Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011		Dec. 31, 2010
Income from continuing operataions before income taxes	$ 622	$ 606		$ 590
Income from continuing operations before income taxes	620	604		587
Income tax expense at statutory rate	217	211		205
MCIT law change, net of federal effect		(32)	[1]
State and local income taxes, net of federal effect	27	21		26
Medicare Part D exempt income law change				3	[2]
Other, net	1	(9)		(10)
Income Tax Expense	245	191		224
Effective income tax rate	39.50%	31.60%		38.20%
Deferred Income Tax Expense (Benefit)	226	169		223
Consumers Energy Company [Member]
Income from continuing operataions before income taxes	736	734		688
Income tax expense at statutory rate	258	257		241
State and local income taxes, net of federal effect	36	24		26
Other, net	3	(14)		(13)
Income Tax Expense	297	267		254
Effective income tax rate	40.40%	36.40%		36.90%
Recognition of regulatory asset related to law change	2,322	2,467
Deferred Income Tax Expense (Benefit)	153	165		250
State Jurisdiction [Member]
MCIT rate		6.00%
Tax Law Change [Member]
Deferred Income Tax Expense (Benefit)		(32)
Tax Law Change [Member] | Consumers Energy Company [Member]
Recognition of regulatory asset related to law change		$ 128

[1] For the year ended December 31, 2011, CMS Energy and Consumers remeasured their Michigan deferred income tax assets and liabilities due to the enactment in May 2011 of the MCIT, which became effective January 1, 2012. The MCIT, a simplified six percent corporate income tax, replaced the MBT, a complex multi-part tax. CMS Energy recognized a one-time non-cash deferred tax benefit of $32 million as a result of this remeasurement. Consumers recognized a $128 million regulatory asset (not including the effects of income tax gross-ups) related to this change in tax law.

[2] For the year ended December 31, 2010, CMS Energy recognized deferred tax expense of $3 million to reflect the enactment of the Health Care Acts. The law change prospectively repealed the tax deduction for the portion of the health care costs reimbursed by the Medicare Part D subsidy for taxable years beginning after December 31, 2012.