Fair value measurement - Financial assets and liabilities carried at fair value measured on recurring basis (Details) - USD ($) $ in Thousands	Oct. 01, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 8,563	$ 7,660
Derivative assets	2,002	1,157
Derivative liabilities	330	2,257
Contingent consideration liabilities	12,117	7,102
Current portion of contingent consideration	591	587
Fair value measurement with unobservable inputs reconciliations other liabilities	11,500	6,500
Quoted prices in active markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	8,563	7,660
Derivative assets	0	0
Derivative liabilities	0	0
Contingent consideration liabilities	0	0
Significant other observable Inputs (Level 2)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	0	0
Derivative assets	2,002	1,157
Derivative liabilities	330	2,257
Contingent consideration liabilities	0	0
Significant unobservable Inputs (Level 3)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	0	0
Derivative assets	0	0
Derivative liabilities	0	0
Contingent consideration liabilities	$ 12,117	$ 7,102