Collaboration and License Agreements - Takeda Pharmaceuticals U.S.A., Inc - Additional Information (Detail) $ in Thousands		3 Months Ended		6 Months Ended
	Oct. 07, 2020 USD ($) Obligation Bundle	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Jan. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenues		$ 32,811	$ 23,529	$ 54,113	$ 52,983
Contract liabilities, current deferred revenue		144,879		144,879			$ 19,291
Deferred revenue, net of current portion		121,530		121,530
Takeda License Agreement | License and Co-Funding Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Agreement date	Oct. 07, 2020
Ratio of Profit Sharing Structure for United States Commercialization	50/50 profit sharing
Cash received as due under collaboration agreement						$ 300,000
Number of distinct bundle | Bundle	1
Number of distinct performance obligations | Obligation	1
Initial transaction price	$ 300,000
Revenues		25,400	$ 0	33,600	$ 0
Contract assets		0		0
Contract liabilities, current deferred revenue		144,900		144,900
Deferred revenue, net of current portion		$ 121,500		$ 121,500
Takeda License Agreement | Minimum | License and Co-Funding Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of eligible to receive tiered royalties on net sales	20.00%
Takeda License Agreement | Maximum | License and Co-Funding Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of eligible to receive tiered royalties on net sales	25.00%
Development regulatory and sales milestones payments	$ 740,000