Business, segment and geographic information - Differences Between Segment Profit and Amounts Included in Net Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	$ 158,285	$ 145,941	$ 161,287
Segment profit	(51,424)	83,318	(6,419)
Insurance recoveries		1,369	464
Corporate income (expenses), net	(11,929)	19,484	(11,770)
Equity in earnings (losses) of Kronos Worldwide, Inc.	(33,940)	26,381	(15,003)
Interest and dividend income	7,000	10,980	9,636
Marketable equity securities gain (loss)	(13,582)	9,821	(8,156)
Settlement loss on pension plan termination and buy-out	(19,670)		(4,911)
Other components of net periodic pension and OPEB cost	(1,140)	(1,210)	(1,368)
Interest expense	(767)	(530)	(746)
Income tax (expense) benefit	16,053	(14,057)	7,001
Net income (loss)	(35,371)	69,261	582
Single reportable segment
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net sales	158,285	145,941	161,287
Segment profit	22,604	17,023	25,435
Insurance recoveries		1,369	464
Corporate income (expenses), net	(11,929)	19,484	(11,770)
Equity in earnings (losses) of Kronos Worldwide, Inc.	(33,940)	26,381	(15,003)
Interest and dividend income	7,000	10,980	9,636
Marketable equity securities gain (loss)	(13,582)	9,821	(8,156)
Settlement loss on pension plan termination and buy-out	(19,670)		(4,911)
Other components of net periodic pension and OPEB cost	(1,140)	(1,210)	(1,368)
Interest expense	(767)	(530)	(746)
Income tax (expense) benefit	16,053	(14,057)	7,001
Net income (loss)	$ (35,371)	$ 69,261	$ 582