Employee Benefit Plans - Schedule of Amortization of Prior Service Cost (Credit) Recognized in AOCI (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credits recognized in Accumulated other comprehensive income	$ (0.1)	$ (0.1)	$ (0.1)
Net periodic benefit loss (income)	(20.0)	14.0	(117.8)
Total pre-tax amount recognized in comprehensive loss (income)	(20.1)	13.9	(117.9)
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service credits recognized in Accumulated other comprehensive income	2.2	2.2	11.9
Net periodic benefit loss (income)	(7.6)	(7.1)	(10.4)
Total pre-tax amount recognized in comprehensive loss (income)	$ (5.4)	$ (4.9)	$ 1.5