Retirement Plans - Summary of Components of Net Pension (Income) Cost and Summary of Components of Postretirement Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Multiemployer pension withdrawals	$ (1.7)	$ 4.2	$ (1.7)	$ 184.2
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	10.2	9.4	32.5	33.5
Interest cost	58.8	51.5	174.0	153.7
Expected return on plan assets	(85.9)	(82.1)	(254.7)	(246.7)
Amortization of net actuarial loss (gain)	6.3	6.7	18.7	15.9
Amortization of prior service cost (credit)	1.3	1.3	3.9	3.5
Curtailment loss	0.1		1.0
Company defined benefit plan income	(9.2)	(13.2)	(24.6)	(40.1)
Multiemployer pension withdrawals	(1.7)	4.2	(1.7)	184.2
Multiemployer and other plans	0.2	0.2	0.5	1.3
Net pension (income) cost	(10.7)	(8.8)	(25.8)	145.4
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.2	0.4	0.8	1.2
Interest cost	1.9	2.1	5.8	6.1
Amortization of net actuarial loss (gain)	(0.7)	0.3	(1.5)	0.2
Amortization of prior service cost (credit)	(0.7)	(1.1)	(2.1)	(3.2)
Net pension (income) cost	$ 0.7	$ 1.7	$ 3.0	$ 4.3