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Derivative Financial Instruments and Fair Value Measurements - Schedule of Changes in Fair Value of Level 3 Investment Assets (Details) - Fair Value, Recurring - Credit facility investment including accrued interest $ in Millions	9 Months Ended
	Jun. 30, 2025 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 21.9
Investment Gains/(Losses)	(0.7)
Interest Earned	0.8
Loans	8.0
Collections	(24.6)
Ending Balance	$ 5.4

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Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

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