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Asset retirement obligations (“ARO”) (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Asset Retirement Obligation [Roll Forward]    
Beginning balance, ARO liability $ 16,124 $ 15,171
Additional ARO liability incurred 764 1,166
ARO liabilities settled (796) (1,380)
Accretion expense 673 1,167
Additional ARO liability from Sebree acquisition 10,591 0
Ending balance, ARO liability $ 27,356 $ 16,124