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Financial Instruments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Financial Instruments [Line Items] |  |  |
Portion of long-term debt supported by third-party credit enhancements | $ 35 | $ 35 |
Consumers Energy Company |  |  |
Financial Instruments [Line Items] |  |  |
Reclassified gain net of tax | $ 0 | (1) | $ 9
DB SERP |  |  |
Financial Instruments [Line Items] |  |  |
Gross reclassified gain |  |  | 2
Reclassified gain net of tax |  |  | 1
DB SERP | Consumers Energy Company |  |  |
Financial Instruments [Line Items] |  |  |
Gross reclassified gain |  |  | 2
Reclassified gain net of tax |  |  | $ 1
CMS Energy Note Payable |  |  |
Financial Instruments [Line Items] |  |  |
Notes payable |  | $ 146 |
Interest rate |  | 4.10% |

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