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Asset retirement obligations (“ARO”) (Details) (USD $)
In Thousands, unless otherwise specified
9 Months Ended 12 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Asset Retirement Obligation [Roll Forward]    
Beginning balance, ARO liability $ 27,113 $ 16,124
Additional ARO liability incurred 1,493 1,730
ARO liabilities settled (3,547) (2,580)
Accretion expense 1,114 1,733
Additional ARO liability from Sebree acquisition 0 10,106
Ending balance, ARO liability $ 26,173 $ 27,113