Derivative Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
Sep. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2011
Forward Contracts [Member] | Sep. 30, 2011
Forward Contracts to Purchase [Member] | Sep. 30, 2011
Forward Contracts to Sell [Member] | Sep. 30, 2011
Option Contract to Sell [Member] | Sep. 30, 2011
Consumers Energy Company [Member]
Financial Transmission Right [Member] | Sep. 30, 2010
Consumers Energy Company [Member]
Financial Transmission Right [Member] | Sep. 30, 2011
Consumers Energy Company [Member]
Financial Transmission Right [Member] | Sep. 30, 2010
Consumers Energy Company [Member]
Financial Transmission Right [Member] | |
| Derivative contracts (GWh/bcf) | 574.0 | 4.4 | 7.5 | 151.0 | ||||||
| Gain Or Loss Deferred In Regulatory Accounts | $ 1 | $ 3 | $ 2 | $ 4 | ||||||
| Derivative Liabilities Subject to Credit-Related-Contingent Features (less than at 9/30/2011) | 1 | 1 | ||||||||
| Loss Exposure to Counterparties for Derivative Instruments less than | $ 1 | |||||||||