Income taxes - Deferred tax (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax assets
Net operating loss carry forwards	$ 11,380	$ 2,899
Research and development tax credit carry forwards	969	231
License fees	3,232	3,669
Stock based compensation	196
Accruals, reserves and other	44	1
Total deferred tax assets	15,821	6,800
Defered tax liabilites
Prepaid expenses	(309)
Total deferred tax liabilities	(309)
Net deferred tax assets	15,512	6,800
Valuation allowance	$ (15,512)	$ (6,800)	$ (1,362)