Employee Benefit Plans - Schedule of Fair Values of Plan Assets by Investment Category (Details) - Pension benefits - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,361.0	$ 1,438.1	$ 1,987.0
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.5	56.9
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,158.2	1,177.6
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	181.3	203.6
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.5	56.9
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21.5	56.9
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Cash and cash equivalents | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
U.S. government securities and futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63.1	73.8
U.S. government securities and futures | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
U.S. government securities and futures | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63.1	73.8
U.S. government securities and futures | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
U.S. government securities and futures | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Other government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33.1	36.1
Other government securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Other government securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	33.1	36.1
Other government securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Other government securities | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,055.4	1,066.9
Corporate debt instruments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Corporate debt instruments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,055.4	1,066.9
Corporate debt instruments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Corporate debt instruments | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Private equity and hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.4	13.3
Private equity and hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Private equity and hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Private equity and hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Private equity and hedge funds | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4.4	13.3
Collective trust funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	176.9	190.3
Collective trust funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Collective trust funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Collective trust funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Collective trust funds | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	176.9	190.3
Other investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.6	0.8
Other investments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Other investments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6.6	0.8
Other investments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Other investments | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0