Summary of Significant Accounting Policies - Net Changes in Capitalized Exploratory Well Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Capitalized Costs Relating To Oil And Gas Properties [Line Items]
Beginning of period	$ 20,207	$ 5,176	$ 427
Exploratory well cost additions (pending determination of proved reserves)	160,813	73,947	39,708
Exploratory well cost reclassifications (successful determination of proved reserves)	(140,091)	(57,646)	(34,959)
Exploratory well dry hole costs (unsuccessful in adding proved reserves)	(505)	(1,270)
End of period	$ 40,424	$ 20,207	$ 5,176

Previously capitalized costs of the drilling of wells that were reclassified as part of the costs of the enterprise's wells and related equipment and facilities at the time that proved reserves were found.

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS19-1

-Paragraph 10

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=8451039&loc=SL6806780-109447

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