Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Cash flow hedging
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 22,659	$ 16,050
Total liability derivatives	7,275	8,581
Foreign exchange contract | Other liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	6,152	7,793
Foreign exchange contract | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	22,659	16,050
Foreign exchange contract | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	1,123	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	1,344	1,216
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	992	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	159	106
Foreign exchange contract | Not Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	131	264
Cross-currency interest rate swap | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	21,156	14,728
Cross-currency interest rate swap | Cash flow hedging | Other liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 6,152	$ 7,793