Segment information - Reconciliation of Total Business Segment Assets to Consolidated Assets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting, Asset Reconciling Item [Line Items]
Cash and cash equivalents	$ 552.9	$ 898.5	$ 538.8
Deferred income taxes	125.3	136.2	114.0
Operating lease right-of-use assets	62.7	79.3	71.4
Prepaid postretirement assets	159.7	187.5	136.2
Assets of discontinued operations	0.0	0.0	243.2
Other corporate assets	207.3	297.0	300.6
Total assets	2,419.5	2,707.9	2,504.0
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,236.6	1,005.3	962.8
Other corporate assets
Segment Reporting, Asset Reconciling Item [Line Items]
Other corporate assets	$ 75.0	$ 104.1	$ 137.0