Retirement Benefits (Schedule Of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension And DB SERP [Member]
Service cost	$ 49	$ 49	$ 45
Interest expense	105	106	104
Expected return on plan assets	(125)	(112)	(92)
Amortization of Net loss	79	65	52
Amortization of Prior service cost	5	5	5
Net periodic cost	113	113	114
Regulatory adjustment			30
Net periodic cost after regulatory adjustment	113	113	144
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Service cost	48	48	44
Interest expense	100	101	99
Expected return on plan assets	(122)	(109)	(89)
Amortization of Net loss	77	63	50
Amortization of Prior service cost	5	5	5
Net periodic cost	108	108	109
Regulatory adjustment			30
Net periodic cost after regulatory adjustment	108	108	139
Pension Plans, Defined Benefit [Member]
Service cost	48	48
Interest expense	99	100
Defined Benefit Plan, Amortization of Net Gains (Losses) from regulatory asset	98
Defined Benefit Plan, Amortization of Net Gains (Losses)	2
Estimated time of amortization of gains losses	11 years	11 years	12 years
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Amortization of Net Gains (Losses) from regulatory asset	98
Estimated time of amortization of gains losses	11 years	11 years	12 years
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	32	27	26
Interest expense	82	77	80
Expected return on plan assets	(66)	(66)	(60)
Amortization of Net loss	46	30	32
Amortization of Prior service cost	(20)	(20)	(17)
Net periodic cost	74	48	61
Regulatory adjustment			5	[1]
Net periodic cost after regulatory adjustment	74	48	66
Defined Benefit Plan, Amortization of Net Prior Service Cost (Credit) from regulatory asset	23
Defined Benefit Plan, Amortization of Net Prior Service Cost (Credit)	(1)
Estimated time of amortization of gains losses	13 years	13 years	14 years
Estimated time of prior service cost			10 years
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Service cost	31	26	25
Interest expense	79	74	77
Expected return on plan assets	(61)	(61)	(56)
Amortization of Net loss	47	31	33
Amortization of Prior service cost	(20)	(20)	(16)
Net periodic cost	76	50	63
Regulatory adjustment			5	[1]
Net periodic cost after regulatory adjustment	76	50	68
Defined Benefit Plan, Amortization of Net Prior Service Cost (Credit) from regulatory asset	23
Estimated time of amortization of gains losses	13 years	13 years	14 years
Estimated time of prior service cost			10 years
Pension And OPEB [Member]
Pension And OPEB Regulatory Asset, less than	1	1
Amortized net gains and losses in excess of PBO or MRV	10.00%	10.00%	10.00%
Pension And OPEB [Member] | Consumers Energy Company [Member]
Pension And OPEB Regulatory Asset, less than	$ 1	$ 1
Amortized net gains and losses in excess of PBO or MRV	10.00%	10.00%	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 asset was less than $1 million at December 31, 2012 and 2011.