Reportable Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Dec. 31, 2011
Operating Revenue	$ 1,507		$ 1,464	$ 4,583		$ 4,883
Total Net Income Available to Common Stockholders	148		139	315		374
Total Plant, Property, and Equipment, Gross	15,103			15,103			14,751
Total Assets	16,608			16,608			16,452
Consumers Energy Company [Member]
Operating Revenue	1,448		1,397	4,405		4,688
Total Net Income Available to Common Stockholders	162		154	359		398
Total Plant, Property, and Equipment, Gross	14,968			14,968			14,621
Total Assets	15,804			15,804			15,662
Electric Utility [Member]
Operating Revenue	1,239		1,180	3,073		3,026
Total Net Income Available to Common Stockholders	165		159	297		309
Total Plant, Property, and Equipment, Gross	10,647			10,647			10,400
Total Assets	10,135	[1]		10,135	[1]		9,938	[1]
Electric Utility [Member] | Consumers Energy Company [Member]
Operating Revenue	1,239		1,180	3,073		3,026
Total Net Income Available to Common Stockholders	165		159	297		309
Total Plant, Property, and Equipment, Gross	10,647			10,647			10,400
Total Assets	10,135	[1]		10,135	[1]		9,938	[1]
Gas Utility [Member]
Operating Revenue	209		217	1,332		1,662
Total Net Income Available to Common Stockholders	(3)		(5)	61		88
Total Plant, Property, and Equipment, Gross	4,306			4,306			4,206
Total Assets	5,032	[1]		5,032	[1]		4,956	[1]
Gas Utility [Member] | Consumers Energy Company [Member]
Operating Revenue	209		217	1,332		1,662
Total Net Income Available to Common Stockholders	(3)		(5)	61		88
Total Plant, Property, and Equipment, Gross	4,306			4,306			4,206
Total Assets	5,032	[1]		5,032	[1]		4,956	[1]
Enterprises [Member]
Operating Revenue	44		56	136		161
Total Net Income Available to Common Stockholders	5		4	9		36
Total Plant, Property, and Equipment, Gross	112			112			109
Total Assets	176			176			242
Other [Member]
Operating Revenue	15		11	42		34
Total Net Income Available to Common Stockholders	(19)		(19)	(52)		(59)
Total Plant, Property, and Equipment, Gross	38			38			36
Total Assets	1,265			1,265			1,316
Other [Member] | Consumers Energy Company [Member]
Total Net Income Available to Common Stockholders				1		1
Total Plant, Property, and Equipment, Gross	15			15			15
Total Assets	$ 637			$ 637			$ 768

[1] In Millions September 30, 2012 December 31, 2011 CMS Energy, including ConsumersPlant, Property, and Equipment, GrossElectric utility$ 10,647$ 10,400Gas utility 4,306 4,206Enterprises 112 109Other 38 36Total Plant, Property, and Equipment, Gross - CMS Energy$ 15,103$ 14,751ConsumersPlant, Property, and Equipment, GrossElectric utility$ 10,647$ 10,400Gas utility 4,306 4,206Other 15 15Total Plant, Property, and Equipment, Gross - Consumers$ 14,968$ 14,621CMS Energy, including ConsumersTotal AssetsElectric utility1$ 10,135$ 9,938Gas utility1 5,032 4,956Enterprises 176 242Other 1,265 1,316Total Assets - CMS Energy$ 16,608$ 16,452ConsumersTotal AssetsElectric utility1$ 10,135$ 9,938Gas utility1 5,032 4,956Other 637 768Total Assets - Consumers$ 15,804$ 15,662Amounts include a portion of Consumers’ other common assets attributable to both the electric and gas utility businesses.