Retirement Benefits (Schedule of Net Benefit Costs) (Details) (USD $) In Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010		Sep. 30, 2011	Sep. 30, 2010
Consumers Energy Company [Member] | Pension Plans, Defined Benefit [Member]
Service cost	$ 12	$ 11		$ 35	$ 32
Interest expense	24	23		73	71
Expected return on plan assets	(27)	(22)		(82)	(67)
Amortization of Net loss	15	13		46	38
Amortization of Prior service cost	1	1		4	4
Net periodic cost	25	26		76	78
Regulatory adjustment		7	[1]		30	[1]
Net periodic cost after regulatory adjustment	25	33		76	108
Pension Plans, Defined Benefit [Member]
Service cost	12	11		36	33
Interest expense	25	25		75	74
Expected return on plan assets	(28)	(24)		(84)	(70)
Amortization of Net loss	16	13		47	39
Amortization of Prior service cost	1	1		4	4
Net periodic cost	26	26		78	80
Regulatory adjustment		7	[2]		30	[2]
Net periodic cost after regulatory adjustment	26	33		78	110
Consumers Energy Company [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	7	6		20	19
Interest expense	19	19		56	59
Expected return on plan assets	(15)	(14)		(46)	(42)
Amortization of Net loss	7	8		23	24
Amortization of Prior service cost	(5)	(5)		(15)	(11)
Net periodic cost	13	14		38	49
Regulatory adjustment		(1)	[1]		5	[1]
Net periodic cost after regulatory adjustment	13	13		38	54
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	7	7		20	20
Interest expense	20	20		58	61
Expected return on plan assets	(17)	(16)		(50)	(45)
Amortization of Net loss	7	8		23	24
Amortization of Prior service cost	(5)	(5)		(15)	(12)
Net periodic cost	12	14		36	48
Regulatory adjustment		(1)	[2]		5	[2]
Net periodic cost after regulatory adjustment	$ 12	$ 13		$ 36	$ 53

[1] 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.

[2] 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.