Retirement Plans - Summary of Components of Net Pension Income and Summary of Components of Postretirement Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 13.1	$ 10.9
Interest cost	49.9	56.5
Expected return on plan assets	(90.8)	(82.9)
Amortization of net actuarial (gain) loss	11.9	7.0
Amortization of prior service cost (credit)	1.4	1.3
Company defined benefit plan income	(14.5)	(7.2)
Multiemployer and other plans	0.3	0.2
Net pension (income) cost	(14.2)	(7.0)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.4	0.3
Interest cost	1.8	1.9
Amortization of net actuarial (gain) loss	(0.2)	(0.4)
Amortization of prior service cost (credit)	(0.7)	(0.7)
Net pension (income) cost	$ 1.3	$ 1.1