Retirement Benefits (Schedule of Net Benefit Costs) (Details) (USD $) In Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010		Jun. 30, 2011	Jun. 30, 2010
Consumers Energy Company [Member] | Pension Plans, Defined Benefit [Member]
Service cost	$ 11	$ 10		$ 23	$ 21
Interest expense	25	24		49	48
Expected return on plan assets	(28)	(22)		(55)	(45)
Amortization of Net loss	16	13		31	25
Amortization of Prior service cost	2	1		3	3
Net periodic cost	26	26		51	52
Regulatory adjustment		21	[1]		23	[1]
Net periodic cost after regulatory adjustment	26	47		51	75
Pension Plans, Defined Benefit [Member]
Service cost	12	11		24	22
Interest expense	25	25		50	49
Expected return on plan assets	(28)	(23)		(56)	(46)
Amortization of Net loss	15	13		31	26
Amortization of Prior service cost	2	1		3	3
Net periodic cost	26	27		52	54
Regulatory adjustment		21	[1]		23	[1]
Net periodic cost after regulatory adjustment	26	48		52	77
Consumers Energy Company [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	7	6		13	13
Interest expense	19	20		37	40
Expected return on plan assets	(16)	(14)		(31)	(28)
Amortization of Net loss	8	8		16	16
Amortization of Prior service cost	(5)	(4)		(10)	(6)
Net periodic cost	13	16		25	35
Regulatory adjustment		6	[1]		7	[1]
Net periodic cost after regulatory adjustment	13	22		25	42
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	6	6		13	13
Interest expense	19	20		38	41
Expected return on plan assets	(16)	(14)		(33)	(29)
Amortization of Net loss	8	8		16	16
Amortization of Prior service cost	(5)	(5)		(10)	(7)
Net periodic cost	12	15		24	34
Regulatory adjustment		6	[1]		7	[1]
Net periodic cost after regulatory adjustment	$ 12	$ 21		$ 24	$ 41

[1] Regulatory adjustments are the differences between amounts included in rates and the periodic benefit cost calculated. These regulatory adjustments were offset by surcharge revenues, resulting in no impact to net income for the periods presented.