Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ 15,651	$ 41,906	$ 31,568
Cash received (paid)	27,380	32,818	58,147
Unrealized gains (losses)	(11,729)	9,088	(26,579)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	14,422	36,693	33,218
Cash received (paid)	23,562	33,480	59,908
Unrealized gains (losses)	(9,140)	3,213	(26,690)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	1,229	5,213	(1,650)
Cash received (paid)	3,818	(662)	(1,761)
Unrealized gains (losses)	$ (2,589)	$ 5,875	$ 111