Schedule I - Condensed Financial Information of Registrant (Condensed Statements Of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net cash provided by operating activities	$ 1,705	$ 1,629	$ 1,640
Net cash used in investing activities	(1,868)	(1,915)	(2,064)
Proceeds from issuance of debt	1,633	1,049	599
Issuance of common stock	83	72	43
Retirement of long-term debt	(980)	(728)	(224)
Debt prepayment costs	(22)	(18)
Change in notes payable	(228)	149	189
Net cash provided by (used in) financing activities	110	255	463
Net Increase in Cash and Cash Equivalents, Including Restricted Amounts	(53)	(31)	39
Cash and Cash Equivalents, Including Restricted Amounts, Beginning of Period	257	288	249
Cash and Cash Equivalents, Including Restricted Amounts, End of Period	204	257	288
CMS Energy [Member]
Net cash provided by operating activities	433	422	209
Investment in subsidiaries	(447)	(275)	(150)
Net cash used in investing activities	(447)	(275)	(150)
Proceeds from issuance of debt	799	603	349
Issuance of common stock	83	72	43
Retirement of long-term debt	(425)	(530)	(100)
Debt prepayment costs	(18)	(18)
Payment of dividends on common stock	(375)	(345)	(320)
Debt issuance costs and financing fees	(3)	(5)	(3)
Change in notes payable	(47)	76	(28)
Net cash provided by (used in) financing activities	14	(147)	(59)
Net Increase in Cash and Cash Equivalents, Including Restricted Amounts
Cash and Cash Equivalents, Including Restricted Amounts, Beginning of Period
Cash and Cash Equivalents, Including Restricted Amounts, End of Period