Retirement Benefits (Schedule Of SERP Trust Assets, ABO And Contributions) (Details) - DB SERP [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
ABO	$ 140	$ 145
Contributions	25
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
ABO	97	99
Contributions	17
DB SERP Trust Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	148	131
DB SERP Trust Assets [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 106	$ 93