Balance Sheet Accounts - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Sep. 30, 2023
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 409,223		$ 409,223			$ 332,213
Less: Accumulated depreciation and amortization	(49,971)		(49,971)			(41,951)
Property, plant and equipment, net	359,252		359,252			290,262
Depreciation and amortization expense for property and equipment	4,100	$ 2,200	7,900	$ 4,500
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	2,996		2,996			2,996
Building
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	75,262		75,262			0
Property, plant and equipment, reclassification			75,300
Useful life					39 years
Research equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	62,926		62,926			56,509
Furniture
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	4,150		4,150			1,540
Computers and software
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	923		923			700
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	104,723		104,723			103,813
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 158,243		158,243			$ 166,655
Property, plant and equipment, reclassification			$ (75,300)