Schedule I - Condensed Financial Information of Registrant (Condensed Statements of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Expenses
Other operating expense									$ (1,403)	$ (1,448)	$ (1,417)
Total operating expenses									(5,318)	(5,606)	(5,711)
Operating Loss	$ 352	$ 369	$ 273	$ 368	$ 311	$ 351	$ 218	$ 359	1,362	1,239	1,162
Other Income and Expenses [Abstract]
Nonoperating retirement benefits, net									118	91	90
Interest income									4	7	11
Other income									6	4	2
Other expense									(62)	(13)	(48)
Total other income									84	109	70
Interest Charges
Interest on long-term debt									483	439	412
Interest on long-term debt									68	75	49
Total interest charges									561	519	458
Income Before Income Taxes									885	829	774
Income tax expense									133	147	115
Net income available to common stockholders	$ 158	$ 218	$ 136	$ 243	$ 167	$ 207	$ 93	$ 213	755	680	657
CMS Energy
Operating Expenses
Other operating expense									(6)	(38)	(7)
Total operating expenses									(6)	(38)	(7)
Operating Loss									(6)	(38)	(7)
Other Income and Expenses [Abstract]
Equity earnings of subsidiaries									909	826	780
Nonoperating retirement benefits, net									(1)	(1)	(1)
Interest income									1	1	2
Other income									1	1	0
Other expense									(19)	0	(17)
Total other income									891	827	764
Interest Charges
Interest on long-term debt									178	156	135
Interest on long-term debt									7	10	7
Total interest charges									185	166	142
Income Before Income Taxes									700	623	615
Income tax expense									(55)	(57)	(42)
Net income available to common stockholders									$ 755	$ 680	$ 657