Derivative Instruments (Effect of Derivative Instruments on Consolidated Statements of Income) (Details) (USD $) In Millions | 3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010 | |
| Gain/(Loss) Recognized in Income on Derivative Instruments | $ (1) | $ 4 | $ (1) | $ 10 |
Commodity Contracts [Member] | Operating Revenue [Member] | ||||
| Gain/(Loss) Recognized in Income on Derivative Instruments | (1) | 2 | (1) | 5 |
Commodity Contracts [Member] | Fuel for Electric Generation [Member] | ||||
| Gain/(Loss) Recognized in Income on Derivative Instruments | 1 | 3 | ||
Commodity Contracts [Member] | Purchased And Interchange Power [Member] | ||||
| Gain/(Loss) Recognized in Income on Derivative Instruments | $ 1 | $ 2 | ||