Financial Instruments (Schedule of Contractual Maturity Securities) (Details) (USD $)
In Millions | Sep. 30, 2011 | Dec. 31, 2010 |
|---|
| Available for sale - fair value | $ 113 | $ 90 |
Consumers Energy Company [Member] | Supplemental Executive Retirement Plan [Member] | | |
| Available for sale state and municipal bonds - Due within one year | 1 | |
| Available for sale state and municipal bonds - Due after one year through five years | 6 | |
| Available for sale state and municipal bonds - Due after five year through ten years | 8 | |
| Available for sale state and municipal bonds - Due after ten years | 2 | |
| Available for sale - fair value | 17 | |
Supplemental Executive Retirement Plan [Member] | | |
| Available for sale state and municipal bonds - Due within one year | 2 | |
| Available for sale state and municipal bonds - Due after one year through five years | 9 | |
| Available for sale state and municipal bonds - Due after five year through ten years | 12 | |
| Available for sale state and municipal bonds - Due after ten years | 3 | |
| Available for sale - fair value | 26 | |
Consumers Energy Company [Member] | | |
| Available for sale - fair value | $ 105 | $ 90 |