Collaboration and License Agreements - Horizon Therapeutics Ireland DAC - Additional Information (Detail) $ in Thousands		12 Months Ended
	Jun. 18, 2021 USD ($) Obligation Bundle	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jul. 31, 2021 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues		$ 138,287	$ 87,992	$ 168,796
Contract liabilities		111,055	19,291
Deferred revenue, net of current portion		131,495
Horizon Therapeutics Ireland DAC | ARO-XDH
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Jun. 18, 2021
Cash received as due under collaboration agreement					$ 40,000
Number of distinct performance obligations | Obligation	1
Initial transaction price	$ 40,000
Revenues		6,700	$ 0	$ 0
Contract assets		100
Contract liabilities		26,700
Deferred revenue, net of current portion		$ 6,700
Horizon Therapeutics Ireland DAC | License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of distinct bundle | Bundle	1
Number of distinct performance obligations | Obligation	1
Horizon Therapeutics Ireland DAC | Maximum | ARO-XDH
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development regulatory and sales milestones payments	$ 660,000