Share-based compensation - Return target options activity (Details) - Employee Stock Option, Target-Based - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options (in shares)
Outstanding, beginning balance (in shares)	3,687,664	3,687,664	2,200,189
Granted (in shares)	0	0	1,653,209
Exercised (in shares)	0	0	0
Forfeitures (in shares)	0	0	(165,734)
Outstanding, ending balance (in shares)	3,687,664	3,687,664	3,687,664	2,200,189
Exercisable (in shares)	0
Vested or expected to vest (in shares)	0
Weighted- Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 6.75	$ 6.75	$ 5.49
Granted (in dollars per share)	0	0	8.29
Exercised (in dollars per share)	0	0	0
Forfeitures (in dollars per share)	0	0	5.49
Outstanding, ending balance (in dollars per share)	6.75	$ 6.75	$ 6.75	$ 5.49
Exercisable (in dollars per share)	0
Vested or expected to vest (in dollars per share)	$ 0
Weighted- Average Remaining Contractual Term (Years)
Remaining term, options outstanding	6 years 9 months 18 days	7 years 9 months 18 days	8 years 9 months 18 days	8 years 10 months 24 days
Aggregate Intrinsic Value (in thousands)
Options outstanding, beginning	$ 85,444	$ 29,908	$ 0
Options exercised	0	0	0
Options outstanding, ending	115,278	$ 85,444	$ 29,908	$ 0
Options exercisable	0
Options vested or expected to vest	$ 0
Number of options vested (in shares)	0	0	0
Unrecognized compensation expense
Unrecognized compensation expense	$ 33,000