Organization and Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	12 Months Ended
	Sep. 28, 2016 USD ($) $ / shares shares	Oct. 31, 2018 USD ($) Target $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Aug. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Oct. 03, 2018 USD ($)	Aug. 01, 2018 USD ($)
Organization And Significant Accounting Policies [Line Items]
Stock issued, price per share | $ / shares			$ 23.00	$ 5.25
Proceeds from the issuance of common stock			$ 60,521,739	$ 56,585,035	$ 12,419,119
Cash and cash equivalents			221,804,128	30,133,213
Short term investments			36,899,894	46,400,176
Long-term investments			44,175,993
Net increase (decrease) in cash and investments			226,300,000
Restricted Cash			1,000,000	0
Fair value of contingent consideration obligation due to discontinuation of clinical trials			$ 0	$ 0
Calculation of effect of dilutive stock options and restricted stock units | shares			4,748,958	0	0
Stock Options
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units | shares			1,007,500
Restricted Stock Units
Organization And Significant Accounting Policies [Line Items]
Anti-dilutive excluded for calculation of effect of dilutive stock options and restricted stock units | shares			11,500
JJDC | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement		$ 175,000,000	$ 175,000,000
Proceeds from the issuance of common stock		$ 75,000,000	75,000,000
Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Number of additional targets for development and sales milestone payments | Target		3
Janssen | JNJ-3989 (ARO-HBV) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment			$ 50,000,000			$ 25,000,000	$ 25,000,000	$ 25,000,000
Amgen
Organization And Significant Accounting Policies [Line Items]
Stock issued, price per share | $ / shares					$ 7.16
Amgen | License Collaboration and Stock Purchase Agreement
Organization And Significant Accounting Policies [Line Items]
Cash received as due under agreement	$ 35,000,000
Proceeds from the issuance of common stock	$ 21,500,000
Amgen | AMG-890 (ARO-LPA) Agreement
Organization And Significant Accounting Policies [Line Items]
Milestone payment									$ 10,000,000
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
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
Minimum
Organization And Significant Accounting Policies [Line Items]
Property and equipment, useful life			3 years
Maximum
Organization And Significant Accounting Policies [Line Items]
Amount insured in FDIC per account			$ 250,000
Property and equipment, useful life			7 years
Maximum | Janssen | License Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments		$ 1,600,000,000
Maximum | Janssen | Collaboration Agreement
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments		$ 1,900,000,000
Maximum | Amgen | Collaboration and License agreements
Organization And Significant Accounting Policies [Line Items]
Development regulatory and sales milestones payments	$ 420,000,000
Option payments, and development, regulatory and sales milestone payments	$ 617,000,000