Supplementary balance sheet information - Additional Information (Details) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) building	Dec. 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges	$ 0.0
Inventories	68.8	$ 77.6
Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of property disposed	$ 11.6	12.4
Elektron segment \| Building \| Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties to be disposed \| building	1
Amount of property disposed	$ 3.7
Gas Cylinders segment \| Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Inventories		0.2
Restructuring Charges
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges		4.6
Impairment Charges
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges	$ 5.2	$ 5.2

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