Significant agreements - Ionis Accounting Analysis (Details) $ in Thousands		1 Months Ended			3 Months Ended		12 Months Ended
	Jul. 09, 2021 USD ($) item	Oct. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jan. 31, 2021 USD ($)	Mar. 31, 2024 USD ($) item	Mar. 31, 2023 USD ($)	Dec. 31, 2020 USD ($)	Mar. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenues					$ 19,530	$ 4,896
Deferred revenue					$ 114,547				$ 135,194	$ 61,873
Ionis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of material rights associated with future payments | item					4
Number of collaboration targets | item	4
Transaction price	$ 38,000				$ 37,800
Upfront cash payment	31,000
Estimated amount payable to CROs	600				$ 400			$ 600
Expected period for satisfaction of performance obligations					3 years
Material rights exercise period					4 years
Incremental revenue					$ 3,300
Collaboration revenues					5,854	$ 2,784
Deferred revenue					6,495				12,464	$ 21,489
Ionis | Combined licenses and research and discovery performance obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price	34,100
Ionis | Four material rights associated with credits for IND Acceptance fees
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price	3,900
Ionis | Ionis Share Purchase Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Transaction price	3,400
Ionis Evaluation and Option Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment	$ 3,000			$ 3,000			$ 3,000
Deferred revenue							$ 3,000		$ 12,500
Amended Ionis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment			$ 1,600
Collaboration revenues		$ 800	$ 800
Third Amended Ionis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenues					$ 800