Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ (42,665)	$ 8,588	$ (58,327)	$ 827
Cash received	7,222	(2,233)	(10,279)	5,790
Unrealized gains (losses)	(35,443)	6,355	(48,048)	(4,963)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	(42,665)	8,588	(58,327)	827
Cash received	7,222	(2,233)	(10,279)	5,790
Unrealized gains (losses)	(35,443)	6,355	(48,048)	(4,963)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	0	0	0	0
Cash received	0	0	0	0
Unrealized gains (losses)	$ 0	$ 0	$ 0	$ 0