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Retirement Benefits (Schedule Of Effect Of One-Percentage-Point Change In Assumed Health Care Cost Trend Rates) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components $ 11
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation 168
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components (9)
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation (137)
Consumers Energy Company [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components 11
Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation 164
Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components (9)
Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation $ (133)