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Identifiable net assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Segment Reporting Information [Line Items]    
Assets $ 678,753 $ 536,360
Investments available for sale 4,315 75,609
Cash 163,383 74,105
Asset Management [Member]    
Segment Reporting Information [Line Items]    
Assets 11,127 11,172
Mining Properties [Member]    
Segment Reporting Information [Line Items]    
Assets 37,205 37,387
Development [Member]    
Segment Reporting Information [Line Items]    
Assets 185,500 196,212
Stabilized Joint Venture [Member]    
Segment Reporting Information [Line Items]    
Assets 268,907 130,472
Unallocated Corporate Assets [Member]    
Segment Reporting Information [Line Items]    
Assets $ 8,316 $ 11,403