Financial instruments - (Schedule of interest rate risk) (Details) - Interest rate 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	2.00%	2.00%
Effect of variance increase on after-tax profit	$ (38.0)	$ 2.3
Effect of variance decrease on after-tax profit	$ 48.5	$ (2.6)

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.

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