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Asset retirement obligations (Tables)
 12 Months Ended
Dec. 31, 2014
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, 2014 and 2013. 
 
 
For the year ended December 31,
 
 
2014
 
2013
Beginning balance
 
$
55,179

 
$
49,214

Liabilities incurred and assumed in current period
 
5,011

 
2,313

Revisions in estimated cash flows
 
2,450

 
6,723

Liabilities settled or disposed in current period
 
(19,184
)
 
(7,016
)
Accretion
 
3,968

 
3,945

 
 
47,424

 
55,179

Less current portion (included in “Accounts payable and accrued liabilities”)
 
4,147

 
1,874

 
 
$
43,277

 
$
53,305