Schedule I - Condensed Financial Information of Registrant (Condensed Statements Of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash Flows from Operating Activities
Net cash provided by operating activities	$ 1,276	$ 1,790	$ 1,703
Cash Flows from Investing Activities
Proceeds from DB SERP investments	0	0	146
Net cash used in investing activities	(2,867)	(2,816)	(2,606)
Cash Flows from Financing Activities
Proceeds from issuance of debt	3,179	2,151	2,767
Issuance of common stock, net of issuance costs	253	12	41
Retirement of long-term debt	(2,010)	(1,285)	(1,870)
Debt prepayment costs	(59)	(8)	(36)
Payment of dividends on common stock	(467)	(436)	(407)
Net cash provided by financing activities	1,619	1,008	874
Net Increase (Decrease) in Cash and Cash Equivalents, Including Restricted Amounts	28	(18)	(29)
Cash and Cash Equivalents, Including Restricted Amounts, Beginning of Period	157	175	204
Cash and Cash Equivalents, Including Restricted Amounts, End of Period	185	157	175
CMS Energy
Cash Flows from Operating Activities
Net cash provided by operating activities	507	697	702
Cash Flows from Investing Activities
Investment in subsidiaries	(657)	(683)	(363)
Increase in notes receivable – intercompany	(307)	0	0
Proceeds from DB SERP investments	0	0	22
Net cash used in investing activities	(964)	(683)	(341)
Cash Flows from Financing Activities
Proceeds from issuance of debt	1,225	1,158	560
Issuance of common stock, net of issuance costs	253	12	41
Retirement of long-term debt	(425)	(738)	(675)
Debt prepayment costs	(16)	0	(16)
Payment of dividends on common stock	(465)	(434)	(405)
Debt issuance costs and financing fees	(10)	(18)	(8)
Change in notes payable – intercompany	(105)	6	142
Net cash provided by financing activities	457	(14)	(361)
Net Increase (Decrease) in Cash and Cash Equivalents, Including Restricted Amounts	0	0	0
Cash and Cash Equivalents, Including Restricted Amounts, Beginning of Period	0	0	0
Cash and Cash Equivalents, Including Restricted Amounts, End of Period	$ 0	$ 0	$ 0