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Retirement Benefits (Narrative) (Details) $ in Millions | 6 Months Ended
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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

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