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Fair value measurements and financial instruments - Financial instruments recorded on a fair value (Details) € in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 EUR (€)	Mar. 31, 2025 EUR (€)	Dec. 31, 2024 USD ($)
Fair value measurements and financial instruments
Market value, current	$ 2.4	$ 2.4				$ 1.9
Market value, noncurrent	4.7	4.7				5.7
Currency forward contract	3.2	3.2
Earn-out liability	(4.6)	(4.6)				$ (4.3)
Currency transactions, net		5.0	$ 2.0
Foreign exchange forward \| Kronos \| Kronos International, Inc
Fair value measurements and financial instruments
Currency forward contract	3.2	3.2
Contract to purchase \| €					€ 25
Exchange rate					1.05
Currency transactions, net	$ 2.3	$ 3.2
3.75% Senior Secured Notes due 2025 \| Kronos \| Kronos International, Inc
Fair value measurements and financial instruments
Debt instrument principal amount \| €				€ 75
Debt instrument interest rate	3.75%	3.75%		3.75%		3.75%
Quoted Prices in Active Markets (Level 1)
Fair value measurements and financial instruments
Market value, noncurrent	$ 2.7	$ 2.7				$ 2.6
Currency forward contract	3.2	3.2
Significant Other Observable Inputs (Level 2)
Fair value measurements and financial instruments
Market value, current	2.4	2.4				1.9
Market value, noncurrent	2.0	2.0				3.1
Significant Unobservable Inputs (Level 3)
Fair value measurements and financial instruments
Earn-out liability	$ (4.6)	$ (4.6)				$ (4.3)