Income taxes - Deferred tax (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Deferred tax assets
Net operating loss carry forwards	$ 88,219	$ 68,450
Research and development tax credit carry forwards	12,846	7,704
License fees	5,545	3,355
Stock based compensation	14,286	8,974
Capitalized research and experimentation costs	58,610	20,803
Accruals, reserves and other	242	191
Total deferred tax assets	179,748	109,477
Valuation allowance	(177,061)	(108,690)	$ (66,853)
Net deferred tax assets	2,687	787
Deferred tax liabilities
Prepaid expenses	(2,687)	(787)
Net deferred tax liabilities	$ (2,687)	$ (787)