Derivative Financial Instruments (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Derivative Financial Instruments
Fair value of oil, natural gas, NGL commodity derivative markets	$ 36,100,000	$ 31,200,000	$ 36,100,000	$ 31,200,000
Derivative (gains) losses relating to de-designated commodity hedges reclassified from AOCL to earnings net of tax	(315,000)	4,271,000	(1,465,000)	9,149,000
De-designated Hedges
Derivative Financial Instruments
Gains expected to be reclassifiied into earnings from AOCL after-tax net loss related to de-designated commodity derivative contracts			$ 315,000
Crude oil
Derivative Financial Instruments
Anticipated production covered by derivative contracts	10,800,000		10,800,000
Natural gas
Derivative Financial Instruments
Anticipated production covered by derivative contracts	79,300,000		79,300,000
Natural gas liquids ("NGLs")
Derivative Financial Instruments
Anticipated production covered by derivative contracts	1,500,000		1,500,000