Organization and Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 28, 2016 USD ($)	Oct. 31, 2018 USD ($) Target $ / shares shares	Mar. 31, 2019 USD ($) shares	Mar. 31, 2018 USD ($) shares	Mar. 31, 2019 USD ($) shares	Mar. 31, 2018 USD ($) shares	Apr. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Aug. 01, 2018 USD ($)	Sep. 30, 2017 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock					$ 60,521,729	$ 56,585,035
Cash and cash equivalents			$ 161,639,208	$ 69,805,117	161,639,208	$ 69,805,117		$ 30,133,213		$ 24,838,567
Short term investments			56,560,674		56,560,674			46,400,176
Long term investments			67,536,054		67,536,054
Net increase (decrease) in cash and investments					209,200,000
Restricted Cash			0		0			0
Fair value of contingent consideration obligation due to discontinuation of clinical trials			$ 0		$ 0			$ 0
Calculation of effect of dilutive stock options and restricted stock units | shares			3,927,237	0	4,590,931	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			917,175		989,175
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			2,000		2,000
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
AMG-890 (ARO-LPA) Agreement | Amgen
Organization And Significant Accounting Policies [Line Items]
Milestone payment									$ 10,000,000
Janssen and 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			$ 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 | Subsequent Event
Organization And Significant Accounting Policies [Line Items]
Milestone payment							$ 25,000,000
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 | Janssen and 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
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		$ 250,000
Property and equipment, useful life					7 years
Maximum | Collaboration and License agreements | Amgen
Organization And Significant Accounting Policies [Line Items]
Option payments, and development, regulatory and sales milestone payments.	$ 617,000,000
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