Retirement Benefits (Schedule of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Plans, Defined Benefit [Member]
Service cost	$ 12	$ 12	$ 24	$ 24
Interest expense	25	25	50	50
Expected return on plan assets	(32)	(28)	(63)	(56)
Amortization of Net loss	19	15	38	31
Amortization of Prior service cost	2	2	3	3
Net periodic cost	26	26	52	52
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Service cost	11	11	23	23
Interest expense	24	25	48	49
Expected return on plan assets	(31)	(28)	(61)	(55)
Amortization of Net loss	19	16	37	31
Amortization of Prior service cost	2	2	3	3
Net periodic cost	25	26	50	51
Other Postretirement Benefit Plans, Defined Benefit [Member]
Service cost	8	6	16	13
Interest expense	20	19	41	38
Expected return on plan assets	(16)	(16)	(33)	(33)
Amortization of Net loss	12	8	23	16
Amortization of Prior service cost	(5)	(5)	(10)	(10)
Net periodic cost	19	12	37	24
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Service cost	8	7	16	13
Interest expense	20	19	40	37
Expected return on plan assets	(16)	(16)	(31)	(31)
Amortization of Net loss	12	8	23	16
Amortization of Prior service cost	(5)	(5)	(10)	(10)
Net periodic cost	$ 19	$ 13	$ 38	$ 25