Derivative Instruments and Hedging Activities (Details Textual) (USD $)	3 Months Ended	6 Months Ended			3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Oil [Member]	Jun. 30, 2011 Oil [Member]	Jun. 30, 2012 Oil [Member]	Jun. 30, 2011 Oil [Member]	Jun. 30, 2012 Natural Gas [Member]	Jun. 30, 2011 Natural Gas [Member]	Jun. 30, 2012 Natural Gas [Member]	Jun. 30, 2011 Natural Gas [Member]
Derivative Instruments, Gain (Loss) Recognized in Income, Net
Increase in gas revenue owing to effective hedging contracts									$ 6,500,000	$ 3,800,000	$ 11,300,000	$ 8,400,000
Decrease/Increase in oil revenue owing to effective hedging contracts					2,900,000	14,300,000	2,900,000	22,800,000
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	20,800,000	23,700,000	2,100,000
Accumulated other comprehensive Income, net of tax		63,005,000		21,868,000
Accumulated other comprehensive Income, to be reclassified into earning in the next twelve months		$ 36,900,000