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Fair Value Measurements - U.S. Treasury Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 409.1	$ 23.4	$ 122.6
Level 2 \| US Treasury Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term investments	347.5
Gross unrealized losses	(0.5)
Short-term investments, fair value	347.0
US Treasury Securities \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2.2
Cash and cash equivalents, fair value	$ 2.2