Retirement Benefits - Schedule of Amounts Included in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Millions	Sep. 30, 2024	Sep. 30, 2023
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	$ (153.2)	$ (153.2)
Net actuarial loss	570.8	548.9
Total	417.6	395.7
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	4.4	4.7
Net actuarial loss	9.0	6.7
Total	$ 13.4	$ 11.4

Amount, after tax, of accumulated other comprehensive (income) loss for defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, after tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Amount, after tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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