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Gold and Silver Bullion - Bullion Holdings (Details)
$ in Thousands
Jun. 30, 2018
USD ($)
oz
$ / oz
Dec. 31, 2017
USD ($)
oz
$ / oz
Schedule of Investments [Line Items]    
Total carrying value $ 3,664 $ 3,812
Gold    
Schedule of Investments [Line Items]    
Ounces | oz 1,904 1,905
Carrying value per ounce | $ / oz 1,250 1,291
Total carrying value $ 2,380 $ 2,459
Silver    
Schedule of Investments [Line Items]    
Ounces | oz 80,099 80,224
Carrying value per ounce | $ / oz 16.03 16.87
Total carrying value $ 1,284 $ 1,353