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Fair value of financial instruments (Details) $ in Millions	3 Months Ended		9 Months Ended
Fair value of financial instruments (Details) $ in Millions	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 derivative_instrument	Mar. 31, 2016 derivative_instrument
Debt Instrument [Line Items]
Other operating income (expense)	$ 9.8	$ 6.5	$ 18.4	$ 7.0
Foreign currency transaction gains (losses)			(5.3)	(10.4)
Interest expense	(27.1)	(22.9)	(70.3)	(68.6)
Not Designated as Hedging Instrument \| Foreign Exchange Contract
Debt Instrument [Line Items]
Notional value	282.0		$ 282.0
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	0.7		$ 0.7		$ 0.6
Designated as Hedging Instrument \| Currency Swap
Debt Instrument [Line Items]
Notional value	104.0		$ 104.0
Weighted average maturity			2 years
Foreign currency derivative instrument gains (losses)	0.3	7.6	$ 29.1	6.3
Other operating income (expense)	(0.8)	(8.9)	(30.6)	(10.3)
Foreign currency transaction gains (losses)	(0.8)	(8.9)	(30.6)	(10.3)
Interest expense	(0.5)	(1.3)	(1.5)	(4.0)
Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	30.9		30.9		2.1
Designated as Hedging Instrument \| Interest Rate Swap
Debt Instrument [Line Items]
Notional value	40.0		$ 40.0
Weighted average maturity			3 months 18 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 8 months 12 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	(2.3)		$ (5.2)
Fair value of swap, net	31.8		31.8		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	0.2		0.2		0.8
Prepaid expenses and other \| Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	4.6		4.6
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	0.9		0.9		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	0.2		0.2
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	26.3		26.3		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	22.2		22.2		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	611.7		611.7		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	410.5		410.5		$ 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)	(3.4)	5.8	0.5	8.3
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)	$ (0.5)	$ 0.0	$ (8.6)	$ 0.0