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Asset retirement obligations (“ARO”) (Tables)
 6 Months Ended
Jun. 30, 2015
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:
 
Six months ended June 30, 2015
Year ended December 31, 2014
Beginning balance, ARO liability
$
36,950

$
27,113

Additional ARO liability incurred
2,665

2,548

ARO liabilities settled
(3,798
)
(4,731
)
Accretion expense
913

1,517

Adjustments (1)
(2,541
)
—

ARO liability from Mt. Holly acquisition
—

10,503

Ending balance, ARO liability
$
34,189

$
36,950