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Asset Retirement Obligation
 9 Months Ended
Sep. 30, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligation
Asset Retirement Obligation
Activity related to the Company’s asset retirement obligation is as follows:
(In thousands)
 
Nine Months Ended 
 September 30, 2014
Balance at beginning of period
 
$
75,853

Liabilities incurred
 
4,360

Liabilities settled
 
(411
)
Liabilities divested
 
(899
)
Change in estimate
 
33,810

Accretion expense
 
3,528

Balance at end of period
 
$
116,241


The change in estimate during 2014 is attributable to an increase in costs of materials and services. The increase in cost of materials and services is primarily due to more rigorous plugging and abandonment techniques associated with the Company's horizontal wells in certain areas of its operations and the lack of availability of service providers in areas with minimal activity.
As of both September 30, 2014 and December 31, 2013, approximately $2.0 million is included in accrued liabilities in the Condensed Consolidated Balance Sheet, which represents the current portion of the Company’s asset retirement obligation.