Retirement Plans - Summary of Components of Net Pension Income and Summary of Components of Postretirement Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Multiemployer pension withdrawal income	$ (2.0)	$ (1.7)	$ (1.1)	$ (1.7)
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	11.9	10.2	39.2	32.5
Interest cost	49.7	58.8	149.1	174.0
Expected return on plan assets	(90.1)	(85.9)	(271.7)	(254.7)
Amortization of net actuarial loss (gain)	11.4	6.3	35.1	18.7
Amortization of prior service cost (credit)	1.9	1.3	5.4	3.9
Curtailment loss		0.1	0.4	1.0
Company defined benefit plan income	(15.2)	(9.2)	(42.5)	(24.6)
Multiemployer pension withdrawal income	(2.0)	(1.7)	(1.1)	(1.7)
Multiemployer and other plans	0.3	0.2	1.0	0.5
Net pension (income) cost	(16.9)	(10.7)	(42.6)	(25.8)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.3	0.2	1.0	0.8
Interest cost	1.7	1.9	5.3	5.8
Amortization of net actuarial loss (gain)	0.5	(0.7)	0.1	(1.5)
Amortization of prior service cost (credit)	(0.7)	(0.7)	(2.1)	(2.1)
Net pension (income) cost	$ 1.8	$ 0.7	$ 4.3	$ 3.0