Fair Value Measurements (Policies)

9 Months Ended

Sep. 30, 2011

Fair Value Measurements, Policies [Abstract]

Offsetting fair value amounts of commodity derivative contracts

Certain of our commodity derivative contracts under master netting arrangements include both asset and liability positions. We have elected to offset the fair value amounts recognized for multiple similar derivative instruments executed with the same counterparty, including any related cash collateral asset or obligation; however, fair value amounts by hierarchy level are presented on a gross basis in the tables above.