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Financial Instruments (Schedule of Investment Securities) (Details) (USD $) In Millions			9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Consumers Energy Company [Member] Supplemental Executive Retirement Plan [Member]	Sep. 30, 2011 Supplemental Executive Retirement Plan [Member]	Sep. 30, 2011 Consumers Energy Company [Member] Mutual Fund [Member]	Dec. 31, 2010 Consumers Energy Company [Member] Mutual Fund [Member]	Sep. 30, 2011 Mutual Fund [Member]	Dec. 31, 2010 Mutual Fund [Member]	Sep. 30, 2011 Consumers Energy Company [Member] State and Municipal Bonds [Member]	Dec. 31, 2010 Consumers Energy Company [Member] State and Municipal Bonds [Member]	Sep. 30, 2011 State and Municipal Bonds [Member]	Dec. 31, 2010 State and Municipal Bonds [Member]	Sep. 30, 2011 Consumers Energy Company [Member] CMS Energy Common Stock [Member]	Dec. 31, 2010 Consumers Energy Company [Member] CMS Energy Common Stock [Member]	Sep. 30, 2011 Consumers Energy Company [Member]	Dec. 31, 2010 Consumers Energy Company [Member]
Available for sale - cost					$ 57	$ 39	$ 88	$ 62					$ 7	$ 8
Available for sale - unrealized gains											1		24	26
Available for sale - unrealized losses							1
Available for sale - fair value	113	90	17	26	57	39	87	62	17	17	26	28	31	34	105	90
Available for sale State and municipal bonds - cost									17	17	25	28
Held to maturity Debt securities - cost	7	5
Held to maturity Debt securities - unrealized gains	1	1
Held to maturity Debt securities - unrealized losses
Held to maturity Debt securities - fair value	8	6
SERP Contributions			$ 20	$ 27