Retirement Benefits (Schedule Of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 13	$ 12	$ 40	$ 36
Interest expense	23	24	70	74
Expected return on plan assets	(32)	(31)	(96)	(94)
Amortization of Net loss	24	19	72	57
Amortization of Prior service cost (credit)	1	1	3	4
Net periodic cost (credit)	29	25	89	77
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	13	12	39	35
Interest expense	23	24	69	72
Expected return on plan assets	(31)	(30)	(94)	(91)
Amortization of Net loss	23	18	70	55
Amortization of Prior service cost (credit)	1	1	3	4
Net periodic cost (credit)	29	25	87	75
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	8	23	24
Interest expense	14	21	52	62
Expected return on plan assets	(20)	(17)	(58)	(50)
Amortization of Net loss	3	12	24	35
Amortization of Prior service cost (credit)	(10)	(5)	(20)	(15)
Net periodic cost (credit)	(8)	19	21	56
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	7	23	23
Interest expense	14	20	50	60
Expected return on plan assets	(19)	(15)	(54)	(46)
Amortization of Net loss	3	12	24	35
Amortization of Prior service cost (credit)	(10)	(5)	(20)	(15)
Net periodic cost (credit)	$ (7)	$ 19	$ 23	$ 57