Supplemental Balance Sheet Information - Schedule of Inventory (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Inventory [Line Items]
Raw materials	$ 73,857,000		$ 73,857,000		$ 74,442,000
Work-in-process	296,685,000		296,685,000		349,978,000
Finished goods	330,305,000		330,305,000		255,855,000
Total inventory	700,847,000		700,847,000		$ 680,275,000	[1]
Cost of sales	188,793,000	$ 96,949,000	398,134,000	$ 263,567,000
Pre-Launch Valoctocogene Roxaparvovec
Inventory [Line Items]
Total inventory	0		0
Cost of sales	$ 81,200,000		$ 81,200,000

[1] December 31, 2019 balances were derived from the audited Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019, filed with the SEC on February 27, 2020.