Collaboration and License Agreements - Janssen Pharmaceuticals, Inc - Additional Information (Detail)		1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 03, 2018 USD ($) Obligation	Oct. 31, 2018 USD ($) Target $ / shares shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Aug. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share | $ / shares			$ 23.00				$ 5.25				$ 23.00	$ 5.25
Proceeds from the issuance of common stock											$ 60,521,739	$ 56,585,035	$ 12,419,119
REVENUE			$ 43,292,770	$ 42,696,636	$ 48,148,275	$ 34,657,896	$ 11,255,000	$ 727,375	$ 650,125	$ 3,509,821	168,795,577	16,142,321	$ 31,407,709
Deferred revenue			77,769,629				$ 600				77,769,629	$ 600
Contract liabilities, long -term deferred revenue			5,035,142								5,035,142
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Initial transaction price	$ 252,600,000
Janssen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of distinct performance obligations | Obligation	1
Janssen | Common Stock Purchase Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 03, 2018
Janssen | License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 03, 2018
Janssen | License Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments		$ 1,600,000,000
Janssen | Research Collaboration and Option Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 03, 2018
Janssen | Collaboration Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of additional targets for development, regulatory and sales milestone payments | Target		3
Janssen | Collaboration Agreement | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory and sales milestones payments		$ 1,900,000,000
Janssen | JNJ-3989 (ARO-HBV) Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment	$ 25,000,000		50,000,000								50,000,000			$ 25,000,000	$ 25,000,000
REVENUE											167,500,000
Contract assets			0								0
Deferred revenue			77,800,000								77,800,000
Contract liabilities, long -term deferred revenue			5,000,000								5,000,000
Janssen | ARO-JNJ1
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
REVENUE											1,000,000
Contract assets			700,000								700,000
Contract liabilities			$ 0								0
JJDC | Common Stock Purchase Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Shares issued | shares		3,260,869
Stock issued, price per share | $ / shares		$ 23.00
JJDC | Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock		$ 75,000,000									$ 75,000,000
Janssen and JJDC | Common Stock Purchase Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [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
Janssen and JJDC | Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement		175,000,000
Proceeds from the issuance of common stock		$ 75,000,000