Retirement Plans - Summary of Components of Net Pension Income and Summary of Components of Postretirement Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 52.6	$ 42.8	$ 44.8
Interest cost	198.6	232.6	204.6
Expected return on plan assets	(362.3)	(340.2)	(328.4)
Amortization of net actuarial loss (gain)	46.8	24.5	21.2
Amortization of prior service cost (credit)	7.5	5.2	4.7
Curtailment loss (gain)	0.4	1.0	(0.6)
Settlement loss (gain)	1.4	(0.2)	(0.5)
Company defined benefit plan (income) cost	(55.0)	(34.3)	(54.2)
Multiemployer and other plans	2.0	1.4	1.4
Net pension (income) cost	(53.0)	(32.9)	(52.8)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1.3	1.2	1.5
Interest cost	6.9	7.7	7.9
Amortization of net actuarial loss (gain)	0.1	(2.0)	0.3
Amortization of prior service cost (credit)	(2.7)	(2.8)	(4.4)
Curtailment loss (gain)	0.0	0.0	(0.1)
Net pension (income) cost	$ 5.6	$ 4.1	$ 5.2