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Asset retirement obligations (“ARO”) (Details) (USD $)
In Thousands, unless otherwise specified
6 Months Ended 12 Months Ended
Jun. 30, 2014
Dec. 31, 2013
Asset Retirement Obligation [Roll Forward]    
Beginning balance, ARO liability $ 27,113 $ 16,124
Additional ARO liability incurred 712 1,730
ARO liabilities settled (1,882) (2,580)
Accretion expense 475 1,733
Additional ARO liability from Sebree acquisition 0 10,106
Ending balance, ARO liability $ 26,418 $ 27,113