Retirement Benefits (Narrative) (Details) | 9 Months Ended | 12 Months Ended | |
|---|---|---|---|
Sep. 30, 2011 | Sep. 30, 2011 | Sep. 30, 2010 | |
| Expected long-term rate of return on pension plan assets | 8.00% | ||
| Actual return on pension plan assets | 2.00% | 11.60% | |
Consumers Energy Company [Member] | |||
| Expected long-term rate of return on pension plan assets | 8.00% | ||
| Actual return on pension plan assets | 2.00% | 11.60% | |