Retirement Benefits (Narrative) (Details) $ in Millions	9 Months Ended
	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