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Goodwill and other identifiable intangible assets - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets
Accumulated goodwill impairment losses	$ 9.6	$ 8.3
Goodwill impairment	1.3
Identifiable intangible asset amortization expense	1.2	1.3	$ 1.1
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity
2019	1.2
2020	1.1
2021	1.1
2022	1.1
2023	1.1
Gas Cylinders
Finite-Lived Intangible Assets
Goodwill impairment	$ 1.3
Gas Cylinders \| Technology and trading related
Finite-Lived Intangible Assets
Impairment charges		$ 0.5