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Asset Retirement Obligation | 12 Months Ended
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Dec. 31, 2013
Asset Retirement Obligation |
Asset Retirement Obligation | 8. Asset Retirement Obligation Activity related to the Company's asset retirement obligation is as follows:
(In thousands) |  | Year Ended December 31, 2013 |
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Balance at beginning of period |  | $ | 67,016 |
Liabilities incurred |  |  | 5,295 |
Liabilities settled |  |  | (762 | )
Liabilities divested |  |  | (6,400 | )
Accretion expense |  |  | 3,691 |
Change in estimate |  |  | 7,013 |
​ | ​ | ​ | ​ | ​
Balance at end of period |  | $ | 75,853 |
​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​
The change in estimate during 2013 is attributable to increased costs for services and related materials to plug and abandon wells in certain areas of our operations. As of December 31, 2013, approximately $2.0 million, which represents the current portion of the Company's asset retirement obligation, is included in accrued liabilities in the Consolidated Balance Sheet.

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- Definition The entire disclosure for an asset retirement obligation and the associated long-lived asset. An asset retirement obligation is a legal obligation associated with the disposal or retirement from service 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -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 -Topic 410 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7569-110849 + Details
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