Stockholders' equity - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2021
Class of Stock [Line Items]
Accumulated deficit subject to restriction	$ 10.0
China | Subsidiaries
Class of Stock [Line Items]
Accumulated profits (at least)	10.00%
Stock options
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)		6,694,262
ESPP
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)		1,156,415