FAIR VALUE MEASUREMENTS - Changes in Level 3 Convertible Debt Investments (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Other comprehensive income (loss) before reclassifications	$ (24.8)	$ (96.4)	$ 35.1
Net Unrealized Loss On Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Other comprehensive income (loss) before reclassifications	(34.9)	(102.0)	(3.1)
Fair value, inputs, level 3 \| Fair Value, Measurements, Recurring \| Convertible debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases	0.0	25.0
Total realized / unrealized gains (losses) included in net earnings	(97.6)	3.7
Fair value, inputs, level 3 \| Fair Value, Measurements, Recurring \| Convertible debt investments \| Net Unrealized Loss On Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Other comprehensive income (loss) before reclassifications	66.4	(102.0)
Fair value, inputs, level 3 \| Estimate of Fair Value Measurement \| Fair Value, Measurements, Recurring \| Convertible debt investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of year	117.0	190.3
Fair value at end of year	$ 85.8	$ 117.0	$ 190.3

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount before tax and reclassification adjustments of other comprehensive income (loss) attributable to parent.

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