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Property, Plant and Equipment - Capitalized Costs of Unproved Properties Excluded from Amortization (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Abstract]        
Property acquisition, cumulative $ 23,973      
Property acquisition 2,653 $ 2,353 $ 4,280 $ 14,687
Exploration, cumulative 630      
Exploration 10 16 564 40
Total costs incurred, cumulative 24,603      
Total costs incurred $ 2,663 $ 2,369 $ 4,844 $ 14,727