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Oil and Gas Producing Activities (Unaudited) - Cost incurred in oil and gas exploration and development activities (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property acquisition costs:    
Unproved $ 4 $ 32
Development costs 1,815 0
Total costs incurred 1,824 323
Expensed    
Capitalized Costs Relating to Oil and Gas Producing Activities, by Geographic Area [Line Items]    
Expensed $ 5 $ 291