Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Level 3 Inputs) (Details) (Fair Value, Inputs, Level 3 [Member], USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Balance at beginning of period	$ 3				$ (2)		$ (3)
Total gains (losses) included in earnings	(1)	[1]	(1)	[1]	1	[1]	(1)	[1]
Total gains (losses) offset through regulatory accounting	1		(1)		8		1
Purchases							1
Settlements	(2)		1		(6)		1
Balance at end of period	1		(1)		1		(1)
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at end of period	(1)	[1]	(1)	[1]	1	[1]
Consumers Energy Company [Member]
Balance at beginning of period	5		3		2		1
Total gains (losses) offset through regulatory accounting	1		(1)		8		1
Purchases							1
Settlements	(2)				(6)		(1)
Balance at end of period	$ 4		$ 2		$ 4		$ 2

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