The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 410

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Asset Retirement Obligation

-URI http://asc.fasb.org/extlink&oid=6505190

Name:	us-gaap_AssetRetirementObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Asset Retirement Obligation - Schedule of Consolidated Balance Sheet where Asset Retirement Obligations Recorded (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Asset Retirement Obligations [Line Items]
Asset retirement obligations	$ 5,142	$ 8,882	$ 9,081	$ 9,679
Other Current Liabilities [Member]
Asset Retirement Obligations [Line Items]
Asset retirement obligations	1,200	3,600
Other Long Term Liabilities [Member]
Asset Retirement Obligations [Line Items]
Asset retirement obligations	$ 3,900	$ 5,300