Valhi Stockholders' Equity - Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	$ 682.5
Other comprehensive income (loss)	28.1	$ 1.3	$ (14.5)
Balance at end of year	829.5	682.5
Marketable Securities
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	1.8	1.7	1.7
Other comprehensive income (loss)	(0.1)	0.1	0.0
Balance at end of year	1.7	1.8	1.7
Currency Translation
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(67.4)	(76.8)	(75.6)
Other comprehensive income (loss)	(4.8)	9.4	(1.2)
Balance at end of year	(72.2)	(67.4)	(76.8)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(154.1)	(146.6)	(134.0)
Other comprehensive income (loss)	10.7	9.8	7.2
Net actuarial gain (loss) arising during the year	22.5	(17.3)	(19.8)
Balance at end of year	(120.9)	(154.1)	(146.6)
Accumulated Defined Benefit Plans Adjustment | Other comprehensive income (loss)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	0.3	1.0	1.7
Other comprehensive income (loss)	(0.3)	(0.8)	(0.8)
Net actuarial gain (loss) arising during the year	0.1	0.1	0.1
Balance at end of year	0.1	0.3	1.0
Accumulated other comprehensive loss
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of year	(219.4)	(220.7)	(206.2)
Balance at end of year	$ (191.3)	$ (219.4)	$ (220.7)