Fair Value Measurements - Schedule of investments measured at NAV (Details) - USD ($) $ in Millions |
6 Months Ended | 12 Months Ended | ||||
|---|---|---|---|---|---|---|
Mar. 31, 2024 |
Sep. 30, 2023 |
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| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Unfunded commitments | $ 43.1 | $ 43.1 | ||||
| Nonredeemable investments with unknown liquidation periods | ||||||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Alternative Investment | [1] | 16.4 | 17.4 | |||
| Nonredeemable investments with known liquidation periods | ||||||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Alternative Investment | [1] | $ 36.7 | $ 32.1 | |||
| Liquidation weighted-average period | 2 years 7 months 6 days | 2 years 10 months 24 days | ||||
| Redeemable Investments [Member] | ||||||
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | ||||||
| Alternative Investment | [2] | $ 53.9 | $ 50.2 | |||
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