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Investments - Summary of Investments by Type of Security (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Schedule Of Available For Sale Securities [Line Items]
Cost	$ 165,650	$ 41,890
Gross Unrealized Gains	22	111
Gross Unrealized Losses	(100)	(3)
Fair Market Value	165,572	41,998
Short-term investments | U.S. Treasury
Schedule Of Available For Sale Securities [Line Items]
Cost	7,735	10,245
Gross Unrealized Gains	3	38
Gross Unrealized Losses	0	0
Fair Market Value	7,738	10,283
Short-term investments | Asset-backed securities
Schedule Of Available For Sale Securities [Line Items]
Cost	5,616	4,723
Gross Unrealized Gains	6	36
Gross Unrealized Losses	(1)	0
Fair Market Value	5,621	4,759
Short-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Cost	118,004	24,956
Gross Unrealized Gains	10	37
Gross Unrealized Losses	(59)	0
Fair Market Value	117,955	24,993
Long-term investments | U.S. Treasury
Schedule Of Available For Sale Securities [Line Items]
Cost	6,532
Gross Unrealized Gains	2
Gross Unrealized Losses	(1)
Fair Market Value	6,533
Long-term investments | Asset-backed securities
Schedule Of Available For Sale Securities [Line Items]
Cost	959
Gross Unrealized Gains	0
Gross Unrealized Losses	(1)
Fair Market Value	958
Long-term investments | Corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Cost	26,804	1,966
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(38)	(3)
Fair Market Value	$ 26,767	$ 1,963

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