Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ (7,761)	$ (305)
Cash received	3,557	7,981
Unrealized gains (losses)	(11,318)	(8,286)
Commodity contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	(7,761)	294
Cash received	3,557	7,981
Unrealized gains (losses)	(11,318)	(7,687)
Interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	0	(599)
Cash received	0	0
Unrealized gains (losses)	$ 0	$ (599)

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 28

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

Name:	us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration