Prepaid and other assets - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Miscellaneous other receivables, VAT	$ 21.5	$ 15.7
Miscellaneous other receivables, advance payments	1.2	5.3
Down payments	3.1	4.3
Eco-tax natural gas	3.2	5.9
Miscellaneous other receivables, guarantee deposits	1.4	1.7
Other unamortized transaction costs	3.4	2.4
Transaction costs	$ 2.8	$ 1.8