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Investments - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Feb. 05, 2021
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Other-than-temporary impairment charges recognized	$ 0	$ 0	$ 0	$ 0
Realized gains (losses) from sale of available-for-sale securities	$ 0	$ 0	$ 0	$ 0
2026 Notes | Convertible Debt
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Debt issued					$ 155,300,000
Debt interest rate					0.75%

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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