Fair value measurement Financial assets and liabilities carried at fair value measured on recurring basis (Details) - USD ($) $ in Thousands	Jun. 28, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 6,960	$ 6,863
Derivative assets	419
Derivative liabilities	1,808
Contingent consideration liabilities	27,150	33,433
Quoted prices in active markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	6,960	6,863
Derivative assets	0
Derivative liabilities	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	419
Derivative liabilities	1,808
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
Derivative liabilities	0
Contingent consideration liabilities	$ 27,150	$ 33,433