Derivative Financial Instruments (Schedule of Notional Amounts and Estimated Fair Values of Commodity Derivative Positions) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Notional Amount [Member] | Non-Hedging Commodity Swaps [Member]
Oil commodity	$ 483	$ 321
Natural gas commodity (2)	4,265	510
Natural gas commodity	4,265	510
Oil commodity (2)	483	321
Notional Amount [Member] | Non-Hedging Commodity Options [Member]
Oil commodity	2,473	1,288
Natural gas commodity (2)	2,400	3,820
Natural gas commodity	2,400	3,820
Oil commodity (2)	2,473	1,288
Estimated Fair Value [Member] | Non-Hedging Commodity Swaps [Member]
Oil commodity	2,318	2,502
Natural gas commodity (2)	(1,022)	(174)
Natural gas commodity	1,145	195
Oil commodity (2)	(2,210)	(2,428)
Estimated Fair Value [Member] | Non-Hedging Commodity Options [Member]
Oil commodity	13,916	7,706
Natural gas commodity (2)	(2,039)	(3,774)
Natural gas commodity	2,039	3,774
Oil commodity (2)	$ (13,916)	$ (7,706)
Non-Hedging Commodity Swaps [Member]
Notional Units, MMBTUs (2)	MMBTUs
Notional Units, MMBTUs	MMBTUs
Notional Units, barrels (2)	Barrels
Notional Units, barrels	Barrels
Non-Hedging Commodity Options [Member]
Notional Units, MMBTUs (2)	MMBTUs
Notional Units, MMBTUs	MMBTUs
Notional Units, barrels (2)	Barrels
Notional Units, barrels	Barrels