Supplementary balance sheet information - Additional Information (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) facility	Dec. 31, 2018 USD ($) facility
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Inventories	$ 94.5	$ 93.6
Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of property disposed	3.7	$ 5.5
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 \| facility		2
Amount of property disposed		$ 4.7
Impairment of assets held-for-sale	1.1
Magnesium Elektron CZ s.r.o. Subsidiary \| Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of buildings to be disposed of	$ 0.2	3.4
Elektron segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges		1.5
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 \| facility	1
Amount of property disposed	$ 3.7
Gas Cylinders segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges		5.1
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	6.6
Impairment Charges
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges	$ 5.2	$ 0.0

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Amount classified as property, plant and equipment attributable to disposal group held for sale or disposed of, expected to be disposed of within one year or the normal operating cycle, if longer.

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