Fair value measurement - Financial assets and liabilities carried at fair value measured on recurring basis (Details) - USD ($) $ in Thousands	Sep. 25, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 7,411	$ 6,922
Derivative assets	1,134	329
Derivative liabilities	2,403	1,298
Contingent consideration liabilities	22,368	20,829
Quoted prices in active markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	7,411	6,922
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	1,134	329
Derivative liabilities	2,403	1,298
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	$ 22,368	$ 20,829