Hedge Contracts (Details) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Apr. 30, 2011
Hedge Contracts (Details)
Percentage of proved reserves sold in April 2011			79.00%
Cash paid for liquidation of hedge contract	$ 14,200,000
Derivative Swap Contracts, Oil
Amount of pretax gain reclassified from accumulated other comprehensive income into product sales revenue (effective portion)	0	1,177,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income	0	(7,854,000)
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)	(14,224,000)	(13,947,000)
Derivative Swap Contracts, Natural Gas
Amount of pretax gain reclassified from accumulated other comprehensive income into product sales revenue (effective portion)	0	0
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income	0	0
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)	0	0
Derivative Swap Contracts, Total
Amount of pretax gain reclassified from accumulated other comprehensive income into product sales revenue (effective portion)	0	1,177,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income	0	(7,854,000)
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)	$ (14,224,000)	$ (13,947,000)