Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Feb. 28, 2015	Oct. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 31	$ 2
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	30	1
Consumers Energy Company [Member] | OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions			$ 29	$ 25