Valhi Stockholders' equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Accumulated other comprehensive income (loss) (net of tax and noncontrolling interest):
Balance at beginning of period	$ 1,036.1
Other comprehensive income (loss)	53.4	$ (13.7)	$ (1.6)
Plan settlement	17.5	0.2	4.3
Balance at end of period	1,023.2	1,036.1
Marketable Securities
Accumulated other comprehensive income (loss) (net of tax and noncontrolling interest):
Balance at beginning of period	1.7	1.7	1.6
Other comprehensive income (loss)			0.1
Balance at end of period	1.7	1.7	1.7
Currency Translation
Accumulated other comprehensive income (loss) (net of tax and noncontrolling interest):
Balance at beginning of period	(111.7)	(88.8)	(91.5)
Other comprehensive income (loss)	21.6	(22.9)	2.7
Balance at end of period	(90.1)	(111.7)	(88.8)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss) (net of tax and noncontrolling interest):
Balance at beginning of period	(49.4)	(58.8)	(55.0)
Other comprehensive income (loss)	1.8	2.1	1.9
Net actuarial gain (loss) arising during the year	12.4	7.1	(10.0)
Balance at end of period	(17.7)	(49.4)	(58.8)
Accumulated Defined Benefit Plans Adjustment | OPEB
Accumulated other comprehensive income (loss) (net of tax and noncontrolling interest):
Balance at beginning of period	0.2	0.4	1.0
Other comprehensive income (loss)	(0.3)	(0.3)	(0.7)
Net actuarial gain (loss) arising during the year	0.4	0.1	0.1
Balance at end of period	0.3	0.2	0.4
Total Accumulated Other Comprehensive Loss
Accumulated other comprehensive income (loss) (net of tax and noncontrolling interest):
Balance at beginning of period	(159.2)	(145.5)	(143.9)
Balance at end of period	$ (105.8)	$ (159.2)	$ (145.5)