Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
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	4	5
Supplemental Executive Retirement Plan [Member]
Financial Instruments [Line Items]
Contributions made to the plan	16
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Financial Instruments [Line Items]
Contributions made to the plan	$ 13