Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ (15,495)	$ (42,665)	$ 7,372	$ (58,327)
Cash received (paid)			72,332	(10,279)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Cash Received on Hedge	(34,840)	7,222	72,332	(10,279)
Unrealized gains (losses)	$ (50,335)	$ (35,443)	$ (64,960)	$ (48,048)