Business segment information - Segment Operating Performance (Detail) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
	Jul. 16, 2024	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Business segment information [Line Items]
Net sales		$ 533.6	$ 468.9		$ 1,623.9	$ 1,468.7
Cost of sales		417.3	408.3		1,300.0	1,288.5
Gross margin		116.3	60.6		323.9	180.2
Operating income		67.7	2.5		154.0	17.0
Interest income and other		5.4	4.9		16.6	14.5
Gain on remeasurement of investment in TiO2 manufacturing joint venture	$ 64.5	64.5			64.5	0.0
Insurance recoveries					1.3	2.9
Gain on land sales				$ 1.5	0.0	1.5
Other components of net periodic pension and OPEB expense		(0.6)	(1.3)		(1.8)	(10.0)
Changes in market value of Valhi common stock held by subsidiaries		3.7	0.1		4.3	(2.1)
General expenses, net		(8.5)	(8.8)		(26.8)	(26.3)
Interest expense		(13.4)	(7.2)		(36.6)	(21.4)
Income (loss) before income taxes		119.9	(9.5)		175.5	(26.4)
Chemicals
Business segment information [Line Items]
Net sales		484.7	396.9		1,464.0	1,266.4
Cost of sales		384.4	362.8		1,193.0	1,158.0
Gross margin		100.3	34.1		271.0	108.4
Operating income		42.6	(21.8)		105.9	(39.5)
Insurance recoveries			0.3			2.5
Component products
Business segment information [Line Items]
Net sales		33.6	40.3		107.5	118.1
Cost of sales		24.1	27.7		77.2	82.5
Gross margin		9.5	12.6		30.3	35.6
Operating income		3.3	6.6		12.1	18.0
Real estate management and development
Business segment information [Line Items]
Net sales		15.3	31.7		52.4	84.2
Cost of sales		8.8	17.8		29.8	48.0
Gross margin		6.5	13.9		22.6	36.2
Operating income		$ 21.8	$ 17.7		$ 36.0	$ 38.5