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		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012		Jun. 30, 2011
Balance at beginning of period	$ (1)	$ (2)	$ (2)		$ (3)
Total gains included in earnings			2	[1]
Total gains offset through regulatory accounting	9	2	7		2
Purchases					1
Settlements	(5)		(4)
Balance at end of period	3		3
Unrealized gains included in earnings relating to assets and liabilities still held at end of period			2	[1]	1	[1]
Consumers Energy Company [Member]
Balance at beginning of period	1		2		1
Total gains offset through regulatory accounting	9	2	7		2
Purchases		1			1
Settlements	(5)		(4)		(1)
Balance at end of period	$ 5	$ 3	$ 5		$ 3

[1] 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.