Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended			3 Months Ended								12 Months Ended					12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Consumers Energy Company [Member]	Sep. 30, 2012 Consumers Energy Company [Member]	Jun. 30, 2012 Consumers Energy Company [Member]	Mar. 31, 2012 Consumers Energy Company [Member]	Dec. 31, 2011 Consumers Energy Company [Member]	Sep. 30, 2011 Consumers Energy Company [Member]	Jun. 30, 2011 Consumers Energy Company [Member]	Mar. 31, 2011 Consumers Energy Company [Member]	Dec. 31, 2012 Consumers Energy Company [Member]	Dec. 31, 2011 Consumers Energy Company [Member]	Dec. 31, 2010 Consumers Energy Company [Member]	Dec. 31, 2012 Internal Revenue Service (IRS) [Member]	May 31, 2012 Internal Revenue Service (IRS) [Member]	Dec. 31, 2010 2009 Interest Dispute [Member]	Dec. 31, 2010 2009 Audit [Member]	Dec. 31, 2010 Changes to Taxable Income [Member] 2009 Audit [Member]	Dec. 31, 2010 Decrease Operating Loss Carryforward [Member] 2009 Audit [Member]	Dec. 31, 2010 Increase Taxable Income [Member] 2009 Audit [Member]
Uncertain tax benefits	$ 1				$ 4				$ 1	$ 4	$ 4	$ 1				$ 4				$ 1	$ 4	$ 3
Accrued interest, less than											1
Results of IRS audit																									6	15	(132)	(82)	(50)
Net operating loss carryforward																								45
Net income	$ 67	$ 149	$ 101	$ 67	$ 41	$ 140	$ 101	$ 135	$ 384	$ 417	$ 343	$ 78	$ 163	$ 122	$ 76	$ 67	$ 155	$ 92	$ 153	$ 439	$ 467	$ 434	$ (1)			$ 1