Property and equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 17,878	$ 13,273
Less: accumulated depreciation	(5,107)	(2,990)
Property and equipment, net	12,771	10,283
Depreciation expense	2,117	1,595	$ 612
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	7,035	6,421
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	8,843	5,262
Furniture and office equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,675	1,565
Other
Property, Plant and Equipment [Line Items]
Property and equipment, gross	306	$ 25
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 19