Revenue recognition - Collaboration and Royalty Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Nov. 30, 2011
Collaboration and royalty revenue
Remaining potential milestones that can be achieved	$ 300,000		$ 300,000
Revenue	165,526	$ 116,676	314,262	$ 234,620
Collaboration revenue
Collaboration and royalty revenue
Revenue			7	20,007
Royalty revenue
Collaboration and royalty revenue
Revenue	21,825	13,563	40,721	20,220
SMA License Agreement \| Collaboration revenue
Collaboration and royalty revenue
Revenue	0	0	0	20,000
SMA License Agreement \| Royalty revenue
Collaboration and royalty revenue
Revenue	21,800	13,600	40,700	20,200
SMA License Agreement \| Roche And Sma Foundation \| Collaboration revenue
Collaboration and royalty revenue
Revenue	0	0	0	20,000
Research And Development Event Milestones \| SMA License Agreement
Collaboration and royalty revenue
Remaining potential milestones that can be achieved	0		0
Milestone payments received		20,000		20,000
Sales Milestones \| SMA License Agreement
Collaboration and royalty revenue
Remaining potential milestones that can be achieved	$ 300,000		$ 300,000
Milestone payments received		$ 20,000		$ 20,000
Maximum \| Research And Development Event Milestones \| SMA License Agreement
Collaboration and royalty revenue
Remaining potential milestones that can be achieved					$ 135,000
Maximum \| Sales Milestones \| SMA License Agreement
Collaboration and royalty revenue
Remaining potential milestones that can be achieved					$ 325,000