Derivative Financial Instruments Gains and Losses (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Price Risk Cash Flow Hedge Gain (Loss) Reclassified to Earnings, Net	$ (2,264,000)	$ 12,997,000	$ 6,641,000
Gain (Loss) on Hedging Activity	3,866,000	(20,707,000)	23,465,000
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Cash settlement (gain) and loss	(44,264,000)	25,671,000
Unrealized derivative (gain) loss	(11,366,000)	(62,757,000)	8,899,000
Unrealized and Realized Derivative (Gain) Loss	(55,630,000)	(37,086,000)	8,899,000
Not Designated as Hedging Instrument [Member]
Unrealized and realized gain and loss from derivative cash settlements and changes in fair value of derivative contracts
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	1,100,000
Crude oil
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Cash settlement (gain) and loss	11,893,000	22,633,000
Unrealized derivative (gain) loss	(31,981,000)	(3,391,000)
Natural Gas [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Cash settlement (gain) and loss	(47,270,000)	(10,711,000)
Unrealized derivative (gain) loss	31,777,000	(64,310,000)
Natural Gas Liquids [Member]
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Cash settlement (gain) and loss	(8,887,000)	13,749,000
Unrealized derivative (gain) loss	$ (11,162,000)	$ 4,944,000
3.50% Senior Convertible Notes
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Debt Instrument, Interest Rate, Stated Percentage	3.50%	3.50%	3.50%