Fair value measurement Financial assets and liabilities carried at fair value measured on recurring basis (Details) (USD $) In Thousands, unless otherwise specified	Mar. 29, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	$ 6,954us-gaap_MarketableSecurities	$ 6,863us-gaap_MarketableSecurities
Derivative assets	1,737us-gaap_DerivativeAssets
Derivative liabilities	1,553us-gaap_DerivativeLiabilitiesCurrent
Contingent consideration liabilities	29,825	33,433
Quoted prices in active markets (Level 1)
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments in marketable securities	6,954	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	1,737
Derivative liabilities	1,553
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	$ 29,825	$ 33,433

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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