Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 11
Interest cost	22	22
Expected return on plan assets	(37)	(38)
Amortization of Net loss	18	20
Amortization of Prior service cost (credit)	1	1
Net periodic cost (credit)	16	16
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	11
Interest cost	21	21
Expected return on plan assets	(35)	(37)
Amortization of Net loss	18	20
Amortization of Prior service cost (credit)	1	1
Net periodic cost (credit)	17	16
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5
Interest cost	9	13
Expected return on plan assets	(24)	(22)
Amortization of Net loss	4	8
Amortization of Prior service cost (credit)	(17)	(9)
Net periodic cost (credit)	(24)	(5)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5
Interest cost	8	13
Expected return on plan assets	(23)	(21)
Amortization of Net loss	4	8
Amortization of Prior service cost (credit)	(16)	(9)
Net periodic cost (credit)	$ (23)	$ (4)