Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 50	$ 42	$ 54
Interest expense	108	105	100
Expected return on plan assets	(138)	(135)	(127)
Amortization of Net loss	97	60	101
Amortization of Prior service cost (credit)	1	1	3
Net periodic cost (credit)	118	73	131
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	49	41	52
Interest expense	103	100	96
Expected return on plan assets	(134)	(131)	(124)
Amortization of Net loss	93	59	98
Amortization of Prior service cost (credit)	1	1	3
Net periodic cost (credit)	112	70	125
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	25	20	29
Interest expense	58	56	65
Expected return on plan assets	(91)	(88)	(77)
Amortization of Net loss	21	2	26
Amortization of Prior service cost (credit)	(41)	(41)	(31)
Net periodic cost (credit)	(28)	(51)	12
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	25	20	28
Interest expense	56	54	63
Expected return on plan assets	(86)	(83)	(72)
Amortization of Net loss	22	3	27
Amortization of Prior service cost (credit)	(40)	(40)	(30)
Net periodic cost (credit)	$ (23)	$ (46)	$ 16