Consolidated Statements of Changes in Equity (Parenthetical) (USD $) In Millions | 3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010 | |
| Retirement benefits liability adjustments, tax | $ 1 | $ 0 | $ 1 | $ 1 |
Consumers Energy Company [Member] | ||||
| Unrealized gain (loss) on investments, tax expense (tax benefit) | $ (1) | $ (1) | $ (1) | |