Segment information - Reconciliation of Total Business Segment Assets to Consolidated Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	$ 2,457.6	$ 2,542.4	$ 2,021.6
Cash and cash equivalents	605.0	733.9	370.6
Deferred income taxes	109.3	119.9	146.1
Prepaid postretirement assets	147.6	148.3	33.3
Operating Segments
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,436.6	1,364.5	1,339.0
Corporate
Segment Reporting, Asset Reconciling Item [Line Items]
Total assets	1,021.0	1,177.9	682.6
Other corporate assets	$ 159.1	$ 175.8	$ 132.6