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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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