Income Taxes (Principal Components of Deferred Income Tax Assets) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Employee benefits	$ 3	$ (126)
Gas inventory	(147)	(155)
Property, plant and equipment	(1,783)	(1,668)
Net regulatory tax liability	131	70
Reserves and accruals	71	86
Securitized costs	(73)	(96)
Tax loss and credit carryforwards	733	806
Other	(15)	92
Gross Deferred Tax Liabilities	(1,080)	(991)
Less: valuation allowance	(3)	(20)
Total net deferred income tax liabilities	(1,083)	(1,011)
Deferred tax assets, net of valuation reserves	935	1,034
Deferred tax liabilities	(2,018)	(2,045)
Consumers Energy Company [Member]
Employee benefits	(36)	(158)
Gas inventory	(147)	(155)
Property, plant and equipment	(1,848)	(1,742)
Net regulatory tax liability	131	70
Reserves and accruals	41	44
Securitized costs	(73)	(96)
Tax loss and credit carryforwards	61	67
Other	(13)	81
Gross Deferred Tax Liabilities	(1,884)	(1,889)
Less: valuation allowance	(1)	(1)
Total net deferred income tax liabilities	(1,885)	(1,890)
Deferred tax assets, net of valuation reserves	232	261
Deferred tax liabilities	$ (2,117)	$ (2,151)