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Asset retirement obligations (Tables)
 12 Months Ended
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations
The following table presents the balance and activity of our asset retirement obligations for the years ended December 31, 2013 and 2012. 
 
 
For the year ended December 31,
 
 
2013
 
2012
Beginning balance
 
$
49,214

 
$
46,493

Liabilities incurred and assumed in current period
 
2,313

 
1,079

Revisions in estimated cash flows
 
6,723

 
—

Liabilities settled or disposed in current period
 
(7,016
)
 
(2,303
)
Accretion
 
3,945

 
3,945

 
 
55,179

 
49,214

Less current portion
 
1,874

 
2,900

 
 
$
53,305

 
$
46,314