Commitments and Contingencies - Summary of Expenses Incurred by Company (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Zolgensma Royalties [Member]
Cost Of Revenue [Abstract]
Royalties on net sales of Zolgensma	$ 94,978	$ 61,631	$ 20,829
GlaxoSmithKline LLC [Member]
Other Commitments [Line Items]
Total related party transaction expense	44,676	36,722	9,169
GlaxoSmithKline LLC [Member] | Zolgensma Royalties [Member]
Cost Of Revenue [Abstract]
Royalties on net sales of Zolgensma	43,161	26,278	5,822
GlaxoSmithKline LLC [Member] | Other Costs Of Revenue [Member]
Other Commitments [Line Items]
Total related party transaction expense	626	9,398	2,419
GlaxoSmithKline LLC [Member] | Cost of Revenues [Member]
Other Commitments [Line Items]
Total related party transaction expense	43,787	35,676	8,241
GlaxoSmithKline LLC [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Total related party transaction expense	889	1,046	928
License Fees [Member] | The Trustees of the University of Pennsylvania [Member]
Other Commitments [Line Items]
Total related party transaction expense	8,752	860	1,105
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Cost of Revenues [Member]
Other Commitments [Line Items]
Total related party transaction expense	8,046	39
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | Research and Development [Member]
Other Commitments [Line Items]
Total related party transaction expense			200
License Fees [Member] | The Trustees of the University of Pennsylvania [Member] | General and Administrative [Member]
Other Commitments [Line Items]
Total related party transaction expense	$ 706	$ 821	$ 905