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Retirement Benefits (Schedule Of SERP Trust Assets, ABO And Contributions) (Details) - DB SERP - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Trust assets $ 0 $ 0 $ 0
ABO 149 137  
Contributions 0 8  
Consumers Energy Company      
Defined Benefit Plan Disclosure [Line Items]      
Trust assets 0 0 $ 0
ABO 107 98  
Contributions 0 5  
Trust assets      
Defined Benefit Plan Disclosure [Line Items]      
Trust assets 143 147  
Trust assets | Consumers Energy Company      
Defined Benefit Plan Disclosure [Line Items]      
Trust assets $ 104 $ 106