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Asset retirement obligations (“ARO”) (Tables)
12 Months Ended
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of changes in asset retirement obligations
The reconciliation of the changes in the asset retirement obligations is presented below:
 
 
Year ended December 31,
 
2013
2012
Beginning balance, ARO liability
$
16,124

$
15,171

Additional ARO liability incurred
1,730

1,166

ARO liabilities settled
(2,580
)
(1,380
)
Accretion expense
1,733

1,167

Additional ARO liability from Sebree acquisition
10,106


Ending balance, ARO liability
$
27,113

$
16,124