011 and 2010 and $4 million in 2009 for the Medicare Part D subsidies.

[3] At December 31, 2011, CMS Energy classified $7 million as current liabilities and $1.3 billion as non-current liabilities on its consolidated balance sheets. At December 31, 2010, CMS Energy classified $7 million as current liabilities and $1.1 billion as non-current liabilities on its consolidated balance sheets. Current liabilities relate to SERP projected payments for the next year. At December 31, 2011, Consumers classified $4 million as current liabilities and $1.2 billion as non-current liabilities on its consolidated balance sheets. At December 31, 2010, Consumers classified $4 million as current liabilities and $1.1 billion as non-current liabilities on its consolidated balance sheets.

[4] At December 31, 2011, $414 million of the total funded status of the Pension Plan was attributable to Consumers based on an allocation of expenses. At December 31, 2010, $463 million of the funded status of the Pension Plan was attributable to Consumers based on an allocation of expenses.

[5] At December 31, 2011, CMS Energy classified $7 million as current liabilities and $1.3 billion as non-current liabilities on its consolidated balance sheets. At December 31, 2010, CMS Energy classified $7 million as current liabilities and $1.1 billion as non-current liabilities on its consolidated balance sheets. Current liabilities relate to SERP projected payments for the next year. At December 31, 2011, Consumers classified $4 million as current liabilities and $1.2 billion as non-current liabilities on its consolidated balance sheets. At December 31, 2010, Consumers classified $4 million as current liabilities and $1.1 billion as non-current liabilities on its consolidated balance sheets.

[6] Plan amendments reflect changes resulting from an agreement reached with the UWUA in April 2010 on a new five-year contract for UWUA members.