Summary of significant accounting policies - Investments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2024 USD ($) item	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) item	Dec. 31, 2022 USD ($)
Investments
Unrealized loss of available-for-sale securities with unrealized losses for less than 12 months	$ (28,809)		$ (8,896)
Total unrealized loss of available-for-sale securities with unrealized losses for less than 12 months	(4)		(1)
Cash, cash equivalents & restricted cash	81,316		$ 77,909
Total cash, cash equivalents, restricted cash and investments, Gross Unrealized Gains				$ 13
Total cash, cash equivalents, restricted cash and investments, Gross Unrealized Losses	(4)			(1)
Total cash, cash equivalents, restricted cash and investments, Fair Value	110,100
Accrued interest receivable	100
Realized gains or losses on investments	$ 0	$ 0
Number of investments in unrealized loss position | item	8		2
Number of investments in unrealized loss position for more than 12 months | item	0
Amortized Cost
Investments
Total cash, cash equivalents, restricted cash and investments, Amortized Cost	$ 111,407		$ 138,284
Total
Investments
Total cash, cash equivalents, restricted cash and investments, Fair Value	111,403		138,296
Cash, cash equivalents & restricted cash | Amortized Cost
Investments
Cash, cash equivalents & restricted cash			79,076
Total cash, cash equivalents, restricted cash and investments, Amortized Cost	82,594
Cash, cash equivalents & restricted cash | Total
Investments
Cash, cash equivalents & restricted cash, Fair Value			79,076
Total cash, cash equivalents, restricted cash and investments, Fair Value	82,594
Cash and money market accounts. | Amortized Cost
Investments
Cash, cash equivalents & restricted cash	82,594		79,076
Cash and money market accounts. | Total
Investments
Cash, cash equivalents & restricted cash, Fair Value	82,594		79,076
Investments.
Investments
Gross Unrealized Gains				13
Gross Unrealized Losses	(4)			$ (1)
Investments. | Amortized Cost
Investments
Investments, Amortize Cost	28,813		59,208
Investments. | Total
Investments
Investments, Fair Value	$ 28,809		$ 59,220