Business segment information - Segment Operating Performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Business segment information
Net sales	$ 540.4	$ 559.7	$ 1,079.0	$ 1,090.3
Cost of sales	462.3	438.4	878.4	882.7
Gross margin	78.1	121.3	200.6	207.6
Operating income	35.5	54.8	85.6	86.3
Interest income and other	3.7	5.5	8.0	11.2
Other components of net periodic pension and OPEB expense	(0.7)	(0.6)	(1.5)	(1.2)
Changes in market value of Valhi common stock held by subsidiaries		0.1	(1.7)	0.6
General expenses, net	(9.2)	(10.3)	(17.1)	(18.1)
Interest expense	(13.8)	(11.9)	(27.0)	(23.2)
Income before income taxes	15.5	37.6	46.3	55.6
Chemicals
Business segment information
Net sales	494.4	500.5	984.2	979.3
Cost of sales	432.1	400.7	815.5	808.6
Gross margin	62.3	99.8	168.7	170.7
Operating income	10.3	40.5	51.5	63.3
Component products
Business segment information
Net sales	40.3	35.9	80.6	73.9
Cost of sales	27.4	24.8	55.5	53.1
Gross margin	12.9	11.1	25.1	20.8
Operating income	6.3	5.1	12.2	8.8
Real estate management and development
Business segment information
Net sales	5.7	23.3	14.2	37.1
Cost of sales	2.8	12.9	7.4	21.0
Gross margin	2.9	10.4	6.8	16.1
Operating income	$ 18.9	$ 9.2	$ 21.9	$ 14.2