Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
DB Pension Plans And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 45	$ 42	$ 50
Interest cost	93	90	108
Expected return on plan assets	(153)	(147)	(138)
Amortization of Net loss	82	71	97
Amortization of Prior service cost (credit)	5	4	1
Net periodic cost (credit)	72	60	118
DB Pension Plans And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	44	41	49
Interest cost	90	87	103
Expected return on plan assets	(149)	(143)	(134)
Amortization of Net loss	79	68	93
Amortization of Prior service cost (credit)	4	4	1
Net periodic cost (credit)	68	57	112
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	18	25
Interest cost	51	46	58
Expected return on plan assets	(90)	(85)	(91)
Amortization of Net loss	29	21	21
Amortization of Prior service cost (credit)	(40)	(41)	(41)
Net periodic cost (credit)	(31)	(41)	(28)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	17	25
Interest cost	49	45	56
Expected return on plan assets	(84)	(80)	(86)
Amortization of Net loss	29	22	22
Amortization of Prior service cost (credit)	(39)	(40)	(40)
Net periodic cost (credit)	$ (26)	$ (36)	$ (23)