Financial Instruments (Summary of the Sales Activity for Investment Securities) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011		Jun. 30, 2011		Jun. 30, 2010
Proceeds from Sales of Available-for-Sale Securities	$ 1	[1]	$ 1	[1]	$ 1	[1]
Consumers Energy Company [Member]
Proceeds from Sales of Available-for-Sale Securities			$ 1	[1]

[1] All of the proceeds related to sales of state and municipal bonds that were held within the SERP and classified as available for sale. Realized losses on these sales were insignificant for both CMS Energy and Consumers during each period.