Significant agreements - Summary of Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Contract liabilities:
Beginning Balance			$ 35,156	$ 5,657	$ 5,657
Additions			36,496		35,085
Deductions			(7,926)		(8,028)
Impact of Exchange Rates			(1,219)		2,442
Ending Balance	$ 62,507		62,507		35,156
Deductions	(4,333)	$ (1,267)	(7,926)	(3,496)
Contract assets
Contract assets	0		0		0
Ionis
Contract liabilities:
Beginning Balance			3,000
Additions			34,442
Deductions			(2,066)
Impact of Exchange Rates			(816)
Ending Balance	34,560		34,560		3,000
Genentech
Contract liabilities:
Beginning Balance			27,579	0	0
Additions			2,000		31,000
Deductions			(4,142)		(4,896)
Impact of Exchange Rates			(347)		1,475
Ending Balance	25,090		25,090		27,579
Dementia Discovery Fund
Contract liabilities:
Beginning Balance			821	744	744
Additions			0		500
Deductions			(314)		(436)
Impact of Exchange Rates			(4)		13
Ending Balance	503		503		821
AstraZeneca
Contract liabilities:
Beginning Balance			3,756	4,913	4,913
Additions			54		585
Deductions			(1,404)		(2,696)
Impact of Exchange Rates			(52)		954
Ending Balance	$ 2,354		2,354		3,756
Ionis Evaluation and Option Agreement
Contract liabilities:
Beginning Balance			$ 3,000	$ 0	0
Additions					3,000
Deductions					0
Impact of Exchange Rates					0
Ending Balance					$ 3,000