Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
DB Pension Plans
Net periodic cost (credit)
Service cost	$ 12	$ 12	$ 36	$ 34
Interest cost	22	23	67	67
Expected return on plan assets	(38)	(39)	(112)	(115)
Amortization of:
Net loss	19	20	56	60
Prior service cost (credit)	1	1	2	3
Net periodic cost (credit)	16	17	49	49
DB Pension Plans | Consumers Energy Company
Net periodic cost (credit)
Service cost	12	11	35	33
Interest cost	22	22	64	65
Expected return on plan assets	(35)	(38)	(104)	(112)
Amortization of:
Net loss	17	19	53	58
Prior service cost (credit)	1	1	2	3
Net periodic cost (credit)	17	15	50	47
OPEB Plan
Net periodic cost (credit)
Service cost	4	5	13	15
Interest cost	9	13	27	39
Expected return on plan assets	(24)	(22)	(73)	(67)
Amortization of:
Net loss	3	7	11	23
Prior service cost (credit)	(16)	(8)	(50)	(26)
Net periodic cost (credit)	(24)	(5)	(72)	(16)
OPEB Plan | Consumers Energy Company
Net periodic cost (credit)
Service cost	4	5	12	14
Interest cost	8	12	25	38
Expected return on plan assets	(22)	(21)	(68)	(63)
Amortization of:
Net loss	4	8	12	24
Prior service cost (credit)	(17)	(8)	(49)	(25)
Net periodic cost (credit)	$ (23)	$ (4)	$ (68)	$ (12)