Financings And Capitalization (Major Long-Term Debt Transactions) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal Balance		$ 1,410,000,000
Debt retirement, principal		730,000,000
Loss on extinguishment of debt		18,000,000	$ 18,000,000
CMS Energy [Member]
Debt Instrument [Line Items]
Principal Balance		575,000,000
Debt retirement, principal		200,000,000
Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		835,000,000
Debt retirement, principal		530,000,000
Term Loan Facility Due 2018 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Principal Balance	[1]	$ 225,000,000
LIBOR plus		0.68%
Interest rate at period end		2.28%
Debt issuance date	[1]	December 2017
Maturity date	[1]	December 2018
Senior Notes 2.950% Due 2027 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate		2.95%
Senior Notes [Member] | Senior Notes 3.450% Due 2027 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Principal Balance		$ 350,000,000
Interest rate		3.45%
Debt issuance date		February 2017
Maturity date		August 2027
Senior Notes [Member] | Senior Notes 5.050% Due 2022 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Interest rate		5.05%
Senior Notes [Member] | Senior Notes 8.750% Due 2019 [Member] | CMS Energy [Member]
Debt Instrument [Line Items]
Principal Balance	[2]	$ 200,000,000
Interest rate	[2]	8.75%
Debt retirement date	[2]	December 2017
Maturity date	[2]	June 2019
Loss on extinguishment of debt		$ 18,000,000
Senior Notes [Member] | Senior Notes 6.875% Due March 2018 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 180,000,000
Interest rate		6.875%
Debt retirement date		September 2017
Maturity date		March 2018
First Mortgage Bonds [Member] | FMB's 3.950% Due July 2047 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 350,000,000
Interest rate		3.95%
Debt issuance date		February 2017
Maturity date		July 2047
First Mortgage Bonds [Member] | FMB's 3.180% Due September 2032 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 40,000,000
Interest rate		3.18%
Debt issuance date		September 2017
Maturity date		September 2032
First Mortgage Bonds [Member] | FMB's 3.520% Due September 2037 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 125,000,000
Interest rate		3.52%
Debt issuance date		September 2017
Maturity date		September 2037
First Mortgage Bonds [Member] | FMB's 3.860% Due September 2052 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 20,000,000
Interest rate		3.86%
Debt issuance date		September 2017
Maturity date		September 2052
First Mortgage Bonds [Member] | FMB's 3.180% Due November 2032 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 60,000,000
Interest rate		3.18%
Debt issuance date		November 2017
Maturity date		November 2032
First Mortgage Bonds [Member] | FMB's 3.520% Due November 2037 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 210,000,000
Interest rate		3.52%
Debt issuance date		November 2017
Maturity date		November 2037
First Mortgage Bonds [Member] | FMB's 3.860% Due November 2052 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 30,000,000
Interest rate		3.86%
Debt issuance date		November 2017
Maturity date		November 2052
First Mortgage Bonds [Member] | FMB's 5.150% Due February 2017 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Debt retirement, principal		$ 250,000,000
Interest rate		5.15%
Debt retirement date		February 2017
Maturity date		February 2017
First Mortgage Bonds [Member] | FMB's 3.21% Due 2017 [Member] | Consumers Energy Company [Member]
Debt Instrument [Line Items]
Principal Balance		$ 100,000,000
Interest rate		3.21%
Debt retirement date		October 2017
Maturity date		October 2017

[1] Outstanding borrowings bear interest at an annual interest rate of LIBOR plus 0.68 percent (2.28 percent at December 31, 2017). CMS Energy used these proceeds to retire $200 million of the 8.75 percent senior notes due June 2019.

[2] CMS Energy retired this debt at a premium and recorded a loss on extinguishment of $18 million in other expense on its consolidated statements of income.