Impact of Derivatives and their Location Within the Unaudited Condensed Consolidated Statement of Operations (Detail) (USD $) In Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010	Jun. 30, 2011		Jun. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on Derivatives net	$ 14,242		$ 13,901	$ 27,784		$ 21,866
Unrealized gain (loss) on Derivatives, net	17,422		(2,158)	(35,633)		30,502
Commodity Contracts
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on Derivatives net	11,388	[1]	16,044	27,070	[1]	26,167
Unrealized gain (loss) on Derivatives, net	20,380		(1,189)	(34,181)		32,701
Interest Rate Swaps
Derivative Instruments, Gain (Loss) [Line Items]
Realized gain (loss) on Derivatives net	2,854	[2]	(2,143)	714	[2]	(4,301)
Unrealized gain (loss) on Derivatives, net	$ (2,958)	[2]	$ (969)	$ (1,452)	[2]	$ (2,199)

[1] Realized gains for the three months and six months ended June 30, 2011 exclude $1.4 million and $2.9 million, respectively, related to the initial value of derivatives acquired in our December 2010 acquisition of oil and natural gas properties that have been relieved through the settlement of such derivatives.

[2] In June 2011, we terminated three of our interest rate swaps and reclassified the $4.7 million non-cash gain from "Unrealized gains (losses) on derivatives, net" to "Realized gains on derivatives, net.