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Unevaluated Property (Summary of Unevaluated Properties Excluded from Amortization) (Details 1) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of unevaluated properties excluded from oil and natural gas properties being amortized        
Property acquisition costs $ 0 $ 8,527 $ 0 $ 582,364
Exploration and development 9,849 6,948 20,673 189,890
Capitalized interest 36,510 30,762 25,220 85,957
Total 46,359 46,237 $ 45,893 $ 858,211
Property acquisition costs 590,891      
Exploration and development 227,360      
Capitalized interest 178,449      
Total $ 996,700 $ 951,397