Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ 162,212	$ (20,852)	$ 36,187
Cash received (paid)	(7,424)	(1,042)	29,723
Unrealized gains (losses)	169,636	(19,810)	6,464
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	162,212	(20,852)	34,781
Cash received (paid)	(7,424)	(1,042)	28,317
Unrealized gains (losses)	169,636	(19,810)	6,464
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	0	0	1,406
Cash received (paid)	0	0	1,406
Unrealized gains (losses)	$ 0	$ 0	$ 0