Business segments - Identifiable net assets (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Identifiable net assets	$ 711,856	$ 709,166
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment
Segment Reporting Information [Line Items]
Identifiable net assets	158,420	158,415
Segment Reporting, Reconciling Item, Corporate Nonsegment
Segment Reporting Information [Line Items]
Identifiable net assets	1,337	1,761
Industrial and commercial | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	38,066	38,784
Mining royalty lands | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	48,391	48,072
Development | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	148,131	212,384
Multifamily | Operating Segments
Segment Reporting Information [Line Items]
Identifiable net assets	$ 317,511	$ 249,750