Liability Related to Sale of Future Royalties -Schedule of Activity Within Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Liability Related To Sale Of Future Royalties [Line Items]
Liability related to sale of future royalties	$ 28,807	$ 18,794
Liability related to sale of future royalties	161,722	$ 174,504
Royalty Purchase Agreement [Member]
Liability Related To Sale Of Future Royalties [Line Items]
Liability related to sale of future royalties, beginning balance	193,298
Zolgensma royalties paid to HCR	(9,471)
Non-cash interest expense	6,702
Liability related to sale of future royalties, ending balance	190,529
Liability related to sale of future royalties	(28,807)
Liability related to sale of future royalties	$ 161,722