Retirement Benefits (Schedule Of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 54	$ 49
Interest expense	105	100	105
Expected return on plan assets	(135)	(127)	(125)
Amortization of Net loss	60	101	79
Amortization of Prior service cost (credit)	1	3	5
Net periodic cost (credit)	73	131	113
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	41	52	48
Interest expense	100	96	100
Expected return on plan assets	(131)	(124)	(122)
Amortization of Net loss	59	98	77
Amortization of Prior service cost (credit)	1	3	5
Net periodic cost (credit)	70	125	108
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	41	53
Interest expense	99	94
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	29	32
Interest expense	56	65	82
Expected return on plan assets	(88)	(77)	(66)
Amortization of Net loss	2	26	46
Amortization of Prior service cost (credit)	(41)	(31)	(20)
Net periodic cost (credit)	(51)	12	74
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	20	28	31
Interest expense	54	63	79
Expected return on plan assets	(83)	(72)	(61)
Amortization of Net loss	3	27	47
Amortization of Prior service cost (credit)	(40)	(30)	(20)
Net periodic cost (credit)	$ (46)	$ 16	$ 76