Stockholders' equity - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2020
Class of Stock [Line Items]
Accumulated deficit subject to restriction	$ 9.2
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)		9,273,221
Employee stock purchase plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)		1,717,211