Fair value measurement Financial assets and liabilities carried at fair value measured on recurring basis (Details) (USD $) In Thousands, unless otherwise specified	Sep. 28, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 6,540	$ 6,150
Derivative assets	959
Derivative liabilities	471
Contingent consideration liabilities	12,643	20,313
Level 1
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	6,540	6,150
Derivative assets	0
Derivative liabilities	0
Contingent consideration liabilities	0	0
Level 2
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	0	0
Derivative assets	959
Derivative liabilities	471
Contingent consideration liabilities	0	0
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	$ 12,643	$ 20,313