Income taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Amount
U.S. federal statutory tax rate	$ (9.0)	$ 51.3	$ (4.1)
State income taxes, net of federal income tax effect	(1.5)	2.8	(0.4)
Changes in valuation allowance	13.2	2.5
Other		0.2	(0.2)
Incremental tax expense (benefit) on earnings (losses) of subsidiary	(12.3)	9.3	(3.9)
Other	0.4	3.3	(0.4)
Changes in unrecognized tax benefits		0.5	(0.7)
Pension termination	(5.5)
Income tax expense	$ 11.9	$ 82.9	$ (24.6)
Tax Jurisdiction of Domicile [Extensible Enumeration]	UNITED STATES	UNITED STATES	UNITED STATES
Percent
U.S. federal statutory tax rate (percentage)	21.00%	21.00%	21.00%
State income taxes, net of federal income tax effect (Percentage)	3.50%	1.20%	2.10%
Changes in valuation allowance (Percentage)	(30.70%)	1.00%
Other (Percentage)			0.40%
Incremental tax expense (benefit) on earnings (losses) of subsidiary (Percentage)	28.60%	3.80%	20.10%
Other (Percentage)	(0.90%)	1.40%	2.00%
Changes in unrecognized tax benefits (Percentage)		0.20%	3.80%
Pension termination (Percentage)	12.80%
Income tax expense (benefit) (Percentage)	(27.60%)	33.90%	126.20%
Kronos
Amount
Incremental tax expense (benefit) on investment	$ (7.1)	$ 4.4	$ (5.5)
Percent
Incremental tax expense (benefit) on investment (Percentage)	16.50%	1.80%	28.40%
BMI/LandWell
Amount
Incremental tax expense (benefit) on investment	$ (0.6)	$ (2.6)	$ (2.7)
Percent
Incremental tax expense (benefit) on investment (Percentage)	1.50%	(1.10%)	13.90%
Germany
Amount
State income taxes, net of federal income tax effect	$ (6.1)	$ 0.8	$ (7.5)
Statutory tax rate difference	2.8	(0.4)	3.6
Effect of changes in tax laws enacted in the current period	19.3
Changes in valuation allowance	8.5
Other	$ 2.3	$ 3.5	$ 0.5
Percent
State income taxes, net of federal income tax effect (Percentage)	14.10%	0.30%	38.90%
Statutory tax rate difference (Percentage)	(6.50%)	(0.20%)	(18.40%)
Effect of changes in tax laws enacted in the current period (Percentage)	(45.10%)
Changes in valuation allowance (Percentage)	(20.00%)
Other (Percentage)	(5.10%)	1.50%	(2.80%)
Belgium
Amount
State income taxes, net of federal income tax effect	$ (1.4)	$ (1.1)	$ (2.0)
Changes in valuation allowance	8.6	$ 8.2
Other	$ 0.1		$ 0.2
Percent
Statutory tax rate difference (Percentage)	3.20%	(0.50%)	10.20%
Changes in valuation allowance (Percentage)	(20.00%)	3.40%
Other (Percentage)	(0.10%)		(0.80%)
Canada
Amount
State income taxes, net of federal income tax effect	$ (0.4)	$ (1.4)	$ (2.4)
Statutory tax rate difference	0.2	0.7	1.3
Other	$ (0.4)	$ (0.4)	$ (1.1)
Percent
State income taxes, net of federal income tax effect (Percentage)	1.00%	(0.60%)	12.40%
Statutory tax rate difference (Percentage)	(0.50%)	0.30%	(6.50%)
Other (Percentage)	1.00%	(0.20%)	5.20%
Other foreign jurisdictions
Amount
Other	$ 0.8	$ 1.3	$ 0.7
Percent
Other (Percentage)	(1.90%)	0.60%	(3.70%)