Retirement Benefits (Schedule Of Benefit Obligations In Excess Of Fair Value Of Plan Assets) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period	$ 2,073		$ 2,354
Service cost	41		53
Interest cost	99		94
Actuarial (gain) loss	458		(308)
Benefits paid	(124)		(120)
Benefit obligation at end of period	2,547		2,073
Plan assets at fair value at beginning of period	1,964		1,727
Actual return on plan assets	139		206
Company contributions			150
Actual benefits paid	(124)		(119)
Plan assets at fair value at end of period	1,979		1,964
Funded status	(568)	[1]	(109)	[1]
Pension Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status	(532)		(86)
DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period	132		144
Service cost	1		1
Interest cost	6		6
Actuarial (gain) loss	24		(12)
Benefits paid	(7)		(7)
Benefit obligation at end of period	156		132
Company contributions	7		7
Actual benefits paid	(7)		(7)
Funded status	(156)		(132)
DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period	93		100
Service cost	1		1
Interest cost	4		4
Actuarial (gain) loss	17		(8)
Benefits paid	(4)		(4)
Benefit obligation at end of period	111		93
Company contributions	4		4
Actual benefits paid	(4)		(4)
Funded status	(111)		(93)
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period	1,123		1,729
Service cost	20		29		32
Interest cost	56		65		82
Plan amendments			(208)
Actuarial (gain) loss	230		(440)
Benefits paid	(51)	[2]	(52)	[2]
Benefit obligation at end of period	1,378		1,123		1,729
Plan assets at fair value at beginning of period	1,218		1,047
Actual return on plan assets	72		150
Company contributions	25		72
Actual benefits paid	(50)	[2]	(51)	[2]
Plan assets at fair value at end of period	1,265		1,218		1,047
Funded status	(113)		95
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	4		5		5
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period	1,088		1,670
Service cost	20		28		31
Interest cost	54		63		79
Plan amendments			(200)	[3]
Actuarial (gain) loss	223		(424)
Benefits paid	(49)	[2]	(49)	[2]
Benefit obligation at end of period	1,336		1,088		1,670
Plan assets at fair value at beginning of period	1,141		978
Actual return on plan assets	68		141
Company contributions	25		71
Actual benefits paid	(48)	[2]	(49)	[2]
Plan assets at fair value at end of period	1,186		1,141		978
Funded status	(150)		53
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received	$ 4		$ 4		$ 5

[1] At December 31, 2014, $532 million of the total funded status of the DB Pension Plan was attributable to Consumers, based on an allocation of expenses. At December 31, 2013, $86 million of the total funded status of the DB Pension Plan was attributable to Consumers, based on an allocation of expenses.

[2] CMS Energy received payments of $4 million in 2014 and $5 million in each of 2013 and 2012 for the Medicare Part D subsidies. Consumers received payments of $4 million in each of 2014 and 2013 and $5 million in 2012 for the Medicare Part D subsidies. The Medicare Part D subsidy payments are used to pay OPEB Plan benefits.

[3] Plan amendments resulted from changing the Medicare drug program provided through the OPEB Plan from an employer-sponsored prescription drug plan with a retiree drug subsidy to an EGWP that began on January 1, 2015, and from certain benefit changes to the OPEB Plan, to begin on January 1, 2016.