Income taxes - Textual (Details) $ in Millions	Sep. 28, 2019 USD ($)
Operating Loss Carryforwards [Line Items]
Undistributed earnings	$ 5.9
Withholding tax liability	0.5
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	60.6
Federal | Research tax credit carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	33.8
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	36.2
State | Research tax credit carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	25.9
Foreign
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	16.1
Foreign | Tax year expiration, indefinite
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	1.4
Foreign | Tax year expiration, 2027
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 14.7