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Short-term and Other Investments, Summary (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Schedule of Cost-method Investments [Line Items]          
Gain/(loss) during the period $ (78) $ (234) $ (858) $ 3,586  
Midas Gold Shares [Member]          
Schedule of Cost-method Investments [Line Items]          
Fair value at beginning of period     4,994 $ 1,798 $ 1,798
Gain/(loss) during the period     (858)   3,196
Fair value at end of period $ 4,136   $ 4,136   $ 4,994
Midas Gold Shares held at end of the period 7,802,615   7,802,615   7,802,615