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Schedule I - Condensed Financial Information of Registrant (Narrative) (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Jul. 31, 2018
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CMS Energy |  |
Condensed Financial Statements, Captions [Line Items] |  |
Maximum potential obligation | $ 433 |
CMS Energy Note Payable |  |
Condensed Financial Statements, Captions [Line Items] |  |
Notes payable |  | $ 146
Interest rate |  | 4.10%
CMS Energy Note Payable | Subsidiaries |  |
Condensed Financial Statements, Captions [Line Items] |  |
Notes payable |  | $ 124

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