Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ (20,852)	$ 36,187	$ 15,651
Cash received	(1,042)	29,723	27,380
Unrealized gains (losses)	(19,810)	6,464	(11,729)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	(20,852)	34,781	14,422
Cash received	(1,042)	28,317	23,562
Unrealized gains (losses)	(19,810)	6,464	(9,140)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	0	1,406	1,229
Cash received	0	1,406	3,818
Unrealized gains (losses)	$ 0	$ 0	$ (2,589)