Ordinary shares - Share Purchase Agreement (Details) $ / shares in Units, $ in Thousands				12 Months Ended
	Jul. 17, 2023 USD ($) shares	Oct. 15, 2021 USD ($) $ / shares shares	Jul. 09, 2021 USD ($) $ / shares shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) shares	Dec. 31, 2023 £ / shares shares	Jul. 17, 2023 £ / shares	Jul. 17, 2023 $ / shares	Dec. 31, 2022 £ / shares shares	Dec. 31, 2020 shares
Class of Stock [Line Items]
Ordinary shares, shares issued | shares							42,431,766			29,873,893
Ordinary shares, shares outstanding | shares							42,431,766			29,873,893
Gross proceeds before discounts and commissions				$ 249,345	$ 5,703	$ 290,984
Net proceeds				249,345	5,703	290,984
Offering expenses				16,000	200	$ 16,100
Ordinary shares, nominal value | £ / shares							£ 0.01			£ 0.01
Ordinary Shares
Class of Stock [Line Items]
Ordinary shares, shares outstanding | shares						29,579,364	42,431,766			29,873,893	21,094,557
Gross proceeds before discounts and commissions				$ 162	$ 2	$ 104
Shares issued (in shares) | shares				12,384,706	181,455	7,498,536
Non voting ordinary shares | Ordinary Shares
Class of Stock [Line Items]
Ordinary shares, shares issued | shares							4,705,882
Ordinary shares, shares outstanding | shares							4,705,882
Voting ordinary shares | Ordinary Shares
Class of Stock [Line Items]
Ordinary shares, shares issued | shares							37,725,884
Ordinary shares, shares outstanding | shares							37,725,884
Ionis Share Purchase Agreement
Class of Stock [Line Items]
Gross proceeds before discounts and commissions						$ 7,558
Ionis Share Purchase Agreement | Ordinary Shares
Class of Stock [Line Items]
Gross proceeds before discounts and commissions						$ 4
Shares issued (in shares) | shares						282,485
At the market offering
Class of Stock [Line Items]
Gross proceeds before discounts and commissions				$ 35,300	$ 5,900	$ 105,800
Shares issued (in shares) | shares				1,561,176	181,455	3,771,684
Net proceeds				$ 34,200	$ 5,700	$ 102,600
Offering expenses				$ 1,100	$ 200	$ 3,200
Underwritten Public Offering
Class of Stock [Line Items]
Gross proceeds before discounts and commissions	$ 230,000
Net proceeds	215,100
Offering expenses	$ 14,900
Underwritten Public Offering | American Depositary Shares
Class of Stock [Line Items]
Price per share | $ / shares		$ 54.00							$ 21.25
Gross proceeds before discounts and commissions		$ 201,300
Shares issued (in shares) | shares	6,117,648	3,726,852
Net proceeds		$ 188,400
Ordinary shares, nominal value | £ / shares								£ 0.01
Underwritten Public Offering | American Depositary Shares | Option on securities
Class of Stock [Line Items]
Shares issued (in shares) | shares	1,411,764
Underwritten Public Offering | Non voting ordinary shares
Class of Stock [Line Items]
Shares issued (in shares) | shares	4,705,882
Ordinary shares, nominal value | £ / shares								£ 0.01
Ionis | Ionis Share Purchase Agreement
Class of Stock [Line Items]
Ordinary shares, shares issued | shares			282,485
Price per share | $ / shares			$ 38.94
Gross proceeds before discounts and commissions			$ 11,000
Fair value of shares held			$ 7,600
Ionis | Ionis Share Purchase Agreement | American Depositary Shares
Class of Stock [Line Items]
Price per share | $ / shares			$ 31.11