Retirement Benefits (Schedule Of Benefit Obligations In Excess Of Fair Value Of Plan Assets) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		$ 2,562		$ 2,403
Service cost		45		42
Interest cost		88		85
Actuarial (gain) loss		241	[1]	196
Benefits paid		(156)		(164)
Benefit obligation at end of period		2,780		2,562	$ 2,403
Plan assets at fair value at beginning of period		2,101		2,013
Actual return on plan assets		360		152
Company contributions				100
Actual benefits paid		(156)		(164)
Plan assets at fair value at end of period		2,305		2,101	2,013
Funded status	[2]	(475)		(461)
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status		(455)		(441)
DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		151		150
Interest cost		5		5
Actuarial (gain) loss		7		4
Benefits paid		(9)		(8)
Benefit obligation at end of period		154		151	150
Company contributions		9		8
Actual benefits paid		(9)		(8)
Funded status		(154)		(151)
DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		109		106
Interest cost		4		4
Actuarial (gain) loss		5		4
Benefits paid		(6)		(5)
Benefit obligation at end of period		112		109	106
Company contributions		6		5
Actual benefits paid		(6)		(5)
Funded status		(112)		(109)
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		1,408		1,227
Service cost		19		18	25
Interest cost		51		46	58
Plan amendments		(309)
Actuarial (gain) loss	[1]	(24)		171
Benefits paid		(48)		(54)
Benefit obligation at end of period		1,097		1,408	1,227
Plan assets at fair value at beginning of period		1,264		1,208
Actual return on plan assets		203		109
Actual benefits paid		(47)		(53)
Plan assets at fair value at end of period		1,420		1,264	1,208
Funded status		323		(144)
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of period		1,365		1,188
Service cost		19		17	25
Interest cost		49		45	56
Plan amendments		(303)
Actuarial (gain) loss	[1]	(31)		167
Benefits paid		(46)		(52)
Benefit obligation at end of period		1,053		1,365	1,188
Plan assets at fair value at beginning of period		1,184		1,133
Actual return on plan assets		190		103
Actual benefits paid		(45)		(52)
Plan assets at fair value at end of period		1,329		1,184	$ 1,133
Funded status		$ 276		$ (181)

[1] The actuarial loss for 2017 for the DB Pension Plans was primarily the result of lowering the discount rates. The actuarial gain for 2017 for the OPEB Plan was primarily the result of better claim experience in calculating the plan's funded status. The actuarial loss for 2016 was primarily the result of claims, experience, and lowering the discount rates used in calculating the plans' funded status.

[2] At December 31, 2017, $455 million of the total funded status of the DB Pension Plans was attributable to Consumers, based on an allocation of expenses. At December 31, 2016, $441 million of the total funded status of the DB Pension Plans was attributable to Consumers, based on an allocation of expenses.