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SUBSEQUENT EVENT (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Jan. 17, 2018
Jan. 03, 2018
Dec. 31, 2017
Dec. 31, 2016
Subsequent events [Line Items]        
Proceeds from sales of investments other than investments accounted for using equity method     $ 14,352 $ 12,774
Acquisition of mineral, royalty and other interests     $ 4,409 $ 10,806
Other disposals of assets [member]        
Subsequent events [Line Items]        
Proceeds from sales of investments other than investments accounted for using equity method   $ 18,300    
Major purchases of assets [member]        
Subsequent events [Line Items]        
Net smelter return royalty percentage 2.00%      
Acquisition of mineral, royalty and other interests $ 45,000