Hedge Contracts (Details)	12 Months Ended
	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2012 EUR (€)	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
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income		(7,854,000)	(1,947,000)
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)		(13,947,000)	(152,000)
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
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income		0	9,118,000
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)		1,177,000	48,939,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income		(7,854,000)	7,171,000
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)		(13,947,000)	(152,000)
Cumulative translation adjustment for investment hedge, net of tax	8,000,000		2,600,000
Borrowings designated as a hedge of a net investment in a foreign subsidiary (euros)				10,000,000
Borrowings designated as a hedge of a net investment in a foreign subsidiary (USD)	$ 13,200,000