Business segments - Schedule of Identifiable Net Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Identifiable net assets	$ 726,918	$ 709,166
Cash items
Segment Reporting Information [Line Items]
Identifiable net assets	145,662	158,415
Unallocated corporate assets
Segment Reporting Information [Line Items]
Identifiable net assets	1,500	1,761
Industrial and commercial | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	38,117	38,784
Mining royalty lands | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	47,733	48,072
Development | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	142,269	212,384
Multifamily | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	$ 351,637	$ 249,750