Derivative Instruments and Hedging Activities (Details Textual) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Derivative Instruments and Hedging Activities (Textual) [Abstract]
Maximum correlation between price of oil & natural gas in market and underlying price basis indicative in the derivative contract	100.00%
Derivative Instruments net of related tax effect		$ (18,300,000)
Accumulated other comprehensive loss, net of tax	(197,000)		21,868,000
Accumulated other comprehensive loss, to be reclassified into earning in the next twelve months	2,000,000
Oil [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Decrease/Increase in oil revenue owing to effective hedging contracts	5,800,000	8,400,000
Natural Gas [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Increase in oil revenue owing to effective hedging contracts	$ 4,800,000	$ 4,500,000