Suspended Exploratory Well Costs - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012 Well	Dec. 31, 2011	Dec. 31, 2010
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capitalized exploratory well costs that have been capitalized for more than one year	$ 11,395	$ 9,528	$ 10,727
Number of wells waiting on pipeline	2
Number of wells not operated by the company	3
Total wells	5
Number of wells in marcellus shale area	4

The amount of exploratory well costs that have been capitalized for a period of greater than one year after the completion of drilling.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 932

-SubTopic 235

-Section 50

-Paragraph 1B

-Subparagraph b

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

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS19-1

-Paragraph 10

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

Name:	us-gaap_CapitalizedExploratoryWellCostsThatHaveBeenCapitalizedForPeriodGreaterThanOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant