Fair Value Measurements (Assets and Liabilities Measured At Fair Value on a Recurring Basis using Significant Level 3 Inputs) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011	Sep. 30, 2010
Balance at beginning of period			$ (5)		$ (3)	$ (8)
Total gains (losses) included in earnings	(1)		2		(1)	4
Total gains (losses) offset through regulatory accounting	(1)		3		2	4
Settlements	1		(3)		1	(3)
Balance at end of period	(1)		(3)		(1)	(3)
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at end of period	(1)	[1]	2	[1]		4	[1]
Consumers Energy Company [Member]
Balance at beginning of period	3				1
Total gains (losses) offset through regulatory accounting	(1)		3		2	4
Settlements			(2)		(1)	(3)
Balance at end of period	$ 2		$ 1		$ 2	$ 1

[1] CMS Energy records realized and unrealized gains and losses for Level 3 recurring fair values in earnings as a component of operating revenue or maintenance and other operating expenses on its consolidated statements of income.