Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
DB Pension Plans
Net periodic cost (credit)
Service cost	$ 10	$ 12
Interest cost	25	22
Expected return on plan assets	(40)	(37)
Amortization of:
Net loss	12	18
Prior service cost (credit)	0	1
Net periodic cost (credit)	7	16
DB Pension Plans | Consumers Energy Company
Net periodic cost (credit)
Service cost	10	12
Interest cost	23	21
Expected return on plan assets	(38)	(35)
Amortization of:
Net loss	12	18
Prior service cost (credit)	0	1
Net periodic cost (credit)	7	17
OPEB Plan
Net periodic cost (credit)
Service cost	4	4
Interest cost	10	9
Expected return on plan assets	(22)	(24)
Amortization of:
Net loss	7	4
Prior service cost (credit)	(16)	(17)
Net periodic cost (credit)	(17)	(24)
OPEB Plan | Consumers Energy Company
Net periodic cost (credit)
Service cost	3	4
Interest cost	10	8
Expected return on plan assets	(21)	(23)
Amortization of:
Net loss	7	4
Prior service cost (credit)	(15)	(16)
Net periodic cost (credit)	$ (16)	$ (23)