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Financings And Capitalization (Major Long-Term Debt Transactions) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]      
Principal Balance $ 2,480    
Debt retirement, principal 1,793    
Loss on extinguishment 16 $ 18 $ 18
Consumers Energy Company      
Debt Instrument [Line Items]      
Debt retirement, principal 1,168    
Tax-exempt pollution control revenue bond | Consumers Energy Company      
Debt Instrument [Line Items]      
Debt retirement, principal $ 68    
Debt retirement date April 2018    
Maturity date 2018-04    
FMB's 5.650% Due 2018 | Consumers Energy Company      
Debt Instrument [Line Items]      
Debt retirement, principal $ 250    
Interest rate 5.65%    
Debt retirement date May 2018    
Maturity date 2018-09    
FMB's 6.125% Due 2019 | Consumers Energy Company      
Debt Instrument [Line Items]      
Debt retirement, principal $ 350    
Interest rate 6.125%    
Debt retirement date November 2018    
Maturity date 2019-03    
FMB's 6.700% Due 2019 | Consumers Energy Company      
Debt Instrument [Line Items]      
Debt retirement, principal $ 500    
Interest rate 6.70%    
Debt retirement date November 2018    
Maturity date 2019-09    
Term loans and revolving credit agreements      
Debt Instrument [Line Items]      
Principal Balance $ 100    
Term loans and revolving credit agreements | Term Loan Facility Due 2019      
Debt Instrument [Line Items]      
Principal Balance $ 100    
Debt issuance date October 2018    
Maturity date 2025-09    
First Mortgage Bonds | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 1,900    
First Mortgage Bonds | FMB's 4.050% Due 2048 | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 550    
Interest rate 4.05%    
Debt issuance date May 2018    
Maturity date 2048-05    
First Mortgage Bonds | FMB's 3.680% Due 2027 | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 100    
Interest rate 3.68%    
Debt issuance date October 2018    
Maturity date 2027-10    
First Mortgage Bonds | FMB's 4.010% Due 2038 | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 215    
Interest rate 4.01%    
Debt issuance date October 2018    
Maturity date 2038-10    
First Mortgage Bonds | FMB's 4.280% Due 2057 | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 185    
Interest rate 4.28%    
Debt issuance date October 2018    
Maturity date 2057-10    
First Mortgage Bonds | FMB's 3.800% Due 2028 | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 300    
Interest rate 3.80%    
Debt issuance date November 2018    
Maturity date 2028-11    
First Mortgage Bonds | FMB's 4.350% Due 2049 | Consumers Energy Company      
Debt Instrument [Line Items]      
Principal Balance $ 550    
Interest rate 4.35%    
Debt issuance date November 2018    
Maturity date 2049-04    
First Mortgage Bonds | FMB's 5.650% Due 2018 | Consumers Energy Company      
Debt Instrument [Line Items]      
Interest rate 5.65%    
First Mortgage Bonds | FMB's 6.125% Due 2019 | Consumers Energy Company      
Debt Instrument [Line Items]      
Interest rate 6.125%    
First Mortgage Bonds | FMB's 6.700% Due 2019 | Consumers Energy Company      
Debt Instrument [Line Items]      
Interest rate 6.70%    
CMS Energy      
Debt Instrument [Line Items]      
Debt retirement, principal $ 625    
CMS Energy | Term Loan Facility Due March 2018      
Debt Instrument [Line Items]      
Debt retirement, principal $ 180    
Debt retirement date March 2018    
Maturity date 2018-12    
CMS Energy | Senior Notes 8.750% Due 2019      
Debt Instrument [Line Items]      
Debt retirement, principal $ 100    
Interest rate 8.75%    
Debt retirement date June 2018    
Maturity date 2019-06    
Loss on extinguishment $ 5    
CMS Energy | Term Loan Facility Due August 2018      
Debt Instrument [Line Items]      
Debt retirement, principal $ 45    
Debt retirement date August 2018    
Maturity date 2018-12    
CMS Energy | Senior Notes 6.250% Due 2020      
Debt Instrument [Line Items]      
Debt retirement, principal $ 300    
Interest rate 6.25%    
Debt retirement date October 2018    
Maturity date 2020-02    
Loss on extinguishment $ 11    
CMS Energy | Junior subordinated notes      
Debt Instrument [Line Items]      
Principal Balance 480    
CMS Energy | Junior subordinated notes | Junior Notes 5.625% Due 2078      
Debt Instrument [Line Items]      
Principal Balance $ 200    
Interest rate 5.625%    
Debt issuance date March 2018    
Maturity date 2078-03    
CMS Energy | Junior subordinated notes | Junior Notes 5.875% Due 2078      
Debt Instrument [Line Items]      
Principal Balance $ 250    
Interest rate 5.875%    
Debt issuance date September 2018    
Maturity date 2078-10    
CMS Energy | Junior subordinated notes | Junior Notes 5.875% Due 2078      
Debt Instrument [Line Items]      
Principal Balance $ 30    
Interest rate 5.875%    
Debt issuance date October 2018    
Maturity date 2078-10    
CMS Energy | Senior notes | Senior Notes 8.750% Due 2019      
Debt Instrument [Line Items]      
Interest rate 8.75%    
CMS Energy | Senior notes | Senior Notes 6.250% Due 2020      
Debt Instrument [Line Items]      
Interest rate 6.25%