Retirement Benefits (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($)
DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in service cost and interest expense	$ 6
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in service cost and interest expense	3
Consumers Energy Company [Member] | DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in service cost and interest expense	6
Consumers Energy Company [Member] | OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Decrease in service cost and interest expense	$ 3