Reportable Segments (Reconciliation of Assets from Segment to Consolidated) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Operating Revenue	$ 1,620		$ 1,464	$ 1,364	$ 2,055	$ 1,682		$ 1,443	$ 1,340	$ 1,967	$ 6,503		$ 6,432		$ 6,205
Operating Income	174		316	207	306	158		319	262	239	1,003		978		698
Total Assets	16,452	[1]				15,616	[1]				16,452	[1]	15,616	[1]	15,256	[1]
Consumers Energy Company [Member]
Operating Revenue	1,565		1,397	1,303	1,988	1,620		1,370	1,276	1,890	6,253		6,156		5,963
Operating Income	173		305	207	300	191		304	207	224	985		926		689
Total Assets	15,662	[1]				14,839	[1]				15,662	[1]	14,839	[1]	14,622	[1]
International [Member]
Operating Revenue											0		0		0
Operating Income											0		0		0
Total Assets	1					3					1		3		3
International [Member] | Consumers Energy Company [Member]
Operating Revenue											0		0		0
Operating Income											0		0		0
Total Assets															$ 0

[1] Amounts include a portion of Consumers' other common assets attributable to both the electric and gas utility businesses.