Financings And Capitalization (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financing And Capitalization [Line Items]
Debt issuance	$ 1,410,000,000
Debt retirement, principal	730,000,000
Limitation on payment of stock dividends	4,400,000,000
Common stock dividends from Consumers	$ 522,000,000
Common stock, shares authorized	350,000,000	350,000,000
Par value of Common Stock	$ 0.01
Shares of Preferred Stock authorized	10,000,000
Par value of Preferred Stock	$ 0.01
Stock Offering Program Maximum Value	$ 100,000,000
Number of Shares Issued	1,494,371
Average Price Per Share	$ 47.31
Net Proceeds	$ 70,000,000
Consumers Energy Company [Member]
Financing And Capitalization [Line Items]
Debt issuance	835,000,000
Debt retirement, principal	530,000,000
Unrestricted retained earnings	$ 1,100,000,000
Common stock, shares authorized	125,000,000	125,000,000
Consumers Energy Company [Member] | Commercial Paper [Member]
Financing And Capitalization [Line Items]
Weighted average annual interest rate	1.69%
Short-term debt, authorized borrowings	$ 500,000,000
Short-term borrowings outstanding	170,000,000
CMS Energy [Member]
Financing And Capitalization [Line Items]
Debt issuance	575,000,000
Debt retirement, principal	200,000,000
CMS Energy [Member] | Term Loan Facility Due 2019 [Member]
Financing And Capitalization [Line Items]
Debt issuance	$ 180,000,000
Debt instrument, extension term	1 year
Three-month LIBOR plus a spread	0.80%