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Short-term and Other Investments, Summary (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]      
Fair value at beginning of period $ 3,746    
Gain (loss) during the period 2,028 $ (312)  
Fair value at end of period 5,774   $ 3,746
Midas Gold Shares [Member]      
Debt and Equity Securities, FV-NI [Line Items]      
Fair value at beginning of period 3,746 $ 4,994 4,994
Gain (loss) during the period 2,028   (1,248)
Fair value at end of period $ 5,774   $ 3,746
Midas Gold Shares held at end of the period 7,802,615   7,802,615