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Short-term and Other Investments, Summary (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 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 $ (468) 2,028 $ (780)  
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