Retirement Benefits (Schedule Of SERP Trust Assets, ABO And Contributions) (Details) - DB SERP [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
ABO	$ 149	$ 143
Contributions	7
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
ABO	107	101
Contributions	6
DB SERP Trust Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	146	144
DB SERP Trust Assets [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 106	$ 104