Quarterly Results of Operations - Schedule of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Quarterly Results Of Operations Schedule Of Quarterly Results Of Operations [Abstract]
Net sales	$ 516.8	$ 458.6	$ 415.0	$ 459.3	$ 414.1	$ 475.2	$ 528.6	$ 479.6	$ 1,849.7	$ 1,897.5	$ 1,820.1
Gross margin	118.7	89.7	104.0	99.7	91.2	98.4	123.2	121.8	412.1	434.6	609.2
Operating income	46.9	28.9	39.3	71.0	25.0	41.4	65.9	60.4	186.1	192.7	370.7
Net income from continuing operations	31.6	19.4	(4.2)	37.9	14.4	17.2	18.2	28.4	84.7	78.2	266.9
Amounts attributable to Valhi stockholders:
Income from continuing operations	20.2	15.4	(9.1)	24.4					50.9	49.2	228.1
Income from discontinued operations	4.3								4.3		34.1
Net income attributable to Valhi stockholders	$ 24.5	$ 15.4	$ (9.1)	$ 24.4	$ 10.8	$ 13.1	$ 7.1	$ 18.2	$ 55.2	$ 49.2	$ 262.2
Earnings per share:
Income from continuing operations	$ 0.71	$ 0.54	$ (0.32)	$ 0.86					$ 1.79	$ 1.73	$ 8.00
Income from discontinued operations	0.15								0.15		1.20
Net income per basic and diluted share	$ 0.86	$ 0.54	$ (0.32)	$ 0.86	$ 0.38	$ 0.46	$ 0.25	$ 0.64	$ 1.94	$ 1.73	$ 9.20