Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	$ 10	$ 34	$ 31
Interest cost	23	21	67	64
Expected return on plan assets	(39)	(36)	(115)	(110)
Amortization of Net loss	20	17	60	52
Amortization of Prior service cost (credit)	1	1	3	3
Net periodic cost (credit)	17	13	49	40
DB Pension Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	11	11	33	31
Interest cost	22	20	65	62
Expected return on plan assets	(38)	(35)	(112)	(107)
Amortization of Net loss	19	16	58	50
Amortization of Prior service cost (credit)	1	1	3	3
Net periodic cost (credit)	15	13	47	39
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	15	14
Interest cost	13	12	39	35
Expected return on plan assets	(22)	(22)	(67)	(65)
Amortization of Net loss	7	5	23	16
Amortization of Prior service cost (credit)	(8)	(10)	(26)	(31)
Net periodic cost (credit)	(5)	(10)	(16)	(31)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	4	14	13
Interest cost	12	12	38	34
Expected return on plan assets	(21)	(20)	(63)	(60)
Amortization of Net loss	8	5	24	16
Amortization of Prior service cost (credit)	(8)	(10)	(25)	(30)
Net periodic cost (credit)	$ (4)	$ (9)	$ (12)	$ (27)