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Financings and Capitalization (Major Long-Term Debt Transactions) (Details) - USD ($) | 1 Months Ended | 9 Months Ended
--- | --- | ---
Oct. 29, 2020 | Sep. 30, 2020
Debt Instrument [Line Items] |  |
Principal balance |  | $ 2,334,000,000
Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Principal balance |  | 1,534,000,000
First Mortgage Bonds Due August 2051 | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 575,000,000
Interest Rate |  | 3.50%
First Mortgage Bonds Due May 2060 | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 525,000,000
Interest Rate |  | 2.50%
First Mortgage Bonds Due May 2070 | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 134,000,000
Effective interest rate |  | 0.00%
CMS Energy Corporation |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 800,000,000
CMS Energy Corporation | Junior subordinated notes |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 500,000,000
Interest Rate |  | 4.75%
Debt instrument interest rate reset term |  | 5 years
London Interbank Offered Rate (LIBOR) | First Mortgage Bonds Due May 2070 | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate |  | 0.30%
Interest rate floor |  | 0.00%
Five-year Treasury Rate | CMS Energy Corporation | Junior subordinated notes |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate |  | 4.116%
Term Loan Facility Due February 2021 | CMS Energy Corporation | Term loan facility |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 300,000,000
Effective interest rate |  | 0.606%
Term Loan Facility Due February 2021 | London Interbank Offered Rate (LIBOR) | CMS Energy Corporation | Term loan facility |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate |  | 0.50%
Term loan facility due January 2021 | Term loan facility | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Principal balance |  | $ 300,000,000
Effective interest rate |  | 0.556%
Term loan facility due January 2021 | London Interbank Offered Rate (LIBOR) | Term loan facility |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate |  | 0.45%
Subsequent Event | First Mortgage Bonds Due October 2070 | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Principal balance | $ 127,000,000 |
Subsequent Event | London Interbank Offered Rate (LIBOR) | First Mortgage Bonds Due October 2070 | Consumers Energy Company |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate | 0.30% |
Interest rate floor | 0.00% |

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