Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Cash flow hedging
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 24,287	$ 16,050
Total liability derivatives	5,300	8,581
Foreign exchange contract | Other liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	3,713	7,793
Foreign exchange contract | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	24,276	16,050
Foreign exchange contract | Cash flow hedging | Other assets
Derivatives Fair Value [Line Items]
Total asset derivatives	11	0
Foreign exchange contract | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	1,587	788
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	883	1,216
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	1,054	524
Foreign exchange contract | Not Designated as Hedging Instrument | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	264	106
Foreign exchange contract | Not Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	533	264
Cross-currency interest rate swap | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	23,129	14,728
Cross-currency interest rate swap | Cash flow hedging | Other assets
Derivatives Fair Value [Line Items]
Total asset derivatives	11	0
Cross-currency interest rate swap | Cash flow hedging | Other liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 3,713	$ 7,793