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Stockholders' Equity - Additional Information (Detail) - USD ($) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Aug. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Class Of Stock [Line Items] |  |  |  |  |
Capital stock authorized for issuance |  |  | 150,000,000 |  |
Common stock, shares authorized |  |  | 145,000,000 | 145,000,000 |
Common stock, par value |  |  | $ 0.001 | $ 0.001 |
Preferred stock, shares authorized |  |  | 5,000,000 |  |
Preferred stock, par value |  |  | $ 0.001 |  |
Common stock, shares outstanding |  |  | 102,376,303 | 95,506,271 |
Net proceeds from common stock after deducting advisory fees and offering expenses |  | $ 250,500,000 | $ 250,478,974 | $ 60,521,739 | $ 56,585,035
Public Offering |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |
Shares issued to institutional investors |  | 4,600,000 |  |  |
Aggregate purchase price paid by investors |  | $ 266,800,000 |  |  |
Net proceeds from common stock after deducting advisory fees and offering expenses |  | $ 250,500,000 |  |  |
Stock issued, price per share |  | $ 58.00 |  |  |
ATM Agreement |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |
Shares issued to institutional investors |  |  | 0 |  |
ATM Agreement | Maximum |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |
Common stock shares value reserved for future issuance | $ 250,000,000 |  |  |  |
Percentage of commission to sales agent | 3.00% |  |  |  |
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, and Inducement Grants |  |  |  |  |
Class Of Stock [Line Items] |  |  |  |  |
Common Stock, Share reserve for issuance |  |  | 8,462,148 |  |

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