Liability Related to Sale of Future Royalties -Schedule of Activity Within Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Liability Related To Sale Of Future Royalties [Line Items]
Liability related to sale of future royalties	$ 35,508	$ 18,794
Liability related to sale of future royalties	144,315	$ 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	(33,252)
Non-cash interest expense	19,777
Liability related to sale of future royalties, ending balance	179,823
Liability related to sale of future royalties	(35,508)
Liability related to sale of future royalties	$ 144,315