Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Maximum correlation between price of oil & natural gas in market and underlying price basis indicative in the derivative contract	100.00%				100.00%
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total			$ 40,400,000		$ 4,600,000	$ 44,400,000
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	10,000,000	[1]	1,300,000	[1]	18,400,000	(13,100,000)
Accumulated other comprehensive Income, net of tax	29,839,000				29,839,000		21,868,000
Accumulated other comprehensive Income, to be reclassified into earning in the next twelve months					19,900,000
Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Increase in gas revenue owing to effective hedging contracts	5,900,000		4,600,000		17,300,000	13,000,000
Oil
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/Increase in oil revenue owing to effective hedging contracts	$ 4,100,000		$ 3,300,000		$ 1,100,000	$ 26,100,000

[1] For the three months ended September 30, 2012, effective hedging contracts increased oil revenue by $4.1 million and increased gas revenue by $5.9 million. For the three months ended September 30, 2011, effective hedging contracts decreased oil revenue by $3.3 million and increased gas revenue by $4.6 million.