Leases (Narrative) (Details) (Consumers Energy Company [Member])	12 Months Ended
	Dec. 31, 2012
Coal-Carrying Railcars [Member]
Operating Leases Without Extension Provisions Expiration Time Period Remaining	11 years
Operating Leases With Extension Provisions Expiration Time Period Remaining	14 years
Vehicle Fleet Operations [Member]
Capital Lease Term	120 months
Gas Transportation Pipeline into Karn [Member]
Capital Lease Term	15 years
Capital Lease Remaining Term	9 years
Gas Transportation Pipeline into Zeeland [Member]
Capital Lease Term	12 years
Capital Lease Remaining Term	5 years
Capital lease renewal term	5 years
Minimum [Member] | Long-term PPAs [Member]
Capital Lease Term	1 year
Maximum [Member] | Long-term PPAs [Member]
Capital Lease Term	20 years