Business Segment Information - Segment Operating Performance (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net sales	$ 578.9	$ 458.6	$ 1,613.2	$ 1,332.9
Cost of sales	426.2	368.9	1,226.6	1,039.5
Gross margin	152.7	89.7	386.6	293.4
Operating income	81.2	28.9	188.1	139.2
Securities earnings	0.9	0.9	2.9	3.5
Insurance recoveries				1.6
Gain on land and related sales	10.4		16.0	0.5
Changes in market value of Valhi common stock held by subsidiaries	(0.2)	0.7	2.0	(2.2)
Other components of net periodic pension and OPEB expense	(4.3)	(5.2)	(13.2)	(14.9)
General expenses, net	(8.7)	(8.4)	(26.1)	(26.0)
Interest expense	(7.9)	(8.9)	(25.2)	(27.4)
Income before income taxes	71.4	8.0	144.5	74.3
Chemicals
Segment Reporting Information [Line Items]
Net sales	499.8	416.9	1,443.4	1,223.9
Cost of sales	377.1	337.3	1,116.7	961.8
Gross margin	122.7	79.6	326.7	262.1
Operating income	60.3	21.6	145.4	104.3
Component Products
Segment Reporting Information [Line Items]
Net sales	34.5	28.4	106.7	84.5
Cost of sales	23.6	21.1	73.4	59.4
Gross margin	10.9	7.3	33.3	25.1
Operating income	5.1	2.1	16.7	9.5
Real Estate Management And Development
Segment Reporting Information [Line Items]
Net sales	44.6	13.3	63.1	24.5
Cost of sales	25.5	10.5	36.5	18.3
Gross margin	19.1	2.8	26.6	6.2
Operating income	$ 15.8	$ 5.2	$ 26.0	$ 25.4