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, 2012		Dec. 31, 2011		Dec. 31, 2010
Balance at beginning of period	$ (2)		$ (3)		$ (8)
Total gains (losses) included in earnings	3	[1]	2	[1]	5	[1]
Total gains (losses) offset through regulatory accounting	6		2		3
Purchases	1		1		1
Sales			(4)		(1)
Settlements	(6)				(3)
Balance at end of period	2		(2)		(3)
Unrealized gains (losses) included in earnings relating to assets and liabilities still held at end of period	2	[1]	2	[1]	4	[1]
Consumers Energy Company [Member]
Balance at beginning of period	2		1
Total gains (losses) offset through regulatory accounting	6		2		3
Purchases	1		1		1
Settlements	(7)		(2)		(3)
Balance at end of period	$ 2		$ 2		$ 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.