Earnings Per Share - CMS Energy (Basic And Diluted EPS Computations) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income available to common stockholders
Net income	$ 140	$ 93	$ 381	$ 292
Income Attributable to Noncontrolling Interests	1	1	1	1
Net Income Available to Common Stockholders	$ 139	$ 92	$ 380	$ 291
Average common shares outstanding
Weighted average shares - basic (in shares)	282.1	279.5	281.8	279.2
Dilutive nonvested stock awards (in shares)	0.5	0.8	0.6	0.9
Weighted average shares - diluted (in shares)	282.6	280.3	282.4	280.1
Net income per average common share available to common stockholders
Basic earnings per average common share (in dollars per share)	$ 0.49	$ 0.33	$ 1.35	$ 1.04
Diluted earnings per average common share (in dollars per share)	$ 0.49	$ 0.33	$ 1.35	$ 1.04