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Schedule I - Condensed Financial Information of Registrant (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2019
USD ($)
lawsuit
Dec. 31, 2018
USD ($)
Jul. 31, 2018
USD ($)
CMS Energy      
Condensed Financial Statements, Captions [Line Items]      
Maximum potential obligation $ 430    
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
Class Action Lawsuits      
Condensed Financial Statements, Captions [Line Items]      
Number of lawsuits | lawsuit 4    
Individual Lawsuits      
Condensed Financial Statements, Captions [Line Items]      
Number of lawsuits | lawsuit 1