Retirement Benefits (Schedule Of Plan Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 12	$ 108	$ 262
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions	$ 8	98	$ 243
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions		100
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Postretirement benefits contributions		$ 93