Investments - Summary of Investments by Type of Security (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 175,011	$ 41,890
Gross Unrealized Gains	9	111
Gross Unrealized Losses	(75)	(3)
Fair Market Value	174,945	41,998
Short-term investments | U.S. Treasury
Schedule Of Available For Sale Securities [Line Items]
Cost	5,240	10,245
Gross Unrealized Gains	1	38
Gross Unrealized Losses	0	0
Fair Market Value	5,241	10,283
Short-term investments | Asset-backed securities
Schedule Of Available For Sale Securities [Line Items]
Cost	6,051	4,723
Gross Unrealized Gains	1	36
Gross Unrealized Losses	0	0
Fair Market Value	6,052	4,759
Short-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Cost	127,025	24,956
Gross Unrealized Gains	7	37
Gross Unrealized Losses	(23)	0
Fair Market Value	127,009	24,993
Long-term investments | U.S. Treasury
Schedule Of Available For Sale Securities [Line Items]
Cost	4,999
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Market Value	4,997
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Cost	28,775	1,966
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(48)	(3)
Fair Market Value	28,727	$ 1,963
Long-term investments | Foreign government and agency securities
Schedule Of Available For Sale Securities [Line Items]
Cost	2,921
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Fair Market Value	$ 2,919