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Fair value of financial instruments Fair value of financial instruments (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 derivative_instrument	Mar. 31, 2016 derivative_instrument
Debt Instrument [Line Items]
Other operating income (expense)	$ 3.5	$ (1.7)	$ 8.6	$ 0.5
Foreign currency transaction gains (losses)			(3.5)	(3.4)
Interest expense	(23.2)	(22.7)	(43.2)	(45.7)
Not Designated as Hedging Instrument \| Foreign Exchange Contract
Debt Instrument [Line Items]
Notional value	142.0		$ 142.0
Weighted average maturity			1 month
Not Designated as Hedging Instrument \| Foreign Exchange Contract \| Level 2
Debt Instrument [Line Items]
Fair value of foreign currency contract, net	3.9		$ 3.9		$ 0.6
Designated as Hedging Instrument \| Currency Swap
Debt Instrument [Line Items]
Notional value	110.0		$ 110.0
Weighted average maturity			2 years 1 month 6 days
Foreign currency derivative instrument gains (losses)	3.4	(3.7)	$ 28.8	(1.3)
Other operating income (expense)	(3.7)	2.4	(29.8)	(1.4)
Foreign currency transaction gains (losses)	(3.7)	2.4	(29.8)	(1.4)
Interest expense	(0.3)	(1.3)	(1.0)	(2.7)
Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	29.2		29.2		2.1
Designated as Hedging Instrument \| Interest Rate Swap
Debt Instrument [Line Items]
Notional value	40.0		$ 40.0
Weighted average maturity			4 months 24 days
Number of interest rate swaps \| derivative_instrument							2
Designated as Hedging Instrument \| $400 million interest rate swap
Debt Instrument [Line Items]
Notional value	400.0		$ 400.0
Weighted average maturity			1 year 9 months 18 days
Number of interest rate swaps \| derivative_instrument						10
Designated as Hedging Instrument \| $400 million interest rate swap \| Level 2
Debt Instrument [Line Items]
Interest expense	(2.1)		$ (2.9)
Fair value of swap, net	34.2		34.2		15.0
Prepaid expenses and other \| Not Designated as Hedging Instrument \| Foreign Exchange Contract \| Level 2
Debt Instrument [Line Items]
Fair value of foreign currency contract, net					0.8
Prepaid expenses and other \| Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	3.9		3.9
Prepaid expenses and other \| Designated as Hedging Instrument \| Interest Rate Swap \| Level 2
Debt Instrument [Line Items]
Fair value of swap, net					0.2
Accrued liabilities \| Not Designated as Hedging Instrument \| Foreign Exchange Contract \| Level 2
Debt Instrument [Line Items]
Fair value of foreign currency contract, net					0.2
Accrued liabilities \| Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, liability position					2.8
Accrued liabilities \| Designated as Hedging Instrument \| Interest Rate Swap \| Level 2
Debt Instrument [Line Items]
Fair value of swap, net	0.3		0.3
Accrued liabilities \| Designated as Hedging Instrument \| $400 million interest rate swap \| Level 2
Debt Instrument [Line Items]
Fair value of swap, liability position	9.6		9.6		3.6
Other assets \| Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	25.3		25.3		4.9
Other liabilities \| Designated as Hedging Instrument \| $400 million interest rate swap \| Level 2
Debt Instrument [Line Items]
Fair value of swap, liability position	24.6		24.6		11.4
Six hundred million senior unsecured notes
Debt Instrument [Line Items]
Carrying value	600.0		600.0		600.0
Six hundred million senior unsecured notes \| Level 3
Debt Instrument [Line Items]
Fair value	581.7		581.7		624.7
Four hundred million senior unsecured notes
Debt Instrument [Line Items]
Carrying value	400.0		400.0		0.0
Four hundred million senior unsecured notes \| Level 3
Debt Instrument [Line Items]
Fair value	394.9		394.9		$ 0.0
Other operating income (expense) \| Not Designated as Hedging Instrument \| Foreign Exchange Contract
Debt Instrument [Line Items]
Foreign currency derivative instrument not designated as hedging gains (losses)	2.6	$ (1.4)	3.9	$ 2.5
Interest and other nonoperating expense \| Not Designated as Hedging Instrument \| Foreign Exchange Contract
Debt Instrument [Line Items]
Foreign currency derivative instrument not designated as hedging gains (losses)	$ (0.4)		$ (8.1)