Fair value measurement - Financial assets and liabilities carried at fair value measured on recurring basis (Details) - USD ($) $ in Thousands	Jul. 01, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 9,272	$ 9,045
Derivative assets	1,954	1,221
Derivative liabilities	861	1,426
Contingent consideration liabilities	242,659	272,136
Quoted prices in active markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	9,272	9,045
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	1,954	1,221
Derivative liabilities	861	1,426
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	$ 242,659	$ 272,136