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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) |  |  |

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