Earnings Per Share - CMS Energy (Basic And Diluted EPS Computations) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income available to common stockholders
Net income	$ 169	$ 172	$ 550	$ 464
Income Attributable to Noncontrolling Interests	0	0	1	1
Net Income Available to Common Stockholders	$ 169	$ 172	$ 549	$ 463
Average common shares outstanding
Weighted average shares - basic (in shares)	282.5	280.8	282.1	279.8
Dilutive nonvested stock awards (in shares)	0.7	0.8	0.7	0.8
Weighted average shares - diluted (in shares)	283.2	281.6	282.8	280.6
Net income per average common share available to common stockholders
Basic earnings per average common share (in dollars per share)	$ 0.60	$ 0.61	$ 1.95	$ 1.65
Diluted earnings per average common share (in dollars per share)	$ 0.59	$ 0.61	$ 1.94	$ 1.65