Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ 66,457	$ (24,035)	$ 8,130	$ (23,208)
Cash received (paid)	(7,557)	(4,165)	(17,836)	1,625
Unrealized gains (losses)	74,014	(19,870)	25,966	(24,833)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	66,457	(24,035)	8,130	(23,208)
Derivative, Cash Received on Hedge	7,557	4,165	17,836	(1,625)
Unrealized gains (losses)	74,014	(19,870)	25,966	(24,833)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	0	0	0	0
Cash received (paid)	0	0	0	0
Unrealized gains (losses)	$ 0	$ 0	$ 0	$ 0