Hedge Contracts (Details) (USD $)	3 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	May 31, 2011
Hedge Contracts (Details)
Percentage of proved reserves sold as of May 31, 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)			1,177,000	22,725,000	6,978,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income			(7,854,000)	(1,947,000)	(13,966,000)
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)			(13,947,000)	(152,000)	(408,000)
Derivative Swap Contracts, Natural Gas
Amount of pretax gain reclassified from accumulated other comprehensive income into product sales revenue (effective portion)			0	26,214,000	40,054,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income			0	9,118,000	22,906,000
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)			0	0	(1,321,000)
Derivative Swap Contracts, Total
Amount of pretax gain reclassified from accumulated other comprehensive income into product sales revenue (effective portion)			1,177,000	48,939,000	47,032,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income			(7,854,000)	7,171,000	8,940,000
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)			(13,947,000)	(152,000)	(1,729,000)
Proceeds received from liquidation of cash flow hedging swap contracts		23,100,000
Cumulative translation adjustment for investment hedge, net of tax				2,600,000
Derivatives [Line Items]
Total derivatives designated as hedging instruments				(2,772,000)
Assets [Member]
Derivatives [Line Items]
Natural gas swap contracts				2,436,000
Liabilities [Member]
Derivatives [Line Items]
Oil swap contracts				$ (5,208,000)