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Property and Equipment (Summary of Unevaluated Properties Excluded from Amortization) (Details 1) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Summary of unevaluated properties excluded from oil and natural gas properties being amortized        
Property acquisition costs $ 8,527 $ 0 $ 0 $ 583,418
Exploration and development 6,948 20,675 24,470 165,419
Capitalized interest 30,762 25,220 28,303 57,655
Total 46,237 45,895 $ 52,773 $ 806,492
Property acquisition costs 591,945      
Exploration and development 217,512      
Capitalized interest 141,940      
Total $ 951,397 $ 927,819