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Fair value of financial instruments Fair value of financial instruments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($) derivative_instrument	Dec. 31, 2019 USD ($)	Mar. 31, 2016 derivative_instrument
Debt Instrument [Line Items]
Foreign currency derivative instrument not designated as hedging gains (losses)	$ (7.7)	$ 0.0
Other operating income (expense)	5.1	2.2
Foreign currency transaction gains (losses)	(1.6)	(3.9)
Interest expense	20.0	23.0
Not Designated as Hedging Instrument \| Foreign Exchange Contract
Debt Instrument [Line Items]
Notional value	$ 722.0
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	$ 5.4		$ 0.6
Designated as Hedging Instrument \| Currency Swap
Debt Instrument [Line Items]
Notional value	$ 116.0
Weighted average maturity	2 years 3 months 18 days
Foreign currency derivative instrument gains (losses)	$ 25.4	2.4
Other operating income (expense)	(26.1)	(3.8)
Foreign currency transaction gains (losses)	26.1	3.8
Interest expense	0.7	$ 1.4
Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	28.4		2.1
Designated as Hedging Instrument \| Interest Rate Swap
Debt Instrument [Line Items]
Notional value	$ 40.0
Weighted average maturity	6 months
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
Weighted average maturity	2 years
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	$ 0.8
Fair value of swap, net	33.0		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	3.5		0.8
Prepaid expenses and other \| Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, asset position	3.0
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	8.9		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	0.4
Accrued liabilities \| Designated as Hedging Instrument \| $400 million interest rate swap \| Level 2
Debt Instrument [Line Items]
Fair value of swap, liability position	8.8		3.6
Other assets \| Designated as Hedging Instrument \| Currency Swap \| Level 2
Debt Instrument [Line Items]
Fair value of contract, liability position	25.4		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	24.2		11.4
Six hundred million senior unsecured notes
Debt Instrument [Line Items]
Carrying value	600.0		600.0
Six hundred million senior unsecured notes \| Level 3
Debt Instrument [Line Items]
Fair value	540.4		$ 624.7
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)	1.3	$ 3.9
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)	$ 7.7