Retirement Benefits (Schedule Of Net Loss And Prior Service Cost (Credit) That Will Be Amortized In Next Fiscal Year) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset (liability)	$ 75
AOCI	2
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset (liability)	75
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset (liability)	(49)
AOCI	(2)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory asset (liability)	$ (49)