Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11	$ 10	$ 22	$ 21
Interest cost	22	22	44	43
Expected return on plan assets	(38)	(37)	(76)	(74)
Amortization of Net loss	20	18	40	35
Amortization of Prior service cost (credit)	1	1	2	2
Net periodic cost (credit)	16	14	32	27
DB Pension Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	11	10	22	20
Interest cost	22	21	43	42
Expected return on plan assets	(37)	(36)	(74)	(72)
Amortization of Net loss	19	17	39	34
Amortization of Prior service cost (credit)	1	1	2	2
Net periodic cost (credit)	16	13	32	26
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	4	10	9
Interest cost	13	11	26	23
Expected return on plan assets	(23)	(21)	(45)	(43)
Amortization of Net loss	8	6	16	11
Amortization of Prior service cost (credit)	(9)	(11)	(18)	(21)
Net periodic cost (credit)	(6)	(11)	(11)	(21)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5	9	9
Interest cost	13	11	26	22
Expected return on plan assets	(21)	(20)	(42)	(40)
Amortization of Net loss	8	5	16	11
Amortization of Prior service cost (credit)	(8)	(10)	(17)	(20)
Net periodic cost (credit)	$ (4)	$ (9)	$ (8)	$ (18)