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Asset retirement obligations (“ARO”) (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Asset Retirement Obligation [Roll Forward]    
Beginning balance, ARO liability $ 16,124 $ 15,171
Additional ARO liability incurred 1,730 1,166
ARO liabilities settled (2,580) (1,380)
Accretion expense 1,733 1,167
Additional ARO liability from Sebree acquisition 10,106 0
Ending balance, ARO liability $ 27,113 $ 16,124