Business and geographic information - Differences Between Segment Profit and Amounts Included in Net Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	$ 145,941	$ 161,287	$ 166,562
Segment profit	83,318	(6,419)	39,241
Insurance recoveries	1,369	464	93
Corporate income (expenses), net	19,484	(11,770)	(11,798)
Equity in earnings (losses) of Kronos Worldwide, Inc.	26,381	(15,003)	31,873
Interest and dividend income	10,980	9,636	3,797
Marketable equity securities gain (loss)	9,821	(8,156)	(8,085)
Loss on pension plan termination		(4,911)
Other components of net periodic pension and OPEB cost	(1,210)	(1,368)	(1,134)
Interest expense	(530)	(746)	(941)
Income tax (expense) benefit	(14,057)	7,001	(2,785)
Net income	69,261	582	36,456
Single reportable segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	145,941	161,287	166,562
Insurance recoveries	1,369	464	93
Equity in earnings (losses) of Kronos Worldwide, Inc.	26,381	(15,003)	31,873
Interest and dividend income	10,980	9,636	3,797
Marketable equity securities gain (loss)	9,821	(8,156)	(8,085)
Loss on pension plan termination		(4,911)
Other components of net periodic pension and OPEB cost	(1,210)	(1,368)	(1,134)
Interest expense	(530)	(746)	(941)
Income tax (expense) benefit	(14,057)	7,001	(2,785)
Net income	69,261	582	36,456
Operating Segments | Single reportable segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Segment profit	17,023	25,435	25,436
Corporate expenses | Single reportable segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Corporate income (expenses), net	$ 19,484	$ (11,770)	$ (11,798)