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Financial Instruments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jul. 31, 2018 | Dec. 31, 2015
Consumers Energy Company |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
Portion of long-term debt supported by third-party credit enhancements | $ 35 | $ 103 |  |  |
Reclassified gain net of tax | (1) | 9 | $ 0 |  |
Gain on CMS Energy common stock |  | 14 |  |  |
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% |
US Treasury Securities | DB SERP | Minimum |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
Debt securities term |  | 1 year |  |  |
US Treasury Securities | DB SERP | Maximum |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
Debt securities term |  | 10 years |  |  |
Accumulated Deficit |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | $ 8 | 0 |  | $ 33
Accumulated Deficit | Consumers Energy Company |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | 19 | $ 0 |  | $ 0
Accumulated Deficit | Accounting Standards Update 2016-01 | Consumers Energy Company |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
Cumulative effect of change in accounting principle |  | 19 |  |  |
Common Stock | Consumers Energy Company |  |  |  |  |
Financial Instruments [Line Items] |  |  |  |  |
CMS Energy common stock | $ 1 | $ 21 |  |  |

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