Other noncurrent liabilities - Other noncurrent liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Other noncurrent liabilities
Accrued development costs	$ 24.7	$ 26.8
Operating lease liabilities	$ 17.1	$ 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	16.2	14.3
Deferred income	8.0	11.8
Other postretirement benefits	7.0	6.7
Employee benefits	5.0	4.5
Earn-out liability	4.6	4.3
Other	0.5	0.6
Total	$ 99.7	$ 102.7