Collaboration and License Agreements - Amgen, Inc - Additional Information (Detail)				1 Months Ended		3 Months Ended	9 Months Ended
	Jun. 30, 2017 $ / shares shares	Nov. 18, 2016 USD ($)	Sep. 28, 2016 USD ($) Agreement	Nov. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock							$ 12,419,119
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Stock issued, price per share | $ / shares	$ 7.16					$ 7.16	$ 7.16
Collaboration and License agreements | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Number of agreements | Agreement			2
ARO-AMG1 Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional amount of common stock shares agreed to sell upon exercise of option			$ 5,000,000
Number of trading days, used to calculate share price of Common Stock, surrounding option exercise date			30 days
Cash received as due under collaboration agreement					$ 5,000,000
Deferred revenue					$ 5,000,000
Deferred revenue amortized into revenue						$ 600,000	$ 1,900,000
Collaboration and License agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement			$ 35,000,000
Collaboration and License agreements | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option payments, and development, regulatory and sales milestone payments.			617,000,000
Collaboration and License agreements | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from the issuance of common stock			$ 21,500,000
ARO-LPA Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cash received as due under collaboration agreement				$ 30,000,000
Deferred revenue				$ 30,000,000
Deferred revenue amortized into revenue						7,900,000	19,400,000
Revenue generated from work orders under services agreement						$ 800,000	$ 1,400,000
Common Stock Purchase Agreement | Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Sep. 28, 2016
Shares issued | shares	3,002,793					3,002,793	3,002,793
Stock issued, price per share | $ / shares	$ 7.16					$ 7.16	$ 7.16
Proceeds from the issuance of common stock		$ 12,500,000
Common Stock Purchase Agreement | Amgen | NASDAQ
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of trading days, used to calculate weighted average price of Common Stock, listed in stock market	30 days