Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions | 9 Months Ended |
|---|---|
Sep. 30, 2011 | |
Minimum [Member] | |
| Mark-to-market threshold above which collateral must be posted | $ 0 |
Maximum [Member] | |
| Mark-to-market threshold above which collateral must be posted | $ 55 |