Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				9 Months Ended
	Jan. 31, 2012 Consumers Energy Company [Member]	Jan. 31, 2011 Consumers Energy Company [Member]	Sep. 30, 2012 Consumers Energy Company [Member]	Dec. 31, 2011 Consumers Energy Company [Member]	Sep. 30, 2012 Supplemental Executive Retirement Plan [Member]	Sep. 30, 2012 Supplemental Executive Retirement Plan [Member] Consumers Energy Company [Member]
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements			$ 103	$ 103
SERP Contributions					13	9
Gain on donation of CMS Energy common stock	$ 5	$ 4