Retirement Benefits (Schedule Of SERP Trust Assets, ABO And Contributions) (Details) - DB SERP - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 0	$ 0	$ 0
ABO	137	149
Contributions	8	7
Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
Trust assets	0	0	$ 0
ABO	98	107
Contributions	5	6
Trust assets
Defined Benefit Plan Disclosure [Line Items]
Trust assets	147	146
Trust assets | Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 106	$ 106