Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		9 Months Ended
	Feb. 28, 2015	Oct. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions			$ 35	$ 5
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions			$ 33	$ 3
Consumers Energy Company [Member] | OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 29	$ 25