Organization and Significant Accounting Policies - Additional Information (Detail)			1 Months Ended			12 Months Ended
	Jan. 22, 2018 USD ($) $ / shares shares	Sep. 28, 2016 USD ($) $ / shares shares	Nov. 30, 2018 USD ($)	Oct. 31, 2018 USD ($) Target $ / shares shares	Nov. 30, 2016 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Aug. 01, 2018 USD ($)	Sep. 30, 2015 USD ($)
Organization And Significant Accounting Policies [Line Items]
Stock issued, price per share | $ / shares						$ 5.25
Proceeds from the issuance of common stock						$ 56,585,035	$ 12,419,119	$ 52,231,433
Cash and cash equivalents						30,133,213	24,838,567	$ 85,366,448		$ 81,214,354
Short term investments						46,400,176	40,769,539
Net increase (decrease) in cash and investments						10,900,000
Restricted Cash						0	0
Fair value of contingent consideration obligation due to discontinuation of clinical trials						$ 0	$ 0
Collaboration and License agreements | Amgen
Organization And Significant Accounting Policies [Line Items]
Cash received as due under collaboration agreement		$ 35,000,000
Proceeds from the issuance of common stock		$ 21,500,000
Amgen
Organization And Significant Accounting Policies [Line Items]
Stock issued, price per share | $ / shares							$ 7.16	$ 7.16
Amgen | AMG-890 (ARO-LPA) Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under collaboration agreement					$ 30,000,000
Amgen milestone payment									$ 10,000,000
Common Stock Purchase Agreement | Amgen
Organization And Significant Accounting Policies [Line Items]
Shares issued | shares		3,002,793
Stock issued, price per share | $ / shares		$ 7.16
Underwritten Public Offering
Organization And Significant Accounting Policies [Line Items]
Stock issued, price per share | $ / shares	$ 5.25
Common stock shares sold | shares	11,500,000
Net proceeds, after deducting underwriting commissions and discounts and offering expenses payable	$ 56,600,000
Minimum
Organization And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						3 years
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account						$ 250,000
Property, Plant and Equipment, Useful Life						7 years
Maximum | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Option payments, and development, regulatory and sales milestone payments.		$ 617,000,000
Subsequent Event | JJDC | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Cash received as due under collaboration agreement			$ 175,000,000	$ 175,000,000
Proceeds from the issuance of common stock				$ 75,000,000
Subsequent Event | Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Number of additional targets for development and sales milestone payments | Target				3
Subsequent Event | Common Stock Purchase Agreement | JJDC
Organization And Significant Accounting Policies [Line Items]
Shares issued | shares				3,260,869
Stock issued, price per share | $ / shares				$ 23.00
Proceeds from the issuance of common stock				$ 75,000,000
Subsequent Event | Maximum | Janssen | License Agreement
Organization And Significant Accounting Policies [Line Items]
Development and sales milestone payments				1,600,000,000
Subsequent Event | Maximum | Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Development and sales milestone payments				$ 1,900,000,000