Fair value measurement - Valuation technique and inputs used to determine fair value (Details)	9 Months Ended
	Sep. 25, 2016
Probability Of Payment | Weighted Average
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	71.30%
Probability Of Payment | Minimum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	2.00%
Probability Of Payment | Maximum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	100.00%
Market Approach Valuation Technique | Weighted Average
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	8.40%
Market Approach Valuation Technique | Minimum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	2.30%
Market Approach Valuation Technique | Maximum
Fair Value Assets And Liabilities Measured On Unobservable Inputs [Line Items]
Contingent consideration	10.00%