Supplemental Balance Sheet Information - Schedule of Inventory (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Inventory [Line Items]
Raw materials	$ 68,391	$ 74,616
Work-in-process	321,073	231,064
Finished goods	219,585	225,191
Total inventory	609,049	$ 530,871	[1]
Pre-Launch Valoctocogene Roxaparvovec
Inventory [Line Items]
Total inventory	$ 7,100

[1] December 31, 2018 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, 2018, filed with the SEC on February 28, 2019.