Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ (15,662)	$ (7,761)
Cash received	(3,057)	3,557
Unrealized gains (losses)	(12,605)	(11,318)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	(15,662)	(7,761)
Cash received	3,057	(3,557)
Unrealized gains (losses)	(12,605)	(11,318)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	0	0
Cash received	0	0
Unrealized gains (losses)	$ 0	$ 0