Investments - Summary of Investments by Type of Security (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Sep. 30, 2021
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 70,558	$ 197,165
Gross Unrealized Gains	0	12
Gross Unrealized Losses	(1,155)	(69)
Fair Market Value	69,403	197,108
Short-term investments | U.S. Treasury
Schedule Of Available For Sale Securities [Line Items]
Cost	2,000	4,222
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(20)	0
Fair Market Value	1,980	4,223
Short-term investments | Asset-backed securities
Schedule Of Available For Sale Securities [Line Items]
Cost		4,812
Gross Unrealized Gains		1
Gross Unrealized Losses		(2)
Fair Market Value		4,811
Short-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Cost	33,446	140,042
Gross Unrealized Gains	0	6
Gross Unrealized Losses	(366)	(25)
Fair Market Value	33,080	140,023
Short-term investments | Foreign government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	2,888
Gross Unrealized Gains	0
Gross Unrealized Losses	(38)
Fair Market Value	2,850
Long-term investments | U.S. Treasury
Schedule Of Available For Sale Securities [Line Items]
Cost	7,481	6,996
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(215)	(2)
Fair Market Value	7,266	6,995
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Cost	24,743	38,184
Gross Unrealized Gains	0	3
Gross Unrealized Losses	(516)	(39)
Fair Market Value	$ 24,227	38,148
Long-term investments | Foreign government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost		2,909
Gross Unrealized Gains		0
Gross Unrealized Losses		(1)
Fair Market Value		$ 2,908