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Organization and Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 28, 2016 USD ($)	Oct. 31, 2018 USD ($) Target $ / shares shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 USD ($) shares	Apr. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Aug. 01, 2018 USD ($)
Organization And Significant Accounting Policies [Line Items]
Proceeds from the issuance of common stock					$ 60,521,729	$ 56,585,035
Cash and cash equivalents			$ 188,331,747		188,331,747			$ 30,133,213
Restricted cash			1,000,000		1,000,000			0
Short term investments			48,567,215		48,567,215			46,400,176
Long term investments			57,555,499		57,555,499
Net increase (decrease) in cash and investments					218,900,000
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			3,949,273	0	4,449,917	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			867,175		967,175
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			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
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
Organization And Significant Accounting Policies [Line Items]
Milestone payment			$ 25,000,000		$ 25,000,000		$ 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