Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
DB Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 11	$ 11
Interest cost	22	21
Expected return on plan assets	(38)	(37)
Amortization of Net loss	20	17
Amortization of Prior service cost (credit)	1	1
Net periodic cost (credit)	16	13
DB Pension Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	11	10
Interest cost	21	21
Expected return on plan assets	(37)	(36)
Amortization of Net loss	20	17
Amortization of Prior service cost (credit)	1	1
Net periodic cost (credit)	16	13
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5
Interest cost	13	12
Expected return on plan assets	(22)	(22)
Amortization of Net loss	8	5
Amortization of Prior service cost (credit)	(9)	(10)
Net periodic cost (credit)	(5)	(10)
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	4
Interest cost	13	11
Expected return on plan assets	(21)	(20)
Amortization of Net loss	8	6
Amortization of Prior service cost (credit)	(9)	(10)
Net periodic cost (credit)	$ (4)	$ (9)