Financial instruments - Fair values of derivative instruments designated as hedging instruments (Details) - USD ($) $ in Thousands	Sep. 29, 2019	Dec. 31, 2018
Cash flow hedging
Derivatives Fair Value [Line Items]
Total asset derivatives	$ 51,898	$ 16,050
Total liability derivatives	932	8,581
Cash flow hedging | Other assets
Derivatives Fair Value [Line Items]
Total asset derivatives	23,857	0
Foreign exchange contract | Other liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	0	7,793
Foreign exchange contract | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	28,041	16,050
Foreign exchange contract | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	932	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	998	1,216
Foreign exchange contract | Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	795	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	289	106
Foreign exchange contract | Not Designated as Hedging Instrument | Cash flow hedging | Other current liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	137	264
Cross-currency interest rate swap | Not Designated as Hedging Instrument | Cash flow hedging | Prepaid expenses and other current assets
Derivatives Fair Value [Line Items]
Total asset derivatives	26,754	14,728
Cross-currency interest rate swap | Not Designated as Hedging Instrument | Cash flow hedging | Other assets
Derivatives Fair Value [Line Items]
Total asset derivatives	23,857	0
Cross-currency interest rate swap | Not Designated as Hedging Instrument | Cash flow hedging | Other liabilities
Derivatives Fair Value [Line Items]
Total liability derivatives	$ 0	$ 7,793