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Financial Instruments (Narrative) (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Portion of long-term debt supported by third-party credit enhancements $ 35 $ 35
Consumers Energy Company    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Portion of long-term debt supported by third-party credit enhancements $ 35 $ 35