Financial instruments - Fair value of derivatives (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Foreign Currency Exchange Contracts | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	$ 7,793	$ 0
Cash Flow Hedging
Derivatives Fair Value [Line Items]
Asset derivatives:	16,050	1,221
Liability derivatives:	8,581	1,426
Cash Flow Hedging | Foreign Currency Exchange Contracts | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	16,050	1,221
Cash Flow Hedging | Foreign Currency Exchange Contracts | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	788	1,426
Cash Flow Hedging | Foreign Currency Exchange Contracts | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	1,216	914
Cash Flow Hedging | Foreign Currency Exchange Contracts | Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	524	1,373
Cash Flow Hedging | Foreign Currency Exchange Contracts | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	106	307
Cash Flow Hedging | Foreign Currency Exchange Contracts | Not Designated as Hedging Instrument | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	264	53
Cash Flow Hedging | Cross Currency Interest Rate Contract | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Asset derivatives:	14,728	0
Cash Flow Hedging | Cross Currency Interest Rate Contract | Other current liabilities
Derivatives Fair Value [Line Items]
Liability derivatives:	$ 7,793	$ 0