Retirement Benefits (Schedule Of SERP Trust Assets, ABO And Contributions) (Details) - DB SERP - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 0	$ 0	$ 0
ABO	159	149
Contributions	8	0
Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
Trust assets	0	0	$ 0
ABO	115	107
Contributions	5	0
Trust assets
Defined Benefit Plan Disclosure [Line Items]
Trust assets	146	143
Trust assets | Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 107	$ 104