Impact of Derivative Activities on the Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ 11,498	$ 15,113	$ 19,827	$ 44,410
Realized and unrealized impact:
Cash received (paid)	8,527	6,803	20,302	24,287
Unrealized gains (losses)	2,971	8,310	(475)	20,123
Derivatives	11,498	15,113	19,827	44,410
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	11,293	13,381	18,598	38,733
Realized and unrealized impact:
Cash received (paid)	5,607	7,433	16,484	25,257
Unrealized gains (losses)	5,686	5,948	2,114	13,476
Derivatives	11,293	13,381	18,598	38,733
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	205	1,732	1,229	5,677
Realized and unrealized impact:
Cash received (paid)	2,920	(630)	3,818	(970)
Unrealized gains (losses)	(2,715)	2,362	(2,589)	6,647
Derivatives	$ 205	$ 1,732	$ 1,229	$ 5,677