Fair Value of Derivatives (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	$ 89,564	$ 123,655
Derivative asset, Netting arrangements	(11,773)	(17,058)
Net recorded fair value	77,791	106,597
Location of derivatives in our unaudited condensed consolidated balance sheets:
Derivative asset	51,215	55,100
Long-term derivative asset	26,576	51,497
Net recorded fair value	77,791	106,597
Derivative liability, fair value	19,545	19,785
Derivative liability, Netting arrangements	(11,773)	(17,058)
Net recorded fair value	7,772	2,727
Location of derivatives in our unaudited condensed consolidated balance sheets:
Derivative liability	1,178	1,943
Long-term derivative liability	6,594	784
Net recorded fair value	7,772	2,727
Commodity Contracts
Derivatives, Fair Value [Line Items]
Derivative asset, fair value	89,564	123,655
Location of derivatives in our unaudited condensed consolidated balance sheets:
Derivative liability, fair value	10,620	7,633
Interest Rate Swaps
Location of derivatives in our unaudited condensed consolidated balance sheets:
Derivative liability, fair value	$ 8,925	$ 12,152