Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		6 Months Ended
	Feb. 28, 2015	Oct. 31, 2014	Jun. 30, 2015	Jun. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions			$ 33	$ 4
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions			$ 31	$ 2
Consumers Energy Company [Member] | OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 29	$ 25