Earnings Per Share - CMS Energy (Basic And Diluted EPS Computations) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Earnings Per Share - CMS Energy [Abstract]
Income from continuing operations	$ 149	$ 140	$ 310	$ 374
Less income attributable to noncontrolling interest	1	1	2	2
Income from Continuing Operations Available to Common Stockholders - Basic and Diluted	$ 148	$ 139	$ 308	$ 372
Weighted average shares - basic	262.9	251.3	259.9	250.5
Add dilutive contingently convertible securities	5.1	11.3	7.2	11.4
Add dilutive non-vested awards and options	1.0	0.6	1.0	0.4
Add dilutive convertible debentures		0.7
Weighted average shares - diluted	269.0	263.9	268.1	262.3
Basic	$ 0.56	$ 0.55	$ 1.18	$ 1.48
Diluted	$ 0.55	$ 0.53	$ 1.14	$ 1.42