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KEY SOURCES OF ESTIMATION UNCERTAINTY AND CRITICAL ACCOUNTING JUDGMENTS (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
KEY SOURCES OF ESTIMATION UNCERTAINTY AND CRITICAL ACCOUNTING JUDGMENTS [Abstract]    
Mineral, royalty and other interests $ 365,477 $ 402,785
Mineral, royalty and other interests impairments $ 9,104 $ 2,507