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Financial Instruments (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2018 | Jul. 31, 2018 | Dec. 31, 2017
Financial Instruments [Line Items] |  |  |  |
Portion of long-term debt supported by third-party credit enhancements |  | $ 35 |  | $ 103
Consumers Energy Company |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Portion of long-term debt supported by third-party credit enhancements |  | $ 35 |  | 103
DB SERP | Minimum |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Debt security term |  | 1 year |  |
DB SERP | Maximum |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Debt security term |  | 10 years |  |
CMS Energy Note Payable |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Notes payable |  |  | $ 146 |
Interest Rate |  |  | 4.10% |
Common Stock | Consumers Energy Company |  |  |  |
Financial Instruments [Line Items] |  |  |  |
Equity investment | [1] | $ 1 |  | $ 21
[1] | 1 All assets and liabilities were classified as Level 1 with the exception of some commodity contracts, which were classified as Level 3.

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