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, 2012		Dec. 31, 2011		Dec. 31, 2010
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	$ 1,451		$ 1,289
Consumers Energy Company [Member]
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	1,385		1,225
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	127	[1]	118
Defined Benefit Plan, Service Cost	1		1
Defined Benefit Plan, Interest Cost	6		6
Defined Benefit Plan, Actuarial Net (Gains) Losses	16		8
Defined Benfefit Plan Benefits Paid Plus Cash Payments	(6)		(6)
Defined Benefit Plan, Benefit Obligation, Ending Balance	144	[1]	127	[1]
Defined Benefit Plan, Contributions by Employer	6		6
Defined Benefit Plan, Benefits Paid	(6)		(6)
Defined Benefit Plan, Funded Status of Plan	(144)		(127)
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	7		7
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	137		120
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	85	[1]	77
Defined Benefit Plan, Service Cost	1		1
Defined Benefit Plan, Interest Cost	4		4
Defined Benefit Plan, Actuarial Net (Gains) Losses	13		6
Defined Benfefit Plan Benefits Paid Plus Cash Payments	(3)		(3)
Defined Benefit Plan, Benefit Obligation, Ending Balance	100	[1]	85	[1]
Defined Benefit Plan, Contributions by Employer	3		3
Defined Benefit Plan, Benefits Paid	(3)		(3)
Defined Benefit Plan, Funded Status of Plan	(100)		(85)
Pension and Other Postretirement Defined Benefit Plans, Current Liabilities	4		4
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	96		81
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	2,072		1,896
Defined Benefit Plan, Service Cost	48		48
Defined Benefit Plan, Interest Cost	99		100
Defined Benefit Plan, Actuarial Net (Gains) Losses	249		107
Defined Benefit Plan, Benefit Obligation, Ending Balance	2,354		2,072
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	1,626		1,401
Defined Benefit Plan, Actual Return on Plan Assets	215		54
Defined Benefit Plan, Contributions by Employer			250
Defined Benefit Plan, Benefits Paid	(114)		(79)
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	1,727		1,626
Defined Benefit Plan, Funded Status of Plan	(627)	[2]	(446)	[2]
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	627		446
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Funded Status of Plan	(590)		(414)
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	590		414
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	1,641	[1]	1,410
Defined Benefit Plan, Service Cost	32		27		26
Defined Benefit Plan, Interest Cost	82		77		80
Defined Benefit Plan, Actuarial Net (Gains) Losses	25		180
Defined Benfefit Plan Benefits Paid Plus Cash Payments	(51)	[3]	(53)	[3]
Defined Benefit Plan, Benefit Obligation, Ending Balance	1,729	[1]	1,641	[1]	1,410
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	924		887
Defined Benefit Plan, Actual Return on Plan Assets	108		23
Defined Benefit Plan, Contributions by Employer	65		67
Defined Benefit Plan, Benefits Paid	(50)	[3]	(53)	[3]
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	1,047		924		887
Defined Benefit Plan, Funded Status of Plan	(682)		(717)
Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	20		26		28
Capitalization Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	7		9		10
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	5		5		5
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	682		717
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	1,585	[1]	1,358
Defined Benefit Plan, Service Cost	31		26		25
Defined Benefit Plan, Interest Cost	79		74		77
Defined Benefit Plan, Actuarial Net (Gains) Losses	24		178
Defined Benfefit Plan Benefits Paid Plus Cash Payments	(49)	[3]	(51)	[3]
Defined Benefit Plan, Benefit Obligation, Ending Balance	1,670	[1]	1,585	[1]	1,358
Defined Benefit Plan, Fair Value of Plan Assets, Beginning Balance	861		825
Defined Benefit Plan, Actual Return on Plan Assets	101		21
Defined Benefit Plan, Contributions by Employer	64		66
Defined Benefit Plan, Benefits Paid	(48)	[3]	(51)	[3]
Defined Benefit Plan, Fair Value of Plan Assets, Ending Balance	978		861		825
Defined Benefit Plan, Funded Status of Plan	(692)		(724)
Prescription Drug Benefit, Effect of Subsidy on Net Periodic Postretirement Benefit Cost	19		25		26
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	5		5		5
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	$ 692		$ 724

[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 $20 million for 2012, $26 million for 2011, and $28 million for 2010. Consumers’ Medicare Part D annualized reduction in net OPEB costs was $19 million for 2012, $25 million for 2011, and $26 million for 2010. The reduction for CMS Energy and Consumers included $7 million for 2012, $9 million for 2011, and $10 million for 2010 in capitalized OPEB costs.

[2] At December 31, 2012, $590 million of the total funded status of the Pension Plan was attributable to Consumers based on an allocation of expenses. At December 31, 2011, $414 million of the funded status of the Pension Plan was attributable to Consumers based on an allocation of expenses.

[3] CMS Energy received payments of $5 million in each of 2012, 2011, and 2010 for the Medicare Part D subsidies. Consumers received payments of $5 million in each of 2012, 2011, and 2010 for the Medicare Part D subsidies. The Medicare Part D subsidy payments are used to pay OPEB plan benefits.