Consolidated Statements Of Changes In Equity (USD $) In Millions, except Share data	Consumers Energy Company [Member] CMS Energy Common Stock [Member]	Consumers Energy Company [Member] Other Paid-in Capital [Member]	Consumers Energy Company [Member] Accumulated Other Comprehensive Income (Loss) [Member]	Consumers Energy Company [Member] Retirement benefits liability [Member]	Consumers Energy Company [Member] Investments [Member]	Consumers Energy Company [Member] Retained Earnings (Accumulated Deficit) [Member]	Consumers Energy Company [Member] Preferred Stock [Member]	Consumers Energy Company [Member]	CMS Energy Common Stock [Member]	Other Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retirement benefits liability [Member]	Investments [Member]	Derivative instruments [Member]	Retained Earnings (Accumulated Deficit) [Member]	Preferred Stock [Member]	Noncontrolling Interest [Member]	Total
Total Equity, beginning at Dec. 31, 2009	$ 841	$ 2,582	$ 2	$ (11)	$ 13	$ 389	$ 44	$ 3,858	$ 2	$ 4,560	$ (33)	$ (32)		$ (1)	$ (1,927)	$ 239	$ 97	$ 2,938
Beginning of period, shares at Dec. 31, 2009										227,891,000
Shares of common stock issued										22,090,000
Common stock issued										22
Common stock reissued, shares
Common stock reissued
Common stock repurchased, shares										(148,000)
Common stock repurchased										(2)
Common stock reacquired, shares										(205,000)
Charge for deferred issuance costs										8					(8)
Stockholder contribution		250						250
Net loss arising during the period				(5)				(5)				(9)						(9)
Amortization of net actuarial loss												1						1
Prior service credit adjustment												1						1
Reclassification adjustments included in net income
Unrealized gain on investments, net of tax					3			3
Net income						434		434										343
Net income attributable to CMS Energy															340			340
Common stock dividends declared						(358)									(154)
Preferred stock dividends declared						(2)									(8)
Income attributable to noncontrolling interests																	3	3
Distributions and other changes in noncontrolling interests																	(56)
Conversion of preferred stock																(239)
Total Equity, end at Dec. 31, 2010	841	2,832		(16)	16	463	44	4,180	2	4,588	(40)	(39)		(1)	(1,757)		44	2,837
End of period, shares at Dec. 31, 2010										249,628,000
Net income								153										135
Net income attributable to CMS Energy																		135
Total Equity, end at Mar. 31, 2011
Total Equity, beginning at Dec. 31, 2010	841	2,832		(16)	16	463	44	4,180	2	4,588	(40)	(39)		(1)	(1,757)		44	2,837
Beginning of period, shares at Dec. 31, 2010										249,628,000
Shares of common stock issued										4,541,000
Common stock issued									1	40
Common stock reissued, shares										269,000
Common stock reissued										5
Common stock repurchased, shares										(323,000)
Common stock repurchased										(6)
Common stock reacquired, shares										(15,000)
Stockholder contribution		125						125
Net loss arising during the period				(4)				(4)				(11)						(11)
Amortization of net actuarial loss				1				1				2						2
Reclassification adjustments included in net income
Unrealized gain on investments, net of tax					1			1
Net income						467		467										417
Net income attributable to CMS Energy															415			415
Common stock dividends declared						(374)									(211)
Preferred stock dividends declared						(2)
Income attributable to noncontrolling interests																	2	2
Distributions and other changes in noncontrolling interests																	(2)
Total Equity, end at Dec. 31, 2011	841	2,957	(2)	(19)	17	554	44	4,394	3	4,627	(49)	(48)		(1)	(1,553)		44	3,072
End of period, shares at Dec. 31, 2011										254,100,000
Total Equity, beginning at Sep. 30, 2011
Net income								67										41
Net income attributable to CMS Energy																		41
Total Equity, end at Dec. 31, 2011	841		(2)				44	4,394			(49)			(1)				3,072
Net income								76										67
Net income attributable to CMS Energy																		67
Total Equity, end at Mar. 31, 2012
Total Equity, beginning at Dec. 31, 2011	841	2,957	(2)	(19)	17	554	44	4,394	3	4,627	(49)	(48)		(1)	(1,553)		44	3,072
Beginning of period, shares at Dec. 31, 2011										254,100,000
Shares of common stock issued										10,107,000
Common stock issued										45
Common stock reissued, shares										272,000
Common stock reissued										6
Common stock repurchased, shares										(389,000)
Common stock repurchased										(9)
Common stock reacquired, shares										(18,000)
Stockholder contribution		150						150
Net loss arising during the period				(8)				(8)				(10)						(10)
Amortization of net actuarial loss				2				2				2						2
Reclassification adjustments included in net income					(3)			(3)
Unrealized gain on investments, net of tax					3			3					2					2
Net income						439		439										384
Net income attributable to CMS Energy															382			382
Common stock dividends declared						(393)									(252)
Preferred stock dividends declared						(2)
Income attributable to noncontrolling interests																	2	2
Distributions and other changes in noncontrolling interests																	(2)
Total Equity, end at Dec. 31, 2012	841	3,107	(8)	(25)	17	598	44	4,582	3	4,669	(55)	(56)	2	(1)	(1,423)		44	3,238
End of period, shares at Dec. 31, 2012										264,072,000
Total Equity, beginning at Sep. 30, 2012
Net income								78										67
Net income attributable to CMS Energy																		67
Total Equity, end at Dec. 31, 2012	$ 841		$ (8)				$ 44	$ 4,582			$ (55)			$ (1)				$ 3,238