Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Nominal or face amount used to calculate payment on derivative.

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Period remaining until the derivative contract matures, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of interest rate derivative instruments held by the entity at the reporting date.

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Derivative Instruments - Interest rate swap agreement (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) agreement	Dec. 31, 2019 USD ($)	Jan. 31, 2021 USD ($) agreement	Oct. 31, 2019 USD ($)	Dec. 31, 2016 USD ($)
Interest rate contracts
Derivative instruments
Borrowings at variable interest rates	$ 783.1	$ 404.3
Number of interest rate swap agreements \| agreement	2
Notional amount outstanding				$ 100.0	$ 100.0
Net fair value of swap liability	$ 4.0	1.7
Interest rate contracts \| Subsequent Event
Derivative instruments
Number of interest rate swap agreements \| agreement			4
Notional amount outstanding			$ 100.0
Interest rate contracts \| Level 2 \| Fair Value, Recurring
Derivative instruments
Loss recorded to net finance costs	$ 2.0	$ (0.7)
2016 Swap Agreement
Derivative instruments
Remaining maturity term	4 months
2019 Swap Agreement
Derivative instruments
Remaining maturity term	42 months
Swap agreement maturing January 2024 \| Subsequent Event
Derivative instruments
Number of interest rate swap agreements \| agreement			2
Swap agreement maturing January 2026 \| Subsequent Event
Derivative instruments
Number of interest rate swap agreements \| agreement			2