Derivative Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Financial Instruments
Fair value of oil, natural gas, NGL commodity derivative markets	$ (3,600,000)	$ 38,700,000
Derivative (gains) losses relating to de-designated commodity hedges reclassified from AOCL to earnings net of tax	61,000	(1,034,000)
Derivatives not designated as hedging instruments
Derivative Financial Instruments
Gains expected to be reclassifiied into earnings from AOCL after-tax net loss related to de-designated commodity derivative contracts	$ 1,100,000
Crude oil
Derivative Financial Instruments
Portion of Future Oil and Gas Production Being Hedged	13,000,000
Natural Gas
Derivative Financial Instruments
Portion of Gas Production Being Hedged	119,000,000
Natural Gas Liquids
Derivative Financial Instruments
Portion of Future Oil and Gas Production Being Hedged	1,500,000