Financial Instruments (Summary of the Sales Activity for Investment Securities) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Proceeds from Sales of Available-for-Sale Securities	$ 29	[1]	$ 1	[1]	$ 53	[1]
Available-for-sale Securities, Gross Realized Gains					8
Other Comprehensive Income Reclassifcation Adjustment for Sale of Securities Included in Net Income Net of Tax					5
Gross Realized Losses from Sales of Available-for-Sale Securities Less Than	1		1		1
Consumers Energy Company [Member]
Proceeds from Sales of Available-for-Sale Securities	19	[1]			32	[1]
Available-for-sale Securities, Gross Realized Gains					5
Other Comprehensive Income Reclassifcation Adjustment for Sale of Securities Included in Net Income Net of Tax					3
Gross Realized Losses from Sales of Available-for-Sale Securities Less Than	$ 1		$ 1		$ 1

[1] All proceeds related to sales of investments that were held within the SERP and classified as available for sale. Realized losses on these sales were less than $1 million for both CMS Energy and Consumers during each period.