Derivative Instruments (Details) (USD $)	12 Months Ended			3 Months Ended				1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2010 Interest rate contract Termination of Derivatives	Dec. 31, 2012 Natural Gas 2013	Dec. 31, 2012 Natural Gas 2014	Dec. 31, 2012 Natural Gas Full 2015 Term	Mar. 31, 2012 Natural Gas Termination of Derivatives	Mar. 31, 2011 Natural Gas Termination of Derivatives	Dec. 31, 2012 Crude Oil 2013	Dec. 31, 2012 Crude Oil 2014	Dec. 31, 2012 Crude Oil Full 2015 Term
Notional amount of derivative instruments
Notional amount of derivatives (Bbl/MMBtu)					10,000	0	0		15,000	18,800	7,500	3,000
Gain (Loss) on Contract Termination				$ (1,900,000)				$ (1,900,000)
Accumulated Other Comprehensive Loss
Expected derivative payments during next twelve months	11,800,000
Unrealized losses, before tax, on contracts previously designated as cash flow hedges		(8,900,000)
Unrealized losses, net of tax, on contracts previously designated as cash flow hedges	0	(5,517,000)
Reclassified unrealized losses, before tax from AOCL into earnings	8,900,000	61,800,000
Reclassified unrealized losses, after tax from AOCL into earnings	$ 5,517,000	$ 38,289,000	$ 16,566,000