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Retirement Benefits (Schedule Of Expected Benefit Payments) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
DB Pension Plans [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Benefit Payments, 2018 $ 157
Benefit Payments, 2019 163
Benefit Payments, 2020 168
Benefit Payments, 2021 169
Benefit Payments, 2022 170
Benefit Payments, 2023-2027 457
DB Pension Plans [Member] | Consumers Energy Company [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Benefit Payments, 2018 153
Benefit Payments, 2019 159
Benefit Payments, 2020 163
Benefit Payments, 2021 164
Benefit Payments, 2022 166
Benefit Payments, 2023-2027 457
DB SERP [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Benefit Payments, 2018 10
Benefit Payments, 2019 10
Benefit Payments, 2020 10
Benefit Payments, 2021 10
Benefit Payments, 2022 10
Benefit Payments, 2023-2027 47
DB SERP [Member] | Consumers Energy Company [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Benefit Payments, 2018 7
Benefit Payments, 2019 7
Benefit Payments, 2020 7
Benefit Payments, 2021 7
Benefit Payments, 2022 7
Benefit Payments, 2023-2027 32
OPEB Plan [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Benefit Payments, 2018 56
Benefit Payments, 2019 58
Benefit Payments, 2020 60
Benefit Payments, 2021 62
Benefit Payments, 2022 62
Benefit Payments, 2023-2027 312
OPEB Plan [Member] | Consumers Energy Company [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Benefit Payments, 2018 54
Benefit Payments, 2019 55
Benefit Payments, 2020 57
Benefit Payments, 2021 59
Benefit Payments, 2022 60
Benefit Payments, 2023-2027 $ 298