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Asset retirement obligations
 6 Months Ended
Jun. 30, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations
Asset retirement obligations
The following table provides a summary of our asset retirement obligation activity during the six months ended June 30, 2014 and 2013. 
 
 
Six months ended June 30,
 
 
2014
 
2013
Beginning balance
 
$
55,179

 
$
49,214

Liabilities incurred in current period
 
2,966

 
262

Liabilities settled and disposed in current period
 
(7,823
)
 
(2,867
)
Accretion expense
 
2,063

 
2,008

Ending balance
 
52,385

 
48,617

Less current portion included in accounts payable and accrued liabilities
 
6,792

 
2,900

 
 
$
45,593

 
$
45,717


See Note 6—Fair value measurements for additional information regarding fair value assumptions associated with our asset retirement obligations.