Stockholders' equity - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Jun. 30, 2019
Class of Stock [Line Items]
Accumulated deficit subject to restriction	$ 9.0
Minimum
Class of Stock [Line Items]
Accumulated profits	10.00%
Employee stock options
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)		8,554,197
Employee stock purchase plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)		1,820,744