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Retirement Benefits (Narrative) (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2016 USD ($)
DB Pension Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Decrease in service cost and interest expense | $ 17
OPEB [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Decrease in service cost and interest expense | 9
Consumers Energy Company [Member] | DB Pension Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Decrease in service cost and interest expense | 17
Consumers Energy Company [Member] | OPEB [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Decrease in service cost and interest expense | $ 9

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