Fair value measurement Valuation technique and inputs used to determine fair value of assets or liabilities (Details)	3 Months Ended
	Mar. 29, 2015
Probability Of Payment
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration, weighted average	50.20%
Probability Of Payment | Minimum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	0.00%
Probability Of Payment | Maximum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	100.00%
Market Approach Valuation Technique [Member]
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration, weighted average	7.30%
Market Approach Valuation Technique [Member] | Minimum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	1.70%
Market Approach Valuation Technique [Member] | Maximum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	10.00%