Financial instruments - (Schedule of share price risk) (Details) - Share price risk [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Disclosure of detailed information about financial instruments [line items]
Sensitivity analysis, variance, percentage	25.00%	25.00%
Effect of variance increase on after-tax profit	$ 3.9	$ 2.8
Effect of variance decrease on after-tax profit	$ (3.9)	$ (2.8)

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

No definition available.

Name:	ifrs-full_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration