Supplementary balance sheet information - Schedule of Condensed Balance Sheet (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Accounts and other receivables
Trade receivables, net	$ 33.6	$ 40.8
Related parties	0.2	2.7
Prepayments and accrued income	5.5	4.8
Derivative financial instruments	0.2	0.3
Deferred consideration	0.2	0.0
Other receivables	3.4	4.2
Total accounts and other receivables	43.1	52.8
Inventories
Raw materials and supplies	26.2	27.9
Work-in-process	19.7	24.2
Finished goods	22.9	25.5
Total inventories	68.8	77.6
Other current assets
Income tax receivable	1.5	1.1
Total other current assets	1.5	1.1
Property, Plant and Equipment [Abstract]
Total property plant and equipment	328.3	316.0
Accumulated depreciation and impairment	(242.3)	(225.8)
Total property, plant and equipment, net	86.0	90.2
Other current liabilities
Contingent liabilities	10.1	6.6
Derivative financial instruments	0.4	0.0
Operating lease liability	2.9	2.3
Other current liabilities	0.1	1.8
Total other current liabilities	13.5	10.7
Other non-current liabilities
Contingent liabilities	1.0	0.9
Operating lease liability	6.7	8.9
Other non-current liabilities	0.0	0.1
Total other non-current liabilities	$ 7.7	$ 9.9
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesCurrent	us-gaap:OtherLiabilitiesCurrent
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Land, buildings and leasehold improvements
Property, Plant and Equipment [Abstract]
Total property plant and equipment	$ 65.2	$ 61.4
Machinery and equipment
Property, Plant and Equipment [Abstract]
Total property plant and equipment	255.3	245.7
Construction in progress
Property, Plant and Equipment [Abstract]
Total property plant and equipment	$ 7.8	$ 8.9