Retirement Benefits (Schedule Of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Plans, Defined Benefit [Member]
Service cost	$ 12	$ 12	$ 36	$ 36
Interest expense	24	25	74	75
Expected return on plan assets	(31)	(28)	(94)	(84)
Amortization of Net loss	19	16	57	47
Amortization of Prior service cost	1	1	4	4
Net periodic cost	25	26	77	78
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Service cost	12	12	35	35
Interest expense	24	24	72	73
Expected return on plan assets	(30)	(27)	(91)	(82)
Amortization of Net loss	18	15	55	46
Amortization of Prior service cost	1	1	4	4
Net periodic cost	25	25	75	76
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	8	7	24	20
Interest expense	21	20	62	58
Expected return on plan assets	(17)	(17)	(50)	(50)
Amortization of Net loss	12	7	35	23
Amortization of Prior service cost	(5)	(5)	(15)	(15)
Net periodic cost	19	12	56	36
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Service cost	7	7	23	20
Interest expense	20	19	60	56
Expected return on plan assets	(15)	(15)	(46)	(46)
Amortization of Net loss	12	7	35	23
Amortization of Prior service cost	(5)	(5)	(15)	(15)
Net periodic cost	$ 19	$ 13	$ 57	$ 38