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Summary of significant accounting policies - Expected recognition of deferred revenue (Details)
$ in Thousands
Sep. 29, 2018
USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue expected to be recognized $ 11,615
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue expected to be recognized 10,905
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue expected to be recognized 9,602
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue expected to be recognized 7,756
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-12-31  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue expected to be recognized 11,089
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: (nil)  
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue expected to be recognized $ 50,967