Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 50	$ 42
Interest cost	90	108	105
Expected return on plan assets	(147)	(138)	(135)
Amortization of Net loss	71	97	60
Amortization of Prior service cost (credit)	4	1	1
Net periodic cost (credit)	60	118	73
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	41	49	41
Interest cost	87	103	100
Expected return on plan assets	(143)	(134)	(131)
Amortization of Net loss	68	93	59
Amortization of Prior service cost (credit)	4	1	1
Net periodic cost (credit)	57	112	70
DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	42	49
Interest cost	85	102
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	18	25	20
Interest cost	46	58	56
Expected return on plan assets	(85)	(91)	(88)
Amortization of Net loss	21	21	2
Amortization of Prior service cost (credit)	(41)	(41)	(41)
Net periodic cost (credit)	(41)	(28)	(51)
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	17	25	20
Interest cost	45	56	54
Expected return on plan assets	(80)	(86)	(83)
Amortization of Net loss	22	22	3
Amortization of Prior service cost (credit)	(40)	(40)	(40)
Net periodic cost (credit)	$ (36)	$ (23)	$ (46)