Investments - Summary of Fair Value of Investments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020
Assets
Short-term investments	$ 131,314	$ 40,035
Long-term investments	34,258	1,963
Convertible senior notes hedge	34,719
Total assets at fair value	200,291	41,998
Liabilities
Embedded conversion derivative	34,719
Acquisition-related contingent consideration		753
Total liabilities at fair value	34,719	753
U.S. Treasury
Assets
Short-term investments	7,738	10,283
Long-term investments	6,533	0
Asset-backed securities
Assets
Short-term investments	5,621	4,759
Long-term investments	958
Corporate debt securities
Assets
Short-term investments	52,354	9,619
Long-term investments	23,838	1,963
Commercial paper
Assets
Short-term investments	65,601	15,374
Foreign government and agency securities
Assets
Long-term investments	2,929
Quoted Prices in Active Markets (Level 1)
Assets
Short-term investments	7,738	10,283
Long-term investments	6,533	0
Convertible senior notes hedge	0
Total assets at fair value	14,271	10,283
Liabilities
Embedded conversion derivative	0
Acquisition-related contingent consideration		0
Total liabilities at fair value	0	0
Quoted Prices in Active Markets (Level 1) \| U.S. Treasury
Assets
Short-term investments	7,738	10,283
Long-term investments	6,533	0
Quoted Prices in Active Markets (Level 1) \| Asset-backed securities
Assets
Short-term investments	0	0
Long-term investments	0
Quoted Prices in Active Markets (Level 1) \| Corporate debt securities
Assets
Short-term investments	0	0
Long-term investments	0	0
Quoted Prices in Active Markets (Level 1) \| Commercial paper
Assets
Short-term investments	0	0
Quoted Prices in Active Markets (Level 1) \| Foreign government and agency securities
Assets
Long-term investments	0
Significant Other Observable Inputs (Level 2)
Assets
Short-term investments	123,576	29,752
Long-term investments	27,725	1,963
Convertible senior notes hedge	34,719
Total assets at fair value	186,020	31,715
Liabilities
Embedded conversion derivative	34,719
Acquisition-related contingent consideration		0
Total liabilities at fair value	34,719	0
Significant Other Observable Inputs (Level 2) \| U.S. Treasury
Assets
Short-term investments	0	0
Long-term investments	0	0
Significant Other Observable Inputs (Level 2) \| Asset-backed securities
Assets
Short-term investments	5,621	4,759
Long-term investments	958
Significant Other Observable Inputs (Level 2) \| Corporate debt securities
Assets
Short-term investments	52,354	9,619
Long-term investments	23,838	1,963
Significant Other Observable Inputs (Level 2) \| Commercial paper
Assets
Short-term investments	65,601	15,374
Significant Other Observable Inputs (Level 2) \| Foreign government and agency securities
Assets
Long-term investments	2,929
Significant Unobservable Inputs (Level 3)
Assets
Short-term investments	0	0
Long-term investments	0	0
Convertible senior notes hedge	0
Total assets at fair value	0	0
Liabilities
Embedded conversion derivative	0
Acquisition-related contingent consideration		753
Total liabilities at fair value	0	753
Significant Unobservable Inputs (Level 3) \| U.S. Treasury
Assets
Short-term investments	0	0
Long-term investments	0	0
Significant Unobservable Inputs (Level 3) \| Asset-backed securities
Assets
Short-term investments	0	0
Long-term investments	0
Significant Unobservable Inputs (Level 3) \| Corporate debt securities
Assets
Short-term investments	0	0
Long-term investments	0	0
Significant Unobservable Inputs (Level 3) \| Commercial paper
Assets
Short-term investments	0	$ 0
Significant Unobservable Inputs (Level 3) \| Foreign government and agency securities
Assets
Long-term investments	$ 0

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