Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 103	$ 103
Consumers Energy Company [Member]
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	103	103
Gain on donation of CMS Energy common stock	5	4
Supplemental Executive Retirement Plan [Member]
Financial Instruments [Line Items]
Contributions made to the plan	13	27
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Financial Instruments [Line Items]
Contributions made to the plan	$ 9	$ 20