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Organization and Significant Accounting Policies - Additional Information (Detail) - USD ($) |  | 12 Months Ended |
--- | --- | --- | ---
Nov. 18, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2014
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Cash and cash equivalents |  | $ 24,838,567 | $ 85,366,448 | $ 81,214,354 | $ 132,510,610
Short term investments |  | 40,769,539 |  |  |
Net increase (decrease) in cash and investments |  | (19,800,000) |  |  |
Cash outflows related to operating activities excluding upfront payment from Amgen |  | (53,900,000) |  |  |
Capital expenditures |  | 7,918,198 | 3,860,237 | $ 1,970,612 |
Proceeds from the issuance of common stock |  | 12,419,119 | 52,231,433 |  |
Restricted Cash |  | 0 | 0 |  |
Fair value of contingent consideration obligation due to discontinuation of clinical trials |  | 0 | $ 0 |  |
Maximum |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Amount insured in FDIC per account |  | $ 250,000 |  |  |
Property, Plant and Equipment, Useful Life |  | 7 years |  |  |
Minimum |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Property, Plant and Equipment, Useful Life |  | 3 years |  |  |
Amgen |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Proceeds from upfront payments and equity investments |  | $ 42,500,000 |  |  |
Amgen | Common Stock Purchase Agreement |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Proceeds from the issuance of common stock | $ 12,500,000 |  |  |  |
Amgen | ARO-LPA (AMG 890) Agreement |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Upfront payment | $ 30,000,000 |  |  |  |
Amgen | ARO-LPA (AMG 890) Agreement | Common Stock Purchase Agreement |  |  |  |  |
Organization And Significant Accounting Policies [Line Items] |  |  |  |  |
Common stock issued | 1,745,810 |  |  |  |

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