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Asset Retirement Obligations (Details Textual) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligations [Line Items]      
Asset Retirement Obligation $ 2,966 $ 2,497 $ 1,838
Asset Retirement Obligation, Credit Adjusted Risk Free Rate 7.90%    
Asset Retirement Obligation, Inflation Rate 2.40%    
Long Term Debt Premium Paid $ 100 100  
Certificates of Deposit, at Carrying Value 1,000    
Surety Bond [Member]      
Asset Retirement Obligations [Line Items]      
Guarantor Obligations, Current Carrying Value 5,507 4,921  
Bond [Member]      
Asset Retirement Obligations [Line Items]      
Guarantor Obligations, Current Carrying Value 443 443  
Bureau of Land Management [Member]      
Asset Retirement Obligations [Line Items]      
Security Deposit 1,749 1,749  
Lahontan Regional Water Quality Control Board [Member]      
Asset Retirement Obligations [Line Items]      
Security Deposit 2,450 2,450  
Regional Board [Member]      
Asset Retirement Obligations [Line Items]      
Security Deposit $ 320 $ 278