Summary of significant accounting policies - Product Warranty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Movement in Standard and Extended Product Warranty Accrual, Increase (Decrease) [Roll Forward]
Beginning balance	$ 470	$ 219	$ 118
Warranty expenses incurred	1,495	1,160	421
Provisions	1,355	1,411	522
Ending balance	$ 330	$ 470	$ 219
Systems [Member]
Product Warranty Liability [Line Items]
Products Warranty Period	1 year
Reagents [Member]
Product Warranty Liability [Line Items]
Products Warranty Period	60 days