Supplementary balance sheet information - Additional Information (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) facility	Dec. 31, 2017 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges	$ 6.6	$ 2.8
Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of property disposed	5.5
Building \| Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount of property disposed	$ 4.7
Number of properties to be disposed \| facility	2
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	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	1.3
Gas Cylinders segment
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Tangible asset impairment charges	$ 5.1	$ 1.5

Disposal Group, Including Discontinued Operation, Property, Plant And Equipment, Number Of Properties

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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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Amount, before tax, of (gain) loss recognized for the (reversal of write-down) write-down to fair value, less cost to sell, of a disposal group. Excludes discontinued operations.

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