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Retirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Amortization [Abstract] |  |  |
Amortization of prior service cost (credit) | $ (0.1) |  |
Amortization of net actuarial (gain) loss | (0.5) |  |
Pension Benefits |  |  |
Components of net periodic benefit cost |  |  |
Service cost | $ 37.2 | $ 42.0 | $ 70.9
Interest cost | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Interest cost | $ 146.5 | $ 149.7 | $ 135.6
Expected return on plan assets | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Expected return on plan assets | $ (169.5) | $ (190.6) | $ (230.7)
Amortization [Abstract] |  |  |
Amortization of prior service cost (credit) | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Amortization of prior service cost (credit) | $ 0.0 | $ 0.1 | $ 0.6
Amortization of net actuarial (gain) loss | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Amortization of net actuarial (gain) loss | $ (1.1) | $ (2.6) | $ 59.4
Settlement and curtailment charges | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Settlement and curtailment charges | $ 0.1 | $ 123.4 | $ 38.6
Net periodic benefit cost | 13.2 | 122.0 | 74.4
Other Postretirement Benefits |  |  |
Components of net periodic benefit cost |  |  |
Service cost | $ 0.5 | $ 0.6 | $ 0.8
Interest cost | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Interest cost | $ 2.6 | $ 2.2 | $ 1.3
Expected return on plan assets | $ 0.0 | $ 0.0 | $ 0.0
Amortization [Abstract] |  |  |
Amortization of prior service cost (credit) | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Amortization of prior service cost (credit) | $ 0.0 | $ 0.0 | $ (0.8)
Amortization of net actuarial (gain) loss | Other Income (Expense) | Other Income (Expense) | Other Income (Expense)
Amortization of net actuarial (gain) loss | $ 1.6 | $ 0.5 | $ 0.7
Settlement and curtailment charges | 0.0 | 0.0 | 0.0
Net periodic benefit cost | $ 4.7 | $ 3.3 | $ 2.0

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