Segment information - Reconciliation of Total Business Segment Assets to Consolidated Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 1,965.4	$ 2,065.6	$ 2,419.5
Cash and cash equivalents	387.7	391.8
Deferred income taxes	114.0	118.6
Operating lease right-of-use assets	35.4	42.5
Prepaid postretirement assets	38.0	119.5
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,196.6	1,190.6	1,237.2
Segment Reconciling Items
Segment Reporting, Asset Reconciling Item [Line Items]
Other assets	82.1	96.8	206.9
Cash and cash equivalents	387.7	391.8	552.9
Deferred income taxes	114.0	118.6	125.3
Operating lease right-of-use assets	35.4	42.5	62.7
Prepaid postretirement assets	38.0	119.5	159.7
Corporate
Segment Reporting, Asset Reconciling Item [Line Items]
Other assets	$ 111.6	$ 105.8	$ 74.8