Balance sheet components - Expected revenue recognition (Details) $ in Thousands	Jun. 29, 2019 USD ($)
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred revenue expected to be recognized	$ 56,106
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-06-30
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred revenue expected to be recognized	$ 3,561
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligations expected to be satisfied in	3 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-09-29
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred revenue expected to be recognized	$ 12,934
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligations expected to be satisfied in	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-09-27
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred revenue expected to be recognized	$ 11,631
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligations expected to be satisfied in	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-03
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred revenue expected to be recognized	$ 9,785
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligations expected to be satisfied in	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-10-02
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Deferred revenue expected to be recognized	$ 18,195
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Performance obligations expected to be satisfied in