Retirement Benefits (Schedule Of Benefit Obligations In Excess Of Fair Value Of Plan Assets) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		$ 2,547	$ 2,073
Service cost		49	41
Interest cost		102	99
Plan amendments		13
Actuarial (gain) loss		(153)	458
Benefits paid		(155)	(124)
Benefit obligation at end of period		2,403	2,547	$ 2,073
Plan assets at fair value at beginning of period		1,979	1,964
Actual return on plan assets		(36)	139
Company contributions		225
Actual benefits paid		(155)	(124)
Plan assets at fair value at end of period		2,013	1,979	1,964
Funded status	[1],[2]	(390)	(568)
Pension Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status		(368)	(532)
DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		156	132
Service cost		1	1
Interest cost		6	6
Actuarial (gain) loss		(5)	24
Benefits paid		(8)	(7)
Benefit obligation at end of period		150	156	132
Company contributions		8	7
Actual benefits paid		(8)	(7)
Funded status		(150)	(156)
DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		111	93
Service cost		1	1
Interest cost		4	4
Actuarial (gain) loss		(5)	17
Benefits paid		(5)	(4)
Benefit obligation at end of period		106	111	93
Company contributions		5	4
Actual benefits paid		(5)	(4)
Funded status		(106)	(111)
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		1,378	1,123
Service cost		25	20	29
Interest cost		58	56	65
Plan amendments		(25)
Actuarial (gain) loss		(152)	230
Benefits paid	[3]	(57)	(51)
Benefit obligation at end of period		1,227	1,378	1,123
Plan assets at fair value at beginning of period		1,265	1,218
Actual return on plan assets		(29)	72
Company contributions		29	25
Actual benefits paid	[3]	(57)	(50)
Plan assets at fair value at end of period		1,208	1,265	1,218
Funded status		(19)	(113)
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received			4	5
OPEB [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received		1
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		1,336	1,088
Service cost		25	20	28
Interest cost		56	54	63
Plan amendments		(24)
Actuarial (gain) loss		(150)	223
Benefits paid	[3]	(55)	(49)
Benefit obligation at end of period		1,188	1,336	1,088
Plan assets at fair value at beginning of period		1,186	1,141
Actual return on plan assets		(27)	68
Company contributions		29	25
Actual benefits paid	[3]	(55)	(48)
Plan assets at fair value at end of period		1,133	1,186	1,141
Funded status		(55)	(150)
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received			$ 4	$ 4
OPEB [Member] | Consumers Energy Company [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received		$ 1

[1] At December 31, 2015, $368 million of the total funded status of the DB Pension Plan was attributable to Consumers, based on an allocation of expenses. 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.

[2] The actuarial loss for 2014 was primarily the result of lowering the discount rates used in calculating the plans' obligations and using the RP-2014 mortality table during the annual measurement of benefit obligations.

[3] CMS Energy received less than $1 million in 2015, $4 million in 2014, and $5 million in 2013 for the Medicare Part D subsidies. Consumers received less than $1 million in 2015 and $4 million in each of 2014 and 2013 for the Medicare Part D subsidies. The Medicare Part D subsidy payments are used to pay OPEB Plan benefits.