Retirement Benefits (Schedule of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010		Dec. 31, 2009
Pension and SERP [Member]
Service cost	$ 49	$ 45		$ 41
Interest expense	106	104		102
Expected return on plan assets	(112)	(92)		(86)
Amortization of Net loss	65	52		41
Amortization of Prior service cost	5	5		6
Net periodic cost	113	114		104
Regulatory adjustment		30	[1]
Net periodic cost after regulatory adjustment	113	144		104
Pension and SERP [Member] | Consumers Energy Company [Member]
Service cost	48	44		40
Interest expense	101	99		97
Expected return on plan assets	(109)	(89)		(83)
Amortization of Net loss	63	50		40
Amortization of Prior service cost	5	5		5
Net periodic cost	108	109		99
Regulatory adjustment		30	[1]
Net periodic cost after regulatory adjustment	108	139		99
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	27	26		24
Interest expense	77	80		80
Expected return on plan assets	(66)	(60)		(50)
Amortization of Net loss	30	32		33
Amortization of Prior service cost	(20)	(17)		(10)
Net periodic cost	48	61		77
Regulatory adjustment		5	[1]
Net periodic cost after regulatory adjustment	48	66		77
Defined Benefit Plan, Amortization of Net Prior Service Cost (Credit) from regulatory asset	33
Defined Benefit Plan, Amortization of Net Prior Service Cost (Credit)	1
Estimated time of amortization of gains losses	13	14		14
Estimated time of prior service cost		10
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Service cost	26	25		24
Interest expense	74	77		77
Expected return on plan assets	(61)	(56)		(46)
Amortization of Net loss	31	33		33
Amortization of Prior service cost	(20)	(16)		(10)
Net periodic cost	50	63		78
Regulatory adjustment		5	[1]
Net periodic cost after regulatory adjustment	50	68		78
Defined Benefit Plan, Amortization of Net Prior Service Cost (Credit) from regulatory asset	33
Estimated time of amortization of gains losses	13	14		14
Estimated time of prior service cost		10
Pension Plans, Defined Benefit [Member]
Service cost	48	44
Interest expense	100	98
Defined Benefit Plan, Amortization of Net Gains (Losses) from regulatory asset	79
Defined Benefit Plan, Amortization of Net Gains (Losses)	2
Estimated time of amortization of gains losses	11	12		12
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Amortization of Net Gains (Losses) from regulatory asset	79
Estimated time of amortization of gains losses	11	12		12
Pension and OPEB [Member]
Pension and OPEB regulatory liability less than million	1	1
Amortized net gains and losses in excess of PBO or MRV	10.00%
Pension and OPEB [Member] | Consumers Energy Company [Member]
Pension and OPEB regulatory liability less than million	$ 1	$ 1
Amortized net gains and losses in excess of PBO or MRV	10.00%

[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, which resulted in no impact to net income for the years presented. The pension and OPEB regulatory liability was less than $1 million at December 31, 2011 and 2010.