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Asset retirement obligations (Tables)
 3 Months Ended
Mar. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations
The following table provides a summary of our asset retirement obligation activity during the three months ended March 31, 2014 and 2013. 
 
 
Three months ended March 31,
 
 
2014
 
2013
Beginning balance
 
$
55,179

 
$
49,214

Liabilities incurred in current period
 
139

 
76

Liabilities settled and disposed in current period
 
(170
)
 
(811
)
Accretion expense
 
1,074

 
1,007

Ending balance
 
56,222

 
49,486

Less current portion
 
1,874

 
2,900

 
 
$
54,348

 
$
46,586