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Supplementary information (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Property acquisition costs:    
Evaluated $ 0 $ 0
Unevaluated 0 0
Exploration costs 15,543 7,263
Development costs 111,158 81,886
Total costs incurred 126,701 89,149
Asset Retirement Obligations    
Property acquisition costs:    
Development costs $ 100 $ 100