Commitments and contingencies - Lease Contracts (Detail) ft² in Thousands, $ in Thousands			3 Months Ended	6 Months Ended
	Aug. 06, 2021 USD ($) ft²	Jul. 28, 2021 USD ($) ft²	Mar. 31, 2022 USD ($)	Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($)
Other Commitments [Line Items]
Tenant improvements			$ 17,600	$ 17,600
Remainder of 2022			7,140	7,140
Change in right-of-use asset				16,600	$ 2,700
Change in operating lease liabilities.				18,200	2,500
Operating lease expense			3,900	7,400
Operating lease payments			$ 3,200	$ 6,000
Lease weighted-average remaining lease term			15 years 6 months	15 years 6 months
Lease weighted average discount rate			6.45%	6.45%
Future minimum payments			$ 157,302	$ 157,302
South San Francisco, California
Other Commitments [Line Items]
Operating lease term		7 years
Operating lease area | ft²		21
Operating lease security deposit with the lessor		$ 200
Initial annual base rent		$ 1,700
Operating lease base rent annual percentage increase		3.00%
Future minimum payments					13,100
Canton, Massachusetts
Other Commitments [Line Items]
Operating lease term	10 years 5 months
Operating lease area | ft²	22
Operating lease security deposit with the lessor	$ 600
Initial annual base rent	$ 1,200
Operating lease base rent annual percentage increase	2.00%
Future minimum payments					$ 13,200