Business Segment Information - Segment Operating Performance (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net sales	$ 509.0	$ 459.3
Cost of sales	399.7	359.6
Gross margin	109.3	99.7
Operating income	51.3	71.0
Securities earnings	0.9	1.6
Insurance recoveries		1.6
Changes in market value of Valhi common stock held by subsidiaries	1.3	(2.4)
Other components of net periodic pension and OPEB expense	(4.3)	(4.7)
General expenses, net	(8.1)	(8.1)
Interest expense	(8.6)	(9.7)
Income before income taxes	32.5	49.3
Chemicals
Segment Reporting Information [Line Items]
Net sales	465.0	421.0
Cost of sales	369.7	333.3
Gross margin	95.3	87.7
Operating income	37.7	46.8
Component Products
Segment Reporting Information [Line Items]
Net sales	35.9	32.3
Cost of sales	24.9	21.9
Gross margin	11.0	10.4
Operating income	5.8	5.0
Real Estate Management And Development
Segment Reporting Information [Line Items]
Net sales	8.1	6.0
Cost of sales	5.1	4.4
Gross margin	3.0	1.6
Operating income	$ 7.8	$ 19.2