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Retirement Benefits (Schedule Of SERP Trust Assets, ABO And Contributions) (Details) - DB SERP [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]    
ABO $ 140 $ 145
Contributions 25  
Consumers Energy Company [Member]    
Defined Benefit Plan Disclosure [Line Items]    
ABO 97 99
Contributions 17  
DB SERP Trust Assets [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Trust assets 148 131
DB SERP Trust Assets [Member] | Consumers Energy Company [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Trust assets $ 106 $ 93