Retirement Benefits (Schedule Of Net Benefit Costs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended		3 Months Ended	12 Months Ended		3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Consumers Energy Company [Member]	Mar. 31, 2013 Consumers Energy Company [Member]	Mar. 31, 2012 Consumers Energy Company [Member]	Mar. 31, 2013 Pension Plans, Defined Benefit [Member]	Mar. 31, 2012 Pension Plans, Defined Benefit [Member]	Mar. 31, 2013 Pension Plans, Defined Benefit [Member] Consumers Energy Company [Member]	Mar. 31, 2012 Pension Plans, Defined Benefit [Member] Consumers Energy Company [Member]	Mar. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member]	Mar. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member]	Mar. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Consumers Energy Company [Member]	Mar. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member] Consumers Energy Company [Member]
Service cost							$ 13	$ 12	$ 13	$ 12	$ 9	$ 8	$ 9	$ 8
Interest expense							24	25	23	24	19	21	18	20
Expected return on plan assets							(32)	(31)	(31)	(30)	(19)	(17)	(18)	(15)
Amortization of Net loss							24	19	23	18	11	11	11	11
Amortization of Prior service cost							1	1	1	1	(5)	(5)	(5)	(5)
Net periodic cost							$ 30	$ 26	$ 29	$ 25	$ 15	$ 18	$ 15	$ 19
Expected long-term rate of return on pension plan assets	7.75%			7.75%
Actual return on pension plan assets		10.70%	7.40%		10.70%	7.40%