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				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011	Jun. 30, 2010
Fair Value, Inputs, Level 3 [Member]
Beginning balance, April 1	$ (2)	[1]	$ (3)	[1]	$ (3)	$ (8)
Total gains (losses) included in earnings	(1)		(2)			2	[1]
Total gains offset through regulatory accounting	3		1		3	1
Settlements			(1)
Ending balance, June 30			(5)			(5)
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at balance sheet date			(2)		1	2
Consumers Energy Company [Member]
Beginning balance, April 1					1
Total gains offset through regulatory accounting	3		1		3	1
Settlements			(1)		(1)	(1)
Ending balance, June 30	$ 3				$ 3

[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.