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	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Balance at beginning of period	$ (3)		$ (8)		$ (16)
Total gains included in earnings	2		5		7
Total gains offset through regulatory accounting	2		3		9
Purchases	1		1		1
Sales	(4)		(1)
Settlements			(3)		(9)
Balance at end of period	(2)		(3)		(8)
Unrealized gains included in earnings relating to assets and liabilities still held at end of period	2	[1]	4	[1]	6	[1]
Consumers Energy Company [Member]
Balance at beginning of period	1
Total gains offset through regulatory accounting	2		3		9
Purchases	1		1		1
Settlements	(2)		(3)		(10)
Balance at end of period	$ 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.