Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
DB Pension Plans
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	$ 14	$ 12
Interest cost	15	21
Expected return on plan assets	(52)	(48)
Amortization of
Net loss	25	22
Prior service cost (credit)	1	0
Settlement loss	1	0
Net periodic cost (credit)	4	7
DB Pension Plans | Consumers Energy Company
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	13	12
Interest cost	14	20
Expected return on plan assets	(49)	(45)
Amortization of
Net loss	25	21
Prior service cost (credit)	1	0
Settlement loss	1	0
Net periodic cost (credit)	5	8
OPEB Plan
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	4	4
Interest cost	6	8
Expected return on plan assets	(27)	(25)
Amortization of
Net loss	2	4
Prior service cost (credit)	(13)	(14)
Settlement loss	0	0
Net periodic cost (credit)	(28)	(23)
OPEB Plan | Consumers Energy Company
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	4	4
Interest cost	6	8
Expected return on plan assets	(25)	(23)
Amortization of
Net loss	2	4
Prior service cost (credit)	(13)	(14)
Settlement loss	0	0
Net periodic cost (credit)	$ (26)	$ (21)