Acquisition of Working Interests in Natural Gas Properties - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
Oct. 31, 2012 Well	Dec. 31, 2012
Acquisition of Working Interest in Natural Gas Properties [Line Items]
Date of business acquisition		Oct. 31, 2012
Acquired average working interest	9.70%
Acquired net revenue interest	7.70%
Proven, developed producing natural gas wells	14
Proven, developed non-producing natural gas wells	7
Proven undeveloped drilling sites	36
Purchase price of business acquisition	$ 50
Net sales from date of acquisition	1.4
Pre-tax income from date of acquisition	0.9
Acquisition -related costs	$ 0.2

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

-Paragraph 23

-URI http://asc.fasb.org/extlink&oid=21917927&loc=d3e1043-128460

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 141R

-Paragraph 59

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration