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Asset retirement obligations ('ARO')
 12 Months Ended
Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations (“ARO”)
Asset retirement obligations ("ARO")
The reconciliation of the changes in the asset retirement obligations is presented below:
 
Year ended December 31,
 
2016
2015
Beginning balance, ARO liability
$
35,165

$
36,950

Additional ARO liability incurred
1,272

3,797

ARO liabilities settled
(1,930
)
(5,514
)
Accretion expense
1,903

1,818

Adjustments
(992
)
(1,886
)
Ending balance, ARO liability
$
35,418

$
35,165