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Retirement Benefits (Narrative) (Details)
$ in Millions
 6 Months Ended
Jun. 30, 2016
USD ($)
DB Pension Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Decrease in service cost and interest expense $ 12
OPEB [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Decrease in service cost and interest expense 6
Consumers Energy Company [Member] | DB Pension Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Decrease in service cost and interest expense 11
Consumers Energy Company [Member] | OPEB [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Decrease in service cost and interest expense $ 6