Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
DB Pension Plans and DB SERP
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	$ 50	$ 41	$ 48
Interest cost	83	103	95
Settlement loss	1	0	0
Expected return on plan assets	(191)	(162)	(149)
Amortization of
Net loss	95	50	76
Prior service cost (credit)	1	1	3
Settlement loss	2	0	0
Net periodic cost (credit)	41	33	73
DB Pension Plans and DB SERP | Consumers Energy Company
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	49	40	47
Interest cost	78	97	88
Expected return on plan assets	(181)	(153)	(139)
Amortization of
Net loss	90	47	73
Prior service cost (credit)	1	1	3
Settlement loss	2	0	0
Net periodic cost (credit)	39	32	72
OPEB Plan
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	16	14	17
Interest cost	33	41	34
Settlement loss	0	0	0
Expected return on plan assets	(100)	(88)	(97)
Amortization of
Net loss	15	26	15
Prior service cost (credit)	(56)	(62)	(67)
Settlement loss	0	0	0
Net periodic cost (credit)	(92)	(69)	(98)
OPEB Plan | Consumers Energy Company
Defined Benefit Plan, Roll Forwards [Abstract]
Service cost	15	13	16
Interest cost	31	40	33
Expected return on plan assets	(93)	(82)	(91)
Amortization of
Net loss	15	26	16
Prior service cost (credit)	(54)	(61)	(65)
Settlement loss	0	0	0
Net periodic cost (credit)	$ (86)	$ (64)	$ (91)