Segment information - Differences Between Segment Profit and Amounts Included in Net Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	$ 40,272	$ 37,971
Segment profit	1,299	8,303
Corporate expenses	(2,721)	(2,359)
Equity in earnings of Kronos Worldwide, Inc.	5,525	2,476
Interest and dividend income	2,038	2,551
Marketable equity securities	(8,552)	2,383
Other components of net periodic pension and OPEB cost	(285)	(318)
Interest expense	(575)	(145)
Income tax (expense) benefit	23	(988)
Net income	1,322	7,315
Single reportable segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	40,272	37,971
Segment profit	5,869	3,715
Corporate expenses	(2,721)	(2,359)
Equity in earnings of Kronos Worldwide, Inc.	5,525	2,476
Interest and dividend income	2,038	2,551
Marketable equity securities	(8,552)	2,383
Other components of net periodic pension and OPEB cost	(285)	(318)
Interest expense	(575)	(145)
Income tax (expense) benefit	23	(988)
Net income	$ 1,322	$ 7,315