Schedule I - Condensed Financial Information of Registrant (Condensed Statements of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Expenses
Other operating expense									$ (1,417)	$ (1,236)	$ (1,248)
Total operating expenses									(5,711)	(5,245)	(5,143)
Operating Loss	$ 250	$ 294	$ 255	$ 363	$ 379	$ 330	$ 241	$ 388	1,162	1,338	1,256
Other Income and Expenses [Abstract]
Nonoperating retirement benefits, net									90	24	41
Interest income									11	12	6
Other income									2	6	8
Other expense									(48)	(76)	(75)
Total other income (expense)									70	(14)	5
Interest Charges
Interest on long-term debt									412	406	411
Interest on long-term debt									49	34	29
Total interest charges									458	438	435
Income Before Income Taxes									774	886	826
Income tax expense (benefit)									115	424	273
Net income (loss) available to common stockholders	$ 108	$ 169	$ 139	$ 241	$ (3)	$ 172	$ 92	$ 199	657	460	551
CMS Energy
Operating Expenses
Other operating expense									(7)	(9)	(14)
Total operating expenses									(7)	(9)	(14)
Operating Loss									(7)	(9)	(14)
Other Income and Expenses [Abstract]
Equity earnings of subsidiaries									780	633	660
Nonoperating retirement benefits, net									(1)	(1)	(1)
Interest income									2	1	1
Other income									0	2	0
Other expense									17	31	19
Total other income (expense)									764	604	641
Interest Charges
Interest on long-term debt									135	143	150
Interest on long-term debt									7	3	1
Total interest charges									142	146	151
Income Before Income Taxes									615	449	476
Income tax expense (benefit)									(42)	(11)	(75)
Net income (loss) available to common stockholders									$ 657	$ 460	$ 551