Business and geographic segments - Segment Operating Performance (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Jul. 16, 2024	Sep. 30, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Business and geographic segments
Net sales			$ 2,077.0	$ 2,104.8	$ 1,921.7
Cost of sales			1,781.6	1,680.3	1,676.5
Gross margin			295.4	424.5	245.2
Operating income			63.3	210.7	34.2
Interest income and other			16.6	22.0	21.3
Gain on remeasurement of investment in TiO2 manufacturing joint venture	$ 64.5			64.5
Gain on remeasurement of earn-out liability		$ 4.6	4.6
Insurance recoveries				1.4	3.0
Other components of net periodic pension and OPEB expense			(32.7)	(2.6)	(11.8)
Changes in market value of Valhi common stock held by subsidiaries			(2.7)	1.9	(1.7)
General expenses, net			(35.0)	(4.3)	(35.2)
Interest expense			(57.1)	(49.9)	(28.3)
Income (loss) before income taxes			(43.0)	244.2	(19.5)
Restructuring Costs			10.3	2.0	5.8
Fixed asset impairment					3.8
Gain on land sales				0.5	1.5
N L Industries Inc
Business and geographic segments
Insurance recoveries			0.0	1.4	0.5
N L Industries Inc
Business and geographic segments
Insurance recoveries			0.0	1.4	0.5
Chemicals
Business and geographic segments
Net sales			1,859.4	1,887.1	1,666.5
Cost of sales			1,648.4	1,530.1	1,502.7
Gross margin			211.0	357.0	163.8
Operating income			(24.5)	138.5	(41.1)
Component products
Business and geographic segments
Net sales			158.3	145.9	161.3
Cost of sales			110.1	104.6	112.1
Gross margin			48.2	41.3	49.2
Operating income			22.6	17.0	25.4
Real estate management and development
Business and geographic segments
Net sales			59.3	71.8	93.9
Cost of sales			23.1	45.6	61.7
Gross margin			36.2	26.2	32.2
Operating income			$ 65.2	$ 55.2	$ 49.9