Other noncurrent liabilities - Other noncurrent liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Other noncurrent liabilities
Accrued development costs	$ 24.9	$ 26.8
Operating lease liabilities	$ 17.0	$ 17.1
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Total	Total
Insurance claims and expenses	$ 16.6	$ 16.6
Asset retirement obligations	15.1	14.3
Other postretirement benefits	6.7	6.7
Deferred income	4.7	11.8
Employee benefits	4.7	4.5
Earn-out liability	4.4	4.3
Other	0.6	0.6
Total	$ 94.7	$ 102.7