Hedge Contracts (Details)	9 Months Ended		12 Months Ended
	Sep. 30, 2012 USD ($)	Sep. 30, 2011 USD ($)	Dec. 31, 2011 USD ($)	Jul. 31, 2012 USD ($)	Jul. 31, 2012 EUR (€)	May 31, 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)		1,177,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income		(7,854,000)
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)		(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
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income		0
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)		0
Derivative Swap Contracts, Total
Amount of pretax gain reclassified from accumulated other comprehensive income into product sales revenue (effective portion)		1,177,000
Amount of pretax gain (loss) from change in derivative fair value recognized in other comprehensive income		(7,854,000)
Amount of pretax gain (loss) recognized in other income (expense) (ineffective portion)		(13,947,000)
Borrowings designated as a hedge of a net investment in a foreign subsidiary					10,000,000
Borrowings designated as a hedge of a net investment in a foreign subsidiary (USD equivalent)				12,900,000
Cumulative change to cumulative translation adjustment account, net of taxes	$ 500,000