Summary of significant accounting policies - Cash, cash equivalents and restricted cash (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Cash and cash equivalents	$ 81,316	$ 77,909
Restricted cash	241	241
Restricted cash	1,278	1,167
Total cash, cash equivalents and restricted cash	82,594	79,076	$ 98,749	$ 79,076	$ 75,789
Prepaid Expenses and Other Current Assets
Property, Plant and Equipment [Line Items]
Restricted cash	1,000	$ 900
Letter of credit | Office and Laboratory Space in Needham, Massachusetts. | Restricted Cash and Cash Equivalents [Member]
Property, Plant and Equipment [Line Items]
Restricted cash	$ 200