Pension Benefit Obligations - Asset Categories (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Post-retirement plan
Fair values of plan assets	$ 1,390.7	$ 1,295.8
Cash and cash equivalents
Post-retirement plan
Fair values of plan assets	49.8	50.6
Equity and debt securities
Post-retirement plan
Fair values of plan assets	488.5	442.3
Diversified and equity funds
Post-retirement plan
Fair values of plan assets	81.2	31.5
Fixed income funds
Post-retirement plan
Fair values of plan assets	29.3	36.2
Common collective funds
Post-retirement plan
Fair values of plan assets	734.8	707.5
Derivative instruments
Post-retirement plan
Fair values of plan assets	7.1	27.7
Level 1
Post-retirement plan
Fair values of plan assets	599.1	498.6
Level 1 | Cash and cash equivalents
Post-retirement plan
Fair values of plan assets	25.1	20.2
Level 1 | Equity and debt securities
Post-retirement plan
Fair values of plan assets	488.5	442.3
Level 1 | Diversified and equity funds
Post-retirement plan
Fair values of plan assets	60.5	13.0
Level 1 | Fixed income funds
Post-retirement plan
Fair values of plan assets	25.0	23.1
Level 2
Post-retirement plan
Fair values of plan assets	52.8	86.3
Level 2 | Cash and cash equivalents
Post-retirement plan
Fair values of plan assets	24.7	30.4
Level 2 | Diversified and equity funds
Post-retirement plan
Fair values of plan assets	16.7	15.1
Level 2 | Fixed income funds
Post-retirement plan
Fair values of plan assets	4.3	13.1
Level 2 | Derivative instruments
Post-retirement plan
Fair values of plan assets	7.1	27.7
Level 3
Post-retirement plan
Fair values of plan assets	4.0	3.4	$ 26.8
Level 3 | Diversified and equity funds
Post-retirement plan
Fair values of plan assets	4.0	3.4
Investments measured at NAV
Post-retirement plan
Fair values of plan assets	734.8	707.5
Investments measured at NAV | Common collective funds
Post-retirement plan
Fair values of plan assets	$ 734.8	$ 707.5