Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
DB Pension Plans
Net periodic cost (credit)
Service cost	$ 12	$ 11	$ 24	$ 22
Interest cost	23	22	45	44
Expected return on plan assets	(37)	(38)	(74)	(76)
Amortization of:
Net loss	19	20	37	40
Prior service cost (credit)	0	1	1	2
Net periodic cost (credit)	17	16	33	32
DB Pension Plans | Consumers Energy Company
Net periodic cost (credit)
Service cost	11	11	23	22
Interest cost	21	22	42	43
Expected return on plan assets	(34)	(37)	(69)	(74)
Amortization of:
Net loss	18	19	36	39
Prior service cost (credit)	0	1	1	2
Net periodic cost (credit)	16	16	33	32
OPEB Plan
Net periodic cost (credit)
Service cost	5	5	9	10
Interest cost	9	13	18	26
Expected return on plan assets	(25)	(23)	(49)	(45)
Amortization of:
Net loss	4	8	8	16
Prior service cost (credit)	(17)	(9)	(34)	(18)
Net periodic cost (credit)	(24)	(6)	(48)	(11)
OPEB Plan | Consumers Energy Company
Net periodic cost (credit)
Service cost	4	4	8	9
Interest cost	9	13	17	26
Expected return on plan assets	(23)	(21)	(46)	(42)
Amortization of:
Net loss	4	8	8	16
Prior service cost (credit)	(16)	(8)	(32)	(17)
Net periodic cost (credit)	$ (22)	$ (4)	$ (45)	$ (8)