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Fair value measurements and financial instruments - Financial instruments recorded on a fair value (Details) $ in Millions			1 Months Ended	12 Months Ended
	Feb. 12, 2024 USD ($)	Feb. 12, 2024 EUR (€)	Aug. 31, 2025 USD ($)	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2025 EUR (€)	Sep. 30, 2025 EUR (€)	Sep. 15, 2025 EUR (€)	Aug. 30, 2025	Mar. 31, 2025 EUR (€)	Aug. 31, 2024	Jul. 30, 2024 EUR (€)
Fair value measurements and financial instruments
Foreign currency transaction gain				$ 2.8
Currency transactions, net				5.4	$ 1.6	$ 1.4
Kronos term loan from Contran					53.7
Valhi - Contran credit facility				591.7	$ 563.4
VALHI, INC. \| Contran Credit Facility
Fair value measurements and financial instruments
Valhi - Contran credit facility				23.6
Foreign exchange forward \| Kronos \| Kronos International, Inc
Fair value measurements and financial instruments
Contract to purchase \| €											€ 25,000,000
Exchange rate											1.05
Cash received from contract settlement			$ 2.8
Currency forward contract				$ 0.0
3.75% Senior Secured Notes due 2025 \| Kronos
Fair value measurements and financial instruments
Debt instrument principal amount \| €		€ 325,000,000							€ 75,000,000
Debt instrument interest rate		3.75%							3.75%
3.75% Senior Secured Notes due 2025 \| Kronos \| Kronos International, Inc
Fair value measurements and financial instruments
Debt instrument principal amount \| €							€ 75,000,000	€ 75,000,000
Debt instrument interest rate				3.75%	3.75%		3.75%	3.75%
Valhi - Contran credit facility					$ 78.3
9.50% Senior Secured Notes due 2029 \| Kronos
Fair value measurements and financial instruments
Debt instrument principal amount \| €		€ 276,174,000							€ 75,000,000				€ 75,000,000
Debt instrument interest rate		9.50%		9.50%			9.50%		9.50%				9.50%
9.50% Senior Secured Notes due 2029 \| Kronos \| Kronos International, Inc
Fair value measurements and financial instruments
Debt instrument principal amount \| €									€ 351,174,000
Debt instrument interest rate		9.50%		9.50%	9.50%		9.50%
Estimated market price of the notes \| €							€ 937
Principal amount of debt instrument \| €							€ 1,000
Valhi - Contran credit facility				$ 503.7	$ 365.4
Subordinated, unsecured term loan from Contran \| Kronos
Fair value measurements and financial instruments
Debt instrument interest rate										11.50%		9.54%
Kronos term loan from Contran	$ 53.7	€ 50,000,000		$ 53.7