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Property and Equipment (Summary of Unevaluated Properties Excluded from Amortization) (Details 1) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Summary of unevaluated properties excluded from oil and natural gas properties being amortized        
Property acquisition costs $ 7,528 $ 0 $ 4,875 $ 592,184
Exploration and development 21,119 24,812 95,397 70,156
Capitalized interest 25,220 28,302 21,179 37,047
Total 53,867 53,114 $ 121,451 $ 699,387
Property acquisition costs 604,587      
Exploration and development 211,484      
Capitalized interest 111,748      
Total $ 927,819 $ 894,948