Fair value of financial instruments - Financial Instruments Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financial assets
Preferred stock tranche liability	$ 10,200	$ 4,189
Level 3
Financial assets
Preferred stock tranche liability	10,200	4,189
Recurring basis
Financial assets
Cash equivalents	46,680	46,093
Short-term investments	28,809		$ 59,220
Total financial assets	75,489	105,313
Preferred stock tranche liability	10,200	4,189
Recurring basis | Level 1
Financial assets
Cash equivalents	46,680	46,093
Short-term investments		5,992
Total financial assets	46,680	52,085
Recurring basis | Level 2
Financial assets
Short-term investments	28,809		$ 53,228
Total financial assets	28,809	53,228
Recurring basis | Level 3
Financial assets
Preferred stock tranche liability	$ 10,200	$ 4,189