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Asset retirement obligations (“ARO”) (Details) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Asset Retirement Obligation [Roll Forward]    
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) 0
ARO liability from Mt. Holly acquisition 0 10,503
Ending balance, ARO liability $ 34,189 $ 36,950
[1] We adjusted our ARO liability for changes in the estimated amounts and timing of costs associated with the disposal of spent pot liner.