Impact of Derivative Activities on the Condensed Consolidated Statements of Income (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]
Derivatives	$ 7,001	$ (580)	$ 8,329	$ 29,297
Realized and unrealized impact:
Unrealized gains (losses)	1,970	(9,631)	(3,446)	11,813
Cash received (paid)	5,031	9,051	11,775	17,484
Derivatives	7,001	(580)	8,329	29,297
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	5,997	(2,860)	7,305	25,352
Realized and unrealized impact:
Unrealized gains (losses)	1,864	(11,649)	(3,572)	7,528
Cash received (paid)	4,133	8,789	10,877	17,824
Derivatives	5,997	(2,860)	7,305	25,352
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	1,004	2,280	1,024	3,945
Realized and unrealized impact:
Unrealized gains (losses)	106	2,018	126	4,285
Cash received (paid)	898	262	898	(340)
Derivatives	$ 1,004	$ 2,280	$ 1,024	$ 3,945